Issue |
A&A
Volume 521, October 2010
Herschel/HIFI: first science highlights
|
|
---|---|---|
Article Number | L33 | |
Number of page(s) | 5 | |
Section | Letters | |
DOI | https://doi.org/10.1051/0004-6361/201015104 | |
Published online | 01 October 2010 |
Herschel/HIFI: first science highlights
LETTER TO THE EDITOR
Sensitive limits on the abundance of cold water vapor in the DM Tauri protoplanetary disk![[*]](/icons/foot_motif.png)
E. A. Bergin1 - M. R. Hogerheijde2 - C. Brinch2 - J. Fogel1 - U. A. Yildiz2 - L. E. Kristensen2 - E. F. van Dishoeck2,3 - T. A. Bell8 - G. A. Blake4 - J. Cernicharo5 - C. Dominik6,7 - D. Lis8 - G. Melnick9 - D. Neufeld10 - O. Panic11 - J. C. Pearson12 - R. Bachiller13 - A. Baudry14 - M. Benedettini15 - A. O. Benz16 - P. Bjerkeli17 - S. Bontemps18 - J. Braine19 - S. Bruderer16 - P. Caselli19,15 - C. Codella15 - F. Daniel5 - A. M. di Giorgio14 - S. D. Doty20 - P. Encrenaz21 - M. Fich22 - A. Fuente23 - T. Giannini24 - J. R. Goicoechea5 - Th. de Graauw25 - F. Helmich25 - G. J. Herczeg3 - F. Herpin14 - T. Jacq15 - D. Johnstone26,27 - J. K. Jørgensen28 - B. Larsson29 - R. Liseau17 - M. Marseille25 - C. M Coey22,30 - B. Nisini14 - M. Olberg17 - B. Parise31 - R. Plume32 - C. Risacher25 - J. Santiago-García33 - P. Saraceno14 - R. Shipman25 - M. Tafalla13 - T. A. van Kempen9 - R. Visser2 - S. F. Wampfler16 - F. Wyrowski31 - F. van der Tak25,34 - W. Jellema25 - A. G. G. M. Tielens2 - P. Hartogh35 - J. Stützki36 - R. Szczerba37
1 - Department of Astronomy, The University of Michigan, 500 Church Street, Ann Arbor, MI 48109-1042, USA
2 -
Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
3 -
Max Planck Institut für Extraterrestrische Physik, Giessenbachstrasse 1, 85748 Garching, Germany
4 -
California Institute of Technology, Division of Geological and Planetary Sciences, MS 150-21, Pasadena, CA 91125, USA
5
- Centro de Astrobiología. Departamento de Astrofísica, CSIC-INTA,
Carretera de Ajalvir, Km 4, Torrejón de Ardoz, 28850 Madrid, Spain
6 -
Astronomical Institute Anton Pannekoek, University of Amsterdam, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands
7 -
Department of Astrophysics/IMAPP, Radboud University Nijmegen, PO Box 9010, 6500 GL Nijmegen, The Netherlands
8 -
California Institute of Technology, Cahill Center for Astronomy and Astrophysics, MS 301-17, Pasadena, CA 91125, USA
9 -
Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, MS 42, Cambridge, MA 02138, USA
10 -
Department of Physics and Astronomy, Johns Hopkins University, 3400 North Charles Street, Baltimore, MD 21218, USA
11 -
European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching, Germany
12 -
Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA
13 -
Observatorio Astronómico Nacional (IGN), Calle Alfonso XII 3, 28014 Madrid, Spain
14
- INAF - Istituto di Fisica dello Spazio Interplanetario, Area di
Ricerca di Tor Vergata, via Fosso del Cavaliere 100, 00133 Roma, Italy
15 -
INAF - Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
16 -
Institute of Astronomy, ETH Zurich, 8093 Zurich, Switzerland
17 -
Department of Radio and Space Science, Chalmers University of Technology, Onsala Space Observatory, 439 92 Onsala, Sweden
18 -
Université de Bordeaux, Laboratoire d'Astrophysique de Bordeaux, France; CNRS/INSU, UMR 5804, Floirac, France
19 -
School of Physics and Astronomy, University of Leeds, Leeds LS2 9JT, UK
20 -
Department of Physics and Astronomy, Denison University, Granville, OH, 43023, USA
21 -
LERMA and UMR 8112 du CNRS, Observatoire de Paris, 61 Av. de l'Observatoire, 75014 Paris, France
22 -
