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 $Q_{\mathrm{p}}=4.9_{-2.5}^{+3.5}\times 10^5$ 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.

arXiv comment18 pages, 5 Figures, Accepted for publication in AJ