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Cyclicities in the Light Variations of S Doradus Stars

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Cyclicities in the Light Variations of S Doradus Stars A&A 376, 224–231 (2001) Astronomy DOI: 10.1051/0004-6361:20010960 & c ESO 2001 Astrophysics Cyclicities in the light variations of S Doradus stars III. P Cygni M. de Groot1,C.Sterken2,?, and A. M. van Genderen3 1 Armagh Observatory, College Hill, Armagh BT61 9DG, Northern Ireland 2 University of Brussels (VUB), Pleinlaan 2, 1050 Brussels, Belgium 3 Leiden Observatory, Postbus 9513, 2300 RA Leiden, The Netherlands Received 19 April 2001 / Accepted 2 July 2001 Abstract. On the basis of new photometric observations and archived data published since 1907, we discuss the light variations of P Cygni. We conclude that there are α Cygni-type microvariations with a stable (pulsation) quasi-period of 17.3 days. There are also longer cycles of variation with P ∼ 100 d, so-called 100 d-type micro- variations, and with P ∼ 1500–1600 d, a short S Dor-type phase. Key words. stars: individual: P Cygni – stars: variables – stars: oscillations – stars: supergiants 1. Introduction P Cyg is in a “state of marginal dynamical instability” and de Jager (2001), studying its photosphere, used the fol- This is the third detailed study on cyclicities in the light lowing expression: “it finds itself at most at the fringe of variations of a selection of well-monitored S Doradus vari- 1 instability”. Dynamical instability means that the outer ables. The two previous papers dealt with ζ Sco in the layers are subject to a steady expansion or contraction, Galaxy and R 40 in the SMC (Sterken et al. 1997a, 1998, and in the first case eject matter vigorously. A discussion respectively). of the periodicity of the radial velocity and light variations S Doradus stars – also known as Luminous Blue of P Cyg was given by van Gent & Lamers (1986), see also Variables (LBVs) – are found in the upper left-hand corner van Genderen (1991) and van Genderen et al. (1992). A of the Hertzsprung-Russell diagram. They are photomet- long time scale spectroscopic study of the Hα emission line rically variable with a large range of amplitudes (several was compared with simultaneous photometry from differ- hundredths of a magnitude to magnitudes) and on a vast ent sources by Markova (2000, 2001) and Markova et al. range of time scales (hours, over decades, to centuries). (2001a, 2001b). These authors found various correlations The amplitudes of the variations seem to increase with with different time scales between the equivalent width of the time scales at which they occur. Considering the pres- the Hα line and the photometric behaviour. This is the ence of circumstellar ejecta, about 40 % of the S Dor stars first time that such a study of an S Dor variable has been seem to have suffered an η Carinae-type outburst in the made. past. For an extensive review of the light curve proper- ties of S Dor stars, we refer to van Genderen (2001), who P Cygni is one of the very few hypergiants that have classified P Cyg as a weak-active (w-a) S Dor variable a well-documented light history. Discovered by Willem (because it was weak-active in the 20th century). Jansz Blaeu on 18th August 1600 as a third-magnitude star, it was only the second so-called “nova” after Tycho’s P Cygni (HR 7763 = HD 193237) is a notorious S Dor “new” star of 1572. This, and the invention of the tele- star of the η Carinae type, with giant eruption(s), scope at about the same time, ensured a long series of S Doradus phases and microvariability (for a detailed dis- observations of P Cygni, starting in 1600 and still contin- cussion, see de Groot 1969, and for a recent review see uing today. Though the observations of the first three cen- Israelian & de Groot 1999). According to Stothers (1999a), turies suffer from the fact that they were done visually, by Send offprint requests to:C.Sterken, different observers using different comparison stars, and e-mail: [email protected] in the absence of an agreed magnitude scale, it has still ? Research Director, Belgian Fund for Scientific Research been possible to recover P Cygni’s photometric history. (FWO). An investigation of the characteristics of the light curve Article published by EDP Sciences and available at http://www.aanda.org or http://dx.doi.org/10.1051/0004-6361:20010960 M. de Groot et al.: Cyclicities in the light variations of S Doradus stars. III. 225 4.8 5.1 5.0 5.2 mv 5.2 B Johnson 5.3 16000 16500 17000 17500 28000 28200 28400 28600 28800 JD-2400000 JD-2400000 Fig. 1. mv light curve of P Cyg, data from von Prittwitz Fig. 2. B light curve of P Cyg, data from Nikonov (1937, 1938). (1907). 