arXiv subjects

Roger A. Chevalier

Publications and source records attributed to Roger A. Chevalier.

At least 91 records · Page 5Linked to original sources

arXiv2001

Optical and Ultraviolet Spectroscopy of SN 1995N: Evidence for Strong Circumstellar Interaction

Optical and ultraviolet observations of the Type IIn supernova 1995N at epochs between 321 and 1799 days after the explosion show three distinct velocity components. The narrow lines come from circumstellar gas and show both low and high ionization. This component has a low filling factor, and is photoionized by X-rays from the shock. The intermediate component, which is dominated by newly processed oxygen, originates in a shell with velocity of 2500-5000 km/s, and most likely comes from the ejecta. The hydrogen- and helium-dominated gas has a low ionization, a high density, and velocities that extend out to ~10,000 km/s. Strong signatures of Ly-alpha-pumped fluorescence lines of Fe II are seen in the near-infrared and ultraviolet. The He/H ratio, ~ 0.3 by number, and the nitrogen overabundance provide strong evidence for CNO burning products. The fluxes of the broad hydrogen and helium lines decrease considerably faster than the oxygen lines. The H-alpha line profile shows strong evolution, with the red wing decreasing faster than the blue. Possible scenarios, involving either a clumpy circumstellar medium, or an aspherical distribution of the surrounding gas, are discussed based on the line profiles and physical conditions. Finally, we propose that Type IIn supernovae have their origin in red supergiants in a superwind phase.

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arXiv2001

Interacting Stellar Wind and Photoionization Models of the SN 1987A Remnant

We are investigating the SN 1987A remnant by modeling the circumstellar environment of the progenitor star. Interacting stellar winds models have been reasonably successful at reproducing the gross features of the nebula, but some details, such as the early rise of the radio and x-ray emission from the supernova blast and the presence of the outer rings, are not explained in a pure wind model. In this paper we describe preliminary results from 2-D models that incorporate the effects of photoionization due to radiation from the central star. These models have successfully produced a thick HII region, as needed to produce the early radio and X-ray emission. The HII region is thickest away from the equatorial plane. The models have also produced a feature resembling the outer rings, but we suspect this to be an artifact of the 2-D calculations that would not persist in 3-D.

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arXiv2001

The Nature of the Compact Supernova Remnants in Starburst Galaxies

Radio observations of starburst regions in galaxies have revealed groups of compact nonthermal sources that may be radiative supernova remnants expanding in the interclump medium of molecular clouds. Because of the high pressure in starburst regions, the interclump medium may have a density ~ 10^3 H atoms cm^{-3} in a starburst nucleus like M82 and ~ 10^4 H atoms cm^{-3} in an ultraluminous galaxy like Arp 220. In M82, our model can account for the sizes, the slow evolution, the high radio luminosities, and the low X-ray luminosities of the sources. We predict expansion velocities ~ 500 km/s, which is slower than the one case measured by VLBI techniques. Although we predict the remnants to be radiative, the expected radiation is difficult to detect because it is at infrared wavelengths and the starburst is itself very luminous; one detection possibility is broad [OI] 63 micron line emission at the positions of the radio remnants. The more luminous and compact remnants in Arp 220 can be accounted for by the higher molecular cloud density. In our model, the observed remnants lose most of the supernova energy to radiation. Other explosions in a lower density medium may directly heat a hot, low density interstellar component, leading to the superwinds that are associated with starburst regions.

