558 research outputs found
Long-lived, long-period radial velocity variations in Aldebaran: A planetary companion and stellar activity
We investigate the nature of the long-period radial velocity variations in
Alpha Tau first reported over 20 years ago. We analyzed precise stellar radial
velocity measurements for Alpha Tau spanning over 30 years. An examination of
the Halpha and Ca II 8662 spectral lines, and Hipparcos photometry was also
done to help discern the nature of the long-period radial velocity variations.
Our radial velocity data show that the long-period, low amplitude radial
velocity variations are long-lived and coherent. Furthermore, Halpha equivalent
width measurements and Hipparcos photometry show no significant variations with
this period. Another investigation of this star established that there was no
variability in the spectral line shapes with the radial velocity period. An
orbital solution results in a period of P = 628.96 +/- 0.90 d, eccentricity, e
= 0.10 +/- 0.05, and a radial velocity amplitude, K = 142.1 +/- 7.2 m/s.
Evolutionary tracks yield a stellar mass of 1.13 +/- 0.11 M_sun, which
corresponds to a minimum companion mass of 6.47 +/- 0.53 M_Jup with an orbital
semi-major axis of a = 1.46 +/- 0.27 AU. After removing the orbital motion of
the companion, an additional period of ~ 520 d is found in the radial velocity
data, but only in some time spans. A similar period is found in the variations
in the equivalent width of Halpha and Ca II. Variations at one-third of this
period are also found in the spectral line bisector measurements. The 520 d
period is interpreted as the rotation modulation by stellar surface structure.
Its presence, however, may not be long-lived, and it only appears in epochs of
the radial velocity data separated by 10 years. This might be due to an
activity cycle. The data presented here provide further evidence of a planetary
companion to Alpha Tau, as well as activity-related radial velocity variations.Comment: 18 pages, 14 figures. Accepted for publication in Astronomy and
Astrophysic
Spectrum radial velocity analyser (SERVAL). High-precision radial velocities and two alternative spectral indicators
Context: The CARMENES survey is a high-precision radial velocity (RV)
programme that aims to detect Earth-like planets orbiting low-mass stars.
Aims: We develop least-squares fitting algorithms to derive the RVs and
additional spectral diagnostics implemented in the SpEctrum Radial Velocity
Analyser (SERVAL), a publicly available python code.
Methods: We measured the RVs using high signal-to-noise templates created by
coadding all available spectra of each star.We define the chromatic index as
the RV gradient as a function of wavelength with the RVs measured in the
echelle orders. Additionally, we computed the differential line width by
correlating the fit residuals with the second derivative of the template to
track variations in the stellar line width.
Results: Using HARPS data, our SERVAL code achieves a RV precision at the
level of 1m/s. Applying the chromatic index to CARMENES data of the active star
YZ CMi, we identify apparent RV variations induced by stellar activity. The
differential line width is found to be an alternative indicator to the commonly
used full width half maximum.
Conclusions: We find that at the red optical wavelengths (700--900 nm)
obtained by the visual channel of CARMENES, the chromatic index is an excellent
tool to investigate stellar active regions and to identify and perhaps even
correct for activity-induced RV variations.Comment: 13 pages, 13 figures. A&A in press. Code is available at
https://github.com/mzechmeister/serva
The CARMENES search for exoplanets around M dwarfs: Radial-velocity variations of active stars in visual-channel spectra
Previous simulations predicted the activity-induced radial-velocity (RV)
variations of M dwarfs to range from cm/s to km/s, depending on
various stellar and activity parameters. We investigate the observed relations
between RVs, stellar activity, and stellar parameters of M dwarfs by analyzing
CARMENES high-resolution visual-channel spectra (m), which were
taken within the CARMENES RV planet survey during its first months of
operation. During this time, of the CARMENES-sample stars were observed
at least five times. From each spectrum we derived a relative RV and a measure
of chromospheric H emission. In addition, we estimated the chromatic
index (CRX) of each spectrum, which is a measure of the RV wavelength
dependence. Despite having a median number of only measurements per star,
we show that the RV variations of the stars with RV scatter of m/s and a
projected rotation velocity km/s are caused mainly by activity.
We name these stars `active RV-loud stars' and find their occurrence to
increase with spectral type: from for early-type M dwarfs
(MV) through for mid-type M dwarfs (MV) to
for late-type M dwarfs (MV). Their RV-scatter amplitude is
found to be correlated mainly with . For about half of the stars, we
also find a linear RVCRX anticorrelation, which indicates that their
activity-induced RV scatter is lower at longer wavelengths. For most of them we
can exclude a linear correlation between RV and H emission. Our results
are in agreement with simulated activity-induced RV variations in M dwarfs. The
RV variations of most active RV-loud M dwarfs are likely to be caused by dark
spots on their surfaces, which move in and out of view as the stars rotate.Comment: A&A accepte
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