Imaging the circumstellar envelopes of AGB stars

We report the results of an exploratory program to image the extended circumstellar envelopes of asymptotic giant branch (AGB) stars in dust-scattered galactic light. The goal is to characterize the morphology of the envelopes as a probe of the mass-loss process. The observations consist of short exposures with the VLT and longer exposures with 1-2m telescopes, augmented with archival images from the Hubble Space Telescope. We observed 12 AGB stars and detected the circumstellar envelopes in 7. The detected envelopes have mass loss rates more than about 5 10E-6 solar mass per year, and they can be seen out to distances of about 1 kpc. The observations provide information on the mass loss history on time scales up to about 10,000 years. For the five AGB envelopes in which the circumstellar geometry is well determined by scattered light observations, all except one (OH348.2-19.7) show deviations from spherical symmetry. Two (IRC+10216 and IRC+10011) show roughly spherical envelopes at large radii but asymmetry or bipolarity close to the star; one (AFGL 2514) shows an extended, elliptical envelope, and one (AFGL 3068) shows a spiral pattern. The non-spherical structures are all consistent with the effects of binary interactions. Our observations are in accord with a scenario in which binary companions play a role in shaping planetary nebulae, and show that the circumstellar gas is already partly shaped on the AGB, before evolution to the proto-planetary nebula phase.Comment: Accepted by AA 21 Feb 2006; 18 pages, 14 figs; for high resolution images, contact mauron at graal.univ-montp2.f

New observations of cool carbon stars in the halo

We report new results of our search for rare, cool carbon stars located at large distances from the galactic plane. Eighteen new C stars were discovered. Six are remarkable by showing the two peculiarities of a strong infrared excess at 12 microns and a large height above the Galactic plane, from 1.7 to 6 kpc. The number of C stars with these properties has been increased to 16. Mass-loss rates were tentatively estimated by assuming that all these 16 stars are Miras and by using the correlation between Mdot and the K-12 colour index. It is found that several stars have large mass loss, with median Mdot around 4E-06 solar mass per year. It would be desirable to detect their CO emission ... Eight stars might be at more than 30 kpc from the Sun, and two at the unprecedented distance of 150 kpc (this abstract has been abridged).Comment: 15 pages; accepted by Astronomy and Astrophysic

On the Formation of Multiple-Shells Around Asymptotic Giant Branch Stars

Two types of models for the formation of semi-periodic concentric multiple shells (M-shells) around asymptotic giant branch (AGB) stars and in planetary nebulae are compared against observations. Models that attribute the M-shells to processes in an extended wind acceleration zone around AGB stars result in an optically thick acceleration zone, which reduces the acceleration efficiency in outer parts of the extended acceleration zone. This makes such models an unlikely explanation for the formation of M-shells. Models which attribute the M-shell to semi-periodic variation in one or more stellar properties are most compatible with observations. The only stellar variation models on time scales of 50-1500 years that have been suggested are based on an assumed solar-like magnetic cycle. Although ad-hoc, the magnetic cycle assumption fits naturally into the increasingly popular view that magnetic activity plays a role in shaping the wind from upper AGB stars.Comment: 8 pages, Submitted to Ap

Deep optical imaging of AGB circumstellar envelopes

We report results of a program to image the extended circumstellar envelopes of asymptotic giant branch (AGB) stars in dust-scattered Galactic light. The goal is to characterize the shapes of the envelopes to probe the mass-loss geometry and the presence of hidden binary companions. The observations consist of deep optical imaging of 22 AGB stars with high mass loss rates: 16 with the ESO 3.5 m NTT telescope, and the remainder with other telescopes. The circumstellar envelopes are detected in 15 objects, with mass loss rates > 2E-6 Msun/year. The surface brightness of the envelopes shows a strong decrease with Galactic radius, which indicates a steep radial gradient in the interstellar radiation field. The envelopes range from circular to elliptical in shape, and we characterize them by the ellipticity (E = major/minor axis) of iso-intensity contours. We find that about 50 percent of the envelopes are close to circular with E 1.2. We interpret the shapes in terms of populations of single stars and binaries whose envelopes are flattened by a companion. The distribution of E is qualitatively consistent with expectations based on population synthesis models of binary AGB stars. We also find that about 50 percent of the sample exhibit small-scale, elongated features in the central regions. We interpret these as the escape of light from the central star through polar holes, which are also likely produced by companions. Our observations of envelope flattening and polar holes point to a hidden population of companions within the circumstellar envelopes of AGB stars. These companions are expected to play an important role in the transition to post-AGB stars and the formation of planetary nebulae.Comment: 19 pages, 13 figures, color pictures in Appendix, accepted by A&

Cool carbon stars in the halo: new very red or distant objects

The goal of this paper is to present and analyse a new sample of cool carbon (C)stars located in the halo. Twenty three new C stars were discovered. Spectra are typical of N-type stars with C2 and CN bands and sometimes Halpha in emission. ... Four objects are particularly red with J-K > 3, with 2 located at more than 5 kpc. from the Galactic plane. Eight additional objects with similar properties are found in the literature and our previous works. These 12 C stars could be useful to study mass loss at low metallicity. Two objects are at distances of 95 and 110 kpc. They are located in the region with galactocentric Z < -60 kpc in which the model of Law et al. predicts the Sgr stream to have a loop. (Abstact abridged)Comment: 16 pages, 12 figures, accepted by A

