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

The low wind expansion velocity of metal-poor carbon stars in the Halo and the Sagittarius stream

We report the detection, from observations using the James Clerk Maxwell Telescope, of CO J $=$ 3$\to$ 2 transition lines in six carbon stars, selected as members of the Galactic Halo and having similar infrared colors. Just one Halo star had been detected in CO before this work. Infrared observations show that these stars are red (J-K $>$3), due to the presence of large dusty circumstellar envelopes. Radiative transfer models indicates that these stars are losing mass with rather large dust mass-loss rates in the range 1--3.3 $\times$$10^{-8}$M$_{\odot}$yr$^{-1}$, similar to what can be observed in the Galactic disc. We show that two of these stars are effectively in the Halo, one is likely linked to the stream of the Sagittarius Dwarf Spheroidal galaxy (Sgr dSph), and the other three stars certainly belong to the thick disc. The wind expansion velocities of the observed stars are low compared to carbon stars in the thin disc and are lower for the stars in the Halo and the Sgr dSph stream than in the thick disc. We discuss the possibility that the low expansion velocities result from the low metallicity of the Halo carbon stars. This implies that metal-poor carbon stars lose mass at a rate similar to metal-rich carbon stars, but with lower expansion velocities, as predicted by recent theoretical models. This result implies that the current estimates of mass-loss rates from carbon stars in Local Group galaxies will have to be reconsidered.Comment: 10 pages, 7 figures, accepted for publication in MNRA

NaI/KI scattering observations in circumstellar envelopes: constraints on ionization and mass-loss rates

International audienceWe investigate KI or NaI fluorescent emission in circumstellar shells to study their ionization and mass-loss rates. First-time KI detections around the mira-type stars W Hya and R Hya are presented. Together with the previously analysed cases of α Ori, α Her and o Cet (Mauron and Caux 1992A&A...265..711M) and other observations, this gives an extended sample of 10 envelopes which includes μ Cep, CE Tau, β Peg, ρ Per and g Her. A few non-detections are also considered. In order to compare observed with expected values of KI intensities, the ionization model of Glassgold and Huggins (1986ApJ...306..605G) is used, and the relevant parameters such as distance d, mass-loss rate ˙(M), gas temperature, fractional electron abundance x_e_ and stellar photoionizing rates Goplus _have to be known. Using data found in the literature, we estimate these parameters in detail for each case. The values of x_e_ are obtained with a hypothesized simple rule based on the circumstellar abundance of molecules and grains which could lock electron donors, giving either x_e_~3x10^-4^ or ~2x10^-5^. The Goplus _values depend on the rare ultraviolet spectrophotometric data available for red giants. The kinetic temperature was assumed to be 30K at the probed impact parameters r_obs_~0.5 to 5x10^16^cm. Despite real uncertainties, the predicted KI intensities with our best estimates of the parameters are in very reasonable agreement with observations in 8 cases in 10. The largest discrepancy concerns the red supergiant μ Cep (M2Ia): our KI data suggest that, similar to α Ori, CO and dust are incompletely formed; for this object we favor ˙(M)=5x10^-6^Msun_/yr. The wind of β Peg is the second case for which a larger mass-loss and/or a larger x_e_ than primarily believed is suggested, but confirming observations are needed. Finally there is no indication that K or Na might be depleted in silicate grains

NaI/KI scattering observations in circumstellar envelopes: constraints on ionization and mass-loss rates

International audienceWe investigate KI or NaI fluorescent emission in circumstellar shells to study their ionization and mass-loss rates. First-time KI detections around the mira-type stars W Hya and R Hya are presented. Together with the previously analysed cases of α Ori, α Her and o Cet (Mauron and Caux 1992A&A...265..711M) and other observations, this gives an extended sample of 10 envelopes which includes μ Cep, CE Tau, β Peg, ρ Per and g Her. A few non-detections are also considered. In order to compare observed with expected values of KI intensities, the ionization model of Glassgold and Huggins (1986ApJ...306..605G) is used, and the relevant parameters such as distance d, mass-loss rate ˙(M), gas temperature, fractional electron abundance x_e_ and stellar photoionizing rates Goplus _have to be known. Using data found in the literature, we estimate these parameters in detail for each case. The values of x_e_ are obtained with a hypothesized simple rule based on the circumstellar abundance of molecules and grains which could lock electron donors, giving either x_e_~3x10^-4^ or ~2x10^-5^. The Goplus _values depend on the rare ultraviolet spectrophotometric data available for red giants. The kinetic temperature was assumed to be 30K at the probed impact parameters r_obs_~0.5 to 5x10^16^cm. Despite real uncertainties, the predicted KI intensities with our best estimates of the parameters are in very reasonable agreement with observations in 8 cases in 10. The largest discrepancy concerns the red supergiant μ Cep (M2Ia): our KI data suggest that, similar to α Ori, CO and dust are incompletely formed; for this object we favor ˙(M)=5x10^-6^Msun_/yr. The wind of β Peg is the second case for which a larger mass-loss and/or a larger x_e_ than primarily believed is suggested, but confirming observations are needed. Finally there is no indication that K or Na might be depleted in silicate grains

The polarized dust shell around alpha Orionis. II The silicon fractional condensation, mass-loss rate and grain radius

International audienc

The polarized dust shell around alpha Orionis. II The silicon fractional condensation, mass-loss rate and grain radius

International audienc

New asymptotic giant branch carbon stars in the galactic halo

SPRINGER has CopyrightInternational audienceFor the first time the data on the eight confirmed or candidate carbon (C) stars found mainly fromobjective-prism plates are presented. By using the Catalina database of lightcurves, we find that allthese stars are pulsating, allowing a distance to be estimated through the K-band Period-Luminosity(PL) relation. This relation does not depend on spectral type (M or C) and distances are reliable even forC candidates. Seven stars are more than 10 kpc from the galactic plane, suggesting they do not belong tothe galactic disk. We also find one star located at about 180 kpc from the Sun, being one of the mostdistant star in the Galaxy. Many of these new C stars are relatively blue. Some comments are alsoprovided on seven other known halo carbon stars for which either a pulsation period is obtained, orbecause they were not included in previous works on halo C stars

A catalogue of oxygen-rich pulsating giants in the galactic halo and the Sagittarius stream

10 pages, 12 figs, 3 Tables. Accepted for publication by AAInternational audienceTo construct a catalogue of oxygen-rich (M) asymptotic giant branch (AGB) stars in the halo, complementing the catalogues of carbon-rich (C) stars, previous lists of Miras and SRa semi-regulars located in the northern hemisphere are merged and cleaned of various defects. After putting aside known C stars, characteristics such as colours and periods indicate that most of the remaining objects are M stars. Distances are obtained through the period-luminosity relation. By considering their position in the sky, stars lying at |Z| > 5 kpc are confirmed to be in majority in the Sgr tidal arms. The M stars are more numerous than C ones. Our distance scale is supported by two cool variables located in the Pal 4 globular cluster. Along the Sgr arms, there is reasonable agreement on distances of our objects with recent RR Lyrae distances. A few stars may be as distant as 150 kpc, with possibly four at the trailing arm apocentre, and two in the A16 sub-structure, angularly close to two C stars. Ninety radial velocities are collected from Gaia and other sources. A catalogue with 417 M pulsating AGB stars is provided. This catalogue contains ∼260 stars in the halo with |Z| > 5 kpc. Their Ks magnitudes range from 8 up to 13. For comparison, the catalogue also provides ∼150 stars in the disc having 5 <  Ks <  8