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arXiv:1708.09206v2 [astro-ph.SR] 22 Sep 2017
Publ. Astron. Soc. Japan (2014) 00(0), 1–9
doi: 10.1093/pasj/xxx000
1
ASASSN-16eg: New candidate of long-period
WZ Sge-type dwarf nova
Yasuyuki WAKAMATSU,1* Keisuke ISOGAI,1Mariko KIMURA,1Taichi KATO,1
Tonny VANMUNSTE R,2Geoff STONE,3Tam´
as TORDAI,4Michael RICHMOND,5
Ian MILLER,6Arto OKSANEN,7Hiroshi ITOH,8Hidehiko AKA ZAWA,9
Seiichiro KIYOTA,10 Enrique de MIGUEL,11,12 Elena P. PAVLENKO,13 Kirill
A. ANTONYUK,13 Oksana I. ANTONYU K,13 Vitaly V. NEUSTROEV,14,15
George SJOBERG,16,17 Pavol A. DUBOVSKY,18 Roger D. PICKARD,19,20 and
Daisaku NOGAMI1
1Department of Astronomy, Kyoto University, Kyoto 606-8502, Japan
2Center for Backyard Astrophysics Belgium, Walhostraat 1A, B-3401 Landen, Belgium
3Sierra Remote Observatories, 44325 Alder Heights Road, Auberry, CA 93602, USA
4Polaris Observatory, Hungarian Astronomical Association, Laborc utca 2/c, 1037 Budapest,
Hungary
5Physics Department, Rochester Institute of Technology, Rochester, New York 14623, USA
6Furzehill House, Ilston, Swansea, SA2 7LE, UK
7Hankasalmi observatory, Jyvaskylan Sirius ry, Murtoistentie 116, FI-41500 Hankasalmi,
Finland
8Variable Star Observers League in Japan (VSOLJ), 1001-105 Nishiterakata, Hachioji, Tokyo
192-0153, Japan
9Department of Biosphere-Geosphere System Science, Faculty of Informatics, Okayama
