Astrochemical dating of a stellar nursery
An international research team led by scientists from the Coordinated Research Center (CRC) 956 “Conditions and Impact of Star Formation” at the University of Cologne has used observations made with the GREAT instrument on board the SOFIA aircraft observatory and the APEX telescope to date the core of an interstellar cloud that is forming a group of Sun-like stars. This work, to which scientists from the University of Helsinki as well as from the Max-Planck-Institutes for Radio Astronomy (MPIfR) and Extraterrestrial Physics (MPE) contributed, is published in this week’s Nature journal.
Stars like our Sun and their planetary systems are born inside clouds consisting of dust and molecular gas. Stellar evolution begins with the contraction of dense material in these stellar nurseries until an embryonic star, the protostar, is formed. How this collapse happens exactly, and on what timescales, is not very well understood. Is the gas “free-falling” towards the centre due to gravity or is the collapse slowed down by other factors? “Since this process takes much longer than human history, it cannot just be followed as a function of time. Instead, one needs to find an internal clock that allows reading off the age of a particular star forming cloud,” says the leading author Sandra Brünken from the University of Cologne.
The hydrogen molecule (H2), by far the most abundant molecule in space, could act as such an internal, “chemical” clock. Molecular hydrogen exists in two different forms, called ortho and para, which correspond to different orientations of the spins of the two hydrogen nuclei. In the cold and dense molecular clouds out of which stars are formed, the relative abundance of these two forms changes continuously with time by chemical exchange reactions. Therefore the current abundance ratio observed is a measure of the time elapsed since the formation of H2, and thereby the molecular cloud itself. Unfortunately, H2 cannot be directly detected in the very cold interstellar “breeding grounds” of stars. However, H2D+, an ionized variant in which a deuteron particle is attached to the H2 molecule, can be observed. Indeed, the ortho and para forms of H2D+ emit and absorb at characteristic wavelengths, forming “spectral lines” that are observable with different telescopes.
The astronomical observations of the dense core were very challenging. The relevant spectral line of para-H2D+ lies in the far infrared wavelength range (at 219 µm), where the Earth’s atmosphere absorbs most of the radiation. Therefore, the observations had to be carried out by two very special telescopes: with the GREAT instrument on board the SOFIA aircraft, a joint project between NASA and the DLR carrying a 2.7 meter diameter telescope as high as 13.7 km, and with the ground-based APEX telescope located in the Chilean Andes at an altitude of 5100 m. The age of this star-forming cloud, which is located in the Ophiuchus constellation at a distance of around 400 light years, was then determined by comparing the data from the telescopes with extensive computer simulations of the chemistry that is changing with time.
These models have to take into account not only a large number of chemical reactions but also the spin orientation of each of the protons. Moreover, the physical structure of the observed object has been modelled and included in the chemical code. From the first detection of a strong H2D+ signal towards a pre-stellar core in 2003 (by Caselli et al.), new laboratory work and improved chemical models have allowed the astrophysicists to unveil the processes going on in these clouds. “The chemistry in these pre-stellar cores is complex but based on solid rules of quantum chemistry,” points out Olli Sipilä, who is now working at the new Center for Astrochemical Studies (CAS) at MPE. “Over the past years, we have included all relevant reactions for spin state chemistry, which can tell us about the age of a cloud once its physical structure is known.”
Comparison of the H2D+ data with these models has now demonstrated that this molecule is in fact a more accurate tracer of the very early cloud evolution toward star formation than previously used molecules. In particular, this “chemical clock” keeps running, when others have already turned off, showing that the dense cloud cores, where stars are forming, are at least one million years old.
“This finding favours those star formation theories where molecular clouds are rather long-lived and undergo a quasi-static contraction,” explains Paola Caselli, head of the CAS group at MPE. “The data are not consistent with rapid modes of star formation, where collapse occurs on a free-fall time.” This shows that the observed dense core, hosting a small group of Solar-type infant stars, has survived for quite a long time.
SOFIA, the Stratospheric Observatory for Infrared Astronomy, is a joint project of the National Aeronautics and Space Administration (NASA) and the Deutsches Zentrum für Luft- und Raumfahrt e.V. (DLR; German Aerospace Centre, grant: 50OK0901). The German component of the SOFIA project is being carried out under the auspices of DLR, with funds provided by the Federal Ministry of Economics and Technology (Bundesministerium für Wirtschaft und Technologie; BMWi) under a resolution passed by the German Federal Parliament, and with funding from the State of Baden-Württemberg and the University of Stuttgart. Scientific operations are coordinated by the German SOFIA Institute (DSI) at the University of Stuttgart and the Universities Space Research Association (USRA) headquartered in Columbia, Maryland, U.S.A.
German SOFIA Institute: http://www.dsi.uni-stuttgart.de/index.en.html
GREAT, the German Receiver for Astronomy at Terahertz Frequencies, is a receiver for spectroscopic observations in the far infrared spectral regime at frequencies between 1.25 and 5 terahertz (wavelengths of 60 to 220 microns), which are not accessible from the ground due to absorption by water vapour in the atmosphere. GREAT is a first-generation German SOFIA instrument, developed and maintained by the Max-Planck Institute for Radio Astronomy (MPIfR) and KOSMA at the University of Cologne, in collaboration with the Max Planck Institute for Solar System Research and the DLR Institute of Planetary Research. Rolf Guesten (MPIfR) is the principal investigator for GREAT. The development of the instrument was financed by the participating institutes, the Max Planck Society and the German Research Foundation (Deutsche Forschungsgemeinschaft; DFG).
GREAT instrument web-page: http://www3.mpifr-bonn.mpg.de/div/submmtech/heterodyne/great/greatmain.html
APEX, the Atacama Pathfinder Experiment, is a collaboration between the Max Planck Institute for Radio Astronomy (MPIfR), Onsala Space Observatory (OSO), and the European Southern Observatory (ESO) to construct and operate a modified prototype antenna of ALMA (Atacama Large Millimetre Array) as a single dish on the Chajnantor plateau at an altitude of 5,100 meters above sea level (Atacama Desert, Chile). The telescope was manufactured by VERTEX Antennentechnik in Duisburg, Germany. The operation of the telescope is entrusted to ESO.
APEX telescope: http://www.apex-telescope.org