X-shooter is the first 2nd generation instrument of the ESO Very Large Telescope (VLT). It is a very efficient, single-target, intermediate-resolution spectrograph that was installed at the Cassegrain focus of UT2 in 2009. The instrument covers, in a single exposure, the spectral range from 300 to 2500 nm. It is designed to maximize the sensitivity in this spectral range through dichroic splitting in three arms with optimized optics, coatings, dispersive elements and detectors. It operates at intermediate spectral resolution (R ∼ 4000−17 000, depending on wavelength and slit width) with fixed échelle spectral format (prism cross-dispersers) in the three arms. It includes a 1.8 × 4 integral field unit as an alternative to the 11 long slits. A dedicated data reduction package delivers fully calibrated two-dimensional and extracted spectra over the full wavelength range. We describe the main characteristics of the instrument and present its performance as measured during commissioning, science verification and the first months of science operations.
Ultra-hot giant exoplanets receive thousands of times Earth’s
insolation
1
,
2
. Their high-temperature
atmospheres (>2,000 K) are ideal laboratories for studying extreme
planetary climates and chemistry
3
–
5
. Daysides
are predicted to be cloud-free, dominated by atomic species
6
and substantially hotter than
nightsides
5
,
7
,
8
. Atoms are expected to recombine into molecules over the
nightside
9
, resulting
in different day-night chemistry. While metallic elements and a large
temperature contrast have been observed
10
–
14
, no
chemical gradient has been measured across the surface of such an exoplanet.
Different atmospheric chemistry between the day-to-night
(“evening”) and night-to-day (“morning”) terminators
could, however, be revealed as an asymmetric absorption signature during
transit
4
,
7
,
15
. Here, we report the detection of an asymmetric
atmospheric signature in the ultra-hot exoplanet WASP-76b. We spectrally and
temporally resolve this signature thanks to the combination of high-dispersion
spectroscopy with a large photon-collecting area. The absorption signal,
attributed to neutral iron, is blueshifted by −11±0.7 km
s
-1
on the trailing limb, which can be explained by a combination
of planetary rotation and wind blowing from the hot dayside
16
. In contrast, no signal arises
from the nightside close to the morning terminator, showing that atomic iron is
not absorbing starlight there. Iron must thus condense during its journey across
the nightside.
We present a self-consistent empirical model for several plasma parameters of a polar coronal hole near solar minimum, derived from observations with the Solar and Heliospheric Observatory Ultraviolet Coronagraph Spectrometer. The model describes the radial distribution of density for electrons, H , and O and the outflow of O are also significantly larger than the corresponding velocities of H . We discuss the constraints and 5ϩ 0 implications on various theoretical models of coronal heating and acceleration.
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