357 research outputs found
Spectral analysis and abundances of the post-HB star HD 76431
HD76431 is a slow rotating post-HB star that shows an underabundance of
helium by 0.5 dex relative to the solar value. These observational facts
suggest that atomic diffusion could be active in its atmosphere. We have used
the MMT and Bok spectra to estimate the atmospheric parameters of the target
star using the model atmospheres and synthetic spectra calculated with TLUSTY
and SYNSPEC. The derived values of the effective temperature, surface gravity,
helium abundance are consistent with those obtained by Ramspeck et al. (2001b).
It appears that NLTE effect are not important for HD76431. We have used Stokes
I spectra from ESPaDOnS at CFHT to perform an abundance analysis and a search
for observational evidence of vertical stratification of the abundance of
certain elements. The results of our abundance analysis are in good agreement
with previously published data with respect to average abundances. Our
numerical simulations show that carbon and nitrogen reveal signatures of
vertical abundance stratification in the atmosphere of HD76431. It appears that
the carbon abundance increases toward the deeper atmospheric layers. Nitrogen
also shows a similar behaviour, but in deeper atmospheric layers we obtain a
significant dispersion for the estimates of its abundance. To our knowledge,
this is the first demonstration of vertical abundance stratification of metals
in a post-HB star and up to now it is the hottest star to show such
stratification features. We also report the detection of two SiIII and one
TiIII emission lines in the spectra of HD76431 that were not detected in
previous studies.Comment: 7 pages, 5 figures, accepted for publication in MNRA
Observations and asteroseismological analysis of the rapid subdwarf B pulsator EC 09582-1137
We made photometric and spectroscopic observations of the rapidly pulsating
subdwarf B star EC 09582-1137 with the aim of determining the target's
fundamental structural parameters from asteroseismology. The new data comprise
~ 30 hours of fast time-series photometry obtained with SUSI2 at the NTT on La
Silla, Chile, as well as 1 hour of low-resolution spectroscopy gathered with
EMMI, also mounted on the NTT. From the photometry we detected 5 independent
harmonic oscillations in the 135-170 s period range with amplitudes up to 0.5%
of the mean brightness of the star. In addition, we extracted two periodicities
interpreted as components of a rotationally split multiplet that indicate a
rotation period of the order of 2-5 days. We also recovered the first harmonic
of the dominant pulsation, albeit at an amplitude below the imposed 4-sigma
detection threshold. The spectroscopic observations led to the following
estimates of the atmospheric parameters of EC 09582-1137: Teff = 34,806+-233 K,
log g = 5.80+-0.04, and log[N(He)/N(H)] = - 1.68+-0.06.
Using the observed oscillations as input, we searched in model parameter
space for unique solutions that present a good fit to the data. Under the
assumption that the two dominant observed periodicities correspond to radial or
dipole modes, we were able to isolate a well-constrained optimal model that
agrees with the atmospheric parameters derived from spectroscopy. The inferred
structural parameters of EC 09582-1137 are Teff = 34,806 K (from spectroscopy),
log g = 5.788+-0.004, M = 0.485+-0.011 M_solar, log(M_env/M_star) = -
4.39+-0.10, R = 0.147+-0.002 R_solar, and L = 28.6+-1.7 L_solar. We
additionally derive the absolute magnitude M_V = 4.44+-0.05 and the distance d
= 1460+-66 pc.Comment: accepted for publication in Astronomy and Astrophysic
High-resolution spectropolarimetric observations of hot subdwarfs
We report on high-resolution spectropolarimetric observations of the hot
subdwarf stars HD 76431 and Feige 66, using the ESPaDOnS echelle
spectropolarimeter at CFHT. We compute cross-correlation Stokes I and V line
profiles to enhance the signal-to-noise ratio. We then average all available
cross-correlation profiles of each star to further decrease the noise level.
