Selected Supernova Remnants in the LMC

Above is an image of the Large Magellanic Cloud as seen in the infrared from Spitzer. During observing cycle 1, we observed over 40 SNRs in the LMC with the Infrared Array Camera (IRAC) and the Multiband Imaging Photometer for Spitzer (MIPS) instruments onboard the Spitzer Space Telescope. I've put here some of the good detections, with some brief information on each remnant. This is not intended to be an extensive list or catalog of SNRs in the LMC (there are plenty of other pages on the internet for that). My collaborators on this work are Kazik Borkowski and Steve Reynolds (my research advisors), as well as Bill Blair, Parviz Ghavamian, Ravi Sankrit, Knox Long, John Raymond, Sean Hendrick, Frank Winkler, Sean Points, and Chris Smith.

DEM L71 and N132D are what I consider to be two ideal cases for examining the properties of SNRs in infrared wavelengths. They are the only examples from our survey of virtually complete detection in more than one wavelength of shell-type SNRs. I will discuss these two in more detail. All of the remnants on this page have excellent correlation with both X-ray images (showing that the dust is collisionally heated by electrons and ions in the plasma) and Hα images, which show a correlation with non-radiative Balmer-dominated shocks. This is important because it rules out any significant contribution from emissin lines that have been observed in other SNRs where radiative shocks are present.

DEM L71

DEM L71 was the first case in our survey of a shell supernova remnant with clear detection of the entire shell. Up to this point, we had no idea if it was even going to be possible to detect the shell, given the amount of IR background present in pretty much any direction you look. We were completely blown away by this image. The remnant itself is the result of a thermonuclear (type Ia) supernova that occured nearly 4500 years ago. The shell structure in the IR image to the left (MIPS 24 µm) corrresponds perfectly to X-ray images, which shows the emission from hot gas behind the expanding shock wave. This was further confirmation of previous work done over the past few decades regarding the interaction of the shock with the interstellar medium (ISM). The same hot plasma which emits the X-rays seen in Chandra images of the remnant also heats dust grains residing in the ISM to temperatures of ~100 K, which causes them to re-radiate energy in the IR. This emission from ISM grains is what we see in the Spitzer image.

Analysis of this remnant (at both 24 and 70 µm) (Borkowski et al. 2006) also confirmed that destruction of dust grains via sputtering does take place in the shock. As much as 50% of the mass originally contained in dust grains is sputtered and returned to the ISM in gaseous phase. Further study also confirmed that it is possible to use IR observations to constrain the density of the ISM behind of the shock. While densities can be derived from X-ray images, the connection between X-ray emission measure and density rests on a number of assumptions. Neglecting any possible effects of cosmic-ray modification of the shock, we find post-shock densities in the neighborhood of 2.5 cm^-3, which gives pre-shock densities of ~0.6 cm^-3. We are also able to determine the total amount of dust radiating at 24 µm is 0.034 solar masses. Even taking into account the effects of grain sputtering, this amount falls well short of the expected mass in dust grains by a factor of ~5, assuming the canonical value for dust-to-gas mass ratio. This discrepancy is currently unexplained, but has been confirmed in many other SNRs as well, including 1987A (Bouchet et al. 2006) and the SMC remnant E0102.2-7219 (Stanimirovic et al. 2005), as well as the other remnants in our sample detailed on this page.

