A comprehensive analysis of the magnetic standard star HD 94660: Host of a massive compact companion?

Abstract

Aims. Detailed information about the magnetic geometry, atmospheric abundances and radial velocity variations has been obtained for the magnetic standard star HD 94660 based on high-dispersion spectroscopic and spectropolarimetric observations from the UVES, HARPSpol and ESPaDOnS instruments. Methods. We perform a detailed chemical abundance analysis using the spectrum synthesis code ZEEMAN for a total of 17 elements. Using both line-of-sight and surface magnetic field measurements, we derive a simple magnetic field model that consists of dipole, quadrupole and octupole components. Results. The observed magnetic field variations of HD 94660 are complex and suggest an inhomogeneous distribution of chemical elements over the stellar surface. This inhomogeneity is not reflected in the abundance analysis, from which all available spectra are modelled, but only a mean abundance is reported for each element. The derived abundances are mostly non-solar, with striking overabundances of Fe-peak and rare-earth elements. Of note are the clear signatures of vertical chemical stratification throughout the stellar atmosphere, most notably for the Fe-peak elements. We also report on the detection of radial velocity variations with a total range of 35 km s −1 in the spectra of HD 94660. A preliminary analysis shows the most likely period of these variations to be of order 840 d and, based on the derived orbital parameters of this star, suggests the first detection of a massive compact companion for a main sequence magnetic star. Conclusions. HD 94660 exhibits interestingly complex magnetic field variations and remarkable radial velocity variations. Long term monitoring is necessary to provide further constraints on the nature of these radial velocity variations. Detection of a companion will help establish the role of binarity in the origin of magnetism in stars with radiative envelopes.

The remarkably unremarkable global abundance variations of the magnetic Bp star HD 133652

“…This is somewhat surprising because Bailey et al (2014) made no effort to establish a reasonably accurate magnetic field model. A recent study by Bailey et al (2015) of the southern magnetic standard star HD 94660 also indicates that the exact choice of magnetic field model is not as important as simply including the effects of the magnetic field when performing an abundance analysis. This is potentially a very important result that requires more rigorous testing because it implies that detailed knowledge of the magnetic field structure is not necessary to derive usefully accurate global abundances for magnetic stars.…”

Chandra X-ray study confirms that the magnetic standard Ap star KQ Vel hosts a neutron star companion

“…KQ Vel is a binary star. The analysis of radial velocity variations by Bailey et al (2015) suggests a companion of around 2 M . The authors argued that such a companion could not be an optical star because spectral features betraying its nature would have been detected.…”

“…KQ Vel is a chemically peculiar star. For example, Bailey et al (2015) found overabundances among the iron group elements by factors of 1000. However, allowing for nonsolar abundances during spectral fitting does not improve the fits of these data with low spectral resolution, and for now, we adopt solar abundances.…”

“…Upon impinging on the stellar surface at the polar caps, this population of nonthermal particles (electrons/protons) radiates nonthermal X-rays by thick-target bremsstrahlung. The viewing geometry of KQ Vel is favorable for always showing the same stellar magnetic pole (Bailey et al 2015). On the other hand, stellar rotation has a key role in the generation of the nonthermal electrons.…”

“…The star is an exceedingly slow rotator, with a rotation period of about 2800 d (Table 1). Bailey et al (2015) determined that the star has a complex magnetic field, strongly nonsolar abundances, with a large overabundance of Fe-peak and rare-earth elements, and shows remarkable radial velocity variations with a period of ∼840 d. The binary companion is not seen in the optical, which led Bailey et al (2015) to suggest the first detection of a compact companion with mass 2 M for a main-sequence magnetic star.…”