469705 ǂKá̦gára

469705 ǂKá̦gára,[1] official designation (469705) 2005 EF₂₉₈, is a trans-Neptunian object and binary system of the core Kuiper belt, located in the outermost region of the Solar System. It was discovered on 2005 March 11 by Marc William Buie at Kitt Peak Observatory. The primary body measures perhaps 140 kilometers (90 miles) in diameter. Its ≈120-kilometer (75-mile) companion ǃHãunu was discovered with the Hubble Space Telescope in 2009. The ǂKá̦gára–ǃHãunu system is currently undergoing mutual occultation and eclipsing events in which one body casts its shadow upon or obstructs the view of the other.

ǂKá̦gára

Stacked Hubble Space Telescope images of ǂKá̦gára and ǃHãunu
Discovery
Discovered by	Marc William Buie
Discovery site	Kitt Peak Observatory
Discovery date	2005
Designations
MPC designation	(469705) 2005 EF298
Pronunciation	English: /ˈkɑːʔɡɑːrə/ ǀXam: [ǂ͡káˤɡáɾa] (listen)
Named after	ǂKá̦gára (San mythology)[1]
Minor planet category	Cold classical KBO[1]
Orbital characteristics[2][3]
Epoch (JD 2458600.5)
Uncertainty parameter 3
Observation arc	10.10 yr (3688 d)
Aphelion	44.058 AU
Perihelion	40.013 AU
Semi-major axis	44.081 AU
Eccentricity	0.085
Orbital period	292.45 yr (106744.25 d)
Mean anomaly	118.059°
Mean motion	0.00337 0° 0^m 0^s / day
Inclination	1.60°
Longitude of ascending node	118.059°
Argument of perihelion	77.966°
Known satellites	1
Physical characteristics
Mean diameter	138+21 −25 km, assuming same albedo as ǃHãunu
Mass	(1.29±0.07)×10^18 kg, assuming same mass and albedo as ǃHãunu
Mean density	1.1+0.9 −0.4 g/cm^3, assuming equal compositions of the bodies
Absolute magnitude (H)	HV = 6.2±0.5

Names

The proposed names ǂKá̦gára and ǃHãunu are from the mythology of the ǀXam people of South Africa. ǂKá̦gára (also rendered ǂKaʻgara) and his brother-in-law ǃHãunu fought an epic battle in the east using thunder and lightning, producing mountainous clouds and rain. The conflict was over ǂKá̦gára's returning his younger sister, ǃHãunu's wife, to their parents.[1][4] The proposed names must be approved by the International Astronomical Union's Working Group for Small Body Nomenclature before they become official.

In the ǀXam language, the names are thought to have been pronounced [ǂ͡káˤɡáɾa] (listen)[Note 1] and [ᵑ̊ǃʰəunu] (listen),[Note 2] respectively.[5] The initial letters ǂK and !H (and indeed the letter ǀX in 'ǀXam') represent some of the many click consonants that characterize ǀXam and other San languages. The diacritic under or after the first 'a' in ǂKá̦gára/ǂKaʻgara indicates that it is a pharyngealized vowel, which are also characteristic of San languages. Usually, when speaking English, the click consonants in words from ǀXam and other San languages are simply ignored (much as Xhosa is pronounced /ˈkoʊzə/ (KOH-zə) rather than [ǁʰosa]), resulting in /ˈkɑːʔɡɑːrə/ (KAHʼ-gar-ə) and /ˈhaʊnuː/ (HOW-noo). ASCII approximations of the names are =Ka'gara (or =Kagara, #Kagara, etc.) and !Haunu.

Orbit and occultations

ǂKá̦gára is a cold classical Kuiper belt object. It orbits the Sun at a distance of 44 AU once every 290 years. Its orbit has an eccentricity of 0.09 and an inclination of 3° with respect to the ecliptic.[2]

If the two bodies are spheres with equal density, then mutual occultation events between ǂKá̦gára and its satellite ǃHãunu should have begun in 2015 and should continue until 2035. When ǂKá̦gára passes in front of ǃHãunu, events may last as long as 8 hours, but when ǃHãunu passes in front of ǂKá̦gára, they are expected to last as long as 2 days. These latter events are only expected to occur from 2022 to 2027.[1]

Formation

Prograde orbits dominate tight binary systems such as 469705 ǂKá̦gára, those with satellite semimajor axes less than about 5% of their Hill radii. Grundy et al. suggest that this could be "the signature of planetesimal formation through gravitational collapse of local density enhancements such as caused by the streaming instability",[1] which has been suggested as the formative mechanism of the only visited planetesimal, 486958 Arrokoth.

