106 research outputs found
Nonradiative Electronic Deexcitation Time Scales in Metal Clusters
The life-times due to Auger-electron emission for a hole on a deep electronic
shell of neutral and charged sodium clusters are studied for different sizes.
We consider spherical clusters and calculate the Auger-transition probabilities
using the energy levels and wave functions calculated in the
Local-Density-Approximation (LDA).
We obtain that Auger emission processes are energetically not allowed for
neutral and positively charged sodium clusters. In general, the Auger
probabilities in small Na clusters are remarkably different from the
atomic ones and exhibit a rich size dependence.
The Auger decay times of most of the cluster sizes studied are orders of
magnitude larger than in atoms and might be comparable with typical
fragmentation times.Comment: 11 pages, 4 figures. Accepted for publication in Phys. Rev.
LBT transmission spectroscopy of HAT-P-12b: confirmation of a cloudy atmosphere with no significant alkali features
The hot sub-Saturn-mass exoplanet HAT-P-12b is an ideal target for
transmission spectroscopy because of its inflated radius. We observed one
transit of the planet with the multi-object double spectrograph (MODS) on the
Large Binocular Telescope (LBT) with the binocular mode and obtained an
atmosphere transmission spectrum with a wavelength coverage of 0.4 --
0.9 m. The spectrum is relatively flat and does not show any
significant sodium or potassium absorption features. Our result is consistent
with the revised Hubble Space Telescope (HST) transmission spectrum of a
previous work, except that the HST result indicates a tentative detection of
potassium. The potassium discrepancy could be the result of statistical
fluctuation of the HST dataset. We fit the planetary transmission spectrum with
an extensive grid of cloudy models and confirm the presence of high-altitude
clouds in the planetary atmosphere. The fit was performed on the combined LBT
and HST spectrum, which has an overall wavelength range of 0.4 -- 1.6
m. The LBT/MODS spectrograph has unique advantages in
transmission spectroscopy observations because it can cover a wide wavelength
range with a single exposure and acquire two sets of independent spectra
simultaneously.Comment: 14 pages, 12 figures. Accepted for publication in Astronomy &
Astrophysic
Polarisation of very-low-mass stars and brown dwarfs
Ultra-cool dwarfs of the L spectral type (Teff=1400-2200K) are known to have
dusty atmospheres. Asymmetries of the dwarf surface may arise from
rotationally-induced flattening and dust-cloud coverage, and may result in
non-zero linear polarisation through dust scattering.
We aim to study the heterogeneity of ultra-cool dwarfs' atmospheres and the
grain-size effects on the polarisation degree in a sample of nine late M, L and
early T dwarfs.
We obtain linear polarimetric imaging measurements using FORS1 at the Very
Large Telescope, in the Bessel I filter, and for a subset in the Bessel R and
the Gunn z filters.
We measure a polarisation degree of (0.31+/-0.06)% for LHS102BC. We fail to
detect linear polarisation in the rest of our sample, with upper-limits on the
polarisation degree of each object of 0.09% to 0.76% (95% CL). For those
targets we do not find evidence of large-scale cloud horizontal structure in
our data. Together with previous surveys, our results set the fraction of
ultra-cool dwarfs with detected linear polarisation to (30+10-6)% (1-sigma).
For three brown dwarfs, our observations indicate polarisation degrees
different (at the 3-sigma level) than previously reported, giving hints of
possible variations.
Our results fail to correlate with the current model predictions for
ultra-cool dwarf polarisation for a flattening-induced polarisation, or with
the variability studies for a polarisation induced by an hetereneous cloud
cover. This stresses the intricacy of each of those tasks, but may as well
proceed from complex and dynamic atmospheric processes.Comment: 8 pages, 2 figures, accepted by A&A. Reference problem and a few
typos corrected; improved error treatment of Zapatero Osorio et al (2005)
data, leading to minor differences in the result
Ionic structure and photoabsorption in medium sized sodium clusters
We present ground-state configurations and photoabsorption spectra of Na-7+,
Na-27+ and Na-41+. Both the ionic structure and the photoabsorption spectra of
medium-size sodium clusters beyond Na-20 have been calculated self-consistently
with a nonspherical treatment of the valence electrons in density functional
theory. We use a local pseudopotential that has been adjusted to experimental
bulk properties and the atomic 3s level of sodium. Our studies have shown that
both the ionic structure of the ground state and the positions of the plasmon
resonances depend sensitively on the pseudopotential used in the calculation,
which stresses the importance of its consistent use in both steps.Comment: 4 pages, 3 figures. Accepted for publication in PRB, tentatively July
15th, 1998 some typos corrected, brought to nicer forma
