Nebula Designations: Bernes 29, Parsamian-Petrosyan 99, Blitz-Fich-Stark 4
Star Designations: LkHa 120, HBC 302, V1331 Cyg
Cygnus
Reflection Nebula
RA 21:01:09
DEC +50:21:45
VMag of star: 11.8-12.4
Nebula size: 30”
Distance: 1,950 light-years
NOTE TO READER: If I understand correctly, a young stellar object (YSO) is so named because it displays the emission spectrum of an accretion disk and/or a star that has yet to enter the main sequence where it will fuse hydrogen in its core for the bulk of its life. The visibility of extended nebulosity isn’t always a given…
Part I – Discovery, Confirmation, and Study
If you enjoyed my last OotW (June 23rd), then you are in for a treat since this one involves a star of less mass but a little farther along in the evolutionary stage.
As our understanding of so-called “dark nebulae” have evolved, we’ve come to see them as fairly dynamic places. In fact, those that play host to recent or even ongoing star formation are generally referred to as dark molecular clouds. Beverly Lynds published her comprehensive catalog of dark nebulae in 1962, compiled from her examination of the over 800 fields contained within the National Geographic-Palomar Observatory Sky Atlas [01]. That same year, George Herbig published a list of 126 stars, which constituted all the known and “probable” TT (TT) variable stars brighter than a photographic magnitude of about 14.5 [02].
Even at the time, it was noticed that quite a few TT stars were found in or very near nebulosity (either bright or dark). Their namesake was no exception since it was illuminating a nebula (NGC 1555) in the dark cloud later cataloged as Dobashi 4471 [04]. However, Herbig noted that only 15 of those on his lengthy list were either illuminating or exciting bright nebulosity right up next to them. So, association with a nebula was not conclusive evidence for membership in the TT class.
Still, those with an associated nebula were the most studied and one with a truly intriguing looking nebula was found nestled up in northern Cygnus, at the border of the small dark cloud Lynds 981. The star itself (with a photographic magnitude of +12.9 in Herbig’s list) carried the recent designation LkHa 120 after showing up in a special survey looking for objects with Ha line in their emission [05].
v1331_cygni.jpg
Image showing LkHa 120 at the end of the dark streamer emanating from LDN 981. (Credit: Mt. Lemmon SkyCenter/Adam Block)
At the time, the idea that TT stars were very young stars (now referred to as pre-main sequence) of only intermediate mass in the process of gravitational contractions from the diffuse material in which they formed was still novel [02]. In fact, only in 1961 did Herbig present observations that supported the idea that these stars have a circumstellar “envelope” (now dubbed an “accretion disk”) very near them since strong mass outflows directly from their optical spectra [06]. So, it was even more puzzling why these irregular variable stars had spectral characteristics that indicated they were both gaining mass and losing mass [02] [07].
Canadian-American astronomer Leonard Kuhi’s interest in the problem was piqued upon seeing a photograph of LkHa 120 taken by Herbig using the 120-inch reflector atop Mount Hamilton [07]. He described the nebula around the star as a “has a fringe of nebulosity fanning out from one side to a distance of 4”, and still farther away is surrounded by an elliptical shell of nebulosity which may be the interference of material streaming out from the star and the surrounding cloud of gas and dust.” Kuhi, who thought that the shell around LkHa 120 was spherical and a prior mass ejection from the star, attempted to figure out how much mass had been lost and if it could explain the observed velocities in the star’s spectrum.
In 1970 [08], LkHa 120 received the variable star designation V1331 Cygni after G. Zaitseva found it to be so in 1968 [09]. But its associated nebulous arc was only cataloged in 1977 by Swedish astronomer Claes Bernes [10]. The year before, in 1976, Gunnar Welin [11] considered V1331 Cyg to be a pre-outburst candidate of a poorly understood type of variable star known as FU Orionis. These were believed to have been TT stars involved in reflection nebulae before experiencing a large and rapid rise in brightness possibly due to a rapid dissipation of dense dust envelopes around the star [12]. A decade later, a total of five FU Orionis variable stars were known and each was associated with a ring-like nebulae not unlike the one around V1331 Cyg [13].
