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Metallicities of the Β Cephei Stars from Low-Resolution Ultraviolet Spectra

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Metallicities of the Β Cephei Stars from Low-Resolution Ultraviolet Spectra A&A 433, 659–669 (2005) Astronomy DOI: 10.1051/0004-6361:20040396 & c ESO 2005 Astrophysics Metallicities of the β Cephei stars from low-resolution ultraviolet spectra E. Niemczura and J. Daszynska-Daszkiewicz´ Astronomical Institute, Wrocław University, ul. Kopernika 11, 51-622 Wrocław, Poland e-mail: [email protected] Received 5 March 2004 / Accepted 22 November 2004 Abstract. We derive basic stellar parameters (angular diameters, effective temperatures, metallicities) and interstellar reddening for all β Cephei stars observed during the IUE satellite mission, including those belonging to three open clusters. The parameters are derived by means of an algorithmic procedure of fitting theoretical flux distributions to the low-resolution IUE spectra and ground-based spectrophotometric observations. Since the metallicity has a special importance for pulsating B-type stars, we focus our attention in particular on this parameter. Key words. stars: early-type – stars: abundances – stars: variables: general 1. Introduction In this paper we analyze the IUE (International Ultraviolet Explorer) data combined with ground-based spectrophotomet- β Cephei variables are a well-known group of early B-type ric observations of β Cephei stars. The ultraviolet (UV) part pulsating stars. Their pulsations are driven by the classical of the spectra of main-sequence B-type stars is very rich in κ-mechanism, operating in the layer of the metal opacity bump lines of the iron-group elements. Because the greatest amount ≈ × 5 at T 2 10 K caused by the large number of absorption of energy for these objects is emitted in the spectral region be- lines of the iron-group elements. The explanation of oscilla- tween 1216 and 10 000 Å, even the low-resolution spectra can tions in these stars was possible only after the publication of the give credible values of stellar parameters such as the effective new opacity tables by Iglesias et al. (1992), and was given by temperature, Teff, surface gravity, log g, microturbulent veloc- Cox et al. (1992), Moskalik & Dziembowski (1992), Kiriakidis ity, vt, metallicity, [m/H], angular diameter, θ, and the color ex- et al. (1992) and Dziembowski & Pamyatnykh (1993). As was cess, E(B − V). The usefulness of the low-resolution IUE spec- shown by Pamyatnykh (1999), the extent of the theoretical in- tra for this purpose has already been demonstrated by many stability domain in the H-R diagram is very sensitive to the authors. Fitzpatrick & Massa (1999) obtained the mean stel- heavy element abundance, Z. The instability domain decreases lar parameters for fourteen main sequence stars in the spec- ≈ with decreasing value of Z,andforZ 0.01 there are no tral range O9.5−A1. Morossi et al. (2002) used IUE spectra, unstable modes in the observed β Cephei domain. Therefore, combined with the visual spectrophotometric observations, to the determination of metallicities is the first step in testing derive [m/H], Teff and θ for 53 objects in the solar neighbor- the theory of pulsational instability in massive Main Sequence hood. In Niemczura et al. (2002) we presented the results for stars. It would be important to fix the minimum observed value some objects from the paper of Code et al. (1976) and for the / of [m H] for which the oscillations are still present in these Magellanic Clouds B-type stars, obtained by the least-squares stars. Most β Cep pulsators belong to our Galaxy, and they are optimization algorithm. In a previous paper (Daszynska´ et al. supposed to have a metal abundance close to the solar value. 2002), using the above method, we determined the values of Recently, Pigulski & Kołaczkowski (2002) discovered the first the metal abundance parameter in the atmospheres of 31 field extragalactic β Cep stars in the LMC, where the average metal- β Cephei stars and 16 stars belonging to three open clusters licity is much below the solar value. All LMC β Cep stars NGC 3293, NGC 4755 and NGC 6231. Here we use the same are in or close to young associations and they probably have data as in Daszynska´ et al. (2002) but we derive the stellar pa- higher metal abundance than average. Moreover, the question rameters with an improved procedure which allows, in partic- “which pulsational parameters are correlated with the metal ular, for a more objective error estimation. This method was abundance?” is still open. applied recently by Niemczura (2003) to determine the metal- licities, effective temperatures, stellar diameters and interstellar Tables 1, 2, 4 and 5 are only available in electronic form at reddenings for Slowly Pulsating B-type stars. http://www.edpsciences.org Article published by EDP Sciences