985 research outputs found
EChO spectra and stellar activity II. The case of dM stars
EChO is a dedicated mission to investigate exoplanetary atmospheres. When
extracting the planetary signal, one has to take care of the variability of the
hosting star, which introduces spectral distortion that can be mistaken as
planetary signal. Magnetic variability is a major deal in particular for M
stars. To this purpose, assuming a one spot dominant model for the stellar
photosphere, we develop a mixed observational-theoretical tool to extract the
spot's parameters from the observed optical spectrum. This method relies on a
robust library of spectral M templates, which we derive using the observed
spectra of quiet M dwarfs in the SDSS database. Our procedure allows to correct
the observed spectra for photospheric activity in most of the analyzed cases,
reducing the spectral distortion down to the noise levels. Ongoing refinements
of the template library and the algorithm will improve the efficiency of our
algorithm.Comment: Submitted to Experimental Astronom
Time evolution of X-ray coronal activity in PMS stars; a possible relation with the evolution of accretion disks
We investigate the evolution of X-ray stellar activity from the age of the
youngest known star forming regions (SFR), < 1Myr, to about 100 Myr, i.e. the
zero age main sequence (ZAMS) for a ~ 1M_sun star. We consider five SFR of
varying age (Rho Ophiuchi, the Orion Nebula Cluster, NGC 2264, Chamaeleon I,
and Eta Chamaeleontis) and two young clusters (the Pleiades and NGC 2516).
Optical and X-ray data for these regions are retrieved both from archival
observations and recent literature, and reanalyzed here in a consistent manner
so to minimize systematic differences in the results. We study trends of L_X
and L_X/L_bol as a function of stellar mass and association age. For low mass
stars (M < 1M_sun) we observe an increase in L_X/L_bol in the first 3-4 Myr and
a subsequent leveling off at the saturation level (L_X/L_bol ~ -3). Slowly
evolving very low mass stars then retain saturated levels down to the oldest
ages here considered, while for higher mass stars activity begins to decline at
some age after ~10^7 years. We find our data consistent with the following
tentative picture: low mass PMS stars with no circumstellar accretion disk have
saturated activity, consistently with the activity-Rossby number relation
derived for MS stars. Accretion and/or the presence of disks somehow lowers the
observed activity levels; disk dissipation and/or the decrease of mass
accretion rate in the first few Myrs of PMS evolution is therefore responsible
for the observed increase of L_X/L_bol with time.Comment: 17 pages, 8 figures, accepted for publication in A&
Simbol-X capability of detecting the non-thermal emission of stellar flares
We investigate the capability of detecting, with Simbol-X, non-thermal
emission during stellar flares, and distinguishing it from hot thermal
emission. We find that flare non-thermal emission is detectable when at least
~20 cts are detected with the CZT detector in the 20-80 keV band. Therefore
Simbol-X will detect the non-thermal emission from some of the X-ray brightest
nearby stars, whether the thermal vs. non-thermal relation, derived for solar
flares, holds.Comment: 2 pages, 2 postscript figures, proceedings of the workshop "Simbol-X:
the hard X-ray universe in focus", to be published in "Memorie of the Italian
Astronomical Society
A Chandra X-ray study of the young star cluster NGC 6231: low-mass population and initial mass function
NGC6231 is a massive young star cluster, near the center of the Sco OB1
association. While its OB members are well studied, its low-mass population has
received little attention. We present high-spatial resolution Chandra ACIS-I
X-ray data, where we detect 1613 point X-ray sources. Our main aim is to
clarify global properties of NGC6231 down to low masses through a detailed
membership assessment, and to study the cluster stars' spatial distribution,
the origin of their X-ray emission, the cluster age and formation history, and
initial mass function. We use X-ray data, complemented by optical/IR data, to
establish cluster membership. The spatial distribution of different stellar
subgroups also provides highly significant constraints on cluster membership,
as does the distribution of X-ray hardness. We perform spectral modeling of
group-stacked X-ray source spectra. We find a large cluster population down to
~0.3 Msun (complete to ~1 Msun), with minimal non-member contamination, with a
definite age spread (1-8 Myrs) for the low-mass PMS stars. We argue that
low-mass cluster stars also constitute the majority of the few hundreds
unidentified X-ray sources. We find mass segregation for the most massive
stars. The fraction of circumstellar-disk bearing members is found to be ~5%.