University of Science, 1-1 Ridai-cho, Okayama, Okayama 700-0005, Japan
10 VSOLJ, 7-1 Kitahatsutomi, Kamagaya, Chiba 273-0126, Japan
11 Departamento de Ciencias Integradas, Facultad de Ciencias Experimentales, Universidad
de Huelva, 21071 Huelva, Spain
12 Center for Backyard Astrophysics, Observatorio del CIECEM, Parque Dunar,
Matalasca˜
nas, 21760 Almonte, Huelva, Spain
13 Federal State Budget Scientific Institution Crimean Astrophysical Observatory of RAS,
Nauchny, 298409, Republic of Crimea
14 Finnish Centre for Astronomy with ESO (FINCA), University of Turku, V¨
ais¨
al¨
antie 20,
FIN-21500 Piikki¨
o, Finland
15 Astronomy research unit, PO Box 3000, FIN-90014 University of Oulu, Finland
16 The George-Elma Observatory, New Mexico Skies, 9 Contentment Crest, #182, Mayhill,
NM 88339, USA
17 American Association of Variable Star Observers, 49 Bay State Rd., Cambridge, MA
02138, USA
18 Vihorlat Observatory, Mierova 4, 06601 Humenne, Slovakia
19 The British Astronomical Association, Variable Star Section (BAA VSS), Burlington House,
Piccadilly, London, W1J 0DU, UK
c
2014. Astronomical Society of Japan.
2Publications of the Astronomical Society of Japan, (2014), Vol. 00, No. 0
20 3 The Birches, Shobdon, Leominster, Herefordshire, HR6 9NG, UK
∗E-mail: ∗wakamatsu@kusastro.kyoto-u.ac.jp
Received 201 0; Accepted 201 0
Abstract
We report on our photometric observations of the 2016 superoutburst of ASASSN-16eg. This
object showed a WZ Sge-type superoutburst with prominent early superhumps with a period
of 0.075478(8) d and a post-superoutburst rebrightening. During the superoutburst plateau,
it showed ordinary superhumps with a period of 0.077880(3) d and a period derivative of
10.6(1.1) ×10−5in stage B. The orbital period (Porb ), which is almost identical with the period
of early superhumps, is exceptionally long for a WZ Sge-type dwarf nova. The mass ratio (q=
M2/M1) estimated from the period of developing (stage A) superhumps is 0.166(2), which is
also very large for a WZ Sge-type dwarf nova. This suggests that the 2:1 resonance can be
reached in such high-qsystems, contrary to our expectation. Such conditions are considered
to be achieved if the mass-transfer rate is much lower than those in typical SU UMa-type dwarf
novae that have comparable orbital periods to ASASSN-16eg and a resultant accumulation of
a large amount of matter on the disk is realized at the onset of an outburst. We examined
other candidates of long-period WZ Sge-type dwarf novae for their supercycles, which are con-
sidered to reflect the mass-transfer rate, and found that V1251 Cyg and RZ Leo have longer
supercycles than those of other WZ Sge-type dwarf novae. This result indicates that these
long-period objects including ASASSN-16eg have a low mass-transfer rate in comparison to
other WZ Sge-type dwarf novae.
Key words: accretion, accretion disks — stars: novae, cataclysmic variables — stars: dwarf novae —
stars: individual (ASASSN-16eg)
1 Introduction
Cataclysmic variables (CVs) are close binary systems com-
posed of a white dwarf (WD) primary and a secondary that
transfers mass to the primary. The secondary fills its Roche lobe
and its overflowing matter pours onto the primary through the
inner Lagrangian point L1. Dwarf novae (DNe) are a subclass
of CVs and have a property of recurrent outbursts with typically
2–5 mag brightening. The outburst lasts for days or weeks. It
is considered that the outburst results from a sudden release of
gravitational energy which is caused by a rapid increase of the
mass accretion rate on the primary by the thermal instability in
the disk (Osaki 1974).
SU UMa-type DNe are a subclass of DNe characterized by
occasional superoutbursts, which have longer durations than
normal outbursts with superhumps. Superhumps are variations
of small amplitudes typically of 0.1–0.5 mag and are consid-
ered to be a result of the tidal instability that is triggered when
the outer disk reaches the 3:1 resonance radius during the out-
burst (Whitehurst 1988; Osaki 1989; Lubow 1991a; Lubow
1991b; Hirose, Osaki 1990). Kato et al. (2009) proposed that
the superoutburst is divided into three distinct stages by a vari-
ation of the superhump period (PSH): stage A has a longer su-
perhump period, in stage B the superhump period systemati-
cally varies, and stage C is a final stage of superoutburst and
has a shorter superhump period. The amplitude of the super-
humps grows during stage A and then decreases during stage B.
Intervals between the superoutbursts (supercycles) are typically
several hundred days.
WZ Sge-type DNe are a subclass of the SU UMa-type
and show mainly superoutbursts and rarely normal outbursts.
WZ Sge-type DNe are characterized by the large amplitude,
long duration superoutbursts and in some cases the existence
of post-superoutburst brightenings, which are called rebright-
enings (Kato 2015, for more detail). WZ Sge-type DNe are
also characterized by double-wave small variations of magni-
tude, which are called early superhumps and have almost the
same period as the orbital period, before a growth of the stage
A superhumps (Kato 2002; Kato et al. 2014; Ishioka et al.
2001; Ishioka et al. 2002). Although the historical classifica-
tions of WZ Sge-type DNe were mainly based on the amplitude
of superoutbursts (e.g., tremendous outburst amplitude dwarf
novae or TOADs (Howell et al. 1995)), the presence of early
superhumps is now considered to be a key criterion for classifi-
cation of WZ Sge-type DNe (Kato 2015)1. It is considered that
1The definitions of WZ Sge-type DNe by American Association of Variable
Star Observers (AAVSO) International Variable Star Index (VSX) are
Publications of the Astronomical Society of Japan, (2014), Vol. 00, No. 0 3
early superhumps are caused when the outer edge of the disk
reaches the 2:1 resonance radius during the outburst. The 2:1
resonance is considered to suppress the deformation of the disk
caused by the 3:1 resonance, and the eccentricity change due to
the 3:1 resonance grows when the outer edge of the disk falls
below the 2:1 resonance radius (Osaki, Meyer 2002; Lubow
1991a). As a consequence, early superhumps are observed in an
early stage of the superoutburst and then ordinary superhumps
appear subsequently instead of the early superhumps. In order
to reach the 2:1 resonance radius, it is considered that a mass ra-
tio q=M2/M1(M1and M2represent the mass of the primary
and secondary, respectively) should be extremely low. Osaki,
Meyer (2002) proposed that the outer edge of the disk can reach
the 2:1 resonance radius in the low mass-ratio systems with q <
0.08. Indeed, WZ Sge-type DNe have extremely small mass ra-
tios, which are typically 0.06–0.08, and also have short orbital
periods, Porb, which are around 0.054–0.056 d (Kato 2015).
