314 research outputs found
The time-dependent rebrightenings in classical novae outbursts: a late-time episodic fuel burning?
A significant fraction of novae exhibit a series of rebrightenings on the
decline branch of their light curves. We use visual observations to study this
phase in several well-observed novae. We find that these rebrightenings are
isolated flare-like events on otherwise smooth light curves and we show that in
most novae in our sample the time intervals between consecutive flares
gradually increase as a geometric series; rebrightenings are equally spaced in
logarithmic time. We also find a correlation between the rate of increase in
the time between rebrightenings with the speed class of the nova in the sense
that slower novae tend to increase the time intervals faster. We attribute
these rebrightenings to instabilities in the envelope hydrogen burning and we
mention other cases of such timing pattern in astronomy and natural sciences in
general.Comment: 5 pages, 3 figures, accepted to ApJ
Burying a Binary: Dynamical Mass Loss and a Continuous Optically-Thick Outflow Explain the Candidate Stellar Merger V1309 Scorpii
V1309 Sco was proposed to be a stellar merger and a common envelope transient
based on the pre-outburst light curve of a contact eclipsing binary with a
rapidly decaying orbital period. Using published data, I show that the period
decay timescale P/Pdot of V1309 Sco decreased from ~1000 to ~170 years in less
than about 6 years, which implies a very high value of second period
derivative. I argue that V1309 Sco experienced an onset of dynamical mass loss
through the outer Lagrange point, which eventually obscured the binary. The
photosphere of the resulting continuous optically-thick outflow expands as the
mass-loss rate increases, explaining the ~200 day rise to optical maximum. The
model yields the mass-loss rate of the binary star as a function of time and
fits the observed light curve remarkably well. It is also possible to
observationally constrain the properties of the surface layers undergoing the
dynamical mass loss. V1309 Sco is thus a prototype of a new class of stellar
transients distinguished by a slow rise to optical maximum that are driven by
dynamical mass loss from a binary. I discuss implications of these findings for
stellar transients and other suggested common envelope events.Comment: 7 pages, 4 figures. Accepted for publication in ApJ. Significant
changes to the manuscript based on the comments of the referees. For a brief
video explaining the key results of this paper, see
http://youtu.be/WV87UqCenD
A Global Model of The Light Curves and Expansion Velocities of Type II-Plateau Supernovae
We present a new self-consistent and versatile method that derives
photospheric radius and temperature variations of Type II-Plateau supernovae
based on their expansion velocities and photometric measurements. We apply the
method to a sample of 26 well-observed, nearby supernovae with published light
curves and velocities. We simultaneously fit ~230 velocity and ~6800 magnitude
measurements distributed over 21 photometric passbands spanning wavelengths
from 0.19 to 2.2 microns. The light curve differences among the Type II-Plateau
supernovae are well-modeled by assuming different rates of photospheric radius
expansion, which we explain as different density profiles of the ejecta and we
argue that steeper density profiles result in flatter plateaus, if everything
else remains unchanged. The steep luminosity decline of Type II-Linear
supernovae is due to fast evolution of the photospheric temperature, which we
verify with a successful fit of SN1980K. Eliminating the need for theoretical
supernova atmosphere models, we obtain self-consistent relative distances,
reddenings, and nickel masses fully accounting for all internal model
uncertainties and covariances. We use our global fit to estimate the time
evolution of any missing band tailored specifically for each supernova and we
construct spectral energy distributions and bolometric light curves. We produce
bolometric corrections for all filter combinations in our sample. We compare
our model to the theoretical dilution factors and find good agreement for the B
and V filters. Our results differ from the theory when the I, J, H, or K bands
are included. We investigate the reddening law towards our supernovae and find
reasonable agreement with standard R_V ~ 3.1 reddening law in UBVRI bands.
Results for other bands are inconclusive. We make our fitting code publicly
available.Comment: 23 pages, 18 figures, 8 tables. Accepted to ApJ. Our results and the
fitting tool are available at http://www.astro.princeton.edu/~pejcha/iip
Shock-powered light curves of luminous red novae as signatures of pre-dynamical mass loss in stellar mergers
Luminous red novae (LRN) are a class of optical transients believed to
originate from the mergers of binary stars, or "common envelope" events. Their
light curves often show secondary maxima, which cannot be explained in the
previous models of thermal energy diffusion or hydrogen recombination without
invoking multiple independent shell ejections. We propose that double-peaked
light curves are a natural consequence of a collision between
dynamically-ejected fast shell and pre-existing equatorially-focused material,
which was shed from the binary over many orbits preceding the dynamical event.
