The Impact of the Convective Blueshift Effect on Spectroscopic Planetary Transits

TL;DR

This paper investigates how the convective blueshift effect influences spectroscopic measurements during planetary transits, highlighting its potential to bias spin-orbit angle estimates and its usefulness for stellar studies.

Contribution

It introduces the convective blueshift-induced RV signal as a distinct factor affecting transit observations, separate from the Rossiter-McLaughlin effect, and discusses its implications for future high-precision RV studies.

Findings

CB effect causes a small, distinct RV shift during transits.

Ignoring CB can bias spin-orbit angle measurements.

Future high-precision RVs can use transits to study stellar convection.

Abstract

We present here a small anomalous radial velocity (RV) signal expected to be present in RV curves measured during planetary transits. This signal is induced by the convective blueshift (CB) effect --- a net blueshift emanating from the stellar surface, resulting from a larger contribution of rising hot and bright gas relative to the colder and darker sinking gas. Since the CB radial component varies across the stellar surface, the light blocked by the planet during a transit will have a varying RV component, resulting in a small shift of the measured RVs. The CB-induced anomalous RV curve is different than, and independent of, the well known Rossiter-McLaughlin (RM) effect, where the latter is used for determining the sky-projected angle between the host star rotation axis and the planet's orbital angular momentum axis. The observed RV curve is the sum of the CB and RM signals, and they are both superposed on the orbital Keplerian curve. If not accounted for, the presence of the CB RV signal in the spectroscopic transit RV curve may bias the estimate of the spin-orbit angle. In addition, future very high precision RVs will allow the use of transiting planets to study the CB of their host stars.