466 research outputs found
On the origin of HE0107-5240, the most iron deficient star presently known
We show that the "puzzling" chemical composition observed in the extremely
metal poor star HE0107-5240 may be naturally explained by the concurrent
pollution of at least two supernovae. In the simplest possible model a
supernova of quite low mass (~15 Msun), underwent a "normal" explosion and
ejected ~0.06 Msun of 56Ni while a second one was massive enough (~35 Msun) to
experience a strong fall back that locked in a compact remnant all the
carbon-oxygen core. In a more general scenario, the pristine gas clouds were
polluted by one or more supernovae of relatively low mass (less than ~25 Msun).
The successive explosion of a quite massive star experiencing an extended fall
back would have largely raised the abundances of the light elements in its
close neighborhood.Comment: 10 pages; 3 figures; accepted for publication in the The
Astrophysical Journal Letter
Isochrones and Luminosity Functions for Old White Dwarfs
Using a new grid of models of cooling white dwarfs, we calculate isochrones
and luminosity functions in the Johnson-Kron/Cousins and HST filter sets for
systems containing old white dwarfs. These new models incorporate a non-grey
atmosphere which is necessary to properly describe the effects of molecular
opacity at the cool temperatures of old white dwarfs. The various functions
calculated and extensively tabulated and plotted are meant to be as utilitarian
as possible for observers so all results are listed in quantities that
observers will obtain. The tables and plots developed should eventually prove
critical in interpreting the results of HST's Advanced Camera observations of
the oldest white dwarfs in nearby globular clusters, in understanding the
results of searches for old white dwarfs in the Galactic halo, and in
determining ages for star clusters of all ages using white dwarfs. As a
practical application we demonstrate the use of these results by deriving the
white dwarf cooling age of the old Galactic cluster M67.Comment: 7 pages, 8 tables, accepted for publication in the Astrophysical
Journa
The Chemical Evolution of Magnesium Isotopic Abundances in the Solar Neighbourhood
The abundance of the neutron-rich magnesium isotopes observed in metal-poor
stars is explained quantitatively with a chemical evolution model of the local
Galaxy that considers - for the first time - the metallicity-dependent
contribution from intermediate mass stars. Previous models that simulate the
variation of Mg isotopic ratios with metallicity in the solar neighbourhood
have attributed the production of Mg25 and Mg26 exclusively to hydrostatic
burning in massive stars.
These models match the data well for [Fe/H]>-1.0 but severely underestimate
Mg25/Mg24 and Mg26/Mg24 at lower metallicities. Earlier studies have noted that
this discrepancy may indicate a significant role played by intermediate-mass
stars. Only recently have detailed calculations of intermediate-mass stellar
yields of Mg25 and Mg26 become available with which to test this hypothesis. In
an extension of previous work, we present a model that successfully matches the
Mg isotopic abundances in nearby Galactic disk stars through the incorporation
of nucleosynthesis predictions of Mg isotopic production in asymptotic giant
branch stars.Comment: 9 pages, 6 figures, to appear in Publications of the Astronomical
Society of Australia (PASA) in 2003, vol. 20, No.
