Spitzer Imaging of Kepler’s Supernova Remnant

1. Spitzer Imaging of Kepler’s Supernova Remnant W.P. Blair, R. Sankrit, P.Ghavamian (JHU), K.S. Long (STScI), K. Borkowski & S.P. Reynolds (NCSU) Summary:We have obtained Spitzer Space Telescope MIPS and IRAC infrared images of Kepler's supernova remnant. The images, taken within two months of the 400th anniversary of the sighting of the supernova, provide the most detailed information to date on the spatial distribution and character of the dust in this supernova remnant. The MIPS 24 micron image shows the overall structure best. While this image is brightest in the northwest and north (as is the optical), the entire circumference is visible, as are patches of emission seen in projection toward the center of the remnant. These patches of IR emission surround optical knots which, based on their kinematics, arise from the front and back sides of the expanding shell. By association, the IR emission is also due to the shell rather than interior emission. The IRAC 8 micron image has the best spatial resolution, but only shows the brightest regions, almost exactly coincident with the brightest optically emitting material. Likewise, the 70 micron MIPS image shows only the brighter regions from the 24 micron image. Only modest variations in 8:24 and 70:24 micron ratios are seen. The shorter wavelength IRAC images are progressively dominated by stellar emission. Comparison to imaging data at optical, X-ray, and radio wavelengths makes it clear we are seeing dust emission primarily from regions that have encountered the primary blast wave as it strikes circumstellar and interstellar material. We compare the observed IR fluxes and their ratios with theoretical calculations of IR emission from collisionally heated silicate dust in order to determine physical conditions within the hot plasma, infer its dust content, and ultimately study the possible destruction of dust by the expanding blast wave. VLA 6 cm (DeLaney et al. 2002) Johannes Kepler 1571 - 1630 October 9, 2004, marked the 400th anniversary of the first sighting of SN1604, also known as Kepler’s Supernova Remnant. The Figure above shows a detailed comparison of the Spitzer data with Chandra X-ray data and ACS imagery from the Hubble Space Telescope. The spatial distribution of NIR emissions are consistent with that seen in optical and hard X-ray, indicating an origin in the primary shock wave as it interacts with a circumstellar and/or interstellar medium. Left: A composite view of Kepler’s SNR combining data from NASA’s three Great Observatories. Note how the green (soft X-ray) emission, attributable to thermal emission from the SN ejecta, tends to be displaced inward from the outer shell. The blue (harder X-ray), red (Spitzer MIPS 24 micron), and yellow (Hubble ACS Ha) emissions are attributed to the primary SN blast wave as it encounters circumstellar and interstellar material. The scale shown assume the new distance estimate of 3.9 Kpc (~13,000 light-years) derived by Sankrit et al. (2005) from the Hubble data and ground-based shock velocity estimates. Note that the intrinsic spatial resolutions of the individual data sets are very different. The ACS data (0.05”) have the highest resolution, followed by the Chandra data (1.2”) and MIPS 24 micron (10”). The red rim on the N and NW sides is largely due to this effect and is not a real physical difference. The Figure above shows a composite of three Spitzer bands: IRAC 4.5 micron (blue) which shows primarily stars, IRAC 8 micron (yellow), and MIPS 24 micron (red). The more extensive nature of the 24 micron image is primarily attributable to the greater sensitivity in this bandpass. The panels above show various representations of the Spitzer IRAC and MIPS imaging data for Kepler’s SNR. The top three panels show different stretches of the MIPS 24 micron image to show the full dynamic range. Note the faint, extended emission in the panel at upper right. The bottom left and middle panels show two versions of the IRAC 8 micron image. The IRAC 3.6 micron image, which is dominated by stellar emission, was scaled and subtracted from the 8 micron image, although numerous stellar residuals remain. As shown in poster 160.02 (Roellig et al.), the IRS spectrum shows a moderately strong [Ar II] 7.0 micron line