arXiv subjects

Jonathan C. Tan

Publications and source records attributed to Jonathan C. Tan.

At least 199 records · Page 11Linked to original sources

arXiv2014

The dynamical properties of dense filaments in the infrared dark cloud G035.39-00.33

Infrared Dark Clouds (IRDCs) are unique laboratories to study the initial conditions of high-mass star and star cluster formation. We present high-sensitivity and high-angular resolution IRAM PdBI observations of N2H+ (1-0) towards IRDC G035.39-00.33. It is found that G035.39-00.33 is a highly complex environment, consisting of several mildly supersonic filaments (sigma_NT/c_s ~1.5), separated in velocity by <1 km s^-1 . Where multiple spectral components are evident, moment analysis overestimates the non-thermal contribution to the line-width by a factor ~2. Large-scale velocity gradients evident in previous single-dish maps may be explained by the presence of substructure now evident in the interferometric maps. Whilst global velocity gradients are small (<0.7 km s^-1 pc^-1), there is evidence for dynamic processes on local scales (~1.5-2.5 km s^-1 pc^-1 ). Systematic trends in velocity gradient are observed towards several continuum peaks. This suggests that the kinematics are influenced by dense (and in some cases, starless) cores. These trends are interpreted as either infalling material, with accretion rates ~(7 \pm 4)x10^-5 M_sun yr^-1 , or expanding shells with momentum ~24 \pm 12 M_sun km s^-1 . These observations highlight the importance of high-sensitivity and high-spectral resolution data in disentangling the complex kinematic and physical structure of massive star forming regions.

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arXiv2014

The Darkest Shadows: Deep Mid-Infrared Extinction Mapping of a Massive Protocluster

We use deep $8\:μm$ Spitzer-IRAC imaging of a massive Infrared Dark Cloud (IRDC) G028.37+00.07 to construct a Mid-Infrared (MIR) extinction map that probes mass surface densities up to $Σ\:\sim 1\:\rm{g~cm^{-2}}$ ($A_V\sim200\:$mag), amongst the highest values yet probed by extinction mapping. Merging with a NIR extinction map of the region, creates a high dynamic range map that reveals structures down to $A_V\sim1\:$mag. We utilize the map to: (1) Measure a cloud mass $\sim7\times10^4\:M_\odot$ within a radius of $\sim8\:$pc. $^{13}$CO kinematics indicate that the cloud is gravitationally bound. It thus has the potential to form one of the most massive young star clusters known in the Galaxy. (2) Characterize the structures of 16 massive cores within the IRDC, finding they can be fit by singular polytropic spheres with $ρ\propto{r}^{-k_ρ}$ and $k_ρ=1.3\pm0.3$. They have $\overlineΣ\simeq0.1-0.4\:\rm{g~cm^{-2}}$ --- relatively low values that, along with their measured cold temperatures, suggest magnetic fields, rather than accretion-powered radiative heating, are important for controlling fragmentation of these cores. (3) Determine the $Σ$ (equivalently column density or $A_V$) probability distribution function (PDF) for a region that is near complete for $A_V>3\:$mag. The PDF is well fit by a single log-normal with mean $\overline{A}_V\simeq9\:$mag, high compared to other known clouds. It does not exhibit a separate high-end power law tail, which has been claimed to indicate the importance of self-gravity. However, we suggest that the PDF does result from a self-similar, self-gravitating hierarchy of structure being present over a wide range of scales in the cloud.

