arXiv subjects

Daniel Kasen

Publications and source records attributed to Daniel Kasen.

At least 127 records · Page 7Linked to original sources

arXiv2013

Tidal Tail Ejection as a Signature of Type~Ia Supernovae from White Dwarf Mergers

The merger of two white dwarfs may be preceded by the ejection of some mass in "tidal tails", creating a circumstellar medium around the system. We consider the variety of observational signatures from this material, which depend on the lag time between the start of the merger and the ultimate explosion (assuming one occurs) of the system in a Type Ia supernova. If the time lag is fairly short, the interaction of the supernova ejecta with the tails could lead to detectable shock emission at radio, optical, and/or x-ray wavelengths. At somewhat later times, the tails produce relatively broad NaID absorption lines with velocity widths of order the white dwarf escape speed ($\sim 1000$ \kms). That none of these signatures have been detected in normal SNe Ia constrains the lag time to be either very short ($\lesssim 100$ s) or fairly long ($\gtrsim 100$ yr). If the tails have expanded and cooled over timescales $\sim 10^4$ yr, they could be observable through narrow NaID and CaII H&K absorption lines in the spectra, which are seen in some fraction of SNe Ia. Using a combination of 3D and 1D hydrodynamical codes, we model the mass-loss from tidal interactions in binary systems, and the subsequent interactions with the interstellar medium, which produce a slow-moving, dense shell of gas. We synthesize NaID line profiles by ray-casting through this shell, and show that in some circumstances tidal tails could be responsible for narrow absorptions similar to those observed.

astro-ph.HE↗

arXiv2013

Opacities and Spectra of the r-process Ejecta from Neutron Star Mergers

Material ejected during (or immediately following) the merger of two neutron stars may assemble into heavy elements by the r-process. The subsequent radioactive decay of the nuclei can power electromagnetic emission similar to, but significantly dimmer than, an ordinary supernova. Identifying such events is an important goal of future transient surveys, offering new perspectives on the origin of r-process nuclei and the astrophysical sources of gravitational waves. Predictions of the transient light curves and spectra, however, have suffered from the uncertain optical properties of heavy ions. Here we consider the opacity of expanding r-process material and argue that it is dominated by line transitions from those ions with the most complex valence electron structure, namely the lanthanides. For a few representative ions, we run atomic structure models to calculate radiative data for tens of millions of lines. We find that the resulting r-process opacities are orders of magnitude larger than that of ordinary (e.g., iron-rich) supernova ejecta. Radiative transport calculations using these new opacities indicate that the transient emission should be dimmer and redder than previously thought. The spectra appear pseudo-blackbody, with broad absorption features, and peak in the infrared (~1 micron). We discuss uncertainties in the opacities and attempt to quantify their impact on the spectral predictions. The results have important implications for observational strategies to find and study the radioactively powered electromagnetic counterparts to compact object mergers.

astro-ph.HE↗

arXiv2013

Effect of a High Opacity on the Light Curves of Radioactively Powered Transients from Compact Object Mergers

The coalescence of compact objects are a promising astrophysical sources of gravitational wave (GW) signals. The ejection of r-process material from such mergers may lead to a radioactively-powered electromagnetic counterpart which, if discovered, would enhance the science return of a GW detection. 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 report that the presence of heavier elements, particularly the lanthanides, 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 from 0.001 to 0.1 solar masses and velocities from 0.1 to 0.3c). 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 ~ 2500K). We further consider the case in which a second mass outflow, composed of 56Ni, 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 56Ni and an infrared peak due to r-process ejecta. We briefly consider the prospects for detection and identification of these transients.

