2,498 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($R'_{HK}$). 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($R'_{HK}$) index so that a comparison across
stars and instruments can be pursued.Comment: 11 pages, 3 figures, 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

### Impact of stellar companions on precise radial velocities

Context: With the announced arrival of instruments such as ESPRESSO one can
expect that several systematic noise sources on the measurement of precise
radial velocity will become the limiting factor instead of photon noise. A
stellar companion within the fiber is such a possible noise source. Aims: With
this work we aim at characterizing the impact of a stellar companion within the
fiber to radial velocity measurements made by fiber-fed spectrographs. We
consider the contaminant star either to be part of a binary system whose
primary star is the target star, or as a background/foreground star. Methods:
To carry out our study, we used HARPS spectra, co-added the target with
contaminant spectra, and then compared the resulting radial velocity with that
obtained from the original target spectrum. We repeated this procedure and used
different tunable knobs to reproduce the previously mentioned scenarios.
Results: We find that the impact on the radial velocity calculation is a
function of the difference between individual radial velocities, of the
difference between target and contaminant magnitude, and also of their spectral
types. For the worst-case scenario in which both target and contaminant star
are well centered on the fiber, the maximum contamination for a G or K star may
be higher than 10 cm/s, on average, if the difference between target and
contaminant magnitude is $\Delta m$ < 10, and higher than 1 m/s if $\Delta m$ <
8. If the target star is of spectral type M, $\Delta m$ < 8 produces the same
contamination of 10 cm/s, and a contamination may be higher than 1 m/sComment: Accepted for publication in A&A on 29/12/2019 - 14 page

### Line-profile variations in radial-velocity measurements: Two alternative indicators for planetary searches

Aims. We introduce two methods to identify false-positive planetary signals
in the context of radial-velocity exoplanet searches. The first is the
bi-Gaussian cross-correlation function fitting, and the second is the
measurement of asymmetry in radial-velocity spectral line information content,
Vasy.
Methods. We make a systematic analysis of the most used common line profile
diagnosis, Bisector Inverse Slope and Velocity Span, along with the two
proposed ones. We evaluate all these diagnosis methods following a set of
well-defined common criteria and using both simulated and real data. We apply
them to simulated cross-correlation functions created with the program SOAP and
which are affected by the presence of stellar spots, and to real
cross-correlation functions, calculated from HARPS spectra, for stars with a
signal originating both in activity and created by a planet.
Results. We demonstrate that the bi-Gaussian method allows a more precise
characterization of the deformation of line profiles than the standard bisector
inverse slope. The calculation of the deformation indicator is simpler and its
interpretation more straightforward. More importantly, its amplitude can be up
to 30% larger than that of the bisector span, allowing the detection of
smaller-amplitude correlations with radial-velocity variations. However, a
particular parametrization of the bisector inverse slope is shown to be more
efficient on high-signal-to-noise data than both the standard bisector and the
bi-Gaussian. The results of the Vasy method show that this indicator is more
effective than any of the previous ones, being correlated with the
radial-velocity with more significance for signals resulting from a line
deformation. Moreover, it provides a qualitative advantage over the bisector,
showing significant correlations with RV for active stars for which bisector
analysis is inconclusive. (abridged)Comment: 12 pages, 7 figures, accepted for publication in Astronomy and
Astrophysics, comments welcom

