Issue |
A&A
Volume 518, July-August 2010
Herschel: the first science highlights
|
|
---|---|---|
Article Number | L143 | |
Number of page(s) | 11 | |
Section | Letters | |
DOI | https://doi.org/10.1051/0004-6361/201014562 | |
Published online | 16 July 2010 |
Herschel: the first science highlights
LETTER TO THE EDITOR
Silicon in the dust formation zone of IRC +10216
,![[*]](/icons/foot_motif.png)
L. Decin1,2 - J. Cernicharo3 - M. J. Barlow4 - P. Royer1 - B. Vandenbussche1 - R. Wesson4 - E. T. Polehampton5,6 - E. De Beck1 - M. Agúndez3,9 - J. A. D. L. Blommaert1 - M. Cohen8 - F. Daniel3 - W. De Meester1 - K. Exter1 - H. Feuchtgruber10 - J. P. Fonfría7 - W. K. Gear11 - J. R. Goicoechea3 - H. L. Gomez11 - M. A. T. Groenewegen12 - P. C. Hargrave11 - R. Huygen1 - P. Imhof13 - R. J. Ivison14 - C. Jean1 - F. Kerschbaum16 - S. J. Leeks5 - T. Lim5 - M. Matsuura4,17 - G. Olofsson15 - T. Posch16 - S. Regibo1 - G. Savini4 - B. Sibthorpe14 - B. M. Swinyard5 - B. Tercero3 - C. Waelkens1 - D. K. Witherick4 - J. A. Yates4
1 - Instituut voor Sterrenkunde,
Katholieke Universiteit Leuven, Celestijnenlaan 200D, 3001 Leuven, Belgium
2 -
Sterrenkundig Instituut Anton Pannekoek, University of Amsterdam, Science Park 904, 1098 Amsterdam, The Netherlands
3
- Laboratory of Molecular Astrophysics, Department of Astrophysics,
CAB, INTA-CSIC, Ctra de Ajalvir, km 4, 28850 Torrejon de Ardoz, Madrid,
Spain
4 -
Dept of Physics & Astronomy, University College London, Gower St, London WC1E 6BT, UK
5 -
Space Science and Technology Department, Rutherford Appleton Laboratory, Oxfordshire, OX11 0QX, UK
6 -
Department of Physics, University of Lethbridge, Lethbridge, Alberta, T1J 1B1, Canada
7
- Departamento de Astrofísica Molecular e Infrarroja, Instituto de
Estructura de la Materia, CSIC, Serrano 121, 28006 Madrid, Spain
8 -
Radio Astronomy Laboratory, University of California at Berkeley, CA 94720, USA
9 -
LUTH, Observatoire de Paris-Meudon, 5 Place Jules Janssen, 92190 Meudon, France
10 -
Max-Planck-Institut für extraterrestrische Physik, 85748 Giessenbachstrasse, Germany
11 -
School of Physics and Astronomy, Cardiff University, Queens Buildings, The Parade, Cardiff, CF24 3AA, UK
12 -
Royal Observatory of Belgium, Ringlaan 3, 1180 Brussels, Belgium
13 -
Blue Sky Spectroscopy, 9/740 4 Ave S, Lethbridge, Alberta T1J 0N9, Canada
14 -
UK Astronomy Technology Centre, Royal Observatory Edinburgh, Blackford Hill, Edinburgh EH9 3HJ, UK
15
- Dept of Astronomy, Stockholm University, AlbaNova University Center,
Roslagstullsbacken 21, 10691 Stockholm, Sweden
16 -
University of Vienna, Department of Astronomy, Türkenschanzstraße 17, 1180 Vienna, Austria
17 -
Mullard Space Science Laboratory, University College London,
Holmbury St. Mary, Dorking, Surrey RH5 6NT, UK
Received 30 March 2010 / Accepted 27 April 2010
Abstract
The interstellar medium is enriched primarily by matter ejected
from evolved low and intermediate mass stars. The outflows from these
stars create a circumstellar envelope in which a rich gas-phase and
dust-nucleation chemistry takes place. We observed the nearest
carbon-rich evolved star, IRC +10216, using the PACS (55-210 m) and SPIRE (194-672
m) spectrometers on board Herschel. We find several tens of lines from SiS and SiO, including lines from the v=1 vibrational level. For SiS these transitions range up to
J=124-123, corresponding to energies around 6700 K, while the highest detectable transition is J=90-89
for SiO, which corresponds to an energy around 8400 K. Both
species trace the dust formation zone of IRC +10216, and the broad
energy ranges involved in their detected transitions permit us to
derive the physical properties of the gas and the particular zone in
which each species has been formed. This allows us to check the
accuracy of chemical thermodynamical equilibrium models and the
suggested depletion of SiS and SiO due to accretion onto dust grains.
