705 research outputs found
Carbon Nitrogen, and Oxygen Galactic Gradients: A Solution to the Carbon Enrichment Problem
Eleven models of Galactic chemical evolution, differing in the carbon,
nitrogen,and oxygen yields adopted, have been computed to reproduce the
Galactic O/H values obtained from H II regions. All the models fit the oxygen
gradient, but only two models fit also the carbon gradient, those based on
carbon yields that increase with metallicity due to stellar winds in massive
stars (MS) and decrease with metallicity due to stellar winds in low and
intermediate mass stars (LIMS). The successful models also fit the C/O versus
O/H evolution history of the solar vicinity obtained from stellar observations.
We also compare the present day N/H gradient and the N/O versus O/H and the
C/Fe, N/Fe, O/Fe versus Fe/H evolution histories of the solar vicinity
predicted by our two best models with those derived from H II regions and from
stellar observations. While our two best models fit the C/H and O/H gradients
as well as the C/O versus O/H history, only Model 1 fits well the N/H gradient
and the N/O values for metal poor stars but fails to fit the N/H values for
metal rich stars. Therefore we conclude that our two best models solve the C
enrichment problem, but that further work needs to be done on the N enrichment
problem. By adding the C and O production since the Sun was formed predicted by
Models 1 and 2 to the observed solar values we find an excellent agreement with
the O/H and C/H values of the solar vicinity derived from H II regions O and C
recombination lines. One of the most important results of this paper is that
the fraction of carbon due to MS and LIMS in the interstellar medium is
strongly dependent on time and on the galactocentric distance; at present about
half of the carbon in the interstellar medium of the solar vicinity has been
produced by MS and half by LIMS.Comment: 34 pages, 6 tables, 7 figures. Accepted for publication in Ap
Evolution and Yields of Extremely Metal Poor Intermediate Mass Stars
Intermediate mass stellar evolution tracks from the main sequence to the tip
of the AGB for five initial masses (2 to 6Msun) and metallicity Z=0.0001 have
been computed. The detailed 1D structure and evolution models include
exponential overshooting, mass loss and a detailed nucleosynthesis network with
updated nuclear reaction rates. The network includes a two-particle heavy
neutron sink for approximating neutron density in the He-shell flash. It is
shown how the neutron-capture nucleosynthesis is important in models of very
low metallicity for the formation of light neutron-heavy species, like sodium
or the heavy neon and magnesium isotopes. The models have high resolution, as
required for modeling the third dredge-up. All sequences have been followed
from the pre-main sequence to the end of the AGB when all envelope mass is
lost. Detailed structural and chemical model properties as well as yields are
presented. This set of stellar models is based on standard assumptions and
updated input physics. It can be confronted with observations of
extremely-metal poor stars and may be used to assess the role of AGB stars in
the origin of abundance anomalies of some Globular Cluster members of
correspondingly low metallicity.Comment: 40 pages, 11 figures, to appear in ApJS, including 5 electronic
table
Detecting Primordial Stars
We discuss the expected properties of the first stellar generations in the
Universe. We find that it is possible to discern truly primordial populations
from the next generation of stars by measuring the metallicity of high-z star
forming objects. The very low background of the future James Webb Space
Telescope (JWST) will enable it to image and study first-light sources at very
high redshifts, whereas its relatively small collecting area limits its
capability in obtaining spectra of z~10-15 first-light sources to either the
bright end of their luminosity function or to strongly lensed sources. With a
suitable investment of observing time JWST will be able to detect individual
Population III supernovae, thus identifying the very first stars that formed in
the Universe.Comment: [8 pages, 5 figures] Invited Talk, to appear in IMF@50: The Stellar
Initial Mass Function Fifty Years Later, eds E. Corbelli, F. Palla, and H.
Zinnecker (Dordrecht: Kluwer
The evolution of luminosity, colour and the mass-to-luminosity ratio of Galactic open clusters: comparison of discrete vs. continuous IMF models
(abridged) We found in previous studies that standard Simple Stellar
Population (SSP) models are unable to describe or explain the colours of
Galactic open clusters both in the visible and in the NIR spectral range. (...)
