APEX/SABOCA observations of small-scale structure of infrared-dark clouds I. Early evolutionary stages of star-forming cores

Infrared-dark clouds (IRDCs) harbor the early phases of cluster and high-mass star formation and are comprised of cold (~20 K), dense (n > 10$^4$ cm$^{-3}$) gas. The spectral energy distribution (SED) of IRDCs is dominated by the far-infrared and millimeter wavelength regime, and our initial Herschel study examined IRDCs at the peak of the SED with high angular resolution. Here we present a follow-up study using the SABOCA instrument on APEX which delivers 7.8" angular resolution at 350 micron, matching the resolution we achieved with Herschel/PACS, and allowing us to characterize substructure on ~0.1pc scales. Our sample of 11 nearby IRDCs are a mix of filamentary and clumpy morphologies, and the filamentary clouds show significant hierarchical structure, while the clumpy IRDCs exhibit little hierarchical structure. All IRDCs, regardless of morphology, have about 14% of their total mass in small scale core-like structures which roughly follow a trend of constant volume density over all size scales. Out of the 89 protostellar cores we identified in this sample with Herschel, we recover 40 of the brightest and re-fit their SEDs and find their properties agree fairly well with our previous estimates ( ~ 19K). We detect a new population of "cold cores" which have no 70 micron counterpart, but are 100 and 160 micron-bright, with colder temperatures ( ~ 16K). This latter population, along with SABOCA-only detections, are predominantly low-mass objects, but their evolutionary diagnostics are consistent with the earliest starless or prestellar phase of cores in IRDCs.Comment: accepted to A&A. 28 pages, 27 figures. For full-resolution image gallery, see http://www.mpia.de/~ragan/saboca.html (v2 includes only minor typographical corrections, changed to agree with published version

Detection of Structure in Infrared-Dark Clouds with Spitzer: Characterizing Star Formation in the Molecular Ring

We have conducted a survey of a sample of infrared-dark clouds (IRDCs) with the Spitzer Space Telescope in order to explore their mass distribution. We present a method for tracing mass using dust absorption against the bright Galactic background at 8 microns. The IRDCs in this sample are comprised of tens of clumps, ranging in sizes from 0.02 to 0.3 pc in diameter and masses from 0.5 to a few 10 Msun, the broadest dynamic range in any clump mass spectrum study to date. Structure with this range in scales confirms that IRDCs are the the precursors to stellar clusters in an early phase of fragmentation. Young stars are distributed in the vicinity of the IRDCs, but the clumps are typically not associated with stars and appear pre-stellar in nature. We find an IRDC clump mass spectrum with a slope of 1.76 +/- 0.05 for masses from 30 to 3000 Msun. This slope is consistent with numerous studies, culled from a variety of observational techniques, of massive star formation regions and is close to the mass function of Galactic stellar clusters and star clusters in other galaxies. We assert that the shape of the mass function is an intrinsic and universal feature of massive star formation regions, that are the birth sites of stellar clusters. As these clouds evolve and their constituent clumps fragment, the mass spectrum will steepen and eventually assume the form of the core mass function that is observed locally.Comment: Accepted to ApJ. 37 pages, 24 figures. Full-resolution versions of the figures are available at http://www.astro.lsa.umich.edu/~seragan/ftp/irdc_figs

Very Large Array Observations of Ammonia in Infrared-dark Clouds. I. Column Density and Temperature Structure

We present Very Large Array observations of NH 3 (1,1) and (2,2) in a sample of six infrared-dark clouds (IRDCs) with distances from 2 to 5 kpc. We find that ammonia serves as an excellent tracer of dense gas in IRDCs, showing no evidence of depletion, and the average abundance in these clouds is 8.1 _ 10 –7 . Our sample consists of four IRDCs with 24 _m embedded protostars and two that appear starless. We calculate the kinetic temperature of the gas in IRDCs and find no significant difference between starless and star-forming IRDCs. We find that the bulk of the gas is between 8 and 13 K, indicating that any embedded or nearby stars or clusters do not affect the gas temperature dramatically. Though IRDCs have temperatures and volume densities on par with local star formation regions of lower mass, they consist of much more mass which induces very high internal pressures. In order for IRDCs to survive as coherent structures, the internal pressure must be balanced by a confining pressure provided by the high concentration of molecular clouds in the spiral arm in which they reside. The high molecular concentration and pressure are roughly consistent with gas dynamics of a bar galaxy.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/90758/1/0004-637X_736_2_163.pd