in the IRAC 8 micron bandpass, at least in the bright radiative filaments in the NW. Hence, this image places an upper limit on the thermal dust emission at 8 microns. The MIPS 70 micron image is shown at bottom right. A non-uniform background in this image complicates the analysis. Except for differing sensitivities and spatial resolutions, the overall spatial distributions of SNR emission in the three bands looks very similar. Preliminary ratios between the Spitzer 8, 24, and 70 micron bands have been constructed from the PBCD Spitzer data sets in the following manner. The 8 and 24 micron data sets were Gaussian smoothed to match the resolution of the 70 micron data. Then statistics from image subsections corresponding to the same spatial regions were extracted, including nearby background regions for each position. Background-subtracted surface brightnesses (in MJy/ster) in each band were then used to determine the ratios in the table above. In general, very little dispersion in the ratios is seen, with the possible exception of the 70/24 micron ratio on the northern nonradiative rim. (Note the 8/24 micron ratios are upper limits owing to possible significant line emission in the IRAC 8 micron bandpass.) Primary Conclusions/Results: + Spitzer MIPS/IRAC images show warm dust primarily associated with the primary blast wave as it encounters the surrounding CSM/ISM (e.g. ISO results, Douvion et al 2001). + No significant evidence of dust associated with the SN ejecta are seen, and no evidence for a massive cold dust component is seen (cf. Morgan et al. 2003). + Models with a range of temperature and density appear to be necessary to explain the observed NIR flux ratios. + Further spectroscopy of Kepler’s SNR is needed to assess possible emission line contamination and improve our ability to model the relative intensities observed with MIPS and IRAC. Theoretical 70/24 and 8/24 micron flux ratios are shown above for collisionally-heated dust in a hot homogeneous plasma as a function of its density and temperature. Heating of dust by electrons and protons increases with increasing density and temperature. This leads to higher dust temperatures in denser and hotter plasmas, so that the 70/24 and 8/24 micron flux ratios can be used as plasma diagnostics. Inhomogeneous plasma with a wide range in density and temperature might be required to explain both 70/24 and 8/24 micron flux ratios seen in Kepler's SNR because the deduced values of electron density and temperature (color bands in the figures) for each ratio barely overlap. Our dust models consist of a power-law distribution of grain radii from 0.005 to 0.25 microns with exponent -3.5 and optical constants appropriate for "astronomical silicate" dust (Draine & Lee 1984). They include effects of fluctuations in dust temperature, which are most important at low densities. Color corrections appropriate for the IRAC and MIPS photometric bands have been taken into account in the calculation of the theoretical flux ratios. The MIPS 160 micron image above shows no indication of enhanced emission coincident with the SNR. (Contours from the 24 micron image are shown for reference.) This does not bode well for the claim by Morgan et al. (2003) that Kepler’s SNR contains ~1 solar mass of cold (T=17K) dust, based on SCUBA observations. (See also recent paper by Krause et al. 2004.) An overview of the optical properties of Kepler’s SNR (adapted from Blair et al. 1991). Material seen in projection near the center is actually due to front and back-side shell emission. The bright knots in the NW are radiative shocks, and show high N-abundance, indicating mass loss from the precursor system. References: Blair, W. P., et al. 1991, ApJ, 366, 484 DeLaney, T., et al. 2002, ApJ, 580, 914 Douvion, T., et al. 2001, A&A, 373, 281 Draine, B. D., & Lee, H. M. 1984, ApJ, 258, 89 Krause, O., et al. 2004, Nature, 432, 596 Morgan, H. L., et al. 2003, ApJ, 597, L33 Sankrit, R., et al. 2005, COSPAR, in press This work is supported by grant JPL-1264303 to the Johns Hopkins University. Special thanks to the public affairs teams from Spitzer, Hubble, and Chandra for their assistance in producing the composite figures, which were released on October 9, 2004, in celebration of the 400th anniversary of the first sighting of SN 1604.