A Source of Uncertainty in TESS Photometry of High-Velocity M dwarfs

The Doppler beaming effect for cool stars is amplified relative to a blackbody of the same temperature because of deep TiO and VO absorption features. Also, a strong Na feature at 5890A coincides with the short wavelength cutoff of the TESS response function. We simulated Doppler beaming for BT-Settl model spectra ranging from 2600K to 5200K and with radial velocities of ± 400 km/s. For a 3000 K object, the change in flux was up to two times greater than predicted for a blackbody in the TESS filter. We repeated the same simulation for the Kepler/K2 and Gaia filters and obtained a similar result. When using the photometric beaming amplitude to determine stellar properties, an underestimated Doppler beaming coefficient results in mass overestimates. This is especially important for M dwarfs in short period binary systems with massive white dwarf, neutron star or black hole companions. The effect could also impact the photometry of runaway/hypervelocity M dwarfs. We present TESS photometry of the pre-cataclysmic variable triple system Wolf 1130 and confirm that the measured beaming amplitude of Wolf 1130AB is nearly triple the theoretical expectation.