73 research outputs found
First evidence of pulsations in Vega? Results of today's most extensive spectroscopic search
The impact of rapid rotation on stellar evolution theory remains poorly
understood as of today. Vega is a special object in this context as
spectroscopic and interferometric studies have shown that it is a rapid rotator
seen nearly pole one, a rare orientation particularly interesting for seismic
studies. In this paper we present a first systematic search for pulsations in
Vega. The goal of the present work is to detect for the first time pulsations
in a rapidly rotating star seen nearly pole-on. Vega was monitored in
quasi-continuous high-resolution echelle spectroscopy. A total of 4478 spectra
were obtained within 3 individual runs in 2008, 2009 and 2010 at high
resolution. This data set should represent the most extensive high S/N, high
resolution quasi-continuous survey obtained on Vega as of today. Equivalent
photospheric absorption profiles were calculated for the stellar spectrum, but
also for the telluric lines acting as a radial velocity reference. Residual
velocities were analysed and periodic low amplitude variations, potentially
indicative of stellar pulsations, detected. All three data sets revealed the
presence of residual periodic variations: 5.32 and 9.19 c/d, (A approx 6 m/s)
in 2008, 12.71 and 13.25 c/d, (A approx 8 m/s) in 2009 and 5.42 and 10.82 c/d,
(A approx 3-4 m/s) in 2010. A Lomb-Scargle periodogram of each velocity bin of
the equivalent profile was performed for the 2010 run, not showing the presence
of any higher order nrp mode. It is too early to conclude that the variations
are due to stellar pulsations, and a confirmation of the detection with a
highly stable spectrograph is a necessary next step. If pulsations are
confirmed, their very small amplitudes show that the star would belong to a
category of very "quiet" pulsators
Discovery of starspots on Vega - First spectroscopic detection of surface structures on a normal A-type star
The theoretically studied impact of rapid rotation on stellar evolution needs
to be confronted with the results of high resolution spectroscopy-velocimetry
observations. A weak surface magnetic field had recently been detected in the
A0 prototype star Vega, potentially leading to a (yet undetected) structured
surface. The goal of this article is to present a thorough analysis of the line
profile variations and associated estimators in the early-type standard star
Vega (A0) in order reveal potential activity tracers, exoplanet companions and
stellar oscillations. Vega was monitored in high-resolution spectroscopy with
the velocimeter Sophie/OHP. A total of 2588 high S/N spectra was obtained
during 5 nights (August 2012) at R = 75000 and covering the visible domain. For
each reduced spectrum, Least Square Deconvolved (LSD) equivalent photospheric
profiles were calculated with a Teff = 9500 and logg = 4.0 spectral line mask.
Several methods were applied to study the dynamic behavior of the profile
variations (evolution of radial velocity, bisectors, vspan, 2D profiles,
amongst others). We present the discovery of a starspotted stellar surface in
an A-type standard star with faint spot amplitudes Delta F/Fc ~5 10^{-4}. A
rotational modulation of spectral lines with a period of rotation P = 0.68 d
has clearly been exhibited, confirming the results of previous
spectropolarimetric studies. Either a very thin convective layer can be
responsible for magnetic field generation at small amplitudes, or a new
mechanism has to be invoked in order to explain the existence of activity
tracing starspots. This first strong evidence that standard A-type stars can
show surface structures opens a new field of research and asks the question
about a potential link with the recently discovered weak magnetic field
discoveries in this category of stars.Comment: accepted for publication by Astronomy & Astrophysics (23rd of March
2015
Regular modes in rotating stars
Despite more and more observational data, stellar acoustic oscillation modes
are not well understood as soon as rotation cannot be treated perturbatively.
