ALBERT

Description: We present the results of the spectroscopic and photometric monitoring campaign of ASASSN-15ed. The transient was discovered quite young by the All Sky Automated Survey for SuperNovae (ASAS-SN) survey. Amateur astronomers allowed us to sample the photometric SN evolution around maximum light, which we estimate to have occurred on JD = 2457087.4 ± 0.6 in the r band. Its apparent r -band magnitude at maximum was r  = 16.91 ± 0.10, providing an absolute magnitude M r –20.04 ± 0.20, which is slightly more luminous than the typical magnitudes estimated for Type Ibn SNe. The post-peak evolution was well monitored, and the decline rate (being in most bands around 0.1 mag d –1 during the first 25 d after maximum) is marginally slower than the average decline rates of SNe Ibn during the same time interval. The object was initially classified as a Type Ibn SN because early-time spectra were characterized by a blue continuum with superimposed narrow P-Cygni lines of He i , suggesting the presence of a slowly moving (1200–1500 km s –1 ), He-rich circumstellar medium. Later on, broad P-Cygni He i lines became prominent. The inferred velocities, as measured from the minimum of the broad absorption components, were between 6000 and 7000 km s –1 . As we attribute these broad features to the SN ejecta, this is the first time we have observed the transition of a Type Ibn SN to a Type Ib SN.

Description: The physical origin of high-velocity cool gas seen in galactic winds remains unknown. Following work by B. Wang, we argue that radiative cooling in initially hot thermally-driven outflows can produce fast neutral atomic and photoionized cool gas. The inevitability of adiabatic cooling from the flow's initial 10 7 –10 8  K temperature and the shape of the cooling function for T 10 7  K imply that outflows with hot gas mass-loss rate relative to star formation rate of $\beta =\dot{M}_{\rm hot}/\dot{M}_\star \gtrsim 0.5$ cool radiatively on scales ranging from the size of the energy injection region to tens of kpc. We highlight the β and star formation rate surface density dependence of the column density, emission measure, radiative efficiency, and velocity. At r cool , the gas produces X-ray and then UV/optical line emission with a total power bounded by ~10 –2 L * if the flow is powered by steady-state star formation with luminosity L * . The wind is thermally unstable at r cool , potentially leading to a multiphase medium. Cooled winds decelerate significantly in the extended gravitational potential of galaxies. The cool gas precipitated from hot outflows may explain its prevalence in galactic haloes. We forward a picture of winds whereby cool clouds are initially accelerated by the ram pressure of the hot flow, but are rapidly shredded by hydrodynamical instabilities, thereby increasing β, seeding radiative and thermal instability, and cool gas rebirth. If the cooled wind shocks as it sweeps up the circumgalactic medium, its cooling time is short, thus depositing cool gas far out into the halo. Finally, conduction can dominate energy transport in low-β hot winds, leading to flatter temperature profiles than otherwise expected, potentially consistent with X-ray observations of some starbursts.

Description: I show that the temperature of nuclear star clusters, starburst clusters in M82, compact high- z galaxies and some globular clusters of the Galaxy likely exceeded the ice-line temperature ( T Ice 150–170 K) during formation for a time comparable to the planet formation time-scale. The protoplanetary discs within these systems will thus, not have an ice line, decreasing the total material available for building protoplanetary embryos, inhibiting the formation of gas- and ice-giants if they form by core accretion, and prohibiting habitability. Planet formation by gravitational instability is similarly suppressed because Toomre's Q  〉 1 in all but the most massive discs. I show that cluster irradiation can in many cases dominate the thermodynamics and structure of passive and active protoplanetary discs for semi-major axes larger than ~1–5 au. I discuss these results in the context of the observed lack of planets in 47 Tuc. I predict that a similar search for planets in the globular cluster NGC 6366 ([Fe/H] = –0.82) should yield detections, whereas (counterintuitively) the relatively metal-rich globular clusters NGC 6440, 6441 and 6388 should be devoid of giant planets. The characteristic stellar surface density above which T Ice is exceeded in star clusters is ~ 6 x 10 3 M pc – 2 f – 1/2 dg, MW , where f dg, MW is the dust-to-gas ratio of the embedding material, normalized to the Milky Way value. Simple estimates suggest that ~5–50 per cent of the stars in the universe formed in an environment exceeding this surface density. Future microlensing planet searches that directly distinguish between the bulge and disc planet populations of the Galaxy and M31 can test these predictions. Caveats and uncertainties are detailed.

