Gloria Koenigsberger

Context.γ ² Velorum is the closest and visually brightest Wolf-Rayet (WR) binary system. Its eccentric orbit modulates the X-rays observed from the wind-wind interaction, and its large separation enables spatial resolution of both components. Aims. We aim to strengthen the constraints on the properties of γ ² Velorum and, in particular, to resolve the discrepancy between the eccentricity determined from the emission lines and that from the absorption lines. Methods. We obtained VLT/GRAVITY observations and combined them with earlier spatially resolved data covering different orbital phases. Results. We determine strong constraints on all orbital parameters and find that e = 0.322, close to the value derived from the emission lines. The X-ray light curve declines as s ⁻³ after periastron, where s is the separation of the two stars, but its modulation is likely affected by absorption and occultation of the X-ray emitting region at other orbital phases. We find that previous discrepancies in the reddening value can be traced to a brighter K -band magnitude than predicted by the WR wind models. We derive E ( B − V ) = 0.02 ± 0.02 mag. Our more precise mass and radius values, combined with previously determined effective temperatures, provide strong constraints on evolutionary models. The closest match for the O star comes from a rotationally mixed model with initial mass M _init = 28.7 M _⊙ and an M _init = 32 M _⊙ model for the WR star, with negligible accretion onto the O star during the WR progenitor’s Roche lobe overflow phase. However, the temperature of the O star is higher and the mass of the WR star is smaller than predicted by the evolutionary tracks at the current epoch, consistent with the well-known “mass-discrepancy problem” in massive stars. Conclusions.γ ² Velorum’s strongly constrained parameters make it ideal for testing structure, evolution, and stellar wind models.

We present a detailed chemical abundance analysis of the eclipsing binary system V505 Per. High-resolution spectra were analyzed using the MOOG spectrum analysis code, we determined abundances not only for iron and lithium but also for Si, Na, Ca, Mn, and Ni—elements that have not previously been analyzed in detail for this system. Abundances were computed across 15 temperature points using model atmospheres, with stellar parameters refined by minimizing abundance trends with excitation potential. We determined effective temperatures of T _eff = 6650 ± 50 K for the primary and T _eff = 6550 ± 50 K, with iron abundances of [Fe/H] = −0.10 ± 0.06 and [Fe/H] = −0.19 ± 0.07, respectively. Most [X/Fe] ratios are consistent with solar values, though manganese is deficient. Our analysis of the effective temperatures shows that both stars lie on the hot edge of the lithium dip, consistent with G. Koenigsberger et al., which may help resolve the inconsistency noted of the stars lithium abundance within the dip by Baugh et al.

We determine the surface lithium abundance of the eclipsing binary components in V 505 Per (HIP 10961), A(Li)=2.65+/-0.07 and 2.35+/-0.1, which supports the rather unexpected conclusion that their surface Li abundances differ. We find effective temperatures 6600 K + 6550 K (~150 K higher than previously reported), which place the stars at the hot limit of the Lithium Dip, thus aleviating the previously suggested discrepancy with cluster stars of similar ages and temperatures. These temperatures are also more consistent with the system's Gaia spectral energy distribution. Our iron abundances, [Fe/H]=-0.15+\-0.07 and -0.25+\-0.1, agree with predictions of the higher temperatures deduced from our spectra and from evolutionary tracks. The surface rotation rate implied by our line profiles, 12.5+\-1 km/s, is smaller than the synchronous rotation rate, a curious result given the circular orbit and the age of the system.

Context. V424 Cephei is an eclipsing binary system that has been classified as a Maia variable candidate. These objects are pulsators that apparently lie outside the theoretical instability strips. Aims. We determine the properties of V424 Cep and identify the nature of the pulsating variable. Methods. We analyzed photometric data obtained over the past three decades from TESS, Gaia, Hipparcos, and ground-based observations. Times of minimum light were determined, and the light curves and spectral energy distribution were analyzed. We analyzed the radial velocity curves of the double-lined system obtained from spectroscopy at the Observatorio Astronómico Nacional on the Sierra San Pedro Mártir. Results. The two eclipses observed in the light curves yield a refined orbital period P = 4.93 d, an eccentricity e = 0.02, and an apsidal period U = 730 yr. The eclipse O–C curve is not linear. Combined with the apsidal period, this indicates the presence of a third component in the system. This conclusion was confirmed by a comparison of predicted and observed absolute fluxes. The masses of the binary pair are 8.7 M_⊙ and 6.3 M_⊙, their radii are 6.0 R_⊙ and 3.5 R_⊙, and the luminosities are log(L/L_⊙) 3.80 and 3.11. The TESS light curve shows oscillations with a dominant period of 0.17 d. They are coherent throughout the orbital cycles with a stable amplitude, except during the (partial) eclipse of the primary star. This indicates that this is the pulsator. Its temperature is 21 000 K. The third-light component contributes no more than 15% to the total light of the system in the TESS wavelength band and 20% in the K band. Conclusions. V424 Cep is not a Maia variable, but rather a β Cep star. This result highlights the importance of combining photometric light-curve solutions with absolute flux models and spectroscopic observations.

