The coalescence of compact objects is a promising astrophysical source of detectable gravitational wave signals. The ejection of r-process material from such mergers may lead to a radioactively ...powered electromagnetic counterpart signal which, if discovered, would enhance the science returns. As very little is known about the optical properties of heavy r-process elements, previous light-curve models have adopted opacities similar to those of iron group elements. Here we consider the effect of heavier elements, particularly the lanthanides, which increase the ejecta opacity by several orders of magnitude. We include these higher opacities in time-dependent, multi-wavelength radiative transport calculations to predict the broadband light curves of one-dimensional models over a range of parameters (ejecta masses ~10 super(-3)-10 super(-1) M sub(middot in circle) and velocities ~0.1-0.3 c). We find that the higher opacities lead to much longer duration light curves which can last a week or more. The emission is shifted toward the infrared bands due to strong optical line blanketing, and the colors at later times are representative of a blackbody near the recombination temperature of the lanthanides (T ~ 2500 K). We further consider the case in which a second mass outflow, composed of super(56)Ni, is ejected from a disk wind, and show that the net result is a distinctive two component spectral energy distribution, with a bright optical peak due to super(56)Ni and an infrared peak due to r-process ejecta. We briefly consider the prospects for detection and identification of these transients.
We investigate line formation in gas that is outflowing and optically thick to electron scattering, as may be expected following the tidal disruption of a star by a supermassive black hole. Using ...radiative transfer calculations, we show that the optical line profiles produced by expanding TDE outflows most likely are primarily emission features, rather than the P-Cygni profiles seen in most supernova spectra. This is a result of the high line excitation temperatures in the highly irradiated TDE gas. The outflow kinematics cause the emission peak to be blueshifted and have an asymmetric red wing. Such features have been observed in some TDE spectra, and we propose that these may be signatures of outflows. We also show that non-coherent scattering of hot electrons can broaden the emission lines by ∼10,000 km s−1, such that the line width in some TDEs may be set by the electron scattering optical depth rather than the gas kinematics. The scattering-broadened line profiles produce distinct, wing-shaped profiles that are similar to those observed in some TDE spectra. The narrowing of the emission lines over time in these observed events may be related to a drop in density rather than a drop in line-of-sight velocity.
The merger of two neutron stars (NSs) or an NS and a black hole (BH) produces a radioactively powered transient known as a kilonova, first observed accompanying the gravitational wave event GW170817. ...While kilonovae are frequently modeled in spherical symmetry, the dynamical ejecta and disk outflows can be considerably asymmetric. We use Monte Carlo radiative transfer calculations to study the light curves of kilonovae with globally axisymmetric geometries (e.g., an ellipsoid and a torus). We find that the variation in luminosity in these models is most pronounced at early times and decreases until the light curves become isotropic in the late optically thin phase. The light-curve shape and peak time are not significantly modified by the global asymmetry. We show that the projected surface area along the line of sight captures the primary geometric effects, and we use this fact to provide a simple analytic estimate of the direction-dependent light curves of the aspherical ejecta. For the kilonova accompanying GW170817, accounting for asymmetry with an oblate (prolate) ellipsoid of axial ratio 2 (1/2) leads to an ∼40% decrease (increase) in the inferred ejecta mass compared to the spherical case. The pole-to-equator orientation effects are expected to be significantly larger (a factor of ∼5-10) for the more extreme asymmetries expected for some NS-BH mergers.