Description: We study the orbital evolution of hierarchical quadruple systems composed of two binaries on a long mutual orbit, where each binary acts as a Kozai–Lidov (KL) perturber on the other. We find that the coupling between the two binaries qualitatively changes the behaviour of their KL cycles. The binaries can experience coherent eccentricity oscillations as well as excursions to very high eccentricity that occur over a much larger fraction of the parameter space than in triple systems. For a ratio of outer to inner semimajor axes of 10–20, about 30–50 per cent of equal-mass quadruples reach eccentricity 1 – e  〈 10 –3 in one of the binaries. This is about 4–12 times more than for triples with equivalent parameters. Orbital ‘flips’ and collisions without previous tidal interaction are similarly enhanced in quadruples relative to triples. We argue that the frequency of evolutionary paths influenced by KL cycles is comparable in the triple and quadruple populations even though field quadruples are a factor of ~5 less frequent than triples. Additionally, quadruples might be a non-negligible source of triples and provide fundamentally new evolutionary outcomes involving close binaries, mergers, collisions and associated transients, which occur without any fine tuning of parameters. Finally, we study the perturbations to a planetary orbit due to a distant binary and we find that the fraction of orbital flips is a factor of 3–4 higher than for the corresponding triple system given our fiducial parameters with implications for hot Jupiters and star–planet collisions.

Description: We present the results of the spectroscopic and photometric monitoring campaign of ASASSN-15ed. The transient was discovered quite young by the All Sky Automated Survey for SuperNovae (ASAS-SN) survey. Amateur astronomers allowed us to sample the photometric SN evolution around maximum light, which we estimate to have occurred on JD = 2457087.4 ± 0.6 in the r band. Its apparent r -band magnitude at maximum was r  = 16.91 ± 0.10, providing an absolute magnitude M r –20.04 ± 0.20, which is slightly more luminous than the typical magnitudes estimated for Type Ibn SNe. The post-peak evolution was well monitored, and the decline rate (being in most bands around 0.1 mag d –1 during the first 25 d after maximum) is marginally slower than the average decline rates of SNe Ibn during the same time interval. The object was initially classified as a Type Ibn SN because early-time spectra were characterized by a blue continuum with superimposed narrow P-Cygni lines of He i , suggesting the presence of a slowly moving (1200–1500 km s –1 ), He-rich circumstellar medium. Later on, broad P-Cygni He i lines became prominent. The inferred velocities, as measured from the minimum of the broad absorption components, were between 6000 and 7000 km s –1 . As we attribute these broad features to the SN ejecta, this is the first time we have observed the transition of a Type Ibn SN to a Type Ib SN.

Description: We present ground-based and Swift photometric and spectroscopic observations of the candidate tidal disruption event (TDE) ASASSN-14li, found at the centre of PGC 043234 ( d ~= 90 Mpc) by the All-Sky Automated Survey for SuperNovae (ASAS-SN). The source had a peak bolometric luminosity of L ~= 10 44 erg s –1 and a total integrated energy of E ~= 7 x 10 50 erg radiated over the ~6 months of observations presented. The UV/optical emission of the source is well fitted by a blackbody with roughly constant temperature of T ~ 35 000 K, while the luminosity declines by roughly a factor of 16 over this time. The optical/UV luminosity decline is broadly consistent with an exponential decline, $L\propto \text{e}^{-t/t_0}$ , with t 0 ~= 60 d. ASASSN-14li also exhibits soft X-ray emission comparable in luminosity to the optical and UV emission but declining at a slower rate, and the X-ray emission now dominates. Spectra of the source show broad Balmer and helium lines in emission as well as strong blue continuum emission at all epochs. We use the discoveries of ASASSN-14li and ASASSN-14ae to estimate the TDE rate implied by ASAS-SN, finding an average rate of r ~= 4.1 x 10 –5 yr –1 per galaxy with a 90 per cent confidence interval of (2.2–17.0) x 10 –5 yr –1 per galaxy. ASAS-SN found roughly 1 TDE for every 70 Type Ia supernovae in 2014, a rate that is much higher than that of other surveys.

Description: Dynamical scattering of binaries and triple systems of stars, planets, and compact objects may produce highly inclined triple systems subject to Kozai–Lidov (KL) oscillations, potentially leading to collisions, mergers, Type Ia supernovae, and other phenomena. We present the results of more than 400 million gravitational scattering experiments of binary–binary, triple–single, and triple–binary scattering. We compute the cross-sections for all possible outcomes and explore their dependences on incoming velocity, mass, semimajor axis, and eccentricity, including analytic fits and discussion of the velocity dependence. For the production of new triple systems by scattering we find that compact triples are preferred, with ratios of outer to inner semimajor axes of ~few–100, flat or quasi-thermal eccentricity distributions, and flat distributions in cosine of the mutual inclination. Dynamically formed triples are thus subject to strong KL oscillations, the ‘eccentric Kozai mechanism’, and non-secular effects. For single and binary flyby encounters with triple systems, we compute the cumulative cross-section for changes to the mutual inclination, eccentricity, and semimajor axis ratio. We apply these results to scattering events in the field, open clusters, and globular clusters, and explore the implications for Type Ia supernovae via collisions and mergers, stellar collisions, and the lifetime and dynamical isolation of triple systems undergoing KL oscillations. An appendix provides an analysis of the velocity dependence of the collision cross-section in binary–single scattering.

