We study Giant Molecular Cloud (GMC) environments surrounding 10 Infrared Dark Clouds (IRDCs), using $^{13}$CO(1-0) emission from the Galactic Ring Survey. We measure physical properties of these IRDCs/GMCs on a range of scales extending to radii, R,
of 30 pc. By comparing different methods for defining cloud boundaries and for deriving mass surface densities and velocity dispersions, we settle on a preferred CE,$tau$,G method of Connected Extraction in position-velocity space plus Gaussian fitting to opacity-corrected line profiles for velocity dispersion and mass estimation. We examine how cloud definition affects measurements of the magnitude and direction of line-of-sight velocity gradients and velocity dispersions, including associated dependencies on size scale. CE,$tau$,G-defined GMCs show velocity dispersion versus size relations $sigmapropto{s}^{1/2}$, which are consistent with the large-scale gradients being caused by turbulence. However, IRDCs have velocity dispersions that are moderately enhanced above those predicted by this scaling relation. We examine the dynamical state of the clouds finding mean virial parameters $bar{alpha}_{rm{vir}}simeq 1.0$ for GMCs and 1.6 for IRDCs, broadly consistent with models of magnetized virialized pressure-confined polytropic clouds, but potentially indicating that IRDCs have more disturbed kinematics. CE,$tau$,G-defined clouds exhibit a tight correlation of $sigma/R^{1/2}proptoSigma^n$, with $nsimeq0.7$ for GMCs and 1.3 for IRDCs (c.f., a value of 0.5 expected for a population of virialized clouds). We conclude that while GMCs show evidence for virialization over a range of scales, IRDCs may be moderately super virial. Alternatively, IRDCs could be virialized but have systematically different $^{13}$CO gas phase abundances, i.e., due to freeze-out, affecting mass estimations.
High-mass Stars are cosmic engines known to dominate the energetics in the Milky Way and other galaxies. However, their formation is still not well understood. Massive, cold, dense clouds, often appearing as Infrared Dark Clouds (IRDCs), are the nurs
eries of massive stars. No measurements of magnetic fields in IRDCs in a state prior to the onset of high-mass star formation (HMSF) have previously been available, and prevailing HMSF theories do not consider strong magnetic fields. Here, we report observations of magnetic fields in two of the most massive IRDCs in the Milky Way. We show that IRDCs G11.11-0.12 and G0.253+0.016 are strongly magnetized and that the strong magnetic field is as important as turbulence and gravity for HMSF. The main dense filament in G11.11-0.12 is perpendicular to the magnetic field, while the lower density filament merging onto the main filament is parallel to the magnetic field. The implied magnetic field is strong enough to suppress fragmentation sufficiently to allow HMSF. Other mechanisms reducing fragmentation, such as the entrapment of heating from young stars via high mass surface densities, are not required to facilitate HMSF.
Inside-Out Planet Formation (IOPF; Chatterjee & Tan 2014, hereafter CT14) is a scenario for sequential in situ planet formation at the pressure traps of retreating dead zone inner boundaries (DZIBs) motivated to explain the many systems with tightly
packed inner planets (STIPs) discovered by Kepler. The scenario involves build-up of a pebble-dominated protoplanetary ring, supplied by radial drift of pebbles from the outer disk. It may also involve further build-up of planetary masses to gap-opening scales via continued pebble accretion. Here we study radial drift & growth of pebbles delivered to the DZIB in fiducial IOPF disk models.
The spatial morphology and dynamical status of a young, still-forming stellar cluster provide valuable clues on the conditions during the star formation event and the processes that regulated it. We analyze the Orion Nebula Cluster (ONC), utilizing t
he latest censuses of its stellar content and membership estimates over a large wavelength range. We determine the center of mass of the ONC, and study the radial dependence of angular substructure. The core appears rounder and smoother than the outskirts, consistent with a higher degree of dynamical processing. At larger distances the departure from circular symmetry is mostly driven by the elongation of the system, with very little additional substructure, indicating a somewhat evolved spatial morphology or an expanding halo. We determine the mass density profile of the cluster, which is well fitted by a power law that is slightly steeper than a singular isothermal sphere. Together with the ISM density, estimated from average stellar extinction, the mass content of the ONC is insufficient by a factor $sim 1.8$ to reproduce the observed velocity dispersion from virialized motions, in agreement with previous assessments that the ONC is moderately supervirial. This may indicate recent gas dispersal. Based on the latest estimates for the age spread in the system and our density profiles, we find that, at the half-mass radius, 90% of the stellar population formed within $sim 5$-$8$ free-fall times ($t_{rm ff}$). This implies a star formation efficiency per $t_{rm ff}$ of $epsilon_{rm ff}sim 0.04$-$0.07$, i.e., relatively slow and inefficient star formation rates during star cluster formation.
