30 research outputs found
A Catalog of GALEX Ultraviolet Emission from Asymptotic Giant Branch Stars
We have performed a comprehensive study of the UV emission detected from AGB
stars by the Galaxy Evolution Explorer (GALEX). Of the 468 AGB stars in our
sample, 316 were observed by GALEX. In the NUV bandpass (z A), 179 AGB stars were detected and 137 were not detected. Only 38
AGB stars were detected in the FUV bandpass ( A).
We find that NUV emission is correlated with optical to near infrared emission
leading to higher detection fractions among the brightest, hence closest, AGB
stars. Comparing the AGB time-variable visible phased light curves to
corresponding GALEX NUV phased light curves we find evidence that for some AGB
stars the NUV emission varies in phase with the visible light curves. We also
find evidence that the NUV emission, and possibly, the FUV emission are
anti-correlated with the circumstellar envelope density. These results suggest
that the origin of the GALEX-detected UV emission is an inherent characteristic
of the AGB stars that can most likely be traced to a combination of
photospheric and chromospheric emission. In most cases, UV detections of AGB
stars are not likely to be indicative of the presence of binary companions.Comment: Accepted by ApJ; go spurs go
The detached dust shells around the carbon AGB stars R Scl and V644 Sco
Detached shells are believed to be created during a thermal pulse, and
constrain the time scales and physical properties of one of the main drivers of
late stellar evolution. We aim at determining the morphology of the detached
dust shells around the carbon AGB stars R Scl and V644 Sco, and compare this to
observations of the detached gas shells. We observe the polarised,
dust-scattered stellar light around these stars using the PolCor instrument
mounted on the ESO 3.6m telescope. Observations were done with a coronographic
mask to block out the direct stellar light. The polarised images clearly show
the detached shells. Using a dust radiative transfer code to model the
dust-scattered polarised light, we constrain the radii and widths of the shells
to 19.5 arcsec and 9.4 arcsec for the detached dust shells around R Scl and
V644 Sco, respectively. Both shells have an overall spherical symmetry and
widths of approx. 2 arcsec. For R Scl we can compare the observed dust emission
directly with high spatial-resolution maps of CO(3-2) emission from the shell
observed with ALMA. We find that the dust and gas coincide almost exactly,
indicating a common evolution. The data presented here for R Scl are the most
detailed observations of the entire dusty detached shell to date. For V644 Sco
these are the first direct measurements of the detached shell. Also here we
find that the dust most likely coincides with the gas shell. The observations
are consistent with a scenario where the detached shells are created during a
thermal pulse. The determined radii and widths will constrain hydrodynamical
models describing the pre-pulse mass loss, the thermal pulse, and post-pulse
evolution of the star
Molecular line study of the S-type AGB star W Aquilae. ALMA observations of CS, SiS, SiO and HCN
Context. With the outstanding spatial resolution and sensitivity of the
Atacama Large Millimeter/sub-millimeter Array (ALMA), molecular gas other than
the abundant CO can be observed and resolved in circumstellar envelopes (CSEs)
around evolved stars, such as the binary S-type Asymptotic Giant Branch (AGB)
star W Aquilae. Aims. We aim to constrain the chemical composition of the CSE
and determine the radial abundance distribution, the photospheric peak
abundance, and isotopic ratios of a selection of chemically important molecular
species in the innermost CSE of W Aql. The derived parameters are put into the
context of the chemical evolution of AGB stars and are compared with
theoretical models. Methods. We employ one-dimensional radiative transfer
modeling - with the accelerated lambda iteration (ALI) radiative transfer code
- of the radial abundance distribution of a total of five molecular species
(CS, SiS, 30SiS, 29SiO and H13CN) and determine the best fitting model
parameters based on high-resolution ALMA observations as well as archival
single-dish observations. The additional advantage of the spatially resolved
ALMA observations is that we can directly constrain the radial profile of the
observed line transitions from the observations. Results. We derive abundances
and e-folding radii for CS, SiS, 30SiS, 29SiO and H13CN and compare them to
previous studies, which are based only on unresolved single-dish spectra. Our
results are in line with previous results and are more accurate due to
resolution of the emission regions
The shock-heated atmosphere of an asymptotic giant branch star resolved by ALMA
Our current understanding of the chemistry and mass-loss processes in
solar-like stars at the end of their evolution depends critically on the
description of convection, pulsations and shocks in the extended stellar
atmosphere. Three-dimensional hydrodynamical stellar atmosphere models provide
observational predictions, but so far the resolution to constrain the complex
temperature and velocity structures seen in the models has been lacking. Here
we present submillimeter continuum and line observations that resolve the
atmosphere of the asymptotic giant branch star W Hya. We show that hot gas with
chromospheric characteristics exists around the star. Its filling factor is
shown to be small. The existence of such gas requires shocks with a cooling
time larger than commonly assumed. A shocked hot layer will be an important
ingredient in the models of stellar convection, pulsation and chemistry that
underlie our current understanding of the late stages of stellar evolution.Comment: 30 pages, 9 figures, including Supplementary information. Author
manuscript version before editorial/copyediting by Nature Astronomy. Journal
version available via http://rdcu.be/xUW
Structure-guided identification of a family of dual receptor-binding PfEMP1 that is associated with cerebral malaria
Cerebral malaria is a deadly outcome of infection by Plasmodium falciparum, occurring when parasite-infected erythrocytes accumulate in the brain. These erythrocytes display parasite proteins of the PfEMP1 family that bind various endothelial receptors. Despite the importance of cerebral malaria, a binding phenotype linked to its symptoms has not been identified. Here, we used structural biology to determine how a group of PfEMP1 proteins interacts with intercellular adhesion molecule 1 (ICAM-1), allowing us to predict binders from a specific sequence motif alone. Analysis of multiple Plasmodium falciparum genomes showed that ICAM-1-binding PfEMP1s also interact with endothelial protein C receptor (EPCR), allowing infected erythrocytes to synergistically bind both receptors. Expression of these PfEMP1s, predicted to bind both ICAM-1 and EPCR, is associated with increased risk of developing cerebral malaria. This study therefore reveals an important PfEMP1-binding phenotype that could be targeted as part of a strategy to prevent cerebral malaria
Shaping the Outflows of Binary AGB Stars
The required conditions for stars to evolve into planetary nebulae (PNe) continue to puzzle. Since PNe are found in a wide variety of shapes, processes that could sculpt circumstellar envelopes (CSEs) are being investigated. A binary companion will have a strong gravitational effect, but known binary AGB stars are rare. We have observed the CO emission from a small sample of known binary AGB stars (R Aqr, Mira, W Aql, and pi(1). Gru) with ALMA. The stars cover a decisive range in binary separation, necessary to provide essential constraints for 3-D models of the binary interaction. They have previously been observed with Herschel/PACS and VLTI/MIDI allowing the circumstellar morphology to be studied from the very inner CSE out to the very largest scales. The circumstellar gas distribution will strongly depend on how the mass is initially lost from the primary. These observations will therefore not only help us understand the important processes for the binary interaction, but will also provide crucial information needed to understand the mass-loss mechanisms of the primary. The first set of observations has been delivered and preliminary results are presented