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Register a new userMid Infrared Photometry of Mass-Losing AGB Stars
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We present ground-based mid-infrared imaging for 27 M-, S- and C-type Asymptotic Giant Branch (AGB) stars. The data are compared with those of the database available thanks to the IRAS, ISO, MSX and 2MASS catalogues. Our goal is to establish relations between the IR colors, the effective temperature $T_{eff}$, the luminosity $L$ and the mass loss rate $dot M$, for improving the effectiveness of AGB modelling. Bolometric (absolute) magnitudes are obtained through distance compilations, and by applying previously-derived bolometric corrections; the variability is also studied, using data accumulated since the IRAS epoch. The main results are: i) Values of $L$ and $dot M$ for C stars fit relations previously established by us, with Miras being on average more evolved and mass losing than Semiregulars. ii) Moderate IR excesses (as compared to evolutionary tracks) are found for S and M stars in our sample: they are confirmed to originate from the dusty circumstellar environment. iii) A larger reddening characterizes C-rich Miras and post-AGBs. In this case, part of the excess is due to AGB models overestimating $T_{eff}$ for C-stars, as a consequence of the lack of suitable molecular opacities. This has a large effect on the colors of C-rich sources and sometimes disentangling the photospheric and circumstellar contributions is difficult; better model atmospheres should be used in stellar evolutionary codes for C stars. iv) The presence of a long-term variability at mid-IR wavelengths seems to be limited to sources with maximum emission in the 8 -- 20 $mu$m region, usually Mira variables (1/3 of our sample). Most Semiregular and post-AGB stars studied here remained remarkably constant in mid-IR over the last twenty years.
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Context. The asymptotic giant branch (AGB) phase marks the end of the evolution for low- and intermediate-mass stars, which are fundamental contributors to the mass return to the interstellar medium and to the chemical evolution of galaxies. The detailed understanding of mass loss processes is hampered by the poor knowledge of the luminosities and distances of AGB stars. Aims. In a series of papers we are trying to establish criteria permitting a more quantitative determination of luminosities for the various types of AGB stars, using the infrared (IR) fluxes as a basis. An updated compilation of the mass loss rates is also required, as it is crucial in our studies of the evolutionary properties of these stars. In this paper we concentrate our analysis on the study of the mass loss rates for a sample of galactic S stars. Methods. We reanalyze the properties of the stellar winds for a sample of galactic MS, S, SC stars with reliable estimates of the distance on the basis of criteria previously determined. We then compare the resulting mass loss rates with those previously obtained for a sample of C-rich AGB stars. Results. Stellar winds in S stars are on average less efficient than those of C-rich AGB stars of the same luminosity. Near-to-mid infrared colors appear to be crucial in our analysis. They show a good correlation with mass loss rates in particular for the Mira stars. We suggest that the relations between the rates of the stellar winds and both the near-to-mid infrared colors and the periods of variability improve the understanding of the late evolutionary stages of low mass stars and could be the origin of the relation between the rates of the stellar winds and the bolometric magnitudes.
AGB phases mark the end of the evolution for Low- and Intermediate-Mass Stars. Our understanding of the mechanisms through which they eject the envelope and our assessment of their contribution to the chemical evolution of Galaxies are hampered by poor knowledge of their Luminosities and mass loss rates,both for C-rich and for O-rich sources.We plan to establish criteria permitting a more quantitative determination of luminosities for the various types of AGB stars on the basis of IR fluxes.In this paper we concentrate on O-rich and s-element-rich MS, S stars and include a small sample of SC stars.We reanalyze the absolute bolometric magnitudes and colors of MS, S, SC stars on the basis of a sample of intrinsic and extrinsic long period variables.We derive bolometric corrections as a function of near- and mid-IR colors,adopting as references a group of stars for which the SED could be reconstructed in detail over a large wavelength range.We determine the absolute HR diagrams and compare luminosities and colors of S-type giants with those of C-rich AGB stars. Luminosity estimates are also verified on the basis of existing Period-Luminosity relations valid for O-rich Miras.S star bolometric luminosities are almost indistinguishable from those of C-rich AGB stars.Their circumstellar envelopes are thinner and less opaque.Despite this last property the IR wavelengths remain dominant, with the bluest stars having their maximum emission in the H or K bands.Based on Period-Luminosity relations for O-rich Miras and on Magnitude-color relations for the same variables we show how approximate distances for sources of so far unknown parallax can be inferred. We argue that most of the sources have a rather small mass(<2Msun);dredge-up might then be not effective enough to let the C/O ratio exceed unity.