On the temperature structure of the Galactic Centre cloud G0.253+0.016

We present a series of smoothed particle hydrodynamical models of G0.253+0.016 (also known as 'The Brick'), a very dense molecular cloud that lies close to the Galactic Centre. We explore how its gas and dust temperatures react as we vary the strength of both the interstellar radiation field (ISRF) and the cosmic ray ionisation rate (CRIR). As the physical extent of G0.253+0.016 along our line-of-sight is unknown, we consider two possibilities: one in which the longest axis is that measured in the plane of the sky (9.4 pc in length), and one in which it is along the line of sight, in which case we take it to be 17 pc. To recover the observed gas and dust temperatures, we find find that the ISRF must be around 1000 times the solar neighbourhood value, and the CRIR must be roughly 1E-14 /s, regardless of the geometries studied. For such high values of the CRIR, we find that cooling in the cloud's interior is dominated by neutral oxygen, in contrast to standard molecular clouds, which at the same densities are mainly cooled via CO. Our results suggest that the conditions near G0.253+0.016 are more extreme than those generally accepted for the inner 500 pc of the galaxy.Comment: 6 pages, 4 figures, 1 table, accepted for publication in ApJ Letter

Peering into the Heart of Galactic Star Formation: A Detailed Characterization of Infrared-Dark Clouds.

Everything we know about other galaxies is based on light from massive stars, yet, in our own Galaxy, it's the formation of massive stars that is the least understood. Star formation studies to date have focused on nearby, low-mass regions, but the bulk of star formation takes place in massive clusters, which takes place primarily in the inner-Galaxy, where the bulk of the molecular gas resides. To learn about the conditions under which massive clusters form, we seek out their precursors, called infrared-dark clouds (IRDCs). We present the results of a high-resolution multi-wavelength observational study of IRDCs, which vastly improves our knowledge of the initial conditions of cluster formation. Beginning with IRDC candidates identified with Midcourse Science Experiment (MSX) survey data, we map 41 IRDCs in the N2H+ (1-0), CS (2-1) and C18O (1-0) molecular transitions using the Five College Radio Astronomy Observatory. We examine the stellar content and absorption structure with Spitzer Space Telescope observations of eleven IRDCs, and we use Very Large Array ammonia observations to probe the kinematics and chemistry of six IRDCs. Our comprehensive high-resolution study of IRDCs confirms that these objects are cold and dense precursors to massive stars and clusters. For the first time, we quantify IRDC sub-structure on sub-parsec scales and show the kinematic structure of IRDCs is diverse and depends on associated local star-formation activity. Overall, IRDCs exhibit non-thermal dynamics, suggesting that turbulence and systematic motions dominate. IRDC temperatures are between 8 and 16~K and are mostly flat with hints of a rise near the edges due to external heating. This study shows that IRDCs are a unique star-forming environment, one that dominates the star formation in the Milky Way. Using high-resolution observations, we have quantified the structure, star formation, kinematics, and chemistry of infrared-dark clouds. Our study of sub-structure in particular shows that IRDCs are undergoing fragmentation and are the precursors to star clusters, and thus we have placed IRDCs in context with Galactic star formation. The characterization presented here offers new constraints on theories of molecular cloud fragmentation and clustered star formation.Ph.D.Astronomy and AstrophysicsUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/64748/1/seragan_1.pd

Very Large Array Observations of Ammonia in Infrared-Dark Clouds II: Internal Kinematics

Infrared-dark clouds (IRDCs) are believed to be the birthplaces of rich clusters and thus contain the earliest phases of high-mass star formation. We use the Green Bank Telescope (GBT) and Very Large Array (VLA) maps of ammonia (NH3) in six IRDCs to measure their column density and temperature structure (Paper 1), and here, we investigate the kinematic structure and energy content. We find that IRDCs overall display organized velocity fields, with only localized disruptions due to embedded star formation. The local effects seen in NH3 emission are not high velocity outflows but rather moderate (few km/s) increases in the line width that exhibit maxima near or coincident with the mid-infrared emission tracing protostars. These line width enhancements could be the result of infall or (hidden in NH3 emission) outflow. Not only is the kinetic energy content insufficient to support the IRDCs against collapse, but also the spatial energy distribution is inconsistent with a scenario of turbulent cloud support. We conclude that the velocity signatures of the IRDCs in our sample are due to active collapse and fragmentation, in some cases augmented by local feedback from stars.Comment: 15 pages, 12 figures, accepted for publication in Ap

Hierarchical fragmentation and differential star formation in the Galactic "Snake": infrared dark cloud G11.11-0.12