Compact object mergers can produce a thermal electromagnetic counterpart (a "kilonova") powered by the decay of freshly synthesized radioactive isotopes. The luminosity of kilonova light curves ...depends on the efficiency with which beta-decay electrons are thermalized in the ejecta. Here we derive a simple analytic solution for thermalization by calculating how accumulate electrons lose energy adiabatically and via plasma interactions. The thermalization efficiency is well described by where the timescale is a function of the ejecta mass and velocity and the exponent depends on the electron energies and the thermalization cross-sections. For a statistical distribution of r-process isotopes with radioactive power and n = 1, the late time kilonova luminosity asymptotes to and depends super-linearly on the ejecta mass, . If a kilonova is instead powered by a single dominate isotope, we show that the late time luminosity can deviate substantially from the underlying exponential decay and the heating from the accumulation of trapped electrons eventually exceeds the instantaneous radioactivity. Applied to the kilonova associated with the gravitational wave source GW170817, these results imply that a possible steepening of the light curve at 7 days is unrelated to thermalization effects and instead could mark the onset of translucency in a high opacity component of ejecta. The analytic results should be convenient for estimating the properties of observed kilonovae and assessing the potential late time detectability of future events.
Simplified analytic methods are frequently used to model the light curves of supernovae and other energetic transients and to extract physical quantities, such as the ejecta mass and amount of ...radioactive heating. The applicability and quantitative accuracy of these models, however, have not been clearly delineated. Here we carry out a systematic study comparing certain analytic models to numerical radiation transport calculations. We show that the neglect of time-dependent diffusion limits the accuracy of common Arnett-like analytic models, and that the widely applied Arnett's rule for inferring radioactive mass does not hold in general, with an error that increases for models with longer diffusion times or more centralized heating. We present new analytic relations that accurately relate the peak time and luminosity of an observed light curve to the physical ejecta and heating parameters. We further show that recombination and spatial distribution of heating modify the peak of the light curve and that these effects can be accounted for by varying a single dimensionless parameter in the new relations. The results presented should be useful for estimating the physical properties of a wide variety of transient phenomena.
The cosmic origin of elements heavier than iron has long been uncertain. Theoretical modelling shows that the matter that is expelled in the violent merger of two neutron stars can assemble into ...heavy elements such as gold and platinum in a process known as rapid neutron capture (r-process) nucleosynthesis. The radioactive decay of isotopes of the heavy elements is predicted to power a distinctive thermal glow (a 'kilonova'). The discovery of an electromagnetic counterpart to the gravitational-wave source GW170817 represents the first opportunity to detect and scrutinize a sample of freshly synthesized r-process elements. Here we report models that predict the electromagnetic emission of kilonovae in detail and enable the mass, velocity and composition of ejecta to be derived from observations. We compare the models to the optical and infrared radiation associated with the GW170817 event to argue that the observed source is a kilonova. We infer the presence of two distinct components of ejecta, one composed primarily of light (atomic mass number less than 140) and one of heavy (atomic mass number greater than 140) r-process elements. The ejected mass and a merger rate inferred from GW170817 imply that such mergers are a dominant mode of r-process production in the Universe.
We undertake a theoretical study of the near-infrared (NIR) light curves of Type Ia supernovae (SNe Ia). In these bands, the light curves are distinguished by a secondary maximum occurring roughly ...20-30 days after the initial one. Using time-dependent multigroup radiative transfer calculations, we calculate the UBVRIJHK-band light curves of model SN Ia ejecta structures. Our synthetic NIR light curves show distinct secondary maxima and provide favorable fits to observed SNe Ia. We offer a detailed explanation of the origin of the NIR secondary maximum, which is shown to relate directly to the ionization evolution of iron group elements in the ejecta. This understanding provides immediate insight into the dependence of the NIR light curves on the physical properties of the ejecta and in particular explains why brighter supernovae have a later and more prominent secondary maximum. We demonstrate the dependence of the NIR light curves on the mass of super(56)Ni, the degree of super(56)Ni mixing, the mass of electron capture elements, the progenitor metallicity, and the abundance of intermediate-mass elements (especially calcium). The secondary maximum is shown to be a valuable diagnostic of these important physical parameters. The models further confirm that SNe Ia should be excellent standard candles in the NIR, with a dispersion of 0.2 mag even when the physical properties of the ejecta are varied widely. This study emphasizes the consummate value of NIR observations in probing the structure of SNe Ia and in furthering their cosmological utility.