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arXiv2001

Pulsar Wind Nebulae in Evolved Supernova Remnants

For pulsars similar to the one in the Crab Nebula, most of the energy input to the surrounding wind nebula occurs on a timescale of less than 1000 years; during this time, the nebula expands into freely expanding supernova ejecta. On a timescale 10,000 years, the interaction of the supernova with the surrounding medium drives a reverse shock front toward the center of the remnant, where it crushes the PWN (pulsar wind nebula). One- and two-dimensional, two-fluid simulations of the crushing and re-expansion phases of a PWN show that (1) these phases are subject to Rayleigh-Taylor instabilities that result in the mixing of thermal and nonthermal fluids, and (2) asymmetries in the surrounding interstellar medium give rise to asymmetries in the position of the PWN relative to the pulsar and explosion site. These effects are expected to be observable in the radio emission from evolved PWN because of the long lifetimes of radio emitting electrons. The scenario can explain the chaotic and asymmetric appearance of the Vela X PWN relative to the Vela pulsar without recourse to a directed flow from the vicinity of the pulsar. The displacement of the radio nebulae in G327.1--1.1, MSH15--56 (G326.3--1.8), G0.9+0.1, and W44 relative to the X-ray nebulae may be due to this mechanism. On timescales much greater than the nebular crushing time, the initial PWN may be mixed with thermal gas and become unobservable, so that even the radio emission is dominated by recently injected particles.

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arXiv2001

Theoretical Implications of the Gamma-Ray Burst - Supernova Connection

Although observations of circumstellar shock interactions around supernovae are generally consistent with a 1/r**2 density profile wind surrounding the progenitor star, this is not true for GRB (gamma-ray burst) afterglows. However, GRB 991208 and GRB 000301C may be consistent with wind interaction if the injection particle spectrum is a broken power law. Circumstellar dust echos can place constraints on supernova and GRB progenitors, but have been clearly observed only around SN 1987A. Excess emission observed in two GRB afterglows is more likely to have a supernova origin. An interstellar dust echo, causing the light curve to flatten out, is a possibility for GRB afterglows, but is not likely to be observable.

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arXiv2001

Supernovae in Molecular Clouds

Supernovae are expected to occur near the molecular material in which the massive progenitor star was born, except in cases where the photoionizing radiation and winds from the progenitor star and its neighbors have cleared out a region. The clumpy structure in molecular clouds is crucial for the remnant evolution; the supernova shock front can become radiative in the interclump medium and the radiative shell then collides with molecular clumps. The interaction is relevant to a number of phenomena: the hydrodynamics of a magnetically supported dense shell interacting with molecular clumps; the molecular emission from shock waves, including the production of the OH 1720 MHz maser line; the relativistic particle emission, including radio synchrotron and gamma-ray emission, from the dense radiative shell; and the possible gravitational instability of a compressed clump.

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arXiv2000

Wind Interaction Models for the Afterglows of GRB 991208 and GRB 000301C

The simplest model of the afterglows of the gamma-ray bursts (GRBs) envisions a spherical blast wave with a power-law distribution of electron energy above some cutoff running into a constant density medium. A refinement involves a narrow jet, often invoked to explain the steep decline and/or steepening of light curves observed in some afterglows. The constant (ambient) density jet model has been applied to GRBs 991208 and 000301C, based to a large extent on radio observations. We show that, for these two sources, a spherical wind model (with an $r^{-2}$ density ambient medium) can fit the radio data as well as the jet model. %%We demonstrate that the The relatively steep decline and the fairly abrupt steepening of the R-band light curves of, respectively, GRB 991208 and GRB 000301C can be accounted for with a non-standard, broken power-law distribution of electron energy. Our model predicts a slower late decline for the radio flux than does the jet model.

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arXiv2000

The X-ray Remnant of SN1987A

We present high resolution Chandra observations of the remnant of SN1987A in the Large Magellanic Cloud. The high angular resolution of the Chandra X-ray Observatory (CXO) permits us to resolve the X-ray remnant. We find that the remnant is shell-like in morphology, with X-ray peaks associated with some of the optical hot spots seen in HST images. The X-ray light curve has departed from the linear flux increase observed by ROSAT, with a 0.5-2.0 keV luminosity of 1.5 x 10^35 erg/s in January 2000. We set an upper limit of 2.3 x 10^34 ergs/s on the luminosity of any embedded central source (0.5 - 2 keV). We also present a high resolution spectrum, showing that the X-ray emission is thermal in origin and is dominated by highly ionized species of O, Ne, Mg, and Si.

