Title: The Herschel - PACS guaranteed time key program on evolved stars: its preparation, execution and initial data exploitation.
Other Titles: Het Herschel-PACS gegarandeerde tijdprogramma rond geëvolueerde sterren: voorbereiding, uitvoering en initiële resultaten.
Authors: Ladjal, Djazia; S0176578
Issue Date: 4-Apr-2011
Abstract: The Herschel Space Observatory (HSO) is the fourth cornerstone in ESA's Horizon 2000 programme. It was successfuly launched in May 2009. The HSO has three instruments on board : HIFI, SPIRE and PACS. Belgium is involved in the HSO through a partnership at the co-PI level (lead by Prof. Waelkens from the Institute of Astronomy) for a 20% share in the Photodetector Array Camera and Spectrometer (PACS). This instrument is designed for imaging through 3 filters at 70, 110 and 170 micron using a bolometer array, and medium resolution spectroscopy using an imaging spectrometer, between 60 and 200 micrometer. The Belgian contribution to PACS is at a level of 20%. This implies that Belgium has GT Observations at a level of approximately 400 hours. The majority of time will be invested into a GT Key Program "The circumstellar environment in post-main-sequence objects". Currently there are agreed contributions by Belgium (approximately 140 hours), Austria (approximately 50 h), and a mission scientist (approximately 25 h). It is clear that Belgium will take the lead in developing this GT KP and this will allow a high visibility in the scientific return of the HSO. The main aims of this GT KP are twofold, namely (1) to study the structure of the circumstellar envelope and time evolution of the mass loss rate, and (2) to study molecular and solid state features in the spectra of a representative sample of both low- and intermediate- post-main sequence objects (i.e. Asymptotic Giant Branch (AGB), post-AGB, Planetary Nebulae(PN) and high mass (i.e. Red Supe Giants(RSG), Wolf-Rayet (WR), Luminous Blue Variables(LBV)). Mass-loss is the dominating factor in the post-main sequence evolution of almost all stars. For low- and intermediate mass stars (initial mass less than 8 solar masses) this takes place mainly on the thermallypulsing AGB (asymptotic giant branch) in a slow (typically 5-25 km/s) dust driven wind with large mass loss rates (up to 10&#104857 6;4 solar masses per year), which is also the driving mechanism for the slightly more massive stars in the Red Supergiants (RSG) phase, while for massive stars (initial mass more than 15 solar masses) the mass loss takes place in a fast (hundreds to a few 1000 km/s) wind driven by radiation pressure on lines at a moderate rate of typically a few 10􀀀6 solar masses per year. Although mass loss is such an important process and has been studied since the late 1960's with the advent of infrared astronomy many basic questions remain unanswered even in the post-ISO era : what is the time evolution of the mass loss rate, what is the geometry of this process and how does this in uence the shaping of the nebulae seen around the central stars of PN and LBVs, what kind of dust species are formed at exactly what location in the wind. With its high spatial resolution, large Field-of-View (FoV) and sensitivity, PACS will be able to make important contributions in th is field. With a typical AGB lifetime of 106 year and a typical expansion velocity of 10 km/s this implies that the effects of the mass loss process could, in principle, be traced over 3. 1014 km, or 10 pc, or about 30 arcmin at 1 kpc distance. Direct observational evidence for a change in mass loss over time has been very limited, mainly to the nearby and luminous star CW Leo. Roughly spherical shells can be identified in optical images with separations of 5-20 arcsec corresponding to timescales of 200-800 years. Another, much longer, timescale for the variations in the mass loss rate is expected from the thermal pulses which occur, depending on the core mass, typically on a time scale of 104 years. This is the interpretation of the interferometric observations of a detached shell around the star TT Cyg (Olofsson et al., 2000, A&A 353, 583) in the CO(1-0) line. Direct observational evidence from infra-red dats for a change in mass loss has been poor. Previous observations with IRAS have only revealed that some shells were extended relative to the arcminute size beam (Young et al. 1993, ApJS 86, 517), but even the better spatial resolution of ISOPHOT at 45 arcseconds was insuficient to really resolve the CSE (Izumiura 1996, A&A 315, L221). Clearly what is lacking at the moment are sensitive observations in the dust continuum over a larger FoV at a spatial resolution of a few arcsec. This would make it possible to trace details of the mass loss history over a large timescale. PACS (and SPIRE) are ideal instruments to perform this. Regarding spectroscopy, The spectra of both AGB stars and PNe are ideal tracers of the solid-state and molecular species that form in the cool CSE. Most of the astronomical solid state features are found in the NIR and MIR ranges. ISO, especially with its ShortWavelength Spectrometer (SWS, 2-45 micron) and LongWavelength Spectrometer (LWS, 40-200 micron) revolutionised our knowledge of dust and ice around stars. In the LWS range, overlapping with Herschel PACS, most of ISOs spectroscopic dust observations were really suffering from S/N-problems in all but the brightest AGB stars. The sensitivity of the HSO will allow to make real progress here, although the short wavelength end of PACS (about 60 micron) is a limitation. Nevertheless dust-species like Forsterite (Mg2SiO4) at 69 micron (sharp, dust thermometer, see Molster et al., 2002, A&A 382, 241), Calcite CaCO3 at 92.6 micron, Crystalline water-ice at 61 micron, CaAl12O19 feature at 78 micron are expected or have already been detected by ISO. Other measured features lack an identification e.g. the 62---63 micron with candidate substances like Dolomite (CaMg(CO3)2), Ankerite, or Diopside. Additionally, in PNe, fine structure lines in the PACS domain include : [NII] 121 micron and 205 micron [O III] 88 micron, [O I] 146 micron, and [CII] 157 micron. In addition there are many high-transition rotational lines of 12CO, 13CO, HCN observable in the PACS domain. In summary, the much improved capabilities in terms of FoV, spatial resolution and sensitivity of PACS compared to previous (far-)infrared missions will allow for a much improved understanding of the scientific questions we want to address in this Belgian lead GT KP, namely the time evolution of mass loss, and the spatial distribution of the circumstellar dust shell, and its chemical composition.
ISBN: 978-90-8649-413-2
Publication status: published
KU Leuven publication type: TH
Appears in Collections:Institute of Astronomy

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