1
No.107

Feb. 2023

ISSN 0917-2211

Variable Star Bulletin
Genuine standstill in the AM CVn star CR Boo
Taichi Kato1 Yutaka Maeda,2 Masayuki Moriyama3
tkato@kusastro.kyoto-u.ac.jp
1 Department of Astronomy, Kyoto University, Sakyo-ku, Kyoto 606-8502, Japan
2 VSOLJ, Kaminishiyamamachi 12-2, Nagasaki, Nagasaki 850-0006, Japan
3 VSOLJ, 290-383, Ogata-cho, Sasebo, Nagasaki 858-0926, Japan
Received 2023 Feb. 09
Abstract
CR Boo is one of the brightest and most famous AM CVn stars showing dwarf nova-type outbursts.
Previous studies showed diﬀerent modes of outbursts in this object ranging from the one equivalent to ahydrogen-rich ER UMa star or WZ Sge star to a low-amplitude oscillating state. We for the ﬁrst time
identiﬁed a bona ﬁde standstill in this object in 2022 and we consider that CR Boo is a helium analog
of Z Cam stars in addition to its SU UMa/ER UMa-type classiﬁcation. The standstill lasted for ∼60 dwith variations typically less than 0.2 mag and ended with fading. This standstill was not preceded by asuperoutburst and was diﬀerent from a post-superoutburst phenomenon. The brightness after the standstill
was similar to those after superoutbursts and the standstill appears to have acted like a superoutburst ineﬀectively accreting the disk mass. The existence of a standstill in an AM CVn star be a challenge to theories
of a helium disk or a degenerate secondary to explain how such a state could be maintained.

CR Boo is one of the brightest and most famous AM CVn stars containing a white dwarf and a masstransferring helium white dwarf [for a review of AM CVn stars, see e.g., Solheim (2010)]. CR Boo has received
much attention since this object exhibits dwarf nova-type outbursts [see e.g., Osaki (1996)] and its behavior can
provide observational tests for the disk instability theory in helium disks (Tsugawa and Osaki 1997; Solheim 2010;
Kotko et al. 2012). CR Boo has been studied by many authors, starting from the identiﬁcation of large-amplitude
variations with a quasi-period of four to ﬁve days by Wood et al. (1987). Subsequent research included Wenzel(1994); Patterson et al. (1997); Provencal et al. (1997); Kato et al. (2000, 2001a); Ramsay et al. (2012); Honeycutt
et al. (2013); Kato et al. (2013, 2014); Levitan et al. (2015); Isogai et al. (2016); Duﬀy et al. (2021); Boneva et al.
(2022).
We used the All-Sky Automated Survey for Supernovae (ASAS-SN) Sky Patrol data (Shappee et al. 2014;
Kochanek et al. 2017) and the Asteroid Terrestrial-impact Last Alert System (ATLAS: Tonry et al. 2018) forced
photometry (Shingles et al. 2021) and our snapshot CCD photometry in addition to observations reported to
VSOLJ and VSNET (Kato et al. 2004). Y. M. used a 25-cm RC telescope with a Canon EOS Kiss X3 DSLR
camera. M. M. used a 25-cm SCT telescope with an SBIG ST-10XME camera.
In ﬁgure 1, we show the light curve of CR Boo in the last four seasons. In the ﬁrst panel (2018–2019 season),
CR Boo showed a regular pattern of supercycles resembling an ER UMa star among hydrogen-rich systems (Katoet al. 2000). This is probably a typical state in this object.
In 2019–2020 (second panel of ﬁgure 1), the object initially showed a regular pattern as seen in the preceding
season, followed by a phase with low-amplitude (∼1.5 mag) frequent outbursts with increasing cycle length toward
the end. Such a pattern is more frequently seen in V803 Cen (sometimes considered as the CR Boo “cousin”):
see e.g., Duﬀy et al. (2021) ﬁgure 1, although Duﬀy et al. (2021) apparently did not pay special attention to thisphenomenon and was not seen in CR Boo during the interval discussed by Duﬀy et al. (2021). Although thisstate may be similar to oscillating state as observed by Patterson et al. (1997) and “standstill” with low-amplitude(∼0.5–1 mag) variations reported in V803 Cen by Kato et al. (2001b), the amplitudes were apparently larger

