5 Attachment(s)
Object of the Week February 12th, 2023 – Proplyd 159-350 in the Great Orion Nebula
V2279 Ori, Brun 591, Parenago 1869, [OW94] 159-350
Papa Proplyd
Orion
Ionized protoplanetary disk in naked-eye emission nebula
RA 05:35:16
DEC -05:23:50
Mag +12.73
Size 0.8” x 0.8”
It seems history did not pass down to us any record of the star Theta Orionis (labeled as such in Johann Bayer’s 1603 star atlas Uranometria) looking particularly noteworthy to the naked-eye before the 17th-century. Which is a little odd when you consider it’s one of the brightest emission nebulae in the sky, is visible as a small smudge naked-eye, and its namesake star is able to be split naked-eye by a rare few (yours truly included1). But records show it wasn’t until the first dozen years after the advent of the telescope that it was seen as a nebula with not one but three stars embedded.
The central star, still known as Theta1 Orionis, is the brightest of a very young open cluster2 that’s emerging into view3. To see more members, one needs at least a quality pair of tripod-mounted 10x50 binoculars4. This will allow you to just see the same three stars (known as “A”, “C”, and “D”) that Galileo Galilei was able to in February of 1617 with his telescope5. To see the next member star (“B”), discovered by Jean Picard in 1673, it takes binoculars with a magnification of at least 15x4.
French comet hunter Charles Messier made the nebula around Theta1 Ori the 42nd entry in his famous catalog in 1769. But it seems it was William Herschel, in his first catalog of double stars thirteen years later, who first used the name “Trapezium” to describe the four brightest members of Theta1 Ori6. He wrote “Theta Orionis. Quadruple. It is the small telescopic Trapezium in the Nebula. Considerably unequal."
Attachment 5027
Image Credit: Franz Hofmann & Wolfgang Paech
The count stood at four until 1826, when Wilhelm Struve spotted the “E” star (+11.1)7. Then, just four years later, John Herschel made out the even more difficult “F” star (+10.1)7. Amazingly, these stars have been seen by many skilled amateurs with telescopes smaller than 100mm in excellent “seeing”.
The last stars to be discovered visually were done with the 36-inch Lick refractor atop Mt. Hamilton in central California. The optics for the telescope had been ground by the firm Alvan Clark & Sons and on January 7th, 1888 – the first full night of operation8 – Alvan G. Clark (the youngest son of Alvan Clark, who had passed away five months prior9) spotted star “G” (+13.7) while using M42 to test the image quality of the instrument. Finally, after years of other stars having been claimed to be seen inside the Trapezium, this was the first one whose existence was firmly established. Later that year, keen-eyed Edward E. Barnard used the telescope to discover a close double now known as “H” (+15.8) along with the star “I” (+16.3)10. But once again, intrepid amateurs under extremely favorable conditions have seen these last three stars with much less than the Lick Refractor. The smallest sighting I’ve heard of is one by veteran southern California amateur astronomer and small-business owner Donald Pensack11. On one occasion, he managed to see all three at 304x while not even specifically looking for them in his finely crafted 12.5-inch dob at an altitude of over 8,000 feet in superb transparency and “seeing”.
Attachment 5028
Image Credit: C.R. O’Dell, and S.K. Wong (Rice University) and NASA/ESA
In the following decades, photography revealed more and more stars scattered around the Trapezium. And they were believed to be just that – stars – because of their stellar appearance on even the best images. It wasn’t until 1979 when French astronomers Pierre Laque and Jean-Louis Vidal12, using interference filtered photographs taken at Pic-du-Midi Observatory in southern France, discovered that several of the stars near Theta1 Ori C displayed nebula-like emission-lines. Their interpretation was that instead of being stars, they were actually very compact, partially ionized globules with the ionization coming from the ultraviolet radiation emanating from Theta1 Ori C.
Then, in 1987, two papers were published (Garay et al.13 and Churchwell et al.14) in which compact radio sources were observed from all six of the objects found by Laque & Vidal. Garay suggested that “[these] neutral condensations are dense fragments remaining from a massive molecular cloud after it was blown out by the action of the ultraviolet radiation and stellar winds from…Theta1 Ori C.” However, Churchwell offered a second, more tantalizing theory in that they could be “low-mass stars surrounded by an evaporating protostellar accretion disk.”
Churchwell’s theory was born out in 1993 when, less than three years after its launch aboard Space Shuttle Discovery and despite its flawed primary mirror, the Hubble Space Telescope (HST) imaged part of the area south of Theta1 Ori C. In those images taken by O’Dell, Wen, and Hu (1993)15, entire protoplanetary disks could be seen in silhouette against the backdrop of the nebula while being illuminated on the end facing towards Theta1 Ori C. These objects quickly became known as proplyds (short for ionized protoplanetary disks).
Attachment 5029
Image Credit: NASA, C.R. O’Dell, and S.K. Wong (Rice University)
After the HST’s optics were corrected, the next images16 were detailed enough to show that the proplyds appeared different depending on their viewing angle with respect to Theta1 Ori C. Those lying directly behind the star were nearly stellar with little more than their ionized side being visible while those lying at the same distance to Theta1 Ori C displayed dark, ragged disks with the end nearest the star being illuminated. These high-resolution images showed the shocking truth that some of the brightest proplyds were those that had been discovered over a century earlier with the Lick Refractor! The Trapezium member stars “G”, “H”, and “I” (Laque & Vidal 2, 5, and 3, respectively) are actually newborn stars shrouded by gas and dust and only visible due to their “sunlit side” being illuminated.
