135 research outputs found
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
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
NIGHT: a compact, near-infrared, high-resolution spectrograph to survey helium in exoplanet systems
Among highly irradiated exoplanets, some have been found to undergo
significant hydrodynamic expansion traced by atmospheric escape. To better
understand these processes in the context of planetary evolution, we propose
NIGHT (the Near-Infrared Gatherer of Helium Transits). NIGHT is a
high-resolution spectrograph dedicated to surveying and temporally monitoring
He I triplet absorption at 1083nm in stellar and planetary atmospheres. In this
paper, we outline our scientific objectives, requirements, and cost-efficient
design. Our simulations, based on previous detections and modelling using the
current exoplanet population, determine our requirements and survey targets.
With a spectral resolution of 70,000 on a 2-meter telescope, NIGHT can
accurately resolve the helium triplet and detect 1% peak absorption in 118
known exoplanets in a single transit. Additionally, it can search for
three-sigma temporal variations of 0.4% in 66 exoplanets in-between two
transits. These are conservative estimates considering the ongoing detections
of transiting planets amenable to atmospheric characterisation. We find that
instrumental stability at 40m/s, less stringent than for radial velocity
monitoring, is sufficient for transmission spectroscopy in He I. As such, NIGHT
can utilize mostly off-the-shelf components, ensuring cost-efficiency. A
fibre-fed system allows for flexibility as a visitor instrument on a variety of
telescopes, making it ideal for follow-up observations after JWST or
ground-based detections. Over a few years of surveying, NIGHT could offer
detailed insights into the mechanisms shaping the hot Neptune desert and
close-in planet population by significantly expanding the statistical sample of
planets with known evaporating atmospheres. First light is expected in 2024.Comment: 15 pages, 20 figures, this manuscript has been accepted for
publication in MNRAS. This is a pre-copyedited, author-produced PD
Deriving High-Precision Radial Velocities
This chapter describes briefly the key aspects behind the derivation of
precise radial velocities. I start by defining radial velocity precision in the
context of astrophysics in general and exoplanet searches in particular. Next I
discuss the different basic elements that constitute a spectrograph, and how
these elements and overall technical choices impact on the derived radial
velocity precision. Then I go on to discuss the different wavelength
calibration and radial velocity calculation techniques, and how these are
intimately related to the spectrograph's properties. I conclude by presenting
some interesting examples of planets detected through radial velocity, and some
of the new-generation instruments that will push the precision limit further.Comment: Lecture presented at the IVth Azores International Advanced School in
Space Sciences on "Asteroseismology and Exoplanets: Listening to the Stars
and Searching for New Worlds" (arXiv:1709.00645), which took place in Horta,
Azores Islands, Portugal in July 201
Could we identify hot Ocean-Planets with CoRoT, Kepler and Doppler velocimetry?
Planets less massive than about 10 MEarth are expected to have no massive
H-He atmosphere and a cometary composition (50% rocks, 50% water, by mass)
provided they formed beyond the snowline of protoplanetary disks. Due to inward
migration, such planets could be found at any distance between their formation
site and the star. If migration stops within the habitable zone, this will
produce a new kind of planets, called Ocean-Planets. Ocean-planets typically
consist in a silicate core, surrounded by a thick ice mantle, itself covered by
a 100 km deep ocean. The existence of ocean-planets raises important
astrobiological questions: Can life originate on such body, in the absence of
continent and ocean-silicate interfaces? What would be the nature of the
atmosphere and the geochemical cycles ?