### 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

### Evaluating the stability of atmospheric lines with HARPS

Context: In the search for extrasolar systems by radial velocity technique, a
precise wavelength calibration is necessary for high-precision measurements.
The choice of the calibrator is a particularly important question in the
infra-red domain, where the precision and exploits still fall behind the
achievements of the optical.
Aims: We investigate the long-term stability of atmospheric lines as a
precise wavelength reference and analyze their sensitivity to different
atmospheric and observing conditions.
Methods: We use HARPS archive data on three bright stars, Tau Ceti, Mu Arae
and Epsilon Eri, spanning 6 years and containing high-cadence measurements over
several nights. We cross-correlate this data with an O2 mask and evaluate both
radial velocity and bisector variations down to a photon noise of 1 m/s.
Results: We find that the telluric lines in the three data-sets are stable
down to 10 m/s (r.m.s.) over the 6 years. We also show that the radial velocity
variations can be accounted for by simple atmospheric models, yielding a final
precision of 1-2 m/s.
Conclusions: The long-term stability of atmospheric lines was measured as
being of 10 m/s over six years, in spite of atmospheric phenomena. Atmospheric
lines can be used as a wavelength reference for short-time-scales programs,
yielding a precision of 5 m/s "out-of-the box". A higher precision, down to 2
m/s can be reached if the atmospheric phenomena are corrected for by the simple
atmospheric model described, making it a very competitive method even on long
time-scales.Comment: 7 pages, accepted for publication in A&

### Controlling high-harmonic generation and above-threshold ionization with an attosecond-pulse train

We perform a detailed analysis of how high-order harmonic generation (HHG)
and above-threshold ionization (ATI) can be controlled by a time-delayed
attosecond-pulse train superposed to a strong, near-infrared laser field. In
particular we show that the high-harmonic and photoelectron intensities, the
high-harmonic plateau structure and cutoff energies, and the ATI angular
distributions can be manipulated by changing this delay. This is a direct
consequence of the fact that the attosecond pulse train can be employed as a
tool for constraining the instant an electronic wave packet is ejected in the
continuum. A change in such initial conditions strongly affects its subsequent
motion in the laser field, and thus HHG and ATI. In our studies, we employ the
Strong-Field Approximation and explain the features observed in terms of
interference effects between various electron quantum orbits. Our results are
in agreement with recent experimental findings and theoretical studies
employing purely numerical methods.Comment: 10 pages revtex and 6 figures (eps files

### Telluric correction in the near-infrared: Standard star or synthetic transmission?

Context. The atmospheric absorption of the Earth is an important limiting
factor for ground-based spectroscopic observations and the near-infrared and
infrared regions are the most affected. Several software packages that produce
a synthetic atmospheric transmission spectrum have been developed to correct
for the telluric absorption; these are Molecfit, TelFit, and TAPAS. Aims. Our
goal is to compare the correction achieved using these three telluric
correction packages and the division by a telluric standard star. We want to
evaluate the best method to correct near-infrared high-resolution spectra as
well as the limitations of each software package and methodology. Methods. We
applied the telluric correction methods to CRIRES archival data taken in the J
and K bands. We explored how the achieved correction level varies depending on
the atmospheric T-P profile used in the modelling, the depth of the atmospheric
lines, and the molecules creating the absorption. Results. We found that the
Molecfit and TelFit corrections lead to smaller residuals for the water lines.
The standard star method corrects best the oxygen lines. The Molecfit package
and the standard star method corrections result in global offsets always below
0.5% for all lines; the offset is similar with TelFit and TAPAS for the H2O
lines and around 1% for the O2 lines. All methods and software packages result
in a scatter between 3% and 7% inside the telluric lines. The use of a tailored
atmospheric profile for the observatory leads to a scatter two times smaller,
and the correction level improves with lower values of precipitable water
vapour. Conclusions. The synthetic transmission methods lead to an improved
correction compared to the standard star method for the water lines in the J
band with no loss of telescope time, but the oxygen lines were better corrected
by the standard star method.Comment: 18 pages, 13 figures, Accepted to A&

### Existence criteria for stabilization from the scaling behaviour of ionization probabilities

We provide a systematic derivation of the scaling behaviour of various
quantities and establish in particular the scale invariance of the ionization
probability. We discuss the gauge invariance of the scaling properties and the
manner in which they can be exploited as consistency check in explicit
analytical expressions, in perturbation theory, in the Kramers-Henneberger and
Floquet approximation, in upper and lower bound estimates and fully numerical
solutions of the time dependent Schroedinger equation. The scaling invariance
leads to a differential equation which has to be satisfied by the ionization
probability and which yields an alternative criterium for the existence of
atomic bound state stabilization.Comment: 12 pages of Latex, one figur

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