[Aavso-photometry] [AAVSO-DIS] AB LYR - I'm rarther confused

[arne]

Martin Nicholson wrote:
> AB LYR is one of the brightest stars in the field in a 30 second IR exposure but is hard to see in a 30 second V exposure.
>    
>   Using TASS comparison stars gives a V-I value of >6. This implies a spectral class of around M9 rather than the M4e listed in VSX. Am I correct in thinking that the quoted value represents a typical value - remembering that at the moment AB LYR is close to minimum V magnitude - and that as the star brightens the V-I value will get smaller since the I band amplitude is smaller than the V band amplitude?
>    
>   http://www.tass-survey.org/tass/catalogs/uvby.calib.1 
>    
This estimate of (V-Ic) is likely in error for several reasons. I don't
think you have transformed your results; the Ic measure in the TASS
catalog has systematic errors; you have to make sure you are using
conversion tables for giants rather than dwarfs; and most importantly,
the Ic filter used by most amateurs has a long red tail that
systematically makes red stars even brighter than they really are
(and therefore systematically overestimates the size of V-Ic).
Molecular bands do some interesting things to broad-band filter
measurements, so don't assume you have a reliable color estimate.
Use it as a guide; obviously you are dealing with a red star rather
than a blue one!  I would also question your exposures; equal
exposure time for a red object means either saturation in the
Ic filter or low signal/noise in the V filter, so the photometric
error in your color is probably quite high.

If you use 2MASS colors instead, AB Lyr is about (J-K) = 1.2.
Table II from Bessell and Brett (1988PASP..100.1134B) indicates
that this is about M4-M5 for a giant.  The M4e spectral type comes
from the catalog by Walt Bidelman (1954ApJS....1..175B), so is
probably pretty good.  Pulsating stars do change spectral type
through their cycle, since you are seeing a temperature change
on the surface.  They are redder at minimum than at maximum, so
it is true that an M4e classification would depend on what phase
the spectra were obtained, but I really think the major cause is
your Ic filter.
Arne