d’Arrest #75 / GC 1475 / NGC 2313 / P17 / PP67 / GN 06.55.6.01
Monoceros
Reflection nebula around pre-main sequence star V565 Mon
RA 06:58:02.7
DEC -07:56:43.6
Vmag 15th
Size 0.75’ x 0.5’
Dist 3,750 light-years
Part I – 19th Century Discovery
In September of 1846, while in his second year as an assistant astronomer at the Berlin Observatory, Heinrich Ludwig d’Arrest greatly aided Johann Gottfried Galle in his visual discovery of Neptune [1]. This, along with his skill as a visual observer, helped him get accepted at the Leipzig University Observatory, which had a 4.6-inch Fraunhofer refractor. With this telescope, he discovered comet 6P/d’Arrest in 1851 and in 1855 started making careful positional measurements of nebulae [1].
He moved his family to Denmark in 1857 to teach at Copenhagen University and be director of the observatory. However, he was unsatisfied with the location of the observatory, so had a new one erected outside the city. The Østervold Observatory was finished in 1861 with the installation of an 11-inch f/17.5 Merz refractor on a mount constructed by Emil Jünger [2]. D’Arrest’s ambition, at the age of 35, was to observe all of the nebulae (not star clusters) in the catalogues of William and John Herschel that would be visible in that telescope. He started in September 1861 and soon found that with such a fine instrument, he was able to see even the most difficult of those discovered by the Herschel’s. D’Arrest valued it “exactly in the middle between Herschel’s 20ft reflectors [18-inch] in their best conditions, and the excellent telescope [24-inch] used with great success by Lassell in the years 1852-54.” [3] On the surface, such a statement sounds incredible. But with today’s OotW, we are going to test it by visiting an original find of d’Arrest with that very telescope that even by today’s standards is considered extremely faint!
On the night of January 4, 1862 d’Arrest recorded that it was “nox placate, frigida, serena” (a calm, cold, serene night). He aimed his refractor less than 30° above the horizon and into southern Monoceros to study Messier 50 and NGC 2316. It was while observing the latter that something “small, weak, circular, barely class II” caught his eye 27’ west-southwest [4]. Two nights later he reobserved it at 240x and mentions that it’s “much more difficult than NGC 2316”. In Astronomische Nachrichten (Astronomical Notes) 1500 [5], d’Arrest lists it as his seventy-fifth nebulous discovery and records two observations and positions for it. His first position is 56” east-southeast while what was actually his third measured position lands a mere 45” north-northwest. Sir John Herschel would go on to include it in his 1864 General Catalogue of Nebulae and Clusters of Stars as #1475 [6]. Using the 72-inch “Leviathan of Parsonstown” in 1877, Dreyer confirmed its existence and recorded “pretty faint, pretty small, irregularly round, fades away north following, two faint stars following about 1’.” [7]
However, Dreyer was unable to share the observation with his former teacher and lifetime idol because in 1875, at the young age of 52, d’Arrest died of a heart attack [8]. Only months before, the Royal Astronomical Society had awarded d’Arrest the Gold Medal for his Leipzig and Copenhagen observations [9]. While not his only discovery while observing Herschel’s finds, his nebula in Monoceros would go on to receive the designation NGC 2313 in Dreyer’s 1888 New General Catalogue [10].
Part II – The Modern Era
In a 1965 publication in Russian titled List of Cometary Nebulae Found on Palomar Charts, author Elma Parsamian presented 23 objects [11]. On her description of #17, she wrote “A bright cometary nebula like NGC 2261, with sharply defined borders.” Now, thanks to work done by Victor van Wulfen and others, we know that 12 of them were original discoveries while the other 11 were not. And NGC 2313 would surely have been in the former group if it wasn’t for d’Arrest’s keen eyes since Parsamian 17 = NGC 2313. Two years later, German observational astronomer Cuno Hoffmeister discovered on plates that the lone star lodged in it was an irregular variable with a recorded photographic range from 14.0 – 15.5 [12]. However, it was the last compilation of newly discovered variable stars he would ever publish since he wasn’t healthy enough to write the introduction and died on January 2, shortly before submitting it and a month shy of turning 76 years old. Two years later, in 1970, it received the variable star designation V565 Monocerotis [13] and has a listed photographic magnitude range of 14.2-15.7 in the AAVSO Variable Star Index.