Satellite, size and mass

ǃHãunu Discovery Discovery date 2009 Designations Pronunciation English: /ˈhaʊnuː/ ǀXam: [ᵑ̊ǃʰəunu] (listen) Named after ǃHãunu (San mythology)[1] Alternative names ǂKá̦gára 1 S/2009 (2005 EF298) 1 Orbital characteristics[1][6] Semi-major axis 7670±140 km Eccentricity 0.694±0.013 Orbital period 128.107±0.027 d (prograde) Inclination 33.33°±0.41° ref'd to J2000 equatorial frame 11.17°±0.41° to heliocentric orbit Physical characteristics Mean diameter 122+16 −19 km, assuming same albedo as ǂKá̦gára Mass (0.89±0.05)×10^18 kg, assuming same density and albedo as ǂKá̦gára Apparent magnitude ΔH = 0.59

469705 ǂKá̦gára has one known satellite, ǃHãunu. The magnitude difference between ǂKá̦gára and ǃHãunu is 0.59 mag. This corresponds to a difference in diameter of 13%, if the two bodies have the same albedo.[1]

The system mass is (2.18±0.12)×10¹⁸ kg, and, given the albedo, the bodies are equivalent to a single sphere of diameter 174+27
−32 km. This corresponds to a density of 1.1+0.9
−0.4 g/cm³. This does not particularly constrain the composition of the bodies, as the error bars cover the densities of both small, porous bodies and larger, collapsed bodies. (See 229762 Gǃkúnǁʼhòmdímà for discussion.) Assuming the two bodies have the same albedo and density, their masses are (1.29±0.07)×10¹⁸ kg and (0.89±0.05)×10¹⁸ kg, and their diameters are 138+21
−25 km and 122+16
−19 km.[1]

Notes

1. Bleek & Lloyd wrote ⟨ǂk⟩ when the posterior release of the click was audible. Based on the sounds attested in the closely related language Nǁng, this [ǂ͡k] may therefore represent a palatal pulmonic-contour click, though this is not certain.
2. Based on the co-occurrence of the letter ⟨H⟩ for aspiration of the click and the tilde for nasalization on the following vowel, together with the patterns of sounds in Nǁng, it is suspected that the click in ǃHãunu has delayed aspiration (i.e., is a long, aspirated, nasalized click), rather than a simple IPA: [ǃʰə̃unu], though again this is not certain.

References

1. Grundy, W.M.; Noll, K.S.; Roe, H.G.; Buie, M.W.; Porter, S.B.; Parker, A.H.; Nesvorný, D.; Levison, H.F.; Benecchi, S.D.; Stephens, D.C.; Trujillo, C.A. (April 2019). "Mutual orbit orientations of transneptunian binaries". Icarus. doi:10.1016/j.icarus.2019.03.035. Retrieved 6 April 2019. (heliocentric parameters averaged over 10Myr)
2. "JPL Small-Body Database Browser: 469705 (2005 EF298)" (2015-04-06 last obs.). Jet Propulsion Laboratory. Retrieved 7 April 2019.
3. "469705 (2005 EF298)". Minor Planet Center. Retrieved 7 April 2019.
4. ≠kagara's fight with !haunu in the east. Lucy Lloyd |xam notebooks, vol. VIII-30, July 1879.
5. Will Grundy: 469705 (how to say)
6. "ǂKá̦gára and ǃHãunu (469705 2005 EF298)". Mutual Orbits of Binary Transneptunian Objects. Retrieved 7 April 2019. (heliocentric parameters averaged over 10Myr)

External links

• 469705 ǂKá̦gára at AstDyS-2, Asteroids—Dynamic Site
  • Ephemeris · Observation prediction · Orbital info · Proper elements · Observational info