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At least 1,243 records · Page 69
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DOE OSTI2012

INVERSE COMPTON X-RAY EMISSION FROM SUPERNOVAE WITH COMPACT PROGENITORS: APPLICATION TO SN2011fe

We present a generalized analytic formalism for the inverse Compton X-ray emission from hydrogen-poor supernovae and apply this framework to SN 2011fe using Swift X-Ray Telescope (XRT), UVOT, and Chandra observations. We characterize the optical properties of SN 2011fe in the Swift bands and find them to be broadly consistent with a 'normal' SN Ia, however, no X-ray source is detected by either XRT or Chandra. We constrain the progenitor system mass-loss rate M-dot < 2 x 10{sup -9} M{sub Sun }yr{sup -1} (3{sigma} c.l.) for wind velocity v{sub w} = 100 km s{sup -1}. Our result rules out symbiotic binary progenitors for SN 2011fe and argues against Roche lobe overflowing subgiants and main-sequence secondary stars if {approx}> 1% of the transferred mass is lost at the Lagrangian points. Regardless of the density profile, the X-ray non-detections are suggestive of a clean environment (n{sub CSM} < 150 cm{sup -3}) for 2 Multiplication-Sign 10{sup 15} {approx}< R {approx}< 5 Multiplication-Sign 10{sup 16} cm around the progenitor site. This is either consistent with the bulk of material being confined within the binary system or with a significant delay between mass loss and supernova explosion. We furthermore combine X-ray and radio limits from Chomiuk et al. to constrain the post-shock energy density in magnetic fields. Finally, we searched for the shock breakout pulse using gamma-ray observations from the Interplanetary Network and find no compelling evidence for a supernova-associated burst. Based on the compact radius of the progenitor star we estimate that the shock breakout pulse was likely not detectable by current satellites.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

THE ELM SURVEY. IV. 24 WHITE DWARF MERGER SYSTEMS

We present new radial velocity and X-ray observations of extremely low mass (ELM, {approx}0.2 M{sub Sun }) white dwarf (WD) candidates in the Sloan Digital Sky Survey Data Release 7 area. We identify seven new binary systems with 1-18 hr orbital periods. Five of the systems will merge due to gravitational wave radiation within 10 Gyr, bringing the total number of merger systems found in the ELM Survey to 24. The ELM Survey has now quintupled the known merger WD population. It has also discovered the eight shortest period detached binary WD systems currently known. We discuss the characteristics of the merger and non-merger systems observed in the ELM Survey, including their future evolution. About half of the systems have extreme mass ratios. These are the progenitors of the AM Canum Venaticorum systems and Type Ia supernovae. The remaining targets will lead to the formation of extreme helium stars, subdwarfs, or massive WDs. We identify three targets that are excellent gravitational wave sources. These should be detected by the Laser Interferometer Space Antenna like missions within the first year of operation. The remaining targets are important indicators of what the Galactic foreground may look like for gravitational wave observatories.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

IMPLODING IGNITION WAVES. I. ONE-DIMENSIONAL ANALYSIS

We show that converging spherical and cylindrical shock waves may ignite a detonation wave in a combustible medium, provided the radius at which the shocks become strong exceeds a critical radius, R{sub crit}. An approximate analytic expression for R{sub crit} is derived for an ideal gas equation of state and a simple (power-law-Arrhenius) reaction law, and shown to reproduce the results of numerical solutions. For typical acetylene-air experiments we find R{sub crit} {approx} 100 {mu}m (spherical) and R{sub crit} {approx} 1 mm (cylindrical). We suggest that the deflagration to detonation transition (DDT) observed in these systems may be due to converging shocks produced by the turbulent deflagration flow, which reaches sub- (but near) sonic velocities on scales >>R{sub crit}. Our suggested mechanism differs from that proposed by Zel'dovich et al., in which a fine-tuned spatial gradient in the chemical induction time is required to be maintained within the turbulent deflagration flow. Our analysis may be readily extended to more complicated equations of state and reaction laws. An order of magnitude estimate of R{sub crit} within a white dwarf at the pre-detonation conditions believed to lead to Type Ia supernova explosions is 0.1 km, suggesting that our proposed mechanism may be relevant for DDT initiation in these systems. The relevance of our proposed ignition mechanism to DDT initiation may be tested by both experiments and numerical simulations.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

