At the start of the Solar System, before the planets were formed, the Solar System consisted  of the proto-Sun which was surrounded by a disk of gas and dust. This disk is often called  the Solar Nebula or the Protoplanetary Solar Disk. As part of the star formation process,  material from the Solar Nebula would have accreted onto the protoSun via solar magnetosphere. A schematic of the process is shown at right.

Accretional flow onto a young star is usually controlled by the stellar magnetosphere. Due to  pressure gradients in the stellar magnetosphere and a disk-induced toroidal field above the  inner disk, it is possible that some of this accretional inflow is diverted to produce a low  pressure, high temperature gaseous atmosphere above the inner accretion disk. The pressure in  this outer disk atmosphere is mainly dependant on the accretion flow rate onto the star. High accretion flow rates imply relatively high pressures, which decrease over time as the  accretion rate decreases.

In the first hundred thousand years after the formation of the Solar Nebula, gas pressures in  this inner disk atmosphere may have been high enough to produce Refractory Metal Nuggets  (RMNs) - the potential precursors to Calcium Aluminium Inclusions (CAIs). Most RMN formation  probably occurred between 20,000 and 40,000 years after the formation of the solar nebula. As  a consequence, the observed bimodal distribution of 26Al in CAIs may be due to the injection  26Al during the CAI formation period, where 26Al was not present when the first CAIs were  formed. In addition, the observed variation of (26Al/27Al)0 between 3X10-5 and 5X10-5 is  probably due to the heterogeneity of the 26Al-rich material that entered the Solar Nebula.

We are attempting to apply this idea to model the formation of RMNs, CAIs, AOAs (Ameboid  Olivine Aggregates) plus also chondrules. All these objects are found in primitive  meteorites. They are the first rock-like materials to form in the Solar System. The recent  Stardust and Genesis missions provide evidence consistent with the idea that these materials  were formed close to the protoSun and then transported from the inner Solar Nebula to the  outer Solar Nebula.

To be specific, Stardust collected CAIs and chondrules in the dust from Comet Wild 2. As CAIs  and chondrules form at temperatures between 1400K and 2000K these materials could not have  formed at the same location as Comet Wild 2. The Genesis mission has, tentatively, found that  the Sun is rich in 16O. CAIs and chondrules also, to a greater or lesser extent, share this  enrichment. Most other planetary materials are poor in 16O. The results of the Genesis  mission are, thus, also consistent with the idea that these meteoritic materials were formed  close to the protoSun and then were transported to the outer regions of the Solar Nebula.

The mechanism by which this transport occurred is not known. It is possible that turbulent  advection moved some of this material away from the Sun. Another potential transport process  was the ejection of material by bipolar jet flows, where the ejected material may reenter the  Solar Nebula at distances far removed from the protoSun. In this way the Solar Nebula was  transformed, over time, from a disk of gas and interstellar medium dust to a disk of gas and  high-temperature-processed, macroscopic particles that became the foundation stones for the  planets.

Liffman, K. and Brown, M. 1995 The motion and size sorting of particles ejected from a protostellar accretion disk. Icarus, 116, p. 275-290
Scott, E. R. D., 2007 Chondrites and the Protoplanetary Disk, Annual Review of Earth and Planetary Sciences, 35, p.577-620
Liffman, K., 2009 A Shocking Solar Nebula? The Astrophysical Journal Letters, 694, pp. L41-L44
Berg, T., Maul, J., Schönhense, G., Marosits, E., Hoppe, P., Ott, U., Palme, H., 2009 Direct Evidence for Condensation in the Early Solar System and Implications for Nebular Cooling Rates. The Astrophysical Journal Letters, 702, L172-L176
Genesis Mission: http://genesismission.jpl.nasa.gov/
Stardust Mission: http://stardust.jpl.nasa.gov/home/index.html

It is now recognized that the bulk of the elements beyond the iron peak were formed primarily by two neutron-capture processes, the s-process and the r-process.  Progress on pinning down the astrophysical sites  that provide the dominant contributions from these processes has been hampered, in the past, by the dearth of stars known that exhibit essentially “pure” s- or r-process abundance patterns, and hence are most directly related to the underlying nuclear physics.  Fortunately, during the past 25 years, ever larger surveys to discover very metal-poor stars (VMP;  [Fe/H] < -2.0) in the Galaxy have now provided sufficient numbers of targets that, with dedicated high-resolution spectroscopic follow-up, this limitation can be lifted.  The HK survey of Beers and colleagues (Beers et al. 1985, 1992) and the Hamburg/ESO survey of Christlieb and collaborators (Christlieb 2003) brought the numbers of VMP stars to over 3000; the SDSS/SEGUE survey (Yanny et al. 2009) has expanded this total  to over 30000 stars.  Among these stars, follow-up high-resolution studies have identified classes of stars that are enhanced in their r-process element abundance ratios by factors of from 3-10 (r-I stars) and > 10 (r-II stars).  The nature of the r-II stars, in particular, will enable detailed understanding of both the origin of the r-process itself as well as the likely astrophysical site(s) associated with its origin.
Already, it is apparent that the r-II stars are grouped in metallicity into a region close to [Fe/H]  = -3.0, while the r-I stars cover a much wider range in metallicity,  -3.0 < [Fe/H] < -1.5 (Barklem et al. 2005).  This may be evidence that, if the r-process originates from the explosion of massive stars, the stars responsible might possess a rather restricted range of mass, e.g., 10-12 Mo.  The metallicity [Fe/H] = -3.0 may correspond to the “chemical time” at which stars of this mass range first appeared in the Galaxy.  A few of the r-II stars present detectable lines of the radioactive species U and Th, from which direct cosmo-chronometric age limits on the age of the Galaxy and the Universe can be derived (Beers & Christlieb 2005; Frebel  & Norris 2011).  Other important results from high-resolution studies to date include the fact that about 1/3rd of the r-II stars exhibit abundances of the actinides, including Th, that are too high to be commensurate with their expected old ages.  Presumably, this is nature telling us that our understanding of the production of the very heaviest elements is still incomplete.    Other interesting recent discoveries include the possible identification of a highly eccentric orbit for the r-II star HE 2327-5642 (Mashonkina et al. 2010), and the discovery of a main-sequence r-II dwarf (Aoki et al. 2010), which eliminates the possibility that the r-process-enhancement phenomenon is an artifact created by peculiarities in the atmospheres of metal-poor giants (all previously known r-II stars were giants).
Workshop Presentation (pdf)

There are presently ~15 known r-II stars, and ~50 known r-I stars, however, these numbers are sure to increase rapidly in the near future, as high-resolution spectroscopic  follow-up of VMP stars identified by SDSS/SEGUE are carried out.  Identification of perhaps an additional 15-30 r-II stars is required for progress to be made with interpretations of the actinide boost phenomenon, and we as to identify more stars with detectable U and Th for cosmo-chronometric studies.  A dedicated program with 6.5m-10m class telescopes , for instance, could target on the order of 600 stars with [Fe/H] < -2.5, and given the known frequency of r-II stars (~5% of stars below [Fe/H] = -2.0), should yield the required numbers.  In addition, high precision radial velocity monitoring of ALL known r-II stars, initiated soon, and lasting a minimum of 5 years (10 is better), is required in order to assess the binary frequency of such objects, and to characterize the nature of their orbits, so one might test the idea that  SNe II are the primary astrophysical origin of the r-process.

