Hidden in Plain Sight II: Characterizing the luminous companion to Kappa Velorum with VLTI/GRAVITY

TL;DR

This study tests whether the bright SB1 kappa Velorum hosts a dark compact companion by directly resolving the system with VLTI/GRAVITY, complemented by archival RVs and UVES spectroscopy. The authors unambiguously detect a luminous main‑sequence B star as the companion and rule out a black hole, deriving a refined orbital solution (e.g., $P=116.795\pm0.002$ d, $e=0.1761\pm0.0004$, $i=74.03\pm0.02^{\circ}$) and dynamical masses that depend on distance, with $M_1$ and $M_2$ shifting from $10^{+4}_{-2}$ and $6.9\pm1.0\ M_\odot$ (original Hipparcos parallax) to $13^{+3}_{-2}$ and $7.7^{+0.9}_{-0.8}\ M_\odot$ (revised parallax). The analysis highlights a potential parallax bias from orbital motion (up to ~22%), emphasizes consistency with a two B‑star system when using a parallax near 6.5 mas, and demonstrates a practical framework—combining interferometry, RVs, UVES spectra, and evolutionary tracks—for vetting Gaia DR4 BH candidates and guiding future interferometric surveys (GRAVITY+, CHARA) to uncover dormant BHs. The work also maps out a path to apply this approach to dozens of Gaia DR4 BH candidates and related SB1 systems identified in large catalogs. Overall, the study confirms a luminous companion in kappa Velorum and showcases a robust, interferometry‑driven route to census hidden BH populations in the Milky Way.

Abstract

Kappa Velorum (Markeb, HD 81188) is one of the brightest stars in the Southern sky and has long been known to be a single-lined spectroscopic binary. The binary mass function is large, $f(M)=1.15\ M_\odot$, suggesting that the bright (V=2.5) B2IV star may host a dark, compact object companion. We use VLTI GRAVITY observations to definitively test this possibility by directly resolving the binary. We detect a main sequence B star companion and rule out the compact object scenario. By combining the relative astrometric orbit and archival radial velocities, we report an updated precise characterization of the orbit (period $P=116.795\pm0.002$ d, eccentricity $e=0.1764\pm0.0004$, inclination $i=74.04\pm0.01^{\circ}$) and estimate the masses of the B stars. Using the original Hipparcos parallax measurement $\varpi = 6.05\pm0.48$ mas, we find $M_1 = 10^{+4}_{-2}\ M_\odot$ and $M_2 = 6.9\pm1.0\ M_\odot$. The uncertainties on the masses are primarily driven by the uncertain parallax, which we find is likely biased by the orbital motion. We use an archival UVES spectrum and MIST evolutionary tracks to refine our mass estimates. Finally, we discuss how interferometry and high-contrast imaging may be used to characterize other candidate star+compact object binaries, including those that will be discovered with Gaia DR4, as part of a larger effort to uncover the hidden population of black holes in the Milky Way.

Figures (9)

• Figure 1: Minimum companion masses, $M_{2,\rm{min}}$, and maximum projected angular separation for binaries in the Gaia SB1s and SB9 catalogs. $\kappa$ Vel is marked in blue. The vertical line marks the projected angular separation resolvable with VLTI GRAVITY.
• Figure 2: Companion mass, $M_2$, as a function of the orbital inclination for different values of the primary mass, $M_1$. Even if the primary mass is $8\ M_\odot$, the companion must be $\gtrsim 6\ M_\odot$.
• Figure 3: Top: RV orbit model and residuals. The red lines show random samples from the MCMC posteriors. Bottom left: VLTI GRAVITY astrometry measurements (black points) and random samples for the astrometric orbit model (blue). Bottom right: residuals for the astrometric orbit model. The solid errorbars give the uncertainty derived from the PMOIRED model, and the dashed errorbars report the scaled uncertainty $\sigma_i^\prime = s_{\text{ast}} \sigma_i$.
• Figure 4: MCMC posteriors for orbital parameters from the simultaneous fit to the RVs and the relative astrometry.
• Figure 5: Mass and mass ratio posteriors computed using the orbit posteriors (Figure \ref{['fig:rv_ast_corner']}) and the original (red) and revised (blue) Hipparcos parallax measurements. Both distance measurements are consistent with the binary being a massive B+B star system.