# AF Leporis

AF Leporis, also known as HD 35850, is a young variable star located 87.5 light-years (26.8 parsecs) away from the Solar System in the constellation of Lepus. With an apparent magnitude of 6.3, it is near the limit of naked eye visibility under ideal conditions. While some studies consider it to be a close spectroscopic binary with a separation of 0.021 AU, other studies show no evidence of binarity, and it is likely that the supposed binarity is an artifact resulting from the presence of starspots. AF Leporis is a member of the Beta Pictoris moving group, with an astronomically young age of about 24 million years. It hosts a circumstellar disk and one known exoplanet.
The stellar classification of AF Leporis is F8V(n)k:, matching an F-type main-sequence star that is generating energy through hydrogen fusion at its core. (The 'n' indicates "nebulous" lines due to spin, while the 'k' means it displays interstellar absorption lines. The ':' suffix is used to note some uncertainty in the classification.) AF Leporis is classified as a RS Canum Venaticorum variable star, which means it has an active surface with large star spots that cause the net luminosity to vary as it rotates.
It is about 24 million years old and is spinning rapidly with a projected rotational velocity of around 50 km/s, giving it a rotation period of less than a day. The star has 9% more mass than the Sun and 1.25 times the Sun's radius. The abundance of elements with mass greater than hydrogen – the star's metallicity – is higher than in the Sun. AF Leporis is radiating 1.84 times the luminosity of the Sun from its photosphere at an effective temperature of 6,130 K.

## Planetary system

In 2023, a super-Jupiter exoplanet, AF Leporis b, was discovered in orbit around AF Leporis by direct imaging using the NIRC2 instrument at the W. M. Keck Observatory and the SPHERE instrument at the Very Large Telescope. It was also detected in astrometric data from the Hipparcos and Gaia spacecraft, allowing an accurate measurement of its mass. AF Leporis b was later precovered in imaging data from 2011, allowing a more accurate determination of its orbit.
There have been multiple studies of AF Leporis b, which have found somewhat different parameters. Dynamical mass measurements range from 2.8 M$_J$ to 5.5 M$_J$. Values for the planet's orbital inclination range from 50°+9° −12° to ~98°, the former consistent with the stellar inclination of 54°+11° −9° and suggesting an aligned system. Initial studies found a fairly eccentric orbit for the planet, but the precovery observations show that its orbit is nearly circular.
AF Leporis b has an effective temperature of about 750 K (477 °C; 890 °F), corresponding to an early-T spectral type. Spectroscopic evidence suggests that it has a metal-rich atmosphere with silicate clouds, though further studies are needed to confirm this.
AF Leporis b was observed with JWST NIRCam. The brightness of F444W is relative faint, indicating significant absorption due to carbon monoxide (CO). The strong CO absorption is explained with disequilibrium chemistry and high metallicity. The observations also rule out additional giant planets in the outer region. The study did not find any variability of AF Leporis b. A study with VLT/GRAVITY confirmed many of the previous observations. The GRAVITY instrument did add high precision astrometry, while at the same time providing a K-band spectrum. The new astrometry together with previous observations was able to constrain the orbit to a circular orbit with an inclination that is aligned with the inclination of the rotation axis of the host star. The K-band spectrum shows prominent methane (CH$_4$) absorption. The spectrum is also consistent with a metal-rich cloudy atmosphere, with ${\begin{smallmatrix}\left[{\ce {Fe}}/{\ce {H}}\right]\end{smallmatrix}}$ = 0.75±0.25, consistent with the formation via core accretion. The temperature was constrained to 800±50 Kelvin and the mass was re-estimated to 3.75 M$_J$. An observation with VLT/ERIS in the K-band detected CO and H$_2$O, but not CH$_4$ or CO$_2$. Observations with VLT/HiRISE in the H-band on the other hand confirmed the detection of CH$_4$.
The AF Leporis planetary system :
| Companion (in order from star) | Mass | Semimajor axis (AU) | Orbital period (years) | Eccentricity | Inclination | Radius |
| b | 3.75±0.5 M$_J$ | 8.98+0.15 −0.08 | 24.38+1.1 −0.41 | 0.015+0.029 −0.011 | 57.5+0.6 −0.7° | 1.30±0.15 R$_J$ |
| Debris disk | 46±9 AU | 46±9 AU | 46±9 AU | 46±9 AU | — | — |