We show that hydrodynamic turbulent cloud simulations naturally produce large filaments made up of a network of smaller and coherent subfilaments. Such simulations resemble observations of filaments ...and fibres in nearby molecular clouds. The subfilaments are dynamical features formed at the stagnation points of the turbulent velocity field where shocks dissipate the turbulent energy. They are a ubiquitous feature of the simulated clouds, which appear from the beginning of the simulation and are not formed by gradual fragmentation of larger filaments. Most of the subfilaments are gravitationally subcritical and do not fragment into cores, however, there is also a significant fraction of supercritical subfilaments which break up into star-forming cores. The subfilaments are coherent along their length, and the residual velocities along their spine show that they are subsonically contracting without any ordered rotation on scales of ∼0.1 pc. Accretion flows along the subfilaments can feed material into star-forming cores embedded within the network. The overall mass in subfilaments and the number of subfilaments increases as the cloud evolves. We propose that the formation of filaments and subfilaments is a natural consequence of the turbulent cascade in the complex multiphase interstellar medium. Subfilaments are formed by the high wavenumber, small-scale modes in the turbulent velocity field. These are then stretched by local shear motions and gathered together by a combination of low wavenumber modes and gravitational contraction on larger scales, and by doing so build up the extended filaments observed in column density maps.
The initial physical conditions of high-mass stars and protoclusters remain poorly characterized. To this end, we present the first targeted ALMA Band 6 1.3 mm continuum and spectral line survey ...toward high-mass starless clump candidates, selecting a sample of 12 of the most massive candidates ( ) within . The joint array maps have a high spatial resolution of ( , θsyn 0 8) and have high point-source mass-completeness down to at (or column density sensitivity of ). We discover previously undetected signposts of low-luminosity star formation from and bipolar outflows and other signatures toward 11 out of 12 clumps, showing that current MIR/FIR Galactic plane surveys are incomplete to low- and intermediate-mass protostars ( ), and emphasizing the necessity of high-resolution follow-up. We compare a subset of the observed cores with a suite of radiative transfer models of starless cores. We find a high-mass starless core candidate with a model-derived mass consistent with when integrated over size scales of . Unresolved cores are poorly fit by radiative transfer models of externally heated Plummer density profiles, supporting the interpretation that they are protostellar even without detection of outflows. A high degree of fragmentation with rich substructure is observed toward 10 out of 12 clumps. We extract sources from the maps using a dendrogram to study the characteristic fragmentation length scale. Nearest neighbor separations, when corrected for projection with Monte Carlo random sampling, are consistent with being equal to the clump average thermal Jeans length ( ; i.e., separations equal to ). In the context of previous observations that, on larger scales, see separations consistent with the turbulent Jeans length or the cylindrical thermal Jeans scale ( ), our findings support a hierarchical fragmentation process, where the highest-density regions are not strongly supported against thermal gravitational fragmentation by turbulence or magnetic fields.
ABSTRACT
The formation of stellar clusters dictates the pace at which galaxies evolve, and solving the question of their formation will undoubtedly lead to a better understanding of the Universe as a ...whole. While it is well known that star clusters form within parsec-scale overdensities of interstellar molecular gas called clumps, it is, however, unclear whether these clumps represent the high-density tip of a continuous gaseous flow that gradually leads towards the formation of stars, or a transition within the gas physical properties. Here, we present a unique analysis of a sample of 27 infrared dark clouds embedded within 24 individual molecular clouds that combine a large set of observations, allowing us to compute the mass and velocity dispersion profiles of each, from the scale of tens of parsecs down to the scale of tenths of a parsec. These profiles reveal that the vast majority of the clouds, if not all, are consistent with being self-gravitating on all scales, and that the clumps, on parsec-scale, are often dynamically decoupled from their surrounding molecular clouds, exhibiting steeper density profiles (ρ∝r−2) and flat velocity dispersion profiles (σ∝r0), clearly departing from Larson’s relations. These findings suggest that the formation of star clusters correspond to a transition regime within the properties of the self-gravitating molecular gas. We propose that this transition regime is one that corresponds to the gravitational collapse of parsec-scale clumps within otherwise stable molecular clouds.
