1No.121

Jul. 2023

Variable Star Bulletin
SDSS J183131.63+420220.2: AM CVn star showing
ER UMa-type behavior and long standstill
Taichi Kato1
tkato@kusastro.kyoto-u.ac.jp
1 Department of Astronomy, Kyoto University, Sakyo-ku, Kyoto 606-8502, Japan
Received 2023 Jul. 14Abstract
SDSS J183131.63+420220.2 is an AM CVn-type cataclysmic variable. Using Asteroid Terrestrial-impact
Last Alert System (ATLAS) and Zwicky Transient Facility (ZTF) data, I found that this object is actuallya helium dwarf nova, which experienced a long (∼6 yr) standstill (2017 to 2022). The object is currently (in2023) in ER UMa-type state with supercycles of 20–30 d and large duty cycles exceeding 0.5. This object
is the second known star among AM CVn stars having characteristics of ER UMa and Z Cam types. Thelong duration of the standstill phase is outstanding among helium dwarf novae. These observations indicatethat the accretion disk in SDSS J183131.63+420220.2 is very close to thermal stability. I detected a periodof 0.01602343(1) d in the ZTF data, which can be the orbital one. Combined with the case of MGAB-V240,
the limit of thermal stability of the disks in AM CVn stars appears to be located around the orbital period of0.0158–0.0160 d.

SDSS J183131.63+420220.2 was selected as a white dwarf candidate from Sloan Digital Sky Survey (SDSS)
colors and was classiﬁed as a cataclysmic variable (CV) by Girven et al. (2011). This object was spectroscopicallyclassiﬁed as a DB white dwarf by Kleinman et al. (2013). The variability of this object was detected by Asteroid
Terrestrial-impact Last Alert System (ATLAS: Tonry et al. 2018) and was listed as ATO J277.8818+42.0389 witha classiﬁcation of “irregular” (Heinze et al. 2018).1 The variation was also detected by Zwicky Transient Facility(ZTF: Masci et al. 2019) and was given a name of ZTF J183131.63+420220.1 (Ofek et al. 2020). Most recently,
Gaia Collaboration et al. (2022) listed this object as a CV (Gaia DR3 2111270034246759424).
Inight et al. (2023) studied this object and classiﬁed it as an AM CVn star with a helium-dominated absorption line spectrum and showing occasional “drop-outs” (fading) in the ZTF and Gaia light curves. Inight et al.
(2023) described that the drop-outs in the Gaia and ZTF light curves are unusual for this type (i.e. AM CVn) ofsystem.
Using ZTF and ATLAS data, I found that this object currently (in 2023) shows ER UMa-type (subclass of
SU UMa stars; see e.g. Kato et al. 1999) variations (vsnet-alert 27896). The long-term light curves are shownin ﬁgures 1 and 2. The enlarged 2023 light curve is shown in ﬁgure 3. Long outbursts (superoutbursts) wereintervened by short fading episodes. Short outbursts are also also evident around BJD 2460131 and 2460133.
The recurrence times of long outbursts varied between 20 and 30 d. The duty cycles of long outbursts were ∼0.5or even larger. The current light curve is very similar to the AM CVn star MGAB-V240 particularly in 2021 to2022 (Kato 2023). It is also similar to the hydrogen-rich ER UMa star RZ LMi when it showed standstills (Katoet al. 2016). MGAB-V240 was the second AM CVn-type object showing true standstills after CR Boo (Katoet al. 2023). SDSS J183131.63+420220.2 has become the third known such an object and is notable in that it
showed a very long standstill lasting almost 6 yr, while the other two objects showed relatively short ones. Thesuggested type is SU UMa(ER UMa)+Z Cam+AM CVn, the same as for MGAB-V240.
The discussion in MGAB-V240 (Kato 2023) also applies to SDSS J183131.63+420220.2. The accretion disk
in this object should be very close to the thermal stability. Using the ZTF data during the standstill, I detected
1 Although I used this as the primary name in vsnet-alert 27896 (<http://ooruri.kusastro.kyoto-u.ac.jp/mailarchive/vsnetalert/27896>), which ﬁrst described variability, I here use the SDSS name knowing that the CV nature had already been recognized.

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Figure 1: Light curve of SDSS J183131.63+420220.2 in 2016–2019. The object was mostly in nearly constantbrightness in 2017–2019. Larger-amplitude variations were present in 2016, as was also evident in Gaia observations shown in Inight et al. (2023).

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Figure 2: Light curve of SDSS J183131.63+420220.2 in 2020–2023. The symbols are the same as in ﬁgure 1.
The object brightened somewhat in 2022 (third panel) with a sudden short drop near the end. The object wasin dwarf nova (ER UMa) phase in 2023 (last panel).

