18 research outputs found
Fine structure of the chromospheric activity in Solar-type stars - The Halpha Line
A calibration of H-alpha as both a chromospheric diagnostic and an age
indicator is presented, complementing the works previously done on this subject
(Herbig 1985, Pasquini & Pallavicini 1991. The chromospheric diagnostic was
built with a statistically significant sample, covering nine years of
observations, and including 175 solar neighborhood stars. Regarding the age
indicator, the presence of stars for which very accurate ages are determined,
such as those belonging to clusters and kinematic groups, lends confidence to
our analysis. We also investigate the possibility that stars of the same age
might have gone through different tracks of chromospheric decay, identifying -
within the same age range - effects of metallicity and mass. These parameters,
however, as well as age, seem to be significant only for dwarf stars, losing
their meaning when we analyze stars in the subgiant branch. This result
suggests that, in these evolved stars, the emission mechanism cannot be
magnetohydrodynamical in nature, in agreement with recent models (Fawzy et al.
2002c, and references therein). The Sun is found to be a typical star in its
H-alpha chromospheric flux, for its age, mass and metallicity. As a byproduct
of this work, we developed an automatic method to determine temperatures from
the wings of H-alpha, which means the suppression of the error inherent to the
visual procedure used in the literature.Comment: 10 pages, 10 figures, accepted for publication in Astronomy &
Astrophysics. Nature of replacement: match astro-ph and ADS title (greek
letter
The magnetic field of the pre-main sequence Herbig Ae star HD 190073
The general context of this paper is the study of magnetic fields in the
pre-main sequence intermediate mass Herbig Ae/Be stars. Magnetic fields are
likely to play an important role in pre-main sequence evolution at these
masses, in particular in controlling the gains and losses of stellar angular
momentum. The particular aim of this paper is to announce the detection of a
structured magnetic field in the Herbig Ae star HD 190073, and to discuss
various scenarii for the geometry of the star, its environment and its magnetic
field. We have used the ESPaDOnS spectropolarimeter at CFHT in 2005 and 2006 to
obtain high-resolution and signal-to-noise circular polarization spectra which
demonstrate unambiguously the presence of a magnetic field in the photosphere
of this star. Nine circular polarization spectra were obtained, each one
showing a clear Zeeman signature. This signature is suggestive of a magnetic
field structured on large scales. The signature, which corresponds to a
longitudinal magnetic field of 74+- 10 G, does not vary detectably on a
one-year timeframe, indicating either an azimuthally symmetric field, a zero
inclination angle between the rotation axis and the line of sight, or a very
long rotation period. The optical spectrum of HD 190073 exhibits a large number
of emission lines. We discuss the formation of these emission lines in the
framework of a model involving a turbulent heated region at the base of the
stellar wind, possibly powered by magnetic accretion. This magnetic detection
brings an important element for our understanding of stellar magnetism at
intermediate masses.Comment: 10 pages, 6 figures. accepted by Astronomy & Astrophysic
The age of the Galactic thin disk from Th/Eu nucleocosmochronology I. Determination of [Th/Eu] abundance ratios
The purpose of this work is to resume investigation of Galactic thin disk
dating using nucleocosmochronology with Th/Eu stellar abundance ratios, a theme
absent from the literature since 1990. A stellar sample of 20 disk
dwarfs/subgiants of F5 to G8 spectral types with -0.8 <= [Fe/H] <= +0.3 was
selected. In stars with such spectral types and luminosity classes, spectral
synthesis techniques must be employed if we wish to achieve acceptably accurate
results. A homogeneous, self-consistent set of atmospheric parameters was
determined. Effective temperatures were determined from photometric
calibrations and H-alpha profile fitting; surface gravities were obtained from
Teff, stellar masses and luminosities; microturbulence velocities and
metallicities were obtained from detailed, differential spectroscopic analysis,
relative to the Sun, using equivalent widths of Fe I and Fe II lines. Chemical
abundances of the elements that contaminate the Th and Eu spectral regions (Ti,
V, Cr, Mn, Co, Ni, Ce, Nd, and Sm) were determined through spectroscopic
analysis. Abundance uncertainties were thoroughly scrutinised, their average
value - (0.10 +/- 0.02) dex - being found to be satisfactorily low. Eu and Th
abundances were determined by spectral synthesis of one Eu II line (4129.72 A)
and one Th II line (4019.13 A), taking into account the detailed hyperfine
structures of contaminating Co lines, as well as the hyperfine structure and
isotope shift of the Eu line. Comparison of our abundances with literature data
shows that our results exhibit a similar behaviour, but a considerably lower
scatter (36% lower for Eu, and 61% lower for Th). The [Th/Eu] abundance ratios
thus obtained were used, in the second paper of this series, to determine the
age of the Galactic disk.Comment: 27 pages, 22 Postscript figures, accepted for publication in
Astronomy & Astrophysics, final versio
The Spectral Energy Distribution and Mass-loss Rate of the A-Type Supergiant Deneb
A stellar wind module has been developed for the PHOENIX stellar atmosphere
code for the purpose of computing non-LTE, line-blanketed, expanding
atmospheric structures and detailed synthetic spectra of hot luminous stars
with winds. We apply the code to observations of Deneb, for which we report the
first positive detections of mm and cm emission (obtained using the SCUBA and
the VLA), as well a strong upper limit on the 850 micron flux (using the HHT).
The slope of the radio spectrum shows that the stellar wind is partially
ionized. We report a uniform-disk angular diameter measurement, 2.40 +/- 0.06
mas, from the Navy Prototype Optical Interferometer (NPOI). The measured
bolometric flux and corrected NPOI angular diameter yield an effective
temperature of 8600 +/- 500 K. Least-squares comparisons of synthetic spectral
energy distributions from 1220 A to 3.6 cm with the observations provide
estimates for the effective temperature and the mass-loss rate of 8400 +/- 100
K and 8 +/- 3 E-7 M_sun/yr, respectively. This range of mass-loss rates is
consistent with that derived from high dispersion UV spectra when non-LTE
metal-line blanketing is considered. We are unable achieve a reasonable fit to
a typical Halpha P-Cygni profile with any model parameters over a reasonable
range. This is troubling because the \ha profile is the observational basis for
Wind Momentum-Luminosity Relationship.Comment: Accepted by the Astrophysical Journal, 43 pages, 23 figure