In this work, we address the fate of Hot Ocean-Planets produced when
migration ends at a closer distance. In this case the liquid/gas interface can
disappear, and the hot H2O envelope is made of a supercritical fluid. Although
we do not expect these bodies to harbor life, their detection and
identification as water-rich planets would give us insight as to the abundance
of hot and, by extrapolation, cool Ocean-Planets.Comment: 47 pages, 6 Fugures, regular paper. Submitted to Icaru
The ESPRI project: astrometric exoplanet search with PRIMA I. Instrument description and performance of first light observations
The ESPRI project relies on the astrometric capabilities offered by the PRIMA
facility of the Very Large Telescope Interferometer for the discovery and study
of planetary systems. Our survey consists of obtaining high-precision
astrometry for a large sample of stars over several years and to detect their
barycentric motions due to orbiting planets. We present the operation
principle, the instrument's implementation, and the results of a first series
of test observations. A comprehensive overview of the instrument infrastructure
is given and the observation strategy for dual-field relative astrometry is
presented. The differential delay lines, a key component of the PRIMA facility
which was delivered by the ESPRI consortium, are described and their
performance within the facility is discussed. Observations of bright visual
binaries are used to test the observation procedures and to establish the
instrument's astrometric precision and accuracy. The data reduction strategy
for astrometry and the necessary corrections to the raw data are presented.
Adaptive optics observations with NACO are used as an independent verification
of PRIMA astrometric observations. The PRIMA facility was used to carry out
tests of astrometric observations. The astrometric performance in terms of
precision is limited by the atmospheric turbulence at a level close to the
theoretical expectations and a precision of 30 micro-arcseconds was achieved.
In contrast, the astrometric accuracy is insufficient for the goals of the
ESPRI project and is currently limited by systematic errors that originate in
the part of the interferometer beamtrain which is not monitored by the internal
metrology system. Our observations led to the definition of corrective actions
required to make the facility ready for carrying out the ESPRI search for
extrasolar planets.Comment: 32 pages, 39 figures, Accepted for publication in Astronomy and
Astrophysic
The Next Generation Transit Survey—Prototyping Phase
We present the prototype telescope for the Next Generation Transit Survey, which was built in the UK in 2008/2009 and tested on La Palma in the Canary Islands in 2010. The goals for the prototype system were severalfold: to determine the level of systematic noise in an NGTS-like system; demonstrate that we can perform photometry at the (sub) millimagnitude level on transit timescales across a wide-field; show that it is possible to detect transiting super-Earth and Neptune-sized exoplanets and prove the technical feasibility of the proposed planet survey. We tested the system for around 100 nights and met each of the goals above. Several key areas for improvement were highlighted during the prototyping phase. They have been subsequently addressed in the final NGTS facility, which was recently commissioned at ESO Cerro Paranal, Chile
NGTS-5b: A highly inflated planet offering insights into the sub-Jovian desert
Context: Planetary population analysis gives us insight into formation and
evolution processes. For short-period planets, the subJovian desert has been
discussed in recent years with regard to the planet population in the
mass/period and radius/period parameter space without taking stellar parameters
into account. The Next Generation Transit Survey (NGTS) is optimised for
detecting planets in this regime, which allows for further analysis of the
sub-Jovian desert.
Aims: With high-precision photometric surveys (e.g. with NGTS and TESS),
which aim to detect short period planets especially around M/K-type host stars,
stellar parameters need to be accounted for when empirical data are compared to
model predictions. Presenting a newly discovered planet at the boundary of the
sub-Jovian desert, we analyse its bulk properties and use it to show the
properties of exoplanets that border the sub-Jovian desert.
Methods: Using NGTS light curve and spectroscopic follow-up observations, we
confirm the planetary nature of planet NGTS-5b and determine its mass. Using
exoplanet archives, we set the planet in context with other discoveries.
Results: NGTS-5b is a short-period planet with an orbital period of 3.3569866
+- 0.0000026 days. With a mass of 0.229 +- 0.037 MJup and a radius of 1.136 +-
0.023 RJup, it is highly inflated. Its mass places it at the upper boundary of
the sub-Jovian desert. Because the host is a K2 dwarf, we need to account for
the stellar parameters when NGTS-5b is analysed with regard to planet
populations.
Conclusions: With red-sensitive surveys (e.g. with NGTS and TESS), we expect
many more planets around late-type stars to be detected. An empirical analysis
of the sub-Jovian desert should therefore take stellar parameters into account
ANDES, the high resolution spectrograph for the ELT: project management and system engineering approaches for mastering its preliminary design phase
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