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arXiv2013

Far-infrared extinction mapping of infrared dark clouds

Progress in understanding star formation requires detailed observational constraints on the initial conditions, i.e. dense clumps and cores in giant molecular clouds that are on the verge of gravitational instability. Such structures have been studied by their extinction of Near-Infrared (NIR) and, more recently, Mid-Infrared (MIR) background light. It has been somewhat more of a surprise to find that there are regions that appear as dark shadows at Far-Infrared (FIR) wavelengths as long as $\sim$100$μm$. Here we develop analysis methods of FIR images from Spitzer-MIPS and Herschel-PACS that allow quantitative measurements of cloud mass surface density, $Σ$. The method builds upon that developed for MIR extinction mapping (MIREX) (Butler and Tan 2012), in particular involving a search for independent saturated, i.e. very opaque, regions that allow measurement of the foreground intensity. We focus on three massive starless core/clumps in IRDC G028.37+00.07, deriving mass surface density maps from 3.5 to 70$μm$. A by-product of this analysis is measurement of the spectral energy distribution of the diffuse foreground emission. The lower opacity at 70$μm$ allows us to probe to higher $Σ$ values, up to $\sim1\:{\rm{g\:cm}^{-2}}$ in the densest parts of the core/clumps. Comparison of the $Σ$ maps at different wavelengths constrains the shape of the MIR-FIR dust opacity law in IRDCs. We find it is most consistent with the thick ice mantle models of Ossenkopf and Henning (1994). There is tentative evidence for grain ice mantle growth as one goes from lower to higher $Σ$ regions.

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arXiv2013

The Dynamics of Massive Starless Cores with ALMA

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 the 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.

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arXiv2013

Inside-Out Planet Formation

The compact multi-transiting planet systems discovered by Kepler challenge planet formation theories. Formation in situ from disks with radial mass surface density, $Σ$, profiles similar to the minimum mass solar nebula (MMSN) but boosted in normalization by factors $\gtrsim 10$ has been suggested. We propose that a more natural way to create these planets in the inner disk is formation sequentially from the inside-out via creation of successive gravitationally unstable rings fed from a continuous stream of small (~cm--m size) "pebbles", drifting inwards via gas drag. Pebbles collect at the pressure maximum associated with the transition from a magneto-rotational instability (MRI)-inactive ("dead zone") region to an inner MRI-active zone. A pebble ring builds up until it either becomes gravitationally unstable to form an $\sim 1\ M_\oplus$ planet directly or induces gradual planet formation via core accretion. The planet may undergo Type I migration into the active region, allowing a new pebble ring and planet to form behind it. Alternatively if migration is inefficient, the planet may continue to accrete from the disk until it becomes massive enough to isolate itself from the accretion flow. A variety of densities may result depending on the relative importance of residual gas accretion as the planet approaches its isolation mass. The process can repeat with a new pebble ring gathering at the new pressure maximum associated with the retreating dead zone boundary. Our simple analytical model for this scenario of inside-out planet formation yields planetary masses, relative mass scalings with orbital radius, and minimum orbital separations consistent with those seen by Kepler. It provides an explanation of how massive planets can form with tightly-packed and well-aligned system architectures, starting from typical protoplanetary disk properties.

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arXiv2013

The Galactic Census of High- and Medium-mass Protostars. II. Luminosities and Evolutionary States of a Complete Sample of Dense Gas Clumps

(Abridged) The Census of High- and Medium-mass Protostars (CHaMP) is the first large-scale (280 degree<l<300 degree, -4 degree<b<2 degree), unbiased, sub-parsec resolution survey of Galactic molecular clumps and their embedded stars. Barnes et al. (2011) presented the source catalog of ~300 clumps based on HCO+(1-0) emission, used to estimate masses M. Here we use archival mid-infrared to mm continuum data to construct spectral energy distributions. Fitting two-temperature grey-body models, we derive bolometric luminosities, L. We find the clumps have 10Lsun<L<1E6.5Lsun and 0.1<L/M<1E3, consistent with theoretical expectations of a clump population that spans a range of instantaneous star formation efficiencies from 0 to ~50%. We thus expect L/M to be a useful, strongly-varying indicator of clump evolution during the star cluster formation process. We find correlations of the ratio of warm to cold component fluxes and of cold component temperature with L/M. We also find a near linear relation between L/M and Spitzer-IRAC specific intensity (surface brightness), which may thus also be useful as a star formation efficiency indicator. The lower bound of the clump $L/M$ distribution suggests the star formation efficiency per free-fall time is epsilon<0.2. We do not find strong correlations of L/M with mass surface density, velocity dispersion or virial parameter. We find a linear relation between L and L_{HCO+(1-0}}, although with large scatter for any given individual clump. Fitting together with extragalactic systems, the linear relation still holds, extending over 10 orders of magnitude in luminosity. The complete nature of the CHaMP survey over a several kiloparsec-scale region allows us to derive a measurement at an intermediate scale bridging those of individual clumps and whole galaxies.