University of Waterloo, Department of Physics and Astronomy, Waterloo, Ontario, Canada
23 -
Observatorio Astronómico Nacional, Apartado 112, 28803 Alcalá de Henares, Spain
24 -
INAF - Osservatorio Astronomico di Roma, 00040 Monte Porzio catone, Italy
25 -
SRON Netherlands Institute for Space Research, PO Box 800, 9700 AV, Groningen, The Netherlands
26 -
National Research Council Canada, Herzberg Institute of Astrophysics, 5071 West Saanich Road, Victoria, BC V9E 2E7, Canada
27 -
Department of Physics and Astronomy, University of Victoria, Victoria, BC V8P 1A1, Canada
28 -
Centre for Star and Planet Formation, Natural History Museum of Denmark, University of Copenhagen,
Øster Voldgade 5-7, 1350 Copenhagen K., Denmark
29 -
Department of Astronomy, Stockholm University, AlbaNova, 106 91 Stockholm, Sweden
30 -
The University of Western Ontario, Department of Physics and Astronomy, London, Ontario, N6A 3K7, Canada
31 -
Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
32 -
Department of Physics and Astronomy, University of Calgary, Calgary, T2N 1N4, AB, Canada
33 -
Instituto de Radioastronomía Milimétrica (IRAM), Avenida Divina Pastora 7, Núcleo Central, 18012 Granada, Spain
34 -
Kapteyn Astronomical Institute, University of Groningen, PO Box 800, 9700 AV, Groningen, The Netherlands
35 -
Max-Planck-Insitut für Sonnensystemforschung, 37191 Katlenburg-Lindau, Germany
36 -
KOSMA, I. Physik. Institut, Universität zu Köln, Zülpicher Str. 77, D 50937 Köln, Germany
37 -
N. Copernicus Astronomical Center, Rabianska 8, 87-100, Torun, Poland
Received 31 May 2010 / Accepted 26 June 2010
Abstract
We performed a sensitive search for the ground-state
emission lines of ortho- and para-water vapor in the DM Tau
protoplanetary disk using the Herschel/HIFI instrument. No
strong lines are detected down to 3 levels in
0.5 km s-1 channels of 4.2 mK for the 110-101 line
and 12.6 mK for the
111-000 line. We report a very
tentative detection, however, of the 110-101 line in the
wide band spectrometer, with a strength of
mK, a
width of 5.6 km s-1 and an integrated intensity of 16.0 mK km s-1. The latter constitutes a
detection. Regardless of the reality of this tentative detection,
model calculations indicate that our sensitive limits on the line
strengths preclude efficient desorption of water in the UV
illuminated regions of the disk. We hypothesize that more than
95-99% of the water ice is locked up in coagulated grains that
have settled to the midplane.
Key words: ISM: abundances - ISM: molecules - protoplanetary disks
1 Introduction
Because of its association with biology on Earth, water is one of the
most important molecules in the solar system and beyond. However, the
origin of water on Earth is highly uncertain.
What is clear is that the distribution of hydrated rocks in the solar
system (Abe et al. 2000) suggests that water resided in the vapor
phase in the warm (
K) inner solar nebula
and is predominantly condensed in the form of ice beyond the so-called
snow-line between 2-2.5 AU (Hayashi 1981; Abe et al. 2000).
The observation of cool water vapor in protoplanetary disks is
hampered by atmospheric attenuation and requires space-based
observations. A number of recent detections of hot (
K) water by the Spitzer Space Telescope has demonstrated
that abundant water vapor is a ubiquitous component in
protoplanetary disks (Carr & Najita 2008; Pontoppidan et al. 2010; Salyk et al. 2008). However, this
emission likely arises within the snow-line of these young systems and
therefore does not provide a complete picture of the distribution of
water (both ice and vapor) in disks with sizes in excess of 100 AU.
