Issue |
A&A
Volume 408, Number 3, September IV 2003
|
|
---|---|---|
Page(s) | 981 - 1007 | |
Section | Interstellar and circumstellar matter | |
DOI | https://doi.org/10.1051/0004-6361:20031030 | |
Published online | 17 November 2003 |
A
m VLT spectroscopic survey of embedded young low mass stars I*,**
Structure of the CO ice
1
Leiden Observatory, PO Box 9513, 2300 RA Leiden, The Netherlands
2
Institut d'Astrophysique Spatiale, Bât. 121, Université Paris XI, 91405 Orsay Cedex, France
3
Astronomical Institute “Anton Pannekoek”, University of Amsterdam, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands
4
Department of Astronomy 105-24, California Institute of Technology, Pasadena, CA 91125, USA
5
Kapteyn Astronomical Institute, PO Box 800, 9700 AV Groningen, The Netherlands
Corresponding author: K. M. Pontoppidan, pontoppi@strw.leidenuniv.nl
Received:
7
April
2003
Accepted:
2
July
2003
Medium resolution () VLT-ISAAC M-band spectra are presented of 39 young
stellar objects in nearby low-mass star forming clouds showing the
stretching vibration mode of solid CO. By
taking advantage of the unprecedentedly large sample, high S/N ratio and high spectral resolution,
similarities in the ice profiles from source to source are identified. It
is found that excellent fits to all the spectra can be obtained
using a phenomenological decomposition of the CO stretching
vibration profile at
into 3 components, centered on
,
and
with fixed widths of 3.0, 3.5 and
, respectively. All observed
interstellar CO profiles can thus be uniquely described by a model
depending on only 3 linear fit parameters, indicating that a maximum of 3 specific molecular environments
of solid CO exist under astrophysical conditions. A simple physical model of
the CO ice is presented, which shows that the
component is indistinguishable from pure CO ice. It is concluded, that in the majority of the observed lines of sight, 60-90% of the CO is in a nearly pure form. In the same model the
component can possibly be explained by the longitudinal optical (LO) component of the vibrational transition in pure crystalline CO ice which appears when the
background source is linearly polarised. The model therefore predicts the polarisation fraction
at
, which can be confirmed by
imaging polarimetry. The
feature characteristic
of CO on or in an unprocessed water matrix is not detected toward
any source and stringent upper limits are given. When this is taken into account, the
component is not consistent with the
available water-rich laboratory mixtures and we suggest that the
carrier is not yet fully understood. A shallow absorption band
centered between
and
is
detected towards 30 sources. For low-mass stars, this band is correlated with
the CO component at
, suggesting the presence of a
carrier different from XCN at
.
Furthermore the absorption band from solid
at
is detected towards IRS 51 in the ρ Ophiuchi
cloud complex and an isotopic ratio of
is derived. It is shown that all the observed solid
profiles, along with the solid
profile, are consistent with grains with an irregularly shaped CO ice mantle
simulated by a Continuous Distribution of Ellipsoids (CDE),
but inconsistent with the commonly used models of spherical grains in the Rayleigh limit.
Key words: astrochemistry / stars: circumstellar matter / ISM: dust, extinction / ISM: molecules / infrared: ISM / stars: pre-main sequence
Based on observations obtained at the European Southern Observatory, Paranal, Chile, within the observing programs 164.I-0605 and 69.C-0441. ISO is an ESA project with instruments funded by ESA Member States (especially the PI countries: France, Germany, The Netherlands and the UK) and with the participation of ISAS and NASA.
© ESO, 2003
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