Trumpler 16-26: A New Centrifugal Magnetosphere Discovered via SDSS/APOGEE H-band Spectroscopy

TL;DR

This paper reports the discovery and characterization of Trumpler 16-26, a rapidly rotating, highly magnetized early B star with a centrifugal magnetosphere, confirmed through spectroscopy, spectropolarimetry, and photometry, expanding knowledge of magnetic massive stars.

Contribution

The study presents the first detailed analysis of Trumpler 16-26 as a centrifugal magnetosphere host, revealing it as the faintest and most rapidly rotating among known such stars.

Findings

Confirmed strong magnetic field with >11 kG dipole strength.

Identified rapid rotation with v sin i=195 km/s.

Detected variable hydrogen emission and helium enhancement.

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 $\sigma$ 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 20$^{\rm th}$ known, the fourth most rapidly rotating, and the faintest CM host star yet discovered. With a projected dipole magnetic field strength of $B_{\rm d}>11$ kG, Tr16-26 is also among the most magnetic CM stars.