In particular, these observations do not probe the cold (
K) outer parts of the disk where
most of the mass resides.
The Herschel Space Observatory (Pilbratt et al. 2010) offers a new opportunity for a characterization of the distribution and evolution of water vapor in protoplanetary disks. We report here a sensitive search for cold water emission in the ground-state lines of ortho- (o) and para- (p) H2O using the high spectral resolution of the HIFI instrument (de Graauw et al. 2010) towards the well studied DM Tau protoplanetary disk. This study is part of the guaranteed time key program ``Water In Star forming regions'' (van Dishoeck et al. in prep.). Stringent limits to the strength of both lines of a few mK suggest that the outer regions of the disk contain little water vapor or water ice. In Sect. 2 we outline the observations. Section 3 presents results from detailed modeling; Sect. 4 summarizes the implications of our result.
2 Observations and results
DM Tau is a T Tauri star located at
7 and
with a disk diameter, estimated from CO emission, of
1800 AU at a systemic velocity of 6 km s-1 and
(Piétu et al. 2007).
The
source is a single M1 star (White & Ghez 2001) with
.
DM Tau has a chemically rich molecular disk
(Dutrey et al. 1997). Accretion from the
disk to the star also provides a source of excess UV
luminosity,
with an overall UV field strength of
(relative to
the standard interstellar radiation field, ISRF; Bergin et al. 2004,2003; Habing 1968). The object is a transition disk
with an inner hole on the order of a few AU, based on models of Spitzer spectra (Calvet et al. 2005).
DM Tau was observed with the HIFI instrument
using the double beam switch observing mode with a throw of
.
On 2010 March 22 spectra were taken in receiver band 1b
with an
on-source integration time of 198 min, and
K.
On 2010 March 4 spectra were taken in receiver
band 4b
and an on-source integration time of 328 min, and
K.
The HIFI beam of 39'' at 556 GHz and 21'' at 1113 GHz is
larger than the DM Tau disk with a diameter of
at 140 pc. The beams are also larger than the pointing
accuracy of Herschel of
2''.
The data were recorded with wide band spectrometer
(WBS)
covering 4.4 GHz
with 1.1 MHz resolution
(0.59 and 0.30 km s-1 at 556 and 1113 GHz, respectively), and the
high-resolution spectrometer (HRS)
covering 230 MHz
at 0.25 MHz resolution (0.13 and 0.067 km s-1 at 556 and 1113 GHz, respectively). Both H- and V-polarizations were measured.
The raw data were calibrated onto the
scale by the in-orbit
system and converted to
assuming a
beam efficiency of 0.74
The data were reduced using HIPE
v3.0. Subsequently, the data were exported to CLASS
. The HIFI flux
calibration is accurate to 10%, while the velocity scale of HIPE v3.0
is accurate to several m s-1. For both lines, the WBS data were
rebinned to 0.54 km s-1 channels (or 1.0 MHz, close to the
instrumental resolution of 1.1 MHz); the HRS data were rebinned to
0.45 km s-1 channels.
All spectra, including the the H- and
V-polarizations, were averaged together weighted by their respective
noise levels. The resulting rms noise levels are 2.9 mK (HRS) and
1.4 mK (WBS) for the H2O
101-101 line, and 7.2 mK (HRS)
and 4.3 mK (WBS) for the H2O
111-000line
. Figure 1 illustrates for band 1b that the noise in
our data decreases as (time)-0.5 up to the full achieved
integration times.
Figure 2 presents the HRS and WBS spectra of the two water
transitions. No strong lines are detected; for comparison, Fig. 2 also
shows the
12CO 1-0 spectrum of DM Tau (Kessler-Silacci 2004;
Panic et al., in prep.) showing a clear emission line with a
width of 2 km s-1
centered on the source velocity of +6.1 km s-1. In the WBS
spectrum of the H2O
110-101 line a weak feature is
present between
+0.5 and +10 km s-1, peaking around
+6.6 km s-1; a similar feature is seen in the noisier HRS spectrum. With
mK, the brightest channel lies at
3
.
