42 research outputs found
Discovery of a compact companion to the hot subdwarf star BD+37 442
We report the results of the first X-ray observation of the luminous and
helium-rich O-type subdwarf BD+37 442, carried out with the XMM-Newton
satellite in August 2011. X-ray emission is detected with a flux of about
3x10^(-14) erg/cm2/s (0.2-1 keV) and a very soft spectrum, well fit by the sum
of a blackbody with temperature kT_BB = 45^(+11)_(-9) eV and a power law with a
poorly constrained photon index. Significant pulsations with a period of 19.2 s
are detected, indicating that the X-ray emission originates in a white dwarf or
neutron star companion, most likely powered by accretion from the wind of BD+37
442.Comment: 5 pages, 4 figures, 1 table. Accepted for publication by The
Astrophysical Journal Letter
On the usefulness of finding charts Or the runaway carbon stars of the Blanco & McCarthy field 37
We have been recently faced with the problem of cross--identifying stars
recorded in historical catalogues with those extracted from recent fully
digitized surveys (such as DENIS and 2MASS). Positions mentioned in the old
catalogues are frequently of poor precision, but are generally accompanied by
finding charts where the interesting objects are flagged. Those finding charts
are sometimes our only link with the accumulated knowledge of past literature.
While checking the identification of some of these objects in several
catalogues, we had the surprise to discover a number of discrepancies in recent
works.The main reason for these discrepancies was generally the blind
application of the smallest difference in position as the criterion to identify
sources from one historical catalogue to those in more recent surveys. In this
paper we give examples of such misidentifications, and show how we were able to
find and correct them.We present modern procedures to discover and solve
cross--identification problems, such as loading digitized images of the sky
through the Aladin service at CDS, and overlaying entries from historical
catalogues and modern surveys. We conclude that the use of good finding charts
still remains the ultimate (though time--consuming) tool to ascertain
cross--identifications in difficult cases.Comment: 4 pages, 1 figure, accepted by A&
Spatial distribution of stellar populations in the Magellanic Clouds: Implementation to Gaia
The main goal of our project is to investigate the spatial distribution of
different stellar populations in the Magellanic Clouds. The results from
modelling the Magellanic Clouds can be useful, among others, for simulations
during the Gaia mission preparation. Isodensity contour maps have been used in
order to trace the morphology of the different stellar populations and estimate
the size of these structures. Moreover, star density maps are constructed
through star counts and projected radial density profiles are obtained. Fitting
exponential disk and King law curves to the spatial distribution allows us to
derive the structural parameters that describe these profiles. The
morphological structure and spatial distributions of various stellar components
in the Magellanic Clouds (young and intermediate age stars, carbon stars) along
with the overall spatial distribution in both Clouds are provided.Comment: 12 pages, 9 figures, to be published in Astronomy & Astrophysics;
typos and language correcte
Luminous AGB stars in nearby galaxies. A study using Virtual Observatory tools
Aims. This study focuses on very luminous Mbol<-6.0 mag AGB stars with
J-Ks>1.5 mag and H-Ks>0.4 mag in the LMC, SMC, M31, and M33 from 2MASS data.
Methods.The data were taken from the 2MASS All-Sky Point Source catalogue
archive. We used Virtual Observatory tools and took advantage of its
capabilities at various stages in the analysis.
Results. It is well known that stars with the colors we selected correspond
mainly to carbon stars. Although the most luminous AGBs detected here contain a
large number of carbon stars,they are not included in existing catalogues
produced from data in the optical domain, where they are not visible since they
are dust-enshrouded. A comparison of the AGB stars detected with combined near
and mid-infrared data from MSX and 2MASS in the LMC shows that 10% of the
bright AGB stars are bright carbon stars never detected before and that the
other 50% are OH/IR oxygen rich stars, whereas the 40% that remain were not
cross-matched.