Characterizing Exoplanets in the Visible and Infrared: A Spectrometer Concept for the EChO Space Mission
Transit-spectroscopy of exoplanets is one of the key observational techniques
to characterize the extrasolar planet and its atmosphere. The observational
challenges of these measurements require dedicated instrumentation and only the
space environment allows an undisturbed access to earth-like atmospheric
features such as water or carbon-dioxide. Therefore, several exoplanet-specific
space missions are currently being studied. One of them is EChO, the Exoplanet
Characterization Observatory, which is part of ESA's Cosmic Vision 2015-2025
program, and which is one of four candidates for the M3 launch slot in 2024. In
this paper we present the results of our assessment study of the EChO
spectrometer, the only science instrument onboard this spacecraft. The
instrument is a multi-channel all-reflective dispersive spectrometer, covering
the wavelength range from 400 nm to 16 microns simultaneously with a moderately
low spectral resolution. We illustrate how the key technical challenge of the
EChO mission - the high photometric stability - influences the choice of
spectrometer concept and drives fundamentally the instrument design. First
performance evaluations underline the fitness of the elaborated design solution
for the needs of the EChO mission.Comment: 20 pages, 8 figures, accepted for publication in the Journal of
Astronomical Instrumentatio
Characterisation of the upper atmospheres of HAT-P-32 b, WASP-69 b, GJ 1214 b, and WASP-76 b through their He I triplet absorption
Characterisation of atmospheres undergoing photo-evaporation is key to
understanding the formation, evolution, and diversity of planets. However, only
a few upper atmospheres that experience this kind of hydrodynamic escape have
been characterised. Our aim is to characterise the upper atmospheres of the hot
Jupiters HAT-P-32 b and WASP-69 b, the warm sub-Neptune GJ 1214 b, and the
ultra-hot Jupiter WASP-76 b through high-resolution observations of their HeI
triplet absorption. In addition, we also reanalyse the warm Neptune GJ 3470 b
and the hot Jupiter HD 189733 b. We used a spherically symmetric 1D
hydrodynamic model coupled with a non-local thermodynamic equilibrium model.
Comparing synthetic absorption spectra with observations, we constrained the
main parameters of the upper atmosphere of these planets and classify them
according to their hydrodynamic regime. Our results show that HAT-P-32 b
photo-evaporates at (13070)10 gs with a hot (12
4002900 K) upper atmosphere; WASP-69 b loses its atmosphere at
(0.90.5)10 gs and 5250750 K; and GJ 1214 b,
with a relatively cold outflow of 3750750 K, photo-evaporates at
(1.31.1)10 gs. For WASP-76 b, its weak absorption
prevents us from constraining its temperature and mass-loss rate significantly;
we obtained ranges of 6000-17 000\,K and 23.521.510
gs. Our reanalysis of GJ 3470 b yields colder temperatures, 3400350
K, but practically the same mass-loss rate as in our previous results. Our
reanalysis of HD 189733 b yields a slightly higher mass-loss rate,
(1.40.5)10 gs, and temperature, 12 700900 K
compared to previous estimates. Our results support that photo-evaporated
outflows tend to be very light
Atmospheric characterization of the ultra-hot Jupiter WASP-33b: Detection of Ti and V emission lines and retrieval of a broadened line profile
Ultra-hot Jupiters are highly irradiated gas giant exoplanets on close-in
orbits around their host stars. We analyzed high-resolution spectra from
CARMENES, HARPS-N, and ESPaDOnS taken over eight observation nights to study
the emission spectrum of WASP-33b and draw conclusions about its atmosphere. By
applying the cross-correlation technique, we detected the spectral signatures
of Ti I, V I, and a tentative signal of Ti II for the first time via emission
spectroscopy. These detections are an important finding because of the
fundamental role of Ti- and V-bearing species in the planetary energy balance.
Moreover, we assessed and confirm the presence of OH, Fe I, and Si I from
previous studies. The spectral lines are all detected in emission, which
unambiguously proves the presence of an inverted temperature profile in the
planetary atmosphere. By performing retrievals on the emission lines of all the
detected species, we determined a relatively weak atmospheric thermal inversion
extending from approximately 3400 K to 4000 K. We infer a supersolar
metallicity close to 1.5 dex in the planetary atmosphere, and find that its
emission signature undergoes significant line broadening with a Gaussian FWHM
of about 4.5 km/s. Also, we find that the atmospheric temperature profile
retrieved at orbital phases far from the secondary eclipse is about 300 K to
700 K cooler than that measured close to the secondary eclipse, which is
consistent with different day- and nightside temperatures. Moreover, retrievals
performed on the emission lines of the individual chemical species lead to
consistent results, which gives additional confidence to our retrieval method.
