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Register a new userSupernova Remnants in the Magellanic Clouds. VII. Infrared Emission from SNRs
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Rosa Murphy Williams
Publication date
2006
fields
Physics
and research's language is
English
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We have used the instruments on the Spitzer Space Telescope to study the Large Magellanic Cloud supernova remnants (SNRs) N11L, N44, N49, N206, N63A, and N157B. The two large SNRs N44 and N206 were not detected in any IRAC or MIPS wavebands; the remainder were detected at one or more wavelengths. In particular, the SNRs N49 and N63A each had features that were evident in all available IRAC and MIPS bands. Each of these two also displayed faint limb emission in the MIPS 24 micron band only. IRS spectra obtained for the N49 SNR showed a number of prominent lines, with little continuum contribution. We therefore suggest that N49, and possibly N63A, are dominated by line emission, with thermal emission from hot dust being at most a secondary component.
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The physical sizes of supernova remnants (SNRs) in a number of nearby galaxies follow an approximately linear cumulative distribution, contrary to what is expected for decelerating shock fronts. This has been attributed to selection effects, or to a majority of SNRs propagating in free expansion, at constant velocity, into a tenuous ambient medium. We compile a list of 77 known SNRs in the Magellanic Clouds (MCs), and argue that they are a fairly complete record of the SNe that have exploded over the last ~20kyr, with most now in the adiabatic, Sedov phase of their expansions. The roughly linear cumulative size distribution (uniform in a differential distribution) can result from the combination of a deceleration during this phase, a transition to a radiation-loss-dominated phase at a radius that depends on the local gas density, and a distribution of ambient densities varying roughly as rho^{-1}. This explanation is supported by the observed -1 power-law distributions of three independent tracers of density: HI column density, Halpha surface brightness, and star formation rate from resolved stellar populations. In this picture, the observed cutoff at r~30 pc in the SNR size distribution is due to a minimum in the mean ambient gas density in the regions where supernovae (SNe) explode. We show that M33 has a SNR size distribution similar to that of the MCs, suggesting these features, and their explanation, may be universal. In a companion paper (Maoz & Badenes 2010), we use our sample of SNRs as an effective SN survey to calculate the SN rate and delay time distribution in the MCs. The hypothesis that most SNRs are in free expansion, rather than in the Sedov phase of their evolution, would result in SN rates that are in strong conflict with independent measurements, and with basic stellar evolution theory.
Supernovae constitute a critical source of energy input to the interstellar medium (ISM). In this short review, we focus on their latest phase of evolution, the supernova remnants (SNRs). We present observations of three old SNRs that have reached the phase where they interact with the ambient ISM: W28, IC443, and 3C391. We show that such objects make up clean laboratories to constrain the physical and chemical processes at work in molecular shock environments. Our studies subsequently allow us to quantify the impact of SNRs on their environment in terms of mass, momentum, and energy dissipation. In turn, their contribution to the energy balance of galaxies can be assessed. Their potential to trigger a further generation of star formation can also be investigated. Finally, our studies provide strong support for the interpretation of gamma-ray emission in SNRs, a crucial step to answer questions related to cosmic rays population and acceleration.
We present the first far ultraviolet (FUV) spectra of the four known Balmer-dominated supernova remnants (SNRs) in the Large Magellanic Cloud, acquired with the Far Ultraviolet Spectroscopic Explorer. The remnants DEM L 71 (0505-67.9), 0509-67.5, 0519-69.0 and 0548-70.4 are all in the non-radiative stages of evolution and exhibit expansion speeds ranging from ~ 500 km/s to ~ 5000 km/s. We have detected broad emission lines of Ly beta, Ly gamma, C III and O VI in DEM L 71 (V(FWHM) ~ 1000 km/s) and have detected broad Ly beta and O VI emission in 0519-69.0, (V(FWHM) ~ 3000 km/s). In addition, broad Ly beta emission (V(FWHM) ~ 3700 km/s) has been observed in 0509-67.5, the first detection of broad line emission from this SNR. No emission was detected in our FUSE spectrum of 0548-70.4, allowing us to place only upper limits on the FUV line fluxes. The spectra of these SNRs are unaffected by postshock cooling, and provide valuable probes of collisionless heating efficiency in high Mach number shocks. We have used the Ly beta / O VI flux ratio and relative widths of the broad Ly beta and O VI lines to estimate the degree of electron-proton and proton-oxygen ion equilibration in DEM L 71, 0509-67.5, and 0519-69.0. Although our equilibration estimates are subject to considerable uncertainty due to the faintness of the FUV lines and contributions from bulk Doppler broadening, our results are consistent with a declining efficiency of electron- proton and proton-oxygen ion equilibration with increasing shock speed. From our shock velocity estimates we obtain ages of 295-585 years for 0509-67.5 and 520-900 years for 0519-69.0, respectively, in good agreement with the ages obtained from SN light echo studies.
We study the non-thermal emission from old shell-type supernova remnants (SNRs) on the frame of a time-dependent model. In this model, the time-dependent non-thermal spectra of both primary electrons and protons as well as secondary electron/positron ($e^{pm}$) pairs can be calculated numerically by taking into account the evolution of the secondary $e^{pm}$ pairs produced from proton-proton (p-p) interactions due to the accelerated protons collide with the ambient matter in an SNR. The multi-wavelength photon spectrum for a given SNR can be produced through leptonic processes such as electron/positron synchrotron radiation, bremsstrahlung and inverse Compton scattering as well as hadronic interaction. Our results indicate that the non-thermal emission of the secondary $e^{pm}$ pairs is becoming more and more prominent when the SNR ages in the radiative phase because the source of the primary electrons has been cut off and the electron synchrotron energy loss is significant for a radiative SNR, whereas the secondary $e^{pm}$ pairs can be produced continuously for a long time in the phase due to the large energy loss time for the p-p interaction. We apply the model to two old SNRs, G8.7$-$0.1 and G23.3$-$0.3, and the predicted results can explain the observed multi-wavelength photon spectra for the two sources.
AIMS. We study and discuss the time-dependent X-ray emission predicted by hydrodynamic modeling of the interaction of a SNR shock wave with an interstellar gas cloud. The scope includes: 1) to study the correspondence between modeled and X-ray emitting structures, 2) to explore two different physical regimes in which either thermal conduction or radiative cooling plays a dominant role, and 3) to investigate the effects of the physical processes at work on the emission of the shocked cloud in the two different regimes. METHODS. We use a detailed hydrodynamic model, including thermal conduction and radiation, and explore two cases characterized by different Mach numbers of the primary shock: M = 30 in which the cloud dynamics is dominated by radiative cooling and M = 50 dominated by thermal conduction. From the simulations, we synthesize the expected X-ray emission, using available spectral codes. RESULTS. The morphology of the X-ray emitting structures is significantly different from that of the flow structures originating from the shock-cloud interaction. The hydrodynamic instabilities are never clearly visible in the X-ray band. Shocked clouds are preferentially visible during the early phases of their evolution. Thermal conduction and radiative cooling lead to two different phases of the shocked cloud: a cold cooling dominated core emitting at low energies and a hot thermally conducting corona emitting in the X-ray band. The thermal conduction makes the X-ray image of the cloud smaller, more diffuse, and shorter-lived than that observed when thermal conduction is neglected.
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