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

Context. 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. All these objects present 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. Most studies suggest that it is the result of a mass flow or transfer in a close binary system. However, the circumstellar matter and the presence of the magnetic field in IRAS 17449+2320 point to the possibility that this star is the result of a merger. Aims. In this paper we present the recent work done to confirm and characterize the magnetic field of this object, and constrain its rotation and binarity. Methods. The sample used for this work consists of 12 spectropolarimetric observations taken with ESPaDOnS at the Canada-France-Hawaii Telescope, obtained from 2006 to 2025. The circular polarization (Stokes V ) was measured in each observation, and linear polarization (Stokes Q and U ) was measured in two of them. 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 the B z dataset to obtain the rotational period of this star. Using the unpolarized (Stokes I ) spectrum, we performed precise measurements of the equivalent widths and radial velocities of various spectral lines. Results. The longitudinal magnetic field presents a sinusoidal variation with a stable period of 36.11 days, with values ranging from 3000 G to −1000 G. The magnetic field modulus measurements depend somewhat on the metallic line used to measure it. For N I , the values range from ∼4600 G to ∼5500 G; from ∼4400 G to ∼5500 G for Si II , and from ∼4300 G to ∼6000 G for Ca II . The magnetic field modulus shows a periodic variation for the three different metallic lines, although they require a second order term to perform a better fit on the datasets. The stable sinusoidal variation of the longitudinal field implies an approximately dipolar surface field topology. Some lines of the spectra show an equivalent width variation with time, following the period of the star. We also obtained the radial velocity from each LSD Stokes I profile, and calculated a mean value of −17.6 km s −1 . Conclusions. 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.