The brightest proplyd in the Orion Nebula shines at magnitude +12.7 and lies 28” south-southwest of Theta1 Ori C. Also known by the variable star designation V2279 Ori17, it was found and cataloged by O’Dell in that first set of HST images as HST 0315. However, in O’Dell & Wen (1994)16 it received the designation 159-350 based on its coordinates. They did this after many more proplyds were found using the newly refurbished HST and wrote “Since the Orion Nebula covers only a limited range of right ascension and declination, the common numbers in the coordinates are dropped, being assumed to be 5 hours 35 minutes of right ascension and -5°20' in declination.” Therefore, an object at 5:35:15.94 -5:23:50.04 would be called proplyd 159-350.
Attachment 5030
Proplyd 159-350 from Bally et al. (1998)
What I believe the next easiest/brightest proplyd (about +13.8) to see is only a little fainter, lies 16” southeast of Theta1 Ori C, and was discovered by Churchwell (1987)14 as a radio source using the Very Large Array. Later, it then was cataloged as proplyd 170-334 by O’Dell & Wen (1994)16.
Attachment 5031
Proplyd 170-334 from Bally et al. (1998)
Inspired by Akarsh Simha to take a stab at observing proplyds late this past year, I readily found my copy of the February 2021 issue of Sky & Telescope and re-read the excellent article on them by Dave Tosteson. I still thought Tosteson had gone too deep (even for the Going Deep column!), but I figured I’d have a decent shot on an upcoming trip to use a friend’s 36-inch f/5.3 alt-az driven dob. After performing my own research, I found that the two brightest ones on amateur images (such as this one by Jerry Lodriguss) were 159-350 and 170-334. However, while Tosteson had mentioned 159-350 in his article, he had oddly failed do the same for 170-334.
On my one night with the 36-inch in late October, I could fairly easily see the two brightest proplyds as stellar with 664x and even suspected the brighter one might be visible under better conditions with my 16-inch. Less than 10 days later, on a dark morning with excellent seeing, I got to test that idea. I was shocked to find that not only was I able to see the brighter one without difficulty at 600x, but the fainter one too! From that observation, it dawned on me that 159-350 might be just bright enough to see in my 10-inch SCT if I could find an equally perfect night.
I avoid observing when the Moon is in the sky. I know it’s a luxury, but my skies are dark enough that even a crescent is enough to brighten my skies. So when I’m inside looking out at the Moon in a clear sky, I try not to think about the fact that I’m possibly passing up otherwise top-notch viewing conditions. But on February 3rd of this year, I chose to set up my 10-inch SCT and observe even though a 13-day-old moon was 36° up at the beginning of night. I already knew I had excellent transparency when I was setting up, but I would have to wait until nightfall to find out about the “seeing”. I figured the Moon couldn’t hurt me anyways considering my primary targets were the recently discovered open cluster Gaia 118 (which lies just 10’ east of brilliant Sirius) and proplyd 159-350 inside brilliant M42! Much to my surprise, the "seeing" was excellent and I saw both at 580x with a 4.8/110° eyepiece producing a 0.45mm exit pupil and an 11’ TFoV.
I look forward to the day I see the next brightest proplyds (106-417 and 167-317 [the “G star”]). Until then,
“Give it a go and let us know!”
For further reading on proplyd 159-350, check out Bally et al. (1998). The breathtaking details in the color images alone are worth the visit. And for further reading on proplyd 170-334, check out Mesa-Delgado et al. (2012).
[1] Astronomy, Feb. 2023, p. 52 (https://www.zinio.com/za/explore/fre...allenge-a39804)
[2] Menten et al. (2007) (https://ui.adsabs.harvard.edu/abs/20....515M/abstract)
[3] Theta1 Ori C is moving deeper into the Orion Molecular Cloud (https://ui.adsabs.harvard.edu/abs/19....696O/abstract)
[4] Amateur Fisk Miles splits Trapezium into three with 10x, four with 15x (https://www.cloudynights.com/topic/8...um/?p=12316434)
[5] Galileo splits Theta1 Ori into three (http://jane.whiteoaks.com/2009/05/31...the-trapezium/)
[6] Page 129 of William Herschel’s first Catalogue of Double Stars (1782) (https://royalsocietypublishing.org/d...rstl.1782.0014)
[7] (https://articles.adsabs.harvard.edu/...str....7..177L)
[8] The Immortal Fire Within by William Sheehan (1995)
[9] Alvan Clark (1804-1887) (https://en.wikipedia.org/wiki/Alvan_Clark)
[10] Burnham (1889) (https://ui.adsabs.harvard.edu/abs/18....352B/abstract)
[11] Private communication with Donald Pensack
[12] Laque & Vidal (1979) (https://ui.adsabs.harvard.edu/abs/1979A&A....73...97L)
[13] Garay et al. (1987) (https://ui.adsabs.harvard.edu/abs/19....535G/abstract)
[14] Churchwell et al. (1987) (https://ui.adsabs.harvard.edu/abs/19....516C/abstract)
[15] O’Dell et al. (1993) (https://ui.adsabs.harvard.edu/abs/19....696O/abstract)
[16] O’Dell & Wen (1994) (https://ui.adsabs.harvard.edu/abs/19....194O/abstract)
[17] SIMBAD (https://simbad.u-strasbg.fr/simbad/s...=SIMBAD+search)
[18] Koposov et al. (2017) (https://ui.adsabs.harvard.edu/abs/20...2702K/abstract)
