44 research outputs found
Hunt for Starspots in HARPS Spectra of G and K Stars
We present a method for detecting starspots on cool stars using the
cross-correlation function (CCF) of high resolution molecular spectral
templates applied to archival high-resolution spectra of G and K stars observed
with HARPS/HARPS-N. We report non-detections of starspots on the Sun even when
the Sun was spotted, the solar twin 18 Scorpii, and the very spotted Sun-like
star HAT-P-11, suggesting that Sun-like starspot distributions will be
invisible to the CCF technique, and should not produce molecular absorption
signals which might be confused for signatures of exoplanet atmospheres. We
detect strong TiO absorption in the T Tauri K-dwarfs LkCa 4 and AA Tau,
consistent with significant coverage by cool regions. We show that despite the
non-detections, the technique is sensitive to relatively small spot coverages
on M dwarfs and large starspot areas on Sun-like stars.Comment: 12 pages, 8 figures, accepted to A
ExoMol Molecular linelists -- XXXIII. The spectrum of Titanium Oxide
Accurate line lists are crucial for correctly modelling a variety of
astrophysical phenomena, including stellar photospheres and the atmospheres of
extra-solar planets. This paper presents a new line database Toto for the main
isotopologues of titanium oxide (TiO): TiO, TiO,
TiO, TiO and TiO. The TiO line list
contains transitions with wave-numbers up to 30,000 cm ie long-wards of
0.33 m. The Toto line list includes all dipole-allowed transitions between
13 low-lying electronic states (X , a , d , E
, A B , C , b , c , f
, e ). Ab initio potential energy curves (PECs) are
computed at the icMRCI level and combined with spin-orbit and other coupling
curves. These PECs and couplings are iteratively refined to match known
empirical energy levels. Accurate line intensities are generated using ab
initio dipole moment curves. The Toto line lists are appropriate for
temperatures below 5000 K and contain 30 million transitions for TiO; it is
made available in electronic form via the CDS data centre and via
www.exomol.com. Tests of the line lists show greatly improved agreement with
observed spectra for objects such as M-dwarfs GJ876 and GL581
Probing Ganymede's atmosphere with HST Ly images in transit of Jupiter
We report results from far-ultraviolet observations by the Hubble Space
Telescope of Jupiter's largest moon Ganymede transiting across the planet's
dayside hemisphere. {Within} a targeted campaign on 9 September 2021 two
exposures were taken during one transit passage to probe for attenuation of
Jupiter's hydrogen Lyman- dayglow above the moon limb. The background
dayglow is slightly attenuated over an extended region around Ganymede, with
stronger attenuation in the second exposure when Ganymede was near the planet's
center. In the first exposure when the moon was closer to Jupiter's limb, the
effects from the Ganymede corona are hardly detectable, likely because the
Jovian Lyman- dayglow is spectrally broader and less intense at this
viewing geometry. The obtained vertical H column densities of around
~cm are consistent with previous results.
Constraining angular variability around Ganymede's disk, we derive an upper
limit on a local HO column density of ~cm, such
as could arise from outgassing plumes in regions near the observed moon limb
A snapshot full-Stokes spectropolarimeter for detecting life on Earth
We present the design of a point-and-shoot non-imaging full-Stokes
spectropolarimeter dedicated to detecting life on Earth from an orbiting
platform like the ISS. We specifically aim to map circular polarization in the
spectral features of chlorophyll and other biopigments for our planet as a
whole. These non-zero circular polarization signatures are caused by
homochirality of the molecular and supramolecular configurations of organic
matter, and are considered the most unambiguous biomarker. To achieve a fully
solid-state snapshot design, we implement a novel spatial modulation that
completely separates the circular and linear polarization channels. The
polarization modulator consists of a patterned liquid-crystal quarter-wave
plate inside the spectrograph slit, which also constitutes the first optical
element of the instrument. This configuration eliminates cross-talk between
linear and circular polarization, which is crucial because linear polarization
signals are generally much stronger than the circular polarization signals.