The mass-transfer rates from the secondary in WZ Sge-type
DNe are very small in comparison with typical SU UMa-type
DNe and the supercycles are extremely long. The supercycles
are typically a few years or decades (Kato 2015). There are,
however, some unusual objects which show superoutbursts with
WZ Sge-type features, i.e., large amplitudes of superoutbursts,
the existence of rebrightenings or in some systems, the exis-
tence of double-wave modulations similar to early superhumps,
although they have longer orbital periods than those of other
WZ Sge-type DNe. These long-period objects are classified as
WZ Sge-type DNe in Kato (2015) based on observational fea-
tures.
According to the standard evolutionary theory of CVs, a bi-
nary separation becomes shorter because of a loss of the total
angular momentum due to the magnetic braking and/or gravi-
tational radiation. If the mass transfer continues, the secondary
becomes degenerate at a certain orbital period and then the bi-
nary evolves as its separation becomes wider. There is, there-
fore, a minimum orbital period of CVs, and the binaries passing
the period minimum are called period bouncers (Kolb, Baraffe
1999; Knigge et al. 2011, and references therein). As CVs
evolve, the mass-transfer rate becomes lower and the supercy-
cles become longer. Therefore, the mass-transfer rates in WZ
Sge-type DNe are considered to be smaller than those in SU
UMa-type DNe (Osaki, Meyer 2002).
In this paper, we present observations of the ASASSN-
16eg. The superoutburst of ASASSN-16eg was detected on
2016 April 9 by All-Sky Automated Survey for Supernovae
(ASAS-SN; Shappee et al. (2014)) and the magnitude was V=
14.4 at the time of detection. The coordinates of this object are
RA: 17h26m10s.3213 and Dec: +42◦20′02′′.660 (J2000.0) in
(i)unusual long supercycle, (ii)existence of early superhumps, (iii)existence
of rebrightenings, (iv)orbital periods with range of 0.05–0.08 d and
(v)outburst amplitudes larger than ∼7 mag.
Gaia Data Release 1 (Gaia Collaboration et al. 2016, for more
detail about Gaia DR1). There is a quiescent counterpart of
G=19.394 in Gaia DR1. ASASSN-16eg was classified in WZ
Sge-type DN since this object showed apparently clear double-
wave modulations having properties of early superhumps and
rebrightening, although its orbital period is particularly long.
We found that ASASSN-16eg has a considerably large mass ra-
tio which is far beyond the upper limit of the mass ratio that
the outer edge of the disk is supposed to be able to reach the
2:1 resonance radius (Osaki, Meyer 2002). We considered why
this object showed a superoutburst that stems from 2:1 reso-
nance despite of its large mass ratio by comparing with other
long-period objects. In section 2, we describe the details of
our observations and the methods of analyses. In section 3, we
present the results of our observations. In section 4, we discuss
the results.
2 Observation and Analysis
Our time-resolved CCD photometry of the superoutburst of
ASASSN-16eg was carried out by the Variable Star Network
(VSNET) collaborations (Kato et al. 2004). Logs of our photo-
metric observations are in table S1. All of the observation times
were described in barycentric Julian date (BJD). We added a
constant to each observer’s magnitude data to adjust the dif-
ference in the zero-point. We used the phase dispersion min-
imization (PDM) method (Stellingwerf 1978) for period anal-
yses. The 1σerror of the best estimated period by the PDM
method was determined by the methods in Fernie (1989) and
Kato et al. (2010). We subtracted the global trend of the light
curve by subtracting a smoothed light curve obtained by locally
weighted polynomial regression (LOWESS: Cleveland 1979)
before making the period analyses. We used O−Cdiagrams,
from which we can derive the slight variation of the superhump
period (see, e.g., Sterken 2005).
3 Result
3.1 Overall light curve
Figure 1 shows the overall light curve of the superoutburst of
ASASSN-16eg. The observation was started on BJD 2457488.
