Spatially resolved spectroscopy across stellar surfaces

II. High-resolution spectra across HD 209458 (G0 V)

¹ Lund Observatory, Box 43, 22100 Lund, Sweden
e-mail: dainis@astro.lu.se
² Zentrum für Astronomie der Universität Heidelberg, Landessternwarte Königstuhl, 69117 Heidelberg, Germany
e-mail: hludwig@lsw.uni-heidelberg.de
³ Present address: Department of Astronomy, AlbaNova University Center, 10691 Stockholm, Sweden

Received: 30 March 2017
Accepted: 17 July 2017

Abstract

Context. High-resolution spectroscopy across spatially resolved stellar surfaces aims at obtaining spectral-line profiles that are free from rotational broadening; the gradual changes of these profiles from disk center toward the stellar limb reveal properties of atmospheric fine structure, which are possible to model with 3D hydrodynamics.

Aims. Previous such studies have only been carried out for the Sun but are now extended to other stars. In this work, profiles of photospheric spectral lines are retrieved across the disk of the planet-hosting star HD 209458 (G0 V).

Methods. During exoplanet transit, stellar surface portions successively become hidden and differential spectroscopy provides spectra of small surface segments temporarily hidden behind the planet. The method was elaborated in Paper I, with observable signatures quantitatively predicted from hydrodynamic simulations.

Results. From observations of HD 209458 with spectral resolution λ/ Δλ ~ 80 000, photospheric Fe I line profiles are obtained at several center-to-limb positions, reaching adequately high S/N after averaging over numerous similar lines.

Conclusions. Retrieved line profiles are compared to synthetic line profiles. Hydrodynamic 3D models predict, and current observations confirm, that photospheric absorption lines become broader and shallower toward the stellar limb, reflecting that horizontal velocities in stellar granulation are greater than vertical velocities. Additional types of 3D signatures will become observable with the highest resolution spectrometers at large telescopes.