ABSTRACT
We present Atacama Large Millimeter/submillimeter Array (ALMA) observations of 85.69- and 99.02-GHz continuum emission and H42α and H40α lines emission from the central 1 kpc of NGC 1808. ...These forms of emission are tracers of photoionizing stars but unaffected by dust obscuration that we use to test the applicability of other commonly star formation metrics. An analysis of the spectral energy distributions shows that free–free emission contributes about 60–90 per cent of the continuum emission in the 85–100-GHz frequency range, dependent on the region. The star formation rate (SFR) derived from the ALMA free–free emission is 3.1 ± 0.3 M⊙ yr−1. This is comparable to the SFRs measured from the infrared emission, mainly because most of the bolometric energy from the heavily obscured region is emitted as infrared emission. The radio 1.5-GHz emission yields an SFR 25 per cent lower than the ALMA value, probably because of the diffusion of the electrons producing the synchrotron emission beyond the star-forming regions. The SFRs measured from the extinction-corrected H α line emission are about 40–65 per cent of the SFR derived from the ALMA data, likely because this metric was not calibrated for high-extinction regions. Some SFRs based on extinction-corrected ultraviolet emission are similar to those from ALMA and infrared data, but given that the ultraviolet terms in the extinction correction equations are very small, these metrics seem inappropriate to apply to this dusty starburst.
ABSTRACT
The physical processes behind the transfer of mass from parsec-scale clumps to massive star-forming cores remain elusive. We investigate the relation between the clump morphology and the ...mass fraction that ends up in its most massive core (MMC) as a function of infrared brightness, i.e. a clump evolutionary tracer. Using Atacama Large Millimeter/submillimeter Array (ALMA) 12 m and Atacama Compact Array, we surveyed six infrared dark hubs in 2.9 mm continuum at ∼3 arcsec resolution. To put our sample into context, we also re-analysed published ALMA data from a sample of 29 high-mass surface density ATLASGAL sources. We characterize the size, mass, morphology, and infrared brightness of the clumps using Herschel and Spitzer data. Within the six newly observed hubs, we identify 67 cores, and find that the MMCs have masses between 15 and 911 M⊙ within a radius of 0.018–0.156 pc. The MMC of each hub contains 3–24 per cent of the clump mass (fMMC), becoming 5–36 per cent once core masses are normalized to the median core radius. Across the 35 clumps, we find no significant difference in the median fMMC values of hub and non-hub systems, likely the consequence of a sample bias. However, we find that fMMC is ∼7.9 times larger for infrared dark clumps compared to infrared bright ones. This factor increases up to ∼14.5 when comparing our sample of six infrared dark hubs to infrared bright clumps. We speculate that hub-filament systems efficiently concentrate mass within their MMC early on during its evolution. As clumps evolve, they grow in mass, but such growth does not lead to the formation of more massive MMCs.
ABSTRACT This study presents a detailed analysis of the GAL045.804 − 0.356 massive star-forming clump. A high-angular resolution and sensitivity observations were conducted using MeerKAT at 1.28 GHz ...and ALMA interferometer at 1.3 mm. Two distinct centimetre radio continuum emissions (source A and source B) were identified within the clump. A comprehensive investigation was carried out on source A, the G45.804 − 0.355 star-forming region (SFR) due to its association with Extended Green Object (EGO), 6.7 GHz methanol maser and the spatial coincidence with the peak of the dust continuum emission at 870 µm. The ALMA observations revealed seven dense dust condensations (MM1–MM7) in source A. The brightest (Sν ∼ 87 mJy) and massive main dense core, MM1, was co-located with the 6.7 GHz methanol maser. Explorations into the kinematics revealed gas motions characterized by a velocity gradient across the MM1 core. Furthermore, molecular line emission showed the presence of an extended arm-like structure, with a physical size of 0.25 pc × 0.18 pc (∼ 50 000 au × 30 000 au) at a distance of 7.3 kpc. Amongst these arms, two arms were prominently identified in both the dust continuum and some of the molecular lines. A blue-shifted absorption P-Cygni profile was seen in the H2CO line spectrum. The findings of this study are both intriguing and new, utilizing data from MeerKAT and ALMA to investigate the characteristics of the AGAL45 clump. The evidence of spiral arms, the compact nature of the EGO and < 2 km s−1 velocity gradient are all indicative of G45.804 − 0.355 being oriented face-on.
ABSTRACT
We analyse the radio-to-submillimetre spectral energy distribution (SED) for the central pseudo-bulge of NGC 1365 using archival data from the Atacama Large Millimeter/submillimeter Array ...(ALMA) and the Very Large Array. This analysis shows that free–free emission dominates the continuum emission at 50–120 GHz and produces about 75 per cent of the 103 GHz continuum emission. However, the fraction of 103 GHz continuum emission originating from free–free emission varies significantly among different subregions in the pseudo-bulge, particularly for an outflow from the active galactic nuclei (AGN) on the eastern pseudo-bulge where the synchrotron emission produces half of the 103 GHz continuum emission. Free–free emission also dominates at 103 GHz within the central 400 pc diameter region, but this emission is associated with the AGN rather than star formation. The star formation rate (SFR) within the pseudo-bulge derived from the ALMA free–free emission is 8.9 ± 1.1 M⊙ yr−1. This is comparable to the SFR from the mid-infrared emission but higher than the SFR from the extinction-corrected H α line emission, mainly because the pseudo-bulge is heavily dust obscured. The 1.5 GHz emission yields a comparable SFR for the pseudo-bulge but may have lower SFRs within subregions of the pseudo-bulge because of the diffusion outside of these regions of the electrons producing the synchrotron radiation. We propose that applying a correction factor of 75 per cent to the 80–110 GHz continuum emission could provide valuable estimates of the free–free emission without performing any SED decomposition, which could derive extinction-free SFRs within 20 per cent accuracy.