The superoutburst lasted about 20 d during BJD 2457489–
2457508 with a slow decline of the brightness, and then rapidly
faded. There was a single rebrightening during BJD 2457512–
2457516. After the rebrightening, the magnitude declined to
around V=19.5 and ASASSN-16eg seemed to be in a quiescent
state.
4Publications of the Astronomical Society of Japan, (2014), Vol. 00, No. 0
57490 57500 57510 57520 57530 57540 57550 57560 57570
14
16
18
20
v
Fig. 1. The overall light curve of the 2016 superoutburst of ASASSN-16eg. The filled square and V-shaped sign represent an observational point and upper
limit by ASAS-SN, respectively.
0.070 0.072 0.074 0.076 0.078 0.080
0.7
0.8
0.9
1.0
(d)
θ
P=0.07548
−0.5 0.0 0.5 1.0 1.5
−0.04
−0.02
0.00
0.02
0.04
Fig. 2. Upper panel: θ-diagram of our PDM analysis of early superhumps
of ASASSN-16eg (BJD 2457489.4–2457494.0). The gray area represents
the 1σerror of the best estimated period by the PDM method. Lower panel:
Phase-averaged profile of early superhumps.
3.2 Early superhumps
We regarded the variations recorded in BJD 2457489.4–
2457494.0 as early superhumps based on the double-wave vari-
ation and the variation of the superhump period. Figure 2
shows the result of PDM analysis of early superhumps (upper
panel) and the mean profile (lower panel) of ASASSN-16eg.
Double-wave variations characterized as early superhumps are
clearly seen. We found the period of early superhumps to be
0.075478(8) d.
3.3 Ordinary superhumps
Figure 3 shows the O−Ccurve (upper panel), the amplitude
of the superhumps (middle panel) and the light curve (lower
panel) of ASASSN-16eg during BJD 2457494–2457508. We
determined the times of maxima of ordinary superhumps in the
same way as in Kato et al. (2009). The resultant times are listed
in table S2. We regarded BJD 2457494.1–2457495.1 (0 ≤E≤
10) as stage A, BJD 2457495.2–2457502.9 (15 ≤E≤106) as
stage B, and BJD 2457503.4–2457508.4 (120 ≤E≤181) as
stage C superhumps from the variation of the superhump period
and the amplitude of superhumps.
Figure 4 shows the result of PDM analysis of stage A super-
humps (upper panel) and the mean profile (lower panel). We
found the stage A superhump period to be PstA = 0.07989(4)
d. We also found the stage B and stage C superhump period
to be 0.077880(3) d and 0.077589(7) d, respectively. Pd ot(≡
˙
Psh/Psh ), which is a derivative of the superhump period during
stage B, was 10.4(0.8) ×10−5.
4 Discussion
4.1 Particularly long orbital period and large mass
ratio
As the period of early superhumps is considered to be almost
equal to the orbital period (Kato et al. 2014), we estimated the
orbital period of ASASSN-16eg to be Porb = 0.075478(8) d.
Publications of the Astronomical Society of Japan, (2014), Vol. 00, No. 0 5
0 50 100 150 200
−0.02
−0.01
0.00
0.01
stage A
stage B stage C
0.0
0.2
0.4
57494 57496 57498 57500 57502 57504 57506 57508
14
16
18
Fig. 3. Upper panel: The O−Ccurve of ASASSN-16eg dur-
ing BJD 2457494–2457508. We used an ephemeris of BJD
2457496.4415+0.0779132Efor drawing this figure. Middle panel: The am-
plitude of superhumps. Lower panel: The light curve. The horizontal axis in
units of BJD and cycle number is common to all of these panels.
This value is particularly large compared with that of other WZ
Sge-type DNe, which are concentrated around 0.054–0.056 d
(Kato 2015). Such long orbital period indicates two possibilities
either ASASSN-16eg is in an earlier stage of CV evolution than
other WZ Sge-type DNe or ASASSN-16eg is in a final stage
of CV evolution as a period bouncer. We excluded, however,
the latter possibility because of its large mass ratio as discussed
below. There are some objects suspected as WZ Sge-type DNe
having long orbital periods like ASASSN-16eg. ASASSN-16eg
may be a new candidate of these long-period objects (see sub-
section 4.3).
