11 research outputs found

    A Circumstellar Disc in a High-Mass Star Forming Region

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    We present an edge-on Keplerian disc model to explain the main component of the 12.2 and 6.7 GHz methanol maser emission detected toward NGC7538-IRS1 N. The brightness distribution and spectrum of the line of bright masers are successfully modeled with high amplification of background radio continuum emission along velocity coherent paths through a maser disc. The bend seen in the position-velocity diagram is a characteristic signature of differentially rotating discs. For a central mass of 30 solar masses, suggested by other observations, our model fixes the masing disc to have inner and outer radii of about 350 AU and 1000 AU.Comment: 11 pages, accepted for publication in ApJ Letter

    Probing the initial conditions of high-mass star formation -- IV. Gas dynamics and NH2_2D chemistry in high-mass precluster and protocluster clumps

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    The initial stage of star formation is a complex area study because of its high density and low temperature. Under such conditions, many molecules become depleted from the gas phase by freezing out onto dust grains. However, the deuterated species could remain gaseous and are thus ideal tracers. We investigate the gas dynamics and NH2_2D chemistry in eight massive pre/protocluster clumps. We present NH2_2D 111_{11}-101_{01} (at 85.926 GHz), NH3_3 (1, 1) and (2, 2) observations in the eight clumps using the PdBI and the VLA, respectively. We find that the distribution between deuterium fractionation and kinetic temperature shows a number density peak at around Tkin=16.1T_{\rm kin}=16.1 K, and the NH2_2D cores are mainly located at a temperature range of 13.0 to 22.0 K. We detect seven instances of extremely high deuterium fractionation of 1.0Dfrac1.411.0 \leqslant D_{\rm frac} \leqslant 1.41. We find that the NH2_2D emission does not appear to coincide exactly with either dust continuum or NH3_3 peak positions, but often surrounds the star-formation active regions. This suggests that the NH2_{2}D has been destroyed by the central young stellar object (YSO) due to its heating. The detected NH2_2D lines are very narrow with a median width of 0.98±0.02km/s\rm 0.98\pm0.02 km/s. The extracted NH2_2D cores are gravitationally bound (αvir<1\alpha_{\rm vir} < 1), are likely prestellar or starless, and can potentially form intermediate-mass or high-mass stars. Using NH3_3 (1, 1) as a dynamical tracer, we find very complicated dynamical movement, which can be explained by a combined process with outflow, rotation, convergent flow, collision, large velocity gradient, and rotating toroids. High deuterium fractionation strongly depends on the temperature condition. NH2_2D is a poor evolutionary indicator of high-mass star formation in evolved stages, but a useful tracer in the starless and prestellar cores.Comment: 27 pages, 25 figures, 6 tables, accepted for publication in A&

    The Bolocam Galactic Plane Survey. XIV. Physical Properties of Massive Starless and Star Forming Clumps

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    We sort 46834683 molecular clouds between 10<<6510^\circ< \ell <65^\circ from the Bolocam Galactic Plane Survey based on observational diagnostics of star formation activity: compact 7070 μm\mu{\rm m} sources, mid-IR color-selected YSOs, H2O{\rm H_2O} and CH3OH{\rm CH_3OH} masers, and UCHII regions. We also present a combined NH3{\rm NH_3}-derived gas kinetic temperature and H2O{\rm H_2O} maser catalog for 17881788 clumps from our own GBT 100m observations and from the literature. We identify a subsample of 22232223 (47.5%47.5\%) starless clump candidates, the largest and most robust sample identified from a blind survey to date. Distributions of flux density, flux concentration, solid angle, kinetic temperature, column density, radius, and mass show strong (>1>1 dex) progressions when sorted by star formation indicator. The median starless clump candidate is marginally sub-virial (α0.7\alpha \sim 0.7) with >75%>75\% of clumps with known distance being gravitationally bound (α<2\alpha < 2). These samples show a statistically significant increase in the median clump mass of ΔM170370\Delta M \sim 170-370 M_\odot from the starless candidates to clumps associated with protostars. This trend could be due to (i) mass growth of the clumps at M˙200440\dot{M}\sim200-440 Msun Myr1^{-1} for an average free-fall 0.80.8 Myr time-scale, (ii) a systematic factor of two increase in dust opacity from starless to protostellar phases, (iii) and/or a variation in the ratio of starless to protostellar clump lifetime that scales as M0.4\sim M^{-0.4}. By comparing to the observed number of CH3OH{\rm CH_3OH} maser containing clumps we estimate the phase-lifetime of massive (M>103M>10^3 M_\odot) starless clumps to be 0.37±0.08 Myr (M/103 M)10.37 \pm 0.08 \ {\rm Myr} \ (M/10^3 \ {\rm M}_\odot)^{-1}; the majority (M<450M<450 M_\odot) have phase-lifetimes longer than their average free-fall time.Comment: Accepted for publication in ApJ; 33 pages; 22 figures; 7 table

