1No.111

Feb. 2023

ISSN 0917-2211

Variable Star Bulletin
MGAB-V240: 23-min AM CVn star showing both 12-d supercycle and standstills
Taichi Kato1
tkato@kusastro.kyoto-u.ac.jp
1 Department of Astronomy, Kyoto University, Sakyo-ku, Kyoto 606-8502, Japan
Received 2023 Feb. 21
Abstract
Using Zwicky Transient Facility (ZTF) data, I noticed that MGAB-V240 = PS1-3PI J185529.82+323017.8
showed two diﬀerent states: regularly outbursting state with a cycle length of 12 d and standstills. I foundthat the regularly outbursting state was in fact a sequence of superoutburst and intervening normal outbursts
comprising a 12-d supercycle. During one of the superoutbursts, superhumps with a period of 0.015824(9) d(=22.79 min) were detected in the ZTF time-resolved data. This period and behavior have conﬁrmed that
MGAB-V240 is an AM CVn-type object with the shortest known supercycle and the second known AM CVnstar showing genuine standstills. The standstills in this system were interrupted by short drops and the system
often brightened after these drops. This phenomenon can be explained by the accumulation of the transferredmatter in the outer part of the disk during the drops. This phenomenon favors a constant mass-transferfrom the secondary combined with the diﬃculty in maintaining the hot state in a helium disk rather than atemporary decrease of the mass-transfer rate as the cause of these drops. MGAB-V240 should be close to theborder of the thermal instability of a helium disk, and the observed superhump period agrees very well withthe activity sequence expected by the disk instability theory and the evolutionary sequence of AM CVn stars.

MGAB-V240 was discovered as a faint SS Cyg-type dwarf nova with frequent outbursts.1 The object had
also been selected as a candidate RR Lyr star (PS1-3PI J185529.82+323017.8, Sesar et al. 2017).2 I used Zwicky
Transient Facility (ZTF: Masci et al. 2019)3 data and found that this object showed standstills in 2020 and 2022.
The long-term light curves containing a regularly outbursting part (2018–2019) and a state with standstills andoutbursts (2020–2022) are shown in ﬁgures 1 and 2, respectively. A naïve look at these ﬁgures would simplyre-classify the object as an Z Cam star [for cataclysmic variables and their subclasses, see e.g., Warner (1995)]. I,
however, noticed that the object showed sudden drops during the 2022 standstill (ﬁgure 3), which is unusual for a
Z Cam-type dwarf nova (vsnet-chat 9317).4 One or two drops were also recorded during the 2020 standstill. The
fading rates of these sudden drops were sometimes close to 2 mag d−1 , whose large value is one of the signaturesof AM CVn-type outbursts (Kato and Kojiguchi 2021) [for a review of AM CVn stars, see e.g., Solheim (2010)].
Upon a closer look at the ZTF light curve of the regularly outbursting part, I found short outbursts between
long outbursts (ﬁgure 4). This is a clear indication of an SU UMa-type supercycle (long outbursts and short
outbursts between them). Superhumps with a period of 0.015824(9) d were indeed detected from the ZTF timeresolved data during one of long outbursts (ﬁgure 5). The error of the period was estimated by the methods ofFernie (1989) and Kato et al. (2010). The overall light curve of the 2018–2019 season resembles that of ASASSN14cc (Kato et al. 2015). Kato et al. (2015) detected a supercycle of 21–33 d together with superhumps with periodof 0.01560–0.01562 d by a network of ground-based small telescopes aiming at ASASSN-14cc under a VSNET
1 <https://www.aavso.org/vsx/index.php?view=detail.top&oid=702830>.
2 Although the name PS1-3PI J185529.82+323017.8 would be adequate considering the priority in discovery, I use MGAB-V240
in this paper because of its brevity.
3 The ZTF data can be obtained from IRSA <https://irsa.ipac.caltech.edu/Missions/ztf.html> using the interface <https://irsa.ipac.caltech.edu/docs/program_interface/ztf_api.html> or using a wrapper of the above IRSA API<https://github.com/MickaelRigault/ztfquery>.
4 <http://ooruri.kusastro.kyoto-u.ac.jp/mailarchive/vsnet-chat/9317>.

217ZTF r

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Figure 1: Light curve of MGAB-V240 in 2018–2019. The object showed a regular pattern of outbursts recurringwith a period of ∼12 d. They turned out to be superoutbursts, not SS Cyg-type outbursts (see text).

