The angular momentum (AM) evolution of stellar interiors, along with the resulting rotation rates of stellar remnants, remains poorly understood. Asteroseismic measurements of red giant stars reveal that their cores rotate much faster than their surfaces, but much slower than theoretically predicted, indicating an unidentified source of AM transport operates in their radiative cores. Motivated by this, we investigate the magnetic Tayler instability and argue that it saturates when turbulent dissipation of the perturbed magnetic field energy is equal to magnetic energy generation via winding. This leads to larger magnetic field amplitudes, more efficient AM transport, and smaller shears than predicted by the classic Tayler-Spruit dynamo. We provide prescriptions for the effective AM diffusivity and incorporate them into numerical stellar models, finding they largely reproduce (1) the nearly rigid rotation of the Sun and main sequence stars, (2) the core rotation rates of low-mass red giants during hydrogen shell and helium burning, and (3) the rotation rates of white dwarfs. We discuss implications for stellar rotational evolution, internal rotation profiles, rotational mixing, and the spins of compact objects.

We present and analyse a new tidal disruption event (TDE), AT2017eqx at redshift z = 0.1089, discovered by Pan-STARRS and ATLAS. The position of the transient is consistent with the nucleus of its host galaxy; the spectrum shows a persistent blackbody temperature T greater than or similar to 20 000 K with broad HI and He II emission; and it peaks at a blackbody luminosity of L approximate to 10(44) erg s(-1). The lines are initially centred at zero velocity, but by 100 d, the HI lines disappear while the He II develops a blueshift of greater than or similar to 5000 km s(-1). Both the early- and late-time morphologies have been seen in other TDEs, but the complete transition between them is unprecedented. The evolution can be explained by combining an extended atmosphere, undergoing slow contraction, with a wind in the polar direction becoming visible at late times. Our observations confirm that a lack of hydrogen a TDE spectrum does not indicate a stripped star, while the proposed model implies that much of the diversity in TDEs may be due to the observer viewing angle. Modelling the light curve suggests AT2017eqx resulted from the complete disruption of a solar-mass star by a black hole of similar to 10(6.3) M-circle dot. The host is another Balmer-strong absorption galaxy, though fainter and less centrally concentrated than most TDE hosts. Radio limits rule out a relativistic jet, while X-ray limits at 500 d are among the deepest for a TDE at this phase.

Although there has recently been tremendous progress in studies of fast radio bursts (FRBs), the nature of their progenitors remains a mystery. We study the fluence and dispersion measure (DM) distributions of the ASKAP sample to better understand their energetics and statistics. We first consider a simplified model of a power-law volumetric rate per unit isotropic energy dN/dE proportional to E-gamma) with a maximum energy E-max in a uniform Euclidean universe. This provides analytic insights for what can be learned from these distributions. We find that the observed cumulative DM distribution scales as N(>DM) proportional to DM5-2 gamma (for gamma > 1) until a maximum DM max above which bursts near E-max fall below the fluence threshold of a given telescope. Comparing this model with the observed fluence and DM distributions, we find a reasonable fit for gamma similar to 1.7 and E-max similar to 10(33) erg Hz(-1). We then carry out a full Bayesian analysis based on a Schechter rate function with cosmological factors. We find roughly consistent results with our analytical approach, although with large errors on the inferred parameters due to the small sample size. The power-law index and the maximum energy are constrained to be gamma similar or equal to 1.6 +/- 0.3 and log E-max (erg Hz(-1) ) similar or equal to 34.1(-0.7)(+1.1) (68% confidence), respectively. From the survey exposure time, we further infer a cumulative local volumetric rate of log N(E > 10(32) erg Hz(-1))(Gpc(-3) yr(-1)) similar or equal to 2.6 +/- 0.4 (68% confidence). The methods presented here will be useful for the much larger FRB samples expected in the near future to study their distributions, energetics, and rates.

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.

An intriguing, growing class of planets are the "super-puffs," objects with exceptionally large radii for their masses and thus correspondingly low densities (less than or similar to 0.3 g cm(-3)). Here we consider whether they could have large inferred radii because 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 similar to 10 ppm and similar to 50 ppm, which roughly scales with the ratio of the planet and star's radii. We conclude with a note about the recently discovered super-puff HIP 41378f.

