Engineering topics

Wheeler, J. C.

Publications and source records attributed to Wheeler, J. C..

At least 19 records

DOE OSTI2020

The shape of SN 1993J re-analysed

SN 1993J is one of the best-studied Type IIb supernovae. Spectropolarimetric data analyses were published over two decades ago at a time when the field of supernova spectropolarimetry was in its infancy. Here, we present a new analysis of the spectropolarimetric data of SN 1993J and an improved estimate of its interstellar polarization (ISP) as well as a critical review of ISP removal techniques employed in the field. The polarization of SN 1993J is found to show significant alignment on the q - u plane, suggesting the presence of a dominant axis and therefore of continuum polarization. We also see strong line polarization features, including H β, He i λ5876, H α, He i λ6678, He i λ7065, and high velocity (HV) components of He i λ5876 and H α. SN 1993J is therefore the second example of a stripped-envelope supernova, alongside iPTF13bvn, with prominent HV helium polarization features, and the first to show a likely HV H α contribution. Overall, we determine that the observed features can be interpreted as the superposition of anisotropically distributed line forming regions over ellipsoidal ejecta. We cannot exclude the possibility of an off-axis energy source within the ejecta. These data demonstrate the rich structures that are inaccessible if solely considering the flux spectra but can be probed by spectropolarimetric observations. In future studies, the new ISP corrected data can be used in conjunction with 3D radiative transfer models to better map the geometry of the ejecta of SN 1993J.

79 ASTRONOMY AND ASTROPHYSICS↗

NASA NTRS2016

Three-Dimensional Distribution of Ejecta in Supernova 1987A at 10,000 Days

Due to its proximity, SN 1987A offers a unique opportunity to directly observe the geometry of a stellar explosion as it unfolds. Here we present spectral and imaging observations of SN 1987A obtained approximately 10,000 days after the explosion with HST/STIS and VLT/SINFONI at optical and near-infrared wavelengths. These observations allow us to produce the most detailed 3D map of H(alpha) to date, the first 3D maps for [Ca II] lambda lambda 7292, 7324 [O I] lambda lambda 6300, 6364, and Mg II lambda lambda 9218, 9244, as well as new maps for [Si I] + [Fe II] 1.644 micrometers and He I 2.058 micrometers. A comparison with previous observations shows that the [Si I] + [Fe II] flux and morphology have not changed significantly during the past ten years, providing evidence that this line is powered by Ti-44. The time evolution of H(alpha) shows that it is predominantly powered by X-rays from the ring, in agreement with previous findings. All lines that have sufficient signal show a similar large-scale 3D structure, with a north-south asymmetry that resembles a broken dipole. This structure correlates with early observations of asymmetries, showing that there is a global asymmetry that extends from the inner core to the outer envelope. On smaller scales, the two brightest lines, Hα and [Si I]+[Fe II] 1.644 micrometers, show substructures at the level of approximately 200-1000 km s(exp -1) and clear differences in their 3D geometries. We discuss these results in the context of explosion models and the properties of dust in the ejecta.

Larsson, J.↗

NASA NTRS2014

A Wolf-Rayet-Like Progenitor of SN 2013cu from Spectral Observations of a Stellar Wind

The explosive fate of massive Wolf-Rayet stars (WRSs) is a key open question in stellar physics. An appealing option is that hydrogen- deficient WRSs are the progenitors of some hydrogen-poor supernova explosions of types IIb, Ib and Ic. A blue object, having luminosity and colours consistent with those of some WRSs, has recently been identified in pre-explosion images at the location of a supernova of type Ib, but has not yet been conclusively determined to have been the progenitor. Similar work has so far only resulted in non-detections. Comparison of early photometric observations of type Ic supernovae with theoretical models suggests that the progenitor stars had radii of less than 10(exp 12) centimetres, as expected for some WRSs. The signature of WRSs, their emission line spectra, cannot be probed by such studies. Here we report the detection of strong emission lines in a spectrum of type IIb supernova 2013cu (iPTF13ast) obtained approximately 15.5 hours after explosion (by 'flash spectroscopy', which captures the effects of the supernova explosion shock breakout flash on material surrounding the progenitor star).We identify Wolf-Rayet-like wind signatures, suggesting a progenitor of the WN(h) subclass (those WRSs with winds dominated by helium and nitrogen, with traces of hydrogen). The extent of this dense wind may indicate increased mass loss from the progenitor shortly before its explosion, consistent with recent theoretical predictions.

