144 research outputs found
The period-luminosity and period-radius relations of Type II and anomalous Cepheids
Method: In an accompanying paper (arXiv: 1705.00886) we determined luminosity
and effective temperature for the 335 T2Cs and ACs in the LMC and SMC
discovered in the OGLE-III survey, by constructing the spectral energy
distribution (SED) and fitting this with model atmospheres and a dust radiative
transfer model (in the case of dust excess). Building on these results we study
the PL and PR relations.
Using existing pulsation models for RR Lyrae and classical Cepheids we derive
the period-luminosity-mass-temperature-metallicity relations, and then estimate
the pulsation mass.
Results: The PL relation for the T2Cs does not appear to depend on
metallicity, and, excluding the dusty RV Tau stars, is (for days). Relations for fundamental and first overtone
LMC ACs are also presented. The PR relation for T2C also shows little or no
dependence on metallicity or period. Our preferred relation combines SMC and
LMC stars and all T2C subclasses, and is .
Relations for fundamental and first overtone LMC ACs are also presented. The
pulsation masses from the RR Lyrae and classical Cepheid pulsation models agree
well for the short period T2Cs, the BL Her subtype, and ACs, and are consistent
with estimates in the literature, i.e. \msol\ and
\msol, respectively. The masses of the W Vir appear
similar to the BL Her. The situation for the pWVir and RV Tau stars is less
clear. For many RV Tau the masses are in conflict with the standard picture of
(single-star) post-AGB evolution, the masses being either too large (
1 \msol) or too small ( 0.4 \msol).Comment: A&A accepte
Luminosities and mass-loss rates of Local Group AGB stars and Red Supergiants
We aim to investigate mass loss and luminosity in a large sample of evolved
stars in several Local Group galaxies with a variety of metalliticies and
star-formation histories: the Small and Large Magellanic Cloud, and the Fornax,
Carina, and Sculptor dwarf spheroidal galaxies. Dust radiative transfer models
are presented for 225 carbon stars and 171 oxygen-rich evolved stars for which
spectra from the Infrared Spectrograph on Spitzer are available. The spectra
are complemented with available optical and infrared photometry to construct
spectral energy distributions. A minimization procedure was used to determine
luminosity and mass-loss rate (MLR). Pulsation periods were derived for a large
fraction of the sample based on a re-analysis of existing data. New deep K-band
photometry from the VMC survey and multi-epoch data from IRAC and
AllWISE/NEOWISE have allowed us to derive pulsation periods longer than 1000
days for some of the most heavily obscured and reddened objects. We derive
(dust) MLRs and luminosities for the entire sample. The estimated MLRs can
differ significantly from estimates for the same objects in the literature due
to differences in adopted optical constants (up to factors of several) and
details in the radiative transfer modelling. Updated parameters for the
super-AGB candidate MSX SMC 055 (IRAS 00483-7347) are presented. Its current
mass is estimated to be 8.5 +- 1.6 \msol, suggesting an initial mass well above
8~\msol. Using synthetic photometry, we present and discuss colour-colour and
colour-magnitude diagrams which can be expected from the James Webb Space
Telescope.Comment: A&A accepted. The full version (100 pages, 12 MB) with complete
tables and all figures of the appendices is available at
http://homepage.oma.be/marting/articlesgroen.htm
The Lutz-Kelker bias in trigonometric parallaxes
The theoretical prediction that trigonometric parallaxes suffer from a
statistical effect, has become topical again now that the results of the
Hipparcos satellite have become available. This statistical effect, the
so-called Lutz-Kelker bias, causes measured parallaxes to be too large. This
has the implication that inferred distances, and hence inferred luminosities
are too small. Published analytic calculations of the Lutz-Kelker bias indicate
that the inferred luminosity of an object is, on average, 30% too small when
the error in the parallax is only 17.5%. Yet, this bias has never been
determined empirically. In this paper we investigate whether there is such a
bias by comparing the best Hipparcos parallaxes which ground-based
measurements. We find that there is indeed a large bias affecting parallaxes,
with an average and scatter comparable to predictions. We propose a simple
method to correct for the LK bias, and apply it successfully to a sub-sample of
our stars. We then analyze the sample of 26 `best' Cepheids used by Feast &
Catchpole (1997) to derive the zero-point of the fundamental mode pulsators and
leads to a distance modulus to the Large Magellanic Cloud - based on Cepheid
parallaxes- of 18.56 +/- 0.08, consistent with previous estimates.Comment: MNRAS Letters in press; 6 pages LaTeX, 6 ps figure
Pulsating stars in the VMC survey
The VISTA survey of the Magellanic Clouds system (VMC) began observations in
2009 and since then, it has collected multi-epoch data at Ks and in addition
multi-band data in Y and J for a wide range of stellar populations across the
Magellanic system. Among them are pulsating variable stars: Cepheids, RR Lyrae,
