235 research outputs found
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
Different stellar rotation in the two main sequences of the young globular cluster NGC1818: first direct spectroscopic evidence
We present a spectroscopic analysis of main sequence (MS) stars in the young
globular cluster NGC1818 (age~40 Myrs) in the Large Magellanic Cloud. Our
photometric survey on Magellanic Clouds clusters has revealed that NGC1818,
similarly to the other young objects with age 600 Myrs, displays not only an
extended MS Turn-Off (eMSTO), as observed in intermediate-age clusters (age~1-2
Gyrs), but also a split MS. The most straightforward interpretation of the
double MS is the presence of two stellar populations: a sequence of
slowly-rotating stars lying on the blue-MS and a sequence of fast rotators,
with rotation close to the breaking speed, defining a red-MS. We report the
first direct spectroscopic measurements of projected rotational velocities
vsini for the double MS, eMSTO and Be stars of a young cluster. The analysis of
line profiles includes non-LTE effects, required for correctly deriving v sini
values. Our results suggest that: (i) the mean rotation for blue- and red-MS
stars is vsini=71\pm10 km/s (sigma=37 km/s) and vsini=202\pm23 km/s (sigma=91
km/s), respectively; (ii) eMSTO stars have different vsini, which are generally
lower than those inferred for red-MS stars, and (iii) as expected, Be stars
display the highest vsini values. This analyis supports the idea that distinct
rotational velocities play an important role in the appearence of multiple
stellar populations in the color-magnitude diagrams of young clusters, and
poses new constraints to the current scenarios.Comment: 16 pages, 1 table, 9 figures. Accepted for publication in AJ
(11/07/2018
Non-gray rotating stellar models and the evolutionary history of the Orion Nebular Cluster
Rotational evolution in the pre-main sequence (PMS) is described with new
sets of PMS evolutionary tracks including rotation, non-gray boundary
conditions (BCs) and either low (LCE) or high convection efficiency (HCE).
Using observational data and our theoretical predictions, we aim at
constraining 1) the differences obtained for the rotational evolution of stars
within the ONC by means of these different sets of models; 2) the initial
angular momentum of low mass stars, by means of their templates in the ONC. We
discuss the reliability of current stellar models for the PMS. While the 2D
radiation hydrodynamic simulations predict HCE in PMS, semi-empirical
calibrations either seem to require that convection is less efficient in PMS
than in the following MS phase or are still contradictory. We derive stellar
masses and ages for the ONC by using both LCE and HCE. The resulting mass
distribution for the bulk of the ONC population is in the range 0.20.3
{\msun} for our non-gray models and in the range 0.10.3{\msun} for models
having gray BCs. In agreement with Herbst et al. (2002) we find that a large
percentage (70%) of low-mass stars (M\simlt 0.5{\msun} for LCE;
M\simlt0.35{\msun} for HCE) in the ONC appears to be fast rotators (P4days).
Three possibilities are open: 1) 70% of the ONC low mass stars lose their
disk at early evolutionary phases; 2)their locking period is shorter; 3) the
period evolution is linked to a different morphology of the magnetic fields of
the two groups of stars. We also estimate the range of initial angular momentum
consistent with the observed periods. The comparisons made indicate that a
second parameter is needed to describe convection in the PMS, possibly related
to the structural effect of a dynamo magnetic field.Comment: 17 pages, 11 figure
The near-IR counterpart of IGR J17480-2446 in Terzan 5
Some globular clusters in our Galaxy are noticeably rich in low-mass X-ray
binaries. Terzan 5 has the richest population among globular clusters of X- and
radio-pulsars and low-mass X-ray binaries. The detection and study of
optical/IR counterparts of low-mass X-ray binaries is fundamental to
characterizing both the low-mass donor in the binary system and investigating
the mechanisms of the formation and evolution of this class of objects. We aim
at identifying the near-IR counterpart of the 11 Hz pulsar IGRJ17480-2446
discovered in Terzan 5. Adaptive optics (AO) systems represent the only
possibility for studying the very dense environment of GC cores from the
ground. We carried out observations of the core of Terzan 5 in the near-IR
bands with the ESO-VLT NAOS-CONICA instrument. We present the discovery of the
likely counterpart in the Ks band and discuss its properties both in outburst
and in quiescence. Archival HST observations are used to extend our discussion
to the optical bands. The source is located at the blue edge of the turn-off
area in the color-magnitude diagram of the cluster. Its luminosity increase
from quiescence to outburst, by a factor 2.5, allows us to discuss the nature
of the donor star in the context of the double stellar generation population of
Terzan 5 by using recent stellar evolution models.Comment: 7 pages, 4 figure
To accrete or not to accrete: the dilemma of the recycling scenario
We study the evolution of a low-mass X-ray binary by coupling a binary
stellar evolution code with a general relativistic code that describes the
behaviour of the neutron star. We find that non-conservative mass transfer
scenarios are required to prevent the formation of submillisecond pulsars
and/or the collapse to a black hole. We discuss the sweeping effects of an
active magneto-dipole rotator on the transferred matter as a promising
mechanism to obtain highly non-conservative evolutions.Comment: 7 pages, including 2 figures. To appear in proceedings of Aspen
Center for Physics Conference on ``Binary Radio Pulsars'' Eds. F. Rasio and
I. Stair
On the Formation of Multiple Stellar Populations in Globular Clusters
Nearly all globular clusters (GCs) studied to date show evidence for multiple
stellar populations, in stark contrast to the conventional view that GCs are a
mono-metallic, coeval population of stars. Building on earlier work, we propose
a simple physical model for the early evolution (several 10^8 yr) of GCs. We
consider the effects of stellar mass-loss, type II and prompt type Ia
supernovae, ram pressure, and accretion from the ambient ISM on the development
of a young GC's own gas reservoir. In our model, type II SNe from a first
generation of star formation clears the GC of its initial gas reservoir. Over
the next several 10^8 yr, mass lost from AGB stars and matter accreted from the
ambient ISM collect at the center of the GC. This material must remain quite
cool (T~10^2K), but does not catastrophically cool on a crossing time because
of the high Lyman-Werner flux density in young GCs. The collection of gas
within the GC must compete with ram pressure from the ambient ISM. After
several 10^8 yr, the Lyman-Werner photon flux density drops by more than three
orders of magnitude, allowing molecular hydrogen and then stars to form. After
this second generation of star formation, type II SNe from the second
generation and then prompt type Ia SNe associated with the first generation
maintain a gas-free GC, thereby ending the cycle of star formation events. Our
model makes clear predictions for the presence or absence of multiple stellar
populations within GCs as a function of GC mass and formation environment.
