294 research outputs found
No planet for HD 166435
The G0V star HD166435 has been observed by the fiber-fed spectrograph ELODIE
as one of the targets in the large extra-solar planet survey that we are
conducting at the Observatory of Haute-Provence. We detected coherent,
low-amplitude, radial-velocity variations with a period of 3.7987days,
suggesting a possible close-in planetary companion. Subsequently, we initiated
a series of high-precision photometric observations to search for possible
planetary transits and an additional series of CaII H and K observations to
measure the level of surface magnetic activity and to look for possible
rotational modulation. Surprisingly, we found the star to be photometrically
variable and magnetically active. A detailed study of the phase stability of
the radial-velocity signal revealed that the radial-velocity variability
remains coherent only for durations of about 30days. Analysis of the time
variation of the spectroscopic line profiles using line bisectors revealed a
correlation between radial velocity and line-bisector orientation. All of these
observations, along with a one-quarter cycle phase shift between the
photometric and the radial-velocity variationss, are well explained by the
presence of dark photospheric spots on HD166435. We conclude that the
radial-velocity variations are not due to gravitational interaction with an
orbiting planet but, instead, originate from line-profile changes stemming from
star spots on the surface of the star. The quasi-coherence of the
radial-velocity signal over more than two years, which allowed a fair fit with
a binary model, makes the stability of this star unusual among other active
stars. It suggests a stable magnetic field orientation where spots are always
generated at about the same location on the surface of the star.Comment: 9 pages, 8 figures, Accepted for publication in A&
Dynamo models and differential rotation in late-type rapidly rotating stars
Increasing evidence is becoming available about not only the surface
differential rotation of rapidly rotating cool stars but, in a small number of
cases, also about temporal variations, which possibly are analogous to the
solar torsional oscillations. Given the present difficulties in resolving the
precise nature of such variations, due to both the short length and poor
resolution of the available data, theoretical input is vital to help assess the
modes of behaviour that might be expected, and will facilitate interpretation
of the observations. Here we take a first step in this direction by studying
the variations in the convection zones of such stars, using a two dimensional
axisymmetric mean field dynamo model operating in a spherical shell in which
the only nonlinearity is the action of the azimuthal component of the Lorentz
force of the dynamo generated magnetic field on the stellar angular velocity.
We consider three families of models with different depths of dynamo-active
regions. For moderately supercritical dynamo numbers we find torsional
oscillations that penetrate all the way down to the bottom of the convection
zones, similar to the case of the Sun. For larger dynamo numbers we find
fragmentation in some cases and sometimes there are other dynamical modes of
behaviour, including quasi-periodicity and chaos. We find that the largest
deviations in the angular velocity distribution caused by the Lorentz force are
of the order of few percent, implying that the original assumed `background'
rotation field is not strongly distorted.Comment: Astronomy and Astrophysics, in pres
A Search for Variability in the Spectral Line Shapes of tau Bootis: Does this Star Really Have a Planet?
An analysis is made of the spectral line shapes of tau Bootis using high
resolution (0.026 A) and high signal-to-noise (S/N~400) data in an effort to
confirm the planet hypothesis for this star. Changes in the line shape are
quantified using spectral line bisectors and line residuals. We detect no
variations in either of these quantities above the level of the noise in the
data. One spectral line, Fe I 6213 A, does show a hint of sinusoidal variations
in the bisector velocity span when phased to the radial velocity period of 3.3
days, but this is not seen in the bisectors for two other lines, nor in the
line residuals. Comparisons of the data to the bisector and residual variations
expected for nonradial pulsations indicate that we can exclude those sectoral
nonradial modes having m>2 and all sectoral modes with k>1, where k is the
ratio of the horizontal to vertical velocities for the pulsations. The lack of
line shape variability and the 469 m/s radial velocity amplitude is still
consistent with nonradial sectoral modes m=1, and possibly m=2, but with k~1,
which is at least 3 orders of magnitude less than the predicted value given the
3.3 day period of tau Bootis. Such low values of k can probably be excluded
given the lack of photometric variations for this star. Although the
measurements presented here do not prove, without any doubt, that tau Boo has a
planetary companion, they do add significantly to the increasing body of
evidence in favor of this hypothesis.Comment: LaTeX. 22 pages, 9 figures. Accepted in the Astrophysical Journa
Fluxes in H\alpha and Ca II H and K for a sample of Southern stars
The main chromospheric activity indicator is the S index, which is esentially
the ratio of the flux in the core of the Ca II H and K lines to the continuum
nearby, and is well studied basically for stars from F to K. Another usual
chromospheric proxy is the H\alpha line, which is beleived to be tightly
correlated with the Ca II index. In this work we characterize both
chromospheric activity indicators, one associated with the H and K Ca II lines
and the other with H\alpha, for the whole range of late type stars, from F to
M. We present periodical medium-resolution echelle observations covering the
complete visual range, which were taken at the CASLEO Argentinean Observatory.
