We use a high-temperature chemical network to derive the molecular abundances in axisymmetric accretion disk models around active galactic nuclei (AGNs) within 100 pc using simple radial and vertical ...density and temperature distributions motivated by more detailed physical models. We explore the effects of X-ray irradiation and cosmic ray ionization on the spatial distribution of the molecular abundances of CO, CN, CS, HCN, HCO+, HC3N, C2H, and c-C3H2 using a variety of plausible disk structures. These simple models have molecular regions with a layer of X-ray dominated regions, a midplane without the strong influence of X-rays, and a high-temperature region in the inner portion with moderate X-ray flux where families of polyynes (C\(_{\rm n}\)H\(_{2}\)) and cyanopolyynes can be enhanced. For the high midplane density disks we explore, we find that cosmic rays produced by supernovae do not significantly affect the regions unless the star formation efficiency significantly exceeds that of the Milky Way. We highlight molecular abundance observations and ratios that may distinguish among theoretical models of the density distribution in AGN disks. Finally, we assess the importance of the shock crossing time and the accretion time relative to the formation time for various chemical species. Vertical column densities are tabulated for a number of molecular species at both the characteristic shock crossing time and steady state. Although we do not attempt to fit any particular system or set of observations, we discuss our models and results in the context of the nearby AGN NGC 1068.
Evolved stars near the tip of the red giant branch (TRGB) show solar-like oscillations with periods spanning hours to months and amplitudes ranging from \(\sim\)1 mmag to \(\sim\)100 mmag. The ...systematic detection of the resulting photometric variations with ground-based telescopes would enable the application of asteroseismology to a much larger and more distant sample of stars than is currently accessible with space-based telescopes such as \textit{Kepler} or the ongoing Transiting Exoplanet Survey Satellite (\textit{TESS}) mission. We present an asteroseismic analysis of 493 M giants using data from two ground-based surveys: the Asteroid Terrestrial-impact Last Alert System (ATLAS) and the All-Sky Automated Survey for Supernovae (ASAS-SN). By comparing the extracted frequencies with constraints from \textit{Kepler}, the Sloan Digital Sky Survey Apache Point Observatory Galaxy Evolution Experiment (APOGEE), and Gaia we demonstrate that ground-based transient surveys allow accurate distance measurements to oscillating M giants with a precision of \(\sim\)15\(\%\). Using stellar population synthesis models we predict that ATLAS and ASAS-SN can provide asteroseismic distances to \(\sim\)2\(\times\)10\(^{6}\) galactic M giants out to typical distances of \(20-50 \; \rm{kpc}\), vastly improving the reach of Gaia and providing critical constraints for Galactic archaeology and galactic dynamics.
ASASSN-18am/SN 2018gk is a newly discovered member of the rare group of luminous, hydrogen-rich supernovae (SNe) with a peak absolute magnitude of \(M_V \approx -20\) mag that is in between normal ...core-collapse SNe and superluminous SNe. These SNe show no prominent spectroscopic signatures of ejecta interacting with circumstellar material (CSM), and their powering mechanism is debated. ASASSN-18am declines extremely rapidly for a Type II SN, with a photospheric-phase decline rate of \(\sim6.0~\rm mag~(100 d)^{-1}\). Owing to the weakening of HI and the appearance of HeI in its later phases, ASASSN-18am is spectroscopically a Type IIb SN with a partially stripped envelope. However, its photometric and spectroscopic evolution show significant differences from typical SNe IIb. Using a radiative diffusion model, we find that the light curve requires a high synthesised \(\rm ^{56}Ni\) mass \(M_{\rm Ni} \sim0.4~M_\odot\) and ejecta with high kinetic energy \(E_{\rm kin} = (7-10) \times10^{51} \) erg. Introducing a magnetar central engine still requires \(M_{\rm Ni} \sim0.3~M_\odot\) and \(E_{\rm kin}= 3\times10^{51} \) erg. The high \(\rm ^{56}Ni\) mass is consistent with strong iron-group nebular lines in its spectra, which are also similar to several SNe Ic-BL with high \(\rm ^{56}Ni\) yields. The earliest spectrum shows "flash ionisation" features, from which we estimate a mass-loss rate of \( \dot{M}\approx 2\times10^{-4}~\rm M_\odot~yr^{-1} \). This wind density is too low to power the luminous light curve by ejecta-CSM interaction. We measure expansion velocities as high as \( 17,000 \) km/s for \(H_\alpha\), which is remarkably high compared to other SNe II. We estimate an oxygen core mass of \(1.8-3.4\) \(M_\odot\) using the OI luminosity measured from a nebular-phase spectrum, implying a progenitor with a zero-age main sequence mass of \(19-26\) \(M_\odot\).
