1No.88

Dec. 2021

ISSN 0917-2211

Variable Star Bulletin
Orbital period of the IW And-type star ST Cha
Taichi Kato1 ,Naoto Kojiguchi1
1

Department of Astronomy, Kyoto University, Sakyo-ku, Kyoto 606-8502, Japantkato@kusastro.kyoto-u.ac.jp

Received 2021 Dec.16

Abstract
We analyzed TESS data of the IW And-type dwarf nova ST Cha during ordinary dwarf nova states. We
have identiﬁed an orbital period of 0.285360(1) d. The object was reported to be eclipsing in the past using
the data obtained in the 1970s, which is not in agreement with the present data. Despite the constant mean
brightness, the strength of the orbital signal varied signiﬁcantly, suggesting that the strength of the orbital
signal does not always reﬂect the mass-transfer rate. During an outburst with a shoulder, we did not ﬁnd
evidence of humps recurring with a period longer than the orbital period which were recorded in V363 Lyr.
This ﬁnding strengthens the idea that V363 Lyr is an unusual object. We found that the strength of theorbital signal increased after an outburst with a shoulder. This outburst may have changed the state of thedisk and the hot spot became more apparent. Such a change in the disk may have triggered a transition
from an ordinary dwarf nova-type state to an IW And-type state and this possibility would require furtherexamination.

1

Introduction

IW And stars are a subgroup of Z Cam type dwarf novae [or general information of dwarf novae, see e.g.
Warner (1995)]. Z Cam stars are characterized by the presence of standstills in addition to ordinary dwarf
nova-type outbursts. Standstills in Z Cam stars usually occur after outbursts and end with fading (see e.g.
Szkody and Mattei 1984). In IW And stars, outbursts and standstills sometimes occur regularly (IW And-typestate) and the light curves are characterized by brightening from a standstill (or standstill-like phase) sometimes
followed by a deep dip (Simonsen 2011; Kato 2019). Accretion disks of the dwarf novae of this type are considered
to be near the thermal stability (see e.g. Kimura et al. 2020).
After recognition of the initial two prototypical objects, IW And and V513 Cas (Simonsen 2011), ST Cha
was the third object of this group (Simonsen et al. 2014; Kato 2019). Kato and Hambsch (2021) studied groundbased time-resolved photometry to characterize the outburst pattern of this object and found that the brightnesslevel of the shoulders (or embedded precursors) during long outbursts is the same as the ones when the object
started brightening at the end of standstills.
In this paper, we identiﬁed the orbital period of this object using the Transiting Exoplanet Survey Satellite(TESS) observations1 . The full light-curve is available at the Mikulski Archive for Space Telescope (MAST2 ).

2Observations

The TESS spacecraft observed ST Cha at a two-minutes cadence during two segments: between 2019 April 24 and2019 June 18 (segment 1) and between 2021 April 29 and 2021 June 24 (segment 2). We used simple-aperture1 <https://tess.mit.edu/observations/>.
2 <http://archive.stsci.edu/>.

2
photometry (SAP) data. The zero point of the TESS data is rather arbitrary and these magnitudes should beregarded as relative ones rather than zero-point calibrated ones. These segments contained ordinary dwarf novastates and not IW And-type states [see Kato and Hambsch (2021) for a light curve containing both states].
Examples of TESS light curves are shown in ﬁgure 1. The upper panel shows a long outburst with a shoulder
in the segment 1. The lower panel shows a short segment (from the segment 2) in which orbital variations weredirectly visible. See also upper panels of ﬁgures 2 and 3 for the entire TESS observations.

3

Results and Analysis

3.1

Orbital period

We made period analysis using Phase Dispersion Minimization (PDM, Stellingwerf 1978) after removing thetrends of outbursts by locally-weighted polynomial regression (LOWESS: Cleveland 1979). The errors of periodsby the PDM method were estimated by the methods of Fernie (1989) and Kato et al. (2010). Both segments 1and 2 yielded almost the same periods [0.28533(3) d for the segment 1 and 0.28533(1) d for the segment 2]. Thesignals were very stable and we identiﬁed this period as the orbital one.
The result of PDM analysis after combination of the two segments is shown in ﬁgure 4. A period of0.285360(1) d, 1σ and 3σ diﬀerent from the periods obtained from the segments 1 and 2, respectively, has been
identiﬁed as the most likely orbital period by this study.

3.2

Eclipses in the past?

The orbital period of ST Cha was suggested to be 0.285 d by Steiner et al. (1988), who analyzed photometricdata by Mauder and Sosna (1975) and concluded that ST Cha is an eclipsing binary with a period of 6.85 hours(=0.285 d) allowing possibilities of aliases of P = 1.9975/(2n + 1) hours, where n = 0, 1, 2, .... Combined witha low-resolution spectrum, Steiner et al. (1988) concluded that ST Cha is either an optically selected low-mass
X-ray binary (LMXB) or a novalike cataclysmic variable. The TESS observations, however, did not show any
sign of eclipses and the orbital variations were in low amplitudes (∼0.02 mag). Such low-amplitude variations
would have been diﬃcult to detect by photographic observations by Mauder and Sosna (1975) and the identityof the periods obtained by this and our studies is likely a chance coincidence, unless ST Cha was an eclipsingbinary in the past which stopped to be eclipsing (such a case may not be unthinkable; there are 10 systems amongclassical eclipsing binaries, see e.g. Zasche and Paschke 2012).