These observations are distributed along 7 years. We use a total of 917
flux-calibrated spectra for 109 stars which range from F6 to M5. We
statistically study these two indicators for stars of different activity levels
and spectral types. We directly derive the conversion factor which translate
the known S index to flux in the Ca II cores, and extend its calibration to a
wider spectral range. We investigate the relation between the activity
measurements in the calcium and hydrogen lines, and found that the usual
correlation observed is basically the product of the dependence of each flux
with stellar colour, and not the product of similar activity phenomena.Comment: 12 pages, including 11 figures and 2 tables. Accepted for publication
in Astronomy and Astrophysic
Polar branches of stellar activity waves: dynamo models and observations
[Abridged abstract:] Stellar activity data provide evidence of activity wave
branches propagating polewards rather than equatorwards (the solar case).
Stellar dynamo theory allows polewards propagating dynamo waves for certain
governing parameters. We try to unite observations and theory, restricting our
investigation to the simplest mean-field dynamo models. We suggest a crude
preliminary systematization of the reported cases of polar activity branches.
Then we present results of dynamo model simulations which contain magnetic
structures with polar dynamo waves, and identify the models which look most
promising for explaining the latitudinal distribution of spots in dwarf stars.
Those models require specific features of stellar rotation laws, and so
observations of polar activity branches may constrain internal stellar
rotation. Specifically, we find it unlikely that a pronounced poleward branch
can be associated with a solar-like internal rotation profile, while it can be
more readily reproduced in the case of a cylindrical rotation law appropriate
for fast rotators. We stress the case of the subgiant component of the active
close binary HR 1099 which, being best investigated, presents the most severe
problems for a dynamo interpretation. Our best model requires dynamo action in
two layers separated in radius. Observations of polar activity branches provide
valuable information for understanding stellar activity mechanisms and internal
rotation, and thus deserve intensive observational and theoretical
investigation. Current stellar dynamo theory seems sufficiently robust to
accommodate the phenomenology.Comment: 13 pages, 10 figures, 4 tables, accepted by Astronomy and
Astrophysic
A possible activity cycle in Proxima Centauri
Several late-type stars present activity cycles resembling the Solar one.
This fact has been observed mostly in stars ranging from F to K, i.e., in stars
with a radiative core and an outer convective layer. This work aims at studying
whether an activity cycle can be detected in the dM5.5e star Proxima Centauri,
which is supposed to be completely convective. We present periodical
medium-resolution echelle observations covering the complete visual range,
which were taken at the CASLEO Argentinean Observatory. These observations are
distributed over 7 years. We discarded the spectra that present flare activity,
and analyze the remaining activity levels using four different statistical
techniques to look for a period of activity. We find strong evidence of a
cyclic activity, with a period of around 442 days. We also estimate that the Ca
II S index varies around 130% due to activity variations outside of flares.Comment: 7 pages, including 8 figures and 2 table
Ages for illustrative field stars using gyrochronology: viability, limitations and errors
We here develop an improved way of using a rotating star as a clock, set it
using the Sun, and demonstrate that it keeps time well. This technique, called
gyrochronology, permits the derivation of ages for solar- and late-type main
sequence stars using only their rotation periods and colors. The technique is
clarified and developed here, and used to derive ages for illustrative groups
of nearby, late-type field stars with measured rotation periods. We first
demonstrate the reality of the interface sequence, the unifying feature of the
rotational observations of cluster and field stars that makes the technique
possible, and extends it beyond the proposal of Skumanich by specifying the
mass dependence of rotation for these stars. We delineate which stars it cannot
currently be used on. We then calibrate the age dependence using the Sun. The
errors are propagated to understand their dependence on color and period.
