Delorme 1 AB b: Accreting Planetary Companion

• Delorme 1 AB b is a directly imaged planetary companion with an estimated mass of 13±5 M_Jup and sustained accretion indicative of a long-lived circumplanetary disk.
• Multi-wavelength spectroscopic diagnostics reveal strong optical, UV, and IR emission lines that support a magnetospheric accretion paradigm similar to T Tauri stars.
• Its wide orbit around a low-mass M5.5+M5.5 binary and unusual CPD chemistry, including a high C/O ratio, offer new insights into planetary formation and disk evolution.

Delorme 1 AB b is a directly imaged, accreting planetary-mass companion orbiting a low-mass M5.5+M5.5 binary in the Tucana–Horologium association at a projected separation of 84 AU and distance 47.2 pc. With an estimated mass of 13 ± 5 M_Jup and an age of 30–45 Myr, Delorme 1 AB b is a benchmark for late-stage planet formation, accretion processes, and circumplanetary disk (CPD) evolution. The system presents sustained accretion rates more typical of much younger objects and exhibits a range of emission features from optical to mid-infrared, challenging canonical protoplanetary disk dispersal timescales and formation models.

1. System Architecture and Basic Properties

Delorme 1 AB b (2MASS J01033563–5515561 (AB) b) was discovered via direct imaging and orbits a tight (12 AU) M5.5+M5.5 binary in the Tucana–Horologium association. The planetary companion's mass, derived from evolutionary tracks for ages of 30–45 Myr and luminosity measurements, is estimated at 12–15 M_Jup. Radius estimates from the system's SED yield R_p ≈ 0.16 R_⊙ (≈1.6 R_Jup). The system is located at 47.2 pc. The separation between the planet and host binary is 1.77″ (projected 84 AU), making Delorme 1 AB b one of the few known wide, circumbinary, planetary-mass companions directly imaged to date (Betti et al., 2022, Teasdale et al., 2024).

The central binary itself is notably overluminous, which may suggest a system age younger than the canonical 30–40 Myr of Tucana–Horologium, but persistent accretion at this age is not unprecedented among "Peter Pan" disks, i.e., low-mass hosts with unusually long-lived disks (Eriksson et al., 2020). The observed accretion rates and disk properties place Delorme 1 AB b within this emerging population of long-lived, actively accreting substellar companions (Ringqvist et al., 2022, Mâlin et al., 8 Oct 2025).

2. Accretion Diagnostics Across the Electromagnetic Spectrum

The accretion regime of Delorme 1 AB b is characterized by strong, variable emission lines spanning the optical, near-UV, and near-IR, as well as robust continuum excess in the UV. The prominent observed transitions include:

• Optical Balmer lines (Hα to H10), with Hα displaying equivalent widths as large as –135 Å, FWHM up to 133 km/s, and log(L_line/L_⊙) ≈ –7.05 (Eriksson et al., 2020, Demars et al., 3 Nov 2025).
• Near-UV Balmer series detections up to H9, and tentative detection of higher series members (H11, H12), plus He I and Ca II H/K lines (Ringqvist et al., 2022).
• Near-IR hydrogen lines, including Paschen β (1.282 μm), Paschen γ (1.094 μm), and Brackett γ (2.166 μm), with line luminosities L_line ≈ (1–6)×10⁻⁸ L_⊙ and equivalent widths of –1 to –2 Å (Betti et al., 2022).
• Optical and near-UV He I lines, with at least seven transitions between 3890 and 6680 Å detected at >5σ; many show asymmetric, multi-component profiles (Viswanath et al., 30 Apr 2026).

Spectroscopic observations using ESO VLT instruments (UVES, MUSE), SOAR/TripleSpec, and JWST/MIRI demonstrate significant epoch-to-epoch variability, with Balmer and He I line fluxes changing on timescales from hours to weeks. All Balmer lines and the UV excess are modulated, with correlated variability patterns between line components and the accretion-powered continuum (Demars et al., 3 Nov 2025).

