[Paper Review] Direct diameter measurement of a star filling its Roche Lobe: The semi-detached binary SS Leporis spatially resolved with VINCI/VLTI

Tijl Verhoelst, E. van Aarle+1 more|Lirias (KU Leuven)|May 30, 2007

Stellar, planetary, and galactic studies22 references10 citations

TL;DR

This study presents the first direct spatial resolution of a star filling its Roche lobe using K-band interferometry with VINCI/VLTI, confirming that the M-type giant in the semi-detached binary SS Leporis is fully Roche-lobe-filling. The observations reveal a circumstellar dust disk and show the A-type primary has expanded due to mass accretion, explaining its high luminosity and shell spectrum, with non-conservative mass transfer confirmed by the disk's geometry and opacity properties.

ABSTRACT

Stellar evolution in close binary systems is strongly influenced by mass transfer from one star to the other when one component fills its zero-velocity surface or Roche Lobe. SS Lep is a fairly nearby close binary showing the Algol paradox and a shell spectrum, both indicative of (past) mass transfer. To study the process of mass transfer and its evolutionary consequences, we aim at a direct characterisation of the spatial dimensions of the different components of SS Lep with IR interferometry. We use VINCI/VLTI interferometric observations in the K band and photometric observations from the UV to the far-IR. The visibilities are interpreted with simple geometrical models and the Spectral Energy Distribution (SED) is decomposed into the three main components: A star, M star and dust shell/disk. From the SED, we find that the main emitters in the K band are the M star and the circumstellar environment. Both are spatially resolved with the VINCI observations, showing the excess to be circumbinary and showing the M star to have a size equal to its Roche Lobe. We conclude that we have, for the first time, directly resolved a star filling its Roche Lobe. The resulting mass transfer is probably the cause of (1) the circumbinary dust disk of which we see the hot inner region spatially resolved in our observations, (2) the unusually high luminosity of the A star and (3) the shell spectrum seen in the UV and optical spectra.

Motivation & Objective

• To directly measure the spatial dimensions of components in the close binary SS Leporis using infrared interferometry.
• To determine whether the M-type star is filling its Roche lobe, a key condition for mass transfer in binary evolution.
• To characterize the circumstellar environment, particularly the nature and geometry of the dust excess in the K band.
• To investigate the evolutionary impact of mass transfer on the A-type primary, including its enhanced luminosity and radius.
• To test the hypothesis of non-conservative mass transfer by analyzing the dust distribution and opacity properties.

Proposed method

• Near-infrared interferometric observations were conducted with VINCI/VLTI on baselines from 4.7 to 121 meters, covering K-band visibilities.
• Photometric data from UV to far-IR were compiled to construct the full Spectral Energy Distribution (SED).
• Geometric models were fitted to the interferometric visibility data to determine the angular size of the M star and circumstellar components.
• The SED was decomposed into contributions from the A star, M star, and dust shell/disk using flux decomposition techniques.
• Dust geometry was constrained by comparing observed K-band excess with theoretical optical depth models using amorphous silicate optical constants.
• The inclination and orbital parameters were used to infer the true size and mass transfer dynamics, assuming face-on disk geometry based on low extinction and lack of short-wavelength excess.

Experimental results

Research questions

• RQ1 Is the M-type star in SS Leporis filling its Roche lobe, as predicted by the semi-detached binary model?
• RQ2 What is the true physical size and structure of the M star and its circumstellar environment in the K band?
• RQ3 What is the geometry and optical depth of the circumstellar dust, and does it support a disk-like or spherical configuration?
• RQ4 How does mass accretion onto the A-type primary affect its radius and luminosity, and is this consistent with theoretical models?
• RQ5 Is the mass transfer process conservative, and what evidence supports or contradicts this based on the observed dust distribution?

Key findings

• The M-type star in SS Leporis was directly resolved and found to have a radius equal to its Roche lobe, confirming it is filling its zero-velocity surface.
• The K-band excess is dominated by the M star and circumstellar dust, with both components spatially resolved, indicating a circumbinary disk rather than a spherical shell.
• The dust geometry is best explained by a face-on, optically thick disk with a temperature of ~1250 K, consistent with a disk containing large grains (~0.1 mm) and a wind of smaller particles.
• The A-type primary has a radius of approximately 18 R⊙, significantly larger than a typical A1V star, due to mass accretion at a rate of ~2×10⁻⁴ M⊙/yr.
• The system's high luminosity and shell spectrum are attributed to ongoing non-conservative mass transfer, with the dust disk formed from material lost by the M star.
• The absence of strong short-wavelength excess rules out a hot, spherically symmetric circumstellar shell, supporting a disk-like structure seen nearly face-on.