# Delorme 1

Delorme 1 (2MASS J01033563-5515561) is a binary star with a planetary-mass companion (PMC) or protoplanet in a circumbinary orbit. The PMC is notable for showing signs of accretion, despite being 30-45 Myr old, making it similar to Peter Pan disks. These disks show characteristics of a gas-rich disk at unexpected high ages.

## The binary system

The star was resolved in 2013 with the Very Large Telescope NACO instrument by Delorme et al. A spectrum of the binary was taken with GMOS at Gemini South, which showed a spectral type of M5.5/M6 and strong Hydrogen-alpha emission. The astrometry showed that this star belongs to the Tucana-Horologium association. The binary is separated by around 12 astronomical units (AU). In 2014 Riedel et al. found a better match with the Carina association, which has a similar age as Tuc-Hor. They also found the system to be over-luminous, which might either hint at a younger age or further multiplicity. Other searches do, however, find a better match with Tuc-Hor. Because the Washington Double Star Catalog named the binary ** DLR 1 after the first author of the discovery paper in 2013, Eriksson et al. suggested the name Delorme 1 for the binary. The binary is named after Philippe Delorme.

## The circumstellar companion

The binary companion was discovered in 2013 as an object with a mass between 12 and 14 M$_J$ and a separation of 84 AU from the central binary. It had a spectrum similar to early L-dwarfs, but redder than field L-dwarfs. In 2020 Eriksson et al. discovered Hydrogen-alpha, -beta and Helium I lines from Delorme 1 (AB)b using MUSE. This is seen as a clear sign of accretion on a planetary-mass object. The spectral type of this object was determined to be L0 with very low gravity due to stronger than expected vanadium oxide absorption. H-alpha can be influenced by chromospheric activity, complicating its interpretation. Betti et al. discovered Paschen and Brackett lines in Delorme 1 (AB)b in the near-infrared, using TripleSpec at SOAR. These observations are in agreement with planetary-shock accretion. In 2023 Ringqvist et al. observed Delorme 1 (AB)b with the VLT UVES, detecting neutral hydrogen in the ultraviolet. Both near-infrared and ultraviolet observations show an accretion rate of about $(2-4)\times 10^{-8}M_{J}yr^{-1}$ (about 1.2 to 2.3 the mass of 10 Hygiea per year). The circumplanetary disk that fuels this accretion around Delorme 1 (AB)b is not detected (as of August 2024). The planet and the star were observed with MIRI/IFU in August 2023, which should reveal any disk around the planet or star in a future work.
Delorme 1 (AB)b has been called a protoplanet candidate and a super-Jupiter. The researchers found that the high accretion is in better agreement with a formation via disk fragmentation, hinting that it might have formed from a circumstellar disk. Giant planets and brown dwarfs are thought to form via disk fragmentation in rare cases in the outer regions of a disk (r>50 AU). Teasdale et al. modelled three formation scenarios in which the planet could have formed. In the first two scenarios the planet forms in a massive disk via gravitational instability. The first two scenarios produce planets that have accretion and separation comparable to the observed ones, but the resulting planets are more massive than Delorme 1 (AB)b. In a third scenario the planet forms via core accretion in a less massive disk much closer to the binary. In this third scenario the mass and accretion are similar to the observed ones, but the separation is smaller.
The Delorme 1 (AB) planetary system :
| Companion (in order from star) | Mass | Semimajor axis (AU) | Orbital period (years) | Eccentricity | Inclination | Radius |
| b | 12 to 14 M$_J$ | 102+47 −27 | 1682+1308 −628 | 0.32+0.27 −0.23 | 127+17 −8° | 1.5764 R$_J$ |