RECONSTRUCTING THE HISTORY OF ENERGY CONDITION VIOLATION FROM OBSERVATIONAL DATA

We study the likelihood of energy condition violations in the history of the universe. Our method is based on a set of functions that characterize energy condition violation. Friedmann-Lemaitre-Robertson-Walker cosmological models are built around these 'indication functions'. By computing the Fisher matrix of model parameters using Type Ia supernova and Hubble parameter data, we extract the principal modes of these functions' redshift evolution. These modes allow us to obtain general reconstructions of energy condition violation history independent of the dark energy model. We find that the data suggest a history of strong energy condition violation, but the null and dominant energy conditions are likely to be fulfilled. Implications for dark energy models are discussed.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

TESTING FOR A LARGE LOCAL VOID BY INVESTIGATING THE NEAR-INFRARED GALAXY LUMINOSITY FUNCTION

Recent cosmological modeling efforts have shown that a local underdensity on scales of a few hundred Mpc (out to z {approx} 0.1) could produce the apparent acceleration of the expansion of the universe observed via Type Ia supernovae. Several studies of galaxy counts in the near-infrared (NIR) have found that the local universe appears underdense by {approx}25%-50% compared with regions a few hundred Mpc distant. Galaxy counts at low redshifts sample primarily L {approx} L* galaxies. Thus, if the local universe is underdense, then the normalization of the NIR galaxy luminosity function (LF) at z > 0.1 should be higher than that measured for z < 0.1. Here we present a highly complete (>90%) spectroscopic sample of 1436 galaxies selected in the H band (1.6 {mu}m) to study the normalization of the NIR LF at 0.1 < z < 0.3 and address the question of whether or not we reside in a large local underdensity. Our survey sample consists of all galaxies brighter than 18th magnitude in the H band drawn from six widely separated fields at high Galactic latitudes, which cover a total of {approx}2 deg{sup 2} on the sky. We find that for the combination of our six fields, the product {phi}*L* at 0.1 < z < 0.3 is {approx}30% higher than that measured at lower redshifts. While our statistical errors in this measurement are on the {approx}10% level, we find the systematics due to cosmic variance may be larger still. We investigate the effects of cosmic variance on our measurement using the COSMOS cone mock catalogs from the Millennium Simulation and recent empirical estimates of cosmic variance. We find that our survey is subject to systematic uncertainties due to cosmic variance at the 15% level (1{sigma}), representing an improvement by a factor of {approx}2 over previous studies in this redshift range. We conclude that observations cannot yet rule out the possibility that the local universe is underdense at z < 0.1. The fields studied in this work have a large amount of publicly available ancillary data and we make available the images and catalogs used here.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

LATERALLY PROPAGATING DETONATIONS IN THIN HELIUM LAYERS ON ACCRETING WHITE DWARFS

Theoretical work has shown that intermediate mass (0.01 M{sub Sun} < M{sub He} < 0.1 M{sub Sun }) helium shells will unstably ignite on the accreting white dwarf (WD) in an AM CVn binary. For more massive (M > 0.8 M{sub Sun }) WDs, these helium shells can be dense enough (>5 Multiplication-Sign 10{sup 5} g cm{sup -3}) that the convectively burning region runs away on a timescale comparable to the sound travel time across the shell, raising the possibility for an explosive outcome rather than an Eddington limited helium novae. The nature of the explosion (i.e., deflagration or detonation) remains ambiguous, is certainly density dependent, and likely breaks spherical symmetry. In the case of detonation, this causes a laterally propagating front whose properties in these geometrically thin and low-density shells we begin to study here. Our calculations show that the radial expansion time of <0.1 s leads to incomplete helium burning, in agreement with recent work by Sim and collaborators, but that the nuclear energy released is still adequate to realize a self-sustaining laterally propagating detonation. These detonations are slower than the Chapman-Jouguet speed of 1.5 Multiplication-Sign 10{sup 9} cm s{sup -1}, but still fast enough at 0.9 Multiplication-Sign 10{sup 9} cm s{sup -1} to go around the star prior to the transit through the star of the inwardly propagating weak shock. Our simulations resolve the subsonic region behind the reaction front in the detonation wave. The two-dimensional nucleosynthesis is shown to be consistent with a truncated one-dimensional Zeldovich-von Neumann-Doering calculation at the slower detonation speed. The ashes from the lateral detonation are typically He rich, and consist of predominantly {sup 44}Ti, {sup 48}Cr, along with a small amount of {sup 52}Fe, with very little {sup 56}Ni and with significant {sup 40}Ca in carbon-enriched layers. If this helium detonation results in a Type Ia supernova, its spectral signatures would appear for the first few days after explosion.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

