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The Discovery of 8.0-Min Radial Velocity Variations in the Strongly ⋆ Magnetic Cool Ap Star HD 154708, a New Roap Star

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The Discovery of 8.0-Min Radial Velocity Variations in the Strongly ⋆ Magnetic Cool Ap Star HD 154708, a New Roap Star Mon. Not. R. Astron. Soc. 372, 286–292 (2006) doi:10.1111/j.1365-2966.2006.10857.x The discovery of 8.0-min radial velocity variations in the strongly ⋆ magnetic cool Ap star HD 154708, a new roAp star D. W. Kurtz,1† V. G. Elkin,1 M. S. Cunha,2 G. Mathys,3 S. Hubrig,3 B. Wolff4 and I. Savanov5 1Centre for Astrophysics, University of Central Lancashire, Preston PR1 2HE 2Centro de Astrofisica da Universidade do Porto, Rua das Estrelas, 4150 Porto, Portugal 3European Southern Observatory, Casilla 19001, Santiago 19, Chile 4European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching, Germany 5Astrophysical Institute Potsdam, an der Sternwarte 16, 14482 Potsdam, Germany Accepted 2006 July 24. Received 2006 July 23; in original form 2006 May 9 ABSTRACT HD 154708 has an extraordinarily strong magnetic field of 24.5 kG. Using 2.5 h of high time resolution Ultraviolet and Visual Echelle Spectrograph (UVES) spectra we have discovered this star to be an roAp star with a pulsation period of 8 min. The radial velocity amplitudes in the rare earth element lines of Nd II,NdIII and Pr III are unusually low – ∼60 m s−1 – for an roAp star. Some evidence suggests that roAp stars with stronger magnetic fields have lower pulsation amplitudes. Given the central role that the magnetic field plays in the oblique pulsator model of the roAp stars, an extensive study of the relation of magnetic field strength to pulsation amplitude is desirable. Key words: stars: individual: HD 154708 – stars: magnetic fields – stars: oscillations – stars: variables: other. those of the roAp stars; in particular, it shows enhanced lines of Pr 1 INTRODUCTION and Nd and apparent abundance differences between Pr II and Pr III The magnetic A stars are important for the study of the interaction of and between Nd II and Nd III. This is a typical signature of the roAp stellar magnetic fields with rotation, pulsation and atomic diffusion, stars (Ryabchikova et al. 2004), although it does not occur for the all of which have implications for many fields of stellar astrophysics. recently discovered, longer period (P = 21 min), more luminous As with many physical systems, extreme cases may provide tests of roAp star HD 116114 (Elkin et al. 2005a). HD 154708, therefore, our understanding of these systems. Recently Hubrig et al. (2005) appears to be a good candidate to be an roAp star, and a most discovered one of the strongest magnetic fields known for an upper interesting one at that, given its strong magnetic field. main sequence chemically peculiar star, Hs=24.5 ± 1.0 kG, in It is clear that there is a close relationship between the mag- the cool Ap star HD 154708 (Teff ≈ 6800 K). The only star known at netic field and pulsation in roAp stars. The oblique pulsator model that time to have a stronger field than this is the much hotter (Teff ≈ (Kurtz 1982) assumed that the pulsation axis is aligned with the mag- 14 500 K; Leckrone 1974) Bp Si star HD 215441 (Babcock’s star) netic axis and was able to explain most pulsation properties of roAp with a mean field modulus of B=34 kG (Babcock 1960). Since stars. Three important consequences result from the direct effect of then Kochukhov (2006a) discovered a strongly non-dipolar field in the magnetic field on pulsations in roAp stars (see e.g. Bigot & the late Bp star HD 137509, from which he inferred a mean surface Dziembowski 2002; Saio & Gautschy 2004; Saio 2005; Cunha field of Hs=29 kG. Another very strong field has been also found 2006). First, the eigenfrequencies are shifted away from their non- in the cool Ap star HD 178898 by Elkin, Kudryavtsev & Romanyuk magnetic values. Secondly, the eigenfunctions near the surface are (2002) for which Ryabchikova et al. (2006) determined a mean modified in such a way that they no longer can be described well surface field of Hs=17.5 kG. by a single spherical harmonic of a given degree, l. Thirdly, slow Hubrig et al. (2005) found that HD 154708 has a luminosity and Alfven´ waves are constantly generated, and later dissipated, taking temperature that place it at the cool end of the range of known roAp away part of the pulsation energy. Some of these features depend stars. They also found that its elemental abundances are similar to mostly on the magnetic field configuration and on the degree of the modes, while others depend essentially on the magnetic field magnitude and on the structure of the surface layers of the star. The ⋆Based on observations collected at the European Southern Observatory, magnetic field is also expected to have important indirect effects on Paranal, Chile, as part of