We use observed radial profiles of mass surface densities of total, $Sigma_g$, & molecular, $Sigma_{rm H2}$, gas, rotation velocity & star formation rate (SFR) surface density, $Sigma_{rm sfr}$, of the molecular-rich ($Sigma_{rm H2}geSigma_{rm HI}/2$
) regions of 16 nearby disk galaxies to test several star formation laws: a Kennicutt-Schmidt law, $Sigma_{rm sfr}=A_gSigma_{g,2}^{1.5}$; a Constant Molecular law, $Sigma_{rm sfr}=A_{rm H2}Sigma_{rm H2,2}$; the turbulence-regulated laws of Krumholz & McKee (KM05) and Krumholz et al. (KMT09), a Gas-$Omega$ law, $Sigma_{rm sfr}=B_OmegaSigma_gOmega$; and a shear-driven GMC Collision law, $Sigma_{rm sfr}=B_{rm CC}Sigma_gOmega(1-0.7beta)$, where $betaequiv d {rm ln} v_{rm circ}/d {rm ln} r$. If allowed one free normalization parameter for each galaxy, these laws predict the SFR with rms errors of factors of 1.4 - 1.8. If a single normalization parameter is used by each law for the entire galaxy sample, then rms errors range from factors of 1.5 - 2.1. Although the Constant Molecular law gives the smallest errors, the improvement over KMT, Kennicutt-Schmidt & GMC Collision laws is not especially significant, particularly given the different observational inputs that the laws utilize and the scope of included physics, which ranges from empirical relations to detailed treatment of interstellar medium processes. We next search for variation of star formation law parameters with local & global galactic dynamical properties of disk shear rate (related to $beta$), rotation speed & presence of a bar. We demonstrate with high significance that higher shear rates enhance star formation efficiency per local orbital time. Such a trend is expected if GMC collisions play an important role in star formation, while an opposite trend would be expected if development of disk gravitational instabilities is the controlling physics.
The enormous radiative and mechanical luminosities of massive stars impact a vast range of scales and processes, from the reionization of the universe, to the evolution of galaxies, to the regulation of the interstellar medium, to the formation of st
ar clusters, and even to the formation of planets around stars in such clusters. Two main classes of massive star formation theory are under active study, Core Accretion and Competitive Accretion. In Core Accretion, the initial conditions are self-gravitating, centrally concentrated cores that condense with a range of masses from the surrounding, fragmenting clump environment. They then undergo relatively ordered collapse via a central disk to form a single star or a small-N multiple. In this case, the pre-stellar core mass function has a similar form to the stellar initial mass function. In Competitive Accretion, the material that forms a massive star is drawn more chaotically from a wider region of the clump without passing through a phase of being in a massive, coherent core. In this case, massive star formation must proceed hand in hand with star cluster formation. If stellar densities become very high near the cluster center, then collisions between stars may also help to form the most massive stars. We review recent theoretical and observational progress towards understanding massive star formation, considering physical and chemical processes, comparisons with low and intermediate-mass stars, and connections to star cluster formation.
We present radiation transfer (RT) simulations of evolutionary sequences of massive protostars forming from massive dense cores in environments of high surface densities. The protostellar evolution is calculated with a detailed multi-zone model, with
the accretion rate regulated by feedback from an evolving disk-wind outflow cavity. Disk and envelope evolutions are calculated self-consistently. In this framework, an evolutionary track is determined by three environmental initial conditions: the initial core mass M_c, the mean surface density of the ambient star-forming clump Sigma_cl, and the rotational-to-gravitational energy ratio of the initial core, beta_c. Evolutionary sequences with various M_c, Sigma_cl, beta_c are constructed. We find that in a fiducial model with M_c=60Msun, Sigma_cl=1 g/cm^2 and beta_c=0.02, the final star formation efficiency >~0.43. For each evolutionary track, RT simulations are performed at selected stages, with temperature profiles, SEDs, and images produced. At a given stage the envelope temperature is highly dependent on Sigma_cl, but only weakly dependent on M_c. The SED and MIR images depend sensitively on the evolving outflow cavity, which gradually wides as the protostar grows. The fluxes at <~100 microns increase dramatically, and the far-IR peaks move to shorter wavelengths. We find that, despite scatter caused by different M_c, Sigma_cl, beta, and inclinations, sources at a given evolutionary stage appear in similar regions on color-color diagrams, especially when using colors at >~ 70 microns, where the scatter due to the inclination is minimized, implying that such diagrams can be useful diagnostic tools of evolutionary stages of massive protostars. We discuss how intensity profiles along or perpendicular to the outflow axis are affected by environmental conditions and source evolution.
