Fitting the Light Curve of 1I/`Oumuamua with a Nonprincipal Axis Rotational Model and Outgassing Torques

In this paper, we investigate the nonprincipal axis (NPA) rotational state of 1I/`Oumuamua -- the first interstellar object discovered traversing the inner Solar System -- from its photometric light curve. Building upon Mashchenko (2019), we develop a model which incorporates NPA rotation and {Sun-induced, time-varying} outgassing torques to generate synthetic light curves of the object. The model neglects tidal forces, which are negligible compared to outgassing torques over the distances that `Oumuamua was observed. We implement an optimization scheme that incorporates the NPA rotation model to calculate the initial rotation state of the object. We find that an NPA rotation state with an average period of $\langle P \rangle\simeq7.34$ hr best reproduces the photometric data. The discrepancy between this period and previous estimates is due to continuous period modulation induced by outgassing torques in the rotational model, {as well as different periods being used}. The best fit to the October 2017 data does not reproduce the November 2017 data (although the later measurements are too sparse to fit). The light curve is consistent with no secular evolution of the angular momentum, somewhat in tension with the empirical correlations between nuclear spin-up and cometary outgassing. The complex rotation of `Oumuamua may be {the result of primordial rotation about the smallest principal axis} if (i) the object experienced hypervolatile outgassing and (ii) our idealized outgassing model is accurate.Comment: 22 pages, 8 figures, 1 animation. Accepted to the Planetary Science Journal. The animation can be found on YouTube (https://youtu.be/f5YEAMTvIeo) and in the online publication by PSJ (when available

Assessing Potential Contributions from Outgassing and Tidal Effects on the Evolving Rotational State of 1I/'Oumuamua

In this paper, we attempt to interpret the photometric light curve of 1I/`Oumuamua, the first interstellar object discovered traversing the inner Solar System. We compare photometric data with synthetic light curves of ellipsoidal bodies for a range of rotational states and observing geometries. While previous work reported an increase in the periodicity of the object during October, we find a $\Delta p\simeq0.21$ hour decrease in the spin period between October and November. We investigate potential contributions to the evolving spin period from both outgassing and tidal effects using a general formalism which may be applied to any elongated object. While sublimation is a stronger effect, tidal deformation could change the moment of inertia and subsequent spin period based on the bulk material properties. We present an open source software which simulates constant-density, constant-viscosity liquid bodies subject to tidal forces for a range of assumed viscosites and sizes ($\texttt{SAMUS}$). These numerical simulations, when applied to `Oumuamua, demonstrate that it may have experienced significant tidal deformation in the presence of sublimation. However, synthetic observations which incorporate tidal effects demonstrate that little deformation is necessary to match the composite light curve. We find that a dynamic viscosity of $\mu\geq10^9$ g cm$^{-1}$ s$^{-1}$, corresponding to a 0.1\% change in moment of inertia, best reproduces the photometric data. It is feasible that tidal deformation contributed to the shorter timescale spin-down in October, while outgassing induced the secular spin-up.Comment: 30 pages, 24 figures, 5 tables. Submitted to AAS Planetary Science Journal. Comments very welcome. Publicly available software at https://github.com/astertaylor/Oumuamu

Interstellar Comets from Post-Main Sequence Systems as Tracers of Extrasolar Oort Clouds

Interstellar small bodies are unique probes into the histories of exoplanetary systems. One hypothesized class of interlopers are "Jurads," exo-comets released into the Milky Way during the post-main sequence as the thermally-pulsing asymptotic giant branch (AGB) host stars lose mass. In this study, we assess the prospects for the Legacy Survey of Space and Time (LSST) to detect a Jurad and examine whether such an interloper would be observationally distinguishable from exo-comets ejected during the (pre-)main sequence. Using analytic and numerical methods, we estimate the fraction of exo-Oort Cloud objects that are released from 1-8 solar mass stars during post-main sequence evolution. We quantify the extent to which small bodies are altered by the increased luminosity and stellar outflows during the AGB, finding that some Jurads may lack hypervolatiles and that stellar winds could deposit dust that covers the entire exo-comet surface. Next, we construct models of the interstellar small body reservoir for various size-frequency distribution slopes, characteristic sizes, and the total mass sequestered in the minor planets of exo-Oort Clouds. Even with the LSST's increased search volume compared to contemporary surveys, we find that detecting a Jurad is unlikely but not infeasible given the current understanding of (exo)planet formation.Comment: 28 pages, 13 figures; accepted to PS

