299 research outputs found
Study of extremely reddened AGB stars in the Galactic bulge
Context. Extremely reddened AGB stars lose mass at high rates of >10^-5
Msun/yr. This is the very last stage of AGB evolution, in which stars in the
mass range 2.0--4.0 Msun (for solar metallicity) should have been converted to
C stars already. The extremely reddened AGB stars in the Galactic bulge are
however predominantly O-rich, implying that they might be either low-mass stars
or stars at the upper end of the AGB mass range. Aims. To determine the mass
range of the most reddened AGB stars in the Galactic bulge. Methods. Using
Virtual Observatory tools, we constructed spectral energy distributions of a
sample of 37 evolved stars in the Galactic bulge with extremely red IRAS
colours. We fitted DUSTY models to the observational data to infer the
bolometric fluxes. Applying individual corrections for interstellar extinction
and adopting a common distance, we determined luminosities and mass-loss rates,
and inferred the progenitor mass range from comparisons with AGB evolutionary
models. Results. The observed spectral energy distributions are consistent with
a classification as reddened AGB stars, except for two stars, which are
proto-planetary nebula candidates. For the AGB stars, we found luminosities in
the range 3000--30,000 Lsun and mass-loss rates 10^-5--3x10^-4 Msun/yr. The
corresponding mass range is 1.1--6.0 Msun assuming solar metallicity.
Conclusions. Contrary to the predictions of the evolutionary models, the
luminosity distribution is continuous, with many O-rich AGB stars in the mass
range in which they should have been converted into C stars already. We suspect
that bulge AGB stars have higher than solar metallicity and therefore may avoid
the conversion to C-rich. The presence of low-mass stars in the sample shows
that their termination of the AGB evolution also occurs during a final phase of
very high mass-loss rate, leading to optically thick circumstellar shells
An infrared study of galactic OH/IR stars. I. An optical/near-IR atlas of the Arecibo sample
In this paper we present optical and near-infrared finding charts, accurate
astrometry (~1") and single-epoch near-infrared photometry for 371 IRAS
sources, 96% of those included in the so-called Arecibo sample of OH/IR stars
(Eder et al. 1988; Lewis et al. 1990a; Chengalur et al. 1993). The main
photometric properties of the stars in the sample are presented and discussed
as well as the problems found during the process of identification of the
optical/near-infrared counterparts. In addition, we also identify suitable
reference stars in each field to be used for differential photometry purposes
in the future.
We find that 39% of the sources (144 in number) have no optical counterpart,
8 of them being invisible even at near infrared wavelengths. The relative
distribution of sources with and without optical counterpart in the IRAS
two-colour diagram and their characteristic near infrared colours are
interpreted as the consequence of the increasing thickness of their
circumstellar shells. Among the objects not detected at near infrared
wavelengths four non-variable sources are proposed to be heavily obscured
post-AGB stars which have just very recently left the AGB. Eight additional
objects with unusually bright and/or blue near-infrared colours are identified
as candidate post-AGB stars and/or proto-planetary nebulae.Comment: 28 pages, 9 figures, for associated finding charts see:
http://www.edpsciences.org/articles/aa/full/2005/08/aa1709/FINDING_CHARTS/are
cibo_index.htm
A catalog of wide binary and multiple systems of bright stars from Gaia-DR2 and the Virtual Observatory
Binary and multiple stars have long provided an effective empirical method of
testing stellar formation and evolution theories. In particular, the existence
of wide binary systems (separations 20,000 au) is particularly challenging
to binary formation models as their physical separations are beyond the typical
size of a collapsing cloud core (5,00010,000 au). We mined the
recently published Gaia-DR2 catalog to identify bright comoving systems in the
five-dimensional space (sky position, parallax, and proper motion). We
identified 3,741 comoving binary and multiple stellar candidate systems, out of
which 575 have compatible RVs for all the members of the system. The candidate
systems have separations between 400 and 500,000 au. We used the analysis
tools of the Virtual Observatory to characterize the comoving system members
and to assess their reliability. The comparison with previous comoving systems
catalogs obtained from TGAS showed that these catalogs contain a large number
of false systems. In addition, we were not able to confirm the ultra-wide
binary population presented in these catalogs. The robustness of our
methodology is demonstrated by the identification of well known comoving star
clusters and by the low contamination rate for comoving binary systems with
projected physical separations 50,000 au. These last constitute a reliable
sample for further studies. The catalog is available online at the Spanish
Virtual Observatory portal
(http://svo2.cab.inta-csic.es/vocats/v2/comovingGaiaDR2/).Comment: Accepted in A
On the evolutionary connection between AGB stars and PNe
The `O-rich AGB sequence' is a sequence of colours describing the location of
O-rich AGB stars in the IRAS two-colour diagram [12]--[25] vs [25]--[60]. We
propose an evolutionary scenario for this sequence in which all stars,
independent of their progenitor mass, start the AGB phase in the blue part of
the `O-rich AGB sequence' and then evolve toward redder colors, although only
the more massive stars would reach the very end of the `O-rich AGB sequence'.