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arXiv2000

Implications of the X-Ray Properties of Pulsar Nebulae

A plausible model for the Crab Nebula is one in which a particle dominated, highly relativistic wind from the pulsar passes through a shock front in which the particles attain a power law energy distribution. The electrons and positrons lose energy radiating synchrotron emission. Here, a one zone version of the model is developed and applied to observations of X-ray pulsar nebulae. Efficient conversion of pulsar spin-down power to X-ray luminosity is expected if the observed electrons are in the synchrotron cooling regime and their energy spectrum is similar to that in the Crab Nebula. In this case, the relation between X-ray luminosity and pulsar spin-down power depends only weakly on the model parameters. The dependence is stronger for a steeper particle spectrum, as appears to be present in N157B, and the efficiency of X-ray production can be lower. If the electrons are not in the cooling regime, the X-ray luminosity can be low, as appears to be the case for the compact nebula around the Vela pulsar. Slow cooling is likely to be caused by a sub-equipartition magnetic field in the radiating region. Observations can place constraints on the uncertain physics of relativistic MHD shocks. The model is related to those developed for gamma-ray burst afterglows.

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arXiv2000

Young Circumstellar Disks Near Evolved Massive Stars and Supernovae

There is increasing evidence that low mass stars with circumstellar disks can be born close to massive stars, in some cases within tenths of a pc. If the disks have lifetimes greater than those of the more massive stars, they are exposed to the radiation fields and gas flows from the late evolutionary phases and supernovae of the massive stars. The fast flows from supernovae are likely to give some stripping of mass from the disks, but do not typically lead to complete disruption of the disks. In the slow wind from a red supergiant star, there is the possibility of gravitational accretion of wind matter onto the circumstellar disk. In the supernova explosion of a red supergiant, the radiative flux at the time of shock breakout can heat and ionize a nearby disk, leading to transient, narrow line emission. There are consequences for the solar nebula if it was born ~0.2 pc from a massive star that became a red supergiant. Accretion from the wind could supply a substantial amount of 26Al to the disk. The high radiative luminosity of the eventual supernova could lead to the melting of dust grains and the formation of chondrules. The passage of the supernova ejecta could drive a shock wave in the disk, heating it.

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arXiv2000

Instabilities and Clumping in Type Ia Supernova Remnants

We present two-dimensional high-resolution hydrodynamical simulations in spherical polar coordinates of a Type Ia supernova interacting with a constant density interstellar medium. The ejecta are assumed to be freely expanding with an exponential density profile. The interaction gives rise to a double-shocked structure susceptible to hydrodynamic instabilities. The Rayleigh-Taylor instability initially grows, but the Kelvin-Helmholtz instability takes over, producing vortex rings. The nonlinear instability initially evolves toward longer wavelengths and eventually fades away when the reverse shock front is in the flatter part of the supernova density distribution. Based on observations of X-ray knots and the protrusion in the southeast outlin of Tycho's supernova remnant, we include clumping in the ejecta. The clump interaction with the reverse shock induces Rayleigh-Taylor and Kelvin-Helmholtz instabilities on the clump surface that facilitate fragmentation. In order to survive crushing and to have a bulging effect on the forward shock, the clump's initial density ratio to the surrounding ejecta must be at least 100 for the conditions in Tycho's remnant. The 56Ni bubble effect may be important for the development of clumpiness in the ejecta. The observed presence of an Fe clump would then require a non-radioactive origin for this Fe, possibly 54Fe. The large radial distance of the X-ray emitting Si and S ejecta from the remnant center indicates that they were initially in clumps.