EFFECTS OF ROTATIONALLY INDUCED MIXING IN COMPACT BINARY SYSTEMS WITH LOW-MASS SECONDARIES AND IN SINGLE SOLAR-TYPE STARS

Many population synthesis and stellar evolution studies have addressed the evolution of close binary systems in which the primary is a compact remnant and the secondary is filling its Roche lobe, thus triggering mass transfer. Although tidal locking is expected in such systems, most studies have neglected the rotationally induced mixing that may occur. Here we study the possible effects of mixing in mass-losing stars for a range of secondary star masses and metallicities. We find that tidal locking can induce rotational mixing prior to contact and thus affect the evolution of the secondary star if the effects of the Spruit-Tayler dynamo are included both for angular momentum and chemical transport. Once contact is made, the effect of mass transfer tends to be more rapid than the evolutionary timescale, so the effects of mixing are no longer directly important, but the mass-transfer strips matter to inner layers that may have been affected by the mixing. These effects are enhanced for secondaries of 1-1.2 M{sub Sun} and for lower metallicities. We discuss the possible implications for the paucity of carbon in the secondaries of the cataclysmic variable SS Cyg and the black hole candidate XTE J1118+480 and for the progenitor evolution of Type Ia supernovae. We also address the issue of the origin of blue straggler stars in globular and open clusters. We find that for models that include rotation consistent with that observed for some blue straggler stars, evolution is chemically homogeneous. This leads to tracks in the H-R diagram that are brighter and bluer than the non-rotating main-sequence turn-off point. Rotational mixing could thus be one of the factors that contribute to the formation of blue stragglers.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

A GRAVITATIONAL REDSHIFT DETERMINATION OF THE MEAN MASS OF WHITE DWARFS: DBA AND DB STARS

We measure apparent velocities (v{sub app}) of absorption lines for 36 white dwarfs (WDs) with helium-dominated atmospheres-16 DBAs and 20 DBs-using optical spectra taken for the European Southern Observatory SN Ia progenitor survey. We find a difference of 6.9 {+-} 6.9 km s{sup -1} in the average apparent velocity of the H{alpha} lines versus that of the He I 5876 A lines for our DBAs. This is a measure of the blueshift of this He line due to pressure effects. By using this as a correction, we extend the gravitational redshift method employed by Falcon et al. to use the apparent velocity of the He I 5876 A line and conduct the first gravitational redshift investigation of a group of WDs without visible hydrogen lines. We use biweight estimators to find an average apparent velocity, (v{sub app}){sub BI}, (and hence average gravitational redshift, (v{sub g}){sub BI}) for our WDs; from that we derive an average mass, (M){sub BI}. For the DBAs, we find (v{sub app}){sub BI} = 40.8 {+-} 4.7 km s{sup -1} and derive (M){sub BI} = 0.71{sup +0.04}{sub -0.05} M{sub Sun }. Though different from (v{sub app}) of DAs (32.57 km s{sup -1}) at the 91% confidence level and suggestive of a larger DBA mean mass than that for normal DAs derived using the same method (0.647{sup +0.013}{sub -0.014} M{sub Sun }; Falcon et al.), we do not claim this as a stringent detection. Rather, we emphasize that the difference between (v{sub app}){sub BI} of the DBAs and (v{sub app}) of normal DAs is no larger than 9.2 km s{sup -1}, at the 95% confidence level; this corresponds to roughly 0.10 M{sub Sun }. For the DBs, we find (v {sup He}{sub app}){sub BI} = 42.9 {+-} 8.49 km s{sup -1} after applying the blueshift correction and determine (M){sub BI} = 0.74{sup +0.08}{sub -0.09} M{sub Sun }. The difference between (v{sup He}{sub app}){sub BI} of the DBs and (v{sub app}) of DAs is {<=}11.5 km s{sup -1} ({approx}0.12 M{sub Sun }), at the 95% confidence level. The gravitational redshift method indicates much larger mean masses than the spectroscopic determinations of the same sample by Voss et al. Given the small sample sizes, it is possible that systematic uncertainties are skewing our results due to the potential of kinematic substructures that may not average out. We estimate this to be unlikely, but a larger sample size is necessary to rule out these systematics.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

