Diversity of chemistry and excitation conditions in the high-mass star forming complex W33

The object W33 is a giant molecular cloud that contains star forming regions at various evolutionary stages from quiescent clumps to developed H II regions. Since its star forming regions are located at the same distance and the primary material of the birth clouds is probably similar, we conducted a comparative chemical study to trace the chemical footprint of the different phases of evolution. We observed six clumps in W33 with the Atacama Pathfinder Experiment (APEX) telescope at 280 GHz and the Submillimeter Array (SMA) at 230 GHz. We detected 27 transitions of 10 different molecules in the APEX data and 52 transitions of 16 different molecules in the SMA data. The chemistry on scales larger than $\sim$0.2 pc, which are traced by the APEX data, becomes more complex and diverse the more evolved the star forming region is. On smaller scales traced by the SMA data, the chemical complexity and diversity increase up to the hot core stage. In the H II region phase, the SMA spectra resemble the spectra of the protostellar phase. Either these more complex molecules are destroyed or their emission is not compact enough to be detected with the SMA. Synthetic spectra modelling of the H$_{2}$CO transitions, as detected with the APEX telescope, shows that both a warm and a cold component are needed to obtain a good fit to the emission for all sources except for W33 Main1. The temperatures and column densities of the two components increase during the evolution of the star forming regions. The integrated intensity ratios N$_{2}$H$^{+}$(3$-$2)/CS(6$-$5) and N$_{2}$H$^{+}$(3$-$2)/H$_{2}$CO(4$_{2,2}$$-$3$_{2,1}$) show clear trends as a function of evolutionary stage, luminosity, luminosity-to-mass ratio, and H$_{2}$ peak column density of the clumps and might be usable as chemical clocks.Comment: 66 pages, 28 figures, 8 tables, accepted for publication at A&

Flickering of 1.3 cm Sources in Sgr B2: Towards a Solution to the Ultracompact HII Region Lifetime Problem

Accretion flows onto massive stars must transfer mass so quickly that they are themselves gravitationally unstable, forming dense clumps and filaments. These density perturbations interact with young massive stars, emitting ionizing radiation, alternately exposing and confining their HII regions. As a result, the HII regions are predicted to flicker in flux density over periods of decades to centuries rather than increasing monotonically in size as predicted by simple Spitzer solutions. We have recently observed the Sgr B2 region at 1.3 cm with the VLA in its three hybrid configurations (DnC, CnB and BnA) at a resolution of 0.25''. These observations were made to compare in detail with matched continuum observations from 1989. At 0.25'' resolution, Sgr B2 contains 41 UC HII regions, 6 of which are hypercompact. The new observations of Sgr B2 allow comparison of relative peak flux densites for the HII regions in Sgr B2 over a 23 year time baseline (1989-2012) in one of the most source-rich massive star forming regions in the Milky Way. The new 1.3 cm continuum images indicate that four of the 41 UC HII regions exhibit significant changes in their peak flux density, with one source (K3) dropping in peak flux density, and the other 3 sources (F10.303, F1 and F3) increasing in peak flux density. The results are consistent with statistical predictions from simulations of high mass star formation, suggesting that they offer a solution to the lifetime problem for ultracompact HII regions.Comment: 12 pages, 3 figures, Accepted for publication in the Astrophysical Journal Letter

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

Young massive clusters (YMCs) have central stellar mass surface densities exceeding $10^{4} M_{\odot} pc^{-2}$. 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