C/O white dwarfs of very low mass: 0.33-0.5 Mo
The standard lower limit for the mass of white dwarfs (WDs) with a C/O core
is roughly 0.5 Mo. In the present work we investigated the possibility to form
C/O WDs with mass as low as 0.33 Mo. Both the pre-WD and the cooling evolution
of such nonstandard models will be described.Comment: Submitted to the "Proceedings of the 16th European White Dwarf
Workshop" (to be published JPCS). 7 pages including 13 figure
The Large Magellanic Cloud globular cluster NGC 1866: new data, new models, new analysis
We present a new deep (down to V ~ 24) photometry of a wide region (6'x 6')
around the LMC globular cluster NGC1866: our sample is complete, down to 3 mag
below the brightest MS star. Detailed comparisons with various theoretical
scenarios using models computed with the evolutionary code FRANEC have been
done reaching the following conclusions: both standard models (i.e. computed by
adopting the Schwarzschild criterion to fix the border of the convective core)
and models with an enlarged convective core (overshooting) lead to a fair fit
of the MS but are not able to reproduce the luminosity and/or the number of He
burning giants. Models including a fraction of 30% of binaries leads to a good
fit both to the MS luminosity function and to the He clump, if standards models
are considered, for a visual distance modulus (m-M)v = 18.8, age t ~ 100 Myr
and mass function slope alpha ~ 2.4, thus largely removing the "classical"
discrepancy between observed and predicted number of stars in the He burning
clump. The fit obtained with models computed with an enlarged convective core
gets worse when a binary component is taken into account, because the presence
of binary systems increases the existing discrepancy between the observed and
predicted clump luminosity. As a consequence of this analysis, we conclude that
the next step towards a proper understanding of NGC 1866, and similar clusters,
must include the accurate determination of the frequency of binary systems that
will be hopefully performed with the incoming Cycle 8 HST observations of
NGC~1866.Comment: AASTEX 5.0, 33 pages, 35 figures. Two tables of photometry and full
resolution figures available on request from the first author
([email protected]). Accepted on A
Evolved stars in the Local Group galaxies - II. AGB, RSG stars and dust production in IC10
We study the evolved stellar population of the Local Group galaxy IC10, with
the aim of characterizing the individual sources observed and to derive global
information on the galaxy, primarily the star formation history and the dust
production rate. To this aim, we use evolutionary sequences of low- and
intermediate-mass () stars, evolved through the asymptotic
giant branch phase, with the inclusion of the description of dust formation. We
also use models of higher mass stars. From the analysis of the distribution of
stars in the observational planes obtained with IR bands, we find that the
reddening and distance of IC10 are mag and Mpc,
respectively. The evolved stellar population is dominated by carbon stars, that
account for of the sources brighter than the tip of the red giant
branch. Most of these stars descend from progenitors,
formed during the major epoch of star formation, which occurred Gyr
ago. The presence of a significant number of bright stars indicates that IC10
has been site of significant star formation in recent epochs and currently
hosts a group of massive stars in the core helium-burning phase. Dust
production in this galaxy is largely dominated by carbon stars; the overall
dust production rate estimated is /yr.Comment: Manuscript accepted for publication in MNRAS on 11 june 2018;17
pages, 10 figure
Heavy element abundances in giant stars of the globular clusters M4 and M5
We present a comprehensive abundance analysis of 27 heavy elements in bright
giant stars of the globular clusters M4 and M5 based on high resolution, high
signal-to-noise ratio spectra obtained with the Magellan Clay Telescope. We
confirm and expand upon previous results for these clusters by showing that (1)
all elements heavier than, and including, Si have constant abundances within
each cluster, (2) the elements from Ca to Ni have indistinguishable
compositions in M4 and M5, (3) Si, Cu, Zn, and all s-process elements are
approximately 0.3 dex overabundant in M4 relative to M5, and (4) the r-process
elements Sm, Eu, Gd, and Th are slightly overabundant in M5 relative to M4. The
cluster-to-cluster abundance differences for Cu and Zn are intriguing,
especially in light of their uncertain nucleosynthetic origins. We confirm that
stars other than Type Ia supernovae must produce significant amounts of Cu and
Zn at or below the clusters' metallicities. If intermediate-mass AGB stars or
massive stars are responsible for the Cu and Zn enhancements in M4, the similar
[Rb/Zr] ratios and (preliminary) Mg isotope ratios in both clusters may be
problematic for either scenario. For the elements from Ba to Hf, we assume that
the s- and r-process contributions are scaled versions of the solar s- and
r-process abundances. We quantify the relative fractions of s- and r-process
material for each cluster and show that they provide an excellent fit to the
observed abundances.Comment: Accepted for publication in Ap
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