130 research outputs found
Measuring precise radial velocities on individual spectral lines. I. Validation of the method and application to mitigate stellar activity
Stellar activity is the main limitation to the detection of Earth-twins using
the RV technique. Despite many efforts in trying to mitigate the effect of
stellar activity using empirical and statistical techniques, it seems that we
are facing an obstacle that will be extremely difficult to overcome using
current techniques. In this paper, we investigate a novel approach to derive
precise RVs considering the wealth of information present in high-resolution
spectra. This new method consists in building a master spectrum from all
observations and measure the RVs of each spectral line in a spectrum relative
to it. When analysing several spectra, the final product is the RVs of each
line as a function of time. We demonstrate on three stars intensively observed
with HARPS that our new method gives RVs that are extremely similar to the ones
derived from the HARPS data reduction software. Our new approach to derive RVs
demonstrates that the non-stability of daily HARPS wavelength solution induces
night-to-night RV offsets with an standard deviation of 0.4 m/s, and we propose
a solution to correct for this systematic. Finally, and this is probably the
most astrophysically relevant result of this paper, we demonstrate that some
spectral lines are strongly affected by stellar activity while others are not.
By measuring the RVs on two carefully selected subsample of spectral lines, we
demonstrate that we can boost by a factor of 2 or mitigate by a factor of 1.6
the red noise induced by stellar activity in the 2010 RVs of Alpha Cen B. By
measuring the RVs of each spectral line, we are able to reach the same RV
precision as other approved techniques. In addition, this new approach allows
to demonstrate that each line is differently affected by stellar activity.
Preliminary results show that studying in details the behaviour of each
spectral line is probably the key to overcome stellar activity.Comment: 14 pages (plus 8 pages of Appendix), 17 figures, 1 table, Accepted
for publication in A&A. Version 2: typo corrected in Equation
Stellar noise and planet detection. I. Oscillations, granulation and sun-like spots
Spectrographs like HARPS can now reach a sub-ms−1 precision in radial-velocity (RV) (Pepe & Lovis 2008). At this level of accuracy, we start to be confronted with stellar noise produced by 3 different physical phenomena: oscillations, granulation phenomena (granulation, meso- and super-granulation) and activity. On solar type stars, these 3 types of perturbation can induce ms−1 RV variation, but on different time scales: 3 to 15 minutes for oscillations, 15 minutes to 1.5 days for granulation phenomena and 10 to 50 days for activity. The high precision observational strategy used on HARPS, 1 measure per night of 15 minutes, on 10 consecutive days each month, is optimized, due to a long exposure time, to average out the noise coming from oscillations (Dumusque et al. 2011a) but not to reduce the noise coming from granulation and activity (Dumusque et al. 2011a and Dumusque et al. 2011b). The smallest planets found with this strategy (Mayor et al. 2009) seems to be at the limit of the actual observational strategy and not at the limit of the instrumental precision. To be able to find Earth mass planets in the habitable zone of solar-type stars (200 days for a K0 dwarf), new observational strategies, averaging out simultaneously all type of stellar noise, are require
New wavelength calibration of the HARPS spectrograph
(abridged) Even if the HARPS spectrograph has been operational for more than
15 years and it provides among the most precise Doppler measurements,
improvements are still possible. One known problem, for instance, is the
non-fully regular block-stitching of the CCDs, which introduces, in some cases,
one-year period parasitic signals in the measured radial velocity.
The aim is to improve the wavelength calibration of HARPS to push further its
planet-detection capabilities.
The properties of the CCD stitching-induced pixel-size anomalies are
determined with LED flat-field frames, and then a physical, gap-corrected map
of the CCDs is used for the fitting model of the spectral orders. We also use a
new thorium line list, based on much higher-accuracy measurements than the one
used up to now. We derive new wavelength solutions for the 15 years of HARPS
data, both before and after the 2015 fibre upgrade.