AN IN-DEPTH STUDY OF THE ABUNDANCE PATTERN IN THE HOT INTERSTELLAR MEDIUM IN NGC 4649

We present our X-ray imaging spectroscopic analysis of data from deep Suzaku and XMM-Newton Observatory exposures of the Virgo Cluster elliptical galaxy NGC 4649 (M60), focusing on the abundance pattern in the hot interstellar medium (ISM). All measured elements show a radial decline in abundance, with the possible exception of O. We construct steady-state solutions to the chemical evolution equations that include infall in addition to stellar mass return and Type Ia supernova (SNIa) enrichment, and consider recently published SNIa yields. By adjusting a single model parameter to obtain a match to the global abundance pattern in NGC 4649, we infer that introduction of subsolar metallicity external gas has reduced the overall ISM metallicity and diluted the effectiveness of SNIa to skew the pattern toward low {alpha}/Fe ratios, and estimate the combination of SNIa rate and level of dilution. Evidently, newly introduced gas is heated as it is integrated into, and interacts with, the hot gas that is already present. These results indicate a complex flow and enrichment history for NGC 4649, reflecting the continual evolution of elliptical galaxies beyond the formation epoch. The heating and circulation of accreted gas may help reconcile this dynamic history with the mostly passive evolution of elliptical stellar populations. In an Appendix, we examine the effects of the recent updated atomic database AtomDB in spectral fitting of thermal plasmas with hot ISM temperatures in the elliptical galaxy range.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

CARNEGIE HUBBLE PROGRAM: A MID-INFRARED CALIBRATION OF THE HUBBLE CONSTANT

Using a mid-infrared calibration of the Cepheid distance scale based on recent observations at 3.6 {mu}m with the Spitzer Space Telescope, we have obtained a new, high-accuracy calibration of the Hubble constant. We have established the mid-IR zero point of the Leavitt law (the Cepheid period-luminosity relation) using time-averaged 3.6 {mu}m data for 10 high-metallicity, Milky Way Cepheids having independently measured trigonometric parallaxes. We have adopted the slope of the PL relation using time-averaged 3.6 {mu}m data for 80 long-period Large Magellanic Cloud (LMC) Cepheids falling in the period range 0.8 < log(P) < 1.8. We find a new reddening-corrected distance to the LMC of 18.477 {+-} 0.033 (systematic) mag. We re-examine the systematic uncertainties in H{sub 0}, also taking into account new data over the past decade. In combination with the new Spitzer calibration, the systematic uncertainty in H{sub 0} over that obtained by the Hubble Space Telescope Key Project has decreased by over a factor of three. Applying the Spitzer calibration to the Key Project sample, we find a value of H{sub 0} = 74.3 with a systematic uncertainty of {+-}2.1 (systematic) km s{sup -1} Mpc{sup -1}, corresponding to a 2.8% systematic uncertainty in the Hubble constant. This result, in combination with WMAP7 measurements of the cosmic microwave background anisotropies and assuming a flat universe, yields a value of the equation of state for dark energy, w{sub 0} = -1.09 {+-} 0.10. Alternatively, relaxing the constraints on flatness and the numbers of relativistic species, and combining our results with those of WMAP7, Type Ia supernovae and baryon acoustic oscillations yield w{sub 0} = -1.08 {+-} 0.10 and a value of N{sub eff} = 4.13 {+-} 0.67, mildly consistent with the existence of a fourth neutrino species.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

