Synthetic Detections of Interstellar Objects with The Rubin Observatory Legacy Survey of Space and Time

The discovery of two interstellar objects passing through the Solar System, 1I/`Oumuamua and 2I/Borisov, implies that a galactic population exists with a spatial number density of order $\sim0.1$ au$^{-3}$. The forthcoming Rubin Observatory Legacy Survey of Space and Time (LSST) has been predicted to detect more asteroidal interstellar objects like 1I/`Oumuamua. We apply recently developed methods to simulate a suite of galactic populations of interstellar objects with a range of assumed kinematics, albedos and size-frequency distributions (SFD). We incorporate these populations into the objectsInField (OIF) algorithm, which simulates detections of moving objects by an arbitrary survey. We find that the LSST should detect between $\sim 0-70$ asteroidal interstellar objects every year (assuming the implied number density), with sensitive dependence on the SFD slope and characteristic albedo of the host population. The apparent rate of motion on the sky -- along with the associated trailing loss -- appears to be the largest barrier to detecting interstellar objects. Specifically, a relatively large number of synthetic objects would be detectable by the LSST if not for their rapid sky-motion ($>0.5^\circ$ d$^{-1}$). Therefore, algorithms that could successfully link and detect rapidly moving objects would significantly increase the number of interstellar object discoveries with the LSST (and in general). The mean diameter of detectable, inactive interstellar objects ranges from $\sim50 - 600$ m and depends sensitively on the SFD slope and albedo.Comment: 13 pages, 11 figures, accepted for publication in the Planetary Science Journa

The Interstellar Interlopers

Interstellar interlopers are bodies formed outside of the solar system but observed passing through it. The first two identified interlopers, 1I/`Oumuamua and 2I/Borisov, exhibited unexpectedly different physical properties. 1I/`Oumuamua appeared unresolved and asteroid-like whereas 2I/Borisov was a more comet-like source of both gas and dust. Both objects moved under the action of non-gravitational acceleration. These interlopers and their divergent properties provide our only window so far onto an enormous and previously unknown galactic population. The number density of such objects is $\sim$ 0.1 AU$^{-3}$ which, if uniform across the galactic disk, would imply 10$^{25}$ to 10$^{26}$ similar objects in the Milky Way. The interlopers likely formed in, and were ejected from, the protoplanetary disks of young stars. However, we currently possess too little data to firmly reject other explanations.Comment: 40 pages, 19 figures, 7 tables, invited review in ARA&A Volume 61, submitted, comments welcom

On the pollution of white dwarfs by exo-Oort cloud comets

A large fraction of white dwarfs (WDs) have metal-polluted atmospheres, which are produced by accreting material from remnant planetary systems. The composition of the accreted debris broadly resembles that of rocky Solar System objects. Volatile-enriched debris with compositions similar to long-period comets (LPCs) is rarely observed. We attempt to reconcile this dearth of volatiles with the premise that exo-Oort clouds (XOCs) occur around a large fraction of planet-hosting stars. We estimate the comet accretion rate from an XOC analytically, adapting the 'loss cone' theory of LPC delivery in the Solar System. We investigate the dynamical evolution of an XOC during late stellar evolution. Using numerical simulations, we show that 1 to 30 per cent of XOC objects remain bound after anisotropic stellar mass loss imparting a WD natal kick of $\sim$1 km/s. We also characterize the surviving comets' distribution function. Surviving planets orbiting a WD can prevent the accretion of XOC comets by the star. A planet's 'dynamical barrier' is effective at preventing comet accretion if the energy kick imparted by the planet exceeds the comet's orbital binding energy. By modifying the loss cone theory, we calculate the amount by which a planet reduces the WD's accretion rate. We suggest that the scarcity of volatile-enriched debris in polluted WDs is caused by an unseen population of 10-100 AU scale giant planets acting as barriers to incoming LPCs. Finally, we constrain the amount of volatiles delivered to a planet in the habitable zone of an old, cool WD.Comment: 18 pages, 12 figures; submitted to MNRAS. Comments welcome

