724 research outputs found
Evidence for the disintegration of KIC 12557548 b
Context. The Kepler object KIC 12557548 b is peculiar. It exhibits
transit-like features every 15.7 hours that vary in depth between 0.2% and
1.2%. Rappaport et al. (2012) explain the observations in terms of a
disintegrating, rocky planet that has a trailing cloud of dust created and
constantly replenished by thermal surface erosion. The variability of the
transit depth is then a consequence of changes in the cloud optical depth.
Aims. We aim to validate the disintegrating-planet scenario by modeling the
detailed shape of the observed light curve, and thereby constrain the cloud
particle properties to better understand the nature of this intriguing object.
Methods. We analysed the six publicly-available quarters of raw Kepler data,
phase-folded the light curve and fitted it to a model for the trailing dust
cloud. Constraints on the particle properties were investigated with a
light-scattering code. Results. The light curve exhibits clear signatures of
light scattering and absorption by dust, including a brightening in flux just
before ingress correlated with the transit depth and explained by forward
scattering, and an asymmetry in the transit light curve shape, which is easily
reproduced by an exponentially decaying distribution of optically thin dust,
with a typical grain size of 0.1 micron. Conclusions. Our quantitative analysis
supports the hypothesis that the transit signal of KIC 12557548 b is due to a
variable cloud of dust, most likely originating from a disintegrating object.Comment: 5 pages, 4 figures. Accepted for publication in Astronomy and
Astrophysic
Hot subdwarf binaries - Masses and nature of their heavy compact companions
Neutron stars and stellar-mass black holes are the remnants of massive stars,
which ended their lives in supernova explosions. These exotic objects can only
be studied in relatively rare cases. If they are interacting with close
companions they become bright X-ray sources. If they are neutron stars, they
may be detected as pulsars. Only a few hundred such systems are presently known
in the Galaxy. However, there should be many more binaries with basically
invisible compact objects in non-interacting binaries. Here we report the
discovery of unseen compact companions to hot subdwarfs in close binary
systems. Hot subdwarfs are evolved helium-core-burning stars that have lost
most of their hydrogen envelopes, often due to binary interactions. Using
high-resolution spectra and assuming tidal synchronisation of the subdwarfs, we
were able to constrain the companion masses of 32 binaries. While most hot
subdwarf binaries have white-dwarf or late-type main sequence companions, as
predicted by binary evolution models, at least 5% of the observed subdwarfs
must have very massive companions: unusually heavy white dwarfs, neutron stars
and, in some cases, even black holes. We present evolutionary models which show
that such binaries can indeed form if the system has evolved through two
common-envelope phases. This new connection between hot subdwarfs, which are
numerous in the Galaxy, and massive compact objects may lead to a tremendous
increase in the number of known neutron stars and black holes and shed some
light on this dark population and its evolutionary link to the X-ray binary
population.Comment: 8 pages, 5 figures, to appear in the Journal of Physics Conference
Proceedings (JPCS) for the 16th European White Dwarf Workshop, Barcelona,
Spain, June 30 - July 11, 200
On the late spectral types of cataclysmic variable secondaries
We investigate why the spectral type of most cataclysmic variable (CV)
secondaries is significantly later than that of a ZAMS star with the same mean
density. Using improved stellar input physics, tested against observations of
low-mass stars at the bottom of the main sequence, we calculate the secular
evolution of CVs with low-mass donors. We consider sequences with different
mass transfer rates and with a different degree of nuclear evolution of the
donor prior to mass transfer. Systems near the upper edge of the gap ( h) can be reproduced by models with a wide range of mass transfer rates
from 1.5 \times 10^{-9} \msolyr to 10^{-8} \msolyr. Evolutionary sequences
with a small transfer rate and donors that are substantially evolved off the
ZAMS (central hydrogen content ) reproduce CVs with late spectral
