1,891 research outputs found
Stellar sources in the ISOGAL intermediate bulge fields
We present a study of ISOGAL sources in the "intermediate" galactic bulge
( 2, 1--4), observed by
ISOCAM at 7 and 15 . In combination with near-infrared (I, J, K) data of DENIS survey, complemented by 2MASS data, we discuss the nature of
the ISOGAL sources, their luminosities, the interstellar extinction and the
mass-loss rates. A large fraction of the 1464 detected sources at 15
are AGB stars above the RGB tip, a number of them show an excess in
([7]-[15]) and (K-[15]) colours, characteristic
of mass-loss. The latter, especially (K-[15]), provide
estimates of the mass-loss rates and show their distribution in the range
10 to 10 M/yr.Comment: 16 pages, accepted for publication in Astronomy and Astrophysic
Interstellar Extinction and Long-Period Variables in the Galactic Center
We use the Spitzer IRAC catalogue of the Galactic Center (GC) point sources
(Ramirez et al. 2008) and combine it with new isochrones (Marigo et al. 2008)
to derive extinctions based on photometry of red giants and asymptotic giant
branch (AGB) stars. This new extinction map extends to much higher values of Av
than previoulsy available. Our new extinction map of the GC region covers 2.0 x
1.4 degree (280 x 200 pc at a distance of 8 kpc). We apply it to deredden the
LPVs found by Glass et al. (2001) near the GC. We make period-magnitude
diagrams and compare them to those from other regions of different metallicity.
The Glass-LPVs follow well-defined period-luminosity relations (PL) in the
IRAC filter bands at 3.6, 4.5, 5.8, and 8.0 micron. The period-luminosity
relations are similar to those in the Large Magellanic Cloud, suggesting that
the PL relation in the IRAC bands is universal. We use ISOGAL data to derive
mass-loss rates and find for the Glass-LPV sample some correlation between
mass-loss and pulsation period, as expected theoretically.The GC has an excess
of high luminosity and long period LPVs compared to the Bulge, which supports
previous suggestions that it contains a younger stellar population.Comment: 12 pages, 11 figures accepted for publication in Astronomy &
Astrophysic
Near-IR spectra of ISOGAL sources in the Inner Galactic Bulge
In this work we present near-IR spectra (HK-band) of a sample of 107 sources
with mid-IR excesses at 7 and 15 m detected during the ISOGAL survey.
Making use of the DENIS interstellar extinction map from Schultheis et al.
(1999) we derive luminosities and find that the vs.~
and diagrams are powerful tools for identifying
supergiants, AGB stars, giants and young stellar objects. The majority of our
sample are AGB stars (~ 80%) while we find four good supergiant candidates,
nine young stellar objects and 12 RGB candidates. We have used the most recent
relation by Jeong et al. (2002) based on recent theoretical
modeling of dust formation of AGB stars to determine mass-loss rates. However,
the uncertainties in the mass-loss rates are rather large. The mass-loss rates
of the supergiants are comparable with those in the solar neighbourhood while
the long-period Variables cover a mass-loss range from . The red giant candidateslie at the lower end of the
mass-loss rate range between . We used the
equivalent width of the CO bandhead at 2.3 , the NaI doublet and the
CaI triplet to estimate metallicities using the relation by Ram\'{\i}rez et al.
(\cite{Ramirez2000}). The metallicity distribution of the ISOGAL objects shows
a mean [Fe/H] -0.25 dex with a dispersion of which is
in agreement with the values of Ram\'{i}rez et al. (\cite{Ramirez2000}) for
Galactic Bulge fields between and . A
comparison with the solar neighbourhood sample of Lan\c{c}on & Wood (LW) shows
that our sample is ~ 0.5 dex more metal-rich on average.Comment: 23 pages, 9 figures, 2 appendix with IR spectra. accepted for A&
AGB Variables and the Mira Period-Luminosity Relation
Published data for large amplitude asymptotic giant branch variables in the
Large Magellanic Cloud are re-analysed to establish the constants for an
infrared (K) period-luminosity relation of the form: Mk=rho[log P-2.38] +
delta. A slope of rho=-3.51+/-0.20 and a zero point of delta=-7.15+/-0.06 are
found for oxygen-rich Miras (if a distance modulus of 18.39+/-0.05 is used for
the LMC). Assuming this slope is applicable to Galactic Miras we discuss the
zero-point for these stars using the revised Hipparcos parallaxes together with
published VLBI parallaxes for OH Masers and Miras in Globular Clusters. These
result in a mean zero-point of delta=-7.25+/-0.07 for O-rich Galactic Miras.
