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Astronomy & Astrophysics Doppler Imaging of Stellar Surface Structure

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Astronomy & Astrophysics Doppler Imaging of Stellar Surface Structure A&A 373, 199–210 (2001) Astronomy DOI: 10.1051/0004-6361:20010594 & c ESO 2001 Astrophysics Doppler imaging of stellar surface structure XVI. A time-series analysis of the moderately-rotating K1-giant σ Geminorum Zs. K˝ov´ari1, K. G. Strassmeier2,?,J.Bartus1, A. Washuettl2,?, M. Weber2,?,andJ.B.Rice3 1 Konkoly Observatory of the Hungarian Academy of Sciences, 1525 Budapest, Hungary e-mail: [email protected]; [email protected] 2 Astrophysikalisches Institut Potsdam, An der Sternwarte 16, 14482 Potsdam, Germany e-mail: kstrassmeier, wasi, [email protected] 3 Department of Physics, Brandon University, Brandon, Manitoba R7A 6A9, Canada e-mail: [email protected] Received 9 June 2000 / Accepted 24 April 2001 Abstract. We present a simultaneous photometric and spectroscopic imaging analysis of the long-period RS CVn binary σ Gem, covering 3.6 consecutive rotation cycles with high time resolution. From six overlapping but consecutive Doppler maps we trace the evolution of individual spots throughout the time range covered. All spots group either along a band at approximately +45◦ latitude and a width of 30◦, or appear centered at the equator. No polar spot is detected. We did not find a conclusive migration pattern from the cross-correlation maps from one rotation to the next and attribute this to a masking effect of short-term spot changes. Key words. stars: activity – stars: imaging – stars: individual: σ Gem – stars: late-type – starspots 1. Introduction such rotational modulation, as reported by Engvold et al. (1988) and Singh et al. (1987), which even suggests an σ Geminorum (75 Gem, HR 2973, HD 62044) is a bright inhomogeneous active corona. Long-term cyclic variation and moderately-rotating K1 giant in a long-period with a probable period of 8.5 years was found by Henry ≈ m RS CVn-type binary (Linsky 1984) with V 4 . 15 and et al. (1995) who analyzed the ≈16 years long photomet- ≈ ≈ Prot Porb 20 days. The photometric light variation is ric history of σ Gem. Variations on time scales from hours due to spots on the K1III primary component. The sec- to years were detected in the ROSAT PSPC bandpass by ondary component is probably a cool main-sequence star Yi et al. (1997). Furthermore, σ Gem is long known as a of G or K spectral type (Ayres et al. 1984) but is not seen radio source (Spangler et al. 1977; Gibson 1980, CABS II in the optical spectrum nor in the photometry. and references therein). The system shows all signs of solar-like magnetic activ- Orbital and rotational period determinations ity. σ Gem is included in the Catalog of Chromospherically (Strassmeier et al. 1988; Henry et al. 1995; Jetsu Active Binary Stars (Strassmeier et al. 1993, hereafter 1996; Strassmeier et al. 1999a) obtained very similar ii CABS II) because of its strong Ca H&K emission orig- values differing by less than 1%. Updated orbital ele- inally discovered by Eberhard & Schwarzschild (1913). ments were obtained by Bopp & Dempsey (1989), and From early IUE observations in the far UV, Ayres et al. recently by D¨ummler et al. (1997) who concluded, that (1984) reported the presence of center-to-limb modula- the secondary component could be a compact object tion effects in the spectra. This was further strenghtened or, alternatively, the primary is not a normal K1III by the finding that the Hα core emission also varied with star. Assuming a circular orbit, an orbital period of the rotational phase (Eker 1986; Bopp et al. 1988) and 19.604 days was determined by D¨ummler et al. (1997). thus established the star’s chromospheric activity due to The optical spectral lines are rotationally broadened plage-like regions. The soft X-ray emission also showed by v sin i =27 2kms−1 (Eaton 1990). D¨ummler et al. (1997) found the same value and also argued for Send offprint requests to:K.G.Strassmeier, e-mail: [email protected] log g =2.5 as the most likely gravity in agreement with ? Visiting Astronomer, National Solar Observatory, operated canonical values for K giants listed, e.g. in Gray (1992). by the Association