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Register a new userToward gas exhaustion in the W51 high-mass protoclusters
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We present new JVLA observations of the high-mass cluster-forming region W51A from 2 to 16 GHz with resolution ${theta}_{fwhm} approx$ 0.3 - 0.5. The data reveal a wealth of observational results: (1) Currently-forming, very massive (proto-O) stars are traced by o-H2CO $2_{1,1}-2_{1,2}$ emission, suggesting that this line can be used efficiently as a massive protostar tracer. (2) There is a spatially distributed population of $sim$mJy continuum sources, including hypercompact H ii regions and candidate colliding wind binaries, in and around the W51 proto-clusters. (3) There are two clearly detected protoclusters, W51e and W51 IRS2, that are gas-rich but may have most of their mass in stars within their inner $sim$ 0.05 pc. The majority of the bolometric luminosity in W51 most likely comes from a third population of OB stars between these clusters. The presence of a substantial population of exposed O-stars coincident with a population of still-forming massive stars, along with a direct measurement of the low mass loss rate via ionized gas outflow from W51 IRS2, together imply that feedback is ineffective at halting star formation in massive protoclusters. Instead, feedback may shut off the large-scale accretion of diffuse gas onto the W51 protoclusters, implying that they are evolving towards a state of gas exhaustion rather than gas expulsion. Recent theoretical models predict gas exhaustion to be a necessary step in the formation of gravitationally bound stellar clusters, and our results provide an observational validation of this process.
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We observed radio recombination lines (RRLs) toward the W51 molecular cloud complex, one of the most active star forming regions in our Galaxy. The UV radiation from young massive stars ionizes gas surrounding them to produce HII regions. Observations of the W51 IRS1 HII region were made with the Arecibo 305 m telescope. Of the full 1-10 GHz database, we have analyzed the observations between 4.5 and 5 GHz here. The steps involved in the analysis were: a) bandpass calibration using on-source/off-source observations; b) flux density calibration; c) removing spectral baselines due to errors in bandpass calibration and d) Gaussian fitting of the detected lines. We detected alpha, beta and gamma transitions of hydrogen and alpha transitions of helium. We used the observed line parameters to 1) measure the source velocity (56.6 $pm$ 0.3 km s$^{-1}$) with respect to the Local Standard of Rest (LSR); 2) estimate the electron temperature (8500 $pm$ 1800 K) of the HII region and 3) derive the emission measure (5.4 $pm$ 2.7 $times$ 10$^{6}$ pc cm$^{-6}$) of the ionized gas.
We observed the W51 high-mass star-forming complex with ALMAs longest-baseline configurations, achieving an angular resolution of $sim$20 milliarcseconds, corresponding to a linear resolution of $sim$100 au at $D_{mathrm{W51}}=5.4$ kpc. The observed region contains three high-mass protostars in which the dust continuum emission at 1.3 mm is optically-thick up to a radius $lesssim$1000 au and has brightness temperatures $gtrsim$200 K. The high luminosity ($gtrsim10^4$ L$_{odot}$) in the absence of free-free emission suggests the presence of massive stars ($Mgtrsim20$ M$_{odot}$) at the earliest stages of their formation. Our continuum images reveal remarkably complex and filamentary structures arising from compact cores. Molecular emission shows no clear signs of rotation nor infall on scales from 150 to 2000 au: we do not detect disks. The central sources drive young ($sim$100 years), fast ($sim 100$ km s$^{-1}$), powerful ($dot{M}>10^{-4}$ M$_{odot} yr^{-1}$), collimated outflows. These outflows provide indirect evidence of accretion disks on scales $rlesssim$100--500 au (depending on the object). The active outflows are connected to fossil flows that have different orientations on larger spatial scales, implying that the orientations of these small disks change over time. These results together support a variant of an accretion model for high-mass star formation in which massive protostars do not form a large, stable Keplerian disk during their early stages, but instead they accrete material from multiple massive flows with different angular momentum vectors. This scenario therefore contrasts with the simplified classic paradigm of a stable disk+jet system, which is the standard model for low-mass star formation, and provides an experimental confirmation of a multi-directional and unsteady accretion model for massive star formation.
We present the first ALMA dust polarization observations towards the high-mass star-forming regions W51 e2, e8, and W51 North in Band 6 (230 GHz) with a resolution around 0.26 ($sim5$mpc). Polarized emission in all three sources is clearly detected and resolved. Measured relative polarization levels are between 0.1% and 10%. While the absolute polarization shows complicated structures, the relative polarization displays the typical anti-correlation with Stokes $I$, though with a large scatter. Inferred magnetic (B) field morphologies are organized and connected. Detailed substructures are resolved, revealing new features such as cometary-shaped B-field morphologies in satellite cores, symmetrically converging B-field zones, and possibly streamlined morphologies. The local B-field dispersion shows some anti-correlation with the relative polarization. Moreover, lowest polarization percentages together with largest dispersions coincide with B-field convergence zones. We put forward $sinomega$, where $omega$ is the measurable angle between a local B-field orientation and local gravity, as a measure of how effectively the B-field can oppose gravity. Maps of $sinomega$ for all three sources show organized structures that suggest a locally varying role of the B-field, with some regions where gravity can largely act unaffectedly, possibly in a network of narrow magnetic channels, and other regions where the B-field can work maximally against gravity.
We report the discovery of maser emission in the two lowest rotational transitions of CS toward the high-mass protostar W51 e2e with ALMA and the JVLA. The masers from CS J=1-0 and J=2-1 are neither spatially nor spectrally coincident (they are separated by ~150 AU and ~30 km/s), but both appear to come from the base of the blueshifted outflow from this source. These CS masers join a growing list of rarely-detected maser transitions that may trace a unique phase in the formation of high-mass protostars.
We studied the distribution of dense gas in a filamentary molecular cloud containing several dense clumps. The center of the filament is given by the dense clump WB673. The clumps are high-mass and intermediate-mass star-forming regions. We observed CS(2-1), 13CO(1-0), C18O(1-0) and methanol lines at 96GHz toward WB673 with the Onsala Space Observatory 20-m telescope. We found CS(2-1) emission in the inter-clump medium so the clumps are physically connected and the whole cloud is indeed a filament. Its total mass is $10^4$ M$_{odot}$ and mass-to-length ratio is 360 M$_{odot}$pc$^{-1}$ from 13CO(1-0) data. Mass-to-length ratio for the dense gas is $3.4-34$ M$_{odot}$pc$^{-1}$ from CS(2-1) data. The PV-diagram of the filament is V-shaped. We estimated physical conditions in the molecular gas using methanol lines. Location of the filament on the sky between extended shells suggests that it could be a good example to test theoretical models of formation of the filaments via multiple compression of interstellar gas by supersonic waves.
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