198 research outputs found
Periodic Halpha variations in GL 581: Further evidence for an activity origin to GL 581d
Radial velocity measurements showed evidence that the M dwarf GL 581 might
host a planet, GL 581d, in the so-called "habitable zone" of the star. A study
of Halpha in GL 581 demonstrated that changes in this activity indicator
correlated with radial velocity variations attributed to GL 581d. An exopplanet
that was important for studies of planet habitability may be an artifact of
stellar activity. Previous investigations analyzing the same activity data have
reached different conclusions regarding the existence of GL 581d. We therfore
investigated the Halpha variations for GL 581 to assess the nature of the
radial velocity variations attributed to the possible planet GL 581d. We
performed a Fourier analysis of the published Halpha measurements for GL 581d
using a so-called pre-whitening process to isolate the variations at the
orbital frequency of GL 581d. The frequency analysis yields five significant
frequencies, one of which is associated with the 66.7 d orbital period of the
presumed planet Gl 581d. The Halpha variations at this period show sine-like
variations that are 180 degrees out-of-phase with the radial velocity
variations of GL 581d. This is seen in thefull data set that spans almost 7
years, as well as a subset of the data that had good temporal sampling over 230
days. Furthermore, No significant temporal variations are found in the ratio of
the amplitudes of the Halpha index and radial velocity variations. This
provides additional evidence that the radial velocity signal attributed to GL
581d is in fact due to stellar activity.Comment: 5 pages, 6 figures, accepted by Astronomy and Astrophysic
On the Nature of the Radial Velocity Variability of Aldebaran: A Search for Spectral Line Bisector Variations
The shape of the Ti I 6303.8 A spectral line of Aldebaran as measured by the
line bisector was investigated using high signal-to-noise, high resolution
data. The goal of this study was to understand the nature of the 643-day period
in the radial velocity for this star reported by Hatzes and Cochran. Variations
in the line bisector with the radial velocity period would provide strong
evidence in support of rotational modulation or stellar pulsations as the cause
of the 643-day period. A lack of any bisector variability at this period would
support the planet hypothesis. Variations in the line asymmetries are found
with a period of 49.93 days. These variations are uncorrelated with 643-day
period found previously in the radial velocity measurements. It is demonstrated
that this 50-day period is consistent with an m=4 nonradial sectoral g-mode
oscillation. The lack of spectral variability with the radial velocity period
of 643 days may provide strong evidence in support of the hypothesis that this
variability stems from the reflex motion of the central star due to a planetary
companion having a mass of 11 Jupiter masses. However, this long-period
variability may still be due to a low order (m=2) pulsation mode since these
would cause bisector variations less than the error measurement.Comment: LaTeX, 8 pages, 10 figures. Accepted in Monthly Notices of the Royal
Astronomical Societ
The way forward
For the last few decades the study of disks around stars young and old and of
different types have progressed significantly. During the same time a
completely new discipline, the study of exoplanets, planets orbiting stars
other than our Sun, have emerged. Both these fields, which are interconnected,
have benefited from the development of new instrumentation, and especially by
telescopes and detectors deployed in space. In this chapter we are describing
the state of the art of such instruments and make an inventory of what is being
currently developed. We also state some of the requirements of the next steps
and what type of instruments will lead the way forward.Comment: 25 pages, 4 figure
The magnetic properties of the planet host star Kepler-78
Kepler-78 is host to a transiting 8.5-hour orbit super-Earth. In this paper,
the rotation and magnetic properties of the planet host star are studied. We
first revisit the Kepler photometric data for a detailed description of the
rotation properties of Kepler-78, showing that the star seems to undergo a
cycle in the spot pattern of ~1,300 day duration. We then use
spectropolarimetric observations with CFHT/ESPaDOnS to measure the circular
polarization in the line profile of the star during its rotation cycle, as well
as spectroscopic proxies of the chromospheric activity. The average field has
an amplitude of 16 G. The magnetic topology is characterized by a poloidal and
a toroidal component, encompassing 60% and 40% of the magnetic energy,
respectively. Differential rotation is detected with an estimated rate of
0.105+-0.039 rad/d. Activity tracers vary with the rotation cycle of the star;
there is no hint that a residual activity level is related to the planetary
orbit at the precision of our data. The description of the star magnetic
field's characteristics then may serve as input for models of interactions
between the star and its close-by planet, e.g., Ohmic dissipation and unipolar
induction
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
Results from the Exoplanet Search Programmes with BEST and TEST
Thueringer Landessternwarte Tautenburg (TLS) has started to operate a small
dedicated telescope - the Tautenburg Exoplanet Search Telescope (TEST) -
searching for transits of extrasolar planets in photometric time series
observations. In a joint effort with the Berlin Exoplanet Search Telescope
(BEST) operated by the Institut fuer Planetenforschung of the "Deutsches
Zentrum fuer Luft- und Raumfahrt (DLR)" at the Observatoire de Haute-Provence
(OHP), France, two observing sites are used to optimise transit search. Here,
we give a short overview of these systems and the data analysis. We describe a
software pipeline that we have set up to identify transit events of extrasolar
planets and variable stars in time series data from these and other telescopes,
and report on some first results.Comment: 6 pages, 5 figures, contributed paper to the "Solar and Stellar
Physics Through Eclipses" conference, eds. O. Demircan, S.O. Selam, B.
Albayrak (Turkey, March 2006
Oscillations and velocity structure of a long-lived cyclonic spot
Dark brown cyclonic spots ('barges') at 14°N were studied by using Voyager 1 and 2 images of Jupiter. Movie sequences were made to study the spots' behavior over intervals of 50 days and longer. These movies revealed that the length and width vary by ±9% with a period of about 15 days, while the area remains approximately constant. The horizontal velocity field was investigated for an interval of about 1 day. Flow around the largest barge (feature 6) occurs as a ring current. The vorticity inferred is about 2½ times that of the ambient cyclonic zonal circulation, and about one-half the value of the local planetary vorticity. Length and width variations appear to be associated with a nonzero horizontal divergence field. If the oscillations are a natural mode of the system, the 15-day period will provide an important datum for testing models of stable closed vortices
Searching for Planets in the Hyades V: Limits on Planet Detection in the Presence of Stellar Activity
We present the results of a radial velocity survey of a sample of Hyades
stars, and discuss the effects of stellar activity on radial velocity
measurements. The level of radial velocity scatter due to rotational modulation
of stellar surface features for the Hyades is in agreement with the predictions
of Saar & Donahue (1997)- the maximum radial velocity rms of up to ~50 m/s,
with an average rms of ~16 m/s. In this sample of 94 stars, we find 1 new
binary, 2 stars with linear trends indicative of binary companions, and no
close-in giant planets. We discuss the limits on extrasolar planet detection in
the Hyades and the constraints imposed on radial velocity surveys of young
stars.Comment: To appear in the June 2004 issue of A
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