15 Cancri

15 Cancri is an α² CVn-type variable star in the zodiac constellation of Cancer, located around 980^[2] light years away. 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]

15 Cancri

A light curve for BM Cancri, plotted from TESS data[1]
Observation data Epoch J2000.0      Equinox J2000.0
Constellation	Cancer
Right ascension	08h 13m 08.87009s[2]
Declination	+29° 39′ 23.5777″[2]
Apparent magnitude (V)	5.53 - 5.65[3]
Characteristics
Spectral type	A0:V:spSiSr[4]
B−V color index	−0.073±0.002[5]
Variable type	α2 CVn[3]
Astrometry
Radial velocity (Rv)	25.0±0.8[6] km/s
Proper motion (μ)	RA: −12.959[2] mas/yr Dec.: −22.232[2] mas/yr
Parallax (π)	3.3279±0.2458 mas[2]
Distance	980 ± 70 ly (300 ± 20 pc)
Absolute magnitude (MV)	−0.97[5]
Orbit[6]
Primary	15 Cancri A
Companion	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
Luminosity	327.24[5] L☉
Surface gravity (log g)	3.22±0.2[7] cgs
Temperature	10,080±200[7] K
Rotational velocity (v sin i)	25[4] km/s
Other designations
15 Cnc, BM Cnc, BD+30°1664, HD 68351, HIP 40240, HR 3215, GSC 01939-01462[8]
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.^[9] The first orbit was calculated in 1973 by Helmut Abt and Michael Snowden with a period of 585 days^[10] 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.^[11][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.^[12] 15 Cancri was given the variable star designation BM Cancri in 1972 as a member of the α² CVn class of variable stars.^[13] The period has since been measured more accurately at 3.3095 d, believed to be the rotational period of the star.^[14]

Notes

1. An alternate period of 475 days is possible but less likely.