2,299 research outputs found
The spectroscopic Hertzsprung-Russell diagram of Galactic massive stars
The distribution of stars in the Hertzsprung-Russell diagram narrates their
evolutionary history and directly assesses their properties. Placing stars in
this diagram however requires the knowledge of their distances and interstellar
extinctions, which are often poorly known for Galactic stars. The spectroscopic
Hertzsprung-Russell diagram (sHRD) tells similar evolutionary tales, but is
independent of distance and extinction measurements. Based on spectroscopically
derived effective temperatures and gravities of almost 600 stars, we derive for
the first time the observational distribution of Galactic massive stars in the
sHRD. While biases and statistical limitations in the data prevent detailed
quantitative conclusions at this time, we see several clear qualitative trends.
By comparing the observational sHRD with different state-of-the-art stellar
evolutionary predictions, we conclude that convective core overshooting may be
mass-dependent and, at high mass (), stronger than previously
thought. Furthermore, we find evidence for an empirical upper limit in the sHRD
for stars with between 10000 and 32000 K and, a strikingly large
number of objects below this line. This over-density may be due to inflation
expanding envelopes in massive main-sequence stars near the Eddington limit.Comment: 5 pages, 2 figures, 1 table; accepted for publication in A&A Letter
Six years of BeppoSAX observations of blazars: a spectral catalog
We present a spectral catalog for blazars based on the BeppoSAX archive. The
sample includes 44 High-energy peaked BL Lacs (HBLs), 14 Low-energy peaked BL
Lacs (LBLs), and 28 Flat Spectrum Radio Quasars (FSRQs). A total of 168 LECS,
MECS, and PDS spectra were analyzed, corresponding to observations taken in the
period 1996--2002. The 0.1--50 keV continuum of LBLs and FSRQs is generally
fitted by a single power law with Galactic column density. A minority of the
observations of LBLs (25%) and FSRQs (15%) is best fitted by more complex
models like the broken power law or the continuously curved parabola. These
latter models provide also the best description for half of the HBL spectra.
Complex models are more frequently required for sources with fluxes F_{2-10
keV} > 10^-11 cm-2 s-1, corresponding to spectra with higher signal-to-noise
ratio. As a result, considering sources with flux above this threshold, the
percentage of spectra requiring those models increases for all the classes. We
note that there is a net separation of X-ray spectral properties between HBLs
on one side, and LBLs and FSRQs on the other, the distinction between LBLs and
FSRQs is more blurry. This is most likely related to ambiguities in the optical
classification of the two classes.Comment: 6 pages, 4 figures, 7 tables. Accepted for publication in A&
Far-Ultraviolet Activity Levels of F, G, K, and M dwarf Exoplanet Host Stars
We present a survey of far-ultraviolet (FUV; 1150 - 1450 Ang) emission line
spectra from 71 planet-hosting and 33 non-planet-hosting F, G, K, and M dwarfs
with the goals of characterizing their range of FUV activity levels,
calibrating the FUV activity level to the 90 - 360 Ang extreme-ultraviolet
(EUV) stellar flux, and investigating the potential for FUV emission lines to
probe star-planet interactions (SPIs). We build this emission line sample from
a combination of new and archival observations with the Hubble Space
Telescope-COS and -STIS instruments, targeting the chromospheric and transition
region emission lines of Si III, N V, C II, and Si IV.
We find that the exoplanet host stars, on average, display factors of 5 - 10
lower UV activity levels compared with the non-planet hosting sample; this is
explained by a combination of observational and astrophysical biases in the
selection of stars for radial-velocity planet searches. We demonstrate that UV
activity-rotation relation in the full F - M star sample is characterized by a
power-law decline (with index ~ -1.1), starting at rotation periods
>~3.5 days. Using N V or Si IV spectra and a knowledge of the star's bolometric
flux, we present a new analytic relationship to estimate the intrinsic stellar
EUV irradiance in the 90 - 360 Ang band with an accuracy of roughly a factor of
~2. Finally, we study the correlation between SPI strength and UV activity in
the context of a principal component analysis that controls for the sample
biases. We find that SPIs are not a statistically significant contributor to
the observed UV activity levels.Comment: ApJS, accepted. 33 pages in emulateapj, 13 figures, 10 table
Chemical composition of A and F dwarfs members of the Hyades open cluster
Aims: Abundances of 15 chemical elements have been derived for 28 F and 16 A stars members of the Hyades open cluster in order to set constraints on self-consistent evolutionary models that include radiative and turbulent diffusion.
Methods: A spectral synthesis, iterative procedure was applied to derive the abundances from selected high-quality lines in high-resolution, high-signal-to-noise spectra obtained with SOPHIE and AURELIE at the Observatoire de Haute Provence.