astro-ph.HE↗

arXiv2012

Supernova Light Curves Powered by Fallback Accretion

Some fraction of the material ejected in a core collapse supernova explosion may remain bound to the compact remnant, and eventually turn around and fall back. We show that the late time (> days) power associated with the accretion of this "fallback" material may significantly affect the optical light curve, in some cases producing super-luminous or otherwise peculiar supernovae. We use spherically symmetric hydrodynamical models to estimate the accretion rate at late times for a range of progenitor masses and radii and explosion energies. The accretion rate onto the proto-neutron star or black hole decreases as Mdot ~ t^-5/3 at late times, but its normalization can be significantly enhanced at low explosion energies, in very massive stars, or if a strong reverse shock wave forms at the helium/hydrogen interface in the progenitor. If the resulting super-Eddington accretion drives an outflow which thermalizes in the outgoing ejecta, the supernova debris will be re-energized at a time when photons can diffuse out efficiently. The resulting light curves are different and more diverse than previous fallback supernova models which ignored the input of accretion power and produced short-lived, dim transients. The possible outcomes when fallback accretion power is significant include super-luminous (> 10^44 ergs / s) Type II events of both short and long durations, as well as luminous Type I events from compact stars that may have experienced significant mass loss. Accretion power may unbind the remaining infalling material, causing a sudden decrease in the brightness of some long duration Type II events. This scenario may be relevant for explaining some of the recently discovered classes of peculiar and rare supernovae.

astro-ph.HE↗

arXiv2012

3-D Radiative Transfer Calculations of Radiation Feedback from Massive Black Holes: Outflow of Mass from the Dusty "Torus"

Observational and theoretical arguments suggest that the momentum carried in mass outflows from AGN can reach several times L / c, corresponding to outflow rates of hundreds of solar masses per year. Radiation pressure on lines alone may not be sufficient to provide this momentum deposition, and the transfer of reprocessed IR radiation in dusty nuclear gas has been postulated to provide the extra enhancement. The efficacy of this mechanism, however, will be sensitive to multi-dimensional effects such as the tendency for the reprocessed radiation to preferentially escape along sight-lines of lower column density. We use Monte Carlo radiative transfer calculations to determine the radiation force on dusty gas residing within approximately 10 parsecs from an accreting super-massive black hole. We calculate the net rate of momentum deposition in the surrounding gas and estimate the mass-loss rate in the resulting outflow as a function of solid angle for different black hole luminosities, sightline-averaged column densities, clumping parameters, and opening angles of the dusty gas. We find that these dust-driven winds carry momentum fluxes of 1-5 times L / c and correspond to mass-loss rates of 10-100 solar masses per year for a 10^8 solar mass black hole radiating at or near its Eddington limit. These results help to explain the origin of high velocity molecular and atomic outflows in local ULIRGs, and can inform numerical simulations of galaxy evolution including AGN feedback.

astro-ph.GA↗

arXiv2012

Pair-Instability Supernovae via Collision Runaway in Young Dense Star Clusters

Stars with helium cores between ~64 and 133 M_sun are theoretically predicted to die as pair-instability supernovae. This requires very massive progenitors, which are theoretically prohibited for Pop II/I stars within the Galactic stellar mass limit due to mass loss via line-driven winds. However, the runaway collision of stars in a dense, young star cluster could create a merged star with sufficient mass to end its life as a pair-instability supernova, even with enhanced mass loss at non-zero metallicity. We show that the predicted rate from this mechanism is consistent with the inferred volumetric rate of roughly ~2x10^-9 Mpc^-3 yr^-1 of the two observed pair-instability supernovae, SN 2007bi and PTF 10nmn, neither of which have metal-free host galaxies. Contrary to prior literature, only pair-instability supernovae at low redshifts z<2 will be observable with the Large Synoptic Survey Telescope (LSST). We estimate the telescope will observe ~10^2 such events per year that originate from the collisional runaway mergers in clusters.

astro-ph.HE↗

arXiv2012

Pair-Instability Supernovae at the Epoch of Reionization

Pristine stars with masses between ~140 and 260 M_sun are theoretically predicted to die as pair-instability supernovae. These very massive progenitors could come from Pop III stars in the early universe. We model the light curves and spectra of pair-instability supernovae over a range of masses and envelope structures. At redshifts of reionization z >= 6, we calculate the rates and detectability of pair-instability and core collapse supernovae, and show that with the James Webb Space Telescope, it is possible to determine the contribution of Pop III and Pop II stars toward reionization by constraining the stellar initial mass function at that epoch using these supernovae. We also find the rates of Type Ia supernovae, and show that they are not rare during reionization, and can be used to probe the mass function at 4-8 M_sun. If the budget of ionizing photons was dominated by contributions from top-heavy Pop III stars, we predict that the bright end of the galaxy luminosity function will be contaminated by pair-instability supernovae.

