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TV Corvi Revisited: Precursor and Superhump Period Derivative Linked to the Disk Instability Model

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TV Corvi Revisited: Precursor and Superhump Period Derivative Linked to the Disk Instability Model A&A 432, 261–271 (2005) Astronomy DOI: 10.1051/0004-6361:20042004 & c ESO 2005 Astrophysics TV Corvi revisited: Precursor and superhump period derivative linked to the disk instability model M. Uemura1,2, R. E. Mennickent1, R. Ishioka3,A.Imada3,T.Kato3,D.Nogami4, R. Stubbings5,S.Kiyota6, P. Nelson 7, K. Tanabe8, B. Heathcote9, and G. Bolt10 1 Departamento de Física, Facultad de Ciencias Físicas y Matemáticas, Universidad de Concepción, Casilla 160-C, Concepción, Chile e-mail: [email protected] 2 Yukawa Institute for Theoretical Physics, Kyoto University, Kyoto 606-8502, Japan 3 Department of Astronomy, Kyoto University, Sakyou-ku, Kyoto 606-8502, Japan 4 Hida Observatory, Kyoto University, Kamitakara, Gifu 506-1314, Japan 5 19 Greenland Drive, Drouin 3818, Victoria, Australia 6 Variable Star Observers League in Japan (VSOLJ), 1-401-810 Azuma, Tsukuba 305-0031, Japan 7 RMB 2493, Ellinbank 3820, Australia 8 Department of Biosphere-Geosphere Systems, Faculty of Informatics, Okayama University of Science, Ridaicho 1-1, Okayama 700-0005, Japan 9 Barfold Observatory, Mia Mia, Victoria, Australia 10 295 Camberwarra Drive, Craigie 6025, Western Autralia Received 14 September 2004 / Accepted 1 November 2004 Abstract. We report optical photometric observations of four superoutbursts of the short-period dwarf nova TV Crv. This ob- ject experiences two types of superoutbursts; one with a precursor and the other without. The superhump period and period excess of TV Crv are accurately determined to be 0.065028 ± 0.000008 d and 0.0342 ± 0.0021, respectively. This large excess implies a relatively large mass ratio of the binary components (M2/M1), though it has a short orbital period. The two types of superoutbursts can be explained by the thermal-tidal instability model for systems having large mass ratios. Our observations reveal that superhump period derivatives are variable in distinct superoutbursts. The variation is apparently related to the pres- ence or absence of a precursor. We propose that the superhump period derivative depends on the maximum disk radius during outbursts. We investigate the relationship of the type of superoutbursts and the superhump period derivative for known sources. In the case of superoutbursts without a precursor, superhump period derivatives tend to be larger than those in precursor-main type superoutbursts, which is consistent with our scenario. Key words. accretion, accretion disks – binaries: close – novae, cataclysmic variables – stars: dwarf novae – stars: individual: TV Crv 1. Introduction Superoutbursts are sometimes associated with a precursor typically lasting one or two days. This precursor phenomenon is actually expected from the TTI model. The precursor is SU UMa-type stars form a sub-group of dwarf novae char- considered to be a normal outburst leading to an expansion acterized by the appearance of long and bright “superout- of the accretion disk over the 3:1 resonance radius and trig- bursts”, during which periodic modulations, “superhumps”, are gering a superoutburst. Growing superhumps have been de- observed (Warner 1985). Superhumps have periods slightly tected during a decay phase from the precursor in T Leo (Kato longer than orbital periods, which can be explained by a beat 1997), V436 Cen (Semeniuk 1980), and GO Com (Imada et al. phenomenon of a precessing tidally-distorted eccentric disk. 2004). These growing superhumps provide evidence for the According to the tidal instability theory, an accretion disk be- TTI model since a system is predicted to reach a supermaxi- comes unstable against a tidal perturbation from a secondary mum with the growth of an eccentric disk. On the other hand, star when the disk reaches the 3:1 resonance radius (Whitehurst the original TTI model cannot explain gradually growing su- 1988). In conjunction with the thermal instability model for perhumps even after supermaxima without a precursor, which (normal) dwarf nova outbursts, the model for superoutbursts is are also frequently observed (Smak 1996). called the thermal-tidal instability (TTI) model (Osaki 1989). Article published by EDP Sciences and available at http://www.aanda.org or http://dx.doi.org/10.1051/0004-6361:20042004 262 M. Uemura et al.: TV Corvi revisited: Precursor and superhump period derivative linked to the disk instability model Osaki & Meyer (2003) propose a refinement of the original Table 1. Journal of observations. TTI model with the idea that the accretion disk can pass