Is molecular gas necessary for star formation

By S. Glover, P. C. Clark. 2011

185 citations recorded by the source corpus.

On galactic scales, the surface density of star formation appears to be well correlated with the surface density of molecular gas. This has led many authors to suggest that there exists a causal relationship between the chemical state of the gas and its ability to form stars – in other words, the assumption that the gas must be molecular before star formation can occur. We test this hypothesis by modelling star formation within a dense cloud of gas with properties similar to a small molecular cloud using a series of different models of the chemistry, ranging from one in which the formation of molecules is not followed and the gas is assumed to remain atomic throughout, to one that tracks the formation of both H2 and CO. We find that the presence of molecules in the gas has little effect on the ability of the gas to form stars: star formation can occur just as easily in atomic gas as in molecular gas. At low densities (<104 cm−3), the gas is able to cool via C+ fine-structure emission almost as efficiently as via CO rotational line emission, while at higher densities, the main cooling process involves the transfer of energy from gas to dust, meaning that the presence of molecules is again unimportant. Cooling by H2 is particularly inefficient, accounting for as little as 1 per cent of the overall cooling in the cloud. Rather than the chemical makeup, we find that the most important factor controlling the rate of star formation is the ability of the gas to shield itself from the interstellar radiation field. As this is also a prerequisite for the survival of molecules within the gas, our results support a picture in which molecule formation and the formation of cold gas are both correlated with the column density of the cloud – and thus its ability to shield itself – rather than being directly correlated with each other.

Subfields: Astronomy and Astrophysics, Instrumentation, Organic Chemistry

View Paper: https://www.semanticscholar.org/paper/efb56a13e69be764672837ed4e71ec87c0d2bff0

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Citations in the corpus

xGASS: total cold gas scaling relations and molecular-to-atomic gas ratios of galaxies in the local Universe (2018, background): “…phase in order to fuel star formation (Blitz & Rosolowsky 2006; Bigiel et al. 2008; Leroy et al. 2008; Krumholz, McKee & Tumlinson 2009), although molecular gas could just be tracing star formation, formed as the by-product of the gravitational collapse of atomic gas (Glover & Clark 2012).”
The big problems in star formation: The star formation rate, stellar clustering, and the initial mass function (2014, background): “Thus the CO abundance is generally always in equilibrium, though that equilibrium value may change as the H2 abundance does (Glover et al. 2010; Glover & Mac Low 2011; Glover & Clark 2012c).”
Turbulent molecular clouds (2012, methodology): “Several methods to follow CO formation in turbulent simulations have been compared by Glover & Clark (2012): they find that, not unexpectedly, the gas dynamics is not sensitive to the details of the chemistry models, but all the adopted models so far fail to reproduce the CO abundances in such…”
CO-dark gas and molecular filaments in Milky Way-type galaxies (2014, methodology): “…the other hand, small-scale models have for some time been able to model both H2 and CO formation in the turbulent ISM (see e.g. Glover et al. 2010; Glover & Clark 2012a), but have not had the dynamical range necessary to model cloud formation self-consistently, or have started from highly…”
Global hierarchical collapse in molecular clouds. Towards a comprehensive scenario (2019, background): “It is important to recall that the presence of molecular gas does not appear necessary to allow the gas to cool and thus keep contracting; at densities lower than 104 cm−3, C+ MNRAS 000, 1–41 (0000) fine-structure emission appears to provide the necessary cooling (Glover & Clark 2012).”
Cool Gas in High-Redshift Galaxies (2013, background): “The implication is that stars can form in CO–poor molecular gas, and that the expected CO luminosities of low metallicity galaxies at high redshift are lower than previously predicted based on standard α values (Glover & Clark 2012; Papadopoulos & Pelupessy 2010).”
The meaning and consequences of star formation criteria in galaxy models with resolved stellar feedback (2013, background): “…is almost no difference in an instantaneous sense between this model and a threshold density – the differences owe to the fact that at these metallicities cooling rates are sufficiently suppressed such that the cooling time is no longer short compared to the dynamical time (Glover & Clark 2012).”
An Overview of the Dwarf Galaxy Survey (2013, background): “This effect of the CO-dark molecular gas in the PDR envelope is shown theoretically to be controlled strongly by AV and should increase in importance as the metallicity decreases (Wolfire et al. 2010; Krumholz & Gnedin 2011; Glover & Clark 2012).”
COMBINED CO AND DUST SCALING RELATIONS OF DEPLETION TIME AND MOLECULAR GAS FRACTIONS WITH COSMIC TIME, SPECIFIC STAR-FORMATION RATE, AND STELLAR MASS (2014, background): “However, it is not clear whether high molecular content as such is causally required for the onset of star formation (Glover & Clark 2012).”
What controls star formation in the central 500 pc of the Galaxy (2013, background): “If this relation is causal in nature, molecularg s may be required to form stars (e.g. Schruba et al. 2011, althoughsee Glover & Clark 2012 and Krumholz 2012 for an alternative view), implying that a star formation threshold could be related tothe phase transition of HI to H2 (Elmegreen & Parravano…”
Physical Processes in the Interstellar Medium (2014, methodology): “At low densities, γ ≈ 0.7 (Larson, 1985; Larson, 2005; Glover & Clark, 2012a), but once the gas and dust temperatures become thermally coupled at ncrit ≈ 105 cm−3 (see Figure 5 and the discussion in Section 3.5), one expects this value to increase, reaching γ ≈ 1.1 at densities n ncrit (Banerjee et…”
The SILCC (SImulating the LifeCycle of molecular Clouds) project – I. Chemical evolution of the supernova-driven ISM (2014, methodology): “Full details of our treatment of these processes can be found in Glover & Mac Low (2007a,b) and Glover & Clark (2012b).”
METALLICITY-DEPENDENT QUENCHING OF STAR FORMATION AT HIGH REDSHIFT IN SMALL GALAXIES (2011, background): “…the H i to H2 transition is an excellent proxy for the transition from gas that is too warm to form stars (T & 100 K) and gas that is cold enough (T ∼ 10 K) to undergo runaway collapse to stellar densities.1 The subsequent numerical simulations of Glover & Clark (2011) confirm this picture.”
FIRE-2 simulations: physics versus numerics in galaxy formation (2017, background): “A more detailed study on sub-galactic scales by Glover & Clark (2012) considering each microphysical cooling mechanism in turn showed that as long as some cooling channel exists, the details of that cooling are largely irrelevant in the dense ISM.”
Theoretical Challenges in Galaxy Formation (2016, background): “…2007, Hennebelle & Iffrig 2014, Walch et al. 2015), are unresolved in most galaxy scale and all cosmological simulations (see, however, Hopkins, Quataert & Murray 2012b) and it is unclear whether H2 formation is the primary driver for star formation (see e.g. Glover & Clark 2012, Krumholz 2013).”
Molecular hydrogen abundances of galaxies in the EAGLE simulations (2015, methodology): “In addition, Glover & Clark (2012) argued against the importance of molecular cooling (e.g. from H2 and CO) as the low temperatures and high-densities needed to form stars are reached even in situations where the gas is forced to remain atomic (see also Schaye 2004).”