We estimated the mass ratio of ASASSN-16eg from the frac-
tional superhump-period excess for the 3:1 resonance, ε∗=
1−Porb/PstA , in the same way as proposed in Kato, Osaki
(2013). We estimated ε∗= 0.0552(6) and then found the mass
ratio to be q= 0.166(2). This value is considerably large for
other WZ Sge-type DNe, which are around 0.06–0.08 (Kato
2015), and thus the mass ratio of ASASSN-16eg is twice or
0.072 0.074 0.076 0.078 0.080 0.082 0.084 0.086 0.088
0.0
0.2
0.4
0.6
0.8
1.0
(d)
θ
P=0.07989
−0.5 0.0 0.5 1.0 1.5
−0.10
−0.05
0.00
0.05
0.10
Fig. 4. Upper panel: θ-diagram of our PDM analysis of stage A superhumps
of ASASSN-16eg (BJD 2457494.1-2457495.1). The gray area represents
the 1σerror of the best estimated period by the PDM method. Lower panel:
Phase-averaged profile of stage A superhumps.
three times as large as these typical values.
Both the orbital period and the mass ratio of ASASSN-16eg
are similar to those of SU UMa-type DNe rather than WZ Sge-
type ones (see figure 5). However, we note that the period of
early superhumps is not exactly but almost equal to the orbital
period. Ishioka et al. (2002) showed that the period of early
superhumps of WZ Sge is 0.05% shorter than the orbital pe-
riod. Kato et al. (2014) showed that the differences between
the periods of early superhumps and the orbital periods in well-
studied WZ Sge-type DNe are very small and periods of early
superhumps can be used as approximate orbital periods with an
accuracy of 0.1%, and they also proposed that we can derive the
orbital period from the period of early superhumps by assuming
fractional excess of early superhumps, ε, of −0.05% if more
accuracy is needed. Considering this difference in ASASSN-
16eg, we obtained an improved orbital period of 0.07544026(8)
d and then estimated the mass ratio to be 0.167(2) by using
the method proposed in Kato, Osaki (2013). This value is very
close to that we estimated from the period of early superhumps,
and thus the difference between the period of early superhumps
and the orbital period is considered to be not important for the
estimation of the mass ratio.
6Publications of the Astronomical Society of Japan, (2014), Vol. 00, No. 0
Table 1. Candidates of long-period objects showing WZ Sge-type superoutbursts
Object Porb (d) PstA (d) qSuperoutbursts Supercycle (yr) References
V1251 Cyg – 0.07616(3) – 1963, 1991, 1994, 1997,
2008∗
3–28 1, 2, 3, 4
RZ Leo 0.07626(7) 0.08072(5) 0.165(6) 1918, 1935, 1952, 1976,
1984, 2000∗, 2006, 2016
6–24 5, 6, 7, 8, 9
BC UMa 0.06251(5) 0.06476(7) 0.096(6) 1960, 1962, 1982, 1990,
1992, 1994, 2000, 2003∗,
2009
2–20 1, 6, 10, 11,
12, 13, 14
MASTER J004527 – 0.08136(7) – 2013∗– 15
QY Per – – – 1999∗, 2005, 2015 6–10 1, 5
ASASSN-16eg 0.075478(8) 0.07989(4) 0.166(2) 2016∗– this paper
References: 1. Kato et al. (2009); 2. Weber (1966); 3. Wenzel (1991); 4. Kato (1995); 5. Kato et al. (2016); 6. Kato (2015); 7.
Wolf (1919); 8. Richter (1985); 9. Ishioka et al. (2001); 10. Kato et al. (2010); 11. Romano (1964); 12. Kunjaya et al. (1998);
13. Boyd (2003); 14. Maehara et al. (2007); 15. Kato et al. (2014)
∗Superoutbursts that we reanalyzed in this paper.