Analyzing intermediate-age LMC clusters, we find evidence for a mass threshold
of ~10^4 Msun below which LMC clusters appear to be truly coeval. This
threshold mass is consistent with our predictions for the mass at which ram
pressure is capable of clearing gas from clusters in the LMC at the present
epoch. (ABRIDGED)Comment: 13 pages, 5 figures, ApJ in pres
Massive binaries and the enrichment of the interstellar medium in globular clusters
Abundance anomalies observed in globular cluster stars indicate pollution
with material processed by hydrogen burning. Two main sources have been
suggested: asymptotic giant branch stars and massive stars rotating near the
break-up limit. We discuss the potential of massive binaries as an interesting
alternative source of processed material.
We discuss observational evidence for mass shedding from interacting
binaries. In contrast to the fast, radiatively driven winds of massive stars,
this material is typically ejected with low velocity. We expect that it remains
inside the potential well of a globular cluster and becomes available for the
formation or pollution of a second generation of stars. We estimate that the
amount of processed low-velocity material that can be ejected by massive
binaries is larger than the contribution of two previously suggested sources
combined.Comment: 6 pages, 2 figures, to appear in the proceedings of IAU Symposium
266, "Star Clusters - Basic Galactic Building Blocks throughout Time and
Space", 10-14 August 2009, at the general assembly in Rio de Janeiro, Brazi
Land degradation assessment for sustainable soil management
Desertification is a complex phenomenon defined as the extreme degree of land degradation induced by human activities and climatic conditions. Climate change is accelerating and widening these areas. Previews analysis and studies assessed the vulnerability to desertification in Italy at national and regional level through a methodological approach based on integrating climate, soil, vege-tation, and socio-economic data (ESA). The studies carried out by ISPRA aim to provide an update of the of land degradation assessment in Italy, based on Trends.Earth methodology and of the three UN-SDGs sub-indicators on Target 15.3.1 (land use/land cover, land productivity and soil organic carbon above and below ground status and trends), together with additional dimensions of land degradation considered crucial for national land characters. Final assessment of the percentage of degraded land is around 36% of national area. This exercise demonstrates the importance to con-sider a larger number of data and include information on other fac-tors, such as climate, physical, chemical data. This integrated approach to the assessment of land degradation will allow to describe also of the loss of related ecosystem services
Debris Disks in NGC 2547
We have surveyed the 30 Myr-old cluster NGC 2547 for planetary debris disks
using Spitzer. At 4.5-8 um we are sensitive to the photospheric level down to
mid-M stars (0.2 Msol) and at 24 um to early-G stars (1.2 Msol). We find only
two to four stars with excesses at 8 um out of ~400-500 cluster members,
resulting in an excess fraction <~1 percent at this wavelength. By contrast,
the excess fraction at 24 um is ~40 percent (for B-F types). Out of four
late-type stars with excesses at 8 um two marginal ones are consistent with
asteroid-like debris disks. Among stars with strong 8 um excesses one is
possibly from a transitional disk, while another one can be a result of a
catastrophic collision. Our survey demonstrates that the inner 0.1-1 AU parts
of disks around solar-type stars clear out very thoroughly by 30 Myrs of age.
Comparing with the much slower decay of excesses at 24 and 70 um, disks clear
from the inside out, of order 10 Myr for the inner zones probed at 8 um
compared with a hundred or more Myr for those probed with the two longer
wavelengths.Comment: Accepted to ApJ, 29 pages, 13 figs. A Note in Proof concerning
cluster's age was added in the original submission of 2007 July 19. Full
Tables 1 and 2 in the electronic form together with the article with full
resolution figures are available at
http://www.astro.ufl.edu/~ngorlova/disksNGC2547
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