DH Cephei is a variable binary star[5] system in the northern circumpolar constellation of Cepheus, positioned about two degrees to the east of the star system Delta Cephei.[10] With an apparent visual magnitude of 8.61,[3] it is too faint to be visible without a telescope. Based on parallax measurements, this system is located at a distance of approximately 9.6 kilolight-years (2.9 kiloparsecs) from the Sun.[2] At present it is moving closer to the Earth with a radial velocity of −33 km/s.[6]

DH Cephei

DH Cephei is at the center of this image of the NGC 7380 complex[1]
Observation data Epoch J2000.0      Equinox J2000.0
Constellation	Cepheus
Right ascension	22h 46m 54.111s[2]
Declination	+58° 05′ 03.53″[2]
Apparent magnitude (V)	8.61[3] (8.63 – 8.70)[4]
Characteristics
Evolutionary stage	Main sequence
Spectral type	O5.5 V + O6 V[5]
B−V color index	0.334±0.041[6]
Variable type	Ellipsoidal[7][4]
Astrometry
Radial velocity (Rv)	−33.4±3.2[6] km/s
Proper motion (μ)	RA: −2.599 mas/yr[2] Dec.: −2.236 mas/yr[2]
Parallax (π)	0.3397 ± 0.0138 mas[2]
Distance	9,600 ± 400 ly (2,900 ± 100 pc)
Absolute magnitude (MV)	−4.66±0.25 (A) −4.55±0.25 (B)[8]
Orbit[3]
Period (P)	2.11095 d
Semi-major axis (a)	≥9.79±0.17 R☉
Eccentricity (e)	0.0 (fixed)
Periastron epoch (T)	2,456,525.564±0.006 HJD
Details[3]
A
Mass	25.0[9] or 38.4±2.5 M☉
Radius	8.31[9] R☉
Luminosity	2.34×105[9] L☉
Surface gravity (log g)	4.3±0.3 cgs
Temperature	44,000 K
Rotational velocity (v sin i)	175 km/s
B
Mass	16.8[9] or 33.4±2.2 M☉
Radius	7.76[9] R☉
Luminosity	1.86×105[9] L☉
Surface gravity (log g)	4.3±0.2 cgs
Temperature	43,000 K
Rotational velocity (v sin i)	160 km/s
Other designations
DH Cep, BD+57° 2607, HD 215835, HIP 112470, WDS 22469+5805[1]
Database references
SIMBAD	data

This is a double-lined spectroscopic binary system consisting of two near-identical, massive, O-type main sequence stars.[5] Evolutionary tracks place the stars close to the zero age main sequence, with an age of less than two million years.[12] This is a detached binary[12] with a close orbit having a period of 2.11 days, and the orbit is assumed to have circularized. The orbital plane is estimated to be inclined by an angle of 47°±1° to the line of sight from the Earth, which yields mass estimates of 38 and 34 times the mass of the Sun.[3] Although initially suspected to be an eclipsing binary and given a variable star designation,[8] it doesn't appear to be eclipsing.[3] Instead, the system displays ellipsoidal light variations that are caused by tidal distortions.[7]

This system lies at the center of the young open cluster NGC 7380. It is the primary ionizing source for the surrounding H II region designated S142. The pair are a source of X-ray emission, which may be the result of colliding stellar winds.[13] Their measured X-ray luminosity is 3.2×10³¹ erg s⁻¹.[9] The location and rare class of these stars make them an important object for astronomical studies.[8]

References