HYDROGEN VARIATIONS IN OMI CET

[Joan Guarro Flo]

Hi,
The graphics showing intensity variations in the Hgamma and Hbeta, on the Omi Cet star. Is that normal ?
J. Guarro.

[James Ley]

Hello Joan,
I've lurked for a while on this website and have always enjoyed your spectra.
I'm certainly no expert, but yes Mira variable stars are pulsating stars and their Balmer emissions vary with their pulsation phase.
I found this recent article very good. Notice Figure 4 on page 5 that shows the differences between carbon-rich and oxygen-rich Mira vairables in their Balmer line equivalent width as a function of phase.
https://arxiv.org/pdf/1708.04703.pdf
I also think this is an interesting article because it offers many Mira targets for amateur spectroscopy. Do you think that Mira variables are as common amateur targets as Be stars and eclipsing and cataclysmic binaries?
Here are two spectra that I took of Mira last year with the Star Analyzer 100.

Mira 1222017.png (15.07 KiB) Viewed 6540 times

Mira 21117.png (16.43 KiB) Viewed 6540 times

[Joan Guarro Flo]

Hi James,

Thank you very much for your clarification, I am only an observer, I am not an analyst and sometimes I ask questions (which may seem very simple), which I do not know the answer.

When the observation of stars like MIra agrees, this may be better answered by François Teyssier, he is the coordinator of symbiotic stars.

Your Omi Cet graphics with Star Analyzer 100 are really good. For my part, I will be watching this star to follow the evolution of the Hydrogen lines.

Friendly, Joan.

[James Foster]

To: Joan,

James L. was helpful to me as well with regards to Omi Ceti....here are some near UV H-lines seen with the LhiresIII spectrograph btw 3700 - 4100 A within the last week:
http://spectro-aras.com/forum/viewtopic.php?f=5&t=1949

James

[Francois Teyssier]

The emission lines in mira stars is the result of the pulsation of the external layer. Typical velocities of the expending and recessing layers are ~ 10 - 20 km/s.
Balmer lines appears near maximum luminosity as result of the propagation of a shock wave which produces locally a strong enhancement of the temperature (from about 2500 K to several 10000 K). This produces dissociation of H2 molecule, ionisation and or excitation of H atoms. In the relaxation zone, H recombines and cascades, producing the emission lines.
The physical conditions (strongly non LTE), optical depth, ... explain the anomalous Balmer decrement which is far from Case A or B
Note that the anomalous
line ratios (e.g., Hy/H<5 ~0.7) may also be explained by
having a moderate optical depth to Balmer line radiation in
the immediate postshock region. The scattering cross section is
lower for higher order members of the Balmer series so that
they will suffer less attenuation than lower order members.
This will cause a reduction in line ratios such as Hy/H<5.
Finally, we note that the weakening of the negative velocity
emission component past maximum is due to the decrease in
the scattering optical depth through the hot immediate postshock
region. Fox & al. 1984

Shock waves in Mira variables (Fox & al.,1984,1985)

http://adsabs.harvard.edu/abs/1984ApJ...286..337F

http://adsabs.harvard.edu/abs/1985ApJ...297..455F

see also a summary in Astrophysics of Emission-Line Stars (2007) p.346

Schematic diagram from Fox &al., 1984

Miras_ShockWave.PNG (128.63 KiB) Viewed 6462 times

See also :
http://www.astronomie-amateur.fr/feuill ... Miras.html
and spectra obtained with LISA (M, C and S miras)
Note the Balmer decrement Ha>Hb>Hg in S and C miras
http://www.astronomie-amateur.fr/feuill ... ctres.html
and a document written by Dave Mais:
http://www.astronomie-amateur.fr/Docume ... bility.PDF

There is very little published quantitive data on the shapes, widths and fluxes of metal emission lines.
Richter ans Wood, 2001

So, mira stars is an excellent project for amateurs, at low and high resolution
The issue at high resolution is the range of luminosity during a plusation. Most of the miras are faint at low luminosity
With Alpy or Lisa, the target should be monitored dunring all the pulsation.
With well calibrated spectra, the study of the variation of the type is possible from the measure of the deepth of TiO bands.
See : http://www.astronomie-amateur.fr/feuill ... ceTiO.html
A very interesting subject is the strong variation from a pulsation to another (see light curves)


About Mira itself.
Of course, it is the prototype if mira stars https://www.aavso.org/media/jaavso/1611.pdf
But it is also a symbiotic mira, say a ‘symbiotic-like’ or ‘weakly symbiotic’ system
the faint B8 companion (Mira B) was discovered by Joy (1926) at 0.6"
Margarita Karovska (1993) deduced an orbital period of 400 years fromp speckles observations
http://adsabs.harvard.edu/abs/1977PASJ...29..319Y
The symbiotic nature of Mira appears only in UV range with high ionized lines
From my current knowledge, the optical spectrum shows only the behaviour of the pulsating red giant Mira A

Miras symbiotics are classified as D or D' symbiotics (with D for dust) an accompt for about 20% of the symbiotics.
One of the best example is R Aqr, well known by ARAS observers

François

[James Ley]

Thank you Francois.
I have read your Low Resolution Spectroscopy Observer's Guide, but I did not know about your website. That is fantastic!

Mira variables are targets that I would like to observe. Because of weather, work and family, long period variable stars like Miras are probably easier for me to follow than other variable stars. However, rather than try to follow one Mira variable only, my idea was to observe several different stars whenever I can and note their phase from the AAVSO. Does that sound like a reasonable approach or are there often differences between calculated and observed phases?

Curious that the pulsation mechanism of Mira variables is thought to result from the ionization and recombination of hydrogen (kappa mechanism) but I guess that we never see those photons directly.

Also interesting is how the theory of Mira variables doesn't address binarity, which has revolutionized the understanding of many other astronomical phenomenon. Perhaps some new observations and understandings await in the future...

[Francois Teyssier]

Hi James,
Nice project!

Quelques suggestions - in french, easiest for me and good for your practice

Plusieurs cibles: oui, d'autant plus que la déclinaison de Mira est faible, d'où une période d'observation réduite dans l'hémisphère nord
Fréquence: je pense qu'un spectre par semaine représente un bon rythme vu les périodes des pulsations
Peut-être augmenter la fréquence lorsqu'apparaissent les raies en émission
Observations sur l'ensemble de la phase orbitale
Choix :
- plutôt des types M au moins pour commencer. Le pseudo continuum des types C est très complexe
- V min ~ 10-12 pour faciliter le suivi au minimum de luminosité, surtout à haute résolution
- déclinaison élevée afin d'augmenter la période d'observation
- choisir parmi les cibles des étoiles dont la courbe de luminosité est assez reproductible d'un cycle à l'autre et d'autres plus variables

La réponse instrumentale doit être très bonne pour un bon suivi des indices TiO
Il peut être intéressant, pour chaque résolution, d'obtenir des spectres de références d'étoiles M "normales"

Tableur Miras : http://www.astronomie-amateur.fr/Docume ... Miras.xlsm
(il faudrait que je mette à jour les périodes et minima/maxima à partir des dernières données AAVSO)

Bien, sûr si d'autres observateurs se joignent au projet, ce serait une excellente nouvelle.

Je te souhaite bonne chance dans ce projet,

François