Photoevaporation of disks under the action of massive stars is suggested by the
spatial distribution of the IR-excess stars. We also find strong Halpha
emission in 9% of cluster PMS stars. The dependence of X-ray properties on
mass, stellar structure, and age agrees with extrapolations based on other
young clusters. The cluster initial mass function, computed over ~2 dex in
mass, has a slope Gamma~-1.14. The total mass of cluster members above 1 Msun
is 2280 Msun, and the inferred total mass is 4380 Msun. We also study the
peculiar, hard X-ray spectrum of the Wolf-Rayet star WR79.Comment: 25 pages, 36 figures, accepted for publication on Astronomy and
Astrophysic
Effects of XUV radiation on circumbinary planets
Several circumbinary planets have recently been discovered. The orbit of a
planet around a binary stellar system poses several dynamic constraints. The
effects that radiation from the host stars may have on the planet atmospheres
must be considered. Because of the configuration of a close binary system,
these stars have a high rotation rate, which causes a permanent state of high
stellar activity and copious XUV radiation. The accumulated effects are
stronger than for exoplanets around single stars, and cause a faster
evaporation of their atmospheres. We evaluate the effects that stellar
radiation has on the evaporation of exoplanets around binary systems and on the
survival of these planets. We considered the XUV spectral range to account for
the photons that are easily absorbed by a planet atmosphere that is mainly
composed of hydrogen. A more complex atmospheric composition is expected to
absorb this radiation more efficiently. We used direct X-ray observations to
evaluate the energy in the X-rays range and coronal models to calculate the
(nondetectable) EUV part of the spectrum. The simulations show that exoplanets
in a close orbit will suffer strong photoevaporation that may cause a total
loss of atmosphere in a short time. A binary system of two solar-like stars
will be highly efficient in evaporating the atmosphere of the planet. These
systems will be difficult to find, even if they are dynamically stable. Still,
planets may orbit around binary systems of low mass stars for wider orbits.
Currently known circumbinary planets are not substantially affected by thermal
photoevaporation processes, unless Kepler-47 b has an inflated atmosphere. The
distribution of the orbital periods of circumbinary planets is shifted to much
longer periods than the average of Kepler planets, which supports a scenario of
strong photoevaporation in close-in circumbinary planets.Comment: Accepted by A&A. 8 pages, 5 figure
The photospheric abundances of active binaries I Detailed analysis of HD 113816 (IS Vir) and HD 119285 (V851 Cen)
The high-resolution optical spectra of the two X-ray active binaries RS CVn
stars HD 113816 (IS Vir) and HD 119285 (V851 Cen) are analysed and their Na,
Mg, Al, Si, Ca, Sc, Ti, Co and Ni contents determined. The analysis of IS Vir
and V851 Cen is performed with three different LTE methods. In the first one,
abundances are derived using measured equivalent widths and Kurucz LTE model
atmospheres as input for the MOOG software package. The input atmospheric
parameters and abundances are iteratively modified until (i) the Fe I
abundances exhibit no trend with excitation potential or equivalent width, (ii)
Fe I and Fe II average abundances are the same and (iii) Fe and Alpha elements
average abundances are consistent with the input values. The second method
follows a similar approach, but relies on the B-V and V-I colour indices to
determine the temperature. The third method relies on fitting the 6162 A Ca I
line wing profiles to derive the surface gravity. The reliability of these
methods is investigated in the context of single line RS CVn stars. It is shown
that the V-I photometric index gives, on a broader sample of stars,
significantly cooler estimates of the effective temperature than the B-V index.
All other approaches give results in good agreement with each other. The
analysis of IS Vir and V851 Cen results in both cases in their primaries being
giant stars of near-solar metallicity. Their parameters as derived with the
first method are respectively Teff = 4720 K, log g = 2.65, [Fe/H] = +0.04 and
Teff = 4700 K, log g = 3.0 and [Fe/H] = -0.13. In the case of V851 Cen the
derived iron content is significantly higher than a previous determination in
the literature. Both stars exhibit relative overabundances of several elements
(e.g. Ca) with respect to the solar mix.Comment: 12 Pages, 2 Figures, Accepted for publication in A&
Connecting substellar and stellar formation. The role of the host star's metallicity
Most of our current understanding of the planet formation mechanism is based
on the planet metallicity correlation derived mostly from solar-type stars
harbouring gas-giant planets. To achieve a far more reaching grasp on the
substellar formation process we aim to analyse in terms of their metallicity a
diverse sample of stars (in terms of mass and spectral type) covering the whole
range of possible outcomes of the planet formation process (from planetesimals
to brown dwarfs and low-mass binaries). Our methodology is based on the use of
high-precision stellar parameters derived by our own group in previous works
from high-resolution spectra by using the iron ionisation and equilibrium
conditions. All values are derived in an homogeneous way, except for the M
dwarfs where a methodology based on the use of pseudo equivalent widths of
spectral features was used. Our results show that as the mass of the substellar
companion increases the metallicity of the host star tendency is to lower
values. The same trend is maintained when analysing stars with low-mass stellar
companions and a tendency towards a wide range of host star's metallicity is
found for systems with low mass planets. We also confirm that more massive
planets tend to orbit around more massive stars. The core-accretion formation
mechanism for planet formation achieves its maximum efficiency for planets with
masses in the range 0.2 and 2 M. Substellar objects with higher
masses have higher probabilities of being formed as stars. Low-mass planets and
planetesimals might be formed by core-accretion even around low-metallicity
stars.Comment: Accepted by A&
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