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arXiv2000

Observations of the Crab Nebula and its pulsar in the far-ultraviolet and in the optical

We present HST/STIS far-UV observations of the Crab nebula and its pulsar. Broad, blueshifted absorption arising in the nebula is seen in C IV 1550, reaching about 2500 km/s. This can be interpreted as evidence for a fast outer shell, and we adopt a spherically symmetric model to constrain the properties of this. We find that the density appears to decrease outward in the shell. A lower limit to the mass is 0.3 solar masses with an accompanying kinetic energy of 1.5EE{49} ergs. A massive 10^{51} erg shell cannot be excluded, but is less likely if the density profile is much steeper than R^{-4} and the velocity is <6000 km/s. The observations cover the region 1140-1720 A. With the time-tag mode of the spectrograph we obtain the pulse profile. It is similar to that in the near-UV, although the primary peak is marginally narrower. Together with the near-UV data, and new optical data from NOT, our spectrum of the pulsar covers the entire region from 1140-9250 A. Dereddening the spectrum gives a flat spectrum for E(B-V)=0.52, R=3.1. This dereddened spectrum of the Crab pulsar can be fitted by a power law with spectral index alpha_ν = 0.11 +/- 0.04. The main uncertainty is the amount and characteristics of the interstel- lar reddening, and we have investigated the dependence of α_ν on E(B-V) and R. In the extended emission covered by our 25" x 0.5" slit in the far-UV, we detect C IV 1550 and He II 1640 emission lines from the Crab nebula. Several interstellar absorption lines are detected toward the pulsar. The Ly alpha absorption indicates a column density of 3.0+/-0.5\EE{21} cm^{-2} of neutral hydrogen, which agrees well with our estimate of E(B-V)=0.52 mag. Other lines show no evidence of severe depletion of metals in atomic gas.

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arXiv2000

Wind Interaction Models for Gamma-Ray Burst Afterglows: The Case for Two Types of Progenitors

Beginning with the $γ$-ray bursts GRB 970228 and GRB 970508, a standard model for the interpretation of GRB afterglows emerged involving synchrotron emission from a constant energy blast wave expanding into a constant density, ``interstellar'' medium. However, a massive star origin for GRBs implies a stellar wind environment, probably a Wolf-Rayet star, and we have previously suggested wind interaction models for the afterglows of GRBs 980326, 980519, and 980425/SN 1998bw. Here, we extend the theory of afterglows in winds, considering strong cooling phases, the transition to nonrelativistic motion, jets, and prompt, reverse shock emission. We find that, compared to the interstellar case, the optical prompt emission in the wind case could have a comparable magnitude, but would die off faster. We examine the afterglows of other well-observed GRBs in the context of wind interaction models, and find that GRBs 970228 and 970508 are likely wind interactors. The revision in the nonthermal afterglow emission from GRB 970228 caused by the recognition of late supernova emission favors wind interaction. The radio evolution of GRB 970508 provides especially strong evidence for wind interaction. For GRB 970508, the observations suggest a density that is compatible with that expected in a Wolf-Rayet star wind. Finally, observations of the afterglow evolution of GRBs 990123 and 990510 and the prompt optical emission from GRB 990123 favor interstellar interaction models, which would suggest compact star merger progenitors for these objects.

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arXiv1999

Supernova Remnants in the Fossil Starburst in M82

We report the discovery of ten compact H-alpha-bright sources in the post-starburst region northeast of the center of M82, ``M82 B.'' These objects have H alpha luminosities and sizes consistent with Type II supernova remnants (SNRs). They fall on the same H alpha surface brightness-diameter (Sigma-D) relation defined by SNRs in other nearby star-forming galaxies, with the M82 candidates lying preferentially at the small diameter end. These are the first candidates for optically-visible SNRs in M82 outside the heavily obscured central starburst within ~250 pc from the galactic center. If these sources are SNRs, they set an upper limit to the end of the starburst in region ``B2,'' about 500 pc from the galaxy's core, of ~50 Myr. Region ``B1,'' about 1000 pc from the core, lacks good SNR candidates and is evidently somewhat older. This suggests star formation in the galaxy has propagated inward toward the present-day intense starburst core.

