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 6150 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