In a way similar to semiclassical theory in quantum physics, we use acoustic
ray dynamics to build an asymptotic theory for the subset of regular modes
which are the easiest to observe and identify. Comparisons with 2D numerical
simulations of oscillations in polytropic stars show that both the frequency
and amplitude distributions of these modes can accurately be described by an
asymptotic theory for almost all rotation rates. The spectra are mainly
characterized by two quantum numbers; their extraction from observed spectra
should enable one to obtain information about stellar interiors.Comment: 5 pages, 4 figures, discussion adde
The tachocline revisited
The solar tachocline is a shear layer located at the base of the solar
convection zone. The horizontal shear in the tachocline is likely turbulent,
and it is often assumed that this turbulence would be strongly anisotropic as a
result of the local stratification. What role this turbulence plays in the
tachocline dynamics, however, remains to be determined. In particular, it is
not clear whether it would result in a turbulent eddy diffusivity, or
anti-diffusivity, or something else entirely. In this paper, we present the
first direct numerical simulations of turbulence in horizontal shear flows at
low Prandtl number, in an idealized model that ignores rotation and magnetic
fields. We find that several regimes exist, depending on the relative
importance of the stratification, viscosity and thermal diffusivity. Our
results suggest that the tachocline is in the stratified turbulence regime,
which has very specific properties controlled by a balance between buoyancy,
inertia, and thermal diffusion.Comment: Invited review for the meeting Dynamics of the Sun and Stars:
Honoring the Life and Work of Michael J. Thompson (Boulder, Colorado, 24-26
September 2019
Measuring mean densities of delta Scuti stars with asteroseismology. Theoretical properties of large separations using TOUCAN
We aim at studying the theoretical properties of the regular spacings found
in the oscillation spectra of delta Scuti stars. We performed a multi-variable
analysis covering a wide range of stellar structure and seismic properties and
model parameters representative of intermediate-mass, main sequence stars. The
work-flow is entirely done using a new Virtual Observatory tool: TOUCAN (the VO
gateway for asteroseismic models), which is presented in this paper. A linear
relation between the large separation and the mean density is predicted to be
found in the low frequency frequency domain (i.e. radial orders spanning from 1
to 8, approximately) of the main-sequence, delta Scuti stars' oscillation
spectrum. We found that such a linear behavior stands whatever the mass,
metallicity, mixing length, and overshooting parameters considered in this
work. The intrinsic error of the method is discussed. This includes the
uncertainty in the large separation determination and the role of rotation. The
validity of the relation found is only guaranteed for stars rotating up to 40
percent of their break-up velocity. Finally, we applied the diagnostic method
presented in this work to five stars for which regular patterns have been
found. Our estimates for the mean density and the frequency of the fundamental
radial mode match with those given in the literature within a 20 percent of
deviation. Asteroseismology has thus revealed an independent direct measure of
the average density of delta Scuti stars, analogous to that of the Sun. This
places tight constraints on the mode identification and hence on the stellar
internal structure and dynamics, and allows a determination the radius of
planets orbiting around delta Scuti stars with unprecedented precision. This
opens the way for studying the evolution of regular patterns in pulsating
stars, and its relation with stellar structure and evolution.Comment: 11 pages, 6 figures, A&A in pres
Magnetism, rotation and accretion in Herbig Ae-Be stars
Studies of stellar magnetism at the pre-main sequence phase can provide
important new insights into the detailed physics of the late stages of star
formation, and into the observed properties of main sequence stars. This is
especially true at intermediate stellar masses, where magnetic fields are
strong and globally organised, and therefore most amenable to direct study.