astro-ph.HE↗

arXiv2012

The Long-Term Evolution of Double White Dwarf Mergers

In this paper, we present a model for the long-term evolution of the merger of two unequal mass C/O white dwarfs (WDs). After the dynamical phase of the merger, magnetic stresses rapidly redistribute angular momentum, leading to nearly solid-body rotation on a viscous timescale of 1e-4 to 1 yr, long before significant cooling can occur. Due to heating during the dynamical and viscous phases, the less massive WD is transformed into a hot, slowly rotating, and radially extended envelope supported by thermal pressure. Following the viscous phase of evolution, the maximum temperature near the envelope base may already be high enough to begin off-center convective carbon-burning. If not, Kelvin-Helmholtz contraction of the inner region of the envelope on a thermal timescale of 1e3-1e4 yr compresses the base of the envelope, again yielding off-center burning. As a result, the long-term evolution of the merger remnant is similar to that seen in previous calculations: the burning shell diffuses inwards over ~1e4 yr, eventually yielding a high-mass O/Ne WD or a collapse to a neutron star. During the cooling and shell-burning phases, the merger remnant radiates near the Eddington limit. Given the double WD merger rate of a few per 1000 yr, tens of these ~1e38 erg/s sources should exist in a Milky Way-type galaxy. While the end result is similar to that of previous studies, the physical picture and the dynamical state of the matter in our model differ from previous work. Furthermore, remaining uncertainties related to the convective structure near the photosphere and mass loss during the thermal evolution may significantly affect our conclusions. Thus, future work within the context of the physical model presented here is required to better address the eventual fate of double WD mergers, including those for which one or both of the components is a He WD.

astro-ph.HE↗

arXiv2011

A Compact Degenerate Primary-Star Progenitor of SN 2011fe

While a white dwarf is, from a theoretical perspective, the most plausible primary star in Type Ia supernova (SN Ia), many other candidates have not been formally ruled out. Shock energy deposited in the envelope of any exploding primary contributes to the early SN brightness and, since this radiation energy is degraded by expansion after the explosion, the diffusive luminosity depends on the initial primary radius. We present a new non-detection limit of the nearby SN Ia 2011fe, obtained what appears to be just 4 hours after explosion, allowing us to directly constrain the initial primary radius, R_p. Coupled with the non-detection of a quiescent X-ray counterpart and the inferred synthesized Ni mass, we show that R_p <~ 0.02 R_sun (a factor of 5 smaller than previously inferred), that the average density of the primary must be rho_p > 10,000 gm cm^{-3}, and that the effective temperature must be less than a few x 10^5 K. This rules out hydrogen burning main sequence stars and giants. Constructing the helium-burning main sequence and carbon-burning main sequence, we find such objects are also excluded. By process of elimination, we find that only degeneracy-supported compact objects---WDs and neutron stars---are viable as the primary star of SN 2011fe. With few caveats, we also restrict the companion (secondary) star radius to R_ c <~ 0.1 R_sun, excluding Roche-Lobe overflowing red giant and main-sequence companions to high significance.

astro-ph.HE↗

arXiv2011

Supernova 2011fe from an Exploding Carbon-Oxygen White Dwarf Star

Type Ia supernovae (SNe Ia) have been used empirically as standardized candles to reveal the accelerating universe even though fundamental details, such as the nature of the progenitor system and how the star explodes, remained a mystery. There is consensus that a white dwarf star explodes after accreting matter in a binary system, but the secondary could be anything from a main sequence star to a red giant, or even another white dwarf. The uncertainty stems from the fact that no recent SN Ia has been discovered close enough to detect the stars before explosion. Here we report early observations of SN 2011fe (PTF11kly) in M101 at a distance of 6.4 Mpc, the closest SN Ia in the past 25 years. We find that the exploding star was likely a carbon-oxygen white dwarf, and from the lack of an early shock we conclude that the companion was most likely a main sequence star. Early spectroscopy shows high-velocity oxygen that varies on a time scale of hours and extensive mixing of newly synthesized intermediate mass elements in the outermost layers of the supernova. A companion paper uses pre-explosion images to rule out luminous red giants and most helium stars as companions.