Aoki, W., Beers, T.C., Honda, S., & Carollo, D. 2010, Extreme Enhancements of r-process Elements in the Cool Metal-poor Main-sequence Star SDSS J2357-0052, Astrophysical Journal, 723, L201
Barklem, P.S., et al., 2005, The Hamburg/ESO R-process Enhanced Star Survey (HERES). II. Spectroscopic Analysis of the Survey Sample, Astronomy & Astrophysics, 439, 129
Beers, T.C., & Christlieb, N. 2005, The Discovery and Analysis of Very Metal-Poor Stars in the Galaxy, Annual Reviews of Astronomy & Astrophysics, 43, 531
Beers, T.C., Preston, G.W., & Shectman, S.A.  1985, A Search for Stars of Very Low Metal Abundance.  I., Astronomical Journal, 90, 2089
Beers, T.C., Preston, G.W., & Shectman, S.A.  1992, A Search for Stars of Very Low Metal Abundance.  II., Astronomical Journal, 103, 1987
Christlieb, N. 2003, Finding the Most Metal-poor Stars of the Galactic Halo with the Hamburg/ESO Objective-prism Survey,  Reviews of Modern Astronomy, 16, 191
Frebel, A., & Norris, J.E.,  Metal-Poor Stars and the Chemical Enrichment of the Universe, in Planets, Stars, and Stellar Systems, Vol. 5, arXiv:1102.1748
Mashonkina, L., Christlieb, N., Barklem, P.S., Hill, V., Beers, T.C., & Velichko, A. 2010, The Hamburg/ESO R-process Enhanced Star Survey (HERES). V. Detailed Abundance Analysis of the r-process Enhanced Star HE 2327-5642, Astronomy & Astrophysics, 516, 46
Yanny, B. et al. 2009, SEGUE: A Spectroscopic Survey of 240,000 Stars with g = 14-20, Astronomical Journal, 137, 4377

Models of primordial and hyper-metal-poor stars that have masses similar to the Sun experience an ingestion of protons into the hot core during the core helium flash phase at the end of their red giant branch evolution. This produces a concurrent secondary flash powered by hydrogen burning that gives rise to further nucleosynthesis in the core. To explore this nucleosynthesis we have performed post-process calculations on a stellar evolution model of a star with mass 1 M_sun and a metallicity of [Fe/H] = -6.5 that suffers a proton ingestion episode.
We find that the mixing and burning of protons into the hot convective core leads to the production of 13C, which then burns via the 13C(a,n)16O reaction, releasing a large number of free neutrons. This gives rise to a prodigious production of s-process elements such as Sr, Ba, and Pb. These nucleosynthetic products are later carried to the stellar surface and ejected via stellar winds.
We compare our nuclesynthesis results with observations of the hyper-metal-poor halo star HE 1327-2326, which shows a strong Sr overabundance. Our model provides the possibility of self-consistently explaining the Sr overabundance in HE 1327-2326 together with its C, N, and O overabundances (all within a factor of ∼ 4) if the material were heavily diluted, for example, via mass transfer in a wide binary system. The model produces at least 18 times too much Ba than observed, but this may be within the large modelling uncertainties.
In this scenario, binary systems of low mass must have formed in the early Universe. If true then this puts constraints on the primordial initial mass function. Furthermore, if this proton ingestion event is the source of the elemental pattern in these stars then it implies that low-mass primordial stars must have formed, as also suggested by recent star formation simulations (eg. Greif at al. 2011).
Workshop Presentation (pdf)

Although our model produces too much Ba this is probably within the model uncertainties. Since the DCF scenario has the potential to self-consistently reproduce most of the strange overabundances in hyper-metal-poor stars we suggest that it warrants further investigation. This would be best done in the framework of multidimensional stellar models, something which is now becoming feasible (Deupree 1996; Dearborn et al. 2006; Mocak et al. 2010; Herwig et al. 2010).
Our model underproduces Na, Mg, Al, Ca, Ti, and Ni. This is suggestive that these elements came with Fe from an early supernova that polluted the protostellar cloud (eg. Joggerst et al. 2010). We note however that Na is underproduced in these models also, thus the uncertainties of the Ne and Na proton-capture reactions should be investigated.
Lithium is another mismatch since the DCF lowers the surface Li abundance by only a factor of two while HE 1327-2326 is heavily depleted in Li. This problem needs to be addressed in terms of mixing processes on the secondary star HE 1327-2326.

Campbell, S. W. & Lattanzio, J. C. 2008, A&A, 490, 769
Campbell, S. W., Lugaro, M., Karakas, A.I., 2010, A&A, 522, 6
Dearborn, D.S.P., Lattanzio, J.C., & Eggleton, P.P. 2006, ApJ, 639, 405
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From quiescent hydrogen burning to violent explosions, nuclear reactions provide the energy that stabilizes stars from gravitational collapse and they provide the mechanism that leads to stellar evolution and the production of elements in the universe. During violent stellar explosions, nuclei that produce characteristic gamma rays are produced. Some of these nuclei have long lifetimes are produced in sufficient quantities to be observed by space-based gamma-ray observatories. The data from these observations provide additional constraints on stellar models and thus help guide our understanding of the underlying processes that are involved in stellar evolution.
Nuclear reaction rates are one of the key inputs to stellar models. Violent processes that lead to gamma-ray emitting isotopes require knowledge of reaction rates that often involve capture reactions on radioactive ions. These rates are very difficult, or in many cases impossible, to measure by the direct techniques that have been developed for capture-reaction rate studies on stable nuclei. Over the past couple of decades, new indirect techniques have been developed to determine these rates. One of these techniques—the measurement of Asymptotic Normalization Coefficients (ANCs)—is discussed in the presentation on ¹⁸F and ²²Na production and destruction rates. ANCs are used to determine reaction rates for proton capture on radioactive ions that are important for both the production of ¹⁸F and the destruction of ²²Na, two nuclei that are candidates for observation of their decay through gamma-ray observatories. Another indirect technique that we have used for this work is the measurement of beta-delayed protons. The proton decays that occur near threshold provide information on resonant states that are important in the proton capture. As an example, proton decays from the nucleus ²³Mg can be used to obtain information about resonance reactions rates for the ²²Na(p,g)²³Mg reaction.
Reaction rates relevant for both systems are found based upon recent measurements of ANCs and beta-delayed protons. These rates must be included in the reaction networks that are being used in stellar models to provide up to date estimates of the yield of the two nuclei that are expected during hydrogen burning in collapsing star systems or rapid proton capture in accretion disks that form in binaries, where one of the members of the binary is a compact object such as a neutron star.
Workshop Presentation (pdf)

A major thrust in low-energy nuclear physics around the world is the development of intense radioactive ion beams. A new generation facility to produce these beams is now operating at RIKEN in Japan. Other facilities are under construction or are in the advanced stages of planning including new facilities in Germany, South Korea, and the United States. In the near future, upgrades of existing facilities will provide additional access to radioactive beams. One such upgrade is nearing completion at the Cyclotron Institute at Texas A&M University. With the availability of more intense radioactive beams, nuclear physicists will be able to extend the measurements of reaction rates involving particle capture on radioactive ions. Thus by the end of this decade, we should be able to provide measured reaction rates for many charged particle capture reactions that occur in novae, supernovae, and X-ray bursts.

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The s- and r-processes are commonly ascribed to separate astrophysical sites (Burbidge et al. 1957). The nucleosynthesis of the s-process occurs in stars of low mass (from 1.3 to 4 solar masses) during their thermally pulsing asymptotic giant branch (TP-AGB) phase. The main neutron source is the 13C(a, n)16O, which burns radiatively at a temperature of about 0.9 x 10⁸ K during the interpulse period in the region between the H- and He-shell (He-intershell). A second neutron source, 22Ne(a, n)25Mg, is marginally activated at the bottom of the recurrent convective thermal instability (thermal pulse, TP) in the He-intershell, mainly affecting the abundance at the branching points that are sensitive to temperature and neutron density. The s-process elements are mixed with the surface during the third dredge-up (TDU) episodes, in which the convective envelope engulfs part of the He-intershell, after the quenching of a TP. We refer to the review by Busso et al. (1999) for major details on the AGB nucleosynthesis. Instead, the physical environment for the r-process is still unknown, although SNII are the most promising candidates.

A quite large number of carbon and s-process enhanced metal-poor (CEMP-s) stars have been detected. CEMP-s are main-sequence/turnoff or giants of low mass (M < 0.9 solar masses). The most plausible explanation for their peculiar high s-element abundances is mass transfer by stellar winds from the most massive AGB companion (now a white dwarf).

