283 research outputs found
Is mass loss along the red giant branch of globular clusters sharply peaked? The case of M3
There is a growing evidence that several globular clusters must contain
multiple stellar generations, differing in helium content. This hypothesis has
helped to interpret peculiar unexplained features in their horizontal branches.
In this framework we model the peaked distribution of the RR Lyr periods in M3,
that has defied explanation until now. At the same time, we try to reproduce
the colour distribution of M3 horizontal branch stars. We find that only a very
small dispersion in mass loss along the red giant branch reproduces with good
accuracy the observational data. The enhanced and variable helium content among
cluster stars is at the origin of the extension in colour of the horizontal
branch, while the sharply peaked mass loss is necessary to reproduce the
sharply peaked period distribution of RR Lyr variables. The dispersion in mass
loss has to be <~ 0.003 Msun, to be compared with the usually assumed values of
~0.02 Msun. This requirement represents a substantial change in the
interpretation of the physical mechanisms regulating the evolution of globular
cluster stars.Comment: Accepted for publication in The Astrophysical Journa
Evolution and nucleosynthesis of primordial low mass stars
We discuss in detail the evolutionary properties of low mass stars M< 1
M(Solar) having metallicity lower than Z=10^(-6) from the pre- main sequence up
to (almost) the end of the Asymptotic Giant Branch phase. We also discuss the
possibility that the large [C,N/Fe] observed on the surface of the most Iron
poor star presently known, HE0107-5240, may be attributed to the autopollution
induced by the penetration of the He convective shell into the H rich mantle
during the He core flash of a low mass, very low metallicity star. On the basis
of a quite detailed analysis, we conclude that the autopollution scenario
cannot be responsible for the observed chemical composition of HE0107-5240
Massive Protoplanetary Disks in the Trapezium Region
(abridged) We determine the disk mass distribution around 336 stars in the
young Orion Nebula cluster by imaging a 2.5' x 2.5' region in 3 mm continuum
emission with the Owens Valley Millimeter Array. For this sample of 336 stars,
we observe 3 mm emission above the 3-sigma noise level toward ten sources, six
of which have also been detected optically in silhouette against the bright
nebular background. In addition, we detect 20 objects that do not correspond to
known near-IR cluster members. Comparisons of our measured fluxes with longer
wavelength observations enable rough separation of dust emission from thermal
free-free emission, and we find substantial dust emission toward most objects.
For the ten objects detected at both 3 mm and near-IR wavelengths, eight
exhibit substantial dust emission. Excluding the high-mass stars and assuming a
gas-to-dust ratio of 100, we estimate circumstellar masses ranging from 0.13 to
0.39 Msun. For the cluster members not detected at 3 mm, images of individual
objects are stacked to constrain the mean 3 mm flux of the ensemble. The
average flux is detected at the 3-sigma confidence level, and implies an
average disk mass of 0.005 Msun, comparable to the minimum mass solar nebula.
The percentage of stars in Orion surrounded by disks more massive than ~0.1
Msun is consistent with the disk mass distribution in Taurus, and we argue that
massive disks in Orion do not appear to be truncated through close encounters
with high-mass stars. Comparison of the average disk mass and number of massive
dusty structures in Orion with similar surveys of the NGC 2024 and IC 348
clusters constrains the evolutionary timescales of massive circumstellar disks
in clustered environments.Comment: 27 pages, including 7 figures. Accepted by Ap
The influence of nova nucleosynthesis on the chemical evolution of the Galaxy
We adopt up-to-date yields of 7Li, 13C, 15N from classical novae and use a
well tested model for the chemical evolution of the Milky Way in order to
predict the temporal evolution of these elemental species in the solar
neighborhood. In spite of major uncertainties due to our lack of knowledge of
metallicity effects on the final products of explosive nucleosynthesis in nova
outbursts, we find a satisfactory agreement between theoretical predictions and
observations for 7Li and 13C. On the contrary, 15N turns out to be overproduced
by about an order of magnitude.Comment: 8 pages, latex, 3 figures. To appear in "The Chemical Evolution of
the Milky Way: Stars versus Clusters", eds. F. Giovannelli and F. Matteucci
(Kluwer: Dordrecht
Detailed Spectroscopic and Photometric Analysis of DQ White Dwarfs
We present an analysis of spectroscopic and photometric data for cool DQ
white dwarfs based on improved model atmosphere calculations. In particular, we
revise the atmospheric parameters of the trigonometric parallax sample of
Bergeron et al.(2001), and discuss the astrophysical implications on the
temperature scale and mean mass, as well as the chemical evolution of these
stars. We also analyze 40 new DQ stars discovered in the first data release of
the Sloan Digital Sky Survey.Comment: 6 pages,3 figures, 14th European Workshop on White Dwarfs, ASP
Conference Series, in pres
PSR J1740-5340: accretion inhibited by radio-ejection in a binary millisecond pulsar in the Globular Cluster NGC 6397
We present an evolutionary scenario for the spin-up and evolution of binary
millisecond pulsars, according to which the companion of the pulsar PSR J
1740-5340, recently discovered as a binary with orbital period of 32.5 hr in
the Globular Cluster NGC 6397, is presently in a phase of ``radio-ejection''
