Object of the Week, June 12, 2022 - Parsamian 21

[akarsh]

Parsamian 21 = HH 221 = IRAS 19266+0932
Young Stellar Object (YSO) in Aquila
RA: 19:29:01
Dec: +09:38:43
Size: 1' x 0.3'
Mag (V): 14.07

Who is Parsamian? [1,2]
Prof. Elma Suren Parsamian is one of the foremost Armenian astronomers affiliated with the famous Byurakan Astrophysical Observatory in Armenia. She got her PhD under Victor Ambartsumian, and has worked at Byurakan, Yerevan State University (Armenia), Tonantzintla Observatory of Mexico, and at the Max Planck Institute for Astronomy at Heidelberg.

Parsamian's work centers around the physics of gas nebulae, investigation of variable stars in clusters and associations, and archeoastronomy. Notably, in 1965, she published her first catalog of cometary nebulae found on Palomar plates, the best-known object in this catalog being the subject of this post [3]. The catalog seems to have been later revised with Petrosyan [4, 5]. Many of the "PP" objects are more familiar through other designations on this forum, like Hubble's Variable Nebula, Hind's Variable Nebula, and NGC 1999. In addition, she worked on flare stars (variable stars that flare up really bright unpredictably), including with Guillermo Haro of Herbig-Haro fame. She is very well-known for identifying the astronomical significance of the "Armenian Stonehenge", Zorats Karer, which has since been challenged and remains controversial [6].

parsamian.jpg
Elma Parsamian, from a website

What is Parsamian 21? [7]
The current model of Parsamian 21 is as follows: HBC 687, an eruptive young star of "FU Orionis" type (a "FUor" for short) lies at its heart . These pre—main sequence stars can brighten up by 4—5 magnitudes in a matter of months and remain bright for several years or decades.

FUOriLightCurve.jpg
FU Orionis Light Curve
Credit: User PopePompus from Wikimedia

These stars are thought to comprise of an accretion disk around a young, low-mass (<~ 2 M_sun) star (a "T Tauri" star). Once the protostar becomes hot enough to ionize hydrogen, the rate of accretion can drastically increase due to an instability, which leads to a sudden inflow of lots of mass onto the star, leading to a hot and bright inner accretion disk. It is this hot accretion disk that is thought responsible for the sudden brightening [7,8]. Eventually, the material in the inner disk gets depleted into the star and the system becomes dim again until new material accumulates. These outbursts are thought to recur over thousands of years, although of course we have no human records of observing such recurrences. Even though no outburst of HBC 687 has been observed, the indirect evidence suggests that it is a FUor.

Parsamian_21_POSSIIBlue.png
POSSII Blue image of Parsamian 21. Credit: DSS/MAST/STScI

Young T Tauri stars with accretion disks are known to eject material along the axis perpendicular to the disk, in a bipolar outflow. Herbig-Haro objects are formed when the bipolar outflow collides with the surrounding interstellar medium (See the OOTWs on HH1, HH2 and HH 222). The H-alpha image presented in [7,9] shows the Herbig-Haro object embedded in the system, extending north and south of the star:

Parsamian_21_H_Alpha.png
Halpha image of Parsmian 21 from [7,9]

Coming back to the cometary nebula, it is bipolar although the southern part is very very weak, with the southeastern jet more prominent than the southwestern jet. The southeastern jet is seen in the near-infrared HST image below, and also vaguely in the above POSSII image. The prominent northern nebulosity is loop-shaped as is seen in both images.

Parsamian_21.jpg
Near-infrared image of Parsamian 21 from the HST

Here's the hypothesis for what causes its structure, explained through a diagram in [7,9]:

Parsamian21_Hypothesis.jpeg
Sketch from [7,9] showing how the morphology of Parsamian 21 may have been formed

The bipolar outflow excavates a cavity in the dense material surrounding the star, which is then illuminated by the star. We see the light scattered from the walls of the cavity as the cometary nebula.

Visual Observation

I came across this amazing object on Larry Mitchell's Texas Star Party Advanced Observing List from 2014. I was at TSP that year with my 18", and was working through Larry's list. Anyone who has had that experience with an 18" knows that typically, Larry's objects can sometimes be exceedingly tough for such apertures. Notably that year, the Serpens Red Object was on the list. So on May 29th 2014, when I landed on Parsamian 21 after logging the Serpens Red Object, I was pleasantly surprised at how bright and actually visible it was! I logged it as "Beautiful! Quite bright. Elongation easily detected". I somehow have this view of Parsamian 21 as a pencil of light shooting out of a bright point burned in my memory, although after 8 years, who knows how accurate it is — it's time for a refresher.

The challenge for large telescopes is to view the loop that was mentioned earlier. If I understood him correctly, Jimi says that he has seen this feature with his 48".

One of the joys of writing (and hopefully reading) these OOTW posts is that it makes you learn so much more about an object, that you can go back to your observations and breathe so much more meaning into those 3 pops of a faint fuzzy blob. But what's even better is when both the visual view and the astrophysics are amazing, as is with this object, so why not

GIVE IT A GO, AND LET US KNOW!

