Goldilocks' Variable


Discovery images One of the most exciting times of my life followed after I realized that a reddish star easily visible on the cover of the June 1990 issue of Astronomy was completely lacking on the cover of the Autumn 1990 issue of Deep Sky which, by a remarkable coincidence, showed apparently the same Dumbbell nebula (M 27, NGC 6853). Reproduced by permission. Copyright © 1990, Kalmbach Publishing Company.

To put it lenghtily, all that began a long time ago, well before the Internet entered my life. Getting acquitanced with the fascinating world of planetary nebulae, I found, much to my surprise, that many of them had their stellar nucleus double. A popular book on astronomy (I can't recall its title now) even said that also the brightest planetary, the Dumbbell nebula (M 27, NGC 6853) belonged to the class. Unfortunately, as it is often with similar sources, no reference or authority was given, and I forgot the episode.

A couple of years later I used to make regular trips to the library of the Astronomical Institute in Ondrejov, a quiet village in a nice hilly landscape some 30 kilometers from Prague, with forests all around and great beer in local pubs. By that time I had already learned to read technical papers and now I got access to the large library full of goodies! Browsing through bibliography of planetary nebulae in hefty volumes of Astronomy and Astrophysics Abstracts, I came across a paper “A probable binary central star in the planetary nebula NGC 6853” (PASP 89, 139, 1977) by Kyle M. Cudworth of the Yerkes Observatory. Comparing positions of a 17th magnitude star next to the nebula's nucleus (separation 6.5", position angle 214°) measured at Lick and Yerkes plates in the 1960s and 1970s, he found that both the stars moved together and suggested they could be physically related to each other.
Finding chart
Finding chart for Goldilocks' variable (arrowed). The common-proper-motion companion of the stellar nucleus is a small star next to (and southwest of) the latter.

I knew that binarity of some other planetary nebulae was much more dramatic (as it is with V 651 Mon, the unique nucleus of NGC 2346), but this was a little known detail I could add to a picture of a well-known object. Having realized that the common-proper-motion companion was visible on many published pictures of the summer planetary, I decided to prepare a map which would show the nebula's extent in various colors (and the light of various ions) and also that pair of the stars. Hoping, of course, that I would find other interesting features to be marked on the chart. I took June 1990 issue of Astronomy with a beautiful blue-red bubble of the Dumbbell (captured with the Canada-France-Hawaii Telescope) on its cover, and started plotting the star field on a tracing paper. To check my creation I reach for the Autumn 1990 cover of Deep Sky which, by a remarkable coincidence, showed apparently the same Dumbbell, with similar orientation and scale. I corrected position of a few small stars when I got to the northwestern corner of the nebula and became puzzled. The place occupied on the cover of Astronomy by an easily visible reddish star was quite empty on the Deep Sky picture! I had never heard about a variable star in M 27 so I began eagerly chasing after photographs of the planetary.

When I emerged from books and journals heaped up during the search, I was sure that the star was really a variable, not an artifact due to some misprint, different magnitude limit or color sensitivity. I was looking forward to add to my chart a detail completely forgotten by authors of deep-sky guidebooks, but first I had to find its coordinates to check them against variable star lists. First aid was given by the image of M 27 reproduced in Perek & Kohoutek's Catalogue of Galactic Planetary Nebulae, the only source at my disposal with clear orientation, angular scale and coordinates of the stellar nucleus. During my next visit at the Department of Theoretical Physics and Astrophysics, Masaryk University, Brno, I went through the General Catalogue of Variable stars (GCVS) that listed variable stars known in early 1980's, as well as its more recent supplements. But none of those authoritative sources mentioned the star of the Astronomy cover! I determined the position again allowing for a pessimistically large error, checked the latest numbers of IBVS (Information Bulletin of Variable Stars), but the result was the same. Finally, I admitted the star was a discovery, and sent a short announcement to IBVS through Attila Mizser (Konkoly Observatory). An enjoyable response – a small card saying that the contribution was accepted and would appear in #3604 – came within a few days.

