Astronomy and Astrophysics, volume 562A, 143-143 (2014/2-1)
Two spotted and magnetic early B-type stars in the young open cluster NGC 2264 discovered by MOST and ESPaDOnS.
FOSSATI L., ZWINTZ K., CASTRO N., LANGER N., LORENZ D., SCHNEIDER F.R.N., KUSCHNIG R., MATTHEWS J.M., ALECIAN E., WADE G.A., BARNES T.G. and THOUL A.A.
Abstract (from CDS):
Star clusters are known as superb tools for understanding stellar evolution. In a quest for understanding the physical origin of magnetism and chemical peculiarity in about 7% of the massive main-sequence stars, we analysed two of the ten brightest members of the ∼10Myr old Galactic open cluster NGC 2264, the early B-dwarfs HD 47887 and HD 47777. We find accurate rotation periods of 1.95 and 2.64-days, respectively, from MOST photometry. We obtained ESPaDOnS spectropolarimetric observations, through which we determined stellar parameters, detailed chemical surface abundances, projected rotational velocities, and the inclination angles of the rotation axis. Because we found only small (<5km/s) radial velocity variations, most likely caused by spots, we can rule out that HD 47887 and HD 47777 are close binaries. Finally, using the least-squares deconvolution technique, we found that both stars possess a large-scale magnetic field with an average longitudinal field strength of about 400 G. From a simultaneous fit of the stellar parameters we determine the evolutionary masses of HD 47887 and HD 47777 to be 9.4+0.6–0.7M☉ and 7.6+0.5–0.5M☉. Interestingly, HD 47777 shows a remarkable helium underabundance, typical of helium-weak chemically peculiar stars, while the abundances of HD 47887 are normal, which might imply that diffusion is operating in the lower mass star but not in the slightly more massive one. Furthermore, we argue that the rather slow rotation, as well as the lack of nitrogen enrichment in both stars, can be consistent with both the fossil and the binary hypothesis for the origin of the magnetic field. However, the presence of two magnetic and apparently single stars near the top of the cluster mass-function may speak in favour of the latter.
stars: fundamental parameters - stars: early-type - stars: individual: HD 47887 - stars: individual: HD 47777 - open clusters and associations: individual: NGC 2264 - stars: magnetic field
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