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
P. Woitke1,2 - W.-F. Thi3 - I. Kamp4 - M. R. Hogerheijde5
1 - UK Astronomy Technology Centre, Royal Observatory, Edinburgh, Blackford Hill, Edinburgh EH9 3HJ, 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
Abstract
The spatial origin and detectability of rotational
H2O emission lines from Herbig Ae type protoplanetary disks
beyond 70 m 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
z/r = 0.1 ... 0.3, 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
1 Introduction
Water is one of the most important species in planet formation, disk evolution and for the origin of life. In protoplanetary disks, water can be abundant either in the gas phase or as solid ice, owing to a high sublimation temperature.
Water vapour is predicted to be abundant inside of the snowline
within a few AU where densities are high and temperatures are too
warm for ice formation (Agúndez et al. 2008; Glassgold et al. 2009). Indeed,
observations in the near- and mid-infrared with Spitzer
(Carr & Najita 2008; Salyk et al. 2008; Eisner et al. 2009; Najita et al. 2009) reveal the presence
of water vapour and other simple organic molecules (OH, C2H2,
HCN, CO2). Simple modelling of the line emissions constrains the
warm gas (a few 100 K) to be located inside of
AU.
The core-accretion model for planet formation is only efficient in the cold midplane of disks where dust grains are covered by water ice. Icy grains are stickier than bare silicate grains and coagulate faster into planetesimals (Ida & Lin 2008). Liquid water is one of the prerequisites for the emergence of life on terrestrial planets. But its origin, either via release of water molecules trapped in hydrated rocks during volcanism or via the impact of comets, is still being debated (Nuth 2008).
The lowest rotational lines of water lie in the far IR and are only observable by satellites, e.g. the Herschel Space Observatory. Other recent works on rotational water lines from protoplanetary disks used X-ray models (without UV photoprocesses) to calculate the thermo-chemical disk structure on top of a pre-described density structure (Meijerink et al. 2008) and applied a multi-zone escape probability method (Poelman & Spaans 2006,2005) to compute the line fluxes. In this Letter, we use the disk code PRODIMO to compute the disk structure, temperature, and water abundance self-consistently, and discuss the prospects for detecting rotational water lines by means of the Monte Carlo code RATRAN (Hogerheijde & van der Tak 2000).
2 The model
Table 1: Herbig Ae type disk model parameter.
We used the recently developed disk code PRODIMO to calculate the
thermo-chemical structure of a protoplanetary disk around a Herbig Ae
type star with parameters listed in Table 1.
PRODIMO combines frequency-dependent 2D dust-continuum radiative
transfer, kinetic gas-phase and UV photo-chemistry, ice formation, and
detailed non-LTE heating & cooling with the consistent calculation of
the hydrostatic disk structure. PRODIMO does not include X-rays at
the moment. X-ray to FUV luminosity ratios of Herbig Ae stars are
often low
(Kamp et al. 2008), so that we
assume that the FUV irradiation provides the main energy input for
the disk. The model is characterised by a high degree of consistency
between the various physical, chemical, and radiative processes, where
the mutual feedbacks are solved by global iterations. For
more details see Woitke et al. (2009), henceforth called Paper I.
Recent updates include an improved treatment of UV photorates
by detailed cross sections in the calculated radiation field
(see Kamp et al. 2009).
![]() |
Figure 1:
Concentration of water molecules
|
Open with DEXTER |
The model results in a flared disk structure with a puffed-up inner
rim and a vertically extended hot atomic layer above
from the inner rim to about r = 20 AU, similar to Fig. 9 in
Paper I (l.h.s.), where
K due to the stellar
UV-irradiation. The major difference between the T Tauri type disk
discussed in Paper I and the Herbig Ae disk discussed here is that in
Herbig disks, the star is much more luminous, so the dust is
warmer in the midplane (here
K inward of
r = 20 AU), which prevents water ice formation.
Water molecules generally form in deeper layers, and the resulting water concentration in these layers is depicted in Fig. 1. The vertical H2O column densities in this model are found to be 1022 cm-2 at 1 AU, still 1019 cm-2 at 10 AU, but then quickly dropping below 1015 cm-2 at 30 AU and beyond.
3 Chemical pathways to water
Table 2: Characteristics of water regions in Herbig Ae disk model.
The formation of H2O follows different chemical pathways in the three different regions shown in Fig. 1. Two of these regions (1 and 3) have been previously identified in vertical slab models for X-ray irradiated T Tauri disks by Glassgold et al. (2009).
