HD 166473
A slowly rotating roAp star with an extremely strong magnetic field.
HD 166473, known by its variable star designation V694 Coronae Australis, is a type of rapidly oscillating Ap star (roAp star) and an α2 CVn variable. It lies roughly 455 light-years away in the southern constellation Corona Australis. At an apparent magnitude of 7.953, it is invisible to the naked eye but can be spotted with binoculars.
Like most roAp stars, HD 166473 has an overabundance of rare-earth elements, chromium, and cobalt, while its iron and nickel levels are similar to the Sun's. It is deficient in carbon and oxygen. Its magnetic field is among the strongest of any Ap star, reaching up to about ten thousand times the strength of Earth's magnetic field (which ranges from 0.25 to 0.65 G). The field's strength and orientation strongly influence where specific elements settle in the star's atmosphere. This effect shows up in radial velocity variations from pulsations: rare-earth elements display amplitudes as high as 110 m/s, whereas elements like iron show no variation at all.
The star is also an extremely slow rotator, taking more than a decade to spin once on its axis—compared to the Sun's rotation period of about 25.38 days. Despite this slow rotation, its magnetic properties are essentially the same as those of faster-rotating Ap stars.
Oscillations in HD 166473 were first detected in 1987, revealing three distinct frequencies with periods between 8.8 and 9.1 minutes and low amplitudes from 0.25 to 0.49 millimagnitudes. Because these frequencies are not evenly spaced, the oblique pulsator model cannot explain them—an unusual situation, as that model typically works well for roAp stars. In 2003, observations using the VLT resolved these pulsations into standing and travelling waves in the stellar atmosphere, making HD 166473 the first star other than the Sun for which such waves were observed. In 2020, its rotation period was determined to be 3836 days, making it only the fourth Ap star with a rotation period exceeding ten years whose magnetic field has been observed through an entire cycle or more.
- Variable star designation
- V694 Coronae Australis (V694 CrA)
- Constellation
- Corona Australis
- Apparent magnitude
- 7.953
- Distance
- ~455 ly
- Rotation period
- 3836 d
- Magnetic field strength
- Up to ~10,000 times Earth's magnetic field (0.25–0.65 G)
- Pulsation periods
- 8.8–9.1 minutes
Lore & Background
HD 166473 is thought to have an overabundance of rare-earth elements, chromium, and cobalt, with solar-like levels of iron and nickel, and deficiencies in carbon and oxygen. Its magnetic field is among the strongest of any Ap star, and its strength and orientation are strongly correlated with the atmospheric layer where specific elements reside. This is evident in radial velocity variations caused by pulsations: rare-earth elements show amplitudes up to 110 m/s, while other elements like iron show no variation at all.
Oscillations were first discovered in 1987, revealing three distinct frequencies with periods between 8.8 and 9.1 minutes and low amplitudes of 0.25 to 0.49 millimagnitudes. The frequencies are not equally spaced, so the oblique pulsator model cannot explain them, which is unusual since that model generally works for roAp stars. In 2003, the pulsations were further resolved into standing and travelling waves in the stellar atmosphere using the VLT, making HD 166473 the first star other than the Sun for which such observations were conducted.
In 2020, the rotation period was determined to be 3836 days, making it only the fourth Ap star with a rotational period exceeding ten years whose magnetic field has been observed for an entire cycle or more. Despite its extremely slow rotation, its magnetic properties are essentially the same as those of faster-rotating Ap stars.
Reader's Guide
HD 166473 holds significance as a benchmark for understanding the interplay between strong magnetic fields, chemical peculiarity, and pulsation in roAp stars. Its extreme magnetic field—roughly ten thousand times stronger than Earth's—and its decade-long rotation period make it a rare laboratory for studying how magnetic fields influence atmospheric structure and element stratification. The correlation between magnetic field orientation and the radial velocity amplitudes of different elements provides direct observational evidence of chemical stratification in a stellar atmosphere. The 2003 detection of standing and travelling waves in its atmosphere marked a milestone, as it was the first star beyond the Sun where such wave behavior could be resolved. The failure of the oblique pulsator model to explain its pulsation frequencies challenges a standard theoretical framework, prompting further refinement of models for roAp stars. Its legacy includes being one of only a handful of Ap stars with a rotation period over ten years for which a full magnetic cycle has been observed, offering a unique window into the long-term behavior of magnetic fields in chemically peculiar stars.
Did You Know?
- HD 166473 has one of the strongest magnetic fields of any Ap star, about ten thousand times stronger than Earth's magnetic field.
- It takes over a decade to rotate once on its axis, compared to the Sun's 25.38-day rotation period.
- In 2003, it became the first star other than the Sun for which standing and travelling waves were resolved in its atmosphere.
Frequently Asked Questions
What is HD 166473?
HD 166473 is a rapidly oscillating Ap star (roAp) catalogued under the variable designation V694 Coronae Australis. It sits in the southern constellation Corona Australis and is also classified as an α2 CVn variable.
How strong is HD 166473's magnetic field?
Its magnetic field reaches roughly 0.25 to 0.65 gauss, which is up to about ten thousand times stronger than Earth's own field. This makes it one of the most magnetically intense Ap stars known.
Can I see HD 166473 with the naked eye?
No; at an apparent magnitude of 7.953 it is far too faint for unaided vision. However, a pair of decent binoculars will reveal it against the night sky in Corona Australis.
What is unusual about HD 166473's chemical composition?
The star shows a pronounced overabundance of rare-earth elements, chromium, and cobalt, while its carbon and oxygen levels are notably depleted. Iron and nickel sit at roughly solar proportions, a pattern typical of roAp stars.
How long does HD 166473 take to complete one rotation?
Its rotation period is approximately 3,836 days, making it a comparatively slow rotator among roAp stars. That sluggish spin, combined with its extreme magnetic field, sets it apart from faster-rotating members of the class.
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