Long-Term Spectroscopic Monitoring of BA Type Supergiants

Astron. Astrophys. 320, 273–286 (1997) ASTRONOMY AND ASTROPHYSICS Long-term spectroscopic monitoring of BA-type supergiants III. Variability of photospheric lines? A. Kaufer1, O. Stahl1, B. Wolf1, A.W. Fullerton4;5, Th. Gang¨ 1;3, C.A. Gummersbach1, I. Jankovics2,J.Kovacs´ 2, H. Mandel1, J. Peitz1, Th. Rivinius1, and Th. Szeifert1 1 Landessternwarte Heidelberg-Konigstuhl,¨ D-69117 Heidelberg, Germany 2 Gothard Astrophysical Observatory, H-9707 Szombathely, Hungary, 3 STScI, Homewood Campus, 3700 San Martin Drive, Baltimore MD 21218, USA 4 Max-Planck-Institut fur¨ Astrophysik, Karl-Schwarzschild-Str. 1, D-85740 Garching, Germany 5 Universitats-Sternwarte¨ Munchen,¨ Scheinerstr. 1, D-81679 Munchen,¨ Germany Received 5 September 1996 / Accepted 26 September 1996 Abstract. We obtained time series of spectra with high S/N 1. Introduction and high resolution in wavelength and time of early-type A and late-type B supergiants (cf. Kaufer et al. 1996a, Paper i, and Our extended spectroscopic monitoring of late B-type and early Kaufer et al. 1996b, Paper ii for the analysis of the variability A-type supergiants (Kaufer et al. 1996a, Paper i, Kaufer et of the stellar envelopes). In this work we inspect the time vari- al. 1996b, Paper ii; Stahl et al. 1995) has revealed for the first ations of the numerous photospheric line profiles in the optical time the complexity of the circumstellar structures of these lu- spectrum. We find complex cyclical variations of the radial ve- minous objects. The time-series analysis of the Hα line-profile 1 locities with a typical velocity dispersion of σ 3kms− . The variability gave strong hints that the envelopes deviate from corresponding equivalent-width variations are≈ less than 1% of spherical symmetry. Further, based on time-scale arguments their mean if we assume a common modulation mechanism for (Paper i) and the observation of reappearing absorption features both radial velocities and equivalent width. We do not find any in the circumstellar envelope (Paper ii), the observed variability depth dependence of the velocity fields in the metallic lines. For was attributed to rotation. α Cyg the Balmer lines show an increase of the radial velocity In this work we want to examine the line-profile variations 1 from H27 to H8 by 3 km s− , which is identified with the on- (LPV) of the numerous photospheric lines in the 4000 6800 A˚ set of the radially accelerating velocity field of the stellar wind. wavelength range, which was covered by our spectrograph− dur- The Cleaned periodograms of the radial-velocity curves show ing all monitoring campaigns in the years from 1990 to 1995. the simultaneous excitation of multiple pulsation modes with Abt (1957), Rosendhal & Wegner (1970a, 1970b), and periods longer and shorter than the estimated radial fundamen- Groth (1972) have found semiperiodic radial-velocity, line- tal periods of the objects, which might indicate the excitation of strength, and width variations with typical time scales from 5 non-radial and radial overtones, respectively. The analysis of the to 50 days and have discussed rotation and photospheric veloc- line-profile variations (LPV) of the photospheric line spectrum ity fields like expansion, macroturbulence and depth-dependent reveals prograde travelling features in the dynamical spectra. microturbulence as possible line-broadening mechanisms. The travelling times of these features are in contradiction to The period analysis by Lucy (1976) of radial-velocity the possible rotation periods of these extended, slowly rotat- curves of α Cyg obtained in 1931/32 by Paddock (1935) re- ing objects. Therefore, we suggest that these features should be vealed the simultaneous excitation of multiple pulsational identified with non-radial pulsation modes, possibly g-modes, modes including radial modes and low-order non-radial of low order (l = m < 5). | | ∼ g modes. Due to the lack of comparable time series until now, this important result has not been verified for further BA-type Key words: stars: early-type, supergiants, emission-line, rota- supergiants. tion, pulsation In the following we want to use our large data set to fill this observational gap. We discuss the measured radial-velocity Send offprint requests to: and line-strength variations and give the amplitudes of the varia- A. Kaufer e-mail: [email protected] tions and the frequency-spectra derived from the radial-velocity ? Based on observations collected at the European Southern Obser- curves. Further, we use dynamical spectra to search for LPV vatory at La Silla, Chile. characteristic for different types of velocity fields. 