1,224 research outputs found
A new list of thorium and argon spectral lines in the visible
Aims. We present a new list of thorium and argon emission lines in the
visible obtained by analyzing high-resolution (R=110,000) spectra of a ThAr
hollow cathode lamp. The aim of this new line list is to allow significant
improvements in the quality of wavelength calibration for medium- to
high-resolution astronomical spectrographs. Methods. We use a series of ThAr
lamp exposures obtained with the HARPS instrument (High Accuracy
Radial-velocity Planet Searcher) to detect previously unknown lines, perform a
systematic search for blended lines and correct individual wavelengths by
determining the systematic offset of each line relative to the average
wavelength solution. Results. We give updated wavelengths for more than 8400
lines over the spectral range 3785-6915 A. The typical internal uncertainty on
the line positions is estimated to be ~10 m/s (3.3 parts in 10^8 or 0.18 mA),
which is a factor of 2-10 better than the widely used Los Alamos Atlas of the
Thorium Spectrum (Palmer & Engleman 1983). The absolute accuracy of the global
wavelength scale is the same as in the Los Alamos Atlas. Using this new line
list on HARPS ThAr spectra, we are able to obtain a global wavelength
calibration which is precise at the 20 cm/s level (6.7 parts in 10^10 or 0.0037
mA). Conclusions. Several research fields in astronomy requiring high-precision
wavelength calibration in the visible (e.g. radial velocity planet searches,
variability of fundamental constants) should benefit from using the new line
list.Comment: 7 pages, 6 figures, accepted for publication in A&
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&
Refined architecture of the WASP-8 system: a cautionary tale for traditional Rossiter-McLaughlin analysis
Probing the trajectory of a transiting planet across the disk of its star
through the analysis of its Rossiter-McLaughlin effect can be used to measure
the differential rotation of the host star and the true obliquity of the
system. Highly misaligned systems could be particularly conducive to these
mesurements, which is why we reanalysed the HARPS transit spectra of WASP-8b
using the 'Rossiter-McLaughlin effect reloaded' (reloaded RM) technique. This
approach allows us to isolate the local stellar CCF emitted by the
planet-occulted regions. As a result we identified a 35% variation in the
local CCF contrast along the transit chord, which might trace a deepening of
the stellar lines from the equator to the poles. Whatever its origin, such an
effect cannot be detected when analyzing the RV centroids of the
disk-integrated CCFs through a traditional velocimetric analysis of the RM
effect. Consequently it injected a significant bias into the results obtained
by Queloz et al. (2010) for the projected rotational velocity (1.59 km/s) and the sky-projected
obliquity (-123.0). Using our
technique, we measured these values to be =
1.900.05 km/s and = -143.0. We
found no compelling evidence for differential rotation of the star, although
there are hints that WASP-8 is pointing away from us with the stellar poles
rotating about 25% slower than the equator. Measurements at higher accuracy
during ingress/egress will be required to confirm this result. In contrast to
the traditional analysis of the RM effect, the reloaded RM technique directly
extracts the local stellar CCFs, allowing us to analyze their shape and to
measure their RV centroids, unbiased by variations in their contrast or FWHM.Comment: Accepted for publication in A&A. 12 page
The Kahler Structure of Supersymmetric Holographic RG Flows
We study the metrics on the families of moduli spaces arising from probing
with a brane the ten and eleven dimensional supergravity solutions
corresponding to renormalisation group flows of supersymmetric large n gauge
theory. In comparing the geometry to the physics of the dual gauge theory, it
is important to identify appropriate coordinates, and starting with the case of
SU(n) gauge theories flowing from N=4 to N=1 via a mass term, we demonstrate
that the metric is Kahler, and solve for the Kahler potential everywhere along
the flow. We show that the asymptotic form of the Kahler potential, and hence
the peculiar conical form of the metric, follows from special properties of the
gauge theory. Furthermore, we find the analogous Kahler structure for the N=4
preserving Coulomb branch flows, and for an N=2 flow. In addition, we establish
similar properties for two eleven dimensional flow geometries recently
presented in the literature, one of which has a deformation of the conifold as
its moduli space. In all of these cases, we notice that the Kahler potential
appears to satisfy a simple universal differential equation. We prove that this
equation arises for all purely Coulomb branch flows dual to both ten and eleven
dimensional geometries, and conjecture that the equation holds much more
generally.Comment: 26 pages. Late
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 < 10, and higher than 1 m/s if <
8. If the target star is of spectral type 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
A new wavelength calibration for echelle spectrographs using Fabry-Perot etalons
The study of Earth-mass extrasolar planets via the radial-velocity technique
and the measurement of the potential cosmological variability of fundamental
constants call for very-high-precision spectroscopy at the level of
\updelta\lambda/\lambda<10^{-9}. Wavelength accuracy is obtained by providing
two fundamental ingredients: 1) an absolute and information-rich wavelength
source and 2) the ability of the spectrograph and its data reduction of
transferring the reference scale (wavelengths) to a measurement scale (detector
pixels) in a repeatable manner. The goal of this work is to improve the
wavelength calibration accuracy of the HARPS spectrograph by combining the
absolute spectral reference provided by the emission lines of a thorium-argon
hollow-cathode lamp (HCL) with the spectrally rich and precise spectral
information of a Fabry-P\'erot-based calibration source. On the basis of
calibration frames acquired each night since the Fabry-P\'erot etalon was
installed on HARPS in 2011, we construct a combined wavelength solution which
fits simultaneously the thorium emission lines and the Fabry-P\'erot lines. The
combined fit is anchored to the absolute thorium wavelengths, which provide the
`zero-point' of the spectrograph, while the Fabry-P\'erot lines are used to
improve the (spectrally) local precision. The obtained wavelength solution is
verified for auto-consistency and tested against a solution obtained using the
HARPS Laser-Frequency Comb (LFC). The combined thorium+Fabry-P\'erot wavelength
solution shows significantly better performances compared to the thorium-only
calibration. The presented techniques will therefore be used in the new HARPS
and HARPS-N pipeline, and will be exported to the ESPRESSO spectrograph.Comment: 15 pages, 8 figure
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
On the long-term correlation between the flux in the Ca II H & K and Halpha lines for FGK stars
The re-emission in the cores of the Ca II H & K and H lines, are well
known proxies of stellar activity. However, these activity indices probe
different activity phenomena, the first being more sensitive to plage
variation, while the other one being more sensitive to filaments. In this paper
we study the long-term correlation between and , two indices based on the Ca II H & K and H lines
respectively, for a sample of 271 FGK stars using measurements obtained over a
9 year time span. Because stellar activity is one of the main obstacles
to the detection of low-mass and long-period planets, understanding further
this activity index correlation can give us some hints about the optimal target
to focus on, and ways to correct for these activity effects. We found a great
variety of long-term correlations between and . Around 20% of our sample has strong positive correlation between
the indices while about 3% show strong negative correlation. These fractions
are compatible with those found for the case of early-M dwarfs. Stars
exhibiting a positive correlation have a tendency to be more active when
compared to the median of the sample, while stars showing a negative
correlation are more present among higher metallicity stars. There is also a
tendency for the positively correlated stars to be more present among the
coolest stars, a result which is probably due to the activity level effect on
the correlation. Activity level and metallicity seem therefore to be playing a
role on the correlation between and . Possible
explanations based on the influence of filaments for the diversity in the
correlations between these indices are discussed in this paper.Comment: 18 pages, 13 figures, 4 tables, accepted for publication in Astronomy
and Astrophysic
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