The fast shell expands freely in the polar directions, powering the initial
optical peak through cooling envelope emission. Radiative shocks from the
collision in the equatorial plane power the secondary light curve peak on the
radiative diffusion timescale of the deeper layers, similar to luminous Type
IIn supernovae and some classical novae. Using a detailed 1D analytic model,
informed by complementary 3D hydrodynamical simulations, we show that
shock-powered emission can explain the observed range of peak timescales and
luminosities of the secondary peaks in LRN for realistic variations in the
binary parameters and fraction of the binary mass ejected. The dense shell
created by the radiative shocks in the equatorial plane provides an ideal
location for dust nucleation consistent with the the inferred aspherical
geometry of dust in LRN. For giant stars, the ejecta forms dust when the
shock-powered luminosity is still high, which could explain the infrared
transients recently discovered by Spitzer. Our results suggest that
pre-dynamical mass loss is common if not ubiquitous in stellar mergers,
providing insight into the instabilities responsible for driving the binary
merger.Comment: 12 pages, 6 figures, submitte
On The Intrinsic Diversity of Type II-Plateau Supernovae
Hydrogen-rich Type II-Plateau supernovae exhibit correlations between the
plateau luminosity , the nickel mass , the explosion
energy , and the ejecta mass . Using our global,
self-consistent, multi-band model of nearby well-observed supernovae, we find
that the covariances of these quantities are strong and that the confidence
ellipsoids are oriented in the direction of the correlations, which reduces
their significance. By proper treatment of the covariance matrix of the model,
we discover a significant intrinsic width to the correlations between , , and , where the uncertainties due to the
distance and the extinction dominate. For fixed , the spread in
is about 0.25 dex, which we attribute to the differences in the
progenitor internal structure. We argue that the effects of incomplete
-ray trapping are not important in our sample. Similarly, the physics
of the Type II-Plateau supernova light curves leads to inherently degenerate
estimates of and , which makes their observed
correlation weak. Ignoring the covariances of supernova parameters or the
intrinsic width of the correlations causes significant biases in the slopes of
the fitted relations. Our results imply that Type II-Plateau supernova
explosions are not described by a single physical parameter or a simple
one-dimensional trajectory through the parameter space, but instead reflect the
diversity of the core and surface properties of their progenitors. We discuss
the implications for the physics of the explosion mechanism and possible future
observational constraints.Comment: 8 pages, 5 figures, accepted to Ap
The Landscape of the Neutrino Mechanism of Core-Collapse Supernovae: Neutron Star and Black Hole Mass Functions, Explosion Energies and Nickel Yields
If the neutrino luminosity from the proto-neutron star formed during a
massive star core collapse exceeds a critical threshold, a supernova (SN)
results. Using spherical quasi-static evolutionary sequences for hundreds of
progenitors over a range of metallicities, we study how the explosion threshold
maps onto observables, including the fraction of successful explosions, the
neutron star (NS) and black hole (BH) mass functions, the explosion energies
(E_SN) and nickel yields (M_Ni), and their mutual correlations. Successful
explosions are intertwined with failures in a complex pattern that is not
simply related to initial progenitor mass or compactness. We predict that
progenitors with initial masses of 15 +/- 1, 19 +/- 1, and 21-26 M_Sun are most
likely to form BHs, that the BH formation probability is non-zero at
solar-metallicity and increases significantly at low metallicity, and that low
luminosity, low Ni-yield SNe come from progenitors close to success/failure
interfaces. We qualitatively reproduce the observed E_SN-M_Ni correlation, we
predict a correlation between the mean and width of the NS mass and E_SN
distributions, and that the means of the NS and BH mass distributions are
correlated. We show that the observed mean NS mass of ~1.33 M_Sun implies that
the successful explosion fraction is higher than 0.35. Overall, we show that
the neutrino mechanism can in principle explain the observed properties of SNe
and their compact objects. We argue that the rugged landscape of progenitors
and outcomes mandates that SN theory should focus on reproducing the wide
ranging distributions of observed SN properties.Comment: 28 pages, 29 figures, 2 tables, accepted to ApJ. Our results are
available at http://www.astro.princeton.edu/~pejcha/synth_sne
The Physics of Massive Star Death
Mathematical and Physical Sciences: 1st Place (The Ohio State University Edward F. Hayes Graduate Research Forum)Neutrino heating may drive explosions of massive stars (core-collapse supernovae). Although it is known that the stalled accretion shock turns into explosion when the neutrino luminosity reaches critical value (L_crit), the reason for existence of L_crit as well as its dependence on parameters of the problem are unknown. We find that there is a direct correspondence between the isothermal accretion flow bounded by a shock and L_crit. We provide an accurate condition for explosion. We find that the luminosity from the cooling of the flow is sub-dominant to the core luminosity for driving the explosion. We suggest that the reduction in L_crit as a function of dimension is due to less efficient cooling in higher dimensions.No embarg
