Issue |
A&A
Volume 518, July-August 2010
Herschel: the first science highlights
|
|
---|---|---|
Article Number | L126 | |
Number of page(s) | 5 | |
Section | Letters | |
DOI | https://doi.org/10.1051/0004-6361/201014591 | |
Published online | 16 July 2010 |
Herschel: the first science highlights
LETTER TO THE EDITOR
The Herschel view of GAS in Protoplanetary Systems (GASPS)
First comparisons with a large grid of models![[*]](/icons/foot_motif.png)
C. Pinte1,2 - P. Woitke3,4,5 - F. Ménard1 - G. Duchêne6,1 - I. Kamp7 - G. Meeus8 - G. Mathews9 - C. D. Howard10 - C. A. Grady11 - W.-F. Thi3,1 - I. Tilling3 - J.-C. Augereau1 - W. R. F. Dent12,13 - J. M. Alacid14,15 - S. Andrews16 - D. R. Ardila17 - G. Aresu7 - D. Barrado18,19 - S. Brittain20 - D. R. Ciardi21 - W. Danchi22 - C. Eiroa8 - D. Fedele8,23,24 - I. de Gregorio-Monsalvo12,13 - A. Heras25 - N. Huelamo19 - A. Krivov26 - J. Lebreton1 - R. Liseau27 - C. Martin-Zaïdi1 - I. Mendigutía19 - B. Montesinos19 - A. Mora28 - M. Morales-Calderon29 - H. Nomura30 - E. Pantin31 - I. Pascucci32 - N. Phillips3 - L. Podio7 - D. R. Poelman5 - S. Ramsay33 - B. Riaz32 - K. Rice3 - P. Riviere-Marichalar19 - A. Roberge34 - G. Sandell34 - E. Solano12,13 - B. Vandenbussche35 - H. Walker36 - J. P. Williams9 - G. J. White36,37 - G. Wright4
1 - Université Joseph-Fourier - Grenoble 1/CNRS, Laboratoire
d'Astrophysique de Grenoble (LAOG) UMR 5571, BP 53, 38041 Grenoble
Cedex 09, France
2 - School of Physics, University of Exeter, UK
3 - SUPA, Institute for Astronomy, University of Edinburgh, Royal Observatory, Blackford Hill, Edinburgh, EH9 3HJ, UK
4 - UK Astronomy Technology Centre, Royal Observatory, Edinburgh, Blackford Hill, Edinburgh EH9 3HJ, UK
5 -
School of Physics & Astronomy, University of St. Andrews, North Haugh, St. Andrews KY16 9SS, UK
6 -
Astronomy Department, University of California, Berkeley, CA 94720-3411, USA
7 -
Kapteyn Astronomical Institute, Postbus 800, 9700 AV Groningen, The Netherlands
8 -
Dep. de Física Teórica, Fac. de Ciencias, UAM Campus Cantoblanco, 28049 Madrid, Spain
9 -
Institute for Astronomy, University of Hawaii at Manoa, Honolulu, HI 96822, USA
10 -
SOFIA-USRA, NASA Ames Research Center, Mailstop 211-3 Moffett Field CA 94035, USA
11
- Eureka Scientific and Exoplanets and Stellar Astrophysics Lab, NASA
Goddard Space Flight Center, Code 667, Greenbelt, MD, 20771, USA
12 -
ALMA, Joint ALMA Office, Avda Apoquindo 3846, Piso 19, Edificio
Alsacia, Las Condes, Santiago, Chile
13 -
European Southern Observatory, Alonso de Córdova 3107, Vitacura, Casilla 19001, Santiago 19, Chile
14
- Unidad de Archivo de Datos, Depto. Astrofísica, Centro de
Astrobiología (INTA-CSIC), P.O. Box 78, 28691 Villanueva de la Cañada,
Spain
15 -
Spanish Virtual Observatory
16 -
Harvard-Smithsonian Center for Astrophysics, 60 Garden St., Cambridge, MA, USA
17 -
NASA Herschel Science Center, California Institute of Technology, Pasadena, USA
18 -
Calar Alto Observatory, Centro Astronómico Hispano-Alemán
C/Jesús Durbán Remón 2-2, 04004 Almería, Spain
19 -
LAEX, Depto. Astrofísica, Centro de Astrobiología (INTA-CSIC), PO Box 78, 28691 Villanueva de la Cañada, Spain
20 -
Clemson University
21 -
NASA Exoplanet Science Institute/Caltech 770 South Wilson Avenue, Mail Code: 100-22, Pasadena, CA USA 91125
22 -
NASA Goddard Space Flight Center, Exoplanets & Stellar Astrophysics, Code 667, Greenbelt, MD 20771, USA
23 -
Max Planck Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