Integrated between +0.5 and +10 km s-1, the feature
contains
mK km s-1, a 5
result. A Gaussian
fit to the feature yields best fit parameters of
km s-1, a FWHM width of
km s-1, an
intensity
mK, and an integrated intensity of
mK km s-1 (6
).
Arguments in favor of interpreting this feature as a positive
detection of the H2O
110-101 line include the facts that
the integrated intensity constitutes a 5-6
detection and that
the feature peaks near the systemic velocity of 6.1 km s-1.
Against the interpretation as a positive detection is
a line peak that is only 2-3
and a linewidth which is twice that of the 12CO line. The width would suggest that the
emission arises from within 10 AU. With respect to the latter, it is
interesting that HCO+ 1-0 line
has a blue wing extending over
3 km s-1 (Dutrey et al. 1997).
Neither set of arguments is clearly stronger, and we interpret the
feature in the H2O
110-101 spectrum as a very
tentative detection of water vapor in the disk of DM Tau.
In the remainder of this Letter, we will work with an intensity of
2.7 mK for the H2O
110-101 line and with an
upper limit to the intensity of the H2O
111-000 line of <12.6 mK.
Smoothing the WBS spectra to 27 km s-1 resolution results
in positive detections of the continuum of DM Tau at 556.9 GHz of
mK (
Jy) and at 1113.3 GHz of
mK (
Jy). These values are
consistent with other continuum measurements
(Dutrey et al. 1996).
![]() |
Figure 1:
Noise level (rms) vs integration time of the WBS and HRS data
of the H2O
110-101 line. The noise decreases as
|
Open with DEXTER |
![]() |
Figure 2:
a)- d) H2O spectra obtained with HIFI. A
tentative feature is present in panel b) for H2O
110-101 detected with WBS between
|
Open with DEXTER |
3 Model predictions
3.1 Chemistry of water vapor in the cold outer disk
Because the midplane temperatures are well below the evaporation
temperature at densities representative of the midplane of >150 K
(D'Alessio et al. 2005; Fraser et al. 2001), we can expect that beyond the snow-line
water is mostly frozen on the surfaces of dust grains.
Therefore, molecular emission arises predominantly from the warm disk
surface that is heated by stellar irradiation (Aikawa et al. 2002).
Beyond 10 AU the dust in this superheated layer is heated to <100 K
(Nomura et al. 2007) and again it is anticipated that water will remain as
ice. However, the upper layers of the disk are exposed to energetic
X-ray and FUV radiation which provide a source for non-thermal
desorption. Models of X-ray induced desorption suggest that X-rays
cannot release significant H2O into the gas (Najita et al. 2001). A
more profitable method to desorb water ice in the cold outer disk is
via photodesorption, which has a measured yield (molecules/photon) of
10-3 (Öberg et al. 2009). This has been suggested as providing a
basal column of water vapor in the disk by Dominik et al. (2005).
Figure 3 presents results from a detailed calculation of
the UV radiation transfer and chemistry of a standard T Tauri disk
with
,
AU and a
standard gas-to-dust ratio (Fogel et al. 2010). A key aspect in the
calculation of the importance of photodesorption is the formation of
water ice on the grain surface via oxygen hydrogenation and also the
2D propagation of UV photons. This includes Ly
radiation.
which is important for H2O (van Dishoeck et al. 2006). As can be
seen a thin layer exists where water vapor is present in moderate
abundance (
10-7-10-6) in the cold outer regions of the
disk.
