Issue |
A&A
Volume 501, Number 1, July I 2009
|
|
---|---|---|
Page(s) | L5 - L8 | |
Section | Letters | |
DOI | https://doi.org/10.1051/0004-6361/200912249 | |
Published online | 04 June 2009 |
Letter to the Editor
Hot and cool water in Herbig Ae protoplanetary disks
A challenge for Herschel
1
UK Astronomy Technology Centre, Royal Observatory, Edinburgh, Blackford Hill, Edinburgh EH9 3HJ, UK e-mail: ptw@roe.ac.uk
2
School of Physics & Astronomy, University of St. Andrews, North Haugh, St. Andrews KY16 9SS, UK
3
SUPA, Institute for Astronomy, University of Edinburgh, Royal Observatory, Blackford Hill, Edinburgh EH9 3HJ, UK
4
Kapteyn Astronomical Institute, Postbus 800, 9700 AV Groningen, The Netherlands
5
Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
Received:
1
April
2009
Accepted:
30
May
2009
The spatial origin and detectability of rotational
H2O emission lines from Herbig Ae type protoplanetary disks
beyond 70 μm is discussed. We use the recently developed
disk code prodimo to calculate the thermo-chemical structure of
a Herbig Ae type disk and apply the non-LTE line radiative
transfer code ratran to predict water line profiles and
intensity maps. The model shows three spatially distinct regions
in the disk where water concentrations are high, related to
different chemical pathways to form the water: (1) a big water
reservoir in the deep midplane behind the inner rim, (2) a belt
of cold water around the distant icy midplane beyond the
“snowline” AU, and (3) a layer of irradiated hot
water at high altitudes
, extending from
about 1 AU to 30 AU, where the kinetic gas temperature ranges
from 200 K to 1500 K. Although region 3 contains only little
amounts of water vapour (~
), it is
this warm layer that is almost entirely responsible for the
rotational water emission lines as observable with Herschel. Only
one ortho and two para H2O lines with the lowest excitation
energies <100 K are found to originate partly from
region 2. We conclude that observations of rotational water lines
from Herbig Ae disks probe first and foremost the conditions in
region 3, where water is predominantly formed via neutral-neutral
reactions and the gas is thermally decoupled from the dust
. The observation of rotational water lines does
not allow for a determination of the snowline, because the
snowline truncates the radial extension of region 1, whereas the
lines originate from the region 3. Different line transfer
approximations (LTE, escape probability, Monte Carlo) are
discussed. A non-LTE treatment is required in most
cases, and the results obtained with the escape probability
method are found to underestimate the Monte Carlo results by
2%–45%.
Key words: astrochemistry / circumstellar matter / stars: formation / radiative transfer / methods: numerical
© ESO, 2009
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