The Habitable Zones of Rapidly Rotating Main Sequence A/F Stars

TL;DR

This study explores how rapid rotation in A/F stars affects their luminosity, spectral energy distribution, and habitable zones, revealing that rotation can make habitable zones closer and reduce UV radiation, impacting exoplanet habitability assessments.

Contribution

It models the effects of rapid stellar rotation on habitable zones and demonstrates its significant influence on planetary habitability around high-mass stars.

Findings

Rapid rotation shifts habitable zones closer to the star.

Gravity darkening reduces stellar UV emission.

Rotation influences star shape and surface temperature gradients.

Abstract

We investigate how rapid stellar rotation commonly seen in A/F stars can influence planet habitability. Specifically, we model how rapid rotation influences a planet's irradiation and determine the location of the habitable zone for stars in the mass range $1.3M_\odot\leq M_\star\leq 2.2M_\odot$. Rapid stellar rotation can dramatically change a star's luminosity and spectral energy distribution and, therefore, can impact the habitability of any surrounding planets. Stars of mass $M_\star\gtrsim1.3M_\odot$ commonly rotate near their breakup speeds, which causes two effects relevant to planet habitability. First, these stars flatten into oblate spheroids with shorter polar radii and elongated equatorial radii. Second, rapid rotation induces a pole-to-equator temperature gradient on the surface of these stars. Using a 1D climate model, we calculate the inner and outer edges of the habitable zone of well-known rapid rotators and average theoretical stars in our stellar mass range. We find that, in general, rapid rotation causes the habitable zone to reside closer in than for a non-rotating equivalent star. We also find that gravity darkening dramatically reduces stellar UV emission, which combats the common assumption that high-mass stars emit too much UV light for habitable worlds. Overall, we determine that rapid stellar rotation has important consequences for the overall habitability of a system and must be accounted for both when modeling exoplanet environments and in observation of planets around high-mass stars.