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From recent spectra secured by Joan Guarro, [NII] 6583 is rising in the red edge of H alpha.
6548 is very weak. The theoretical ratio 6583/6548 is ~ 3 and in the blue part of H alpha absorption are still important.
I propose the following question:
why [NII] 5755 is intense and [NII] 6583, 6583 are still very weak?

François

Good question François,
Is it because the electron temperature is still too high?
The 5755 line is produced by transition from a higher energy ¹S level than the ¹D level that produces the 6583 and 6548 lines. So if the electron temperature is still very high, the higher level (¹S) would be much more populated than the lower level (¹D). The result would be [NII] 5755 (¹S to ¹D) is intense and [NII] 6583, 6548 (¹D to ³P) are still very weak.

After re-reading Osterbrock page 109, the longer lifetime of the 1D term could result in selectively more collisional de-excitation, thus weakening the 6583 and 6548 lines.

Perhaps both density and electron temperature are important in this situation.

Yes, James, this is the good way.
In your first analysis, only electronic temperature was taken into account.
This is the case in the low density approximation (HI regions, planetaries nebulae -but not all-, classical novae in late nebular phase ...)
say when the Ne is < 10^4: the Ne term can be neglected.
In this case: the ratio 5575/6584 is almost independant of the electronic density with horizontal lines.
In its current state, the ejecta of Nova Cas 2021 is still optically very thick (as seen with the absorption components of the low members of the Balmer lines) and the electronic density is high and cannot be neglected.
As you mention we must take it in account.

The same diagnostic diagram, but for a wider range of Ne:
Due to the high Ne, the level 4 is depopulated essentially by electronic collisions *before* it can produce a desexcitation by radiation to level 3: the lines 6548, 6584 are very weak or even absent.
The critical density of level 5 is much higher, thus desexcitation by radiative transfert to level 3 can happen.

Critical densities of the [NII] levels:

Level Critical densities
Level 1: 0.000E+00
Level 2: 3.814E+01
Level 3: 2.382E+02
Level 4: 8.865E+04
Level 5: 1.636E+07
Level 6: 9.019E+09

The current intensities of NII lines allow a rough estimation of the electronic density: between 10^5 and 10^7 cm^-3.


François

Thank you for that wonderful explanation François.
I had a hard time understanding the definition of critical density given by Osterbrock and Ferland. For me, their definition was more opaque than this optically thick nova.

Hello,

That is a general view of Nova Cas 2021 this last night, from SMM Obs.

J.Guarro.

In spite of the English weather I have now captured 33 Alpy spectra of Nova Cas 2021, available in the ARAS and BAA databases. It is interesting to highlight the timing of the observations on the light curve from Vmag data in the AAVSO database:
When compared as a stacked, offset plot then I think the spectra paint an equally interesting story. To help resolve some of the detail I split the plots into two wavelength ranges and cropped the y-axis. The spectra are normalised at 5550-5560A and offset by 0.3, the first spectrum on March 22 at the bottom and the last spectrum on December 06 at the top. The spectrum captured near maximum brightness on May 10 is highlighted in red. I think I have the emission lines correctly identified but would welcome feedback on that:
The explosion velocity can be estimated from the P Cygni profiles on many of the emission lines. The inverse relationship between the velocity and brightness of the nova, already commented on by many amateurs, can be clearly seen:
Cheers
Hugh

Hello,

Good job Hugh!

That is a view of Nova Cas 2021 from last 15th December.

Cheers, J. Guarro.

Hello,

Nova Cas 2021 last night.

J.Guarro.

Hello all,

Great jobs Hugh and Joan !!

I obtained three new high resolution spectra of the Halpha line on December 2 at 23 h 35 TU (resolution of 15203, JJ = 2459551.4825, 6 x 600s), on December 16 at 21 h 37 TU (resolution of 18441, JJ = 2459565.401, 6 x 600s) and on December 21 at 21 h 27 TU (resolution of 18732, JJ = 2459570.3941, 11 x 600s) with my LHIRES III (grating of 2400 lines / mm).

Here are all the spectra obtained between November 8th and December 21th :
The visible structures in the Halpha line profile and the relative intensities of the nitrogen forbidden lines [NII] continue their evolution.

According to the AAVSO website, the decline of the Nova continues with a magnitude now close to 10 !

(Johnson V : green squares) Merry Christmas and clear skies to all of you !!

Christophe BOUSSIN