Similarly to 99Tc (99Tc/99Ru) (Merrill 1952) observed in intrinsic spectral types MS, S, SC, or C(N) stars (Lambert 1985; Smith & Lambert 1990; Plez, Smith, & Lambert 1993), spectroscopic detection of [Zr/Nb] provides a confirmation of binary systems. 93Zr is a long-lived isotope with half life 1.5 x 106 yr produced by the s process. The presence of high Nb (from 93Zr decay) in the envelope of a CEMP-s star indicated that the s process nucleosynthesis is not more present in the star (Wallerstein & Dominy 1988). Another possible "binarity" indicator is the 205Pb-205Tl ratio. 205Pb is long-lived isotope with 1.7 x 107 yr produced by the s-process. Unfortunately Tl observations are not available. 

About half of these CEMP-s stars are also highly enhanced in r-process elements (CEMP-s/r). The observed r-enhancement in these stars reflects the observations of unevolved Galactic stars at low metallicity. CEMP-s/r stars show abundance patterns incompatible with a pure s-process nucleosynthesis. We suggest that the molecular cloud from which the binary system formed was already enriched in r-process elements by local pollution of SNII ejecta (Sneden, Cowan & Gallino 2008; Bisterzo & Gallino 2010). This hypothesis is supported by numerical simulations by Vanhala & Cameron 1998, who found that SNII explosion in a molecular cloud may trigger the formation of binary systems. These simulations may explain the very high fraction of CEMP-s/r (about 50\%) among the CEMP-s. The initial r-enrichment does not affect the s-process nucleosynthesis. However, the s-process indicators [hs/ls] (where ls is defined as the average of Y and Zr; hs as the average of La, Nd, Sm) and [Pb/hs] may depend on the initial r-enhancement. For instance, the hs-peak has to account of an r-process contribution estimated to be 30% for solar La,  40% for solar Nd, and 70% for solar Sm. A large spread of [Eu/Fe] is observed in unevolved halo stars up to [Eu/Fe] about 2 dex. In presence of a very high initial r-enrichment of the molecular cloud, the maximum [hs/Fe] predicted in CEMP-s/r stars may increase up to 0.3 dex.  Instead, the spread of [Y,Zr/Fe] observed in unevolved halo stars reaches a maximum of only about 0.5 dex,  not affecting much the predicted [ls/Fe].

The theoretical interpretation of two CEMP-s stars recently analysed is presented here as example:  BD +04 2466 (Pereira & Drake 2009; Zhang et al. 2009, Ishigaki et al. 2010; Fig.1), CS 29513-032 (Roederer  et al. 2010; Fig.2). The AGB models adopted have been presented in Bisterzo et al. (2010).
 
Among CEMP-s/r, we recall here the main-sequence star CS 29497-030 by Ivans et al. (2005), for which a [Zr/Nb] close to 0 is detected, in agreement with a binary system.

Workshop Presentation (pdf)

The study of chemical composition of CEMP-s and CEMP-s/r stars is fundamental in order to test nucleosynthesis and evolutionary models. Continuous improvements of AGB models of different masses and metallicities provide a complete analysis of the s-process, from CEMP-s stars up to disc metallicities.

We present here a preliminary analysis of a comparison between AGB theoretical predictions and spectroscopic observations of CEMP-s and CEMP-s/r stars. A detailed discussion will be presented in Bisterzo et al., MNRAS, submitted.

Burbidge, E. M., Burbidge, G. R., Fowler, W. A., Hoyle, F., 1957, Rev. Mod. Phys., 29, 547
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Bisterzo, S., Gallino, R., Straniero, O., Cristallo, S., Kaeppeler F., 2010, MNRAS, 404, 1529
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Ivans, I. I., Sneden, C., Gallino, R., Cowan, J. J., Preston, G. W., 2005, ApJ, 627, 145
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One of the surprising results of recent survey work searching for metal-poor stars is that below [Fe/H]=-2 around 20% of stars are rich in carbon, with [C/Fe]>+1. These carbon-enhanced, metal-poor (CEMP) stars can be divided into groups based on their heavy element abundances. Four classes are commonly recognised based on the abundances of barium (thought to represent the s-process) and europium (thought to represent the r-process). Here we shall only concern ourselves with two: the CEMP-s and -r/s classes. The former show enhanced levels of barium, but little enhancement of europium. They are all believed to be binary systems and the most probably scenario for their formation is wind mass transfer (of material containing both carbon and s-process elements) from an asymptotic giant branch (AGB) star. The CEMP-r/s stars show enhanced levels of both barium and europium, and the origin of these stars is something of a mystery. Many scenarios for their formation have been proposed, including: AGB mass transfer in a system that was pre-polluted by a supernova; pollution by a massive AGB star that subsequently exploded as a supernova; accretion induced collapse.
In this work, we examine those scenarios involving AGB stars. We have evolved stellar models in the mass range of 1-6 solar masses at a metallicity of [Fe/H]=-2.3. We compare models computed with two different stellar evolution codes, and determine the effects of different initial compositions on the final s-process signature. In addition, we also vary the extent of the partially mixed zone  that gives rise to the carbon-13 pocket -- the source of neutrons for the s-process and determine its effects on the nucleosynthetic signature. Finally, we compare these models to the observed heavy element abundances of carbon-enhanced metal-poor stars.
Our AGB models do an excellent job of reproducing the [Ba/Fe] and [Eu/Fe] values observed in the CEMP-s stars. For a fixed mass of the partially mixed zone, we find little sensitivity in stellar mass: our models pass through the appropriate range of values (for both evolution codes). Note that we have not taken into account any possible dilution of material into the companion star after mass transfer has taken place. While some of the models are able to reach the location of the less enriched CEMP-r/s stars in [Ba/Fe]-[Eu/Fe] space, they cannot account for the whole range of values and so it is unlikely these objects have come from `normal' AGB stars. Our massive AGB models (evolved without a partially mixed zone) do not produce significant amounts of s-process elements via the neon-22 source and are therefore unlikely to be the progenitors of the CEMP-rs objects (also, they undergo hot bottom burning and so do not produce C-rich ejecta). Finally, we demonstrate that the pre-pollution scenario does not explain the CEMP-r/s objects either. Even with an initial r-process pre-enrichment of +1 dex, the models produce similar values for [Ba/Fe] and [Eu/Fe] to the non-enriched models.
Workshop Presentation (pdf)

In addition to the above comparisons with the observed Ba and Eu abundances of CEMP stars, we plan to compare the models to other observed quantities. These will include the light s-processes isotopes and lead. With these additional constraints we hope to get a more accurate picture of which systems can and cannot be explained by the AGB mass transfer scenario.

The monoisotopic element Nb is produced mostly by the slow neutron-capture process (the s process) via the decay of the long-living radioactive nucleus 93Zr (half life of 1.5 million years), which is on the main path of the s process. We model the s process in asymptotic giant branch (AGB) stars with a nucleosynthesis network of 320 nuclear species from H to Bi and 2,336 nuclear reactions with rates from the USA Joint Institute for Nuclear Astrophysics (JINA) database. Neutron-capture cross sections along the s-process path are fundamental input physics for the models.

The Zr neutron-capture cross sections, including that of 93Zr, have been recently remeasured at the n_TOF (neutron time of flight) n_TOF facility at CERN (see, e.g., Tagliente et al. 2010). The preliminary n_TOF estimate for 93Zr(n,gamma)94Zr is 43% lower than given by the pioneering experiment by Macklin (1985), and outside the previously given 20% 2sigma error bar. The n_TOF experiment allows for a strong reduction of the background induced by neutrons scattered by the sample and captured in the materials constituting the experimental setup. For 93Zr it was possible to extract n_TOF information only below 8 keV of incident neutron energy. To complement n_TOF data at higher energies the evaluation given by the JENDL calculations at high energy was scaled by the same factor extracted in the n_TOF measured range. A lower 93Zr(n,g) value means that more 93Zr is produced. After radiogenic decay of 93Zr
more Nb will result. The final result is ~50% more Nb.

