Trumpler 16-26: a new centrifugal magnetosphere star discovered via SDSS/APOGEE H-band spectroscopy

Abstract

We report the discovery of a new example of the rare class of highly magnetized, rapidly rotating, helium enhanced, early B stars that produce anomalously wide hydrogen emission due to a centrifugal magnetosphere (CM). The star is Trumpler 16-26, a B1.5 V member of the Trumpler 16 open cluster. A CM was initially suspected based on hydrogen Brackett series emission observed in SDSS/APOGEE H-band spectra. Similar to the other stars of this type, the emission was highly variable and at all times remarkable due to the extreme velocity separations of the double peaks (up to 1300 km s−1.) Another clue lay in the TESS lightcurve, which shows two irregular eclipses per cycle when phased with the likely 0.9718115 day rotation period, similar to the behavior of the well known CM host star σ Ori E. To confirm a strong magnetic field and rotation-phase-locked variability, we initiated a follow-up campaign consisting of optical spectropolarimetry and spectroscopy. The associated data revealed a longitudinal magnetic field varying between −3.1 and +1.6 kG with the period found from photometry. The optical spectra confirmed rapid rotation (v sin i = 195 km s−1), surface helium enhancement, and wide, variable hydrogen emission. Tr16-26 is thus confirmed as the 20th known, the fourth most rapidly rotating, and the faintest CM host star yet discovered. With a projected dipole magnetic field strength of Bd > 11 kG, Tr16-26 is also among the most magnetic CM stars.

A magnetic massive star has experienced a stellar merger

“…A strong Balmer emission line (Br γ) is present at 2.16 μm in the GRAVITY spectrum; our fitting of the interferometric data shows that it can only be associated with the primary star, not the secondary. Strong Br γ emission is an indirect sign of stellar winds confined by a magnetic field ( 25 ) and has been used as an infrared indicator of magnetospheres in hot stars ( 26 – 28 ). Because the Br γ line arises from only the primary star, we confirmed the previous suggestion ( 20 ) that only one star in HD 148937 has a strong magnetic field.…”

Variability of Magnetic Hot Stars from the TESS Observations

“…These surveys indicate about ∼10% of the B stars host strong and large-scale magnetic fields, while only ∼6% of O-type (Grunhut et al 2012b;Grunhut et al 2017). Up to now, more and more magnetic hot stars are still detected by the spectrometric or photometric surveys (e.g., Eikenberry et al 2014;Chojnowski et al 2019Chojnowski et al , 2022Jayaraman et al 2022).…”