This talk reviews recent high-precision ESPaDOnS observations of pre-main
sequence Herbig Ae-Be stars, which are yielding qualitatively new information
about intermediate-mass stars: the origin and evolution of their magnetic
fields, the role of magnetic fields in generating their spectroscopic activity
and in mediating accretion in their late formative stages, and the factors
influencing their rotational angular momentum.Comment: 8 page
Spectroscopic Pulsational Frequency Identification and Mode Determination of Gamma Doradus Star HD135825
We present the mode identification of frequencies found in spectroscopic
observations of the Gamma Doradus star HD135825. Four frequencies were
successfully identified: 1.3150 +/- 0.0003 1/d; 0.2902 +/- 0.0004 1/d; 1.4045
+/- 0.0005 1/d; and 1.8829 +/- 0.0005 1/d. These correspond to (l, m) modes of
(1,1), (2,-2), (4,0) and (1,1) respectively. Additional frequencies were found
but they were below the signal-to-noise limit of the Fourier spectrum and not
suitable for mode identification. The rotational axis inclination and vsini of
the star were determined to be 87 degrees (nearly edge-on) and 39.7 km/s
(moderate for Gamma Doradus stars) respectively. A simultaneous fit of these
four modes to the line profile variations in the data gives a reduced chi
square of 12.7. We confirm, based on the frequencies found, that HD135825 is a
bona fide Gamma Doradus star.Comment: Accepted to MNRAS 2012 March
EK Eridani: the tip of the iceberg of giants which have evolved from magnetic Ap stars
We observe the slowly-rotating, active, single giant, EK Eri, to study and
infer the nature of its magnetic field directly. We used the spectropolarimeter
NARVAL at the Telescope Bernard Lyot, Pic du Midi Observatory, and the Least
Square Deconvolution method to create high signal-to-noise ratio Stokes V
profiles. We fitted the Stokes V profiles with a model of the large-scale
magnetic field. We studied the classical activity indicators, the CaII H and K
lines, the CaII infrared triplet, and H\alpha line. We detected the Stokes V
signal of EK Eri securely and measured the longitudinal magnetic field Bl for
seven individual dates spanning 60% of the rotational period. The measured
longitudinal magnetic field of EK Eri reached about 100 G and was as strong as
fields observed in RSCVn or FK Com type stars: this was found to be
extraordinary when compared with the weak fields observed at the surfaces of
slowly-rotating MS stars or any single red giant previously observed with
NARVAL. From our modeling, we infer that the mean surface magnetic field is
about 270 G, and that the large scale magnetic field is dominated by a poloidal
component. This is compatible with expectations for the descendant of a
strongly magnetic Ap star.Comment: 8 pages, 6 figures. Accepted for publication in A&
Discovery of a weak magnetic field in the photosphere of the single giant Pollux
Aims: We observe the nearby, weakly-active single giant, Pollux, in order to
directly study and infer the nature of its magnetic field. Methods: We used the
new generation spectropolarimeters ESPaDOnS and NARVAL to observe and detect
circular polarization within the photospheric absorption lines of Pollux. Our
observations span 18 months from 2007-2009. We treated the spectropolarimetric
data using the Least-Squares Deconvolution method to create high
signal-to-noise ratio mean Stokes V profiles. We also measured the classical
activity indicator S-index for the Ca H&K lines, and the stellar radial
velocity (RV). Results: We have unambiguously detected a weak Stokes V signal
in the spectral lines of Pollux, and measured the related surface-averaged
longitudinal magnetic field Bl. The longitudinal field averaged over the span
of the observations is below one gauss. Our data suggest variations of the
longitudinal magnetic field, but no significant variation of the S-index. We
observe variations of RV which are qualitatively consistent with the published
ephemeris for a proposed exoplanet orbiting Pollux. The observed variations of
Bl appear to mimic those of RV, but additional data for this relationship to be
established. Using evolutionary models including the effects of rotation, we
derive the mass of Pollux and we discuss its evolutionary status and the origin
of its magnetic field. Conclusions: This work presents the first direct
detection of the magnetic field of Pollux, and demonstrates that ESPaDOnS and
NARVAL are capable of obtaining sub-G measurements of the surface-averaged
longitudinal magnetic field of giant stars, and of directly studying the
relationships between magnetic activity, stellar evolution and planet hosting
of these stars.Comment: 8 pages, 6 figures, accepted for publication in Astronomy and
Astrophysic
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