24 -
Johns Hopkins University Dept. of Physics and Astronomy, 3701 San Martin drive Baltimore, MD 21210 USA
25 -
Research and Scientific Support Department-ESA/ESTEC, PO Box 299, 2200 AG Noordwijk, The Netherlands
26
- Astrophysikalisches Institut und Universitätssternwarte,
Friedrich-Schiller-Universität, Schillergäßchen 2-3, 07745 Jena,
Germany
27 -
Department of Radio and Space Science, Chalmers University of Technology, Onsala Space Observatory, 439 92 Onsala, Sweden
28 -
ESA-ESAC Gaia SOC, PO Box 78, 28691 Villanueva de la Cañada, Madrid, Spain
29 -
Spitzer Science Center, California Institute of Technology, 1200 E California Blvd, 91125 Pasadena, USA
30 -
Department of Astronomy, Graduate School of Science, Kyoto University, Kyoto 606-8502, Japan
31 -
CEA/IRFU/SAp, AIM UMR 7158, 91191 Gif-sur-Yvette, France
32 -
Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
33 -
European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching bei München, Germany
34 -
Exoplanets and Stellar Astrophysics Lab, NASA Goddard Space Flight
Center, Code 667, Greenbelt, MD, 20771, USA
35 -
Instituut voor Sterrenkunde, KU Leuven, Celestijnenlaan 200D, 3001 Leuven, Belgium
36 -
The Rutherford Appleton Laboratory, Chilton, Didcot, OX11 OQL, UK
37 -
Department of Physics & Astronomy, The Open University, Milton Keynes MK7 6AA, UK
Received 30 March 2010 / Accepted 5 May 2010
Abstract
The Herschel GASPS key program is a survey of the gas
phase of protoplanetary discs, targeting 240 objects which cover a large
range of ages, spectral types, and disc properties.
To interpret this
large quantity of data
and initiate self-consistent analyses of the
gas and dust properties of protoplanetary discs,
we have combined the
capabilities of the radiative transfer code MCFOST with the gas
thermal balance and chemistry code ProDiMo to compute a grid of 300 000 disc models (DENT).
We present a comparison of the first Herschel/GASPS line and continuum
data with the predictions from the DENT grid of models.
Our objective
is to test some of
the main trends already identified in the DENT grid, as well as to define
better empirical diagnostics to estimate the total gas mass of
protoplanetary discs.
Photospheric UV radiation appears to be the dominant gas-heating
mechanism for Herbig stars, whereas UV excess and/or X-rays emission
dominates for T Tauri stars.
The DENT grid reveals
the complexity in the analysis of far-IR lines and the difficulty to
invert these observations into physical quantities.
The combination of
Herschel line observations with continuum data and/or with
rotational lines in the (sub-)millimetre regime, in particular CO
lines, is required for a detailed characterisation of the physical and
chemical properties of circumstellar discs.
Key words: astrochemistry - circumstellar matter - protoplanetary disks - stars: formation - radiative transfer - line: formation
1 Introduction
The dust phase of circumstellar discs has received a lot of attention in the last few decades, giving us a clearer picture of their structure and dust content through many continuum surveys in various wavelengths regimes (e.g. Evans et al. 2007; Beckwith et al. 1990; Andrews & Williams 2007), complemented by detailed studies of individual objects, combining spectral energy distributions (SEDs) and resolved maps in scattered light and thermal emission (e.g. Duchêne et al. 2010; Pinte et al. 2008).
