Pan-STARRS comet spectrum

[Paolo Berardi]

Hi Robin, thanks for adding this element that I had not considered. It could be a further step to correct continuum shape. I have all the data related to Moon geometry but may be not so simple to find a numeric Moon reflectance curve...

...Na emission in the spectrum in the region below the comet (from the extended coma?)...

I suppose, because there seems to be sodium all around the nucleus up to a large distance. See the 21 march bidimensional spectrum and sky (slit geometry showed in my previous post):



Note field of slit view is much larger on 21 march. In fact I used 520 mm focal length (a 6.2 centimeters refractor) against 1250 mm of 16 march.

Sky spectrum is taken just after comet sets (you see for continuum level > zero on comet spectrum), going back with right ascension. Was impossibile for me in some minutes the comet was visible, take comet and sky spectrum at the same time.

Just a doubt... during OHP 2012 a member of group showed us a particular phenomenon that involve earth atmosphere sodium emission at a precise time. I don't remember when it happens... Would be an incredible coincidence!

As soon as possibile I correlate 2d spectrum taken on 21 march slit view and show the result. It should figure where the maximum emission of sodium is placed with respect to the visible tail.

See you soon,
Paolo

[Paolo Berardi]

Christian , I just send you via e-mail comet raw and calibration lamp spectra.
If necessary, I can share all files in "gmail drive" (but you must have a Gmail account).

Ciao
Paolo

[Christian Buil]

The Na emission seems clearly associated with the comet because the strong line asymmetry between the top and bottom of the slit.
It is possible that the sodium emission is associated with large cloud around the comet. Also, the atmospheric sodium flash is much lower.

Serge Koutchmy (Institut d'Astrophysique, Paris) remarked that comet Ikeya-Seki (1965) also showed a of sodium redshift...
This is a rare observation!

Paolo, I transmit your e-mail to Serge Koutchmy - I am convinced that there is a possibility of scientific publication.
If possibile, more spectral observations are needed from you but also from other observers of course (I am out of course
for the moment). LISA, Alpy 600, Lhires, eShel, ..., everything is usable !

Christian

[Paolo Berardi]

Amazing! Christian, would be a great satisfaction if these observations were useful.

I reduced 21 march bidimensional spectra, once again finding a lot of sodium away from the nucleus.

Following image shows correlated slit view and spectrum images:



Focal lenght is reduced to 520 mm, then this observation covers a larger field of view. From nucleus to far end of slit I measured about half a degree. Sodium emission across the entire slit, presenting a bulge related with the dust tail.

I was surprise when I replaced into slit view a very deep image (with same scale) taken by Lorenzo Comolli. Me and Lorenzo performed on 21 march a coordinated observing session with the aim to better detect which comet area (eventually faint) falls in slit.



Sodium seems to produce a strong emission where the deep image shows a dark background! At this point would be interesting to take a spectrum with a shorter focal length, a telephoto or similar...

Lorenzo full image with slit FOV indication is here (actual spectrum is related to blue square and line, I have another spectrum in position indicated by red line):

http://quasar.teoth.it/html/spectra/Pan ... olli_B.jpg
(TEC140, SBIG STL-11K, Lum: 7x120s Central time 18.56 UT)

I synthetically describe reduction steps.

Comet was observed at twilight time. I have only spectrum of darker sky (taken after the comet was set). So in 2d spectrum remains some solar intensities in the background. During twilight the situation is rapidly evolving and for me was not easy to take all correct calibration frames.



To correlate slit view frame with spectrum frame I took a series of spectrum moving a star along slit. Then I measure vertical coordinate for stars and spectra, finding the respective scales. I rescale on this ratio slit image containing in two separate layers star points and comet view. Finally I superimposed, aligning with star spectra and comet spectrum (also in two layers).



Please tell me if something is not correct.

Ciao
Paolo

[Christian Buil]

About the extension of Na emission, I supect you observe a very large cloud linked to the comet. It is similar to CN emission cloud (3880 A line), not detected on photography in general, but easy to see very far of nucleus position with a slit spectrograph. To be confirmed. A CN observertion is welcome also (ideal target for new Alpy 600 spectrograph, because it give very sharp UV spectrum image, see
http://www.astrosurf.com/buil/alpy600/performances.htm) - but it is a very young product ).

Christian

[Filipe Dias]

We are seeing it in the perspective of the Earth-living observer.. It could be interesting to have another eye out there in the Solar System..
But coming back to Earth, the idea of using a shorter focal length is a good one! But it appears we need even more field of view...

