15 Cancri
Star in the constellation Cancer
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15 Cancri is an α2 CVn-type variable star in the zodiac constellation of Cancer, located around 700 light years away.[1] It has the variable star designation BM Cancri (BM Cnc); 15 Cancri (15 Cnc) is the Flamsteed designation. This system is visible to the naked eye as a faint, white-hued star with an apparent visual magnitude of about 5.6. It is moving away from the Earth with a heliocentric radial velocity of 25 km/s.[6]
 | |
| Observation data Epoch J2000.0 Equinox J2000.0 | |
|---|---|
| Constellation | Cancer |
| Right ascension | 08h 13m 08.86806s[1] |
| Declination | +29° 39′ 23.5368″[1] |
| Apparent magnitude (V) | 5.53 - 5.65[2] |
| Characteristics | |
| Evolutionary stage | main sequence[3] |
| Spectral type | A0:V:spSiSr[4] |
| B−V color index | −0.073±0.002[5] |
| Variable type | α2 CVn[2] |
| Astrometry | |
| Radial velocity (Rv) | 25.0±0.8[6] km/s |
| Proper motion (μ) | RA: −11.265[1] mas/yr Dec.: −19.506[1] mas/yr |
| Parallax (π) | 4.6720±0.2238 mas[1] |
| Distance | 700 ± 30 ly (210 ± 10 pc) |
| Absolute magnitude (MV) | −0.97[5] |
| Orbit[6] | |
| Primary | 15 Cancri A |
| Name | 15 Cancri B |
| Period (P) | 635.73±0.53 days[note 1] |
| Eccentricity (e) | 0.55±0.08 |
| Periastron epoch (T) | 2422045±18 |
| Argument of periastron (ω) (primary) | 196±13° |
| Semi-amplitude (K1) (primary) | 9.6±1.1 km/s |
| Details | |
| 15 Cnc A | |
| Mass | 2.4[3] M☉ |
| Radius | 3.4[3] R☉ |
| Luminosity | 131[3] L☉ |
| Surface gravity (log g) | 3.68[3] cgs |
| Temperature | 10,400[3] K |
| Rotation | 6.563 days[7] |
| Rotational velocity (v sin i) | 25[4] km/s |
| Age | 358[8] Gyr |
| Other designations | |
| 15 Cnc, BM Cnc, BD+30°1664, HD 68351, HIP 40240, HR 3215, GSC 01939-01462[9] | |
| Database references | |
| SIMBAD | data |
Radial velocity measurements taken at the Dominion Astrophysical Observatory in Victoria, British Columbia Canada in 1918 and 1919 led to the determination that 15 Cancri is a single-lined spectroscopic binary system.[11] The first orbit was calculated in 1973 by Helmut Abt and Michael Snowden with a period of 585 days[12] however later measurements showed that the orbital period was 635 days.[6][note 1]
15 Cancri A, the visible component, is an Ap star, a chemically peculiar star with an over-abundance of iron peak elements, particularly silicon, chromium, and strontium, in its spectrum.[13][4]
Like all Ap stars, 15 Cancri has a strong magnetic field. This magnetic field varies as it rotates and in 1968 the visual brightness of the star was shown to vary regularly over about four days.[14] 15 Cancri was given the variable star designation BM Cancri in 1972 as a member of the α2 CVn class of variable stars.[15] The period has since been measured more accurately at 3.3095 d, believed to be the rotational period of the star.[16]
In Johann Bayer’s Uranometria, this star was designated as Psi Geminorum. When the constellation borders were set in 1930, it was moved to Cancer, so the designation has been dropped.[17]
Notes
- An alternate period of 475 days is possible but less likely.