types above P \simgr 6 h. Systems with the most discrepant (late) spectral
type should have the smallest donor mass at any given . Consistency with the
period gap suggests that the mass transfer rate increases with decreasing donor
mass for evolved sequences above the period gap. In this case, a
single-parameter family of sequences with varying \xc and increasing mass
transfer rate reproduces the full range of observed spectral types. This would
imply that CVs with such evolved secondaries dominate the CV population.Comment: 9 pages, Latex file, uses mn.sty, accepted for publication in MNRA
The violent past of Cygnus X-2
Cygnus X-2 appears to be the descendant of an intermediate-mass X-ray binary
(IMXB). Using Mazzitelli's (1989) stellar code we compute detailed evolutionary
sequences for the system and find that its prehistory is sensitive to stellar
input parameters, in particular the amount of core overshooting during the
main-sequence phase. With standard assumptions for convective overshooting a
case B mass transfer starting with a 3.5 M_sun donor star is the most likely
evolutionary solution for Cygnus X-2. This makes the currently observed state
rather short-lived, of order 3 Myr, and requires a formation rate > 1e-7 - 1e-6
per yr of such systems in the Galaxy. Our calculations show that neutron star
IMXBs with initially more massive donors (> 4 M_sun) encounter a delayed
dynamical instability; they are unlikely to survive this rapid mass transfer
phase. We determine limits for the age and initial parameters of Cygnus X-2 and
calculate possible dynamical orbits of the system in a realistic Galactic
potential, given its observed radial velocity. We find trajectories which are
consistent with a progenitor binary on a circular orbit in the Galactic plane
inside the solar circle that received a kick velocity < 200 km/s at the birth
of the neutron star. The simulations suggests that about 7% of IMXBs receiving
an arbitrary kick velocity from a standard kick velocity spectrum would end up
in an orbit similar to Cygnus X-2, while about 10% of them reach yet larger
Galactocentric distances.Comment: 9 pages, 12 figures, accepted for publication in MNRA
Arabidopsis CURVATURE THYLAKOID1 Proteins Modify Thylakoid Architecture by Inducing Membrane Curvature
Chloroplasts of land plants characteristically contain grana, cylindrical stacks of thylakoid membranes. A granum consists of a core of appressed membranes, two stroma-exposed end membranes, and margins, which connect pairs of grana membranes at their lumenal sides. Multiple forces contribute to grana stacking, but it is not known how the extreme curvature at margins is generated and maintained. We report the identification of the CURVATURE THYLAKOID1 (CURT1) protein family, conserved in plants and cyanobacteria. The four Arabidopsis thaliana CURT1 proteins (CURT1A, B, C, and D) oligomerize and are highly enriched at grana margins. Grana architecture is correlated with the CURT1 protein level, ranging from flat lobe-like thylakoids with considerably fewer grana margins in plants without CURT1 proteins to an increased number of membrane layers (and margins) in grana at the expense of grana diameter in overexpressors of CURT1A. The endogenous CURT1 protein in the cyanobacterium Synechocystis sp PCC6803 can be partially replaced by its Arabidopsis counterpart, indicating that the function of CURT1 proteins is evolutionary conserved. In vitro, Arabidopsis CURT1A proteins oligomerize and induce tubulation of liposomes, implying that CURT1 proteins suffice to induce membrane curvature. We therefore propose that CURT1 proteins modify thylakoid architecture by inducing membrane curvature at grana margins
Non-conservative Evolution of Cataclysmic Variables
We suggest a new mechanism to account for the loss of angular momentum in
binaries with non-conservative mass exchange. It is shown that in some cases
the loss of matter can result in increase of the orbital angular momentum of a
binary. If included into consideration in evolutionary calculations, this
mechanism appreciably extends the range of mass ratios of components for which
mass exchange in binaries is stable. It becomes possible to explain the
existence of some observed cataclysmic binaries with high donor/accretor mass
ratio, which was prohibited in conservative evolution models.Comment: LaTeX, 32 pages, to be published in Astron. Z
