🔭 astrophysics

ExoplaNeT accRetion mOnitoring sPectroscopic surveY (ENTROPY) - II. Time series of Balmer line profiles of Delorme 1(AB)b

Using high-resolution VLT/UVES time-series spectroscopy, this study reveals that the accretion signatures in the planetary-mass companion Delorme 1(AB)b consist of a broad, magnetospheric funnel-driven component and a narrow, shock-related component, both exhibiting significant flux variability over timescales ranging from hours to weeks.

Original authors: Dorian Demars, Mickaël Bonnefoy, Catherine Dougados, Gayathri Viswanath, Simon C. Ringqvist, Markus Janson, Yuhiko Aoyama, Thanawuth Thanathibodee, Gabriel-Dominique Marleau, Carlo F. Manara, Elisabet

Published 2026-02-04

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Original authors: Dorian Demars, Mickaël Bonnefoy, Catherine Dougados, Gayathri Viswanath, Simon C. Ringqvist, Markus Janson, Yuhiko Aoyama, Thanawuth Thanathibodee, Gabriel-Dominique Marleau, Carlo F. Manara, Elisabetta Rigliaco, Judith Szulágyi, Aurora Sicilia-Aguilar, Jérôme Bouvier, Evelyne Alecian, Simon Petrus, Mathis Houllé

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

The Big Picture: A Growing Planetary "Baby"

Imagine a planet, roughly 13 times the mass of Jupiter, that is about 30 to 45 million years old. In the world of astronomy, this is considered "middle-aged" for a planet, like a human in their 30s. Usually, by this age, a planet has stopped growing and has settled down.

However, the astronomers in this study found that this specific planet, named Delorme 1 (AB)b, is still actively eating. It is currently swallowing gas and dust from a disk swirling around it, a process called accretion. Think of it like a teenager who should have stopped growing but is still having a massive growth spurt.

The team used a giant telescope (VLT) to take 31 high-resolution "snapshots" of this planet over the course of a year. They were looking at the light emitted by hydrogen gas (the most common element in the universe) to understand how the planet is feeding.

The "Fingerprint" of Feeding

When the planet eats, it doesn't just swallow silently. The gas heats up and glows, creating specific lines of light in the spectrum (like a barcode). The scientists looked at these "Balmer lines" (a specific set of hydrogen fingerprints).

They discovered that these light signatures are not static; they change shape and brightness over time. To make sense of this complex, shifting light, the researchers broke the signal down into two distinct parts, like separating a choir into two different sections:

The "Wings" (The Fast, Chaotic Part):
- What it looks like: This part of the light signal is broad and has deep "dips" or shadows in it.
- The Analogy: Imagine a waterfall crashing down. The water is moving fast and hitting the rocks below, creating a wide spray and a lot of turbulence.
- What it means: The scientists found that this "Wings" component is strongly linked to a burst of ultraviolet (UV) light coming from the planet. This UV light is the heat generated when gas slams into the planet's surface. The shape of the "Wings" perfectly matches computer models of gas falling down a magnetic funnel (like water going down a drain). This proves that magnetospheric accretion is happening: the planet's magnetic field is guiding the gas onto its surface.
The "Core" (The Steady, Central Part):
- What it looks like: This is a narrower, more symmetrical peak of light right in the middle of the signal.
- The Analogy: Imagine a steady, glowing campfire in the center of the waterfall. It's bright and consistent, but not as chaotic as the falling water.
- What it means: This part is trickier. It doesn't correlate as strongly with the UV heat. It looks like it could be caused by the gas hitting the surface (a "shock"), but it also looks a lot like the light produced by active, stormy stars (chromospheric activity). The scientists aren't 100% sure if this is purely from eating or if it's just the planet having a "sunny day" of magnetic storms.

The Mystery of the "Burst"

The team watched the planet for a year and noticed something exciting. For most of the time, the planet was eating at a steady, slow pace. But then, for a few days in October 2022, the planet went into a feeding frenzy.

The Analogy: Think of a person who usually eats a normal lunch, but then suddenly eats a massive Thanksgiving dinner in one sitting.
The Result: During this "burst," the light from the "Wings" (the waterfall) got much brighter and changed shape. The amount of gas the planet was eating jumped up significantly. This shows that planetary growth isn't always a smooth, steady process; sometimes it happens in big, sudden gulps.

Why This Matters

This study is important because it helps us understand how planets form and grow.

The "Old" Planet: Finding a planet this old (30-45 million years) still actively eating is surprising. It suggests that some planets can hold onto their food supply (a circumplanetary disk) for much longer than we thought.
The Method: By breaking the light into "Wings" and "Core," the scientists could tell the difference between gas falling from a magnetic funnel and other types of light. This is like being able to hear the difference between a jet engine and a siren in a noisy city.

What They Didn't Find

The scientists also looked for signs that the planet's rotation was causing the light to wobble (like a lighthouse beam). They didn't find a clear pattern yet. This is because their "snapshots" weren't taken frequently enough to catch the planet spinning. They suggest that future observations need to be taken more often (like taking a video instead of a photo) to catch the rotation.