4.7 after 1700 has led to the conclusion that we are witnessing 4.8 photometric changes due to stellar evolution (de Groot & V Johnson 4.9 Lamers 1992; de Groot et al. 2001). 30800 30850 30900 30950 31000 31050 In this paper we present the analysis of a unique set of J D -2400000 new photometric measurements of P Cyg covering almost Fig. 3. V light curve of P Cyg, data from Groeneveld (1944). two complete decades, combined with archival data from the literature. 2. The data -RKQVRQ 9 2.1. Pre-1982 data -' The data for the years 1902–1907 are from von Prittwitz Fig. 4. V light curve of P Cyg, data from Percy & Welch (1907). The observations were done with a Z¨ollner-type (1983). photometer (see Z¨ollner 1861; Sterken & Staubermann 2000); the comparison star was BD +36 3955 = 29 Cyg, a Table 1. Sources of the post-1982 data, ID is the identification λ Boostarintheδ Scuti instability strip with a pulsation label used in the discussion, N denotes the number of measure- m ∼ amplitude of 0. 03 in V and P 45 min. The resulting ments. APT stands for Automatic Photometric Telescope. mv values were not corrected to the V scale. The mv light curve in Fig. 1 shows four blocks of data, totalling 38 mea- ID N Reference surements. The mean light level is m =5.04 The associ- v P 38 Percy et al. (1988) ated standard deviation (0m. 11) is rather high, and reflects the uncertainties inherent in visual photometry. There is VRI 95 From VRI APT data a rather strong increase in brightness level between the APTA 515 Armagh programme on the APT first group and the following ones; the overall brightness APTL 127 Leiden programme on the APT gradient is about 0m. 02 y−1 over almost 2000 d. The only CAMC 118 Carlsberg Automatic Meridian Circle structure visible is a maximum around JD 2 415 898, and DB 17 Dietmar B¨ohme, Nessa, Germany another one around JD 2 416 700 (Fig. 1). Mi 95 R. Milton, Somes Bar, CA A second set of early data was published by Nikonov ZS 31 E. Zsoldos, Konkoly Observatory Budapest (1937, 1938): 65 data points, B filter close to Johnson B, PS 8 Peter Sterzinger, Australia the standard deviation (0m. 035) is low, and there is a steady increase of the brightness level by ∼0m. 03 y−1 over MT 11 Markova & Tomov (1998) the 800 days spanning the observations. Figure 2 gives the B light curve for the recorded measurements. Two clearly- delined light maxima are present, viz. JD 2 428 074.8 and 2.2. Post-1982 data 2 428 442. The post-1982 dataset is a combination of V data from A third set is by Groeneveld (1944), and consists different sources, as is described in Table 1. The so far of 55 V measurements collected over a time interval of unpublished photometric data used in this study will be about five months. Most remarkable is the fast decline in submitted for publication to the Journal of Astronomical light starting on JD 2 431 007, and lasting for three subse- Data (JAD, 2001). Evidently, one may not anticipate that − quent nights with a gradient of −0m. 023 d 1. These data all these data can be swiftly merged into one composite are illustrated in Fig. 3. light curve: the various equipment, sites, observers and Percy & Welch (1983) published 11 V measurements comparison stars must inevitably lead to inhomogeneities of P Cyg (Fig. 4), apparently, a light maximum occurred and systematic errors. In order to minimize such effects, close to JD 2 445 123.7 (June 1982). we have attempted to determine a transformation from 226 M. de Groot et al.: Cyclicities in the light variations of S Doradus stars. III. B-V 0.3 0.4 V 4.6 4.7 4.8 4.9 45000 46000 47000 48000 49000 50000 51000 H JD -2 4 0 0 0 0 0 Fig. 5. Combined V light curve. The continuous line represents the best-fitting sine curve with P = 1630 days and amplitude 0m. 015 (fit on the basis of all data collected after JD 2446000). Table 2. Differences (APTA minus other) of quasi- The latter figures illustrate a shape of the light curve simultaneous observations. N is the number of such data ava- of P Cyg that seems quite characteristic: there is a pseudo- iable, ∆ is the average difference, σ is s.d. cyclic behaviour with a characteristic time of the order of 16–19 days. The descending branch after maximum seems ID N ∆ σ to be quite smooth while the rising branch frequently dis- VRI 15 −0m. 040 0.051∗ plays some kind of downward bump just preceding max- APTL 44 −0m. 010 0.011 imum – the latter sometimes has the shape of a stillstand CAMC 33 0m.
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