and asymptotic giant branch stars that represent useful tracers of the host
system geometry.Comment: 8 pages, 7 figures, proceeding contribution of invited presentation
at "Wide-field variability surveys: a 21st-century perspective", San Pedro de
Atacama (Chile
A dearth of OH/IR stars in the Small Magellanic Cloud
We present the results of targeted observations and a survey of 1612-, 1665-,
and 1667-MHz circumstellar OH maser emission from asymptotic giant branch (AGB)
stars and red supergiants (RSGs) in the Small Magellanic Cloud (SMC), using the
Parkes and Australia Telescope Compact Array radio telescopes. No clear OH
maser emission has been detected in any of our observations targeting luminous,
long-period, large-amplitude variable stars, which have been confirmed
spectroscopically and photometrically to be mid- to late-M spectral type. These
observations have probed 3 - 4 times deeper than any OH maser survey in the
SMC. Using a bootstrapping method with LMC and Galactic OH/IR star samples and
our SMC observation upper limits, we have calculated the likelihood of not
detecting maser emission in any of the two sources considered to be the top
maser candidates to be less than 0.05%, assuming a similar pumping mechanism as
the LMC and Galactic OH/IR sources. We have performed a population comparison
of the Magellanic Clouds and used Spitzer IRAC and MIPS photometry to confirm
that we have observed all high luminosity SMC sources that are expected to
exhibit maser emission. We suspect that, compared to the OH/IR stars in the
Galaxy and LMC, the reduction in metallicity may curtail the dusty wind phase
at the end of the evolution of the most massive cool stars. We also suspect
that the conditions in the circumstellar envelope change beyond a simple
scaling of abundances and wind speed with metallicity
Luminosities and mass-loss rates of SMC and LMC AGB stars and Red Supergiants
(Abridged) Dust radiative transfer models are presented for 101 carbon stars
and 86 oxygen-rich evolved stars in the Magellanic Clouds for which 5-35 \mum\
{\it Spitzer} IRS spectra are available. The spectra are complemented with
available optical and infrared photometry to construct the spectral energy
distribution. A minimisation procedure is used to fit luminosity, mass-loss
rate and dust temperature at the inner radius. Different effective temperatures
and dust content are also considered. Periods from the literature and from new
OGLE-III data are compiled and derived. The O-rich stars are classified in
foreground objects, AGB stars and Red Super Giants.
For the O-rich stars silicates based on laboratory optical constants are
compared to "astronomical silicates". Overall, the grain type by Volk & Kwok
(1988) fit the data best. However, the fit based on laboratory optical
constants for the grains can be improved by abandoning the small-particle
limit. The influence of grain size, core-mantle grains and porosity are
explored.
Relations between mass-loss rates and luminosity and pulsation period are
presented and compared to the predictions of evolutionary models, those by
Vassiliadis & Wood (1993) and their adopted mass-loss recipe, and those based
on a Reimers mass-loss law with a scaling of a factor of five. The Vassiliadis
& Wood models describe the data better, although there are also some
deficiencies, in particular to the maximum adopted mass-loss rate.
The OGLE-III data reveal an O-rich star in the SMC with a period of 1749
days. Its absolute magnitude of makes it a good candidate
for a super-AGB star.Comment: A&A accepte
Type II and anomalous Cepheids in the Kepler K2 mission
We present the results of the analysis of Type II and anomalous Cepheids
using the data from the Kepler K2 mission. The precise light curves of these
pulsating variable stars are the key to study the details of their pulsation,
such as the period-doubling effect or the presence of additional modes. We
applied the Automated Extended Aperture Photometry (autoEAP) to obtain the
light curves of the targeted variable stars which were observed. The light
curves were Fourier analyzed. We investigated twelve stars observed by the K2
mission, seven Type II and five anomalous Cepheids. Among the Type II Cepheids
EPIC 210622262 shows period-doubling, and four stars have modulation present in
their light curves which are different from the period-doubling effect. We
calculated the high-order Fourier parameters for the short-period Cepheids. We
also determined physical parameters by fitting model atmospheres to the
spectral energy distributions. The determined distances using the parallaxes
measured by the Gaia space telescope have limited precision below 16 mag for
these types of pulsating stars, regardless if the inverse method is used or the
statistical method to calculate the distances. The BaSTI evolutionary models
were compared to the luminosities and effective temperatures. Most of the Type
II Cepheids are modeled with low metallicity models, but for a few of them
solar-like metallicity ([Fe/H]=0.06) model is required. The anomalous Cepheids
are compared to low-metallicity single stellar models. We do not see signs of
binarity among our sample stars.Comment: 21 pages, 13 figures, accepted for publication in MNRA
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