Letters to the Editor

Exoplanet transmission spectroscopy: accounting for the eccentricity and the longitude of periastron

Superwinds in the upper atmosphere of HD 209458b?

¹ Centro de Astrofisica da Universidade do Porto, Rua das Estrelas, 4150-762 Porto, Portugal
e-mail: Marco.Montalto@astro.up.pt
² Departamento de Física e Astronomia, Faculdade de Ciências, Universidade do Porto, Portugal

Received: 10 January 2011
Accepted: 2 February 2011

Abstract

Context. A planet transiting in front of the disk of its parent star offers the opportunity to study the compositional properties of its atmosphere by means of the analysis of the stellar light that is filtered by the planetary atmospheric layers. Several studies have so far placed useful constraints on planetary atmospheric properties with this technique, and for the case of HD 209458b even the radial velocity of the planet during the transit event has been reconstructed. This opens up a new range of possibilities.

Aims. In this contribution we highlight the importance to account for the orbital eccentricity and the longitude of periastron of the planetary orbit to accurately interpret the measured planetary radial velocity during the transit.

Methods. We calculate the radial velocity of a transiting planet in an eccentric orbit.

Results. Given the higher orbital speed of planets with respect to their stellar companions, even small eccentricities can result in detectable blue or redshift radial velocity offsets during the transit with respect to the systemic velocity, the exact value of which also depends on the longitude of the periastron of the planetary orbit. For a hot-jupiter planet, an eccentricity of only e = 0.01 can produce a radial velocity offset on the order of the km s^-1.

Conclusions. We propose an alternative interpretation of the recently claimed radial velocity blueshift (~ 2 km s^-1) of the planetary spectral lines of HD 209458b, which implies that the orbit of this system is not exactly circular. In this case, the longitude of the periastron of the stellar orbit is most likely confined in the first quadrant (and that of the planet in the third quadrant). We highlight that transmission spectroscopy allows us not only to study the compositional properties of planetary atmospheres, but also to refine planetary orbital parameters, and that any conclusion regarding the presence of windflows on planetary surfaces coming from transmission spectroscopy measurements requires precisely known orbital parameters from RV.