# # Program information file # PROGRAM_ID 2022A070 PROGRAM_TITLE Pinpointing the Common Envelope Phase with Precise Masses for HZ 9 PROGRAM_INV1 Philip Muirhead PROGRAM_INV2 Jason Nordhaus PROGRAM_INV3 Maria Drout PROGRAM_INV4 Eunkyu Han PROGRAM_INV5 PROGRAM_SCICAT stellar PROGRAM_ABSTRACT_BEG Binary star evolution models predict that a large fraction of stellar systems will undergo common envelope evolution --- a phase that dramatically shrinks the orbit and changes the binary's ultimate fate. Despite its importance, current theoretical models of the common envelope phase are highly uncertain. Direct observational constraints on the outcome of common envelope evolution for a binary system with a given set of initial conditions are non-existent. The MS-WD binary HZ 9 is a post-common envelope benchmark that provides a critical test of common envelope evolution. As a verified member of the Hyades cluster, the age of the system is well constrained. With additional radial velocity observations of the M-type main sequence component and a measurement of the stellar vsini, we can measure the white dwarf mass to a precision of 0.01 solar masses, and constrain the zero-age-main-sequence progenitor mass to within 0.05 solar masses. This translates to a very tight constraint on the binding energy at the time of common envelope. Thus, HZ 9 would be the first post-CE system to be linked to its pre-CE state and serve as the only benchmark theorists currently have. We request 8 hours of observations of HZ 9 and a calibrator star, spread over 4 consecutive nights in order to adequately sample the radial velocity curve and measure the dM vsini. PROGRAM_ABSTRACT_END