4.2 Conditions of the 2:1 resonance
Osaki, Meyer (2002) proposed that the outer edge of the disk
cannot reach the 2:1 resonance radius in the systems with q >
0.08. This upper limit of qfor the 2:1 resonance may not be a
rigid value since this extension of the disk radius was derived
under an assumption of angular momentum conservation of the
steady hot disk. If accretion onto the primary proceeds during
the outburst, the outer edge of the disk may expand beyond this
radius until the resonance radius or the tidal truncation radius
stops its expansion. Accretion of a large amount of matter onto
the primary will lead to a wide extension of the disk and the disk
may exceed the tidal truncation radius. To achieve such a con-
dition, an extremely low mass-transfer rate would be a key. The
low mass transferring leads to a very low viscosity in quiescence
because of the poor conductivity of the cold disk and resulting
decay of magneto-hydrodynamic turbulence (Gammie, Menou
1998; Osaki et al. 2001). If the viscosity is extremely low, mass
transferred from the secondary would be stored in a torus at the
outer edge of the disk and thus a large amount of matter would
be accumulated on the disk at the onset of an outburst.
In the case of q= 0.166(2), the 2:1 resonance radius for a
circular orbit, R2:1/a =(1/2)2/3(1 + q)−1/3, where ais a sep-
aration of the binary, is 0.599. The Roche-lobe radius that is
approximated by Eggleton (1983) is 0.537 with q= 0.166(2).
These values indicate that the 2:1 resonance radius is larger than
the Roche-lobe radius. The distance of L1, the first Lagrangian
point, from the primary, which is given by R1/a = 0.500–
0.227log qin Warner (1995), is 0.677 and the 2:1 resonance
radius seems to be smaller than L1, which may enable the disk
to reach the 2:1 resonance radius without colliding with the sec-
ondary. Therefore, the outer edge of the disk perhaps can reach
the 2:1 resonance radius depending on conditions. Another pos-
sibility is that the 2:1 resonance can work even if the outer disk
does not strictly reach the resonance radius since the 2:1 reso-
nance is very strong.
4.3 The properties of long-period objects
In Kato (2015), five objects, i.e. V1251 Cyg, RZ Leo,
BC UMa, MASTER OT J004527.52+503213.8 (hereafter
MASTER J004527) and QY Per, are supposed as candidates of
the borderline class between the SU UMa-type DNe and the WZ
Sge-type ones or long-period objects2. These objects showed
superoutbursts with features similar to those of WZ Sge-type
DNe although these objects have unsuitably long orbital peri-
ods for WZ Sge-type ones. MASTER J004527 and QY Per may
be, however, SU UMa-type DNe because no early superhumps
were detected in these two objects (Kato et al. 2014; Kato et al.
2016). Except for QY Per, all of these long-period objects
showed a single rebrightening as in ASASSN-16eg.
We closely re-examined the past superoutbursts of these
long-period objects. We also reanalyzed the mass ratios of
V1251 Cyg (Kato et al. 2009), RZ Leo (Ishioka et al. 2001)
and BC UMa (Maehara et al. 2007) by adding new observa-
tions from the AAVSO database and by using a new method
proposed in Kato, Osaki (2013). In these three objects, mod-
ulations similar to early superhumps are detected3. We sum-
marized our analyses in table 1. We list our estimated values
for Porb and PstA. The details of the analyses are summarized
2Although some period bouncers may have long PSH, they are not consid-
ered here.
3In RZ Leo, although Kato et al. (2009) suggested that variations of mag-
nitude before the onset of ordinary superhumps are probably early super-
humps rather than an extension of ordinary superhumps, Kato et al. (2016)
indicated that these modulations may be different from those of typical WZ
Sge-type DNe. However, the existence of ASASSN-16eg supports the ex-
istence of long-period WZ Sge-type DNe, and the variations of magnitude
similar to early superhumps in RZ Leo may indeed be early superhumps.
Publications of the Astronomical Society of Japan, (2014), Vol. 00, No. 0 7
in the supplementary discussion, figure S1–S10 and table S3–
S6. We should note that these estimations of mass ratios based
on the periods of early superhumps and the stage A superhump
periods involve large uncertainties mainly because of the short
baseline of the data of stage A superhumps. A reliable value
of the period of early superhumps of V1251 Cyg could not be
derived from the 2008 superoutburst and thus we could not esti-
mate the mass ratio. Although the orbital period of BC UMa is
longer than those of other WZ Sge-type DNe, it is rather short
in comparison with that of RZ Leo or ASASSN-16eg. The mass
ratio of BC UMa also quite small in comparison with that of RZ
Leo or ASASSN-16eg. Thus, this object may be an intermedi-
ate class between the SU UMa-type DNe and the WZ Sge-type
ones as mentioned in Maehara et al. (2007). The orbital period
and mass ratio of RZ Leo are similar to those of ASASSN-16eg.