abridged The pressure exerted by the radiation of young stars may be an important feedback mechanism in forming star clusters and the disks of starburst galaxies. However, there is great uncertainty ...in how efficiently radiation couples to matter in these high optical depth environments. In particular, it is unclear what levels of turbulence the radiation can produce, and whether the infrared radiation trapped by the dust opacity can give rise to heavily mass-loaded winds. In this paper we report a series of two-dimensional flux-limited diffusion radiation-hydrodynamics calculations performed with the code ORION in which we drive strong radiation fluxes through columns of dusty matter confined by gravity. We consider both systems where the radiation flux is sub-Eddington throughout the gas column, and where it is super-Eddington at the midplane but sub-Eddington in the atmosphere. In the latter, we find that the radiation-matter interaction gives rise to radiation-driven Rayleigh-Taylor instability, which drives supersonic turbulence at a level sufficient to fully explain the turbulence seen in Galactic protocluster gas clouds, and to make a non-trivial contribution to the turbulence observed in starburst galaxy disks. However, the instability also produces a channel structure in which the radiation-matter interaction is reduced because the radiation field is not fully trapped. For astrophysical parameters relevant to forming star clusters and starburst galaxies, we find that this effect reduces the net momentum deposition rate in the dusty gas by a factor of ~2-6 compared to simple analytic estimates, and that in steady state the Eddington ratio reaches unity and there are no strong winds. We provide an approximation formula, appropriate for implementation in analytic models and non-radiative simulations, for the force exerted by the infrared radiation field in this regime.
We study the physics of core-collapse supernovae and the neutron stars they create. We study the microphysics of neutrino interactions and demonstrate the importance of two processes previously ...ignored in full supernova simulations: inelastic neutrino-nucleon scattering and nucleon-nucleon bremsstrahlung. We show that these processes dominate neutrino-electron scattering and electron-positron annihilation as thermalization and production mechanisms, respectively, for mu- and tau-neutrinos in regimes vital to emergent spectrum formation. In addition, we solve the general-relativistic steady-state eigenvalue problem of neutrino-driven protoneutron star winds, which immediately follow core-collapse supernova explosions. We provide velocity, density, temperature, and composition profiles and explore the systematics and structures generic to such a wind for a variety of protoneutron star characteristics. Furthermore, we derive the entropy, dynamical timescale, and compositions essential in assessing this site as a candidate for r-process nucleosynthesis. Finally, we construct dynamical models of core-collapse supernovae. We employ a full solution to the transport equation for each neutrino species, a realistic high-density nuclear equation of state, and explicit hydrodynamics. We present results from a set of different supernova progenitors. We vary the microphysics and nuclear equation of state and compare our results to those of other groups. We examine the electron-neutrino breakout phenomenon and address the importance of nucleon-nucleon bremsstrahlung and inelastic neutrino-electron scattering in mu and tau neutrino spectrum formation. We convolve the emergent spectra obtained in these models with terrestrial neutrino detectors and find that the electron-neutrino breakout burst can likely be observed and identified uniquely.
I review aspects of the theory of long-duration gamma-ray burst (GRB) central engines. I focus on the requirements of any model; these include the angular momentum of the progenitor, the power, ...Lorentz factor, asymmetry, and duration of the flow, and both the association and the non-association with bright supernovae. I compare and contrast the collapsar and millisecond proto-magnetar models in light of these requirements. The ability of the latter model to produce a flow with Lorentz factor ~100 while simultaneously maintaining a kinetic luminosity of ~10^50 ergs/s for a timescale of ~10-100 s is emphasized.
We calculate the steady-state properties of neutrino-driven winds from strongly magnetized, rotating proto-neutron stars (`proto-magnetars') under the assumption that the outflow geometry is set by ...the force-free magnetic field of an aligned dipole. Our goal is to assess proto-magnetars as sites of r-process nucleosynthesis and gamma-ray burst engines. 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. Proto-magnetars with rotation periods of P~2-5 ms are shown to produce outflows more favorable 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. Proto-magnetars 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 proto-magnetars with P~1-2 ms can achieve maximum Lorentz factors Gamma ~ 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 ultra-high energy cosmic rays are also addressed.
