Identifying close-in Jupiters that arrived via disk migration: Evidence of primordial alignment, preference of nearby companions and hint of runaway migration

Abstract

Two leading hypotheses for hot Jupiter migration are disk migration and high-eccentricity migration (HEM). Stellar obliquity is commonly used to distinguish them, as high obliquity often accompanies HEM. However, low obliquity does not guarantee disk migration, due to possible spin-orbit realignment or coplanar HEM. Seeking a proxy for disk migration, we investigate the idea that when the circularization timescale of a planet on circular orbit is longer than its age ($\tau_\mathrm{cir} > \tau_\mathrm{age}$), HEM would not have had sufficient time to complete, favoring disk migration. We empirically calibrate the reduced planetary tidal quality factor to be $Q_\mathrm{p}=4.9^{+3.5}_{-2.5}\times10^5$ using the eccentricity distribution of 500+ Jovian mass ($0.2M_\mathrm{J}<M_\mathrm{p}<13M_\mathrm{J}$) planets with measured masses and radii, a value consistent with solar system Jupiter. We then calculate $\tau_\mathrm{cir}$ and identify dozens of disk migration candidates ($\tau_\mathrm{cir} > \tau_\mathrm{age}, \ e < 0.1$). These planets show three notable trends. We first find a clear cutoff of obliquity at $\tau_\mathrm{cir} \sim \tau_\mathrm{age}$, suggesting the primordial alignment of protoplanetary disks. Secondly, we find that among hot Jupiters ($a<0.1$ au), nearby companions are preferentially found around disk migration candidates, suggesting that either HEM dominates hot Jupiter formation, or disk migration also disrupts nearby companions at short separations. Finally, we find a possible dearth of disk migration candidates around mass ratio $\log q \sim -3.2$, consistent with a similar dip suggested at longer orbits from microlensing. The lack of planets across different orbital distance, if true, could be interpreted as a hint of runaway migration.