AF Lep b: The lowest-mass planet detected by coupling astrometric and direct imaging data

Abstract

Aims. Using the direct-imaging technique, we searched for low-mass companions around the star AF Lep, which presents a significant proper-motion anomaly (PMa) signal obtained from the comparison of HIPPARCOS and Gaia eDR3 catalogs. Methods. We observed AF Lep in two epochs with VLT/SPHERE using its subsystems IFS and IRDIS in the near-infrared, covering wavelengths ranging from the Y to the K spectral bands (between 0.95 and 2.3 μm). We then reduced the data using the high-contrast imaging techniques angular differential imaging (ADI) and spectral differential imaging in order to be able to retrieve the signal from low-mass companions of the star. Results. A faint companion was retrieved at a separation of ~0.335″ from the star and with a position angle of ~70.5° in the first epoch and with a similar position in the second epoch. This corresponds to a projected separation of ~9 au. The extracted photometry allowed us to estimate a mass for the companion of between 2 and 5 MJup. This mass is in good agreement with astrometric measurements of the dynamic mass of the companion, which give 5.2–5.5 MJup. This is the first companion with a mass well below the deuterium burning limit that was discovered by coupling direct imaging with PMa measurements. Orbital fitting done using the orvara tool allowed us to further confirm the companion mass and to define its main orbital parameters.

Jupiter-like planets are common in a low density environment

“…These four stars have all been observed in HCI without detection of close companions; however, the lesson of HIP 25486 = AF Lep b, detected by Mesa et al 24 after previous unsuccessful attempts, tells that the planets may well be there, but they are not detectable in some observations because at that epoch they were projected too close to the star. Indeed, for these targets the probability that the planet is observed when it is not hidden by the coronagraph is rather low (see Table 2).…”

“…Repeated observations are then needed to increase the chance of observing it at the right phase for detection (see e.g. the case of AF Lep b 24,25 ). The implication is that the search of Jupiter-like planets using HCI is likely severely incomplete as HCI surveys typically do not re-observe a star in case of lack of detection of candidate companions.…”

“…The parameters for the substellar companions were obtained as follows: 51 Eri b 51,93 ; AF Lep b 24 ; β Pic b and c 52,53,94,95 ; HIP 88399 b 42 ; PZ Tel B 19,96 ; η Tel B 54 .…”

Dynamical Mass of the Young Brown Dwarf Companion PZ Tel B

“…The sample of substellar companions with dynamical masses and age constraints now amounts to ≈20 brown dwarf companions (e.g., Cheetham et al 2018;Brandt et al 2019Brandt et al , 2020Brandt et al , 2021cBonavita et al 2022;Franson et al 2022;Kuzuhara et al 2022;Franson et al 2023a;Li et al 2023) and giant planets (Dupuy et al 2019;Lagrange et al 2019;Nowak et al 2020;Brandt et al 2021a;Hinkley et al 2022;Franson et al 2023b;De Rosa et al 2023;Mesa et al 2023). Most of these benchmark companions orbit old field stars (>1 Gyr); only three planets-β Pic b, β Pic c, and AF Lep b-are young (<200 Myr) and have precise ages based on membership in kinematic associations (Dupuy et al 2019;Nowak et al 2020;Brandt et al 2021a;Franson et al 2023b;De Rosa et al 2023;Mesa et al 2023). 2 Age-dating individual field stars is notoriously challenging, which can limit the precision of model tests for benchmark companions around young field stars (see e.g., HD 984 B; Franson et al 2022) compared to objects in young moving groups.…”

Prospects from TESS and Gaia to Constrain the Flatness of Planetary Systems

“…However, this panorama is expected to be changed in the next few years as the Gaia astrometry mission is going to release about 20,000 giant planet detections with its upcoming data release (DR) 4 (Perryman et al 2014). In fact, with the release of the Gaia DR3 (Gaia Collaboration et al 2023a), we already have dozens of astrometric candidates in the substellar regime (see, e.g., Gaia Collaboration et al 2023b), and a few systems have been identified using Hipparcos and Gaia astrometry, and confirmed by direct imaging (e.g., Currie et al 2023;De Rosa et al 2023;Mesa et al 2023). Astrometry will be especially useful as it can provide us with measurements of the orbital inclinations and true masses of planets (see, e.g., Sozzetti et al 2001;Casertano et al 2008;Perryman et al 2014;Sozzetti et al 2014).…”