Brown Dwarfs and the Cataclysmic Variable Period Minimum
Using improved, up-to-date stellar input physics tested against observations
of low-mass stars and brown dwarfs we calculate the secular evolution of
low-mass donor cataclysmic variables (CVs), including those which form with a
brown dwarf donor. Our models confirm the mismatch between the calculated
minimum period (Pmin ~ 70 min) and the observed short-period cut-off (~ 80 min)
in the CV period histogram. We find that tidal and rotational corrections
applied to the one-dimensional stellar structure equations have no significant
effect on the period minimum. Theoretical period distributions synthesized from
our model sequences always show an accumulation of systems at the minimum
period, a feature absent from the observed distribution. We suggest that
non-magnetic CVs become unobservable as they are effectively trapped in
permanent quiescence before they reach Pmin, and that small-number statistics
may hide the period spike for magnetic CVs.Comment: 10 pages; accepted for publication in MNRA
Detached white dwarf main-sequence star binaries
We considered the formation of detached white dwarf main-sequence star (WDMS)
binaries through seven evolutionary channels subdivided according to the
evolutionary process that gives rise to the formation of the white dwarf or its
helium-star progenitor: dynamically stable Roche-lobe overflow (Algol-type
evolution), dynamically unstable Roche-lobe overflow (common-envelope
evolution), or stellar winds (single star evolution). We examine the
sensitivity of the population to changes in the amount of mass lost from the
system during stable Roche-lobe overflow, the common-envelope ejection
efficiency, and the initial mass ratio or initial secondary mass distribution.
In the case of a flat initial mass ratio distribution, the local space density
of WDMS binaries is of the order of 10^{-3}/pc^3. This number decreases to
10^{-4}/pc^3 when the initial mass ratio distribution is approximately
proportional to the inverse of the initial mass ratio. More than 75% of the
WDMS binary population stems from wide systems in which both components evolve
as if they were single stars. The remaining part of the population is dominated
by systems in which the white dwarf is formed in a common-envelope phase. The
birthrate of WDMS binaries forming through a common-envelope phase is about 10
times larger than the birthrate of WDMS binaries forming through a stable
Roche-lobe overflow phase. The ratio of the number of helium white dwarf
systems to the number of carbon/oxygen or oxygen/neon/magnesium white dwarf
systems derived from large samples of observed WDMS binaries by, e.g., future
planet-search missions such as SuperWASP, COROT, and Kepler may furthermore
constrain the common-envelope ejection efficiency.Comment: 22 pages, accepted for publication in A&
V-I characteristics in the vicinity of order-disorder transition in vortex matter
The shape of the V-I characteristics leading to a peak in the differential
resistance r_d=dV/dI in the vicinity of the order-disorder transition in NbSe2
is investigated. r_d is large when measured by dc current. However, for a small
Iac on a dc bias r_d decreases rapidly with frequency, even at a few Hz, and
displays a large out-of-phase signal. In contrast, the ac response increases
with frequency in the absence of dc bias. These surprisingly opposite phenomena
and the peak in r_d are shown to result from a dynamic coexistence of two
vortex matter phases rather than from the commonly assumed plastic depinning.Comment: 12 pages 4 figures. Accepted for publication in PRB rapi
MITS: the Multi-Imaging Transient Spectrograph for SOXS
The Son Of X-Shooter (SOXS) is a medium resolution spectrograph R~4500
proposed for the ESO 3.6 m NTT. We present the optical design of the UV-VIS arm
of SOXS which employs high efficiency ion-etched gratings used in first order
(m=1) as the main dispersers. The spectral band is split into four channels
which are directed to individual gratings, and imaged simultaneously by a
single three-element catadioptric camera. The expected throughput of our design
is >60% including contingency. The SOXS collaboration expects first light in
early 2021. This paper is one of several papers presented in these proceedings
describing the full SOXS instrument
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