AGN ACTIVITY AND THE MISALIGNED HOT ISM IN THE COMPACT RADIO ELLIPTICAL NGC 4278

The analysis of a deep (579 ks) Chandra ACIS pointing of the elliptical galaxy NGC 4278, which hosts a low-luminosity active galactic nucleus (AGN) and compact radio emission, allowed us to detect extended emission from hot gas out to a radius of {approx}5 kpc, with 0.5-8 keV luminosity of 2.4 Multiplication-Sign 10{sup 39} erg s{sup -1}. The emission is elongated in the NE-SW direction, misaligned with respect to the stellar body, and aligned with the ionized gas and with the Spitzer IRAC 8 {mu}m non-stellar emission. The nuclear X-ray luminosity decreased by a factor of {approx}18 since the first Chandra observation in 2005, a dimming that enabled the detection of hot gas even at the position of the nucleus. The gas shows a significantly larger temperature (kT = 0.75 keV) in both the projected and deprojected profiles in the inner {approx}300 pc than in the surrounding region, where it stays at {approx}0.3 keV, a value lower than expected from standard gas heating assumptions. The nuclear X-ray emission is consistent with that of a low radiative efficiency accretion flow, accreting mass at a rate close to the Bondi rate; estimates of the power of the nuclear jets require that the accretion rate is not largely reduced with respect to the Bondi rate. Among possible origins for the central large hot gas temperature, such as gravitational heating from the central massive black hole and a recent AGN outburst, interaction with the nuclear jets seems more likely, especially if the latter remain confined, and heat the nuclear region frequently. The unusual hot gas distribution on the galactic scale could be due to the accreting cold gas triggering the cooling of the hot phase, a process also contributing to the observed line emission from ionized gas, and to the hot gas temperature being lower than expected; alternatively, the latter could be due to the efficiency of the Type Ia supernova heating that is lower than usually adopted.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

SUPERNOVA REMNANT PROGENITOR MASSES IN M31

Using Hubble Space Telescope photometry, we age-date 59 supernova remnants (SNRs) in the spiral galaxy M31 and use these ages to estimate zero-age main-sequence masses (M{sub ZAMS}) for their progenitors. To accomplish this, we create color-magnitude diagrams (CMDs) and employ CMD fitting to measure the recent star formation history of the regions surrounding cataloged SNR sites. We identify any young coeval population that likely produced the progenitor star, then assign an age and uncertainty to that population. Application of stellar evolution models allows us to infer the M{sub ZAMS} from this age. Because our technique is not contingent on identification or precise location of the progenitor star, it can be applied to the location of any known SNRs. We identify significant young star formation around 53 of the 59 SNRs and assign progenitor masses to these, representing a factor of {approx}2 increase over currently measured progenitor masses. We consider the remaining six SNRs as either probable Type Ia candidates or the result of core-collapse progenitors that have escaped their birth sites. In general, the distribution of recovered progenitor masses is bottom-heavy, showing a paucity of the most massive stars. If we assume a single power-law distribution, dN/dM{proportional_to}M{sup {alpha}}, then we find a distribution that is steeper than a Salpeter initial mass function (IMF) ({alpha} = -2.35). In particular, we find values of {alpha} outside the range -2.7 {>=} {alpha} {>=} -4.4 to be inconsistent with our measured distribution at 95% confidence. If instead we assume a distribution that follows a Salpeter IMF up to some maximum mass, then we find that values of M{sub Max} > 26 are inconsistent with the measured distribution at 95% confidence. In either scenario, the data suggest that some fraction of massive stars may not explode. The result is preliminary and requires more SNRs and further analysis. In addition, we use our distribution to estimate a minimum mass for core collapse between 7.0 and 7.8 M{sub Sun }.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

