BCD spectrophotometry for massive stars in transition phases^★

¹ Instituto de Astrofísica La Plata, CCT La Plata, CONICET-UNLP, Paseo del Bosque s/n, B1900FWA La Plata, Argentina
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² Departamento de Espectroscopía, Facultad de Ciencias Astronómicas y Geofísicas, Universidad Nacional de La Plata (UNLP), Paseo del Bosque s/n, B1900FWA, La Plata, Argentina
³ Observatório Nacional, Rua General José Cristino 77, 20921-400 São Cristovão, Rio de Janeiro, Brazil
⁴ Sorbonne Université, CNRS, UPMC, UMR7095 Institut d’Astrophysique de Paris, 98bis Bd. Arago, 75014 Paris, France
⁵ Astronomical Institute, Czech Academy of Sciences, Fričova 298, 251 65 Ondřejov, Czech Republic

Received: 9 September 2022
Accepted: 3 July 2023

Abstract

Context. Stars in transition phases, such as those showing the B[e] phenomenon and luminous blue variables (LBVs), undergo strong, often irregular mass-ejection events. The prediction of these phases in stellar evolution models is therefore extremely difficult, if not impossible. As a result, their effective temperatures, their luminosities, and even their true nature are not fully known.

Aims. A suitable procedure to derive the stellar parameters of these types of objects is to use the BCD spectrophotometric classification system, which is based on the analysis of the Balmer discontinuity (BD). The BCD parameters (λ₁, D) have the advantage that they are independent of interstellar extinction and circumstellar contributions.

Methods. We obtained low-resolution spectra for a sample of 14 stars with the B[e] phenomenon and LBVs. Using the BCD classification system, we derived the stellar and physical parameters. The study was complemented with the information provided by the JHK colour-colour diagram.

Results. For each star under investigation, the BCD system gives a complete set of fundamental parameters and related quantities such as luminosity and distance. Among the 14 studied stars, we confirmed the classification of HK Ori, HD 323771, and HD 52721 as pre-main sequence HAeB[e] stars, AS 202 and HD 85567 as FS CMa-type stars, and HD 62623 as sgB[e]. We also classified Hen 3- 847, CD-245721, and HD 53367 as young B[e] stars or FS CMa-type candidates, and HD58647 as a slightly evolved B[e] star. In addition, Hen 3–1398 is an sgB[e] and MWC 877, CPD–59 2854, and LHA 120-S 65 are LBV candidates. The stellar parameters of the latter two LBVs are determined for the first time. We also used the size-luminosity relation to estimate the inner radius of the dust disc around the pre-main sequence and main sequence B[e] stars.

Conclusions. Our results emphasise that the BCD system is an important and highly valuable tool with which to derive the stellar parameters and physical properties of B-type stars in transition phases. This method can be combined with near-infrared (NIR) colour-colour diagrams to determine or confirm the evolutionary stage of emission-line stars with dust discs.