As noted by Dominik et al. (2005)
the column density of water vapor produced
by photodesorption is independent of the photon flux. This can be seen
by balancing formation by photodesorption with destruction by UV
photodissociation, giving a maximum
water abundance (Hollenbach et al. 2009):
Here
is the integral of the
photon flux given in Fig. 3 and
cm-2(van Dishoeck et al. 2006).
is the cross section of a
given site on the grain (=
,
with grain radius
m
and
).
is a correction for the
fact the UV photons only penetrate the first few monolayers
(Öberg et al. 2009), with a yield of
.
is the fraction of water ice over the
total amount of ice and
is the number of monolayers. Based on this approximation we find
;
for scaling relations see Hollenbach et al. (2009). This could be lowered if the
grains have a reduced fraction of water ice or perhaps less than a
monolayer of coverage.
![]() |
Figure 3:
Abundance of water vapor ( Top) and Ly |
Open with DEXTER |
3.2 Comparison to observations
Using the above chemical model calculation as input, we use the molecular excitation and radiative transfer code LIME (Brinch et al., in prep.) to calculate the line intensity in both observed water lines. We use the collision rates of water with p-H2 from Faure et al. (2007) as provided by the LAMDA database (Schöier et al. 2005)![[*]](/icons/foot_motif.png)
Because of the appreciable abundance of H2O in the model
(disk-averaged column density of
cm-2), it is not surprising that significant line intensities are
predicted of
mK for the
110-101 line and
300 mK for the
111-000 line with a Gaussian spectral profile. Clearly, our observations rule
out the presence of the amounts of water vapor predicted by
photodesorption regardless of the details of our model.
Absorption by low-excitation water from foreground material cannot
explain the absence of detected emission: the cloud seen in 12CO
by (Dutrey et al. 1997) has narrow emission centered 3 km s-1 away
from the DM Tau disk. Only when the column density of water is scaled
down by a factor of 130 to a disk-averaged value of
cm-2 does the predicted strength of the
110-101 line
becomes consistent with the observed limits; the limits on the
111-000 line are less strict because of the higher noise of
these observations.
Our model could predict lines that are too intense if we overestimate
the collisional excitation of water.
Dick et al. (2010) suggested that existing collisional excitation rates for water are overestimated at temperatures below 50-80 K.
Decreasing the collisional
excitation rates has little effect on the line strengths for the
original column density. In that case, the lines are still highly
optically thick, with maximum optical depth of 2000, and line trapping effectively excites the
line. However, for collision rates lower by a factor of 10 compared to
the adopted rates, reducing the disk averaged column density by a
factor of 20 to
cm-2 is sufficient to comply
with the observational constraints.
Our generic model disk contains 0.03
similar to DM Tau, but
its 400 AU radius is only half that of DM Tau. The increased beam
dilution only strengthens our conclusions. For a DM Tau specific
model, Dominik et al. (2005) predict the
110-101 line to be in
absorption. Line profiles with combinations of emission and strong
absorption naturally arise in disk models with strong temperature,
density and water abundance gradients viewed under non-zero
inclinations. An example is provided by Cernicharo et al. (2009) for the
HD 97048 disk. These models have water abundances peaking at much
larger height above the midplane, explaining the low excitation
resulting in strong absorption. For our discussion here, we stress
that all these models predict emission or absorption lines that are
inconsistent with our observational limits. Future work will focus on
more detailed models for DM Tau, and also explore the effect of dust
settling and non-standard gas-to-dust ratios (Brinch et al. in prep.).
Finally, we note that Ceccarelli et al. (2005)
have claimed a 4
detection HDO in the DM Tau. This
detection has been cast into doubt based on line formation
considerations (Guilloteau et al. 2006).
Our models predict water will be in emission, and not absorption. At
face value this would argue against the reality of the absorption. If
we derive a column density from the HDO observations and the limits
here, the D/H ratio would also be exceedingly high >0.2. Deeper HDO
observations
are needed to settle this issue.
4 Implications
We have presented the results from a deep search for the ground state
emission lines of o-H2O and p-H2O towards the DM Tau disk.
Based on the best theoretical knowledge we have to date, water vapor
should be present in the outer disk and presumably emissive. However,
our sensitive observations show that, at least for this object, it is
not. Our limit on the p-H2O line precludes an extreme
o/p ratio as explanation for the low o-H2O emission strength.