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arXiv1999

Radio Supernova SN 1998bw and Its Relation to GRB 980425

SN 1998bw is an unusual Type Ic supernova that may be associated with the $γ$-ray burst GRB 980425. We use a synchrotron self-absorption model for its radio emission to deduce that the synchrotron-emitting gas is expanding into a circumstellar medium of approximately $r^{-2}$ density profile, at a speed comparable to the speed of light. We assume that the efficiencies of production of relativistic electrons and magnetic field are constant through the evolution. The circumstellar density is consistent with that expected around the massive star core thought to be the progenitor of SN 1998bw. The explosion energy in material moving with velocity $>0.5c$ is $\sim 10^{49}- 3\times 10^{50}$ ergs, with some preference for the high values. The rise in the radio light curves observed at days 20-40 is inferred to be the result of a rise in the energy of the blast wave by a factor $\sim 2.5$. Interaction with a jump in the ambient density is not consistent with the observed evolution. We infer that the boost in energy is from a shell of matter from the explosion that catches up with the decelerating shock front. Both the high explosion energy and the nature of the energy input to the blast wave are difficult to reconcile with energy input from the shock-accelerated high velocity ejecta from a supernova. The implication is that there is irregular energy input from a central engine, which is the type of model invoked for normal $γ$-ray bursts. The link between SN 1998bw and GRB 980425 is thus strengthened.

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arXiv1998

Late-Time Optical and UV Spectra of SN 1979C and SN 1980K

A low-dispersion Keck I spectrum of SN 1980K taken in August 1995 (t = 14.8 yr after explosion) and a November 1997 MDM spectrum (t = 17.0 yr) show broad 5500 km s^{-1} emission lines of Hα, [O I] 6300,6364 A, and [O II] 7319,7330 A. Weaker but similarly broad lines detected include [Fe II] 7155 A, [S II] 4068,4072 A, and a blend of [Fe II] lines at 5050--5400 A. The presence of strong [S II] 4068,4072 A emission but a lack of [S II] 6716,6731 A emission suggests electron densities of 10^{5-6} cm^{-3}. From the 1997 spectra, we estimate an Hαflux of 1.3 \pm 0.2 \times 10^{-15} erg cm^{-2} s^{-1} indicating a 25% decline from 1987--1992 levels during the period 1994 to 1997, possibly related to a reported decrease in its nonthermal radio emission.

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arXiv1998

Hubble Space Telescope WFPC2 Imaging of SN 1979C and Its Environment

The locations of supernovae in the local stellar and gaseous environment in galaxies contain important clues to their progenitor stars. As part of a program to study the environments of supernovae using Hubble Space Telescope (HST) imaging data, we have examined the environment of the Type II-L SN 1979C in NGC 4321 (M100). We place more rigorous constraints on the mass of the SN progenitor, which may have had a mass M \approx 17--18 M_sun. Moreover, we have recovered and measured the brightness of SN 1979C, m=23.37 in F439W (~B; m_B(max) = 11.6), 17 years after explosion. .

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arXiv1998

Supernova Remnants in Molecular Clouds

Molecular clouds are known to be clumpy, with dense molecular clumps occupying only a few percent of the volume. A supernova remnant then evolves primarily in the interclump medium, and becomes radiative at a radius of about 6 pc, forming a shell that is magnetically supported. When this shell interacts with the dense clumps, the molecular shock fronts are driven by a considerable overpressure compared to the pressure in the rest of the remnant. Observations of the remnants W44 and IC 443 can be understood in this model. Both remnants are shell sources of radio synchrotron emission, which can be attributed to relativistic electrons in the cool radiative shell. If ambient cosmic ray electrons are accelerated by the shock front and by the postshock compression, the radio fluxes and the flat spectral indices of W44 and IC 443 can be explained, as well as the high energy gamma ray emission from bremsstrahlung.

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