467 research outputs found
The GTC exoplanet transit spectroscopy survey X. Stellar spots versus Rayleigh scattering: the case of HAT-P-11b
Rayleigh scattering in a hydrogen-dominated exoplanet atmosphere can be
detected from ground or space based telescopes, however, stellar activity in
the form of spots can mimic Rayleigh scattering in the observed transmission
spectrum. Quantifying this phenomena is key to our correct interpretation of
exoplanet atmospheric properties. We obtained long-slit optical spectroscopy of
two transits of HAT-P-11b with the Optical System for Imaging and
low-Intermediate-Resolution Integrated Spectroscopy (OSIRIS) at Gran Telescopio
Canarias (GTC) on August 30 2016 and September 25 2017. We integrated the
spectrum of HAT-P-11 and one reference star in several spectroscopic channels
across the 400-785 nm region, creating numerous light curves of
the transits. We fit analytic transit curves to the data taking into account
the systematic effects and red noise present in the time series in an effort to
measure the change of the planet-to-star radius ratio
() across wavelength. By fitting both transits
together, we find a slope in the transmission spectrum showing an increase of
the planetary radius towards blue wavelengths. A closer inspection to the
transmission spectrum of the individual data sets reveals that the first
transit presents this slope while the transmission spectrum of the second data
set is flat. Additionally we detect hints of Na absorption in the first night,
but not in the second. We conclude that the transmission spectrum slope and Na
absorption excess found in the first transit observation are caused by
unocculted stellar spots. Modeling the contribution of unocculted spots to
reproduce the results of the first night we find a spot filling factor of
and a spot-to-photosphere temperature difference
of K.Comment: Accepted for publication in Astronomy & Astrophysics, 13 page
Rotational modulation of the linear polarimetric variability of the cool dwarf TVLM 51346546
Aims: We aimed to monitor the optical linear polarimetric signal of the
magnetized, rapidly rotating M8.5 dwarf TVLM 51346546.
Methods: - and -band linear polarimetry images were collected with the
ALFOSC instrument of the 2.56-m Nordic Optical Telescope on two consecutive
nights covering about 0.5 and 4 rotation cycles in the and filters,
respectively. We also obtained simultaneous intensity curves by means of
differential photometry. The typical precision of the data is 0.46\%
(), 0.35\% () in the linear polarization degree and 9 mmag
(), 1.6 mmag () in the differential intensity curves.
Results: Strong and variable linear polarization is detected in the and
filters, with values of maximum polarization ( = 1.300.35 \%)
similar for both bands. The intensity and the polarimetric curves present a
sinusoid-like pattern with a periodicity of 1.98 h, which we ascribe to
structures in TVLM 51346's surface synchronized with rotation. We found that
the peaks of the intensity and polarimetric curves occur with a phase
difference of 0.180.01, and that the maximum of the linear polarization
happens nearly half a period (0.590.03) after the radio pulse. We
discussed different scenarios to account for the observed properties of the
light curves.Comment: Accepted for publication in Astronomy and Astrophysic
Earthshine observations of an inhabited planet
Earthshine is sunlight that has been reflected from the dayside Earth onto
the dark side of the Moon and back again to Earth. In recent times, there has
been renewed interest in ground-based visible and near-infrared measurements of
earthshine as a proxy for exoplanet observations. Observations of earthshine
allow us to explore and characterize the globally integrated photometric,
spectral and polarimetric features of the Earth, and to extract precise
information on the distinctive characteristics of our planet, and life in
particular. They also allow us to quantify how this feature changes with time
and orbital configuration. Here we present a brief review of the main
earthshine observations and results.Comment: To appear in the proceedings of the Les Houches Winter School
"Physics and Astrophysics of Planetary Systems",(EDP Sciences: EAS
Publications Series
The impact of the Kasatochi eruption on the Moon's illumination during the August 2008 lunar eclipse
The Moon's changeable aspect during a lunar eclipse is largely attributable
to variations in the refracted unscattered sunlight absorbed by the terrestrial
atmosphere that occur as the satellite crosses the Earth's shadow. The
contribution to the Moon's aspect from sunlight scattered at the Earth's
terminator is generally deemed minor. However, our analysis of a published
spectrum of the 16 August 2008 lunar eclipse shows that diffuse sunlight is a
major component of the measured spectrum at wavelengths shorter than 600 nm.
