Volume 466, Number 3, May II 2007
|Page(s)||977 - 988|
|Section||Interstellar and circumstellar matter|
|Published online||24 April 2007|
Tracing high energy radiation with molecular lines near deeply embedded protostars*
Institute of Astronomy, ETH Zurich, 8092 Zurich, Switzerland e-mail: email@example.com
2 Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
3 Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
4 Department of Physics and Astronomy, Denison University, Granville, OH 43023, USA
5 SRON National Institute for Space Research, Landleven 12, 9747 AD Groningen, The Netherlands
Accepted: 2 January 2007
Aims.The aim is to probe high energy radiation emitted by deeply embedded protostars.
Methods.Submillimeter lines of CN, NO, CO+ and SO+, and upper limits on SH+ and N2O are observed with the James Clerk Maxwell Telescope in two high-mass and up to nine low-mass young stellar objects and compared with chemical models.
Results.Constant fractional abundances derived from radiative transfer modeling of the line strengths are a few 10-11–10-8, –10-8 and –10-10. SO+ has abundances of a few in the high-mass objects and upper limits of ≈10-12–10-11 in the low-mass sources. All abundances are up to 1–2 orders of magnitude higher if the molecular emission is assumed to originate mainly from the inner region (≲1000 AU) of the envelope. For high-mass sources, the CN, SO+ and CO+ abundances and abundance ratios are best explained by an enhanced far-ultraviolet (FUV) field impacting gas at temperatures of a few hundred K. The observed column densities require that this region of enhanced FUV has scales comparable to the observing beam, such as in a geometry in which the enhanced FUV irradiates outflow walls. For low-mass sources, the required temperatures within the FUV models of K are much higher than found in models, so that an X-ray enhanced region close to the protostar ( AU) is more plausible. Gas-phase chemical models produce more NO than observed, suggesting an additional reduction mechanism not included in current models.
Conclusions.The observed CN, CO+ and SO+ abundances can be explained with either enhanced X-rays or FUV fields from the central source. High-mass sources likely have low opacity regions that allow the FUV photons to reach large distances from the central source. X-rays are suggested to be more effective than FUV fields in the low-mass sources. The observed abundances imply X-ray fluxes for the Class 0 objects of –1031 erg s-1, comparable to those observed from low-mass Class I protostars. Spatially resolved data are needed to clearly distinguish the effects of FUV and X-rays for individual species.
Key words: stars: formation / stars: low-mass, brown dwarfs / ISM: molecules / X-rays: ISM
© ESO, 2007
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