BC Cassiopeiae: First Detection of IW And-Type Phenomenon Among Post-Eruption Novae
aa r X i v : . [ a s t r o - ph . S R ] S e p Publ. Astron. Soc. Japan (2014) 00(0), 1–4doi: 10.1093/pasj/xxx000 BC Cassiopeiae: First Detection of IW And-TypePhenomenon Among Post-Eruption Novae
Taichi K
ATO , Naoto K
OJIGUCHI Department of Astronomy, Kyoto University, Kyoto 606-8502, Japan ∗ E-mail: ∗ [email protected] Received 201 0; Accepted 201 0
Abstract
IW And-type dwarf novae are recently recognized group of cataclysmic variables which arecharacterized by a sequence of brightening from a standstill-like phase with damping oscilla-tions often followed by a deep dip. We found that the supposed classical nova BC Cas whicherupted in 1929 experienced a state of an IW And-type dwarf nova in 2018, 89 yr after theeruption. This finding suggests that high mass-transfer rate following the nova eruption is as-sociated with the IW And-type phenomenon. The mass of the white dwarf inferred from thedecline rate of the nova is considerably higher than the average mass of the white dwarfs incataclysmic variables and the massive white dwarfs may be responsible for the manifestationof the IW And-type phenomenon.
Key words: accretion, accretion disks — stars: novae, cataclysmic variables — stars: dwarf novae —stars: individual (BC Cassiopeiae)
Cataclysmic variables (CVs) are composed of a whitedwarf and a mass-transferring secondary. The transferredmatter forms an accretion disk around the white dwarf,and in some cases, the accreted matter on the surface ofthe white dwarf causes thermonucular runaway, whichis observed as a nova. Dwarf novae (DNe) are a class ofCVs, which show outbursts resulting from instabilities inthe accretion disk [for general information of CVs, novaeand DNe see e.g. Warner (1995)].One of the long-standing questions about the relationbetween novae and DNe is whether novae between erup-tions become detached systems (and hence not observedas CVs) or become DNe. This possibility was highlightedby Patterson (1984) after a comparison of the space densi-ties of novae and known CVs, and Patterson (1984) sug-gested that there are “dead” novae ∼
100 times of CVs.Shara et al. (1986) extended this discussion and suggestedthe “hibernation” scenario by assuming that the mass-transfer stops following angular momentum loss duringa nova eruption resulting in the binary to be detached. The relationship between novae and DNe have only re-cently directly confirmed by observations: discoveries ofremnant nova shells around the DNe Z Cam (Shara et al.2007) and AT Cnc (Shara et al. 2012), DN in AD 483 novashell Te 11 (Miszalski et al. 2016), DN state before a novaeruption (Mróz et al. 2016) and DN in AD 1437 nova shell(Shara et al. 2017).Quite recently, a nova eruption was recorded from thepreviously known DN V392 Per by Y. Nakamura in 2018 These observations indicate that at least some novae eruptfrom DNe and some very old ( > ∼ < > .c (cid:13) Publications of the Astronomical Society of Japan , (2014), Vol. 00, No. 0, (2014), Vol. 00, No. 0
100 times of CVs.Shara et al. (1986) extended this discussion and suggestedthe “hibernation” scenario by assuming that the mass-transfer stops following angular momentum loss duringa nova eruption resulting in the binary to be detached. The relationship between novae and DNe have only re-cently directly confirmed by observations: discoveries ofremnant nova shells around the DNe Z Cam (Shara et al.2007) and AT Cnc (Shara et al. 2012), DN in AD 483 novashell Te 11 (Miszalski et al. 2016), DN state before a novaeruption (Mróz et al. 2016) and DN in AD 1437 nova shell(Shara et al. 2017).Quite recently, a nova eruption was recorded from thepreviously known DN V392 Per by Y. Nakamura in 2018 These observations indicate that at least some novae eruptfrom DNe and some very old ( > ∼ < > .c (cid:13) Publications of the Astronomical Society of Japan , (2014), Vol. 00, No. 0, (2014), Vol. 00, No. 0
BC Cas was originally discovered as a long-period vari-able by Beljawsky (1931) on 1929 Simeis plates. Duerbeck(1984a) studied Harvard plates and presented a lightcurve. The maximum brightness was 10.7 mag (photo-graphic) on 1929 August 1. The object was not detected14 d before. Since the outburst amplitude was small andsince an old “nova” (HV Vir, 1929) similarly recognizedby the same author (Duerbeck 1984b) turned out to bea large-amplitude DN (Leibowitz et al. 1994; Kato et al.2001), BC Cas was suspected to be a candidate large-amplitude DN and a search for a further outburst wasconducted [see a remark in Kato et al. (2001)]. This sit-uation continued until Ringwald et al. (1996) publisheda spectrum. The spectrum showed relatively weak H α emission and a red continuum, which preferred a mod-erately reddened nova rather (the object is indeed the di-rection of a star-forming region SFR G115.80 − Bp − Rp =1.326) color (Gaia Collaboration et al. 2018).According to Schaefer (2018), the distance and Galacticextinction were estimated to be 2114( − + A V =3.7, respectively. We used Public Data Release 3 of the Zwicky TransientFacility (ZTF, Masci et al. 2019) observations. We foundthat BC Cas showed DN-type outbursts in 2018 (figure 1).In E-section 1, we also presented an analysis of the relia-bility of the ZTF data in response to the reviewer’s opin-ion. The ZTF data for BC Cas are given in E-section 2.The three outbursts (BJD 2458288, 2458367 and2458429) were separated by 60–80 d and the brightnessbetween the outbursts gradually increased at least be-fore the first and second outbursts. Following the secondoutburst, the object showed a deep dip. The total am-plitude of the outburst including this dip was 0.9 mag.There was also a shallower dip after the third outburst.Following the dip after the second outburst, there was ahint of damping oscillation, though the details were notsufficiently clear due to the observational gaps. 