We present Submillimeter Array (SMA) $\lambda =$ 0.88 and 1.3 mm broad band observations, and the Jansky Very Large Array (VLA) observations in $\rm{NH_3}$ $(J,K) = (1,1)$ up to $(5,5)$, $\rm{H_2O}$ and $\rm{CH_3OH}$ maser lines toward the two most massive molecular clumps in infrared dark cloud (IRDC) G11.11-0.12. Sensitive high-resolution images reveal hierarchical fragmentation in dense molecular gas from the $\sim 1$ pc clump scale down to $\sim 0.01$ pc condensation scale. At each scale, the mass of the fragments is orders of magnitude larger than the Jeans mass. This is common to all four IRDC clumps we studied, suggesting that turbulence plays an important role in the early stages of clustered star formation. Masers, shock heated $\rm{NH_3}$ gas, and outflows indicate intense ongoing star formation in some cores while no such signatures are found in others. Furthermore, chemical differentiation may reflect the difference in evolutionary stages among these star formation seeds. We find $\rm{NH_3}$ ortho/para ratios of $1.1\pm0.4$, $2.0\pm0.4$, and $3.0\pm0.7$ associated with three outflows, and the ratio tends to increase along the outflows downstream. Our combined SMA and VLA observations of several IRDC clumps present the most in depth view so far of the early stages prior to the hot core phase, revealing snapshots of physical and chemical properties at various stages along an apparent evolutionary sequence.Comment: 21 pages, 11 figures, 8 tables, accepted to MNRAS; this version includes minor typo corrections from proo

Young stars as tracers of a barred-spiral Milky Way

Identifying the structure of our Galaxy has always been fraught with difficulties, and while modern surveys continue to make progress building a map of the Milky Way, there is still much to understand. The arm and bar features are important drivers in shaping the interstellar medium, but their exact nature and influence still require attention. We present results of smoothed particle hydrodynamic simulations of gas in the Milky Way including star formation, stellar feedback, and ISM cooling, when exposed to different arm and bar features, with the aim of better understanding how well newly formed stars trace out the underlying structure of the Galaxy. The bar is given a faster pattern speed than the arms, resulting in a complex, time-dependent morphology and star formation. Inter-arm branches and spurs are easily influenced by the bar, especially in the two-armed spiral models where there is a wide region of resonance overlap in the disc. As the bar over-takes the spiral arms it induces small boosts in star formation and enhances spiral features, which occur at regularly spaced beat-like intervals. The locations of star formation events are similar to those seen in observational data, and do not show a perfect 1:1 correspondence with the underlying spiral potential, though arm tangencies are generally well traced by young stars. Stellar velocity fields from the newly formed stars are compared to data from Gaia DR2, showing that the spiral and bar features can reproduce many of the non-axisymmetric features seen in the data. A simple analytical model is used to show many of these feature are a natural response of gas to rigidly rotating spiral and bar potentials

Characterizing the Youngest Herschel-detected Protostars I. Envelope Structure Revealed by CARMA Dust Continuum Observations

We present CARMA 2.9 mm dust continuum emission observations of a sample of 14 Herschel-detected Class 0 protostars in the Orion A and B molecular clouds, drawn from the PACS Bright Red Sources (PBRS) sample (Stutz et al.). These objects are characterized by very red 24 \micron\ to 70 \micron\ colors and prominent submillimeter emission, suggesting that they are very young Class 0 protostars embedded in dense envelopes. We detect all of the PBRS in 2.9 mm continuum emission and emission from 4 protostars and 1 starless core in the fields toward the PBRS; we also report 1 new PBRS source. The ratio of 2.9 mm luminosity to bolometric luminosity is higher by a factor of $\sim$5 on average, compared to other well-studied protostars in the Perseus and Ophiuchus clouds. The 2.9 mm visibility amplitudes for 6 of the 14 PBRS are very flat as a function of uv-distance, with more than 50\% of the source emission arising from radii $<$ 1500 AU. These flat visibility amplitudes are most consistent with spherically symmetric envelope density profiles with $\rho$~$\propto$~R$^{-2.5}$. Alternatively, there could be a massive unresolved structure like a disk or a high-density inner envelope departing from a smooth power-law. The large amount of mass on scales $<$ 1500 AU (implying high average central densities) leads us to suggest that that the PBRS with flat visibility amplitude profiles are the youngest PBRS and may be undergoing a brief phase of high mass infall/accretion and are possibly among the youngest Class 0 protostars. The PBRS with more rapidly declining visibility amplitudes still have large envelope masses, but could be slightly more evolved.Comment: Accepted to ApJ, 40 pages, 9 Figures, 4 Table