Unraveling the mystery of McNeil's nebula.
FOR VETERAN AMATEUR ASTRONOMER Jay McNeil, the night of January 23, 2004, was a typical winter evening in western Kentucky. A cold front had just passed, bringing clear skies and temperatures that quickly plunged into the teens. A brisk westerly wind gusting up to 20 miles an hour added to the bone-chilling cold. That night McNeil decided to test his latest telescope--a 76-millimeter (3-inch) Takahashi apochromatic refractor--that he had bought used a few weeks earlier. "I wanted to shoot something that would sort of show off the instrument's wide-field capabilities with my SBIG ST-10XME CCD camera," he recalls, "so I chose the area containing the familiar Messier 78 in Orion."
This region to the northeast of the celestial Hunter's Sword is known for its complex of bright reflection nebulae--vast clouds of dust that glow by reflecting the light of hot young stars nearby or embedded within--as well as large swaths of dark interstellar molecular clouds. McNeil decided to take an unfiltered exposure of M78 lasting 90 minutes, along with separate 20-minute exposures through red, green, and blue filters.
Due to his workload and family commitments, it wasn't until the evening of the 29th that McNeil finally sat down in front of his computer and began the painstaking task of processing and stacking his CCD images to create a color composite. He noticed right away a "funky-looking" elongated object in his images, shining feebly between magnitudes 15 and 16. "Having observed this area on many occasions, I soon realized that I had never noted this peculiar object before," he explains. "It was rather small in appearance but quite conspicuous--a little too conspicuous not to have been picked up by other observers!"
McNeil immediately began searching the Internet, downloading a total of seven photographic images from the Digitized Sky Survey that showed the suspect's location. Since these red- and blue-filtered images were taken from 1951 to 1991 with the 1.2-meter Palomar and UK Schmidt telescopes, he knew that the diffuse object would show up there. To his great surprise, however, it didn't. "Holy cow!" he exclaimed. "Could I have just made a new discovery?"
A quick check of the SIMBAD astronomical database turned up only one cataloged object in the vicinity of his "new" nebula. It was an extremely faint haze called Herbig-Haro 22, or HH 22. This object was all but invisible on the sky-survey images but quite obvious in his shots with the 3-inch scope. "I knew right away that something quite spectacular had to be occurring in this part of the sky," says McNeil. He e-mailed his findings to his friend Brian Skiff at Lowell Observatory in Flagstaff, Arizona, who then urged him to contact Bo Reipurth at the University of Hawaii's Institute for Astronomy in Honolulu.
A Star Is Born
Reipurth, who is one of the world's leading researchers in the area of early stellar evolution, quickly confirmed McNeil's find. He replied that he suspected that a newborn star deeply embedded within the dense molecular cloud called Lynds 1630 was responsible for this new nebula. Apparently, this star, previously noted only as infrared source IRAS 05436-0007 and radio source LMZ 12, underwent an outburst and had dispersed its obscuring layer of dust. This allowed the star's light to shine through and get reflected off the surrounding dust particles, creating a fan-shaped cometary reflection nebula. McNeil's Nebula, as the new object is now known, encompasses HH 22, but the two may not be physically related.
Reipurth adds that this is a very rare event, apparently similar to one involving IRAS 05380-0728 that he himself had discovered in Orion back in 1986. To date, less than a dozen such outbursts have ever been recorded. That's why he urged McNeil to immediately report his discovery to the International Astronomical Union so it could be published in the Circulars and alert other astronomers worldwide before Orion disappeared into the evening twilight.
"At this point the chain of events became almost surreal," says McNeil. "Within 24 hours of my initial discovery, Bo and a colleague were imaging it with the University of Hawaii's 2.2-meter reflector. Next thing I know, he and Colin Aspin of Gemini Observatory were requesting that the massive 8-meter-diameter optics of the Gemini North Telescope atop Mauna Kea be pointed toward 'my' object! The idea of the whole thing evolving from my 3-inch scope, which I can easily hold with one hand, to an instrument weighing 342 metric tons is absolutely staggering. And the fact that it all happened in less than 48 hours is even more incredible!"
Born in Greenville, Mississippi, in 1971, McNeil joined the US Navy after graduating from high school. In 1991 he received his discharge from the military and moved to Houston, becoming a regular attendee at the Texas Star Party (S&T: August 1998, page 86).
"My initial interest in the sky was sparked by my grandfather, who used to show me the Moon through his 50-millimeter refractor when I was five," he remembers. "At age 12, our town clerk showed me Saturn through his 10-inch Cave reflector. At that very moment, I knew exactly what my true eternal passion would be: the night sky."
To earn money in order to support this eternal passion, McNeil currently installs satellite-television dishes at residential homes. He lives with his wife, Jerri, and two children, Amber and Nicholas, in a suburb of Paducah, Kentucky, about 200 miles southwest of Louisville.
Until recently, McNeil was an avid visual observer of planetary nebulae. Energetic as a quasar, he could recite by heart the names and positions of all planetaries that had recently appeared in professional journals. His list of 450 challenging, obscure planetaries included those with such names as Jonckheere 320, Peimbert-Batiz 4, and ManchadoGarcia-Pottasch 2. McNeil now prefers to image deep-sky objects with his ST-10XME camera (April issue, page 78).