Key words: techniques: spectroscopic - stars: AGB and post-AGB - stars: carbon - circumstellar matter - stars: mass-loss - stars: individual: IRC +10216
1 Introduction
IRC +10216 (IRC +10216) is the brightest non-Solar System object in the sky at 5




![]() |
Figure 1: Continuum-subtracted PACS
and SPIRE spectrum of IRC +10216. In the three upper panels, the
PACS spectrum of IRC +10216 (black) is compared to the ISO-LWS
spectrum (grey, Cernicharo et al. 1996).
The fourth and fifth panels show the SPIRE spectrum of IRC +10216
(black). The bottom panel zooms in on the 141-146.8 |
Open with DEXTER |
2 Observations and data reduction
Thanks to its high infrared brightness, IRC +10216 is an ideal target for observation with Herschel (Pilbratt et al. 2010). PACS and SPIRE spectroscopic observations were obtained in the context of the guaranteed time key programme ``Mass-loss of Evolved StarS'' (Groenewegen et al., in prep.).
The PACS instrument, its in-orbit performance and calibration,
and its scientific capabilities are described in Poglitsch et al. (2010). The PACS spectroscopic observations of IRC +10216 consist of full SED scans between 52 and 210 m obtained in a
raster, i.e. a pointing
on the central object, and two pointings 30
either side. The observations were performed on 2009 Nov. 12 (OD 182). The position angle was 110 degrees.
The instrument mode was a non-standard version of the chop-nod PACS-SED AOT, used with a large chopper throw (6
).
The spectral resolving power varies between 1000 and 4500. A
description of the observing mode and of the data reduction process can
be found in Royer et al. (2010). The only difference with the data reduction of VY CMa as presented in Royer et al. (2010)
is that the ground-based calibration was used for IRC +10216. The
estimated calibration uncertainty on the line fluxes is 50%.
The SPIRE FTS measures the Fourier transform of the source spectrum across short (SSW, 194-313 m) and long (SLW, 303-671
m) wavelength bands simultaneously. The FWHM beamwidths of the SSW and SLW arrays vary between 17-19
and 29-42
respectively.
The source spectrum, including the continuum, is restored by taking the
inverse transform of the observed interferogram. The absolute flux
calibration uncertainty is 15-20% in the SSW band and 20-30% in the SLW
band above 20 cm-1 (up to 50% below 20 cm-1). For more details on the SPIRE FTS and its calibration see Griffin et al. (2010) and Swinyard et al. (2010).
IRC +10216 was observed with the high-resolution mode of the SPIRE FTS on the 2009 Nov. 19 (OD 189). Twenty repetitions were used, each of which consisted of one forward and one reverse scan of the FTS, with each scan taking 66.6 s. The total on-source integration time was therefore 2664s. The unapodized spectral resolution is 1.4 GHz (0.048 cm-1), and this is 2.1 GHz (0.07 cm-1) after apodization (using extended Norton-Beer function 1.5; Naylor & Tahic 2007).
PACS and SPIRE photometry observations of IRC +10216 are presented in Ladjal et al. (2010).
3 Results
Currently, more than 500 molecular emission lines have been identified in the PACS and SPIRE spectra of IRC +10216 (see Fig. 1), belonging to 10 different molecules and their isotopologues (12CO, 13CO, C18O, H12CN, H13CN, H2O, NH3, SiS, SiO, CS, C34S, 13CS, C3, C2H, HCl, and H37Cl). The detection of this last molecule is discussed by Cernicharo et al. (2010). In the ISO-LWS spectrum shown in Fig. 1, 57 lines belonging to CO and HCN were identified by Cernicharo et al. (1996). The number of identified lines increases to 280 in the PACS spectrum thanks to its higher spectral resolution. Most of the lines in the PACS and SPIRE spectrum arise from HCN, with the strongest lines from 12CO. HCN is one of the most abundant molecular species in the CSEs of carbon stars (Willacy & Cherchneff 1998) and it is known to show maser action in various vibrational states. The strength of the 12CO lines are diagnostics for the thermophysical structure (see Sect. 3.1). In this paper, we focus on the silicon-bearing molecules SiS and SiO, two refractory species that are formed in the inner envelope. As soon as the temperature of the gas falls below a certain critical value, the molecules can start to condense and form dust grains.
