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

Published online by Cambridge University Press:  09 November 2026

Iris Bermejo LozanoOpen the ORCID record for Iris Bermejo Lozano[Opens in a new window],
Gregg A. Wade,
Colin P. Folsom and
Daniela Korčáková
Iris Bermejo Lozano*
Affiliation:
Cambridge University Press
Gregg A. Wade
Affiliation:
Cambridge University Press
Colin P. Folsom
Affiliation:
Cambridge University Press
Daniela Korčáková
Affiliation:
Cambridge University Press
*
email: irisberloz@gmail.com
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Abstract

IRAS17449+2320 is the first B[e] star with a detected magnetic field. It belongs to a subgroup of peculiar hot B-type stars called FS CMa stars. These stars are surrounded by a huge amount of gas and dust, whose origin is still unclear. The most accepted scenario is binarity. However, a new scenario - the post-merger — was introduced as a consequence of the detection of the Zeeman splitting in some metal lines in the spectrum of this star. Theoretical models suggest that magnetic fields are created during the merging process and can remain stable on evolutionary timescales.

In this presentation, we describe recent work to confirm and characterize the magnetic field of this star, and to refine its physical properties. After analyzing spectropolarimetric observations obtained with ESPaDOnS at the Canada-France-Hawaii Telescope, we have confirmed the detection and variability of the star’s magnetic field, and determined its rotational period. We have not detected any traces of binarity, which points to a post-merger origin for this object. Identifying more magnetic objects among FS CMa stars is of special interest, as it may lead to better understanding of the merging process.

Keywords

stars: magnetic fieldstars: emission-lineBestars: formationstars: binariesstars: mergersstars: chemically peculiarstars: mass-losscircumstellar matter

Information

Type
Poster Paper
Information
Proceedings of the International Astronomical Union , Volume 21 , Symposium S402: Massive Stars Across Redshifts in the Era of JWST and Large Surveys , January 2025, pp. 254 - 258
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Creative Commons
Creative Common License - CCCreative Common License - BY
This is an Open Access article, distributed under the terms of the Creative Commons Attribution licence (https://creativecommons.org/licenses/by/4.0/), which permits unrestricted re-use, distribution and reproduction, provided the original article is properly cited.
Copyright
© The Author(s), 2026. Published by Cambridge University Press on behalf of The International Astronomical Union

1. Introduction

IRAS 17449+2320 belongs to a subgroup of peculiar stars called FS CMa stars. This group consists of (mainly) B-type stars that present the B[e] phenomenon, i.e. permitted and forbidden emission lines, and infrared excess (Miroshnichenko Reference Miroshnichenko2007). For the permitted emission lines, the Balmer lines are especially strong; for the forbidden emission lines, FS CMa stars present low ionization metallic lines, such as [FeII] or [OI] (Fig. 1).

Figure 1.

H alpha $\alpha$ (left panel) and [OI] 6300, 6363 Å (right panel) lines of IRAS 17449+2320.

Two line graphs depicting spectral data with normalized flux on the y-axis and wavelength on the x-axis.

These peculiar characteristics are produced by a huge amount of gas and dust surrounding the star. The origin of this circumstellar matter is unclear, although there are two main hypotheses that explain it: binarity and post-merger. The binarity scenario suggests that the circumstellar matter is produced due to the mass-transfer process in a binary system. During the mass-transfer process, an overflow of the Roche Lobe occurs, and the matter is released around the binary (Miroshnichenko et al. Reference Miroshnichenko, Bjorkman and Chentsov2002). The second hypothesis suggests that the circumstellar matter is created during the merger of the components of a binary system. It is supported by theoretical simulations (Scheneider et al. 2019) and the presence of a magnetic field in one of the members of this group, IRAS 17449+2320 (Korčáková et al. Reference Korčáková, Sestito and Manset2022).

Our study case is the magnetic field of IRAS 17449+2320, the first magnetic B[e] star detected to date.

2. Results

We have reduced 12 Stokes upper V plus upper I $V + I$ spectropolarimetric observations taken during the last 19 years with ESPaDOnS at the Canadian-France-Hawaii Telescope (CFHT). We obtained the LSD Stokes V and I from the spectropolarimetric observations (Fig. 2), and calculated the longitudinal magnetic field (B Subscript z $_z$ ) from them. The longitudinal magnetic field ranges from 3 000 G to -1 000 G. The rotational period of the star is calculated by performing a Lomb-Scargle (LS) periodogram over B Subscript z $_z$ . This data set follows a first-order sinusoidal fit with a period of 36.11 plus or minus $\pm$ 0.01 days, as shown in Fig. 3. This variation of the longitudinal magnetic field is compatible with a rotational modulation.

Figure 2.

LSD Stokes I and V profiles of the 12 spectropolarimetric observations, phased using the rotational period of 36.11 days. The vertical dashed lines are the limits used to calculate B Subscript z $_z$ , and the pink lines mark the radial velocity of each observation.

Two line graphs depict velocity profiles of spectropolarimetric observations.

Figure 3.

The longitudinal magnetic field phased using a period of 36.11 days. Observations from different years are plotted with different symbols.

Line graph showing longitudinal magnetic field over phase with data from different years.

