arXiv subjects

A. Wyttenbach

Publications and source records attributed to A. Wyttenbach.

40 records · Page 3Linked to original sources

arXiv2017

A cautionary tale: limitations of a brightness-based spectroscopic approach to chromatic exoplanet radii

Determining wavelength-dependent exoplanet radii measurements is an excellent way to probe the composition of exoplanet atmospheres. In light of this, Borsa et al. (2016) sought to develop a technique to obtain such measurements by comparing ground-based transmission spectra to the expected brightness variations during an exoplanet transit. However, we demonstrate herein that this is not possible due to the transit light curve normalisation necessary to remove the effects of the Earth's atmosphere on the ground-based observations. This is because the recoverable exoplanet radius is set by the planet-to-star radius ratio within the transit light curve; we demonstrate this both analytically and with simulated planet transits, as well as through a reanalysis of the HD 189733b data.

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arXiv2016

Refined architecture of the WASP-8 system: a cautionary tale for traditional Rossiter-McLaughlin analysis

Probing the trajectory of a transiting planet across the disk of its star through the analysis of its Rossiter-McLaughlin effect can be used to measure the differential rotation of the host star and the true obliquity of the system. Highly misaligned systems could be particularly conducive to these mesurements, which is why we reanalysed the HARPS transit spectra of WASP-8b using the 'Rossiter-McLaughlin effect reloaded' (reloaded RM) technique. This approach allows us to isolate the local stellar CCF emitted by the planet-occulted regions. As a result we identified a $\sim$35% variation in the local CCF contrast along the transit chord, which might trace a deepening of the stellar lines from the equator to the poles. Whatever its origin, such an effect cannot be detected when analyzing the RV centroids of the disk-integrated CCFs through a traditional velocimetric analysis of the RM effect. Consequently it injected a significant bias into the results obtained by Queloz et al. (2010) for the projected rotational velocity $v_{eq} \sin i_{\star}$ (1.59$\stackrel{+0.08}{_{-0.09}}$ km/s) and the sky-projected obliquity $λ$ (-123.0$\stackrel{+3.4}{_{-4.4}}^{\circ}$). Using our technique, we measured these values to be $v_{eq} \sin i_{\star}$ = 1.90$\pm$0.05 km/s and $λ$ = -143.0$\stackrel{+1.6}{_{-1.5}}^{\circ}$. We found no compelling evidence for differential rotation of the star, although there are hints that WASP-8 is pointing away from us with the stellar poles rotating about 25% slower than the equator. Measurements at higher accuracy during ingress/egress will be required to confirm this result. In contrast to the traditional analysis of the RM effect, the reloaded RM technique directly extracts the local stellar CCFs, allowing us to analyze their shape and to measure their RV centroids, unbiased by variations in their contrast or FWHM.

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arXiv2015

The HARPS search for southern extra-solar planets. XXXVIII. Bayesian re-analysis of three systems. New super-Earths, unconfirmed signals, and magnetic cycles

We present the analysis of the entire HARPS observations of three stars that host planetary systems: HD1461, HD40307, and HD204313. The data set spans eight years and contains more than 200 nightly averaged velocity measurements for each star. This means that it is sensitive to both long-period and low-mass planets and also to the effects induced by stellar activity cycles. We modelled the data using Keplerian functions that correspond to planetary candidates and included the short- and long-term effects of magnetic activity. A Bayesian approach was taken both for the data modelling, which allowed us to include information from activity proxies such as $\log{(R'_{\rm HK})}$ in the velocity modelling, and for the model selection, which permitted determining the number of significant signals in the system. The Bayesian model comparison overcomes the limitations inherent to the traditional periodogram analysis. We report an additional super-Earth planet in the HD1461 system. Four out of the six planets previously reported for HD40307 are confirmed and characterised. We discuss the remaining two proposed signals. In particular, we show that when the systematic uncertainty associated with the techniques for estimating model probabilities are taken into account, the current data are not conclusive concerning the existence of the habitable-zone candidate HD40307 g. We also fully characterise the Neptune-mass planet that orbits HD204313 in 34.9 days.

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arXiv2015

Spectrally resolved detection of sodium in the atmosphere of HD189733b with the HARPS spectrograph

Atmospheric properties of exoplanets can be constrained with transit spectroscopy. The signature of atomic sodium NaI, known to be present above the clouds, is a powerful probe of the upper atmosphere, where it can be best detected and characterized at high spectral resolution. Our goal is to obtain a high-resolution transit spectrum of HD189733b in the region around the resonance doublet of NaI at 589 nm, to characterize the absorption signature previously detected from space at low resolution. We analyze archival transit data of HD189733b obtained with the HARPS spectrograph. We retrieve the transit spectrum and light curve of the planet, implementing corrections for telluric contamination and planetary orbital motion. We spectrally resolve the NaI D doublet and measure line contrasts of $0.64\pm0.07\%$ (D2) and $0.40\pm0.07\%$ (D1) and FWHMs of $0.52\pm0.08~Å$. This corresponds to a detection at the 10-$σ$ level of excess of absorption of $0.32\pm0.03\%$ in a passband of $2\times0.75\ Å$ centered on each line. We derive temperatures of $2\,600\pm600$ K and $3270\pm330$ K at altitudes of $9\,800\pm2\,800$ km and $12\,700\pm2\,600$ km in the NaI D1 and D2 line cores, respectively. We measure a temperature gradient of $\sim0.2$ K km$^{-1}$ from comparison with theoretical models. We also detect a blueshift of $0.16\pm0.04\ Å$ (4 $σ$) in the line positions. This blueshift may be due to winds blowing at $8\pm2$ km s$^{-1}$ in the upper layers of the atmosphere. We demonstrate the relevance of studying exoplanet atmospheres with high-resolution spectrographs mounted on 4-meter-class telescopes. Our results pave the way towards in-depth characterization of physical conditions in the atmospheres of many exoplanetary systems with future spectrographs such as ESPRESSO on the VLT or HiReS and METIS on the E-ELT.

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