Kashiv et al. (2010) used synchrotron X-ray fluorescence to measure the abundances of Zr and Nb in single stardust silicon carbide (SiC) grains from AGB stars. Chemically, Zr and Nb are very similar: they are both very refractory elements. This means that they condense from gas into solid at high temperatures. So, they should be both present in SiC. Since the timescale for dust condensation around AGB stars is less than a million year, 93Zr was still alive at the time the grains condensed in the source AGB stars, so most Nb in SiC would have derived from the decay of 93Zr inside the grains. As compared to AGB s-process predictions, the grains confirm that all 93Zr produced in AGB stars turned into Nb in the grains. But, using the older 93Zr neutron-capture cross section the data are generally to the left of the prediction lines. With the new 93Zr(n,g) cross section the prediction lines go right through the data. The importance of the new nuclear experimental data is emphasised because the Nb/Zr ratio predicted by the s process depends only on the the neutron-capture cross sections, and is virtually independent of the stellar models and its uncertainties.

Workshop Presentation (pdf)

1) The preliminary n_TOF 93Zr(n,g) cross section used here needs to be confirmed.

2) There are still a few grains that are outside 2 sigma of the model predictions. Those grains with low Nb and Zr abundances are likely explained by selective contamination of solar system Nb. Selective removal from the gas of 80% of Zr or of 90% of Nb has to be invoked to explain another two grains. We need to understand if this feasible.

3) AGB stars do not produce Zr and Nb in solar proportions, but they make more Nb than Zr, relatively to solar. Hence, a prediction arises to be verified: that the Light Elements Primary Process (LEPP, e.g. Montes et al. 2007), should make more Zr than Nb, relatively to solar.

Tagliente, G. et al. 2010, Phys. Rev. C., 81, id.055801
Macklin, R. 1985, Astrophys. Space Sci. 115, 71
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The abundance of most nuclei heavier than Fe are a result of neutron-capture processes occurring in stars. The s- (slow) neutron-capture process operates under typical conditions of low neutron densities (~10⁷ neutrons cm^-3) for unstable nuclei decay. Unstable nuclei with long half-lives, under certain conditions of neutron density and temperature, can capture a neutron and not decay so that branching points open on the s-process path. The s-process takes place in the deep He-rich layers of Asymptotic Giant Branch (AGB) stars and is responsible for the production of about half of the solar abundances of tungsten (W) and hafnium (Hf). These elements have been found in trace amounts in stardust silicate-carbide (SiC) grains from AGB stars. Their isotopic ratios show the signature of the s-process and can be used to test s-process stellar models and better our understanding of neutron production in AGB stars. Of particular interest is the s-process path from Hf to Os (slide 3) where there are some reported problems in the production of the solar W and Os isotopes. The r-residual of 182W is well above the r-residual smooth line, which means that more 182W should be made by the s-process. Theoretical models predict a significant overproduction of 186Os s-process abundance with respect to its solar abundance, which is not allowed because 186Os is an s-only isotope. These abundances may reflect uncertainties related to neutron-capture reaction and beta decay rates. Several branching points on the s-process path at 182Ta, 183Ta and 185W, affect the isotopic abundance of W.

Slides 9, 10 and 11 present SiC data compared with s-process predictions (Avila et al. 2011 in prep.) model predictions. Plots were made using the FRANEC code and the Monash code. The symbols are plotted when C>O. The black squares are the SiC-enriched sample and the white squares are single SiC grains.  The grey dashed lines give the best linear fit through all grains. The black solid lines give the best linear fit through the SiC-enriched grains. Black dotted lines are the solar values. We find that the models produce less 182W and 183W than expected by simply taking the inverse ratio of the neutron-capture cross section (the red dot on the graphs) because the branching point at 182Ta and 183Ta have been activated (slide 9). The 4 solar mass model is able to reach the same height as the grain LU-41 (slide 10), and in slide 11 the predicted Hf isotopic ratios correspond to the inverse ratio of the neutron-capture cross section.

We performed some tests by varying the neutron capture cross section of the W isotopes (slides 12 and 13). Multiplying the 182W(n,γ)183W cross-section by 30% as suggested by Vockenhuber et al. (2007) to produce a smaller r-residual made our comparison with the grains much worse. Multiplying the 185W(n,γ)186W cross-section by a factor of 2 did not help reaching the 186W/184W measure in grain LU-41. Note that Sonnabend et al. (2003) states that the 186Os abundance can be reproduced by the stellar model by increasing the 185W(n,γ) 186W cross-section by 60%.

Further testing of the effect neutron capture reaction rates is needed. We also need to perform testing of the main uncertainty in the AGB s-process models: the size of the region where the main neutron source 13C is expected to form at the end of each third dredge-up episode via proton diffusion from the envelope in the intershell. The mechanism by which this diffusion occurs is still a matter of debate and hence the features of this 13C pocket are still uncertain.

Avila, J., Ireland, T., Holden, P., Gyngard, F., Bennett, V., Amari, S., & Zinner, E. 2008 ‘Tungsten isotopic compositions in presolar silicon carbide grains', Meteoritics and Planetary Science Supplement, 43, 5120-+.
Cristallo, S., Straniero, O., Gallino, R., Piersanti, L., Dominguez, I., & Lederer, M.T. 2010, Evolution, Nucleosynthesis, and Yields of Low-Mass Asymptotic Giant Branch Stars at Different Metallicities, ApJ, 696, 797.
Marganiec, J., Dilmann, I., Dmingo, Pardo C., & Käppeler, F. 2009 'Neutron capture cross sections of W184 and W186', Physical Review C, 80, 2.
Sonnabend, K., Mengoni, A., Mohr, P., Rauscher, T., Vogt, K., & Zilges, A. 2003 'Determination of the (n, γ) reaction rate of unstable 185W in the astrophysical s-process via its inverse reaction', Nuclear Physics A, 718, 533-535.
Wisshak, K., Voss, F., Käppeler, F., Kazakov, L., Bečvář, F., Krtička, M., Gallino, R., & Pignatari, M. 2006 'Fast neutron capture on the Hf isotopes: Cross sections, isomer production, and stellar aspects', Physical Review C, 73, 4.

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In collaboration with: K. Buczak, I. Dillmann, T. Faestermann, J. Feige, F. Käppeler, K. Knie, G. Korschinek, C. Lederer, A. Mengoni, S. Merchel, U.Ott, M. Paul, M. Poutivtsev, G. Rugel, P. Steier.

A key ingredient to our understanding of nucleosynthesis is the accurate knowledge of cross-section data. For specific reactions the sensitivity of AMS offers a unique tool to pin down uncertainties, thus elucidating current open questions e.g. within the s- and p-process path. Measurements of neutron- and charged particle induced cross sections have become a main research topic at the VERA (Vienna Environmental Research Accelerator) facility (Wallner 2010): a series of samples was irradiated at Karlsruhe Institute of Technology with neutrons closely resembling a Maxwell-Boltzmann distribution of 25 keV, and also with quasi-monoenergetic neutrons of energies up to 500 keV. After neutron activation the amount of some radionuclides was quantified by AMS. New results have been obtained for the neutron capture reactions of 13C(n,γ)14C and 14N(n,p)14C; both acting as neutron poisons in s-process nucleosynthesis, while 14N(n,p) also serves as a proton donator, leading to a delayed neutron recycling, and it affects the production of 19F as well. New precise data were obtained for 13C(n,γ) for 25 keV, and first experimental results for 130 and 170 keV neutron energy, where a strong resonance is predicted. The reaction 14N(n,p) was measured for neutron energies between 25 and 180 keV. The new precise results obtained with AMS on the reaction product 14C, result in slightly lower values than most previous experimental results indicate. However, they do not confirm the only previous measurement with substantially lower cross section in keV energy range (see Fig.).
Recent data from a series of AMS results obtained for (n,γ) reactions in the mass range between Be and Fe, allow a systematic comparison of AMS results with those from other independent techniques and as such allow to address systematic uncertainties in the measurements.
AMS allows also to search for minute traces of live extraterrestrial radionuclides which might have been produced in the late stages of stellar evolution or in the event of a supernova (SN)-explosion: Dust formed in the ejecta of a SN contains freshly produced long-lived radionuclides, and – attached to dust particles – might be deposited live in terrestrial archives. The search for other SN-radionuclides like 26Al, 182Hf, 244Pu and 247Cm will complement the 60Fe discovery at TU Munich (Knie et al. 2004) and will provide direct clues on the nucleosynthesis of massive stars. A detailed search for live 244Pu in the very same crust as used for 60Fe detection has been performed at the VERA laboratory. This facility offered a ten times higher overall detection efficiency compared to previous searches. In combination with ten times more material available, a two orders of magnitude higher sensitivity for detection of 244Pu was given for this project; compared to previous work, where e.g. one count of 244Pu has been registered in a manganese crust sample (C. Wallner 2004). This experimental result and also theoretical considerations from r-process yields combined with extrapolations of U concentrations measured in interplanetary dust, suggest a clear 244Pu signal in these new AMS measurements. Interestingly, preliminary results indicate a much lower than expected 244Pu count rate.

