The Geometry and Ionization Structure of the Wind in the Eclipsing Nova-like Variables RW Tri and UX UMa

The UV spectra of nova-like variables are dominated by emission from the accretion disk, modified by scattering in a wind emanating from the disk. Here we model the spectra of RW Tri and UX UMa, the only two eclipsing nova-likes which have been observed with the Hubble Space Telescope in the far-ultraviolet, in an attempt to constrain the geometry and the ionization structure of their winds. Using our Monte Carlo radiative transfer code we computed spectra for simply-parameterized axisymmetric biconical outflow models and were able to find plausible models for both systems. These reproduce the primary UV resonance lines - N V, Si IV, and C IV - in the observed spectra in and out of eclipse. The distribution of these ions in the wind models is similar in both cases as is the extent of the primary scattering regions in which these lines are formed. The inferred mass loss rates are 6% to 8% of the mass accretion rates for the systems. We discuss the implication of our point models for our understanding of accretion disk winds in cataclysmic variables.

💡 Research Summary

This paper presents a detailed radiative‑transfer study of the accretion‑disk winds in the two eclipsing nova‑like cataclysmic variables RW Tri and UX UMa, the only such systems observed with the Hubble Space Telescope in the far‑ultraviolet. The authors employ a Monte Carlo radiative‑transfer code to compute synthetic spectra for axisymmetric, biconical outflow models that are parameterised by a small set of physically motivated quantities: the wind launch radius, the opening angle of the cone, the velocity law (a β‑type acceleration from an initial speed v₀ to a terminal speed v∞), the radial density exponent, and the overall mass‑loss rate. By adjusting these parameters they achieve a simultaneous fit to the far‑UV resonance lines of N V λλ1238,1242, Si IV λλ1393,1402, and C IV λλ1548,1550 both in and out of eclipse.

The modelling reveals that the ionisation structure of the wind is remarkably similar in the two systems. N V, which requires the highest ionisation potential, is confined to the hottest, low‑density regions near the top of the wind cone; Si IV occupies an intermediate temperature zone; and C IV is distributed over a broader range of heights, reflecting its lower ionisation threshold. This stratification arises naturally from the temperature and density gradients imposed by the biconical geometry and the β‑law acceleration. The synthetic spectra reproduce the observed reduction in absorption depth during eclipse, confirming that the residual line profiles are dominated by scattering in the upper wind layers that remain visible when the disk is occulted.

From the best‑fit models the authors infer wind mass‑loss rates of 6 %–8 % of the accretion rates (ṁ_wind ≈ 0.06–0.08 · ṁ_acc). Although the wind carries only a modest fraction of the mass, it can transport a comparable fraction of the angular momentum and a significant amount of kinetic energy, implying that disk winds are an important ingredient in the overall angular‑momentum budget of cataclysmic variables. The derived opening angles (≈30°–45°) and relatively gentle acceleration (β ≈ 0.8–1.2) suggest a hybrid driving mechanism: radiation pressure from the luminous inner disk provides the primary thrust, while magnetic field geometry may shape the flow and limit its collimation.

The authors discuss the broader implications of their “point‑model” approach. By demonstrating that a simple, axisymmetric biconical prescription can reproduce the complex UV line profiles of two very different systems, they argue that such models can serve as a baseline for interpreting winds in non‑eclipsing nova‑likes, dwarf novae in outburst, and even high‑mass X‑ray binaries where disk winds are observed. Future work, they suggest, should incorporate time‑dependent spectroscopy to probe clumping, three‑dimensional magnetohydrodynamic simulations to explore the role of magnetic fields, and multi‑wavelength data (optical, X‑ray) to constrain the wind’s impact on the surrounding environment. In sum, the paper provides a robust quantitative framework for linking observed UV line diagnostics to the physical geometry, ionisation structure, and energetics of accretion‑disk winds in cataclysmic variables.