Post-AGB stars are key objects for the study of the dramatic morphological changes of low- to intermediate-mass stars on their evolution from the Asymptotic Giant Branch (AGB) towards the Planetary Nebula stage. There is growing evidences that binary interaction processes may very well have a determining role in the shaping process of many objects, but so far direct evidence is still weak. We aim at a systematic study of the dust distribution around a large sample of Post-AGB stars as a probe of the symmetry breaking in the nebulae around these systems. We used imaging in the mid-infrared to study the inner part of these evolved stars to probe direct emission from dusty structures in the core of Post-AGB stars in order to better understand their shaping mechanisms. We imaged a sample of 93 evolved stars and nebulae in the mid-infrared using VISIR/VLT, T-Recs/Gemini South and Michelle/Gemini North. We found that all the the Proto-Planetary Nebulae we resolved show a clear departure from spherical symmetry. 59 out of the 93 observed targets appear to be non resolved. The resolved targets can be divided in two categories. The nebulae with a dense central core, that are either bipolar and multipolar. The nebulae with no central core have an elliptical morphology.The dense central torus observed likely host binary systems which triggered fast outflows that shaped the nebulae.
To study the nature of Bulge AGB stars and in particular their circumstellar dust, we have analysed mid-infrared spectra obtained with the ISOCAM CVF spectrometer in three Bulge fields. The ISOCAM 5-16.5 micron CVF spectra were obtained as part of the ISOGAL infrared survey of the inner Galaxy. A classification of the shape of the 10 micron dust feature was made for each case. The spectra of the individual sources were modelled using a radiative transfer model. Different combinations of amorphous silicates and aluminium-oxide dust were used in the modelling. Spectra were obtained for 29 sources of which 26 are likely to be Bulge AGB stars. Our modelling shows that the stars suffer mass loss rates in the range of 10^{-8} - 5 x 10^{-7} Msun / yr, which is at the low end of the mass-loss rates experienced on the Thermally Pulsing AGB. The luminosities range from 1,700 to 7,700 Lsun as expected for a population of AGB stars with Minit of 1.5 - 2Msun. In agreement with the condensation sequence scenario, we find that the dust content is dominated by Al_2O_3 grains in this sample of low mass-loss rate stars.
We have carried out an infrared search for obscured AGB stars in the Magellanic Clouds. The survey uncovered a number of obscured AGB stars as well as some supergiants with infrared excess. We present photometry of the sources and discuss the colour diagrams and bolometric luminosities. Most of the AGB stars are luminous, often close to the classical limit of $M_{rm bol}=-7.1$. To determine whether the stars are oxygen-rich or carbon-rich, we have acquired narrow-band mid-infrared photometry with the ESO TIMMI camera for several sources. All but one are found to show the silicate feature and therefore to have oxygen-rich dust: the colours of the remaining source are consistent with either an oxygen-rich or a carbon-rich nature. A method to distinguish carbon and oxygen stars based on H$-$K versus K$-$[12] colours is presented. We discuss several methods of calculating the mass-loss rate: for the AGB stars the mass-loss rates vary between approximately 5 times 10**-4 and 5 times 10**-6 solar masses per year, depending on assumed dust-to-gas mass ratio. We present a new way to calculate mass-loss rates from the OH-maser emission. We find no evidence for a correlation of the mass-loss rates with luminosity in these obscured stars. Neither do the mass-loss rates for the LMC and SMC stars differ in any clear systematic way from each other. Expansion velocities appear to be slightly lower in the LMC than in the Galaxy. Period determinations are discussed for two sources: the periods are comparable to those of the longer-period galactic OH/IR stars. All of the luminous stars for which periods are available, have significantly higher luminosities than predicted from the period--luminosity relations.
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