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HD 53705 / 53706 / 53680 AB
	A star chart of the constellation of Puppis showing the position of HD 53705/53706/53680 (circled)
Observation data; Epoch J2000.0 Equinox J2000.0
Constellation	Puppis
	HD 53705
Right ascension	07h 03m 57.317s ± 11.59
Declination	−43° 36′ 28.94″ ± 2.46
Apparent magnitude (V)	5.7033 ± 0.0320
	HD 53706
Right ascension	07h 03m 58.911s ± 110.53
Declination	−43° 36′ 40.56″ ± 79.27
Apparent magnitude (V)	7.0459 ± 0.0908
	HD 53680
Right ascension	07 03h 50.236m ± 12.82
Declination	−43° 33′ 40.82″ ± 8.53
Apparent magnitude (V)	8.8041 ± 0.0017
Characteristics
HD 53705 A
Spectral type	G0V
B−V color index	0.624 ± 0.009
HD 53706 B
Spectral type	K0V
B−V color index	0.779 ± 0.020
HD 53680 AB
Spectral type	K5V/M(MS)
B−V color index	1.180 ± 0.012
Astrometry
HD 53705 A
Radial velocity (Rv)	89.5 km/s
Proper motion (μ)	RA: -104.10 ± 0.91 mas/yr ; Dec.: 389.07 ± 1.32 mas/yr
Parallax (π)	60.55 ± 1.04 mas
Distance	53.9 ± 0.9 ly ; (16.5 ± 0.3 pc)
HD 53706 B
Radial velocity (Rv)	89.0 km/s
Proper motion (μ)	RA: -113.80 ± 9.01 mas/yr ; Dec.: 417.98 ± 12.58 mas/yr
Parallax (π)	47.99 ± 9.89 mas
Distance	68.0 ± 14.6 ly ; (20.8 ± 4.5 pc)
HD 53680 AB
Radial velocity (Rv)	89.065 ± 0.005 km/s
Proper motion (μ)	RA: -75.64 ± 0.97 mas/yr ; Dec.: 393.50 ± 1.46 mas/yr
Parallax (π)	58.2 ± 0.8 mas
Distance	56.0 ± 0.8 ly ; (17.2 ± 0.2 pc)
Absolute magnitude (MV)	7.81 ± 0.03
Orbit
Primary	HD 53680 A
Companion	HD 53680 B
Period (P)	1688.6 ± 1.1 days
Semi-major axis (a)	34.9 ± 3.2 mas
Eccentricity (e)	0.475 ± 0.002
Inclination (i)	163.6 +1.4; −1.7°
Longitude of the node (Ω)	238.9 ± 2.9°
Argument of periastron (ω); (secondary)	-133.2 ± 0.3°
Semi-amplitude (K1); (primary)	1.2398 +0.0041; −0.0040 km/s
Details
HD 53705
Mass	0.98 +0.02; −0.03 M☉
Radius	1.14 +0.04; −0.03 R☉
Luminosity	1.34 +0.11; −0.10 (log 0.127 ± 0.035) L☉
Surface gravity (log g)	4.34 +0.03; −0.04 cgs
Temperature	5827 ± 44 K
Metallicity [Fe/H]	-0.21 ± 0.03 dex
Rotational velocity (v sin i)	1.6 ± 0.5 km/s
Age	8.56 +1.44; −1.72 Gyr
HD 53706
Mass	0.81 +0.04; −0.03 M☉
Radius	0.79 +0.03; −0.03 R☉
Luminosity	0.40 +0.25; −0.15 (log -0.40 ± 0.21) L☉
Surface gravity (log g)	4.59 +0.03; −0.05 cgs
Temperature	5245 ± 44 K
Metallicity [Fe/H]	-0.28 ± 0.03 dex
Rotational velocity (v sin i)	0.3 ± 0.5 km/s
Age	11.7 +3.3; −9.6 (weakly constrained) Gyr
HD 53680 AB
Mass	0.79 ± 0.02 / 0.22 ± 0.02 M☉
Radius	0.64 ± 0.05 R☉
Temperature	4460 ± 100 K
Metallicity [Fe/H]	-0.29 ± 0.08 dex
Rotational velocity (v sin i)	2.08 ± 0.31 km/s
Age	0.7 - 9.4 (weakly constrained) Gyr
Other designations
	HD 53705: HD 53705, HIP 34065, HR 2667
	HD 53706: HD 53706, HIP 34069, HR 2668
	HD 53680: HD 53680, HIP 34052
Database references
HD 53705
SIMBAD	data
HD 53706
SIMBAD	data
HD 53680 AB
SIMBAD	data

HD 53705/53706/53680 is a star system that lies approximately 54 light-years away in the constellation of Puppis. The system consists of four stars in two binaries, making it one of the nearest quadruple star systems.

Component discovery

A diagram of HD 53706 and HD 53680, relative to HD 53705 (Click to enlarge).

HD 53705 was discovered to be a visual binary very early on, owing to the brightness of the two components. The earliest observation in the Washington Double Star Catalog (WDS) dates to 1826 and was made by James Dunlop, stating a position angle of 119 degrees and a separation of 21.5 arcseconds for the companion.^[6] The two stars have moved very little relative to each other since, with the most recent measurement from 1999 stating a position angle of 126 degrees and a separation of 20.9 arcseconds. This is related to the nearby location of the system: a separation of 21 arcseconds translates to a physical separation perpendicular to the line of sight of approximately 480 AU,^[7] so the orbit of the stars lasts somewhere on the order of millennia.

