146 research outputs found
UV chromospheric and circumstellar diagnostic features among F supergiant stars
A survey of F supergiant stars to evaluate the extension of chromospheric and circumstellar characteristics commonly observed in the slightly cooler G, K, and M supergiant is discussed. An ultraviolet survey was elected since UV features of Mg II and Fe II might persist in revealing outer atmosphere phenomena even among F supergiants. The encompassed spectral types F0 to G0, and luminosity classes Ib, Ia, and Ia-0. In addition, the usefulness of the emission line width-to-luminosity correlation for the G-M stars in both the Ca II and Mg II lines is examined
High resolution absolute flux profiles of the MC 2 h and k lines in evolved F8 to M5 stars
The central results of a survey of the Mg II resonance line emission in a sample of over 50 evolved late type stars, including spectral-luminosity type F8 to M5 and La to IV are presented. Observed and surface fluxes are derived and correlations noted. The major findings include: (1) Mg II k emission core asymmetry transition near K1 III, analogous to that known for Ca II K; (2) a small gravity and temperature dependence of the Mg II chromospheric radiative loss rate
Direct UV observations of the circumstellar envelope of alpha Orionis
Observations were made in the IUE LWP camera, low dispersion mode, with alpha Ori being offset various distances from the center of the Long Wavelength Large Aperture along its major axis. Signal was acquired at all offset positions and is comprised of unequal components of background/dark counts, telescope-scattered light, and scattered light emanating from the extended circumstellar shell. The star is known from optical and infrared observations to possess an extended, arc-minute sized, shell of cool material. Attempts to observe this shell with the IUE are described, although the deconvolution of the stellar signal from the telescope scattered light requires further calibration effort
Taming the Invisible Monster: System Parameter Constraints for Epsilon Aurigae from the Far-Ultraviolet to the Mid-Infrared
We have assembled new Spitzer Space Telescope Infrared Array Camera
observations of the mysterious binary star Epsilon Aurigae, along with archival
far-ultraviolet to mid-infrared data, to form an unprecedented spectral energy
distribution spanning three orders of magnitude in wavelength from 0.1 microns
to 100 microns. The observed spectral energy distribution can be reproduced
using a three component model consisting of a 2.2+0.9/-0.8 Msun F type
post-asymptotic giant branch star, and a 5.9+/-0.8 Msun B5+/-1 type main
sequence star that is surrounded by a geometrically thick, but partially
transparent, disk of gas and dust. At the nominal HIPPARCOS parallax distance
of 625 pc, the model normalization yields a radius of 135+/-5 Rsun for the F
star, consistent with published interferometric observations. The dusty disk is
constrained to be viewed at an inclination of i > 87 deg, and has effective
temperature of 550+/-50 K with an outer radius of 3.8 AU and a thickness of
0.95 AU. The dust content of the disk must be largely confined to grains larger
than ~10 microns in order to produce the observed gray optical-infrared
eclipses and the lack of broad dust emission features in the archival Spitzer
mid-infrared spectra. The total mass of the disk, even considering a potential
gaseous contribution in addition to the dust that produces the observed
infrared excess, is << 1 Msun. We discuss evolutionary scenarios for this
system that could lead to the current status of the stellar components and
suggests possibilities for its future evolution, as well as potential
observational tests of our model.Comment: 13 pages, 3 figures. Accepted for publication in The Astrophysical
Journal
Taming the Invisible Monster: System Parameter Constraints for Ñ” Aurigae from the Far-ultraviolet to the Mid-infrared
We have assembled new Spitzer Space Telescope IRAC observations of the mysterious binary star є Aurigae, along with archival far-ultraviolet to mid-infrared data, to form an unprecedented spectral energy distribution (SED) spanning 3 orders of magnitude in wavelength from 0.1 μm to 100 μm. The observed SED can be reproduced using a three-component model consisting of a 2.2^(+0.9)_(–0.8) M_☉ F-type post-asymptotic giant branch star, and a 5.9 ± 0.8 M_☉ B5±1 type main-sequence star that is surrounded by a geometrically thick, but partially transparent, disk of gas and dust. At the nominal HIPPARCOS parallax distance of 625 pc, the model normalization yields a radius of 135 ± 5 R_☉ for the F star, consistent with published interferometric observations. The dusty disk is constrained to be viewed at an inclination of i ≳87°, and has an effective temperature of 550 ± 50 K with an outer radius of 3.8 AU and a thickness of 0.95 AU. The dust content of the disk must be largely confined to grains larger than ~10 μm in order to produce the observed gray optical-infrared eclipses and the lack of broad dust emission features in the archival Spitzer mid-infrared spectra. The total mass of the disk, even considering a potential gaseous contribution in addition to the dust that produces the observed infrared excess, is « 1 M_☉. We discuss evolutionary scenarios for this system that could lead to the current status of the stellar components and suggest possibilities for its future evolution, as well as potential observational tests of our model
Tycho 2 stars with infrared excess in the MSX Point Source Catalogue
Stars of all evolutionary phases have been found to have excess infrared
emission due to the presence of circumstellar material. To identify such stars,