observations↗

NASA NTRS2013

A Panchromatic View of the Restless SN 2009ip Reveals the Explosive Ejection of a Massive Star Envelope

The double explosion of SN 2009ip in 2012 raises questions about our understanding of the late stages of massive star evolution. Here we present a comprehensive study of SN 2009ip during its remarkable rebrightenings. High-cadence photometric and spectroscopic observations from the GeV to the radio band obtained from a variety of ground-based and space facilities (including the Very Large Array, Swift, Fermi, Hubble Space Telescope, and XMM) constrain SN 2009ip to be a low energy (E approximating 10(exp 50) ergs for an ejecta mass approximating 0.5 M solar mass) and asymmetric explosion in a complex medium shaped by multiple eruptions of the restless progenitor star. Most of the energy is radiated as a result of the shock breaking out through a dense shell of material located at approximately 5 times 10 (exp 14) cm with M approximating 0.1 solar mass, ejected by the precursor outburst approximately 40 days before the major explosion. We interpret the NIR (Near Infrared) excess of emission as signature of material located further out, the origin of which has to be connected with documented mass-loss episodes in the previous years. Our modeling predicts bright neutrino emission associated with the shock break-out if the cosmic-ray energy is comparable to the radiated energy. We connect this phenomenology with the explosive ejection of the outer layers of the massive progenitor star, which later interacted with material deposited in the surroundings by previous eruptions. Future observations will reveal if the massive luminous progenitor star survived. Irrespective of whether the explosion was terminal, SN 2009ip brought to light the existence of new channels for sustained episodic mass loss, the physical origin of which has yet to be identified.

massive↗

NASA NTRS2003

Near-Infrared Spectra of Type Ia Supernovae

We report near-infrared (NIR) spectroscopic observations of 12 'branch-normal' Type Ia supernovae (SNe Ia) that cover the wavelength region from 0.8 to 2.5 microns. Our sample more than doubles the number of SNe Ia with published NIR spectra within 3 weeks of maximum light. The epochs of observation range from 13 days before maximum light to 18 days after maximum light. A detailed model for a Type Ia supernovae is used to identify spectral features. The Doppler shifts of lines are measured to obtain the velocity and thus the radial distribution of elements. The NIR is an extremely useful tool to probe the chemical structure in the layers of SNe Ia ejecta. This wavelength region is optimal for examining certain products of the SNe Ia explosion that may be blended or obscured in other spectral regions. We identify spectral features from Mg II, Ca II, Si II, Fe II, Co II, Ni II, and possibly Mn II. We find no indications for hydrogen, helium, or carbon in the spectra. The spectral features reveal important clues about the physical characteristics of SNe Ia. We use the features to derive upper limits for the amount of unburned matter, to identify the transition regions from explosive carbon to oxygen burning and from partial to complete silicon burning, and to estimate the level of mixing during and after the explosion. Elements synthesized in the outer layers during the explosion appear to remain in distinct layers. That provides strong evidence for the presence of a detonation phase during the explosion as it occurs in delayed detonation or merger models. Mg II velocities are found to exceed 11,000 - 15,000 km/s, depending on the individual SNe Ia. That result suggests that burning during the explosion reaches the outermost layers of the progenitor and limits the amount of unburned material to less than 10% of the mass of the progenitor. Small residuals of unburned material are predicted by delayed detonation models but are inconsistent with pure deflagration or merger models. Differences in the spectra of the individual SNe Ia demonstrate the variety of these events.

Marion, G. H.↗

NASA NTRS1997

Simplifying silicon burning: Application of quasi-equilibrium to (alpha) network nucleosynthesis

While the need for accurate calculation of nucleosynthesis and the resulting rate of thermonuclear energy release within hydrodynamic models of stars and supernovae is clear, the computational expense of these nucleosynthesis calculations often force a compromise in accuracy to reduce the computational cost. To redress this trade-off of accuracy for speed, the authors present an improved nuclear network which takes advantage of quasi- equilibrium in order to reduce the number of independent nuclei, and hence the computational cost of nucleosynthesis, without significant reduction in accuracy. In this paper they will discuss the first application of this method, the further reduction in size of the minimal alpha network. The resultant QSE- reduced alpha network is twice as fast as the conventional alpha network it replaces and requires the tracking of half as many abundance variables, while accurately estimating the rate of energy generation. Such reduction in cost is particularly necessary for future generation of multi-dimensional models for supernovae.

