[vsnet-chat 56] Re: SU UMa's and TOADs

[Taichi Kato <tkato>]

In response to Steve Howell's comment in vsnet-chat 38:

> TOAD is not just another name for WZ Sge stars. These in fact. were fisrt
> defined by their dounle-peaked spectra (due to WZ Sge being at high i).
> TOADs have a number of defining properties.

    I believe the "WZ Sge-type dwarf novae" were first proposed in Bailey
1979 (MNRAS 189, 41p).  Bailey summarized in this article:

>   The cataclysmic variables UZ Boo and WX Cet both resemble WZ Sge in
> showing large outburst amplitudes (~8 mag). slow declines from outbursts,
> and long intervals between outbursts.  It is proposed that these three
> stars form a distinct sub-group of the dwarf novae and that they may
> all have similar binary structure.

    How differs this description from the current TOAD definition?
I have up to now understood there is only one point that TOADs have amplitudes
larger than 6 magnitudes...  There seems to be no mentioning about the
double-lined emission spectrum as a part of his definition.  Insightful
was the inclusion in his list of UZ Boo!

Eric Kuulkers wrote (vsnet-chat 36):

> because WZ Sge is not a very good representative of its class.
> Only during one of its outbursts it showed a dip in the outburst light curve 
> after a long main superoutburst and a subsequent rebrightening ala AL Com.
> The other two outbursts didn't show evidence for such dips in brightness.

    I can't fully understand this notion.  Is WZ Sge a poor representative
because it showed a dip and re-brightetning only one of its outbursts?
I think this is only a matter of chance.  Depending on the instabilities
occurring after the main superoutbursts, stars may behave either as
WZ Sge in 1913 and 1946 (no re-brighetnings), EG Cnc in 1996-1997 (repeated
re-brightenings) or AL Com in 1995 and WZ Sge in 1978 (double superoutbursts).
Dips or re-brightenings are common in WZ Sge stars, but should not be
regarded as necessary conditions.

----------------------------------------------------------------------------

     For reader's interest, I have attached the text of the poster we have
presented at the Keele CV Conference, 1995, discussing the matter of
re-brightenings in WZ Sge stars.  The details have been and will probably
be published in Kato et al. PASJ 48, L21 (1996) and Nogami et al. submitted
to ApJ.  Possible connection to USXT-outbursts was already poined out here.

        Observation of WZ Sge-type Dwarf Nova AL Comae Berenices
             (presented at the Keele CV Conference, June 1995)

        Taichi Kato, Daisaku Nogami, Hajime Baba (Kyoto University)
             K. Matsumoto, J. Arimoto, K. Tanabe and K. Ishikawa
                        (Osaka Kyoiku University)

1. Introduction

   Although presently classified as a dwarf nova, WZ Sagittae (WZ Sge)
is peculiar in many respects.  The star exhibited three historical
outbursts separated by 33 years.  The light curves of its ourbursts
resemble a fast nova in that they show 1) rather rapid initial decline,
2) subsequent slower decline which sometimes lasts 60 to 100 days, and
3) large (~8 mag) outburst amplitudes.
From these features, this object had been long believed to be
a recurrent nova.
   However, spectroscopic and photometric observations in 1978 outburst
confirmed the object to be a dwarf nova, not a classical nova
(Patterson et al., 1981, and references therein).  Most striking was the
discovery of superhumps which are one of the defining characteristics of
SU UMa-type dwarf novae.

   There are two major competing theories to explain the peculiar
nature of WZ Sge.  One is the mass-transfer burst theory, which is
supported by the discovery of large-amplitude periodic humps early in its
1978 outburst (Patterson et al. 1981). The period of the humps was equal
to the orbital period, and they were considered to reflect the hot spot
enhanced by the mass-transfer burst, although the hump maxima occured
0.17 orbital phase prior to the orbital humps in quiescence.

   The other is the extension of thermal and tidal instability theory
of SU UMa-stars towards the lowest mass-transfer rate (Osaki 1995).
Numerical simulations have shown that thermal instability of the
accretion disk occurs rarely under combination of very low mass-transfer
rate and very low viscosity of the accretion disk during quiescence.
It is shown that a normal outburst in such condition always leads
to the tidal instability to trigger a superoutburst.

   We here report new findings and its implications obtained through
the intensive observation of the very recent spectacular outburst of
AL Com, which has proven to be an almost perfect twin to WZ Sge in many
respects.