Millimeter multiplicity in DR21(OH): outflows, molecular cores and envelopes

We present sensitive high angular resolution ($\sim$ 1$"$) millimeter continuum and line observations from the massive star forming region DR21(OH) located in the Cygnus X molecular cloud. Within the well-known dusty MM1-2 molecular cores, we report the detection of a new cluster of about ten compact continuum millimeter sources with masses between 5 and 24 M$_\odot$, and sizes of a few thousands of astronomical units. These objects are likely to be large dusty envelopes surrounding massive protostars, some of them most probably driving several of the outflows that emanate from this region. Additionally, we report the detection of strong millimeter emission of formaldehyde (H$_2$CO) and methanol (CH$_3$OH) near 218 GHz as well as compact emission from the typical outflow tracers carbon monoxide and silicon monoxide (CO and SiO) toward this massive star-forming region. The H$_2$CO and CH$_3$OH emission is luminous ($\sim$ 10$^{-4}$ L$_{\odot}$), well resolved, and found along the collimated methanol maser outflow first identified at centimeter wavelengths and in the sources SMA6 and SMA7. Our observations suggest that this maser outflow might be energized by a millimeter source called SMA4 located in the MM2 dusty core. The CO and SiO emission traces some other collimated outflows that emanate from MM1-2 cores, and are not related with the low velocity maser outflow.Comment: Accepted to Ap

Time Variability in Simulated Ultracompact and Hypercompact HII Regions

Ultracompact and hypercompact HII regions appear when a star with a mass larger than about 15 solar masses starts to ionize its own environment. Recent observations of time variability in these objects are one of the pieces of evidence that suggest that at least some of them harbor stars that are still accreting from an infalling neutral accretion flow that becomes ionized in its innermost part. We present an analysis of the properties of the HII regions formed in the 3D radiation-hydrodynamic simulations presented by Peters et al. as a function of time. Flickering of the HII regions is a natural outcome of this model. The radio-continuum fluxes of the simulated HII regions, as well as their flux and size variations are in agreement with the available observations. From the simulations, we estimate that a small but non-negligible fraction (~ 10 %) of observed HII regions should have detectable flux variations (larger than 10 %) on timescales of ~ 10 years, with positive variations being more likely to happen than negative variations. A novel result of these simulations is that negative flux changes do happen, in contrast to the simple expectation of ever growing HII regions. We also explore the temporal correlations between properties that are directly observed (flux and size) and other quantities like density and ionization rates.Comment: Monthly Notices of the Royal Astronomical Society, in press. The movie of free-free optical depth can be found at http://www.ita.uni-heidelberg.de/~tpeters/tau.av

A highly-collimated SiO jet in the HH212 protostellar outflow

We mapped the HH212 Class 0 outflow in SiO(2--1, 5--4) and continuum using the PdBI in its extended configurations. The unprecedented angular resolution (down to 0.34") allows accurate comparison with a new, deep H2 image obtained at the VLT. The SiO emission is confined to a highly-collimated bipolar jet (width 0.35") along the outflow axis. The jet can be traced down to within 500 AU of the protostar, in a region that is heavily obscured in H2 images. Where both species are detected, SiO shows the same overall kinematics and structure as H2, indicating that both molecules are tracing the same material. We find that the high-velocity SiO gas near the protostar is not tracing a wide-angle wind but is already confined to a flow inside a narrow cone of half-opening angle < 6 deg.Comment: Astronomy and Astrophysics Letter, in pres

High Resolution CO Observations of Massive Star Forming Regions

Context. To further understand the processes involved in the formation of massive stars, we have undertaken a study of the gas dynamics surrounding three massive star forming regions. By observing the large scale structures at high resolution, we are able to determine properties such as driving source, and spatially resolve the bulk dynamical properties of the gas such as infall and outflow. Aims. With high resolution observations, we are able to determine which of the cores in a cluster forming massive stars is responsible for the large scale structures. Methods. We present CO observations of three massive star forming regions with known HII regions and show how the CO traces both infall and outflow. By combining data taken in two SMA configurations with JCMT observations, we are able to see large scale structures at high resolution. Results. We find large (0.26-0.40 pc), massive (2-3 M_sun) and energetic (13-17 \times 10^44 erg) outflows emanating from the edges of two HII regions suggesting they are being powered by the protostar(s) within. We find infall signatures in two of our sources with mass infall rates of order 10-4 M_sun/yr. Conclusions. We suggest that star formation is ongoing in these sources despite the presence of HII regions. We further conclude that the source(s) within a single HII region are responsible for the observed large scale structures; that these large structures are not the net effect of multiple outflows from multiple HII regions and hot cores.Comment: 8 pages,2 figures, accepted for publication in A&

