SiO outflows in high-mass star forming regions: A potential chemical clock?

Astronomy and Astrophysics (Impact Factor: 5.08). 11/2010; DOI: 10.1051/0004-6361/201015827
Source: arXiv

ABSTRACT Some theoretical models propose that O-B stars form via accretion, in a similar fashion to low-mass stars. Jet-driven molecular outflows play an important role in this scenario, and their study can help to understand the process of high-mass star formation and the different evolutionary phases involved. Observations towards low-mass protostars so far favour an evolutionary picture in which jets are always associated with Class 0 objects while more evolved Class I/II objects show less evidence of powerful jets. The present study aims at checking whether an analogous picture can be found in the high-mass case. The IRAM 30-m telescope (Spain) has been used to perform single-pointing SiO(2-1) and (3-2) observations towards a sample of 57 high-mass molecular clumps in different evolutionary stages. Continuum data at different wavelengths, from mid-IR to 1.2 mm, have been gathered to build the spectral energy distributions of all the clumps and estimate their bolometric luminosities. SiO emission at high velocities, characteristic of molecular jets, is detected in 88% of our sources, a very high detection rate indicating that there is ongoing star formation activity in most of the sources of our sample. The SiO(2-1) luminosity drops with L/M, which suggests that jet activity declines as time evolves. This represents the first clear evidence of a decrease of SiO outflow luminosity with time in a homogeneous sample of high-mass molecular clumps in different evolutionary stages. The SiO(3-2) to SiO(2-1) integrated intensity ratio shows only minor changes with evolutionary state. Comment: 12 pages, 10 figures

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    ABSTRACT: We study the production of SiO in the gas phase of molecular outflows, through the sputtering of Si--bearing material in refractory grain cores, which are taken to be olivine; we calculate also the rotational line spectrum of the SiO. The sputtering is driven by neutral particle impact on charged grains, in steady--state C-type shock waves, at the speed of ambipolar diffusion. The emission of the SiO molecule is calculated by means of an LVG code. A grid of models has been generated. We compare our results with those of an earlier study (Schilke et al. 1997). Improvements in the treatment of the coupling between the charged grains and the neutral fluid lead to narrower shock waves and lower fractions of Si being released into the gas phase. More realistic assumptions concerning the initial fractional abundance of O2 lead to SiO formation being delayed, so that it occurs in the cool, dense postshock flow. Good agreement is obtained with recent observations of SiO line intensities in the L1157 and L1448 molecular outflows. The inferred temperature, opacity, and SiO column density in the emission region differ significantly from those estimated by means of LVG `slab' models. The fractional abundance of SiO is deduced. Observed line profiles are wider than predicted and imply multiple, unresolved shock regions within the beam. Comment: 1 tex doc, 19 figures
    Astronomy and Astrophysics 03/2008; · 5.08 Impact Factor
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    ABSTRACT: We report the results of a SEST 1.2 mm continuum emission survey toward regions previously identified as harbouring a methanol maser and/or an UC HII region, typically indicative of massive star formation. Emission is detected toward all of the methanol maser and UC HII regions targeted, implying that these objescts are associated with cold, deeply embedded objects. We have also identified a large number (253) of sources within the SIMBA maps, which are devoid of maser and radio continuum emission. These `mm-only' cores may be an entirely new class of source that represents an earlier stage in the evolution of massive stars, prior to the onset of methanol maser emission. Or, they may harbour protoclusters, which do not contain any high mass stars (i.e. below the HII). Alternatively, they may represent a cross-section of both, where the more massive mm-only cores are a precursor to the methanol maser and the least massive mm-only cores will form intermediate mass stars. Analysis of the mm-only sources shows that they are less massive ({\= M} = 0.9 x 10^{3} Msun), and smaller ({\= R} = 0.4 pc) than sources with methanol maser and/or radio continuum emission, which collectively have a mean mass of 2.5 x 10^{3} Msun and a mean radius of 0.7 pc. Comment: 29 pages + 20 page image appendix (not included here), 5 figures. Submitted to MNRAS
    Monthly Notices of the Royal Astronomical Society 06/2005; · 5.52 Impact Factor

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