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What's this OIII signal in Sh2-86? (If it's in Sh2-86)

OIII SHO NGC6823 Sh2-86 NGC Objects
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Björn Arnold avatar
Björn Arnold avatar
Hi,

I recently did a narrowband imaging project in SHO:


Sh2-86, NGC6823 and NGC6820, >>the lonely pillar of creation<<


While I was preparing the image, I have already noticed some signal in OIII that caught my attention and which I'm currently analyzing right now in more detail.
The following collage shows a cropped section of the final image's narrowband channels. 


I'm interested in the indicated OIII patches.
Does somebody know what this is? Is it stars embedded in the nebula and ionizing the oxygen? Are these planetary nebula?
Maybe there are some members here who are also digging a bit deeper into the astrophysics? 

In order to get closer to an answer, I did some analysis:
In Ha and SII, the position is identified through stars which aren't (easily) visible in OIII. Instead at their position, I see the diffuse OIII signal.
To see if they are artifacts, I did some checks. The patches are even slightly visible in each sub exposure. I also prepared an OIII stack without clipping to confirm that the data shows actual signal and to show that the star is not removed. The OIII was captured over two consecutive nights.

I've checked the SIMBAD database and could identify the stars through the GAIA EDR3 catalogue. The stars' ids are provided within the image.

For the stars themselves, I could extract following R, G and B magnitudes from the GAIA database (https://gea.esac.esa.int/archive/):
Star, R. mag, G.max, B. mag
2020090592985593344, 11.8, 13.5, 18.5
2020084442592330496, 12.1, 13.7, 18.4
2020038086994047744, 10.9, 12.6, 17.9

These numbers reflect the fact that we cannot see them in OIII.

Through the associated database for the estimated star distances, (Bailer-Jones et al. (2021)), the distances (pc) of these stars are estimated to be
Star, avg. dist, avg. dist - sigma (LCL), adv. dist + sigma (UCL)
2020090592985593344, 5790.0, 3880.7, 8703.1
2020084442592330496, 5408.2, 3636.0, 8203.8
2020038086994047744, 1879.0, 1365.1, 2922.0

The distance estimates show quite significant spread. At least for the last star (2020038086994047744) it seems to be located within the nebula Sh2-86 (dist. approx. 1840pc. source: https://apod.nasa.gov/apod/ap041004.html).

For the more distant stars, it would either mean that the extension of the Sh2-68 nebula is very far, with a possible reach over 2 to 3 kpc. The alternative I see is that they are planetary nebulae.

Feel free to comment!

Clear skies!
Björn
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andrea tasselli avatar
andrea tasselli avatar
Hi Björn,

They could be infrared halos if the OIII is somewhat leaking in the IR band. I doubt them being PNs but the only dead certain way is to measure the Hbeta flux since they are supposedly bright in the OIII bands.This short of full head-on spectrometry. Against the hypothesis of them being PNs is that they look suspiciously very similar in size and morphology and only different in brightness, in tune with the supposed parent star. If they are true PNs they should be easily identifiable in integrated/green light. Finally, what happened to the star flux in the OIII image? Besides the fact that PN residual cores tend to be white hot objects not dim red ones and your stars are all having a strong red colour.
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Björn Arnold avatar
Björn Arnold avatar
Hi Andrea,
Against the hypothesis of them being PNs is that they look suspiciously very similar in size and morphology and only different in brightness, in tune with the supposed parent star

I agree on that. It would also be extremely unlikely.
Finally, what happened to the star flux in the OIII image? Besides the fact that PN residual cores tend to be white hot objects not dim red ones and your stars are all having a strong red colour.

Since I am using full false color imaging, you can't go for my star color. But the numerics from the GAIA satellite support the fact that blue is missing and also green. Hence the stars are red.

For the filter, I would refer to the information from Optolong: https://www.optolong.com/cms/document/detail/id/62.html
Seems to be flat in IR.

Björn

EDIT: I did a followup on Aladin-Light. The stars seems to very bright in IR (GLIMPSE360 data)
andrea tasselli avatar
andrea tasselli avatar
Björn Arnold:
Since I am using full false color imaging, you can't go for my star color. But the numerics from the GAIA satellite support the fact that blue is missing and also green. Hence the stars are red.

For the filter, I would refer to the information from Optolong: https://www.optolong.com/cms/document/detail/id/62.html
Seems to be flat in IR.

I wasn't. I was using the reported magnitudes. As for the transmission curves they seem a bit "artificial" to me rather than the output of a spectrograph. If you see what I mean...