The zero-point for Miras in the Galactic Bulge is not significantly different
from this value.
Carbon-rich stars are also discussed and provide results that are consistent
with the above numbers, but with higher uncertainties. Within the uncertainties
there is no evidence for a significant difference between the period-luminosity
relation zero-points for systems with different metallicity.Comment: 15 pages, 3 figures, accepted for MNRA
Galactic bulge giants: probing stellar and galactic evolution I. Catalogue of Spitzer IRAC and MIPS sources
Aims: We aim at measuring mass-loss rates and the luminosities of a
statistically large sample of Galactic bulge stars at several galactocentric
radii. The sensitivity of previous infrared surveys of the bulge has been
rather limited, thus fundamental questions for late stellar evolution, such as
the stage at which substantial mass-loss begins on the red giant branch and its
dependence on fundamental stellar properties, remain unanswered. We aim at
providing evidence and answers to these questions. Methods: To this end, we
observed seven 15 times 15 arcmin^2 fields in the nuclear bulge and its
vicinity with unprecedented sensitivity using the IRAC and MIPS imaging
instruments on-board the Spitzer Space Telescope. In each of the fields, tens
of thousands of point sources were detected. Results: In the first paper based
on this data set, we present the observations, data reduction, the final
catalogue of sources, and a detailed comparison to previous mid-IR surveys of
the Galactic bulge, as well as to theoretical isochrones. We find in general
good agreement with other surveys and the isochrones, supporting the high
quality of our catalogue.Comment: 21 pages, accepted for publication in A&A. A version with
high-resolution figures, as well as the data catalogues (including cross-id
with GLIMPSE and GALCEN) and image mosaics are available at the anonymous
ftp://ftp.ster.kuleuven.be/dist/stefan/Spitzer
On the Floquet Theory of Delay Differential Equations
We present an analytical approach to deal with nonlinear delay differential
equations close to instabilities of time periodic reference states. To this end
we start with approximately determining such reference states by extending the
Poincar'e Lindstedt and the Shohat expansions which were originally developed
for ordinary differential equations. Then we systematically elaborate a linear
stability analysis around a time periodic reference state. This allows to
approximately calculate the Floquet eigenvalues and their corresponding
eigensolutions by using matrix valued continued fractions
Synchronous Behavior of Two Coupled Electronic Neurons
We report on experimental studies of synchronization phenomena in a pair of
analog electronic neurons (ENs). The ENs were designed to reproduce the
observed membrane voltage oscillations of isolated biological neurons from the
stomatogastric ganglion of the California spiny lobster Panulirus interruptus.
The ENs are simple analog circuits which integrate four dimensional
differential equations representing fast and slow subcellular mechanisms that
produce the characteristic regular/chaotic spiking-bursting behavior of these
cells. In this paper we study their dynamical behavior as we couple them in the
same configurations as we have done for their counterpart biological neurons.
The interconnections we use for these neural oscillators are both direct
electrical connections and excitatory and inhibitory chemical connections: each
realized by analog circuitry and suggested by biological examples. We provide
here quantitative evidence that the ENs and the biological neurons behave
similarly when coupled in the same manner. They each display well defined
bifurcations in their mutual synchronization and regularization. We report
briefly on an experiment on coupled biological neurons and four dimensional ENs
which provides further ground for testing the validity of our numerical and
electronic models of individual neural behavior. Our experiments as a whole
present interesting new examples of regularization and synchronization in
coupled nonlinear oscillators.Comment: 26 pages, 10 figure
Socioeconomic status and treatment outcomes for individuals with HIV on antiretroviral treatment in the UK: cross-sectional and longitudinal analyses
Background: Few studies have assessed the effect of socioeconomic status on HIV treatment outcomes in settings with universal access to health care. Here we aimed to investigate the association of socioeconomic factors with antiretroviral therapy (ART) non-adherence, virological non-suppression, and virological rebound, in HIV-positive people on ART in the UK.
Methods: We used data from the Antiretrovirals, Sexual Transmission Risk and Attitudes (ASTRA) questionnaire study, which recruited participants aged 18 years or older with HIV from eight HIV outpatient clinics in the UK between Feb 1, 2011, and Dec 31, 2012. Participants self-completed a confidential questionnaire on sociodemographic, health, and lifestyle issues. In participants on ART, we assessed associations of financial hardship, employment, housing, and education with: self-reported ART non-adherence at the time of the questionnaire; virological non-suppression (viral load >50 copies per mL) at the time of questionnaire in those who started ART at least 6 months ago (cross-sectional analysis); and subsequent virological rebound (viral load >200 copies per mL) in those with initial viral load of 50 copies per mL or lower (longitudinal analysis).
