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Model Atmospheres of Magnetic Chemically Peculiar Stars. Alecian, G., & Stift, M.J

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Model Atmospheres of Magnetic Chemically Peculiar Stars. Alecian, G., & Stift, M.J Astronomy & Astrophysics manuscript no. shulyak c ESO 2018 November 4, 2018 Model atmospheres of magnetic chemically peculiar stars A remarkable strong-field Bp SiCrFe star HD 137509 D. Shulyak1, O. Kochukhov2, and S. Khan3,4 1 Institute of Astronomy, Vienna University, Turkenschanzstrasse 17, 1180 Vienna, Austria 2 Department of Astronomy and Space Physics, Uppsala University, Box 515, 751 20, Uppsala, Sweden 3 Physics and Astronomy Department, University of Western Ontario, London, ON, N6A 3K7, Canada 4 Institute for Computational Astrophysics, Saint Mary’s University, 923 Robie Street, Halifax, B3H 3C3, Nova Scotia, Canada Received / Accepted ABSTRACT Context. In the last few years we have developed stellar model atmospheres which included effects of anomalous abundances and strong magnetic field. In particular, the full treatment of anomalous Zeeman splitting and polarized radiative transfer were introduced in the model atmosphere calculations for the first time. The influence of the magnetic field on the model atmosphere structure and various observables were investigated for stars of different fundamental parameters and metallicities. However, these studies were purely theoretical and did not attempt to model real objects. Aims. In this investigation we present results of modelling the atmosphere of one of the most extreme magnetic chemically peculiar stars, HD 137509. This Bp SiCrFe star has the mean surface magnetic field modulus of about 29 kG. Such a strong field, as well as clearly observed abundance peculiarities, make this star an interesting target for application of our newly developed model atmosphere code. Methods. We use the recent version of the line-by-line opacity sampling stellar model atmosphere code LLmodels, which incorporates the full treatment of Zeeman splitting of spectral lines, detailed polarized radiative transfer and arbitrary abundances. We compare model predictions with photometric and spectroscopic observations of HD 137509, aiming to reach a self-consistency between the abundance pattern derived from high-resolution spectra and abundances used for model atmosphere calculation. Results. Based on magnetic model atmospheres, we redetermined abundances and fundamental parameters of HD 137509 using spectroscopic and photometric observations. This allowed us to obtain a better agreement between observed and theoretical parameters compared to non-magnetic models with individual or scaled-solar abundances. Conclusions. We confirm that the magnetic field effects lead to noticeable changes in the model atmosphere structure and should be taken into account in the stellar parameter determination and abundance analysis. Key words. stars: chemically peculiar – stars: magnetic fields – stars: atmospheres – stars: individual: HD 137509 1. Introduction mentioned above and to answer the question how does the mag- netic field act at different temperatures and what one could ex- The atmospheric structure of magnetic chemically peculiar (CP) pect if the magnetic field is ignored in calculations of model at- stars deviates from that of normal stars with similar funda- mosphere of magnetic CP stars. It was shown that the strength of arXiv:0806.1296v1 [astro-ph] 7 Jun 2008 mental parameters due to unusual chemistry, abundance inho- the magnetic field is the key characteristic controlling the mag- mogeneities and the presence of strong magnetic field. These nitude of the magnetic field effects and the polarized radiative effects are not considered in the standard model atmosphere transfer should be taken into account. In contrast, the orienta- calculations, possibly leading to errors in the stellar parame- tion of the magnetic field vector does not have much influence ter determination and abundance analysis. To circumvent this on any of the observed stellar characteristics and, thus, can be long-standing problem of stellar astrophysics, we have devel- safety ignored in the analysis routines. oped a new line-by-line opacity sampling model atmosphere code LLmodels (Shulyak et al. 2004). Using this tool in the se- So far our models with the magnetic field effects included ries of recent papers, we investigated in detail the effects of have been developed and applied only in the context of purely anomalous Zeeman splitting (Kochukhov et al. 2005), polarized theoretical studies. Here we make the first attempt to model the models radiative transfer (Khan & Shulyak 2006a) and inclination of the atmosphere of a real star. In this work we use the LL stel- magnetic field vector (Khan & Shulyak 2006b) on the model lar model atmosphere code to investigate the atmospheric struc- structure, energy distribution, hydrogen line profiles, photomet- ture of the star, taking into account individualchemical composi- ric colors and the magnitude of bolometric corrections for a grid tion, anomalous Zeeman splitting and polarized radiative trans- of model atmospheres with different effective temperatures and fer. metallicities. For the first time we were able to obtain