Issue |
A&A
Volume 466, Number 3, May II 2007
|
|
---|---|---|
Page(s) | 977 - 988 | |
Section | Interstellar and circumstellar matter | |
DOI | https://doi.org/10.1051/0004-6361:20065762 | |
Published online | 24 April 2007 |
Tracing high energy radiation with molecular lines near deeply embedded protostars*
1
Institute of Astronomy, ETH Zurich, 8092 Zurich, Switzerland e-mail: pascalst@astro.phys.ethz.ch
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
Received:
6
June
2006
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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