摘要：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.
关键词：stars: abundances;stars: atmospheres;stars:
evolution;stars: magnetic field;stars: individual:
CPD-57°3509;stars: massive
