The Period Changes of the Cepheid RT Aurigae
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The Period Changes of the Cepheid RT Aurigae David G. Turner Department of Astronomy and Physics, Saint Mary’s University, Halifax, Nova Scotia B3H 3C3, Canada [email protected] Ivan S. Bryukhanov, Igor I. Balyuk, Alexey M. Gain, Roman A. Grabovsky, Valery D. Grigorenko, Igor V. Klochko, Attila Kosa-Kiss, Alexey S. Kosinsky, Ivan J. Kushmar, Vyacheslav T. Mamedov, Natalya A. Narkevich, Andrey J. Pogosyants, Andrey S. Semenyuta, Ivan M. Sergey, Vladimir V. Schukin, Jury B. Strigelsky, and Valentina G. Tamello Group “Betelgeuse,” Republic Center of Technical Creativity of Pupils, 12 Makaionak Street, Minsk 220023, Belarus betelgeize [email protected] David J. Lane and Daniel J. Majaess Department of Astronomy and Physics, Saint Mary’s University, Halifax, Nova Scotia B3H 3C3, Canada ABSTRACT Observations of the light curve for the 3.7-day Cepheid RT Aur both before and since 1980 indicate that the variable is undergoing an overall period increase, amounting to +0.082±0.012 s yr−1, rather than a period decrease, as implied by all observations prior to 1980. Superposed on the star’s O–C variations is a sinusoidal trend that cannot be attributed to random fluctuations in pulsation period. Rather, it appears to arise from light travel time effects in a binary system. The derived orbital period for the system is P = 26, 429 ± 89 days (72.36 ± 0.24 years). The inferred orbital parameters from the O–C residuals differ from those indicated by existing radial 8 velocity data. The latter imply the most reasonable results, namely a1sini = 9.09(±1.81) × 10 km and a minimum secondary mass of M2 = 1.15 ± 0.25 M⊙. Continued monitoring of the arXiv:0709.3085v1 [astro-ph] 19 Sep 2007 brightness and radial velocity changes in the Cepheid are necessary to confirm the long-term trend and to provide data for a proper spectroscopic solution to the orbit. Subject headings: Stars 1. Introduction radius as post main-sequence stars of 3 − 20 M⊙ evolve through the instability strip in the H-R Every well-studied Cepheid undergoes changes diagram (Turner et al. 2006). Parabolic trends in pulsation period: some rapidly, others ex- in Cepheid O–C diagrams — temporal plots of tremely slowly, and ∼ 10% in irregular fashion, at- the differences between Observed and Computed tributable to random fluctuations in pulsation pe- times of light maxima — are diagnostic features riod, generally superposed upon parabolic evolu- of stars undergoing slow changes in mean radius tionary trends, e.g. SV Vul (Turner & Berdnikov (Parenago 1958; Struve 1959). 2004). For the large majority the effect can be The case for the 3.7-day Cepheid RT Aur is attributed directly to gradual changes in mean most unusual. Summaries by Szabados (1977, 1 1991) and Fernie (1993) of observed times of in pulsation period for RT Aur, and argue that maximum light between 1897 and 1980 provide the long-term brightness changes of the Cepheid a strong case for a regular period decrease in are actually more consistent with a period increase the Cepheid (Turner 1998), although Szabados than a period decrease. There is, in fact, convinc- (1977) preferred to interpret the O–C data as evi- ing evidence for a superposed sinusoidal trend that dence for a discontinuous period change, contrary hints at more complex behavior generally consis- to the arguments for evolution (Szabados 1983; tent with orbital motion in a binary system. The Turner et al. 2006). The available O–C data to main point to be made, however, is that the true 1980, from Szabados (1977, 1991), Fernie (1993), situation will only be established by further mon- Wunder (1992), and an unpublished list by Vi- itoring of the star. The last few decades of ob- taly Goransky of the Sternberg Astronomical In- servation merely hint at the interesting changes stitute, cited by Kosinsky et al. (2006), of ob- occurring in the system. served times of maximum light are shown in Fig- ure 1 (upper). The weighting scheme for the data 2. Observational Data used throughout this paper is that employed by Szabados (1977), with weights assigned to sources We obtained a selection of new O–C data points not cited by Szabados (1977, 1991) on the basis of for RT Aur through analysis of a variety of un- the perceived quality of the result. The negative published observations for the Cepheid, which in- parabolic trend is the signature of a regular pe- clude observations by members of the American riod decrease (Struve 1959), and the inferred rate Association of Variable Star Observers (AAVSO) of −0.123 ± 0.018 s yr−1 is close to what is pre- from Henden (2006) and data from Group “Betel- dicted from stellar evolutionary models for a star geuse.” The latter include individual observations in