X-rays Trace the Volatile Content of Interstellar Objects

The non-detection of a coma surrounding 1I/`Oumuamua, the first discovered interstellar object (ISO), has prompted a variety of hypotheses to explain its nongravitational acceleration. Given that forthcoming surveys are poised to identify analogues of this enigmatic object, it is prudent to devise alternative approaches to characterization. In this study, we posit X-ray spectroscopy as a surprisingly effective probe of volatile ISO compositions. Heavily ionized metals in the solar wind interact with outgassed neutrals and emit high-energy photons in a process known as charge exchange, and charge exchange induced X-rays from comets and planetary bodies have been observed extensively in our Solar System. We develop a model to predict the X-ray flux of an ISO based on its chemical inventory and ephemeris. We find that while standard cometary constituents, such as H$_2$O, CO$_2$, CO, and dust are best probed via optical or infrared observations, we predict strong X-ray emission generated by charge exchange with extended comae of H$_2$ and N$_2$ -- species which lack strong infrared fluorescence transitions. We find that XMM-Newton would have been sensitive to charge exchange emission from 1I/`Oumuamua during the object's close approach to Earth, and that constraints on composition may have been feasible. We argue for follow-up X-ray observations of newly discovered ISOs with close-in perihelia. Compositional constraints on the general ISO population could reconcile the apparently self-conflicting nature of 1I/`Oumuamua, and provide insight into the earliest stages of planet formation in extrasolar systems.Comment: Accepted to The Astrophysical Journal. 18 pages, 6 figure

Avalanches and the Distribution of Reconnection Events in Magnetized Circumstellar Disks

Cosmic rays produced by young stellar objects can potentially alter the ionization structure, heating budget, chemical composition, and accretion activity in circumstellar disks. The inner edges of these disks are truncated by strong magnetic fields, which can reconnect and produce flaring activity that accelerates cosmic radiation. The resulting cosmic rays can provide a source of ionization and produce spallation reactions that alter the composition of planetesimals. This reconnection and particle acceleration are analogous to the physical processes that produce flaring in and heating of stellar coronae. Flaring events on the surface of the Sun exhibit a power-law distribution of energy, reminiscent of those measured for Earthquakes and avalanches. Numerical lattice-reconnection models are capable of reproducing the observed power-law behavior of solar flares under the paradigm of self-organized criticality. One interpretation of these experiments is that the solar corona maintains a nonlinear attractor -- or ``critical'' -- state by balancing energy input via braided magnetic fields and output via reconnection events. Motivated by these results, we generalize the lattice-reconnection formalism for applications in the truncation region of magnetized disks. Our numerical experiments demonstrate that these nonlinear dynamical systems are capable of both attaining and maintaining criticality in the presence of Keplerian shear and other complications. The resulting power-law spectrum of flare energies in the equilibrium attractor state is found to be nearly universal in magnetized disks. This finding indicates that magnetic reconnection and flaring in the inner regions of circumstellar disks occur in a manner similar to activity on stellar surfaces

The Influence of Tidal Heating on the Habitability of Planets Orbiting White Dwarfs

In recent years, there have been a growing number of observations indicating the presence of rocky material in short-period orbits around white dwarfs. In this Letter, we revisit the prospects for habitability around these post-main-sequence star systems. In addition to the typically considered radiative input luminosity, potentially habitable planets around white dwarfs are also subjected to significant tidal heating. The combination of these two heating sources can, for a narrow range of planetary properties and orbital parameters, continuously maintain surface temperatures amenable for habitability for planets around white dwarfs over time scales up to 10 Gyr. We show that for a specific locus of orbital parameter space, tidal heating can substantially extend the timescale of continuous habitability for a planet around a white dwarf.Comment: Accepted to ApJ

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