Gas phase atomic metals in the circumstellar envelope of IRC+10216

We report the results of a search for gas phase atomic metals in the circumstellar envelope of the AGB carbon star IRC+10216. The search was made using high resolution (R=50000) optical absorption spectroscopy of a backgound star that probes the envelope on a line of sight 35" from the center. The metal species that we detect in the envelope include NaI, KI, CaI, CaII, CrI, and FeI, with upper limits for AlI, MnI, TiI, TiII, and SrII. The observations are used to determine the metal abundances in the gas phase and the condensation onto grains. The metal depletions range from a factor of 5 for Na to 300 for Ca, with some similarity to the depletion pattern in interstellar clouds. Our results directly constrain the condensation efficiency of metals in a carbon-rich circumstellar envelope and the mix of solid and gas phase metals returned by the star to the ISM. The abundances of the uncondensed metal atoms that we observe are typically larger than the abundances of the metal-bearing molecules detected in the envelope. The metal atoms are therefore the major metal species in the gas phase and likely play a key role in the metal chemistry.Comment: 11 pages, 8 Figures. Accepted by Astronomy and Astrophysic

Les circonstances de l'infection par le VIH

Aujourd'hui encore, en Suisse, nous pouvons recenser chaque année pas moins de 600 nouveaux diagnostics de VIH (virus de l'immunodéficience humaine) [1]. Selon les définitions de l'ONUSIDA, l'épidémie du VIH en Suisse est dite concentrée [2] ; c'est-à-dire que la prévalence de l'infection au sein de la population générale est faible (0.4%), alors qu'elle touche plus fortement les groupes cibles, que sont les hommes ayant des rapports sexuels avec d'autres hommes (HSH), les consommateurs de drogues par voie intraveineuse (IDU) et les migrants en provenance de pays à haute prévalence (PHP). Il est donc primordial que l'épidémie du VIH soit surveillée et évaluée. La recherche sur le VIH ainsi que l'évaluation de son épidémie sont menées par plusieurs organismes, dont l'Etude suisse de cohorte VIH (SHCS), qui collecte avant tout des données cliniques sur le sujet. Une des tâches de cette surveillance est de pouvoir définir les groupes à risque accru d'exposition au VIH, afin qu'ils puissent bénéficier des mesures de prévention et de dépistage adéquates permettant de ralentir la propagation dudit virus [1]. En 2007, le « Clinics and Laboratories Committee » de la SHCS décide d'intégrer de nouvelles questions ciblant les circonstances de l'infection par le VIH aux formulaires adressés aux patients nouvellement inclus dans l'étude (cf annexe 1). Quatre questions sont alors sélectionnées pour explorer ce sujet : - Selon le médecin, quelle est la source probablement responsable de l'infection ? - Est-ce que le patient connaît une ou plusieurs sources potentielles de son infection ? - Est-ce que le patient connaît la période durant laquelle il a contracté le virus ? - Selon lui, où l'infection a-t-elle vraisemblablement eu lieu

The Shapes of AGB Envelopes as Probes of Binary Companions

We describe how the large scale geometry of the circumstellar envelopes of asymptotic giant branch stars can be used to probe the presence of unseen stellar companions. A nearby companion modifies the mass loss by gravitationally focusing the wind towards the orbital plane, and thereby determines the shape of the envelope at large distances from the star. Using available simulations, we develop a prescription for the observed shapes of envelopes in terms of the binary parameters, envelope orientation, and type of observation. The prescription provides a tool for the analysis of envelope images at optical, infrared, and millimetre wavelengths, which can be used to constrain the presence of companions in well observed cases. We illustrate this approach by examining the possible role of binary companions in triggering the onset of axi-symmetry in planetary nebula formation. If interaction with the primary leads to axi-symmetry, the spherical halos widely seen around newly formed nebulae set limits on the companion mass. Only low mass objects may orbit close to the primary without observable shaping effects: they remain invisible until the interaction causes a sudden change in the mass loss geometry.Comment: 11 pages, 7 figures, to appear in MNRA

The envelope of IRC+10216 reflecting the galactic light: UBV surface brightness photometry and interpretation

We present and analyse new optical images of the dust envelope surrounding the high mass-loss carbon star IRC+10216. This envelope is seen due to external illumination by galactic light. Intensity profiles and colors of the nebula were obtained in the UBV bandpasses. The data are compared with the results of a radiative transfer model calculating multiple scattering of interstellar field photons by dust grains with a single radius. The data show that the observed radial shape of the nebula, especially its half maximum radius, does not depend on wavelength (within experimental errors), suggesting that grains scatter in the grey regime, etc, etc (this abstract has been shortened)Comment: accepted by A

Cool carbon stars in the halo and in dwarf galaxies: Halpha, colours, and variabiity

The population of cool carbon (C) stars located far from the galactic plane is probably made of debris of small galaxies such as the Sagittarius dwarf spheroidal galaxy (Sgr), which are disrupted by the gravitational field of the Galaxy. We aim to know this population better through spectroscopy, 2MASS photometric colours, and variability data. When possible, we compared the halo results to C star populations in the Fornax dwarf spheroidal galaxy, Sgr, and the solar neighbourhood. We first present a few new discoveries of C stars in the halo and in Fornax. The number of spectra of halo C stars is now 125. Forty percent show Halpha in emission. The narrow location in the JHK diagram of the halo C stars is found to differ from that of similar C stars in the above galaxies. The light curves of the Catalina and LINEAR variability databases were exploited to derive the pulsation periods of 66 halo C stars. A few supplementary periods were obtained with the TAROT telescopes. We confirm that the period distribution of the halo strongly resembles that of Fornax, and we found that is it very different from the C stars in the solar neighbourhood. There is a larger proportion of short period Mira/SRa variables in the halo than in Sgr, but the survey for C stars in this dwarf galaxy is not complete, and the study of their variability needs to be continued to investigate the link between Sgr and the cool halo C stars.Comment: 11 pages, 10 figures + one appendix of 26 pages; accepted for publication in Astronomy and Astrophysic