VARIABLE STAR CLASSIFICATION AND LIGHT CURVES III
- PAPER REVIEW
REVIEW OF - THE SHAPE OF LONG OUTBURSTS IN U GEM TYPE DWARF NOVAE
FROM AAVSO DATA
BY JOHN K. CANNIZZO
Link to original paper http://arxiv.org/pdf/1208.5477v1.pdf
Motivation
I find Cataclysmic Variables (CV) an interesting group of objects and I have limited
knowledge of superhumps which are used to categorise superoutbursts of SU UMa stars.
Although this paper does not directly deal with superhumps it does explain the mechanism
by which they appear so this is of interest to me.
Overview
The aim is to find embedded precursors to superoutbursts that have been found in the
Kepler space observatory data for short period dwarf novae (orbital periods <2 hours) in
dwarf novae with orbital periods above the period gap (orbital periods >3 hours). This
would reinforce the finding that long outbursts in dwarf novae above the period gap and
superoutbursts in systems below the period gap constitute a unified class. Thermal-tidal
instability predicts embedded precursors for short orbital period dwarf novae, the presence
of embedded precursors in long orbital period systems would call for a more general
mechanism to explain long outbursts.
Observations
The high quality data obtained from the Kepler space observatory was used to identify
characteristics in short period dwarf novae. For the study of long outburst dwarf novae the
two cataclysmic variables U Geminorum (UG) and SS Cyg (UGSS) were chosen as the AAVSO
observational data gave good coverage for a 100 years. With U Gem in the order of 115,000
observations and SS Cyg some 455,000. The data in the early years are single visual
observations so have a low cadence and are prone to visual errors due to the way
individual observers see colour. Whereas observations over the last 10 years from
amateurs using CCD time series photometry have a high cadence with, grater accuracy and
by the use of V band photometry is independent on colour. This study is based on the lone
outburst observations in the AAVSO database of U Gem in 2005 and 2007 and for SS Cyg in
2005.
Evidence of embedded precursors in CV with orbital period > 3hours

Two cataclysmic variables U Geminorum (UG) and SS Cyg (UGSS) were chosen as
their orbital period were Ph=4.25 and Ph=6.60 respectively.

The data used was for the final analysis was restricted to V band and on just three
days (out of the 10 years of data which was studied) which gave the best
characterisation of a long outburst at near outburst onset.

There were two such outbursts by U Gem (2005 & 2007) and one by SS Cyg (2005).

The U Gem outburst of 2005 showed a failed outburst (embedded precursors) at the
start of the long outburst but the 2007 outburst did not.
Implications
In considering the mechanisms that cause outbursts in CV's whose orbital period are either
side of the periods gap have been considered to be independent. This paper argues that the
presence of an embedded precursor found in long outbursts calls for a more general
mechanism to explain long outbursts which spans the period gap.
Conclusions
I found this paper very interesting and it is clear from the analysis of the presence of
embedded precursors in two of the three long outbursts studied, that there is a relationship
in CV outbursts that spans the period gap.
I did not understand the relationship of the thermal tidal instability model and the accretion
disk thermal limit cycle instability but that, I feel, was down to my limited knowledge of this
subject.
Some follow up study on the thermal, tidal instability and accretion discs will help me
understand this more.
Graham Darlington
13th February 2013