[vsnet-chat 538] Re: Red variables and high B-V values

[Fraser Farrell <fraserf@dove.net.au>]

G'day all,

(Berto's discussion of blackbody radiation omitted..)

   >When the star surface cools, we see a fainter star, partly due to
   >the lower bolometric (over the total spectrum) radiance (though
   >the effective radiating surface area might be larger) but mainly due
   >to its spectrum shifting towards longer wavelengths and outside
   >our visibility range.  Further temperature decreases will cause
   >dramatic drops in visual magnitude, but IRAS would only notice
   >small drops in its signal.

The visual magnitude of long period variables is also affected by changes in the
chemical composition of the star's surface and atmosphere.  As the star expands
and cools, molecules (eg: TiO, FeO, carbon chains) can form and accumulate.
When the star contracts, these molecules are destroyed by the increase in
surface temperature.

Many of these molecular compounds absorb visible light.  Spectroscopically, some
will appear as broadband absorptions, others as a multitude of narrow spectral
lines.  The absorbed energy is re-radiated away from our line of sight and/or at
longer wavelengths (eg: infrared); thereby increasing the visual amplitude of
the star.  I read somewhere that changing chemistry adds 1.8 magnitudes to the V
amplitude of a Mira of solar composition.

In addition, the light radiated in a Mira's emission lines will change during
a pulsation cycle.  I don't know if this is significant for a visual observer
but it's something else to consider.


   >All the above implications apply to photometric (read: radiometric)
   >measurements in the different 'photometric' systems and
   >wavelength bands, and to photographic plates, using different
   >emulsions (read: of different makes and from different batches and
   >ages).
   >The 'new' variable near M27: could it be such a case?

I think not.  Gianluca Masi used a CCD.  I assume the same optics, CCD chip, and
software were used for all the images!?

I suspect the reasons this variable has been missed is because of its faintness
and/or the distraction of a lovely planetary nebula.


   >PS: Another phenomenon occurs when we are staring at a red
   >star. The star seems to grow in brightness, Is this change in

Part of the Purkinje Effect...




cheers,
Fraser Farrell

http://vsnet.dove.net.au/~fraserf/   email: fraserf@dove.net.au
traditional: PO Box 332, Christies Beach, SA  5165, Australia

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