References:
[1] Biography of Elma Parsamian: https://www.aras.am/Books/books/Elma-Parsamian-Eng.pdf
[2] https://www.aras.am/FamousAstronomers/parsamian.html
[3] Parsamian's original catalog: only abstract is available in English https://ui.adsabs.harvard.edu/abs/19....146P/abstract
[4] Parsamian-Petrosyan catalog: https://arar.sci.am/dlibra/publicati...110062/content
[5] Presumably an English translation of [4] with contemporary digitized imagery: https://arar.sci.am/dlibra/publicati...295324/content
[6] Armenian stonehenge: https://www.smithsonianmag.com/trave...nge-180964207/
[7] Agnes Kospal's thesis: http://kisag.konkoly.hu/Group/events..._theses_en.pdf
[8] https://www.annualreviews.org/doi/pd...astro.34.1.207
[9] https://academic.oup.com/mnras/artic...3/1015/1035688

[Clear Skies]

A fascinating object and one I have single observation logged for. 27 August 2019 in the French Ardennes, 14" SCT @ 168x/29':

Using AV the SSE part of the nebula surrounding a mag. 13.5 star is visible along with, very subtly, a faint streak to the NNW.
To the NNW is a mag. 14 star. The faint NNW part of the nebula is not visible. No components of the double star Li 3 (WDS19290+0939) can be discerned.

In an attempt to find the double's faint components B & C, I upped the magnification to 316x, but to no avail.

parsamyan21-1.jpg parsamyan21-2.jpg

Click here for OOTW observing guide, here's the complete Parsamyan Cometary Nebulae edition.

[akarsh]

Victor, which double-star are you referring to? SIMBAD is unable to understand either identifier you specify.

[Clear Skies]

SIMBAD does not contain the complete WDS.

As noted in the observing guide, it's the double star Li 3 = WDS19290+0939.

The goto site for WDS data in a readable format is stelledoppie.it. Li 3: https://www.stelledoppie.it/index2.p...iddoppia=81220

Also, I would like to note that some double stars that are associated with the more obscure nebulae, are sometimes brighter knots in the nebula, as is the case with the VY Canis Majoris nebula (that was Steve's OOTW back in February 2021). Often, their listed magnitudes are brighter than they really are, even if they really are stars. Anyhow, the best way is of course to go take a look.

[Jimi Lowrey]

The Loop I am talking about is the dark void between the jets. I am not sure if it is dust or a void? I have seen this void on good nights at high power at 816X. On the Hubble image below you can see stars in the void but they might be foreground? I would be interested to know if it is a void or dust.

C699DB06-68AF-4735-9927-97B0484BD3A4.jpeg

[lamperti]

"...you can go back to your observations and breathe so much more meaning into those 3 pops of a faint fuzzy blob."

I had forgotten that I did see this back in 2015 with the 22" at 450x. My notes: "Found by entering coordinates in Argo Navis to get the star field matching finder chart. Best without a filter but needed averted vision 2. Two faint stars in the field and the "upper" one was elongated." NELM = 6.0 (SQM = 20.8)

Thanks for the reminder!

[Raul Leon]

Hi, here's my observation from 9/22/2017: Parsamian 21 : magnitude: 14 ; size: 1' ; YSO in Aquila ; small and faint ; elongated oblong shaped ; it blows me away that I can see this through my scope!! I used a 4mm Radian at 436x with my 14.5 Starstructure f/4.3

Scan0607.jpg

[Jimi Lowrey]

That is an impressive observation Raul. Way to Go!!!

[akarsh]

The Loop I am talking about is the dark void between the jets. I am not sure if it is dust or a void? I have seen this void on good nights at high power at 816X. On the Hubble image below you can see stars in the void but they might be foreground? I would be interested to know if it is a void or dust.

C699DB06-68AF-4735-9927-97B0484BD3A4.jpeg

Based on the model of how the nebula was formed shown in the illustration I picked from reference [7] and [9], I imagine it is a void and not a dark nebula: the gas and dust in that region is hypothesized
to have been blown away by the bipolar outflow from the young star as seen below:

Parsamian21_Hypothesis.jpeg

Quoting from [7]:

"the star drives an approximately north-south oriented bipolar outflow, which had excavated a conical cavity in the dense circumstellar material [Fig. aforementioned sketch]. The star illuminates this cavity and the light is scattered towards us mainly from the walls of the cavity."
...
"the morphology and limb brightening suggest a hollow cavity (as opposed to an “outflow nebula”, where the lobes are composed of dense material ejected by the central source)"
...
"We characterised the opening of the upper lobe by marking the ridge along the northeastern and northwestern arcs which we interpret as the walls of the cavity. As viewed from the star northwards, the cavity starts as a cone with an opening angle of ? 60 degrees, giving the nebula in Fig. 4.7 a characteristic equilateral triangle-shape. Farther away from the star the cavity deviates from the conical shape, becomes narrower. The whole cavity occupies an area of 8 000 × 24 000 AU (at a distance of 400 pc). The sharp outer boundary of the nebula implies a significant density contrast between the cavity and the surrounding envelope."


