[vsnet-chat 1926] Re: Perplexing questions, UV delay?

[Taichi Kato <tkato>]

Re: [vsnet-chat 1922] Re: Perplexing questions, UV delay?

   Yes, your interpretation of the UV delay seems to correspond to what is
most supported by astronomers.  Outbursts of dwarf novae, esp. rapidly rising
outbursts, are believed to start in the outer region of the accretion disk
(as you know, the viscosity instability theory is the best accepted
theoretical explanation).  The outer region of the accretion disk is cool
enough to emit visual light, leading to the optical maximum.  The hot-state
transition of the disk then propagates inward, finally reaching the innermost
region of the disk, where matter falling in a steep gravitational potential
prefominantly emit UV radiation.  The UV delay is thus considered to be the
travelling time of transition wave through the disk from outside to inside.
(There have been, however, a number of arguments or modifications to this
rather simplified explanation).

   In anomalous outbursts (slowly rising outbursts), the instability is
usually considered to start from the inner region of the disk, leading to
a shorter UV delay.

   You might find it interesting to read the most comprehensive CV book,
Warner's "Cataclysmic Variable Stars" (Cambridge Astrophysics Series 28,
1995).  The beautiful UV delay light curve is given in page 181.

Regards,
Taichi Kato

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