A family of binaries with an extreme mass ratio

TL;DR

This work targets nascent short-period binaries with extreme mass ratios ($q<0.2$) among OB stars to distinguish primordial from post-interaction origins and to understand their role in fast stellar rotation. By combining high-quality TESS photometry with extensive ground-based spectroscopy and applying a two-Gaussian plus reflection model to the light curves, the authors securely identify a dozen new systems and extract orbital and physical parameters for the primaries and their cool companions. Spectral analyses and, where needed, spectral disentangling yield primary Teff, log $g$, and $v\sin i$, while RVs constrain masses and inclinations; several systems show apsidal motion (AN Dor) and LTTE-induced eclipse-time shifts, revealing outer companions in a subset. The results show primaries are fast rotators in most cases, supporting natal fast rotation scenarios and providing stringent constraints for star-formation models and early binary evolution; the expanded sample strengthens the link between extreme mass ratios, eclipses/reflection signals, and rapid rotation in massive-star formation contexts.

Abstract

Multiplicity is ubiquitous among massive stars. While the stellar components usually display similar masses, some binaries with extremely low mass ratios were also observed. Some of them are primordial, while others arise from binary interactions. The identification of systems with extreme mass ratios brings valuable information, notably on the origin of fast rotation in massive stars. We identify new short-period systems with extreme mass ratios through the detection of eclipses and reflection effects. The physical properties of a dozen newly identified cases were precisely evaluated through high-quality photometry and spectroscopy. In addition to characterizing these binaries, we found a clear signature of apsidal motion in one system, and three other systems display long-term shifts in eclipse times. All systems we reported here are composed of a massive star and a cool low-mass companion. They are therefore primordial cases. This doubles the known number of these systems in the Galaxy. In this context, it is important to note that most massive stars in these systems, as well as in previous systems reported in the literature, rotate fast (supersynchronous compared to the orbital motion). The high incidence of fast rotation in these nascent binaries provides strong constraints for star formation models.

Figures (15)

• Figure 1: Line profiles of He i$\lambda$4388Å in the most blue- and redshifted spectra (blue and red lines, respectively) for AN Dor, HD 309317, and V1061 Cen. The spectra were corrected for the measured RVs, and the black line provides their difference (multiplied by four and then shifted by +1 to facilitate the comparison with the lines).
• Figure 2: Light curve observed with ASAS (four epochs; see text) and TESS for AN Dor (three sectors shown). The separation between the primary and secondary eclipses evolves.
• Figure 3: Best-fit adjustment to the differences between the observed times of minima of AN Dor and the linear ephemerides $O-C_{\rm linear}$ for $i = 80^{\circ}$. The orange and pink symbols show the eclipses by the primary and secondary stars, respectively. Data prior to 2010 are taken from the ASAS survey, and data after 2018 are taken from TESS photometry. The long and short dashed lines yield our best-fit theoretical relations for the times of minima. In this plot, the linear ephemerides are $T_0=2\,460\,204.6031$ and $P^{sid}=2.0326782$ d.
• Figure 4: Comparison between the measured RVs of AN Dor (blue symbols with error bars) and the best-fit orbital solution explicitly accounting for apsidal motion.
• Figure 5: Left: Differences between observed and predicted eclipse times based on the ephemerides of Table \ref{['phot1']}. The data for the deeper eclipse are shown by blue dots, and the data for the shallower eclipse are shown as red circles. Close-ups are provided for TYC 1881-933-1. Right: Fourier spectrum of the $O-C$ values of the times of primary minimum of TYC 1881-933-1 (top panel) and the associated spectral window (bottom panel).