10 research outputs found
Elliptic and hyperelliptic magnetohydrodynamic equilibria
The present study is a continuation of a previous one on "hyperelliptic"
axisymmetric equilibria started in [Tasso and Throumoulopoulos, Phys. Plasmas
5, 2378 (1998)].
Specifically, some equilibria with incompressible flow nonaligned with the
magnetic field and restricted by appropriate side conditions like "isothermal"
magnetic surfaces, "isodynamicity" or P + B^2/2 constant on magnetic surfaces
are found to be reducible to elliptic integrals. The third class recovers
recent equilibria found in [Schief, Phys. Plasmas 10, 2677 (2003)]. In contrast
to field aligned flows, all solutions found here have nonzero toroidal magnetic
field on and elliptic surfaces near the magnetic axis.Comment: 9 page
Nightside condensation of iron in an ultra-hot giant exoplanet
Ultra-hot giant exoplanets receive thousands of times Earth's insolation.
Their high-temperature atmospheres (>2,000 K) are ideal laboratories for
studying extreme planetary climates and chemistry. Daysides are predicted to be
cloud-free, dominated by atomic species and substantially hotter than
nightsides. Atoms are expected to recombine into molecules over the nightside,
resulting in different day-night chemistry. While metallic elements and a large
temperature contrast have been observed, 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. 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. 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.Comment: Published in Nature (Accepted on 24 January 2020.) 33 pages, 11
figures, 3 table
Future wavelength calibration standards at ESO : the Laser Frequency Comb
A new technique for precise wavelength calibration of high-resolution spectrographs using frequency combs has recently been proposed. After introducing the basic concepts and advantages of this technique, we describe the ongoing development between ESO and the Max-Planck Institute for Quantum Optics for a novel wavelength calibration system that aims, within three years, to construct a laboratory demonstrator
EMIR: the GTC NIR multi-object imager spectrograph
We present the final global design and performances of EMIR, the NIR multi-object spectrograph of the GTC, as well as the plan for its early scientific exploitation
Codex
International audienceMany talks in this conference have shown the power of high resolution spectroscopy, coupled to high precision. A new spectrograph, fed by an Extremely Large Telescope, will have such a huge collecting power that it will allow for the first time some outstanding applications. We argue here that such a new instrument should therefore aim at the highest long term stability and precision. High resolution and high accuracy need photons; the ELTs will give us a tremendous opportunity, and we shall make use of it
ESPRESSO: the Echelle spectrograph for rocky exoplanets and stable spectroscopic observations
International audienceESPRESSO, the Echelle SPectrograph for Rocky Exoplanets and Stable Spectroscopic Observations, will combine the efficiency of modern echelle spectrograph design with extreme radial-velocity precision. It will be installed on ESO's VLT in order to achieve a gain of two magnitudes with respect to its predecessor HARPS, and the instrumental radialvelocity precision will be improved to reach cm/s level. Thanks to its characteristics and the ability of combining incoherently the light of 4 large telescopes, ESPRESSO will offer new possibilities in various fields of astronomy. The main scientific objectives will be the search and characterization of rocky exoplanets in the habitable zone of quiet, nearby G to M-dwarfs, and the analysis of the variability of fundamental physical constants. We will present the ambitious scientific objectives, the capabilities of ESPRESSO, and the technical solutions of this challenging project
Fundamental physics with ESPRESSO: Precise limit on variations in the fine-structure constant towards the bright quasar HE 0515â4414
The strong intervening absorption system at redshift 1.15 towards the very bright quasar HE 0515â4414 is the most studied absorber for measuring possible cosmological variations in the fine-structure constant, α. We observed HE 0515â4414 for 16.1 h with the Very Large Telescope and present here the first constraint on relative variations in α with parts-per-million (ppm) precision from the new ESPRESSO spectrograph: Îα/αâ=â1.3â
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1.3statâ
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0.4sysâppm. The statistical uncertainty (1Ï) is similar to the ensemble precision of previous large samples of absorbers and derives from the high signal-to-noise ratio achieved (â105 per 0.4 km sâ1 pixel). ESPRESSOâs design, and the calibration of our observations with its laser frequency comb, effectively removed wavelength calibration errors from our measurement. The high resolving power of our ESPRESSO spectrum (Râ=â145â000) enabled the identification of very narrow components within the absorption profile, allowing a more robust analysis of Îα/α. The evidence for the narrow components is corroborated by their correspondence with previously detected molecular hydrogen and neutral carbon. The main remaining systematic errors arise from ambiguities in the absorption profile modelling, effects from redispersing the individual quasar exposures, and convergence of the parameter estimation algorithm. All analyses of the spectrum, including systematic error estimates, were initially blinded to avoid human biases. We make our reduced ESPRESSO spectrum of HE 0515â4414 publicly available for further analysis. Combining our ESPRESSO result with 28 measurements, from other spectrographs, in which wavelength calibration errors have been mitigated yields a weighted mean Îα/αâ=ââ0.5â
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0.5statâ
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0.4sys ppm at redshifts 0.6â2.4
Fundamental physics with E
Observations of metal absorption systems in the spectra of distant quasars allow one to constrain a possible variation of the fine-structure constant throughout the history of the Universe. Such a test poses utmost demands on the wavelength accuracy and previous studies were limited by systematics in the spectrograph wavelength calibration. A substantial advance in the field is therefore expected from the new ultra-stable high-resolution spectrograph ESPRESS