🔭 astrophysics

ExoplaNeT accRetion mOnitoring sPectroscopic surveY (ENTROPY) III. Optical He I line profiles of the accreting super Jupiter Delorme 1 (AB)b

This study utilizes high-resolution VLT/UVES spectroscopy to detect asymmetric helium emission lines in the accreting super-Jupiter Delorme 1 (AB)b, revealing accretion-driven post-shock and shock-front geometries analogous to classical T Tauri stars but with distinct kinematic properties.

Original authors: Gayathri Viswanath, Mickaël Bonnefoy, Catherine Dougados, Simon C. Ringqvist, Markus Janson, Dorian Demars, Aurora Sicilia-Aguilar, Jérôme Bouvier, Gabriel-Dominique Marleau, Evelyne Alecian, Gaël Cha

Published 2026-05-04

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CC BY 4.0

Original authors: Gayathri Viswanath, Mickaël Bonnefoy, Catherine Dougados, Simon C. Ringqvist, Markus Janson, Dorian Demars, Aurora Sicilia-Aguilar, Jérôme Bouvier, Gabriel-Dominique Marleau, Evelyne Alecian, Gaël Chauvin

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

Imagine a giant, glowing baby planet named Delorme 1 (AB)b. It's about 13 times heavier than Jupiter, orbiting a pair of stars far away (about 47 light-years). Even though this planet is roughly 40 million years old—ancient by human standards but a toddler in cosmic time—it is still hungry. It is actively swallowing gas from a disk swirling around it, a process called accretion.

This paper is like a high-speed, high-definition video camera capturing the moment this planet eats. The astronomers used a massive telescope (VLT) to take 33 "snapshots" of the planet over several months, looking specifically at the light emitted by Helium atoms.

Here is the story of what they found, explained simply:

1. The "Fingerprint" of Eating

When a planet (or a star) eats gas, the gas falls in at incredible speeds. When it hits the surface, it slams into a "shock," getting super hot and glowing. This glow creates specific lines of light, like a fingerprint.

The team found seven distinct helium fingerprints (specific colors of light) coming from this planet. This is a big deal because helium lines are usually very hard to see in small, faint objects like planets. Finding them is like hearing a whisper in a hurricane; it proves the planet is definitely accreting (eating) material.

2. The "Two-Tone" Voice

The most interesting discovery is how these helium lines look. They aren't just simple, smooth bumps. They have a weird, lopsided shape, like a voice that has two distinct tones:

The Narrow Tone: A sharp, quiet spike right in the middle.
The Broad Tone: A wider, louder hump that is slightly shifted to the "red" side (meaning the gas is moving slightly away from us, or falling in).

The Analogy: Imagine a drummer hitting a drum.

The Narrow Tone is the sharp crack of the drumstick hitting the skin. This happens right at the surface where the gas crashes down.
The Broad Tone is the rumble of the sound traveling through the air just above the drum. This happens slightly higher up in the atmosphere, where the gas is still falling but hasn't hit the surface yet.

The paper argues that both sounds are coming from the same "kitchen" (the shock zone where the gas hits the planet), just at different heights. The narrow sound is right at the "floor," and the broad sound is a bit higher up in the "ceiling."

3. The "Dance" of the Planet

The astronomers watched this planet over time and noticed the helium light was dancing.

Sometimes the light was bright; sometimes it was dim.
The shape of the "voice" changed slightly every few days.

This variability is a smoking gun for accretion. If the light were just coming from the planet's own internal heat or magnetic storms (like solar flares), it would be much more steady. The fact that it changes so quickly means the "food supply" (the gas falling in) is fluctuating, like a faucet that is being turned on and off.

4. Is it Eating or Just Hiccuping? (Activity vs. Accretion)

A major question in astronomy is: Is this light from the planet eating, or is it just the planet having a "hiccup" (magnetic activity) like a star?

The team compared the planet's light to:

Older, inactive planets: They are very quiet.
Young, active stars: They are loud and chaotic.