Although both targets were previously reported to host kilo-gauss magnetic
fields, we do not derive any evidence of large-scale photospheric fields from
our sets of observations, in spite of tight error bars on the longitudinal
field of the order of 60 gauss for HD 76431 and 200 gauss for Feige 66. A new
analysis of FORS1 observations of HD 76431, which provided the basis for the
original claim of field detection, confirms the absence of any detectable
Zeeman signature, with an error bar of about 100 gauss on the longitudinal
magnetic field.Comment: Proceedings of the fifth meeting on hot subdwarf stars and related
object
Mode identification from monochromatic amplitude and phase variations for the rapidly pulsating subdwarf B star EC 20338-1925
We obtain time-series spectrophotometry observations at the VLT with the aim
of partially identifying the dominant oscillation modes in the rapidly
pulsating subdwarf B star EC 20338-1925 on the basis of monochromatic amplitude
and phase variations. From the data gathered, we detect four previously known
pulsations with periods near 147, 168, 126 and 140 s and amplitudes between 0.2
and 2.3 % of the star's mean brightness. We also determine the atmospheric
parameters of EC 20338-1925 by fitting our non-LTE model atmospheres to an
averaged combined spectrum. The inferred parameters are Teff = 34,153+-94 K,
log g =5.966+-0.017 and log[N(He)/N(H)] = - 1.642+-0.022, where the uncertainty
estimates quoted refer to the formal fitting errors. Finally, we calculate the
observed monochromatic amplitudes and phases for the periodicities extracted
using least-squares fitting to the light curves obtained for each wavelength
bin. These observed quantities are then compared to the corresponding
theoretical values computed on the basis of dedicated model atmosphere codes
and also taking into account non-adiabatic effects. We find that the quality of
the data is sufficient to identify the dominant pulsation at 146.9 s as a
radial mode, while two of the lower amplitude periodicities must be low-degree
modes with l=0-2. This is the first time that monochromatic amplitudes and
phases have been used for mode identification in a subdwarf B star, and the
results are highly encouraging.Comment: 11 pages. Accepted for publication in Astronomy & Astrophysic
Kepler detection of a new extreme planetary system orbiting the subdwarf-B pulsator KIC10001893
KIC10001893 is one out of 19 subdwarf-B (sdB) pulsators observed by the
Kepler spacecraft in its primary mission. In addition to tens of pulsation
frequencies in the g-mode domain, its Fourier spectrum shows three weak peaks
at very low frequencies, which is too low to be explained in terms of g modes.
The most convincing explanation is that we are seeing the orbital modulation of
three Earth-size planets (or planetary remnants) in very tight orbits, which
are illuminated by the strong stellar radiation. The orbital periods are
P1=5.273, P2=7.807, and P3=19.48 hours, and the period ratios P2/P1=1.481 and
P3/P2=2.495 are very close to the 3:2 and 5:2 resonances, respectively. One of
the main pulsation modes of the star at 210.68 {\mu}Hz corresponds to the third
harmonic of the orbital frequency of the inner planet, suggesting that we see,
for the first time in an sdB star, g-mode pulsations tidally excited by a
planetary companion. The extreme planetary system that emerges from the Kepler
data is very similar to the recent discovery of two Earth-size planets orbiting
the sdB pulsator KIC05807616 (Charpinet et al. 2011a).Comment: 6 pages, 5 figures, accepted for publication in Astronomy and
Astrophysic
Ground-based follow-up observations of TRAPPIST-1 transits in the near-infrared
The TRAPPIST-1 planetary system is a favorable target for the atmospheric
characterization of temperate earth-sized exoplanets by means of transmission
spectroscopy with the forthcoming James Webb Space Telescope (JWST). A possible
obstacle to this technique could come from the photospheric heterogeneity of
the host star that could affect planetary signatures in the transit
transmission spectra. To constrain further this possibility, we gathered an
extensive photometric data set of 25 TRAPPIST-1 transits observed in the
near-IR J band (1.2 m) with the UKIRT and the AAT, and in the NB2090 band
(2.1 m) with the VLT during the period 2015-2018. In our analysis of these
data, we used a special strategy aiming to ensure uniformity in our
measurements and robustness in our conclusions. We reach a photometric
precision of (RMS of the residuals), and we detect no significant
temporal variations of transit depths of TRAPPIST-1 b, c, e, and g over the
period of three years. The few transit depths measured for planets d and f hint
towards some level of variability, but more measurements will be required for
confirmation. Our depth measurements for planets b and c disagree with the
stellar contamination spectra originating from the possible existence of bright
spots of temperature 4500 K. We report updated transmission spectra for the six
inner planets of the system which are globally flat for planets b and g and
some structures are seen for planets c, d, e, and f.Comment: accepted for publication in MNRA
A polarity reversal in the large-scale magnetic field of the rapidly rotating Sun HD 190771
Aims. We investigate the long-term evolution of the large-scale photospheric
magnetic field geometry of the solar-type star HD 190771. With fundamental
parameters very close to those of the Sun except for a shorter rotation period
of 8.8 d, HD 190771 provides us with a first insight into the specific impact
of the rotation rate in the dynamo generation of magnetic fields in 1
stars.
Methods. We use circularly polarized, high-resolution spectra obtained with
the NARVAL spectropolarimeter (Observatoire du Pic du Midi, France) and compute
cross-correlation line profiles with high signal-to-noise ratio to detect
polarized Zeeman signatures. From three phase-resolved data sets collected
during the summers of 2007, 2008, and 2009, we model the large-scale
photospheric magnetic field of the star by means of Zeeman-Doppler imaging and
follow its temporal evolution.
Results. The comparison of the magnetic maps shows that a polarity reversal
of the axisymmetric component of the large-scale magnetic field occurred
between 2007 and 2008, this evolution being observed in both the poloidal and
toroidal magnetic components. Between 2008 and 2009, another type of global
evolution occured, characterized by a sharp decrease of the fraction of
magnetic energy stored in the toroidal component. These changes were not
accompanied by significant evolution in the total photospheric magnetic energy.
Using our spectra to perform radial velocity measurements, we also detect a
very low-mass stellar companion to HD 190771.Comment: Accepted by Astronomy and Astrophysics (Letter to the Editor
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