Rotationally modulated variability and pulsations of the He-rich star CPD −62°2124 with an extraordinarily strong magnetic field

Abstract

A longitudinal magnetic field with a strength of 5.2 kG was recently detected in CPD −62 • 2124, which has a fractional main-sequence lifetime of about 60%. Strongly magnetic early-B type chemically peculiar stars in an advanced evolutionary state are of special interest to understand the evolution of the angular momentum and spin-down timescales in the presence of a global magnetic field. We exploited 17 FORS 2 low-resolution spectropolarimetric observations and 844 ASAS3 photometric measurements for the determination of the rotation period, pulsations, and the magnetic field geometry of the star. We calculated periodograms and applied phenomenological models of photometric, spectral and spectropolarimetric variability. We found that all quantities studied, specifically equivalent widths, the mean longitudinal magnetic field B z , and the flux in the V filter, vary with the same period P = 2.628 d, which was identified as the rotation period. The observed variations can be fully explained by a rigidly rotating main-sequence star with an uneven distribution of chemical elements, photometric spots, and a stable, nearly dipolar magnetic field with a polar field strength of about 21 kG, frozen into the body of the star. The magnetic field of CPD −62 • 2124 is tilted to the rotation axis by β = 28 • ± 7 • , while the inclination of the rotation axis towards the line of sight is only i = 20 • ± 5 • . In the acquired FORS 2 spectra, we detect short-term line profile variations indicating the presence of β Cephei type pulsations. As of today, no other pulsating star of this type is known to possess such a strong magnetic field.

Discovery of Resolved Magnetically Split Lines in SDSS/APOGEE Spectra of 157 Ap/Bp Stars

“…The only other nondegenerate star known to approach 30 kG is HD 75049 (Elkin et al 2010), for which B ranges from 24 to 30 kG over an orbital period. Among the more massive magnetic stars, which are He-rich early-type B stars, the strongest known magnetic field is only 21 kG (Hubrig et al 2017). We can speculate that the magnetic field value of 34 kG likely represents a critical field strength above which stable magnetic fields do not exist in Ap stars.…”

The Magnetic Early B-type Stars IV: Breakout or Leakage? H$α$ emission as a diagnostic of plasma transport in centrifugal magnetospheres

“…CPD−62 • 2124: the extremely strong (B d ∼ 25 kG) magnetic field and Hα emission of this star were first reported by Castro et al (2017). Hubrig et al (2017b) published followup observations with FORS2 and determined the rotational period. The star is a member of the IC 2944 open cluster, and the line core is contaminated with nebular emission.…”

“…This basic emission morphology and pattern of variation has been reported for several individual stars, most notably the extensively studied σ Ori E, which was the first star in which Hα emission was associated with a magnetosphere (Landstreet & Borra 1978), which inspired the RRM model Oksala et al 2012) as well as its immediate precursors (Nakajima 1985;Preuss et al 2004), and which remains the only star for which custom RRM models have been calculated via extrapolation of Zeeman Doppler Imaging maps (Oksala et al 2015). Other stars shown to display CM-type Hα emission include HD 36485 (Leone et al 2010), HD 182180 (Oksala et al 2010;Rivinius et al 2010Rivinius et al , 2013b, HD 176582 (Bohlender & Monin 2011), HD 142184 (Grunhut et al 2012), HD 23478 (Eikenberry et al 2014;Sikora et al 2015;Hubrig et al 2015;Wisniewski et al 2015), HD 35502 (Sikora et al 2016), HD 345439 (Eikenberry et al 2014;Hubrig et al 2015;Wisniewski et al 2015), ALS 3694 (Shultz et al 2016), HD 164492C (Wade et al 2017;González et al 2017), and CPD−62 • 2124 (Castro et al 2017;Hubrig et al 2017b). CM-type emission has also been found in a tidally locked binary star, HD 156324 (Shultz et al 2018a), which shows a single, rather than double, humped emission line morphology consistent with distortion of the gravitocentrifugal potential by the presence of a close companion.…”

Spectropolarimetry of Stars Across the H-R Diagram

“…The strongest longitudinal magnetic field in an O star of more than 5 kG was detected in the Of?p star NGC 1624-2, suggesting a dipole component of about 20 kG (Wade et al 2012). Among the early B-type stars, the record holder is the He-rich star CPD −62 • 2124 with a dipole component of at least 21 kG (Hubrig et al 2017).…”