Sarah Ragan

The seeds of star formation in the filamentary infrared-dark cloud G011.11-0.12

Context. Infrared-dark clouds (IRDCs) are the precursors to massive stars and stellar clusters. G011.11–0.12 is a well-studied filamentary IRDC, though, to date, the absence of far-infrared data with sufficient spatial resolution has limited the understanding of the structure and star-formation activity. Aims. We use Herschel to study the embedded population of young pre- and protostellar cores in this IRDC. Methods. We examine the cloud structure, which appears in absorption at short wavelength and in emission at longer wavelength. We derive the properties of the massive cores from the spectral energy distributions of bright far-infrared point sources detected with the PACS instrument aboard Herschel. Results. We report on the detection and characterization of pre- and protostellar cores in a massive filamentary infrared-dark cloud G011.11–0.12 using PACS. We characterize 18 cores directly associated with the filament, two of which have masses over 50 M� , making them the best candidates to become massive stars in G011.11−0.12. These cores are likely at various stages of protostar formation, showing elevated temperature (� T �∼ 22 K) with respect to the ambient gas reservoir. The core masses (� M �∼ 24 M� )a re small compared to that in the cold filament. The mean core separation is 0.9 pc, well in excess of the Jeans length in the filament. Conclusions. We confirm that star formation in IRDCs is underway and diverse, and IRDCs have the capability of forming massive stars and clusters.

The Earliest Phases of Star Formation (EPoS): a Herschel key program - The precursors to high-mass stars and clusters

Context. Stars are born deeply embedded in molecular clouds. In the earliest embedded phases, protostars emit the bulk of their radiation in the far-infrared wavelength range, where Herschel is perfectly suited to probe at high angular resolution and dynamic range. In the high-mass regime, the birthplaces of protostars are thought to be in the high-density structures known as infrared-dark clouds (IRDCs). While massive IRDCs are believed to have the right conditions to give rise to massive stars and clusters, the evolutionary sequence of this process is not well-characterized. Aims: As part of the Earliest Phases of Star formation (EPoS) Herschel guaranteed time key program, we isolate the embedded structures within IRDCs and other cold, massive molecular clouds. We present the full sample of 45 high-mass regions which were mapped at PACS 70, 100, and 160 μm and SPIRE 250, 350, and 500 μm. In the present paper, we characterize a population of cores which appear in the PACS bands and place them into context with their host molecular cloud and investigate their evolutionary stage. Methods: We construct spectral energy distributions (SEDs) of 496 cores which appear in all PACS bands, 34% of which lack counterparts at 24 μm. From single-temperature modified blackbody fits of the SEDs, we derive the temperature, luminosity, and mass of each core. These properties predominantly reflect the conditions in the cold, outer regions. Taking into account optical depth effects and performing simple radiative transfer models, we explore the origin of emission at PACS wavelengths. Results: The core population has a median temperature of 20 K and has masses and luminosities that span four to five orders of magnitude. Cores with a counterpart at 24 μm are warmer and bluer on average than cores without a 24 μm counterpart. We conclude that cores bright at 24 μm are on average more advanced in their evolution, where a central protostar(s) have heated the outer bulk of the core, than 24 μm-dark cores. The 24 μm emission itself can arise in instances where our line of sight aligns with an exposed part of the warm inner core. About 10% of the total cloud mass is found in a given clouds core population. We uncover over 300 further candidate cores which are dark until 100 μm. These are possibly starless objects, and further observations will help us determine the nature of these very cold cores.

Hierarchical fragmentation and differential star formation in the Galactic ‘Snake’: infrared dark cloud G11.11−0.12

We present Submillimeter Array (SMA) λ = 0.88 and 1.3 mm broad-band observations, and Very Large Array (VLA) observations in NH_3 (J, K) = (1,1) up to (5,5), H_2O and CH_3OH maser lines towards 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 ∼1 pc clump scale down to ∼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 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 NH_3 ortho/para ratios of 1.1 ± 0.4, 2.0 ± 0.4, and 3.0 ± 0.7 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.

The Earliest Phases of Star formation (EPoS) observed with Herschel : the dust temperature and density distributions of B68

Context. Isolated starless cores within molecular clouds can be used as a testbed to investigate the conditions prior to the onset of fragmentation and gravitational proto-stellar collapse. Aims. We aim to determine the distribution of the dust temperature and the density of the starless core B68. Methods. In the framework of the Herschel guaranteed-time key programme “The Earliest Phases of Star formation” (EPoS), we have imaged B68 between 100 and 500 μm. Ancillary data at (sub)millimetre wavelengths, spectral line maps of the 12 CO (2–1), and 13 CO (2–1) transitions, as well as an NIR extinction map were added to the analysis. We employed a ray-tracing algorithm to derive the 2D mid-plane dust temperature and volume density distribution without suffering from the line-of-sight averaging effects of simple SED fitting procedures. Additional 3D radiative transfer calculations were employed to investigate the connection between the external irradiation and the peculiar crescent-shaped morphology found in the FIR maps. Results. For the first time, we spatially resolve the dust temperature and density distribution of B68, convolved to a beam size of 36. �� 4. We find a temperature gradient dropping from (16.7 +1.3 −1.0 ) K at the edge to (8.2

