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HD 158450: the Magnetic Chemically Peculiar Star in a Young Stellar Group

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HD 158450: the Magnetic Chemically Peculiar Star in a Young Stellar Group HDHD 158450:158450: aa magneticmagnetic chemicallychemically peculiarpeculiar starstar inin aa youngyoung stellarstellar groupgroup 1 2,3 2 2 2 N.A. Drake , E.G. Jilinski , V.G. Ortega , R. de la Reza , B. Bazzanella 1 Sobolev Astronomical Institute, St. Petersburg State University, Russia 2 Observatório Nacional/MCT, Rio de Janeiro, Brazil 3 Pulkovo Observatory, Russian Academy of Sciences, St. Petersburg, Russia Introduction The star HD 158450 The Stellar Parameters We report on the discovery of the Zeeman resolved spectral lines, One of the stars of Mamajek 2 group, HD 158450, shows a highly We used the B2-G Geneva index (Rufener 1988) to estimate corresponding to the very large magnetic field modulus <H> = peculiar spectrum. This star was included in the list of “the brighter the effective temperature of HD158450. Estimating the reddening 11.1 kG, in the spectrum of the chemically peculiar star HD 158450 stars of astrophysical interest in the southern sky” by Bidelman & of the star as E(B-V) = 0.31 (Mamajek 2006) and emploing the belonging to the recently discovered Mamajek 2 stellar group. We MacConnell (1973) as a peculiar A star of the Sr- relation E(B2-G) ~ E(B-V) we find the dereddened index (B2-G)0 = - determined fundamental parameters and projected rotation Cr-Eu type. 0.477 which gives, with the calibration of Hauck & North (1993), an velocity vsin i = 9 ± 1 km/s. effective temperature of Observations Teff = 8880 K. Dynamical 3D mean orbit calculations showed that Mamajek 2 Placing the star on the isochrone for a cluster age of 135 Myr, we stellar group has an age of 135 ± 5 Myr. Based on this age, we We have one high-resolution spectrum of this star obtained at June 2, determine stellar mass estimate that HD 158450 has completed about 12% of its main- 2007 (MJD = 54253.31316) with the FEROS spectrograph attached to the sequence life. 2.2 m telescope of ESO at La Silla, Chile, with a resolution R = 48000 and M = 1.96Mo. a spectral range coverage from 3800 to 8800 Å. Analysis of the connection between magnetic fields and stellar The Magnetic Field evolution is a very actual problem (see, e.g., Landstreet et al. The Stellar Age 2008). The analysis of this spectrum indicates the presence of a strong Based on the Schaller et al. (1992) evolution tracks, we estimate magnetic field resulting in the magnetic splitting of some spectral lines. The importance of the detailed study of HD 158450, a member of a that The most prominent spectral feature is the Fe II 6149.258 Å line (see stellar group with a well determined age, for understanding the Figure 2), commonly used for magnetic field strength determination HD 158450 has completed only about origin and evolution of stellar magnetic fields is discussed. (Mathys 1997). In unpolarized light the profile of this line in the presence 12% of its main-sequence life. of a magnetic field is a simple doublet. The mean magnetic field modulus (the line-intensity weighted average over the visible stellar Mamajek 2 stellar group hemisphere of the modulus of the magnetic vector) can be estimated by The Radial Velocity the equation (Stütz et al. 2003): This new stellar group was discovered by Mamajek (2006) on the Our determination of the radial velocity of this star basis of common parallel motions and similar trigonometric -13 2 H> v = -17.3 km/s>,٠ λ ٠ 10 ۰ geff ٠ ΔλZ = 4.67 ٠ ½ parallaxes of the stars. It contains the bright B8 giant μ Oph and rad eight further B and A-type stars. In his work Mamajek (2006) is in perfect agreement with the value of -17.2 ± 1.4 km/s obtained where Δλ – is the measured Zeeman splitting in Å, – the proposes the coevality of this group located at a present distance Z geff = 1.35 by Kudryavtsev et al. (2007). Previously, Grenier et al. (1999) of 170 pc from the Sun with an age of 120 ±25 Myr. This author effective Landé factor, λ – the central wavelength of the unshifted line found the value of -22.0 ± 4.2 km/s. Although the difference in suggests that the Mamajek 2 group may have formed in the same in Å, and <H> - the mean magnetic modulus in Gauss. radial velocity is rather large, we note that Grenier et al. have star forming region as the Pleiades, NGC 2516 and α Persei The measured mean magnetic field modulus of measured radial velocity of this star by correlation with templates cluster and the AB Dor stellar group. HD 158450 turns to be <H> = 11.1 kG. of the same spectral class. The peculiar nature of the spectrum of HD 158450 could lead to some discrepancy. That is why the The presence of a magnetic field on the surface of this star was recently double or multiple nature