1
No.109Feb. 2023

ISSN 0917-2211

Variable Star Bulletin
ASASSN-15cm: an SU UMa star with an orbital period of 5.0 hours
Taichi Kato1
tkato@kusastro.kyoto-u.ac.jp
1 Department of Astronomy, Kyoto University, Sakyo-ku, Kyoto 606-8502, Japan
Received 2023 Feb. 15
Abstract
ASASSN-15cm had been identiﬁed as a dwarf nova with an orbital period of 5.0 hours and a hot, luminous
secondary of of a spectral type around K2.5 in the previous study. Using the Zwicky Transient Facility(ZTF) data, the Asteroid Terrestrial-impact Last Alert System (ATLAS) forced photometry and the AllSky Automated Survey for Supernovae (ASAS-SN) Sky Patrol data, I found that this object underwent
a superoutburst in 2019. I obtained a reﬁned orbital period of 0.2084652(3) d and a superhump period of
0.2196(1) d, which gave a mass ratio q=0.22. Modeling of quiescent ellipsoidal variations yielded an inclinationof i=55◦ , consistent with the lack of eclipses during outbursts. The object adds another example of SU UMastars above or in the period gap containing a secondary with an evolved core, and has the longest orbital
period among the established ones. ASASSN-15cm showed relatively regular superoutbursts with a supercycleof 849(18) d between 2015 and 2022, and the next superoutburst is expected to occur in the early half of 2024.
Coordinated detailed observations during the next superoutburst are expected to better clarify the nature ofthis object.

1

Introduction

ASASSN-15cm was discovered as a dwarf nova by the All-Sky Automated Survey for Supernovae (ASAS-SN:
Shappee et al. 2014) at V =15.8 on 2015 January 31. The ASAS-SN team noted that the Catalina Real-time Transient Survey (CRTS, Drake et al. 2008)1 had detected previous outbursts. Thorstensen et al. (2016) performed
spectroscopic and photometric studies of this object and obtained an orbital period (Porb ) of 0.208466(2) d using
the CRTS data. Spectroscopy by Thorstensen et al. (2016) indicated a secondary of a spectral type of K2.5±2.5,
hot and luminous for a cataclysmic variable (CV) with this Porb Based on the long Porb , the object was implicitly
assumed to be an SS Cyg star and no special attention has been paid to search for an SU UMa-type signature.
[For general information of CVs and subclasses, see e.g., Warner (1995). For a review of dwarf novae, including
SU UMa stars, see e.g., Osaki (1996)].

2

Data

I used the Asteroid Terrestrial-impact Last Alert System (ATLAS: Tonry et al. 2018) forced photometry (Shingles
et al. 2021) and the Zwicky Transient Facility (ZTF: Masci et al. 2019)2 data together with ASAS-SN Sky Patrol
data (Kochanek et al. 2017).

3

Results and Discussions
1 <http://nesssi.cacr.caltech.edu/catalina/>.

2 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>.

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Figure 1: Light curve of ASASSN-15cm in 2015–2018.

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Figure 2: Light curve of ASASSN-15cm in 2019–2022. The symbols are the same as in ﬁgure 1.

