76 research outputs found
Linear polarization of rapidly rotating ultracool dwarfs
Aims. We aim at studying the near infrared linear polarization signal of
rapidly rotating ultracool dwarfs with spectral types ranging from M7 through
T2 and projected rotational velocities vsini >= 30 km s^{-1}. All these dwarfs
are believed to have dusty atmospheres and oblate shapes, an appropriate
scenario to produce measurable linear polarization of the continuum light.\\
Methods. Linear polarimetric images were collected in the J-band for a sample
of 18 fast-rotating ultracool dwarfs, five of which were also observed in the
Z-band using the LIRIS spectrograph on the Cassegrain focus of the 4.2-m
William Herschel Telescope. The measured median uncertainty in the linear
polarization degree is +/-0.13% for our sample, which allowed us to detect
polarization signatures above ~0.39% with a confidence of >=3\sigma.\\ Results.
About 40+/-15% of the sample is linearly polarized in the Z- and J-bands. All
positive detections have linear polarization degrees ranging from 0.4% to 0.8%
in both filters independently of spectral type and spectroscopic rotational
velocity. However, simple statistics point at the fastest rotators (vsini >=60
km s^{-1}) having a larger fraction of positive detections and a larger
averaged linear polarization degree than the moderately rotating dwarfs
(vsini=30--60 km s^{-1}). Our data suggest little linear polarimetric
variability on short time scales (i.e., observations separated by a few ten
rotation periods), and significant variability on long time scales (i.e.,
hundred to thousand rotation cycles), supporting the presence of "long-term
weather" in ultracool dwarf atmospheres.Comment: 12 pages, 7 figures, recommended for publication in Astronomy and
Astrophysic
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&
Updating and extending the “dimming” and “brightening” over Spain
PĂłster elaborado para la European Geosciences Union General Assembly, celebrada en Viena del 3 al 8 de abril de 2011Ministerio de Ciencia e InnovaciĂłn. Proyecto NUCLIERSOL (CGL2010-
18546
Commissioning and First Observations with Wide FastCam at the Telescopio Carlos S\'anchez
The FastCam instrument platform, jointly developed by the IAC and the UPCT,
allows, in real-time, acquisition, selection and storage of images with a
resolution that reaches the diffraction limit of medium-sized telescopes.
FastCam incorporates a specially designed software package to analyse series of
tens of thousands of images in parallel with the data acquisition at the
telescope. Wide FastCam is a new instrument that, using the same software for
data acquisition, does not look for lucky imaging but fast observations in a
much larger field of view. Here we describe the commissioning process and first
observations with Wide FastCam at the Telescopio Carlos S\'anchez (TCS) in the
Observatorio del Teide.Comment: 7 pages, 8 figures, Proc. SPIE. 9908, Ground-based and Airborne
Instrumentation for Astronomy VI, 99082O. (August 09, 2016
- …