2. 1995 Superoutburst of AL Com

   AL Com was discovered by Rosino during its outburst in 1962.  Its
dwarf nova nature was confirmed by subsequent spectroscopy during
outburst, and by discovery of historical outbursts back to 1892.
The existence of a deep dip in the middle of the fading stage was
noticed during this outburst (Bertola 1964).
The next impressive outburst was observed in 1975, when the object
showed a similar long-lasting outburst and possible re-brightening
after the decline to near quiescence.
Its exceptionally large outburst amplitude (~8 mag) for a dwarf
nova has attracted a number of investigators.  For example, Howell and
Szkody (1987) showed a large amplitude variation with a period near
40 min.  They suggested from this period that AL Com may belong to
either DQ Her-type magnetic systems or AM CVn-type double degenerate
systems.  Further spectroscopic observation by Mukai (1990) precluded
the latter possibility.  More extensive photometry by Abbott et al.
(1992) showed two distinct periodicities of 41 min and 87 -- 90 min,
the latter was suggested to be the orbital period, and the former
the rotation period of the white dwarf.  From these periodicities
they concluded that AL Com bears the properties of both an enigmatic
dwarf nova WZ Sge and a unique intermediate polar EX Hya.

   After 20 years' dormancy, AL Com was again observed by D. York (AAVSO)
during its rise to an outburst in early April 1995.  The news was soon
relayed through Internet, accompanied by a burst of e-mails full of
astonishment.  The object reached its historical record of mv=12.2
at maximum, and started fading.  We then started time-resolved CCD
photometry during the full course of the outburst.  Most of the observations
were done by a 60-cm telescope at Ouda Station (Kyoto University) and
by a 50-cm telescope at Osaka Kyoiku University.  The overall light curve
is given in Fig. 1.

2.1 Early Stage of Outburst

   Within two days of the outburst maximum, periodic modulation was
discovered independently from many observatories.  The first night of
the Ouda data showed doubly humped modulation (Fig. 2, Apr. 7).  This
feature showed a continuous decaly until Apr. 12.  A period analysis
using PDM (Phase Dispersion Minimization) gives a very stable period
of 0.05666 +/- 0.00002 day (Fig. 3).  This period was later confirmed
to be exactly twice (within the error of each estimates) of 41-min
periodicity observed during quiescence.
From this perfect agreement and later discovery of typical superhumps
with a significantly longer period, we have identified this period
as the orbital period.  A similar feature was discovered in WZ Sge,
which was assumed to originate from the enhanced hot spot
(Patterson et al. 1981).  The suggested orbital period differs from
that of WZ Sge only by 2 sec!.

2.2 Appearance of Superhumps

   After the decay of initial periodic modulation, a new distinct feature
appeared.  The Ouda data on Apr. 17 showed a large amplitude modulation
(Fig. 4) whose profile was very characteristic to superhumps in usual
SU UMa-type dwarf novae.  A period analysis yielded a period of 0.05722
+/- 0.00010 day, which is 1.0 +/- 0.2% longer than the "orbital superhumps"
observed in the early stage of outburst.  The superhump profile is shown
in Fig. 5.  This observation first firmly established the SU UMa-type
nature of AL Com.  After the development of superhumps, the object entered
a stage of more gradual decline.  The outburst amplitude and historical
behavior, the course of the outburst, orbital and superhump periods, and
the time lag before development of superhumps already made us convincing
AL Com as a perfect twin of WZ Sge.

2.3 The "Dip"

   The "dip" is one of the unique characterstics of the outburst light
curve of WZ Sge.  The dips occur in the middle stage of the outburst,
and are characterized by a short (a few days) excursion to the faint
state, although still enough brigter than usual quiescence.  There seems
to be some evidence that such dips are rather common among SU UMa-type
dwarf novae with very large outburst amplitudes.  The same feature was
naturally sought in the case of AL Com.  The dip actually occurred as
expected (Fig. 1).

   Although our observations were unfortunately hindered by the unfavorable
weather, the latter half of the main dip was clearly visualized.
Fllowing the dip, AL Com brightened rather slowly to the second maximum,
then started a rapid fading.  During this stage, we could no longer find
any evidence of superhumps or orbital humps.  After this
"second dip", the object showed a recovery which led to a
plateau stage.

2.4 The Plateau Stage

   After recovery from dip episodes, the time-resolved light curve of
AL Com became flat again.  However, within a week from recovery, the
object again started to show a low-amplitude modulation.  An analysis
gives a period consistent with the superhump period observed earlier
(Fig. 6, Osaka Kyoiku University data).  This finding first clearly
demonstrate that "after-the-dip" behavior traces the usual course of
a superoutburst.

2.5 The Final Decline

   After the plateau phase with superhumps, AL Com started to show
rather irregular variability with a time-scale of hours to a day.
The superhumps seemed to have completely disappeared (Fig. 7).
Soon after this stage, the object showed a precipitous decline.
The quiescent magnitude was not yet reached during our observations.
AL Com stayed at about V=18.6 at least for some days after the rapid
decline.

3. Discussion

   The present observations clearly show that AL Com is a genuine
member of SU UMa-type dwarf novae, and that this object is almost
a twin of an enigmatic dwarf nova WZ Sge in many respects.
We here try to explain several peculiar features common to these
objects (hereafter "WZ Sge-type dwarf novae", cf. Bailey 1979).