Aladin and Vizier report nothing that might hint to PNs in the zone, AFAIK. In a molecular cloud, besides?
andrea tasselli avatar
andrea tasselli avatar
andrea tasselli:
EDIT: I did a followup on Aladin-Light. The stars seems to very bright in IR (GLIMPSE360 data)

I suspected as such. See if it is I-Band.
Björn Arnold avatar
Björn Arnold avatar
Hi Andrea, 

I've worked a bit with Aladin and tried to collect more data:

I'm doing a quick summary on the datasets in the above image (source: Aladin v11.0):
DDS2 color 400nm..600nm
JSPR1 850nm (no bandwidth provided)
SPITZER IRAC 1 3.1um..4um
DSS2 NIR 700nm..950nm
andrea tasselli:
As for the transmission curves they seem a bit "artificial" to me rather than the output of a spectrograph. If you see what I mean...

On second glance, they're even saying that they are rather illustrative. But now to the IR point:
If the filter were significantly transmissive in IR, wouldn't I see a star instead of this blurry "something"? I'm just trying to figure out by which mechanism the halo should come to be?

I'm curious about the type of star as I start getting the impression that it may have an intensity maximum in the IR (certainly a correct spectrogram would help).

Björn
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andrea tasselli avatar
andrea tasselli avatar
Hi Björn,

Seems quite nice assembly over there. I'd discount the Spitzer imagery as, if I recall my AstroPhysics right the atmosphere is rather opaque at those wavelengths. Plus, you'd need a sensor sensitive at thermal wavelength which I doubt is what our CMOS got. You could try to map stars with a significant excess of IR/R vs B (e.g. OIII as you don't Hbeta) and see what the final match is going to be.
Björn Arnold avatar
Björn Arnold avatar
Hi Andrea, 

The atmosphere is still transparent around 3um but it won’t matter for the sensor as you’ve said. I only put it there to see what’s special about these locations in space.

If you look at the SPITZER signal, the three stars are outstanding w.r.t. to the other „background“ stars. If we go to the NIR they aren‘t too special anymore. Let’s take a look at the three neighbors of star 1. They seem to have a similar NIR brightness but show this imaging artifact. In the whole image there are stars with apparently similar NIR intensities.

While I agree with you that the things point towards an issue with the IR transmission, it’s strange why other comparably bright stars do not show this behavior. Do you have a hypothesis for this?  

EDIT:
I will have to make an experiment to confirm the hypothesis. I could place an L filter in front (i.e. towards the aperture of the scope) of the OIII. The L should remove the NIR and leave the OIII nearly untouched.

Björn
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Björn Arnold avatar
Björn Arnold avatar
Hi!

I did a test and placed the camera in front of some strong IR-LEDs (from a home surveillance camera). Here are the results:


The numbers give the intensity of the respective image center (intensity on a scale of 16bits, linear image). In order to test the IR-blocking capability of the Ha filter, I've attached the Ha in front (towards light source) of the OIII. The IR LEDs create too much red light which could not be distinguished from IR hence the OIII. Although I probably should have increased the exposure time for the filter combo to make a more detailed assessment. However, from the sky imaging, the stars were crisp clear so the Ha and SII seems to be fine in this respect.

(The camera was about 120mm away from the light source, unity gain and exposure time of 50ms. The room was darkened for the test and no other light sources except the PC display).

The famous question: claim that the filter is defective and contact the vendor or live with this fact, knowing the strong IR sources might give image artifacts?

Cheers and thanks for your help!

Björn
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andrea tasselli avatar
andrea tasselli avatar
Björn Arnold:
The famous question: claim that the filter is defective and contact the vendor or live with this fact, knowing the strong IR sources might give image artifacts?

Glad you worked out the issue of the OIII filter. I wouldn't hesitate a second and return the filter. I have, to the last count, 3 OIII filters (one visual, Lumicon) and not one ever gave me this sort of issue, which is *very* annoying in my book.
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Björn Arnold avatar
Björn Arnold avatar
I'm approaching the vendor. I've found an apparently more realistic spectrum of the filters:
https://www.astroshop.de/linienfiltersets/optolong-sho-filter-kit-2-/p,67530

One can see some transmission at 820nm and something more serious starting at 950nm.

I could imagine that this bit of transmission is sufficient for a permanent issue with this OIII filter and I should probably change to a different manufacturer. You opinion?
andrea tasselli avatar
andrea tasselli avatar
As I said before that is a very first for me, in terms of actually witnessing the issues "second hand" so to say. In my book is very bad. Not just the spillage but also the defocusing issue at those wavelengths. A total no-no. Stick with either Astronomik, Baader or Astrodon. I have/had all of them they are just good as they get.