An analysis of the rapidly rotating Bp star HD 133880★

Abstract

HD 133880 is a rapidly rotating chemically peculiar B-type (Bp) star (v sin i 103 km s −1 ) and is host to one of the strongest magnetic fields of any Ap/Bp star. A member of the Upper Centaurus Lupus association, it is a star with a well-determined age of 16 Myr. 12 new spectra, four of which are polarimetric, obtained from the FEROS, ESPaDOnS and HARPS instruments, provide sufficient material from which to re-evaluate the magnetic field and obtain a first approximation to the atmospheric abundance distributions of He, O, Mg, Si, Ti, Cr, Fe, Ni, Pr and Nd.An abundance analysis was carried out using ZEEMAN, a program which synthesizes spectral line profiles for stars with permeating magnetic fields. The magnetic field structure was characterized by a colinear multipole expansion from the observed variations of the longitudinal and surface fields with rotational phase. Both magnetic hemispheres are clearly visible during the stellar rotation, and thus a three-ring abundance distribution model encompassing both magnetic poles and magnetic equator with equal spans in colatitude was adopted.Using the new magnetic field measurements and optical photometry together with previously published data, we refine the period of HD 133880 to P = 0.877 476 ± 0.000 009 d. Our simple axisymmetric magnetic field model is based on a predominantly quadrupolar component that roughly describes the field variations. Using spectrum synthesis, we derived mean abundances for O, Mg, Si, Ti, Cr, Fe and Pr. All elements, except Mg, are overabundant compared to the Sun. Mg appears to be approximately uniform over the stellar surface, while all other elements are more abundant in the negative magnetic hemisphere than in the positive magnetic hemisphere. In contrast to most Ap/Bp stars which show an underabundance in O, in HD 133880 this element is clearly overabundant compared to the solar abundance ratio.In studying the Hα and Paschen lines in the optical spectra, we could not unambiguously detect information about the magnetosphere of HD 133880. However, radio emission data at both 3 and 6 cm suggest that the magnetospheric plasma is held in rigid rotation with the star by the magnetic field and further supported against collapse by the rapid rotation. Subtle differences in the shapes of the optically thick radio light curves at 3 and 6 cm suggest that the large-scale magnetic field is not fully axisymmetric at large distances from the star.

Abundances determined using Si ii and Si iii in B-type stars: evidence for stratification

“…They interpret this discrepancy as being due to vertical stratification of silicon in the atmosphere, but point out that increasing T eff from the assumed value also brings the abundances derived from lines of Si ii and Si iii into better agreement. Bailey et al (2012) found a similar discordance in the derived abundances of the first and second ionisation states of Si in HD 133 880, another strongly magnetic Bp star ( B z varies from about −4000 to +2000 G) having T eff = 13 000 K and log g = 4.34. They find that an enhancement of about 1 dex above the Si ii abundance is necessary to adequately fit the Si iii lines.…”

“…For the magnetic stars HD 137 509, HD 318 107, and HD 133 880, detailed colinear multipole magnetic field geometries have been reported in the literature that could be used for modelling (see Kochukhov 2006;Bailey et al 2011Bailey et al , 2012. For other stars, the line-of-sight magnetic field measurements, B z , of Bagnulo et al (2006); ; Landstreet et al (2008) and available Table 2.…”

“…Modelling and abundance determination was carried out using the fortran spectrum synthesis code zeeman (see Landstreet 1988;Landstreet et al 1989;Wade et al 2001a;Bailey et al 2012). zeeman accepts as input an assumed magnetic field geometry for stars with magnetic fields (such as the magnetic Ap/Bp stars), but functions as a conventional line synthesis program for modelling of non-magnetic stars (e.g.…”

Discovery of new magnetic early-B stars within the MiMeS HARPSpol survey

“…They are mainly dipolar or low-order multi-polar fields, with polar strengths ranging from 300 G to 30 kG, most having fields of order 1 kG (e.g. Landstreet & Borra 1978;Landstreet et al 1989;Bohlender et al 1987Bohlender et al , 1993Mathys & Lanz 1997;Wade et al 2000aWade et al , 2006Kochukhov 2006;Elkin et al 2010;Bailey et al 2012). These fields are stable over many years, and even decades A&A 567, A28 (2014) (for stars with sufficient observations, e.g.…”

Discovery of secular variations in the atmospheric abundances of magnetic Ap stars

“…The detail of these studies vary from coarse models that roughly describe the magnetic field geometry and abundance variations over the stellar surface (e.g. Landstreet 1988;Bailey et al 2011Bailey et al , 2012 to more detailed maps of both the magnetic field structure and abundance distributions using spectroscopic observations in all four Stokes parameters (e.g. Kochukhov et al 2004;Kochukhov & Wade 2010).…”