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No.87

Nov. 2021

ISSN 0917-2211

Variable Star Bulletin
Study of the low-amplitude Z Cam star IX Vel
Taichi Kato1
1

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

Received 2021 Nov. 30

Abstract
IX Vel has been considered as one of the prototypical novalike cataclysmic variables with thermally stableaccretion disks. Using All-Sky Automated Survey (ASAS-3) and All-Sky Automated Survey for Supernovae(ASAS-SN) observations, I found that IX Vel is a low-amplitude dwarf nova showing standstills. This object
has been re-classiﬁed as a Z Cam-type dwarf nova. This conclusion is consistent with the mass-transfer rateusing the Gaia parallax which places the object near the lower limit of the thermal stability. Using twodimensional Least Absolute Shrinkage and Selection Operator (Lasso), I found that the cycle lengths of dwarf
nova outbursts varied between 13 and 20 d. Analysis of the ASAS-3 data suggested that the cycle lengths areshorter and the amplitudes are smaller when the system is bright. Standstills occurred when the system wasbright. These results support the idea that a subtle variation in the mass-transfer rate from the secondary
causes transitions between outbursting state and standstills in Z Cam stars.

1

Introduction

Despite its apparent brightness (V ∼9.5), IX Vel (=CD−48◦ 3636 = CPD−48◦ 1577) has been relatively new tothe ﬁeld of cataclysmic variables (CVs) [for general information of cataclysmic variables and dwarf novae, see e.g.
Warner (1995)]. This object was spectroscopically identiﬁed as a UX UMa-type novalike CV by Garrison et al.
(1984). Eggen and Niemela (1984) performed radial-velocity observations and obtained a possible orbital period(0.1220 d, which was not a correct value). Wargau et al. (1983) obtained a better orbital period of 0.187(3) d.
Although Wargau et al. (1983) reported the similarity of the spectrum of IX Vel with that of the Z Cam star
HL CMa, the lack of outbursts in the archival plates suggested a novalike variable. This reference was the ﬁrst toclassify IX Vel by long-term photometric history. The modern binary parameters (orbital period = 0.193927 d)were determined by Beuermann and Thomas (1990); Kubiak et al. (1999); Linnell et al. (2007).
Using the data in All-Sky Automated Survey (ASAS-3: Pojmański 2002), I found in 2018 that IX Vel is a
low-amplitude Z Cam star (vsnet-chat 8199).1 In this paper, I report analysis of IX Vel as a dwarf nova.

2

Results

2.1

Overall light curve

The light curve based on the ASAS-3 data is shown in ﬁgure 1. Most of the time, this object showed low-amplitude(up to 0.5 mag) quasi-cyclic variations. The object, however, showed standstills such as BJD 2453030–2453080
and 2454500–2454620. The light curve based on the All-Sky Automated Survey for Supernovae (ASAS-SN:
1 <http://ooruri.kusastro.kyoto-u.ac.jp/mailarchive/vsnet-chat/8199>.

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Shappee et al. 2014; Kochanek et al. 2017) is shown in ﬁgure 2. Although some g-band observations of the
ASAS-SN data were apparently aﬀected by saturation (particularly after BJD 2459300 when there were a numberof measurements below 10.0 mag), the overall variations are composed of phases of low-amplitude, quasi-cyclic
variations and standstills as in the ASAS-3 data. The presence of standstills are more apparent in the ASAS-SN
data (such as BJD 2457630–2457690 and BJD 2458440–2458630; the amplitudes in more recent data were also
small, but this may have been aﬀected by saturation). The overall features are suﬃcient to classify IX Vel as a
Z Cam star.