I would like to understand why it is that we can "see through" the part of the cone that is in our way in the center, but the edges are brightened. I'm guessing that this is what is referred to as "limb brightening" here, but I can't find a good resource on what causes it. It seems to have something to do with the nature of light scattered by a highly-reflective but thin cloud of dust.

Of course [7] is one possible hypothesis. They support it by observing polarization etc. But I could not find any papers refuting this point of view, so I guess this is our best model: that the center is a conical void surrounded by a shell that scatters light from the edges.

[Steve Gottlieb]

Great write-up, Akarsh!

I've made 5 observations since 1999 with three different scopes (17.5" to 24"). One example ...

24" (7/24/14): Picked up at 260x unfiltered, but a better view at 375x. The "head" of this cometary nebula is the mag 13 star HBC 687 (FU Orionis type). It was clearly non-stellar, though only 3"-5" in diameter. A low surface brightness "tail" extended about 30"x10" to the north-northwest and fans very little.

[akarsh]

SIMBAD does not contain the complete WDS.

As noted in the observing guide, it's the double star Li 3 = WDS19290+0939.

The goto site for WDS data in a readable format is stelledoppie.it. Li 3: https://www.stelledoppie.it/index2.p...iddoppia=81220

Also, I would like to note that some double stars that are associated with the more obscure nebulae, are sometimes brighter knots in the nebula, as is the case with the VY Canis Majoris nebula (that was Steve's OOTW back in February 2021). Often, their listed magnitudes are brighter than they really are, even if they really are stars. Anyhow, the best way is of course to go take a look.

Ah, I see. Not sure why or how they associated a double-star with this nebulosity, but I imagine that is something that could be detected from a spectrum, perhaps?

I searched through the thesis [7] for "binary" and found this:

"There are several theories about what can trigger the thermal instability in the disk [of an FU Orionis star]. One idea is that the close passing of a binary companion may produce these episodic outbursts."

So apparently the dust hasn't settled on the cause of the thermal instability mentioned in the original post, and it could be due to a binary star. But digging into [7] also says:

"The surroundings of Parsamian 21 have been searched for companions several times, but close-by stars seen in J, H and K-band images proved to be field stars (Li et al., 1994), and no close-by sources have been found at longer wavelengths so far (Polomski et al., 2005)."

[akarsh]

Raul, impressive observation to see this in a 14.5".

Great write-up, Akarsh!

I've made 5 observations since 1999 with three different scopes (17.5" to 24"). One example ...

24" (7/24/14): Picked up at 260x unfiltered, but a better view at 375x. The "head" of this cometary nebula is the mag 13 star HBC 687 (FU Orionis type). It was clearly non-stellar, though only 3"-5" in diameter. A low surface brightness "tail" extended about 30"x10" to the north-northwest and fans very little.

Steve, your "non-stellar" description brings me to a question. (My notes are very meager, so I don't have a way of figuring out what I saw, but I vaguely remember a HSB point.) Can we even hope to see the FU orionis type star, or is it going to be shrouded in its nebulosity? I imagine we even have no way of seeing the accretion disk... The Hubble image, even shorter exposures, don't seem to have a sharp stellaring:
Parsamian21.jpeg.jpe

[Uwe Glahn]

Nice write-up Akarsh.

I want to ad the GN designation - GN 19.26.6 as a nomenclature if the "Atlas of Galactic Nebulae" by Thorsten Neckel and Hans Vehrenberg.

I could also catch both vans and a slightly brighter knot at the NE end of the shorter van as perhaps a result of the diffuse patch. Transparency and darkness were exceptionally good, seeing was good, but not perfect.

sketch: 27", 586x, NELM 7m0+, seeing III
Par21.jpg
home

[akarsh]

That is an extremely impressive sketch with a 27" Uwe. I hope to see that kind of view some day.

[akarsh]

I have found that when I pick a OOTW object that I have only seen many years ago, all the exciting learning about the object from the preparation of the OOTW kindles my interest in taking a closer, fresh look at the object. That's what I did late last month on the night of June 26, 2022 from the Warner Mountains in northeastern California.

Parsamian_21.jpg

This time, I was able to see one of the "jet"s as distinctly curved and bright. It started out heading NW from the bright point, and then eventually curved counterclockwise, heading north, as shown in the sketch. The opposite "jet" on the NE side was also sensed but very intermittently. The seeing was very good at the 7300ft location, and my mirror had cooled down as this observation was made towards the end of the night. I believe I also re-collimated just before observing.

Later at Golden State Star Party, observing the same through Steve Gottlieb's 24", I was not able to detect the NE jet. However, the NW jet showed the same clear curvature described in the 18" sketch.