ASASSN-16eg also may have the same properties as RZ Leo,
such as the long recurrence time of superoutbursts.
4.4 Supercycles of long-period objects
As we mentioned in subsection 4.2, the mass-transfer rate may
be low in ASASSN-16eg for causing a WZ Sge-type super-
outburst. Similarly, other long-period objects showing WZ
Sge-type superoutbursts may have similarly low mass-transfer
rates. The recurrence time of the superoutburst is considered to
be proportional to the inverse of the mass-transfer rate (Osaki
1995). Therefore, if there is no overlooked superoutburst, the
length of supercycle would reflect the mass-transfer rate and the
exceptionally long supercycle suggests the exceptionally low
mass-transfer rate.
We excluded MASTER J004527 from this discussion since
this object showed only one superoutburst. The shortest super-
cycle of BC UMa is 2 yr as seen in table 1 and this value is rather
short for typical WZ Sge-type DNe, which are about decades,
although it is long for typical SU UMa-type ones, which are
several hundred days. As Maehara et al. (2007) mentioned,
however, this object may be an intermediate class and seems
to be different from other long-period objects (see also figure
5). Thus we also excluded this object from this discussion.
The other three objects, V1251 Cyg, RZ Leo and QY Per,
have long supercycles comparable to those of other WZ Sge-
type DNe. Although the shortest supercycle of V1251 Cyg is
3 yr and seems to be fairly short, the 1994 superoutburst was
not observed well enough (Kato et al. 2009) and this superout-
burst might be one before mass accumulated sufficiently at the
onset of the outburst. These long supercycles indicate that in
these long-period objects the mass-transfer rates are low. From
comparison with these long-period objects, it might be indicated
that ASASSN-16eg also has a long supercycle and thus a low
mass-transfer rate.
We also searched the past outburst of ASASSN-16eg by us-
0.050 0.055 0.060 0.065 0.070 0.075 0.080
0.05
0.10
0.15
0.20
RZ Leo
BC UMa
ASASSN−16eg
SU UMa−type
WZ Sge−type
Fig. 5. Mass ratio, q, versus orbital period, Porb . The dashed and solid
curves represent the standard and optimal evolutionary tracks in Knigge
et al. (2011), respectively. The triangle and filled circle represent SU UMa-
type and WZ Sge-type DNe, respectively, which are listed in Kato, Osaki
(2013) and Kato (2015). The star represents ASASSN-16eg. The diamonds
represent RZ Leo and BC UMa, respectively. There are two candidates of
period bouncers at the lower right in this panel. They have long orbital pe-
riods but extremely small mass ratios in comparison with ASASSN-16eg or
RZ Leo.
ing Harvard astronomical plate digitalized by Digital Access to
a Sky Century @ Harvard (DASCH; Grindlay et al. (2009);
Laycock et al. (2010)) project. These plates records objects
brighter than B ∼14 – 17, and could detect the superoutbursts of
ASASSN-16eg if this object showed superoutbursts and it was
recorded in plates. There is, however, no record of brightening
of ASASSN-16eg and we could not investigate the supercycle
of ASASSN-16eg.
4.5 CV evolution
We show the Porb –qrelation in figure 5, in which we showed
SU UMa-type and WZ Sge-type DNe with qvalues estimated
by Kato, Osaki (2013) and Kato (2015). We also included
ASASSN-16eg and other long-period objects, RZ Leo and BC
UMa.
As described in section 1, the binary separation of CVs be-
comes shorter because of a loss of the total angular momentum.
Because of Roche overflow, the mass ratio of the binary also re-
duces. Thus CVs generally evolve with the mass ratio decreas-
ing and the separation decreasing. Therefore, it is considered
that DNe evolve into WZ Sge-type DNe through SU UMa-type
8Publications of the Astronomical Society of Japan, (2014), Vol. 00, No. 0
DNe. This may indicate that the long-period objects includ-
ing ASASSN-16eg are at an earlier stage of CV evolution than
other WZ Sge-type DNe. However, as mentioned in section 1,
the mass-transfer rate decreases as CVs evolve. In this picture,
the mass-transfer rate is considered a function of orbital period.