1) The big inner water reservoir. The deep midplane regions from just behind the inner rim to a distance of about 10 AU in this model host the majority of the water in the disk, see Table 2. This region is almost completely shielded from the stellar and interstellar radiation (AV > 10), is still too warm for water to freeze out


2) The distant water belt.
Region 2 at
AU and
is
characterised by particle densities
109-1010 cm-3, temperatures
K, and UV-strengths
(see Eq. (41) in Paper I) below 500. These conditions allow water to
freeze out, and there is a tight equilibrium between water adsorption
and photodesorption
,
where
H2O
designates water ice. The formation of
gaseous water in this region is mainly controlled by the following two
photoreactions
Therefore, the strength of the UV field

![[*]](/icons/foot_motif.png)





![]() |
Figure 2:
Monte Carlo simulations of three ortho H2O lines with increasing excitation energy for a distance of 140 pc and inclination |
Open with DEXTER |
3) The hot water layer. At
distances
AU and relative heights
,
the model is featured by an additional layer
of warm water-rich gas that is thermally decoupled from the dust
.
The particle density is about
108-1010 cm-3 in this layer, the dust temperature
K, the gas temperature 200-1500 K,
and the UV radiation field strength
.
Above region 3, the high gas temperatures in combination with the direct
UV irradiation from the star efficiently destroys all OH and
H2O. In region 3, the medium is shielded from the direct stellar
irradiation by the puffed-up inner rim. In the shadow of the inner
rim,
drops quickly by about two orders of magnitude (the
remaining UV photons are scattered stellar photons), and water forms
via the following chain of surface and neutral-neutral reactions
counterbalanced by the photo-dissociation reactions



Table 3:
Properties of rotational water lines, and calculated line fluxes
for different line transfer methods.
4 Spectral appearance of rotational water lines
Having calculated the density structure, the molecular abundances, the dust and gas temperatures and the continuous radiation field in the disk, we performed axisymmetric non-LTE line transfer calculations for selected rotational water lines as summarised in Table 3. The non-LTE input data for ortho (para) H2O is taken from the Leiden LAMBDA database (Schöier et al. 2005), which includes 45 (45) levels, 158 (157) lines, and 990 (990) collisional transitions with H2. A scaled version of the last data is also applied to collisions with H, which is essential as region 3 is partly H2-poor and H-rich. The velocity field is assumed to be Keplerian. We add a turbulent line width of 0.15 km s-1 to the thermal line width throughout the disk. The ratio between para and ortho H2O is assumed to be as in LTE.
We used three different methods of increasing complexity to calculate
the water population numbers: local thermal equilibrium (LTE), a
simple escape probability method (ES, see Sect. 6.1 of Paper I) and a
modified version of the 2D Monte Carlo code RATRAN
(Hogerheijde & van der Tak 2000), see (Kamp et al. 2009) for modifications. The
LTE and ES methods used the full
PRODIMO
output directly as thermo-chemical input model. The grid size of the MC model
needed to be somewhat reduced for practical reasons. We decided to run
80
80 MC models, which need about
photon packages to converge to a signal/noise ratio better than 5 for the worst population number in the worst cell, which takes about 13 CPU hours on a 2.66 GHz Linux machine.
To investigate the role of the hot water layer (region 3) for the
spectral appearance of the rotational lines, we calculated two sets
for each model. The first set includes the full chemical input
model. In the second set, we artificially put the water abundance in
region 3 to zero (if
cm-3 and
K). The results of the two sets of MC models are compared in Fig. 2.
![]() |
Figure 3:
Comparison between the results of different line transfer
methods in application to the high-excitation para-H2O line at 89.99 |
Open with DEXTER |
We generally observe double-peaked line profiles typical for rotating
gas in emission. In the case of the full input model, the peak separation
generally measures the radial extension of region 3 (Kepler velocity
is 8.8 km s-1 at 25 AU in this model, inclined to
gives
4.4 km s-1). However, the peak separation of the lowest o-H2O
excitation line (538.29
m) and the two lowest p-H2O
excitation lines (269.27
m and 303.46
m, see
Table 3) correspond to the full radial extent of
the model, 150 AU.