An Astrophysical Laboratory
The object responsible for McNeil's Nebula is likely a young solar-mass star that is probably around a hundred thousand years old. Such stars are still encircled by massive, rotating disks of gas and dust. Material slowly spirals through the disk, gently heating the disk in the process. At the disk's inner edge, gas locks onto magnetic-field lines that connect the disk and the star and free-falls to the star's surface, where energy is released and heats the surface. At the same time, strong winds lift off from the inner disk regions and form a bipolar outflow that burrows through the surrounding dark cloud, thus forming two large outflow cavities.
Occasionally, a sudden increase in the accretion rate causes the inner disk to heat up and become very bright. At the same time, large amounts of material crash onto the star's surface and form luminous "hot spots" up to 20,000[degrees] Kelvin. These trigger a dramatic increase in the outflow activity. Such violent accretion episodes are known as EXor or FUor eruptions, which are very similar, except that EXors last from months to a few years, whereas FUors can last many decades. Both EXors and FUors are very rare events, which may occur repeatedly in a given star, increasing its visual brightness by up to five or six magnitudes.
Apparently, the young star at the base of McNeil's Nebula has undergone either an EXor or a FUor eruption. Observations indicate that its powerful winds have reached velocities of more than 600 kilometers (370 miles) per second. Apparently, a layer of gas and dust had built up and surrounded the star before the eruption, but the winds have punched a hole in this dust cover, allowing the star's much-increased light to burst forth like a lighthouse beacon on a foggy night. This light now illuminates the inside walls of the cone-shaped cavity carved into the molecular cloud by earlier outflow episodes, thus creating McNeil's Nebula.
Only the star's northern outflow is visible --its southern one is still most likely buried deep inside a dense, opaque cloud of dust.
The star itself is probably of spectral type K, but scientists are not quite certain since it's still too heavily veiled by emission from the hot spots to allow for detailed spectroscopic examination.
A few days after news of McNeil's discovery broke, Gianluca Masi (University of Rome "Tor Vergata") and John Welch (Phoenix, Arizona) noted that the object was clearly visible in another, older image--a 15-minute unfiltered exposure taken by Evered Kreimer from his observatory in Prescott, Arizona, on October 22, 1966, using a 12 1/2-inch f/7 Cave reflector and Kodak Tri-X film chilled to -109[degrees]F with dry ice. That photo was published in S&T in January 1970 (page 27) and again eight years later in the Sky Publishing book The Messier Album.
A search of S&T's photo archives by senior editor Dennis di Cicco revealed that the nebula has been playing a game of cosmic hide-and-seek for quite some time now. A Palomar Observatory Sky Survey plate of M78 obtained through a red filter on November 29, 1951, revealed no trace of the nebula. Kreimer's 1966 shot shows it quite prominently, appearing virtually as bright as it was last January. An examination of a 90-minute exposure obtained by California astro imager Kim Zussman on October 15, 1988, turned up nothing. Other images suggest that the nebula only started to "turn on" again around late October 2003. Since then, more prediscovery images have been uncovered (see Michael Richmond's Web site at http://spiff.rit .edu/classes/phys440/lectures/new_star/new_star.html).
Rather than being a newly emerging nebula, McNeil's Nebula is apparently highly variable. How long this current brightening will last is anybody's guess.
"Based on our preliminary data, this event can be short-lived, fading to its previous brightness in a matter of months or years, which would be consistent with an EXor eruption," Reipurth told Sky & Telescope. "There is the potential for occasional flare-ups such as what is being observed at the moment." The star is still to be assigned a permanent variable-star designation in the General Catalogue of Variable Stars.
McNeil's Nebula is somewhat similar to other variable cometary reflection nebulae, which exhibit changes in brightness and shape over time. These include NGC 2261, Hubble's Variable Nebula, which reflects light from the young star R Monocerotis, and Gyulbudaghian's Nebula, illuminated by PV Cephei (see the images on the facing page).
Astronomers will continue to monitor the development of McNeil's Nebula. Observations should resume in August as Orion emerges from the Sun's glare.
Doors of Opportunity
Amateur skygazers have been discovering scores of comets, asteroids, novae, and supernovae, but finding a new nebula is quite rare. The last amateur nebula discovery is probably "Lower's Nebula." Also known as Sharpless 2-261, this hydrogen emission nebula was found on photographs taken in 1939 by the San Diego, California, father-and-son team of Charles and Harold A. Lower with their homemade 8-inch f/1 Schmidt camera. Coincidentally, Lower's Nebula is also located in Orion, at right ascension [6.sup.h] [08.9.sup.m], declination +15[degrees] 49' (about 17[degrees] north-northeast of McNeil's Nebula).
McNeil's discovery has opened new doors for him. As part of its public-outreach program, Gemini Observatory invited him to visit Hawaii's Big Island in early April. In addition to touring Gemini's facility, he was scheduled to give a talk regarding his find to a local school and astronomy club. He was also expected to discuss with University of Hawaii astronomy undergraduate students the role that amateurs play in the science.
"It's truly great to know that the gap between backyard amateurs like myself and professionals like Brian and Bo can, on occasion, be bridged to allow such a potentially remarkable discovery to be realized essentially overnight!" he muses. "I personally believe that this event showcases pro-am collaboration at its finest, and I'm absolutely thrilled to be a part of such a great effort."
Reipurth adds, "I think Jay has proven that what counts is not the size of the equipment but the care with which he examined his images. Most amateurs would have enjoyed their pictures and put them aside, so I'm delighted and impressed that Jay made such a careful examination, which clearly paid off."
Associate editor EDWIN L. AGUIRRE also hopes to discover something--anything--from his backyard observatory.