Conclusions. The catalogues of the most luminous AGB stars compiled here are
an important complement to existing data. In the LMC, these bright AGB stars
are centrally located, whereas they are concentrated in an active
star-formation ring in M31. In the SMC and M33, there are not enough of them to
draw definite conclusions, although they tend to be centrally located. Their
luminosity functions are similar for the four galaxies we studied.Comment: 16 pages, 12 figures, 4 tables (Appendix A), accepted in A&
Modelling of intermediate-age stellar populations: III Effects of dust-shells around AGB stars
In this paper,we present single stellar population models of intermediate age
stellar populations where dust-enshrouded AGB stars are introduced. The
formation of carbon stars is also accounted for, and is taken to be a function
of both initial mass and metallicity. The effect of the dusty envelopes around
AGB stars on the optical/near-infrared spectral energy distribution were
introduced using semi-emipirical models where the mass-loss and the
photospheric chemistry determine the spectral properties of a star along the
AGB sequence. The spectral dichotomy between O-rich stars and C-rich stars is
taken into account in the modelling. We have investigated the AGB sequence
morphology in he near-infrared CMD as a function of time and metallicity. We
show that this diaggram is characterized by three morphological features,
occupied by optically bright O-rich stars, optically bright C-rich stars, and
dust-enshrouded O-rich and C-rich stars respectively. Our models are able to
reproduce the distribution of the three AGB subtype stellar populations in
colour-colour diagrams. Effects of dusty envelopes on the luminosity function
are also investigated (Abriged).Comment: 11 pages, 9 figures, accepted for publication in A&
Three aspects of red giant studies in the Magellanic Clouds
There are three important aspects concerning the study of the red giant and
in particular of the asymptotic giant branch (AGB) stars in the Magellanic
Clouds. These are: the surface distribution, the luminosity function and the
variability. The spatial distribution of AGB stars is an efficient tool to
study the structure of the galaxies and their metalicity by analysing the ratio
between carbon- and oxygen-rich AGB stars. The shape of the luminosity function
carries informations about the star formation rate in the Clouds and it can be
mathematically related to their history. Most AGB stars vary their magnitude in
a few to several hundred years time; the one epoch DENIS magnitudes for both
Large and Small Magellanic Cloud AGB stars outline the same relations as a
function of period.Comment: 8 pages, 6 figures, invited talk, to be published in: Mass-Losing
Pulsating Stars and their Circumstellar Matter, Y. Nakada & M. Honma (eds)
Kluwer ASSL serie
ROSAT X-ray sources in the field of the LMC. II.Statistics of background AGN and X-ray binaries
About 200 X-ray sources from a sample of spectrally hard ROSAT PSPC sources,
given in the catalog of Haberl & Pietsch (1999), and observed in a ~60 square
degree field of the LMC during several archival pointed observations with a
wide range of exposure times have been reanalyzed. For these sources accurate
count rates and hardness ratios have been recalculated. In comparison to Haberl
& Pietsch (1999) we used merged data from all available observations and we
derived average source parameters by investigating each source individually.
From a simulation powerlaw spectral tracks have been derived in the HR1 - HR2
plane and ~170 sources have been classified as background X-ray sources or as
LMC X-ray binaries. 80% of the spectrally hard X-ray sources with more than 50
observed counts have been found to be consistent with background X-ray sources
and 20% with LMC X-ray binaries (53 sources with AGN and 15 with X-ray
binaries). The discovery of a new supersoft source RX J0529.4-6713 at the
southern HI boundary of the supergiant shell LMC4 is reported. We find two new
candidate X-ray binary systems which are associated with the optical bar of the
LMC and additional candidate X-ray binaries which are associated with
supergiant shells.Comment: 13 pages, accepted for publication in A&A, March 22 200
Evolution, nucleosynthesis and yields of low mass AGB stars at different metallicities (II): the FRUITY database
By using updated stellar low mass stars models, we can systematically
investigate the nucleosynthesis processes occurring in AGB stars, when these
objects experience recurrent thermal pulses and third dredge-up episodes. In
this paper we present the database dedicated to the nucleosynthesis of AGB
stars: the FRUITY (FRANEC Repository of Updated Isotopic Tables & Yields)
database. An interactive web-based interface allows users to freely download
