598 research outputs found
Technetium and the third dredge up in AGB stars. I. Field stars
We searched for Tc in a sample of long period variables selected by stellar
luminosity derived from Hipparcos parallaxes. Tc, as an unstable s-process
element, is a good indicator for the evolutionary status of stars on the
asymptotic giant branch (AGB). In this paper we study the occurrence of Tc as a
function of luminosity to provide constraints on the minimum luminosity for the
third dredge up as estimated from recent stellar evolution models.
A large number of AGB stars above the estimated theoretical limit for the
third dredge up are found not to show Tc. We confirm previous findings that
only a small fraction of the semiregular variables show Tc lines in their
spectra. Contrary to earlier results by Little et al. (1987) we find also a
significant number of Miras without Tc.
The presence and absence of Tc is discussed in relation to the mass
distribution of AGB stars. We find that a large fraction of the stars of our
sample must have current masses of less than 1.5 M_{\sun}. Combining our
findings with stellar evolution scenarios we conclude that the fraction of time
a star is observed as a SRV or a Mira is dependent on its mass.Comment: 10 pages, 4 figures, accepted for publication in A&
Field #3 of the Palomar-Groningen Survey II. Near-infrared photometry of semiregular variables
Near-infrared photometry (JHKL'M) was obtained for 78 semiregular variables
(SRVs) in field #3 of the Palomar-Groningen survey (PG3, l=0, b=-10). Together
with a sample of Miras in this field a comparison is made with a sample of
field SRVs and Miras. The PG3 SRVs form a sequence (period-luminosity
& period-colour) with the PG3 Miras, in which the SRVs are the short period
extension to the Miras. The field and PG3 Miras follow the same P/(J--K)o
relation, while this is not the case for the field and PG3 SRVs. Both the PG3
SRVs and Miras follow the SgrI period-luminosity relation adopted from Glass et
al. (1995, MNRAS 273, 383). They are likely pulsating in the fundamental mode
and have metallicities spanning the range from intermediate to approximately
solar.Comment: 14 pages LaTeX (2 tables, 8 figures), to appear in A&A 338 (1998);
minor modifications in tex
Cox regression survival analysis with compositional covariates: application to modelling mortality risk from 24-h physical activity patterns
Survival analysis is commonly conducted in medical and public health research to assess the association of an exposure or intervention with a hard end outcome such as mortality. The Cox (proportional hazards) regression model is probably the most popular statistical tool used in this context. However, when the exposure includes compositional covariables (that is, variables representing a relative makeup such as a nutritional or physical activity behaviour composition), some basic assumptions of the Cox regression model and associated significance tests are violated. Compositional variables involve an intrinsic interplay between one another which precludes results and conclusions based on considering them in isolation as is ordinarily done. In this work, we introduce a formulation of the Cox regression model in terms of log-ratio coordinates which suitably deals with the constraints of compositional covariates, facilitates the use of common statistical inference methods, and allows for scientifically meaningful interpretations. We illustrate its practical application to a public health problem: the estimation of the mortality hazard associated with the composition of daily activity behaviour (physical activity, sitting time and sleep) using data from the U.S. National Health and Nutrition Examination Survey (NHANES)
The complex environment of the bright carbon star TX Psc as probed by spectro-astrometry
Context: Stars on the asymptotic giant branch (AGB) show broad evidence of
inhomogeneous atmospheres and circumstellar envelopes. These have been studied
by a variety of methods on various angular scales. In this paper we explore the
envelope of the well-studied carbon star TX Psc by the technique of
spectro-astrometry. Aims: We explore the potential of this method for detecting
asymmetries around AGB stars. Methods:We obtained CRIRES observations of
several CO v=1 lines near 4.6 m and HCN lines near 3 m in
2010 and 2013. These were then searched for spectro-astrometric signatures. For
the interpretation of the results, we used simple simulated observations.
Results: Several lines show significant photocentre shifts with a clear
dependence on position angle. In all cases, tilde-shaped signatures are found
where the positive and negative shifts (at PA 0deg) are associated with blue
and weaker red components of the lines. The shifts can be modelled with a
bright blob 70 mas to 210 mas south of the star with a flux of several percent
of the photospheric flux. We estimate a lower limit of the blob temperature of
1000 K. The blob may be related to a mass ejection as found for AGB stars or
red supergiants. We also consider the scenario of a companion object.