The sources located in the blue part of the sequence are mainly Mira variables,
whose mean period is increasing with the IRAS colours. Most of them will evolve
into O-rich Type II (and III) Planetary Nebulae. Part of the stars located in
the red part of the sequence will change their chemical composition from O-rich
to C-rich during their evolution in the AGB phase, and might evolve into C-rich
Type II Planetary Nebulae. Hot bottom burning may prevent the conversion to
carbon stars of the rest of sources located in the red part of the sequence and
they will end up as N-rich Type I Planetary Nebulae.Comment: 4 pages, 6 figures, proceedings of the conference 'Planetary Nebulae
as Astrophysical Tools', held in Gdansk, Poland (June 28 - July 2, 2005
A white dwarf catalogue from Gaia-DR2 and the Virtual Observatory
We present a catalogue of 73Âż221 white dwarf candidates extracted from the astrometric and photometric data of the recently published Gaia-DR2 catalogue. White dwarfs were selected from the Gaia HertzsprungâRussell diagram with the aid of the most updated population synthesis simulator. Our analysis shows that Gaia has virtually identified all white dwarfs within 100Âżpc from the Sun. Hence, our sub-population of 8555 white dwarfs within this distance limit and the colour range considered, -0.52<(GBP-GRP)<0.80Âż, is the largest and most complete volume-limited sample of such objects to date. From this sub-sample, we identified 8343 CO-core and 212 ONe-core white dwarf candidates and derived a white dwarf space density of 4.9±0.4Ă10-3pc-3Âż. A bifurcation in the HertzsprungâRussell diagram for these sources, which our models do not predict, is clearly visible. We used the Virtual Observatory SED Analyzer tool to derive effective temperatures and luminosities for our sources by fitting their spectral energy distributions, that we built from the ultraviolet to the near-infrared using publicly available photometry through the Virtual Observatory. From these parameters, we derived the white dwarf radii. Interpolating the radii and effective temperatures in hydrogen-rich white dwarf cooling sequences, we derived the surface gravities and masses. The Gaia 100Âżpc white dwarf population is clearly dominated by cool (~8000ÂżK) objects and reveals a significant population of massive (ÂżM~0.8M¿¿) white dwarfs, of which no more than ~30--40 per cent can be attributed to hydrogen-deficient atmospheres, and whose origin remains uncertain.Peer ReviewedPreprin
Nebular emission lines in IRAS 17347-3139
We report the detection of nebular emission lines in the optical and
mid-infrared spectra of IRAS 17347-3139, a heavily obscured OH/IR star which
may be rapidly evolving from the AGB to the PN stage. The presence of emission
lines is interpreted as a clear indication that the ionization of its
circumstellar envelope has already started. This source belongs to the rare
class of objects known as `OHPNe' displaying both OH maser and radio continuum
emission. However, unlike the rest of stars in this class, prominent C-rich
dust features are detected in its mid-infrared spectrum, which makes the
analysis of this star particularly interesting.Comment: 2 pages, 2 figures, Proc. IAU Symp. 234, Planetary Nebulae in Our
Galaxy and Beyond (3-7 Apr 2006), eds. M.J. Barlow & R.H. Mendez (Cambridge
Univ. Press
Random Forest identification of the thin disk, thick disk and halo Gaia-DR2 white dwarf population
Gaia-DR2 has provided an unprecedented number of white dwarf candidates of
our Galaxy. In particular, it is estimated that Gaia-DR2 has observed nearly
400,000 of these objects and close to 18,000 up to 100 pc from the Sun. This
large quantity of data requires a thorough analysis in order to uncover their
main Galactic population properties, in particular the thin and thick disk and
halo components. Taking advantage of recent developments in artificial
intelligence techniques, we make use of a detailed Random Forest algorithm to
analyse an 8-dimensional space (equatorial coordinates, parallax, proper motion
components and photometric magnitudes) of accurate data provided by Gaia-DR2
within 100 pc from the Sun. With the aid of a thorough and robust population
synthesis code we simulated the different components of the Galactic white
dwarf population to optimize the information extracted from the algorithm for
disentangling the different population components. The algorithm is first
tested in a known simulated sample achieving an accuracy of 85.3%. Our
methodology is thoroughly compared to standard methods based on kinematic
criteria demonstrating that our algorithm substantially improves previous
approaches. Once trained, the algorithm is then applied to the Gaia-DR2 100 pc
white dwarf sample, identifying 12,227 thin disk, 1,410 thick disk and 95 halo
white dwarf candidates, which represent a proportion of 74:25:1, respectively.