Monoceros Variable.png
Hoffmeister chart
In Wolfgang Steinicke’s 2010 masterpiece Observing and Cataloguing Nebulae and Star Clusters: From Herschel to Dreyer’s New General Catalogue [14], he writes that “Both the reflection nebula NGC 2064 and the emission nebula NGC 2313 were later catalogued as ‘planetary nebulae’ by Perek and Kohoutek (PK).” Well, the first edition of the Catalogue of Galactic Planetary Nebulae was published in 1967 and produced by Czechoslovakian astronomers Luboš Perek and Luboš Kohoutek’s [15]. But it doesn’t include either NGC 2064 or NGC 2313 even though Steinicke equates NGC 2064 = PK 2+2.1 (Kohoutek 5-11) and NGC 2313 = PK 26-2.2 (Kohoutek 4-7). Since learning this, I’ve reported it to Steinicke because K 5-11 is a planetary nebula in southeastern Ophiuchus while K 4-7 is a symbiotic star in Scutum.
While the reflection nebula is fairly uninteresting scientifically, the same cannot be said for its illuminating star. In 2021, Hasmik Andreasyan [16] undertook a study of V565 Mon and found that “its significant luminosity (higher than a great majority of T Tau stars) suggests that it has intermediate mass and, consequently, belongs to the HAeBe star class. On the other hand, its spectral type is probably too late for such classification and its spectrum corresponds to T Tau stars.” He also noted that “The spectral type of V565 Mon and the scarcity of emission lines in its spectrum are in favor of the FUor hypothesis. Also, it is well known that the existence of Ba II (especially Lambda 6497) lines is one of the most typical features in spectra of FUors. However, there are several reasons against such a suggestion…”
Part III – Observations
I had NGC 2313 on my list of objects to view last winter, but simply never got around to doing so. Finally, last month, I swung my 16-inch dobsonian towards it and was immediately surprised by how hard it was to see! Using 300x, I saw a very faint star in a very small, faint haze. Only with careful scrutiny could I detect that the feeble glow heads a minuscule ways north and east. A few days later, I made an attempt with my vintage 10-inch SCT after learning that d’Arrest had discovered it using an 11-inch reflector. At 260x, I could discern a very faint star with partial averted vision while full averted vision revealed a tiny, feeble halo around it (it looked larger than a single star with full averted vision). Frankly, it’s incredible that d’Arrest could discover such a nebula since even under 21.55 mpsas skies and at a latitude that places it 20° higher in the sky, I can barely discern it in a 10-inch even after knowing exactly where to look. NGC 2313 is a true testament to d’Arrest’s skill and the quality of that 11-inch refractor!
potw2119a rotate 50deg.jpg
Hubble image taken through ACS released in May 2021
Credit: ESA/Hubble, R. Sahai
As always, "Give it a go and let us know!"
References:
[1] Steinicke, Wolfgang 2010 “Observing and Cataloguing Nebulae and Star Clusters”
[2] Østervold Observatory (https://web.astronomicalheritage.net...=1&tmpl=system)
[3] d’Arrest, Heinrich 1962c (https://ui.adsabs.harvard.edu/abs/18....337D/abstract)
[4] d’Arrest, Heinrich 1867 (https://books.google.com/books?id=cg...page&q&f=false)
[5] d’Arrest, Heinrich 1865 AD1500 (https://babel.hathitrust.org/cgi/pt?...5391948&seq=99)
[6] Herschel, John 1864 (https://ui.adsabs.harvard.edu/abs/18......1H/abstract)
[7] Parsons, Lawrence 1880 (https://ui.adsabs.harvard.edu/abs/18......1R/abstract)
[8] d’Arrest 1822 – 1875 (https://www.findagrave.com/memorial/...s-d'arrest)
[9] Steinicke, Wolfgang 2010 “Observing and Cataloguing Nebulae and Star Clusters”
[10] Dreyer, John L. E. 1888 (https://ui.adsabs.harvard.edu/abs/18......1D/abstract)
[11] Parsamian, Elma 1965 (https://clearskies.eu/astronomy/articles/parsamian/)
[12] Hoffmeister, Cuno 1968 (https://ui.adsabs.harvard.edu/abs/19....277H/abstract)
[13] Kukarkin et al. 1970 (https://ui.adsabs.harvard.edu/abs/19......1K/abstract)
[14] Steinicke, Wolfgang 2010 “Observing and Cataloguing Nebulae and Star Clusters”
[15] Perek & Kohoutek 1967 (https://ui.adsabs.harvard.edu/abs/19.......P/abstract)
[16] Andreasyan 2021 (https://ui.adsabs.harvard.edu/abs/20.....64A/abstract)
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Originally Posted by wvreeven
