Magnetic field, rotation, and binarity of the first magnetic B[e] star, IRAS 17449+2320

Abstract

IRAS 17449+2320 is the first identified magnetic B[e] star. It belongs to a subgroup of peculiar hot B-type stars, called FS~CMa stars. presenting the B[e] phenomenon, i.e., permitted and forbidden emission lines, and infrared excess as a result of the large amount of gas and dust surrounding the central star. The origin of this circumstellar material is unclear, with binarity being the most accepted scenario. However, the presence of a magnetic field in IRAS 17449+2320 points to the merger origin. In this paper, we confirm and characterize the magnetic field of this object, and constrain its rotation and binarity. We used a sample of 12 spectropolarimetric observations taken with ESPaDOnS at the CFHT. We obtained the least squares deconvolution (LSD) profile of the Stokes $V$ parameter of each spectropolarimetric observation. From the LSD profiles, we measured the longitudinal magnetic field (B$_z$), and obtained the magnetic field modulus ($|B|$) from the Zeeman splitting of some metallic lines of the spectra. We calculated a Lomb-Scargle periodogram over B$_z$ to obtain the rotational period of this star. B$_z$ presents a sinusoidal variation with a stable period of 36.11 days, with values ranging from 3000 G to -1000 G. $|B|$ measurements range from $\sim$4400~G to $\sim$6000~G, and it shows a second order term periodic variation. The stable sinusoidal variation of B$_z$ implies an approximately dipolar surface field topology. IRAS~17449+2320 is the first magnetic B[e] star detected. It presents Zeeman splitting in individual lines and consistent Stokes $V$ profiles. We see no evidence of binarity in this object, with stable radial velocities and no detectable lines of a secondary star in the spectrum. We consider the possibility that this star is the result of a merger, which could have generated the strong magnetic field and the large amount of circumstellar material.