Giant molecular filaments in the Milky Way

Throughout the Milky Way, molecular clouds typically appear filamentary, and mounting evidence indicates that this morphology plays an important role in star formation. What is not known is to what extent the dense filaments most closely associated with star formation are connected to the surrounding diffuse clouds up to arbitrarily large scales. How are these cradles of star formation linked to the Milky Way’s spiral structure? Using archival Galactic plane survey data, we have used multiple datasets in search of large-scale, velocity-coherent filaments in the Galactic plane. In this paper, we present our methods employed to identify coherent filamentary structures first in extinction and confirmed using Galactic Ring Survey data. We present a sample of seven giant molecular filaments (GMFs) that have lengths on the order of ~100 pc, total masses of 104–105 M⊙, and exhibit velocity coherence over their full length. The GMFs we study appear to be inter-arm clouds and may be the Milky Way analogs to spurs observed in nearby spiral galaxies. We find that between 2 and 12% of the total mass (above ~1020 cm-2) is “dense” (above 1022 cm-2), where filaments near spiral arms in the Galactic midplane tend to have higher dense gas mass fractions than those further from the arms.

The dynamics and star-forming potential of the massive Galactic centre cloud G0.253+0.016

Context. The massive infrared dark cloud G0.253+0.016 projected ∼45 pc from the Galactic centre contains ∼10 5 Mof dense gas whilst being mostly devoid of observed star-formation tracers. Aims. Our goals are therefore to scrutinise the physical properties, dynamics and structure of this cloud with reference to its star- forming potential. Methods. We have carried out a concerted SMA and IRAM 30 m study of this enigmatic cloud in dust continuum, CO isotopologues, several shock tracing molecules, as well as H2CO to trace the gas temperature. In addition, we include ancillary far-IR and sub-mm Herschel and SCUBA data in our analysis. Results. We detect and characterise a total of 36 dust cores within G0.253+0.016 at 1.3 mm and 1.37 mm, with masses between 25 and approximately 250 M� , and find that the kinetic temperature of the gas traced by H2CO ratios is >320 K on size-scales of ∼0.15 pc. Analysis of the position-velocity diagrams of our observed lines shows broad linewidths and strong shock emission in the south of the cloud, indicating that G0.253+0.016 is colliding with another cloud at vLSR ∼ 70 km s −1 . We confirm via an analysis of the observed dynamics in the Central Molecular Zone that it is an elongated structure, orientated with Sgr B2 closer to the Sun than Sgr A*, however our results suggest that the actual geometry may be more complex than an elliptical ring. We find that the column density probability distribution function of G0.253+0.016 derived from SMA and SCUBA dust continuum emission is log-normal with no discernible power-law tail, consistent with little star formation, and that its width can be explained in the framework of theory predicting the density structure of clouds created by supersonic, magnetised turbulence. We also present the Δ-variance spectrum of this region, a proxy for the density power spectrum of the cloud, and show it is consistent with that expected for clouds with no current star formation. Finally, we show that even after determining a scaled column density threshold for star formation by incorporating the effects of the increased turbulence in the cloud, we would still expect ten stars with masses >15 Mto form in G0.253+0.016. If these cannot be accounted for by new radio continuum observations, then further physical aspects may be important, such as the background column density level, which would turn an absolute column density threshold for star formation into a critical over-density. Conclusions. We conclude that G0.253+0.016 contains high-temperatures and wide-spread shocks, displaying evidence of interaction with a nearby cloud which we identify at v LSR ∼ 70 km s −1 . Our analysis of the structure of the cloud can be well-explained by theory of magnetised turbulence, and is consistent with little or no current star formation. Using G0.253+0.016 as a test-bed of the conditions required for star formation in a different physical environment to that of nearby clouds, we also conclude that there is not one column density threshold for star formation, but instead this value is dependant on the local physical conditions.

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 and Very Large Array 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-1) increases in the linewidth that exhibit maxima near or coincident with the mid-infrared emission tracing protostars. These linewidth 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.

Fragmentation and dynamical collapse of the starless high-mass star-forming region IRDC 18310-4

Context. Because of their short evolutionary time-scales, the earliest stages of high-mass star formation prior to the existence of any embedded heating source have barely been characterized until today. Aims: We study the fragmentation and dynamical properties of a massive starless gas clump at the onset of high-mass star formation. Methods: Based on Herschel continuum data we identify a massive gas clump that remains far-infrared dark up to 100 μm wavelengths. The fragmentation and dynamical properties are investigated by means of Plateau de Bure Interferometer and Nobeyama 45 m single-dish spectral line and continuum observations. Results: The massive gas reservoir (between ~800 and ~1600 M⊙, depending on the assumed dust properties) fragments at spatial scales of ~18 000 AU in four cores. Comparing the spatial extent of this high-mass region with intermediate- to low-mass starless cores from the literature, we find that linear sizes do not vary significantly over the whole mass regime. However, the high-mass regions squeeze much more gas into these similar volumes and hence have orders of magnitude larger densities. The fragmentation properties of the presented low-to high-mass regions are consistent with gravitational instable Jeans fragmentation. Furthermore, we find multiple velocity components associated with the resolved cores. Recent radiative transfer hydrodynamic simulations of the dynamic collapse of massive gas clumps also result in multiple velocity components along the line of sight because of the clumpy structure of the regions. This result is supported by a ratio between viral and total gas mass for the whole region <1. Conclusions: This apparently still starless high-mass gas clump exhibits clear signatures of early fragmentation and dynamic collapse prior to the formation of an embedded heating source. A comparison with regions of lower mass reveals that the linear size of star-forming regions does not necessarily have to vary much for different masses, however, the mass reservoirs and gas densities are orders of magnitude enhanced for high-mass regions compared to their lower-mass siblings.

