230 research outputs found
Revisiting the correlation between stellar activity and planetary surface gravity
Aims: We re-evaluate the correlation between planetary surface gravity and
stellar host activity as measured by the index log(). This
correlation, previously identified by Hartman (2010), is now analyzed in light
of an extended measurements dataset, roughly 3 times larger than the original
one.
Methods: We calculated the Spearman's rank correlation coefficient between
the two quantities and its associated p-value. The correlation coefficient was
calculated for both the full dataset and the star-planet pairs that follow the
conditions proposed by Hartman (2010). In order to do so, we considered
effective temperatures both as collected from the literature and from the
SWEET-Cat catalog, which provides a more homogeneous and accurate effective
temperature determination.
Results: The analysis delivers significant correlation coefficients, but with
a lower value than those obtained by Hartman (2010). Yet, the two datasets are
compatible, and we show that a correlation coefficient as large as previously
published can arise naturally from a small-number statistics analysis of the
current dataset. The correlation is recovered for star-planet pairs selected
using the different conditions proposed by Hartman (2010). Remarkably, the
usage of SWEET-Cat temperatures leads to larger correlation coefficient values.
We highlight and discuss the role of the correlation betwen different
parameters such as effective temperature and activity index. Several additional
effects on top of those discussed previously were considered, but none fully
explains the detected correlation. In light of the complex issue discussed
here, we encourage the different follow-up teams to publish their activity
index values in the form of log() index so that a comparison across
stars and instruments can be pursued.Comment: 11 pages, 3 figures, accepted for publication in A&
Detecting transit signatures of exoplanetary rings using SOAP3.0
CONTEXT. It is theoretically possible for rings to have formed around
extrasolar planets in a similar way to that in which they formed around the
giant planets in our solar system. However, no such rings have been detected to
date.
AIMS: We aim to test the possibility of detecting rings around exoplanets by
investigating the photometric and spectroscopic ring signatures in
high-precision transit signals.
METHODS: The photometric and spectroscopic transit signals of a ringed planet
is expected to show deviations from that of a spherical planet. We used these
deviations to quantify the detectability of rings. We present SOAP3.0 which is
a numerical tool to simulate ringed planet transits and measure ring
detectability based on amplitudes of the residuals between the ringed planet
signal and best fit ringless model.
RESULTS: We find that it is possible to detect the photometric and
spectroscopic signature of near edge-on rings especially around planets with
high impact parameter. Time resolution 7 mins is required for the
photometric detection, while 15 mins is sufficient for the spectroscopic
detection. We also show that future instruments like CHEOPS and ESPRESSO, with
precisions that allow ring signatures to be well above their noise-level,
present good prospects for detecting rings.Comment: 13 pages, 16 figures, 2 tables , accepted for publication in A&
Probing the effect of gravitational microlensing on the measurements of the Rossiter-McLaughlin effect
In general, in the studies of transit light-curves and the
Rossiter-McLaughlin (RM), the contribution of the planet's gravitational
microlensing is neglected. Theoretical studies, have, however shown that the
planet's microlensing can affect the transit light-curve and in some extreme
cases cause the transit depth to vanish. In this letter, we present the results
of our quantitative analysis of microlening on the RM effect. Results indicate
that for massive planets in on long period orbits, the planet's microlensing
will have considerable contribution to the star's RV measurements. We present
the details of our study, and discuss our analysis and results.Comment: 6 pages, 3 figures, accepted for publication in Astronomy &
Astrophysic
Improvements on analytic modelling of stellar spots
In this work we present the solution of the stellar spot problem using the
Kelvin-Stokes theorem. Our result is applicable for any given location and
dimension of the spots on the stellar surface. We present explicitely the
result up to the second degree in the limb darkening law. This technique can be
used to calculate very efficiently mutual photometric effects produced by
eclipsing bodies occulting stellar spots and to construct complex spot shapes.Comment: Resubmitted to MNRAS after accounting for minor comments of second
review, 9 pages, 5 figures, software available at
http://eduscisoft.com/KSINT
Can stellar activity make a planet seem misaligned?