N132D

N132D is the remnant of a core-collapse supernova that exploded ~2500 years ago. It is one of the brightest and most studied X-ray remnants in the Magellanic Clouds, and has been the subject of extensive study in optical and IR wavelengths as well. This remnant is a bright IR remnant as well, with a total 24 µm flux of ~3 Jy. As with DEM L71, we see no evidence for dust associated with ejecta from the supernova. Regions of ejecta identified by X-ray spectra in SNRs have not shown strong IR emission in any of the supernova remnants we have analyzed. Does this pose a problem for supernova theories that predict large amounts of dust condensation in ejecta? Only time and more observations will tell, but clearly this is an issue that needs to be resolved, particularly as it relates to studies of supernovae in the early universe. In Williams et al. (2006), we analyzed both the bright southern rim and the somewhat fainter but still easily visible northern rim separately. We found significantly higher densities in this remnant compared to DEM L71, consistent with the core-collapse origin, which explains the higher fluxes and lower 70/24 µm ratio seen in N132D. A similar discrepancy in the amount of dust found versus the amount of dust expected was found in this remnant as well, ruling out the possibility of something unusual going on with thermonuclear or core-collapse supernovae. We have evidence for similar differences in measured and expected values in both cases. Interestingly, Tappe et al. (2006) have claimed detection of polycyclic aromatic hydrocarbons (PAHs) in IRAC images and spectra of this remnant.

Here are some images of other SNRs we detected in our survey, with only brief information about each. See Borkowski et al. (2006) and Williams et al. (2006) for more information on several of these remnants.

Type Ia

0548-70.4

SNR 0548-70.4 is the remnant of somewhat older (~7000 yrs.) thermonuclear supernova. It is only fainly detected here at 24 µm (with the stretch on this image enhanced to show the "wings" of the remnant, clearly visible on the east and west sides. This remnant is overall extremely faint, with a total flux at 24 µm of ~3 milliJanskys (mJy). It also contains relatively cool dust in a low-density environment, resulting in a high 70/24 µm flux ratio.

0509-67.5

SNR 0509-67.5 is the youngest of the SNRs observed in our sample (other than 1987A, which we serendipitously obtained IRAC images of in the field of view of another pointing). At an age of only ~400 years, this object is nearly an identical age to Kepler's SNR in our own galaxy, and is of similar origin, being the remnant of a thermonuclear supernova. The resolution issues of MIPS can be clearly seen in this image, as the SNR is only ~30 arcseconds in diameter! The faint eastern rim can also be seen on this stretch. In Borkowski et al. (2006), we explored the possibility of this being due to a contrast in densities between the eastern and western edges of the remnant.

0519-69.0

0519-69.0 is in some ways a "cousin" to 0509-67.5. Both are remnants of thermonuclear supernovae, and both are at similar stages in their development. 0519 is slightly older, at ~600 years, and is expanding into a slightly higher density medium, and thus is brighter at 24 µm by a factor of a few. The most striking feature of this remnant is the three bright knots in the shell. These knots are seen in X-ray and Hα images as well, and are believed to be regions of higher density than the rest of the remnant. I've maintained since the first time I saw this image that this supernova remnant looks like a spaceship viewed from below. The aliens are coming! They'll be here in 160,000 years.

Core-Collapse

N49B

N49B (0525-66.0) represents the oldest supernova remnant in this sample (~11,000 yrs.) and is seen here in its entirety. It is also the largest, at over 2 arcminutes in diameter. The somewhat lower density environment compared to some other core-collapse remnants (post-shock n_p ~ 1 cm^-3) combined with the age yield a low dust temperature, and a high 70/24 µm flux ratio. This remnant contains nearly 1/10 of a solar mass of swept-up dust in the shell, however, that number is still short of expectations.

N23

N23 (0506-68.0) appears to be an odd-shaped remnant, but we are really only seeing half of the shell. The other half is only faintly identified at any wavelength, and is not visible in the IR images. The higher density of the medium surrounding this remnant lead to warmer dust and a significantly higher 24 µm luminosity than N49B. As an interesting side note, a compact central object has recently been identified in this remnant by Hayato et al. (2006).

0453-68.5

SNR 0453-68.5 was detected in its entirety at 24 µm, albeit rather faintly. This remnant contains the coolest dust of any in this sample, with dust temperatures ranging from 40-55 K. We always report a range of dust temperatures, because we model IR emission with a distribution of grain sizes, and not simply a single grain size, single temperature model. For our analysis, we adopted the grain size distributions of Weingartner and Draine (2001). This remnant also has the most dust of any in our sample, with 1/10 of a solar mass.

References

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