Publications (9)34.52 Total impact

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ABSTRACT: Hydrogenrich Type IIPlateau supernovae are known to exhibit correlations between the plateau luminosity L_pl, the nickel mass M_Ni, the explosion energy E_exp, and the ejecta mass M_ej. Using our global, selfconsistent, multiband model of nearby wellobserved supernovae, we find that the uncertainty covariances of these quantities are significant 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 L_pl, E_exp and M_Ni, where the uncertainties due to the distance and the extinction dominate. For fixed E_exp, the spread in M_Ni is about 0.25 dex, which we explain as differences in the progenitor internal structure. We argue that the effects of incomplete gammaray trapping are not important in our sample. Similarly, the physics of the Type IIPlateau supernova light curves leads to inherently degenerate estimates of E_exp and M_ej, 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 and other parameters of the fitted relations. Our results imply that Type IIPlateau supernova explosions are not described by a single physical parameter or a simple onedimensional 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. 
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ABSTRACT: We present a new selfconsistent and versatile method that derives photospheric radius and temperature variations of Type IIPlateau supernovae based on their expansion velocities and photometric measurements. We apply the method to a sample of 26 wellobserved, 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. We show that the light curve differences among the Type IIPlateau supernovae are wellmodeled by assuming different rates of photospheric radius expansion, which we explain as differences in the density profiles of the ejecta between supernovae. We argue that the steep luminosity decline of Type IILinear 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 selfconsistent 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. 
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ABSTRACT: If the neutrino luminosity from the protoneutron star formed during a massive star core collapse exceeds a critical threshold, a supernova (SN) results. The normalization of this critical threshold and its dependencies on the parameters of the system remain uncertain. Using spherical quasistatic evolutionary sequences for many hundreds of progenitors over a wide range of metallicities, we study how the explosion threshold maps onto observables  (1) fraction of successful explosions, (2) remnant neutron star (NS) and black hole (BH) mass functions, (3) explosion energies (E_SN), (4) nickel yields (M_Ni)  and their mutual correlations. We show that the neutrino mechanism can in principle explain the observed properties of SNe and the compact objects they produce. Successful explosions are intertwined with failures in a complex but welldefined pattern that is not well described by the progenitor initial mass and is not simply related to compactness. In the context of singlestar progenitors, we show that the neutrino mechanism predicts that at solar metallicity, initial masses of 15+/1, 19+/1, and 2126 M_Sun are most likely to form BHs, that the BH formation probability is significantly higher for low metallicity progenitors, and that low luminosity, low M_Ni SNe come from progenitors close to success/failure interfaces. We qualitatively reproduce the correlation between E_SN and M_Ni and we predict a correlation between both the mean and width of the NS mass distribution and E_SN distribution. We use the observed properties of NSs, BHs, and SN explosions to study the likelihood of many parameterizations of the neutrino mechanism. We find a distinct region of high probability favoring the existence of failed supernovae. We argue that the rugged landscape of progenitor structures mandates performing internally consistent simulations for large sets of progenitors. (Abridged) 
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ABSTRACT: We study the orbital evolution of hierarchical quadruple systems composed of two binaries on a long mutual orbit, where each binary acts as a KozaiLidov (KL) perturber on the other. We find that the coupling between the two binaries qualitatively changes the behavior 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 to 20, about 30 to 50% of equalmass quadruples reach eccentricity 1e < 10^{3} in one of the binaries. This is about 4 to 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 nonnegligible 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 to 4 higher than for the corresponding triple system given our fiducial parameters with implications for hot Jupiters and starplanet collisions.Monthly Notices of the Royal Astronomical Society 04/2013; 435(2). DOI:10.1093/mnras/stt1281 · 5.23 Impact Factor 
Article: The observed neutron star mass distribution as a probe of the supernova explosion mechanism
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ABSTRACT: The observed distribution of neutron star (NS) masses reflects the physics of corecollapse supernova explosions and the structure of the massive stars that produce them at the end of their evolution. We present a Bayesian analysis that directly compares the NS mass distribution observed in double NS systems to theoretical models of NS formation. We find that models with standard binary mass ratio distributions are strongly preferred over independently picking the masses from the initial mass function, although the strength of the inference depends on whether current assumptions for identifying the remnants of the primary and secondary stars are correct. Second, NS formation models with no mass fallback are favored because they reduce the dispersion in NS masses. The double NS system masses thus directly point to the mass coordinate where the supernova explosion was initiated, making them an excellent probe of the supernova explosion mechanism. If we assume no fallback and simply vary the mass coordinate separating the remnant and the supernova ejecta, we find that for solar metallicity stars the explosion most likely develops at the edge of the iron core at a specific entropy of about 2.8 k_B. The primary limitations of our study are the poor knowledge of the supernova explosion mechanism and the lack of broad range of SN model explosions of LMC to solar metallicity.Monthly Notices of the Royal Astronomical Society 04/2012; 424(2). DOI:10.1111/j.13652966.2012.21369.x · 5.23 Impact Factor 
Article: A Global Physical Model for Cepheids
