5,307 research outputs found
The center-to-limb variation across the Fraunhofer lines of HD 189733; Sampling the stellar spectrum using a transiting planet
The center-to-limb variation (CLV) describes the brightness of the stellar
disk as a function of the limb angle. Across strong absorption lines, the CLV
can vary quite significantly. We obtained a densely sampled time series of
high-resolution transit spectra of the active planet host star HD 189733 with
UVES. Using the passing planetary disk of the hot Jupiter HD 189733 b as a
probe, we study the CLV in the wings of the Ca II H and K and Na I D1 and D2
Fraunhofer lines, which are not strongly affected by activity-induced
variability. In agreement with model predictions, our analysis shows that the
wings of the studied Fraunhofer lines are limb brightened with respect to the
(quasi-)continuum. The strength of the CLV-induced effect can be on the same
order as signals found for hot Jupiter atmospheres. Therefore, a careful
treatment of the wavelength dependence of the stellar CLV in strong absorption
lines is highly relevant in the interpretation of planetary transit
spectroscopy.Comment: Accepted in A&
Time-resolved UVES observations of a stellar flare on the planet host HD 189733 during primary transit
HD 189733 is an exoplanetary system consisting of a transiting hot Jupiter
and an active K2V-type main sequence star. We aim to use VLT/UVES high
resolution echelle spectra to study a stellar flare. We have performed
simultaneous analyses of the temporal evolution in several chromospheric
stellar lines, namely, the Ca II H and K lines, Halpha, Hbeta, Hgamma, Hdelta,
Hepsilon, the Ca II infrared triplet line, and He I D3. Observations were
carried out with a time resolution of approximately 1 min for a duration of
four hours, including a complete planetary transit. We determine the energy
released during the flare in all studied chromospheric lines combined to be
about 8.7e31 erg, which puts this event at the upper end of flare energies
observed on the Sun. Our analysis does not reveal any significant delay of the
flare peak observed in the Balmer and Ca II H and K lines, although we find a
clear difference in the temporal evolution of these lines. The He I D3 shows
additional absorption possibly related to the flare event. Based on the flux
released in Ca II H and K lines during the flare, we estimate the soft X-ray
flux emission to be 7e30 erg. The observed flare can be ranked as a moderate
flare on a K-type star and confirms a rather high activity level of HD 189733
host star. The cores of the studied chromospheric lines demonstrate the same
behavior and let us study the flare evolution. We demonstrate that the activity
of an exoplanet host star can play an important role in the detection of
exoplanet atmospheres, since these are frequently discovered as an additional
absorption in the line cores. A possible star-planet interaction responsible
for a flare occurrence during a transit can neither be confirmed nor ruled out.Comment: 12 pages, 9 figures, accepted for publication in A&
The REX survey: a search for Radio Emitting X-ray sources
We present the scientific goals, the strategy and the first results of the
REX project, an effort aimed at creating a sizable and statistically complete
sample of Radio Emitting X-ray sources (REX) using the available data from a
VLA survey and the ROSAT PSPC archive. Through a positional cross-correlation
of the two data sets we have derived a sample of about 1600 REX. Among the 393
REX identified so far a high fraction is represented by AGNs, typically radio
loud QSOs and BL Lacs. The remaining sources are galaxies, typically radio
galaxies isolated or in cluster. Thanks to the low flux limits in the radio and
in the X-ray band and the large area of sky covered by the survey, we intend to
derive a new complete and unbiased sample of BL Lacs which will contain both
``RBL'' and ``XBL'' type objects. In this way, the apparent dichotomy resulting
from the current samples of BL Lacs will be directly analyzed in a unique
sample. Moreover, the high number of BL Lacs expected in the REX sample (about
200) will allow an accurate estimate of their statistical properties. To date,
we have discovered 15 new BL Lacs and 11 BL Lac candidates with optical
properties intermediate between those of a typical elliptical galaxy and those
of a typical BL Lac object. These objects could harbour weak sources of
non-thermal continuum in their nuclei and, if confirmed, they could represent
the faint tail of the BL Lac population. The existence of such ``weak'' BL Lacs
is matter of discussion in recent literature and could lead to a re-assessment
of the defining criteria of a BL Lac and, consequently, to a revision of their
cosmological and statistical properties.Comment: 30 pages, 13 figures, accepted for publication to Ap
Emission Line AGNs from the REX survey: Results from optical spectroscopy
We present 71 Emission Line objects selected from the REX survey. Except for
3 of them, for which the presence of an active nucleus is dubious, all these
sources are Active Galactic Nuclei (QSOs, Seyfert galaxies, emission line
radiogalaxies). In addition, we present the spectra of other 19 AGNs included
in a preliminary version of the REX catalog but not in the final one. The
majority (80) of the 90 sources presented in this paper is newly discovered.
Finally, we present the general properties in the radio and in the X-ray band
of all the AGNs discovered so far in the REX survey.Comment: 27 pages. To be published in Astronomy and Astrophysics, Supplement
Series. Better quality figures can be asked to the autho
Module extraction via query inseparability in OWL 2 QL
We show that deciding conjunctive query inseparability for OWL 2 QL ontologies is PSpace-hard and in ExpTime. We give polynomial-time (incomplete) algorithms and demonstrate by experiments that they can be used for practical module extraction
Isospin effects on sub-threshold kaon production at intermediate energies
We show that in collisions with neutron rich heavy ions at energies around
the production threshold K^0 and K^+ yields might probe the isospin dependent
part of the nuclearEquation of State (EoS) at high baryon densities. In
particular we suggest the K^0/K^+ ratio as a promising observable. Results
obtained in a fully covariant relativistic transport approach are presented for
central Au+Au collisions in the beam energy range 0.8-1.8~AGeV. The focus is
put on the EoS influence which goes beyond the "collision-cascade" picture. The
isovector part of the in-medium interaction affects the kaon multiplicities via
two mechanisms: i) a "symmetry potential" effect, i.e. a larger neutron
repulsion in n-rich systems (isospin fractionation); ii) a "threshold" effect,
due to the change in the self-energies of the particles involved in inelastic
processes. Genuine relativistic contributions are revealed, that could allow to
directly ``measure'' the Lorentz structure of the effective isovector
interaction.Comment: 5 pages, 2 figures, revtex
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