of this star, as pointed out in the Dynamical Evolution discovered by Kudryavtsev et al. (2006) from the spectropolarimetric SIMBAD database, needs further investigation by monitoring its observations of a sample of chemically peculiar stars at the 6-m radial velocity. Calculations telescope of the SAO RAS, Russia. These authors found the mean longitudinal component of the magnetic field to be <B 2>½ = 1570 +/- 180 The Rotational Velocity To study the dynamical evolution of stellar groups it is necessary l G, whereas the individual values of the longitudinal magnetic field vary to integrate back in time the 3D orbits of the stars. The dynamical for different dates from -2920 +/- 200 to +810 +/- 240 G, indicating strong The approximation of the Fe II 6149.258 Å magnetically splitted age is then determined as the time elapsed since orbital variation of the magnetic field strength with stellar rotation. line by a synthetic spectrum showed that this star has the low convergence. The resulting age is called dynamical because the projected rotational velocity of orbits are calculated under a modeled Galactic potential, composed of the Plummer potential for the bulge, the Miyamoto- v sin i = 9 +/- 1 km/s. Nagai potential for the disk, and the logarithmic potential for the halo. More details of the adopted methodology can be found in Ortega et al. (2002, 2004) and Jilinski et al. (2005). Discussion and Conclusion The dynamical results showed that Mamajek 2 group The origin and evolution of the strong magnetic fields of upper- and and the open cluster NGC 2516 may have had a middle-main-sequence stars continue to be the subject of long debates. common origin at the age of 135 +/- 5 Myr (see Figure Two different theories have been suggested: according to the one of 1). them, the stars would acquire their fields at the time of their formation – this is the fossil field theory; according to the other theory, the fields On the other hand our calculation shows that the α Persei cluster has a completely different past dynamical evolution when would be generated and maintained by a dynamo mechanism acting compared with Mamajek 2 group. inside the star. Hubrig et al. (2000) showed that the distribution of the magnetic Ap stars of masses below 3Mo in the H-R diagram differs from that of the normal stars. They found that magnetic fields appear only in stars that have already completed at least approximately 30% of their main- sequence lifetimes. Analyzing the relation between the magnetic flux and the stellar rotation velocity, they suggested that a dynamo origin of Figure 2. Spectrum of the Ap star HD 158450 the magnetic field in Ap stars may be possible. Later, Hubrig et al. showing the lines Cr II 6147.154 Å, Fe II 6147.741 (2005) carrying out spectropolarimetric observations with FORS 1 at the Å, and Fe II 6149.258 Å. Note the direct magnetic VLT confirmed that magnetic stars are concentrated towards the center splitting of the Fe II 6149.258 Å line. The spectrum of the of the main-sequence band. normal low rotating A star HD160142, member of the Stępień (2000) suggested that the fields of Ap-Bp stars are already Mamajek 2 group, is shown below (shifted in intensity by 0.2) for comparison purposes. present during the pre-main-sequence phase. Moss (2001) argued that the fields are fossils, left from the pre-main-sequence evolution of the star, rather than dynamo fields being generated during the main- sequence phase. The behaviour of magnetic fields during the main- References sequence evolution of the star was analyzed by Braithwaite & Nordlund Bagnulo, S., Landstreet, J.D., Lo Curto, G. et al. 2003, A&A, 403, 645 (2006). They showed that fossil fields of magnetic Ap stars are stable on Bidelman, W.P., MacConnell, D.J. 1973, AJ, 78, 687 an evolutionary time scale, and that during the main-sequence phase Brathwaite, J., Nordlund, Å. 2006, A&A, 450, 1077 the typical magnetic fields of Ap stars should increase. Jilinski, E., Ortega, V.G., de la Reza, R. 2005, ApJ, 619, 945 Jilinski, E.G., Ortega, V.G., de la Reza, R., Drake, N.A., Bazzanella, B., The study of the evolution of magnetic fields across the main sequence 2008, in press will provide important observational constraints for testing theoretical Hauck, B., North, P. 1993, A&A, 269, 403 predictions. Magnetic field measurements for stars, members of stellar Hubrig, S., North, P., Mathys, G.2000, ApJ, 539, 352 clusters or groups having well-determined ages, are very important. Hubrig, S., North, P., Szeifert, T. ASPC, 2005, 343, 374 Kudryavtsev, D.O., Romanyuk, I.I., Elkin, V.G., Paunzen, E. 2006, Recent discoveries of stars with strong magnetic fields MNRAS, 372, 1804 close to ZAMS may be seen as an argument in favour of the Kudryavtsev, D.O., Romanyuk, I.I., Semenko, E.A., Solov’ev, G.A., 2007, Figure 1.
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