Although gas represents 99% of the initial mass of discs, it has been more difficult to observe and is mostly restricted to millimetre lines probing the cold outer disc, where the freeze-out of molecules is important (e.g. Schaefer et al. 2009; Dent et al. 2005), and near-IR lines which are only emitted from the hot inner parts of discs (e.g. Brittain et al. 2007; Najita et al. 2003). The high sensitivity of Herschel (Pilbratt et al 2010) opens an opportunity to systematically probe the gas phase of discs, in particular the warm atomic and molecular layer responsible for the bright gas emission lines in the far-IR. The GASPS open time key program (see Dent et al., in prep.; and Mathews et al 2010) is a large survey of gas in discs with a gas mass sensitivity comparable to the dust surveys. GASPS will observe several atomic and molecular lines in about 240 protoplanetary disc systems with ages in the critical 1 to 30 million year age range during which planets form and the gas seems to dissipate.
The interpretation of gas observations is complicated by the large number of processes at play: processing of radiation by dust grains, disc thermal structure, chemistry, excitation and destruction of molecules, freeze-out and desorption on the dust grains, etc. To estimate the relative importance of these mechanisms as a function of age, stellar properties, disc structure, and dust content, we have computed a large grid of synthetic SEDs and gas emission lines, named Disc Evolution with Neat Theory (DENT, Woitke et al. 2010). Here, we confront the trends identified in the DENT grid with the first GASPS observations.
![]() |
Figure 1:
[OI] 63 |
Open with DEXTER |
2 The DENT grid and initial GASPS data
The DENT grid is intended as a statistical tool to investigate the influence of stellar, disc, and dust properties on the various continuum and line observables, and to study to what extent these dependencies can be inverted to retrieve disc properties. The grid relies on the combined capabilities of the 3D radiative transfer code MCFOST (Pinte et al. 2009,2006) and the gas thermal balance and chemistry code ProDiMo (Woitke et al. 2009; Kamp et al. 2010). Spectral energy distributions and line fluxes of [OI], [CII], 12CO, ortho-H2O and para-H2O are predicted for more than 300 000 discs models. The DENT grid was built by systematically exploring an 11-dimension parameter space (see Woitke et al. 2010, Table 1). In particular the DENT grid explores the effect of varying the central star (age, mass, UV excess), disc dust mass and gas-to-dust mass ratio, inner and outer radii, flaring and surface density exponents, grain sizes, and presence of dust settling. It is important to keep in mind that even with the large number of calculated models, the sampling of each parameter remains coarse and that the DENT grid does not reflect the statistics of objects in GASPS (each parameter has been sampled uniformly and not following the distributions of the GASPS target list). We refer the reader to Woitke et al. (2010) for details about the grid properties and computational implementation.
We include here data obtained during the science demonstration phase (SDP, Mathews et al 2010), as well as GASPS data reduced prior to 2010 April 23, which will be presented in detail in following papers. Due to the limited number of sources, statistical analyses remain premature, but initial comparisons with the model predictions are necessary to ensure that the range of models cover the GASPS observations.
3 Results and discussion
One of the main reasons to compute the DENT grid was to estimate the degeneracies between parameters, i.e. how they influence the various lines and how far Herschel line observations can be inverted to assess the physical and chemical conditions of the disc. Not surprisingly, the DENT grid revealed that many parameters affect the predicted line fluxes and SEDs, and degeneracies between parameters are common and complex, which makes the interpretation of lines fluxes difficult.
3.1 Gas heating processes
![]() |
Figure 2:
Correlations between line fluxes. Symbols as in
Fig. 1. Left panel: [OI]
145 |
Open with DEXTER |
Figure 1 plots the [OI] 63 m line flux as a
function of the stellar luminosity and accretion luminosity.
The DENT grid predicts a correlation between the line
flux and the stellar luminosity. All observational points (except
sources with a large outflow or envelope)
lie within the 1-sigma envelope of the models.
For T Tauri stars, the detected line fluxes are well reproduced by
models with a high UV excess (
,
see Woitke et al. 2010),
suggesting that UV emission produced by accretion onto the star is one
of the main gas-heating processes.
The right panel of
Fig. 1 indeed suggests a trend between [OI] line flux
and accretion luminosity.
On the other hand, Herbig Ae/Be stars show a strong correlation between the line
flux and stellar luminosities, with a much smaller scatter than for
T Tauri stars. Large UV excesses do not seem necessary to reproduce
the Herbig observations (data points lie between models with high- and
low-UV excess).
