MICHELLE Peers at the Dust and Gas of the Planetary Nebula NGC 7027
July 23, 2003
As a part of commissioning and engineering runs at the Frederick C. Gillett Gemini North Telescope in June and July 2003, the mid-infrared imager and spectrograph MICHELLE was used to image the young planetary nebula NGC 7027.
Planetary nebula is the name given to the gas ejected from, and heated by, a low to intermediate mass star as the star makes the transition from red giant to white dwarf. As the radiation pressure from the contracting and heating core of the red giant causes the star to lose its outer atmospheric layers, the intense ultraviolet radiation from the core raises the temperature of those layers to tens of thousands of degrees Kelvin, causing the gas to glow. The planetary nebula phase of stellar evolution lasts only a few tens of thousands of years before the gas is so distant from the star that it no longer glows. NGC 7027 is believed to be one of the youngest known planetary nebulae, having formed within the last thousand years. Despite its young age, the nebula is still 14,000 times larger than the Earth-Sun distance. Our sun is expected to become such an object in roughly five billion years. While optical images of planetary nebulae allow views of the hot gas that has been blown off the outer layers of the star, these views are often partially obscured by dust grains that can form in the cooler and outer regions of the nebula. The mid-infrared MICHELLE images presented here map where those dust grains are in the nebula by detecting the thermal emission the grains emit as they are heated by the intense radiation given off by the central star. Since the longer wavelength infrared filters used in MICHELLE are sensitive to cold material (T ~ 300 K), the intensely hot white dwarf (T ~ 200,000 K) at the core of the nebula is not visible in these images.
On the broadest scale this MICHELLE images are similar to previous mid-infrared observations of the source. That is, they show that the dust in NGC 7027 is distributed in a ring-like structure in the outer regions of the nebula. However, the high angular resolution offered by MICHELLE and Gemini has revealed small-scale structure in the dust distribution, particularly in the 7.9-micron image (left). The images show that the dust ring is 'broken' in both the northwest and southeast, directly along the axis of the two outflows known to originate from the central star. In these images, we can see that the outflows are 'punching through' the dust shell and re-distributing the grains along their paths.
NGC 7027 as seen through the 7.9- and 18.5-micron narrowband (10%) filters of MICHELLE. North is up and East is left in both panels. For each filter, the on-source exposure time was approximately 30 seconds. The data were obtained using the standard chop/nod technique with a chop throw of 15” at a PA=90. The data were calibrated using observations of Vega. The color table is scaled the same in the sense that the lowest color (dark blue) represents the same percentage level of the peak brightness in both images. During reduction, a fwhm = 0.25 Gaussian smooth was applied to the data. Click on the image for the high resolution graphic.
The MICHELLE commissioning team - Tom Kerr, Scott Fisher, Tom Geballe, Matthieu Bec and Chris Carter - carried out these observations. MICHELLE was designed and built by the Astronomy Technology Center (ATC) in Edinburgh, UK. It was first commissioned at the United Kingdom Infrared Telescope (UKIRT) on Mauna Kea in late 2001 and used until September 2002. It was transferred to the Gemini North Telescope in late 2002.
Gemini North's mid-infrared imager and spectrograph MICHELLE mounted on the upward-looking port of the instrument support structure (ISS). During the recent commissioning run, MICHELLE was mounted on a side port of the ISS.
