Abstract

The classic Cepheid eta Aql was not included in past Leavitt Law work (Benedict et al. 2007) because of a presumed complicating orbit due to a known B9.8V companion. In an effort to determine an orbit and mass for eta Aql B, and obtain a parallax for eta Aql, we analyze a significant number of radial velocity measures (RV) from eight sources, including four more recent. With these we establish the radial velocity variation due to Cepheid pulsation, using a twelve Fourier coefficient  description of the Cepheid pulsation, while solving for velocity offsets required  to bring the eight RV data sets into coincidence.  Residuals to the Cepheid pulsation model provide no evidence of significant orbital motion, suggesting a nearly face on A-B orbit orientation. Our astrometric re-analysis of Hubble Space Telescope Fine Guidance Sensor data now includes reference star parallax and proper motion priors from Gaia EDR3. A model without EDR3 priors for eta Aql yields parallax and proper motion values inconsistent with EDR3, and the Benedict 2007 Leavitt Law. Large  residuals  hint at unmodeled orbital motion.  However, we have an insufficient number of epochs with which to establish an orbit. Introducing EDR3 parallax and proper motion priors reduces the residuals and produces a parallax in agreement with EDR3.We hypothesize an eta Aql A-B orbit with a one year period. We use the parallax mismatch to estimate a perturbation size, and find a mass for eta Aql B agreeing with  past estimates.