Several studies have shown that the occultation of stellar active regions by
the transiting planet can generate anomalies in the high-precision transit
light curves, and these anomalies may lead to an inaccurate estimate of the
planetary parameters (e.g., the planet radius). Since the physics and geometry
behind the transit light curve and the Rossiter- McLaughlin effect
(spectroscopic transit) are the same, the Rossiter-McLaughlin observations are
expected to be affected by the occultation of stellar active regions in a
similar way. In this paper we perform a fundamental test on the spin-orbit
angles as derived by Rossiter-McLaughlin measurements, and we examine the
impact of the occultation of stellar active regions by the transiting planet on
the spin-orbit angle estimations. Our results show that the inaccurate
estimation on the spin-orbit angle due to stellar activity can be quite
significant (up to 30 degrees), particularly for the edge-on, aligned, and
small transiting planets. Therefore, our results suggest that the aligned
transiting planets are the ones that can be easily misinterpreted as misaligned
owing to the stellar activity. In other words, the biases introduced by
ignoring stellar activity are unlikely to be the culprit for the highly
misaligned systems.Comment: 8 pages, 8 figures, accepted for publication in Astronomy &
Astrophysic
SOAP-T: A tool to study the light-curve and radial velocity of a system with a transiting planet and a rotating spotted star
We present an improved version of SOAP (Boisse et al. 2012) named "SOAP-T",
which can generate the radial velocity variations and light-curves for systems
consisting of a rotating spotted star with a transiting planet. This tool can
be used to study the anomalies inside transit light-curves and the
Rossiter-McLaughlin effect, to better constrain the orbital configuration and
properties of planetary systems and active zones of their host stars. Tests of
the code are presented to illustrate its performance and to validate its
capability when compared with analytical models and real data. Finally, we
apply SOAP-T to the active star, HAT-P-11, observed by the NASA Kepler space
telescope and use this system to discuss the capability of this tool in
analyzing light-curves for the cases where the transiting planet overlaps with
the star's spots.Comment: 9 pages, 7 figures, accepted for publication in Astronomy and
Astrophysic
A new analysis of the WASP-3 system: no evidence for an additional companion
In this work we investigate the problem concerning the presence of additional
bodies gravitationally bounded with the WASP-3 system. We present eight new
transits of this planet and analyse all the photometric and radial velocity
data published so far. We did not observe significant periodicities in the
Fourier spectrum of the observed minus calculated (O-C) transit timing and
radial velocity diagrams (the highest peak having false-alarm probabilities of
56 per cent and 31 per cent, respectively) or long-term trends. Combining all
the available information, we conclude that the radial velocity and transit
timing techniques exclude, at 99 per cent confidence limit, any perturber more
massive than M \gtrsim 100 M_Earth with periods up to 10 times the period of
the inner planet. We also investigate the possible presence of an exomoon on
this system and determined that considering the scatter of the O-C transit
timing residuals a coplanar exomoon would likely produce detectable transits.
This hypothesis is however apparently ruled out by observations conducted by
other researchers. In case the orbit of the moon is not coplanar the accuracy
of our transit timing and transit duration measurements prevents any
significant statement. Interestingly, on the basis of our reanalysis of SOPHIE
data we noted that WASP-3 passed from a less active (log R'_hk=-4.95) to a more
active (log R'_hk=-4.8) state during the 3 yr monitoring period spanned by the
observations. Despite no clear spot crossing has been reported for this system,
this analysis claims for a more intensive monitoring of the activity level of
this star in order to understand its impact on photometric and radial velocity
measurements.Comment: MNRAS accepted (14/08/2012
Distinguishing the albedo of exoplanets from stellar activity
Light curves show the flux variation from the target star and its orbiting
planets as a function of time. In addition to the transit features created by
the planets, the flux also includes the reflected light component of each
planet, which depends on the planetary albedo. This signal is typically
referred to as phase curve and could be easily identified if there were no
additional noise. As well as instrumental noise, stellar activity, such as
spots, can create a modulation in the data, which may be very difficult to
distinguish from the planetary signal. We analyze the limitations imposed by
the stellar activity on the detection of the planetary albedo, considering the
limitations imposed by the predicted level of instrumental noise and the short
duration of the observations planned in the context of the CHEOPS mission. As
initial condition, we have assumed that each star is characterized by just one
orbiting planet. We built mock light curves that included a realistic stellar
activity pattern, the reflected light component of the planet and an
instrumental noise level, which we have chosen to be at the same level as
predicted for CHEOPS. We then fit these light curves to try to recover the
reflected light component, assuming the activity patterns can be modeled with a
Gaussian process.We estimate that at least one full stellar rotation is
necessary to obtain a reliable detection of the planetary albedo. This result
is independent of the level of noise, but it depends on the limitation of the
Gaussian process to describe the stellar activity when the light curve
time-span is shorter than the stellar rotation. Finally, in presence of typical
CHEOPS gaps in the simulations, we confirm that it is still possible to obtain
a reliable albedo.Comment: Accepted for publication in A&A, 14 pages, 12 figure
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