If the Sodium is near the comet, it will not fade away very quickly, I suppose.. Well, it will fade mostly due to it going away from the Sun, but not much due to it spreading around the comet, because it is already spread around the comet... So I think there is still time to catch it. This has an advantage, because the comet is moving away from the horizon, each night (though the sun is also setting later), so I think it is a case of Sodium flash, it should fade away faster, than if it is not.. or at least it should fade depending on local observation time. If it is a sodium flash, it will depend much more on observation time, I suppose, and not so much with comet distance to us.

A suggestion would be to place the comet tail on one side of the slit, so that the slit can reach far away from it.. Or, we could simply try to image sky at a known distance from the comet. (perhaps astrometrically determined by visible stars). For instance, trying to find out where the sodium disappears.

We can think in an opposite direction: from computing its distance to the Earth in a given interval, it is going away by around 13.9 km/s. The gas cloud around the comet has to be moving away from us slightly faster than that. For the Sodium wavelength (5893A?), this corresponds to noticing a shift compared to the sodium of our atmosphere (if present) of more than 0,27A. This is another challenge for a Lhires and a 2400l/mm grating, but now we need not use a large FOV, only an F/8-or-slower (for the Lhires) telescope instead ... And since we want resolution, we will need to choose slit size carefully, so exposure-time is still "decent"...
Is the Sodium emission "thin" enough to detect this shift in this way? Is it enough to take a well calibrated spectrum of the emission on the comet, if we assume there is no sodium flash? Will the sodium flash + comet sodium form any detectable variation of a double peak?

Weather, here, has been frustrating!!!

[Martin Dubs]

Hi Paolo and Fil,

very nice spectra, Paolo! About the Na emission over the whole length of the slit I am inclined to the opinion of Fil, that it may be caused by the Na flash in the upper atmosphere. A thin Na layer is located at about 90 km altitude and the yellow resonance fluorescence excited by sunlight. It is only observed a short time during dusk, when the sky brightness is low enough and the sun still illuminates the layer. Those at OHP may have seen the impressive demonstration of Peter Schlatter when the twilight Na absorption suddenly switched into emission and later into absorption again.
Based on the date and time of the spectrum I did arough calculation of the geometric situation.

sky view for L'Aquila at time of spectrum

PanStarrs130321sky.PNG (122.76 KiB) Viewed 13162 times

At time of the spectrum (21st March, 18:21 UT) the sun was about 11.5° below the horizon at L'Aquila, the comet about 7.5° above the horizon according to CdC.
At an assumed altitude of 90 km the horizon is at an elevation of -9.5° or about 1000 km to the west. Since the sun is already 11.5 ° below the horizon, the sun would not illuminate the Na layer in the zenith. Howwever the spectrometer looks at an elevation of 7.5° to the west. The view ray intersects the Na layer about 4.5° to the west, as a little bit of trigonometry shows. At that point the Na layer seems still illuminated by the sun:

geometry of observation

PanStarrs_Na-layer.PNG (22.11 KiB) Viewed 13162 times

The diagram (not to scale) shows the approximate situation. Apparently the visible range of the Na flash would extend to an elevation of the sun of 14° (9.5 + 4.5) below the horizon. This would be around 18:35 UT. If the sky background spectrum was taken later than that it would be clear that the Na flash has disappeared and cannot be subtracted from the comet spectrum. That would explain the almost constant Na emisssion strength over the length of the slit, where comet tail intensity is low. Crossing the tail one sees the increase in intensity of the Na emission from the comet.
These arguments make it plausible that the background Na emission is indeed of terrestric origin. It is not a proof. A test would be to record a sky spectrum at the same elevation with the sun the same elevation below the horizon or even better a time series and see if the Na emission is there without a comet. Unfortunately the weather has not been very good around here, I did not even see the comet yet.

Regards,

Martin

[Lars Zielke]

Hi there

I present a few spectra from 25th and 27th March taken with my Star Analyser.

Na emission is present on all images.

The elevation of the comet and Sun is listed for each record. The stars I used for instrument calibration is also listed, they are taken just after the final comet image. I don't think there's any evidence of the Na emission, but maybe the Sun is to low at that time?

Poster with the results
http://www.nightsky.dk/Astronomi/comets ... -25-27.jpg

The ref. stars for instrument calibration. (Some problems with them, For the inst. calibration I used the wrong binding )
http://www.nightsky.dk/Astronomi/comets ... _stars.jpg

Cheers
Lars Zielke

[Christian Buil]

Lars,

Good job!

My conclusion is inverse of your: the spectra reveal presence of significant Na emission!

The problem concern the use of SA, unable to reveal clealy fine spectral features on angular extanded
object. But the Na signature is evident.

Christian

[Lars Zielke]

Thanks Christian

About your conclusion, do you see evidence of Na emission in the ref. star spectra?