Del Rio's CCD image

It required 24 individual exposures of six fields to create this CCD mosaic of the Dumbbell Nebula. Daniel Del Rio (Fort Lauderdale, Florida) took them on November 23, 1994, with a ST-5 camera attached at his 14.5-inch reflector.

That happened in the spring of 1991. What is known about the variable today, six years later? The most precise piece of information is its position. Soon after the discovery, Jan Manek (Stefanik Observatory, Prague) found that my original coordinates were erroneous. Applying standard astrometric procedures to a pair of photographs of the Dumbbell which appeared in print, he derived new declination and right ascension: 19 h 59 m 29.8 s, +22° 45' 14'' (2000.0, precessed from the original 1950.0 equinox using the NED online calculator). However strange his method may seem, it worked surprisingly well. In the summer of 1992, Petr Pravec (Astronomical Institute, Ondrejov) took at my request several CCD frames of the planetary nebula and measured them with the following result: 19 h 59 m 29.7 s, +22° 45' 12'' (2000.0). The third (and so far the last) position I am aware of is that published by M. Morel (Rankin Park, Australia) in the IBVS #4037 (Improved Astrometry for Variable Stars): 19 h 59 m 29.8 s, +22° 45' 13'' (2000.0). As you can see, the three sets of coordinates agree perfectly with each other, given that the first one is based on the SAO catalog and the others on the GSC data.

Unfortunately, situation with the variable's photometry is much worse. It seems most likely that it belongs to the class of long-term variables of which Mira (Omicron Ceti) is the most popular representative. This is indicated by the red color of the star, the reason it's so bright on frames taken by common CCD cameras and on the near-infrared image of the Dumbbell nebula reproduced in the September 1996 issue of the Sky & Telescope, page 16. More importantly, the classification is supported by character of light changes documented by a series of CCD frames made by Daniel Del Rio (Fort Lauderdale, Florida) during the fall of 1994. Using the ST-5 camera attached at his 14.5-inch Newtonian, he got so far the best brightness measurements. Alhough relative and unfiltered, Daniel's photometry show that the variable has brightened by some two magnitudes between the early October and the late November of that year (the check star nearby was constant within a few hundredths of magnitude). This is too much for other sorts of red variables (those various semiregular and irregular beasts). In addition, the real range may be even greater, because the images of the variable and comparison star turned out to be saturated.

But that all isn't enough for giving the star (which I privately named Goldilocks' variable after a ravishing young lady) the definitive designation V XXX Vul. With Vulpecula the Little Fox rising again above the morning horizon (now decorated with the comet), it should be quite easy for an experienced variable star observer equipped with a CCD camera (and ideally also with the standard R or V filter) to follow its changes closely thorough a cycle or two. Needless to say, I would much appreciate any results. Clear skies!

(March 1997)

Light curve April - July 1997

First update

While the comet mentioned above is finally gone, Goldilocks' variable is still out there, and what is most important, you can enjoy a preliminary light curve prepared by Rudolf “Karel” Novak (Nicholas Copernicus Observatory and Planetarium, Brno).

Between April and early July 1997, he took several frames with a ST-7 CCD camera (R-band Kron-Cousins filter) attached at the Brno Observatory 40 cm (f/4.4) reflector, and processed them by the MIDAS aperture photometry function magnitude/circle. The results are presented in the accompanying diagram. The top panel shows the magnitude difference between the variable and a comparison star, the bottom panel the same for the comparison and a check star.

Rudolf's most recent measurements (not included in the diagram yet) indicate that Goldilocks' variable has already reached its minimum and is brightnening.

(September 1997)

Thanks!

To Attila Mizser (Konkoly Observatory) who arranged publication of my discovery report in IBVS. To Petr Pravec (Astronomical Institute, Ondrejov) and Jan Manek (Stefanik Observatory, Prague) for their careful positional measurements. To Daniel Del Rio for an envelope full of goodies related to the Dumbbell Nebula and nice CCD images. To Dave Bruning and Terry Conley (Astronomy) for permission to reproduce the discovery images. And to Rudolf Novak for his present and future photometry of my favorite variable star.


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