Migrating Planets into Ultra-short-period Orbits during Episodic Accretion Events

Abstract

Ultra-short-period (USP) planets reside inside the expected truncation radius for typical T Tauri disks. As a result, their current orbital locations require an explanation beyond standard disk migration or in situ formation. Modern theories of planet-disk interactions indicate that once a planet migrates close to the disk's inner truncation radius, Type I torques vanish or switch direction, depending on the stellar and disk conditions, so that the planet is expected to stop its orbital decay and become trapped. In this work, we show that that magnetically driven sub-Keplerian gas flow in the inner disk can naturally counteract these effects and produce systems with USP planets at their observed orbital radii. The sub-Keplerian gas flow provides a headwind to small planets, and the resulting torque can overcome the effects of outward Type I migration near the corotation radius. For suitable disk and planet parameters, the torques due to the sub-Keplerian gas flow lead to inward migration on a rapid timescale. Over the time span of an FU Ori outburst, which moves the disk truncation radius inward, the rapid headwind migration can place planets in USP orbits. The combination of headwind migration and FU Ori outbursts thus provides a plausible mechanism to move small planets from a = 0.05-0.1 au down to a = 0.01-0.02 au. This effect is amplified for low-mass planets, consistent with existing observations.

Mutual Inclination of Ultra-Short-Period Planets with Time Varying Stellar J2-moment

“…showed that the initial obliquity needs to be about ∼ 20 − 40 • to form the USP, which is similar to the required tilts in the top row of Figure 9, corresponding to the inner-aligned configuration. Moreover, if the USP formed during episodic accretion events (Becker et al 2021), the planet tends to arrive early because the USP can migrate in a short timescale with this mechanism. This indicates a large initial inclination of the outer planet (e.g., ∼ 34 • if arrived around 10 Myr).…”

“…Moreover, dynamical migrations could lead to the configuration of an aligned outer planet with a misaligned inner planet (Petrovich et al 2019;Pu & Lai 2019). Finally, the migration during episodic accretion (Becker et al 2021) can also contribute to the formation of USPs and the migration timescale is much shorter than the other mechanisms mentioned above.…”

“…Planet obliquity tides can also produce USP planets with low initial semi-major axes (a 0.05 au) . Moreover, during episodic accretion events, the planet migrates into a USP orbit in a very short timescale by headwind torques (Becker et al 2021).…”

Mutual Inclination of Ultra-short-period Planets with Time-varying Stellar J 2 Moments

“…Planet obliquity tides can also produce USP planets with low initial semimajor axes (a  0.05 au ;. Moreover, during episodic accretion events, the planet migrates into a USP orbit in a very short timescale by headwind torques (Becker et al 2021).…”

A General Origin for Multiplanetary Systems With Significantly Misaligned USP Planets

“…USP planets (defined as those with orbital periods of less than a day) occur around ∼0.5% of G-dwarf stars (Sanchis-Ojeda et al 2014), and when they occur in STIPs, they often exist in apparently decoupled dynamical modes as compared to the rest of the system, sometimes in a coplanar configuration (ex: Swift et al 2013;Becker et al 2015;MacDonald et al 2016) and sometimes in a misaligned configuration (e.g., Rodriguez et al 2018;Quinn et al 2019). Since the (sometimes near-resonant) geometry of STIPs indicates that they formed via disk migration (Rein 2012;Batygin 2015;Deck & Batygin 2015), the presence of an interior planet in a different dynamical mode requires an explanation (Petrovich et al 2019;Pu & Lai 2019;Becker et al 2021). Dai et al (2018) noted that USP planets' orbital inclinations have a wider range of values with respect to nearby, exterior planets than do similar but longer-period planets.…”