Findings: Of the 3258 people who completed the questionnaire, 2771 (85%) reported being on ART at the time of the questionnaire, and 2704 with complete data were included. 873 (32%) of 2704 participants reported non-adherence to ART and 219 (9%) of 2405 had virological non-suppression in cross-sectional analysis. Each of the four measures of lower socioeconomic status was strongly associated with non-adherence to ART, and with virological non-suppression (prevalence ratios [PR] adjusted for gender/sexual orientation, age, and ethnic origin: greatest financial hardship vs none 2·4, 95% CI 1·6–3·4; non-employment 2·0, 1·5–2·6; unstable housing vs homeowner 3·0, 1·9–4·6; non-university education 1·6, 1·2–2·2). 139 (8%) of 1740 individuals had subsequent virological rebound (rate=3·6/100 person-years). Low socioeconomic status was predictive of longitudinal rebound risk (adjusted hazard ratio [HR] for greatest financial hardship vs none 2·3, 95% CI 1·4–3·9; non-employment 3·0, 2·1–4·2; unstable housing vs homeowner 3·3, 1·8–6·1; non-university education 1·6, 1·1–2·3).
Interpretation: Socioeconomic disadvantage was strongly associated with poorer HIV treatment outcomes in this setting with universal health care. Adherence interventions and increased social support for those most at risk should be considered
Evolution of the dust mass loss with luminosity along the giant branch of the globular cluster 47 Tuc
The paper investigates the properties of the dust mass loss in stars
populating the giant branch of the globular cluster 47 Tuc, by combining ISOCAM
and DENIS data. Raster maps of 5 fields covering areas ranging from 4 x 4 to 15
x 15 arcmin2 at different distances from the center of the cluster have been
obtained with ISOCAM at 11.5 mum (LW10 filter). The covered fields include most
of the red variables known in this cluster. A detection threshold of about 0.2
mJy is achieved, allowing to detect giant stars at 11.5 mum all the way down to
the horizontal branch. No dust-enshrouded asymptotic giant branch stars have
been found in the observed fields, contrary to the situation encountered in
LMC/SMC globular clusters with larger turnoff masses. The color index [12]-[2]
(based on the ISO 11.5 mum flux and on the DENIS Ks magnitude) is used as a
diagnostic of dust emission (and hence dust mass loss). Its evolution with
luminosity along the giant branch reveals that dust mass loss is only present
in V3 (the only cluster Mira variable observed in the present study) and in
V18, a star presenting intermittent variability. This conclusion confirms the
importance of stellar pulsations in the dust formation and ensuing mass loss.Comment: 16 pages, accepted in Astronomy & Astrophysic
Reprocessing the Hipparcos data for evolved stars III Revised Hipparcos period-luminosity relationship for galactic long-period variable stars
We analyze the K band luminosities of a sample of galactic long-period
variables using parallaxes measured by the Hipparcos mission. The parallaxes
are in most cases re-computed from the Hipparcos Intermediate Astrometric Data
using improved astrometric fits and chromaticity corrections. The K band
magnitudes are taken from the literature and from measurements by COBE, and are
corrected for interstellar and circumstellar extinction. The sample contains
stars of several spectral types: M, S and C, and of several variability
classes: Mira, semiregular SRa, and SRb. We find that the distribution of stars
in the period-luminosity plane is independent of circumstellar chemistry, but
that the different variability types have different P-L distributions. Both the
Mira variables and the SRb variables have reasonably well-defined
period-luminosity relationships, but with very different slopes. The SRa
variables are distributed between the two classes, suggesting that they are a
mixture of Miras and SRb, rather than a separate class of stars. New
period-luminosity relationships are derived based on our revised Hipparcos
parallaxes. The Miras show a similar period-luminosity relationship to that
found for Large Magellanic Cloud Miras by Feast et al. (1989). The maximum
absolute K magnitude of the sample is about -8.2 for both Miras and
semi-regular stars, only a little fainter than the expected AGB limit. We show
that the stars with the longest periods (P>400d) have high mass loss rates and
are almost all Mira variables.Comment: Comments welcome. Submitted to A&A 11 pages, 7 figs, 3 table
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