We construct a numerical SSP-model, with an underlying Salpeter IMF, valid
within an upper and lower stellar mass range, and with total masses
typical of open clusters. We assume that the mass
loss from a cluster is provided by mass loss from evolved stars and by the
dynamical evaporation of low-mass members due to two-body relaxation. The data
for the latter process were scaled to the models from high-resolution N-body
calculations. We also investigate how a change of the -limit influences
magnitudes and colours of clusters of a given mass and derive a necessary
condition for a luminosity and colour flash. The discreteness of the IMF leads
to bursts in magnitude and colour of model clusters at moments when red
supergiants or giants appear and then die. The amplitude of the burst depends
on the cluster mass and on the spectral range; it is strongly increased in the
NIR compared to optical passbands. In the discrete case, variations of the
parameter are able to substantially change the magnitude-age and
-age relations. For the colours, the lowering of considerably
amplifies the discreteness effect. The influence of dynamical mass loss on
colour and magnitude is weak, although it provides a change of the slopes of
the considered relations, improving their agreement with observations. For the
Galactic open clusters we determined luminosity and tidal mass independent of
each other. The derived mass-to-luminosity ratio shows, on average, an increase
with cluster age in the optical, but gradually declines with age in the NIR.
The observed flash statistics can be used to constrain in open clusters.Comment: 15 pages, 13 figures, accepted for publication in Astronomy and
Astrophysic
The s-Process in Rotating Asymptotic Giant Branch Stars
(abridged) We model the nucleosynthesis during the thermal pulse phase of a
rotating, solar metallicity AGB star of 3M_sun. Rotationally induced mixing
during the thermal pulses produces a layer (~2E-5M_sun) on top of the CO-core
where large amounts of protons and C12 co-exist. We follow the abundance
evolution in this layer, in particular that of the neutron source C13 and of
the neutron poison N14. In our AGB model mixing persists during the entire
interpulse phase due to the steep angular velocity gradient at the
core-envelope interface. We follow the neutron production during the interpulse
phase, and find a resulting maximum neutron exposure of tau_max =0.04 mbarn^-1,
which is too small to produce any significant s-process. In parametric models,
we then investigate the combined effects of diffusive overshooting from the
convective envelope and rotationally induced mixing. Models with overshoot and
weaker interpulse mixing - as perhaps expected from more slowly rotating stars
- yield larger neutron exposures. We conclude that the incorporation of
rotationally induce mixing processes has important consequences for the
production of heavy elements in AGB stars. Through a distribution of initial
rotation rates it may lead to a natural spread in the neutron exposures
obtained in AGB stars of a given mass - as appears to be required by
observations. Our results suggest that both processes, diffusive overshoot and
rotational mixing, may be required to obtain a consistent description of the
s-process in AGB stars which fulfils all observational constraints. Finally, we
find that mixing due to rotation within our current framework does increase the
production of N15 in the partial mixing zone, however still falling short of
what seems required by observations.Comment: 50 pages, 13 figures, ApJ in press, tentatively scheduled for v593 n2
August 20, 200
A New Look At Carbon Abundances In Planetary Nebulae. IV. Implications For Stellar Nucleosynthesis
This paper is the fourth and final report on a project designed to study
carbon abundances in a sample of planetary nebulae representing a broad range
in progenitor mass and metallicity. We present newly acquired optical
spectrophotometric data for three Galactic planetary nebulae IC 418, NGC 2392,
and NGC 3242 and combine them with UV data from the IUE Final Archive for
identical positions in each nebula to determine accurate abundances of He, C,
N, O, and Ne at one or more locations in each object. We then collect
abundances of these elements for the entire sample and compare them with
theoretical predictions of planetary nebula abundances from a grid of
intermediate mass star models. We find some consistency between observations
and theory, lending modest support to our current understanding of
nucleosynthesis in stars below 8 M_o in birth mass. Overall, we believe that
observed abundances agree with theoretical predictions to well within an order
of magnitude but probably not better than within a factor of 2 or 3. But even
this level of consistency between observation and theory enhances the validity
of published intermediate-mass stellar yields of carbon and nitrogen in the
study of the abundance evolution of these elements.Comment: 41 pages, 11 figures. Accepted for publication in the Astrophysical
Journa
A new method to identify subclasses among AGB stars using Gaia and 2MASS photometry
Aims: We explore the wealth of high quality photometric data provided by data
release 2 of the Gaia mission for long period variables (LPVs) in the Large
Magellanic Cloud. Our goal is to identify stars of various types and masses
along the Asymptotic Giant Branch.