programme 075.D-0145. pulsations, through the influence it has on convection. In fact, the †E-mail: [email protected] magnetic suppression of convection in the outer layers of roAp stars C 2006 The Authors. Journal compilation C 2006 RAS The discovery of 8.0-min variations in HD 154708 287 seems to be necessary in order to account for the excitation of the version of table 2 in Kurtz & Martinez (2000), lists them with some high overtone pulsations observed (Balmforth et al. 2001; Cunha useful data, and with references to a recent photometric study and 2002; Saio 2005). Thus, the intensity and configuration of the mag- a recent spectroscopic study, as well as a discovery paper, or early netic field are likely to be determinant also for the excitation of the study. oscillations. Cunha & Gough (2000) (see also Cunha 2001; Saio & Gautschy 2004) showed that the dissipation of pulsation energy through slow 2 OBSERVATIONS AND DATA REDUCTION Alfven´ waves is maximal for particular combinations of pulsation Our spectroscopic observations were obtained at the Very Large frequency and magnetic field intensity, and suggested that such dissi- Telescope (VLT) using the Ultraviolet and Visual Echelle Spectro- pation could be important for mode selection in roAp stars. Recently, graph (UVES). We observed HD 154708 for 2.5 h on 2005 May Saio (2005) performed non-adiabatic calculations taking into ac- 19–20 (JD 245 3510) using 40-s exposures with 25-s readout and count the direct effect of the magnetic field on pulsations and found overhead times, giving a time resolution of 65 s and a total number that for some magnetic field intensities the energy losses through of spectra of 140. The CCD images were independently processed the dissipation of slow Alfen´ waves are indeed sufficient to stabilize by VGE and BW using UVES pipeline recipes and the ESO MIDAS high overtone p modes, even when convection in the outer layers package to extract one-dimensional (1D) spectra. Both reductions of roAp stars is assumed to be suppressed by the magnetic field. gave similar results. The extracted spectra were normalized to the Additionally, he found that the dissipation through Alfven´ waves continuum and each spectrum was corrected in velocity to the Solar leads to the stabilization of the low overtone p modes. Along with System Barycentre. The spectra range from 4970 to 7010 Å with a the helium settling due to diffusion in the absence of convection small gap about 60 Å wide near 6000 Å. The signal-to-noise ratio in (Theado,´ Vauclair & Cunha 2005), this stabilizing effect gives a the individual spectra ranges from 35 to 180, where we define that good explanation of why δ Scuti pulsations have not been found in signal-to-noise ratio as the ratio of the measured noise level in a (rel- these stars. Finally, Saio found excitation of dipole and quadrupole atively) line-free section of continuum compared to the continuum (ℓ = 1, 2) modes primarily for higher overtones, in good agreement level. This value varies across the orders of the spectrum. with observations of roAp stars (see e.g. Kurtz & Martinez 2000). Precise radial velocities were measured for a number of spectral All these results suggest that magnetic field strength and configu- lines by the centre-of-gravity method and by fitting Gaussians, sep- ration are important in both mode selection and excitation in roAp arately for each line, and for each of the Zeeman components for stars. Whether the magnetic field also influences the amplitude of lines for which this is possible. Generally, the central π components the oscillations and how such influence may depend on the mag- are sharper than the σ components and give more precise radial ve- netic field properties, is an issue that still needs to be looked at locity measurements than the whole line. Gaussian fitting was only closely. used for unblended lines which are well approximated with Gaus- There is no known correlation between magnetic field strength for sians, and it gives similar results to the centre-of-gravity method. For roAp stars and observed pulsation frequencies (from which mode spectral line identification we used several data bases and other pub- overtone may be estimated with an appropriate model), or with lished sources: the Vienna Atomic Line Data Base (VALD)1 (Kupka pulsation amplitude. Testing for such correlations is not yet easy. et al. 1999), the Atomic Spectra Data Base NIST,2 the Data Base Amplitudes in roAp stars are strongly a function of atmospheric on Rare Earths At Mons University (DREAM)3 (Biemont,´ Palmeri height; in the case of HD 99563, for example, the radial velocity & Quinet 1999; Quinet & Biemont´ 2004), the line lists for roAp amplitude increases from about zero to nearly 5 km s−1 within the stars from the Vienna Asteroseismology along the Main Sequence core of the Hα line alone (Elkin, Kurtz & Mathys 2005b).
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