Issue |
A&A
Volume 425, Number 3, October III 2004
|
|
---|---|---|
Page(s) | 955 - 972 | |
Section | Interstellar and circumstellar matter | |
DOI | https://doi.org/10.1051/0004-6361:200400026 | |
Published online | 28 September 2004 |
Organic molecules in protoplanetary disks around T Tauri and Herbig Ae stars*
1
Leiden Observatory, PO Box 9513, 2300 RA Leiden, The Netherlands e-mail: ewine@strw.leidenuniv.nl
2
Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, UK
3
Sterrenkundig Instituut Anton Pannekoek, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands
4
Koninklijk Nederlands Meteorologisch Instituut, PO Box 201, 3730 AE De Bilt, The Netherlands
Received:
14
January
2003
Accepted:
15
June
2004
The results of single-dish observations of low- and high-J
transitions of selected molecules from protoplanetary disks around
two T Tauri stars (LkCa 15 and TW Hya) and two
Herbig Ae stars (HD 163296 and MWC 480) are
reported. Simple molecules such as CO, CO, HCO+, CN
and HCN are detected. Several lines of H2CO are found
toward the T Tauri star LkCa 15 but not in other objects.
No CH3OH has been detected down to abundances of
10-9–10-8 with respect to H2. SO and CS lines have
been searched for without success. Line ratios indicate that the
molecular emission arises from dense (10
–10
cm
) and moderately warm (
20–40 K)
intermediate height regions of the disk atmosphere between the
midplane and the upper layer, in accordance with predictions from
models of the chemistry in disks. The sizes of the disks were
estimated from model fits to the 12CO 3–2 line profiles. The
abundances of most species are lower than in the envelope around the
solar-mass protostar IRAS 16293-2422. Freeze-out in the
cold midplane and photodissociation by stellar and interstellar
ultraviolet photons in the upper layers are likely causes of the
depletion. CN is strongly detected in all disks, and the CN/HCN
abundance ratio toward the Herbig Ae stars is even higher than that
found in galactic photon-dominated regions, testifying to the
importance of photodissociation by radiation from the central object
in the upper layers. DCO+ is detected toward TW Hya,
but not in other objects. The high inferred DCO+/HCO+ ratio
of ~0.035 is consistent with models of the deuterium
fractionation in disks which include strong depletion of CO. The
inferred ionization fraction in the intermediate height regions as
deduced from HCO+ is at least 10-11–10-10, comparable to
that derived for the midplane from recent H2D+ observations.
Comparison with the abundances found in cometary comae is made.
Key words: ISM: molecules / stars: circumstellar matter / stars: pre-main-sequence / astrochemistry
© ESO, 2004
Current usage metrics show cumulative count of Article Views (full-text article views including HTML views, PDF and ePub downloads, according to the available data) and Abstracts Views on Vision4Press platform.
Data correspond to usage on the plateform after 2015. The current usage metrics is available 48-96 hours after online publication and is updated daily on week days.
Initial download of the metrics may take a while.