3.1 Origin of Early Stage Modulation

   As described in Chap. 2.1, we consider the period of this modulation
has the same period with the orbital period.  In the case of WZ Sge,
the singly humped profile suggested the hot spot as the origin.
However, in the case of AL Com (and HV Vir in our study), the
profile of this modulation is doubly peaked, and sometimes dominated
by dip features rather than humps.  This finding seems to preclude the
hot spot as the source of light.  A possibility of reprocessed light
by the secondary is also precluded, because this interpretation should
predict a sinusoidal light variation.  We therefore suspect that this
modulation originates from the accretion disk itself (either by changing
energy output from the disk or the effect of geometrical obscuration).
In either cases, we have no need to think of enhanced mass-transfer
as was assumed in explaining WZ Sge-type phenomenon.  One attractive
possibility is that this modulation represents "immature" superhumps.
Numerical simulations have shown that during the growing stage of the
tidal instability, the period of superhumps tends to be close to the
orbital period.  It is, however, a mystery why such growing stages are
observed only in WZ Sge-type dwarf novae.

3.2 Superhumps

   Fully grown superhumps were observed in AL Com.  The good seasonal
condition allowed us unambiguously determine the period.  The superhump
period of AL Com is 1.0 % longer than the suggested orbital period.
This value of superhump excess is the second smallest among SU UMa-type
dwarf novae (the smallest: WZ Sge 0.8 %).  Since the superhump excess is
known to correlate with the binary mass ratio, the mass ratio of AL Com
should be the second extreme one among established SU UMa-type dwarf novae.
A small difference from WZ Sge in this respect may be a cause of small
difference in the outburst behavior between these two systems.

3.3 The Dip

   We don't have enough observational evidence about the origin of the
dip.  However, the reported rate of decline of AL Com from reported
observations (mostly from visual observers of AAVSO) suggests that
the dip may be produced by a thermal transition wave starting from the
outer accretion disk.  The magnitudes during the dips were well above
quiescence in both AL Com and WZ Sge.
This implies that the inner part of the accretion disk remained hot
even during the dip.  The recovery from the dip was rather slow.
This implies that the recovery represents the "inside-out" type
propagation of the thermal transition wave.  Nonexistence of orbital
humps during this stage strongly supports that the recovery from the
dip was not powered by a mass-transfer event, but by the thermal
instability of the accretion disk.

3.4 The Plateau Phase

  After the dip, AL Com showed a short-lived maximum followed by a
rapid decline.  After this "secondary dip" the star entered a new
stage of the "plateau phase".  The same features can be traced in the
light curve of WZ Sge during 1978 outburst.  As already described
in Chap. 2.4, the superhumps grew during this phase.  We can therefore
identify this stage as a start of a new superoutburst.  A short-lived
maximum after the main dip may represent a normal outburst which
triggered this superoutburst.

3.5 The Final Decline

  The final decline can be again considered to represent the propagation
of a thermal transition wave through the accretion disk.  The disappearance
of superhumps a few days before this decline implies that the cessation
of the tidal instability is responsible for the decline.  After the
decline, AL Com stayed about 2 mag brighter than the reported quiescence.
Although the observational materials are limited due to its faintness,
it is likely that some inner part of the accretion disk remained hot
even after the final decline.  This mechanism may be responsible for the
long fading tail of WZ Sge, which still remains poorly understood.
A similar phenomenon was also observed after the main (or super-) outburst
of GRO J0422+32, a USXT (ultrasoft X-ray transient) which bears a number
of similarities with WZ Sge-type dwarf novae.
Measurement of the contribution of the accretion disk and the hot spot
to the observed light may be a clue in understanding this poorly understood
unique feature of WZ Sge-type dwarf novae and USXTs.
The persitent "superhump" periodicity observed in HV Vir, another
object very similar to WZ Sge, during the same stage by Leibowitz et al.
(1994) supports a significant contribution to the light from the accretion
disk this stage. [Although this modulation was originally ascribed to an
orbital modulation (Leibowitz et al. 1994), our new analysis strongly
favors the persistent superhumps.]

References:

Abbott T. M. C., Robinson E. L., Hill G. J., Haswell C. A. 1992,
         ApJ 399, 680

Bailey J., 1979, MNRAS 189, 41P

Bertola F. 1964, Ann d'Astrophys. 27, 298

Howell S. B., Szkody P. 1987, PASP 100, 224

Leibowitz E. M., Mendelson H., Bruch A., Duerbeck H. W., Seitter W. C.,
Richter G. A. 1994, ApJ 421, 771

Mukai K. et al. 1990, MNRAS 245, 385

Osaki Y. 1995, PASJ 47, 47

Patterson J., McGraw J. T., Coleman L., Africano J. L. 1981,
          ApJ 248, 1067

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