In 1985, Hartmann and Kenyon [14] published a paper arguing that FU Orionis wasn’t a class of variable as much as it was just one of many outbursts that TT stars go through during a period of elevated accretion from a circumstellar disk. Stuart McMuldroch and his colleagues in 1993 discovered a radially expanding torus-shaped molecular gaseous cloud just beyond the observed arc-shaped nebula via carbon monoxide emission synthesis maps [15]. They were also the first to figure out that since the torus inclination is about 30°, we are viewing the star more pole-on – near its outflow axis. Using this, they came to the conclusion that V1331 Cyg might have experienced an FU Orionis outburst some 4,000 years ago and could be currently between outbursts.
10_1086_300461_fg6_hr.gif
The [S II] and continuum image of V1331 Cyg from Mundt and Eislöffel (1998) showing a jet and the Herbig-Haro object at its end.
In their 1998 paper, authors Reinhard Mundt and Jochen Eislöffel managed to photograph several new Herbig-Haro objects around TT stars using a special filter that isolated the emission of singly-ionized sulfur [16]. The brightest one they found around V1331 Cyg lies 3’ south while the jet (more aptly a “flow”) powering it could also be faintly seen. The Herbig-Haro object was included in the Second Edition of the General Catalogue of Herbig-Haro Objects as HH 389 [17].
But you may be wondering what exactly a Herbig-Haro object is. Well, for decades after the discovery of these unusual nebulae in the 1940s, their physical nature and excitation remained a mystery. Our current understanding – which still needs refinement – was only reached step by step with many important discoveries and ideas gradually revealing their true nature. Basically, outflow activity is associated with all stages of early stellar evolution, from deeply embedded protostellar objects to visible young stars. The reason for this is because to allow accretion to proceed in the first place, there must be a mechanism that removes angular momentum from the system. Now, the exact driving mechanism is still unclear [18], but there seems to be a “disk wind” that is created from the interaction of the young star and the accretion disk. And some part of the wind can be collimated into jets along their axis with very small opening angles. Herbig-Haro objects are the optical manifestations of this powerful mass loss and occur when these jets of ionized gas collide with nearby clouds of gas and dust at high speeds, creating bright patches of nebulosity.
V1331Cyg.jpg
This Hubble image was processed by Judy Schmidt to better reveal the jet aimed at us and the inner ring of this helix-shaped nebula. The inner one has a radii of about 3,500 AU while the outer one has a radii around 9,000 AU. (Credit: NASA/ESA/J. Schmidt)
Upon our entering the new century, the Hubble Space Telescope was turned towards V1331 Cyg. From those high-resolution images, Quanz et al (2007) discovered an inner dust ring around 12” in diameter that extends almost half a circle around the star [19]. They also agreed with earlier findings that our view of the system was nearly pole-on along the axis of a conical outflow and that the outer ringlike reflection nebula was likely the remnants of the molecular core that V1331 Cyg formed from. However, they stated that “V1331 Cyg can currently not be classified as an FU Orionis object and seems to be in a more quiescent evolutionary phase.”
Before Petrov et al 2014, a direct spectrum of the star (known as the photospheric spectrum) had not been detected. Instead, the spectral type and luminosity had been estimated from its spectral energy distribution, interstellar extinction, and distance. In historical order of their estimates, it was considered a B0.5, then A8-F0, F0-F2, G0, and G5. They managed to get a spectral type of G7-K0 IV and a mass of 2.8 suns. Upon closely studying the emission line spectrum of the star and detecting the signature of post-shocked gas in a jet, they concluded that “the star is seen through its jet, so it may be considered as a stellar analogue of blazar.” Therefore, what Mundt and Eislöffel (1998) found is the jet as it deviates or is diverted on its course at larger distances from the star.
aa27547-15-fig1.jpg
Taken by Hubble in 2009, this image reveals the nebulous arc around V1331 Cyg. It lies at the eastern tip of the dark streamer Dobashi 2860, which links it to the dark cloud LDN 981.