In the seconds after collapse of a massive star, the newborn proto-neutron star (PNS) radiates neutrinos of all flavors. The absorption of electron-type neutrinos below the radius of the stalled ...shockwave may drive explosions (the "neutrino mechanism"). Because the heating rate is proportional to the square of neutrino energy, flavor conversion of mu and tau neutrinos to electron-type neutrinos via collective neutrino oscillations (CnuO) may in principle increase the heating rate and drive explosions. In order to assess the potential importance of CnuO for the shock revival, we solve the steady-state boundary value problem of spherically-symmetric accretion between the PNS surface (r_nu) and the shock (r_S), including a scheme for flavor conversion via CnuO. For a given r_nu, PNS mass (M), accretion rate (Mdot), and assumed values of the neutrino energies from the PNS, we calculate the critical neutrino luminosity above which accretion is impossible and explosion results. We show that CnuO can decrease the critical luminosity by a factor of at most ~1.5, but only if the flavor conversion is fully completed inside r_S and if there is no matter suppression. The magnitude of the effect depends on the model parameters (M, Mdot, and r_nu) through the shock radius and the physical scale for flavor conversion. We quantify these dependencies and find that CnuO could lower the critical luminosity only for small M and Mdot, and large r_nu. However, for these parameter values CnuO are suppressed due to matter effects. By quantifying the importance of CnuO and matter suppression at the critical neutrino luminosity for explosion, we show in agreement with previous studies that CnuO are unlikely to affect the neutrino mechanism of core-collapse supernovae significantly.
We present the first spatially and spectrally resolved image of the molecular outflow in the western nucleus of Arp\,220. The outflow, seen in HCN~(1--0) by ALMA, is compact and collimated, with ...extension \(\lesssim\) 120\,pc. Bipolar morphology emerges along the minor axis of the disk, with redshifted and blueshifted components reaching maximum inclination-corrected velocity of \(\sim\,\pm\)\,840\,km\,s\(^{-1}\). The outflow is also seen in CO and continuum emission, the latter implying that it carries significant dust. We estimate a total mass in the outflow of \(\geqslant\)\,10\(^{6}\)\,M\(_{\odot}\), a dynamical time of \(\sim\)\,10\(^{5}\)\,yr, and mass outflow rates of \(\geqslant55\)\,M\(_{\odot}\)\,yr\(^{-1}\) and \(\geqslant\,15\)\,M\(_{\odot}\)\,yr\(^{-1}\) for the northern and southern lobes, respectively. Possible driving mechanisms include supernovae energy and momentum transfer, radiation pressure feedback, and a central AGN. The latter could explain the collimated morphology of the HCN outflow, however we need more complex theoretical models, including contribution from supernovae and AGN, to pinpoint the driving mechanism of this outflow.
I consider the physics of gravitational instabilities in the presence of dynamically important radiation pressure and gray radiative diffusion, governed by a constant opacity, kappa. For any non-zero ...radiation diffusion rate on an optically-thick scale, the medium is unstable unless the classical gas-only isothermal Jeans criterion is satisfied. When diffusion is "slow," although the dynamical Jeans instability is stabilized by radiation pressure on scales smaller than the adiabatic Jeans length, on these same spatial scales the medium is unstable to a diffusive mode. In this regime, neglecting gas pressure, the characteristic timescale for growth is independent of spatial scale and given by (3 kappa c_s^2)/(4 pi G c), where c_s is the adiabatic sound speed. This timescale is that required for a fluid parcel to radiate away its thermal energy content at the Eddington limit, the Kelvin-Helmholz timescale for a radiation pressure supported self-gravitating object. In the limit of "rapid" diffusion, radiation does nothing to suppress the Jeans instability and the medium is dynamically unstable unless the gas-only Jeans criterion is satisfied. I connect with treatments of Silk damping in the early universe. I discuss several applications, including photons diffusing in regions of extreme star formation (starburst galaxies & pc-scale AGN disks), and the diffusion of cosmic rays in normal galaxies and galaxy clusters. The former (particularly, starbursts) are "rapidly" diffusing and thus cannot be supported against dynamical instability in the linear regime by radiation pressure alone. The latter are more nearly "slowly" diffusing. I speculate that the turbulence in starbursts may be driven by the dynamical coupling between the radiation field and the self-gravitating gas, perhaps mediated by magnetic fields. (Abridged)
