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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)
Johannes Kepler
1571 - 1630
October 9, 2004, marked the 400th
anniversary of the first sighting of
SN1604, also known as Kepler’s
Supernova Remnant.
VLA 6 cm (DeLaney et al. 2002)
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.
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.
Position
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.
8/24 Ratio
70/24 Ratio
NW (radiative)
NNW
0.031
0.022
0.29
0.30
D45 Central
D50 E Central
0.034
0.020
0.27
0.30
NE
0.024
0.27
N Rim (nonrad)
0.021
0.49
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. Backgroundsubtracted 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.)
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 Nabundance, indicating mass loss from the precursor system.
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).
The MIPS 160 micron image above shows no indication of
+ Models with a range of temperature and density appear to be
enhanced emission coincident with the SNR. (Contours from the
necessary to explain the observed NIR flux ratios.
24 micron image are shown for reference.) This does not bode well
+ Further spectroscopy of Kepler’s SNR is needed to assess possible
for the claim by Morgan et al. (2003) that Kepler’s SNR contains
emission line contamination and improve our ability to model the
~1 solar mass of cold (T=17K) dust, based on SCUBA
relative intensities observed with MIPS and IRAC.
observations. (See also recent paper by Krause et al. 2004.)
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.
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.
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