4

3.1

Superoutbursts and superhumps

In ﬁgures 1 and 2, I show the light curve of ASASSN-15cm in the 2015–2022 seasons. ASAS-SN data did notreach 18.0 mag and quiescent parts was not recorded. In all these surveys, there were false bright detections andthey were removed by comparing with the data obtained on the same nights by the same or other surveys. Upper
limit observations were not plotted to avoid the ﬁgures to be too crowded.
The initial outburst in 2015 (ﬁrst panel of ﬁgure 1) corresponds to the initial announcement of the outburst
detection by the ASAS-SN team. I noticed that a long outburst in 2019 (ﬁrst panel of ﬁgure 2) had a shape similar
to an SU UMa-type superoutburst. This outburst was composed of a linearly fading initial part and a subsequent
part with a scatter in the light curve (ﬁgure 3). Using these data after removing the trend by locally-weighted
polynomial regression (LOWESS: Cleveland 1979), a phase dispersion minimization (PDM, Stellingwerf 1978)
analysis yielded a strong superhump signal (ﬁgure 4) having a period of 0.2196(1) d with its error determinedby the methods of Fernie (1989) and Kato et al. (2010). This period analysis had been proven to be useful forsparse, but with multiple samples in the same night, data such as ATLAS observations (Kato 2023). Note thatthe entire part was analyzed and possible variation in the superhump period (e.g. Kato et al. 2009) may haveaﬀected the result. There was no hint of the orbital modulation during the superoutburst.
ASASSN-15cm showed long outbursts (likely superoutbursts) relatively regularly: in 2015 (BJD 2457054),
in 2017 (BJD 2457830), in 2019 (BJD 2458706) and in 2022 (BJD 2459591), where BJD values refer to the peaksof the outbursts (ﬁgures 1, 2). These long outbursts could be expressed by a period (supercycle) of 849(18) dwithin errors of 41 d. This result suggests that superbursts in ASASSN-15cm occur very regularly, and the 2019superoutburst was unlikely an exceptional phenomenon. The next superoutburst is expected to occur in the early
half of 2024 (around BJD 2460417) and coordinated detailed observations are desirable. The supercycle of 849 d
is fairly long and this implies a low mass-transfer rate.
The object also showed normal outbursts. These outbursts were relatively slowly rising and could be insideout outbursts. The small number of normal outbursts also supports the low mass-transfer rate (Ichikawa andOsaki 1994; Osaki 1996).

3.2

Orbital period and variation in quiescence

Using ATLAS and ZTF data (all bands combined), a PDM analysis of the quiescent part yielded an orbital
period of 0.2084652(3) d (ﬁgure 5), in very good agreement with the value by Thorstensen et al. (2016). The
quiescent light curve consists of prominent ellipsoidal variations, which are expected from the hot and luminous
secondary (Thorstensen et al. 2016). The zero phase was chosen to be BJD 2457553.8461 to match ﬁgure 15 in(Thorstensen et al. 2016). This zero phase corresponds to the inferior conjunction of the secondary. Multicolor
phase-averaged light curves are shown in ﬁgure 6. The maximum at phase 0.7–0.8 (possibly corresponding to the
hot spot, but the O’Connell eﬀect cannot be excluded) becomes brighter in the r and g bands than in the i band.

3.3

Mass ratio and orbital parameters

The observed fractional superhump excess ϵ ≡ PSH /Porb −1, where PSH represents the superhump period, is 0.053.
Assuming that PSH represents the period of stage B superhumps [for superhumps and stages, see Kato et al.
(2009)], the semi-empirically derived q = M2 /M1 is 0.22 (table 4 in Kato 2022). Although I do not explicitly givean error estimate, the error is expected to be an order of 0.01 [see Kato (2022) for the derivation process]. Thisappears to ﬁt the observations fairly well since q <0.25–0.33 is required to show superhumps during outbursts(Whitehurst 1988; Murray et al. 2000). The regularity of superoutbursts (subsection 3.1) provides a support tothe q value not too close to the borderline, i.e. not close to 0.3 or larger.
This q is very diﬀerent from q ∼0.6 suggested in Thorstensen et al. (2016).3 This constraint by Thorstensen
et al. (2016) on q was based on a combination of the observed K2 velocity and the constraint on the inclination
allowing a large amplitude of ellipsoidal variations, which could not be comfortably modeled without introducing
artiﬁcially increasing gravity darkening and decreasing the disk contribution excessively (Thorstensen et al.
2016). The second assumption (negligible contribution from the disk) required for modeling at least appears to
be satisﬁed considering the low mass-transfer rate inferred from the long supercycle (subsection 3.1).
Using Ellipsoidal Modulation Light Curve Generator by M. Uemura (2006), which was based on Orosz and
Bailyn (1997a,b), I obtained ellipsoidal variations similar to the ZTF light curves in ﬁgure 6. I used Teﬀ =5000 K,
3 The modest lower limit of q=0.1 by Thorstensen et al. (2016), however, turned out to be an excellent choice after the identiﬁcationof this system as an SU UMa star.