The International Astronomical Union Symposium Number 402 focused on massive stars across redshifts in the era of JWST and large-scale surveys, and brought together specialists from a broad range of topics. I briefly summarize a few of the highlights from the excellent presentations and rich discussions and, because recent observations suggest that binary interactions dominated star formation and evolution during the early universe, I venture a few personal reflections on this topic.

The multiple Small Magellanic Cloud system HD 5980 contains two very massive stars displaying a Wolf-Rayet type spectrum and which appear to be following chemically homogeneous evolutionary tracks. This means that they will remain very massive and compact prior to ending their lives as supernovae, and are thus excellent binary black hole progenitor prototypes formed in low-metallicty environments. In this paper we summarize some of HD 5980’s extraordinary properties and refine its orbital period for the 2018–2025 timespan, based on the times of secondary minima in TESS observations.

Context. Binary stars cannot be fully understood without assessing the interaction effects between the two components and the impact of these effects on observational diagnostics. Periastron brightening events, also known as the heartbeat phenomenon, are a clear manifestation of this type of interaction. Aims. We aim to explore the role of tidal shear energy dissipation in stars undergoing periastron brightening events. Methods. We performed a computation from first principles that uses a quasi-hydrodynamic Lagrangian scheme to simultaneously solve the orbital motion and the equations of motion of a 3D grid of volume elements covering the inner, rigidly rotating “core” of a tidally perturbed star. The equations of motion include the gravitational acceleration of both stars, the centrifugal, Coriolis, gas pressure accelerations, and viscous coupling between volume elements. The method is illustrated for a grid of model binary systems with a 10 M_⊙ primary that is perturbed by a 6.97 M_⊙ companion in eccentric orbits (e = 0 − 0.7). The model is then applied to the heartbeat star MACHO 80.7443.1718. Results. We find an increase by factors ∼10⁻⁶–10⁻³ in tidal shear energy dissipation at periastron, consistent with the majority of observed heartbeat stars. The magnitude of the periastron effect correlates with the degree of departure from synchronicity: stars rotating much faster or much slower than the synchronous rate at periastron present the strongest effect. We confirm that for eccentricities ≤0.3, pseudo-synchronization occurs for 0.8 < ω/Ω_ave < 1, where Ω_ave is the average orbital angular velocity. The minimum energy rotation rate (pseudo-synchronism) for e = 0.5 and 0.7 occurs for 1.0 < ω/Ω_ave < 1.15. The tidal shear energy dissipation model reproduces from first principles the ∼23% maximum brightness enhancement at periastron of MACHO 80.7443.1718. Conclusions. Our results suggest that the magnitude and shape of the heartbeat signal may serve as diagnostics for the internal stellar rotation and turbulent viscosity values.

The kinetic energy in tidal flows, when converted into heat, can affect the internal structure of a star and shift its location on a color–magnitude diagram from that of standard models. In this paper we explore the impact of injecting heat into stars with masses near the main-sequence turnoff mass (1.26 M _⊙) of the open cluster M67. The heating rate is obtained from the tidal shear energy dissipation rate, which is calculated from first principles by simultaneously solving the equations that describe orbital motion and the response of a star’s layers to the gravitational, Coriolis, centrifugal, gas pressure, and viscous forces. The stellar structure models are computed with MESA. We focus on the effects of injecting heat in pulses lasting 0.01 Gyr, a time frame consistent with the synchronization timescale in binary systems. We find that the location of the tidally perturbed stars in the M67 color–magnitude diagram is shifted to significantly higher luminosities and effective temperatures than predicted by the standard model isochrone and includes locations corresponding to some of the blue straggler stars. Because tidal heating takes energy from the orbit, causing it to shrink, blue straggler stars could be merger or mass transfer progenitors, as well as products of these processes.

Chemically homogeneous evolution (CHE) is a promising channel for forming massive binary black holes. The enigmatic, massive Wolf–Rayet binary HD 5980 A&B has been proposed to have formed through this channel. We investigate this claim by comparing its observed parameters with CHE models. Using MESA, we simulate grids of close massive binaries, then use a Bayesian approach to compare them with the stars’ observed orbital period, masses, luminosities, and hydrogen surface abundances. The most probable models, given the observational data, have initial periods ∼3 days, widening to the present-day ∼20 days orbit as a result of mass loss—correspondingly, they have very high initial stellar masses (≳150 M _⊙). We explore variations in stellar-wind mass loss and internal mixing efficiency, and find that models assuming enhanced mass loss are greatly favored to explain HD 5980, while enhanced mixing is only slightly favored over our fiducial assumptions. Our most probable models slightly underpredict the hydrogen surface abundances. Regardless of its prior history, this system is a likely binary black hole progenitor. We model its further evolution under our fiducial and enhanced wind assumptions, finding that both stars produce black holes with masses ∼19–37 M _⊙. The projected final orbit is too wide to merge within a Hubble time through gravitational waves alone. However, the system is thought to be part of a 2+2 hierarchical multiple. We speculate that secular effects with the (possible) third and fourth companions may drive the system to promptly become a gravitational-wave source.