274 A. Kaufer et al.: Long-term spectroscopic monitoring of BA-type supergiants. III Table 1. The complete data set for all objects. spectra/nights t Object sp exp S=N 1990 1991 1992 1993 1994 1995 [min] HD 91619 B7 Iae – – – – 52/60 85/124 120 100 20/27 86/104 88/110 80/110 5–10 350–500 HD 34085 (β Ori) B8 Iae –– 4/26 10–40 10 300–1200 HD 96919 B9 Iae – – – 83/116 102/130 83/124× 60 150 HD 92207 A0 Iae – – – 86/115 107/129 48/124 60 120 HD 100262 A2 Iae – – – – 98/130 46/124 60 150 49/173 74/155 – 10 350 HD 197345 (α Cyg) A2 Iae –– – –– 1/1 120 1200 2. Data set Table 2. Number of lines for the cross-correlation templates, which In the years from 1990 to 1995 we obtained extended time se- cover the wavelength range from 4100 to 6700 A.˚ All Balmer lines are ries of spectra in the wavelength range 4 000 6 800 A˚ with excluded. The three groups (w = weak, m = medium, s = strong) have high S/N and high resolution in wavelength (λ/−∆λ 20 000) been built according to the theoretical equivalent widths computed with ≈ the given stellar parameters and solar abundances for the Kurucz mod- and time (∆t 1 d) of the late-type B and early-type A su- 1 els and a microturbulence of ξ =15kms− for the line-formation pergiants HD 91619≈ (B7 Ia), β Ori (B8 Ia), HD 96919 (B9 Ia), micro code. HD 92207 (A0 Ia), HD 100262 (A2 Ia) and α Cyg (A2 Ia) with the Flash spectrograph. This instrument was upgraded to the wms sp Teff log g 50 200 200 500 > 500 Heros spectrograph between 1994 and 1995 (cf. Paper i and − − [K] mAm˚ Am˚A˚ Paper ii for a description of these instruments and the data re- B7 12000 1.6 66 20 1 duction procedures). Table 1 gives a summary of the number of B8 11000 1.4 68 27 4 spectra obtained for each object in the different years including A0 10000 1.4 109 30 10 the recent observations of 1995. In addition, typical exposure A2 9000 1.2 127 53 13 times (texp) and signal-to-noise ratios (S/N)inV are given. 3. Measurements of lines in the blue region (mainly due to Feii lines) for later spectral types is clearly seen. 3.1. Cross-correlation technique To be sensitive to possible depth-dependent effects on the The optical spectra of BA-type supergiants display several hun- lines we have constructed for each spectral type three templates dred photospheric lines with equivalent widths Wλ > 50 mA,˚ from three groups of lines with different strength. The group of which are available for analysis of LPV. weak lines (w) includes all lines from the spectrum synthesis To exploit this large number of lines, we use the cross- with equivalent widths 50 <Wλ 200 mA,˚ the medium strong ≤ correlation of the individual spectra of a time series with a group (m) the lines with 200 <Wλ 500 mA,˚ and the strong ≤ synthetic template spectrum. To do this, we construct a cross- group (s) all lines with Wλ > 500 mA.˚ In Table 2 we give the correlation profile, which is a weighted mean of all photospheric respective number of lines in the templates of the w, m, and s lines included in the synthetic template. The template is con- group for the different stellar parameters. If we assume that on structed by computing LTE-model atmospheres (Kurucz 1979) average weak lines form deeper in the atmosphere than strong for different stellar parameters, which roughly match the spec- lines, a possible depth dependence of, e.g., the radial velocities tral types of the objects. With the LTE line-formation code of should be found (cf. below). With the three templates per spec- Baschek et al. (1966) in the version of Peytremann et al. (1967) tral type we further receive three independent measurements of the equivalent widths of all lines available in the line list of Ku- the LPV by using the cross-correlation method. rucz & Peytremann (1975) are computed subsequently for solar The cross-correlation method has several advantages: 1 abundances and a value of ξmicro =15kms− for the microtur- Basically this method constructs a high-S/N weighted mean bulence parameter in the wavelength range from 4100 6700 A.˚ profile with weights according to the relative line strengths. It These computed equivalent widths are finally used as− peak in- is capable of handling the large number of lines available in the tensities of Dirac delta functions placed at the laboratory wave- extended wavelength region of our instrument simultaneously. lengths given by the line list. All Balmer lines are excluded Compared to the red wavelength region of BA-type supergiants, from the templates because of their obviously strong sensitiv- the number of metallic lines in the blue is large (cf. Fig. 1), ity to the wind variability, which is not the focus of this study. i.e., in the spectral region where the S/N is already lowered Fig. 1 shows the computed template spectra for the different by the decreasing efficiency of our CCD.

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