of Universities for Research in Astronomy, The detailed photometric studies with starspot- Inc. under contract with the National Science Foundation. modelling techniques by Fried et al. (1983), Article published by EDP Sciences and available at http://www.aanda.org or http://dx.doi.org/10.1051/0004-6361:20010594 200 Zs. K˝ov´ari et al.: Doppler imaging of stellar surface structure. XVI. Strassmeier et al. (1988), Ol´ah et al. (1989), Dempsey et Table 1. Observing log and radial velocities. al. (1992), Henry et al. (1995), and, more recently, Jetsu (1996) and Berdyugina & Tuominen (1998) found the HJD phase vr σvr presence of at least two long-living large active regions (2 400 000+) (Eq. (1)) (km s−1)(kms−1) (spots) on the primary. These studies are qualitatively 50389.017 0.033 76.4 1.2 in agreement with Eker’s (1986) early spectroscopic 50391.024 0.135 64.1 1.3 study, where an estimation of the inclination angle of 50391.920 0.181 59.6 1.7 ◦ ≤ ≤ ◦ 45 i 73 was also given. 50392.885 0.230 43.4 1.4 Finally, Hatzes (1993) presented the first Doppler im- 50393.875 0.281 40.6 1.5 age of σ Gem and found five spots in a band centered at a 50394.838 0.330 27.9 0.7 medium latitude which he interpreted as an active-latitude 50395.899 0.384 16.9 1.4 belt. No evidence of a polar cap-like spot was seen. The 50396.895 0.435 9.4 1.8 v sin i of 27 km s−1 and the inclination of i =60◦ adopted 50399.057 0.545 7.6 1.4 by Hatzes (1993) yield a radius of ≈12 R , which is con- 50400.004 0.594 15.2 1.5 sistent with a spectral type of a normal K1III giant. 50400.993 0.644 23.8 0.9 The present paper is the 16th paper in our series of 50401.988 0.695 34.8 1.4 a stellar surface structure. The aim of this series is to find 50404.938 0.845 60.8 1.9 a evidence for systematic changes of the surface temperature 50405.862 0.892 66.7 2.1 50406.942a 0.947 75.8 1.5 distribution that are related to basic astrophysical param- a eters like age, mass, and rotation, and to detect specific 50408.886 1.047 77.9 1.7 50411.960 1.203 55.2 1.7 surface phenomena like differential rotation or meridional 50412.945 1.254 42.8 1.3 flows. In this paper, we present a Doppler-imaging anal- 50415.893 1.404 15.2 1.4 ysis with new high-resolution spectra and by photometry 50416.864 1.454 9.4 1.2 that covered 3.6 consecutive stellar rotations of σ Gem. 50417.952 1.509 9.4 1.1 50418.949 1.560 12.0 1.7 50419.890 1.608 16.8 1.4 2. Observations 50420.931 1.661 25.1 1.5 50421.926 1.712 35.2 1.9 2.1. Spectroscopy 50422.912 1.762 45.6 1.7 All spectroscopic observations were obtained with the 50423.880 1.811 55.3 1.7 McMath-Pierce telescope at Kitt Peak National Solar 50424.999 1.869 66.1 1.6 Observatory (NSO). Our 52 spectra were taken during a 50425.980 1.919 75.2 1.5 70 nights long observing run between 1 November 1996−9 50426.985 1.970 78.7 1.8 50428.959 2.071 75.0 1.6 January 1997, covering 3.57 consecutive stellar rotations. 50429.973 2.122 68.9 1.4 We used the stellar spectrograph with the 800 × 800 TI-4 ˚ 50430.972 2.173 61.8 1.7 CCD camera at a dispersion of 0.10 A/pixel and a resolv- 50431.846 2.218 48.4 1.4 ing power of 42 000 as judged from the width of several Th- 50432.878 2.270 38.4 1.9 Ar comparison-lamp lines. The observations covered the 50433.841 2.320 28.7 1.6 6410–6460 A˚ wavelength range but include only two lines 50434.898 2.373 19.2 1.4 suitable for Doppler imaging (Ca i 6439 and Fe i 6430) due 50435.934 2.426 13.1 1.3 to a cosmetic defect on the detector. The average signal-to- 50436.913 2.476 7.8 1.2 noise (S/N) ratio is ≈400:1 but many spectra have more 50437.868 2.525 10.2 1.2 than 500:1 and several also near 300:1. 50438.833 2.574 14.7 1.2 Data reductions were done using the NOAO/IRAF 50439.923 2.630 19.3 1.7 software package and followed our standard procedure 50440.865 2.678 26.4 1.7 for coud´e spectra (see, e.g., Weber & Strassmeier 1998). 50441.942 2.733 41.2 1.9 b Nightly observations of the radial-velocity standard α Ari 50444.893 2.883 68.0 2.2 were used to obtain the radial velocities for σ Gem, except 50446.900 2.986 78.7 2.6 for one night where β Gem was used instead. We adopted 50447.914 3.037 77.2 1.5 −1 −1 50450.015 3.145 65.7 1.9 vαAri = −14.51 km s and vβGem =+3.23 km s (Scarfe et al.
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