High-J rotational lines have been detected from both molecules. For SiO, 80 rotational transitions in the ground-state from J=11-10 to J=90-89 (
K), and 99 lines from
J = 26-25 to
J = 124-123 (
)
for SiS are clearly detected. From the detected lines,
45% of both species is unblended (see Table A.1 in the online Appendix, which also lists the detected 12CO and 13CO lines).
The emission lines of higher-J
transitions and rotational transitions in the first vibrational state
are very weak, but their line contribution can be deduced from the
theoretical modelling (see Sect. 3.2, and Table A.1). The line formation region of the highest-J lines of SiO (SiS) is within the first 5
(10
), i.e., tracing the recently identified dust formation region (Fonfría et al. 2008).
3.1 Thermophysical structure of the envelope
The large number of optically thick 12CO and optically thin 13CO
lines enabled us to perform a tomographical study of the CSE.
Properties of the circumstellar gas, such as the kinetic temperature,
velocity, and density structure, were determined through a non-local
thermodynamic equilibrium (non-LTE) radiative transfer modelling of the
12CO lines. The 12CO lines cover energy levels from J = 3 (at 31 K) to J=47 (at 5853 K) and trace the envelope for radii
cm (R<2000
).
The GASTRoNOoM code was used to calculate the kinetic temperature and
velocity structure in the envelope and to solve the non-LTE radiative
transfer equations (Decin et al. 2010,2006). The rate equations were solved for the ground and first excited vibrational state, with
.
The CO line list and collisional rates are discussed in Decin et al. (2010).
The terminal velocity was deduced from ground-based observations of low-J 12CO lines (De Beck et al. 2010).
The GASTRoNOoM code computes the velocity structure by solving the
momentum equation and the temperature structure from the equation
expressing the conservation of energy (see Eq. (6) in Decin et al. 2006). However, the resulting temperature was slightly too low beyond 60
to correctly predict the lower excitation 12CO lines, which mainly reflects uncertainties in the gas-grain collisional heating. Therefore, we opted to use
for R>60
.
The best-fit model was determined using the log-likelihood function as described in Decin et al. (2007).
The derived (circum)stellar parameters are given in Table 1, the deduced thermodynamical structure is displayed in Fig. 2, and the line predictions are shown in Fig. 1. Specifically, we obtained a mass loss rate of
/yr (with an uncertainty of a factor 2) and a 12CO/13CO ratio of
.
The latter is on the lower side of the range of 12C/13C ratios quoted in the literature, going from 20 (Barnes et al. 1977) to 50 (Schöier & Olofsson 2000).
The lowest value is obtained from vibra-rotational transitions in the
fundamental band of CO, and higher values are often obtained from
low-excitation CO or CS lines. The accuracy of isotopologue ratios
obtained from low-excitation rotational transitions is often limited by
the uncertain effect of photodissociation by interstellar UV photons
and chemical fractionation (e.g., Mamon et al. 1988), effects that are not hampering the high-excitation 12CO and 13CO lines in the PACS and SPIRE spectra.
Table 1: Parameters for the best-fit model, where numbers in italics indicate input parameters that have been kept fixed at the given value.
![]() |
Figure 2: Thermodynamical structure in the envelope of IRC +10216 as derived from the 12CO rotational line transitions. The vertical dotted line represents the dust condensation radius. |
Open with DEXTER |
3.2 Abundance profiles of SiO and SiS
The SiO and SiS emission lines are modelled with the thermodynamical structure as deduced in Sect. 3.1. Linelists and (available) collisional rates are described in Decin et al. (2010). However, the lack of collisional rates for high-J transitions of both molecules with He or H2 led us calculate the level populations in LTE. This approach is justified since most of the detected high-J lines originate in the stellar photosphere and in the inner wind envelope, where the high gas density and temperature ensure thermal equilibrium for the level populations. Pulsation driven shocks in the inner envelope may alter abundances predicted from equilibrium chemistry. The estimated uncertainty on the derived abundances is a factor of 5, when taking the line flux uncertainty into account.
SiO:
Using an outer radius value of 560
![$_2]=1\times10^{-7}$](/articles/aa/full_html/2010/10/aa14562-10/img26.png)

















![]() |
Figure 3:
Comparison between few PACS and SPIRE SiO v=0
lines (black) and theoretical line predictions (grey). Full grey lines
represent theoretical line profiles using a constant SiO fractional
abundance of [SiO/H2] =
|
Open with DEXTER |
SiS:
The SiS fractional abundances derived from the PACS and SPIRE observations is [SiS/H![$_2]=4\times10^{-6}$](/articles/aa/full_html/2010/10/aa14562-10/img36.png)





![$_2]=7.5\times10^{-6}$](/articles/aa/full_html/2010/10/aa14562-10/img40.png)






![$_2]=1.5\times10^{-5}$](/articles/aa/full_html/2010/10/aa14562-10/img45.png)

4 Conclusion
The PACS and SPIRE spectroscopic observations of IRC +10216 have
been shown to be of excellent quality for studying the thermodynamical
and chemical structure of the envelope, created by its copious mass
loss. The temperature and mass-loss rate of the envelope are derived
from the 12CO lines.