How do stars that are more massive than the Sun form, and thus how is the stellar initial mass function (IMF) established? Such intermediate- and high-mass stars may be born from relatively massive pre-stellar gas cores, which are more massive than t
he thermal Jeans mass. The Turbulent Core Accretion model invokes such cores as being in approximate virial equilibrium and in approximate pressure equilibrium with their surrounding clump medium. Their internal pressure is provided by a combination of turbulence and magnetic fields. Alternatively, the Competitive Accretion model requires strongly sub-virial initial conditions that then lead to extensive fragmentation to the thermal Jeans scale, with intermediate- and high-mass stars later forming by competitive Bondi-Hoyle accretion. To test these models, we have identified four prime examples of massive (~100Msun) clumps from mid-infrared extinction mapping of infrared dark clouds (IRDCs). Fontani et al. found high deuteration fractions of N2H+ in these objects, which are consistent with them being starless. Here we present ALMA observations of these four clumps that probe the N2D+(3-2) line at 2.3 resolution. We find six N2D+ cores and determine their dynamical state. Their observed velocity dispersions and sizes are broadly consistent with the predictions of the Turbulent Core model of self-gravitating, magnetized (with Alfven Mach number m_A~1) and virialized cores that are bounded by the high pressures of their surrounding clumps. However, in the most massive cores, with masses up to ~60Msun, our results suggest that moderately enhanced magnetic fields (so that m_A~0.3) may be needed for the structures to be in virial and pressure equilibrium. Magnetically regulated core formation may thus be important in controlling the formation of massive cores, inhibiting their fragmentation, and thus helping to establish the stellar IMF.
Measuring the mass distribution of infrared dark clouds (IRDCs) over the wide dynamic range of their column densities is a fundamental obstacle in determining the initial conditions of high-mass star formation and star cluster formation. We present a
new technique to derive high-dynamic-range, arcsecond-scale resolution column density data for IRDCs and demonstrate the potential of such data in measuring the density variance - sonic Mach number relation in molecular clouds. We combine near-infrared data from the UKIDSS/Galactic Plane Survey with mid-infrared data from the Spitzer/GLIMPSE survey to derive dust extinction maps for a sample of ten IRDCs. We then examine the linewidths of the IRDCs using 13CO line emission data from the FCRAO/Galactic Ring Survey and derive a column density - sonic Mach number relation for them. For comparison, we also examine the relation in a sample of nearby molecular clouds. The presented column density mapping technique provides a very capable, temperature independent tool for mapping IRDCs over the column density range equivalent to A_V=1-100 mag at a resolution of 2. Using the data provided by the technique, we present the first direct measurement of the relationship between the column density dispersion, sigma_{N/<N>}, and sonic Mach number, M_s, in molecular clouds. We detect correlation between the variables with about 3-sigma confidence. We derive the relation sigma_{N/<N>} = (0.047 pm 0.016) Ms, which is suggestive of the correlation coefficient between the volume density and sonic Mach number, sigma_{rho/<rho>} = (0.20^{+0.37}_{-0.22}) Ms, in which the quoted uncertainties indicate the 3-sigma range. When coupled with the results of recent numerical works, the existence of the correlation supports the picture of weak correlation between the magnetic field strength and density in molecular clouds (i.e., B ~ rho^{0.5}).
The initial conditions of massive star and star cluster formation are expected to be cold, dense and high column density regions of the interstellar medium, which can reveal themselves via near, mid and even far-infrared absorption as Infrared Dark C
louds (IRDCs). Elucidating the dynamical state of IRDCs thus constrains theoretical models of these complex processes. In particular, it is important to assess whether IRDCs have reached virial equilibrium, where the internal pressure balances that due to the self-gravitating weight of the cloud plus the pressure of the external environmental. We study this question for the filamentary IRDC G035.39-00.33 by deriving mass from combined NIR & MIR extinction maps and velocity dispersion from C18O (1-0) & (2-1) line emission. In contrast to our previous moderately super-virial results based on 13CO emission and MIR-only extinction mapping, with improved mass measurements we now find that the filament is consistent with being in virial equilibrium, at least in its central parsec-wide region where ~1000 M_Sun snakes along several parsecs. This equilibrium state does not require large-scale net support or confinement by magnetic fields.