new re- HD137509 (HIP76011, NN Aps) is a B9p SiCrFe chemi- sults applying direct and self-consistent modeling of all effects cally peculiar star with a strong reversing longitudinal field and variable lines of Si ii and iron-peak elements (Mathys 1991; Send offprint requests to: D. Shulyak, Mathys & Lanz 1997). Kochukhov (2006) (hereafter Paper I) e-mail: [email protected] has detected resolved Zeeman split lines in the spectrum of 2 D. Shulyak et al.: Model atmospheres of magnetic chemically peculiar stars HD137509, showing that this star is characterized by a non- Table 1. Element abundances of HD137509 compared to the dipolar magnetic field geometry with a mean surface field solar values (Asplundet al. 2005). The second and the third strength of about 29kG. This is the second-largestmagnetic field columns give, respectively, abundances derived using the scaled- ever found in a CP star (the first place is occupied by the well- solar abundance model atmosphere (Paper I) and using model known Babcock’s star, Preston (1969)). The atmospheric param- with Teff = 13750K, log g = 4.2 with magnetic field included eters, Teff = 12750 ± 500 K and log g = 3.8 ± 0.1, were derived (see Sec. 3.4). Abundances are given in the logarithmic scale in Paper I using theoretical fit to the observed Hβ and Hγ line log(Nel/Ntotal). profiles based on the ATLAS9 (Kurucz 1993a) model with en- hanced metallicity, [M/H] = +1.0. The appearance of such a Element t12750g3.8 (Paper I) t13750g42 Sun strong magnetic field and the presence of outstanding abundance He < −3.50 < −3.50 −1.10 anomaliesinferredin Paper I allow us to use HD137509as a test Si −3.73 −3.58 −4.53 ground for the application of the new generation magnetic model Fe −3.19 −3.00 −4.59 atmospheres. Cr −4.20 −3.90 −6.40 − − − In the next section we briefly describe the techniques em- Ti 4.54 4.20 7.14 Ca −7.93 −7.50 −5.73 ployed to construct magnetic model atmospheres. Results of the Mg −5.71 −5.50 −4.51 calculations for HD137509 are presented in Sect. 3. Main con- clusions of our study are summarized in Sect. 4. 29 kG. The approach for calculating magnetic models used here 2. Model atmosphere calculations is equivalent to the one described in Sect. 2 in Khan & Shulyak To calculate magnetic model atmospheres we used the cur- (2006a). rent version of the LLmodels code, originally developed by For all models presented in this study we assume a force- Shulyak et al. (2004). The LLmodels is a LTE, 1-D, plane- free configuration of the surface magnetic field. This agrees parallel, hydrostatic stellar model atmosphere code for early and with the results of multipolar modeling of magnetic topology intermediate-type stars. The direct line-by-line calculation of the (Kochukhov 2006) and implies that possible modification of the bound-bound opacities implemented in this code allows one to hydrostatic equilibrium by the Lorentz force (see Shulyak et al. account for arbitrary individual and stratified stellar abundance (2007) and references therein) is absent in HD137509. Thus, all patterns and include various effects caused by the magnetic differences in the pressure structure between magnetic and non- field. The VALD database (Piskunov et al. 1995; Kupka et al. magnetic models that we obtain are caused only by additional 1999) was used as a source of atomic line parameters. The ex- opacity in the Zeeman components. tracted lines were subjected to a preselection procedure inside The necessity to perform polarized radiative transfer calcu- LLmodels, with the default criterion αline/αcont > 1% in at least lations over a wide wavelength range makes the models with one atmospheric layer allowing to select only those lines that magnetic field very computationally expensive. To reduce com- significantly contribute to the opacity (here αline and αcont are putational costs, we used the following approach. Fixing funda- the line center and continuum opacity coefficients, respectively). mental parameters and abundance pattern we calculated a non- The numerical experiments showed that this criterion is suffi- magnetic model. After this model is converged, a model with cient for accurate representation of the energy distribution and T- magnetic field is iterated using the temperature-pressure struc- P structure of the models. Decreasing this value leads to signifi- ture of the initial non-magnetic model as a first approximation. cant increase of the number of selected lines but without produc- The abundances of seven chemical elements for HD137509 ing noticeable changes in overall blanketing (see Shulyak et al. were derivedin Paper I and are listed in Table 1 second column). 2004). The preselection procedure was performed twice per Due to the abnormal weakness of the He lines, the value of the model atmospherecalculation: at the first iteration and at the iter- He abundance should be considered as an upper limit only. One ation when the temperature correction at each model atmosphere can note the strong overabundance of Si, Fe, Cr and Ti, whereas layer is less than few tens of K. The remaining model iterations Ca and Mg are underabundantby more than 1 dex relative to the are performed with the later line list, thus ensuring a consistency solar chemical composition.
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