A new study of the spectroscopic binary 7 Vul with a Be star primary

TL;DR

This study confirms the binary nature of 7 Vul, refines its orbital parameters, analyzes spectral and photometric variations, and suggests the secondary may be a hot subdwarf, contributing new insights into Be star binaries.

Contribution

It provides the first detailed spectroscopic orbit of 7 Vul and proposes the secondary as a potential hot subdwarf, expanding understanding of Be star binary systems.

Findings

Confirmed binary nature of 7 Vul with a 69.42-day orbit.

Detected long-term spectral variations and rapid light changes.

Suggested the secondary might be a hot subdwarf star.

Abstract

We confirmed the binary nature of the Be star 7~Vul, derived a~more accurate spectroscopic orbit with an orbital period of (69.4212+/-0.0034) d, and improved the knowledge of the basic physical elements of the system. Analyzing available photometry and the strength of the \ha emission, we also document the long-term spectral variations of the Be primary. In addition, we confirmed rapid light changes with a~period of 0.5592 d, which is comparable to the expected rotational period of the Be primary, but note that its amplitude and possibly its period vary with time. We were able to disentangle only the He I 6678 A line of the secondary, which could support our tentative conclusion that the secondary appears to be a hot subdwarf. A search for this object in high-dispersion far-UV spectra could provide confirmation. Probable masses of the binary components are ($6\pm1$)~Mnom \ and ($0.6\pm0.1$)~Mnom. If the presence of a hot subdwarf is firmly confirmed, 7 Vul might be identified as a rare object with a B4-B5 primary; all Be + hot subdwarf systems found so far contain B0-B3 primaries.