astro-ph.CO↗

arXiv2011

Absorption line systems in simulated galaxies fed by cold streams

Hydro cosmological simulations reveal that massive galaxies at high redshift are fed by long narrow streams of merging galaxies and a smoother component of cold gas. We post-process seven high-resolution simulated galaxies with radiative transfer to study the absorption characteristics of the gas in galaxies and streams, in comparison with the statistics of observed absorption-line systems. We find that much of the stream gas is ionized by UV radiation from background and local stellar sources, but still optically thick (N_HI > 10^17 cm^-2) so that the streams appear as Lyman-limit systems (LLSs). At z>3, the fraction of neutral gas in streams becomes non-negligible, giving rise to damped Lyman-alpha (DLA) absorbers as well. The gas in the central and incoming galaxies remains mostly neutral, responsible for DLAs. Within one (two) virial radii, the covering factor of optically thick gas is <25% (10%) for LLSs and <5% (1%) for DLAs, slowly declining with time following the universal expansion. Nevertheless, galaxies and their cold streams in the studied mass range, M_vir = 10^10 - 10^12 Msun, account for >30% of the observed absorbers in the foreground of quasars, the rest possibly arising from smaller galaxies or the intergalactic medium. The mean metallicity in the streams is ~1% solar, much lower than in the galaxies. The simulated galaxies reproduce the Lyalpha-absorption equivalent widths observed around Lyman-break galaxies, but they severely underpredict the equivalent widths in metal lines, suggesting that the latter may arise from outflows. We conclude that the observed metal-poor LLSs are likely detections of the predicted cold streams. Revised analysis of the observed LLSs kinematics and simulations with more massive outflows in conjunction with the inflows may enable a clearer distinction between the signatures of the various gas modes.

astro-ph.CO↗

arXiv2011

The Most Slowly Declining Type Ia Supernova 2001ay

We present optical and near-infrared photometry, as well as ground-based optical spectra and Hubble Space Telescope ultraviolet spectra, of the Type Ia supernova (SN) 2001ay. At maximum light the Si II and Mg II lines indicated expansion velocities of 14,000 km/sec, while Si III and S II showed velocities of 9,000 km/sec There is also evidence for some unburned carbon at 12,000 km/sec. SN 2001ay exhibited a decline-rate parameter Delta m_15(B) = 0.68 \pm 0.05 mag; this and the B-band photometry at t > +25 d past maximum make it the most slowly declining Type Ia SN yet discovered. Three of four super-Chandrasekhar-mass candidates have decline rates almost as slow as this. After correction for Galactic and host-galaxy extinction, SN 2001ay had M_B = -19.19 and M_V = -19.17 mag at maximum light; thus, it was not overluminous in optical bands. In near-infrared bands it was overluminous only at the 2-sigma level at most. For a rise time of 18 d (explosion to bolometric maximum) the implied Ni-56 yield was (0.58 \pm 0.15)/alpha M_Sun, with alpha = L_max/E_Ni probably in the range 1.0 to 1.2. The Ni-56 yield is comparable to that of many Type Ia supernovae. The "normal" Ni-56 yield and the typical peak optical brightness suggest that the very broad optical light curve is explained by the trapping of the gamma rays in the inner regions.

