Pan-STARRS comet spectrum

[Martin Dubs]

Hello,

I described previously the terrestric origin of Na-airglow visible in some spectra. In the meantime I also looked at the literature (with the help of Peter Schlatter) and found some supporting literature. Very profound and giving all the relevant details is the following review by D.M. Hunten:
http://adsabs.harvard.edu/abs/1967SSRv....6..493H
From this the geometry of the Na layer illumination can be seen in more detail:

geometry of sun Na-layer and observer

twilight geometry.png (135.08 KiB) Viewed 10536 times

together with some equations for the calculation of the shadow effect. It confirms my basic assumptions. To be more precise I should have talked about resonance scattering instead of fluoroscence.
Much more about the phenomenon can be found in the article. Nowadays the observation of the Na airglow is not so important any more, more precise measurements of the height and density of the layer can be made with LIDAR especially also for the Na layer as a guide star for adaptive optics.
Since the effect varies quite fast during twilight, it is important to subtract the background taken under similar conditions (same sun elevation at the same sky elevation).

Regards, Martin

[Robin Leadbeater]

There is however a pair of clear emission lines about 90A to the red of the Na D lines

The emission lines at the red edge are at 5976.91 and 5995.65 with an estimated uncertainty of 0.3A
a quick check against the line list in VSpec and with a high resolution solar absorption spectrum has not thrown up any obvious candidates. (There is a strong Fe line at 5976.8 in the solar absorption but nothing for the other line.)
Any ideas?

Robin

[Robin Leadbeater]

Since the effect varies quite fast during twilight, it is important to subtract the background taken under similar conditions (same sun elevation at the same sky elevation).

Hi Martin,

This morning, as soon as the sky became too bright for taking comet spectra, I slewed away from the comet and kept taking sky spectra at 1 min cadence. It was fascinating to watch the Na D emission give way to absorption, though the emission may still have been there in the core of the absorption line. Even at ~2.5A resolution the narrowness of the emission lines compared with the absorption lines was discernable. We still have a lot of low pressure sodium street lighting in this area so some of the emission was undoubtedly from this source (apparent at all times of the night) This too will disappear at dawn of course as the lights switch off.

To reduce this spectrum I think I will probably subtract the sky background based on the area below the obvious region containing the comet spectrum.

Cheers
Robin

[Robin Leadbeater]

There is however a pair of clear emission lines about 90A to the red of the Na D lines

The emission lines at the red edge are at 5976.91 and 5995.65 with an estimated uncertainty of 0.3A
a quick check against the line list in VSpec and with a high resolution solar absorption spectrum has not thrown up any obvious candidates. (There is a strong Fe line at 5976.8 in the solar absorption but nothing for the other line.)

Looking at this line list
http://www.gps.caltech.edu/~mbrown/comet/echelle.html
the most likely candidates seem to be NH2, perhaps redshifted by ~0.5A

Robin

[Robin Leadbeater]

I will need to do some careful background subtraction to see if there is still any emission. There is however a pair of clear emission lines about 90A to the red of the Na D lines. More later when I have reduced it.

Here is the reduced spectrum. 2.7A resolution, background subtracted, divided by G2v star (15UMa) There is still a small amount of Na D (and NH2, C2) emission detectable in the most condensed part of the coma

Cheers
Robin

[Paolo Berardi]

Excellent Robin! Interesting the NH2 lines presence. Thanks also for the link to lines catalogue (would be useful to create a dat file profile for a more immediate recognition).

Your observation shows a dramatic decrease in comet sodium emission compared to 16 March observation. This confirms Lars remark about emission D1D2 absence in SA100 spectra (now it is probably too weak to appear in such a spectrum). The distance of the comet from the sun is increased from 0.36 to 0.80 AU. Probably the mechanisms underlying sodium emission processes are impacted. I don't know if the comet may have a sodium "emptying" as a result of perihelion passage.

I would like to thank Martin again (and Peter Schlatter) for detailed considerations about Na-airglow phenomenon. In my experience I can only say that it's very difficult to produce an optimal sky spectrum to be subtracted from the comet one in the same observing session. Twilight is rapidly evolving and there is not much time to capture comet and related sky spectrum. It should perform exposures pointing alternately comet and a portion of the sky away from it. Not so easy, especially with some clouds! Perhaps it is now possible, having more time available.

Ciao
Paolo

[Filipe Dias]

After what has been presented, I feel somewhat ashamed for this post, because it is a very noisy and bad spectrum.. But I tried to go after the tail, and it is difficult.. I also think the resolution I have of the light pollution is very low, so something was likely done wrong.. (collimator focus? Too fast F/6.8 refractor?)
WO-80mm F/6.8 (I know I am wasting light, but I wanted the focal length)
LhiresIII + 300l/mm
KAF-402 binned 3x3 3x300s

slit view + 2D spectrum + sky subtracted spectrum

20130407_panstarrs_spectrum.jpg (120.57 KiB) Viewed 10509 times

slit view + 2D spectrum + sky-subtracted spectrum

[Paolo Berardi]

Fil, as you can see on a light curve:

http://www.aerith.net/comet/catalog/2011L4/2011L4.html

Pan-STARRS comet decreased its brightness from mag 0-1 at mid march to mag 4-5 at the time of your observation. Ok for decreased twilight but there is a 40x factor in brightness intensity! Furthermore you observed outside false nucleus and coma.

Anyway your observation confirms that now there isn't any relevant D1D2 emission in comet tail and around coma (I note no intensity into HPS self-absorption). Na emission was also weak (EW -1.8A) at nucleus in last Robin observation.

Concerning resolution I suspect can be caused by the thermal gradient if you spent a lot of time between the doublet set-up and the start of spectrum capturing. Otherwise I have no idea...

Ciao
Paolo

[Paolo Berardi]

I update the topic with a feedback concerning sodium flash. This is a quick reply, I just come back from an observing session aimed to capture sodium flash event. This is the first opportunity to perform a test with west horizon completely free of clouds since 21 march. Actually I had another chance on 3 april but there was some clouds in sky target area and I had to change position (a little) to avoid them. So, I waited for a better session. For present observation I had to move to another site (not far from 16 and 21 march site) in order to have unobstructed sky at precise azimut/altitude (to respect same geometry).

Maximum intensity sodium flash event was recorded at 18:56 UT with Sun at 12.9 degree below horizon. In single 60 sec frame (SXVR-H694 in binning 2x2), emission became visible at 18:47 UT (Sun depression 11.1°) and has disappeared at 19:02 (Sun depression 13.9°).

Intensity level of Na emission line at maximum is very low compared to 21 march comet observation (single 60 sec frame):




The difference is even more pronounced in the similar geometry comparison:



This is the maximum sodium flash intensity level of 3 april 2013:
http://quasar.teoth.it/html/spectra/SF3.jpg

All images are 60 sec exposure frames.

Sky condition on testing evening (14 april):
http://quasar.teoth.it/html/spectra/sky_cond.jpg

I don't know how much sodium flash intensity can vary, but from what I have observed it would seem that sodium emission line, outside the bright part of tail, of 21 march comet spectra is very low correlated with telluric phenomenon (maybe only a small percentage), as Christian suggested.

Ciao
Paolo

[Ken Harrison]

Paolo,
Great follow up observation!
I think you've made your point very clearly.
Well done.