TOADS Tremendous outburst amplitude dwarf novae

- In 1995, Howell, Szkody and Cannizzo distinguished a particular type of dwarf novae, characterized by the very large outburst amplitudes of their optical outbursts (6 to 10 magnitudes) and very long intervals between the outbursts (months to decades). These TOADs are a subset of the SU UMa systems (dwarf novae that show both "normal outbursts" and "super-outburst"). Apart from the very long intervals and the very large amplitudes, TOADs also differ from the other SU UMa systems in that almost all TOAD outbursts are super-outbursts. not recognized within the GCVS

With such an intriguing name, TOADs must be interesting objects. Like all CVs, they are members of semidetached binary systems that have orbital periods ranging from approximately one to twelve hours. When studying dwarf novae, distinctions are made using orbital periods. Those stars with orbital periods below the CV period gap (~2-3 hours), belong to the SU UMa group.

Astronomers Steve Howell and Paula Szkody have launched a multiyear project to study the fainter CVs with periods below this gap. Their research has discovered many binary systems that share some of the characteristics of the SU UMa systems, but that have additional unique properties.

These dwarf novae have been named TOADs because of their unusually large outburst amplitudes. Whether or not these systems are really the same thing as SU UMa stars at different stages of their evolution or they are a separate subclass of dwarf novae is still to be determined. TOADs and SU UMa systems can both be classified as dwarf novae with orbital periods less than ~2.5 hours. Both types of systems exhibit super-outbursts.

The unique traits of TOADs are: longer, brighter and less frequent super-outbursts, likely low mass transfer rate (less than 10"" solar masses/year), possibly very low viscosity disk material (lO-lOOx below normal) at minimum, accretion disks may be advected, secondary stars may be degenerate.

There is much debate regarding these stars with suggestions made that they are simply a subset of the SU UMa stars. Dr. Joe Patterson of Columbia University, runs the Center for Backyard Astrophysics, a group of amateur astronomers dedicated to the observation of various cataclysmic variable stars. I have found his comments interesting:

There has been much mention of TOADs and I thought it was worth entering a dissent on this term, which seems to me is astronomically and zoologically poor. Dwarf nova fans are familiar with the term SU UMa stars and the meaning is clear: dwarf novae whose eruptions divide very distinctly into long and short, and which show super-humps in their long eruptions. Some people also use the term WZ Sge stars to refer to the subset of SU UMa stars that either: (a) show the longest outburst intervals, (b) have few or no short outbursts, or (c) qualify according to both (a) and (b). Because this is somewhat vague and because Nature provides no dividing line, many of us either do not use the term WZ Sge stars or use it only as a convenient shorthand for a more cumbersome phrase, the most infrequently erupting SU UMa stars. Whether it is a useful subclass is harder to say.

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