we have positionally correlated the infrared MSX point source catalogue and the
Tycho 2 optical catalogue. A near/mid infrared colour criteria has been
developed to select infrared excess stars. The search yielded 1938 excess
stars, over half (979) have never previously been detected by IRAS. The excess
stars were found to be young objects such as Herbig Ae/Be and Be stars, and
evolved objects such as OH/IR and carbon stars. A number of B type excess stars
were also discovered whose infrared colours could not be readily explained by
known catalogued objects.Comment: Added Comment: 16 pages, 9 figures, accepted for publication in MNRA
3D Simulations of Betelgeuse's Bow Shock
Betelgeuse, the bright, cool red supergiant in Orion, is moving
supersonically relative to the local interstellar medium. The star emits a
powerful stellar wind which collides with this medium, forming a cometary
structure, a bow shock, pointing in the direction of motion. We present the
first 3D hydrodynamic simulations of the formation and evolution of
Betelgeuse's bow shock. The models include realistic low temperature cooling
and cover a range of plausible interstellar medium densities and stellar
velocities between 0.3 - 1.9 cm-3 and 28 - 73 km/s. We show that the flow
dynamics and morphology of the bow shock differ substantially due to the
preferential growth of Rayleigh-Taylor or Kelvin-Helmholtz instabilities in the
models. The former dominate the models with slow stellar velocities resulting
in a clumpy bow shock sub-structure, whereas the latter produce a smoother,
more layered sub-structure in the fast models. If the mass in the bow shock
shell is low, as seems to be implied by the AKARI luminosities (~0.003 Msun),
then Betelgeuse's bow shock is very young and is unlikely to have reached a
steady state. The circular nature of the bow shock shell is consistent with
this conclusion. Thus, our results suggest that Betelgeuse entered the red
supergiant phase only recently.Comment: Minor revisions, replaced Fig. 1, 15, and 16, added movies. For a pdf
version with higher resolution, see A&A: Forthcomin
Reovirus-Induced Apoptosis in the Intestine Limits Establishment of Enteric Infection
Several viruses induce intestinal epithelial cell death during enteric infection. However, it is unclear whether proapoptotic capacity promotes or inhibits replication in this tissue. We infected mice with two reovirus strains that infect the intestine but differ in the capacity to alter immunological tolerance to new food antigen. Infection with reovirus strain T1L, which induces an inflammatory immune response to fed antigen, is prolonged in the intestine, whereas T3D-RV, which does not induce this response, is rapidly cleared from the intestine. Compared with T1L, T3D-RV infection triggered apoptosis of intestinal epithelial cells and subsequent sloughing of dead cells into the intestinal lumen. We conclude that the infection advantage of T1L derives from its capacity to subvert host restriction by epithelial cell apoptosis, providing a possible mechanism by which T1L enhances inflammatory signals during antigen feeding. Using a panel of T1L × T3D-RV reassortant viruses, we identified the viral M1 and M2 gene segments as determinants of reovirus-induced apoptosis in the intestine. Expression of the T1L M1 and M2 genes in a T3D-RV background was sufficient to limit epithelial cell apoptosis and enhance viral infection to levels displayed by T1L. These findings define additional reovirus gene segments required for enteric infection of mice and illuminate the antiviral effect of intestinal epithelial cell apoptosis in limiting enteric viral infection. Viral strain-specific differences in the capacity to infect the intestine may be useful in identifying viruses capable of ameliorating tolerance to fed antigen in autoimmune conditions like celiac disease
Structure of the outer layers of cool standard stars
Context: Among late-type red giants, an interesting change occurs in the
structure of the outer atmospheric layers as one moves to later spectral types
in the Hertzsprung-Russell diagram: a chromosphere is always present, but the
coronal emission diminishes and a cool massive wind steps in.
Aims: Where most studies have focussed on short-wavelength observations, this
article explores the influence of the chromosphere and the wind on
long-wavelength photometric measurements.
Methods: The observational spectral energy distributions are compared with
the theoretical predictions of the MARCS atmosphere models for a sample of 9 K-
and M-giants. The discrepancies found are explained using basic models for flux
emission originating from a chromosphere or an ionized wind.
Results: For 7 out of 9 sample stars, a clear flux excess is detected at
(sub)millimeter and/or centimeter wavelengths. The precise start of the excess
depends upon the star under consideration. The flux at wavelengths shorter than
about 1 mm is most likely dominated by an optically thick chromosphere, where
an optically thick ionized wind is the main flux contributor at longer
wavelengths.
Conclusions: Although the optical to mid-infrared spectrum of the studied K-
and M-giants is well represented by a radiative equilibrium atmospheric model,
the presence of a chromosphere and/or ionized stellar wind at higher altitudes
dominates the spectrum in the (sub)millimeter and centimeter wavelength ranges.
The presence of a flux excess also has implications on the role of these stars
as fiducial spectrophotometric calibrators in the (sub)millimeter and
centimeter wavelength range.Comment: 13 pages, 6 figures, 7 pages of online material, submitted to A&
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