This suggests that stellar radiation is the dominant gas-heating source
for Herbig stars. Because these sources radiate large phostospheric UV
emission, the accretion luminosity represents a smaller fraction of the UV
luminosity and is not as critical a gas heating mechanism as for
T Tauri stars. This is also consistent with the small fraction of
large accretors (
)
among Herbig stars
(Garcia Lopez et al. 2006).
X-ray irradiation, which is not yet included in the DENT grid, can also contribute significantly to the gas heating and chemistry for low-mass objects (e.g. Meijerink et al. 2008; Glassgold et al. 2004; Semenov et al. 2004; Hollenbach & Gorti 2009; Ercolano & Owen 2010) and higher fluxes can be expected for sources with typical T Tauri X-ray emission. The small number of sources observed by GASPS so far prevents us from distinguishing between UV and X-rays for the main heating process for low mass objects. The full GASPS survey should provide detailed answers on these aspects.
Figure 2 presents the [OI] 145 m and [CII]
158
m line fluxes as a function of the [OI] 63
m line. The models
are in excellent agreement with the GASPS observations.
The DENT grid predicts a correlation between the [OI] 63 and 145
m line fluxes.
A regression fit of all the DENT points indicates that both line fluxes
are almost proportional (
), with a [OI] 145/63 line
ratio around 0.05 on average.
The presence of scatter in the plot illustrates the wide range of
physical conditions encountered in the DENT grid. The correlation, however,
is very strong (Pearson correlation coefficient of 0.97) and holds for several
orders of magnitude in line flux.
These results agree with the prediction of Tielens & Hollenbach (1985)
for a 1D photodissociation region (see their Fig. 2).
Our detailed line modelling of the 2D PDR disc surface with varying
density and irradiation confirms the picture drawn from these 1D
models: the oxygen lines are optically
thick and originate in
a relatively high-temperature gas (
100 K).
As a
consequence, this line ratio does not provide
constraints on the local gas density and temperature in most cases.
Deviations from this average ratio of 0.05 are interesting though. In particular, two Herbig Ae with known outflows observed by GASPS present a small line ratio around 0.025. According to Tielens & Hollenbach (1985), this value cannot be obtained for optically thick lines. This suggests that a significant fraction of the line fluxes originates from an optically thin region above the bulk of the disc (potentially the outflow) with a temperature between 40 and 200 K. These sources will be studied in detail in following GASPS papers.
The [CII] 158 m line also presents a correlation with the [OI] 63
m, but with a much larger scatter. A regression fit of the
DENT models indicates
that on average
,
with a decreasing line ratio as
the line flux increases. This suggests an increasing gas temperature
(Fig. 3 in Tielens & Hollenbach 1985) in the disc towards
more luminous objects. Most of the DENT models lie in a region where
the line ratio is between 0.01 and 1, suggesting gas temperatures higher
than 100 K. More detailed analyses of the [CII] line
are complicated by several
factors: our disc models show that the line originates from larger radii
and lower density regions than the [OI] lines, and it is very sensitive to the
amount of UV radiation.
3.2 Gas mass and gas-to-dust mass ratio
In addition to the main trend of an increasing line flux with
(UV) luminosity,
there is also a trend with gas mass,
where the synthetic line flux increases with mass,
but the correlation seems to saturate above 10-4 (see also Woitke et al. 2010), preventing direct inversion of the line
flux into a gas mass (Fig. 3).
![]() |
Figure 3:
[OI] 63 |
Open with DEXTER |
The right panel of Fig. 3 shows the correlation between the [OI]
63 m line flux and the adjacent continuum.
Because the line and continuum emissions are optically thick in
most cases, these fluxes give an indication of the relative
temperatures and projected surface area of the emitting regions (gas and dust). As the
stellar luminosity increases, the region of the disc which is warm enough to
contribute significantly to the emission also increases, resulting in
larger fluxes.
This behaviour is observed for most GASPS sources where the line flux
roughly increases with the continuum level, but with a significant
scatter. As a consequence, this
indicates that a large fraction of discs with a significant far-IR
excess will be detected in [OI] by Herschel.
The separation of the DENT models according to their
gas-to-dust mass ratio
suggests that most objects are gas-rich (gas/dust mass ratio >10).
