18 research outputs found

    The Millimeter Astronomy Legacy Team 90 GHz Survey (MALT90) and ALMA

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    ALMA will revolutionize our understanding of star formation within our galaxy, but before we can use ALMA we need to know where to look. The Millimeter Astronomy Legacy Team 90 GHz (MALT90) Survey is a large international project to map the molecular line emission of over 2,000 dense clumps in the Galactic plane. MALT90 serves as a pathfinder for ALMA, providing a large public database of dense molecular clumps associated with high-mass star formation. In this proceedings, we describe the survey parameters and share early science highlights from the survey, including (1) a comparison between galactic and extragalactic star formation relations, (2) chemical trends in MALT90 clumps, (3) the distribution of high-mass star formation in the Milky Way, and (4) a discussion of the Brick, the target of successful ALMA Cycle 0 and Cycle 1 proposals

    Star and Stellar Cluster Formation: ALMA-SKA Synergies

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    Over the next decade, observations conducted with ALMA and the SKA will reveal the process of mass assembly and accretion onto young stars and will be revolutionary for studies of star formation. Here we summarise the capabilities of ALMA and discuss recent results from its early science observations. We then review infrared and radio variability observations of both young low-mass and high-mass stars. A time domain SKA radio continuum survey of star forming regions is then outlined. This survey will produce radio light-curves for hundreds of young sources, providing for the first time a systematic survey of radio variability across the full range of stellar masses. These light-curves will probe the magnetospheric interactions of young binary systems, the origins of outflows, trace episodic accretion on the central sources and potentially constrain the rotation rates of embedded sources

    Tracing the Conversion of Gas into Stars in Young Massive Cluster Progenitors

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    Whilst young massive clusters (YMCs; MM \gtrsim 104^{4} M_{\odot}, age \lesssim 100 Myr) have been identified in significant numbers, their progenitor gas clouds have eluded detection. Recently, four extreme molecular clouds residing within 200 pc of the Galactic centre have been identified as having the properties thought necessary to form YMCs. Here we utilise far-IR continuum data from the Herschel Infrared Galactic Plane Survey (HiGAL) and millimetre spectral line data from the Millimetre Astronomy Legacy Team 90 GHz Survey (MALT90) to determine their global physical and kinematic structure. We derive their masses, dust temperatures and radii and use virial analysis to conclude that they are all likely gravitationally bound -- confirming that they are likely YMC progenitors. We then compare the density profiles of these clouds to those of the gas and stellar components of the Sagittarius B2 Main and North proto-clusters and the stellar distribution of the Arches YMC. We find that even in these clouds -- the most massive and dense quiescent clouds in the Galaxy -- the gas is not compact enough to form an Arches-like (MM = 2x104^{4} M_{\odot}, Reff_{eff} = 0.4 pc) stellar distribution. Further dynamical processes would be required to condense the resultant population, indicating that the mass becomes more centrally concentrated as the (proto)-cluster evolves. These results suggest that YMC formation may proceed hierarchically rather than through monolithic collapse

    G337.342-0.119 (the "Pebble"): A Cold, Dense, High-Mass Molecular Cloud with Unusually Large Linewidths and a Candidate High-Mass Star Cluster Progenitor

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    Exactly how high-mass star clusters form, especially the young massive clusters (YMCs: age 10410^4 solar masses), remains an open problem, largely because they are so rare that examples of their cold, dense, molecuar progenitors remain elusive. The molecular cloud G337.342-0.119, the `Pebble,' is a candidate for such a cold progenitor. Although G337.342-0.119 was originally identified as four separate ATLASGAL clumps, the similarity in their molecular line velocities and linewidths in the MALT90 dataset demonstrate that these four clumps are in fact one single, coherent cloud. This cloud is unique in the MALT90 survey for its combination of both cold temperatures (Tdust14T_{dust} \sim 14 K) and large linewidths (ΔV10(\Delta V \sim 10 km s1^{-1}). The near/far kinematic distance ambiguity is difficult to resolve for G337.342-0.119. At the near kinematic distance (4.7 kpc), the mass is 5,000 solar masses and the size is 7×27\times2 pc. At the far kinematic distance (11 kpc), the mass is 27,000 solar masses and the size is 15×415 \times 4 pc. The unusually large linewidths of G337.342-0.119 are difficult to reconcile with a gravitationally bound system in equilibrium. If our current understanding of the Galaxy's Long Bar is approximately correct, G337.342-0.119 cannot be located at its end. Rather, it is associated with a large star-forming complex that contains multiple clumps with large linewidths. If G337.342-0.119 is a prototypical cold progenitor for a high-mass cluster, its properties may indicate that the onset of high-mass star cluster formation is dominated by extreme turbulence

