Nicolas Mauron

The mass-loss rates of red supergiants and the de Jager prescription

Mass loss of red supergiants (RSG) is important for the evolution of massive stars, but is not fully explained. Several empirical prescriptions have been proposed, trying to express the mass-loss rate ( u M) as a function of fundamental stellar parameters (mass, luminosity, effective temperature). Our goal is to test whether the de Jager et al. (1988, A&AS, 72, 259) prescription, which is used in some stellar evolution models, is still valid in view of more recent mass-loss determinations. By considering 40 Galactic RSGs with an infrared excess and an IRAS 60-μm flux larger than 2 Jy, and assuming a gas-to-dust mass ratio of 200, we find that the de Jager rate agrees within a factor 4 with most u M estimates based on the 60-μm signal. It also agrees with six of the only eight Galactic RSGs for which u M can be measured more directly through observations of the circumstellar gas. The two objects that do not follow the de Jager prescription (by an order of magnitude) are μ Cep and NML Cyg. We also considered the RSGs of the Magellanic Clouds. Thanks to the results of previous research, we find that the RSGs of the Small Magellanic Cloud have mass-loss rates consistent with the de Jager rate scaled by (Z/Z� ) α ,w hereZ is the metallicity and α is 0.7. The situation is less clear for the RSGs of the Large Magellanic Cloud. In particular, for L > 1.6 × 10 5 L� , one finds numerous RSGs (except WOH-G64) with u M significantly smaller than the de Jager rate and indicating that u M would no longer increase with L. Before this odd situation is confirmed through further analyses of LMC RSGs, we suggest to keep the de Jager prescription unchanged at solar metallicity in the stellar evolutionary models and to apply a( Z/Z� ) 0.7 dependence.

New observations of cool carbon stars in the halo

Aims. We report new results of our search for rare, cool carbon stars located at large distances from the Galactic plane. Methods. Candidate stars were selected in the 2MASS point source catalogue with JHKs colours typical of N-type carbon stars, with Ks > 6.0 and with Galactic latitude |b| > 20 ◦ . Low resolution slit spectroscopy was carried out on 58 candidates. Results. Eighteen new carbon stars were discovered. Six are remarkable by showing the two peculiarities of a strong infrared excess at 12 µm 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 u M and the K − [12] colour index. It is found that several stars have large mass loss, with a median u

The shapes of asymptotic giant branch 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 axisymmetry in planetary nebula formation. If interaction with the primary leads to axisymmetry, the spherical haloes 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.

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 asymptotic giant branch carbon star IRC+10216. The search was made using high resolution (λ/∆λ = 50 000) optical absorption spectroscopy of a background star that probes the envelope on a line of sight 35 from the center. The metal species that we detect in the envelope include Na i, K i, Ca i, Ca ii, Cr i, and Fe i, with upper limits for Al i, Mn i, Ti i, Ti ii, and Sr ii. The observations are used to determine the metal abundances in the gas phase and the condensation onto grains. The metal depletions in the envelope 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 interstellar medium. 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.We report the results of a search for gas phase atomic metals in the circumstellar envelope of the asymptotic giant branch carbon star IRC+10216. The search was made using high resolution (λ/Δλ = 50 000) optical absorption spectroscopy of a background star that probes the envelope on a line of sight 35 �� from the center. The metal species that we detect in the envelope include Na i ,K i ,C ai, Ca ii ,C ri ,a nd Fei, with upper limits for Al i ,M ni ,T ii ,T iii ,a nd Srii. The observations are used to determine the metal abundances in the gas phase and the condensation onto grains. The metal depletions in the envelope 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 interstellar medium. 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.

VLT/UVES and WHT/UES absorption spectroscopy of the circumstellar envelope of IRC +10° 216 using background stars: First results and a search for DIBs