Both SiO and SiS are refractory species, and the PACS and SPIRE data
can provide a strong diagnostic tool for determining their role in the
dust formation process. Analysing the high-J SiO and SiS lines yields a constant fractional abundance of
and
,
respectively.
However, we detect onlyv=0 and v=1
transitions for both species, mainly because of the densely populated
spectrum of IRC+10216, while it is known from ground-based observations
that levels of SiS up to v=8 have been detected (Agúndez et al. 2010, in prep.). Moreover, the low-J
transitions of SiO and SiS, which are more sensitive to the external
envelope, are not accesible to PACS and SPIRE. Since the high-J lines in the ground-state and the v=1 lines
of both molecules are very weak, we cannot put strong constraints on
the fractional abundance in the inner envelope (
).
For SiO, 1/3 at most is estimated to take part in dust formation
process, while we deduce a fraction of 1/2 for SiS.
Only a merged set of millimeter, submillimeter, and far-infrared
observations of SiO and SiS can provide a detailed analysis of the
abundance of these species from the photosphere to the
photodissociation zone (Agúndez et al. 2010, in prep.).
PACS was developed by a consortium of institutes led by MPE (Germany) and including UVIE (Austria); KUL, CSL, IMEC (Belgium); CEA, OAMP (France); MPIA (Germany); IFSI, OAP/AOT, OAA/CAISMI, LENS, SISSA (Italy); IAC (Spain). This development has been supported by the funding agencies BMVIT (Austria), ESA-PRODEX (Belgium), CEA/CNES (France), DLR (Germany), ASI (Italy), and CICT/MCT (Spain). SPIRE has been developed by a consortium of institutes led by Cardiff Univ. (UK) and including Univ. Lethbridge (Canada); NAOC (China); CEA, LAM (France); IFSI, Univ. Padua (Italy); IAC (Spain); Stockholm Observatory (Sweden); Imperial College London, RAL, UCL-MSSL, UKATC, Univ. Sussex (UK); Caltech, JPL, NHSC, Univ. Colorado (USA). This development has been supported by national funding agencies: CSA (Canada); NAOC (China); CEA, CNES, CNRS (France); ASI (Italy); MCINN (Spain); SNSB (Sweden); STFC (UK); and NASA (USA). L.D. acknowledges financial support from the Fund for Scientific Research - Flanders (FWO). M.G., D.L,. J.B., W.D.M., K.E., R.H., C.H., S.R., P.R., and B.V. acknowledge support from the Belgian Federal Science Policy Office via the PRODEX Programme of ESA. F.K. acknowledges funding by the Austrian Science Fund FWF under project number P18939-N16 and I163-N16
References
- Barnes, T. G., Hinkle, K. H., Lambert, D. L., & Beer, R. 1977, ApJ, 213, 71 Bieging, J. H., & Nguyen-Quang-Rieu. 1989, ApJ, 343, L25 [NASA ADS] [CrossRef] [Google Scholar]
- Boyle, R. J., Keady, J. J., Jennings, D. E., et al. 1994, ApJ, 420, 863 [NASA ADS] [CrossRef] [Google Scholar]
- Cernicharo, J., Barlow, M. J., Gonzalez-Alfonso, E., et al. 1996, A&A, 315, L201 [NASA ADS] [Google Scholar]
- Cernicharo, J., Guélin, M., & Kahane, C. 2000, A&AS, 142, 181 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
- Cernicharo, J., Decin, L., Barlow, M., et al. 2010, A&A, 518, L136 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
- Crosas, M., & Menten, K. M. 1997, ApJ, 483, 913 [NASA ADS] [CrossRef] [Google Scholar]
- De Beck, E., Decin, L., de Koter, A., et al. 2010, A&A, submitted [Google Scholar]
- Decin, L., Hony, S., de Koter, A., et al. 2006, A&A, 456, 549 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
- Decin, L., Hony, S., de Koter, A., et al. 2007, A&A, 475, 233 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
- Decin, L., De Beck, E., Brunken, S., et al. 2010, A&A, 516, A69 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