    The link between turbulence, magnetic fields, filaments, and star formation in the Central Molecular Zone cloud G0.253+0.016

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    Star formation is primarily controlled by the interplay between gravity, turbulence, and magnetic fields. However, the turbulence and magnetic fields in molecular clouds near the Galactic Center may differ substantially from spiral-arm clouds. Here we determine the physical parameters of the central molecular zone (CMZ) cloud G0.253+0.016, its turbulence, magnetic field and filamentary structure. Using column-density maps based on dust-continuum emission observations with ALMA+Herschel, we identify filaments and show that at least one dense core is located along them. We measure the filament width W_fil=0.17±\pm0.08pc and the sonic scale {\lambda}_sonic=0.15±\pm0.11pc of the turbulence, and find W_fil~{\lambda}_sonic. A strong velocity gradient is seen in the HNCO intensity-weighted velocity maps obtained with ALMA+Mopra, which is likely caused by large-scale shearing of G0.253+0.016, producing a wide double-peaked velocity PDF. After subtracting the gradient to isolate the turbulent motions, we find a nearly Gaussian velocity PDF typical for turbulence. We measure the total and turbulent velocity dispersion, 8.8±\pm0.2km/s and 3.9±\pm0.1km/s, respectively. Using magnetohydrodynamical simulations, we find that G0.253+0.016's turbulent magnetic field B_turb=130±\pm50μ\muG is only ~1/10 of the ordered field component. Combining these measurements, we reconstruct the dominant turbulence driving mode in G0.253+0.016 and find a driving parameter b=0.22±\pm0.12, indicating solenoidal (divergence-free) driving. We compare this to spiral-arm clouds, which typically have a significant compressive (curl-free) driving component (b>0.4). Motivated by previous reports of strong shearing motions in the CMZ, we speculate that shear causes the solenoidal driving in G0.253+0.016 and show that this reduces the star formation rate (SFR) by a factor of 6.9 compared to typical nearby clouds

    Comparing Young Massive Clusters and their Progenitor Clouds in the Milky Way

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    Young massive clusters (YMCs) have central stellar mass surface densities exceeding 104Mpc210^{4} M_{\odot} pc^{-2}. It is currently unknown whether the stars formed at such high (proto)stellar densities. We compile a sample of gas clouds in the Galaxy which have sufficient gas mass within a radius of a few parsecs to form a YMC, and compare their radial gas mass distributions to the stellar mass distribution of Galactic YMCs. We find that the gas in the progenitor clouds is distributed differently than the stars in YMCs. The mass surface density profiles of the gas clouds are generally shallower than the stellar mass surface density profiles of the YMCs, which are characterised by prominent dense core regions with radii ~ 0.1 pc, followed by a power-law tail. On the scale of YMC core radii, we find that there are no known clouds with significantly more mass in their central regions when compared to Galactic YMCs. Additionally, we find that models in which stars form from very dense initial conditions require surface densities that are generally higher than those seen in the known candidate YMC progenitor clouds. Our results show that the quiescent, less evolved clouds contain less mass in their central regions than in the highly star-forming clouds. This suggests an evolutionary trend in which clouds continue to accumulate mass towards their centres after the onset of star formation. We conclude that a conveyor-belt scenario for YMC formation is consistent with the current sample of Galactic YMCs and their progenitor clouds

