B fields in OB stars (BOB): Detection of a magnetic field in the He-strong star CPD −57° 3509

Abstract

Aims. We report the detection of a magnetic field in the helium-strong star CPD −57 • 3509 (B2 IV), a member of the Galactic open cluster NGC 3293, and characterise the star's atmospheric and fundamental parameters. Methods. Spectropolarimetric observations with FORS2 and HARPSpol are analysed using two independent approaches to quantify the magnetic field strength. A high-S/N FLAMES/GIRAFFE spectrum is analysed using a hybrid non-LTE model atmosphere technique. Comparison with stellar evolution models constrains the fundamental parameters of the star. Results. We obtain a firm detection of a surface averaged longitudinal magnetic field with a maximum amplitude of about 1 kG. Assuming a dipolar configuration of the magnetic field, this implies a dipolar field strength larger than 3.3 kG. Moreover, the large amplitude and fast variation (within about 1 day) of the longitudinal magnetic field implies that CPD −57 • 3509 is spinning very fast despite its apparently slow projected rotational velocity. The star should be able to support a centrifugal magnetosphere, yet the spectrum shows no sign of magnetically confined material; in particular, emission in Hα is not observed. Apparently, the wind is either not strong enough for enough material to accumulate in the magnetosphere to become observable or, alternatively, some leakage process leads to loss of material from the magnetosphere. The quantitative spectroscopic analysis of the star yields an effective temperature and a logarithmic surface gravity of 23 750 ± 250 K and 4.05 ± 0.10, respectively, and a surface helium fraction of 0.28 ± 0.02 by number. The surface abundances of C, N, O, Ne, S, and Ar are compatible with the cosmic abundance standard, whereas Mg, Al, Si, and Fe are depleted by about a factor of 2. This abundance pattern can be understood as the consequence of a fractionated stellar wind. CPD −57 • 3509 is one of the most evolved He-strong stars known with an independent age constraint due to its cluster membership.

B field in OB stars (BOB): The outstandingly strong magnetic field in the evolved He-strong star CPD −62° 2124

“…Despite the similarities between the He-strong stars CPD −62 • 2124 and CPD −57 • 3509, the latter does not show any spectral feature indicative of circumstellar material (see Fig. 2), although on theoretical grounds it could also host a centrifugal magnetosphere ( B z = 1.1 kG; Przybilla et al 2016). CPD −62 • 2124 therefore provides a further empirical constrain on the physical conditions (e.g.…”

“…The HARPSpol data were independently reduced and analysed using different techniques by the Bonn and Potsdam groups, as described in previous BOB works (Hubrig et al 2014a;Przybilla et al 2016).…”

“…Przybilla et al (2016) presented the magnetic field detection and parameters for CPD −57 • 3509, which is also a He-strong B2IV star, though with a weaker magnetic field ( B z ≈ 1.1 kG) compared to that of CPD −62 • 2124.…”

“…A description of the technique, atomic models and limitations is given by Przybilla et al (2016), with the difference being that in this analysis we did not consider the hydrogen Balmer lines that are contaminated by circumstellar emission (see Sect. 5).…”

A comprehensive study of young B stars in NGC 2264

“…The validity of the approach was verified by direct comparison with hydrostatic and hydrodynamic line-blanketed non-LTE model atmospheres (Nieva & Przybilla 2007;Nieva & Simón-Díaz 2011;Przybilla et al 2011). Non-LTE level populations and synthetic spectra of all elements were computed using updated model atoms as summarized in Table 2 of Przybilla et al (2016). An additional challenge for the analysis of mid-/late B-type stars as discussed here are the overall reduced number of metal lines, which are also typically much weaker than in the earlier B-type stars.…”