908 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&
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
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&
Consequences of spectrograph illumination for the accuracy of radial-velocimetry
For fiber-fed spectrographs with a stable external wavelength source,
scrambling properties of optical fibers and, homogeneity and stability of the
instrument illumination are important for the accuracy of radial-velocimetry.
Optical cylindric fibers are known to have good azimuthal scrambling. In
contrast, the radial one is not perfect. In order to improve the scrambling
ability of the fiber and to stabilize the illumination, optical double
scrambler are usually coupled to the fibers. Despite that, our experience on
SOPHIE and HARPS has lead to identified remaining radial-velocity limitations
due to the non-uniform illumination of the spectrograph. We conducted tests on
SOPHIE with telescope vignetting, seeing variation and centering errors on the
fiber entrance. We simulated the light path through the instrument in order to
explain the radial velocity variation obtained with our tests. We then
identified the illumination stability and uniformity has a critical point for
the extremely high-precision radial velocity instruments (ESPRESSO@VLT,
CODEX@E-ELT). Tests on square and octagonal section fibers are now under
development and SOPHIE will be used as a bench test to validate these new feed
optics.Comment: to appear in the Proceedings conference "New Technologies for Probing
the Diversity of Brown Dwarfs and Exoplanets", Shanghai, 200
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
The HARPS search for Earth-like planets in the habitable zone: I -- Very low-mass planets around HD20794, HD85512 and HD192310
In 2009 we started an intense radial-velocity monitoring of a few nearby,
slowly-rotating and quiet solar-type stars within the dedicated HARPS-Upgrade
GTO program. The goal of this campaign is to gather very-precise
radial-velocity data with high cadence and continuity to detect tiny signatures
of very-low-mass stars that are potentially present in the habitable zone of
their parent stars. Ten stars were selected among the most stable stars of the
original HARPS high-precision program that are uniformly spread in hour angle,
such that three to four of them are observable at any time of the year. For
each star we recorded 50 data points spread over the observing season. The data
points consist of three nightly observations with a total integration time of
10 minutes each and are separated by two hours. This is an observational
strategy adopted to minimize stellar pulsation and granulation noise. We
present the first results of this ambitious program. The radial-velocity data
and the orbital parameters of five new and one confirmed low-mass planets
around the stars HD20794, HD85512, and HD192310 are reported and discussed,
among which is a system of three super-Earths and one that harbors a 3.6
Earth-mass planet at the inner edge of the habitable zone. This result already
confirms previous indications that low-mass planets seem to be very frequent
around solar-type stars and that this may occur with a frequency higher than
30%Comment: 18 pages, 22 figures, accepted by A&A on 15/08/2011 with reference
AA/2011/17055. Radial velocity data will be available through CD
Radial Velocities with CRIRES: Pushing precision down to 5-10 m/s
With the advent of high-resolution infrared spectrographs, Radial Velocity
(RV) searches enter into a new domain. As of today, the most important
technical question to address is which wavelength reference is the most
suitable for high-precision RV measurements.
In this work we explore the usage of atmospheric absorption features. We make
use of CRIRES data on two programs and three different targets. We re-analyze
the data of the TW Hya campaign, reaching a dispersion of about 6 m/s on the RV
standard in a time scale of roughly 1 week. We confirm the presence of a
low-amplitude RV signal on TW Hya itself, roughly 3 times smaller than the one
reported at visible wavelengths. We present RV measurements of Gl 86 as well,
showing that our approach is capable of detecting the signal induced by a
planet and correctly quantifying it.
Our data show that CRIRES is capable of reaching a RV precision of less than
10 m/s in a time-scale of one week. The limitations of this particular approach
are discussed, and the limiting factors on RV precision in the IR in a general
way. The implications of this work on the design of future dedicated IR
spectrographs are addressed as well.Comment: 9 pages, accepted for publication in A&
Formation and structure of the three Neptune-mass planets system around HD69830
Since the discovery of the first giant planet outside the solar system in
1995 (Mayor & Queloz 1995), more than 180 extrasolar planets have been
discovered. With improving detection capabilities, a new class of planets with
masses 5-20 times larger than the Earth, at close distance from their parent
star is rapidly emerging. Recently, the first system of three Neptune-mass
planets has been discovered around the solar type star HD69830 (Lovis et al.
2006). Here, we present and discuss a possible formation scenario for this
planetary system based on a consistent coupling between the extended core
accretion model and evolutionary models (Alibert et al. 2005a, Baraffe et al.
2004,2006). We show that the innermost planet formed from an embryo having
started inside the iceline is composed essentially of a rocky core surrounded
by a tiny gaseous envelope. The two outermost planets started their formation
beyond the iceline and, as a consequence, accrete a substantial amount of water
ice during their formation. We calculate the present day thermodynamical
conditions inside these two latter planets and show that they are made of a
rocky core surrounded by a shell of fluid water and a gaseous envelope.Comment: Accepted in AA Letter
The HARPS search for southern extra-solar planets XLI. A dozen planets around the M dwarfs GJ 3138, GJ 3323, GJ 273, GJ 628, and GJ 3293
Context. Low mass stars are currently the best targets for searches for rocky
planets in the habitable zone of their host star. Over the last 13 years,
precise radial velocities measured with the HARPS spectrograph have identified
over a dozen super-Earths and Earth-mass planets (msin i<10Mearth ) around M
dwarfs, with a well understood selection function. This well defined sample
informs on their frequency of occurrence and on the distribution of their
orbital parameters, and therefore already constrains our understanding of
planetary formation. The subset of these low-mass planets that were found
within the habitable zone of their host star also provide prized targets for
future atmospheric biomarkers searches. Aims. We are working to extend this
planetary sample to lower masses and longer periods through dense and long-term
monitoring of the radial velocity of a small M dwarf sample. Methods. We
obtained large numbers of HARPS spectra for the M dwarfs GJ 3138, GJ 3323, GJ
273, GJ 628 and GJ 3293, from which we derived radial velocities (RVs) and
spectroscopic activity indicators. We searched them for variabilities,
periodicities, Keplerian modulations and correlations, and attribute the
radial-velocity variations to combinations of planetary companions and stellar
activity. Results. We detect 12 planets, of which 9 are new with masses ranging
from 1.17 to 10.5 Mearth . Those planets have relatively short orbital periods
(P<40 d), except two of them with periods of 217.6 and 257.8 days. Among these
systems, GJ 273 harbor two planets with masses close to the one of the Earth.
With a distance of 3.8 parsec only, GJ 273 is the second nearest known
planetary system - after Proxima Centauri - with a planet orbiting the
circumstellar habitable zone.Comment: 19 pages, 24 figures. Astronomy and Astrophysics in pres
Traveling interface modulations in the NH 3 + O 2 reaction on a Rh(110) surface
A new type of traveling interface modulation has been observed in the NH 3 + O 2 reaction on a Rh(110) surface. A model is set up which reproduces the effect, which is attributed to diffusional mixing of two spatially separated adsorbates causing an excitability which is strictly localized to the vicinity of the interface of the adsorbate domains. © 2012 the Owner Societies
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