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and the long duration of this state suggests a form of quasi-stable state in helium dwarf novae, which may be avariety of quasi-periodic variations reported in Kato et al. (2001a).
In the third panel (2020–2021) of ﬁgure 1, the object showed a relatively regular pattern of outbursts withlarger amplitudes (up to 2.0 mag) with cycle lengths of 15–20 d. This pattern corresponds to the second supercycle
described in Kato et al. (2001a). In the fourth panel (2021–2022), the object initially showed a pattern similar to
the 2020–2021 season. The amplitudes decreased and entered a standstill at around BJD 2459710. The standstill
lasted for ∼60 d and faded to 17 mag showing short outbursts. This faint state lasted for ∼20 d.
The details of the standstill in 2022 is shown in ﬁgure 2. The object was essentially almost constant within
0.2 mag during the standstill, as best shown by the ASAS-SN g and ATLAS o data. Based on the constancy ofthe brightness for a long time (∼60 d), we identiﬁed this phenomenon to be a true standstill in this system. Sucha phenomenon was never reported in this system (Patterson et al. 1997; Honeycutt et al. 2013; Levitan et al.
2015; Duﬀy et al. 2021), and probably none in other helium dwarf novae (Levitan et al. 2015; Duﬀy et al. 2021).
In ﬁgure 1 of Levitan et al. (2015), V803 Cen possibly showed a similar state around JD 2454500–2456700 but
only with a small number of measurements. The data by ASAS-3 (Pojmański 2002) and VSNET observations(mainly by Rod Stubbings) of the corresponding interval could not conﬁrm the constancy and we consider that
this was unlikely a true standstill.
There was some hint of a standstill-like phenomenon in CR Boo following a superoutburst and subsequent
dips (ﬁgure 3 in Duﬀy et al. 2021). The same feature was observed in some superoutbursts of V803 Cen (initialpart of V803 Cen shown in ﬁgure 1 of Duﬀy et al. 2021). This feature was ﬁrst discovered during observing
campaigns run by VSNET (PI: K. Isogai) in 2016 and 2017 (the main observers were Berto Monard, Josch
Hambsch, Peter Starr and Gordon Myers), see e.g., vsnet-alert 198301 and 198552 . This feature is similar to
damping oscillations seen in some hydrogen-rich WZ Sge-type dwarf novae following a superoutburst: ASASSN15po (Namekata et al. 2017) and PQ And (Tampo et al. 2021). These phenomena in V803 Cen and hydrogen-rich
WZ Sge-type dwarf novae have much shorter (less than ∼10 d) durations and were preceded by a superoutburst,
and they are usually considered as a part of the rebrightening phenomenon of WZ Sge stars (see Kato 2015).
The 2022 standstill of CR Boo had a much longer duration and was not preceded by a superoutburst and is
diﬀerent from a part of the rebrightening phenomenon. We therefore consider that CR Boo is a helium Z Cam
star [for classiﬁcation of cataclysmic variables, see Warner (1995)]. As in many hydrogen-rich Z Cam stars, thestandstill ended with fading. The brightness after the standstill was similar to those after superoutbursts [seealso Isogai et al. (2016) for a study of a typical superoutburst]. The standstill in CR Boo may have had aneﬀect similar to a superoutburst on the disk – sweeping out a signiﬁcant fraction of the mass stored in the disk
as in (hydrogen-rich) SU UMa-type superoutbursts (Osaki 1989). Although the presence of superhumps was
unfortunately not conﬁrmed during this standstill, they were likely present considering the low mass ratio of thissystem, and the eﬀective removal of the angular momentum by tidal instability may have worked just as in an
SU UMa-type superoutburst.
The existence of a standstill in itself may not be surprising for an object near the thermal stability (Tsugawa
and Osaki 1997; Kotko et al. 2012). In hydrogen-rich Z Cam stars, a subtle change in the mass transfer isconsidered to responsible for standstills (Meyer and Meyer-Hofmeister 1983). Whether this is also the case inhelium dwarf novae requires further investigation, and, if it is the case, it would be an interesting question how adegenerate helium white dwarf (without a solar-type magnetic cycle as expected for secondaries in hydrogen-rich
Z Cam stars) can produce such a variation in mass transfer. If it is not the case, it would be an interestingquestion how the disk can produce various states without variation in the mass transfer from the secondary.

