Sunday, September 27, 2009

The Garnet Star and the Suspected Planetary

A couple of weeks ago I was talking to a friend of mine who brought up the subject of imaging Carbon Stars. A Carbon star is a type of star whose atmosphere is rich in carbon. They are quite rare with surface temperatures ranging from 2600 to about 5000K. It is the presence of carbon that gives these stars a very red appearance.I thought that was a neat idea also because months ago I ran into a list of Red Stars published by the Saguaro Astronomy Club located in Arizona and colorful red stars would certainly make beautiful astrophotos. The Saguaro list is not restricted to Carbon Stars, but it includes stars of late spectral types which appear orange or red. Regardless I thought it would be a good starting point. Poring over the Saguaro's list of Red Stars I decided to image a relatively easy red star, μ Cephei, also know as the Herschel's Garnet star. Cepheus is very high in the sky these days from my location in Western Canada and remains visible for many hours, avoiding roofs and fast-growing trees that obstruct the view from my backyard. The Garnet star (so called because of its intense red colour) is not a Carbon star, but it is remarkable in its own right since it is one of the largest stars in the Milky Way. If placed where the Sun is, it would extend past the orbit of Jupiter and would almost reach the orbit of Saturn! It is not difficult to understand then why the Garnet star is classified as a Red Supergiant. This behemot is close to the end of its life which likely started few millions years ago as a Main Sequence star of 20 to 50 solar masses. Stars these big burn their fuel (hydrogen) very fast and when they approach the end swell to gargantuan proportions before concluding their existence as supernovae. We don't know when the Garnet star goes off with a bang, it could be tomorrow or in a million years, but when that happens a bright new object of magnitude -6 will appear in the sky (by comparison Venus reaches magnitude -4.4 at its maximum).

To get to the Garnet Star from my +4.8 limiting magnitude backyard sky is very easy since it is visible naked eye: at the moment the Garnet Star is a +4 magnitude star (more or less) placed at the vertex of a triangle the base of which is the line connecting α and ζ:

The first attempt at imaging the Garnet star ended after collecting 220x30s subframes for a total of 1hr and 50min exposure using my 10" f/4.7 Newtonian. The camera used was my unmodified Canon XSi at 1600ISO with no light pollution filter. The scope was mounted on a Losmandy G-11 mount and the exposure was autoguided using an Orion Starshoot Autoguider attached to an Orion ST80 guidescope. A Baader Planetarium coma corrector was used to contain coma at the edges of the field of view. Subframes were aligned and stacked in DeepSkyStacker The final image was processed in Photoshop CS2, but half way through I shelved it to come back at a later time and moved to other targets.

After a couple of imaging sessions I grew mildly frustrated with the performance of my 10". It seemed that getting reasonably round star was getting too hard so I decided to move back to my 8" f/4.9 Newtonian. A difference of about 10lbs. would have helped for sure. For some reason I decided to re-image the Garnet Star using the 8". This time around I collected 240x30s subframes for a total of 2hrs exposure. Camera settings were identical and so were the tools used for processing. This time I was pleased with the overall star shape, but I realized how much of a difference 2" make when it comes to collecting light! The image obtained with the 8" was still aestethically pleasing, but not on par with the one obtained with the 10". The region of sky around the Garnet star is very rich because of the presence of the Milky Way so the larger aperture was able to capture more stars and make the bright ones stand out more. For this reason I decided to shelve the final product.

It seemed that I was going nowhere with my project until I ran into a post on the Amateur Astronomy Mailing List mentioning a newly confirmed planetary nebula PM 1-333. A follow-up to that post indicated the new planetary being located only 23 arcmin away from the Garnet star! Since the field of view of my reflectors through my Canon spans about a degree, I realized that PM 1-333 must have been captured in my images. The article that confirms the planetary nature (in the sense of a nebula, of course) of PM 1-333 and other two objects (PM 1-242 and PM 1-318) can be found here.