We present a radiative transfer code to model the nebular phase spectra of supernovae (SNe) in non-LTE (NLTE). We apply it to a systematic study of SNe Ia using parameterized 1D models and show how ...nebular spectral features depend on key physical parameters, such as the time since explosion, total ejecta mass, kinetic energy, radial density profile, and the masses of 56Ni, intermediate-mass elements, and stable iron-group elements. We also quantify the impact of uncertainties in atomic data inputs. We find the following. (1) The main features of SN Ia nebular spectra are relatively insensitive to most physical parameters. Degeneracy among parameters precludes a unique determination of the ejecta properties from spectral fitting. In particular, features can be equally well fit with generic Chandrasekhar mass ( ), sub- , and super- models. (2) A sizable ( 0.1 ) central region of stable iron-group elements, often claimed as evidence for models, is not essential to fit the optical spectra and may produce an unusual flat-top Co iii profile. (3) The strength of S iii emission near 9500 can provide a useful diagnostic of explosion nucleosynthesis. (4) Substantial amounts ( 0.1 ) of unburned C/O mixed throughout the ejecta produce O iii emission not seen in observations. (5) Shifts in the wavelength of line peaks can arise from line-blending effects. (6) The steepness of the ejecta density profile affects the line shapes, offering a constraint on explosion models. (7) Uncertainties in atomic data limit the ability to infer physical parameters.
We present stellar evolution calculations of the remnant of the merger of two carbon-oxygen white dwarfs (CO WDs). We focus on cases that have a total mass in excess of the Chandrasekhar mass. After ...the merger, the remnant manifests as an ... source for ... yr. A dusty wind may develop, leading these sources to be self-obscured and to appear similar to extreme asymptotic giant branch (AGB) stars. Roughly ~10 such objects should exist in the Milky Way and M31 at any time. As found in previous work, off-centre carbon fusion is ignited within the merger remnant and propagates inwards via a carbon flame, converting the WD to an oxygen-neon (ONe) composition. By following the evolution for longer than previous calculations, we demonstrate that after carbon-burning reaches the centre, neutrino-cooled Kelvin-Helmholtz contraction leads to off-centre neon ignition in remnants with masses ... The resulting neon-oxygen flame converts the core to a silicon WD. Thus, super-Chandrasekhar WD merger remnants do not undergo electron-capture induced collapse as traditionally assumed. Instead, if the remnant mass remains above the Chandrasekhar mass, we expect that it will form a low-mass iron core and collapse to form a neutron star. Remnants that lose sufficient mass will end up as massive, isolated ONe or Si WDs. (ProQuest: ... denotes formulae/symbols omitted.)
ABSTRACT We study the emission from tidal disruption events (TDEs) produced as radiation from black hole accretion propagates through an extended, optically thick envelope formed from stellar debris. ...We analytically describe key physics controlling spectrum formation, and present detailed radiative transfer calculations that model the spectral energy distribution and optical line strengths of TDEs near peak brightness. The steady-state transfer is coupled to a solver for the excitation and ionization states of hydrogen, helium, and oxygen (as a representative metal), without assuming local thermodynamic equilibrium. Our calculations show how an extended envelope can reprocess a fraction of soft X-rays and produce the observed optical fluxes of the order of 1043 erg s−1, with an optical/UV continuum that is not described by a single blackbody. Variations in the mass or size of the envelope may help explain how the optical flux changes over time with roughly constant color. For high enough accretion luminosities, X-rays can escape to be observed simultaneously with the optical flux. Due to optical depth effects, hydrogen Balmer line emission is often strongly suppressed relative to helium line emission (with He ii-to-H line ratios of at least 5:1 in some cases) even in the disruption of a solar-composition star. We discuss the implications of our results to understanding the type of stars destroyed in TDEs and the physical processes responsible for producing the observed flares.
PDF