The conclusion is supported by two distinct features, namely the spectrum's
tail at short wavelengths and the unequal absorption by an oxygen collisional
complex at two nearby bands. Our findings are consistent with the presence of
the volcanic cloud reported at high northern latitudes following the 7-8 August
2008 eruption in Alaska of the Kasatochi volcano. The cloud both attenuates the
unscattered sunlight and enhances moderately the scattered component, thus
modifying the contrast between the two contributions.Comment: Accepted for publication in Geophysical Research Letter
The centre-to-limb variations of solar Fraunhofer lines imprinted upon lunar eclipse spectra - Implications for exoplanet transit observations
The atmospheres of exoplanets are commonly studied by observing the transit
of the planet passing in front of its parent star. The obscuration of part of
the stellar disk during a transit will reveal aspects of its surface structure
resulting from general centre-to-limb variations (CLVs). These become apparent
when forming the ratio between the stellar light in and out of transit. These
phenomena can be seen particularly clearly during the progress of a penumbral
lunar eclipse, where the Earth transits the solar disk and masks different
regions of the solar disk as the eclipse progresses. When inferring the
properties of the planetary atmosphere, it is essential that this effect
originating at the star is properly accounted for. Using the data observed from
the 2014-April-15 lunar eclipse with the ESPaDOnS spectrograph mounted on the
Canada France Hawaii Telescope (CFHT), we have obtained for the first time a
time sequence of the penumbral spectra. These penumbral spectra enable us to
study the centre-to-limb variations of solar Fraunhofer lines when the Earth is
transiting Sun. The Na i and Ca ii absorption features reported from previous
lunar eclipse observations are demonstrated to be CLV features, which dominate
the corresponding line profiles and mask possible planetary signal. Detecting
atmospheric species in exoplanets via transit spectroscopy must account for the
CLV effect.Comment: 9 pages, 11 figures, accepted, A&
The effect of the stellar absorption line centre-to-limb variation on exoplanet transmission spectrum observations
Transit spectroscopy is one of the most commonly used techniques for
exoplanet atmosphere characterisation. This technique has been used to detect
ionized and neutral species in exoplanet atmospheres by comparing the observed
stellar lines in and out of transit. The centre-to-limb variation (CLV) of the
stellar lines across the stellar disk is an important effect for transmission
spectroscopy, since it results in a change of stellar line depth when the
planet transits different parts of the stellar disk. We reanalyse the transit
data of HD 189733b taken with the HARPS spectrograph to study the CLV effect
during transit. The transmission light curve of the Na i D line so obtained
shows a clear imprint of the CLV effect. We use a one-dimensional non-LTE
stellar spectral model to simulate the CLV effect. After applying the
correction, the measurement of the Na i absorption in the atmosphere of HD
189733b becomes better determined. We compare the CLV effect of HD 189733b to
that of HD 209458b. The CLV effects are different for these two benchmark
planetary systems and this is attributed to their different stellar effective
temperatures and transit impact parameters. We then explore the general CLV
effect that occurs during exoplanet transits. Normally, a star with a lower
effective temperature exhibits a stronger CLV effect and its CLV feature
extends over a relatively broad wavelength range. The transit impact parameter
(b) describes the transit trajectory on the stellar disk and thus determines
the actual manifestation of the CLV effect. We introduce a b-diagram which
describes the behavior of the CLV effect as the function of different impact
parameters. With improving observational precision, a careful modeling and
correction of the CLV effect is necessary for exoplanet atmosphere
characterisation using transit spectroscopy.Comment: Accepted for publishing on A&
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