Revisiting symbiotic binaries with interferometry: II. New PIONIER data

TL;DR

This study leverages VLTI/PIONIER interferometry and Gaia distances to precisely measure the angular diameters and hence radii of red giants in seven symbiotic binaries. By placing these giants on the HR diagram with MIST tracks, the authors estimate their masses and compare $R$ to the Roche-lobe radii $R_{\\rm Roche}$ to infer mass-transfer modes, finding wind-driven transfer for most systems and a near Roche-lobe filling in ZZ CMi. The work provides concrete constraints on the evolutionary state (primarily AGB) and orbital interactions of these binaries, highlighting that many will approach Roche-lobe contact as they evolve, and it underscores the critical role of accurate distances, temperatures, and metallicities for robust interpretation. The results offer valuable benchmarks for binary evolution and mass-transfer models and set the stage for Gaia DR4 to refine distances and orbital inclinations, crucial for finalizing Roche-lobe filling assessments.

Abstract

Symbiotic stars, which generally comprise a red giant and an accreting white dwarf, are excellent laboratories to understand mass transfer in wide binaries, with application to a wide family of systems. One of the fundamental questions is how mass is transferred from the red giant to the white dwarf. We use interferometric measurements made with the VLTI/PIONIER instrument, combined with Gaia data, to measure the radius of the giant in seven symbiotic systems. We further place the giants in the H-R diagramme, which allows us to estimate their mass and to show that they are all very evolved and likely on the asymptotic giant branch. We compare our measured giant radii to their Roche-lobe radius and show that, except for ZZ CMi, all giants are well within their Roche lobe and that mass transfer likely takes place via stellar wind. Our interferometric data provide further evidence that the giant in ZZ CMi (nearly) fills its Roche lobe. Our conclusions are still hampered by the poor characterisation of some of the giants or their binary orbit, and we encourage the community to make an effort to provide these.

Figures (13)

• Figure 1: Reduced PIONIER interferometric data for ZZ CMi. The left plot shows the squared visibilities ($V^2$), while the right one shows the closure phases (T3PHI) in degrees. The colours correspond to the various baselines ($B$, expressed in terms of the wavelengths, $\lambda$), while the black line is the simple fit of a uniform diameter. The turquoise stars are the fit corresponding to a Roche-lobe filling (and, hence, deformed) star. They are mostly different from the black line in the closure phases.
• Figure 2: Squared visibilities for the remaining six symbiotic systems as a function of the baselines. The colours correspond to the various baselines, while the black line is the fit. The angular diameter that is then obtained is also indicated.
• Figure 3: Comparison between our derived properties and MIST stellar models for V1044 Cen for various masses. The left panels show the radius (upper) and effective temperature (lower) as a function of age, for various stellar masses, with the derived quantities indicated with the black lines and the gray shaded regions. For the radius, we also show the Roche lobe radius for a circular orbit (red dotted line) and at periastron for $e=0.16$ (purple dotted line). The right panel shows the H-R diagramme, with the inset showing the whole region. Our derived quantities are indicated with the black point and the error bars.
• Figure 4: Same as Fig. \ref{['fig:HR_V1044']} for V417 CMa, assuming a solar metallicity.
• Figure 5: The Gaia light curve of V417 CMa (in the three bands, $G$, $B_{\rm P}$, $R_{\rm P}$), phase folded on a period of 397.5 days.