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Register a new userDisc fragmentation and oligarchic growth of protostellar systems in low-metallicity gas clouds
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We study low-metallicity star formation with a set of high-resolution hydrodynamics simulations for various gas metallicities over a wide range $0$--$10^{-3} {rm Z}_{bigodot}$. Our simulations follow non-equilibrium chemistry and radiative cooling by adopting realistic elemental abundances and dust size distribution. We examine the condition for the fragmentation of collapsing clouds (cloud fragmentation; CF) and of accretion discs (disc fragmentation; DF). We find that CF is suppressed due to rapid gas heating accompanied with molecular hydrogen formation, whereas DF occurs in almost all our simulations regardless of gas metallicities. We also find that, in the accretion discs, the growth of the protostellar systems is overall oligarchic. The primary protostar grows through the accretion of gas, and secondary protostars form through the interaction of spiral arms or the break-up of a rapidly rotating protostar. Despite vigorous fragmentation, a large fraction of secondary protostars are destroyed through mergers or tidal disruption events with other protostars. For a few hundred years after the first adiabatic core formation, only several protostars survive in a disc, and the total mass of protostars is $0.52$--$3.8 {rm M}_{bigodot}$.
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We study gravitational collapse of low-metallicity gas clouds and the formation of protostars by three-dimensional hydrodynamic simulations. Grain growth, non-equilibrium chemistry, molecular cooling, and chemical heating are solved in a self-consistent manner for the first time. We employ the realistic initial conditions for the abundances of metal and dust, and the dust size distribution obtained from recent Population III supernova calculations. We also introduce the state-of-the-art particle splitting method based on the Voronoi tessellation and achieve an extremely high mass resolution of ~10^{-5} Msun (10 earth masses) in the central region. We follow the thermal evolution of several early clouds with various metallicities. We show that the condition for cloud fragmentation depends not only on the gas metallicity but also on the collapse timescale. In many cases, the cloud fragmentation is prevented by the chemical heating owing to molecular hydrogen formation even though dust cooling becomes effective. Meanwhile, in several cases, efficient OH and H2O cooling promotes the cloud elongation, and then cloud filamentation is driven by dust thermal emission as a precursor of eventual fragmentation. While the filament fragmentation is driven by rapid gas cooling with >10^{-5} Zsun, fragmentation occurs in a different manner by the self-gravity of a circumstellar disk with <10^{-5} Zsun. We use a semi-analytic model to estimate the number fraction of the clouds which undergo the filament fragmentation to be a several percents with 10^{-5}--10^{-4} Zsun. Overall, our simulations show a viable formation path of the recently discovered Galactic low-mass stars with extremely small metallicities.
The Magellanic Clouds provide the only laboratory to study the effect of metallicity and galaxy mass on molecular gas and star formation at high (~20 pc) resolution. We use the dust emission from HERITAGE Herschel data to map the molecular gas in the Magellanic Clouds, avoiding the known biases of CO emission as a tracer of H$_{2}$. Using our dust-based molecular gas estimates, we find molecular gas depletion times of ~0.4 Gyr in the LMC and ~0.6 SMC at 1 kpc scales. These depletion times fall within the range found for normal disk galaxies, but are shorter than the average value, which could be due to recent bursts in star formation. We find no evidence for a strong intrinsic dependence of the molecular gas depletion time on metallicity. We study the relationship between gas and star formation rate across a range in size scales from 20 pc to ~1 kpc, including how the scatter in molecular gas depletion time changes with size scale, and discuss the physical mechanisms driving the relationships. We compare the metallicity-dependent star formation models of Ostriker, McKee, and Leroy (2010) and Krumholz (2013) to our observations and find that they both predict the trend in the data, suggesting that the inclusion of a diffuse neutral medium is important at lower metallicity.
