919 research outputs found
Post-main sequence evolution of A star debris discs
While the population of main sequence debris discs is well constrained,
little is known about debris discs around evolved stars. This paper provides a
theoretical framework considering the effects of stellar evolution on debris
discs, particularly the production and loss of dust within them. Here we repeat
a steady state model fit to disc evolution statistics for main sequence A
stars, this time using realistic grain optical properties, then evolve that
population to consider its detectability at later epochs. Our model predicts
that debris discs around giant stars are harder to detect than on the main
sequence because radiation pressure is more effective at removing small dust
around higher luminosity stars. Just 12% of first ascent giants within 100pc
are predicted to have discs detectable with Herschel at 160um. However this is
subject to the uncertain effect of sublimation on the disc, which we propose
can thus be constrained with such observations. Our model also finds that the
rapid decline in stellar luminosity results in only very young white dwarfs
having luminous discs. As such systems are on average at larger distances they
are hard to detect, but we predict that the stellar parameters most likely to
yield a disc detection are a white dwarf at 200pc with cooling age of 0.1Myr,
in line with observations of the Helix Nebula. Our model does not predict
close-in (<0.01AU) dust, as observed for some white dwarfs, however we find
that stellar wind drag leaves significant mass (~10^{-2}Msolar), in bodies up
to ~10m in diameter, inside the disc at the end of the AGB phase which may
replenish these discs
Inner edges of compact debris disks around metal-rich white dwarfs
A number of metal-rich white dwarfs (WDs) are known to host compact, dense
particle disks, which are thought to be responsible for metal pollution of
these stars. In many such systems the inner radii of disks inferred from their
spectra are so close to the WD that particles directly exposed to starlight
must be heated above 1500 K and are expected to be unstable against
sublimation. To reconcile this expectation with observations we explore
particle sublimation in H-poor debris disks around WDs. We show that because of
the high metal vapor pressure the characteristic sublimation temperature in
these disks is 300-400 K higher than in their protoplanetary analogues,
allowing particles to survive at higher temperatures. We then look at the
structure of the inner edges of debris disks and show that they should
generically feature superheated inner rims directly exposed to starlight with
temperatures reaching 2500-3500 K. Particles migrating through the rim towards
the WD (and rapidly sublimating) shield the disk behind them from strong
stellar heating, making the survival of solids possible close to the WD. Our
model agrees well with observations of WD+disk systems provided that disk
particles are composed of Si-rich material such as olivine, and have sizes in
the range ~(0.03-30) cm.Comment: 12 pages, 6 figures, submitted to Ap
Near-ultraviolet and optical effects of Debris Disks around White Dwarfs
Studies of debris disks around white dwarfs (WDs) have focused on infrared
wavelengths because debris disks are much colder than the star and are believed
to contribute to the spectrum only at longer wavelengths. Nevertheless, these
disks are made of dust grains which absorb and scatter near-UV and optical
photons from the WD, leaving a fingerprint that can be used to further
constrain disk properties. Our goal is to show that it is possible to detect
near-UV and optical effects of debris disks in the star + disk integrated
spectrum. We make theoretical calculations and discuss the necessary
observational conditions to detect the near-UV and optical effects. We show how
these effects can be used to infer the disk mass, composition, optical depth,
and inclination relative to the line of sight. If the IR excess is due to a
disk, then near-UV and optical effects should be observed in only some systems,
not all of them, while for dust shells the effects should be observed in all
systems.Comment: 6 pages, 5 figure
Inhibition of the EGF receptor by binding of MIG6 to an activating kinase domain interface.
Members of the epidermal growth factor receptor family (EGFR/ERBB1, ERBB2/HER2, ERBB3/HER3 and ERBB4/HER4) are key targets for inhibition in cancer therapy. Critical for activation is the formation of an asymmetric dimer by the intracellular kinase domains, in which the carboxy-terminal lobe (C lobe) of one kinase domain induces an active conformation in the other. The cytoplasmic protein MIG6 (mitogen-induced gene 6; also known as ERRFI1) interacts with and inhibits the kinase domains of EGFR and ERBB2 (refs 3-5). Crystal structures of complexes between the EGFR kinase domain and a fragment of MIG6 show that a approximately 25-residue epitope (segment 1) from MIG6 binds to the distal surface of the C lobe of the kinase domain. Biochemical and cell-based analyses confirm that this interaction contributes to EGFR inhibition by blocking the formation of the activating dimer interface. A longer MIG6 peptide that is extended C terminal to segment 1 has increased potency as an inhibitor of the activated EGFR kinase domain, while retaining a critical dependence on segment 1. We show that signalling by EGFR molecules that contain constitutively active kinase domains still requires formation of the asymmetric dimer, underscoring the importance of dimer interface blockage in MIG6-mediated inhibition
A multi-wavelength view on the dusty Wolf-Rayet star WR 48a
We present results from the first attempts to derive various physical
characteristics of the dusty Wolf-Rayet star WR 48a based on a multi-wavelength
view of its observational properties. This is done on the basis of new optical
and near-infrared spectral observations and on data from various archives in
the optical, radio and X-rays. The optical spectrum of WR 48a is acceptably
well represented by a sum of two spectra: of a WR star of the WC8 type and of a
WR star of the WN8h type. The strength of the interstellar absorption features
in the optical spectra of WR 48a and the near-by stars D2-3 and D2-7 (both
members of the open cluster Danks 2) indicates that WR 48a is located at a
distance of ~4 kpc from us. WR 48a is very likely a thermal radio source and
for such a case and smooth (no clumps) wind its radio emission suggests a
relatively high mass-loss rate of this dusty WR star (dM/dt = a few x 10^(-4)
solar masses per year). Long timescale (years) variability of WR 48a is
established in the optical, radio and X-rays. Colliding stellar winds likely
play a very important role in the physics of this object. However, some
LBV-like (luminous blue variable) activity could not be excluded as well.Comment: Accepted for publication in MNRAS; 16 pages, 16 figures, 6 table
Dust-enshrouded Asymptotic Giant Branch Stars in the Solar Neighbourhood
A study is made of a sample of 58 dust-enshrouded Asymptotic Giant Branch
(AGB) stars (including 2 possible post AGB stars), of which 27 are carbon-rich
and 31 are oxygen-rich. These objects were originally identified by Jura &
Kleinmann as nearby (within about 1 kpc of the sun) AGB stars with high
mass-loss rates, greater than 1E-6 solar masses per year. Ground-based
near-infrared photometry, data obtained by IRAS and kinematic data from the
literature are combined to investigate the properties of these stars. The light
amplitude in the near-infrared is found to be correlated with period, and this
amplitude decreases with increasing wavelength. Statistical tests show that
there is no reason to suspect any difference in the period distributions of the
carbon- and oxygen-rich stars for periods less than 1000 days, and no
carbon-rich star has a period longer than 1000 days. The colours are consistent
with those of cool stars with evolved circumstellar dust-shells. Luminosities
and distances are estimated using a period-luminosity relation. Mass-loss
rates, estimated from the 60 micron fluxes, show a correlation with pulsation
period and is tightly correlated with the K-[12] colour. The kinematics and
scale-height of the sample shows that the sources with periods less than 1000
days must have low mass main-sequence progenitors. It is argued that the three
oxygen-rich stars with periods over 1000 days probably had intermediate mass
main-sequence progenitors with remaining stars having an average progenitor
mass of about 1.3 solar masses. The average lifetime of stars in this phase is
estimated to be about 4.0E4 years, indicating they will undergo at most one
more thermal pulse before leaving the AGB.Comment: 27 pages, 20 figures, accepted for MNRA
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