Additionally, we measured the Zeeman splitting of NI 8703 Å, SII 6371 Å and CaII 8497 Å, and obtained the magnetic field modulus ( StartAbsoluteValue upper B EndAbsoluteValue $|B|$ ). The StartAbsoluteValue upper B EndAbsoluteValue $|B|$ values range from tilde $\sim$ 6 000 G from the CaII line to tilde $\sim$ 5 000 G for the NI line (Fig. 4). These values of the magnetic field modulus were also fitted using a first-order sinusoidal function and considering a period of 36.11 days. However, as can be seen in Fig. 4, the first-order sinusoidal function was not performing a good fit over the data sets, and we used a second-order sinusoidal function to improve it. This second-order fit for the magnetic field modulus, together with the first-order fit of the longitudinal magnetic field, point to a mainly dipolar configuration of the magnetic field of the star. Moreover, by comparing StartAbsoluteValue upper B EndAbsoluteValue $|B|$ and B Subscript z $_z$ , it is noticeable that the magnetic field modulus is a factor of a few times the longitudinal field. This is consistent with a globally organized magnetic field that has remained stable during the last 19 years.

Figure 4.

Magnetic field modulus obtained from three different lines showing Zeeman splitting. The data are phased with a rotational period of 36.11 days. The solid line corresponds to a first order sinusoid function, and the dashed line to a second order sinusoid function.

Three line graphs depict magnetic field modulus over phase.

Some metallic lines and the LSD Stokes I profiles show a equivalent width (EW) variation with time (Fig. 5). It varies sinusoidally when phased with the 36.11 d period. The maximum EW corresponds with the maximum of B Subscript z $_z$ . This equivalent width variation could be generated by inhomogeneities in the abundances of elements on the surface of the star. However, it could also be another manifestation of the Zeeman effect, called magnetic intensification; less likely scenarios include veiling or emission filling in the absorption lines. Further analysis is needed to conclude the origin of this EW variation (Bermejo Lozano et al., in preparation).

Figure 5.

Plots of the equivalent width variation of some metallic lines and the LSD Stokes I profile. All the data sets are phased with the 36.11 days period.

Multiple line graphs depict variations in equivalent width and LSD Stokes I profile phased with a 36.11-day period.

In order to obtain more information about the possible origin of this object, we studied the radial velocity (RV) variation of this star. To achieve this, we calculated the RV by fitting a Gaussian to the LSD Stokes I profiles of each observation. The radial velocities are marked as vertical pink lines in Fig. 2. The radial velocity is shown in Fig. 6, it varies from minus $-$ 18.06 plus or minus $\pm$ 0.08 to minus $-$ 16.72 plus or minus $\pm$ 0.15 km s Superscript negative 1 $^{-1}$ , with minus $-$ 17.6 plus or minus $\pm$ 0.36 km s Superscript negative 1 $^{-1}$ being the most likely value for the radial velocity of the star. The RV does not show any significant variation, and therefore, it shows no traces of binarity for this object.

Figure 6.

Radial velocity variation of the LSD I profiles over the rotation phase (upper panel). The bottom panel shows a histogram of these radial velocities.

Two graphs depict radial velocity variation and its histogram over rotation phase.

3. Conclusions

IRAS 17449+2320 is the first B[e] star with a magnetic field detected. The Zeeman splitting can be observed in multiple lines along the spectrum, and it is consistent with the LSD Stokes V profiles. The magnetic field configuration points to a mainly dipolar one, although we need further analysis to confirm or discard the quadrupolar contribution to it. The radial velocity of this star does not show any traces of binarity. Therefore, we suggest that the origin of this object could be a post-merger, as the magnetic field together with the large amount of gas and dust surrounding the star could have been formed during this merger process.

Supplementary material

To view supplementary material for this article, please visit https://doi.org/10.1017/S1743921326105547.

References

Miroshnichenko, A. S. 2007, The Astrophysical Journal, 667, 497 CrossRefGoogle Scholar
Miroshnichenko, A. S., Bjorkman, K. S., Chentsov, E. L., et al. 2002, A&A, 388, 563 Google Scholar
Schneider, F. R. N., Ohlmann, S. T., Podsiadlowski, P., et al. 2019, Natur, 574, 211 CrossRefGoogle Scholar
Korčáková, D., Sestito, F., Manset, N., et al. 2022, Astronomy and Astrophysics, 659, 1 CrossRefGoogle Scholar
Figure 0

Figure 1.Hα$\alpha$ (left panel) and [OI] 6300, 6363 Å (right panel) lines of IRAS 17449+2320.

Figure 1

Figure 2.LSD Stokes I and V profiles of the 12 spectropolarimetric observations, phased using the rotational period of 36.11 days. The vertical dashed lines are the limits used to calculate Bz$_z$, and the pink lines mark the radial velocity of each observation.

Figure 2

Figure 3.The longitudinal magnetic field phased using a period of 36.11 days. Observations from different years are plotted with different symbols.

Figure 3

Figure 4.Magnetic field modulus obtained from three different lines showing Zeeman splitting. The data are phased with a rotational period of 36.11 days. The solid line corresponds to a first order sinusoid function, and the dashed line to a second order sinusoid function.

Figure 4

Figure 5.Plots of the equivalent width variation of some metallic lines and the LSD Stokes I profile. All the data sets are phased with the 36.11 days period.

Figure 5

Figure 6.Radial velocity variation of the LSD I profiles over the rotation phase (upper panel). The bottom panel shows a histogram of these radial velocities.

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