The Verdict: The planet is definitely eating. The amount of light it emits is far too bright to be just a "hiccup." However, the paper notes that there might be a tiny bit of "hiccuping" mixed in with the eating, but the meal is the main event.

5. Why This Matters

For a long time, we thought planets stopped eating gas when they were a few million years old. This planet is 40 million years old and is still eating! It's like finding a human teenager who is still drinking baby formula.

This discovery helps scientists understand how giant planets form and how long they keep their "nursery" (the gas disk) around them. It also proves that we can use helium lines to study these tiny, distant worlds, opening a new window to see how planets grow up.

In a nutshell: This paper is a detailed report card showing that a 40-million-year-old "super-Jupiter" is still actively eating gas from its surroundings, and we can hear the "sound" of that meal through the unique way helium light behaves.

1. Problem and Motivation

While the magnetospheric accretion paradigm is well-established for Classical T Tauri Stars (CTTS), its application to planetary-mass objects (PMOs, <15 $M_{Jup}$ ) remains poorly understood due to the faintness of accretion signatures at these masses. Previous studies of PMOs have largely relied on low-resolution ( $R_\lambda < 10,000$ ) hydrogen line emission (e.g., H $\alpha$ , Pa $\beta$ ).

Helium (He i) lines are critical diagnostics because they have higher excitation potentials than hydrogen, requiring high temperatures or proximity to ionizing sources, thus constraining the line-forming region more strictly. However, high-resolution ( $R_\lambda > 10,000$ ) helium line profiles from accreting PMOs had not been systematically studied. This paper addresses the gap by analyzing high-resolution optical He i lines from Delorme 1 (AB)b), a $\sim13 M_{Jup}$ companion to a binary system, to infer the geometry and physical conditions of its accretion flow.

2. Methodology

Target: Delorme 1 (AB)b, a $\sim13 M_{Jup}$ companion at $\sim84$ au from an M5.5 binary, located 47 pc away in the Tucana–Horologium association ( $\sim40$ Myr old).
Observations:
- Instrument: VLT/UVES (Ultraviolet and Visual Echelle Spectrograph).
- Epochs: 33 exposures collected over 11 nights between October 2022 and January 2023, combined with archival data from October 2021.
- Resolution: $R_\lambda \sim 50,000$ .
- Wavelength: Near-UV to optical (3300–6800 Å).
Data Reduction:
- Used the ESO/UVES pipeline and a custom CLEAN-like procedure for extracting the companion spectrum.
- Flux calibrated using the primary star's Gaia DR3 magnitude.
- Corrected for radial velocity ( $7.3 \pm 2.6$ km s $^{-1}$ ) and barycentric motion.
- Generated a "grand median" spectrum (high S/N) and individual night medians to assess variability.
Analysis Tools:
- STAR-MELT: Python package used for line identification and Gaussian fitting.
- Decomposition: Line profiles were fitted with single or double Gaussian components (Narrow Component - NC, Broad Component - BC).
- Scaling Relations: Accretion luminosity ( $L_{acc}$ ) and mass accretion rates ( $\dot{M}_{acc}$ ) were derived using empirical $L_{line}$ – $L_{acc}$ relations from Fiorellino et al. (2025) based on young stellar objects (YSOs).

3. Key Contributions

This work provides the first high-resolution report of resolved optical He i line profiles from an accreting planetary-mass companion. It extends the study of accretion geometry from the stellar regime (CTTS) to the planetary regime, specifically investigating the kinematics and physical origins of He i emission components.

4. Key Results

A. Detection and Line Characteristics

Seven He i lines detected with high confidence ( $>5\sigma$ ): $\lambda\lambda$ 3890, 4027, 4473, 4923, 5017, 5877, and 6680.
No He ii emission was detected, indicating the absence of highly ionized regions typical of very strong shocks or high-mass accretion.
Asymmetry: The lines $\lambda\lambda$ $λλ$ 5877, 4923, 4473, and 4027 exhibit clear asymmetry, decomposable into:
- Narrow Component (NC): Centered near $0$ km s $^{-1}$ (slightly redshifted, mean $\sim 0.1$ km s $^{-1}$ ) with FWHM $\sim 14-16$ km s $^{-1}$ .
- Broad Component (BC): Redshifted by $\sim 10-16$ km s $^{-1}$ with FWHM $\sim 16-33$ km s $^{-1}$ .
Variability: Significant flux variations ( $\sim 3-4\times$ ) were observed between epochs, correlating with accretion bursts. The BC/NC flux ratio varies, suggesting the asymmetry is intrinsic to the accretion process rather than a spectral resolution artifact of triplet lines.