The truncated model generally results in
smaller line fluxes with a broader, often unclear profile. If the water in region 3 is missing, the lines originate mainly from region 1, which is optically thick even in the continuum. Since the gas
is in thermal balance with the dust in region 1 (
),
the lines do not go much into emission in region 1. Again, the
lowest three excitation lines behave differently, and region 2
contributes by
30% for these lines. There is
furthermore one intermittent case (o-H2O 179.5
m with
Eu = 114 K) where the truncated model reveals a
small contribution of the extended region 2 with the character of
low-excitation lines (middle column in Fig. 2).
All rotational water lines of the Herbig Ae disk discussed in
this letter are above the 1
detection limit of the PACS
spectrometer (
1-
W/m2, depending on
). The strongest water lines at
m and
89.99
m are above the
detection limit. However, the
lines sit on a strong dust continuum, which possibly complicates the
detection by Herschel.
5 LTE vs. escape probability vs. Monte Carlo
Figure 3 compares the results obtained by three
different line transfer methods for the high-excitation para-H2O
line at 89.99 m. The results for the other lines are listed in
Table 3. We consider the deviations from the
results of the most advanced method (MC) as a measure of the quality
of the other methods. The LTE predictions are generally too high, by
up to a factor of 3.5, although continuum flux, line width, and peak
separation are similar. Since the densities in region 3 are lower
than the critical density
1010 cm-3, the levels tend
to depopulate radiatively, which explains the overpredictions by LTE.
Deviations between ES and MC are between 2% and 45%, and increase
with excitation energy Eu. Our ES method tends to underestimate the
line fluxes in general. The levels in region 3 are pumped by line
radiation from distant regions in LTE that have larger line source
functions. This effect is difficult, if not impossible, to be properly
account for in the ES approximation.
6 Conclusions
The rotational water lines from Herbig Ae disks beyond 70 m
originate predominantly from a warm molecular layer at relative
altitudes
where H2O is formed via
neutral-neutral reactions in a thermally decoupled gas
(
). The more distant cold water around the icy
midplane, where the ice is photodesorbed, contributes only to the
lowest excitation lines. The peak separation of all other lines
measures the radial extension of the warm molecular layer, which is
about 40 AU in the discussed model. In contrast, the vast majority
of water vapour in the disk is situated in the deep midplane,
extending from just behind the inner rim outward to the snowline,
where water freezes out to form water ice. The gas in this massive
deep water reservoir is in thermal balance (
)
with
optically thick dust and, therefore, no strong line emissions are
produced with respect to the continuum from this deep region. Thus, no
information about the position of the snowline can be deduced from
the rotational water lines. A similar conclusion was reached by
Meijerink et al. (2008). The line analysis generally requires a non-LTE
treatment. Our escape probability method is found to underestimate
the water line fluxes with respect to the more expensive Monte Carlo method
by about 2%-45%.
Acknowledgements
We thank Dr. Rowin Meijerink for an open discussion about water in disks and Dr. Dieter Poelman for internal benchmark tests of different non-LTE line transfer methods.
References
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- Kamp, I., Tilling, I., Woitke, P., Thi, W.-F., & Hogerheijde, M. R. 2009, A&A, in prep. (In the text)
- Meijerink, R., Poelman, D. R., Spaans, M., Tielens, A. G. G. M., & Glassgold, A. E. 2008, ApJ, 689, L57 [NASA ADS] [CrossRef] (In the text)
- Najita, J. R., Doppmann, G. W., Carr, J. S., Graham, J. R., & Eisner, J. A. 2009, ApJ, 691, 738 [NASA ADS] [CrossRef]
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Footnotes
- ... concentration
- Our analysis is restricted to the case of kinetic chemical equilibrium. Thus, we cannot discuss the history of water ice formation with our model. Water ice may furthermore be photodesorbed directly into OH, see e.g. Andersson & van Dishoeck (2008).
All Tables
Table 1: Herbig Ae type disk model parameter.
Table 2: Characteristics of water regions in Herbig Ae disk model.
Table 3:
Properties of rotational water lines, and calculated line fluxes
for different line transfer methods.
All Figures
![]() |
Figure 1:
Concentration of water molecules
|
Open with DEXTER | |
In the text |
![]() |
Figure 2:
Monte Carlo simulations of three ortho H2O lines with increasing excitation energy for a distance of 140 pc and inclination |
Open with DEXTER | |
In the text |
![]() |
Figure 3:
Comparison between the results of different line transfer
methods in application to the high-excitation para-H2O line at 89.99 |
Open with DEXTER | |
In the text |
Copyright ESO 2009
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