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 IRSAAPI < https://github.com/MickaelRigault/ztfquery > . The features of the light curve of BC Cas 89-yr after the(most likely) nova eruption in 1929 described in section 3very well match the characteristics of IW And-type DNe[see Kato (2019) and references therein], which have beena recently recognized type of DNe. The IW And-typeDNe show outbursts starting from (quasi-)standstills andthese outbursts are often followed by dips and dampingoscillations. These DNe exhibit such outbursts only a frac-tion of time and such a state is referred to as IW And-type state or phenomenon. The most remarkable featureis that this sequence is often semi-periodically repeated.The semi-regular occurrence of this cycle suggests a yetunidentified type of limit-cycle oscillation (Kato 2019).BC Cas is the first (supposed) classical nova that experi-enced the IW And-type state.The origin of the unusual variations of IW And-typeDNe is still poorly known. Szkody et al. (2013) suggestedperiodic increase of the mass-transfer from the secondary.Kimura et al. (2020b) suggested that the tilted accretiondisk could cause a limit-cycle oscillation similar to IWAnd DNe. The tilted disk, however, has proven not tobe a universal explanation for IW And stars by the lack ofnegative superhumps, a signature of a tilted disk, in manyIW And stars [e.g. IM Eri Kato et al. (2020)]. Most recently,a detailed analysis of the Kepler data of KIC 9406652,an IW And DN with prominent negative superhumps,by Kimura et al. (2020a) indicated that neither models ofSzkody et al. (2013) and Kimura et al. (2020b) could ex-plain the variation of the disk radius. The cause of theunusual pattern of variation of IW And DNe still remainsa mystery.
In the hibernation scenario of classical novae, the grad-ual reduction of the mass-transfer rate ( ˙ M ) from the sec-ondary Shara et al. (1986) enables the accretion disks inpost-novae to become thermally unstable to cause DNoutbursts. BC Cas showed the IW And-type phenomenon89 yr after the supposed nova eruption, which is one ofthe shortest among classical novae to exhibit DN activ-ity (no other classical novae showed this phenomenon inthe ZTF data, and the case of BC Cas should be rare).The condition, such as ˙ M , enabling the IW And-typephenomenon is still unknown. Considering that BC Casshowed this phenomenon during the cooling phase after These objects are also called “anomalous” Z Cam stars (Szkody et al.2013). ublications of the Astronomical Society of Japan , (2014), Vol. 00, No. 0 BC Cas
ZTF rZTF g
IM Eri
Fig. 1. (Upper) Light curve of BC Cas from ZTF observations. The filled squares and circles represent r and g observations, respectively. (Lower) Light curveof IM Eri (typical IW And-type dwarf nova) for comparison. The data were from Kato et al. (2020). the nova eruption, it is likely that the IW And-type phe-nomenon in this object was achieved in high- ˙ M conditionprobably close to the border of thermal instability of theaccretion disk [see e.g. Warner (1995)]. Whether this isapplicable to all IW And-type DNe requires further studyin other systems.According to Duerbeck (1984a), BC Cas was a moder-ately fast nova with t (time required to fade by 3 mag)of 50–75 d. This translates to t of 29–44 d using theformula in Hachisu and Kato (2006). It is widely ac-cepted that nova eruptions occurring on heavier whitedwarfs have shorter t or t [see e.g. Hachisu and Kato(2006)]. Using the model grid in Shara et al. (2018), themass of the white dwarf in BC Cas is expected to be 1.02–1.09 M ⊙ . Zorotovic et al. (2011) obtained an average massof 0.82(3) M ⊙ for white dwarfs in CVs with an intrinsicscatter of white dwarf masses of 0.15 M ⊙ . A more re-cent study of eclipsing CVs yielded a consistent result(McAllister et al. 2019). The inferred mass of the whitedwarf in BC Cas is more than 1 σ larger than the averagewhite dwarf mass in CVs and it may be that the IW And-type phenomenon associated with the high mass of thewhite dwarf. This might explain why the IW And-typephenomenon is seen only in limited number of DNe, andwhy the same object repeatedly shows this phenomenonwhile others never showed it. Since no reliable estimatesof white dwarf masses are available in IW And-type DNe,more effort should be paid to determine orbital parame-ters of these objects. Acknowledgments
Based on observations obtained with the Samuel Oschin48-inch Telescope at the Palomar Observatory as partof the Zwicky Transient Facility project. ZTF is sup-ported by the National Science Foundation under GrantNo. AST-1440341 and a collaboration including Caltech,IPAC, the Weizmann Institute for Science, the OskarKlein Center at Stockholm University, the Universityof Maryland, the University of Washington, DeutschesElektronen-Synchrotron and Humboldt University, LosAlamos National Laboratories, the TANGO Consortiumof Taiwan, the University of Wisconsin at Milwaukee, andLawrence Berkeley National Laboratories. Operations areconducted by COO, IPAC, and UW.The ztfquery code was funded by the EuropeanResearch Council (ERC) under the European Union’sHorizon 2020 research and innovation programme (grantagreement n ◦ Supporting information
Additional supporting information can be found in theonline version of this article. Supplementary data is avail-able at PASJ Journal online.
References
Beljawsky, S. 1931, Astron. Nachr., 243, 115Duerbeck, H. W. 1984a, IBVS, 2490Duerbeck, H. W. 1984b, IBVS, 2502Gaia Collaboration, et al. 2018, A&A, 616, A1
Publications of the Astronomical Society of Japan , (2014), Vol. 00, No. 0, (2014), Vol. 00, No. 0