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Sub-brown dwarf in the constellation Hydra From Wikipedia, the free encyclopedia
WISE 0855−0714 (full designation WISE J085510.83−071442.5,[6] or W0855 for short) is a sub-brown dwarf 2.28±0.01 parsecs (7.43±0.04 light-years)[2] from Earth, therefore the fourth-closest star or (sub-) brown dwarf system to the Sun,[7] the discovery of which was announced in April 2014 by Kevin Luhman using data from the Wide-field Infrared Survey Explorer (WISE).[7] As of 2014[update], WISE 0855−0714 has the third-highest proper motion (8,151.6±1.8 mas/yr)[2] after Barnard's Star (10,300 mas/yr) and Kapteyn's Star (8,600 mas/yr)[6] and the fourth-largest parallax (439.0±2.4 mas)[2] of any known star or brown dwarf. It is also the coldest object of its type found in interstellar space, having a temperature of about 285 K (12 °C; 53 °F).[4]
Time-lapsed photo sequence of WISE 0855−0714's movement in the sky using captured images from the JWST | |
| Observation data Epoch J2000 Equinox J2000 | |
|---|---|
| Constellation | Hydra |
| Right ascension | 08h 55m 10.83168s[1] |
| Declination | −07° 14′ 42.5256″[1] |
| Characteristics | |
| Evolutionary stage | Sub-brown dwarf |
| Spectral type | Y4V[2][3] |
| Apparent magnitude (J) | 25.00±0.53[1] |
| Apparent magnitude (H) | 23.83±0.24[1] |
| Astrometry | |
| Proper motion (μ) | RA: −8,123.7±1.3 mas/yr[2] Dec.: 673.2±1.3 mas/yr[2] |
| Parallax (π) | 439.0 ± 2.4 mas[2] |
| Distance | 7.43 ± 0.04 ly (2.28 ± 0.01 pc) |
| Details[4] | |
| Mass | ~3–10 MJup |
| Radius | 0.89[lower-alpha 1] RJup |
| Radius | 63,500 km |
| Luminosity | 4.9545×10−8[lower-alpha 2] L☉ |
| Surface gravity (log g) | ~4 cgs |
| Temperature | 285 K |
| Metallicity [Fe/H] | ~0 dex |
| Age | 1–10 Gyr |
| Other designations | |
| Database references | |
| SIMBAD | data |
  WISE 0855−0714 Location of WISE 0855−0714 in the constellation Hydra | |
The WISE object was detected in March 2013, and follow-up observations were taken by the Spitzer Space Telescope and the Gemini North telescope.[7]
Based on direct observations, WISE 0855−0714 has a large parallax, which specifically relates to its distance from the Solar System. This phenomenon results in a distance of around 7.43±0.04 light-years,[2] with a small margin of error due to the strength of the parallax effect and the clarity of observations. WISE 0855−0714 also has an exceptionally high proper motion.[2]
Its luminosity in different bands of the thermal infrared in combination with its absolute magnitude—because of its known distance—was used to place it in context of different models; the best characterization of its brightness was in the W2 band of 4.6 μm at an apparent magnitude of 13.89±0.05, though it was brighter into the deeper infrared.[6] Infrared images taken with the Magellan Baade Telescope suggest evidence of sulfide clouds below water ice clouds.[8]
Near- and mid-infrared spectra in the L- and M-band were taken with the GNIRS instrument on the Gemini North Telescope. The M-band (4.5–5.1 μm) spectrum is dominated by water vapour (H2O) absorption. The L-band (3.4–4.14 μm) spectrum is dominated by methane absorption. Both the M- and L-band surprisingly have no detection of phosphine (PH3), which appears in the atmosphere of Jupiter. The M-band spectrum shows evidence for water ice clouds and the near-infrared photometry WISE 0855 is faint compared to models, suggesting an additional absorber, probably clouds made of ammonium dihydrogen phosphate (NH4)(H2PO4), which are below the water ice clouds.[9][10] An approved JWST proposal describes how the team is planning to use a near-infrared time-series to study the hydrological cycle in the atmosphere of WISE 0855 with NIRSpec.[11]
Observations with NIRSpec detected methane (CH4), water vapor (H2O), ammonia (NH3) and carbon monoxide (CO) in the atmosphere, but was not able to confirm any phosphine (PH3) or carbon dioxide (CO2) in the atmosphere. Water ice clouds are also not confirmed and the spectrum is well matched with a cloudless model.[4] Observations with MIRI showed a water vapor depletion and a water abundance that is variable with pressure. This is consistent with water condensing out in the upper atmosphere. The observations did however not detect any water ice clouds, which were predicted in previous studies. This discrepancy is explained with the rainout of the water: Water condenses into particles in the upper atmosphere, which quickly sink into the lower atmosphere. Clouds only form if upward mixing is present. A similar process is present for alkali metals in L- and T-dwarfs. A direct rainout would suggest weak mixing, but disequilibrium chemistry suggest rigours mixing. Future variable studies might resolve if upward mixing or settling is the dominant process. Cloud models however potentially detected deep ammonium dihydrogen phosphate (NH4)(H2PO4) clouds. The observations also detected 15NH3 for the first time in WISE 0855. The atmosphere has a mass fraction of 14NH3/15NH3 = 332+63
−43, meaning it has about 99.7% 14N and about 0.3% 15N. Compared to solar values and the ratio of WISE 1828, the atmosphere of WISE 0855 is enriched in 15N. The nitrogen isotope ratio is closer to today's 15N-enriched interstellar medium. This could mean that WISE 0855 formed from a younger cloud, but more measurements of 15N in other brown dwarfs are needed to establish evolutionary trends.[12]
Variability of WISE 0855 in the infrared was measured with Spitzer IRAC. A relative small amplitude of 4–5% was measured. Water ice cloud models predicted a large amplitude. This small amplitude might suggest that the hemispheres of WISE 0855 have very small deviation in cloud coverage. The light curve is too irregular to produce a good fit and rotation periods between 9.7 and 14 hours were measured.[13]
Based on models of brown dwarfs WISE 0855−0714's is estimated to have a mass of 3 to 10 MJup.[7] This mass is in the range of a sub-brown dwarf or other planetary-mass object.
As of 2003, the International Astronomical Union considers an object with a mass above 13 MJup, capable of fusing deuterium, to be a brown dwarf. A lighter object and one orbiting another object is considered a planet.[14] However, if the distinction is based on how the object formed then it might be considered a failed star, a theory advanced for the object Cha 110913-773444.[15]
Combining its luminosity, distance, and mass it is estimated to be the coldest-known brown dwarf, with a modeled effective temperature of 225 to 260 K (−48 to −13 °C; −55 to 8 °F), depending on the model.[7] Models matching the NIRSpec spectrum are well fitted with a temperature of 285 K (12°C; 53 °F).[4]
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