# HD 28736

HD 28736 (HIP 21152, HR 1436) is a triple star system in the constellation of Taurus. It is composed of an F-type main-sequence star, an orbiting low-mass brown dwarf or giant planet, and a high-mass brown dwarf or low-mass star. Located some 141 light-years (43 parsecs) away according to Gaia EDR3 parallax measurements, it is a member of the Hyades cluster, moving away from Earth at a heliocentric radial velocity of 39.58 km/s. With an apparent magnitude of 6.352, it is near the limit for naked eye visibility under dark skies.

## Stellar components
### HD 28736 A

HD 28736 A is a young star, aged 650 ± 100 million years. This was estimated from the age of the Hyades cluster itself, which is about 625 million years. It is about 40% more massive than the Sun and also hotter at 6,655 K (6,382 °C; 11,519 °F).
The star is enriched in many elements heavier than hydrogen and helium compared to the Sun. In particular, the concentrations of strontium, barium, lanthanum, cerium, samarium, and gadolinium are at least 150% greater. A similar pattern is observed in other F-type dwarfs belonging to the cluster.

### HD 28736 B

In 2022, three teams of astronomers independently announced the discovery of a brown dwarf orbiting HD 28736 A via direct imaging, the first brown dwarf to be discovered by this method around main-sequence stars in the Hyades and the first substellar object of any kind to be found in the cluster to orbit stars with a spectral type of F, G, or K.
The object, HD 28736 B, is near the border between L and T dwarfs, with an estimated spectral type of T0 ± 1. It has a mass of 24 M$_J$ (~2% of the host star mass), substantially lower than evolutionary model predictions and close to the planet-brown dwarf boundary. Indeed, some organizations classify it as a planet instead, such as the NASA Exoplanet Science Institute, which includes the object in the NASA Exoplanet Archive since it weighs less than 30 M$_J$. It completes one orbit around the star every 60 years at a distance of 17 AU (2.5×10^$^9$ km), slightly closer than Uranus is to the Sun (19.165 AU).

### HD 28736 C

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 at 250000±140000 AU (4.0±2.2 ly), comparable to the distance between the Sun and Alpha Centauri. Despite this, there is a high likelihood that the object is gravitationally bound to the system.
It has a mass of 73±7 M$_J$, placing it right at the hydrogen burning limit. As such, it is uncertain whether the object is a massive brown dwarf or a low-mass star.

## Nearby interstellar clouds

Observations via the Space Telescope Imaging Spectrograph, installed on the Hubble Space Telescope, revealed three distinct velocity components in the interstellar absorption seen in HD 28736's spectra, specifically in the emission lines of singly ionized magnesium (Mg II in spectroscopic notation). This corresponds to three interstellar clouds occupying the space between the system and Earth, all moving in different directions and speeds.
The closest of the three is the Local Interstellar Cloud (LIC), which the Solar 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.