Two spotted and magnetic early B-type stars in the young open cluster NGC 2264 discovered by MOST and ESPaDOnS

Abstract

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 ∼10 Myr 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 (<5 km s −1 ) 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 −0.7 M and 7.6 +0.5 −0.5 M . 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.

Smooth X-ray variability fromρOphiuchi A+B

“…Were the spot at the equator, a more complex toroidal field would have to be involved. Rho Ophiuchi is not the first case of spotted B stars as spots have been identified in other B-early stars, e.g., two young B2 stars in NGC 2264 (Fossati et al 2014), as we observed before. In these two cases, the magnetic origin of the surface spots is inferred by means of spectropolarimetric observations, with magnetic fields of a few hundred Gauss in intensity and configurations as simple and double dipoles, respectively.…”

“…In these two cases, the magnetic origin of the surface spots is inferred by means of spectropolarimetric observations, with magnetic fields of a few hundred Gauss in intensity and configurations as simple and double dipoles, respectively. Fossati et al (2014) discuss the origin of such magnetic fields, preferring the hypothesis of a merging event during the star formation rather than a fossil origin. The relevance of ρ Ophiuchi in this context is that its close distance allows further detailed investigations.…”

“…Significant examples of X-ray emitters among late O-early B type stars are τ Sco (Cohen et al 2003), β Crucis , ζ Puppis (Nazé et al 2013), and θ 1 Ori C (Gagné et al 1997). The detection of spots on the surface of late O/early B-type stars has been reported in X-ray band by Gagné et al (1997) on θ 1 Ori C (O7 V, rotation period p = 15.4) with ROSAT, and recently by Fossati et al (2014) in two early B-type stars of the NGC 2264 star forming region. Another case of modulated variability and hard X-ray spectrum in early B-type star which hints at a magnetic origin is σ Ori E (Skinner et al 2008).…”

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

“…This is consistent with an evolutionary age (see Table 3) that compares reasonably well with the age of the parent open cluster NGC 3293, 10.7 Myr (Lotkin & Matkin 1994). There is also reasonably good agreement between the spectroscopic distance of CPD −57 • 3509 (d spec = 2630 ± 370 pc) and the cluster distance of ∼2460 pc (Briquet et al 2013;Petit et al 2013;Alecian et al 2014;Neiner et al 2014;Fossati et al 2014Fossati et al , 2015aSikora et al 2015;Castro et al 2015, grey circles), and CPD −57 • 3509 (black dot). Error bars are 1σ uncertainties.…”

Detection of magnetic field in the B2 star ρ Ophiuchi A with ESO FORS2

“…Two spotted and magnetic early B stars in the young open cluster of NGC 2264 have been discovered by Fossati et al (2014), namely HD 47887 and HD 47777. For HD 47887, there is a X-ray detection in Chandra with a flux of 6.03 × 10 −15 erg s −1 cm −2 , at a distance of 800 pc this means a X-ray luminosity of ∼ 4.2 × 10 29 erg s −1 , which is lower than that observed on average in ρ Oph A.…”