A unique and novel set of observations has been undertaken to probe the circumstellar envelope (CSE) of the nearby (130 pc) carbon star IRC +10 216 using optical absorption spectroscopy towards background stars lying beyond the envelope. The primary aim of the observations is to search for diuse band (DIB) carriers in the CSE, for which the mass-losing envelopes of carbon stars are a likely place of origin. Our principal target is a V = 16 G-type star located 37 00 from IRC +10 216 and was observed with VLT/UVES. A detailed model atmosphere and abundance analysis shows that it is somewhat metal-poor and has conrmed that it lies far beyond IRC +10 216. The circumstellar H+2H2 column density expected along the line of sight towards this target is relatively high, 2 10 21 cm 2 , and is large compared to that derived from the small interstellar extinction estimated in the zone of IRC +10 216 at b =+ 43, EB V < 0:03 mag. The CSE is certainly detected in the K i resonance lines, which are centred at the heliocentric velocity of IRC +10 216 and have FWHM 30 km s 1 , consistent with twice the terminal expansion velocity of the circumstellar gas. The data show also that circumstellar Na i is very probably detected, as seen towards two background stars. The strongest DIB (6284 A) present in the UVES wavelength coverage is detected but very probably arises in the foreground ISM. No DIB is detected at 6614 A, or elsewhere. Overall, the data suggest that the DIB carriers, if present in the CSE, have a low abundance relative to H in the C-rich envelope of IRC +10 216, in comparison with this ratio in the ISM.

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

Aims. The goal of this paper is to present and analyse a new sample of cool carbon (C) stars located in the Galactic halo. Methods. These rare objects are discovered by searching the 2MASS point-source catalogue for candidates having near-infrared colours typical of C stars. Optical spectroscopy is subsequently performed. Results. Twenty-three new C stars were discovered, with their Ks magnitude in the range 6 to 13.3. Spectra are typical of N-type carbon stars with C2 and CN bands and sometimes Hα in emission. One object is a S-type star. When the objects are bright enough (V ≤ 15.5), the data of the Northern Sky Variability Survey can be exploited. In all cases, stars belonging to this survey show light variations confirming that they are AGB stars. Distances and galactocentric XYZ coordinates have been estimated by assuming these stars to be similar in luminosity to those of the Sagittarius dwarf galaxy. Four objects are particularly red with J-Ks > 3, with two 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 other objects are remarkably far from the Sun, at distances of 95 and 110 kpc. They are located, together with two other C stars previously found, in the region Z < −60 kpc in which the model of Law et al. (2005) predicts the Sgr Stream to have a loop.

Small scale structure in circumstellar envelopes and the origin of globules in planetary nebulae

We analyze the small scale structure in the circumstellar envelopes of NGC 7027 and IRC+10216, using high resolution optical images in dust scattered light. We use the observations to test the proposal that globules in planetary nebulae, typified by globules in the Helix nebula, originate in high density contrast proto-globules in the atmosphere of the progenitor AGB star and are carried out in the circumstellar wind (Dyson et al. 1989). We find no evidence for the presence of proto- globules in the extended envelopes of NGC 7027 and IRC+10216 with the expected sizes and masses>10 5 M which are needed to produce globules like those in the Helix nebula. We do find azimuthal structure in the envelopes on size scales of l> 0:1d where d is the radial distance, consistent with the smoothing out of small scale structure by thermal motions in the wind acceleration region. Unless globules require special conditions not found in NGC 7027 or IRC+10216, which are among the most likely precursors of Helix-like nebulae, our results argue against their origin in the atmosphere of the central star. We suggest alternative scenarios for globule formation, including the fragmentation of the neutral circumstellar gas at the transition phase by directed outflows or jets.

HST IMAGES REVEAL DRAMATIC CHANGES IN THE CORE OF IRC+10216

IRC+10216 is the nearest carbon star with a very high mass-loss rate. The existence of a binary companion has been hinted by indirect observational evidence, such as the bipolar morphology of its nebula and a spiral-like pattern in its circumstellar material; however, to date, no companion has been identified. We have examined archival Hubble Space Telescope images of IRC+10216, and find that the images taken in 2011 exhibit dramatic changes in its innermost region from those taken at earlier epochs. The scattered light is more spread out in 2011. After proper motion correction, the brightest peak in 2011 is close to, but not coincident with, the dominant peak in previous epochs. A fainter point-like object was revealed at about 0.5 arcsec from this brightest peak. We suggest that these changes at the core of IRC+10216 are caused by dissipation of intervening circumstellar dust, as indicated by the brightening trend in the lightcurve extracted from the Catalina photometric survey. We tentatively identify the brightest peak in 2011 as the primary star of IRC+10216 and the fainter point-like source as a companion. The cause of non-detections of the companion candidate in earlier epochs is uncertain. These identifications need to be verified by monitoring of the core of IRC+10216 at high resolution in the future.

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.

Cool carbon stars in the halo and in dwarf galaxies: Hα, colours, and variability

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 Hα 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 it is 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.