Discovery of Two Rare Rigidly-Rotating Magnetosphere Stars in the APOGEE Survey

Abstract

The Apache Point Observatory Galactic Evolution Experiment (APOGEE) - one of the Sloan Digital Sky Survey III programs -- is using near-infrared spectra of $\sim 100,000$ red giant branch star candidates to study the structure of the Milky Way. In the course of the survey, APOGEE also acquires spectra of hot field stars to serve as telluric calibrators for the primary science targets. We report the serendipitous discovery of two rare, fast-rotating B stars of the $\sigma$ Ori E type among those blue field stars observed during the first year of APOGEE operations. Both of the discovered stars display the spectroscopic signatures of the rigidly rotating magnetospheres (RRM) common to this class of highly-magnetized ($B\sim 10$ kiloGauss) stars, increasing the number of known RRM stars by $\sim 10\%$. One (HD 345439) is a main-sequence B star with unusually strong He absorption (similar to $\sigma$ Ori E), while the other (HD 23478) fits a "He-normal" B3IV classification. We combine the APOGEE discovery spectra with other optical and near-infrared spectra of these two stars, and of $\sigma$ Ori E itself, to show how near-infrared spectroscopy can be a uniquely powerful tool for discovering more of these rare objects, which may show little/no RRM signatures in their optical spectra. We discuss the potential for further discovery of $\sigma$ Ori E type stars, as well as the implications of our discoveries for the population of these objects and insights into their origin and evolution.

arXiv comment14 pages, incl. 5 figures; accepted for publication in ApJ Letters

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• Discovery of Resolved Magnetically Split Lines in SDSS/APOGEE Spectra of 157 Ap/Bp Stars
• A combined multiwavelength VLA/ALMA/Chandra study unveils the complex magnetosphere of the B-type star HR5907
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• Characterizing the Rigidly Rotating Magnetosphere Stars HD 345439 and HD 23478
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• News from Gaia on sigma Ori E: a case study for the wind magnetic braking process
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• MOBSTER -- VII. Using light curves to infer magnetic and rotational properties of stars with centrifugal magnetospheres