This leads to a quite unorthodox optical concept for the spectrograph, in which
the object and the pupil are switched. We discuss the general design
requirements and trade-offs of LSDpol (Life Signature Detection polarimeter), a
prototype instrument that is currently under development
Physically-motivated basis functions for temperature maps of exoplanets
Thermal phase curves of exoplanet atmospheres have revealed temperature maps as a function of planetary longitude, often by sinusoidal decomposition of the phase curve. We construct a framework for describing two-dimensional temperature maps of exoplanets with mathematical basis functions derived for a fluid layer on a rotating, heated sphere with drag/friction, which are generalisations of spherical harmonics. These basis functions naturally produce physically-motivated temperature maps for exoplanets with few free parameters. We investigate best practices for applying this framework to temperature maps of hot Jupiters by splitting the problem into two parts: (1) we constrain the temperature map as a function of latitude by tuning the basis functions to reproduce general circulation model outputs, since disk-integrated phase curve observations do not constrain this dimension; and (2) we infer the temperature maps of real hot Jupiters using original reductions of several Spitzer phase curves, which directly constrain the temperature variations with longitude. The resulting phase curves can be described with only three free parameters per bandpass – an efficiency improvement over the usual five or so used to describe sinusoidal decompositions of phase curves. Upon obtaining the hemispherically averaged day side and night side temperatures, the standard approach would be to use zero-dimensional box models to infer the Bond albedo and redistribution efficiency. We elucidate the limitation of these box models by demonstrating that negative Bond albedos may be obtained due to a choice of boundary condition on the night side temperature. We propose generalized definitions for the Bond albedo and heat redistribution efficiency for use with two-dimensional (2D) temperature maps. Open-source software called kelp is provided to efficiently compute the 2D temperature maps, phase curves, albedos and redistribution efficiencies
A strong H- opacity signal in the near-infrared emission spectrum of the ultra-hot Jupiter KELT-9b
We present the analysis of a spectroscopic secondary eclipse of the hottest
transiting exoplanet detected to date, KELT-9b, obtained with the Wide Field
Camera 3 aboard the Hubble Space Telescope.
We complement these data with literature information on stellar pulsations
and Spitzer/Infrared Array Camera and Transiting Exoplanet Survey Satellite
eclipse depths of this target to obtain a broadband thermal emission spectrum.
Our extracted spectrum exhibits a clear turnoff at 1.4m. This points to
H bound-free opacities shaping the spectrum.
To interpret the spectrum, we perform grid retrievals of self-consistent 1D
equilibrium chemistry forward models, varying the composition and energy
budget.
The model with solar metallicity and C/O ratio provides a poor fit because
the H signal is stronger than expected, requiring an excess of electrons.
This pushes our retrievals toward high atmospheric metallicities
() and a C/O ratio that is subsolar by 2.4.
We question the viability of forming such a high-metallicity planet, and
therefore provide other scenarios to increase the electron density in this
atmosphere.
We also look at an alternative model in which we quench TiO and VO. This fit
results in an atmosphere with a slightly subsolar metallicity and subsolar C/O
ratio (, log(C/O)).
However, the required TiO abundances are disputed by recent high-resolution
measurements of the same planet.Comment: 8 pages, 4 figures, Accepted for publication in A&A letter
Biosignatures of the Earth
ontext. Homochirality is a generic and unique property of life on Earth and is considered a universal and agnostic biosignature. Homochirality induces fractional circular polarization in the incident light that it reflects. Because this circularly polarized light can be sensed remotely, it can be one of the most compelling candidate biosignatures in life detection missions. While there are also other sources of circular polarization, these result in spectrally flat signals with lower magnitude. Additionally, circular polarization can be a valuable tool in Earth remote sensing because the circular polarization signal directly relates to vegetation physiology.
Aims. While high-quality circular polarization measurements can be obtained in the laboratory and under semi-static conditions in the field, there has been a significant gap to more realistic remote sensing conditions.
Methods. In this study, we present sensitive circular spectropolarimetric measurements of various landscape elements taken from a fast-moving helicopter.
Results. We demonstrate that during flight, within mere seconds of measurements, we can differentiate (S∕N > 5) between grass fields, forests, and abiotic urban areas. Importantly, we show that with only nonzero circular polarization as a discriminant, photosynthetic organisms can even be measured in lakes.