    The Hi-GAL compact source catalogue - II. The 360\ub0 catalogue of clump physical properties

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    We present the 360\ub0 catalogue of physical properties of Hi-GAL compact sources, detected between 70 and 500 μ\mum. This release not only completes the analogous catalogue previously produced by the Hi-GAL collaboration for -71\ub0 2 \ue1 2 67\ub0, but also meaningfully improves it because of a new set of heliocentric distances, 120 808 in total. About a third of the 150 223 entries are located in the newly added portion of the Galactic plane. A first classification based on detection at 70 μ\mum as a signature of ongoing star-forming activity distinguishes between protostellar sources (23 per cent of the total) and starless sources, with the latter further classified as gravitationally bound (pre-stellar) or unbound. The integral of the spectral energy distribution, including ancillary photometry from λ = 21 to 1100 μ\mum, gives the source luminosity and other bolometric quantities, while a modified blackbody fitted to data for λ160μ\lambda \ge 160∼\mum yields mass and temperature. All tabulated clump properties are then derived using photometry and heliocentric distance, where possible. Statistics of these quantities are discussed with respect to both source Galactic location and evolutionary stage. No strong differences in the distributions of evolutionary indicators are found between the inner and outer Galaxy. However, masses and densities in the inner Galaxy are on average significantly larger, resulting in a higher number of clumps that are candidates to host massive star formation. Median behaviour of distance-independent parameters tracing source evolutionary status is examined as a function of the Galactocentric radius, showing no clear evidence of correlation with spiral arm positions

    Methanol masers as tools to study high-mass star formation

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    Original paper can be found at: http://journals.cambridge.org/ Copyright International Astronomical Union DOI : 10.1017/S1743921307012628In this contribution I will attempt to show that the study of Galactic 6.7 and 12.2GHz methanol masers themselves, as opposed to the use of methanol masers as signposts, can yield important conclusions contributing to the understanding of high-mass star formation. Due to their exclusive association with star formation, methanol masers are the best tools to do this, and their large number allows us to probe the entire Galaxy. In particular I will focus on the determination of the luminosity function of methanol masers and on the determination of an unambiguous signature for a circumstellar masing disc seen edge-on. Finally I will try to point out some future fields of research in the study of methanol masers

    A pilot study for the SCUBA-2 'All-Sky' survey

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    The definitive version can be found at : http://onlinelibrary.wiley.com/ Copyright Royal Astronomical SocietyWe have carried out a pilot study for the Submillimetre Common-User Bolometer Array 2 (SCUBA-2) 'All-Sky' Survey (SASSy), a wide and shallow mapping project at 850 mu m, designed to find rare objects, both Galactic and extragalactic. Two distinct sets of exploratory observations were undertaken and used to test the SASSy approach and data-reduction pipeline. The first was a 0 degrees.5 x 0 degrees.5 map around the nearby galaxy NGC 2559. The galaxy was easily detected at 156 mJy, but no other convincing sources are present in the map. Comparison with other galaxies with similar wavelength coverage indicates that NGC 2559 has relatively warm dust. The second observations cover 1 deg(2) around the W5-E H II region. As well as diffuse structure in the map, a filtering approach was able to extract 27 compact sources with signal-to-noise ratio greater than 6. By matching with data at other wavelengths we can see that the SCUBA-2 data can be used to discriminate the colder cores. Together these observations show that the SASSy project will be able to meet its original goals of detecting new bright sources which will be ideal for follow-up observations with other facilities.Peer reviewe