  | |
| Observation data EpochJ2000 EquinoxJ2000 | |
|---|---|
| Constellation | Taurus[1] |
| HD 28736 A | |
| Right ascension | 04h 32m 04.80859s[2] |
| Declination | +05° 24′ 36.1482″[2] |
| Apparent magnitude (V) | 6.352[3] |
| HD 28736 C | |
| Right ascension | 04h 33m 56.59566s[4] |
| Declination | +05° 37′ 23.5351″[4] |
| Characteristics | |
| Spectral type | (F5V + L9-T1) + L0/1[5] |
| B−Vcolor index | +0.431[6] |
| Astrometry | |
| HD 28736 A | |
| Radial velocity (Rv) | 39.5813 ± 0.0216[7] km/s |
| Proper motion (μ) | RA: 112.174[2]mas/yr Dec.: 7.756[2]mas/yr |
| Parallax (π) | 23.1089±0.0278 mas[2] |
| Distance | 141.1 ± 0.2 ly (43.27 ± 0.05 pc) |
| Absolute magnitude (MV) | 3.18[1] |
| HD 28736 C | |
| Proper motion (μ) | RA: 107.173[4] mas/yr Dec.: 7.595[4] mas/yr |
| Parallax (π) | 22.6203 ± 0.4505 mas[4] |
| Distance | 144 ± 3 ly (44.2 ± 0.9 pc) |
| Orbit[5] | |
| Primary | HD 28736 A |
| Companion | HD 28736 B |
| Period (P) | 60+30 −16yr |
| Semi-major axis (a) | 0.37" (17+5 −4 AU) |
| Eccentricity (e) | 0.36+0.37 −0.25 |
| Inclination (i) | 95.3+3.9 −2.1° |
| Orbit[5] | |
| Primary | HD 28736 A |
| Companion | HD 28736 C |
| Semi-major axis (a) | 1837" (79000 AU) |
| Details[5] | |
| HD 28736 A | |
| Mass | 1.40±0.05 M☉ |
| Surface gravity (log g) | 4.3±0.2 cgs |
| Temperature | 6655±125 K |
| Metallicity [Fe/H] | 0.13±0.05 dex |
| Rotational velocity (v sin i) | 45.6±1.8 km/s |
| Age | 650±100 Myr |
| HD 28736 B | |
| Mass | 24+6 −4 MJup |
| Surface gravity (log g) | 4.5 cgs |
| Temperature | 1300±50 K |
| HD 28736 C | |
| Mass | 73±7 MJup |
| Radius | 0.10[4] R☉ |
| Luminosity | 2.40+0.23 −0.21×10−4 L☉ |
| Other designations | |
| HD 28736 AB:58 G. Tauri,AG+05°479,BD+05°674,Gaia DR3 3285426613077584384,GC 5531,HD 28736,HIP 21152,HR 1436,SAO 111879,PPM 147652,TIC 452767166,TYC 90-33-1,GSC 00090-00033,2MASS J04320481+0524359[8][9] | |
| HD 28736 C:Gaia DR3 3285527699426683264,2MASS J04335658+0537235,WISE J043356.65+053723.5[10] | |
| Database references | |
| SIMBAD | HD 28736 |
| B | |
| C | |
HD 28736 (HIP 21152,HR 1436) is atriple star system in theconstellation ofTaurus. It is composed of anF-type main-sequence star, an orbiting low-massbrown dwarf orgiant planet, and a high-mass brown dwarf or low-mass star. Located some 141 light-years (43 parsecs) away according toGaia DR3parallax measurements, it is a member of theHyades cluster, moving away fromEarth at a heliocentricradial velocity of 39.58 km/s. With anapparent magnitude of 6.352, it is near the limit for naked eye visibility under dark skies.
HD 28736 A is a young star, aged 650 ± 100 million years. This was estimated from the age of the Hyades cluster itself,[5] which is about 625 million years.[11] It is about 40% more massive than theSun and also hotter at 6,655 K (6,382 °C; 11,519 °F).
The star is enriched in many elements heavier thanhydrogen andhelium compared to the Sun. In particular, the concentrations ofstrontium,barium,lanthanum,cerium,samarium, andgadolinium are at least 150% greater. A similar pattern is observed in other F-type dwarfs belonging to the cluster.[12]
In 2022, three teams of astronomers independently announced the discovery of abrown dwarf orbiting HD 28736 A viadirect imaging, the first brown dwarf to be discovered by this method aroundmain-sequence stars in the Hyades[5][13] and the firstsubstellar object of any kind to be found in the cluster to orbit stars with aspectral type of F, G, or K.[14]
The object, HD 28736 B, is near the border betweenL andT dwarfs, with an estimated spectral type of T0 ± 1. It has a mass of 24MJ (~2% of the host star mass[13]), substantially lower than evolutionary model predictions[5] and close to the planet-brown dwarf boundary.[13] Indeed, some organizations classify it as a planet instead, such as theNASA Exoplanet Science Institute, which includes the object in theNASA Exoplanet Archive[15][a] since it weighs less than 30MJ.[16] It completes one orbit around the star every 60 years at a distance of 17 AU (2.5×10^9 km), slightly closer thanUranus is to the Sun (19.165 AU[17]).
One of the teams that discovered HD 28736 B reported that another object, 2MASS J04335658+0537235, was found to be a co-moving companion to the HD 28736 system, at a very wide projected separation of 79,000 AU (1.25 ly) at the host star's distance. The 3D separation is even larger at250000±140000 AU (4.0±2.2 ly), comparable to the distance between the Sun andAlpha Centauri. Despite this, there is a high likelihood that the object isgravitationally bound to the system.[5]
It has a mass of73±7MJ, placing it right at thehydrogen burning limit. As such, it is uncertain whether the object is a massive brown dwarf or a low-mass star.[5]
Observations via theSpace Telescope Imaging Spectrograph, installed on theHubble Space Telescope, revealed three distinctvelocity components in theinterstellar absorption seen in HD 28736'sspectra, specifically in theemission lines of singlyionizedmagnesium (Mg II inspectroscopic notation). This corresponds to threeinterstellar clouds occupying the space between the system and Earth, all moving in different directions and speeds.[18]
The closest of the three is theLocal Interstellar Cloud (LIC), which theSolar System is within and thus absorbs light emitted by all Hyades members. A secondary cloud, dubbed the Hyades Cloud, is located farther than the LIC and possesses a more filament-like structure, absorbing light from a substantial portion of Hyades stars. A third cloud has been identified that affects light from HD 28736 but not other stars in the vicinity, meaning it only covers a small patch of the sky and hence is probably situated farther away than the Hyades Cloud.[18]