Representative age errors associated with the technique are estimated at ~15%
(plus possible systematic errors) for late-F, G, K, & early-M stars. Ages
derived via gyrochronology for the Mt. Wilson stars are shown to be in good
agreement with chromospheric ages for all but the bluest stars, and probably
superior. Gyro ages are then calculated for each of the active main sequence
field stars studied by Strassmeier and collaborators where other ages are not
available. These are shown to be mostly younger than 1Gyr, with a median age of
365Myr. The sample of single, late-type main sequence field stars assembled by
Pizzolato and collaborators is then assessed, and shown to have gyro ages
ranging from under 100Myr to several Gyr, and a median age of 1.2Gyr. Finally,
we demonstrate that the individual components of the three wide binaries
XiBooAB, 61CygAB, & AlphaCenAB yield substantially the same gyro ages.Comment: 58 pages, 18 color figures, accepted for publication in The
Astrophysical Journal; Age uncertainties slightly modified upon correcting an
algebraic error in Section
About the p-mode frequency shifts in HD 49933
We study the frequency dependence of the frequency shifts of the low-degree p
modes measured in the F5V star HD 49933, by analyzing the second run of
observations collected by the CoRoT satellite. The 137-day light curve is
divided into two subseries corresponding to periods of low and high stellar
activity. The activity-frequency relationship is obtained independently from
the analysis of the mode frequencies extracted by both a local and a global
peak-fitting analyses, and from a cross-correlation technique in the frequency
range between 1450 microHz and 2500 microHz. The three methods return
consistent results. We show that the frequency shifts measured in HD 49933
present a frequency dependence with a clear increase with frequency, reaching a
maximal shift of about 2 microHz around 2100 microHz. Similar variations are
obtained between the l=0 and l=1 modes. At higher frequencies, the frequency
shifts show indications of a downturn followed by an upturn, consistent between
the l=0 and 1 modes. We show that the frequency variation of the p-mode
frequency shifts of the solar-like oscillating star HD 49933 has a comparable
shape to the one observed in the Sun, which is understood to arise from changes
in the outer layers due to its magnetic activity.Comment: 5 pages, 3 figures, 1 table, Accepted for publication in A\&
Probable detection of starlight reflected from the giant exoplanet orbiting tau Bootis
Giant planets orbiting stars other than the Sun are clearly detectable
through precise radial-velocity measurements of the orbital reflex motion of
the parent star. In the four years since the discovery of the companion to the
star 51 Peg, similar low-amplitude ``Doppler star wobbles'' have revealed the
presence of some 20 planets orbiting nearby solar-type stars. Several of these
newly-discovered planets are very close to their parent stars, in orbits with
periods of only a few days. Being an indirect technique, however, the
reflex-velocity method has little to say about the sizes or compositions of the
planets, and can only place lower limits on their masses. Here we report the
use of high-resolution optical spectroscopy to achieve a probable detection of
the Doppler-shifted signature of starlight reflected from one of these objects,
the giant exoplanet orbiting the star tau Bootis. Our data give the planet's
orbital inclination i=29 degrees, indicating that its mass is some 8 times that
of Jupiter, and suggest strongly that the planet has the size and reflectivity
expected for a gas-giant planet.Comment: 15 pages, 4 figures. (Fig 1 and equation for epsilon on p1 para 2
revised; changed from double to single spacing
The Extrasolar Planet epsilon Eridani b - Orbit and Mass
Hubble Space Telescope observations of the nearby (3.22 pc), K2 V star
epsilon Eridani have been combined with ground-based astrometric and radial
velocity data to determine the mass of its known companion. We model the
astrometric and radial velocity measurements simultaneously to obtain the
parallax, proper motion, perturbation period, perturbation inclination, and
perturbation size. Because of the long period of the companion, \eps b, we
extend our astrometric coverage to a total of 14.94 years (including the three
year span of the \HST data) by including lower-precision ground-based
astrometry from the Allegheny Multichannel Astrometric Photometer. Radial
velocities now span 1980.8 -- 2006.3. We obtain a perturbation period, P = 6.85
+/- 0.03 yr, semi-major axis, alpha =1.88 +/- 0.20 mas, and inclination i =
30.1 +/- 3.8 degrees. This inclination is consistent with a previously measured
dust disk inclination, suggesting coplanarity. Assuming a primary mass M_* =
0.83 M_{\sun}, we obtain a companion mass M = 1.55 +/- 0.24 M_{Jup}. Given the
relatively young age of epsilon Eri (~800 Myr), this accurate exoplanet mass
and orbit can usefully inform future direct imaging attempts. We predict the
next periastron at 2007.3 with a total separation, rho = 0.3 arcsec at position
angle, p.a. = -27 degrees. Orbit orientation and geometry dictate that epsilon
Eri b will appear brightest in reflected light very nearly at periastron.
Radial velocities spanning over 25 years indicate an acceleration consistent
with a Jupiter-mass object with a period in excess of 50 years, possibly
responsible for one feature of the dust morphology, the inner cavity
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