419 research outputs found
Age spreads in star forming regions?
Rotation periods and projected equatorial velocities of pre-main-sequence
(PMS) stars in star forming regions can be combined to give projected stellar
radii. Assuming random axial orientation, a Monte-Carlo model is used to
illustrate that distributions of projected stellar radii are very sensitive to
ages and age dispersions between 1 and 10 Myr which, unlike age estimates from
conventional Hertzsprung-Russell diagrams, are relatively immune to
uncertainties due to extinction, variability, distance etc. Application of the
technique to the Orion Nebula cluster reveals radius spreads of a factor of
2--3 (FWHM) at a given effective temperature. Modelling this dispersion as an
age spread suggests that PMS stars in the ONC have an age range larger than the
mean cluster age, that could be reasonably described by the age distribution
deduced from the Hertzsprung-Russell diagram. These radius/age spreads are
certainly large enough to invalidate the assumption of coevality when
considering the evolution of PMS properties (rotation, disks etc.) from one
young cluster to another.Comment: To appear in "The Ages of Stars", E.E. Mamajek, D.R. Soderblom,
R.F.G. Wyse (eds.), IAU Symposium 258, CU
Discovery of Par 1802 as a Low-Mass, Pre-Main-Sequence Eclipsing Binary in the Orion Star-Forming Region
We report the discovery of a pre-main-sequence, low-mass, double-lined,
spectroscopic, eclipsing binary in the Orion star-forming region. We present
our observations including radial velocities derived from optical
high-resolution spectroscopy, and present an orbit solution that permits the
determination of precise empirical masses for both components of the system. We
measure that Par 1802 is composed of two equal mass (0.39+-0.03, 0.40+-0.03
Msun) stars in a circular, 4.7 day orbit. There is strong evidence, such as the
system exhibiting strong Li lines and a center-of-mass velocity consistent with
cluster membership, that this system is a member of the Orion star-forming
region and quite possibly the Orion Nebula Cluster, and therefore has an age of
only a few million years. As there are currently only a few empirical mass and
radius measurements for low-mass, PMS stars, this system presents an
interesting test for the predictions of current theoretical models of pre-main
sequence stellar evolution.Comment: 21 pages, 6 figures, 2 tables; Fig 2 caption edite
The role of General Relativity in the evolution of Low Mass X-ray Binaries
We study the evolution of Low Mass X-ray Binaries (LMXBs) and of millisecond
binary radio pulsars (MSPs), with numerical simulations that keep into account
the evolution of the companion, of the binary system and of the neutron star.
According to general relativity, when energy is released, the system loses
gravitational mass. Moreover, the neutron star can collapse to a black hole if
its mass exceeds a critical limit, that depends on the equation of state. These
facts have some interesting consequences: 1) In a MSP the mass-energy is lost
with a specific angular momentum that is smaller than the one of the system,
resulting in a positive contribution to the orbital period derivative. If this
contribution is dominant and can be measured, we can extract information about
the moment of inertia of the neutron star, since the energy loss rate depends
on it. Such a measurement can therefore help to put constraints on the equation
of state of ultradense matter. 2) In LMXBs below the bifurcation period (\sim
18 h), the neutron star survives the period gap only if its mass is smaller
than the maximum non-rotating mass when the companion becomes fully convective
and accretion pauses. Therefore short period (P < 2h) millisecond X-ray pulsar
like SAX J1808.4-3658 can be formed only if either a large part of the
accreting matter has been ejected from the system, or the equation of state of
ultradense matter is very stiff. 3) In Low Mass X-ray binaries above the
bifurcation period, the mass-energy loss lowers the mass transfer rate. As side
effect, the inner core of the companion star becomes 1% bigger than in a system
with a non-collapsed primary. Due to this difference, the final orbital period
of the system becomes 20% larger than what is obtained if the mass-energy loss
effect is not taken into account.Comment: 7 pages, 3 figures, accepted by the MNRA
Why are massive O-rich AGB stars in our Galaxy not S-stars?
We present the main results derived from a chemical analysis carried out on a
large sample of galactic O-rich AGB stars using high resolution optical
spectroscopy (R~40,000-50,000) with the intention of studying their lithium
abundances and/or possible s-process element enrichment. Our chemical analysis
shows that some stars are lithium overabundant while others are not. The
observed lithium overabundances are interpreted as a clear signature of the
activation of the so-called ``Hot Bottom Burning'' (HBB) process in massive
galactic O-rich AGB stars, as predicted by the models. However, these stars do
not show the zirconium enhancement (taken as a representative for the s-process
element enrichment) associated to the third dredge-up phase following thermal
pulses. Our results suggest that the more massive O-rich AGB stars in our
Galaxy behave differently from those in the Magellanic Clouds, which are both
Li- and s-process-rich (S-type stars). Reasons for this unexpected result are
discussed. We conclude that metallicity is probably the main responsible for
the differences observed and suggest that it may play a more important role
than generally assumed in the chemical evolution of AGB stars.Comment: 4 pages, 2 figures, to appear in the proceedings of the conference
"Planetary Nebulae as astronomical tools" held in Gdansk, Poland, jun 28/jul
02, 200
The stellar origin of 7Li - Do AGB stars contribute a substantial fraction of the local Galactic lithium abundance?
