A distant pair of stars orbiting so closely that their atmospheres merge has been found to be unexpectedly stable, challenging theories about when such systems should spiral toward a merger. The system, known as V717 Andromedae, was found to have a very low mass ratio, a characteristic that typically points to an unstable future. However, a new analysis indicates the binary is not a candidate for a stellar merger, forcing a refinement of the criteria that make these systems vulnerable to collapse.
The research, based on a multi-band photometric analysis, provides a detailed portrait of an eclipsing contact binary, a type of system where two stars share a common outer envelope of gas. Led by a team from Western Sydney University, the study examined the physical properties of V717 Andromedae and concluded that despite the lopsided masses of its two stars, it remains in a stable orbital configuration. The findings suggest that factors like stellar age and chemical composition play a more crucial role in determining the fate of contact binaries than previously thought, offering a more nuanced view of their evolution.
System Properties Revealed
Through detailed photometric observations, astronomers have painted a precise picture of the V717 Andromedae system. It is classified as an extreme low mass ratio contact binary, with the smaller star having a mass only 19.7% that of the primary star. This is a significant disparity in a class of binary systems where stars are physically touching. The two stars orbit each other with a high inclination of 84.5 degrees as viewed from Earth, meaning we see the system nearly edge-on, which produces pronounced eclipses as one star passes in front of the other.
The analysis also determined that the system has a moderate degree of contact, calculated at 27%. This figure describes how much the stars are overfilling their Roche lobes, the gravitational boundary that defines each star’s individual volume. Despite the large difference in mass, the two stars have nearly equal surface temperatures, with the secondary component measured at 5813 K. This temperature sharing across a common envelope is a hallmark of contact binaries.
A Question of Orbital Stability
Binary star systems with very low mass ratios have long been considered prime candidates for mergers. Theoretical models predict that below a certain mass ratio threshold, the orbital configuration becomes unstable, leading the smaller star to be absorbed by the larger one in an event that could produce a luminous red nova. V717 Andromedae, with its low ratio, appeared to fit this profile. However, the new orbital analysis demonstrates that the system is stable and not in danger of merging.
Calculating the Tipping Point
The researchers calculated that the specific instability mass ratio for V717 Andromedae would be in the range of 0.100 to 0.118. The system’s actual measured mass ratio of 0.197 is well above this critical threshold. This indicates that the gravitational and tidal forces are in a stable equilibrium. Recent research has shown there is no single, universal instability ratio for all contact binaries; instead, it varies for each system based on unique properties. Factors such as the system’s age and metallicity—the abundance of elements heavier than hydrogen and helium—can significantly alter the point at which a system becomes unstable. For older or more metal-poor stars, the instability range is even lower, making them more stable than younger, metal-rich systems.
Evidence for Chromospheric Activity
The study also uncovered strong evidence that V717 Andromedae is chromospherically active. This means the chromosphere, a layer in the stars’ atmospheres, shows signs of dynamic magnetic phenomena, such as spots or plages, similar to the sunspots on our own sun. While the system’s light curves do not show a significant O’Connell effect—a common phenomenon where the brightness maxima of the eclipses are unequal—other photometric markers point toward underlying magnetic activity. Such activity is common in close, tidally locked binary systems where the rapid rotation and convection within the stars can generate powerful magnetic fields.
Long-Term Observational Analysis
The conclusions were reached using a combination of new observations and historical data. The core of the research involved a multi-band photometric analysis, which measures the brightness of the star system in different colors or wavelengths of light. By observing how the light changes as the stars eclipse each other, astronomers can deduce their geometric and physical properties, including their masses, radii, temperatures, and the shape of their orbit.
To assess the system’s long-term stability, the team conducted a period analysis using survey photometry data collected over more than 6,300 days. This extensive dataset allowed them to check for any minute changes in the timing of the eclipses, which could indicate that the orbit is decaying or otherwise changing. The analysis revealed no significant period change over the nearly 17-year span, providing strong evidence that the orbit of V717 Andromedae is not shrinking and that the system is stable.