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arXiv2013

Properties of dense cores in clustered massive star-forming regions at high angular resolution

We aim at characterising dense cores in the clustered environments associated with massive star-forming regions. For this, we present an uniform analysis of VLA NH3(1,1) and (2,2) observations towards a sample of 15 massive star-forming regions, where we identify a total of 73 cores, classify them as protostellar, quiescent starless, or perturbed starless, and derive some physical properties. The average sizes and ammonia column densities are 0.06 pc and 10^15 cm^-2, respectively, with no significant differences between the starless and protostellar cores, while the linewidth and rotational temperature of quiescent starless cores are smaller, 1.0 km/s and 16 K, than those of protostellar (1.8 km/s, 21 K), and perturbed starless (1.4 km/s, 19 K) cores. Such linewidths and temperatures for these quiescent starless cores in the surroundings of massive stars are still significantly larger than the typical values measured in starless cores of low-mass star-forming regions, implying an important non-thermal component. We confirm at high angular resolutions the correlations previously found with single-dish telescopes between the linewidth, the temperature of the cores, and the bolometric luminosity. In addition, we find a correlation between the temperature of each core and the incident flux from the most massive star in the cluster, suggesting that the large temperatures measured in the starless cores of our sample could be due to heating from the nearby massive star. A simple virial equilibrium analysis seems to suggest a scenario of a self-similar, self-graviting, turbulent, virialised hierarchy of structures from clumps (0.1-10 pc) to cores (0.05 pc). A closer inspection of the dynamical state taking into account external pressure effects, reveal that relatively strong magnetic field support may be needed to stabilise the cores, or that they are unstable and thus on the verge of collapse.

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arXiv2013

A Massive Protostar Forming by Ordered Collapse of a Dense, Massive Core

We present 30 and 40 micron imaging of the massive protostar G35.20-0.74 with SOFIA-FORCAST. The high surface density of the natal core around the protostar leads to high extinction, even at these relatively long wavelengths, causing the observed flux to be dominated by that emerging from the near-facing outflow cavity. However, emission from the far-facing cavity is still clearly detected. We combine these results with fluxes from the near-infrared to mm to construct a spectral energy distribution (SED). For isotropic emission the bolometric luminosity would be 3.3x10^4 Lsun. We perform radiative transfer modeling of a protostar forming by ordered, symmetric collapse from a massive core bounded by a clump with high mass surface density, Sigma_cl. To fit the SED requires protostellar masses ~20-34 Msun depending on the outflow cavity opening angle (35 - 50 degrees), and Sigma_cl ~ 0.4-1 g cm-2. After accounting for the foreground extinction and the flashlight effect, the true bolometric luminosity is ~ (0.7-2.2)x10^5 Lsun. One of these models also has excellent agreement with the observed intensity profiles along the outflow axis at 10, 18, 31 and 37 microns. Overall our results support a model of massive star formation involving the relatively ordered, symmetric collapse of a massive, dense core and the launching bipolar outflows that clear low density cavities. Thus a unified model may apply for the formation of both low and high mass stars.