Potential Melting of Extrasolar Planets by Tidal Dissipation

Tidal heating on Io due to its finite eccentricity was predicted to drive surface volcanic activity, which was subsequently confirmed by the $\textit{Voyager}$ spacecrafts. Although the volcanic activity in Io is more complex, in theory volcanism can be driven by runaway melting in which the tidal heating increases as the mantle thickness decreases. We show that this runaway melting mechanism is generic for a composite planetary body with liquid core and solid mantle, provided that (i) the mantle rigidity, $\mu$, is comparable to the central pressure, i.e. $\mu/ (\rho g R_{\rm P})\gtrsim0.1$ for a body with density $\rho$, surface gravitational acceleration $g$, and radius $R_{\rm P}$, (ii) the surface is not molten, (iii) tides deposit sufficient energy, and (iv) the planet has nonzero eccentricity. We calculate the approximate liquid core radius as a function of $\mu/ (\rho g R_{\rm P})$, and find that more than $90\%$ of the core will melt due to this runaway for $\mu/ (\rho g R_{\rm P})\gtrsim1$. From all currently confirmed exoplanets, we find that the terrestrial planets in the L98-59 system are the most promising candidates for sustaining active volcanism. However, uncertainties regarding the quality factors and the details of tidal heating and cooling mechanisms prohibit definitive claims of volcanism on any of these planets. We generate synthetic transmission spectra of these planets assuming Venus-like atmospheric compositions with an additional 5, 50, and $98\%$ SO$_2$ component, which is a tracer of volcanic activity. We find a $\gtrsim 3 \sigma$ preference for a model with SO$_2$ with 5-10 transits with $\textit{JWST}$ for L98-59bcd.Comment: 16 pages, 8 Figures, accepted for publication in Ap

Dark Comets? Unexpectedly Large Nongravitational Accelerations on a Sample of Small Asteroids

We report statistically significant detections of nonradial, nongravitational accelerations based on astrometric data in the photometrically inactive objects 1998 KY _26 , 2005 VL _1 , 2016 NJ _33 , 2010 VL _65 , 2016 RH _120 , and 2010 RF _12 . The magnitudes of the nongravitational accelerations are greater than those typically induced by the Yarkovsky effect, and there is no radiation-based, nonradial effect that can be so large. Therefore, we hypothesize that the accelerations are driven by outgassing and calculate implied H _2 O production rates for each object. We attempt to reconcile outgassing-induced acceleration with the lack of visible comae or photometric activity via the absence of surface dust and low levels of gas production. Although these objects are small, and some are rapidly rotating, the surface cohesive forces are stronger than the rotational forces, and rapid rotation alone cannot explain the lack of surface debris. It is possible that surface dust was removed previously, perhaps via outgassing activity that increased the rotation rates to their present-day value. We calculate dust production rates of order ∼10 ^−4 g s ^−1 in each object, assuming that the nuclei are bare, within the upper limits of dust production from a sample stacked image of 1998 KY _26 of ${\dot{M}}_{\mathrm{Dust}}\lt 0.2$ g s ^−1 . This production corresponds to brightness variations of order ∼0.0025%, which are undetectable in extant photometric data. We assess the future observability of each of these targets and find that the orbit of 1998 KY _26 —which is also the target of the extended Hayabusa2 mission—exhibits favorable viewing geometry before 2025