Is NP Aqr a new near-contact binary?

TL;DR

This study analyzes the spectroscopic and photometric data of NP Aqr, revealing it as a near-contact binary with a tertiary component, and provides detailed stellar parameters and evolutionary insights.

Contribution

First detailed analysis of NP Aqr as a near-contact binary with a tertiary star, including spectroscopic and light curve data, and comparison with evolutionary models.

Findings

NP Aqr is a non-eclipsing near-contact binary.

Presence of a tertiary star with similar properties to the primary.

Discrepancy in secondary star's location in the HR diagram.

Abstract

We present radial velocities of the double-lined spectroscopic binary NP Aqr. The radial velocities and the optical light curves obtained by Hipparcos and ASAS-3 are analyzed separately. The masses of the primary and secondary components have been found to be 1.65$\pm$0.09 and 0.99$\pm$0.05 M$_{\odot}$, respectively. The cross-correlation functions indicate triple peaks which show presence of a tertiary star. The spectroscopic properties of this additional component resemble to that of the primary star. The analysis of the light curves yielded that the more massive primary star fills its corresponding Roche lobe. The secondary component is at or near Roche lobe indicating a new $\beta$ Lyrae-type near-contact binary. The orbital inclination is about 40$^{\circ}$ and, therefore, the observed light variations are produced only by the proximity effects. Due to the absence of eclipses, the effective temperature of the secondary star and the radii of the components could not be determined accurately. We conclude that NP Aqr is a non-eclipsing double-lined spectroscopic binary with a distance of about 134$\pm$7 pc. The absolute parameters of the components are also compared with the evolutionary models. While the location of the primary star seems to be suitable with respect to its mass in the Hertzsprung-Russell diagram, the secondary component is located as if a star having a mass less than 0.6 M$_{\odot}$. This discrepancy is originated from very low effective temperature determined only from the light curve produced by proximity effects. The distance to the third star appears to be very close to that of the close binary which indicates that it may be dynamically bounded to the binary.