The onset of high-mass star formation in the direct vicinity of the Galactic mini-starburst W43

Context. The earliest stages of high-mass star formation are still poorly characterized. Densities, temperatures and kinematics are crucial parameters for simulations of high-mass star formation. It is also unknown whether the initial conditions vary with environment. Aims: We want to investigate the youngest massive gas clumps in the environment of extremely active star formation. Methods: We selected the IRDC 18454 complex, directly associated with the W43 Galactic mini-starburst, and observed it in the continuum emission between 70 μm and 1.2 mm with Herschel, APEX and the 30 m telescope, and in spectral line emission of N2H+ and 13CO with the Nobeyama 45 m, the IRAM 30 m and the Plateau de Bute Interferometer. Results: The multi-wavelength continuum study allows us to identify clumps that are infrared dark even at 70 μm and hence the best candidates to be genuine high-mass starless gas clumps. The spectral energy distributions reveal elevated temperatures and luminosities compared to more quiescent environments. Furthermore, we identify a temperature gradient from the W43 mini-starburst toward the starless clumps. We discuss whether the radiation impact of the nearby mini-starburst changes the fragmentation properties of the gas clumps and by that maybe favors more high-mass star formation in such an environment. The spectral line data reveal two different velocity components of the gas at 100 and 50 km s-1. While chance projection is a possibility to explain these components, the projected associations of the emission sources as well as the prominent location at the Galactic bar - spiral arm interface also allow the possibility that these two components may be spatially associated and even interacting. Conclusions: High-mass starless gas clumps can exist in the close environment of very active star formation without being destroyed. The impact of the active star formation sites may even allow for more high-mass stars to form in these 2nd generation gas clumps. This particular region near the Galactic bar - spiral arm interface has a broad distribution of gas velocities, and cloud interactions may be possible.

THOR: The H I, OH, Recombination line survey of the Milky Way. The pilot study: H I observations of the giant molecular cloud W43

To study the atomic, molecular, and ionized emission of giant molecular clouds (GMCs) in the Milky Way, we initiated a large program with the Karl G. Jansky Very Large Array (VLA): “THOR: The H i, OH, Recombination line survey of the Milky Way”. We map the 21 cm H i line, 4 OH lines, up to 19 Hα recombination lines and thecontinuum from 1 to 2 GHz of a significant fraction of the Milky Way (l = 15°−67°, | b | ≤ 1°) at an angular resolution of ~ 20″. Starting in 2012, as a pilot study we mapped 4 square degrees of the GMC associated with the W43 star formation complex. The rest of the THOR survey area was observed during 2013 and 2014. In this paper, we focus on the H i emission from the W43 GMC complex. Classically, the H i 21 cm line is treated as optically thin with properties such as the column density calculated under this assumption. This approach might yield reasonable results for regions of low-mass star formation, however, it is not sufficient to describe GMCs. We analyzed strong continuum sources to measure the optical depth along the line of sight, and thus correct the H i 21 cm emission for optical depth effects and weak diffuse continuum emission. Hence, we are able to measure the H i mass of this region more accurately and our analysis reveals a lower limit for the H i mass of M = 6.6-1.8 × 106 M⊙ (vLSR = 60−120 km s-1), which is a factor of 2.4 larger than the mass estimated with the assumption of optically thin emission. The H i column densities are as high as NH i ~ 150 M⊙ pc-2 ≈ 1.9 × 1022 cm-2, which is an order of magnitude higher than for low-mass star formation regions. This result challenges theoretical models that predict a threshold for the H i column density of ~10 M⊙ pc-2, at which the formation of molecular hydrogen should set in. By assuming an elliptical layered structure for W43, we estimate the particle density profile. For the atomic gas particle density, we find a linear decrease toward the center of W43 with values decreasing from nH i = 20 cm-3 near the cloud edge to almost 0 cm-3 at its center. On the other hand, the molecular hydrogen, traced via dust observations with the Herschel Space Observatory, shows an exponential increase toward the center with densities increasing to nH2> 200 cm-3, averaged over a region of ~10 pc. While atomic and molecular hydrogen are well mixed at the cloud edge, the center of the cloud is strongly dominated by H2 emission. We do not identify a sharp transition between hydrogen in atomic and molecular form. Our results, which challenge current theoretical models, are an important characterization of the atomic to molecular hydrogen transition in an extreme environment.