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 light curve, which shows two irregular eclipses per cycle when phased with the likely 0.971 8115-d rotation period, similar to the behaviour 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.
We review magnetic-field measurements of nondegenerate stars across the Hertzprung–Russell diagram for main sequence, premain sequence, and postmain sequence stars. For stars with complex ...magnetic-field morphologies, which includes all G–M main sequence stars, the analysis of spectra obtained in polarized vs unpolarized light provides very different magnetic measurements because of the presence or absence of cancellation by oppositely directed magnetic fields within the instrument’s spatial resolution. This cancellation can be severe, as indicated by the spatially averaged magnetic field of the Sun viewed as a star. These averaged fields are smaller by a factor of 1000 or more compared to spatially resolved magnetic-field strengths. We explain magnetic-field terms that characterize the fields obtained with different measurement techniques. Magnetic fields typically control the structure of stellar atmospheres in and above the photosphere, the heating rates of stellar chromospheres and coronae, mass and angular momentum loss through stellar winds, chemical peculiarity, and the emission of high energy photons, which is critically important for the evolution of protoplanetary disks and the habitability of exoplanets. Since these effects are governed by the star’s magnetic energy, which is proportional to the magnetic-field strength squared and its fractional surface coverage, it is important to measure or reliably infer the true magnetic-field strength and filling factor across a stellar disk. We summarize magnetic-field measurements obtained with the different observing techniques for different types of stars and estimate the highest magnetic-field strengths. We also comment on the different field morphologies observed for stars across the H–R diagram, typically inferred from Zeeman-Doppler imaging and rotational modulation observations,
Abstract
We report the detection and characterization of a new magnetospheric star, HD 135348, based on photometric and spectropolarimetric observations. The TESS light curve of this star exhibited ...variations consistent with stars known to possess rigidly rotating magnetospheres (RRMs), so we obtained spectropolarimetric observations using the Robert Stobie Spectrograph (RSS) on the South African Large Telescope (SALT) at four different rotational phases. From these observations, we calculated the longitudinal magnetic field of the star 〈
B
z
〉, as well as the Alfvén and Kepler radii, and deduced that this star contains a centrifugal magnetosphere. However, an archival spectrum does not exhibit the characteristic “double-horned” emission profile for H
α
and the Brackett series that has been observed in many other RRM stars. This could be due to the insufficient rotational phase coverage of the available set of observations, as the spectra of these stars significantly vary with the star’s rotation. Our analysis underscores the use of TESS in photometrically identifying magnetic star candidates for spectropolarimetric follow-up using ground-based instruments. We are evaluating the implementation of a machine-learning classifier to search for more examples of RRM stars in TESS data.
The Very Large Telescope Interferometer on Cerro Paranal in Northern Chile is one of the largest optical facilities used in astronomy. It can combine two or three of the four 8.2
m and four movable ...1.8
m telescopes, which span baselines between 8 and 202
m. Observations are carried out in the near- and mid-infrared, covering the wavelength range from 1 to 10
μm. In this paper, we describe the status of the interferometric facility at the end of 2006 and provide an overview about the challenges encountered when operating this optical interferometer.
Aims. Only for very few β Cephei stars has the behaviour of the magnetic field been studied over the rotation cycle. During the past two years we have obtained multi-epoch polarimetric spectra of the ...β Cephei star V1449 Aql with SOFIN at the Nordic Optical Telescope to search for a rotation period and to constrain the geometry of the magnetic field. Methods. The mean longitudinal magnetic field is measured at 13 different epochs. The new measurements, together with the previous FORS 1 measurements, have been used for the frequency analysis and the characterization of the magnetic field. Results. V1449 Aql so far possesses the strongest longitudinal magnetic field of up to 700 G among the β Cephei stars. The resulting periodogram displays three dominant peaks with the highest peak at f = 0.0720 d-1 corresponding to a period P = 13ḍ893. The magnetic field geometry can likely be described by a centred dipole with a polar magnetic field strength Bd around 3 kG and an inclination angle β of the magnetic axis to the rotation axis of 76 ± 4°. As of today, the strongest longitudinal magnetic fields are detected in the β Cephei stars V1449 Aql and ξ1 CMa with large radial velocity amplitudes. Their peak-to-peak amplitudes reach ~90 km s-1 and ~33 km s-1, respectively. Concluding, we briefly discuss the position of the currently known eight magnetic β Cephei and candidate β Cephei stars in the Hertzsprung-Russell (H-R) diagram.
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