CIRCUMSTELLAR SHELL FORMATION IN SYMBIOTIC RECURRENT NOVAE

We present models of spherically symmetric recurrent nova shells interacting with circumstellar material (CSM) in a symbiotic system composed of a red giant (RG) expelling a wind and a white dwarf accreting from this material. Recurrent nova eruptions periodically eject material at high velocities ({approx}> 10{sup 3} km s{sup -1}) into the RG wind profile, creating a decelerating shock wave as CSM is swept up. High CSM densities cause the shocked wind and ejecta to have very short cooling times of days to weeks. Thus, the late-time evolution of the shell is determined by momentum conservation instead of energy conservation. We compute and show evolutionary tracks of shell deceleration, as well as post-shock structure. After sweeping up all the RG wind, the shell coasts at a velocity {approx}100 km s{sup -1}, depending on system parameters. These velocities are similar to those measured in blueshifted CSM from the symbiotic nova RS Oph, as well as a few Type Ia supernovae that show evidence of CSM, such as 2006X, 2007le, and PTF 11kx. Supernovae occurring in such systems may not show CSM interaction until the inner nova shell gets hit by the supernova ejecta, days to months after the explosion.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

THE NATURE OF GAMMA-RAY EMISSION OF TYCHO'S SUPERNOVA REMNANT

The nature of the recently detected high-energy and very high-energy {gamma}-ray emission of Tycho's supernova remnant (SNR) is studied. A nonlinear kinetic theory of cosmic-ray (CR) acceleration in SNRs is employed to investigate the properties of Tycho's SNR and their correspondence with the existing experimental data, taking into account that the ambient interstellar medium (ISM) is expected to be clumpy. It is demonstrated that the overall steep {gamma}-ray spectrum observed can be interpreted as the superposition of two spectra produced by the CR proton component in two different ISM phases: the first {gamma}-ray component, extending up to about 10{sup 14} eV, originates in the diluted warm ISM, whereas the second component, extending up to 100 GeV, comes from numerous dense, small-scale clouds embedded in this warm ISM. Given the consistency between acceleration theory and the observed properties of the nonthermal emission of Tycho's SNR, very efficient production of nuclear CRs in Tycho's SNR is established. The excess of the GeV {gamma}-ray emission due to the clouds' contribution above the level expected in the case of a purely homogeneous ISM is inevitably expected in the case of Type Ia SNe.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

NUCLEAR DOMINATED ACCRETION FLOWS IN TWO DIMENSIONS. I. TORUS EVOLUTION WITH PARAMETRIC MICROPHYSICS

We explore the evolution of radiatively inefficient accretion disks in which nuclear reactions are dynamically important ('Nuclear Dominated Accretion Flows' or NuDAFs). Examples of such disks are those generated by the merger of a white dwarf with a neutron star or black hole, or by the collapse of a rotating star. Here, we present two-dimensional hydrodynamic simulations that systematically explore the effect of adding a single nuclear reaction to a viscous torus. The equation of state, anomalous shear stress, and nuclear reactions are given parametric forms. Our results point to the existence of two qualitatively different regimes of NuDAF evolution: (1) steady accretion with quiescent burning or (2) detonation of the disk. These outcomes are controlled primarily by the ratio {Psi} of the nuclear energy released to the enthalpy at the burning radius. Disks detonate if {Psi} exceeds a critical value {Psi}{sub crit} {approx} 1, and if burning occurs in regions where neutrino cooling is unimportant. Thermonuclear runaways are seeded by the turbulent mixing of hot ash with cold fuel at the burning front. Disks with {Psi} < {Psi}{sub crit} do not explode, but instead power a persistent collimated outflow of unbound material composed primarily of ash, with a mass-loss rate that increases with {Psi}. We discuss the implications of our results for supernova-like counterparts from astrophysical events in the NuDAF regime. In particular, detonations following a white dwarf-neutron star merger could account for some subluminous Type Ia supernovae, such as the class defined by SN 2002cx.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

