350 research outputs found
The early evolution of Globular Clusters: the case of NGC 2808
Enhancement and spread of helium among globular cluster stars have been
recently suggested as a way to explain the horizontal branch blue tails, in
those clusters which show a primordial spread in the abundances of CNO and
other elements involved in advanced CNO burning (D'Antona et al. 2002). In this
paper we examine the implications of the hypothesis that, in many globular
clusters, stars were born in two separate events: an initial burst (first
generation), which gives origin to probably all high and intermediate mass
stars and to a fraction of the cluster stars observed today, and a second,
prolonged star formation phase (second generation) in which stars form directly
from the ejecta of the intermediate mass stars of the first generation. In
particular, we consider in detail the morphology of the horizontal branch in
NGC 2808 and argue that it unveils the early cluster evolution, from the birth
of the first star generation to the end of the second phase of star formation.
This framework provides a feasible interpretation for the still unexplained
dichotomy of NGC 2808 horizontal branch, attributing the lack of stars in the
RR Lyr region to the gap in the helium content between the red clump, whose
stars are considered to belong to the first stellar generation and have
primordial helium, and the blue side of the horizontal branch, whose minimum
helium content reflects the helium abundance in the smallest mass
(~4Msun)contributing to the second stellar generation. This scenario provides
constraints on the required Initial Mass Function, in a way that a great deal
of remnant neutron stars and stellar mass black holes might have been produced.Comment: 23 pages, 7 figures, in press on The Astrophysical Journa
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
Modeling the Near-Infrared Luminosity Functions of Young Stellar Clusters
We present the results of numerical experiments designed to evaluate the
usefulness of near-infrared luminosity functions for constraining the Initial
Mass Function (IMF) of young stellar populations. From this numerical modeling,
we find that the luminosity function of a young stellar population is
considerably more sensitive to variations in the underlying initial mass
function than to either variations in the star forming history or assumed
pre-main-sequence (PMS) mass-to-luminosity relation. To illustrate the
potential effectiveness of using the KLF of a young cluster to constrain its
IMF, we model the observed K band luminosity function of the nearby Trapezium
cluster. Our derived mass function for the Trapezium spans two orders of
magnitude in stellar mass (5 Msun to 0.02 Msun), has a peak near the hydrogen
burning limit, and has an IMF for Brown Dwarfs which steadily decreases with
decreasing mass.Comment: To appear in ApJ (1 April 2000). 37 pages including 11 figures, AAS:
ver 5.
Lithium Depletion Boundary in a Pre-Main Sequence Binary System
A lithium depletion boundary is detected in HIP 112312 (GJ 871.1 A and B), a
\~12 Myr old pre-main sequence binary system. A strong (EW 300 mA) Li 6708 A
absorption feature is seen at the secondary (~M4.5) while no Li 6708 A feature
is detected from the primary (~M4). The physical companionship of the two stars
is confirmed from common proper motions. Current theoretical pre-main sequence
evolutionary models cannot simultaneously match the observed colors,
brightnesses, and Li depletion patterns of this binary system. At the age upper
limit of 20 Myr, contemporary theoretical evolutionary models predict too slow
Li depletion. If true Li depletion is a faster process than predicted by
theoretical models, ages of open clusters (Pleiades, alpha Persei, and IC 2391)
estimated from the Li depletion boundary method are all overestimated. Because
of the importance of the open cluster age scale, development of self-consistent
theoretical models to match the HIP 112312 data is desirable.Comment: Accepted in ApJL. 5 pages total (3 tables, 3 figures
Towards a working model for the abundance variations within Globular Clusters stars
A popular self--enrichment scenario for the formation of globular clusters
assumes that the abundance anomalies shown by the stars in many clusters are
due to a second stage of star formation occurring from the matter lost by the
winds of massive asymptotic giant branch (AGB) stars. Until today, the
modellizations of the AGB evolution by several different groups failed, for
different reasons, to account for the patterns of chemical anomalies. Here we
show that our own modelling can provide a consistent picture if we constrain
the three main parameters which regulate AGB evolution: 1) adopting a high
efficiency convection model; 2) adopting rates of mass loss with a high
dependence on the stellar luminosity; 3) assuming a very small overshooting
below the formal convective regions during the thermal pulse (TP) phase. The
first assumption is needed to obtain an efficient oxygen depletion in the AGB
envelopes, and the second one is needed to lose the whole stellar envelope
within few thermal pulses, so that the sum of CNO elements does not increase
too much, consistently with the observations. The third assumption is needed to
fully understand the sodium production. We also show that the Mg - Al
anticorrelation is explained adopting the higher limit of the NACRE rates for
