229 research outputs found
Atmospheric Stellar Parameters from Cross-Correlation Functions
The increasing number of spectra gathered by spectroscopic sky surveys and
transiting exoplanet follow-up has pushed the community to develop automated
tools for atmospheric stellar parameters determination. Here we present a novel
approach that allows the measurement of temperature (),
metallicity () and gravity () within a few seconds
and in a completely automated fashion. Rather than performing comparisons with
spectral libraries, our technique is based on the determination of several
cross-correlation functions (CCFs) obtained by including spectral features with
different sensitivity to the photospheric parameters. We use literature stellar
parameters of high signal-to-noise (), high-resolution HARPS
spectra of FGK Main Sequence stars to calibrate , and as a function of CCFs parameters. Our technique is validated
using low spectra obtained with the same instrument. For FGK
stars we achieve a precision of K, and at , while the precision for observation with
and the overall accuracy are constrained by the
literature values used to calibrate the CCFs. Our approach can be easily
extended to other instruments with similar spectral range and resolution, or to
other spectral range and stars other than FGK dwarfs if a large sample of
reference stars is available for the calibration. Additionally, we provide the
mathematical formulation to convert synthetic equivalent widths to CCF
parameters as an alternative to direct calibration. We have made our tool
publicly available.Comment: Accepted by MNRAS. 12 pages, 12 figures. The code to retrieve the
atmospheric stellar parameters from HARPS and HARPS-N spectra is available
"at this url, https://github.com/LucaMalavolta/CCFpams
Spectroscopic direct detection of reflected light from extra-solar planets
At optical wavelengths, an exoplanet's signature is essentially reflected
light from the host star - several orders of magnitude fainter. Since it is
superimposed on the star spectrum its detection has been a difficult
observational challenge. However, the development of a new generation of
instruments like ESPRESSO and next generation telescopes like the E-ELT put us
in a privileged position to detect these planets' reflected light as we will
have access to extremely high signal-to-noise ratio spectra. With this work, we
propose an alternative approach for the direct detection of the reflected light
of an exoplanet. We simulated observations with ESPRESSO@VLT and HIRES@E-ELT of
several star+planet systems, encompassing 10h of the most favourable orbital
phases. To the simulated spectra we applied the Cross Correlation Function to
operate in a much higher signal-to-noise ratio domain than when compared with
the spectra. The use of the Cross-Correlation Function permitted us to recover
the simulated the planet signals at a level above 3 \sigma_{noise} significance
on several prototypical (e.g., Neptune type planet with a 2 days orbit with the
VLT at 4.4 \sigma_{noise} significance) and real planetary systems (e.g., 55
Cnc e with the E-ELT at 4.9 \sigma_{noise} significance). Even by using a more
pessimistic approach to the noise level estimation, where systematics in the
spectra increase the noise 2-3 times, the detection of the reflected light from
large close-orbit planets is possible. We have also shown that this kind of
study is currently within reach of current instruments and telescopes (e.g., 51
Peg b with the VLT at 5.2 \sigma_{noise} significance), although at the limit
of their capabilities.Comment: Accepted for Publication on MNRAS: 2013 August 29; Online Article:
http://mnras.oxfordjournals.org/content/early/2013/09/27/mnras.stt1642; 5
Figures, 11 page
The Earth transiting the Sun as seen from Jupiter's moons: detection of an inverse Rossiter-McLaughlin effect produced by the Opposition Surge of the icy Europa
We report on a multi-wavelength observational campaign which followed the
Earth's transit on the Sun as seen from Jupiter on 5 Jan the 2014. Simultaneous
observations of Jupiter's moons Europa and Ganymede obtained with HARPS from La
Silla, Chile, and HARPS-N from La Palma, Canary Islands, were performed to
measure the Rossiter-McLaughlin effect due to the Earth's passage using the
same technique successfully adopted for the 2012 Venus Transit (Molaro et al
2013). The expected modulation in radial velocities was of about 20 cm/s but an
anomalous drift as large as 38 m/s, i.e. more than two orders of magnitude
higher and opposite in sign, was detected instead. The consistent behaviour of
the two spectrographs rules out instrumental origin of the radial velocity
drift and BiSON observations rule out the possible dependence on the Sun's
magnetic activity. We suggest that this anomaly is produced by the Opposition
Surge on Europa's icy surface, which amplifies the intensity of the solar
radiation from a portion of the solar surface centered around the crossing
Earth which can then be observed as a a sort of inverse Rossiter-McLaughling
effect. in fact, a simplified model of this effect can explain in detail most
features of the observed radial velocity anomalies, namely the extensions
before and after the transit, the small differences between the two
observatories and the presence of a secondary peak closer to Earth passage.
This phenomenon, observed here for the first time, should be observed every
time similar Earth alignments occur with rocky bodies without atmospheres. We
predict it should be observed again during the next conjunction of Earth and
Jupiter in 2026.Comment: 9 pages, 7 figure
A Non-isothermal Theory for Interpreting Sodium Lines in Transmission Spectra of Exoplanets
We present a theory for interpreting the sodium lines detected in
transmission spectra of exoplanetary atmospheres. Previous analyses employed
the isothermal approximation and dealt only with the transit radius. By
recognising the absorption depth and the transit radius as being independent
observables, we develop a theory for jointly interpreting both quantities,
which allows us to infer the temperatures and number densities associated with
the sodium lines. We are able to treat a non-isothermal situation with a
constant temperature gradient. Our novel diagnostics take the form of
simple-to-use algebraic formulae and require measurements of the transit radii
(and their corresponding absorption depths) at line center and in the line wing
for both sodium lines. We apply our diagnostics to the HARPS data of HD
189733b, confirm the upper atmospheric heating reported by Huitson et al.
(2012), derive a temperature gradient of K km and
find densities to cm.Comment: Accepted by ApJ Letters. 6 pages, 3 figure
Star-planet interactions. IV. Possibility of detecting the orbit-shrinking of a planet around a red giant
The surface rotations of some red giants are so fast that they must have been
spun up by tidal interaction with a close companion, either another star, a
brown dwarf, or a planet. We focus here on the case of red giants that are spun
up by tidal interaction with a planet. When the distance between the planet and
the star decreases, the spin period of the star decreases, the orbital period
of the planet decreases, and the reflex motion of the star increases. We study
the change rate of these three quantities when the circular orbit of a planet
of 15 M that initially orbits a 2 M star at 1 au shrinks under
the action of tidal forces during the red giant phase. We use stellar evolution
models coupled with computations of the orbital evolution of the planet, which
allows us to follow the exchanges of angular momentum between the star and the
orbit in a consistent way. We obtain that the reflex motion of the red giant
star increases by more than 1 m s per year in the last 40 years
before the planet engulfment. During this phase, the reflex motion of the star
is between 660 and 710 m s. The spin period of the star increases by
more than about 10 minutes per year in the last 3000 y before engulfment.
During this period, the spin period of the star is shorter than 0.7 year.
During this same period, the variation in orbital period, which is shorter than
0.18 year, is on the same order of magnitude. Changes in reflex-motion and spin
velocities are very small and thus most likely out of reach of being observed.
The most promising way of detecting this effect is through observations of
transiting planets, that is, through{\it } changes of the beginning or end of
the transit. A space mission like PLATO might be of great interest for
detecting planets that are on the verge of being engulfed by red giants.Comment: 4 pages, 4 figure
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&
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