2.2

Analysis of dwarf nova-type outbursts using two-dimensional Lasso

I used two-dimensional Least Absolute Shrinkage and Selection Operator (Lasso) method (Kato and Uemura
2012; Kato 2021) to study the variation of cycle lengths in the dwarf nova phases. The result for the ASAS-3
data is shown in ﬁgure 3. I used the window size and shift value of 200 d and 10 d, respectively. I had to use a
relatively large window size due to the limited sampling rate of observations. During the interval BJD 2453400–
2454300, dwarf nova-type variations were most apparent at frequencies 0.056–0.062 c/d, corresponding to cycle
lengths of 16–18 d. The cycle lengths of dwarf nova-type variations varied fairly strongly, and they were aroundfrequencies 0.071–0.078 c/d, corresponding to cycle lengths of 13–14 d in the interval BJD 2453000–2453200.
During the latter interval, the signal in the two-dimensional Lasso spectrum was weaker compared to the former
interval, which can be also seen as smaller amplitudes in the upper panel of the ﬁgure.
The middle panel of the ﬁgure shows the trend of the light curve obtained by locally-weighted polynomial
regression (LOWESS: Cleveland 1979). This demonstrates a positive correlation between the brightness trend
and the frequency of the dwarf nova-type variations. When the system was bright, there was a tendency that thecycle lengths were shorter and the amplitudes were smaller. It is also apparent from the ﬁgure that standstills
occurred when the system was bright. It was a slight pity that the two-dimensional Lasso spectrum was rather
fragmentary due to the presence of observational gaps. It looks likely that the cycle lengths would have been
longer when the system was faint around BJD 2452300, but the lack of the data prevented me from drawing aconclusion.
The same type of analysis for the ASAS-SN data is shown in ﬁgure 4. In order to avoid the eﬀect ofsaturation, I used only V data. Readers should keep in mind that the entire time interval of the ﬁgure is much
shorter than the ASAS-3 data (ﬁgure 3). Thanks to the higher sampling rate of the ASAS-SN observations, I
could use a smaller window size and shift value of 130 d and 5 d, respectively.
In the upper panel of the ﬁgure, the dwarf nova-type variations are clearly seen after BJD 2457700. The
frequencies varied between 0.05 c/d (corresponding to a cycle length of 20 d) to 0.075 c/d (13 d). In contrast to
the ASAS-3 data, this variation of the frequencies does not look like to positively correlate with the trend of thelight curve. This result is based on a relatively short segment of data (700 d, compared to 2900 d in the ASAS-3
data) and needs to be treated with caution. I consider that the result from the ASAS-3 data is more reliable indiscussing the correlation.

3

Discussion

The mechanism of transition between outbursting state and standstills in Z Cam stars is not still well understood.
There has been a consensus that a subtle variation in the mass-transfer rate from the secondary causes transitions
between outbursting state and standstills (Meyer and Meyer-Hofmeister 1983). This mechanism implies that themean brightness during standstills is brighter than during outbursting state. In four of ﬁve “classical” Z Cam stars
studied by Honeycutt et al. (1998), the standstills were as bright as or brighter (by 0.2 mag in some systems)
than the mean brightness during outbursting states. The unusual standstills of SY Cnc were reported to be
fainter than the mean brightness during outbursting intervals (Honeycutt et al. 1998). Whether standstills in
Z Cam stars are universally brighter than outbursting states still needs to be investigated. As I have shown,
the ASAS-3 data of IX Vel indeed show this tendency and this would be an additional support to the idea by
Meyer and Meyer-Hofmeister (1983).
In the case of IX Vel, the mass-transfer rate appears to be close to the limit of thermal instability and whenthe mass-transfer rate is slightly below the critical value, dwarf nova-type outbursts occur. The mass-transferrate, however, is only slightly below the critical value and only the outer part of the disk is thermally unstable (orthe cooling wave cannot easily reach deeper into the inner region of the disk). This explains why the amplitudesof dwarf nova-type outbursts in IX Vel are so small. According to Dubus et al. (2018), the mass-transfer rate of

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Figure 1: Light curve of IX Vel using the ASAS-3 database.

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Figure 2: Light curve of IX Vel using the ASAS-SN database. Blue and green symbols represent V and gobservations, respectively. Note that the scales are diﬀerent from ﬁgure 1.

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Figure 3: Period analysis of IX Vel using the ASAS-3 database. (Upper) Light curve. (Middle) Trend of thelight curve using LOWESS. (Lower) Two-dimensional Lasso power spectrum. The window size and shift value
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Figure 4: Period analysis of IX Vel using the ASAS-SN V data. (Upper) Light curve. (Middle) Trend of thelight curve using LOWESS. (Lower) Two-dimensional Lasso power spectrum. The window size and shift value
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IX Vel using Gaia DR2 data (Gaia Collaboration et al. 2018)2 is near the lower limit of the thermal stability andthe above interpretation is consistent with the disk-instability theory.
Low-amplitude outbursts during standstills of “classical” Z Cam stars have also been documented, such asin Szkody and Mattei (1984); Kato (2001). These phenomena are likely the same as low-amplitude outbursts
recorded in IX Vel.
Kimura et al. (2020) performed numerical simulations of dwarf nova outbursts (both in tilted and non-tilted
disks) based on the thermal-viscous instability model (the simulation scheme was based on Ichikawa and Osaki
1992). In their simulations (ﬁgure 18 in Kimura et al. 2020), tilted disks are prone to thermal instability evenunder the condition of the mass-transfer rate high enough to keep the entire disk hot in a non-tilted disk. Theirresultant light curve looks similar to that of IX Vel and it would be worth studying whether there is a signatureof a disk tilt in this system.
The present observational knowledge of the behavior of the accretion disk near the border of the thermal instability might also be helpful in understanding the unusual behavior of IW And stars (Simonsen 2011;
Hameury and Lasota 2014; Kato 2019; Kimura et al. 2020), which are also considered to have accretion disksnear the border of the thermal instability.

Acknowledgements
This work was supported by JSPS KAKENHI Grant Number 21K03616. I am grateful to ASAS-3 and ASAS-SN
teams for for making the database available to the public.

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VSOLJ
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Editor Seiichiro Kiyota
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