proton captures by Mg25 and Mg26, and the models are consistent with the
recently discovered F-Al correlation. Problems remain to fully explain the
observed Mg isotopes ratios
A Double Main Sequence in the Globular Cluster NGC 6397
High-precision multi-band HST photometry reveals that the main sequence (MS)
of the globular cluster NGC 6397 splits into two components, containing ~30%
and ~70% of the stars. This double sequence is consistent with the idea that
the cluster hosts two stellar populations: (i) a primordial population that has
a composition similar to field stars, and containing ~30% of the stars, and
(ii) a second generation with enhanced sodium and nitrogen, depleted carbon and
oxygen, and a slightly enhanced helium abundance (Delta Y~0.01). We examine the
color difference between the two sequences across a variety of color baselines
and find that the second sequence is anomalously faint in m_F336W. Theoretical
isochrones indicate that this could be due to NH depletion.Comment: 19 pages, 11 figures, accepted for pubblication in Ap
A helium spread among the main sequence stars in NGC 2808
We studied the color distribution of the main sequence of the Globular
Cluster NGC 2808, based on new deep HST-WFPC2 photometry of a field in the
uncrowded outskirts of the cluster. The color distribution of main sequence
stars is wider than expected for a single stellar population, given our
(carefully determined) measurement errors. About 20% of the sample stars are
much bluer than expected and are most plausibly explained as a population
having a much larger helium abundance than the bulk of the main sequence. We
estimate that the helium mass fraction of these stars is Y ~ 0.4. NGC 2808 may
have suffered self-enrichment, with different stellar populations born from the
ejecta of the intermediate mass asymptotic giant branch (AGB) stars of the
first generation. In addition to the Y=0.40 stars, roughly 30% of the stars
should have Y distributed between 0.26-0.29 while 50% have primordial Y, to
explain also the peculiar horizontal branch morphology. Three main stages of
star formation are identified, the first with primordial helium content Y ~
0.24, the second one born from the winds of the most massive AGBs of the first
stellar generation (6-7msun), having Y ~ 0.4, and a third one born from the
matter ejected from less massive AGBs (~ 3.5-4.5msun) with Y ~ 0.26-0.29. For a
long hiatus of time (several 10^7yr) between the second and third generation,
star formation might have been inhibited by the explosion of late Supernovae II
deriving from binary evolution.Comment: accepted for publication on "The Astrophysical Journal
Light Element Production in the Circumstellar Matter of Energetic Type Ic Supernovae
We investigate energetic type Ic supernovae as production sites for Li6 and
Be in the early stages of the Milky Way. Recent observations have revealed that
some very metal-poor stars with [Fe/H]<-2.5 possess unexpectedly high
abundances of Li6. Some also exbihit enhanced abundances of Be as well as N.
From a theoretical point of view, recent studies of the evolution of
metal-poor massive stars show that rotation-induced mixing can enrich the outer
H and He layers with C, N, and O (CNO) elements, particularly N, and at the
same time cause intense mass loss of these layers. Here we consider energetic
supernova explosions occurring after the progeniter star has lost all but a
small fraction of the He layer. The fastest portion of the supernova ejecta can
interact directly with the circumstellar matter (CSM), both composed of He and
CNO, and induce light element production through spallation and He-He fusion
reactions. The CSM should be sufficiently thick to energetic particles so that
the interactions terminate within its innermost regions. We calculate the
resulting Li6/O and Be9/O ratios in the ejecta+CSM material out of which the
very metal-poor stars may form. We find that they are consistent with the
observed values if the mass of the He layer remaining on the pre-explosion core
is 0.01-0.1 solar mass, and the mass fraction of N mixed in the He layer is
about 0.01. Further observations of Li6, Be and N at low metallicity should
provide critical tests of this production scenario.Comment: 12 pages, 2 figures, revised with referee suggestions, final version
accepted in ApJ Letter
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
Timing an Accreting Millisecond Pulsar: Measuring the Accretion Torque in IGR J00291+5934
We present here a timing analysis of the fastest accreting millisecond pulsar
IGR J00291+5934 using RXTE data taken during the outburst of December 2004. We
corrected the arrival times of all the events for the orbital (Doppler) effects
and performed a timing analysis of the resulting phase delays. In this way we
find a clear parabolic trend of the pulse phase delays showing that the pulsar
is spinning up as a consequence of accretion torques during the X-ray outburst.
The accretion torque gives us for the first time an independent estimate of the
mass accretion rate onto the neutron star, which can be compared with the
observed X-ray luminosity. We also report a revised value of the spin period of
the pulsar.Comment: Proceedings of the Frascati Workshop 2005: Multifrequency Behaviour
of High Energy Cosmic Sources, Vulcano, May 23-28. 7 pages including 1 figur
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