Planetary Nebulae with Lhires

[Filipe Dias]

Hi,

I am excited with a project I started on the 29th and 30th of October, the two last nights before the stormy weather here.. For these two consecutive nights, I left my equipment assembled for 36h outside, so it was my first glimpse for what having an observatory feels like, although without a roof/dome..
I would like to ask a few questions about what is visible in the spectra, and ask for feedback on the (bad) quality of the spectra.

Idea
I tried to take spectra and measure the expansion rate of two Planetary Nebulae (the huge Helix Nebula NGC7293, and the tiny NGC1514), and also looked at part of the Rosette Nebula (vicinity of Oe star HD46150) for a taste of comparison..
I imagined a Planetary Nebula as a semitransparent balloon that is inflating. I though that in a spectrum across the diameter of a PN, I would see a ring shape: at the centre of the PN I should be observing the front surface moving towards me, and the back surface moving away. Indeed I see this in the spectra, but with some unexpected twists!

Results
Dispersion is 0.17 Å/pixel horizontally, and spacial resolution is roughly 3.2"/pixel vertically, East is up on all images.

NGC 7293 - 27x1200s (rigt-click: "view image" or "show picture" to see the complete image)

NGC7293 Spectrum and Doppler shift

NGC7293_complete.jpg (98.23 KiB) Viewed 5580 times

NGC 1514 - 3x1200s

NGC1514 2D Spectrum (Ha, no [NII]!)

n1514_scale.jpg (50.42 KiB) Viewed 5580 times

NGC Rosette - 10x1200s (rigt-click: "view image" or "show picture" to see the complete image)

Rosette 2D Spectrum (Ha + [NII])

rosette_scale.jpg (45.26 KiB) Viewed 5580 times

I found particularly intriguing the appearance of the forbidden [NII] lines (at 6583.5Å and 6548.1Å). Why do these lines appear? Is it due to collision of stellar wind with material, or is it due to the intense UV radiation from the White Dwarf? Why are they present in NGC 7293 and the Rosette and not in NGC 1514 ? Could NGC 1514 be too young or of a different origin ?

In the Helix Nebula, the [NII] lines appear to be originating on the expanding surface of the nebula. In the Rosette, they also appear to be on the "contact" surface between the Hydrogen and the Light coming from the bright stars.. This makes them very handy for giving clues to the relative movement of the gas, there. We can clearly identify a knot in the Rosette, travelling at a slower speed than the background nebula.
I was intrigued to find the H-alpha emission did not follow the same "ring" pattern of only expanding walls of the PN! There is more gas in the same line of view travelling at other (slower) speeds, to fill the inside of the expected "ring" shape of the 2D spectrum, (or a wider emission combined with a bad spectrum?).

Speed
Finally, an article about knots in Planetary Nebulae states that the expansion rate for the Helix Nebula is 31~32 km/s (astrometricly measured). Based on my spectrum, with a shift of around 1.067 ~ 1.087 Å between inbound and outbound PN walls in the [SII] emission, I would estimate the expansion not to be far from 24~25 km/s, as I am measuring near the line of sign of the central star. Wikipedia states that NGC 7293 seems to be oriented at an angle of 21 to 37 degrees, and I noticed that 25km/s divided by cos(37º) = ~30km/s. Is this just a coincidence? The article only says that "spectroscopic measurements confirmed" the speed, but they do not say what speed they measured spectroscopically, nor do they show any didactic spectrum. What explains this difference ? Bad spectrum or something else? A faster movement on the "equatorial plane", between the rings than towards the observer? Could I be looking at wrong wavelength for this?

Quality
Spectrum quality is poor, I think. I got very low signal, despite the 35um slit and having combined 27 frames of 1200s over two nights! I used a 4" refractor (102mm) for two reasons: focal length would make the Helix nebula fit aesthetically inside the silt, and F/8 would be the fastest scope I have to go with the LhiresIII for capturing the extended nebulosity. The problem with a small aperture then was to see the faint central star !! Guiding had to be done on a star outside the slit. (Watec-120N+ on a mag 10 star)

Not heaving a stellar spectrum, with a weak signal compared to hotpixels, forced me to manually align the images to compensate for flexure and the two nights of observation.

Images also have some visible residual image left behind from the calibration lamp.. Only darks + flats used, no correction for the atmospheric lines.

Does it look like my spectrum could be out-of-focus ? Are these targets too far-fetched for a 102mm aperture?

Cheers,

[Robin Leadbeater]

Hi Filipe

That is remarkable to get high resolution spectra on such faint objects with such a small aperture.
I know it is easier to do with emission spectra, but 9 hours exposure - wow !!
I will add your observation to my list of "I would not believe it if I had not seen it with my own eyes" examples. It is good fun showing these sorts of observations to some sceptical professionals and watching their face

Cheers
Robin

[Olivier Thizy]

9h combined exposure... I believe this is the longuest I have ever seen!!!

Anyway, result is really nice!

Cordialement,
Olivier Thizy
Vous ne verrez plus des étoiles comme avant !
http://www.shelyak.com/en/

[Filipe Dias]

Thank you Robin and Olivier.. Perhaps I was worried about doing something wrong, or not having focused correctly because I thought the signal was very weak.. But if the Helix is faint, that explains it... I am not yet acquainted with the differences in exposure from normal take-a-pic to spectr-a-pic
For observing stars, where the amateur can always amplify more and still not resolve it, aperture rules! 4-inch may sound small, but I chose the scope based on focal ratio in hopes that it would benefit me in taking spectra from a supposedly "uniformly out-of-detail extended surface" (the ionization front on the nebula).

I was also worried about something else.. What I did suggest the expansion rate to be different (slower) from the rate that I read in the wikipedia/article..
From what I know, 31km/s would imply a difference of 1.46 Angstroms between the two observed emission lines, give or take. That is over 36% greater than what I measure. I would not mind thinking that my measurement has an error of 37%, the problem is that the [NII] photons should have hit my sensor way further apart than they did!

After reading the article more carefully, I found that the person who cited this article when writing the info in wikipedia, was hasty! I was also hasty in interpreting the information.
The 31~32 km/s expansion speed is mentioned in the article (section 2.4) when the authors describe the NGC7293 as what was known at the time of writing (i.e. a different article from 1997) and referring to measuring a different thing. What the authors have measured in this article was 24 +/- 1 km/s, and related to the [NII] emission on the knots (section 4.4).
I would risk saying: that was precisely what I measured and that makes me happy, after all!

So, it looks there is something expanding at 31~32 km/s as a reminder of the most recent "Kaboom" (also known as mass ejection). There is, however, other stuff that is being ionized by the star, and that produces [NII] emission, and that is moving at 24 +/-1 km/s, matching my measurement!
Also I could get hints from the article that if radiation from the central star can be blocked to a point where it will not be able to ionize "stuff" in an [NII]-spectacular way as if does in NGC7293, thus providing a possible answer to my question about "why would NGC1514 not have [NII] visible?".

It may have taken my camera 9h to reach the same conclusion professional cameras reach in 51x48 seconds (13x less!!!)... But I am sure all my equipment was more than 13x cheaper! And it gave me more than 13x more fun to learn than it would have if someone had told me beforehand.