Spatially resolved spectroscopy across stellar surfaces

Abstract

Context. Spectroscopy across spatially resolved stellar surfaces reveals spectral line profiles free from rotational broadening, whose gradual changes from disk center toward the stellar limb reflect an atmospheric fine structure that is possible to model by 3D hydrodynamics. Aims. Previous studies of photospheric spectral lines across stellar disks exist for the Sun and HD 209458 (G0 V) and are now extended to the planet-hosting HD 189733A to sample a cooler K-type star and explore the future potential of the method. Methods. During exoplanet transit, stellar surface portions successively become hidden and differential spectroscopy between various transit phases uncovers spectra of small surface segments temporarily hidden behind the planet. The method was elaborated in Paper I, in which observable signatures were predicted quantitatively from hydrodynamic simulations. Results. From observations of HD 189733A with the ESO HARPS spectrometer at λ/Δλ~ 115 000, profiles for stronger and weaker Fe I lines are retrieved at several center-to-limb positions, reaching adequate S/N after averaging over numerous similar lines. Conclusions. Retrieved line profile widths and depths are compared to synthetic ones from models with parameters bracketing those of the target star and are found to be consistent with 3D simulations. Center-to-limb changes strongly depend on the surface granulation structure and much greater line-width variation is predicted in hotter F-type stars with vigorous granulation than in cooler K-types. Such parameters, obtained from fits to full line profiles, are realistic to retrieve for brighter planet-hosting stars, while their hydrodynamic modeling offers previously unexplored diagnostics for stellar atmospheric fine structure and 3D line formation. Precise modeling may be required in searches for Earth-analog exoplanets around K-type stars, whose more tranquil surface granulation and lower ensuing microvariability may enable such detections.

Probing the atmosphere of HD189733b with the Na i and K i lines

“…Czesla et al 2015;Yan et al 2017), the RM effect (see e.g. Cameron et al 2010;Cegla et al 2016;Dravins, Gustavsson & Ludwig 2018;Bourrier et al 2018), or stellar activity (see e.g. Cauley et al 2018), whereas the later is of main importance especially for this work and demonstrated in Fig.…”

“…where g 2 is the statistical weight of the upper level, A 21 the Einstein A-coefficient, ν 0 the wavenumber of the transition, E 1 the energy of the lower level, Q the partition function, and T the temperature. The values for g 2 and A 21 are taken from Draine (2011), while E 1 is zero because the resonance lines are ground-level transitions. The line profile is modelled by a Voigt profile, consisting of a Doppler core and Lorentzian line wings, where the standard deviation of the Doppler profile σ D is given by ) with the molecular mass of the species m. The Na I and K I resonance lines are well known for possessing a far-wing line profile that is non-Lorentzian (e.g.…”

Spatially resolved spectroscopy across stellar surfaces

“…For the cooler K1 V star HD 189733A (Alopex), such an effect is neither expected nor observed (Paper III; Dravins et al 2018). Further modeling constraints may come from the strongest (partly chromospheric) lines, such as Ca II H and K, or Na I D 1 and D 2 , where limb brightening in their wings relative to the adjacent continuum has been seen (Czesla et al 2015).…”

Planet and star synergy at high-spectral resolution. A rationale for the characterization of exoplanet atmospheres

“…Cegla et al (2018) inspected the impact of the granulation on the resultant line profiles and found induced center-to-limb variations in shape and net position. Dravins et al (2017Dravins et al ( , 2018 scrutinised the possibility to study stellar microvariability highlighting small surface segments, hidden behind the planet transits, with high spectral resolution observations.…”