8,500 research outputs found
Abundance studies of sdB stars using UV echelle HST/STIS spectroscopy
Aims: We test the hypothesis that the pulsations in sdB stars are correlated
with the surface abundances of iron-group elements. Any correlation might
explain why, when given two spectroscopically similar stars, one will pulsate
while the other will not. Methods: We have obtained high-resolution ultraviolet
spectra two pulsating and three non-pulsating sdB stars using the Space
Telescope Imaging Spectrograph onboard the Hubble Space Telescope. We
determined abundances for 25 elements including the iron group and even heavier
elements such as tin and lead using LTE curve-of-growth and spectrum synthesis
techniques. Results: We find no clear correlation between pulsations and metal
abundances, and we comment on the resulting implications, including whether it
is possible to determine the difference between a pulsating and a non-pulsating
sdB spectroscopically. In addition to the main goal of our observations, we
have also investigated the effect of supersolar metallicity on fundamental
parameter determination, possible trends with iron abundance, and the
hypothesis that weak winds may be selectively removing elements from the
stellar envelopes. These effects provide challenges to stellar atmosphere
modelling and diffusion models for sdB stars.Comment: 14 pages, 12 figures, accepted for publication in A&
Equilibrium States and SRB-like measures of C1 Expanding Maps of the Circle
For any C1 expanding map f of the circle we study the equilibrium states for
the potential -log |f'|. We formulate a C1 generalization of Pesin's Entropy
Formula that holds for all the SRB measures if they exist, and for all the
(necessarily existing) SRB-like measures. In the C1-generic case Pesin's
Entropy Formula holds for a unique SRB measure which is not absolutely
continuous with respect to Lebesgue. The result also stands in the non generic
case for which no SRB measure exists.Comment: Un this version we include some addings and corrections that were
suggested by the referees. The final version will appear in Portugaliae
Mathematica and will be available at
http://www.ems-ph.org/journals/journal.php?jrn=p
Discovery of magnetic fields in hot subdwarfs
We present initial results of a project to measure mean longitudinal magnetic
fields in a group of sdB/OB/O stars. The project was inspired by the discovery
of three super-metal-rich sdOB stars, each having metals (e.g. Ti, V) enhanced
by factors of 10^3 to 10^5. Similar behaviour is observed in chemically
peculiar A stars, where strong magnetic fields are responsible for the
enrichment. With this in mind, we obtained circularly polarised spectra of two
of the super-metal-rich sdOBs, two "normal" sdBs and two sdOs using FORS1 on
the ESO/VLT. By examining circular polarisation in the hydrogen Balmer lines
and in helium lines, we have detected magnetic fields with strengths of 1-2 kG
in most of our targets. This suggests that such fields are relatively common in
hot subdwarfs.Comment: 4 pages, to appear in White Dwarfs, eds. D. Koester, S. Moehler, ASP
Conf. serie
Spatial gradients of GCR protons in the inner heliosphere derived from Ulysses COSPIN/KET and PAMELA measurements
During the transition from solar cycle 23 to 24 from 2006 to 2009, the Sun
was in an unusual solar minimum with very low activity over a long period.
These exceptional conditions included a very low interplanetary magnetic field
(IMF) strength and a high tilt angle, which both play an important role in the
modulation of galactic cosmic rays (GCR) in the heliosphere. Thus, the radial
and latitudinal gradients of GCRs are very much expected to depend not only on
the solar magnetic epoch, but also on the overall modulation level. We
determine the non-local radial and the latitudinal gradients of protons in the
rigidity range from ~0.45 to 2 GV. This was accomplished by using data from the
satellite-borne experiment Payload for Antimatter Matter Exploration and
Light-nuclei Astrophysics (PAMELA) at Earth and the Kiel Electron Telescope
(KET) onboard Ulysses on its highly inclined Keplerian orbit around the Sun
with the aphelion at Jupiter's orbit. In comparison to the previous A>0 solar
magnetic epoch, we find that the absolute value of the latitudinal gradient is
lower at higher and higher at lower rigidities. This energy dependence is
therefore a crucial test for models that describe the cosmic ray transport in
the inner heliosphere
Galactic cosmic ray hydrogen spectra and radial gradients in the inner heliosphere measured by the HELIOS Experiment 6
Context: The HELIOS solar observation probes provide unique data regarding
their orbit and operation time. One of the onboard instruments, the Experiment
6 (E6), is capable of measuring ions from 4 to several hundred MeV/nuc.
Aims: In this paper we aim to demonstrate the relevance of the E6 data for
the calculation of galactic cosmic ray (GCR), anomolous cosmic ray (ACR), and
solar energetic particle (SEP) fluxes for different distances from the sun and
time periods.
Methods: Several corrections have been applied to the raw data: determination
of the Quenching factor of the scintillator, correction of the temperature
dependent electronics, degradation of the scintillator as well as the effects
on the edge of semi-conductor detectors.
Results: Fluxes measured by the E6 are in accordance with the force field
solution for the GCR and match models of the anomalous cosmic ray propagation.
GCR radial gradients in the inner heliosphere show a different behaviour than
in the outer heliosphereComment: 8 pages, 10 figure
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