- Fonfría, J. P., Cernicharo, J., Richter, M. J., & Lacy, J. H. 2008, ApJ, 673, 445 [NASA ADS] [CrossRef] [Google Scholar]
- González-Alfonso, E., Neufeld, D. A., & Melnick, G. J. 2007, ApJ, 669, 412 [NASA ADS] [CrossRef] [Google Scholar]
- Griffin, M. J., Abergel, A., Abreu, A., et al. 2010, A&A, 518, L3 [Google Scholar]
- He, J. H., Dinh-V-Trung, K. S., Müller, H. S. P., et al. 2008, ApJS, 177, 275 [NASA ADS] [CrossRef] [Google Scholar]
- Keady, J. J., & Ridgway, S. T. 1993, ApJ, 406, 199 [NASA ADS] [CrossRef] [Google Scholar]
- Ladjal, D., Barlow, M., Groenewegen, M., et al. 2010, A&A, 518, L141 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
- Mamon, G. A., Glassgold, A. E., & Huggins, P. J. 1988, ApJ, 328, 797 [NASA ADS] [CrossRef] [Google Scholar]
- Naylor, D. A., & Tahic, M. K. 2007, J. of Optical Soc. of America A, 24, 3644 [Google Scholar]
- Olofsson, H., Johansson, L. E. B., Hjalmarson, A., & Nguyen-Quang-Rieu. 1982, A&A, 107, 128 [NASA ADS] [Google Scholar]
- Pilbratt, G. L., Riedinger, J. R., Passvogel, T., et al. 2010, A&A, 518, L1 [CrossRef] [EDP Sciences] [Google Scholar]
- Poglitsch, A., Waelkens, C., Geis, N., et al. 2010, A&A, 518, L2 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
- Ridgway, S., & Keady, J. J. 1988, ApJ, 326, 843 [NASA ADS] [CrossRef] [Google Scholar]
- Royer, P., Decin, L., Wesson, R., et al. 2010, A&A, 518, L145 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
- Schöier, F. L., & Olofsson, H. 2000, A&A, 359, 586 [NASA ADS] [Google Scholar]
- Schöier, F. L., Fong, D., Olofsson, H., et al. 2006, ApJ, 649, 965 [NASA ADS] [CrossRef] [Google Scholar]
- Schöier, F. L., Bast, J., Olofsson, H., & Lindqvist, M. 2007, A&A, 473, 871 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
- Skinner, C. J., Meixner, M., & Bobrowsky, M. 1998, MNRAS, 300, L29 [NASA ADS] [CrossRef] [Google Scholar]
- Swinyard, B. M., Ade, P., Baluteau, J.-P., et al. 2010, A&A, 518, L4 [Google Scholar]
- Willacy, K., & Cherchneff, I. 1998, A&A, 330, 676 [NASA ADS] [Google Scholar]
- Zuckerman, B., & Dyck, H. M. 1986, ApJ, 304, 394 [NASA ADS] [CrossRef] [Google Scholar]
Online Material
Appendix A: Identified lines of 12C16O, 13C16O, 28Si16O, and 28Si32S
Table A.1: Identified lines of 12C16O, 13C16O, 28Si16O, and 28Si32S in the PACS and SPIRE spectrum of IRC 10216.
Footnotes
- ... IRC +10216
- Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.
- ...
- Appendix is only available in electronic form at http://www.aanda.org
All Tables
Table 1: Parameters for the best-fit model, where numbers in italics indicate input parameters that have been kept fixed at the given value.
Table A.1: Identified lines of 12C16O, 13C16O, 28Si16O, and 28Si32S in the PACS and SPIRE spectrum of IRC 10216.
All Figures
![]() |
Figure 1: Continuum-subtracted PACS
and SPIRE spectrum of IRC +10216. In the three upper panels, the
PACS spectrum of IRC +10216 (black) is compared to the ISO-LWS
spectrum (grey, Cernicharo et al. 1996).
The fourth and fifth panels show the SPIRE spectrum of IRC +10216
(black). The bottom panel zooms in on the 141-146.8 |
Open with DEXTER | |
In the text |
![]() |
Figure 2: Thermodynamical structure in the envelope of IRC +10216 as derived from the 12CO rotational line transitions. The vertical dotted line represents the dust condensation radius. |
Open with DEXTER | |
In the text |
![]() |
Figure 3:
Comparison between few PACS and SPIRE SiO v=0
lines (black) and theoretical line predictions (grey). Full grey lines
represent theoretical line profiles using a constant SiO fractional
abundance of [SiO/H2] =
|
Open with DEXTER | |
In the text |
Copyright ESO 2010
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.