Friday, July 19, 2002

The Search for Gomez's Hamburger
A Stop at a Hamburger Stand on the way to the Restaurant at the end of the Universe. (r.i.p. Douglas Adams)

by Barbara Wilson
George Observatory, Houston Museum of
Natural Science

Author's note: 10 years ago this month I wrote an observing article about this object (Deep Sky Journal #2 Autumn 1992) and thought it would be fun to revisit this challenging little object. Dana Lambert and I imaged Gomez' Hamburger using the 36" at the George and the Star 1 CCD camera. After I started to write this in early June 2002, I learned that the Hubble Space Telescope has imaged Gomez' Hamburger for the Hubble Heritage Project.

In May 1985 several astronomers decided to conduct a research survey deep in the teeming dense star fields of Sagittarius. In most areas near the galactic center of our barred spiral, distant stars and objects are hidden from view or deeply obscured by countless dust molecules. These astronomers did a search for RR Lyrae stars by taking advantage of a small area of clarity that is free from gas and dust called Baade's Window. Their idea was to search Baades' Window and find RR Lyrae stars for use as distance probes.

Using a 1.5-meter telescope at Cerro Tololo, Chile, survey plates were taken of these RR Lyrae stars in the galactic bulge. On one survey plate Arturo Gomez found a tiny extended object. This was unexpected, since Baade's Window was known to the professional astronomers to be clear of emission nebulae and dark clouds. They initially thought the object might be a background galaxy according to Andrew Phillips, a team member.

The object's glowing gas lobes (the buns) are cut in two by a dark equatorial absorption band resembling a hamburger patty. It was promptly named appropriately, Gomez's Hamburger. It is an unusual looking object. It is about .08pc by .05 pc in physical size, the size of a planetary nebula. At an estimated distance of 2.9 kiloparsecs, Gomez' Hamburger is only 3.5 by 5.5 arc seconds, and fairly dim at magnitude 14.4.

In the journal article (Maria Teresa Ruiz et al., Astrophysical Journal, 316, L21 (1 May 1987), the discoverers point out that Gomez' Hamburger could be a proto-planetary nebula. It was described as a torus of hot dust encircling an A0III evolved giant central star. The giant star illuminates the dust. The hot circular dust ring is also buried within an outer cloud of colder dust. The light from the nebula is increasingly polarized away from the center. Gomez's Hamburger was detected and catalogued previously with IRAS (Infrared Astronomical Satellite) and designated as IRAS 18059 -3211. Its alternate designation is: PK 359-6.2.

Any object with such a delightful name has to be fair game at the eyepiece for an amateur astronomer. Sky and Telescope had a news note "Chewing on Gomez' Hamburger" (Sky and Telescope, Nov 1987, pg 462.) on its discovery. The photograph in the magazine showed a strange object unlike any seen before. It resembled a hamburger patty viewed edge on and the news item referred to the discovery article. I found the discovery article at the Rice University Library and photocopied it. As a bonus, the discovery article contained a finder photograph with the proverbial arrow (like the one you see on photographs of Pluto) pointing to tiny Gomez' Hamburger.

The spectra in the article showed no emission lines, so led me to believe that this object should behave like a reflection nebula (unlike a classical planetary nebula) at the eyepiece.
Normal nebular filters like the UHC or OIII filters would not enhance its visibility but actually cause it to disappear! Gomez' Hamburger joins objects like Minkowski's Footprint and the Red Rectangle as a special class of bipolar reflection nebulae believed to be proto-planetaries.
Unfortunately the journal photograph showed only a small 8' x 6' field, and was a deep one, going much further than my Atlas Stellarum, which I used to seek out faint objects. I precessed the coordinates from 1950 to 2000 using a computer program and plotted the position in the then new
Sky-charting program by Emil Bonnano called MEGASTAR. MegaStar was the first charting program to use the entire Guide Star Catalogue, with stars to 15.5 magnitude in some areas.
Using MegaStar I printed a 7.5' x 9.2' arc minute sky chart to augment the much fainter ESO B atlas photo from the Astrophysical Journal.

At the April 1992 Texas Star Party, searching on consecutive nights, nothing was visible at the exact precessed position at powers ranging from 220x to 450X with my 20 inch f/4 Newtonian mounted on an equatorial platform. One complication was that I could never reconcile the ESO chart with the real sky. It seemed the Hamburger stand was closed for business!

I drew the field carefully noting all the stars visible at high power at what was to be our last clear night at the 1992 TSP. Back in Houston, I could not reconcile the eyepiece drawing with the ESO survey plate. Was the position wrong? Using another precession program, with a more precise algorithm, it appeared that the previous program had placed the elusive chiliburger 1 minute in declination too far south. To come so far, and miss by so little! Now everything became clear. I could now correlate my eyepiece drawing with the ESO chart's brighter stars. Replotting the position in MegaStar, the star fields now matched the ESO stars. The realization came that I had definitely been observing the correct field, but studying the wrong position. The J2000 position for Gomez' Hamburger is 18 hours 9 minutes 13.3 seconds - 32 degrees declination 10 minutes 47 seconds.

The next attempt on May 23rd, a hazy humid southeast Texas night, I examined the field carefully at high power, and was confident of having the correct field using the ESO chart. No Hamburger!
To observe the tiny almost stellar burger would require a better sky. Would it require a sky like we had in West Texas?

On Saturday June 20th, the moon would rise at midnight, and the weather forecast predicted a front would arrive at 11 p.m. The observing window to search out this elusive object was only an hour, if the forecast was correct, and Sagittarius would not yet have reached the meridian. I took the chance anyway, going 80 miles to the Houston Astronomical Society's observing site.

The Sky was gloriously clear all evening as we waited for Sagittarius to rise. Finally I could wait no longer despite that the object was still east of the meridian. Just before 11 pm as the predicted clouds started slowly moving in from the northwest, I meticulously studied each star I could see in the eyepiece to the stars plotted on the ESO finder chart in order to positively identify the position of the protoplanetary nebula.

Using extreme averted vision, finally I saw the faintly glowing barely above stellar Gomez's Hamburger! It did not respond to a nebula filter, which was expected. The polarizing filter did not help. The best view came at 750 power, where the tiny object showed the most contrast. Gomez's object was visually fainter that I imagined. It never responded to direct vision. It could be seen at 220 power, however, not even a hint of its unusual structure was visible.

Buster Wilson, Matt Delevoryas, Paula and Ken Drake, and Paul Sventek all observed this protoplantary nebula with me that night, and I relished our stop at the Hamburger Stand.
Maybe our Australian observing friends can give us a better visual description, since it passes nearly overhead for them. But for now this occasion called for a celebration. Matt and I ate a cold hamburger.. Now on to that restaurant…

Epilogue: Gomez's Hamburger has been barely detected in telescopes as small as 8 inches, by a very experienced amateur since that date 10 years ago when I first tried to see it.