The search for supernova-produced radionuclides has been continued and also extended within the European Eurogenesis Research Programme ‘Cosmic Dust as a Diagnostic for Massive Stars’ (CoDustMas), through the subproject “SUPRATEAMS – supernova-produced radionuclides and trace elements studies by AMS” (Wallner 2011). It comprises the laboratory study of cosmic dust via accelerator mass spectrometry (AMS) measurements at the Vienna Environmental Research Accelerator (VERA) of the University of Vienna in collaboration with TU Munich, ETH Zurich, MPI Mainz, Hebrew University, Jerusalem and others. Moreover, this project tackles two aspects:
(1) the measurement of trace element isotope ratios in presolar nanodiamonds isolated from meteorites (stardust), e.g. isotope ratios of Pt isotopes to extract r-process nucleosynthesis signatures (Ott 2011, Wallner 2011); and (2) the search for live supernova (SN)-produced radionuclides in terrestrial deep-sea archives, more specifically search for the isotopes of e.g. 26Al, 182Hf, 244Pu and 247Cm.
Ultra-sensitive techniques such as accelerator mass spectrometry (AMS) allow the analysis of rare trace elements in presolar dust through isotope ratio measurements being free of molecular isobaric background. This project aims at better understanding of nucleosynthesis and mixing in supernovae through studies of nanodiamonds isolated from meteorites (in cooperation with MPI Mainz, Germany (Ott 2011)) with state-of-the-art AMS techniques. Such laboratory studies will assess the isotopic signature of SN dust and its chemical nature.
In addition, as dust formed in the ejecta of a SN contains freshly produced long-lived radionuclides, there might be a chance of finding such radionuclides live in dust particles deposited into terrestrial archives. Stellar nucleosynthesis processes and SN dust formation and its transport into the solar system can be traced through the search for live radionuclides in terrestrial archives using AMS. Similar to the analysis of stable isotopes in presolar grains, AMS techniques at VERA will be exploited in the search for minute traces of such long-lived isotopes. An example is the recent discovery of live 60Fe on the Pacific ocean floor showing that isotopes produced in SN explosions were able to find their way to Earth (see above). The search for other SN-radionuclides like 26Al, 182Hf, 244Pu and 247Cm will complement the 60Fe discovery at TU Munich and will provide direct clues on the nucleosynthesis of massive stars.
In a parallel effort we pursue laboratory studies of nucleosynthesis via AMS. Here we aim at measurements of nuclear reactions by analysing the product isotopes via AMS for studying s-process related neutron-capture reactions, and charged-particle induced reactions of relevance to the p-process. The feasibility of reactions will be explored where extremely low cross sections might be accessible. In a few cases AMS provides a sensitive tool for such low cross sections, which should help to tune and test model calculations.

Knie 2004: K. Knie, G. Korschinek, T. Faestermann, E. A. Dorfi, G. Rugel, and A. Wallner: “60Fe Anomaly in a Deep-Sea Manganese Crust and Implications for a Nearby Supernova Source”, Phys. Rev. Lett. 93 (2004) 171103
C. Wallner 2004: C. Wallner, T. Faestermann, U. Gerstmann, K. Knie, G. Korschinek, C. Lierse and G. Rugel, Supernova produced and anthropogenic 244Pu in deep-sea manganese encrustations, New Astron. Rev. 48 (2004) 145.
Wallner 2010: A. Wallner, Nuclear Astrophysics and AMS – probing nucleosynthesis in the lab, Nucl. Instr. and Meth. B 268 (2010) 1277.
Ott 2011: U. Ott et al., New attempts to understand nanodiamond stardust, these proceedings.
Wallner 2011: A. Wallner et al., Nuclear astrophysics and nuclear physics programme at VERA, submitted, Nucl. Instr. and Meth. B (2011); see also the EuroGenesis Web Page

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Understanding stardust nanodiamonds present in primitive meteorites has not progressed as much as understanding other types of stardust, e.g. silicon carbide and oxide grains. A problem is the small size (average ~2.6 nm), which except for maybe carbon does not permit useful single grain isotopic analysis. “Bulk” samples (i.e. millions of grains) yield 12C/13C ratios within the range of solar system materials, and also nitrogen isotopes resemble the solar 14N/15N ratio as newly re-defined by analyses of solar wind implanted in target materials of the Genesis mission. Hence, although nominally more abundant than any other known type of stardust mineral (except maybe the silicates), we cannot be certain what abundance fraction of the diamonds is true stardust.
Diagnostic isotopic features are present in several trace elements and suggest a connection to supernovae. This includes xenon-HL (enhancements in p- and r- isotopes), krypton-H (heavy isotopes enhanced) and tellurium (enhancement of the r-only isotopes). Processes suggested to account for the r-isotope (H) enhancements include a neutron burst (Meyer et al. 2000), but also a “regular” r-process augmented by an “early” separation between stable end products and radioactive precursors (Ott 1996). While the latter provides a formally better match to the observed Xe and Te isotopic patterns and in principle is also applicable to Kr and to Xe-L, it lacks a credible setting for the separation process to occur.
In a concerted effort we are looking – using secondary ion microprobe mass spectrometry (SIMS) - for the decay products (26Mg, 44Ca) of radioactivities (26Al, 44Ti) diagnostic for type II SN and – using accelerator mass spectrometry (AMS) – for further diagnostic features in trace elements. We also explore whether variations in the high entropy wind (HEW) scenario for the standard r-process may result in the observed unusual enhancements of r-isotopes by the r-process proper.
Results obtained so far indicate: a) there is no evidence for the former presence of now extinct 26Al and 44Ti in our diamond samples other than what can be attributed to SiC “impurities” (Besmehn et al. 2011); b) analysis by AMS of platinum in “bulk diamond” yields an overabundance of r-only 198Pt (Ott et al. 2010; Fig. 1) that is more consistent with the neutron burst than with the separation model; c) if the Xe-H pattern was directly established by an r-process, it must have been a very strong variant of the main r-process (Ott et al. 2009).
The Figure shows a three-isotope plot for platinum measured by AMS in “bulk samples” of nanodiamonds from the Allende meteorite. Ratios are shown as δ-values, i.e. deviations from normal ratios in per mill. The data indicate an overabundance of r-only 198Pt, but without the deficit in 194Pt expected if “average r-process” platinum had been added.

Workshop Presentation (pdf)

1)    Since the fraction of true diamond that is true stardust is not known, it would be useful to set more stringent limits on the possible abundance of radioactivities 26Al and 44Ti. This may be achieved by the analysis of purer diamond separates.
2)    The AMS data for platinum need to be confirmed. AMS measurements will also be performed on Rare Earth elements as well as uranium to search for diagnostic features there.
3)    Detailed model calculations for r-process nucleosynthesis via the HEW scenario will be performed using updated nuclear physics. The effect of variations of the input parameters electron fraction will be studied as well and the dependence on the range of entropies included.

