AstroSat Picture of the Month (December 2017)

NGC 40: A Planetary Nebula with an Ultra-Violet Halo

UV image of NGC 40 taken by UVIT onboard AstroSat.

The red gas cloud is the Bow-Tie nebula, which is being illuminated by the central hot star.The gold coloured diffuse light surrounding the nebula is the newly discovered far ultra-violet halo.

Image credit: Kameswara Rao et.al.

NGC 40, or the Bow-Tie Nebula, is a Planetary Nebula about 3500 light years away from us, in the northern constellation of Cepheus. Discovered by William Herschel in 1788, it can be seen in a moderate sized telescope by amateur astronomers. Earlier optical images of NGC 40 show a central star as hot as about 70000 K surrounded by expanding gas that gives it its characteristic shape. The central hot star is blowing a fast hot wind into this surrounding gas at 1700 km/s, and heating it up.

Kameswara Rao and his colleagues used the AstroSat to image this object in many regions of the ultra-violet. First, however, let us look at what a planetary nebula is. Some old Red Giant stars  throw out their outer layers of gas, which expands away from the star. This exposes the hot inner part of the star, whose radiation makes the outer gas layers shine brightly as a planetary nebula. Our Sun too, will meet this same fate. With time, the inner star will evolve to become a White Dwarf, a very strange object indeed. And the heavier elements cooked inside the stars, thrown out into space, go back into forming newer stars and planets like ours.

The astronomers who looked at the ultra-violet images of NGC 40 using AstroSat, were not only able to study the central hot star and the surrounding gas, but also made a new discovery. As the image shows, they discovered, for the first time, a large halo of ultra-violet radiation in the far UV band surrounding the entire nebula. This halo,they figure out, is due to molecules energized due to the light from the central star.

The paper describing these results can be downloaded from https://arxiv.org/abs/1711.07698