the second crossing of the Cepheid instability of RT Aur by group members, as well as data ob- strip (Turner et al. 2006). tained from visual inspection of photographic im- ages in the plate archives at Minsk and Odessa. Regular photoelectric monitoring of the bright- ness variations of RT Aur by professional observers All data listed only by Julian Date were con- ceased over a decade ago, with the exception of verted into heliocentric equivalents, and were Berdnikov et al. (1997), Barnes et al. (1997), Kiss phased using a new ephemeris given by: (1998), and observations by the Hipparcos satel- HJD = 2441723.6925+ 3.72824 E, (1) lite (ESA 1997). A variety of other observations max of the star, primarily by amateur astronomers, where E is the number of elapsed cycles. We also has generated additional times of light maximum made use of a standard light curve for RT Aur, that are listed by Wunder (1992), Kosinsky et al. in B and V , constructed from the detailed pho- (2006), and Meyer (2004, 2006). They are plotted tometry of Winzer (1973) supplemented by data in Figure 1 (lower), using the weighting scheme from Moffett & Barnes (1984) that were matched described above, but are generally of lower qual- in both phase and magnitude to the observations ity than most observations by professional ob- of Winzer (1973). servers, as indicated by the larger scatter in the Since RT Aur is a fifth magnitude Cepheid, more recent times of light maxima. Nevertheless, its brightness is typically monitored optically by it is clear that current O–C data do not confirm means of binoculars or low power oculars, al- the regular period decrease evident prior to 1980. though magnitude estimates without optical aid A similar trend is indicated in an O–C plot for would likely be more accurate given the eye’s in- RT Aur generated by Berdnikov et al. (2003) us- creasing precision for estimating brightness levels ing low quality observations by members of the when functioning near the visibility limit (Turner American Association of Variable Star Observers 2000). Stellar brightness is more difficult to es- (AAVSO), who, in conjunction with European ob- tablish optically when it falls well above the eye servers, have been the primary observers of the limit, which may partly explain the large scat- Cepheid in the current era. ter in the AAVSO estimates for RT Aur (Henden Here we present additional data that confirm 2006), as indicated in Figure 2. The large num- the more recent observations of the curious change ber of individual AAVSO estimates compensates 2 for the large scatter, however, and results in very for the AAVSO values in Figure 4 (upper). The in- precise O–C estimates. The AAVSO database for dividual light curves from the Group “Betelgeuse” RT Aur is relatively sparse prior to 1969, however data exhibit slightly smaller scatter, but gener- (cf., Berdnikov et al. 2003), which restricts its use- ally result in less accurate O–C values because of fulness mainly to the last four decades. the smaller number of individual estimates, ac- The Group “Betelgeuse” brightness estimates cording to Figure 4 (lower). Both sets of obser- for RT Aur are a mix of different sources: individ- vations confirm the trend indicated by the O–C ual eye estimates obtained between 1989 and 2000, data derived by other observers, primarily ama- as well as from 2005 to 2007, by individual group teur astronomers (Figure 1, lower). It is not clear members, using low power oculars and wide field from the O–C estimates cited by Wunder (1992), telescopes, and eye estimates from photographs Kosinsky et al. (2006), and Meyer (2004, 2006) in the plate archives of Odessa and Minsk. The how the times of light maximum were derived, but archival photographic material dates from 1988 to the techniques are apparently less robust than the 1996, and consists of panchromatic GZS-2 film variant of Hertzsprung’s method employed here. with a magnitude limit of V = 9.0, and A500 The new photometry obtained here (Table 1), film exposed through a UV filter with a magni- as well as visual observations of the Cepheid by tude limit of B = 9.0 − 9.5. Typically 2–4 refer- Bryukhanov between December 2006 and April ence stars differing in brightness by ∼ 0.5 mag- 2007, also display a phase shift relative to the nitude were used for comparison purposes, with standard light curve, as evident from Figure 5, individual estimates made using the step method. that confirms the O–C trend of the other observa- Some typical light curves are illustrated in Fig- tions. A compilation of all O–C estimates from the ure 3, where the superior quality of eye estimates present study is given in Table 3, including, where from single observers over those of inhomogeneous possible, reworkings of older data sets available in groups is evident.