A&A 415, 241–249 (2004) Astronomy DOI: 10.1051/0004-6361:20034142 & c ESO 2004 Astrophysics Asteroseismology of the β Cep star HD 129929 I. Observations, oscillation frequencies and stellar parameters C. Aerts1, C. Waelkens1, J. Daszy´nska-Daszkiewicz1,3,, M.-A. Dupret2,4,, A. Thoul2,†, R. Scuflaire2, K. Uytterhoeven1, E. Niemczura3, and A. Noels2 1 Instituut voor Sterrenkunde, Katholieke Universiteit Leuven, Celestijnenlaan 200 B, 3001 Leuven, Belgium 2 Institut d’Astrophysique et de G´eophysique, Universit´edeLi`ege, all´ee du Six Aoˆut 17, 4000 Li`ege, Belgium 3 Astronomical Institute of the Wrocław University, ul. Kopernika 11, 51-622 Wrocław, Poland 4 Instituto de Astrof´ısica de Andaluc´ıa-CSIC, Apartado 3004, 18080 Granada, Spain Received 1 August 2003 / Accepted 25 September 2003 Abstract. We have gathered and analysed a timeseries of 1493 high-quality multicolour Geneva photometric data of the B3V β Cep star HD 129929. The dataset has a time base of 21.2 years. The occurrence of a beating phenomenon is evident from the data. We find evidence for the presence of at least six frequencies, among which we see components of two frequency multiplets with an average spacing of ∼0.0121 c d−1 which points towards very slow rotation. This result is in agreement with new spectroscopic data of the star and also with previously taken UV spectra. We provide the amplitudes of the six frequencies in all seven photometric filters. The metal content of the star is Z = 0.018 ± 0.004. All these observational results will be used to perform detailed seismic modelling of this massive star in a subsequent paper. Key words. stars: oscillations – stars: variables: general – stars: individual: HD 129929 1. Introduction Waelkens & Rufener (1983). HD 129929 is not included in the list of runaway stars composed from HIPPARCOS data = The star HD 129929 (spectral type B3V, V mag 8.1) is quite (Lindblad et al. 1997). However, the new HIPPARCOS dis- a peculiar object, as it is situated at intermediate galactic lati- tance estimate requires a mean vertical velocity of “only” = ◦ tude with b 20.21 (Hill et al. 1974), which is unusual for 13 km s−1 should the star have formed in the galactic plane such a massive object. This is the reason why Rufener (1981) (where we have used the age estimate of 17 million years re- included it in the Geneva database as a standard, which led him cently found by Aerts et al. 2003). We compare this value with to discover its variability. our new estimate of the radial velocity of the star in Sect. 2. The parallax of HD 129929 has meanwhile been measured by HIPPARCOS, albeit with a large uncertainty. The mea- Waelkens & Rufener (1983) made the first detailed study sured π = 1.48 ± 1.03 corresponds to a distance estimate of of the variability of HD 129929 by means of Geneva photom- etry and found the star to vary triperiodically with frequen- some 675 parsec. This value leads to a distance of 233 par- −1 sec perpendicular to the Galactic plane, which is less than cies 6.460965, 6.979940 and 6.449041 c d . The amplitudes half the value derived earlier from multicolour photometry by of these three frequencies were found to range between 10 to 18 millimagnitudes. The star was hence classified as a new Send offprint requests to: M.-A. Dupret, e-mail: [email protected] β Cep star. More recently, Heynderickx (1992) also established Based on data gathered with the Swiss 0.7m telescope equipped three frequencies in a more extensive dataset that included the with the photometer P7 of the Geneva Observatory and with the one used by Waelkens & Rufener (1983). However, only two of FEROS spectrograph attached to the ESO 2.2 m telescope, both these are in common with those found by Waelkens & Rufener situated at La Silla in Chile; the reduced photometric multicolour data (1983) and moreover the values he lists are slightly different: are provided in Table 1, which is only available in electronic form at 6.98670, 6.45610 and 6.97697 c d−1. the CDS via anonymous ftp to With such closely spaced frequencies, HD 129929 is a cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/415/241 very interesting massive pulsating star to try and perform seis- Postdoctoral Fellow, Belgian Federal Office for Scientific, mic modelling, once the frequencies are firmly established and Technical and Cultural Affairs. the modes are well identified. This situation now occurs for Marie-Curie Postdoctoral Fellow, European Union. HD 129929 and the main results of our seismic study based † Chercheur Qualifi´e au Fonds National de la Recherche upon the multicolour photometry were summarized