We consider the situation where the luminosity from a transient event is reprocessed by an optically thick wind. Potential applications are the tidal disruption of stars by black holes, engine-powered supernovae, and unique fast transients found by c
urrent and future wide-field surveys. We derive relations between the injected and observed luminosity for steady and time dependent winds, and discuss how the temperature is set for scattering-dominated radiative transport. We apply this framework to specific examples of tidal disruption events and the formation of a black hole by a massive star, as well as discuss other applications such as deriving observables from detailed hydrodynamic simulations. We conclude by exploring what is inferred about the mass loss rate and underlying engine powering AT2018cow if it is explained as a wind-reprocessed transient, demonstrating that its optical emission is consistent with reprocessing of the observed soft X-rays.
An intriguing, growing class of planets are the super-puffs, objects with exceptionally large radii for their masses and thus correspondingly low densities ($lesssim0.3rm,g,cm^{-3}$). Here we consider whether they could have large inferred radii beca
use they are in fact ringed. This would naturally explain why super-puffs have thus far only shown featureless transit spectra. We find that this hypothesis can work in some cases but not all. The close proximity of the super-puffs to their parent stars necessitates rings with a rocky rather than icy composition. This limits the radius of the rings, and makes it challenging to explain the large size of Kepler 51b, 51c, 51d, and 79d unless the rings are composed of porous material. Furthermore, the short tidal locking timescales for Kepler 18d, 223d, and 223e mean that these planets may be spinning too slowly, resulting in a small oblateness and rings that are warped by their parent star. Kepler 87c and 177c have the best chance of being explained by rings. Using transit simulations, we show that testing this hypothesis requires photometry with a precision of somewhere between ~10 ppm and ~50 ppm, which roughly scales with the ratio of the planet and stars radii. We conclude with a note about the recently discovered super-puff HIP 41378f.
Tidal interactions can play an important role as compact white dwarf (WD) binaries are driven together by gravitational waves (GWs). This will modify the strain evolution measured by future space-based GW detectors and impact the potential outcome of
the mergers. Surveys now and in the near future will generate an unprecedented population of detached WD binaries to constrain tidal interactions. Motivated by this, I summarize the deviations between a binary evolving under the influence of only GW emission and a binary that is also experiencing some degree of tidal locking. I present analytic relations for the first and second derivative of the orbital period and braking index. Measurements of these quantities will allow the inference of tidal interactions, even when the masses of the component WDs are not well constrained. Finally, I discuss tidal heating and how it can provide complimentary information.
In our solar system, the presence of rings is exclusive to the gas giants, but is this the case for all planetary systems? In principle, it seems that rocky exoplanets could also have rings, which could be searched for by studying their subtle imprin
t on the ingress and egress of transits. Unfortunately, such effects are difficult to measure and require high precision photometric and/or spectroscopic observations. At the most basic level though, the presence of rings would result in an increased transit depth that could be mistaken as an anonymously large radius. Motivated by this, I consider how a population of exoplanets with rings would impact radius measurements, focusing on Earth-like exoplanets. It is found that this population introduces an enhancement of inferred radii in the range of $sim2-3R_oplus$, not unlike the sub-Neptunes that have been identified in recent transit surveys. Whether rings can explain all or most sub-Neptunes seems difficult, since it would require a large fraction of rocky planets to have rings ($gtrsim40%$) and/or a factor of $sim2-3$ increase in the number of planets with radii $lesssim1.2R_oplus$. Even if rings cannot explain all sub-Neptunes, this work suggests that focusing on those planets currently classified as sub-Neptunes may be a good starting place for finding rocky planets with rings.
In recent years, there has been interest in Earth-like exoplanets in the habitable zones of low mass stars ($sim0.1-0.6,M_odot$). Furthermore, it has been argued that a large moon may be important for stabilizing conditions on a planet for life. If t
hese two features are combined, then an exoplanet can feel a similar tidal influence from both its moon and parent star, leading to potentially interesting dynamics. The moons orbital evolution depends on the exoplanets initial spin period $P_0$. When $P_0$ is small, transfer of the exoplanets angular momentum to the moons orbit can cause the moon to migrate outward sufficiently to be stripped by the star. When $P_0$ is large, the moon migrates less and the stars tidal torques spin down the exoplanet. Tidal interactions then cause the moon to migrate inward until it is likely tidally disrupted by the exoplanet and potentially produces rings. While one may think that these findings preclude the presence of moons for the exoplanets of low mass stars, in fact a wide range of timescales are found for the loss or destruction of the moon; it can take $sim10^6-10^{10},{rm yrs}$ depending on the system parameters. When the moon is still present, the combined tidal torques force the exoplanet to spin asynchronously with respect to both its moon and parent star, which tidally heats the exoplanet. This can produce heat fluxes comparable to those currently coming through the Earth, arguing that combined tides may be a method for driving tectonic activity in exoplanets.
Binary white dwarf (WD) coalescences driven by gravitational waves or collisions in triple systems are potential progenitors of Type Ia supernovae (SNe Ia). We combine the distribution of 56Ni inferred from observations of SNe Ia with the results of
both sub-Chandrasekhar detonation models and direct collision calculations to estimate what mass WDs should be exploding in each scenario to reproduce the observations. These WD mass distributions are then compared with the observed Galactic WD mass distribution and Monte Carlo simulations of WD-WD binary populations. For collisions, we find that the average mass of the individual components of the WD-WD binary must be peaked at ~0.75Msun, significantly higher than the average WD mass in binaries or in the field of ~0.55-0.60Msun. Thus, if collisions produce a large fraction of SNe Ia, then a mechanism must exist that favors large mass WDs. On the other hand, in an old stellar population, collisions would naturally result in a class of low luminosity SNe Ia, and we suggest these may be related to 1991bg-like events. For sub-Chandrasekhar detonations, we find that the average mass of the exploding WDs must be peaked at ~1.1Msun. This is interestingly similar to the average sum of the masses in WD-WD binaries, but it is not clear (and should be further explored) whether double degenerate mergers would be sufficiently efficient at synthesizing 56Ni to match the observed yields. If not, then actual ~1.1Msun WDs would be needed for sub-Chandrasekhar detonations. Since such high mass WDs are produced relatively quickly in comparison to the age of the environments where SNe Ia are found, this would require either accretion onto lower mass WDs prior to ignition or a long timescale between formation of the ~1.1Msun WD and ignition (such as set by gravitational wave emission or binary interactions).
The mode of explosive burning in Type Ia SNe remains an outstanding problem. It is generally thought to begin as a subsonic deflagration, but this may transition into a supersonic detonation (the DDT). We argue that this transition leads to a breakou
t shock, which would provide the first unambiguous evidence that DDTs occur. Its main features are a hard X-ray flash (~20 keV) lasting ~0.01 s with a total radiated energy of ~10^{40} ergs, followed by a cooling tail. This creates a distinct feature in the visual light curve, which is separate from the nickel decay. This cooling tail has a maximum absolute visual magnitude of M_V = -9 to -10 at approximately 1 day, which depends most sensitively on the white dwarf radius at the time of the DDT. As the thermal diffusion wave moves in, the composition of these surface layers may be imprinted as spectral features, which would help to discern between SN Ia progenitor models. Since this feature should accompany every SNe Ia, future deep surveys (e.g., m=24) will see it out to a distance of approximately 80 Mpc, giving a maximum rate of ~60/yr. Archival data sets can also be used to study the early rise dictated by the shock heating (at about 20 days before maximum B-band light). A similar and slightly brighter event may also accompany core bounce during the accretion induced collapse to a neutron star, but with a lower occurrence rate.