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arXiv2013

Radiation Transfer of Models of Massive Star Formation. II. Effects of the Outflow

(Abridged) We present radiation transfer simulations of a massive (8 Msun) protostar forming from a massive (Mc=60 Msun) protostellar core, extending the model developed by Zhang & Tan (2011). The two principal improvements are (1) developing a model for the density and velocity structure of a disk wind that fills the bipolar outflow cavities; and (2) solving for the radially varying accretion rate in the disk due to a supply of mass and angular momentum from the infall envelope and their loss to the disk wind. One consequence of the launching of the disk wind is a reduction in the amount of accretion power that is radiated by the disk. For the transition from dusty to dust-free conditions where gas opacities dominate, we now implement a gradual change as a more realistic approximation of dust destruction. We study how the above effects, especially the outflow, influence the SEDs and the images of the protostar. Dust in the outflow cavity significantly affects the SEDs at most viewing angles. It further attenuates the short-wavelength flux from the protostar, controlling how the accretion disk may be viewed, and contributes a significant part of the near- and mid-IR fluxes. These fluxes warm the disk, boosting the mid- and far-IR emission. We find that for near face-on views, the SED from the near-IR to about 60 micron is very flat, which may be used to identify such systems. We show that the near-facing outflow cavity and its walls are still the most significant features in images up to 70 micron, dominating the mid-IR emission and determining its morphology. The thermal emission from the dusty outflow itself dominates the flux at ~20 micron. The detailed distribution of the dust in the outflow affects the morphology, for example, even though the outflow cavity is wide, at 10 to 20 micron, the dust in the disk wind can make the outflow appear narrower than in the near-IR bands.

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arXiv2012

Molecular Clouds: Internal Properties, Turbulence, Star Formation and Feedback

All stars are born in molecular clouds, and most in giant molecular clouds (GMCs), which thus set the star formation activity of galaxies. We first review their observed properties, including measures of mass surface density, Sigma, and thus mass, M. We discuss cloud dynamics, concluding most GMCs are gravitationally bound. Star formation is highly clustered within GMCs, but overall is very inefficient. We compare properties of star-forming clumps with those of young stellar clusters (YSCs). The high central densities of YSCs may result via dynamical evolution of already-formed stars during and after star cluster formation. We discuss theoretical models of GMC evolution, especially addressing how turbulence is maintained, and emphasizing the importance of GMC collisions. We describe how feedback limits total star formation efficiency, epsilon, in clumps. A turbulent and clumpy medium allows higher epsilon, permitting formation of bound clusters even when escape speeds are less than the ionized gas sound speed.

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arXiv2012

Kiloparsec-Scale Simulations of Star Formation in Disk Galaxies. I. The unmagnetized and zero-feedback limit

We present hydrodynamic simulations of the evolution of self-gravitating dense gas on scales of 1 kiloparsec down to < parsec in a galactic disk, designed to study dense clump formation from giant molecular clouds (GMCs). These structures are expected to be the precursors to star clusters and this process may be the rate limiting step controling star formation rates in galactic systems as described by the Kennicutt-Schmidt relation. We follow the thermal evolution of the gas down to ~5K using extinction-dependent heating and cooling functions. We do not yet include magnetic fields or localized stellar feedback, so the evolution of the GMCs and clumps is determined solely by self-gravity balanced by thermal and turbulent pressure support and the large scale galactic shear. While cloud structures and densities change significantly during the simulation, GMC virial parameters remain mostly above unity for time scales exceeding the free-fall time of GMCs indicating that energy from galactic shear and large-scale cloud motions continuously cascades down to and within the GMCs. We implement star formation at a slow, inefficient rate of 2% per local free-fall time, but even this yields global star formation rates that are about two orders of magnitude larger than the observed Kennicutt-Schmidt relation due to over-production of dense gas clumps. We expect a combination of magnetic support and localized stellar feedback is required to inhibit dense clump formation to ~1% of the rate that results from the nonmagnetic, zero-feedback limit.