The table below summarizes representative emission line luminosities:

Transition	log(L_line/L_⊙)	Reference
Hα	–7.05 ± 0.06	(Eriksson et al., 2020)
Pa β	–7.32 to –7.68	(Betti et al., 2022)
He I (5877Å)	–6.68 ± 0.29	(Viswanath et al., 30 Apr 2026)

Line profiles are typically decomposed into two Gaussian components: a broad component (BC) with FWHM ∼80–160 km/s and a narrow component (NC) with FWHM ∼10–50 km/s. The BCs are generally redshifted by 10–20 km/s, while the NCs are centered close to systemic velocity (Ringqvist et al., 2022, Viswanath et al., 30 Apr 2026). The broad components correlate more strongly with instantaneous UV excess, identifying them with magnetospheric funnel flows; narrow components trace post-shock cooling zones or potentially chromospheric activity (Demars et al., 3 Nov 2025, Viswanath et al., 30 Apr 2026).

3. Accretion Rates and Geometry

Mass accretion rates ($\dot{M}$) derived from multi-wavelength line luminosities and empirical or theoretical scaling relations consistently indicate ongoing accretion at rates $$\dot{M} ≈ 3×10^{-10} - 6×10^{-8}\ M_\mathrm{Jup}\mathrm{\,yr}^{-1}$$ across methods and epochs (Eriksson et al., 2020, Betti et al., 2022, Ringqvist et al., 2022, Viswanath et al., 30 Apr 2026). The most robust estimates for the current epoch cluster near $$\dot{M} ≈ 0.2–5×10^{-8}\ M_\mathrm{Jup}\ \mathrm{yr}^{-1}$$.

Accretion geometry is inferred from the dichotomous line profile structure and their correlations with UV/optical continuum excess. Near-UV and optical lines favor a magnetospheric accretion paradigm: localized accretion columns channel material along the magnetic field onto the planetary surface, producing both high-velocity wings (BC), and hot-spot post-shock emission (NC) (Ringqvist et al., 2022, Demars et al., 3 Nov 2025, Viswanath et al., 30 Apr 2026).

The fraction of the planetary surface emitting (filling factor $f_\mathrm{fill}$) is small: $f_\mathrm{fill} ≲ 1\%$, supporting the scenario of highly localized accretion shocks, analogous to classical T Tauri stars (CTTS), but with lower velocities and smaller widths due to the object's lower mass and radius (Ringqvist et al., 2022, Viswanath et al., 30 Apr 2026).

He I lines provide further constraints. The triplet/singlet ratio, small NC widths (∼16 km/s), NC centroids near systemic velocity, and tight correlation with UV excess indicate an origin in the dense, hot base of accretion shocks. The BC, with persistent mild redshift (∼16 km/s), likely forms in the outermost layer of the accretion shock structure (Viswanath et al., 30 Apr 2026).

4. Circumplanetary Disk, Chemistry, and Outflows

The spectral energy distribution (SED) of Delorme 1 AB b at λ > 10 μm is dominated by a circumplanetary disk (CPD), as inferred from JWST/MIRI-MRS spectra (Mâlin et al., 8 Oct 2025). Modeling the IR excess with a blackbody yields T_dust = 295 ± 27 K and an emitting region size of R_bb ≈ 19 R_Jup, confirming the presence of a dusty, warm CPD.

Molecular emission features from HCN (14.0 μm) and C₂H₂ (13.7 μm) are pronounced, with slab-fit column densities of N_HCN = 2.15×10^17 cm⁻² and N_C₂H₂ = 1×10^17 cm⁻². No CO, CO₂, or H₂O lines are detected down to N_O < 10^16 cm⁻², implying an elevated circumplanetary gas C/O ratio (C/O ≫ 1, conservatively ≳1), which is unprecedented among CPDs (Mâlin et al., 8 Oct 2025).

Pure-rotational H₂ lines (e.g., S(3) 9.67 μm, S(5) 6.91 μm) exhibit spatial extension up to ~40 AU, twice the planet's Hill radius, signifying a warm molecular outflow or disk wind. The corresponding mass-loss rate is $$\dot{M}_{wind} ≃ 2×10^{-10} M_\mathrm{Jup}\ \mathrm{yr}^{-1}$$, with wind-to-accretion ratios compatible with T Tauri disk winds.