Results: The abundance patterns found for A and F stars in the Hyades resemble those observed in Coma Berenices and Pleiades clusters. In graphs representing the abundances versus the effective temperature, A stars often display much more scattered abundances around their mean values than the coolest F stars do. Large star-to-star variations are detected in the Hyades A dwarfs in their abundances of C, Na, Sc, Fe, Ni, Sr, Y, and Zr, which we interpret as evidence of transport processes competing with radiative diffusion. In A and Am stars, the abundances of Cr, Ni, Sr, Y, and Zr are found to be correlated with that of Fe as in the Pleiades and in Coma Berenices. The ratios C/Fe and O/Fe are found to be anticorrelated with Fe/H as in Coma Berenices. All Am stars in the Hyades are deficient in C and O and overabundant in elements heavier than Fe but not all are deficient in Ca and/or Sc. The F stars have solar abundances for almost all elements except for Si. The overall shape of the abundance pattern of the slow rotator HD 30210 cannot be entirely reproduced by models including radiative diffusion and different amounts of turbulent diffusion.
Conclusions: While part of the discrepancies between derived and predicted abundances could come from non-LTE effects, including competing processes such as rotational mixing and/or mass loss seems necessary in order to improve the agreement between the observed and predicted abundance patterns
High-precision photometry of WASP-12 b transits
The transiting extrasolar planet WASP-12 b was found to be one of the most
intensely irradiated exoplanets. It is unexpectedly bloated and is losing mass
that may accrete into the host star. Our aim was to refine the parameters of
this intriguing system and search for signs of transit timing variations. We
gathered high-precision light curves for two transits of WASP-12 b. Assuming
various limb-darkening laws, we generated best-fitting models and redetermined
parameters of the system. Error estimates were derived by the prayer bead
method and Monte Carlo simulations. System parameters obtained by us are found
to agree with previous studies within one sigma. Use of the non-linear
limb-darkening laws results in the best-fitting models. With two new
mid-transit times, the ephemeris was refined to BJD(TDB)=(2454508.97682 +/-
0.00020) + (1.09142245 +/- 0.00000033) E. Interestingly, indications of transit
timing variation are detected at the level of 3.4 sigma. This signal can be
induced by an additional planet in the system. Simplified numerical simulations
shows that a perturber could be a terrestrial-type planet if both planets are
in a low-order orbital resonance. However, we emphasise that further
observations are needed to confirm variation and to constrain properties of the
perturber.Comment: 5 pages, 3 figures, accepted for publication in A&
Spectropolarimetric Variability and Co-Rotating Structure in HD 92207
We report on low resolution (R~3000) spectropolarimetry of the A0 supergiant
star HD 92207. This star is well-known for significant spectral variability.
The source was observed on seven different nights spanning approximately 3
months in time. With a rotation period of approximately 1 year, our data covers
approximately a quarter of the star's rotational phase. Variability in the
continuum polarization level is observed over this period of time. The
polarization across the Halpha line on any given night is typically different
from the degree and position angle of the polarization in the continuum.
Interestingly, Hbeta is not in emission and does not show polarimetric
variability. We associate the changes at Halpha as arising in the wind, which
is in accord with the observed changes in the profile shape and equivalent
width of Halpha along with the polarimetric variability. For the continuum
polarization, we explore a spiral shaped wind density enhancement in the
equatorial plane of the star, in keeping with the suggestion of Kaufer etal
(1997). Variable polarization signatures across Halpha are too complex to be
explained by this simple model and will require a more intensive polarimetric
follow-up study to interpret properly.Comment: to appear in A
Suppressed Far-UV stellar activity and low planetary mass-loss in the WASP-18 system
WASP-18 hosts a massive, very close-in Jupiter-like planet. Despite its young age (R′HK activity parameter lies slightly below the basal level; there is no significant time-variability in the log R′HK value; there is no detection of the star in the X-rays. We present results of far-UV observations of WASP-18 obtained with COS on board of HST aimed at explaining this anomaly. From the star’s spectral energy distribution, we infer the extinction (E(B − V) ≈ 0.01mag) and then the ISM column density for a number of ions, concluding that ISM absorption is not the origin of the anomaly. We measure the flux of the four stellar emission features detected in the COS spectrum (C II, C III, C IV, Si IV). Comparing the C II/C IV flux ratio measured for WASP-18 with that derived from spectra of nearby stars with known age, we see that the far-UV spectrum of WASP-18 resembles that of old (>5Gyr), inactive stars, in stark contrast with its young age. We conclude that WASP-18 has an intrinsically low activity level, possibly caused by star-planet tidal interaction, as suggested by previous studies. Re-scaling the solar irradiance reference spectrum to match the flux of the Si IV line, yields an XUV integrated flux at the planet orbit of 10.2 erg s−1 cm−2. We employ the rescaled XUV solar fluxes to model of the planetary upper atmosphere, deriving an extremely low thermal mass-loss rate of 10−20MJ Gyr−1. For such high-mass planets, thermal escape is not energy limited, but driven by Jeans escape
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