KELT-16b: A highly irradiated, ultra-short period hot Jupiter nearing tidal disruption

We announce the discovery of KELT-16b, a highly irradiated, ultra-short period hot Jupiter transiting the relatively bright (\(V = 11.7\)) star TYC 2688-1839-1. A global analysis of the system shows KELT-16 to be an F7V star with \(T_\textrm{eff} = 6236\pm54\) K, \(\log{g_\star} = 4.253_{-0.036}^{+0.031}\), [Fe/H] = -0.002\(_{-0.085}^{+0.086}\), \(M_\star = 1.211_{-0.046}^{+0.043} M_\odot\), and \(R_\star = 1.360_{-0.053}^{+0.064} R_\odot\). The planet is a relatively high mass inflated gas giant with \(M_\textrm{P} = 2.75_{-0.15}^{+0.16} M_\textrm{J}\), \(R_\textrm{P} = 1.415_{-0.067}^{+0.084} R_\textrm{J}\), density \(\rho_\textrm{P} = 1.20\pm0.18\) g cm\(^{-3}\), surface gravity \(\log{g_\textrm{P}} = 3.530_{-0.049}^{+0.042}\), and \(T_\textrm{eq} = 2453_{-47}^{+55}\) K. The best-fitting linear ephemeris is \(T_\textrm{C} = 2457247.24791\pm0.00019\) BJD\(_{tdb}\) and \(P = 0.9689951 \pm 0.0000024\) d. KELT-16b joins WASP-18b, -19b, -43b, -103b, and HATS-18b as the only giant transiting planets with \(P < 1\) day. Its ultra-short period and high irradiation make it a benchmark target for atmospheric studies by HST, Spitzer, and eventually JWST. For example, as a hotter, higher mass analog of WASP-43b, KELT-16b may feature an atmospheric temperature-pressure inversion and day-to-night temperature swing extreme enough for TiO to rain out at the terminator. KELT-16b could also join WASP-43b in extending tests of the observed mass-metallicity relation of the Solar System gas giants to higher masses. KELT-16b currently orbits at a mere \(\sim\) 1.7 Roche radii from its host star, and could be tidally disrupted in as little as a few \(\times 10^{5}\) years (for a stellar tidal quality factor of \(Q_*' = 10^5\)). Finally, the likely existence of a widely separated bound stellar companion in the KELT-16 system makes it possible that Kozai-Lidov oscillations played a role in driving KELT-16b inward to its current precarious orbit.