K. G. Johnston

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.

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.

The standard model of low-mass star formation applied to massive stars: a multi-wavelength picture of AFGL 2591

Context. While it is currently unclear from a theoretical standpoint which forces and processes dominate the formation of high-mass stars, and hence determine the mode in which they form, much of the recent observational evidence suggests that massive stars are born in a similar manner to their low-mass counterparts. Aims. This paper aims to investigate the hypothesis that the embedded luminous star AFGL 2591-VLA 3 (2.3 × 10 5 Lat 3.33 kpc) is forming according to a scaled-up version of a low-mass star formation scenario. Methods. We present multi-configuration Very Large Array (VLA) 3.6 cm and 7 mm, as well as Combined Array for Research in Millimeter Astronomy C 18 O and 3 mm continuum observations to investigate the morphology and kinematics of the ionized gas, dust, and molecular gas around AFGL 2591. We also compare our results to ancillary Gemini North near-IR images, and model the near-IR to sub-mm spectral energy distribution (SED) and Two Micron All Sky Survey (2MASS) image profiles of AFGL 2591 using a Monte-Carlo dust continuum radiative transfer code. Results. The observed 3.6 cm images uncover for the first time that the central powering source AFGL 2591-VLA 3 has a compact core plus collimated jet morphology, extending 4000 AU eastward from the central source with an opening angle of <10 ◦ at this radius. However, at 7 mm VLA 3 does not show a jet morphology, but instead compact (<500 AU) emission, some of which (<0.57 mJy of 2.9 mJy) is estimated to be from dust emission. The spectral index of AFGL 2591-VLA 3 between 3.6 cm and 7 mm was found to be between 0.4 and 0.5, similar to that of an ionized wind. If the 3.6 cm emission is modelled as an ionized jet, the jet has almost enough momentum to drive the larger-scale flow. However, assuming a shock efficiency of 10%, the momentum rate of the jet is not sufficient to ionize itself via only shocks, and thus a significant portion of the emission is instead likely created in a photoionized wind. The C 18 O emission uncovers dense entrained material in the outflow(s) from these young stars. The main features of the SED and 2MASS images of AFGL 2591-VLA 3 are also reproduced by our model dust geometry of a rotationally flattened envelope with and without ad isk. Conclusions. The above results are consistent with a picture of massive star formation similar to that seen for low-mass protostars. However, within its envelope, AFGL 2591-VLA 3 contains at least four other young stars, constituting a small cluster. Therefore it appears that AFGL 2591-VLA 3 may be able to source its accreting material from a shared gas reservoir while still exhibiting the phenomena expected during the formation of low-mass stars.

Giant molecular filaments in the Milky Way - II. The fourth Galactic quadrant

Filamentary structures are common morphological features of the cold, molecular interstellar medium (ISM). Recent studies have discovered massive, hundred-parsec-scale filaments that may be connected to the large-scale, Galactic spiral arm structure. Addressing the nature of these Giant Molecular Filaments (GMFs) requires a census of their occurrence and properties. We perform a systematic search of GMFs in the fourth Galactic quadrant and determine their basic physical properties. We identify GMFs based on their dust extinction signatures in near- and mid-infrared and velocity structure probed by ^{13}CO line emission. We use the ^{13}CO line emission and ATLASGAL dust emission data to estimate the total and dense gas masses of the GMFs. We combine our sample with an earlier sample from literature and study the Galactic environment of the GMFs. We identify nine GMFs in the fourth Galactic quadrant; six are located in the Centaurus spiral arm and three in inter-arm regions. Combining this sample with an earlier study using the same identification criteria in the first Galactic quadrant results in 16 GMFs, nine of which are located within spiral arms. The GMFs have sizes of 80-160 pc and ^{13}CO-derived masses between 5-90 x 10^{4} Msun. Their dense gas mass fractions are between 1.5-37%, being higher in the GMFs connected to spiral arms. We also compare the different GMF-identification methods and find that emission and extinction based techniques overlap only partially, highlighting the need to use both to achieve a complete census.

Resolving the fragmentation of high line-mass filaments with ALMA: the integral shaped filament in Orion A

We study the fragmentation of the nearest high line-mass filament, the integral shaped filament (ISF, line-mass ~400 M ⊙  pc -1 ) in the Orion A molecular cloud. We have observed a 1.6 pc long section of the ISF with the Atacama Large Millimetre/submillimeter Array (ALMA) at 3 mm continuum emission, at a resolution of ~3″ (1200 AU). We identify from the region 43 dense cores with masses about a solar mass. 60% of the ALMA cores are protostellar and 40% are starless. The nearest neighbour separations of the cores do not show a preferred fragmentation scale; the frequency of short separations increases down to 1200 AU. We apply a two-point correlation analysis on the dense core separations and show that the ALMA cores are significantly grouped at separations below ~17 000 AU and strongly grouped below ~6000 AU. The protostellar and starless cores are grouped differently: only the starless cores group strongly below ~6000 AU. In addition, the spatial distribution of the cores indicates periodic grouping of the cores into groups of ~30 000 AU in size, separated by ~50 000 AU. The groups coincide with dust column density peaks detected by Herschel . These results show hierarchical, two-mode fragmentation in which the maternal filament periodically fragments into groups of dense cores. Critically, our results indicate that the fragmentation models for lower line-mass filaments (~16 M ⊙  pc -1 ) fail to capture the observed properties of the ISF. We also find that the protostars identified with Spitzer and Herschel in the ISF are grouped at separations below ~17 000 AU. In contrast, young stars with disks do not show significant grouping. This suggests that the grouping of dense cores is partially retained over the protostar lifetime, but not over the lifetime of stars with disks. This is in agreement with a scenario where protostars are ejected from the maternal filament by the slingshot mechanism, a model recently proposed for the ISF. The separation distributions of the dense cores and protostars may also provide an evolutionary tracer of filament fragmentation.

Chasing discs around O-type (proto)stars: Evidence from ALMA observations

This work is supported by a H2020 Marie Sklodowska-Curie Action (GESTATE 661249) funded by the European Research Commission. Reproduced with permission from Astronomy & Astrophysics.

First Millimeter Detection of the Disk around a Young, Isolated, Planetary-mass Object

OTS44 is one of only four free-floating planets known to have a disk. We have previously shown that it is the coolest and least massive known free-floating planet (

A survey for hydroxyl in the THOR pilot region around W43

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ABSORPTION FILAMENTS TOWARD THE MASSIVE CLUMP G0.253+0.016

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