Cool and Luminous Transients from Mass-Losing Binary Stars
We study transients produced by equatorial disk-like outflows from
catastrophically mass-losing binary stars with an asymptotic velocity and
energy deposition rate near the inner edge which are proportional to the binary
escape velocity v_esc. As a test case, we present the first smoothed-particle
radiation-hydrodynamics calculations of the mass loss from the outer Lagrange
point with realistic equation of state and opacities. The resulting spiral
stream becomes unbound for binary mass ratios 0.06 < q < 0.8. For synchronous
binaries with non-degenerate components, the spiral-stream arms merge at a
radius of ~10a, where a is the binary semi-major axis, and the accompanying
shock thermalizes about 10% of the kinetic power of the outflow. The
mass-losing binary outflows produce luminosities reaching up to ~10^6 L_Sun and
effective temperatures spanning 500 < T_eff < 6000 K, which is compatible with
many of the class of recently-discovered red transients such as V838 Mon and
V1309 Sco. Dust readily forms in the outflow, potentially in a catastrophic
global cooling transition. The appearance of the transient is viewing
angle-dependent due to vastly different optical depths parallel and
perpendicular to the binary plane. We predict a correlation between the peak
luminosity and the outflow velocity, which is roughly obeyed by the known red
transients. Outflows from mass-losing binaries can produce luminous (10^5
L_Sun) and cool (T_eff < 1500 K) transients lasting a year or longer, as has
potentially been detected by Spitzer surveys of nearby galaxies.Comment: Accepted to MNRAS, 24 pages, 24 figure
Binary Stellar Mergers with Marginally-Bound Ejecta: Excretion Disks, Inflated Envelopes, Outflows, and their Luminous Transients
We study mass loss from the outer Lagrange point (L2) in binary stellar
mergers and their luminous transients by means of radiative hydrodynamical
simulations. Previously, we showed that for binary mass ratios 0.06 < q < 0.8,
synchronous L2 mass loss results in a radiatively inefficient, dust-forming
unbound equatorial outflow. A similar outflow exists irrespective of q if the
ratio of the sound speed to the orbital speed at the injection point is
sufficiently large, \epsilon = c_T/v_orb > 0.15. By contrast, for cold L2
mass-loss (\epsilon 0.8, the
equatorial outflow instead remains marginally-bound and falls back to the
binary over tens to hundreds of binary orbits, where it experiences additional
tidal torqueing and shocking. As the bound gas becomes virialized with the
binary, the luminosity of the system increases slowly at approximately constant
photosphere radius, causing the temperature to rise. Subsequent evolution
depends on the efficiency of radiative cooling. If the bound atmosphere is able
to cool efficiently, as quantified by radiative diffusion time being shorter
than the advection time (t_diff/t_adv < 1), then the virialized gas collapses
to an excretion disk, while for t_diff/t_adv > 1 an isotropic wind is formed.
Between these two extremes, an inflated envelope transports the heat generated
near the binary to the surface by meridional flows. In all cases, the radiated
luminosity reaches a fraction ~0.01 to 0.1 of Mdot v_orb^2/2, where Mdot is the
mass outflow rate. We discuss the implications of our results for transients in
the luminosity gap between classical novae and supernovae, such as V1309 Sco
and V838 Mon.Comment: 14 pages, 13 figures, 1 table. Accepted to MNRAS. Movie version of
Figure 1 is available at http://www.astro.princeton.edu/~pejcha/ltwo
Pre-explosion spiral mass loss of a binary star merger
Binary stars commonly pass through phases of direct interaction which result
in the rapid loss of mass, energy, and angular momentum. Though crucial to
understanding the fates of these systems, including their potential as
gravitational wave sources, this short-lived phase is poorly understood and has
thus far been unambiguously observed in only a single event, V1309 Sco. Here we
show that the complex and previously-unexplained photometric behavior of V1309
Sco prior to its main outburst results naturally from the runaway loss of mass
and angular momentum from the outer Lagrange point, which lasts for thousands
of orbits prior to the final dynamical coalescence, much longer than predicted
by contemporary models. This process enshrouds the binary in a "death spiral"
outflow, which affects the amplitude and phase modulation of its light curve,
and contributes to driving the system together. The total amount of mass lost
during this gradual phase () rivals the mass lost during the
subsequent dynamical interaction phase, which has been the main focus of
"common envelope" modeling so far. Analogous features in related transients
suggest that this behavior is ubiquitous.Comment: 16 pages, 8 figures, 1 table. Accepted to ApJ. Accompanying videos
are available at
https://www.youtube.com/playlist?list=PLHhoUnCGbH6APNlIq8weLbURoytTjZez
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