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arXiv2012

Kiloparsec-Scale Simulations of Magnetised Molecular Clouds in Disk Galaxies

We present simulations of the evolution of self-gravitating dense gas on kiloparsec-size scales in a galactic disk, designed to study dense clump formation from giant molecular clouds (GMCs). These dense clumps are expected to be the precursors to star clusters and this process may be the rate limiting step controling star formation rates in galactic systems as described by the Kennicutt-Schmidt relation. The evolution of these simulated GMCs and clumps is determined by self-gravity balanced by turbulent pressure support and the large scale galactic shear. While the cloud structures and densities significantly change during their evolution, they remain roughly in virial equilibrium for time scales exceeding the free-fall time of GMCs, indicating that energy from the galactic shear continuously cascades down. We implement star formation at a slow, inefficient rate of 2% per local free-fall time, but this yields global star formation rates that are more than ~two orders of magnitude larger than the observed Kennicutt-Schmidt relation due to the over-production of dense clump gas. To explain this discrepancy, we anticipate magnetic fields to provide additional support. Low-resolution simulations indeed show that the magnetic field reduces the star formation rate.

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arXiv2012

High-dynamic-range extinction mapping of infrared dark clouds: Dependence of density variance with sonic Mach number in molecular clouds

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, σ_{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 σ_{N/ } = (0.047 \pm 0.016) Ms, which is suggestive of the correlation coefficient between the volume density and sonic Mach number, σ_{ρ/<ρ>} = (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 ~ ρ^{0.5}).

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arXiv2012

Complex, Quiescent Kinematics in a Highly Filamentary Infrared Dark Cloud

Infrared Dark Clouds (IRDCs) host the initial conditions under which massive stars and stellar clusters form. We have obtained high sensitivity and high spectral resolution observations with the IRAM 30m antenna, which allowed us to perform detailed analysis of the kinematics within one IRDC, G035.39-00.33. We focus on the 1-0 and 3-2 transitions of N2H+, C18O (1-0), and make comparison with SiO (2-1) observations and extinction mapping. Three interacting filaments of gas are found. We report large-scale velocity coherence throughout the cloud, evidenced through small velocity gradients and relatively narrow line widths. This suggests that the merging of these filaments is somewhat "gentle", possibly regulated by magnetic fields. This merging of filaments may be responsible for the weak parsec-scale SiO emission detected by Jimenez-Serra et al. 2010, via grain mantle vaporization. A systematic velocity shift between the N2H+ (1-0) and C18O (1-0) gas throughout the cloud of 0.18 +/- 0.04 kms^{-1} is also found, consistent with a scenario of collisions between filaments which is still ongoing. The N2H+ (1-0) is extended throughout the IRDC and it does not only trace dense cores, as found in nearby low-mass star-forming regions. The average H2 number density across the IRDC is ~ 5 x 10^4 cm^{-3}, at least one order of magnitude larger than in nearby molecular clouds where low-mass stars are forming. A temperature gradient perpendicular to the filament is found. From our study, we conclude that G035.39-00.33 (clearly seen in the extinction map and in N2H+) has been formed via the collision between two relatively quiescent filaments with average densities of ~ 5 x 10^3 cm^{-3}, moving with relative velocities of ~ 5 kms^{-1}. The accumulation of material at the merging points started > 1 Myr ago and it is still ongoing.

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arXiv2012

A Virialized Filamentary Infrared Dark Cloud

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 Clouds (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.