    The link between solenoidal turbulence and slow star formation in G0.253+0.016

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    Star formation in the Galactic disc is primarily controlled by gravity, turbulence, and magnetic fields. It is not clear that this also applies to star formation near the Galactic Centre. Here we determine the turbulence and star formation in the CMZ cloud G0.253+0.016. Using maps of 3mm dust emission and HNCO intensity-weighted velocity obtained with ALMA, we measure the volume-density variance σρ/ρ0=1.3±0.5\sigma_{\rho/\rho_0} = 1.3 \pm 0.5 and turbulent Mach number M=11±3\mathcal{M} = 11 \pm 3. Combining these with turbulence simulations to constrain the plasma β=0.34±0.35\beta = 0.34 \pm 0.35, we reconstruct the turbulence driving parameter b=0.22±0.12b = 0.22 \pm 0.12 in G0.253+0.016. This low value of bb indicates solenoidal (divergence-free) driving of the turbulence in G0.253+0.016. By contrast, typical clouds in the Milky Way disc and spiral arms have a significant compressive (curl-free) driving component (b>0.4b > 0.4). We speculate that shear causes the solenoidal driving in G0.253+0.016 and show that this may reduce the star formation rate by a factor of 7 compared to nearby clouds

    Infall Signatures in a Prestellar Core embedded in the High-Mass 70 μμm Dark IRDC G331.372-00.116

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    Using Galactic Plane surveys, we have selected a massive (1200 M_\odot), cold (14 K) 3.6-70 μ\mum dark IRDC G331.372-00.116. This IRDC has the potential to form high-mass stars and, given the absence of current star formation signatures, it seems to represent the earliest stages of high-mass star formation. We have mapped the whole IRDC with the Atacama Large Millimeter/submillimeter Array (ALMA) at 1.1 and 1.3 mm in dust continuum and line emission. The dust continuum reveals 22 cores distributed across the IRDC. In this work, we analyze the physical properties of the most massive core, ALMA1, which has no molecular outflows detected in the CO (2-1), SiO (5-4), and H2_2CO (3-2) lines. This core is relatively massive (MM = 17.6 M_\odot), subvirialized (virial parameter αvir=Mvir/M=0.14\alpha_{vir}=M_{vir}/M=0.14), and is barely affected by turbulence (transonic Mach number of 1.2). Using the HCO+^+ (3-2) line, we find the first detection of infall signatures in a relatively massive, prestellar core (ALMA1) with the potential to form a high-mass star. We estimate an infall speed of 1.54 km s1^{-1} and a high accretion rate of 1.96 ×\times 103^{-3} M_\odot yr1^{-1}. ALMA1 is rapidly collapsing, out of virial equilibrium, more consistent with competitive accretion scenarios rather than the turbulent core accretion model. On the other hand, ALMA1 has a mass \sim6 times larger than the clumps Jeans mass, being in an intermediate mass regime (MJ=2.7<MM_{J}=2.7<M\lesssim 30 M_\odot), contrary to what both the competitive accretion and turbulent core accretion theories predict

    Young massive star cluster formation in the Galactic Centre is driven by global gravitational collapse of high-mass molecular clouds

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    Young massive clusters (YMCs) are the most compact, high-mass stellar systems still forming at the present day. The precursor clouds to such systems are, however, rare due to their large initial gas mass reservoirs and rapid dispersal timescales due to stellar feedback. Nonetheless, unlike their high-z counterparts, these precursors are resolvable down to the sites of individually forming stars, and hence represent the ideal environments in which to test the current theories of star and cluster formation. Using high angular resolution (1^{\prime\prime} / 0.05pc) and sensitivity ALMA observations of two YMC progenitor clouds in the Galactic Centre, we have identified a suite of molecular line transitions -- e.g. c-C3_{3}H2_{2} (767-6) -- that are believed to be optically thin, and reliably trace the gas structure in the highest density gas on star-forming core scales. We conduct a virial analysis of the identified core and proto-cluster regions, and show that half of the cores (5/10) and both proto-clusters are unstable to gravitational collapse. This is the first kinematic evidence of global gravitational collapse in YMC precursor clouds at such an early evolutionary stage. The implications are that if these clouds are to form YMCs, then they likely do so via the "conveyor-belt" mode, whereby stars continually form within dispersed dense gas cores as the cloud undergoes global gravitational collapse. The concurrent contraction of both the cluster-scale gas and embedded (proto)stars ultimately leads to the high (proto)stellar density in YMCs
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