A VLBI study of the wind-wind collision region in the massive multiple HD 167971

Context. Colliding winds in massive binaries are able to accelerate particles up to relativistic speeds as the result of the interaction between the winds of the different stellar components. HD 167971 exhibits this phenomenology which makes it a strong radio source. Aims. We aim at characterizing the morphology of the radio emission and its dependence on the orbital motion, traced independently by NIR-interferometry, of the spectroscopic binary and the tertiary component that conforms HD 167971. Methods. We analyze 2006 and 2016 very long baseline interferometric data at C and X bands. We complement our analysis with a geometrical model of the wind-wind collision region, and with an astrometric description of the system. Results. We confirm that the detected non-thermal radio emission is associated with the wind-wind collision region of the spectroscopic binary and the tertiary component in HD 167971. The wind-wind collision region changes orientation in agreement with the orbital motion of the tertiary around the spectroscopic binary. The total intensity also changes between the two observing epochs in a way inversely proportional to the separation between the SB and T, with a negative-steep spectral index typical of an optically thin synchrotron emission possibly steepened by an inverse Compton cooling effect. The wind-wind collision bow-shock shape and its position with respect to the stars indicates that the wind momentum from the spectroscopic binary is stronger than that of the tertiary. Finally, the astrometric solution derived for the stellar system and the wind-wind collision region is consistent with independent Gaia data.Comment: Accepted for publication in Astronomy and Astrophysics, 7 pages, 6 figure

Toward gas exhaustion in the W51 high-mass protoclusters

Reproduced with permission from Astronomy & Astrophysics, © 2016 ESOWe present new JVLA observations of the high-mass cluster-forming region W51A from 2 to 16 GHz with resolution θfwhm ≈ 0.3−0.5″. The data reveal a wealth of observational results: (1) Currently forming, very massive (proto-O) stars are traced by o - H2CO21,1−21,2 emission, suggesting that this line can be used efficiently as a massive protostar tracer; (2) there is a spatially distributed population of ≲mJy continuum sources, including hypercompact H ii regions and candidate colliding wind binaries, in and around the W51 proto-clusters; and (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 ≲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, together with a direct measurement of the low mass loss rate via ionized gas outflow from W51 IRS2, implies 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 toward 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.Peer reviewedFinal Published versio

Flux Density Variations at 3.6 cm in the Massive Star-Forming Region W49A

A number of ultracompact H II regions in Galactic star forming environments have been observed to vary significantly in radio flux density on timescales of 10-20 years. Theory predicted that such variations should occur when the accretion flow that feeds a young massive star becomes unstable and clumpy. We have targeted the massive star-forming region W49A with the Karl G. Jansky Very Large Array (VLA) for observations at 3.6 cm with the B-configuration at 0.8'' resolution, to compare to nearly identical observations taken almost 21 years earlier (February 2015 and August 1994). Most of the sources in the crowded field of ultracompact and hypercompact H II regions exhibit no significant changes over this time period. However, one source, W49A/G2, decreased by 20% in peak intensity (from 71+/-4 mJy/beam to 57+/-3 mJy/beam), and 40% in integrated flux (from 0.109+/-0.011 Jy to 0.067+/-0.007 Jy), where we cite 5 sigma errors in peak intensity, and 10% errors in integrated flux. We present the radio images of the W49A region at the two epochs, the difference image that indicates the location of the flux density decrease, and discuss explanations for the flux density decrease near the position of W49A/G2.Comment: 9 pages, 2 figures, accepted for publication, The Astrophysical Journal Letters, July 26, 201