313 research outputs found
MS, S and C Stars in the Infrared. Luminosities and Mass Loss Rates
In this note I present an outline of infrared (IR) photometric AGB
properties, based on two samples of Galactic Long Period Variables (C- and
S-type respectively). I show the various selection criteria used during the
choice of the sources and describe the motivations of observing them at near-
and mid-IR wavelengths. I discuss the problems encountered in estimating their
luminosity and distance and motivate the methods I choose for this purpose.
Properties of the luminosity functions and of the Hertzsprung-Russell (HR)
diagrams obtained from the analysis are discussed. Finally, the choices made
for estimating of the mass loss rates are described and preliminary results
concerning them are shown.Comment: 10 pages, 6 figures, contribution from the IX Torino Workshop, to be
published by AI
Infrared Properties Of AGB Stars: from Existing Databases to Antarctic Surveys
We present here a study of the Infrared properties of Asymptotic Giant Branch
stars (hereafter AGB) based on existing databases, mainly from space-borne
experiments. Preliminary results about C and S stars are discussed, focusing on
the topics for which future Infrared surveys from Antarctica will be crucial.
This kind of surveys will help in making more quantitative our knowledge of the
last evolutionary stages of low mass stars, especially for what concerns
luminosities and mass loss.Comment: 6 pages, 3 figures. Contribution from the 1st ARENA Conference on
"Large Astronomical Infrastructures at CONCORDIA, prospects and constraints
for Antarctic Optical/IR Astronomy" held 16-19 October 2006 at Roscoff,
Franc
Nucleosynthesis of light element isotopes in evolved stars experiencing extended mixing
We present computations of nucleosynthesis in red giants and asymptotic giant
branch stars of Population I experiencing extended mixing. The assumed physical
cause for mass transport is the buoyancy of magnetized structures, according to
recent suggestions. The peculiar property of such a mechanism is to allow for
both fast and slow mixing phenomena, as required for reproducing the spread in
Li abundances displayed by red giants and as discussed in an accompanying
paper. We explore here the effects of this kind of mass transport on CNO and
intermediatemass nuclei and compare the results with the available evidence
from evolved red giants and from the isotopic composition of presolar grains of
AGB origin. It is found that a good general accord exists between predictions
and measurements; in this framework we also show which type of observational
data best constrains the various parameters. We conclude that magnetic
buoyancy, allowing for mixing at rather different speeds, can be an interesting
scenario to explore for explaining together the abundances of CNO nuclei and of
Li.Comment: 8 pages, 7 figures, proceeding of 'The Origin of the Elements Heavier
than Fe' September 24-28, 2008, Torino, Italy. PASA (accepted for
publication
The revenues of local governments in the statistical register for public administrations: inequality decomposition by sources
During the last decade, the Italian national institute of statistics (Istat) has been
engaged in a modernization program involving the revision of the statistical
production model. The main goal behind this program is the use of statistical
registers integrated into a single logical environment, the Italian Integrated System
of Statistical Registers (ISSR), for supporting the consistency of statistical
production processes and improving the quality of information for users. One object
of the ISSR is the satellite statistical REgister for Public Administrations (REPA)
that contains information on structural and economic variables on a subset of the
Italian PA. The public institutions are different from each other in structures, dimensions,
and scopes. In the present paper, these differences are analysed by looking at their
revenues. For this aim, the inequality in the revenues, measured with the Gini index,
is decomposed “by sources”
Extra-Mixing in Luminous Cool Red Giants. Hints from Evolved Stars with and without Li
We present an analysis of Li abundances in low mass stars (LMS) during the
Red Giant Branch (RGB) and Asymptotic Giant Branch (AGB) stages, based on a new
determination of their luminosities and evolutionary status. By applying
recently suggested models for extra-mixing, induced by magnetic buoyancy, we
show that both Li-rich and Li-poor stars can be accounted for. The simplest
scenario implies the development of fast instabilities on the RGB, where Li is
produced. When the fields increase in strength, buoyancy slows down and Li is
destroyed. 3He is consumed, at variable rates. The process continues on the
AGB, where however moderate mass circulation rates have little effect on Li due
to the short time available. O-rich and C-rich stars show different histories
of Li production/destruction, possibly indicative of different masses. More
complex transport schemes are allowed by magnetic buoyancy, with larger effects
on Li, but most normal LMS seem to show only the range of Li variation
discussed here.Comment: 8 pages, 6 figures, accepted for publication in PAS
The VMC survey III : Mass-loss rates and luminosities of LMC AGB stars
Context. Asymptotic giant branch (AGB) stars are major contributors to both the chemical enrichment of the interstellar medium and the integrated light of galaxies. Despite its importance, the AGB is one of the least understood phases of stellar evolution. The main difficulties associated with detailed modelling of the AGB are related to the mass-loss process and the 3rd dredge-up efficiency Aims. We provide direct measures of mass-loss rates and luminosities for a complete sample of AGB stars in the Large Magellanic Cloud, disentangling the C- and O-rich stellar populations. Methods. Dust radiative transfer models are presented for all 374 AGB stars candidates in one of the fields observed by the new VISTA survey of the Magellanic Clouds (VMC). Mass-loss rates, luminosities and a classification of C-and O-rich stars are derived by fitting the models to the spectral energy distribution (SED) obtained by combining VMC data with existing optical, near-, and mid-infrared photometry. Results. The classification technique is reliable at a level of - at worst -75% and significantly better for the reddest dusty stars. We classified none of the stars with a relevant mass-loss rate as O-rich, and we can exclude the presence of more than one dusty O-rich star at a similar to 94% level. The bolometric luminosity function we obtained is fully consistent with most of the literature data on the LMC and with the prediction of theoretical models, with a peak of the C-star distribution at M-bol similar or equal to -4.8 mag and no stars brighter than the classical AGB tip, at M-bol = -7.1 mag. Conclusions. This exploratory study shows that our method provides reliable mass-loss rates, luminosities and chemical classifications for all AGB stars. These results offer already important constraints to AGB evolutionary models. Most of our conclusions, especially for the rarer dust-enshrouded extreme AGB stars, are however strongly limited by the relatively small area covered by our study. Forthcoming VMC observations will easily remove this limitation.Peer reviewe
Mass loss in AGB stars from infrared colors
Abstract. 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 leading to the formation of their extended dusty envelopes and of planetary nebulae. Our sample includes more than 250 sources in our Galaxy, distributed along the evolutionary sequence that gradually changes the spectral characteristics of M giants, forming MS, S and then C-stars. Thanks to a re-analysis 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 mid-infrared colors. We show the existence of clear correlations suggesting that mass loss can be inferred from photometric colors 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 including mass loss in stellar models. In this paper we discuss in particular, as an example, 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
, the luminosity and the mass loss rate , 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 and 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 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 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.Comment: Accepted for publication in the Astronomical Journal - 35 pages (in
preprint), 9 figures, 5 table
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