The companionship of HD 53680 to the closer binary was recognised later, with the first measurement in the WDS dating to 1900.^[6] With a position angle of 337 degrees and a separation of 185.7 arcseconds, HD 53680 lies on the opposite side of HD 53705 when compared to B, and is about nine times more distant. This separation results in a physical separation perpendicular to the line of sight of 4390 AU,^[7] which is atypically distant for a stellar companion but still close enough to be strongly gravitationally bound.

While all three components have similar proper motion, HD 53680's proper motion as measured by HIPPARCOS is sizeably discrepant from the proper motions of the other two components. A clue to the cause of this is that HD 53680's Tycho-2 proper motion is different from the HIPPARCOS values, which indicates that the star is being perturbed by a close companion.^[8] A fit of the HIPPARCOS astrometric data found a weakly constrained fit found a period of 1500 days, an inclination of 180 degrees (a face-on orbit) and a semimajor axis of 30.6 milliarcseconds.^[9] The fit is weakly constrained because HIPPARCOS observations do not span the full orbit of the companion, but the fit does adjust HD 53680's proper motion to be consistent with the proper motion of HD 53705/53706.

The low inclination of HD 53680 B's orbit reduces the amplitude of the radial velocity variation that it caused on HD 53680 A. In this case, the effect reduced the minimum mass of the companion into the brown dwarf regime as deduced from observations with the CORALIE spectrograph.^[3] The spectroscopic orbit produces far stronger constraint compared to the astrometry-only orbit.

Stellar properties

HD 53705, with a spectral type of G0V, is a G-type main-sequence star that is slightly hotter, larger and brighter than the Sun. Meanwhile, HD 53706 and HD 53680 A are both K-type main-sequence stars, with spectral types of K0V and K5V, respectively. Both of these stars are substantially cooler, smaller and dimmer than the Sun.

The three stars with observed spectra in the system have similar metallicity values: [Fe/H] = -0.21 ± 0.03 and -0.28 ± 0.03 for HD 53705 and B,^[2] and [Fe/H] = -0.29 ± 0.08 for HD 53680 A.^[3] The average value, -0.26 ± 0.04, results in an iron abundance of 55 ± 5% solar, a value typical for field stars.

The sub-solar metallicity of the stars has the effect of heating up their chromospheres; Though HD 53705 has a mass that is approximately solar, its effective temperature is about fifty degrees hotter.^{[citation needed]}

The kinematics of the stars, with large proper motion and radial velocity, suggests that the system is a member of the thick disk,^[7]^[9] the population of stars that comprise most of the older members of the Milky Way's spiral arms. This is supported by the parameters of HD 53705; the surface gravity of 4.34 is somewhat low for a G0V star, indicating that it is relatively old and moving towards the end of its main sequence lifetime - which, when coupled with the solar mass, mean that estimates for the star's age are approximately 9 billion years old, approximately twice the solar age. With a peculiar velocity of 75.7 km/s, The orbit of the system about the galaxy has an eccentricity of 0.31 and brings the system up to 151 parsecs away from the galactic plane - again indicative of a thick disk system.^[7]

Planet searches

Being bright, solar-type and nearby, HD 53705 and B are attractive targets for radial velocity (RV)-based planet searches.

HD 53705 was one of the 37 targets of the first RV-based planet search in the southern hemisphere, the ESO CES survey.^[10] This survey did not detect any companion with several Jovian masses out to a few AU. An extension of this survey to the HARPS spectrograph provides further constraint, suggesting that there are no Jupiter-mass companions out to about 5 AU.^[11]

HD 53705 can be presumed to be on the CORALIE sample, as it satisfies the criteria of parallax = ≥20 mas with error ≤5 mas, and spectral type between F8 and M1.^[12] HD 53706 fails the criteria due to the large error on its parallax, while HD 53680 satisfies them.^[3]

The two stars are also on the Anglo-Australian Telescope sample,^[13] which has found that they are stable to 4.5 and 2.9 m/s, respectively. This excludes the presence of giant planets at separations of a few AU around either star.^[14]

Notes

^ The poor quality of HD 53706's HIPPARCOS data is due to being in such close vicinity to HD 53705; the light from the relatively bright primary contaminated the data on the secondary, which is a common effect for close visual binaries of similar brightnesses observed by HIPPARCOS such as HD 80606 / HD 80607.
^ The proper motions of HD 53680 are not corrected for the motion induced by its companion.
^ The parameters of HD 53706, in particular its luminosity, possess large errors due to the poor quality of the HIPPARCOS parallax.
^ The derived (not observed) parameters of HD 53680 A from Sahlmann et al. (2010) are unreliable, because they used a parallax 30.1 ± 0.6 mas of unclear source (The parallax from the original HIPPARCOS reduction is close to the current value, so the 30 mas value is likely a typographical error) which yields a somewhat high mass by about 0.1 M_☉ and extremely high log g of 5.39 ± 0.29 cgs, a value similar to that of Proxima Centauri. The derived mass of HD 53680 B is therefore also somewhat higher than it should be.