No 10 μm silicate features are detected, indicative of a cavity between the sublimation radius (R_sub ≈ 0.0006 AU) and inner dust rim (R_bb ≈ 0.016 AU), possibly due to grain growth, drift, or incomplete dust replenishment. The inferred gas mass in the CPD is 10⁻³–10⁻² M_Jup, sufficient to sustain accretion at observed rates for a few Myr (Mâlin et al., 8 Oct 2025). The disk survives for ≳30 Myr, classifying it as a "Peter Pan" disk.

5. Time-domain Variability and Accretion Stability

Time-series high-resolution optical spectroscopy over 31 epochs reveals significant variability in Balmer line fluxes and shapes on week-to-month timescales, but limited variability within individual night (hour) sequences. The broad component of Balmer lines varies by up to ∼100% in flux, tightly correlated with UV excess; the narrow component is less variable and less strongly correlated with UV, pointing to differing physical origins (Demars et al., 3 Nov 2025). This behavior is analogous to that seen in CTTS, supporting the universality of magnetospheric accretion across the stellar–planetary mass range.

High-cadence monitoring could reveal rotational modulation of accretion hotspots and help constrain rotational period and the geometry of the magnetic field, as anticipated from the funnel-flow paradigm. The current rotation period is estimated to be ≲5 h, with v sin i ≳ 15 km/s for R_p = 1.5 R_Jup (Demars et al., 3 Nov 2025).

6. Formation Pathways and Evolutionary Context

The presence of a $\sim$13 M_Jup companion at 84 AU, persistently accreting at 40 Myr, poses challenges for classical planet formation models. Smoothed Particle Hydrodynamics (SPH) simulations including disk instability (GI), core-accretion, and migration/scattering were tested against the observed mass, separation, and accretion rate (Teasdale et al., 2024):

1. In-situ GI in a massive disk places a planet at the correct separation with high accretion rates but tends to overshoot the observed mass (M_p > 13 M_Jup).
2. Close-in GI + outward migration/scattering also reproduces separation and $\dot M$ but again yields masses above the observed value.
3. Core-accretion plus scattering can reproduce M_p ≈ 6–9 M_Jup and late-time $$\dot M ≈ 10^{-9}–10^{-8} M_\mathrm{Jup}\ \mathrm{yr}^{-1}$$, but it is difficult to reach a_p ≳80 AU without coincidental dynamical interactions.

All scenarios require a long-lived CPD, at odds with standard ∼5 Myr dispersal timescales. A plausible implication is that either disk lifetimes in such low-mass systems are systematically underestimated or that some replenishment mechanism (for instance, capture or transfer of material) operates. More precise age, mass, and $\dot{M}$ constraints are required to discriminate between these scenarios (Teasdale et al., 2024).

7. Significance, Open Questions, and Future Directions

Delorme 1 AB b exemplifies late-stage planetary accretion, magnetospheric funnel flows, and long-lived CPDs in a mass/separation regime previously thought inhospitable to ongoing disk-mediated accretion. Resolved multi-epoch spectroscopy and IR imaging have established Delorme 1 AB b as a prototype for:

• Magnetospheric-shock accretion in planetary-mass companions, with narrow and broad line components as diagnostics of geometry and kinematics.
• Carbon-rich CPDs with elevated C/O ratios, potentially influencing satellite composition and disk chemistry.
• Outflow phenomena (disk winds) at planetary mass scales.
• Challenges to both core-accretion and GI-based planetary formation models at wide binary separations and for disk-dispersal timescales.

Remaining uncertainties include the precise CPD mass and structure, the mechanism of disk longevity, the potential chromospheric contribution to narrow line components, and the planet's precise rotation and inclination. Further progress is expected from coordinated high-cadence multiwavelength spectroscopy and deep JWST imaging, with special focus on time-domain and spatially resolved diagnostics, to fully constrain accretion geometry and disk evolution in similar systems.

Key References: (Ringqvist et al., 2022, Betti et al., 2022, Demars et al., 3 Nov 2025, Viswanath et al., 30 Apr 2026, Mâlin et al., 8 Oct 2025, Teasdale et al., 2024, Eriksson et al., 2020)