Dust-enshrouded asymptotic giant branch stars in the solar neighbourhood (original) (raw)
Related papers
Infrared photometry and evolution of mass-losing AGB stars
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.
EVOLUTION OF ASYMPTOTIC GIANT BRANCH STARS
Annual Review of Astronomy and Astrophysics, 2005
We present a large set of theoretical isochrones, whose distinctive features mostly reside on the greatly improved treatment of the thermally pulsing asymptotic giant branch (TP-AGB) phase. Essentially, we have coupled the TP-AGB tracks described in Paper I, at their stages of pre-flash quiescent H-shell burning, with the evolutionary tracks for the previous evolutionary phases from . Theoretical isochrones for any intermediate value of age and metallicity are then derived by interpolation in the grids. We take care that the isochrones keep, to a good level of detail, the several peculiarities present in these TP-AGB tracks -e.g. the cool tails of C-type stars owing to the use of proper molecular opacities as convective dredge up occurs along the TP-AGB, the bell-shaped sequences in the Hertzsprung-Russell (HR) diagram for stars with hotbottom burning, the changes of pulsation mode between fundamental and first overtone, the sudden changes of mean mass-loss rates as the surface chemistry changes from M-to C-type, etc. Theoretical isochrones are then converted to about 20 different photometric systems -including traditional ground-based systems, and those of recent major wide-field surveys such as SDSS, OGLE, DENIS, 2MASS, UKIDSS, etc., -by means of synthetic photometry applied to an updated library of stellar spectra, suitably extended to include C-type stars. Finally, we correct the predicted photometry by the effect of circumstellar dust during the mass-losing stages of the AGB evolution, which allows us to improve the results for the optical-to-infrared systems, and to simulate mid-and far-IR systems such as those of Spitzer and AKARI. We illustrate the most striking properties of these isochrones by means of basic comparisons with observational data for the Milky Way disk and the Magellanic Clouds. Access to the data is provided both via a web repository of static tables (http://stev.oapd.inaf.it/dustyAGB07 and CDS), and via an interactive web interface (http://stev.oapd.inaf.it/cmd) that provides tables for any intermediate value of age and metallicity, for several photometric systems, and for different choices of dust properties.
Mid-Infrared Photometry of Mass-losing Asymptotic Giant Branch Stars
Astronomical Journal, 2007
We present ground-based mid-IR 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, Infrared Space Observatory, Midcourse Space Experiment, and Two Micron All Sky Survey catalogs. Our goal is to establish relations between the IR colors, the effective temperature T eff , the luminosity L, and the mass-loss rateṀ , for improving the effectiveness of AGB modeling. 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 as follows: (1) Values of L andṀ for C stars fit relations previously established by us, with Mira variables being on average more evolved and mass-losing than semiregular variables. (2) 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. (3) A larger reddening characterizes C-rich Mira variables and post-AGB stars. 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. (4) The presence of a long-term variability at mid-IR wavelengths seems to be limited to sources with maximum emission in the 8Y20 m region, usually Mira variables (one-third of our sample). Most of the semiregular and post-AGB stars studied here have remained remarkably constant in the mid-IR over the last 20 years.
Mass-losing AGB stars: infrared observations and evolutionary implications
Memorie Della Societa Astronomica Italiana, 2004
We have selected a wide database of mid-infrared observations, for AGB stars from both ground-based and space-borne observatories, with the aim of characterizing the efficient mass loss mechanisms that lead to the formation of their extended dusty envelopes. Our sample includes more than 400 sources in our Galaxy, distributed along the evolutionary sequence that gradually changes the spectral characteristics of M giants to MS, S and then C-stars. Thanks to a reanalysis of existing estimates of mass loss at radio frequencies and to improved measurements of distance (often provided by the Hipparcos mission), we compile a homogeneous set of corrected mass-loss rates and of near and mid-infrared colours. We show the existence of clear correlations. This suggests that mass loss can be inferred from photometric colours in mid-infrared, once these have been suitably calibrated. This provides a tool to predict the efficiency of stellar winds for other less known sources and is a decisive step in view of the determination of observationally based criteria for inclusion of mass loss in stellar models. In this paper we discuss in particular our sample of C-rich stars.
Mid Infrared Photometry of Mass-Losing AGB Stars
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 TeffT_{eff}Teff, the luminosity LLL and the mass loss rate dotM\dot MdotM, 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 LLL and dotM\dot MdotM 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...