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Figure 3: Enlargement of the light curve of SDSS J183131.63+420220.2 in 2023. The symbols are the same as inﬁgure 1. Long outbursts were intervened by short fading episodes. Short outbursts are also also evident aroundBJD 2460131 and 2460133.

a period of 0.01602343(1) d (ﬁgure 4) using the phase dispersion minimization (PDM: Stellingwerf 1978) method
after removing long-term trends by using locally-weighted polynomial regression (LOWESS: Cleveland 1979).
The error was estimated by the methods of Fernie (1989); Kato et al. (2010). There was no other candidate
period in the period region acceptable for an AM CVn star. This period is likely either the orbital or superhump
period of this system. I consider the former more likely based on the long coherence. If the period is the orbital
period, a superhump signal ∼1% diﬀerent from this can be expected. I could not ﬁnd such a signal. The period
in SDSS J183131.63+420220.2 is very close to the superhump period [0.015824(9) d] of MGAB-V240. It is most
likely that the limit of the thermal stability in the disks of AM CVn stars is around these periods, as expectedfrom the disk-instability theory (Tsugawa and Osaki 1997; Solheim 2010; Kotko et al. 2012).
The long-term variation during the standstill probably requires long-term slight variation of the mass-transferrate, as is usually considered as the origin of standstills in hydrogen-rich Z Cam stars (Meyer and MeyerHofmeister 1983). I also point out that the gradually rising standstill (2017 to 2021) and brightening in late 2022
before its termination are somewhat similar to the IW And stars (hydrogen-rich systems) (Szkody et al. 2013;
Kato 2019). The time scales in SDSS J183131.63+420220.2, however, would be too long compared to the viscoustime scale of the disk to be considered as the same origin as in IW And stars.

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 ATLAS and ZTF teams formaking their data available to the public.
This work has made use of data from the Asteroid Terrestrial-impact Last Alert System (ATLAS) project.
The ATLAS project is primarily funded to search for near earth asteroids through NASA grants NN12AR55G,
80NSSC18K0284, and 80NSSC18K1575; byproducts of the NEO search include images and catalogs from thesurvey 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 Instituteof Astrophysics (MAS), Chile.
Based on observations obtained with the Samuel Oschin 48-inch Telescope at the Palomar Observatory as

5

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1.5Figure 4: Period analysis of SDSS J183131.63+420220.2 during the standstill. (Upper): PDM analysis. The
bootstrap result using randomly contain 50% of observations is shown as a form of 90% conﬁdence intervals inthe resultant θ statistics. (Lower): Phase plot.

6part 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, Deutsches
Elektronen-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, RZ LMi, SU UMa, ER UMa, ATO J277.8818+42.0389, Gaia DR3 2111270034246759424,
MGAB-V240, SDSS J183131.63+420220.2, ZTF J183131.63+420220.1

References
Cleveland, W. S. (1979) Robust locally weighted regression and smoothing scatterplots. J. Amer. Statist. Assoc.
74, 829
Fernie, J. D. (1989) Uncertainties in period determinations. PASP 101, 225
Gaia Collaboration et al. (2022) Gaia Data Release 3. Summary of the contents and survey properties. preprint(arXiv:2208.00211)
Girven, J., Gänsicke, B. T., Steeghs, D., & Koester, D. (2011) DA white dwarfs in Sloan Digital Sky Survey DataRelease 7 and a search for infrared excess emission. MNRAS 417, 1210
Heinze, A. N. et al. (2018) A ﬁrst catalog of variable stars measured by the Asteroid Terrestrial-impact Last
Alert System (ATLAS). AJ 156, 241
Inight, K. et al. (2023) A catalogue of cataclysmic variables from 20 years of the Sloan Digital Sky Survey withnew classiﬁcations, periods, trends and oddities. MNRAS (arXiv:2304.06749)
Kato, T. (2019) Three Z Cam-type dwarf novae exhibiting IW And-type phenomenon. PASJ 71, 20
Kato, T. (2023) MGAB-V240: 23-min AM CVn star showing both 12-d supercycle and standstills. VSOLJ
Variable Star Bull. 111, (arXiv:2302.11148)
Kato, T. et al. (2016) RZ Leonis Minoris bridging between ER Ursae Majoris-type dwarf nova and nova-likesystem. PASJ 68, 107
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., Nogami, D., Baba, H., Masuda, S., Matsumoto, K., & Kunjaya, C. (1999) in Disk Instabilities in Close
Binary Systems, ed. S. Mineshige, & J. C. Wheeler (Tokyo: Universal Academy Press) p. 45
Kleinman, S. J. et al. (2013) SDSS DR7 white dwarf catalog. ApJS 204, 5
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
Meyer, F., & Meyer-Hofmeister, E. (1983) A model for the standstill of the Z Camelopardalis variables. A&A121, 29

7Ofek, E. O., Soumagnac, M., Nir, G., Gal-Yam, A., Nugent, P., Masci, F., & Kulkarni, S. R. (2020) A catalogueof over 10 million variable source candidates in ZTF Data Release 1. MNRAS 499, 5782
Solheim, J. (2010) AM CVn stars: Status and challenges. PASP 122, 1133
Stellingwerf, R. F. (1978) Period determination using phase dispersion minimization. ApJ 224, 953
Szkody, P. et al. (2013) A study of the unusual Z Cam systems IW Andromedae and V513 Cassiopeia. PASP
125, 1421
Tonry, J. L. et al. (2018) ATLAS: A High-cadence All-sky Survey System. PASP 130, 064505
Tsugawa, M., & Osaki, Y. (1997) Disk instability model for the AM Canum Venaticorum stars. PASJ 49, 75

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