OU Andromedae
A fast-rotating giant star with X-ray flaring activity.
OU Andromedae, also cataloged as HR 9024, is a rotationally variable star located in the constellation Andromeda. Its brightness shifts between magnitudes 5.87 and 5.94, and it has been tentatively classified as an FK Comae Berenices variable, though this classification remains uncertain. The star is a G1IIIe giant, meaning it shows emission lines in its spectrum, and it is currently on the subgiant branch, contracting across the Hertzsprung gap toward the red giant branch.
The star's variability was discovered in 1985 by Jeffrey Hopkins and colleagues, who found a period of roughly 23.3 days. It received the variable star designation OU Andromedae the following year. In 2007, Paola Testa and others reported that the star exhibits X-ray flare activity.
OU Andromedae spins unusually fast for an evolved star of its type, with a projected rotational velocity of 21.5 km/s. One possible explanation is that it may have swallowed a nearby giant planet, such as a hot Jupiter, as an infrared excess has been observed. Another explanation involves its strong magnetic field: if the star was an Ap star during its main-sequence phase, it could have retained both the fast rotation and the strong magnetic field typical of such stars.
The star is a bright X-ray source, a result of coronal activity driven by its strong magnetic field. It is estimated that solar-like active regions cover about 30% of its surface. This magnetic field produces continuous flaring, generating a large volume of hot plasma at coronal temperatures around 7.5 million Kelvin.
- Magnitude range
- 5.87 – 5.94
- Spectral classification
- G1IIIe
- Variable type
- FK Comae Berenices (uncertain)
- Period
- ~23.3 days
- Projected rotational velocity
- 21.5 km/s
- Coronal temperature
- ~7.5×10^6 K
- Discovery year
- 1985
- Designation year
- 1986
Lore & Background
OU Andromedae was discovered to be a variable star in 1985 by Jeffrey Hopkins et al., with a period of approximately 23.3 days. It received the variable star designation OU Andromedae in 1986. The star has a spectral classification of G1IIIe, indicating it is a giant star that shows emission lines in its spectrum. It is considered to be on the subgiant branch, contracting across the Hertzsprung gap towards the red giant branch.
The star's spin rate is unusually high for an evolved star of this type, with a projected rotational velocity of 21.5 km/s. One possible explanation is that it may have engulfed a nearby giant planet, such as a hot Jupiter, since an infrared excess has been observed. Another explanation relies on its strong magnetic field; if OU Andromedae was an Ap star during its main sequence stage, it could have retained both the strong magnetic field and the fast rotation of Ap stars.
OU Andromedae is a bright X-ray source due to the activity of its corona; it is estimated that solar-like active regions cover 30% of the surface. This is another effect of the strong magnetic field, which produces an uninterrupted flaring activity that generates a large volume of hot plasma at coronal temperatures of approximately 7.5×10^6 K. Paola Testa et al. reported that the star showed X-ray flare activity in 2007.
Reader's Guide
OU Andromedae's significance lies in its combination of rapid rotation, strong magnetic field, and X-ray activity, which challenge standard models of stellar evolution for giant stars. Its classification as an FK Comae Berenices variable remains uncertain, reflecting ongoing debate about the nature of such stars. The star's fast spin—projected at 21.5 km/s—is anomalous for an evolved G1IIIe giant, and two competing hypotheses have been proposed: engulfment of a hot Jupiter (supported by an observed infrared excess) or a retained Ap-star magnetic field from its main-sequence phase. The strong magnetic field drives continuous coronal flaring, producing hot plasma at ~7.5×10^6 K and covering 30% of the surface with solar-like active regions. This makes OU Andromedae a bright X-ray source and a valuable laboratory for studying magnetic activity in post-main-sequence stars. The discovery of its variability by Jeffrey Hopkins et al. in 1985 and the subsequent X-ray flare report by Paola Testa et al. in 2007 highlight its long-term observational interest. Its legacy includes informing models of planet engulfment, magnetic field retention, and coronal heating in evolved stars.
Did You Know?
- OU Andromedae varies between magnitudes 5.87 and 5.94 with a period of ~23.3 days.
- Its projected rotational velocity of 21.5 km/s is unusually high for an evolved giant star.
- The star's strong magnetic field produces uninterrupted flaring and hot plasma at ~7.5×10^6 K.
- One hypothesis for its fast rotation is that it may have engulfed a hot Jupiter.
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