Acknowledgements
This work was supported by JSPS KAKENHI Grant Number 21K03616.
We are grateful to the ATLAS, ASAS-SN and ASAS-3 teams for making their data available to the public.
We are also grateful to observers (particularly Masao Funada and Gary Poyner) who provided data of CR Boo
in 2018–2022, and observers who contributed to campaigns of V803 Cen (particularly Berto Monard, Josch
Hambsch, Peter Starr, Gordon Myers and Rod Stubbings).
This work has made use of data from the Asteroid Terrestrial-impact Last Alert System (ATLAS) project.
The Asteroid Terrestrial-impact Last Alert System (ATLAS) project is primarily funded to search for near earth
asteroids through NASA grants NN12AR55G, 80NSSC18K0284, and 80NSSC18K1575; byproducts of the NEO
1 <http://ooruri.kusastro.kyoto-u.ac.jp/mailarchive/vsnet-alert/19830>.
2 <http://ooruri.kusastro.kyoto-u.ac.jp/mailarchive/vsnet-alert/19855>.

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ASN g/V

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59300

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Figure 1: Light curve of CR Boo in 2018–2022. ASAS-SN (ASN) g and (V , only one night in the initial part)were combined together with V observations reported to VSOLJ and VSNET (ﬁve observations). The category
CCD includes unﬁleted snapshot CCD observations mainly by Y.M. and M.M. In the ﬁrst panel (2018–2019
season), CR Boo showed a regular pattern of supercycles resembling an ER UMa star among hydrogen-richsystems (Kato et al. 2000). In the second panel (2019–2020), the object initially showed a regular pattern as seenin the preceding season, followed by a phase with low-amplitude (∼1.5 mag) frequent outbursts with increasing
cycle length toward the end. In the third panel (2020–2021), the object showed a relatively regular pattern ofoutbursts with larger amplitudes (up to 2.0 mag) with cycle lengths of 15–20 d. This pattern corresponds to thesecond supercycle described in Kato et al. (2001a). In the fourth panel (2021–2022), the object initially showeda pattern similar to the 2020–2021 season. The amplitudes decreased and entered a standstill at around BJD
2459710. The standstill lasted for ∼60 d and faded to 17 mag showing short outbursts. This faint state lasted
for ∼20 d.

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59720

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Figure 2: Enlargement of the light curve including the 2022 standstill. The symbols are the same as in ﬁgure
1. The object was essentially almost constant within 0.2 mag during the standstill starting from around BJD
2459710, as best shown by the ASAS-SN g and ATLAS o data.

search include images and catalogs from the survey area. This work was partially funded by Kepler/K2 grant
J1944/80NSSC19K0112 and HST GO-15889, and STFC grants ST/T000198/1 and ST/S006109/1. The ATLAS
science products have been made possible through the contributions of the University of Hawaii Institute for
Astronomy, the Queen’s University Belfast, the Space Telescope Science Institute, the South African Astronomical
Observatory, and The Millennium Institute of Astrophysics (MAS), Chile.

List of objects in this paper
PQ And, CR Boo, Z Cam, V803 Cen, AM CVn, WZ Sge, SU UMa, ER UMa, ASASSN-15po

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VSOLJ
c/o Keiichi Saijo National Science Museum, Ueno-Park, Tokyo Japan
Editor Seiichiro Kiyota
e-mail:skiyotax@gmail.com