Many Type II supernovae (SNe) show hot early (similar to 30 days) emission, and a diversity in their light curves extending from the Type IIP to the Type IIL, which can be explained by interaction with dense and confined circumstellar material (CSM). We perform hydrodynamical simulations of red supergiants to model the ejection of CSM caused by wave heating during late-stage nuclear burning. Even a small amount of deposited energy (10(46)-10(47) erg), which is roughly that expected due to waves excited by convection in the core, is sufficient to change the shapes of SN light curves and bring them into better agreement with observations. As a test case, we consider the specific example of supernova (SN) 2017eaw, which shows that a nuclear burning episode is able to explain the light curve if it occurs similar to 150-450 days prior to core collapse. Due to the long timescale that it takes for the low-energy shock to traverse the star, this would manifest as a pre-SN outburst similar to 50-350 days prior to the full-fledged SN. Applying work like this to other SNe will provide a direct connection between the SN and pre-SN outburst properties, which can be tested by future wide field surveys. In addition, we show that our models can qualitatively explain the short-lived "flash-ionization" lines seen in the early spectra of many Type II SNe.

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 (BHs), engine-powered supernovae, and unique fast transients found by current 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 BH 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 AT 2018cow if it is explained as a wind-reprocessed transient, demonstrating that its optical emission is consistent with reprocessing of the observed soft X-rays.

The progenitor system of the compact binary merger GW190425 had a total mass of 3.41(-0.1)(+0.3) M-circle dot (90th-percentile confidence region) as measured from its gravitational wave signal. This mass is significantly different from the Milky Way (MW) population of binary neutron stars (BNSs) that are expected to merge in a Hubble time and from that of the first BNS merger, GW170817. Here, we explore the expected electromagnetic (EM) signatures of such a system. We make several astrophysically motivated assumptions to further constrain the parameters of GW190425. By simply assuming that both components were NSs, we reduce the possible component masses significantly, finding m(1) = 1.85(-0.19)(+0.27) M-circle dot and m(2) = 1.471(-0.18)(+0.16) M-circle dot. However, if the GW190425 progenitor system was an NS-black hole (BH) merger, we find best-fitting parameters m(1) = 2.19(-0.17)(+0.21) M-circle dot and m(2) = 1.26(-0.08)(+0.10) M-circle dot. For a well-motivated BNS system where the lighter NS has a mass similar to the mass of non-recycled NSs in MW BNS systems, we find m(1) = 2.03(-0.14)(+0.1)5 M-circle dot and m(2) = 1.35 +/- 0.09 M-circle dot, corresponding to only 7 per cent mass uncertainties. For all scenarios, we expect a prompt collapse of the resulting remnant to a BH. Examining detailed models with component masses similar to our best-fitting results, we find the EM counterpart to GW190425 is expected to be significantly redder and fainter than that of GW170817. We find that almost all reported search observations were too shallow to detect the expected counterpart to GW190425. If the LIGO-Virgo Collaboration promptly provides the chirp mass, the astronomical community can adapt their observations to improve the likelihood of detecting a counterpart for similarly 'high-mass' BNS systems.

We present a wide-field optical imaging search for electromagnetic counterparts to the likely neutron star-black hole (NS-BH) merger GW190814/S190814bv. This compact binary merger was detected through gravitational waves by the LIGO/Virgo interferometers, with masses suggestive of an NS-BH merger. We imaged the LIGO/Virgo localization region using the MegaCam instrument on the Canada-France-Hawaii Telescope (CFHT). We describe our hybrid observing strategy of both tiling and galaxy-targeted observations, as well as our image differencing and transient detection pipeline. Our observing campaign produced some of the deepest multiband images of the region between 1.7 and 8.7 days post-merger, reaching a 5 sigma depth of g > 22.8 (AB mag) at 1.7 days and i > 23.1 and i > 23.9 at 3.7 and 8.7 days, respectively. These observations cover a mean total integrated probability of 67.0% of the localization region. We find no compelling candidate transient counterparts to this merger in our images, which suggests that the lighter object was tidally disrupted inside of the BH's innermost stable circular orbit, the transient lies outside of the observed sky footprint, or the lighter object is a low-mass BH. We use 5 sigma source detection upper limits from our images in the NS-BH interpretation of this merger to constrain the mass of the kilonova ejecta to be M-ej less than or similar to 0. 015M(circle dot) for a "blue" (kappa=0.5cm(2) g(-1)) kilonova and M-ej less than or similar to 0. 04M(circle dot) for a "red" (kappa=5-10 cm(2) g(-1)) kilonova. Our observations emphasize the key role of large-aperture telescopes and wide-field imagers such as CFHT MegaCam in enabling deep searches for electromagnetic counterparts to gravitational-wave events.