The coordinate of PM 1-333 are given at p.19 and are:
R.A. 21h 40m 59.1s
Dec +58° 58' 37"

Displaying the field of view of the 10" centered on the Garnet star in Cartes du Ciel and using the coordinates given above I was able to determine where the planetary should have been in my images:

When I looked at the processed images obtained with both the 10" and the 8" I could identify a bluish smudge exactly where it was supposed to be. The smudge was obvious in the image obtained with the 10" (once I knew where to look...this is not exactly a bright object!) and barely discernible in the image obtained with the 8". The fact is that I needed more data to bring the object well above the noise, but the excitement for the "discovery" convinced me to take a different route. I had already collected almost 4hrs of data, although with two different telescopes and it felt like a big waste not being able to combine the two sets of subframes. Without being sure whether it could be done, I loaded a total of 440 subframes in DeepSkyStacker and let it go. After about 8hrs of processing the final stacked unprocessed image was ready for further tweaking in Photoshop. I was pleased to see that DeepSkyStacker had not been confused by two different sets of images: no fake "double stars" appearing anywhere. I can't praise enough the amazing job that the author Luc Coiffier has done by providing the community with such a great (free!) tool.

Anyway this is the final image. I added an inset to make PM 1-333 more obvious.

The nebula's colour is an electric blue which makes a nice contrast with the deep orange of the Hershel's Garnet star. The amount of faint stars surrounding the two objects adds another aestheric element to the final result. I tried to estimate what the magnitude of the faintest stars in the final image is by using Cartes du Ciel. I was able to identify for certain +16 stars, but there are many others that did not show up in Cartes du Ciel that are definititely fainter. I would say that something around +17.5 is a reasonable estimate.

As for PM 1-333: this object was found for the first time during a survey conducted by IRAS (Infrared Astronomical Satellite). According to Wikipedia about 350,000 objects were found, many of which are still awaiting identification. PM 1-333 was one them until earlier this year when a systematic spectroscopic analysis combined with narrowband images was done on this object. According to what it is known at the moment, it seems that PM 1-333 is an evolved planetary nebula, mainly because of the absence of a well structured shell morphology typical of popular planetaries like the Ring Nebula or the Dumbbell. When the strong winds from the central star of a planetary slow down and eventually stop, the nebular material begins to backfill the cavity surrounding the central star. When the planetary is still young the bubble dominated by the star winds is sharply defined, but not so anymore when the planetary gets older. This planetary is reported to be about 105"x50" in size with an almost circular main body of about 40" in diameter. The latter has been confirmed by visual observations.

I tried to match my image to narrowband images of this object. The overall morphology of PM 1-333 is clearly visible in my image when compared to the OIII image although at a much lower resolution:

For the record the narrowband images were taken using the ALFOSC camera mounted on the 2.6m NOT (Nordic Optical Telescope) located at the Roque de los Muchachos in La Palma (Canary Islands, Spain) so my cheap reflectors didn't do too bad after all!
The arrow in the Red narrowband image indicates where the candidate for the central star is located. I believe the yellow arrow in my image points to a knot that corresponds to the location of the central star. The string of three star to the right of the central star is clearly visible in my image and it can be used as a guide to locate the central star (magnitude +17.8, so my preliminary estimate on the limiting magnitude achieved by my setup was pretty close).
Then I compared my image with the NII narroband image taken by the professionals:

The two smaller nebular objects seen in the narroband image are classified as LIS (Low Ionization Structures). The stellar winds from the central stars are energetic enough to strip off electrons from the atoms that make up the nebular material and that's why planetaries are usually highly ionized. For some reasons atoms in these two regions of the nebula are able to retain more electrons than the atoms located around them and that is why they are referred to as low ionization structures. On a related note, some planetaries like the Blinking Nebula (NGC 6826) exhibit FLIERs (Fast Low Ionization Emission Regions) which look similar to the LIS regions in PM 1-333. The formation of FLIERs is difficult to explain, let alone their possible relationship with the LIS regions in planetaries like PM 1-333 so we'll leave that up to the professionals to find out. As fas as I am concerned I am quite thrilled to see that both LIS regions in PM 1-333 were captured in my image above (see yellow arrows).

This is the third time this year that I discover something hidden in my images. The first was when I realized that Triton had been captured by my camera while imaging the Jupiter-Neptune conjunction. The second was when I was imaging Uranus and its moons and discovered that the Compact Group of Galaxies Hickson 97 was sitting right beside Uranus and now this one. Hopefully that doesn't mean months of rain or a constant -30C winter ahead...:-)