Model parameters for the CU Hya system The omegas and f describe the surface structures of the two stars relative to their Roche lobes. The r features describe the radii of the stars (towards and away from each other, and normal to their line of centres).
Briefly, the more massive star evolves off the main sequence first, swelling in the sub-giant phase until it overflows its inner critical surface or Roche lobe, depositing the overflowed mass on the secondary.
The "radii" of both components, under these circumstances, would include the filled Roche lobes
, as the system is an over contact system.
Since the only other available supply of angular momentum is the binary's orbit, the stars become closer and their Roche lobes shrink.
What is a star's Roche lobe? Roche lobes form a unique gravitational "surface" surrounding any two objects that orbit one another.
As a result, it eventually overflows the latter's Roche lobe as well.
Key factors in their classification are the position of the components relative to the barycentre of the system, their colour, magnitude and in particular the degree to which their Roche lobes have been filled.
Its short period of less than one day, plus the degree to which the components fill their respective Roche lobes, results in a classification of near-contact binary that may possess close evolutionary connections with the W UMa systems.1 In these systems the onset of eclipses is difficult to pinpoint exactly from their lightcurves due to the component stars' ellipsoidal shapes, resulting from their mutually strong gravitational interaction.
Figure 5 shows the Roche lobes
of V448 Cen at phase 0.15.
The binary system is at an inclination of 72[degrees] and is in a close overcontact configuration in which both stars fill their Roche lobes
and share a common envelope.
It is also possible to have contact or common-envelope systems, where both stars overflow their Roche lobes.
In close binaries (or interacting binaries), which have the shortest orbital periods, a time will come when one star becomes a giant or supergiant and fills its Roche lobe. This is the point around a star (for our purposes) where the gravitational field is constant and any particle at this point can fall equally either back onto its parent star or onto the companion star.