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ABSTRACT: We perform a global fit to ~5,000 radial velocity and ~177,000 magnitude measurements in 29 photometric bands covering 0.3 to 8.0 microns distributed among 287 Galactic, LMC, and SMC Cepheids with P > 10 days. We assume that the Cepheid light curves and radial velocities are fully characterized by distance, reddening, and timedependent radius and temperature variations. We construct phase curves of radius and temperature for periods between 10 and 100 days, which yield light curve templates for all our photometric bands and can be easily generalized to any additional band. With only 4 to 6 parameters per Cepheid, depending on the existence of velocity data and the amount of freedom in the distance, the models have typical rms light and velocity curve residuals of 0.05 mag and 3.5 km/s. The model derives the mean Cepheid spectral energy distribution and its derivative with respect to temperature, which deviate from a black body in agreement with metalline and molecular opacity effects. We determine a mean reddening law towards the Cepheids in our sample, which is not consistent with standard assumptions in either the optical or nearIR. Based on stellar atmosphere models we predict the biases in distance, reddening, and temperature determinations due to the metallicity and we quantify the metallicity signature expected for our fit residuals. The observed residuals as a function of wavelength show clear differences between the individual galaxies, which are compatible with these predictions. In particular, we find that metalpoor Cepheids appear hotter. Finally, we provide a framework for optimally selecting filters that yield the smallest overall errors in Cepheid parameter determination, or filter combinations for suppressing or enhancing the metallicity effects on distance determinations. We make our templates publicly available.The Astrophysical Journal 12/2011; 748(2). DOI:10.1088/0004637X/748/2/107 · 6.28 Impact Factor 
Article: Effect of Collective Neutrino Oscillations on the Neutrino Mechanism of CoreCollapse Supernovae
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ABSTRACT: In the seconds after collapse of a massive star, the newborn protoneutron star (PNS) radiates neutrinos of all flavors. The absorption of electrontype 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 electrontype 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 steadystate boundary value problem of sphericallysymmetric 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 corecollapse supernovae significantly.Monthly Notices of the Royal Astronomical Society 06/2011; 425(2). DOI:10.1111/j.13652966.2012.21443.x · 5.23 Impact Factor 
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ABSTRACT: (Abridged) Neutrino heating may drive corecollapse supernova explosions. Although it is known that the stalled accretion shock turns into explosion when the neutrino luminosity from the collapsed core exceeds a critical value (L_crit) (the "neutrino mechanism"), the physics of L_crit, as well as its dependence on the properties of the protoneutron star (PNS) and changes to the microphysics has never been systematically explored. We solve the onedimensional steadystate accretion problem between the PNS surface and the accretion shock. We quantify the deep connection between the solution space of steadystate accretion flows with bounding shocks and the neutrino mechanism. We show that there is a maximum, critical sound speed above which it is impossible to maintain accretion with a standoff shock, because the shock jump conditions cannot be satisfied. The physics of this critical sound speed is general and does not depend on a specific heating mechanism. For the simple model of pressureless freefall onto a shock bounding an isothermal accretion flow with sound speed c_T, we show that if c_T^2/v_escape^2 > 3/16 explosion results. We generalize this result to the more complete supernova problem, showing explicitly that the same physics determines L_crit. We find that the critical condition for explosion can be written as c_S^2/v_escape^2 = 0.19, where c_S is the adiabatic sound speed. This "antesonic" condition describes L_crit over a broad range in accretion rate and microphysics. We show that the addition of the accretion luminosity (L_acc) reduces L_crit nontrivially. As in previous work, we find that L_crit is always significantly higher than the maximum possible value of L_acc. Finally, we provide evidence that the reduction in L_crit seen in recent multidimensional simulations results from a reduction in the efficiency of cooling, rather than an increase in the heating rate.The Astrophysical Journal 03/2011; 746(1). DOI:10.1088/0004637X/746/1/106 · 6.28 Impact Factor 
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ABSTRACT: We explore the sensitivity of twopointmass gravitational microlensing to the extended nature of the source star, as well as the related sensitivity to its limb darkening. We demonstrate that the sensitive region, usually considered to be limited to a sourcediameterwide band along the caustic, is strongly expanded near cusps, most prominently along their outer axis. In the case of multicomponent caustics, facing cusps may form a region with a nonnegligible extendedsource effect spanning the gap between them. We demonstrate that for smaller sources the size of the sensitive region extending from a cusp measured in units of source radii increases, scaling as the inverse cube root of the radius. We study the extent of different sensitivity contours and show that for a microlensed Galactic bulge giant the probability of encountering at least a 1% extendedsource effect is higher than the probability of caustic crossing by 4060% when averaged over a typical range of lenscomponent separations, with the actual value depending on the mass ratio of the components. We derive analytical expressions for the extendedsource effect and chromaticity for a source positioned off the caustic. These formulae are more generally applicable to any gravitational lens with a sufficiently small source. Using exactly computed amplifications we test the often used linearfold caustic approximation and show that it may lead to errors on the level of a few percent even in nearideal causticcrossing events. Finally, we discuss several interesting cases of observed binary and planetary microlensing events and point out the importance of our results for the measurement of stellar limb darkening from microlensing light curves.The Astrophysical Journal 01/2008; DOI:10.1088/0004637X/690/2/1772 · 6.28 Impact Factor
Publication Stats
83  Citations  
34.52  Total Impact Points  
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2011

The Ohio State University
Columbus, Ohio, United States