astro-ph.CO↗

arXiv2011

Sub-Chandrasekhar Mass Models For Type Ia Supernovae

For carbon-oxygen white dwarfs accreting hydrogen or helium at rates in the range ~1-10 x 10^(-8) Msun/y, a variety of explosive outcomes is possible well before the star reaches the Chandrasekhar mass. These outcomes are surveyed for a range of white dwarf masses (0.7 - 1.1 Msun), accretion rates (1 - 7 x 10^(-8) Msun/y), and initial white dwarf temperatures (0.01 and 1 Lsun). The results are particularly sensitive to the convection that goes on during the last few minutes before the explosion. Unless this convection maintains a shallow temperature gradient, and unless the density is sufficiently high, the accreted helium does not detonate. Below a critical helium ignition density, which we estimate to be 5 - 10 x 10^5 g cm^(-3), either helium novae or helium deflagrations result. The hydrodynamics, nucleosynthesis, light curves, and spectra of a representative sample of detonating and deflagrating models are explored. Some can be quite faint indeed, powered at peak for a few days by the decay of 48Cr and 48V. Only the hottest, most massive white dwarfs considered with the smallest helium layers, show reasonable agreement with the light curves and spectra of common Type Ia supernovae. For the other models, especially those involving lighter white dwarfs, the helium shell mass exceeds 0.05 Msun and the mass of the 56Ni that is synthesized exceeds 0.01 Msun. These explosions do not look like ordinary Type Ia supernovae, or any other frequently observed transient.

astro-ph.HE↗

arXiv2011

Peculiar Type II Supernovae from Blue Supergiants

The vast majority of Type II supernovae (SNe) are produced by red supergiants (RSGs), but SN 1987A revealed that blue supergiants (BSGs) can produce members of this class as well, albeit with some peculiar properties. This best studied event revolutionized our understanding of SNe, and linking it to the bulk of Type II events is essential. We present here optical photometry and spectroscopy gathered for SN 2000cb, which is clearly not a standard Type II SN and yet is not a SN 1987A analog. The light curve of SN 2000cb is reminiscent of that of SN 1987A in shape, with a slow rise to a late optical peak, but on substantially different time scales. Spectroscopically, SN 2000cb resembles a normal SN II but with ejecta velocities that far exceed those measured for SN 1987A or normal SNe II, above 18000 km/s for H-alpha at early times. The red colours, high velocities, late photometric peak, and our modeling of this object all point toward a scenario involving the high-energy explosion of a small-radius star, most likely a BSG, producing 0.1 solar masses of Ni-56. Adding a similar object to the sample, SN 2005ci, we derive a rate of about 2% of the core-collapse rate for this loosely defined class of BSG explosions.

astro-ph.CO↗

arXiv2011

Simple Models of Metal-Line Absorption and Emission from Cool Gas Outflows

We analyze the absorption and emission-line profiles produced by a set of simple, cool gas wind models motivated by galactic-scale outflow observations. We implement monte carlo radiative transfer techniques that track the propagation of scattered and fluorescent photons to generate 1D spectra and 2D spectral images. We focus on the MgII 2796,28303 doublet and FeII UV1 multiplet at ~2600A, but the results are applicable to other transitions that trace outflows (e.g. NaI, Lya, SiII). By design, the resonance transitions show blue-shifted absorption but one also predicts strong resonance and fine-structure line-emission at roughly the systemic velocity. This line-emission `fills-in' the absorption reducing the equivalent width by up to 50%, shift the absorption-lin centroid by tens of km/s, and reduce the effective opacity near systemic. Analysis of cool gas outflows that ignores this line-emission may incorrectly infer that the gas is partially covered, measure asignificantly lower peak optical depth, and/or conclude that gas at systemic velocity is absent. Because the FeII lines are connected by optically-thin transitions to fine-structure levels, their profiles more closely reproduce the intrinsic opacity of the wind. Together these results naturally explain the absorption and emission-line characteristics observed for star-forming galaxies at z<1. We also study a scenario promoted to describe the outflows of z~3 Lyman break galaxies and find prfiles inconsistent with the observations due to scattered photon emission. Although line-emission complicates the analysis of absorption-line profiles, the surface brightness profiles offer a unique means of assessing the morphology and size of galactic-scale winds. Furthermore, the kinematics and line-ratios offer powerful diagnostics of outflows, motivating deep, spatially-extended spectroscopic observations.