There are two potential explanations for this result. Either water
vapor is unemissive as would be the case if the excitation at low
temperature is lower than generally assumed
(Dick et al. 2010). Cernicharo et al. (2009) also show that the water
excitation sensitively depends on the adopted collision rates with
o- and p-H2. However, our calculations suggest that
line trapping is sufficiently effective at the predicted water
abundances to still produce detectable lines. Alternatively, our
physical/chemical understanding may be incorrect. The photodesorption
yield is measured in the laboratory at low temperature and the
abundance is independent of the photon flux so these aspects appear
unlikely to provide the answer. One intriguing possibility is that
the upper layers of the outer disk are ``dry'' which could be the case
if only bare grains are present in the region where UV photons are
present. A well known key aspect of disk physical evolution is the
coagulation and settling of dust grains to the disk midplane
(Furlan et al. 2006; Weidenschilling & Cuzzi 1993; Dullemond & Dominik 2004). Icy grains present a more
favorable surface for grain coagulation and would therefore become
larger and settle to deeper layers than their bare silicate
counterparts (Dominik & Tielens 1997). While upward mixing of gas and small
grains may occur, larger ice-bearing grains remain in the midplane.
In addition, the total
will be reduced, thereby
reducing the efficiency of grain surface formation of H2O upon
which the photodesorption model depends for water vapor creation
(Hollenbach et al. 2009). This ``cold-finger'' effect was also proposed by
Meijerink et al. (2009) to explain the truncation of warm
water vapor beyond
1 AU seen in Spitzer
measurements. Thus, Herschel and Spitzer both suggest
that the disk around DM Tau is settled. In summary, our
Herschel results suggest that less than 1-5% of the water ice
reservoir survives in the UV-illuminated outer disk regions
around DM Tau. If this finding is confirmed by more detailed models
and by additional observations, Herschel may be
telling us something entirely new about the chemical structure of
protoplanetary disks.
HIFI has been designed and built by a consortium of institutes and university departments from across Europe, Canada and the United States under the leadership of SRON Netherlands Institute for Space Research, Groningen, The Netherlands and with major contributions from Germany, France and the US. Consortium members are: Canada: CSA, U.Waterloo; France: CESR, LAB, LERMA, IRAM; Germany: KOSMA, MPIfR, MPS; Ireland, NUI Maynooth; Italy: ASI, IFSI-INAF, Osservatorio Astrofisico di Arcetri- INAF; Netherlands: SRON, TUD; Poland: CAMK, CBK; Spain: Observatorio Astronómico Nacional (IGN), Centro de Astrobiología (CSIC-INTA). Sweden: Chalmers University of Technology - MC2, RSS & GARD; Onsala Space Observatory; Swedish National Space Board, Stockholm University - Stockholm Observatory; Switzerland: ETH Zurich, FHNW; USA: Caltech, JPL, NHSC. Support for this work was provided by NASA through an award issued by JPL/Caltech. E.A.B. acknowledges support by NSF Grant 0707777, M.R.H. by NWO grant 639.042.404.
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Footnotes
- ... disk
- Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with participation important from NASA.
- ... CLASS
- http://www.iram.fr/IRAMFR/GILDAS
- ...
111-000line
- Although the channel spacing in both bands in similar
(0.54 vs. 0.45 km s-1), the noise in the WBS is 1.7-2.0 times
lower than the noise in the HRS because of the larger noise
bandwidth of the WBS and a
loss factor in the HRS autocorrelator.
- ...(Schöier et al. 2005)
- http://www.strw.leidenuniv.nl/ moldata
All Figures
![]() |
Figure 1:
Noise level (rms) vs integration time of the WBS and HRS data of the H2O
110-101 line.
The noise decreases as |
Open with DEXTER | |
In the text |
![]() |
Figure 2:
a)- d) H2O
spectra obtained with HIFI. A tentative feature is present in
panel b) for H2O 110-101 detected
with WBS between |
Open with DEXTER | |
In the text |
![]() |
Figure 3:
Abundance of water vapor ( Top) and Ly |
Open with DEXTER |
In the text
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