Hix, W. R.↗

NASA NTRS1995

Delayed detonation models for normal and subluminous type Ia sueprnovae: Absolute brightness, light curves, and molecule formation

We compute optical and infrared light curves of the pulsating class of delayed detonation models for Type Ia supernovae (SN Ia's) using an elaborate treatment of the Local Thermodynamic Equilbrium (LTE) radiation transport, equation of state and ionization balance, expansion opacity including the cooling by CO, Co(+), and SiO, and a Monte Carlo gamma-ray deposition scheme. The models have an amount of Ni-56 in the range from approximately or equal to 0.1 solar mass up to 0.7 solar mass depending on the density at which the transition from a deflagration to a detonation occurs. Models with a large nickel production give light curves comparable to those of typical Type Ia supernovae. Subluminous supernovae can be explained by models with a low nickel production. Multiband light curves are presented in comparison with the normally bright event SN 1992bc and the subluminous events Sn 1991bg and SN 1992bo to establish the principle that the delayed detonation paradigm in Chandrasekhar mass models may give a common explosion mechanism accounting for both normal and subluminous SN Ia's. Secondary IR-maxima are formed in the models of normal SN Ia's as a photospheric effect if the photospheric radius continues to increase well after maximum light. Secondary maxima appear later and stronger in models with moderate expansion velocities and with radioactive material closer to the surface. Model light curves for subluminous SN Ia's tend to show only one 'late' IR-maximum. In some delayed detonation models shell-like envelopes form, which consist of unburned carbon and oxygen. The formation of molecules in these envelopes is addressed. If the model retains a C/O-envelope and is subluminous, strong vibration bands of CO may appear, typically several weeks past maximum light. CO should be very weak or absent in normal Sn Ia's.

Hoflich, P.↗

NASA NTRS1995

Late-time spectra and type Ia supernova models: New clues from the Hubble Space Telescope

Calculated late-time spectra of two classical hydrodynamical models for Type Ia supernovae (deflagration model W7 of Nomoto, Thielemann, & Yokoi, and delayed detonation model DD4 of Woosley & Weaver) are compared with observations of SN 1992A and other spectroscopically normal SNe Ia. An important new piece of information is provided by observations done with the Hubble Space Telescope (HST) which cover the ultraviolet range at the nebular phase of a SN Ia: SN 1992A in NGC 1380. For the first time a picture of SN Ia emission from the ultraviolet through the optical is obtained at these phases. Predictions of the classical model (W7 and DD4) are compared with the observed spectrum of SN 1992A and with the optical spectra of SN 1989M in NGC 4579 and SN 1990N in NGC 4639 at similar epochs. The absolute B and V magnitudes of the models are also estimated at these late phases. Taken at face value the nebular spectra of these 'classical' models are more consistent with the long extragalactic distance scale, pointing to distances to NGC 4579 around 21 +/- 3 Mpc and a slightly larger distance, 22 +/- 3 Mpc, to NGC 4639, on the back side of the Virgo Cluster. However, the calculated Fe(+3) luminosity as predicted from the models exceeds the observed limit from the HST data of SN 1992A. Other differences in the ratios of the line intensities between calculated and observed spectra, show some disagreement with the observed spectra at the nebular phases. They may not be the best choice for spectroscopically normal SNe Ia, and their use as an independent calibration of the extragalactic distance scale should be viewed with caution.

Ruiz-Lapuente, P.↗

NASA NTRS1994

SN 1994I in M51 and the nature of type Ibc supernovae

Early spectra of SN 1994I in M51 (NGC 5194) are presented along with arguments that is a member of the class of helium-poor Type Ic supernovae. The issue of H and He in the spectra of Type Ib and Ic events is reexamined with the conclusion that Type Ib eject substantially less H than even transition events like SN 1993J and that Type Ic eject substantially less He than Type Ib and their optical spectra are consistent with no H or He. Type Ic show and absorption of the Si II lambda 6355 blend that characterizes Type Ia. This feature requires only a solar abundance of Si. Some Type Ic show an absorption that is probably C II lambda 6580. IR spectra are presented of SN 1990W which show the line of He I lambda 10830. The strength of this feature and the rather slow decay of the late time light curve suggest that this event could be better classified as a Type Ib. Even if trace abundances of H are present in Type Ib and of He in Type Ic the spectral differences still imply a distinctly different evolution for the progenitors of Type Ib from Type Ic and events like SN 1993J.

