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Register a new userA model for the formation of stellar associations and clusters from giant molecular clouds
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We present a large suite of MHD simulations of turbulent, star-forming giant molecular clouds(GMCs) with stellar feedback, extending previous work by simulating 10 different random realizations for each point in the parameter space of cloud mass and size. It is found that oncethe clouds disperse due to stellar feedback, both self-gravitating star clusters and unbound stars generally remain, which arise from the same underlying continuum of substructured stellar density, ie. the hierarchical cluster formation scenario. The fraction of stars that are born within gravitationally-bound star clusters is related to the overall cloud star formation efficiency set by stellar feedback, but has significant scatter due to stochastic variations in the small-scale details of the star-forming gas flow. We use our numerical results to calibrate a model for mapping the bulk properties (mass, size, and metallicity) of self-gravitating GMCs onto the star cluster populations they form, expressed statistically in terms of cloud-level distributions. Synthesizing cluster catalogues from an observed GMC catalogue in M83, we find that this model predicts initial star cluster masses and sizes that are in good agreement with observations, using only standard IMF and stellar evolution models as inputs for feedback. Within our model, the ratio of the strength of gravity to stellar feedback is the key parameter setting the masses of star clusters, and of the various feedback channels direct stellar radiation(photon momentum and photoionization) is the most important on GMC scales.
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Young massive star clusters (YMCs, with M $geq$10$^4$ M$_{odot}$) are proposed modern-day analogues of the globular clusters (GCs) that were products of extreme star formation in the early universe. The exact conditions and mechanisms under which YMCs form remain unknown -- a fact further complicated by the extreme radiation fields produced by their numerous massive young stars. Here we show that GC-sized clusters are naturally produced in radiation-hydrodynamic simulations of isolated 10$^7$ M$_{odot}$ Giant Molecular Clouds (GMCs) with properties typical of the local universe, even under the influence of radiative feedback. In all cases, these massive clusters grow to GC-level masses within 5 Myr via a roughly equal combination of filamentary gas accretion and mergers with several less massive clusters. Lowering the heavy-element abundance of the GMC by a factor of 10 reduces the opacity of the gas to radiation and better represents the high-redshift formation conditions of GCs. This results in higher gas accretion leading to a mass increase of the largest cluster by a factor of ~4. When combined with simulations of less massive GMCs (10$^{4-6}$ M$_{odot}$), a clear relation emerges between the maximum YMC mass and the mass of the host GMC. Our results demonstrate that YMCs, and potentially GCs, are a simple extension of local cluster formation to more massive clouds and do not require suggested exotic formation scenarios.
Star formation in a filamentary infrared dark cloud (IRDC) is simulated over a dynamic range of 4.2 pc to 28 au for a period of $3.5times 10^5$ yr, including magnetic fields and both radiative and outflow feedback from the protostars. At the end of the simulation, the star formation efficiency is 4.3 per cent and the star formation rate per free fall time is $epsilon_{rm ff}simeq 0.04$, within the range of observed values (Krumholz et al. 2012a). The total stellar mass increases as $sim,t^2$, whereas the number of protostars increases as $sim,t^{1.5}$. We find that the density profile around most of the simulated protostars is $sim,rhopropto r^{-1.5}$, as predicted by Murray & Chang (2015). At the end of the simulation, the protostellar mass function approaches the Chabrier (2005) stellar initial mass function. We infer that the time to form a star of median mass $0.2,M_odot$ is about $1.4times 10^5$~yr from the median mass accretion rate. We find good agreement among the protostellar luminosities observed in the large sample of Dunham et al. (2013), our simulation, and a theoretical estimate, and conclude that the classical protostellar luminosity problem Kenyon et al. (1990) is resolved. The multiplicity of the stellar systems in the simulation agrees to within a factor 2 of observations of Class I young stellar objects; most of the simulated multiple systems are unbound. Bipolar protostellar outflows are launched using a sub-grid model, and extend up to 1 pc from their host star. The mass-velocity relation of the simulated outflows is consistent with both observation and theory.
We investigate the formation of star clusters in an unbound GMC, where the supporting kinetic energy is twice as large as the clouds self-gravity. This cloud manages to form a series of star clusters and disperse, all within roughly 2 crossing times (10 Myr), supporting recent claims that star formation is a rapid process. Simple assumptions about the nature of the star formation occurring in the clusters allows us to place an estimate for the star formation efficiency at about 5 to 10 %, consistent with observations. We also propose that unbound clouds can act as a mechanism for forming OB associations. The clusters that form in the cloud behave as OB subgroups. These clusters are naturally expanding from one another due to unbound nature of the flows that create them. The properties of the cloud we present here are are consistent with those of classic OB associations.
We present a simple, self-consistent model to predict the maximum masses of giant molecular clouds (GMCs), stellar clusters and high-redshift clumps as a function of the galactic environment. Recent works have proposed that these maximum masses are set by shearing motions and centrifugal forces, but we show that this idea is inconsistent with the low masses observed across an important range of local-Universe environments, such as low-surface density galaxies and galaxy outskirts. Instead, we propose that feedback from young stars can disrupt clouds before the global collapse of the shear-limited area is completed. We develop a shear-feedback hybrid model that depends on three observable quantities: the gas surface density, the epicylic frequency, and the Toomre parameter. The model is tested in four galactic environments: the Milky Way, the Local Group galaxy M31, the spiral galaxy M83, and the high-redshift galaxy zC406690. We demonstrate that our model simultaneously reproduces the observed maximum masses of GMCs, clumps and clusters in each of these environments. We find that clouds and clusters in M31 and in the Milky Way are feedback-limited beyond radii of 8.4 and 4 kpc, respectively, whereas the masses in M83 and zC406690 are shear-limited at all radii. In zC406690, the maximum cluster masses decrease further due to their inspiral by dynamical friction. These results illustrate that the maximum masses change from being shear-limited to being feedback-limited as galaxies become less gas-rich and evolve towards low shear. This explains why high-redshift clumps are more massive than GMCs in the Local Universe.
We present Submillimeter Array (SMA) observations of seven massive molecular clumps which are dark in the far-infrared for wavelengths up to 70 $mu$m. Our 1.3 mm continuum images reveal 44 dense cores, with gas masses ranging from 1.4 to 77.1 M$_{odot}$. Twenty-nine dense cores have masses greater than 8 M$_{odot}$ and the other fifteen dense cores have masses between 1.4 and 7.5 M$_{odot}$. Assuming the core density follows a power-law in radius $rho propto r^{-b}$, the index $b$ is found to be between 0.6 and 2.1 with a mean value of 1.3. The virial analysis reveals that the dense cores are not in virial equilibrium. CO outflow emission was detected toward 6 out of 7 molecular clumps and associated with 17 dense cores. For five of these cores, CO emissions appear to have line-wings at velocities of greater than 30 km s$^{-1}$ with respect to the source systemic velocity, which indicates that most of the clumps harbor protostars and thus are not quiescent in star formation. The estimated outflow timescale increase with core mass, which likely indicates that massive cores have longer accretion timescale than that of the less massive ones. The fragmentation analysis shows that the mass of low-mass and massive cores are roughly consistent with thermal and turbulent Jeans masses, respectively.
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