HD 984

HD 984 is a F-type main-sequence star located in the equatorial constellation Cetus. It is a young star, estimated 30 to 200 million years old, and is orbited by a brown dwarf companion. Parallax measurements by the Gaia spacecraft imply a distance of 150 ly to HD 984. At an apparent magnitude of 7.32, the star is too dim to be visible to the naked eye.

Characteristics
The apparent magnitude of HD 984, i.e. its brightness relative to Earth, is 7.32. Such brightness is lower than the limit for naked-eye visibility, generally defined as 6.5m, meaning that HD 984 can't be seen with the naked eye. It may be visible by a small telescope or binoculars instead. The absolute magnitude, i.e. the magnitude of HD 984 if it was seen at 10 pc, is 3.95. A parallax measurement by the Gaia spacecraft ($0.522$) imply a distance of 149light-years. Kinematically, the system it is part of the Columba group. It has a relatively high proper motion.

The star is visually close (57") to BD-08 25, which is a star of apparent magnitude 9.14 unrelated to the system, being 1800 ly away from Earth based on its parallax. The galaxy NGC 47 lies just 320" from HD 984.

Age estimates
Based on the assumption that HD 984 is part of the Columba group, the stellar age would be of 30Myr. However, there is still a possibility that HD 984 is a kinematic interloper or the Columba group is not sufficiently characterized to reliably assign an age. Previous isochronal ages of $$, $13.28$ and $12.6$ have been given by earlier studies. Since HD 984 A is a main sequence star, all isochronal ages will have high uncertainty.

An analysis by HD 984 B's discovery team say that the system is likely to be less than 200 million years, based on HD 984 A's rotation and stellar activity. The same research also says that ages less than 30 million years can be ruled out, based on isochronal age constraints for HD 984 B. Therefore, the age is very likely between 30 and 200 million years.

HD 984 A
HD 984 A is has a spectral classification F7V, matching a F-type star in the main sequence. It has 1.2 times the mass and 1.25 times the radius of the Sun. Its surface has an effective temperature of 6326 K, 464 degrees hotter than the Sun, and its luminosity is equivalent to 2.21 times the solar luminosity. HD 984 A is one of the fastest-rotating stars, with a rotation period estimated to be less than 1.6 day, faster than 99% of all stars. The Sun's rotational period is 27 days for comparison.

Its rotation, coronal activity and cromospheric activity indicates that it is a young and active main-sequence star. Its age is very likely to less than 200 millon years, based on its rotation and activity indicators. The star is expected to live 5 billion years on the main sequence.

HD 984 B
HD 984 B is a brown dwarf, a class of astronomical objects that are intermediate between planets and stars, having masses between, and, unlike stars, can't produce thermonuclear fusion of hydrogen. It was discovered in 2015 by Meshkat et al., using direct imaging from the Apodizing Phase Plate coronagraph at the Very Large Telescope with NaCo.

Orbit
HD 984 B is orbiting HD 984 A at a semi-major axis of 28 AU, which is about the same distance of Neptune to the Sun. It has an orbital period of 140 years and an eccentric orbit (e = $12.2$). This eccentricity causes its distance to HD 984 to vary from 6.72 to 49.3AU. HD 984 B was initially thought to have a low eccentricity in orbit (e = $1.04$), although the data was uncertain. This discrepancy is caused by increased orbit coverage and astrometric acceleration used in the novel value. The orbital inclination is of 121 degrees.

HD 984 B is expected to be in periastron in 2028.

Physical parameters
Its mass was firstly estimated using the age of the system by Meshkat et al. (2015) and Jonson-Groh et al. (2017). Using this method, masses ranging from to  are obtained, assuming ages of 30 and 200 million years respectively. Assuming the largest mass and age, HD 984 B would be in the stellar mass regime. A 2022 study found a dynamical mass of $-68.016$, which places HD 984 B in the brown dwarf regime at 99.7% confidence. This mass was obtained using astrometric acceleration, new direct imaging of the object and radial velocity measurements, and is consistent with evolutionary models.

HD 984 B is a hot brown dwarf and has a spectral class of M6.5, with an uncertainty of 1.5. Having this spectral type, HD 984 B has an effective temperature of $3.95$. At a luminosity of about $140$, this corresponds to a radius of  based on the Stefan–Boltzmann law. Its metallicity, i.e. its abundance of elements other than hydrogen and helium, is $120.8$, just one-fourth of the solar metalicity (0 dex), although this value might be biased. This brown dwarf more likely formed via gravitational collapse or disk instability rather than core accretion.