3.3

Strength of orbital signal and mass-transfer rate

We performed two-dimensional PDM analysis of the two segments (ﬁgures 5, 6) (see Kato 2021a). The orbital
signal was persistently recorded during the segment 1, but was weak during the initial half of the segment 2.
Although the strength of orbital humps is often considered to be an indicator the mass-transfer rate, this resultsuggests that it is not always the case since the mean brightness did not signiﬁcantly varied during the segment
2. It may have been that the hot spot was sometimes not formed such as by stream overﬂow as suggested for
SW Sex stars (Hellier 2000). There was no evidence of positive or negative superhumps.

3.4

Enhanced orbital signal after outburst with shoulder?

Kato (2021a) reported that humps recurring with a period slightly longer than the orbital one were recording
by Kepler observations of the dwarf nova V363 Lyr during a long, bright outburst associated with a shoulder.
The nature of these humps in V363 Lyr was inconclusive: (1) V363 Lyr may have an anomalously undermassive
secondary and this object is an SU UMa star above the period gap or (2) The humps were a result of deformationof the disk at the tidal truncation radius. In the case of ST Cha, an outburst associated with a shoulder inthe segment 1 did not show any sign of the humps similar to V363 Lyr. The case of V363 Lyr looks like tobe special in that it showed brightness increase by 0.5 mag after the shoulder in contrast to ST Cha. After an
outburst associated with a shoulder in the segment 2 (ﬁgure 6), however, the orbital signal became stronger. It
may be that the outburst with a shoulder changed the state of the disk and the hot spot may have become more
apparent. Such a change in the disk may have triggered a transition from an ordinary dwarf nova-type state toan IW And-type state and this possibility would require further examination.

3

13.5

14.0

14.5

58634

58636

58638

58640

58642

58644

58646

13.5

14.0

14.559364

59366

59368

59370

59372

Figure 1: Examples of TESS light curves of ST Cha. The data were binned to 0.02 d for better visibility ofvariations. (Upper): Long outburst with a shoulder (or embedded precursor). (Lower): A short segment inwhich orbital variations were directly visible.

4

θ
1.00

0.98

0.96

P=0.28533
0.94
0.270

0.275

0.280

0.285

0.290

0.295

0.300

(d)
−0.02

−0.01

0.00

0.01

−0.5

0.0

0.5

1.0

1.5

Figure 2: Period analysis of the segment 1. (Upper): We analyzed 100 samples which randomly contain 50% ofobservations, 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.

5

θ
1.00

0.98

0.96

P=0.28533
0.270

0.275

0.280

0.285

0.290

0.295

0.300

(d)
−0.02

−0.01

0.00

0.01

−0.5

0.0

0.5

1.0

1.5

Figure 3: Period analysis of the segment 2. (Upper): PDM analysis. (Lower): Orbital variation.

61.00
θ

0.99

0.98

0.97

0.96

P=0.28536
0.95
0.28500.28520.28540.2856

(d)
−0.02

−0.01

0.00

0.01

−0.5

0.0

0.5

1.0

Figure 4: Period analysis of all the data. (Upper): PDM analysis. (Lower): Orbital variation.

1.5

7

13.5

14.0

14.5

15.0

4.0

Porb

3.5

3.0

58600

58610

58620

58630

58640

58650

Figure 5: (Upper): TESS light curve of the segment 1. The data were binned to 0.05 d. (Lower): Two-dimensional
PDM analysis. The width of the sliding window and the time step used are 10 d and 1 d, respectively. Darkcolors represent signals (lower θ in the PDM statistics). The orbital signal was persistently recorded.

813.5

14.0

14.5

15.0

4.0Porb

3.5

3.0

59340

59350

59360

59370

59380

59390

Figure 6: (Upper): TESS light curve of the segment 2. The data were binned to 0.05 d. (Lower): Two-dimensional
PDM analysis. The strength of the orbital signal varied signiﬁcantly.

93.5

Are IW And-type states brighter than dwarf nova-type states?

We used All-Sky Automated Survey for Supernovae (ASAS-SN: Shappee et al. 2014; Kochanek et al. 2017) gband observations to see whether IW And-type states are brighter than dwarf nova-type states. The method isthe same as described in Kato (2021b) using LOWESS to obtain the trends (the trends were determined fromﬂuxes, not magnitudes). The result in ﬁgure 7 did not show a strong tendency that the object was brighter
during IW And-type states, although a very faint state (BJD 2458600–2458700) corresponded to a long-lasting
dwarf nova-type state.

Acknowledgements
This work was supported by JSPS KAKENHI Grant Number 21K03616. This paper includes data collectedwith the TESS mission, obtained from the MAST data archive at the Space Telescope Science Institute (STScI).
Funding for the TESS mission is provided by the NASA Explorer Program. STScI is operated by the Associationof Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555. We are grateful to theASAS-SN team for for making the database available to the public.

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IW And−type states
14.1

14.2

14.3

14.4

14.5
58200

58400

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58800

59000

59200

59400

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