mass loss from the system. At present, Roche lobe overflow due to the nuclear
evolution of the pulsar companion and to systemic angular momentum losses by
magnetic braking is still going on, but accretion is inhibited by the momentum
exerted by the radiation of the pulsar on the matter at the inner Lagrangian
point. The presence of this matter around the system is consistent with the
long lasting irregular radio eclipses seen in the system. Roche lobe
deformation of the mass losing component is also necessary to be compatible
with the optical light curve. The "radio-ejection" phase had been recently
postulated by us to deal with the problem of the lack of submillisecond pulsars
(Burderi et al. 2001, ApJ, 560, L71).Comment: Accepted for publication in The Astrophysical Journa
A Surprising Reversal of Temperatures in the Brown-Dwarf Eclipsing Binary 2MASS J05352184-0546085
The newly discovered brown-dwarf eclipsing binary 2MASS J05352184-0546085
provides a unique laboratory for testing the predictions of theoretical models
of brown-dwarf formation and evolution. The finding that the lower-mass brown
dwarf in this system is hotter than its higher-mass companion represents a
challenge to brown-dwarf evolutionary models, none of which predict this
behavior. Here we present updated determinations of the basic physical
properties of 2M0535-05, bolstering the surprising reversal of temperatures
with mass in this system. We compare these measurements with widely used
brown-dwarf evolutionary tracks, and find that the temperature reversal can be
explained by some models if the components of 2M0535-05 are mildly non-coeval,
possibly consistent with dynamical simulations of brown-dwarf formation.
Alternatively, a strong magnetic field on the higher-mass brown dwarf might
explain its anomalously low surface temperature, consistent with emerging
evidence that convection is suppressed in magnetically active, low-mass stars.
Finally, we discuss future observational and theoretical work needed to further
characterize and understand this benchmark system.Comment: 31 pages, 7 figures, accepted by Ap
Predictions for self-pollution in globular cluster stars
Full evolutionary models have been built to follow the phases of asymptotic
giant branch evolution with mass loss for metal mass fractions from Z=0.0002 to
Z=0.004. For the first time, we find that temperatures close to or even larger
than 10^8 K are achieved at low Z; the full CNO cycle operates at the base of
the envelope, the Oxygen abundance for the most metal-poor models of 4 and 5
solar masses is drastically reduced, and sodium and aluminum production by by
proton capture on neon and magnesium can occur. These results may be relevant
for the evolution of primordial massive globular clusters: we suggest that the
low-mass stars may have been polluted at the surface by accretion from the gas
that was lost from the evolving intermediate-mass stars at early ages.Comment: 14 pages, 6 figure
Evolution of low-mass metal-free stars including effects of diffusion and external pollution
We investigate the evolution of low-mass metal-free Population III stars.
Emphasis is laid upon the question of internal and external sources for
CNO-elements, which - if present in sufficient amounts in the hydrogen-burning
regions - lead to a strong modification of the stars' evolutionary behavior.
For the production of carbon due to nuclear processes inside the stars, we use
an extended nuclear network, demonstrating that hot pp-chains do not suffice to
produce enough carbon or are less effective than the triple3-alpha-process. As
an external source of CNO-elements we test the efficiency of pollution by a
nearby massive star combined with particle diffusion. For all cases
investigated, the additional metals fail to reach nuclear burning regions
before deep convection on the Red Giant Branch obliterates the previous
evolution. The surface abundance history of the polluted Pop III stars is
presented. The possibilities to discriminate between a Pop II and a polluted
Pop III field star are also discussed.Comment: Accepted for publication in Ap
Quantitative Evidence for an Intrinsic Age Spread in the Orion Nebula Cluster
Aims. We present a study of the distribution of stellar ages in the Orion
Nebula Cluster (ONC) based on accurate HST photometry taken from the HST
Treasury Program observations of the ONC utilizing the most recent estimate of
the cluster's distance (Menten et al. 2007). We investigate the presence of an
intrinsic age spread in the region and a possible trend of age with the spatial
distribution. Methods. We estimate the extinction and accretion luminosity
towards each source by performing synthetic photometry on an empirical
calibration of atmospheric models (Da Rio et al. 2010) using the package
Chorizos (Maiz-Apellaniz 2004). The position of the sources in the HR-diagram
is compared with different theoretical isochrones to estimate the mean cluster
age and age dispersion. Through Monte Carlo simulations we quantify the amount
of intrinsic age spread in the region, taking into account uncertainties on the
distance, spectral type, extinction, unresolved binaries, accretion and
photometric variability. Results. According to Siess et al. (2000) evolutionary
models the mean age of the Cluster is 2.2 Myr with a scatter of few Myrs. With
Monte Carlo simulations we find that the observed age spread is inconsistent
with a coeval stellar population, but is in agreement with a star formation
activity between 1.5 and 3.5 Myrs. We also observe light evidence for a trend
of ages with spatial distribution.Comment: 12 pages, 12 figures, Accepted for publication in Astronomy and
Astrophysic
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