(Kato et al. 2004) campaign. This period was conﬁrmed by TESS photometry (Pichardo Marcano et al. 2021).
Kato et al. (2015) suggested that ASASSN-14cc showed a supercycle similar to the hydrogen-rich system RZ LMi.
RZ LMi typically has a supercycle of 19 d (Robertson et al. 1995; Nogami et al. 1995; Olech et al. 2008), butshowed a short standstill in 2016 (Kato et al. 2016). Such a short supercycle could not be naturally reproduced(Osaki 1995a,b) by the thermal-tidal instability model (Osaki 1989, 1996), which successfully explained thesupercycles of most SU UMa stars. Osaki (1995b) explained the short supercycle by artiﬁcially quenching thesuperoutburst when the accretion disk is still large. This treatment led to an idea of decoupling between thethermal and tidal instabilities (Hellier 2001).
The present observations of MGAB-V240 make this object as a perfect helium analog (although no spectrum
has been obtained, the short superhump period is an unambiguous signature of an AM CVn-type object) of thehydrogen-rich RZ LMi: a short-period supercycle and standstills. True standstills in AM CVn stars have beenvery rare and this object becomes the second well-established example after CR Boo (Kato et al. 2023). The
suggested type is SU UMa(ER UMa)+Z Cam+AM CVn.
In AM CVn stars, an RZ LMi-like short supercycle would be more easily achieved than in hydrogen-richsystems for two reasons: (1) Helium disks require a higher temperature to maintain the hot state and a coolingwave starts more easily than in hydrogen-rich systems. (2) AM CVn stars have (usually) lower mass ratios thanin hydrogen-rich systems, which would make decoupling between the thermal and tidal instabilities easier tohappen [For the disk-instability model of AM CVn stars, see Tsugawa and Osaki (1997); Solheim (2010); Kotkoet al. (2012)]. Drops from standstills may be caused by the diﬃculty (relative to hydrogen-rich systems) inmaintaining the hot state. In contrast to most (hydrogen-rich) Z Cam stars, these drops were often followed byshort brightening (after the initial three drops in ﬁgure 4). This phenomenon can be interpreted as follows: Themass transfer from the secondary and the mass accretion to the primary are balanced during the standstill. Oncea cooling wave starts, the mass accretion to the primary decreases and the mass accumulates (if the mass-transferrate is constant) during these drops in the outer part of the disk, and this extra mass causes brightening when
the disk becomes hot again. The phenomenon observed in standstills of MGAB-V240 excludes the possibility ofa temporary decrease of the mass-transfer rate as the cause of the drops; rather the constant mass-transfer rateis favored to explain brightening after the drops.
The superhump period of 0.015824 d, which is usually ∼1% longer than the orbital period (Porb ) in AM CVnstars, in MGAB-V240 is between the thermally unstable dwarf nova-type CR Boo (Porb =0.017029 d: Provencal

3

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Figure 2: Light curve of MGAB-V240 in 2020–2022. The symbols are the same as in ﬁgure 1. Standstills werepresent in addition to outbursts.

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Figure 3: Enlargement of the 2022 standstill of MGAB-V240. The symbols are the same as in ﬁgure 1. Sudden
drops from the standstill were recorded. The object usually brightened after these drops.

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Figure 4: Enlargement of the 2018 outbursts of MGAB-V240. The symbols are the same as in ﬁgure 1. In
addition to relatively regular long outbursts, short outbursts (with tick marks) were also present. Some outbursts
between the long ones apparently had durations more than 1 d and they were not marked by the ticks.

5

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Figure 5: Superhumps on 2018 June 30–July 1 recorded by ZTF time-resolved photometry. (Upper): PDManalysis. The bootstrap result using randomly contain 50% of observations is shown as a form of 90% conﬁdence
intervals in the resultant θ statistics. (Lower): Phase plot.

6et al. 1997) and the thermally stable novalike-type HP Lib (Porb =0.012763 d: Patterson et al. 2002; Roelofs et al.
2007) among AM CVn stars and follows the activity sequence expected by the disk instability theory and theevolutionary sequence of AM CVn stars (Tsugawa and Osaki 1997; Kotko et al. 2012).

Acknowledgements
This work was supported by JSPS KAKENHI Grant Number 21K03616.
I am grateful to Naoto Kojiguchi for helping downloading the ZTF data and the ZTF team for making theirdata available to the public.
Based on observations obtained with the Samuel Oschin 48-inch Telescope at the Palomar Observatory as
part of the Zwicky Transient Facility project. ZTF is supported by the National Science Foundation under Grant
No. AST-1440341 and a collaboration including Caltech, IPAC, the Weizmann Institute for Science, the Oskar
Klein Center at Stockholm University, the University of Maryland, the University of Washington, DeutschesElektronen-Synchrotron and Humboldt University, Los Alamos National Laboratories, the TANGO Consortiumof Taiwan, the University of Wisconsin at Milwaukee, and Lawrence Berkeley National Laboratories. Operations
are conducted by COO, IPAC, and UW.
The ztfquery code was funded by the European Research Council (ERC) under the European Union’s Horizon
2020 research and innovation programme (grant agreement n◦ 759194 – USNAC, PI: Rigault).

