444 research outputs found
The stratified evolution of a cool star
A low mass star usually experiences stratification and abundance anomalies
during its evolution. A 0.95 solar mass star with a metallicity Z = 0.004 is
followed from the main-sequence to the Horizontal Branch (HB). On the
main-sequence the larger effects of stratification may come from accretion as
was suggested in relation to metallicity and planet formation. As it evolves
through the giant branch, stratification appears around the hydrogen burning
shell. It may create hydrodynamic instabilities and be related to abundance
anomalies on the giant branch. After the He flash the star evolves to the HB.
If it loses enough mass, it ends up a hot HB star (or in the field an sdB star)
with effective temperatures larger than 11000 K. All sdB stars are observed to
have an approximately solar iron abundance whatever their original metallicity,
implying overabundances by factors of up to 100. So should the 0.95 solar mass
star. How its internal hydrodynamic properties on the main sequence may
influence its fate on the HB is currently uncertain.Comment: Astronomische Nachrichten - Astronomical Notes (AN) papers presented
at the Cool Stars 17 conference 2012 (AN 334, issue 1-2
Abundance anomalies in pre-main-sequence stars: Stellar evolution models with mass loss
The effects of atomic diffusion on internal and surface abundances of A and F
pre-main-sequence stars with mass loss are studied in order to determine at
what age the effects materialize, as well as to further understand the
processes at play in HAeBe and young ApBp stars. Self-consistent stellar
evolution models of 1.5 to 2.8Msun with atomic diffusion (including radiative
accelerations) for all species within the OPAL opacity database were computed
and compared to observations of HAeBe stars. Atomic diffusion in the presence
of weak mass loss can explain the observed abundance anomalies of
pre-main-sequence stars, as well as the presence of binary systems with metal
rich primaries and chemically normal secondaries such as V380 Ori and HD72106.
This is in contrast to turbulence models which do not allow for abundance
anomalies to develop on the pre-main-sequence. The age at which anomalies can
appear depends on stellar mass. For A and F stars, the effects of atomic
diffusion can modify both the internal and surface abundances before the onset
of the MS. The appearance of important surface abundance anomalies on the
pre-main-sequence does not require mass loss, though the mass loss rate affects
their amplitude. Observational tests are suggested to decipher the effects of
mass loss from those of turbulent mixing. If abundance anomalies are confirmed
in pre-main-sequence stars they would severely limit the role of turbulence in
these stars.Comment: 9 pages, 6 figures, accepeted for publicatio
Abundance anomalies in pre-main-sequence stars: Stellar evolution models with mass loss
The effects of atomic diffusion on internal and surface abundances of A and F
pre-main-sequence stars with mass loss are studied in order to determine at
what age the effects materialize, as well as to further understand the
processes at play in HAeBe and young ApBp stars. Self-consistent stellar
evolution models of 1.5 to 2.8Msun with atomic diffusion (including radiative
accelerations) for all species within the OPAL opacity database were computed
and compared to observations of HAeBe stars. Atomic diffusion in the presence
of weak mass loss can explain the observed abundance anomalies of
pre-main-sequence stars, as well as the presence of binary systems with metal
rich primaries and chemically normal secondaries such as V380 Ori and HD72106.
This is in contrast to turbulence models which do not allow for abundance
anomalies to develop on the pre-main-sequence. The age at which anomalies can
appear depends on stellar mass. For A and F stars, the effects of atomic
diffusion can modify both the internal and surface abundances before the onset
of the MS. The appearance of important surface abundance anomalies on the
pre-main-sequence does not require mass loss, though the mass loss rate affects
their amplitude. Observational tests are suggested to decipher the effects of
mass loss from those of turbulent mixing. If abundance anomalies are confirmed
in pre-main-sequence stars they would severely limit the role of turbulence in
these stars.Comment: 9 pages, 6 figures, accepeted for publicatio
Measurement of the electron drift velocity for directional dark matter detectors
Three-dimensional track reconstruction is a key issue for directional Dark
Matter detection. It requires a precise knowledge of the electron drift
velocity. Magboltz simulations are known to give a good evaluation of this
parameter. However, large TPC operated underground on long time scale may be
characterized by an effective electron drift velocity that may differ from the
value evaluated by simulation. In situ measurement of this key parameter is
hence a way to avoid bias in the 3D track reconstruction. We present a
dedicated method for the measurement of the electron drift velocity with the
MIMAC detector. It is tested on two gas mixtures : and . We also show that adding allows us to lower the
electron drift velocity while keeping almost the same Fluorine content of the
gas mixture.Comment: Proceedings of the 4th international conference on Directional
Detection of Dark Matter (CYGNUS 2013), 10-12 June 2013, Toyama, Japa
In situ measurement of the electron drift velocity for upcoming directional Dark Matter detectors
Three-dimensional track reconstruction is a key issue for directional Dark
Matter detection and it requires a precise knowledge of the electron drift
velocity. Magboltz simulations are known to give a good evaluation of this
parameter. However, large TPC operated underground on long time scale may be
characterized by an effective electron drift velocity that may differ from the
value evaluated by simulation. In situ measurement of this key parameter is
hence needed as it is a way to avoid bias in the 3D track reconstruction. We
present a dedicated method for the measurement of the electron drift velocity
with the MIMAC detector. It is tested on two gas mixtures: CF4 and CF4 + CHF3.