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arXiv2012

Gravitational Slingshot of Young Massive Stars in Orion

The Orion Nebula Cluster (ONC) is the nearest region of massive star formation and thus a crucial testing ground for theoretical models. Of particular interest amongst the ONC's ~1000 members are: θ^1 Ori C, the most massive binary in the cluster with stars of masses 38 and 9 MSun (Kraus et al. 2009); the Becklin-Neugebauer (BN) object, a 30 km/s runaway star of ~8 MSun (Tan 2004); and the Kleinmann-Low (KL) nebula protostar, a highly-obscured, ~15 MSun object still accreting gas while also driving a powerful, apparently "explosive" outflow (Allen & Burton 1993). The unusual behavior of BN and KL is much debated: How did BN acquire its high velocity? How is this related to massive star formation in the KL nebula? Here we report the results of a systematic survey using ~ 10^7 numerical experiments of gravitational interactions of the θ^1C and BN stars. We show that dynamical ejection of BN from this triple system at its observed velocity leaves behind a binary with total energy and eccentricity matching those observed for θ^1C. Five other observed properties of θ^C are also consistent with it having ejected BN and altogether we estimate there is only a <~ 10^{-5} probability that θ^1C has these properties by chance. We conclude that BN was dynamically ejected from the θ^1C system about 4,500 years ago. BN has then plowed through the KL massive-star-forming core within the last 1,000 years causing its recently-enhanced accretion and outflow activity.

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arXiv2012

Mid-Infrared Extinction Mapping of Infrared Dark Clouds II. The Structure of Massive Starless Cores and Clumps

(abridged) We develop the mid-infrared extinction (MIREX) mapping technique of Butler & Tan (2009, Paper I), presenting a new method to correct for the Galactic foreground emission based on observed saturation in independent cores. Using Spitzer GLIMPSE 8 micron images, this allows us to accurately probe mass surface densities, Sigma, up to ~0.5g/cm^2 with 2" resolution. We then characterize the structure of 42 massive starless and early-stage IRDC cores and their surrounding clumps, measuring Sigma_cl(r) from the core/clump centers. We first assess the properties of the core/clump at a scale where the total enclosed mass as projected on the sky is M_cl=60Msun. We find these objects have a mean radius of R_cl~0.1pc, mean Sigma_cl=0.3g/cm^2 and, if fit by a power law density profile rho_cl ~ r^{-k_{rho,cl}}, a mean value of k_{rho,cl}=1.1. If we assume a core is embedded in each clump and subtract the surrounding clump envelope to derive the core properties, we find a mean core density power law index of k_{rho,c} = 1.6. We repeat this analysis as a function of radius and derive the best-fitting power law plus uniform clump envelope model for each of the 42 core/clumps. The cores have typical masses of M_c~100Msun and mean Sigma_c~0.1g/cm^2, and are embedded in clumps with comparable mass surface densities. We conclude massive starless cores exist and are well-described by singular polytropic spheres. Their relatively low values of Sigma and the fact that they are IR dark may imply that their fragmentation is inhibited by magnetic fields rather than radiative heating. Comparing to massive star-forming cores, there is tentative evidence for an evolution towards higher densities and steeper density profiles as star formation proceeds.

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arXiv2011

Mapping Large-Scale CO Depletion in a Filamentary Infrared Dark Cloud

Infrared Dark Clouds (IRDCs) are cold, high mass surface density and high density structures, likely to be representative of the initial conditions for massive star and star cluster formation. CO emission from IRDCs has the potential to be useful for tracing their dynamics, but may be affected by depleted gas phase abundances due to freeze-out onto dust grains. Here we analyze C18O J=1-0 and J=2-1 emission line data, taken with the IRAM 30m telescope, of the highly filamentary IRDC G035.39.-0033. We derive the excitation temperature as a function of position and velocity, with typical values of ~7K, and thus derive total mass surface densities, Sigma_C18O, assuming standard gas phase abundances and accounting for optical depth in the line, which can reach values of ~1. The mass surface densities reach values of ~0.07 g/cm^2. We compare these results to the mass surface densities derived from mid-infrared (MIR) extinction mapping, Sigma_SMF, by Butler & Tan, which are expected to be insensitive to the dust temperatures in the cloud. With a significance of >10sigma, we find Sigma_C18O/Sigma_SMF decreases by about a factor of 5 as Sigma increases from ~0.02 to ~0.2 g/cm^2, which we interpret as evidence for CO depletion. Several hundred solar masses are being affected, making this one of the most massive clouds in which CO depletion has been observed directly. We present a map of the depletion factor in the filament and discuss implications for the formation of the IRDC.

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