SY Equulei information

SY Equulei
	; A light curve for SY Equulei, plotted from Hipparcos data
Observation data; Epoch J2000.0 Equinox J2000.0 (ICRS)
Constellation	Equuleus
Right ascension	21h 23m 28.8086s
Declination	+09° 55′ 54.9204″
Apparent magnitude (V)	8.52 - 8.58
Characteristics
Spectral type	B0.5 IIIn
U−B color index	−1.00
B−V color index	−0.20
Variable type	β Cephei
Astrometry
Radial velocity (Rv)	48±5.5 km/s
Proper motion (μ)	RA: −0.185 mas/yr ; Dec.: −9.049 mas/yr
Parallax (π)	0.3869 ± 0.0558 mas
Distance	approx. 8,000 ly ; (approx. 2,600 pc)
Absolute bolometric; magnitude (Mbol)	−6.04
Details
Mass	14 M☉
Radius	10.44 R☉
Luminosity	2,490 L☉
Surface gravity (log g)	3.8±0.2 cgs
Temperature	28,184+1,328; −1,268 K
Metallicity [Fe/H]	0.00 dex
Rotational velocity (v sin i)	180 km/s
Age	~7.5 Myr
Other designations
	SY Equueli, AG+09°2978, BD+09°4793, HD 203664, HIP 105614, SAO 126757
Database references
SIMBAD	data

SY Equulei, also known as HD 203664, is a single variable star located in the equatorial constellation Equuleus. It has an average apparent magnitude of about 8.5, varying by a few hundredths of a magnitude, making it readily visible in binoculars and small telescopes, but not to the naked eye. The star is relatively far away at a distance of 8,000 light years^[2] and is receding with a heliocentric radial velocity of 48 km/s.^[7] At that distance, SY Equulei is dimmed by 0.19 magnitudes due to interstellar dust.^[16]

Throughout the late 20th century, HD 203664 was known to have a dust cloud surrounding it. Subsequent observations from Kenneth R. Sembach (1995) reveal it to contain high abundances of calcium as well as traces of magnesium, aluminum, and silicon.^[17] The cloud probably came from outside the galactic plane and is moving towards the star at a rate of 70 km/s.^[17] The star has a high galactic latitude, indicating its location in the galactic halo. HD 203664 was most likely ejected from its birthplace to its current distance.^[9] However, its status as a Beta Cephei variable wasn't discovered until a survey of 2000 using Hipparcos data.^[8] It was then given the designation SY Equulei.^[18]

SY Equulei has a stellar classification of B0.5 IIIn,^[4] indicating an evolved B-type star with nebulous (broad) absorption lines due to rapid rotation. Unlike most stars of its type, it spins rapidly with a projected rotational velocity of 180 km/s,^[14] which is 40% of its break-up velocity.^[12] It has 14 times the mass of the Sun^[9] and a radius of 10.4 radius.^[10] It radiates at 2,490 times the luminosity of the Sun^[11] from its photosphere at an effective temperature of 28,184 K,^[12] giving a whitish blue hue.

SY Equueli is a variable star with an amplitude of 0.07 magnitudes^[6] and an average period of 3.98 hours.^[19] In later observations, SY Equulei was found to have multiple periods.^[12]

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Observation data Epoch J2000.0 Equinox J2000.0 (ICRS)
A light curve for SY Equulei, plotted from Hipparcos data^[1]
Constellation	Equuleus
Right ascension	21^h 23^m 28.8086^s^[2]
Declination	+09° 55′ 54.9204″^[2]
Apparent magnitude (V)	8.52 - 8.58^[3]
Characteristics
Spectral type	B0.5 IIIn^[4]
U−B color index	−1.00^[5]
B−V color index	−0.20^[5]
Variable type	β Cephei^[6]
Astrometry

Radial velocity (R_v)	48±5.5^[7] km/s
Proper motion (μ)	RA: −0.185 mas/yr^[2] Dec.: −9.049 mas/yr^[2]
Parallax (π)	0.3869 ± 0.0558 mas^[2]
Distance	approx. 8,000 ly (approx. 2,600 pc)
Absolute bolometric magnitude (M_bol)	−6.04^[8]

Details

Mass	14^[9] `M`_☉
Radius	10.44^[10] `R`_☉
Luminosity	2,490^[11] `L`_☉
Surface gravity (log g)	3.8±0.2^[12] cgs
Temperature	28,184^+1,328 _−1,268^[12] K
Metallicity [Fe/H]	0.00^[13] dex
Rotational velocity (v sin i)	180^[14] km/s
Age	~7.5^[12] Myr

Other designations
SY Equueli, AG+09°2978, BD+09°4793, HD 203664, HIP 105614, SAO 126757^[15]
Database references
SIMBAD	data

SY Equulei information

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