1No.100

Apr. 2022

ISSN 0917-2211

Variable Star Bulletin
Z Cam star PY Per in SU UMa state?
Taichi Kato1
tkato@kusastro.kyoto-u.ac.jp
1 Department of Astronomy, Kyoto University, Sakyo-ku, Kyoto 606-8502, Japan

Received 2022 Apr. 25

Abstract
PY Per has been known as a Z Cam star. Using VSOLJ, VSNET, AAVSO, ASAS-SN and ZTF observations, I found that this object experienced faint states and classiﬁed it to be a Z Cam + VY Scl star.
Furthermore, the object changed the outburst behavior since 2020 and it has been showing regularly recurring
long, bright outbursts with cycles of 110–160 d and short, faint outbursts between them. The 2021 December–2022 January outburst particularly resembled a superoutburst with a gradually fading plateau phase having
a duration more than 25 d and two rebrightenings in the fading part. This object has a long orbital period of0.15468(5) d based on the TESS data and the previous radial-velocity study. The existence of a superoutburst
in a system with a long orbital period would be unusual, but not unprecedented. Since the phenomenon
may still be ongoing, I draw attention to this object when it emerges from in the morning sky after the solar
conjunction.

PY Per was discovered by Hoﬀmeister (1966) (S 9160) as an RW Aur star (the type currently known as
pre-main sequence rapid irregular variables or the GCVS type “IS”) with a photographic range of 14–16.5 mag.
Hoﬀmeister (1966) made a remark that the star was not particularly colored and that the amplitudes on Sonneberg
plates were 1.5–2 mag. Bond (1978) systematically studied high-latitude blue variables and found that PY Per
had diﬀuse (i.e. broad) hydrogen emission lines, superposed on a blue continuum. Bond (1978) classiﬁed PY Per
as a possible dwarf nova. Zwitter and Munari (1994) obtained a low-resolution spectrum and conﬁrmed theﬁnding by Bond (1978). Taylor and Thorstensen (1996) obtained a radial-velocity study and found an orbital
period (Porb ) of 0.15480(17) d. Taylor and Thorstensen (1996) pointed out that PY Per was very faint (V ∼19.8)
on an image in Downes and Shara (1993).
Although this object has been known as a Z Cam star and was relatively well observed by the American
Association of Variable Stars (AAVSO) members particularly between 2003 and 2017, the coverage has not been
good as in the past in recent years. During the period of intensive observations by the AAVSO, the object
showed both bright and faint states and the faint states apparently lasted more than a year, particularly in late2008 to 2009. During this faint state, there was no detected outburst and the object can be classiﬁed as a Z
Cam+VY Scl star. During bright states, the range of variation was relatively small (14.0–17.0), which probably
corresponded to the state observed by Hoﬀmeister (1966). The faint record on an image in Taylor and Thorstensen(1996) was not that faint and was brighter than 19.0 compared to Gaia EDR3 magnitudes of nearby stars(Gaia Collaboration et al. 2021). The state when the image in Downes and Shara (1993) was obtained wasprobably similar to faint states recently observed.
I noticed that the outburst behavior changed dramatically since 2020, when relatively regularly recurring long,bright outbursts with cycles of 110–160 d and short, faint outbursts between them dominated in the light curve(ﬁgure 1). I used the All-Sky Automated Survey for Supernovae (ASAS-SN) Sky Patrol data (Shappee et al. 2014;

2Kochanek et al. 2017), the Zwicky Transient Facility (ZTF: Masci et al. 2019) data1 , observations by AAVSOInternational Database2 , Variable Star Observers League in Japan (VSOLJ) and VSNET (Kato et al. 2004). The
pattern even looks like that of an SU UMa star. The long outburst in 2021 December–2022 January gradually
faded and had a duration more than 25 d, which was unexpectedly long, and even had two rebrightenings on thefading phase (ﬁgure 2). This outburst particularly looks like a superoutburst.
I also analyzed Transiting Exoplanet Survey Satellite (TESS) observations.3 The full light-curve is available
at the Mikulski Archive for Space Telescope (MAST4 ). The TESS observations were obtained in 2019 November(BJD 2458790–2458814), when the amplitudes of dwarf nova-type variations were small. The Porb was determined
to be 0.15468(5) d by using the Phase Dispersion Minimization (PDM, Stellingwerf 1978) method after removing
long-term trends by locally-weighted polynomial regression (LOWESS: Cleveland 1979) (ﬁgure 3). The errorsof periods by the PDM method were estimated by the methods of Fernie (1989) and Kato et al. (2010). This
result conﬁrmed and slightly reﬁned the value in Taylor and Thorstensen (1996). There was no hint of positiveor negative superhumps.
If PY Per indeed showed a superoutburst, it is unusual for an object with Porb of 0.15468(5) d. There is,however, a conﬁrmed case of BO Cet with Porb =0.1398 d (Kato et al. 2021). A suspected case of ASASSN14ho [Porb =0.24315(10) d, Gasque et al. 2019] with multiple rebrightenings has also been reported (Kato 2020).
Although there was apparently no time-resolved photometry during the 2021 December–2022 January outburst
and the object has already gone into solar conjunction, I would like to call attention to this object to see whether
this state continues when the object emerges in the morning sky and if it is the case, time-resolved photometry
during a long, bright outburst is desired. It is known that SU UMa stars with long Porb can have three types ofoutbursts (normal, long normal and superoutbursts) as in TU Men (Warner 1995; Bateson et al. 2000), NY Ser(Pavlenko et al. 2014) and V1006 Cyg (Kato et al. 2016; Pavlenko et al. 2018). The diﬀerence in the durationsof long outbursts in PY Per after 2020 may reﬂect diﬀerent types of outbursts and only the 2021 December–2022
January might have been a genuine superoutburst.

Acknowledgements
This work was supported by JSPS KAKENHI Grant Number 21K03616. The author is grateful to the ASAS-SNand ZTF teams for making their data available to the public. I am grateful to VSOLJ, AAVSO and VSNET
observers for reporting observations and to Naoto Kojiguchi for helping downloading the ZTF data.
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, 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
RW Aur, BO Cet, Z Cam, V1006 Cyg, TU Men, PY Per, VY Scl, ASASSN-14hoReferences
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3 <https://tess.mit.edu/observations/>.
4 <http://archive.stsci.edu/>.

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Figure 1: Long-term light curve of PY Per using VSOLJ, VSNET, AAVSO, ASAS-SN and ZTF observations.
The object showed low-amplitude outbursts before 2020 (the ﬁrst and second panels). The outburst patternchanged since then and the behavior in the bottom panel resembles that of an SU UMa star in that long, brightoutbursts occur in addition to short, faint outbursts.

4121416

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Figure 2: Enlargement of the 2021 December–2022 January long and bright outburst of PY Per. There wereapparently two rebrightenings during the fading phase of the outburst. The symbols are the same as in ﬁgure 1.

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5θ
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Figure 3: Period analysis of the TESS data. (Upper): We analyzed 100 samples which randomly contain 50%of observations, and performed the PDM analysis for these samples. The bootstrap result is shown as a form of90% conﬁdence intervals in the resultant PDM θ statistics. (Lower): Orbital variation.

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