recently Scientifique, Belgium. in Aerts et al. (2003). They established firm evidence of the Article published by EDP Sciences and available at http://www.aanda.org or http://dx.doi.org/10.1051/0004-6361:20034142 242 C. Aerts et al.: Asteroseismology of HD 129929. I. Only high-quality data, with a typical individual error less than 5 mmag, were retained in our analysis. An example of the lightcurve in the Geneva V filter for an observing run of 11 days is shown in Fig. 2. It is evident from this graph that HD 129929 behaves multiperiodically and that large beating effects occur on a timescale of a week. Such beat periods are typical for multiperiodic β Cep stars. In order to confirm the important result that the star exhibits non-rigid rotation with a very low equatorial rotation velocity of some 2 km s−1 (Aerts et al. 2003), we have taken one high- resolution (wavelength step of 0.0298 Å)echelle ´ spectrum of HD 129929 with the FEROS spectrograph attached to the ESO 2.2 m telescope. We integrated over 2000 s to reach a signal- to-noise ratio of some 200 at 4550 Å. This spectrum leads to a radial velocity of 64±1kms−1, from which we derive a velocity Fig. 1. The spectral window of our photometric data of HD 129929. of 22 km s−1 perpendicular to the Galactic plane. This is largely sufficient to bring the star to its current position and suggests that it was kicked out of the Galactic plane, rather than having occurrence of core overshooting in the star with an overshoot- formed outside of the plane. If so, it must have been kicked ing parameter α = 0.1. Moreover, they have shown the star to ov when it was 10 million years old if we consider it to move with have non-rigid slow rotation. constant speed ever since. The current paper is the first one in a series of three de- We show some particular selected absorption lines of the voted to the in-depth physical interpretation of the variability spectrum in Fig. 3. We see that the star is indeed sharp-lined. of HD 129929. In this first paper we provide the full frequency + + content based on all available measurements of this star. We An upper limit of the overall (thermal pulsational rotational) broadening can be derived from the FWHM of the different show that at least six oscillation modes at millimagnitude level metal lines in the spectrum. This leads to a value of some are excited in HD 129929. Moreover, we compare the rotation −1 velocity predicted by Aerts et al. (2003) from the photometric 17 km s . Taking into account the thermal broadening of a B3V star (some 10 km s−1) and the fact that considerable pul- variability with a recently obtained high-resolution spectrum sational broadening must occur, the spectrum provides inde- of the star. Our extensive seismic study will be published in a pendent evidence for the slow rotation of the star. We find an subsequent paper in this series (Dupret et al. 2004, hereafter upper limit of ∼13 km s−1 (assuming no pulsational broaden- termed Paper II) while a detailed comparison between inde- v i pendent evolution and oscillation codes, with HD 1299929 as ing nor any turbulence) for sin which is compatible with the estimate of Ω R = 2kms−1 provided by Aerts et al. (2003). a case study, constitutes a third paper in the series (Daszy´nska- Daszkiewicz et al., in preparation, Paper III). Finally, we mention that some low-resolution IUE spectra of HD 129929 are publicly available and have led previously to a radial-velocity estimate of 66 km s−1 which is entirely com- 2. Observations patible with our new result. These UV spectra also already showed that the star is sharp-lined but the resolution of these ff In an e ort to resolve and understand the discrepant fre- data is much lower than that of our FEROSechelle ´ spectrum. quency determinations by Heynderickx (1992) and Waelkens & Rufener (1983), HD 129929 was kept in the long-term pro- gramme of photometric monitoring of variable B stars per- 3. Frequency analysis formed by the Institute of Astronomy of the University of Leuven with the 0.7 m Swiss photometric telescope. This tele- For seismic analyses of stars to be successful, accurate fre- scope was operational at the La Silla observatory until 1997. It quency determinations of as many oscillation modes as pos- was equipped with the Geneva multicolour photometer P7. sible have to be derived. Heynderickx (1992) already pointed Many members of the Institute of Astronomy gathered in out, on the basis of some 800 datapoints with a time base of total 1493 good-quality data points with a total time base of some 15 years, that the star has closely spaced frequencies.