ABSTRACT We describe a new mm-wave molecular-line mapping survey of the southern Galactic Plane and its first data releases. The Three-mm Ultimate Mopra Milky Way Survey (ThrUMMS) maps a 60° × 2° ...sector of our Galaxy's fourth quadrant, using a combination of fast mapping techniques with the Mopra radio telescope, simultaneously in the lines of 12CO, 13CO, C18O, and CN near 112 GHz at ∼arcminute and ∼0.3 km s−1 resolution, with ∼2 K channel−1 sensitivity for 12CO and ∼1 K channel−1 for the other transitions. The calibrated data cubes from these observations are made available to the community after processing through our pipeline. Here, we describe the motivation for ThrUMMS, the development of new observing techniques for Mopra, and how these techniques were optimized to the objectives of the survey. We showcase some sample data products and describe the first science results on CO-isotopologue line ratios. These vary dramatically across the Galactic Plane, indicating a very wide range of optical depth and excitation conditions, from warm and translucent to cold and opaque. The population of cold clouds in particular have optical depths for 12CO easily exceeding 100. We derive a new, nonlinear conversion law from 12CO integrated intensity to column density, , which suggests that the molecular mass traced by CO in the Galactic disk may have been substantially underestimated. This further suggests that some global relationships in disk galaxies, such as star formation laws, may need to be recalibrated. The large ThrUMMS team is proceeding with several other science investigations.
Abstract
We present Very Large Array (VLA) and Atacama Large Millimeter/submillimeter Array (ALMA) observations of the close (0.″3 = 90 au separation) protobinary system SVS 13. We detect two small ...circumstellar disks (radii ∼12 and ∼9 au in dust, and ∼30 au in gas) with masses of ∼0.004–0.009
M
☉
for VLA 4A (the western component) and ∼0.009–0.030
M
☉
for VLA 4B (the eastern component). A circumbinary disk with prominent spiral arms extending ∼500 au and a mass of ∼0.052
M
☉
appears to be in the earliest stages of formation. The dust emission is more compact and with a very high optical depth toward VLA 4B, while toward VLA 4A the dust column density is lower, allowing the detection of stronger molecular transitions. We infer rotational temperatures of ∼140 K, on scales of ∼30 au, across the whole source, and a rich chemistry. Molecular transitions typical of hot corinos are detected toward both protostars, being stronger toward VLA 4A, with several ethylene glycol transitions detected only toward this source. There are clear velocity gradients, which we interpret in terms of infall plus rotation of the circumbinary disk, and pure rotation of the circumstellar disk of VLA 4A. We measured orbital proper motions and determined a total stellar mass of 1
M
☉
. From the molecular kinematics, we infer the geometry and orientation of the system, and stellar masses of ∼0.26
M
☉
for VLA 4A and ∼0.60
M
☉
for VLA 4B.
We present one of the first Shanghai Tian Ma Radio Telescope (TMRT) K Band observations towards a sample of 26 infrared dark clouds (IRDCs). We observed the (1,1), (2,2), (3,3), and (4,4) transitions ...of NH
3
together with CCS (2
1
–>1
0
) and HC
3
N
J
= 2–1, simultaneously. The survey dramatically increases the existing CCS-detected IRDC sample from 8 to 23, enabling a better statistical study of the ratios of carbon-chain molecules (CCM) to N-bearing molecules in IRDCs. With the newly developed hyperfine group ratio (HFGR) method of fitting NH
3
inversion lines, we found the gas temperature to be between 10 and 18 K. The column density ratios of CCS to NH
3
for most of the IRDCs are less than 10
−2
, distinguishing IRDCs from low-mass star-forming regions. We carried out chemical evolution simulations based on a three-phase chemical model NAUTILUS. Our measurements of the column density ratios between CCM and NH
3
are consistent with chemical evolutionary ages of ≲10
5
yr in the models. Comparisons of the data and chemical models suggest that CCS, HC
3
N, and NH
3
are sensitive to the chemical evolutionary stages of the sources.
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