Hence, the numerical spatial densities are , and
for the thin disk, thick disk and
halo components, respectively. The populations thus obtained represent the most
complete and volume-limited samples to date of the different components of the
Galactic white dwarf population.Comment: 18 pages, 11 figures and 3 tables. Accepted for publication in MNRA
Testing the chemical tagging technique with open clusters
Context. Stars are born together from giant molecular clouds and, if we
assume that the priors were chemically homogeneous and well-mixed, we expect
them to share the same chemical composition. Most of the stellar aggregates are
disrupted while orbiting the Galaxy and most of the dynamic information is
lost, thus the only possibility of reconstructing the stellar formation history
is to analyze the chemical abundances that we observe today.
Aims. The chemical tagging technique aims to recover disrupted stellar
clusters based merely on their chemical composition. We evaluate the viability
of this technique to recover co-natal stars that are no longer gravitationally
bound.
Methods. Open clusters are co-natal aggregates that have managed to survive
together. We compiled stellar spectra from 31 old and intermediate-age open
clusters, homogeneously derived atmospheric parameters, and 17 abundance
species, and applied machine learning algorithms to group the stars based on
their chemical composition. This approach allows us to evaluate the viability
and efficiency of the chemical tagging technique.
Results. We found that stars at different evolutionary stages have distinct
chemical patterns that may be due to NLTE effects, atomic diffusion, mixing,
and biases. When separating stars into dwarfs and giants, we observed that a
few open clusters show distinct chemical signatures while the majority show a
high degree of overlap. This limits the recovery of co-natal aggregates by
applying the chemical tagging technique. Nevertheless, there is room for
improvement if more elements are included and models are improved.Comment: accepted for publication in Astronomy and Astrophysics. Corrected
typo
Near-IR variability properties of a selected sample of AGB stars
We present the results of a near-infrared monitoring programme of a selected
sample of stars, initially suspected to be Mira variables and OH/IR stars,
covering more than a decade of observations. The objects monitored cover the
typical range of IRAS colours shown by O-rich stars on the Asymptotic Giant
Branch and show a surprisingly large diversity of variability properties. 16
objects are confirmed as large-amplitude variables. Periods between 360 and
1800 days and typical amplitudes from 1 to 2 magnitudes could be determined for
nine of them. In three light curves we find a systematic decrease of the mean
brightness, two light curves show pronounced asymmetry. One source, IRAS
07222-2005, shows infrared colours typical of Mira variables but pulsates with
a much longer period (approx. 1200 days) than a normal Mira. Two objects are
ither close to (IRAS 03293+6010) or probably in (IRAS 18299-1705) the post-AGB
phase. In IRAS 16029-3041 we found a systematic increase of the H-K colour of
approximately 1 magnitude, which we interpret as evidence of a recent episode
of enhanced mass loss. IRAS 18576+0341, a heavily obscured Luminous Blue
Variable was also monitored. The star showed a continued decrease of brightness
over a period of 7 years (1995 - 2002).Comment: 9 pages + 3 appendix, 36 figures, photometry table, accepted in
Astronomy & Astrophysic
Infrared-excess white dwarfs in the Gaia 100 pc sample
We analyse the 100Âżpc Gaia white dwarf volume-limited sample by means of VOSA (Virtual Observatory SED Analyser) with the aim of identifying candidates for displaying infrared excesses. Our search focuses on the study of the spectral energy distribution (SED) of 3733 white dwarfs with reliable infrared photometry and GBP - GRP colours below 0.8 mag, a sample that seems to be nearly representative of the overall white dwarf population. Our search results in 77 selected candidates, 52 of which are new identifications. For each target, we apply a two-component SED fitting implemented in VOSA to derive the effective temperatures of both the white dwarf and the object causing the excess. We calculate a fraction of infrared-excess white dwarfs due to the presence of a circumstellar disc of 1.6 ± 0.2 perÂżcent, a value that increases to 2.6 ± 0.3 perÂżcent if we take into account incompleteness issues. Our results are in agreement with the drop in the percentage of infrared excess detections for cool (20Âż000ÂżK) white dwarfs obtained in previous analyses. The fraction of white dwarfs with brown dwarf companions we derive is Âż0.1â0.2 perÂżcent.Peer ReviewedPostprint (author's final draft
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