EBLM J0555-57

EBLM J0555-57 is a triple star system approximately 670 light-years from Earth. The system's discovery was released on July 12, 2017. EBLM J0555-57Ab, the smallest star in the system, orbits its primary star with a period of 7.8 days, and currently is the smallest known star with a mass sufficient to enable the fusion of hydrogen in its core.

EBLM J0555-57AB

Resolved image of EBLM J0555-57A (left) and EBLM J0555-57B (right) taken with the Leonhard Euler Telescope at the ESO's La Silla Observatory[1] Credit: ESO
Observation data Epoch J2000      Equinox J2000
Constellation	Pictor
EBLM J0555-57A
Right ascension	05h 55m 32.6885s[2]
Declination	−57° 17′ 26.067″[2]
Apparent magnitude (V)	9.98[1]
EBLM J0555-57B
Right ascension	05h 55m 32.3944s[3]
Declination	−57° 17′ 26.748″[3]
Apparent magnitude (V)	10.76[1]
Characteristics
EBLM J0555-57A
Spectral type	F8[4]
Variable type	Eclipsing[1]
EBLM J0555-57B
Spectral type	G0V[citation needed]
Astrometry
EBLM J0555-57A
Radial velocity (Rv)	18.66±0.74[5] km/s
Proper motion (μ)	RA: 2.972[2] mas/yr Dec.: −39.532[2] mas/yr
Parallax (π)	4.8611±0.0137 mas[2]
Distance	671 ± 2 ly (205.7 ± 0.6 pc)
EBLM J0555-57B
Proper motion (μ)	RA: 2.838[3] mas/yr Dec.: −38.344[3] mas/yr
Parallax (π)	4.8747 ± 0.0175 mas[3]
Distance	669 ± 2 ly (205.1 ± 0.7 pc)
Position (relative to A)
Component	B
Epoch of observation	1998
Angular distance	2.50″ [4]
Position angle	254° [4]
Projected separation	479 AU [1]
Orbit[1]
Primary	Aa
Companion	Ab
Period (P)	7.757676+0.000029 −0.000025 d
Semi-major axis (a)	0.0895+0.0018 −0.0019 au
Eccentricity (e)	0.0894+0.0035 −0.0036
Inclination (i)	89.3+0.9 −1.1°
Argument of periastron (ω) (secondary)	−53.7+1.5 −1.8°
Details[1]
EBLM J0555-57Aa
Mass	1.180+0.082 −0.079[6] M☉
Radius	1.00+0.14 −0.07[6] R☉
Luminosity	3.245[7] L☉
Surface gravity (log g)	4.10±0.21[6] cgs
Temperature	6,368±124[6] K
Metallicity [Fe/H]	−0.24±0.16[1] or −0.04±0.14[6] dex
Rotation	6.92±1.61[6] days
Rotational velocity (v sin i)	7.60±0.28[1] km/s
Age	1.6±1.2[6] Gyr
EBLM J0555-57Ab
Mass	0.0839±0.0038[6] M☉
Mass	87.9±4.0[6] MJup
Radius	0.0844+0.0131 −0.0060[6] R☉
Radius	0.82+0.13 −0.06[6] RJup
Surface gravity (log g)	5.51+0.06 −0.12[6] cgs
EBLM J0555-57B
Mass	1.01[8] M☉
Radius	0.94±0.08[1] R☉
Luminosity	1.501[9] L☉
Surface gravity (log g)	4.37[8] cgs
Temperature	5,717±124[1] K
Metallicity [Fe/H]	−0.38[8] dex
Other designations
CD−57 1311, CPD−57 913, WDS J05555-5717, TYC 8528-926, 2MASS J05553262-5717261
Database references
SIMBAD	data

System

A z' band light curve for EBLM J0555-57, adapted from von Boetticher et al. (2017)^[1]

EBLM J0555-57, also known as CD−57 1311, is a triple star system^[1][10] in the constellation Pictor, which contains a visual binary system consisting of two sun-like stars separated by 2.5": EBLM J0555-57Aa, a magnitude 9.98 spectral type F8 star, and EBLM J0555-57B, a magnitude 10.76 star. No orbital motion has been detected but they have almost identical radial velocities and are assumed to be gravitationally bound.^[1]

Component A of the system is itself an eclipsing binary (EBLM J0555-57Ab orbiting EBLM J0555-57Aa). Eclipses, also known as transits in the context of planetary searches, have been detected in the near infrared, with brightness drops of 0.05% during the eclipse. The shape and duration of the transits allow the radii of the two stars to be determined. A full solution of the orbit gives a period of 7 days and 18 hours, with a low eccentricity of 0.09, an almost edge-on inclination of 89.84°, and a semi-major axis of 0.08 AU.^[1]

EBLM J0555-57Ab

EBLM J0555-57Ab has a mass of about 88±4 Jupiter masses, or 0.084 solar masses. Its radius is 0.084 solar radii (about 59,000 km), comparable to Saturn, which has an equatorial radius of 60,268 km. The star is about 290 times more massive than Saturn.^[6] Current stellar models put its mass at the lower limit for hydrogen-burning stars. EBLM J0555-57Ab was discovered by a group of scientists at the University of Cambridge associated with the EBLM project (Eclipsing Binary, Low Mass),^[1] using data collected by the WASP project. WASP (Wide Angle Search for Planets) is searching for exoplanets using the transit method. ^[11] The findings were released on July 12th, 2017, though the exact date of the discovery is unknown. Additional properties of the star were determined using Doppler spectroscopy, to measure the periodic radial velocity variation of the primary star due to the gravitational influence of its companion.^[1]

EBLM J0555-57Ab is the smallest hydrogen burning star currently known.

See also

• 2MASS J0523−1403
• OGLE-TR-122 - This binary stellar system contained one of the smallest red dwarfs known when it was discovered.
• OGLE-TR-123
• TRAPPIST-1
• SSSPM J0829-1309
• GJ 1245
• List of smallest stars