Pencil Nebula: Knot D pictures (APOD 2009 January 8)
Posted: Thu Jan 08, 2009 1:48 pm
http://apod.nasa.gov/apod/ap090108.html
http://violet.pha.jhu.edu/~wpb/hstvela/hstvela.html
<<The Vela Knot D region is interesting, and my colleagues Ravi Sankrit, John Raymond and I have recently published a paper on the ultraviolet emissions from this region (links to a pdf file). The X-ray Knot D is bounded by a beautiful "fan" of optical filaments know commonly as
the Pencil Nebula (NGC 2736).
Briefly, when first discovered the Knot D region (as well as the other protrusions seen in X-rays) was thought to represent a place where "ejecta" (chunks of the precursor star) were being shot out at high velocity from the explosion site. However, even with extensive observations of the region, it has been difficult to confirm (or refute!) this claim. The ultraviolet and optical observations are consistent with the region being a "breakout" from the main shell of the SNR. That is, a weaker, lower density portion of the shell may have allowed the main shock wave to "run ahead" in this region, and the shock wave is now encountering a denser region again, causing the X-ray and optical/UV emission we see. We do not find evidence for peculiar abundances or physical conditions that would indicate stellar ejecta are involved in creating this structure Future observations, including some with the Chandra X-ray Observatory, are being pursued to reconcile which of these models might be correct.
One final surprise discovered fairly recently is that yet another supernova remnant is present along this line of sight! When the Rosat X-ray data shown above were processed in a way that showed only the higher energy X-rays, a separate, smaller diameter shell popped out:
Caption: These two images show low (left) and high (right) energy X-ray images of the same piece of sky (from Aschenbach, B. 1998, Nature, 396, 141). In the right panel, a separate supernova remnant, about 2 degrees across, is revealed.
The distance to this new supernova remnant (known formally as RX J0852.0-4622, but known affectionately as "Vela Jr.") is a matter of hot debate. It could be a background object, similar to the Puppis A SNR mentioned above. A more tantalizing possibility, though, is that it is physically associated with the Vela SNR, and represents an even younger supernova from within the same generation of stars! The discovery image above even raises the possibility that the Vela D/Pencil Nebula region could be associated with THIS OBJECT instead of the Vela SNR itself.
It's amazing sometimes how surprises pop out of new observations, and how difficult is can be to discern the true situation!>>