Discovery of new magnetic early-B stars within the MiMeS HARPSpol survey

Abstract

Context. The Magnetism in Massive Stars (MiMeS) project aims at understanding the origin of the magnetic fields in massive stars as well as their impact on stellar internal structure, evolution, and circumstellar environment. Aims. One of the objectives of the MiMeS project is to provide stringent observational constraints on the magnetic fields of massive stars; however, identification of magnetic massive stars is challenging, as only a few percent of high-mass stars host strong fields detectable with the current instrumentation. Hence, one of the first objectives of the MiMeS project was to search for magnetic objects among a large sample of massive stars, and to build a sub-sample for in-depth follow-up studies required to test the models and theories of fossil field origins, magnetic wind confinement and magnetospheric properties, and magnetic star evolution. Methods. We obtained high-resolution spectropolarimetric observations of a large number of OB stars thanks to three large programs (LP) of observations that have been allocated on the high-resolution spectropolarimeters ESPaDOnS, Narval, and the polarimetric module HARPSpol of the HARPS spectrograph. We report here on the methods and first analysis of the HARPSpol magnetic detections. We identified the magnetic stars using a multi-line analysis technique. Then, when possible, we monitored the new discoveries to derive their rotation periods, which are critical for follow-up and magnetic mapping studies. We also performed a first-look analysis of their spectra and identified obvious spectral anomalies (e.g., surface abundance peculiarities, Hα emission), which are also of interest for future studies. Results. In this paper, we focus on eight of the 11 stars in which we discovered or confirmed a magnetic field from the HARPSpol LP sample (the remaining three were published in a previous paper). Seven of the fields were detected in early-type Bp stars, while the last field was detected in the Ap companion of a normal early B-type star. We report obvious spectral and multiplicity properties, as well as our measurements of their longitudinal field strengths, and their rotation periods when we are able to derive them. We also discuss the presence or absence of Hα emission with respect to the theory of centrifugally-supported magnetospheres.

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.…”

B fields in OB stars (BOB): on the detection of weak magnetic fields in the two early B-type starsβ CMa andϵ CMa

“…We then re-normalised all spectra to the intensity of the continuum obtaining a spectrum of Stokes I (I/I c ) and V (V/I c ), plus a spectrum of the diagnostic The luminosities for the magnetic stars published by Alecian et al (2014) have been computed on the basis of the stars' magnitude and distance, the latter given by Alecian et al (2014). The evolutionary tracks for Milky Way metallicity by Brott et al (2011) and the evolutionary tracks for solar metallicity by Georgy et al (2013) are overplotted.…”

“…The terminal wind velocity is in good agreement with the previous determinations (e.g., Snow & Morton 1976), while the massloss rate is about a factor of ten lower than crudely estimated by Drew et al (1997). Figure 2 shows the position of β CMa and CMa in the Hertzsprung-Russell diagram (HRD), together with that of most of the magnetic massive stars published so far (Petit et al 2013;Fossati et al 2014;Alecian et al 2014). For reference, Fig.…”

Weak metallic emission lines in early B-type stars

“…Data of rotational velocities (v sin i ) are taken from Abt et al (2002). Data of v sin i for five stars (HD 3360, HD 29248, HD 35039, HD35468, and HD36591) have been replaced with new data taken from Simón-Díaz and Herrero (2014) and that of HD 133518 is taken from Alecian et al (2014). We could find no published data of v sin i for HD 89587.…”

“…We adopt data given in Takeda et al (2010) and Nieva (2013). When no data can be found in these two papers, we use T eff and log g values given in Lefever et al (2010) (HD 35468 and HD 214993), Aerts et al (2014) (HD 163472), Morel and Butler (2008) (HD 170580), and Alecian et al (2014) (HD 133518). We can find neither published data of T eff and log g nor uvby and β photometric data for the southern star HD 89587.…”