the δ 3:1 resonance radius and reach the tidal truncation radius. The ID Tstart TN Site dammed matter at the tidal truncation radius causes a gradual 01-01 1957.1607 4.75 361 Kyoto decay without a precursor. This refined TTI model predicts that 01-02 1958.1373 5.28 243 Tsukuba 01-03 1958.2718 2.12 176 Kyoto SU UMa stars having a large mass ratio (q = M /M ,whereM 2 1 1 01-04 1959.0578 7.21 542 Kyoto and M2 are the masses of a white dwarf and a secondary star, re- 01-05 1960.3133 0.75 64 Kyoto spectively) can show both types of superoutbursts, that is, those 01-06 1961.1367 4.90 374 Kyoto with and without a precursor. This idea should be examined by 01-07 1961.1470 3.04 70 Tsukuba observations of the early evolution of superoutbursts and super- 01-08 1963.2215 1.96 138 Kyoto humps. 01-09 1964.0792 2.78 73 Tsukuba 01-10 1965.1255 5.17 436 Kyoto The superhump period (PSH) in SU UMa stars generally decreases through a superoutburst with a period derivative of 01-11 1965.1483 3.81 94 Tsukuba − 01-12 1966.1710 0.56 39 Kyoto order P˙ /P ∼−10 5 (Warner 1985; Patterson et al. 1993). SH SH 01-13 1969.1343 4.44 303 Kyoto A simple dynamical treatment for the tidal instability shows 02-01 2427.9767 2.18 132 Kyoto that the precession rate of the eccentricity wave is proportional 02-02 2428.0206 1.47 119 Kyoto 1.5 to r ,wherer is the disk radius (Osaki 1985). The shorten- 02-03 2428.9583 2.23 195 Kyoto ing of PSH can, hence, be understood with the shrink of the 02-04 2428.9960 1.86 51 Kyoto disk during a superoutburst. Hydrodynamical simulations also 02-05 2429.0071 1.56 147 Okayama show that the precessing eccentricity wave propagates inward, 02-06 2430.9581 1.16 103 Kyoto which causes the period shortening of superhumps (Lubow 02-07 2430.9627 2.34 182 Kyoto 1992; Whitehurst 1994). 02-08 2434.9773 2.03 161 Kyoto 03-01 2769.9589 6.18 327 Craigie On the other hand, several short-period SU UMa stars 03-02 2776.9232 1.34 71 Ellinbank ˙ / showing positive PSH PSH have been discovered since mid- 03-03 2777.0997 2.88 31 Kyoto 90’s (Howell et al. 1996a; Kato et al. 2003c). WZ Sge-type 03-04 2777.8617 2.18 111 Ellinbank stars, in particular, tend to show positive P˙ SH/PSH (e.g. Howell 03-05 2780.1247 1.35 37 Kyoto et al. 1996a; Kato et al. 1997). The situation becomes more 03-06 2781.0285 1.82 86 Hida complicated because ultra-short period systems, V485 Cen 04-01 3160.9673 2.46 237 Kyoto and EI Psc also show positive P˙SH/PSH (Olech 1997; Uemura 04-02 3161.9689 2.05 142 Kyoto et al. 2002a). These two sources have quite large mass ratios 04-03 3162.4693 5.84 210 Concepción (q ∼ 0.2), though WZ Sge stars have quite small mass ra- 04-04 3163.4698 6.18 238 Concepción tio (q ∼ 0.01). Based on the discussions for ordinary negative 04-05 3164.9970 0.70 34 Barfold 04-06 3166.5550 3.81 182 Concepción P˙ /P , the positive P˙ /P has been proposed to arise due SH SH SH SH 04-07 3167.5709 3.40 195 Concepción to an expansion of the disk or an outward-propagation of the 04-08 3168.5882 2.86 176 Concepción eccentricity wave (Baba et al. 2000; Kato et al. 2004a). It is, 04-09 3169.5472 1.67 57 Concepción however, poorly understood why the outward propagation can 04-10 3169.9654 2.63 123 Kyoto occur only in the short-period systems regardless of their mass 04-11 3170.9610 3.06 189 Kyoto ratio (Ishioka et al. 2003). Tstart = HJD−2 450 000. TV Crv is known as an SU UMa-type dwarf nova having a δT = Period of observations in hours. short orbital period of 0.06288 ± 0.00013 d (Woudt & Warner N = Number of images. 2003). The historical discovery of this object is summarized in Levy et al. (1990). Howell et al. (1996b) reported superhumps results with those for other known systems. Finally, we sum- . ± . with a period of 0 0650 0 0008 d from observations of a marize our findings in Sect. 7. superoutburst in 1994 June. This PSH provides a superhump period excess ε = (PSH − Porb)/Porb = 0.033 ± 0.009. This value of the excess implies that TV Crv may be a peculiar ob- 2. Observations ject regarding its possibly large period excess compared with We conducted observational campaigns for four superoutbursts ε other short period systems (Patterson 2001). The error of is, of TV Crv which occurred in 2001 February–March, 2002 ε however, so large that the large is not conclusive. June, 2003 May, and 2004 July, through VSNET Collaboration Here we report observations of four superoutbursts of (Kato et al. 2004b). Photometric observations were performed TV Crv. Our observations on TV Crv provide new clues to un- with unfiltered CCD cameras attached to 30-cm class tele- derstand the superhump period evolution related to the precur- scopes at Concepción (2004), Kyoto (2001, 2002, 2003, and sor phenomenon and the TTI model.
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