the full (from H to Bi) isotopic composition, as it changes after each third
dredge-up episode and the stellar yields the models produce. A first set of AGB
models, having masses in the range 1.5 < M/Msun < 3.0 and metallicities 1e-3 <
Z < 2e-2, is discussed here. For each model, a detailed description of the
physical and the chemical evolution is provided. In particular, we illustrate
the details of the s-process and we evaluate the theoretical uncertainties due
to the parametrization adopted to model convection and mass loss. The resulting
nucleosynthesis scenario is checked by comparing the theoretical [hs/ls] and
[Pb/hs] ratios to those obtained from the available abundance analysis of
s-enhanced stars. On the average, the variation with the metallicity of these
spectroscopic indexes is well reproduced by theoretical models, although the
predicted spread at a given metallicity is substantially smaller than the
observed one. Possible explanations for such a difference are briefly
discussed. An independent check of the third dredge-up efficiency is provided
by the C-stars luminosity function. Consequently, theoretical C-stars
luminosity functions for the Galactic disk and the Magellanic Clouds have been
derived. We generally find a good agreement with observations.Comment: Accepted for Publication on The Astrophysical Journal Supplement
Oxygen- and carbon-rich variable red giant populations in the Magellanic Clouds from EROS, OGLE, MACHO, and 2MASS photometry
The carbon-to-oxygen (C/O) ratio of asymptotic giant branch (AGB) stars
constitutes an important index of evolutionary and environment/metallicity
factor. We develop a method for mass C/O classification of AGBs in photometric
surveys without using periods. For this purpose we rely on the slopes in the
tracks of individual stars in the colour-magnitude diagram. We demonstrate that
our method enables the separation of C-rich and O-rich AGB stars with little
confusion. For the Magellanic Clouds we demonstrate that this method works for
several photometric surveys and filter combinations. As we rely on no period
identification, our results are relatively insensitive to the phase coverage,
aliasing, and time-sampling problems that plague period analyses. For a
subsample of our stars, we verify our C/O classification against published C/O
catalogues. With our method we are able to produce C/O maps of the entire
Magellanic Clouds. Our purely photometric method for classification of C- and
O-rich AGBs constitutes a method of choice for large, near-infrared photometric
surveys. Because our method depends on the slope of colour-magnitude variation
but not on magnitude zero point, it remains applicable to objects with unknown
distances.Comment: 14 pages, 16 figures, 1 table, accepted for publication in Astronomy
& Astrophysic
Circumstellar masers in the Magellanic Clouds
We have searched for 22 GHz H2O and 43/86/129 GHz SiO masers in bright IRAS
point sources in the SMC and LMC, to test whether the kinematics of the mass
loss from these stars depends on metallicity. H2O masers were detected in the
red supergiants IRAS04553-6825 and IRAS05280-6910, and tentatively in the
luminous IR object IRAS05216-6753 and the AGB star IRAS05329-6708. SiO masers
were detected in IRAS04553-6825. The outflow velocity increases between the H2O
masing zone near the dust-formation region and the more distant OH masing zone
from 18 to 26 km/s for IRAS04553-6825 and from 6 to 17 km/s for IRAS05280-6910.
The total sample of LMC targets is analysed in comparison with circumstellar
masers in the Galactic Centre. The photon fluxes of circumstellar masers in the
LMC are very similar to those in the Galactic Centre. The expansion velocities
in the LMC appear to be 20% lower than for similarly bright OH masers in the
Galactic Centre, but the data are consistent with no difference in expansion
velocity. OH/IR stars in the LMC appear to have slower accelerating envelopes
than OH/IR stars in the Galactic Centre. Masers in the LMC have blue-asymmetric
emission profiles. This may be due to the amplification of stellar and/or
free-free radiation, rather than the amplification of dust emission, and may be
more pronounced in low metallicity envelopes. SiO maser strength increases with
the photometric amplitude at 2.2 micron but is independent of the photometric
amplitude at 10 micron. This suggests a strong connection between shocks in the
dust-free SiO masing zone and the dust formation process. Appendices describe
H2O maser emission from R Dor in the Milky Way, optical echelle spectroscopy of
IRAS04553-6825, and the properties of masers in the Galactic Centre (Abridged).Comment: 19 pages, 17 figures. Accepted for publication in Astronomy &
Astrophysics Main Journa