HD 1976
HD 1976 is a hierarchical triple system in the deep northern constellation of Cassiopeia, somewhere around from Earth. It has the variable-star designation V746 Cassiopeiae. The system is faintly visible to the naked eye under dark skies, having an apparent magnitude of 5.580. It consists of an inner pair between a B-type subgiant and a less massive unknown-type star, which is distantly orbited by another B-type subgiant. It is currently moving closer towards the Solar System at a heliocentric radial velocity of −9.70 km/s.
Measurement discrepancies
Several measurements have been made on the distance to the star system, namely,, and, but all of them have large errors and differ substantially from one another. The first two agree within the wide error bars, while the third value is thought to be too low due to the noisy radial velocity orbit swaying the semi-amplitude. In a 2022 study, the total mass of the inner pair could only be constrained poorly at because of this uncertainty, and the masses of the individual stars were estimated from a mass ratio of and an adopted mass figure of 6.348.Stellar parameter estimates via astronomical spectroscopy also yield different results depending on whether the spectral data near the Balmer lines H-β, H-γ, and H-δ is included in calculations, which are often affected by instrumental problems and rectification systematics.
Stellar components
HD 1976 Aa
HD 1976 Aa is a B-type subgiant star with a spectral type of B5IV. It is thought to be very young, at only about 60 million years old, a little over one-eightieth the age of the Sun. It emits 70% of the total light from the system. Two solutions exist on its mass, namely 4.71 and 6.45. The latter, deduced excluding data near the Balmer lines, seems to agree better with the newer 2019 estimate of 6.348.The Aa/Ab pair is part of an SB2 spectroscopic binary with HD 1976 B, denoting that the spectral lines from both components are visible, and is itself an SB1 spectroscopic binary, meaning that only Aa's spectral lines are visible.
File:V746CasLightCurve.png|thumb|left|A light curve for V746 Cassiopeiae, plotted from TESS data. One of the dominant periods, 1.065 days, is marked in red.
HD 1976 was found to be a variable star when the Hipparcos data was analyzed. It was given its variable star designation in 1999. The star was reported to show multiperiodic pulsations with periods ranging between 0.83 and 2.50 days. As such, the star was classified as a slowly pulsating B-type star. However, a 2017 study identified the two dominant photometric periods as the rotation periods of the tertiary and primary stars, which, if confirmed, would throw the SPB classification into question.
In 2014, it was announced that the star possessed a strong magnetic field, detected through spectropolarimetric observations, though the magnetic field is now thought to belong to the third star instead.
HD 1976 Ab
The only component whose spectrum cannot be directly observed, HD 1976 Ab is in a nearly circular 25-day orbit with Aa. Radial velocity variations caused by the star were observed as far back as 1912, and its orbital parameters were first determined in 1963.As is the case with the other two stars, its physical properties are very uncertain. A 2017 study presented two sets of possible characteristics, each corresponding to an A-type and F-type main-sequence star. In 2022, however, a far higher mass of was reported, which resembles that of a late B-type main-sequence star.