Wheeler, J. C.↗

NASA NTRS1994

Disk instability and the spectral evolution of the 1992 outburst of the intermediate polar GK Persei

The disk instability model can explain the previous history of dwarf-nova-like outbursts in the intermediate polar GK Per, which occur about once every three years. Disk models that reproduce the recurrence time and outburst light curves suggest that GK Per has a large effective inner disk radius (approx. 30-40 white dwarf radii) truncated by a strong magnetic field (10(exp 7) G). In this context, the effective radius is that of the portion of the disk that participates in the disk thermal instability. The radius derived is larger than the corotation radius, which must be an upper limit on the true dynamical inner radius of the disk. Disk instability models with this large effective inner radius predict that the ultraviolet continuum should be rather flat. Here we compare the predictions of the disk instability model to IUE observations of the 1981 outburst and to IUE and ROSAT observations of the recent 1992 outburst of GK Per. The model disk continuum spectral evolution is consistent with the observed UV and optical spectra, especially at maximum and in the early decay phase of the outburst. The consistency of the model with the observed UV spectra suggests that the effective inner radius of the disk is almost constant, independent of mass accretion rate, and that whatever structure lies between the effective inner radius and the corotation radius neither participates in the disk instability nor radiates substantially in the UV. The related physics of the inner disk region will be briefly discussed.

Kim, S.-W.↗

NASA NTRS1994

Disk irradiation and light curves of x ray novae

We study the disk instability and the effect of irradiation on outbursts in the black hole X-ray nova system. In both the optical and soft X-rays, the light curves of several X-ray novae, A0620-00, GH 2000+25, Nova Muscae 1991 (GS 1124-68), and GRO J0422+32, show a main peak, a phase of exponential decline, a secondary maximum or reflare, and a final bump in the late decay followed by a rapid decline. Basic disk thermal limit cycle instabilities can account for the rapid rise and overall decline, but not the reflare and final bump. The rise time of the reflare, about 10 days, is too short to represent a viscous time, so this event is unlikely to be due to increased mass flow from the companion star. We explore the possibility that irradiation by X-rays produced in the inner disk can produce these secondary effects by enhancing the mass flow rate within the disk. Two plausible mechanisms of irradiation of the disk are considered: direct irradiation from the inner hot disk and reflected radiation from a corona or other structure above the disk. Both of these processes will be time dependent in the context of the disk instability model and result in more complex time-dependent behavior of the disk structure. We test both disk instability and mass transfer burst models for the secondary flares in the presence of irradiation.

Kim, S.-W.↗

NASA NTRS1994

A constraint on the pair-density ratio (Z+) in an electron-positron pair wind

We derive a constraint on the pair density ratio, z(sub +) = n(sub +)/n(sub p), in an electron-positron pair wind flowing away from the central region of an accretion disk around a compact object under the assumption of a coupling between electrons, positrons, and protons. The minimum rate at which positrons are injected into the annihilation volume is given by the observed annihilation flux per unit volume. This rate is then used to determine a minimum mass loss rate per unit area, M(dot)(sub *) for a given pair density ratio at the base of the streamline. The requirement that M(dot)(sub *) less than M(dot)(sub *)(sub Edd) (the mean Eddington mass loss rate per unit area) then places a lower limit on the pair density ratio, z(sub +,)(sub min). A positron annihilation line was observed in Nova Muscae 1991 by GRANAT/SIGMA. The narrow width and redshift of the line suggest that the pair production and annihilation regions are physically distinct. We hypothesize that an electron-positron pair wind transports the pairs from the production to the annihilation region and calculate z(sub +),(sub min). We then determine constraints on the physical parameters on the pair production region by comparing z(sub +),(sub min) with previous studies of two-temperature and one-temperature accretion disks with electron-positron pairs.

Moscoso, M. D.↗

NASA NTRS1994

The physics of black hole x ray novae

X-ray transients that are established or plausible black hole candidates have been discovered at a rate of about one per year in the galaxy for the last five years. There are now well over a dozen black hole candidates, most being in the category of X-ray novae with low-mass companions. There may be hundreds of such transient systems in the galaxy yet to be discovered. Classic black hole candidates like Cygnus X-1 with massive companions are in the minority, and their census in the galaxy and magellanic clouds is likely to be complete. The black hole X-ray novae (BHXN) do not represent only the most common environment in which to discover black holes. Their time dependence gives a major new probe with which to study the physics of accretion into black holes. The BHXN show both a soft X-ray flux from an optically thick disk and a hard power law tail that is reminiscent of AGN spectra. The result may be new insight into the classical systems like Cyg X-1 and LMC X-1 that show similar power law tails, but also to accretion into supermassive black holes and AGN.