Description: Kozai–Lidov (KL) oscillations can accelerate compact object mergers via gravitational wave radiation by driving the inner binaries of hierarchical triples to high eccentricities. We perform direct three-body integrations of high-mass-ratio compact object triple systems using fewbody including post-Newtonian terms. We find that the inner binary undergoes rapid eccentricity oscillations (REOs) on the time-scale of the outer orbital period which drive it to higher eccentricities than secular theory would otherwise predict, resulting in substantially reduced merger times. For a uniform distribution of tertiary eccentricity ( e 2 ), ~40 per cent of systems merge within ~1–2 eccentric KL time-scales whereas secular theory predicts that only ~20 per cent of such systems merge that rapidly. This discrepancy becomes especially pronounced at low e 2 , with secular theory overpredicting the merger time by many orders of magnitude. We show that a non-negligible fraction of systems have eccentricity 〉0.8 when they merge, in contrast to predictions from secular theory. Our results are applicable to high-mass-ratio triple systems containing black holes or neutron stars. In objects in which tidal effects are important, such as white dwarfs, stars, and planets, REOs can reduce the tidal circularization time-scale by an order of magnitude and bring the components of the inner binary into closer orbits than would be possible in the secular approximation.

Description: We calculate the steady-state properties of neutrino-driven winds from strongly magnetized, rotating protoneutron stars (PNSs; ‘protomagnetars’) under the assumption that the outflow geometry is set by the force-free magnetic field of an aligned dipole. Our goal is to assess protomagnetars as sites of r -process nucleosynthesis and gamma-ray burst engines using a more realistic outflow geometry than assumed in previous works. One-dimensional solutions calculated along flux tubes corresponding to different polar field lines are stitched together to determine the global properties of the flow at a given neutrino luminosity and rotation period. Protomagnetars with rotation periods of P  ~ 2–5 ms are shown to produce outflows more favourable for the production of third-peak r -process nuclei due to their much shorter expansion times through the seed nucleus formation region, yet only moderately lower entropies, as compared to normal spherical PNS winds. Protomagnetars with moderately rapid birth periods P  ~ 3–5 ms may thus represent a promising galactic r -process site which is compatible with a variety of other observations, including the recent discovery of possible magnetar-powered supernovae in metal-poor galaxies. We also confirm previous results that the outflows from protomagnetars with P  ~ 1–2 ms can achieve maximum Lorentz factors max  ~ 100–1000 in the range necessary to power gamma-ray bursts (GRBs). The implications of GRB jets with a heavy nuclei-dominated composition as sources of ultrahigh energy cosmic rays are also addressed.

Description: It is typically assumed that radiation-pressure-driven winds are accelerated to an asymptotic velocity of v ~= v esc , where v esc is the escape velocity from the central source. We note that this is not the case for dusty shells and clouds. Instead, if the shell or cloud is initially optically thick to the UV emission from the source of luminosity L , then there is a significant boost in v that reflects the integral of the momentum absorbed as it is accelerated. For shells reaching a generalized Eddington limit, we show that v ~= (4 R UV L / M sh c ) 1/2 , in both point-mass and isothermal-sphere potentials, where R UV is the radius where the shell becomes optically thin to UV photons, and M sh is the mass of the shell. The asymptotic velocity significantly exceeds v esc for typical parameters, and can explain the ~1000–2000 km s –1 outflows observed from rapidly star-forming galaxies and active galactic nuclei (AGN) if the surrounding halo has low gas density. Similarly fast outflows from massive stars can be accelerated on ~few–10 3  yr time-scales. These results carry over to clouds that subtend only a small fraction of the solid angle from the source of radiation and that expand as a consequence of their internal sound speed. We further consider the dynamics of shells that sweep up a dense circumstellar or circumgalactic medium. We calculate the ‘momentum ratio’ $\dot{M} v/(L/c)$ in the shell limit and show that it can only significantly exceed ~2 if the effective optical depth of the shell to re-radiated far-infrared photons is much larger than unity. We discuss simple prescriptions for the properties of galactic outflows for use in large-scale cosmological simulations. We also briefly discuss applications to the dusty ejection episodes of massive stars, the disruption of giant molecular clouds, and AGN.