Meyer, B.S. et al. 2000, Astrophys. J. Lett., 540, L49
Ott, U. 1996, Astrophys. J., 463, 344
Besmehn, A. et al. 2011, subm. to Meteoritics and Planetary Science
Ott U. et al. 2010, Meteoritics and Planetary Science, 45, A159
Ott U et al., 2009, Meteoritics and Planetary Science, 44, A162

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Historically it has been assumed that the necessary condition for carbon solids to grow in a cooling gas is higher bulk C abundance than O abundance. That condition is observed to be necessary and valid in AGB red-giant stars, inducing more general acceptance of that requirement. But for supernovae, that bulk-abundance condition is too restrictive. Within their expanding and cooling interiors, radioactivity maintains a prolific source for new free C. Very abundant CO molecules can not retain oxidized carbon owing to their dissociation by energetic free electrons, which are created continuously by Compton scattering of gamma rays from newly created radioactive 56Co nuclei.  As the local gas temperature cools below 2000K the free C atoms condense as carbonaceous grains, even in the presence of more abundant O atoms. The necessary conditions within supernovae are expressible by three new rules:
I present original calculations for these expectations elaborating on figures from my published works (see the pdf version). The chemical dynamics provide a new discipline of astronomy with radioactivity in supernovae because carbonaceous supernova grains (see Clayton and Nittler 2004 ARAA) extracted from meteorites contain abundant isotopic evidence of details of young supernova remnants. But expertise from the field of molecular and chemical dynamics is needed to bring the theory to fruition.

Workshop Presentation (pdf)

Overwhelmingly the most important issue to be decided is whether new rules for thermal condensation of carbon in cooling oxygen-rich environments are needed to replace an outdated rule-of-thumb. If New Rules are indeed warranted, are these the correct ones? Do these new rules provide necessary conditions for C condensation within expanding and cooling oxygen-rich supernovae II? Are they sufficient conditions? It will be necessary for stardust chemists to speak up on these questions. Their decade-long silence could be interpreted as indifference, but more likely it represents a hunch that C condensation in O-rich environments is a misguided wish. But silence leaves a dark cloud of doubt over stardust research and needs be replaced by active interaction.
Secondly, to advance these questions will require expert chemists and molecular dynamicists. The issue within SNII is as significant and as puzzling as the rapid condensation of buckyballs was two decades ago, and to solve it requires chemical competence comparable to that used by Smalley, Kroto, Curl and Heath. The present work was aided only by Alex Dalgarno and Weihong Liu among such experts. This writer along with other astrophysicists, nucleosynthesis experts, supernova experts, dust experts, equilibrium theorists, reaction-network theorists, and stardust experimenters are only adjuncts to the main scientific questions, which are kinetic rates for chemical reactions. How can stardust astrophysics show itself to creative chemical theorists to be a chemical frontier of huge significance meriting their attention?
Turn now to eight specific questions of high importance for the future. Each appears to need expert chemical treatment for the future of carbon SUNOCONs. (To read my anticipated answers to each question, click here)
1. What is the most significant consequence for Astronomy with Radioactivity?
2. Is linear C_n the appropriate start in hot gaseous C + O mixture?
3. Do spontaneous transition from linear C_n to ringed C_N* provide the effective seed molecules for growth of large graphitic grains?
4. Is the radioactive lifetime of CO adequately computed from the energy per pair in pure C+O gas?
5. What is the role of population control? How does it work?
See here for the argument that the large SUNOCONs size restricts their growth to oxidizing environments.
6. How does SiC condense? Is linear C_n t the right start in hot gaseous C+O+Si mixtures?
7. How does gaseous atomic oxygen attack condensing graphite?
8. How do the 13C and the 14N components enter the SUNOCONs?
(Extended Prospects notes as pdf )

Clayton and Nittler 2004 ARAA

The neutrino process was proposed as origin of some rare isotopes of light and heavy elements (Woosley 1990) is of importance for studying neutrino spectra from the supernovae and for discussing neutrino oscillation (Heger 2005, Yoshida 2006). Among many heavy elements, only two isotopes of 138La and 180Ta are considered to be synthesized primarily by the neutrino process. These two isotopes have similar features: they cannot produced by beta decays since two stable isobars shield these decays, and their isotopic abundances are rare 0.0902% (0.012%) for 138La (180Ta). Heger et al., (2005) calculated the solar abundances of these two isotopes and concluded that they cannot reproduce consistently both abundances without an isomer residual ratio of 180Ta after supernova explosions. The ground state of 180Ta decays through beta-decay with a half-life of about 8 h, whereas an isomer at 75 keV is meta-stable. These two states are linked by (gamma, gamma’) reactions in high temperature environment. The transition rate depends on the temperature and the temperature at SNe suddenly decreases. We propose a time-dependent model to calculate the isomer residual ratio. We treat the ground state and the isomer as independent nuclei and they are linked with weak transitions depending on temperature. We calculate the isomer ratio using this model with measured 9 linking transition rates (Belic 2001). The final isomer ratio is 0.39, which is independent with the astrophysical parameters such as explosion energies, peak temperatures, or neutrino spectrum (Hayakawa 2010). We modified the solar abundances using previous calculations (Heger 2005). The solar abundances of 138La and 180Ta can be reproduced with electron neutrino of 4 MeV. The astrophysical origin of an unstable isotope 92Nb is an open question. This nucleus cannot be produced by beta decays because of stable isobars. This situation is similar to those of 138La and 180Ta. We propose the neutrino process origin of 92Nb. We have calculated reaction rates on neutrino-induced reactions and the neutrino process.
Workshop Presentation (pdf)

1)    We propose a new time-dependent model to calculate the isomer ratio of 180Ta in supernova explosions.

2)    We can reproduce the solar abundances of 138La and 180Ta with the electron neutrino of 4 MeV.

3)    We propose a neutrino process origin of an unstable isotope 92Nb.

Belic et al. 2002 PRC 65, 035801
Hayakawa et al. 2010, PRC 81, 052801(R)
Heger et al. 2005, Phys. Lett. B 606, 258
Yoshida et al, 2006, PRL 96, 091101
Woosley et al. 1990, ApJ 356, 272

Thermonuclear (type-I) bursts arise from ignition of accumulated H/He fuel on the surface of neutron stars accreting from stellar companions. Early theoretical work identified three possible cases of ignition, and established the expected pattern of variation of burst properties as a function of accreted fuel composition and the accretion rate. However, thirty years of observations of such events have revealed an unexpectedly rich spectrum of behavior. Bursts are observed with extremely short (minutes) recurrence times, far too short to reach the critical ignition conditions. Burst activity ceases for most systems at a critical accretion rate about 30% of the predicted threshold. Additionally, for many systems, in some ranges of accretion rate the burst frequency decreases with increasing accretion rate, the opposite of the theoretically predicted behaviour. These deviations from the theory suggest there is additional physics contributing to the burst behaviour. Additionally, some new classes of bursts have emerged in recent years, including so-called "super" bursts, likely powered by unstable ignition of carbon, and intermediate-duration bursts which likely require a large accreted reservoir of pure helium.
A few sources which have been studied intensively offer confirmed examples of two of the three classes of ignition predicted theoretically. In these cases we have established fairly confidently the accretion rate, and fuel composition (both accreted and at ignition), and we find that the behaviour of these systems matches theoretical expectations. These systems serve as crucial test-cases for numerical models, but also serve as a way to probe in detail the extensive and unique nuclear reaction networks. In this talk I have attempted to summarise the observational status of thermonuclear bursts, and discuss what present efforts are underway to better understand the influence of nuclear reactions on the burst properties.  I described the observations of a remarkable new source, Terzan 5 X-2, an 11 Hz pulsar which provides a link between the burst behaviour and mHz oscillations (the latter thought to indicate quasi-stable nuclear burning). Finally I described an ongoing project, the Multi-Instrument Burst ARchive (MINBAR), which aims to collate all bursts observed by recent instruments to enable comprehensive future studies of rare events and broad-scale behavior.
The Figure on the right shows a comparison of observed burst lightcurves, from GS 1826-24, and lightcurves calculated from the KEPLER code (Woosley et al. 2004). The histogram in each panel shows the average lightcurve from the bursts observed during the year 2000 when the recurrence time was ≈ 4 hours (Galloway et al. 2004, Figure 2). The error bars are the 1σ variations from burst to burst. The solid and dashed curves are the average burst profiles from models A3 (CNO mass fraction Z = 0.02) and B3 (Z = 0.001), which have ∆t = 3.9 and 4.0 hours respectively. The lower panel magnifies the rise and the initial part of the decay. The agreement between the observed and predicted lightcurves for solar H-abundance and CNO metallicity is excellent.
Workshop Presentation (pdf)