Models of galaxy formation predict that gas accretion from the cosmic web is a primary driver of star formation over cosmic history. Except in very dense environments where galaxy mergers are also important, model galaxies feed from cold streams of gas from the web that penetrate their dark matter haloes. Although these predictions are unambiguous, the observational support has been indirect so far. Here we report spectroscopic evidence for this process in extremely metal-poor galaxies (XMPs) of the local Universe, taking the form of localized starbursts associated with gas having low metallicity. Detailed abundance analyses based on Gran Telescopio Canarias (GTC) optical spectra of ten XMPs show that the galaxy hosts have metallicities around 60 % solar on average, while the large star-forming regions that dominate their integrated light have low metallicities of some 6 % solar. Because gas mixes azimuthally in a rotation timescale (a few hundred Myr), the observed metallicity inhomogeneities are only possible if the metal-poor gas fell onto the disk recently. We analyze several possibilities for the origin of the metal-poor gas, favoring the metal-poor gas infall predicted by numerical models. If this interpretation is correct, XMPs trace the cosmic web gas in their surroundings, making them probes to examine its properties.
Stars form out of the densest parts of molecular clouds. Far-IR emission can be used to estimate the Star Formation Rate (SFR) and high dipole moment molecules, typically HCN, trace the dense gas. A strong correlation exists between HCN and Far-IR emission, with the ratio being nearly constant, over a large range of physical scales. A few recent observations have found HCN to be weak with respect to the Far-IR and CO in subsolar metallicity (low-Z) objects. We present observations of the Local Group galaxies M33, IC10, and NGC6822 with the IRAM 30meter and NRO 45m telescopes, greatly improving the sample of low-Z galaxies observed. HCN, HCO$^+$, CS, C$_2$H, and HNC have been detected. Compared to solar metallicity galaxies, the Nitrogen-bearing species are weak (HCN, HNC) or not detected (CN, HNCO, N$_2$H$^+$) relative to Far-IR or CO emission. HCO$^+$ and C$_2$H emission is normal with respect to CO and Far-IR. While $^{13}$CO is the usual factor 10 weaker than $^{12}$CO, C$^{18}$O emission was not detected down to very low levels. Including earlier data, we find that the HCN/HCO$^+$ ratio varies with metallicity (O/H) and attribute this to the sharply decreasing Nitrogen abundance. The dense gas fraction, traced by the HCN/CO and HCO$^+$/CO ratios, follows the SFR but in the low-Z objects the HCO$^+$ is much easier to measure. Combined with larger and smaller scale measurements, the HCO$^+$ line appears to be an excellent tracer of dense gas and varies linearly with the SFR for both low and high metallicities.
The early evolution of protostellar disks with metallicities in the $Z=1.0-0.01~Z_odot$ range was studied with a particular emphasis on the strength of gravitational instability and the nature of protostellar accretion in low-metallicity systems. Numerical hydrodynamics simulations in the thin-disk limit were employed that feature separate gas and dust temperatures, and disk mass-loading from the infalling parental cloud cores. Models with cloud cores of similar initial mass and rotation pattern, but distinct metallicity were considered to distinguish the effect of metallicity from that of initial conditions. The early stages of disk evolution in low-metallicity models are characterized by vigorous gravitational instability and fragmentation. Disk instability is sustained by continual mass-loading from the collapsing core. The time period that is covered by this unstable stage is much shorter in the $Z=0.01~Z_odot$ models as compared to their higher metallicity counterparts thanks to the higher mass infall rates caused by higher gas temperatures (that decouple from lower dust temperatures) in the inner parts of collapsing cores. Protostellar accretion rates are highly variable in the low-metallicity models reflecting a highly dynamical nature of the corresponding protostellar disks. The low-metallicity systems feature short, but energetic episodes of mass accretion caused by infall of inward-migrating gaseous clumps that form via gravitational fragmentation of protostellar disks. These bursts seem to be more numerous and last longer in the $Z=0.1~Z_odot$ models in comparison to the $Z=0.01~Z_odot$ case. Variable protostellar accretion with episodic bursts is not a particular feature of solar metallicity disks. It is also inherent to gravitationally unstable disks with metallicities up to 100 times lower than solar.
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