Wheeler, J. C.↗

NASA NTRS1993

Supernova 1993J as a spectroscopic link between type II and type Ib supernovae

Supernova 1993J in the nearby galaxy M81 is one of the closest - and hence brightest - supernovae to be witnessed this century. The early spectrum of SN1993J showed the characteristic hydrogen signature of type II supernovae, but its subsequent evolution is atypical for this class of supernova. Here we present optical and infrared spectra of SN1993J up to 43 days after outburst, which reveal the onset of the helium absorption and emission features more commonly associated with hydrogen-free type Ib supernovae. Corresponding model spectra show that the progenitor star must have possessed an unusually thin (for type II supernovae) hydrogen-rich envelope overlying a helium-rich mantle. Moreover, the supernova ejecta must have remained compositionally stratified, with little transport of the hydrogen-rich material down into the underlying helium layer or mixing of heavier elements outwards. SN1993J therefore represents a transition object between hydrogen-dominated type II supernovae, and hydrogen-free, helium-dominated type Ib supernovae.

Swartz, D. A.↗

NASA NTRS1993

Spectra of low-mass helium star models and the type Ic supernova SN 1987M

The question of whether the low-mass helium star models reproduce the helium-poor Type Ic spectrum of SN 1987M is investigated by comparing the theoretical spectrum from the 3.3-solar-mass model of Shigeyama et al. (1990) to observations of SN 1987M reported by Filippenko et al. (1990). The results indicate a number of positive correlations between the observed and the computed spectra. In both types of spectra, Ca II, O I, and Mg I emission lines dominate at late times, and, if extensive composition mixing is imposed, good fits can be made to the shapes of the emission features and good agreement can be found with the observed ratio of the forbidden and IR triplet blends of Ca II. The former indicates that the 3.3-solar-mass model predicts the proper density/velocity structure, and the latter indicates that the computed temperature structure is consistent with the observation though the lines are too strong relative to the continuum. This result is roughly independent of the He abundance. The computed and observed spectra are consistent with a small Ni-56 production in contrast to Type Ia thermonuclear events whose spectra are dominated by Fe-group lines at late times.

Swartz, Douglas A.↗

NASA NTRS1992

Explosion of a supernova with a red giant companion

Two-dimensional numerical simulations of the collision between spherical ejecta from a supernova and a red giant companion are presented. In contrast to previous numerical studies, in which the companion was a main-sequence star or a compact object, the collision consequences are found to have a dramatic impact upon the red giant. In most cases the red giant companion loses most of its envelope in a time scale of 10 exp 7 s with typical velocities about an order of magnitude less than those of the expanding velocity of the supernova shell. We confirm the conclusion of Chugai (1986) that the stripped hydrogen tends to come off as a low-velocity component interior to the supernova ejecta. Possible observational consequences of the results are discussed.

Livne, E.↗

NASA NTRS1992

Disk instability and the time-dependent X-ray emission from the intermediate polar GK Persei

The correlation between the disk instability model for the 1981-1989 optical outbursts of the intermediate polar GK Per and the accompanying X-ray emission is examined, and the self-consistency of the combined optical-X-ray model is investigated. Special attention is given to the nature of the transition in the X-ray emission due to the time-dependent accretion rates in the simple column accretion model. The large variation in the efficiency of hard X-ray production is explained.

Yi, Insu↗

NASA NTRS1992

Disk instability and outburst properties of the intermediate polar GK Persei

The present study reproduces the observed recurrence time scales and duration of the 1981-1989 dwarf nova-type outbursts of the intermediate polar GK Per with a model based on the time-dependent disk instability model. The model gives approximately symmetric (type B, inside-out) outbursts, as observed. This agreement suggests that the outbursts cannot result from a mass transfer instability. A modest variation of the mass transfer rate from the companion is sufficient to account for the variable outburst recurrence time scale. Contamination from the companion and the hot spot dominate the light from the disk except during a bright outburst. An intermediate temperature stagnation wave followed by a full heating wave is consistent with the observed rapid rise and double-peaked structure in the UV and optical light curves. It is argued that the hard X-ray efficiency is dime-dependent, and it is found to be much smaller in outburst than in quiescence.

Kim, Soon-Wook↗