The non-thermal radio emitter HD 93250 resolved by long baseline interferometry

As the brightest O-type X-ray source in the Carina nebula, HD 93250 (O4 III(fc)) is X-ray overluminous for its spectral type and has an unusually hard X-ray spectrum. Two different scenarios have been invoked to explain its X-ray properties: wind-wind interaction and magnetic wind confinement. Yet, HD 93250 shows absolutely constant radial velocities over time scales of years suggesting either a single star, a binary system seen pole-one or a very long period and/or highly eccentric system. Using the ESO Very Large Telescope Interferometer, we resolved HD 93250 as a close pair with similar components. We measured a near-infrared flux ratio of 0.8+/-0.1 and a separation of 1.5+/-0.2 x 10E-03 arcsec. At the distance of Carina, this corresponds to a projected physical distance of 3.5 A.U. While a quantitative investigation would require a full characterization of the orbit, the binary nature of HD 93250 allows us to qualitatively explain both its X-ray flux and hardness and its non-thermal radio emission in the framework of a colliding wind scenario. We also discuss various observational biases. We show that, due to line-blending of two similar spectral components, HD 93250 could have a period as short as one to several years despite the lack of measurable radial velocity variations.

💡 Research Summary

HD 93250, an O4 III(fc) star located in the Carina Nebula, has long puzzled astronomers because it is the brightest O‑type X‑ray source in the region, exhibits an X‑ray luminosity (L_X ≈ 1.5 × 10³³ erg s⁻¹) that is a factor of two to three above the canonical L_X–L_bol relation, shows a very hard X‑ray spectrum (kT ≈ 2.3 keV), and is a known non‑thermal (synchrotron) radio emitter. Two main explanations have been proposed: (i) magnetically confined winds producing hard X‑rays, and (ii) colliding winds in a massive binary. However, extensive optical spectroscopy over more than a decade revealed essentially constant radial velocities (RVs) with a dispersion of only ~1 km s⁻¹, suggesting either a single star, a pole‑on binary, or a very long‑period/high‑eccentricity system. To resolve this ambiguity, the authors employed the ESO Very Large Telescope Interferometer (VLTI) with the AMBER beam combiner, observing HD 93250 in the H and K bands on two epochs (December 2010 and March 2011) using three baselines (A0‑K0, A0‑G1, K0‑G1) that sampled a range of position angles.

The interferometric data (visibilities and closure phases) could not be fitted with a single uniform disc (χ²_red ≈ 10) but were excellently reproduced by a binary model consisting of two unresolved point sources with a constant flux ratio across H and K. The best‑fit parameters are a projected separation ρ = 1.5 ± 0.2 mas, a position angle θ ≈ 73° (measured east of north), and a K‑band flux ratio L₂/L₁ = 0.8 ± 0.1. At the adopted Carina distance of 2.35 kpc, this angular separation corresponds to a physical projected separation of ≈3.5 AU. The near‑identical fluxes imply that the two components have very similar bolometric luminosities and, using a mass‑luminosity relation L ∝ M²·⁰⁴, a mass ratio M₂/M₁ ≈ 0.9. Assuming a primary mass of ~47 M_⊙ (appropriate for an O4 III star), a circular orbit with a semi‑major axis of 3.5 AU would have an orbital period of roughly 250 days and orbital velocities of ~70 km s⁻¹.

Such velocities should produce detectable RV shifts, yet the spectroscopic data show no variations larger than ~5 km s⁻¹. The authors demonstrate that this apparent contradiction can be resolved by two effects. First, the system could be seen at a very low inclination (i ≈ 2.5°), which would suppress the line‑of‑sight component of the orbital motion; however, the probability of such an orientation is only ~10⁻⁴, making it unlikely. Second, and more plausibly, the two stars have nearly identical spectra, causing severe line blending. Synthetic spectra generated with the non‑LTE FASTWIND code for O4 III+O4 III binaries show that, for RV separations up to ~55–70 km s⁻¹ (corresponding to physical separations of ~70 km s⁻¹), a single‑Gaussian fit to the blended lines yields RVs that change by less than the detection threshold (≈5 km s⁻¹) and line‑profile parameters (peak‑to‑peak amplitude, FWHM) that vary by <5 % and <10 % respectively. Consequently, the lack of observed RV variability does not necessarily imply a long period or high eccentricity; it can be a natural consequence of spectral similarity and observational bias.

The colliding‑wind scenario is then examined quantitatively. Using the formalism of Usov (1992) adapted for O+O binaries, the kinetic power dissipated in the wind–wind collision of two O4 III stars of comparable strength can readily produce an X‑ray luminosity of order 10³³ erg s⁻¹, matching the observed excess. Moreover, the same shock region can accelerate electrons to relativistic energies, which, in the presence of magnetic fields, emit synchrotron radiation, accounting for the observed non‑thermal radio emission. The binary nature thus provides a unified explanation for both the X‑ray over‑luminosity/hardness and the non‑thermal radio output.

The paper also discusses the broader implications. It highlights that long‑baseline interferometry can uncover binaries with separations (≈1–2 mas) inaccessible to traditional spectroscopy or imaging, especially when the components are nearly identical and RV signatures are hidden. The authors caution that many O‑type stars previously classified as single based on stable RVs may in fact be close, equal‑mass binaries, potentially revising statistics on massive star multiplicity and influencing models of massive star evolution, wind physics, and particle acceleration.

In conclusion, the VLTI/AMBER observations definitively resolve HD 93250 into a close, near‑equal‑mass O‑type binary with a projected separation of ~3.5 AU. This discovery reconciles its anomalous X‑ray and radio properties within the colliding‑wind framework and demonstrates the critical role of high‑resolution interferometry in revealing hidden massive binaries.