Although there is much about thermonuclear burning on neutron stars that we don’t understand, there is at least one source where this lack of understanding doesn’t seem to matter: 1826-24. Observationally, it is a high priority to
  * understand better what is special about 1826-like bursts
  * gather and analyse additional examples
  * compare in increasing detail with numerical models
This work has begun last year at Monash and hopefully will continue (via the Multi-INstrument Burst ARchive project – see http://users.monash.edu.au/~dgallow/minbar) and related modelling efforts (see references)

1826-24 & other bursters:
Nuclear reactions & burst models:
Intermediate-duration bursts, superbursts, & He-rich bursts:
Terzan 5 X-2:

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The impacts of massive stars on their surroundings during their evolution and through the final supernova are not only the origin of a variety of spectacular phenomena, they also are the dominant drivers of the evolution of interstellar medium, hence of the evolution of galaxies. The kinetic energy of stellar winds and supernova explosions stirs the ISM and stimulates parts thereof to form stars. The starlight and supernova radiation ionizes the ISM. Winds and supernovae carry products of nucleosynthesis, i.e. freshly-produced isotopes, into the surrounding ISM, eventually mixing on large scales and thus enriching the gas with metals. Each of these actions impacts on the properties of ISM and of the next generation of stars to form, hence it is called 'feedback'.
We can observe different signatures of these different actions, and compare those to models of massive-star feedback. Thus, the morphology of the ISM and in particular inetrstellar loops, cavities, and shells carry information about the kinetic energy of winds and supernovae. The effects of ionizing radiation create free electrons in interstellar space, whose radio emission from their interactions with interstellar magnetic fields has been measured. Finally, nucleosynthesis ejecta include radioactive isotopes, the decay of some of those leading to characteristic gamma-ray lines which can be measured with gamma-ray telescopes.
We have developed a model for massive-star feedback which predicts all of these observables as a function of time, over the evolution time scale of a group of massive stars which are assumed to be coeval and formed within a short time compared to the 20-30 My tracked by our model. We employ a statistical Monte Carlo approach to generate a random group of stars of adopted total number and a mass spectrum given by the Salpeter power-law distribution. Then we make use of several alternative stellar-evolution models (Geneva, Frascati; e.g. Meynet et al. 1997, Palacios et al. 2005, Limongi & Chieffi 2006) to track their evolution, their characteristic phases (in particular the Wolf-Rayet phase), and their time until collapsing as a supernova; these models aslo describe the ionizing starlight intensity in different evolutionary phases. Wind and supernova energetic models are employed for tracking the kimetic energy ejected into the surroundings.
Observations have been collected for several suitable and well-known stellar groups. Stellar richness and age estimates are obtained from a variety of optical and IR surveys of stars. Free-free emission of inetrstellar electrons is mapped in detail from the WMAP measurements. Interstellar shells and loops have been derived by several groups from HI radio surveys such as the Leiden-Dwingeloo survey, supplemented by diffuse X-ray emission measured e.g. with the ROSAT and XMM-Newton missions from the interiors of some of these cavities. Finally, gamma-ray line measurements of 26Al radioactivity is provided from the Compton Observatory (specifically COMPTEL) database and from INTEGRAL/SPI measurements.
We show that the Cygnus region in general appears to confirm our general model, now that the stellar census had been measured more precisely through 2MASS and other recent deep surveys; however, the nucleosynthesis production of 26Al appears somewhat on the low side of the INTEGRAL/SPI measured value, if the nucleosynthesis yields are estimated applying a lower metallicity than solar, such as suggested from observations. The stellar groups in Orion had been analyzed with respect to our feedback model from COMPTEL; from INTEGRAL/SPI, data are still of insufficient sensitivity to confirm and improve on the 26Al signal. For the nearby Scorpius-Centaurus groups, a first study has recently been possible, as an 26Al signal from this region of the sky could be found with INTEGRAL/SPI; other required observations for our comparison are excellent for these stellar groups due to their small distance at 115-145 pc only, making Sco-Cen an ideal laboratory for massive star feedback. In this case, different stellar groups of different ages have been identified and related through teh concenpt of 'triggered star formation', which will be subject to more refined follow-up studies.

Workshop Presentation (pdf)

We currently work on the Carina, Sco-Cen, and Orion regions, aiming at sensitive comparisons of our model predictions for 26Al gamma-rays, for free-free emission, for cumulative kinetic energy as scaled by the constrained stellar content. For most data types, observations are available in above-mentioned surveys. We plan to enhance our kinetic energy constraints using Argentina/Bonn/Parkes refinements of the interstellar HI surveys, employ XMM-Newton diffuse X-ray emission, and INTEGRAL/SPI gamma-ray data. INTEGRAL will continue collecting data at least till end of 2014, according to ESA plans and bi-annual extension reviews.

Voss R. et al., Astron.&Astroph. 504, 531 (2009)
Martin P. et al., Astron. & Astroph. 506, 703 (2009)
Diehl R. et al., Astron. & Astroph. 522, A51 (2010)
Voss R. et al., Astron.&Astroph. 520, A51 (2010)
Diehl R. et al., Springer LNP 812, Ch.7 (2010)

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The deep oceans crust 237KD analysed by Knie et al. (2004) shows a significant increase of the radioisotope 60Fe 2.2 Myr ago. Since 60Fe is produced exclusively in massive stars and ejected by supernova explosions, it is assumed that one or more supernovae must have exploded in the solar vicinity to eject enough enriched material to be deposited on Earth. The Local Bubble, an X-ray emitting, HI-deficient cavity in the local interstellar medium (ISM) was presumably produced by 14 - 20 SN explosions in a moving group around 14 Myr ago (Fuchs et al. 2006), passing by the solar neighborhood. The still living stars belong today to the subgroups Upper Centaurus Lupus (UCL) and Lower Centaurus Crux (LCC) of the Scorpius OB2 association. Calculating the trajectories back in time, we find a minimal distance to Earth of 65 pc about 2.2 Myr, which coincides with the time span of the 60Fe peak in the ocean's crust.
In order to determine the amount of 60Fe arriving on Earth, the explosion times, the 60Fe yields and the time the remant takes to reach the Earth have to be known. We determine the explosion times of the individual stars by fitting an IMF for young,  massive stars (Massey et al. 1995) to the remaining stars. Nucleosynthesis computations are very sensitive to a variety of factors, e.g. if mass loss is included, which reaction rates are used, what is the initial composition and where the mass cut is situated. Therefore we considered the yields of Limongi & Chieffi (2006) as a lower and Woosley & Heger (2007) as an upper limit. To determine the time the remnant takes to hit the Earth we tested a SN model developed by Kahn in 1998. Instead of describing the evolution of a SN remnant into a homogeneous ambient medium, we calculate the expansion into a medium that has already been shaped by a previous SN explosion. This is reasonable, because not only one, but many SN explosions formed the Local Bubble. Then the fluence, which is the number of atoms that reach the Earth per cm¬≤, can be calculated.

We find that the 60Fe deposition in the Earth's crust, inferred from our analytical SN remnant evolution model, is in very good agreement with measurements of the ferromanganese crust. In our model the closest explosion took place 2.3 Myr ago at a distance of 86 pc, which intersects with the path of the stars of LCC. We are able not only to fit the peak position, but the whole 60Fe distribution in the ocean's crust.

