The eta Aquilae System: Radial Velocities and Astrometry in Search of eta Aql B

TL;DR

This study combines radial velocity and astrometric data to investigate the eta Aquilae system, aiming to detect and characterize its companion eta Aql B, but finds no definitive evidence of orbital motion due to observational limitations.

Contribution

It provides a comprehensive analysis of eta Aql's radial velocities and astrometry, applying Gaia EDR3 data, and explores possible properties of eta Aql B despite inconclusive orbital evidence.

Findings

No clear orbital motion detected in radial velocities.

Astrometric analysis suggests possible long-period companion.

Consistent parallax and proper motion with Gaia EDR3 and previous studies.

Abstract

The classical 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. To determine the orbit of eta Aql B, we analyze a significant number of radial velocity measures (RV) from eight sources. With these we establish the RV variation due to Cepheid pulsation, using a twelve Fourier coefficient model, while solving for velocity offsets required to bring the RV data sets into coincidence. RV residuals provide no evidence of orbital motion, suggesting either nearly face-on orientation or very long period. Reanalysis of Hubble Space Telescope Fine Guidance Sensor astrometry now includes reference star parallax and proper motion priors from Gaia EDR3. As modeling confirmation, we reanalyze zeta Gem in parallel, deriving zeta Gem parallax and proper motion values consistent with Gaia EDR3, and consistent with the Benedict 2007 Leavitt Law. In an effort to further characterize eta Aql B, we hypothesize that eta Aql residuals larger than those of the associated reference stars or a parallax inconsistent with EDR3 and the Benedict 2007 Leavitt Law indicate unmodeled orbital motion. Using the astrometric noise or parallax mismatch with EDR3 we estimate possible periods and mass for eta Aql B. Ascribing photocenter motion to the photometric variation of the Cepheid, eta Aql A, yields a plausible separation, consistent with a long period, explaining the lack of RV variation. None of these approaches yields an unassailable characterization of the eta Aql A-B system