13 research outputs found
Luminosities of Carbon-rich Asymptotic Giant Branch stars in the Milky Way
Stars evolving along the Asymptotic Giant Branch can become Carbon-rich in
the final part of their evolution. They replenish the inter-stellar medium with
nuclear processed material via strong radiative stellar winds. The
determination of the luminosity function of these stars, even if far from being
conclusive, is extremely important to test the reliability of theoretical
models. In particular, strong constraints on the mixing treatment and the
mass-loss rate can be derived.
We present an updated Luminosity Function of Galactic Carbon Stars obtained
from a re-analysis of available data already published in previous papers.
Starting from available near- and mid-infrared photometric data, we
re-determine the selection criteria. Moreover, we take advantage from updated
distance estimates and Period-Luminosity relations and we adopt a new
formulation for the computation of Bolometric Corrections. This leads us to
collect an improved sample of carbon-rich sources from which we construct an
updated Luminosity Function.
The Luminosity Function of Galactic Carbon Stars peaks at magnitudes around
-4.9, confirming the results obtained in a previous work. Nevertheless, the
Luminosity Function presents two symmetrical tails instead of the larger high
luminosity tail characterizing the former Luminosity Function. The derived
Luminosity Function of Galactic Carbon Stars matches the indications coming
from recent theoretical evolutionary Asymptotic Giant Branch models, thus
confirming the validity of the choices of mixing treatment and mass-loss
history. Moreover, we compare our new Luminosity Function with its counterpart
in the Large Magellanic Cloud finding that the two distributions are very
similar for dust-enshrouded sources, as expected from stellar evolutionary
models. Finally, we derive a new fitting formula aimed to better determine
Bolometric Corrections for C-stars.Comment: 7 pages, 5 figures. To be published in Astronomy and Astrophysic
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
Dynamos and Chemical Mixing in Evolved Stars
In low-mass Red Giant Branch (RGB) and Asymptotic Giant Branch (AGB) stars,
anomalous mixing must transport material near the hydrogen-burning shell to the
convective envelope. Recently, it was suggested that buoyant magnetic flux
tubes could supply the necessary transport rate (Busso et al. 2007). The fields
are assumed to originate from a dynamo operating in the stellar interior. Here,
we show what is required of an dynamo in the envelope of an AGB
star to maintain these fields. Differential rotation and rotation drain via
turbulent dissipation and Poynting flux, so if shear can be resupplied by
convection, then large-scale toroidal field strengths of
\left\simeq3\times10^4 G can be sustained at the base of the
convection zone.Comment: 7 pages, 3 figures. To appear in AIP Proceedings of the IXth Torino
Workshop on AGB Nucleosynthesi
Low temperature mean opacities for the carbon-rich regime
Asymptotic Giant Branch (AGB) stars undergo a change in their chemical
composition during their evolution. This in turn leads to an alteration of the
radiative opacities, especially in the cool layers of the envelope and the
atmosphere, where molecules are the dominant opacity sources. A key parameter
in this respect is the number ratio of carbon to oxygen atoms (C/O). In terms
of low temperature mean opacities, a variation of this parameter usually cannot
be followed in stellar evolution models, because up to now tabulated values
were only available for scaled solar metal mixtures (with C/O ~ 0.5). We thus
present a set of newly generated tables containing Rosseland mean opacity
coefficients covering both the oxygen-rich (C/O
1) regime. We compare our values to existing tabular data and investigate the
relevant molecular opacity contributors.Comment: 8 pages, 5 figures. To appear in the AIP Proceedings of the IXth
Torino Workshop on AGB Nucleosynthesi
Spitzer/IRAC Observations of AGB stars
We present here the first observation of galactic AGB stars with the InfraRed
Array Camera (IRAC) onboard the Spitzer Space Telescope. Our sample consists of
48 AGB stars of different chemical signature, mass loss rate and variability
class. For each star we have measured IRAC photometry and colors. Preliminary
results shows that IRAC colors are sensitive to spectroscopic features
associated to molecules and dust in the AGB wind. Period is only loosely
correlated to the brightness of the stars in the IRAC bands. We do find,
however, a tight period-color relation for sources classified as semiregular
variables. This may be interpreted as the lack of warm dust in the wind of the
sources in this class, as opposed to Mira variables that show higher infrared
excess in all IRAC bands.Comment: 8 pages, to be published in proceedings "IX Torino Workshop on
Evolution and Nucleosynthesis in AGB Stars", 22-26 October 2007, Perugia,
Ital
The connection between mass loss and nucleosynthesis
I discuss the relationship between mass loss and nucleosynthesis on the
Asymptotic Giant Branch (AGB). Because of thermal pulses and possibly other
mixing processes, products of nucleosynthesis can be brought to the surface of
AGB stars, increasingly so as the star becomes more luminous, cooler, and
unstable against pulsation of its tenuous mantle. As a result, mass loss is at
its most extreme when dredge-up is too. As the high rate of mass loss truncates
AGB evolution, it determines the enrichment of interstellar space with the AGB
nucleosynthesis products. The changing composition of the stellar atmosphere
also affects the mass-loss process, most obviously in the formation of dust
grains - which play an important role in driving the wind of AGB stars.Comment: 8 pages, including 3 colour figures. To appear in the AIP proceedings
of "The IXth Torino Workshop on Evolution and Nucleosynthesis in AGB Stars".
Also available from http://www.astro.keele.ac.uk/~jacco/research/ecology.htm
The VMC survey - XI : Radial Stellar Population Gradients in the Galactic Globular Cluster 47 Tucanae
Copyright American Astronomical SocietyWe present a deep near-infrared color-magnitude diagram of the Galactic globular cluster 47 Tucanae, obtained with the Visible and Infrared Survey Telescope for Astronomy (VISTA) as part of the VISTA near-infrared Y, J, Ks survey of the Magellanic System (VMC). The cluster stars comprising both the subgiant and red giant branches exhibit apparent, continuous variations in color-magnitude space as a function of radius. Subgiant branch stars at larger radii are systematically brighter than their counterparts closer to the cluster core; similarly, red-giant-branch stars in the cluster's periphery are bluer than their more centrally located cousins. The observations can very well be described by adopting an age spread of ~0.5 Gyr as well as radial gradients in both the cluster's helium abundance (Y) and metallicity (Z), which change gradually from (Y = 0.28, Z = 0.005) in the cluster core to (Y = 0.25, Z = 0.003) in its periphery. We conclude that the cluster's inner regions host a significant fraction of second-generation stars, which decreases with increasing radius; the stellar population in the 47 Tuc periphery is well approximated by a simple stellar population.Peer reviewe
The VMC ESO Public Survey
The VISTA near-infrared YJKs survey of the Magellanic Clouds system (VMC) has entered its core phase: about 50% of the observations across the Large and Small Magellanic Clouds (LMC, SMC), the Magellanic Bridge and Stream have already been secured and the data are processed and analysed regularly. The initial analyses, concentrated on the first two completed tiles in the LMC (including 30 Doradus and the South Ecliptic Pole), show the superior quality of the data. The photometric depth of the VMC survey allows the derivation of the star formation history (SFH) with unprecedented quality compared to previous wide-area surveys, while reddening maps of high angular resolution are constructed using red clump stars. The multi-epoch Ks-band data reveal tight period-luminosity relations for variable stars and permit the measurement of accurate proper motions of the stellar populations. The VMC survey continues to acquire data that will address many issues in the field of star and galaxy evolution