Workshop Presentation (pdf)

- Breitschwerdt & de Avillez 2006, A&A, 452:1
- Fuchs et al. 2006, MNRAS, 373, 993
- Knie et al. 2004, PhRvL, 93:17
- Kahn 1998, LNP, 506, 483
- A. Wallner 2011, this meeting; see also the EuroGenesis Web Page

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There are many young associations and clusters of massive stars in the solar vicinity that are potential birth places of neutron stars, hence supernova hosts. We investigate the origin of young nearby neutron stars kinematically to identify their birth places and determine their kinematic ages. The comparison of such potential birth places with sources of radioactive isotopes is a good indicator in order to confirm or reject a particular supernova scenario.
Ages of neutron stars are important to probe cooling curves since there is still a large variety depending on e.g. neutron star mass, composition and cooling mechanism (e.g. Yakovlev et al. 2008). The comparison of theoretical models with observations is crucial to constrain the equation-of-state of matter under extreme conditions. Here, we need a lot more observational data (precise ages). Kinematic ages better represent the true age of a neutron star than its characteristic age as the latter may be influenced by pulsar winds (e.g. Wu et al. 2003) and emission of gravitational waves (e.g. Wette et al. 2008). Moreover, kinematic ages seem to fit well with cooling curves (Tetzlaff et al. 2010).
Most massive stars form in binary or multiple systems (Pfalzner & Olczak 2007; Gies 2008). Investigations show that about 80 per cent of the systems of which the primary component experiences a supernova will get disrupted (Kuranov, Popov & Postnov 2009). The result of the supernova is a fast moving neutron star and a so-called runaway star (Blaauw 1965).
By tracing back in time young neutron stars, young runaway stars (Tetzlaff et al. 2011a) and young associations/clusters, we search for close encounters between the neutron star and a runaway star and/or the neutron star and an association. To account for the errors on the observables as well as the for neutron stars unknown radial velocity, we perform Monte Carlo simulations (as suggested by Hoogerwerf et al. 2001).
If we find that a neutron star and a runaway star could have been at the same place and time inside an association/ cluster, it is well possible that the neutron star was born at that place at that time (that gives the kinematic age of the neutron star) in a supernova. From the association age and assuming contemporaneous star formation, we can also obtain the mass of the supernova progenitor from its life time and evolutionary models.
Since the identification of birth places is often not unique, we need further indicators to decide on a particular scenario. Beside the association with a runaway star, one strategy is to look for radioactive isotopes which are formed during a supernova and are detectable due to their decay. Such isotopes are 60Fe and 26Al with a lifetime of 0.1 to 1 Myr, i.e. much longer visible than a supernova remnant (~0.01 Myr). A hot cooling neutron star is visible for ~1 Myr, i.e. a similar time span.
The comparison between maps of gamma ray emission, probable origins of runaway stars and neutron stars as well as the supernova rate shows that there are suspicious regions on the sky where more supernovae/neutron stars are present than average (Fig.). This is also an important input for gravitational wave searches.
We also try to find the neutron star, which was born in the nearby recent supernova, which may have ejected the 60Fe found in the Earth?s crust. We can then test and calibrate supernova ejecta models.
Recently, we updated the results of the two radio-quiet neutron stars RX J1856.5-3754 and RX J0720.4-3125 using most recent measurements of the parallax and proper motion (Tetzlaff et al. 2011b, submitted). For RX J1856.5-3754 we found that it was born in the Upper Scorpius association 0.46+/-0.05 Myr ago. Its bow shock suggests a radial velocity close to zero (van Kerkwijk & Kulkarni 2001) which we also found using a probability distribution for the radial velocity of the neutron star in our Monte Carlo simulations. From the flight time of the neutron star and adopting an age of Upper Sco of 8-10 Myr (Sartori et al. 2003), we estimate the mass of the progenitor star to be ~20MSun.
For RX J0720.4-3125, the solution is not unique. Even with the identification of a former companion candidate, there are still three possible birth scenarios (Tr 10, Tuc-Hor, beta Pic-Cap). The parallax that we predict from our calculations seems to be too large for a scenario in Tuc-Hor or beta Pic-Cap, hence Tr 10 is more probable the birth place of RX J0720.4-3125 with HIP 43158 being the former companion candidate. However, to confirm this, we need more evidences, e.g. a gamma ray source at the particular position.

Workshop Presentation (pdf)

We aim to identify the birth associations of all neutron stars that are suitable for our investigation, i.e. those neutron stars with distances and proper motion measurements with sufficient precision are available (e.g. ATNF pulsar database, http://www.atnf.csiro.au/research/pulsar/psrcat/). We will take into account additional lines of evidence like runaway stars, gamms-ray sources, and the location of supernova remnants. We will also update our runaway star catalogue (Tetzlaff et al. 2011a) for non-Hipparcos stars. For those stars, that we find to be of particular interest, we will perform follow-up observations.

Tetzlaff et al., 2011b, submitted to MNRAS
Neuhäuser et al. 2011, NIC-11 Inv. Talk Conf. Abstract, in press
Tetzlaff et al., 2011a, MNRAS, 410, 190
Tetzlaff et al., 2010, MNRAS, 402, 2369
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We briefly review the observed properties of the Galactic positron annihilation radiation and their explanation in terms of the propagation within the various phases of the Galactic interstellar medium of positrons, produced by the decay of radioactive 56Ni, 44Ti and 25Al from Galactic supernovae. We have shown (Higdon, Lingenfelter & Rothschild 2009) that such an explanation can fully account for all seventeen of INTEGRAL/SPI’s measured properties of this annihilation radiation (e.g. Knodlseder et al 2005, Churazov et al. 2005, Jean et al. 2006, Wiedenspointner et al. 2006, 2007, 2008a,b, Prantzos et al. 2010). Thus there is in fact no real mystery as to its origin, if ordinary positron propagation is considered. Most important, we have shown that the Galactic Bulge spatial distribution, the positronium annihilation fraction, and the 511 keV annihilation line widths in the warm phases of the ISM in the two component (Central Molecular Zone and Tilted Disk, e.g. Ferriere et al. 2007) Bulge, expected from this explanation, all match those of the model independent (Richardson-Lucy) spatial fit (Knodlseder et al 2005), and the spectral features measured by INTEGRAL/SPI. Lastly, we have shown (Lingenfelter, Higdon & Rothschild, 2009) that the many exotic Dark Matter sources proposed (e.g. Boehm, et al. 2004, and about 150 other papers) to explain the so-called “mystery” of this radiation, all assumed that positron propagation was negligible and were thus not consistent with nearly all of its measured properties.

We also briefly discuss the new observational analyses by Churazov et al. 2011 and a cooling gas annihilation model that they propose. They report an oblateness and tilt in the annihilation emission from the Galactic Bulge, assuming a single oblate spheroidal emission region, which seems consistent with both the model independent analysis and that expected from our explanation, since their one-component best-fit tilt (~12 deg) is essentially that of the two-component average of ~14.5 deg. They also explore the possibility that the warm-phase annihilation of the Bulge emission may result from in-situ annihilation of supernova-produced positrons that annihilate in the cooling gas within the supernova remnants, rather than propagating to the observed, longer-lived, preexisting warm gas regions. The flaw in this model, however, is that it requires no further heating of the supernova remnant gas, so that it can cool before the positrons annihilate in the initially hot gas within it. But, as we show here, the supernova rate in the Bulge is high enough that the remnants will be repeatedly reheated before they can cool, so such a model cannot work there.
We suggest, nevertheless, that it might be quite applicable to the gas of the Galactic Bulge wind expanding into the Halo, where it could provide warm diffuse gas for annihilation there. This can be tested when the positronium fraction and 511 keV line width are better measured in the Halo.

CR propagation studies; deeper observations at 511 keV

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Churazov et al. 2005, MNRAS, 357, 377
Churazov et al. 2011, MNRAS, 411, 1727
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Knoedlseder et al 2005, A&A, 441, 513
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Weidenspointner et al. 2008b, Nature, 451, 159

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