B. Kinematics and Comparison to CTTS

Narrow Component (NC):
- Widths are much narrower than typical CTTS ( $\sim 16$ km s $^{-1}$ vs. $\sim 47$ km s $^{-1}$ ).
- Velocities are near-zero/slightly redshifted, consistent with emission from the post-shock region immediately above the planet's surface.
- The triplet-to-singlet line ratio ( $\lambda$ 5877/ $\lambda$ 6680 $\approx 1.7$ ) suggests high densities ( $>10^{11}$ cm $^{-3}$ ) and near-thermal equilibrium, typical of shock-heated gas.
Broad Component (BC):
- Consistently redshifted (unlike the often blueshifted BCs in CTTS which trace winds).
- Widths are significantly narrower than CTTS BCs ( $\sim 20-30$ km s $^{-1}$ vs. $>100$ km s $^{-1}$ ).
- Strong temporal correlation with the NC velocity ( $r=0.8$ ), implying a common origin mechanism.
- The redshift and narrow width suggest the BC originates in the shock structure closer to the shock front (higher temperature, infalling gas) rather than in an accretion column wind or Keplerian disk.

C. Accretion Parameters

Accretion Luminosity: Derived median $L_{acc} \approx 1.3^{+1.6}_{-0.7} \times 10^{-5} L_\odot$ .
Mass Accretion Rate: Derived $\dot{M}_{acc} \approx 0.7^{+0.9}_{-0.4} \times 10^{-8} M_{Jup}$ yr $^{-1}$ ( $\sim 7 \times 10^{-12} M_\odot$ yr $^{-1}$ ).
These values are comparable to, but slightly higher than, estimates derived from UV excess emission and H $\alpha$ scaling relations, though discrepancies exist depending on the assumed inner disk truncation radius ( $R_{in}$ ).

D. Accretion vs. Activity

The fractional He i luminosity ( $\log(L_{He}/L_{bol}) \approx -3.74$ ) is significantly higher than the "chromospheric noise" threshold for non-accreting M-dwarfs and Class III YSOs.
While chromospheric activity may contribute, the emission is dominated by accretion.
The primary star (Delorme 1 AB) shows symmetric, non-variable He i lines consistent with activity, providing a stark contrast to the asymmetric, variable lines of the companion.

5. Significance and Conclusions

Validation of Magnetospheric Accretion: The study confirms that magnetospheric accretion models, previously applied to stars, are applicable to planetary-mass objects, albeit with scaled-down velocities and widths due to the lower mass and gravity of the accretor.
Geometry of Accretion: The results support a model where He i emission arises from a temperature-stratified shock structure:
- NC: Forms in the cooler, post-shock region close to the planet surface.
- BC: Forms in the hotter, pre-shock or immediate shock region where infalling gas is still accelerating.
Episodic Accretion: The strong variability and the fact that the object was activity-dominated in a 2018 MUSE epoch (but accretion-dominated in 2022/2023) suggest that accretion in wide-orbit PMOs may be highly episodic.
Future Outlook: The detection of these lines at planetary masses highlights the need for more low-mass Class III samples to distinguish activity from accretion. Future instruments like ANDES (ELT) and CRIRES (VLT) will be crucial for observing near-IR He i $\lambda$ 10833 to further constrain accretion geometries and outflows in protoplanets.

In summary, this paper establishes that high-resolution helium spectroscopy is a powerful tool for probing the accretion physics of forming planets, revealing that the physical mechanisms driving accretion in planetary-mass objects are fundamentally similar to those in stars, but scaled to the specific gravitational and thermal environments of the planet.