We adopt up-to-date 7Li yields from asymptotic giant branch stars in order to study the temporal evolution of 7Li in the solar neighbourhood in the context of a revised version of the two-infall model for the chemical evolution of our galaxy. We consider several lithium stellar sources besides the asymptotic giant branch stars such as Type II supernovae, novae, low-mass giants as well as Galactic cosmic rays and low-mass X-ray binaries. We conclude that asymptotic giant branch stars cannot be considered as important 7Li producers as believed in so far and that the contribution of low-mass giants and novae is necessary to reproduce the steep rise of the 7Li abundance in disk stars as well as the meteoritic 7Li abundance. Lithium production in low-mass X-ray binaries hardly affects the temporal evolution of 7Li in the solar neighbourhood
Dissecting the Spitzer color-magnitude diagrams of extreme LMC AGB stars
We trace the full evolution of low- and intermediate-mass stars () during the Asymptotic Giant Branch (AGB) phase in the
{\it Spitzer} two-color and color-magnitude diagrams. We follow the formation
and growth of dust particles in the circumstellar envelope with an
isotropically expanding wind, in which gas molecules impinge upon pre--existing
seed nuclei, favour their growth. These models are the first able to identify
the main regions in the {\it Spitzer} data occupied by AGB stars in the Large
Magellanic Cloud (LMC). The main diagonal sequence traced by LMC extreme stars
in the [3.6]-[4.5] vs. [5.8]-[8.0] and [3.6]-[8.0] vs. [8.0] planes are nicely
fit by carbon stars models; it results to be an evolutionary sequence with the
reddest objects being at the final stages of their AGB evolution. The most
extreme stars, with [3.6]-[4.5] 1.5 and [3.6]-[8.0] 3, are 2.5-3
stars surrounded by solid carbon grains. In higher mass () models dust formation is driven by the extent of Hot Bottom Burning
(HBB) - most of the dust formed is in the form of silicates and the maximum
obscuration phase by dust particles occurs when the HBB experienced is
strongest, before the mass of the envelope is considerably reduced.Comment: 5 pages, 2 figures, accepted for publication in MNRAS Letter
Pre-main-sequence Lithium Depletion
In this review I briefly discuss the theory of pre-main-sequence (PMS) Li
depletion in low-mass (0.075<M<1.2 Msun) stars and highlight those uncertain
parameters which lead to substantial differences in model predictions. I then
summarise observations of PMS stars in very young open clusters, clusters that
have just reached the ZAMS and briefly highlight recent developments in the
observation of Li in very low-mass PMS stars.Comment: 8 pages, invited review at "Chemical abundances and mixing in stars
in the Milky Way and its satellites", eds. L. Pasquini, S. Randich. ESO
Astrophysics Symposium (Springer-Verlag
A WFC3/HST view of the three stellar populations in the Globular Cluster NGC6752
Multi-band Hubble Space Telescope photometry reveals that the main sequence,
sub-giant, and the red giant branch of the globular cluster NGC6752 splits into
three main components in close analogy with the three distinct segments along
its horizontal branch stars. These triple sequences are consistent with three
stellar groups: a stellar population with a chemical composition similar to
field halo stars (population a), a population (c) with enhanced sodium and
nitrogen, depleted carbon and oxygen and enhanced helium abundance (Delta Y
~0.03), and a population (b) with an intermediate (between population a and c)
chemical composition and slightly helium enhanced (Delta Y ~0.01). These
components contain ~25% (population a), ~45% (population b), and ~30%
(population c) of the stars. No radial gradient for the relative numbers of the
three populations has been identified out to about 2.5 half mass radii.Comment: 42 pages, 24 figures, accepted for publication in Ap
Strength training in elderly: An useful tool against sarcopenia
The loss of muscle mass and strength in elderly population (especially after the age of 65-70) represents a public health problem. Due to the high prevalence of frailty in older adults, cardiovascular or low-intensity exercise is implemented as first choice option. Although beneficial these training schemes are not as effective as strength-based resistance training for increasing muscle strength and hypertrophy. In fact, when performed progressively and under professional supervision, strength-based training has been proposed as an important and valid methodology to reduce sarcopenia-related problems. In this mini-review, we not only summarize the benefits of weight resistance training but also highlight practical recommendations and other non-conventional methods (e.g., suspension training) as part of an integral anti-sarcopenia strategy. Future directions including cluster set configurations and high-speed resistance training are also outlined
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