List of objects in this paper
CR Boo, Z Cam, AM CVn, SS Cyg, HP Lib, RZ LMi, SU UMa, ER UMa, ASASSN-14cc, MGAB-V240,
PS1-3PI J185529.82+323017.8

References
Fernie, J. D. (1989) Uncertainties in period determinations. PASP 101, 225
Hellier, C. (2001) On echo outbursts and ER UMa supercycles in SU UMa-type cataclysmic variables. PASP113, 469
Kato, T., Hambsch, F.-J., & Monard, B. (2015) ASASSN-14cc: A likely helium analog of RZ Leonis Minoris.
PASJ 67, L2
Kato, T. et al. (2016) RZ Leonis Minoris bridging between ER Ursae Majoris-type dwarf nova and nova-likesystem. PASJ 68, 107
Kato, T., & Kojiguchi, N. (2021) New candidates for AM Canum Venaticorum stars among ASAS-SN transients.
PASJ 73, 1375
Kato, T., Maeda, Y., & Moriyama, M. (2023) Genuine standstill in the AM CVn star CR Boo. VSOLJ Variable
Star Bull. 107, (arXiv:2302.04454)
Kato, T. et al. (2010) Survey of period variations of superhumps in SU UMa-type dwarf novae. II. The secondyear (2009–2010). PASJ 62, 1525
Kato, T., Uemura, M., Ishioka, R., Nogami, D., Kunjaya, C., Baba, H., & Yamaoka, H. (2004) Variable Star
Network: World center for transient object astronomy and variable stars. PASJ 56, S1
Kotko, I., Lasota, J.-P., Dubus, G., & Hameury, J.-M. (2012) Models of AM Canum Venaticorum star outbursts.
A&A 544, A13
Masci, F.-J. et al. (2019) The Zwicky Transient Facility: Data processing, products, and archive. PASP 131,018003
Nogami, D., Kato, T., Masuda, S., Hirata, R., Matsumoto, K., Tanabe, K., & Yokoo, T. (1995) Photometricobservations of an extreme ER UMa star, RZ Leonis Minoris. PASJ 47, 897

7Olech, A., Wisniewski, M., Zloczewski, K., Cook, L. M., Mularczyk, K., & Kedzierski, P. (2008) Curious Variables
Experiment (CURVE). RZ LMi – the most active SU UMa star. Acta Astron. 58, 131
Osaki, Y. (1989) A model for the superoutburst phenomenon of SU Ursae Majoris stars. PASJ 41, 1005
Osaki, Y. (1995a) A model for a peculiar SU Ursae Majoris-type dwarf nova ER Ursae Majoris. PASJ 47, L11
Osaki, Y. (1995b) Why does RZ Leonis Minoris, and unusual SU UMa star, have such a short supercycle? PASJ47, L25
Osaki, Y. (1996) Dwarf-nova outbursts. PASP 108, 39Patterson, J. et al. (2002) Superhumps in cataclysmic binaries. XXI. HP Librae (=EC 15330−1403). PASP 114,65
Pichardo Marcano, M., Rivera Sandoval, L. E., Maccarone, T. J., & Scaringi, S. (2021) TACOS: TESS AM CVnoutbursts survey. MNRAS 508, 3275
Provencal, J. L. et al. (1997) Whole Earth Telescope observations of the helium interacting binary PG 1346+082(CR Bootis). ApJ 480, 383
Robertson, J. W., Honeycutt, R. K., & Turner, G. W. (1995) RZ Leonis Minoris, PG 0943+521, and V1159
Orionis: Three cataclysmic variables with similar and unusual outburst behavior. PASP 107, 443
Roelofs, G. H. A., Groot, P. J., Nelemans, G., Marsh, T. R., & Steeghs, D. (2007) On the orbital periods of theAM CVn stars HP Librae and V803 Centauri. MNRAS 379, 176
Sesar, B. et al. (2017) Machine-learned identiﬁcation of RR Lyrae stars from sparse, multi-band data: The PS1
sample. AJ 153, 204
Solheim, J. (2010) AM CVn stars: Status and challenges. PASP 122, 1133
Tsugawa, M., & Osaki, Y. (1997) Disk instability model for the AM Canum Venaticorum stars. PASJ 49, 75
Warner, B. (1995) Cataclysmic Variable Stars (Cambridge: Cambridge University Press)

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