The Rate of Planet-star Coalescences Due to Tides and Stellar Evolution

TL;DR

This paper models the rate of star-planet coalescences caused by tidal interactions and stellar evolution, predicting their frequency and transient brightness, with implications for future observations.

Contribution

It provides a population synthesis analysis of star-planet mergers, estimating their rates and transient luminosities across different stellar types and planetary masses.

Findings

Most mergers involve red giants but are not detectable as transients.

Merger rate with main sequence stars is about once every 70 years in a Milky Way-like galaxy.

Bright transients from massive planets occur roughly once every 350 years.

Abstract

Orbits of close-in planets can shrink significantly due to dissipation of tidal energy in a host star. This process can result in star-planet coalescence within the Galactic lifetime. In some cases, such events can be accompanied by an optical or/and UV/X-ray transient. Potentially, these outbursts can be observed in near future with new facilities such as LSST from distances about few Mpc. We use a population synthesis model to study this process and derive the rate of star-planet mergers of different types. Mostly, planets are absorbed by red giants. However, these events, happening with the rate about 3 per year, mostly do not produce detectable transients. The rate of mergers with main sequence stars depends on the effectiveness of tidal dissipation; for reasonable values of stellar tidal quality factor, such events happen in a Milky Way-like galaxy approximately once in 70 yrs or more rarely. This rate is dominated by planets with low masses. Such events do not produce bright transients having maximum luminosities $\lesssim 10^{36.5}$erg s$^{-1}$. Brighter events, related to massive planets, with maximum luminosity $\sim 10^{37.5}$--$10^{38}$erg s$^{-1}$, have the rate nearly five times smaller.