HD 1976 is a hierarchical triple system in the deep northern constellation of Cassiopeia, somewhere around 1,100 light-years (340 parsecs) from Earth. It has the variable-star designationV746 Cassiopeiae (abbreviated to V746 Cas). 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-typesubgiant 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 406±54 pc,[1]307±59 pc,[7] and 186±24 pc,[6] 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 9±5M☉ because of this uncertainty, and the masses of the individual stars were estimated from a mass ratio of 1.57±0.28 and an adopted[a] mass figure of 6.348 M☉.[6]
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.[4]
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 (4.6 Gyr[9]). It emits 70% of the total light from the system. Two solutions exist on its mass, namely 4.71 and 6.45 M☉. The latter, deduced excluding data near the Balmer lines,[4] seems to agree better with the newer 2019 estimate of 6.348 M☉.[8]
The Aa/Ab pair is part of an SB2 spectroscopic binary with HD 1976 B, denoting that the spectral lines from both components (A, B) are visible, and is itself an SB1 spectroscopic binary, meaning that only Aa's spectral lines are visible.[3]
A light curve for V746 Cassiopeiae, plotted from TESS data.[10] One of the dominant periods, 1.065 days, is marked in red.
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 (SPB). However, a 2017 study identified the two dominant photometric periods (2.503867 and 1.0649524 days) as the rotation periods of the tertiary and primary stars (albeit the latter is tentative), which, if confirmed, would throw the SPB classification into question.[4]
In 2014, it was announced that the star possessed a strong magnetic field, detected through spectropolarimetric observations,[3] though the magnetic field is now thought to belong to the third star (B) instead.[4]
HD 1976 Ab
The only component whose spectrum cannot be directly observed, HD 1976 Ab is in a nearly circular (eccentricity 0.05) 25-day orbit with Aa.[4] Radial velocity variations caused by the star were observed as far back as 1912,[11] and its orbital parameters were first determined in 1963.[12]
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 (1.87 M☉) and F-type main-sequence star (1.31 M☉).[4] In 2022, however, a far higher mass of 4.0±0.7M☉ was reported,[6] which resembles that of a late B-type main-sequence star.[b]
HD 1976 B
HD 1976 B is a B-type subgiant much like HD 1976 Aa but slightly less luminous, radiating about 30% of the total light from the system. It distantly orbits the inner Aa/Ab pair at a period of about 170 years.[4] Its orbit was first determined in 1986, though at the time the period was underestimated at 104 years.[14] The mass estimate differs significantly depending on the aforementioned inclusion of the Balmer lines: 2.65 M☉ if included and 6.10 M☉ if not. The star has a strong bipolar magnetic field that varies with a period of 2.504 days, which is most likely its rotational period.[4]
^ abHøg, E.; et al. (February 2000). "The Tycho-2 Catalogue of the 2.5 Million Brightest Stars". Astronomy and Astrophysics. 355 (1): L27–L30. Bibcode:2000A&A...355L..27H.
^Docobo, J. A.; Costa, J. M. (1986). "First orbits for the visual double stars ADS 328, ADS 9043, and ADS 13048". The Astrophysical Journal Supplement Series. 60. American Astronomical Society: 945. Bibcode:1986ApJS...60..945D. doi:10.1086/191106. ISSN0067-0049.
