The role of metallicity and radiation in molecular clouds. Stars are born from giant molecular clouds of gas and dust. A complicated interplay exists at this stage between the physical processes and the environmental conditions. In Hocuk & Spaans (2010a), I studied the dependence of cloud fragmentation on the ambient conditions as they pertain to starburst and dwarf galaxy regions. My findings show that the amount of fragmentation and the compressibility of the gas scales with increasing metallicity. In this, the cooling by gas phase and grain surface chemical species play a subtle, but fundamental role. Low metallicity gas leads to the formation of massive cloud cores, which eventually induce massive star formation (cf. Lada et al., 2008; Dabringhausen et al., 2009).
Following this work, I investigated the impact of X-rays, cosmic rays, and UV on cloud dynamics and the initial mass function (IMF). It is known that star formation is regulated through a variety of feedback processes (e.g. Bate, 2009; Krumholz et al., 2010). In order to achieve my goals, I integrated a radiative transfer code (Meijerink & Spaans, 2005) into the adaptive mesh refinement hydro code FLASH (Fryxell et al., 2000). In Hocuk & Spaans (2010b), I discovered that when there is a strong X-ray source, the star forming cloud fragments into larger clumps whereby fewer, but more massive protostellar cores are formed. Accretion proved to have a significant impact on the mass function and a near-flat, non-Salpeter IMF was the result. Further extending the study in Hocuk & Spaans (2011), I showed that an increased cosmic ray rate leads to a reduced formation efficiency of low-mass stars. While the shape of the mass function is preserved, high cosmic ray rates increase the average mass of stars, thereby shifting the turn-over mass of the IMF to higher values.
Variations in the IMF in active galaxies. The conditions that affect the formation of stars in active environments are quite different, and more extreme, than the conditions in our local neighborhood. Yet, even in the inner parsec of our Galaxy, i.e., Sgr A*, as well as in M31, young stars have been found at distances of up to 0.3 pc (Genzel et al., 2003; Paumard et al., 2006; Levin, 2007).This raises the question of how it is possible that even in extreme environments stars can form.
To investigate this matter, I performed a numerical study on the formation of stars at 10 pc of a supermassive black hole in Hocuk & Spaans (2011). To this end, I created an outside-in ray-tracing method in order to compute the column density to each cell. One aspect of the study was to explore the importance of shear that is imposed by the 106 − 108 M⊙ black hole. I found that the turbulence caused by shearing effects strongly reduces the star-formation efficiency, but that X-rays can still incite star formation. Together with the X-ray impact and the cosmic ray results, the inevitable conclusion was that the IMF inside active galaxies is different and, therefore, not universal in nature.
In continuation of this work, I investigated in Hocuk et al. (2012) the impact of magnetic fields in similar environments. It is often contemplated that magnetic fields play a crucial role in the distribution of stellar masses inside star-forming molecular clouds. My general finding was that the collapse of a gravitationally unstable cloud is indeed slowed down with an increasing magnetic field strength. The main component of the lognormal IMF had a peak shifted to sub-solar masses (≤ 0.3 M⊙ ) in ideal magnetized cores. This was due to a decrease in the accretion rates from the gas reservoir, with an emerging flat
component of the IMF above ∼ 2 M⊙ (also see Li et al., 2010; Krumholz et al., 2016).
Chemical influences during the evolution of interstellar clouds. Atoms and molecules, and in particular C+ and CO, are important coolants during the evolution of interstellar gas clouds. The presence of dust grains, which effectively catalyze many chemical reactions, strongly impacts the molecular composition of a cloud. Depletion of these molecules affects the thermal balance of molecular clouds and with that their whole evolution. While on dust surfaces, water is the main constituent of the icy mantle in which a complex chemistry is taking place.
In Hocuk et al. (2014) and in Hocuk et al. (2016), I investigated the significance of freeze-out on cloud evolution. The focus was on the impact on cloud thermodynamics. For this work, I created a time-dependent gas-grain chemical model in 3D based on the rate equations method. My results showed that the freeze-out of CO, which is the main coolant around the critical densities of 103 − 104 H/cm3 , increases the gas temperature, while the equation of state becomes softer. However, due to the reactivity of exothermic reactions, a non-thermal route is made available for frozen species to be released into the gas phase (e.g., Garrod et al., 2007; Dulieu et al., 2013; Minissale et al., 2016), reducing the effect of freeze-out. My conclusion was that while surface chemistry enhances low-mass star formation to a small degree, the impact is negligible as far as the IMF is concerned.
While the other study was ongoing, I was able to witness the formation of the first ice layer on dust in Hocuk & Cazaux (2015). In my unprecedented study, the gas-dust interplay was fully considered by including the fine details of grain surface processes. The difference in binding energies of chemical species on bare and icy surfaces was also treated. My novel result was that CO is well mixed and strongly present within the first ice layer in cold (and sub G0) environments, in contrast to the common notion that water ice always forms first.
The interstellar dust temperature. The temperature of dust plays a crucial role in the thermodynamics of interstellar clouds, because of the gas-dust collisional heat transfer. It is also a key parameter in astrochemical studies that governs the rate at which molecules form on dust. In hydrodynamic simulations often a simple expression for the dust temperature is adopted, because of computational constraints, while modelers tend to keep the temperature constant over a large range of parameter space. This is often a less than ideal assumption.
In Hocuk et al. (2017), my motivation was to provide a simple and accurate parametric expression for the community. To this end, I calculated the dust temperature from basic principles for dust in thermal equilibrium by considering in detail the interstellar radiation field, the attenuation of radiation, the dust composition, and the dust opacities. The parametric expression I provide match the range of observed dust temperatures excellently at low and at high optical depths (AV). Considering the impact of ices, I found that ice formation changes the opacity of dust significantly enough to reduce the effective cooling at high AV. This allows the dust to be slightly warmer (∼15%) in highly embedded regions, which may be crucial in avoiding the freeze-out of H2 molecules in numerical models.
Observing H2O and CO ices in prestellar cores. Detecting ices was difficult in the past, but with the advent of advanced infrared (IR) telescopes, this research field has strongly developed in the last decade. At the time of writing, only a handful of ice species are detected in space, with some of them still being uncertain or controversial detections. I expect that the future flagship space telescope JWST will usher in a new era of ice chemistry.
As a Co-I, I have taken part in two recent successful observational proposals. The proposals were based on the predictions done in my work of Hocuk & Cazaux (2015). To this end, we observed carbon monoxide (CO) and water (H2O) ices in the quiescent regions of the Pipe Nebula with the SPEX / IRTF and NIRSPEC / KECKⅡ telescope instruments. A collaborative paper has been submitted from this project (Goto et al., 2017) and a follow-up observation has been proposed. In this observational program, we have positively detected the water ice absorption bands at 3.05 μm on seven sources in the Pipe Nebula. Our findings show that the peak optical depth τ3μm of the water ice is significantly lower than the canonical relation of τ3μm versus visual extinction (AV) in Taurus. I believe that exploring cosmic ices is going to be the next big thing in astronomy with the aid of future telescopes, especially JWST.