HD 1976
Triple B-type subgiant system with disputed pulsation classification.
HD 1976, also designated V746 Cassiopeiae, is a hierarchical triple star system in the constellation Cassiopeia, located roughly 1100 light-years from Earth. It is faintly visible to the naked eye under dark skies with an apparent magnitude of 5.580, and is moving closer to the Solar System at a heliocentric radial velocity of −9.70 km/s.
Quick Facts
- Variable-star designation
- V746 Cassiopeiae (V746 Cas)
- Apparent magnitude
- 5.580
- Heliocentric radial velocity
- −9.70 km/s
- Orbital period (aa/ab)
- 25 days
Facts from the source article.
Measurement discrepancies
Distance measurements to the system differ substantially and carry large errors; two agree within their error bars, while a third is considered too low due to the noisy radial velocity orbit affecting the semi-amplitude. The total mass of the inner pair was poorly constrained in a 2022 study because of this uncertainty, with individual masses estimated from a mass ratio and an adopted figure of 6.348. Stellar parameter estimates from spectroscopy also vary depending on whether Balmer line data (H-β, H-γ, H-δ) are included, as those lines are often affected by instrumental problems and rectification systematics.
Stellar components
HD 1976 Aa is a B-type subgiant (B5IV) about 60 million years old, emitting 70% of the system's total light. Two mass solutions exist: 4.71 and 6.45, with the latter, derived excluding Balmer line data, agreeing better with a 2019 estimate of 6.348. The Aa/Ab pair is an SB1 spectroscopic binary (only Aa's lines visible) and together with HD 1976 B forms an SB2 binary (lines of both A and B visible). HD 1976 was identified as a variable star from Hipparcos data and designated in 1999, showing multiperiodic pulsations with periods from 0.83 to 2.50 days, leading to a classification as a slowly pulsating B-type star. However, a 2017 study identified two dominant photometric periods (2.503867 and 1.0649524 days) as rotation periods of the tertiary and primary stars, which would challenge the SPB classification. A strong magnetic field was announced in 2014 from spectropolarimetric observations, but is now thought to belong to the third star (B). HD 1976 Ab, whose spectrum cannot be directly observed, orbits Aa in a nearly circular 25-day orbit (eccentricity 0.05); its radial velocity variations were noted as early as 1912, with orbital parameters first determined in 1963. Its physical properties are very uncertain: a 2017 study gave two sets of characteristics (A-type at 1.87 and F-type at 1.31), while a 2022 study reported a far higher mass resembling a late B-type main-sequence star.
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