and available at http://www.aanda.org or http://dx.doi.org/10.1051/0004-6361:20040396 660 E. Niemczura and J. Daszynska-Daszkiewicz:´ Metallicities of the β Cephei stars from low-resolution UV spectra In Sect. 2, all stars we analyze are presented and spec- the GCPD database (General Catalogue of Photometric Data, trophotometric observations together with their calibration are Mermilliod et al. 1997). shown. The procedure of fitting the theoretical spectra to the All stars selected for the analysis are presented in the observations and the method of bootstrap resampling adopted Tables 1 and 2, where their names, HD numbers (ID numbers for determining the errors of parameters are briefly described for cluster stars), numbers of IUE spectra, parallaxes and ref- in Sect. 3. The results and interparametric correlations are pre- erences for the ground-based observations of field objects are sented in Sect. 4. In Sect. 5, we discuss the values of metallici- shown. Many β Cephei stars we analyze belong to binary or ties we obtained. Conclusions are given in Sect. 6. multiple systems. Some of them are wide visual binaries with separation greater than 20 arcsec and can be resolved by the 2. The observational material IUE cameras. In this group we have eight stars: ν Eri, 12 Lac, The observational material consists of IUE observations β CMa, ξ1 CMa, Cen, α Lup, BU Cir and γ Peg. obtained with the large or small aperture. We connected In some cases the parameters of the companion indicate that the data from both the long-wavelength (LWP and LWR, it does not influence the spectrum of the much hotter β Cephei 1950−3200 Å) and the short-wavelength (SWP, 1100−1950 Å) star. BW Vul is a binary system (Porb = 33.3 ± 0.3 y, Pigulski cameras. We used observations with high (0.1−0.3 Å) and 1992) with the secondary component 6−10 mag fainter than low (6−7 Å) spectral resolutions. The data were processed the primary. EN Lac is an eclipsing binary with orbital pe- by means of two reduction packages: IUE/NEWSIPS (New riod 12.0968 d (Pigulski & Jerzykiewicz 1988; Lehmann et al. Spectral Image Processing System, Nichols & Linsky 1996; 2001, and references therein). The secondary component is Gahart et al. 1997) and IUE/INES (IUE Newly Extracted fainter than the primary by about 5 mag. ν Cen is a spec- Spectra, Rodríguez Pascual et al. 1999; Gonzáles-Riestra et al. troscopic binary (SB) system with an orbital period equal to 2001, and the references therein). A detailed comparison of 2.622 ± 0.018 d (Schrijvers & Telting 2002) and a separation the IUE low-dispersion spectra extracted by the INES and of 5.588 ± 0.061 arcsec (Shatsky & Tokovinin 2002). Merezhin NEWSIPS procedures made by Schartel & Skillen (1998) (1994) determined the masses of the components to be equal to showed an excellent agreement in most cases. The main dif- 10.20 ± 0.10 and 1.57 ± 0.25 M. β Cep is a triple system with ferences were found for underexposed spectra and strong spec- a distant visual component (separation equal to 13.4 arcsec). tral lines. Niemczura (2003) carried out a comparison of the Gezari et al. (1972) found a close component, separated from IUE spectra processed by NEWSIPS and INES packages for the primary by about 0.25 arcsec. By 1992 this value decreased SPB and reference stars. A similar comparison for the hotter to 0.03 arcsec. The orbital elements of this system were deter- stars made in this work gives the same results. The systematic mined by Pigulski & Boratyn (1992) who derived a period of differences appear in strong lines and at the wavelengths where 91.6 ± 3.7 y from the light-time effect. the LWP/LWR and SWP spectra overlap. Large differences In some cases we have not enough information about the also occur in the wavelength range 2000−2300 Å for highly secondary component. 27 CMa is a binary system resolved by reddened stars. Other discrepancies are probably not system- speckle interferometry (see for example Hartkopf et al. 2000). atic. As found by Niemczura (2003), better agreement exists The separation of the system is growing, the last determined for high-resolution data. value being equal to 0.133 ± 0.008 arcsec. Abt & Cardona All data points with a flag indicating some data-quality (1984) found the orbital period of the system, Porb ≈ 40 y. problems were excluded from further analysis. Because of the θ Oph has a close companion with a separation of 0.300 ± presence of interstellar Lyα absorption, the spectral region 0.025 arcsec (Shatsky & Tokovinin 2002). τ1 Lup is a visual bi- around Lyα was omitted. The IUE observations for λ>3000 Å nary with a separation of 158.2 arcsec and a difference in mag- were also excluded from the analysis because of the very nitude of 4.7, therefore the visual component does not influ- low signal-to-noise ratio at these wavelengths. Thus, we used ence the spectrum of the β Cephei star.
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