astro-ph.GA↗

arXiv2011

Pair Instability Supernovae: Light Curves, Spectra, and Shock Breakout

For the initial mass range (140 < M < 260 Msun) stars die in a thermonuclear runaway triggered by the pair-production instability. The supernovae they make can be remarkably energetic (up to ~10^53 ergs) and synthesize considerable amounts of radioactive isotopes. Here we model the evolution, explosion, and observational signatures of representative pair-instability supernovae (PI SNe) spanning a range of initial masses and envelope structures. The predicted light curves last for hundreds of days and range in luminosity, from very dim to extremely bright, L ~ 10^44 ergs/s. The most massive events are bright enough to be seen at high redshift, but the extended light curve duration (~1 year) -- prolonged by cosmological time-dilation -- may make it difficult to detect them as transients. An alternative approach may be to search for the brief and luminous outbreak occurring when the explosion shock wave reaches the stellar surface. Using a multi-wavelength radiation-hydrodynamics code we calculate that, in the rest-frame, the shock breakout transients of PI SNe reach luminosities of 10^45-10^46 ergs/s, peak at wavelengths ~30-170 Angstroms, and last for several hours. We explore the detectability of PI SNe emission at high redshift, and discuss how observations of the light curves, spectra, and breakout emission can be used to constrain the mass, radius, and metallicity of the progenitor.

astro-ph.HE↗

arXiv2010

Measuring Ejecta Velocity Improves Type Ia Supernova Distances

We use a sample of 121 spectroscopically normal Type Ia supernovae (SNe Ia) to show that their intrinsic color is correlated with their ejecta velocity, as measured from the blueshift of the Si II 6355 feature near maximum brightness, v_Si. The SN Ia sample was originally used by Wang et al. (2009) to show that the relationship between color excess and peak magnitude, which in the absence of intrinsic color differences describes a reddening law, was different for two subsamples split by v_Si (defined as "Normal" and "High-Velocity"). We verify this result, but find that the two subsamples have the same reddening law when extremely reddened events (E(B-V) > 0.35 mag) are excluded. We also show that (1) the High-Velocity subsample is offset by ~0.06 mag to the red from the Normal subsample in the (B_max - V_max) - M_V plane, (2) the B_max - V_max cumulative distribution functions of the two subsamples have nearly identical shapes, but the High-Velocity subsample is offset by ~0.07 mag to the red in B_max - V_max, and (3) the bluest High-Velocity SNe Ia are ~0.10 mag redder than the bluest Normal SNe Ia. Together, this evidence indicates a difference in intrinsic color for the subsamples. Accounting for this intrinsic color difference reduces the scatter in Hubble residuals from 0.190 mag to 0.130 mag for SNe Ia with A_V < 0.7 mag. The scatter can be further reduced to 0.109 mag by exclusively using SNe Ia from the Normal subsample. Additionally, this result can at least partially explain the anomalously low values of R_V found in large SN Ia samples. We explain the correlation between ejecta velocity and color as increased line blanketing in the High-Velocity SNe Ia, causing them to become redder. We discuss some implications of this result, and stress the importance of spectroscopy for future SN Ia cosmology surveys, with particular focus on the design of WFIRST.

astro-ph.CO↗

arXiv2010

Single or Double Degenerate Progenitors? Searching for Shock Emission in the SDSS-II Type Ia Supernovae

From the set of nearly 500 spectroscopically confirmed type~Ia supernovae and around 10,000 unconfirmed candidates from SDSS-II, we select a subset of 108 confirmed SNe Ia with well-observed early-time light curves to search for signatures from shock interaction of the supernova with a companion star. No evidence for shock emission is seen; however, the cadence and photometric noise could hide a weak shock signal. We simulate shocked light curves using SN Ia templates and a simple, Gaussian shock model to emulate the noise properties of the SDSS-II sample and estimate the detectability of the shock interaction signal as a function of shock amplitude, shock width, and shock fraction. We find no direct evidence for shock interaction in the rest-frame $B$-band, but place an upper limit on the shock amplitude at 9% of supernova peak flux ($M_B > -16.6$ mag). If the single degenerate channel dominates type~Ia progenitors, this result constrains the companion stars to be less than about 6 $M_{\odot}$ on the main sequence, and strongly disfavors red giant companions.

astro-ph.CO↗