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Figure 3: Enlargement of the light curve of the 2019 superoutburst. The symbols are the same as in ﬁgure 1.

log g=4.5 and gravitational darkening and limb-darkening coeﬃcients given in Claret and Bloemen (2011) (solar
metallicity and a microturbulent velocity of 2 km s−1 as typical values were used, since I do not have information
for them; however, they have little eﬀect on the result). With these parameters, the primary with a mass of
0.7 M⊙ can reproduce the observed K2 =229 km s−1 .

3.4

SU UMa stars above or in the period gap

SU UMa stars above the CV period gap are relatively rare, since CVs with long periods have larger q values,
which make the 3:1 resonance in the disk diﬃcult to achieve. The present ﬁnding in ASASSN-15cm adds anotherexample of SU UMa stars above or in the period gap containing a secondary with an evolved core. The known
such systems include OT J002656.6+284933 (=CSS101212:002657+284933, PSH =0.13225 d, Kato et al. 2017),
ASASSN-18aan (Porb =0.149454 d, Wakamatsu et al. 2021) and likely ASASSN-19ax (PSH =0.1000 d, Kato et al.
2021a) following the identiﬁcation of QZ Ser (but near the lower border of the period gap with Porb =0.08316 d)
as an SU UMa star with an anomalously hot and luminous secondary (Thorstensen et al. 2002). V363 Lyr(Porb =0.185723 d, Kato 2021) might be another example, although the superhump-like signal recorded in thissystem may have not been the same as in other SU UMa stars. Another group of SU UMa stars above the period
gap, but apparently lacking evidence of a secondary with an evolved core, contains BO Cet (Porb =0.139835 d,
Kato et al. 2021b; Kato 2023) and possibly MisV1448 (PSH =0.2275 d, vsnet-alert 24912,4 Kojiguchi et al. in
preparation). The status of ASASSN-14ho (Porb =0.24315 d, Kato 2020) still requires observational conﬁrmation.
Lists of long-Porb SU UMa stars or CVs with an anomalously warm secondary can be found in Wakamatsu et al.
(2021) and Kato et al. (2021a). A search for superhumps in long-Porb systems with a secondary with an evolved
core would be an interesting challenge.

Acknowledgements
This work was supported by JSPS KAKENHI Grant Number 21K03616.
I am grateful to Makoto Uemura for sharing the code of Ellipsoidal Modulation Light Curve Generator,
Naoto Kojiguchi for helping downloading the ZTF data and the ZTF, ATLAS and ASAS-SN teams for making
their data 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
4 <http://ooruri.kusastro.kyoto-u.ac.jp/mailarchive/vsnet-alert/24912>.

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θ
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Figure 4: Superhumps of ASASSN-15cm in 2019 (interval BJD 2458705.6–2458726.7). (Upper): PDM analysis.
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.

7

θ
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Figure 5: Orbital proﬁle of ASASSN-15cm in quiescence using ATLAS and ZTF data. (Upper): PDM analysis.
(Lower): Phase plot. Prominent ellipsoidal variations are present. The zero phase was chosen to be BJD
2457553.8461 to match ﬁgure 15 in Thorstensen et al. (2016). This zero phase corresponds to the inferior
conjunction of the secondary.

8

−0.2

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Figure 6: Orbital proﬁle of ASASSN-15cm in quiescence using multicolor ZTF data. The zero phase is deﬁned
as the same as in ﬁgure 5. The plots were shifted by 0.3 mag between diﬀerent bands. The solid curves represent
ellipsoidal variations expected for q=0.22 and i=55◦ (see text).

9
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).
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
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
BO Cet, V363 Lyr, QZ Ser, SU UMa, ASASSN-14ho, ASASSN-15cm, ASASSN-18aan, ASASSN-19ax,
CSS101212:002657+284933, MisV1448, OT J002656.6+284933

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VSOLJ
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Editor Seiichiro Kiyota
e-mail:skiyotax@gmail.com