- Title: Optical and X-ray Transients from Planet-Star Mergers
- Authors: B.D. Metzger, D. Giannios, D.S. Spiegel
- First Author’s Institution: Department of Astrophysical Sciences, Princeton University
One of the biggest headlines in the era of exoplanets has been the discovery of a population of Jupiter-sized, gaseous planets orbiting their host star at a fraction of an AU – the hot Jupiters. The current idea is that these objects form in dense part of the host star’s protoplanetary disk several AU out and then migrate to a close-in, stable orbit. There’s a catch though: this close-in, stable orbit quickly becomes unstable when tides are considered. If the exoplanet orbits asynchronously — with an orbital period different than the host star’s rotation period — the exoplanet will raise synchronization tides in the atmosphere its host star. Tide-raising is an expensive business and we’re on a tight budget, so the radius of our exoplanet’s orbit must shrink. This tidal dissipation process will drive the planet closer and closer to its host star until the planet is destroyed by a violent interaction with the host star. What the authors of this study want to know is: what would this event look like?
While this question is certainly an interesting one, it only merits detailed theoretical investigation if we can hope to observe such an event. For this reason, the authors use the sample of exoplanets found by Kepler to predict the rate of galactic planet-star mergers. Previous theoretical studies of synchronous tides tell us what timescale the orbital migration will happen over. It is a simple function of the planet-star mass ratio, stellar radius, orbital separation, and a parameter encoding the efficiency of tidal dissipation. The authors calculate this timescale for every planet found by the Kepler survey. Comparing the observed distribution of merger timescales to models of this distribution (and considering that we have only observed 0.001% of Milky Way stars with transit surveys!), the authors find that their best-fit model predicts 1-10 galactic planet-star merger events per year. Fortunately, we live not only in an era of exoplanets but also a time of transients: several teams are (and will be) carefully monitoring the transient sky. Provided that a planet-star merger has a bright optical component, we should be able to see it with current or planned transient surveys. The next question to answer is then, what does the electromagnetic radiation from a planet-star merger look like, how long is its duration, and is it bright enough to see from across the galaxy?
In fact, there is more than one answer, depending on when in its inward migration the planet fills its Roche lobe – the approximately spherical surface beyond which the planet’s atmosphere is gravitationally bound to the star rather than the planet. As the planet’s orbit shrinks due to tidal dissipation, its Roche lobe shrinks. In the case of a fluffy planet, with density comparable to that of its host star, the outer layers of the planetary atmosphere are loosely bound to their host body. As such, the planet can stably donate its mass to the host star.
The fate of a slightly denser planet is dramatically different. The planet will not fill its Roche-lobe until it is just barely above the stellar surface. So when it does, the accreted gas accretes unstably, ripping apart the planet over a few short hours in a tidal disruption event. Initially, the accretion rate exceeds the Eddington accretion rate, and therefore must drive a bright, optically visible wind. The authors also find that this wind will be composed of planetary ashes: the wind metallicity will be five times greater than the host-star metallicity! After several months, the accretion rate decreases such that it no longer drives winds, and the accretion disk itself is revealed optically.
Planets more than five times as dense as their host star comprise the third regime. Planets in this group remain gravitationally bound until after they have entered the stellar atmosphere: this is called a direct-impact merger. As the planet enters the stellar atmosphere, it will shock-heat the stellar atmosphere in its path to millions of degrees. The heated gas will producing abundant UV and X-ray photons, and the intensity of the X-ray emission will oscillate with each orbit the engulfed planet makes within the star. The planet will continue to orbit for several weeks in this way until the it finally merges with the star, releasing the remaining orbital energy into the stellar atmosphere. This energy powers the bright optical component of the transient. Figure 1 shows (in color) which of the three merger types each of the Kepler-detected exoplanetary systems is expected to undergo, as a function of the host-star and and planet densities.
Ultimately, the authors conclude that although the frequency, brightness, and duration of these transients lends them to observability, obscuration by Galactic dust may foil efforts to observe a planet-star merger within our own galaxy. Instead, they suggest that an optical, UV, or X-ray survey of massive nearby galaxies such as M31 would be fruitful hunting grounds. The authors end their study with the exciting idea that given our current and near-future survey capabilities, “it seems promising that planet-star mergers should be detected within the next decade.”