Mass-loss rates and luminosity functions of dust-enshrouded AGB stars and red supergiants in the LMC
Arxiv preprint astro-ph/ …, 1999
A radiative transfer code is used to model the spectral energy distributions of 57 mass-losing Asymptotic Giant Branch (AGB) stars and red supergiants (RSGs) in the Large Magellanic Cloud (LMC) for which ISO spectroscopic and photometric data are available. As a result we derive mass-loss rates and bolometric luminosities. A gap in the luminosity distribution around M bol = −7.5 mag separates AGB stars from RSGs. The luminosity distributions of optically bright carbon stars, dust-enshrouded carbon stars and dust-enshrouded M-type stars have only little overlap, suggesting that the dust-enshrouded AGB stars are at the very tip of the AGB and will not evolve significantly in luminosity before mass loss ends their AGB evolution. Derived mass-loss rates span a range fromṀ ∼ 10 −7 to 10 −3 M yr −1. More luminous and cooler stars are found to reach higher mass-loss rates. The highest mass-loss rates exceed the classical limit set by the momentum of the stellar radiation field, L/c, by a factor of a few due to multiple scattering of photons in the circumstellar dust envelope. Mass-loss rates are lower than the mass consumption rate by nuclear burning,Ṁ nuc , for most of the RSGs. Two RSGs havė M Ṁ nuc , however, suggesting that RSGs shed most of their stellar mantles in short phases of intense mass loss. Stars on the thermal pulsing AGB may also experience episodes of intensified mass loss, but their quiescent mass-loss rates are usually already higher thanṀ nuc .
The Astrophysical Journal, 2016
Over two decades ago, a prominent, mysterious emission band peaking at ∼20.1 μm was serendipitously detected in four preplanetary nebulae (PPNe; also known as "protoplanetary nebulae"). To date, this spectral feature, designated as the "21 μm" feature, has been seen in 27 carbon-rich PPNe in the Milky Way and the Magellanic Clouds. The nature of its carriers remains unknown although many candidate materials have been proposed. The 21 μm sources also exhibit an equally mysterious, unidentified emission feature peaking at 30 μm. While the 21 μm feature is exclusively seen in PPNe, a short-lived evolutionary stage between the end of the asymptotic giant branch (AGB) and planetary nebula (PN) phases, the 30 μm feature is more commonly observed in all stages of stellar evolution from the AGB through PPN to PN phases. We derive the stellar mass-loss rates (Ṁ) of these sources from their infrared (IR) emission, using the "2-DUST" radiative transfer code for axisymmetric dusty systems which allows one to distinguish the mass-loss rates of the AGB phase (M AGḂ) from that of the superwind (M SẆ) phase. We examine the correlation between M AGḂ or M SẆ and the fluxes emitted from the 21 and 30 μm features. We find that both features tend to correlate with M AGḂ , suggesting that their carriers are probably formed in the AGB phase. The nondetection of the 21 μm feature in AGB stars suggests that, unlike the 30 μm feature, the excitation of the carriers of the 21 μm feature may require ultraviolet photons which are available in PPNe but not in AGB stars.
Mid-infrared period-magnitude relations for AGB stars
Monthly Notices of the Royal Astronomical Society: Letters, 2009
Asymptotic giant branch (AGB) variables are found to obey period-luminosity relations in the mid-infrared (mid-IR) similar to those seen at K S (2.14 μm), even at 24 μm where emission from circumstellar dust is expected to be dominant. Their loci in the M, log P diagrams are essentially the same for the Large Magellanic Cloud (LMC) and for NGC 6522 in spite of different ages and metallicities. There is no systematic trend of slope with wavelength. The offsets of the apparent magnitude versus log P relations imply a difference between the two fields of 3.8 in distance modulus. The colours of the variables confirm that a principal period with log P > 1.75 is a necessary condition for detectable mass loss. At the longest observed wavelength, 24 μm, many semiregular variables have dust shells comparable in luminosity to those around Miras. There is a clear bifurcation in LMC colour-magnitude diagrams involving 24 μm magnitudes.
of Extremely Hot Post-AGB Stars: Constraints for Evolutionary Theory
2013
Abstract. Spectral analysis by means of Non-LTE model-atmosphere techniques has arrived at a high level of sophistication: fully line-blanketed model atmospheres which consider opacities of all elements from H to Ni allow the reliable determination of photospheric parameters of hot, compact stars. Such models provide a crucial test of stellar evolutionary theory: recent abundance determinations of trace elements like, e.g., F, Ne, Mg, P, S, Ar, Fe, and Ni are suited to investigate on AGB nucleosynthesis. E.g., the strong Fe depletion found in hydrogen-deficient post-AGB stars is a clear indication of an efficient s-process on the AGB where Fe is transformed into Ni or even heavier trans iron-group elements. We present results of recent spectral analyses based on high-resolution UV observations of hot stars.