We demonstrate that we correct the gap anomalies by computing the wavelength
solutions of laser frequency comb exposures, both with and without taking the
gap correction into account. By comparing the rms of the most stable stars of
the HARPS sample, we show that we globally decrease the radial velocity
dispersion of the data, especially for the data acquired after the change of
fibres. Finally, the comparative analysis of several individual systems shows
that we manage to attenuate the periodogram power at one year in most cases.
The analysis of the RVs derived from individual stellar lines also shows that
we correct the stitching-induced RV variation.
This improved calibration of the HARPS spectrograph allows to go deeper in
the search for low-amplitude radial-velocity signals. It will be further
improved by combining the thorium calibration spectra with laser frequency comb
and Fabry-Perot calibration spectra, and not only for HARPS but notably also
for HARPS-N and ESPRESSO.Comment: Accepted for publication in A&
Stellar noise and planet detection. II. Radial-velocity noise induced by magnetic cycles
For the 451 stars of the HARPS high precision program, we study correlations between the radial-velocity (RV) variation and other parameters of the Cross Correlated Function (CCF). After a careful target selection, we found a very good correlation between the slope of the RV-activity index (log(R'HK)) correlation and the Teff for dwarf stars. This correlation allow us to correct RV from magnetic cycles given the activity index and the Tef
Spectral Line Depth Variability in Radial Velocity Spectra
Stellar active regions, including spots and faculae, can create radial
velocity (RV) signals that interfere with the detection and mass measurements
of low mass exoplanets. In doing so, these active regions affect each spectral
line differently, but the origin of these differences is not fully understood.
Here we explore how spectral line variability correlated with S-index (Ca H & K
emission) is related to the atomic properties of each spectral line. Next we
develop a simple analytic stellar atmosphere model that can account for the
largest sources of line variability with S-index. Then we apply this model to
HARPS spectra of {\alpha} Cen B to explain Fe I line depth changes in terms of
a disk-averaged temperature difference between active and quiet regions on the
visible hemisphere of the star. This work helps establish a physical basis for
understanding how stellar activity manifests differently in each spectral line,
and may help future work mitigating the impact of stellar activity on exoplanet
RV surveys.Comment: 13 pages, 7 figures, submitted to The Astrophysical Journal, August
202
RASSINE: Interactive tool for normalising stellar spectra I. Description and performance of the code
Aims: We provide an open-source code allowing an easy, intuitive, and robust
normalisation of spectra. Methods: We developed RASSINE, a Python code for
normalising merged 1D spectra through the concepts of convex hulls. The code
uses six parameters that can be easily fine-tuned. The code also provides a
complete user-friendly interactive interface, including graphical feedback,
that helps the user to choose the parameters as easily as possible. To
facilitate the normalisation even further, RASSINE can provide a first guess
for the parameters that are derived directly from the merged 1D spectrum based
on previously performed calibrations. Results: For HARPS spectra of the Sun
that were obtained with the HELIOS solar telescope, a continuum accuracy of
0.20% on line depth can be reached after normalisation with RASSINE. This is
three times better than with the commonly used method of polynomial fitting.
For HARPS spectra of Cen B, a continuum accuracy of 2.0% is reached.
This rather poor accuracy is mainly due to molecular band absorption and the
high density of spectral lines in the bluest part of the merged 1D spectrum.
When wavelengths shorter than 4500 \AA are excluded, the continuum accuracy
improves by up to 1.2%. The line-depth precision on individual spectrum
normalisation is estimated to be 0.15%, which can be reduced to the
photon-noise limit (0.10%) when a time series of spectra is given as input for
RASSINE. Conclusions: With a continuum accuracy higher than the polynomial
fitting method and a line-depth precision compatible with photon noise, RASSINE
is a tool that can find applications in numerous cases, for example stellar
parameter determination, transmission spectroscopy of exoplanet atmospheres, or
activity-sensitive line detection.Comment: 13 pages, 9 pages appendix, 9 figure
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