Spatially resolved spectroscopy across stellar surfaces

Abstract

Context. High-precision stellar analyses require hydrodynamic modeling to interpret chemical abundances or oscillation modes. Exoplanet atmosphere studies require stellar background spectra to be known along the transit path while detection of Earth analogs require stellar microvariability to be understood. Hydrodynamic 3D models can be computed for widely different stars but have been tested in detail only for the Sun with its resolved surface features. Model predictions include spectral line shapes, asymmetries, and wavelength shifts, and their center-to-limb changes across stellar disks. Aims. We observe high-resolution spectral line profiles across spatially highly resolved stellar surfaces, which are free from the effects of spatial smearing and rotational broadening present in full-disk spectra, enabling comparisons to synthetic profiles from 3D models. Methods. During exoplanet transits, successive stellar surface portions become hidden and differential spectroscopy between various transit phases provides spectra of small surface segments temporarily hidden behind the planet. Planets cover no more than ∼1% of any main-sequence star, enabling high spatial resolution but demanding very precise observations. Realistically measurable quantities are identified through simulated observations of synthetic spectral lines. Results. In normal stars, line profile ratios between various transit phases may vary by ∼0.5%, requiring S /N 5000 for meaningful spectral reconstruction.While not yet realistic for individual spectral lines, this is achievable for cool stars by averaging over numerous lines with similar parameters. Conclusions. For bright host stars of large transiting planets, spatially resolved spectroscopy is currently practical. More observable targets are likely to be found in the near future by ongoing photometric searches.

Mass-loss rate and local thermodynamic state of the KELT-9 b thermosphere from the hydrogen Balmer series

“…Hence, it is necessary to separate and to measure accurately each component to retrieve the unknown parameters. The present figure is partially inspired by figures in Dravins et al (2015Dravins et al ( , 2017; Dumusque et al (2014); Cegla et al (2016); Bourrier et al (2017a). Bourrier et al 2017aBourrier et al , 2018:…”

Spatially resolved spectroscopy across stellar surfaces

“…However, direct comparisons between theory and spectral-line observations have in the past only been possible for the spatially resolved Sun (e.g., Lind et al 2017). In Paper I (Dravins et al 2017), we examined theoretically predicted spatially resolved signatures for a group of main-sequence stellar models with temperatures between 6730 and 3960 K. Corresponding observations are feasible during exoplanet transits when small stellar surface portions successively become hidden, and differential spectroscopy between different transit phases can provide spectra of small surface segments temporarily hidden behind the planet. The observational requirements were elaborated in Paper I, in which observable parameters were predicted quantitatively.…”

The Transit Light Source Effect. II. The Impact of Stellar Heterogeneity on Transmission Spectra of Planets Orbiting Broadly Sun-like Stars

“…http://keplergo.arc.nasa.gov/Instrumentation.shtml 3 Observations of transiting exoplanets, though, offer a promising probe of stellar photospheres that can shed light on this problem (e.g.,Dravins et al 2017aDravins et al ,b, 2018Rackham et al 2017;Espinoza et al 2019). …”