Summary

In short, the paper tells us that Delorme 1 (AB)b is a "late bloomer" planet that is still growing. It eats gas in two ways: a fast, magnetic funnel flow (the "Wings") and a steady, shock-driven flow (the "Core"). Occasionally, it goes on a massive eating binge. By studying the light from this planet, astronomers are learning that the final stages of planet formation can be much more active and variable than previously imagined.

Technical Summary: ENTROPY II. Time series of Balmer line profiles of Delorme 1(AB)b

Problem and Context
Accretion processes in the planetary-mass regime remain poorly constrained, despite their critical role in the formation and evolution of planets and circumplanetary disks (CPDs). While emission lines indicative of active accretion have been detected in a few tens of planetary-mass objects (PMOs), the mechanisms driving these accretion flows—specifically whether they arise from magnetospheric accretion funnels or localized accretion shocks—are not yet fully understood. Furthermore, the variability timescales of these lines, which could inform detection strategies for protoplanets, are poorly characterized for planetary-mass companions. Previous studies on objects like PDS 70 b and c, and GQ Lup b, have shown variability, but high-resolution, multi-epoch monitoring of resolved line profiles in this mass regime is scarce.

Methodology
This study presents a high-resolution (R ≈ 50,000) optical spectroscopic monitoring campaign of the ∼13 $M_{Jup}$ companion Delorme 1 (AB)b, conducted with the VLT/UVES instrument over 31 epochs between October 2022 and January 2023. The observations cover the wavelength range 330–680 nm, probing variability from hours to years.

The data reduction and analysis involved several key technical steps:

Extraction and Contamination Removal: Due to the proximity of the host binary (separation ∼1.77″), a two-stage process was developed to subtract the host star's halo. This involved multi-Gaussian fitting of the primary signal within the 2D spectral traces, followed by a post-processing step to model and remove residual stellar contamination using a master spectrum and a wavelength-dependent contamination factor.
Line Decomposition: Instead of assuming Gaussian profiles, a custom algorithm based on the L-BFGS-B optimization method was employed to decompose the Balmer emission lines (H $\alpha$ through H10) into two static components (fixed shape) with variable amplitudes. This empirical approach was chosen to handle the complex line shapes without assuming specific physical profiles a priori.
Modeling: The decomposed line components were compared against two theoretical frameworks:
- Magnetospheric Accretion Models: Using the funnel flow models of Thanathibodee et al. (2019).
- Shock Models: Using the accretion shock models of Aoyama et al. (2018).
UV Excess Analysis: The ultraviolet continuum excess (below ∼3700 Å) was modeled using slab models of hydrogen emission in local thermal equilibrium (LTE) to derive accretion luminosities and rates, which were then compared to those derived from line fluxes.

Key Results

Detection and Variability: The study detected Balmer lines from H $\alpha$ to H10 and a variable UV continuum excess, confirming ongoing accretion. All lines and the UV excess exhibited variability.
Two-Component Structure: The emission lines were successfully decomposed into two distinct components:
- A broad "wings" component characterized by deep absorption features and extended velocity wings (up to ±500 km/s).
- A narrower "core" component with a more symmetrical, plateau-like profile.
Physical Origins:
- The wings component shows a strong correlation with the UV excess and is qualitatively reproduced by magnetospheric accretion funnel models. Its profile features (asymmetries, broad wings) are inconsistent with simple shock models.
- The core component correlates less strongly with the UV excess. While it can be reproduced by 1D accretion shock models, its profile similarity to active M-dwarfs suggests it may also originate from chromospheric activity.
Variability Timescales:
- On hour-timescales, the lines show low relative flux variability (∼8–10%).
- On timescales of weeks to years, variability increases significantly, reaching up to ∼100%.
- The broad wings component exhibits the highest relative variability and is responsible for the changes in line profile shape, particularly during an accretion outburst observed in October 2022. The narrower core component drives the majority of the absolute flux variability.
Accretion Rates: Modeling the UV excess yields accretion rates of $\dot{M} \approx (1–3) \times 10^{-12} M_{\odot}/yr$ for most epochs, with a peak of $\sim 6.6 \times 10^{-12} M_{\odot}/yr$ during the October 2022 outburst. These rates are consistent with the wings component modeling but significantly lower than rates derived from shock modeling of the core component.

Significance and Conclusions
The paper concludes that the properties of the broad wings component strongly support a magnetospheric accretion scenario for Delorme 1 (AB)b, providing the first resolved line profile evidence of this mechanism in a planetary-mass companion. The narrow core component's origin remains ambiguous, potentially representing a mix of accretion shocks and chromospheric activity.

The study highlights that while the object is old (30–45 Myr), it retains a mass reservoir (likely a "Peter Pan" disk) and continues to accrete. The variability behavior, characterized by low short-term variability but significant long-term changes, mirrors that of T Tauri stars. The authors note that the moderate variability on hour-timescales is likely insufficient to explain the low detection yield of H $\alpha$ imaging surveys for protoplanets, suggesting that longer-term variability or other factors may be more relevant. Finally, the work underscores the necessity of multi-epoch, multi-wavelength analysis to disentangle the complex accretion geometries and mechanisms operating in planetary-mass objects.