Increasing the number of species included in the retrieval and expanding the
set of retrieved atmospheric parameters will further advance our understanding
of exoplanet atmospheres.Comment: Accepted for publication in A&
The CARMENES search for exoplanets around M dwarfs. Line-by-line sensitivity to activity in M dwarfs
Radial velocities (RVs) measured from high-resolution stellar spectra are
routinely used to detect and characterise orbiting exoplanet companions. The
different lines present in stellar spectra are created by several species,
which are non-uniformly affected by stellar variability features such as spots
or faculae. Stellar variability distorts the shape of the spectral absorption
lines from which precise RVs are measured, posing one of the main problems in
the study of exoplanets. In this work we aim to study how the spectral lines
present in M dwarfs are independently impacted by stellar activity. We used
CARMENES optical spectra of six active early- and mid-type M dwarfs to compute
line-by-line RVs and study their correlation with several well-studied proxies
of stellar activity. We are able to classify spectral lines based on their
sensitivity to activity in five M dwarfs displaying high levels of stellar
activity. We further used this line classification to compute RVs with
activity-sensitive lines and less sensitive lines, enhancing or mitigating
stellar activity effects in the RV time series. For specific sets of the least
activity-sensitive lines, the RV scatter decreases by ~ 2 to 5 times the
initial one, depending on the star. Finally, we compare these lines in the
different stars analysed, finding the sensitivity to activity to vary from star
to star. Despite the high density of lines and blends present in M dwarf
stellar spectra, we find that a line-by-line approach is able to deliver
precise RVs. Line-by-line RVs are also sensitive to stellar activity effects,
and they allow for an accurate selection of activity-insensitive lines to
mitigate activity effects in RV. However, we find stellar activity effects to
vary in the same insensitive lines from star to star.Comment: Accepted for publication in A&
Discriminating between hazy and clear hot-Jupiter atmospheres with CARMENES
Context: Relatively large radii of some hot Jupiters observed in the
ultraviolet (UV) and blue-optical are generally interpreted to be due to
Rayleigh scattering by high-altitude haze particles. However, the haze
composition and its production mechanisms are not fully understood, and
observational information is still limited. Aims: We aim to study the presence
of hazes in the atmospheres of HD 209458 b and HD 189733 b with high spectral
resolution spectra by analysing the strength of water vapour cross-correlation
signals across the red optical and near-infrared wavelength ranges. Methods: A
total of seven transits of the two planets were observed with the CARMENES
spectrograph at the 3.5 m Calar Alto telescope. Their Doppler-shifted signals
were disentangled from the telluric and stellar contributions using the
detrending algorithm SYSREM. The residual spectra were subsequently
cross-correlated with water vapour templates at 0.70-0.96 m to measure the
strength of the water vapour absorption bands. Results: The optical water
vapour bands were detected at in HD 209458 b in one transit,
whereas no evidence of them was found in four transits of HD 189733 b.
Therefore, the relative strength of the optical water bands compared to those
in the near-infrared were found to be larger in HD 209458 b than in HD 189733
b. Conclusions: We interpret the non-detection of optical water bands in the
transmission spectra of HD 189733 b, compared to the detection in HD 209458 b,
to be due to the presence of high-altitude hazes in the former planet, which
are largely absent in the latter. This is consistent with previous measurements
with the Hubble Space Telescope. We show that currently available CARMENES
observations of hot Jupiters can be used to investigate the presence of haze
extinction in their atmospheres.Comment: 13 pages; accepted for publication in A&
The CARMENES search for exoplanets around M dwarfs -- Planet occurrence rates from a subsample of 71 stars
The CARMENES exoplanet survey of M dwarfs has obtained more than 18 000
spectra of 329 nearby M dwarfs over the past five years as part of its
guaranteed time observations (GTO) program. We determine planet occurrence
rates with the 71 stars from the GTO program for which we have more than 50
observations. We use injection-and-retrieval experiments on the radial-velocity
(RV) time series to measure detection probabilities. We include 27 planets in
21 planetary systems in our analysis. We find 0.06+0.04-0.03 giant planets (100
M_Earth < M_pl sin i < 1000 M_Earth) per star in periods of up to 1000 d, but
due to a selection bias this number could be up to a factor of five lower in
the whole 329-star sample. The upper limit for hot Jupiters (orbital period of
less than 10 d) is 0.03 planets per star, while the occurrence rate of planets
with intermediate masses (10 M_Earth < M_pl sin i < 100 M_Earth) is
0.18+0.07-0.05 planets per star. Less massive planets with 1 M_Earth < M_pl sin
i < 10 M_Earth are very abundant, with an estimated rate of 1.32+0.33-0.31
planets per star for periods of up to 100 d. When considering only late M
dwarfs with masses M_star < 0.34 M_sol, planets more massive than 10 M_Earth
become rare. Instead, low-mass planets with periods shorter than 10 d are
significantly overabundant. For orbital periods shorter than 100 d, our results
confirm the known stellar mass dependences from the Kepler survey: M dwarfs
host fewer giant planets and at least two times more planets with M_pl sin i <
10 M_Earth than G-type stars. In contrast to previous results, planets around
our sample of very low-mass stars have a higher occurrence rate in short-period
orbits of less than 10 d. Our results demonstrate the need to take into account
host star masses in planet formation models.Comment: 15 pages, 12 figures. Accepted for publication in Astronomy &
Astrophysic
- …