The large scatter in
the models, and the optical depth in the continuum and the line,
precludes however,
in most cases, a precise estimate of the gas-to-dust
ratio for individual sources. For
instance, no direct ratio (or upper limit for HD 181327) can be estimated from this diagnostic alone for
HD 169142 and TW Hydra,
for which the
line fluxes can be reproduced by any ratio between 1 and 1000.
In addition, the contribution of an
outflow to the line flux may affect the estimation of the disc gas-to-dust
ratio and needs to be accounted for.
Greater observational constraints
and more detailed modelling is
required to estimate the gas mass and gas-to-dust
ratio. In particular,
the combination of low rotational level transitions of CO with oxygen
lines offers a valuable proxy to estimate the amount of gas in
discs.
Figure 4 plots the 12CO
line flux
as a function of the [OI] 63
m line flux. We stress that the accuracy in the calculated CO abundances
is limited by our approximate treatment of self-shielding (see
Woitke et al. 2009), but this does not affect our conclusions. For low-mass
discs, this diagram allows a clear distinction of the gas disc
mass.
As the mass increases, lines become optically thick and the
corresponding fluxes saturate, preventing determination of the gas
mass. Current CO surveys can only reach sources in this
saturation regime (see for instance data from
Dent et al. 2005 in Fig. 4), but
this perspective is particularly interesting in the context of
ALMA, which will offer high sensitivity for CO lines (
10-23 W m-2).
Similar diagrams combining 13CO (not included in DENT, but see
Meeus et al 2010 and Thi et al. 2010),
C18O and [OI] 145
m, which saturate at higher masses
due to lower optical depths, will further help to overcome this
degeneracy. As oxygen lines are sensitive to warm
gas in the inner 10-30 AU (for T Tauri stars), they offer
complementary views to the low-J CO lines which probe regions outside
of 20-40 AU, especially
when resolved maps
of the CO emission are available.
4 Summary and conclusions
![]() |
Figure 4:
12CO
|
Open with DEXTER |
The GASPS survey will offer unique views of the gas and dust phases of protoplanetary discs. In order to provide statistical tools to help the interpretation of the survey results, we interfaced the MCFOST and ProDiMo codes and calculated a large grid of models sampling the range of discs observed by GASPS. This will allow us to determine some of the physical conditions within discs. The initial results from the GASPS survey tend to confirm the predictions of the DENT grid, illustrating the main parameters affecting the line fluxes, namely the UV excess and/or X-ray emission for T Tauri stars and the UV stellar irradiation for Herbig stars. This is a highly relevant point to be considered in subsequent open time programs on discs.
The interpretation of line results remains difficult and their inversion
into physical parameters must be performed
with caution, because the DENT grid highlights considerable degeneracies between
parameters and the complex interplay between various physical processes.
The [OI] 63m is crucialfor breaking some of the degeneracies. By
combining this line with continuum and/or (sub)mm rotational lines, we can
possibly distinguish various parameters.
Meeus et al (2010) and Thi et al. (2010) illustrate how far this inversion
can be performed when high quality data sets with a wide range of
observational techniques are available.
C. Pinte acknowledges funding from the European Commission's 7th Framework Program as a Marie Curie Intra-European Fellow (PIEF-GA-2008-220891). The members of LAOG, Grenoble acknowledge PNPS, CNES and ANR (contract ANR-07-BLAN-0221) for financial support. W.F. Thi acknowledges a SUPA astrobiology fellowship. G. Meeus, C. Eiroa, I. Mendigutía and B. Montesinos are partly supported by Spanish grant AYA 2008-01727. D.R. Ardila, S.D. Brittain, W. Danchi, C.A. Grady, C.D. Howard, G.S. Mathews, I. Pascucci, A. Roberge, B. Riaz, G. Sandell and J.P. Williams acknowledge NASA/JPL for funding support. J.M. Alcid and E. Solano acknowledges funding from the Spanish MICINN (grant AYA2008-02156).
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Footnotes
- ... models
- Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.
All Figures
![]() |
Figure 1:
[OI] 63 |
Open with DEXTER | |
In the text |
![]() |
Figure 2:
Correlations between line fluxes. Symbols as in
Fig. 1. Left panel: [OI]
145 |
Open with DEXTER | |
In the text |
![]() |
Figure 3:
[OI] 63 |
Open with DEXTER | |
In the text |
![]() |
Figure 4:
12CO
|
Open with DEXTER | |
In the text |
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