Precise spectral subtype and rotational broadening of the companion

Determine the precise spectral subtype and projected rotational velocity of FQ Circini’s B-type companion through high signal-to-noise, high-resolution spectroscopy, enabling reliable conversion of line widths to equatorial rotation speed.

Background

The authors classify the companion as B1 V(n)(e) but note that uncertainties in spectral subtype, inclination, and emission fill-in hinder accurate derivation of the rotation speed from line widths.

A precise determination of spectral subtype and v sin i is important for quantifying spin-up, Be-disk formation, and the role of ellipsoidal modulation in the observed light curve.

References

These measures cannot be reliably converted to $v_{\rm rot}$, because we do not know the exact spectral class, we only have a rough limit on the inclination, and because the rotation effect is not large compared to the pressure broadening and thus is hard to pull out in a noisy, variable spectra with partially-filled-in profiles.

— FQ Circini: An Ordinary Nova with a High-mass B1 V(n)(e) Companion Whose Decretion Disk Transfers Mass to the White Dwarf via Roche-Lobe Overflow  (2511.16594 - Schaefer et al., 20 Nov 2025) in Section 2.17 (Companion Rotation Velocity)