Review of
“Photometric Observations and Numerical Simulation of Early Superhumps in BC UMa during 2003
Outburst”
Hiroyuki Maehara, Izumi Hachisu
Department of General Systems Studies, Graduate School of Arts and Sciences,
University of Tokyo, 3-8-1 Komaba, Meguro, Tokyo 153-8902, Japan
maehara@provence.c.u-tokyo.ac.jp, hachisu@chianti.c.u-tokyo.ac.jp
Kazuhiro Nakajima
Variable Star Observers League in Japan (VSOLJ), 124 Isatotyo Teradani, Kumano, Mie, Japan
K.Nakajima@ztv.ne.jp
PASJ: Publ. Astron. Soc. Japan, 1–??,
c 2008. Astronomical Society of Japan.
arXiv:astro-ph/0611519v1
I chose a paper on BC UMa, an SU UMa type dwarf nova, because I didn’t know a superhump from
Humpty-Dumpty. Also, long term light curve information for this paper was taken from AAVSO, VSOLJ,
and VSNET and the paper seemed to have excellent explanations and descriptions of the physical
processes underlying the light curves. Although, as is common with most reviewed and published papers
in scientific journals, this paper assumes that the reader is familiar with the literature on the topic, it
provided a lot of prerequisite background information that allowed someone who knows little about this
kind of variable gain a good general understanding of concepts and nomenclature without a large
amount of additional research. A notable exception to this was the Theta-period diagram output from
PDM analysis. Further, BC UMa seems to have some unique characteristics that make it a more
interesting target than the average SU UMa, and it is at a good location in the sky for northern
hemisphere observers. Last but not least, reviewing this paper gave me an excellent opportunity to suck
up to Mike because I know he is interested in these stars.
In this paper, observations of the BC UMa 2003 super outburst were compared to earlier observations
from the 2000 super outburst of the same star and to earlier observations of WZ Sge. BC UMA is
classified as a SU UMa type dwarf nova because it exhibits two types of outbursts: Normal, which has
comparatively short duration with a faint maximum (mv ~13 for BC UMa), and super outbursts, which
have relatively long duration and a bright maximum and have superhumps. Superhumps are small
amplitude periodic modulations of the light curve fractionally longer than the orbital period of the
binary stars. Superhumps are only present during super outbursts. The BC UMa shows a unique feature
in that it exhibits two distinct types of super outbursts one longer and brighter than the other (~10 days
and mv ~12 and ~20 days and mv ~11 – 11.5). SU UMa has a subtype, WZ Sge stars, which exhibit super
outbursts but no normal outbursts, have exceptionally long periods between outbursts for a dwarf nova
(~ 10 years), exceptionally long outbursts (a month or more) and exceptionally large ∆mv during an
outburst (~ 6 magnitudes). WZ Sge type dwarf novae also exhibit early superhumps (a.k.a. outburst
orbital humps) characterized by unequal double humps with total period for the double humped curve
indistinguishable from, or very slightly different (shorter in the case of WZ Sge than the orbital period of
the binary stars (but the difference is a small fraction of the superhump excess). In addition to having an
abnormally large super outburst magnitude (∆mv ~ 7), BC UMa also exhibits an unusually long period
between super outbursts, in the range of 600 to 1000 days and exhibits early superhumps with a period
indistinguishable from the orbital period of the binary pair. The authors suggest that BC UMa may be an
intermediate type between SU UMa and WZ Sge dwarf novae. BC UMa also exhibits early superhumps in
the first 2 days of a super outburst.
Light curve features observed and parameters observed and derived had very close agreement with
earlier observations and showed deviations from typical SU UMa characteristics and similarities to WZ
Sge characteristics, which is why the authors claim this star may be an intermediate type between the
two. Classification into types is often arbitrary when, in actuality, objects may fall into more of a
continuum between types or subtypes having similar underlying physical processes. Interesting
additional background information given for BC UMa in the paper includes the following:
Discovery was announced in a 1964 paper by G Romano as a dwarf nova with ∆m ~ 7.
1. Humps of ~ 91 minutes and amplitude of 0.25 mag during quiescence were reported by Howell
et al. in 1990
2. Mukai et al reported in 1990 that both emission and absorption spectral characteristics are
present in its Balmer lines and TiO band features at 760 nm. They estimated that the spectral
type is M5 or later, distance is in the range of 130 to 400 pc, absolute magnitude is Mv 11.013.5.
3. Patterson et al. (2003) reported time series photometry of the 2000 super outburst and
obtained radial velocity measurements that allowed them to determine orbital period of the
binary stars. These were compared to the observations from the observed super outburst.
4. Earlier cited work showed that accretion disks in dwarf novae with extremely small mass ratio
can extend to greater than the 2:1 resonance ratio. Two theories had been proposed to explain
the double peaks of early superhumps. The first is that the 2:1 resonance causes a pair of spiral
arms to form. The second was that irradiation of elevated accretion disk caused by tidal
distortions.
Specific results published in the paper are the following:
1. The 2003 super outburst that was observed for this paper was reported at Feb 1.205, 2003.
Observations were started 9 hours after the report and the magnitude of BC UMa was reported
at less than 13.7 magnitude 7 hours before the outburst detection.
2. Double peaked early superhumps similar to WZ Sge with ∆mv ~ 0.04 were observed during the
first two days of the super outburst. Common Superhumps were present starting with the third
night
3. The best fit period of the early superhumps was 0.06258 +/- 0.00013 d – indistinguishable within
error limits from the orbital period of 062625 +/- 0.000011 d and in agreement with the early
superhump period of .06256 +/- 00008 d observed in the 2000 super outburst
4. The observed superhump period (Psh) was 0.06448 +/- 0.00006 d which agrees with the
observed period from the 2000 super outburst, Psh = 0.06452 +/- 0.00009.
5. The period excess of the observed common superhumps is 3% +/- 0.1%
6. Mean amplitude of the common superhumps was ∆mv ~ 0.2
7. This super outburst was of the shorter type.
8. Timing of superhump maxima excluding the last 5 gave a parabolic fit to the O-C diagram
implying that the period of the superhumps was increasing. However, No explanation was given
as to why the last 5 observations beginning at cycle count 125 and ending at cycle count 143,
showed much shorter periods than predicted by parabolic fit and would be much more
consistent with a constant period.
9. Comparison of the superhump excess period of BC UMa to WZ Sge using a well-established
relationship between superhump period excess and mass ratio yields a mass ratio of 0.13 for BC
UMa, which is more than twice the mass ratio of WZ Sge (0.06).
10. A 3D numerical model of the two stars, and accretion disk was conducted with assumptions of
circular orbit, black body radiation spiral structures temperature estimates and a number of
other factors determined from the literature. Excellent agreement of the best fit of the model to
the early superhump light curve was obtained. An image of the 3D model was presented that
was very helpful in understanding the physical process underlying the light curve.
The end result of reading the paper is that I came away with a good basic understanding of dwarf novae
light curves in general, SU UMa dwarf novae in particular and the relationships between observable
features of the light curve and the underlying physical processes. I thought it was a good paper.