The latter has been chosen for the MIMAC detector as we expect that adding CHF3
to pure CF4 will lower the electron drift velocity. This is a key point for
directional Dark Matter as the track sampling along the drift field will be
improved while keeping almost the same Fluorine content of the gas mixture. We
show that the drift velocity at 50 mbar is reduced by a factor of about 5 when
adding 30% of CHF3.Comment: 19 pages, 14 figures. Minor corrections, matches published version in
JINS
Dedicated front-end and readout electronics developments for real time 3D directional detection of dark matter with MIMAC
A complete dedicated electronics, from front-end to back-end, was developed
to instrument a MIMAC prototype. A front end ASIC able to monitor 64 strips of
pixels and to provide their individual "Time Over Threshold" information has
been designed. An associated acquisition electronics and a real time track
reconstruction software have been developed to monitor a 512 channel prototype.
This auto-triggered electronic uses embedded processing to reduce the data
transfer to its useful part only, i.e. decoded coordinates of hit tracks and
corresponding energy measurements. The electronic designs, acquisition software
and the results obtained are presented.Comment: Proceedings of the 3rd International conference on Directional
Detection of Dark Matter (CYGNUS 2011), Aussois, France, 8-10 June 201
Development of a front end ASIC for Dark Matter directional detection with MIMAC
A front end ASIC (BiCMOS-SiGe 0.35 \mum) has been developed within the
framework of the MIMAC detector project, which aims at directional detection of
non-baryonic Dark Matter. This search strategy requires 3D reconstruction of
low energy (a few keV) tracks with a gaseous \muTPC. The development of this
front end ASIC is a key point of the project, allowing the 3D track
reconstruction. Each ASIC monitors 16 strips of pixels with charge
preamplifiers and their time over threshold is provided in real time by current
discriminators via two serializing LVDS links working at 320 MHz. The charge is
summed over the 16 strips and provided via a shaper. These specifications have
been chosen in order to build an auto triggered electronics. An acquisition
board and the related software were developed in order to validate this
methodology on a prototype chamber. The prototype detector presents an anode
where 2 x 96 strips of pixels are monitored.Comment: 12 pages, 10 figure
Development and validation of a 64 channel front end ASIC for 3D directional detection for MIMAC
A front end ASIC has been designed to equip the {\mu}TPC prototype developed
for the MIMAC project, which requires 3D reconstruction of low energy particle
tracks in order to perform directional detection of galactic Dark Matter. Each
ASIC is able to monitor 64 strips of pixels and provides the "Time Over
Threshold" information for each of those. These 64 digital informations,
sampled at a rate of 50 MHz, can be transferred at 400MHz by eight LVDS serial
links. Eight ASIC were validated on a 2x256 strips of pixels prototype.Comment: proceedings of TWEPP-11, Vienna, Austria, 26-30 September 201
Horizontal Branch evolution, metallicity and sdB stars
Context. Abundance anomalies have been observed in field sdB stars and in
nearly all Horizontal Branch (HB) stars of globular clusters with Teff > 11
000K whatever be the cluster metallicity. Aims. The aim is to determine the
abundance variations to be expected in sdB stars and in HB stars of
metallicities Z \geq 0.0001 and what observed abundances teach us about
hydrodynamical processes competing with atomic diffusion. Methods. Complete
stellar evolution models, including the effects of atomic diffusion and
radiative acceleration, have been computed from the zero age main-sequence for
metallicities of Z0 = 0.0001, 0.001, 0.004 and 0.02. On the HB the masses were
selected to cover the Teff interval from 7000 to 37000K. Some 60 evolutionary
HB models were calculated. The calculations of surface abundance anomalies
during the horizontal branch depend on one parameter, the surface mixed mass.
Results. For sdB stars with Teff 11 000K
in all observed clusters, independent of metallicity, it was found that most
observed abundance anomalies (even up to ~ x 200) were compatible, within error
bars, with expected abundances. A mixed mass of ~1.E-7 M\odot was determined by
comparison with observations. Conclusions. Observations of globular cluster HB
stars with Teff > 11 000K and of sdB stars with Teff < 37 000K suggest that
most observed abundance anomalies can be explained by element separation driven
by radiative acceleration occuring at a mass fraction of ~1.E-7 M\odot. Mass
loss or turbulence appear to limit the separation between 1.E-7 M\odot and the
surface.Comment: Accepted for publication by A&
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