Orbital solutions study of Subdwarf B wide binary systems

Subdwarf B (sdB) stars are post main-sequence stars that paradoxically are small (0.10-0.25 solar radii), low mass (~0.5 solar masses), very hot (20,000 - 40,000 K), and extremely hydrogen deficient, being composed almost entirely of helium. These exotic objects are the dominant hot stellar population in globular clusters, spiral bulges, and elliptical galaxies, and are thought to form in binary systems through common envelope ejection (CEE), Roche-lobe overflow (RLOF), or white-dwarf mergers (WD2).

Of the three formation "channels" only the CEE is easily observed using radial velocity measurements due to their short (hours to few-day) orbital periods. A research team at Embry-Riddle Aeronautical University will use small (1-m) telescopes to measure periodic arrival time variations in the pulsations of sdB stars, the same approach used by Ole Romer in 1676 to determine the speed of light, to increase the number of confirmed long period sdB systems and solitary sdB stars to gauge the formation channels.

This technique is also capable of detecting Jupiter-like planets should they exist in these systems. This research will support two undergraduate researchers and one graduate student.

To improve theoretical models of sdB star formation and evolution many more long-period sdB binary systems with a variety of orbital parameters will be needed. However, this would require many nights on large telescopes for an accurate radial velocity analysis. The pulsation timing method is much more efficient approach as 30% of sdB stars exhibit stable pulsations, with periods (p-mode) ranging from 1.5 - 5 minutes.

Pulse arrival times change periodically as the sdB star’s reflex motion is manifested by the changing distance along the line of sight. This method is particularly effective for finding orbital solutions for long period binary (or planetary) systems, so this can be efficiently used for observational studies for RLOF and white-dwarf merger (WD2) channels.

Frequent observations with 1-m class telescope are required, but the research team has access to the three 1-m telescopes of the Southeastern Association for Research in Astronomy (SARA) Observatory as well as a 1-m telescope on the ERAU campus. They will also search for pulsation period changes resulting due to binary evolution to estimate the current stage of each sdB star.

The results will lead to a better understanding of sdB star formation and so a more complete understanding of the stellar populations in our own and other galaxies.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.