Conclusions. Circular spectropolarimetry can be a powerful technique to detect life beyond Earth, and we emphasize the potential of utilizing circular spectropolarimetry as a remote sensing tool to characterize and monitor in detail the vegetation physiology and terrain features of Earth itself
A Spectral Survey of WASP-19b with ESPRESSO
High resolution precision spectroscopy provides a multitude of robust
techniques for probing exoplanetary atmospheres. We present multiple
VLT/ESPRESSO transit observations of the hot-Jupiter exoplanet WASP-19b with
previously published but disputed atmospheric features from low resolution
studies. Through spectral synthesis and modeling of the Rossiter-McLaughlin
(RM) effect we calculate stellar, orbital and physical parameters for the
system. From narrow-band spectroscopy we do not detect any of H\,I, Fe\,I,
Mg\,I, Ca\,I, Na\,I and K\,I neutral species, placing upper limits on their
line contrasts. Through cross correlation analyses with atmospheric models, we
do not detect Fe\,I and place a 3 upper limit of
on its mass
fraction, from injection and retrieval. We show the inability to detect the
presence of HO for known abundances, owing to lack of strong absorption
bands, as well as relatively low S/N ratio. We detect a barely significant peak
(3.02\,\,0.15\,) in the cross correlation map for TiO, consistent
with the sub-solar abundance previously reported. This is merely a hint for the
presence of TiO and does \textit{not} constitute a confirmation. However, we do
confirm the presence of previously observed enhanced scattering towards blue
wavelengths, through chromatic RM measurements, pointing to a hazy atmosphere.
We finally present a reanalysis of low resolution transmission spectra of this
exoplanet, concluding that unocculted starspots alone cannot explain previously
detected features. Our reanalysis of the FORS2 spectra of WASP-19b finds a
\,100 sub-solar TiO abundance, precisely constrained to
, consistent with the TiO hint
from ESPRESSO. We present plausible paths to reconciliation with other
seemingly contradicting results.Comment: 24 pages, 16 figures, submitted to MNRA
Design of the Life Signature Detection Polarimeter LSDpol
Many biologically produced chiral molecules such as amino acids and sugars
show a preference for left or right handedness (homochirality). Light reflected
by biological materials such as algae and leaves therefore exhibits a small
amount of circular polarization that strongly depends on wavelength. Our Life
Signature Detection polarimeter (LSDpol) is optimized to measure these
signatures of life. LSDpol is a compact spectropolarimeter concept with no
moving parts that instantaneously measures linear and circular polarization
averaged over the field of view with a sensitivity of better than 1e-4. We
expect to launch the instrument into orbit after validating its performance on
the ground and from aircraft.
LSDpol is based on a spatially varying quarter-wave retarder that is
implemented with a patterned liquid-crystal. It is the first optical element to
maximize the polarimetric sensitivity. Since this pattern as well as the
entrance slit of the spectrograph have to be imaged onto the detector, the slit
serves as the aperture, and an internal field stop limits the field of view.
The retarder's fast axis angle varies linearly along one spatial dimension. A
fixed quarter-wave retarder combined with a polarization grating act as the
disperser and the polarizing beam-splitter. Circular and linear polarization
are thereby encoded at incompatible modulation frequencies across the spectrum,
which minimizes the potential cross-talk from linear into circular
polarization.Comment: 10 pages, 10 figures, SPIE Proceedings 11443-16
HELIOS-K 2.0 Opacity Calculator and Open-source Opacity Database for Exoplanetary Atmospheres
Computing and using opacities is a key part of modeling and interpreting data of exoplanetary atmospheres. Since the underlying spectroscopic line lists are constantly expanding and currently include up to ∼1010–1011 transition lines, the opacity calculator codes need to become more powerful. Here we present major upgrades to the HELIOS-K GPU-accelerated opacity calculator and describe the necessary steps to process large line lists within a reasonable amount of time. Besides performance improvements, we include more capabilities and present a toolbox for handling different atomic and molecular data sets, from downloading and preprocessing the data to performing the opacity calculations in a user-friendly way. HELIOS-K supports line lists from ExoMol, HITRAN, HITEMP, NIST, Kurucz, and VALD3. By matching the resolution of 0.1 cm−1 and cutting length of 25 cm−1 used by the ExoCross code for timing performance (251 s excluding data read-in time), HELIOS-K can process the ExoMol BT2 water line list in 12.5 s. Using a resolution of 0.01 cm−1, it takes 45 s, equivalent to about 107 lines s−1. As a wavenumber resolution of 0.01 cm−1 suffices for most exoplanetary atmosphere spectroscopic calculations, we adopt this resolution in calculating opacity functions for several hundred atomic and molecular species and make them freely available on the open-access DACE database. For the opacity calculations of the database, we use a cutting length of 100 cm−1 for molecules and no cutting length for atoms. Our opacities are available for downloading from https://dace.unige.ch/opacityDatabase and may be visualized using https://dace.unige.ch/opacity