In cases of long-period objects with long supercycles, the mass-
transfer rate appears to be lower than that expected from this
picture. Such systems may be in a state with temporary de-
creased mass-transfer rate. However, the mechanism to realize
a low mass-transferring state is still unclear.
As one possibility of realizing the temporarily decreased
mass-transferring state, we propose the hibernation scenario
(Shara et al. 1986; Livio 1992), which assumes that after a nova
eruption, mass transfers from the secondary decreases because
of the increase in binary separation and the weakening of irradi-
ation from the primary, and then the binary system would be in
the temporally low mass-transferring state. Although nova-like
stars below the period gap are very rare (Patterson et al. 2013),
this hypothesis could explain the reason why the mass-transfer
rate can be low even in long-period objects.
Another possibility to explain the low mass-transfer rate
in these long-period WZ Sge-type objects is that these ob-
jects trace a different evolutionary track from the standard one.
Goliasch, Nelson (2015) showed that the evolutionary track of
CVs depends on the initial conditions of the primary or the sec-
ondary. The objects discussed here, however, appear to be on
the standard evolutionary track as judged from the Porb –qre-
lation (figure 5), and this possibility appears to be less likely.
5 Summary
We report on our photometric observations of the 2016 super-
outburst of ASASSN-16eg. This object showed a WZ Sge-
type superoutburst with clear early superhumps and a post-
superoutburst rebrightening. We derived the period of early su-
perhumps to be 0.075478(8) d. The orbital period, which is
almost identical with the period of early superhumps, is excep-
tionally long for a WZ Sge-type DN. The mass ratio estimated
from the period of stage A superhumps is 0.166(2), which is
also very large for a WZ Sge-type DN. Osaki, Meyer (2002)
proposed that in systems with q > 0.08 the outer edge of the
disk cannot reach the 2:1 resonance radius. However, if the
accretion onto the primary proceeds, the disk continues to ex-
pand until the resonance radius or tidal truncation radius stops
its expansion. If the mass-transfer rate is low and thus a large
amount of matter accumulates on the disk before the onset of
an outburst, the outer edge of the disk may reach to or close to
the 2:1 resonance radius beyond the tidal truncation radius by
violent outburst.
For candidates of long-period objects showing WZ Sge-type
superoutbursts, we examined their supercycles, which are con-
sidered to reflect the mass-transfer rates. We found that V1251
Cyg and RZ Leo have long supercycles in comparison to other
WZ Sge-type DNe. This suggests that these long-period objects
have low mass-transfer rates. From comparison to these long-
period objects, it is indicated that ASASSN-16eg also has a long
supercycle and thus a low mass-transfer rate.
The long orbital period suggests that ASASSN-16eg is in
an earlier stage of CV evolution than other WZ Sge-type DNe.
Although CVs evolve as their mass-transfer rate decreases,
long-period objects appear to have a low mass-transfer rate
comparable to other WZ Sge-type DNe. As a mechanism to
realize a low mass-transfer rate, we propose the hibernation
scenario or possibility that long-period objects trace a different
evolutionary track from the standard one.
Acknowledgement
This work was supported by a Grant-in-Aid Initiative for
High-Dimensional Data-Driven Science through Deepening of
Sparse Modeling from the Ministry of Education, Culture,
Sports, Science and Technology (MEXT) of Japan. We are
grateful to the All-Sky Automated Survey for Supernovae
(ASAS-SN) for detecting a large number of DNe and
the superoutburst of ASASSN-16eg. We are thankful
to AAVSO and the many amateur observers for provid-
ing much of the data used in this research. This work
has made use of data from the European Space Agency
(ESA) mission Gaia (http://www.cosmos.esa.int/gaia), pro-
cessed by the Gaia Data Processing and Analysis Consortium
(DPAC, http://www.cosmos.esa.int/web/gaia/dpac/consortium).
Funding for the DPAC has been provided by national insti-
tutions, in particular the institutions participating in the Gaia
Multilateral Agreement. This work has made use of the VizieR
catalogue access tool provided by CDS, Strasbourg, France, and
of Astrophysics Data System (ADS) provided by NASA, USA.
This work also has made use of Digital Access to a Sky Century
@ Harvard (DASCH) project and we thank Denis Denisenko for
his help with use of DASCH.
Supporting information
Supplementary discussion, figure S1–S10 and table S1–S6 are
reported in the online version.
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