Conclusions: Although there is clear spectro-astrometric evidence of a rather
prominent structure near TX Psc, it does not seem to relate to the other
evidence of asymmetries, so no definite explanation can be given. Our data thus
underline the very complex structure of the environment of this star, but
further observations that sample the angular scales out to a few hundred
milli-arcseconds are needed to get a clearer picture
Modelling the atmosphere of the carbon-rich Mira RU Vir
Context. We study the atmosphere of the carbon-rich Mira RU Vir using the
mid-infrared high spatial resolution interferometric observations from
VLTI/MIDI. Aims. The aim of this work is to analyse the atmosphere of the
carbon-rich Mira RU Vir, with state of the art models, in this way deepening
the knowledge of the dynamic processes at work in carbon-rich Miras. Methods.
We compare spectro-photometric and interferometric measurements of this
carbon-rich Mira AGB star, with the predictions of different kinds of modelling
approaches (hydrostatic model atmospheres plus MOD-More Of Dusty,
self-consistent dynamic model atmospheres). A geometric model fitting tool is
used for a first interpretation of the interferometric data. Results. The
results show that a joint use of different kind of observations (photometry,
spectroscopy, interferometry) is essential to shed light on the structure of
the atmosphere of a carbon-rich Mira. The dynamic model atmospheres fit well
the ISO spectrum in the wavelength range {\lambda} = [2.9, 25.0] {\mu}m.
Nevertheless, a discrepancy is noticeable both in the SED (visible), and in the
visibilities (shape and level). A possible explanation are intra-/inter-cycle
variations in the dynamic model atmospheres as well as in the observations. The
presence of a companion star and/or a disk or a decrease of mass loss within
the last few hundred years cannot be excluded but are considered unlikely.Comment: 15 pages. Accepted in A&
Catching the fish - Constraining stellar parameters for TX Psc using spectro-interferometric observations
Stellar parameter determination is a challenging task when dealing with
galactic giant stars. The combination of different investigation techniques has
proven to be a promising approach. We analyse archive spectra obtained with the
Short-Wavelength-Spectrometer (SWS) onboard of ISO, and new interferometric
observations from the Very Large Telescope MID-infrared Interferometric
instrument (VLTI/MIDI) of a very well studied carbon-rich giant: TX Psc. The
aim of this work is to determine stellar parameters using spectroscopy and
interferometry. The observations are used to constrain the model atmosphere,
and eventually the stellar evolutionary model in the region where the tracks
map the beginning of the carbon star sequence. Two different approaches are
used to determine stellar parameters: (i) the 'classic' interferometric
approach where the effective temperature is fixed by using the angular diameter
in the N-band (from interferometry) and the apparent bolometric magnitude; (ii)
parameters are obtained by fitting a grid of state-of-the-art hydrostatic
models to spectroscopic and interferometric observations. We find a good
agreement between the parameters of the two methods. The effective temperature
and luminosity clearly place TX Psc in the carbon-rich AGB star domain in the
H-R-diagram. Current evolutionary tracks suggest that TX Psc became a C-star
just recently, which means that the star is still in a 'quiet' phase compared
to the subsequent strong-wind regime. This is in agreement with the C/O ratio
being only slightly larger than 1.Comment: 11 pages, 9 figures, 5 table
Deltaretroviruses infect a broad range of mammalian species and have circulated since the Paleogene
The Deltaretrovirus genus of retroviruses (family Retroviridae) includes the human T cell leukemia viruses and bovine leukemia virus (BLV). Relatively little is known about the biology and evolution of these viruses, because only a few species have been identified and the genomic ‘fossil record’ is relatively sparse. Here, we report the discovery of multiple novel endogenous retroviruses (ERVs) derived from ancestral deltaretroviruses. These sequences—two of which contain complete or near complete internal coding regions—reside in genomes of several distinct mammalian orders, including bats, carnivores, cetaceans, and insectivores. We demonstrate that two of these ERVs contain unambiguous homologs of the tax gene, indicating that complex gene regulation has ancient origins within the Deltaretrovirus genus. ERVs demonstrate that the host range of the deltaretrovirus genus is much more extensive than suggested by the relatively small number of exogenous deltaretroviruses described so far, and allow the evolutionary timeline of deltaretrovirus-mammal interaction to be more accurately calibrated
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