Methods: For this endeavour, we developed a new multi-band approach combining
Wesenheit functions W_{RP,BP-RP} and W_{K_s,J-K_s} in the Gaia BP, RP and 2MASS
J, K_s spectral ranges, respectively, and use a new diagram
(W_{RP,BP-RP}-W_{K_s,J-K_s}) versus K_s to distinguish between different kinds
of stars in our sample of LPVs. We used stellar population synthesis models to
validate our approach.
Results:We demonstrate the ability of the new diagram to discriminate between
O-rich and C-rich objects, and to identify low-mass, intermediate-mass and
massive O-rich red giants, as well as extreme C-rich stars. Stellar evolution
and population synthesis models guide the interpretation of the results,
highlighting the diagnostic power of the new tool to discriminate between
stellar initial masses, chemical properties and evolutionary stages.Comment: accepted for publication in A&A Letters; 7 figures, 2 appendice
Uncertainties in stellar evolution models: convective overshoot
In spite of the great effort made in the last decades to improve our
understanding of stellar evolution, significant uncertainties remain due to our
poor knowledge of some complex physical processes that require an empirical
calibration, such as the efficiency of the interior mixing related to
convective overshoot. Here we review the impact of convective overshoot on the
evolution of stars during the main Hydrogen and Helium burning phases.Comment: Proc. of the workshop "Asteroseismology of stellar populations in the
Milky Way" (Sesto, 22-26 July 2013), Astrophysics and Space Science
Proceedings, (eds. A. Miglio, L. Girardi, P. Eggenberger, J. Montalban
Ionization of the diffuse gas in galaxies: Hot low-mass evolved stars at work
We revisit the question of the ionization of the diffuse medium in late type
galaxies, by studying NGC 891, the prototype of edge-on spiral galaxies. The
most important challenge for the models considered so far was the observed
increase of [OIII]/Hbeta, [OII]/Hbeta, and [NII]/Halpha with increasing
distance to the galactic plane. We propose a scenario based on the expected
population of massive OB stars and hot low-mass evolved stars (HOLMES) in this
galaxy to explain this observational fact. In the framework of this scenario we
construct a finely meshed grid of photoionization models. For each value of the
galactic latitude z we look for the models which simultaneously fit the
observed values of the [OIII]/Hbeta, [OII]/Hbeta, and [NII]/Halpha ratios. For
each value of z we find a range of solutions which depends on the value of the
oxygen abundance. The models which fit the observations indicate a systematic
decrease of the electron density with increasing z. They become dominated by
the HOLMES with increasing z only when restricting to solar oxygen abundance
models, which argues that the metallicity above the galactic plane should be
close to solar. They also indicate that N/O increases with increasing z.Comment: Accepted for publication in MNRA
Global survey of star clusters in the Milky Way: III. 139 new open clusters at high Galactic latitudes
Context. An earlier analysis of the Milky Way Star Cluster (MWSC) catalogue revealed an apparent lack of old (t � 1 Gyr) open clusters in the solar neighbourhood (d � 1 kpc). Aims. To fill this gap we undertook a search for hitherto unknown star clusters, assuming that the missing old clusters reside at high Galactic latitudes | b | > 20°. Methods. We were looking for stellar density enhancements using a star count algorithm on the 2MASS point source catalogue. To increase the contrast between potential clusters and the field, we applied filters in colour-magnitude space according to typical colour-magnitude diagrams of nearby old open clusters. The subsequent comparison with lists of known objects allowed us to select thus far unknown cluster candidates. For verification they were processed with the standard pipeline used within the MWSC survey for computing cluster membership probabilities and for determining structural, kinematic, and astrophysical parameters. Results. In total we discovered 782 density enhancements, 524 of which were classified as real objects. Among them 139 are new open clusters with ages 8.3 < log (t [yr]) < 9.7, distances d< 3 kpc, and distances from the Galactic plane 0.3 <Z< 1 kpc. This new sample has increased the total number of known high latitude open clusters by about 150%. Nevertheless, we still observe a lack of older nearby clusters up to 1 kpc from the Sun. This volume is expected to still contain about 60 unknown clusters that probably escaped our detection algorithm, which fails to detect sparse overdensities with large angular size
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