THE CURIOUS CASE OF GLASS I: HIGH IONIZATION AND VARIABILITY OF DIFFERENT TYPES

Our Spitzer IRS observation of the infrared companion Glass Ib revealed fine-structure emission with high ionization ([Ne III]/[Ne II] = 2.1 and [S IV]/[S III] = 0.6) that indicates that the gas is likely illuminated by hard radiation. While models suggest that extreme-ultraviolet radiation could be present in T Tauri stars, this is the first detection of [S IV] and such a high [Ne III]/[Ne II] ratio in a young star. We also find that Glass Ib displays the molecules HCN, CO{sub 2}, and H{sub 2}O in emission. Here we investigate the Glass I binary system and consider possible mechanisms that may have caused the high ionization, whether from an outflow or disk irradiation. We also model the spectral energy distributions of Glass Ia and Ib to test if the system is a young member of the Chameleon I star-forming region, and we consider other possible classifications for the system. We find that Glass Ib is highly variable, showing changes in continuum strength and emission features at optical, near-infrared, and mid-infrared wavelengths. The optical light curve indicates that a central stellar component in Glass Ib became entirely visible for 2.5 years beginning in mid-2002 and possibly displayed periodic variability with repeated, short-period dimming during that time. As the fine-structure emission was not detected in observations before or after our Spitzer IRS observation, we explore whether the variable nature of Glass Ib is related to the gas being highly ionized, possibly due to variable accretion or an X-ray flare.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

HUBBLE PARAMETER MEASUREMENT CONSTRAINTS ON DARK ENERGY

We use 21 Hubble parameter versus redshift data points from Simon et al., Gaztanaga et al., Stern et al., and Moresco et al. to place constraints on model parameters of constant and time-evolving dark energy cosmologies. The inclusion of the eight new measurements results in H(z) constraints more restrictive than those derived by Chen and Ratra. These constraints are now almost as restrictive as those that follow from current Type Ia supernova (SNIa) apparent magnitude versus redshift data, which now more carefully account for systematic uncertainties. This is a remarkable result. We emphasize, however, that SNIa data have been studied for a longer time than the H(z) data, possibly resulting in a better estimate of potential systematic errors in the SNIa case. A joint analysis of the H(z), baryon acoustic oscillation peak length scale, and SNIa data favors a spatially flat cosmological model currently dominated by a time-independent cosmological constant but does not exclude slowly evolving dark energy.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

WHAT CAN WE LEARN FROM THE RISING LIGHT CURVES OF RADIOACTIVELY POWERED SUPERNOVAE?

The light curve of the explosion of a star with a radius {approx}< 10-100 R{sub Sun} is powered mostly by radioactive decay. Observationally, such events are dominated by hydrogen-deficient progenitors and classified as Type I supernovae (SNe I), i.e., white dwarf thermonuclear explosions (Type Ia), and core collapses of hydrogen-stripped massive stars (Type Ib/c). Current transient surveys are finding SNe I in increasing numbers and at earlier times, allowing their early emission to be studied in unprecedented detail. Motivated by these developments, we summarize the physics that produces their rising light curves and discuss ways in which observations can be utilized to study these exploding stars. The early radioactive-powered light curves probe the shallowest deposits of {sup 56}Ni. If the amount of {sup 56}Ni mixing in the outermost layers of the star can be deduced, then it places important constraints on the progenitor and properties of the explosive burning. In practice, we find that it is difficult to determine the level of mixing because it is hard to disentangle whether the explosion occurred recently and one is seeing radioactive heating near the surface or whether the explosion began in the past and the radioactive heating is deeper in the ejecta. In the latter case, there is a ''dark phase'' between the moment of explosion and the first observed light emitted once the shallowest layers of {sup 56}Ni are exposed. Because of this, simply extrapolating a light curve from radioactive heating back in time is not a reliable method for estimating the explosion time. The best solution is to directly identify the moment of explosion, either through observing shock breakout (in X-ray/UV) or the cooling of the shock-heated surface (in UV/optical), so that the depth being probed by the rising light curve is known. However, since this is typically not available, we identify and discuss a number of other diagnostics that are helpful for deciphering how recently an explosion occurred. As an example, we apply these arguments to the recent SN Ic PTF 10vgv. We demonstrate that just a single measurement of the photospheric velocity and temperature during the rise places interesting constraints on its explosion time, radius, and level of {sup 56}Ni mixing.

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