In a 2016 study published by Arpita Choudhary, they performed a proper motion analysis on the outer and inner dust rings/arcs. Remarkably, they accomplished it using HST images with a time gap of just under ten years. From this, they confirmed the radial expansion rate of the inner and outer rings, demonstrating that we are in fact looking at rings and not limb-brightening of a shell (aka the “soap bubble effect”). They also dove into the question as to why the rings aren’t seen to be complete if we are looking “down” up them. It makes visual sense in hindsight, but they were the first to prove that the “missing arc” section to the northwest is likely caused by shadowing from the (unseen) circumstellar disk and not due to foreground extinction from LDN 891 like Quanz et al. (2007) thought.
Part II – Observations
For the most part, V1331 Cyg stays around magnitude +12.0, making it an easy find in any telescope if you’re good at star-hopping westward of M39. I took my first swing at this unique nebula in mid-November last year with my friend’s 36-inch f/5.1. On the first night, even though the scintillation was terrible, I believe I detected the part of the main arc around the star at 332x. However, it was an odd-looking detection. The next night, conditions were better and at 332x I could confirm seeing a peculiar glow on the southeast side of the star. It didn’t seem to start right at the star and was linear, not curved.
I’m anxious to hear from others because I really am not sure what the smallest aperture sighting may be to see any portion of the helix-shaped nebula. But I can say that it will take an experienced observer to make a confident observation considering the unusual shape of the nebula. A cool second-place prize is the fact that when you see V1331 Cyg, you are simultaneously looking through the partially ionized material known as an HH object and the accretion disk around the star!
So, as always, “Give it a go and let us know!”
[01] Lynds 1962 (https://ui.adsabs.harvard.edu/abs/19......1L/abstract)
[02] Herbig 1962 (https://ui.adsabs.harvard.edu/abs/19...1409K/abstract)
[04] Dobashi 2011 (https://ui.adsabs.harvard.edu/abs/20......1D/abstract)
[05] Herbig 1954 (https://ui.adsabs.harvard.edu/abs/19....483H/abstract)
[06] Herbig 1961 (https://ui.adsabs.harvard.edu/abs/19....337H/abstract)
[07] Kuhi 1964 (https://ui.adsabs.harvard.edu/abs/19...1409K/abstract)
[08] Kukarkin et al 1970 (https://ui.adsabs.harvard.edu/abs/19......1K/abstract)
[09] Zaitseva 1968 (https://ui.adsabs.harvard.edu/abs/19....435Z/abstract)
[10] Bernes 1977 (https://ui.adsabs.harvard.edu/abs/19.....65B/abstract)
[11] Welin 1976 (https://ui.adsabs.harvard.edu/abs/19....145W/abstract)
[12] Herbig 1977 (https://ui.adsabs.harvard.edu/abs/19....693H/abstract)
[13] Goodrich 1987 (https://ui.adsabs.harvard.edu/abs/19....116G/abstract)
[14] Hartmann & Kenyon 1985 (https://ui.adsabs.harvard.edu/abs/19....462H/abstract)
[15] McMuldroch 1993 (https://ui.adsabs.harvard.edu/abs/19...2477M/abstract)
[16] Mundt & Eislöffel 1998 (https://ui.adsabs.harvard.edu/abs/19....860M/abstract)
[17] Reipurth 2000 (https://ui.adsabs.harvard.edu/abs/20......0R/abstract)
[18] Günther 2013 (https://ui.adsabs.harvard.edu/abs/20.....67G/abstract)
[19] Quanz et al 2007 (https://ui.adsabs.harvard.edu/abs/20....287Q/abstract)
[20] Petrov et al 2014 (https://ui.adsabs.harvard.edu/abs/20...3643P/abstract)
[21] Choudhary et al 2016 (https://ui.adsabs.harvard.edu/abs/20....106C/abstract)
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