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

TL;DR

This work presents a multi-epoch, high-resolution spectroscopic study of Delorme 1(AB)b to dissect Balmer-line emission and variability. By decomposing H i lines into wings and core components and comparing them to magnetospheric accretion and shock models, the authors find the wings are best explained by magnetospheric accretion funnels while the core resembles accretion shocks or chromospheric activity. UV-excess slab modeling provides robust accretion rates of order 10^{-12} M_⊙ yr^{-1}, with a notable outburst around 2022-10-14 to 2022-10-15 that amplified the UV excess and line wings. Across hours-to-years timescales, the Balmer lines exhibit limited hourly variability but substantial longer-term flux changes, underscoring a complex accretion geometry and the importance of multi-epoch, multi-wavelength campaigns for planetary-mass companions. The results support magnetospheric accretion as a key mechanism in PMCs and highlight the need for higher-cadence monitoring to constrain rotation and funnel geometry.

Abstract

Accretion processes in the planetary-mass regime remain poorly constrained, yet they strongly influence planet formation, evolution, and the composition of circumplanetary disks (CPDs). We investigate the resolved Balmer hydrogen emission-line profiles and their variability in the ~13Mjup, 30-45 Myr-old companion Delorme to constrain the underlying accretion mechanisms. Using VLT/UVES, we obtained 31 new epochs of high-resolution optical spectra (330-680 nm, R = 50,000), probing variability from hours to years. We analyze the shape and flux variability of hydrogen emission lines and compare them to two proposed origins: magnetospheric accretion funnels and localized accretion shocks. We detect Balmer lines from Halpha to H10 (6564-3799 AA) and a UV continuum excess, both indicative of ongoing accretion. All features are variable. The hydrogen lines decompose into two static components that vary only in flux. The broader velocity component correlates strongly with the UV excess and is qualitatively consistent with magnetospheric funnel models, but not with shock models. This component dominates the shape variability. The narrower component, which correlates less with the UV excess, is better matched by shock-emission models and drives most of the flux variability. Line fluxes show low variability on hour timescales but up to ~100% over weeks, similar to T Tauri stars. Our findings support magnetospheric accretion as the origin of the broad component. The narrow component may arise from accretion shocks or chromospheric activity. Higher-cadence observations could reveal rotational modulations and help constrain the object's rotation period and accretion geometry.

Figures (37)

• Figure 1: Illustration of the extraction process for one of the exposures, in the RED arm. Left: 2D reconstructed slit, before (top) and after (bottom) subtraction of the PSF models for both the primary and companion. Right: illustration of the extraction process at a single wavelength bin within $\mathrm{H}{\alpha}$. The blue and green shaded regions are the various Gaussian components used in the PSF fitting process for the primary and companion respectively (3 Gaussian components here for the RED arm). In both cases, the lower panel shows the modeling residuals.
• Figure 2: Illustration of the residual contamination correction, for the exposure 2022-11-05 UT 01:22:25. This corresponds to the case of highest contamination. Top panel: residual contamination factor (black) smoothed with a 10-px wide moving Gaussian box (red) and its associated error bars (black shaded region). Middle panel: companion spectrum as the output of the extraction process (black) with the contamination contribution (red) and contamination-corrected companion spectrum (green). All black-shaded regions were excluded from the fit, they correspond to emission lines and detector edges. Third row: contamination contribution within emission lines (same colors). The contamination removal process is able to remove most of the residual primary flux in the companion spectrum, which accounts for up $\sim10$ % of the lines flux depending on the exposure.
• Figure 3: Final spectra over the full spectral range. All Delorme 1 (AB)b spectra, overlaid in black, with the mean spectrum in red. All spectra were smoothed to $R=5000$ for clarity. The UV excess of Delorme 1 (AB)b is clearly visible below $\sim$3700 Å, as well as the generally flat continuum shape. The line marked as an artifact is due to a bad pixels row of the detector.
• Figure 4: Balmer lines of Delorme 1 (AB)b. Each panel shows the mean line of each observing epoch (11 epochs), from $\mathrm{H}{\alpha}$ to $\mathrm{H}{\epsilon}$. The last panel shows the mean line (over all epochs) for each transition order, all the way up to H$_\uptheta$/H10. All lines are plotted after subtracting the local continuum baseline, averaged between $(-750;-400)$ and $(+400;+750)$$\mathrm{km/s}$. The lines show great profile and amplitude variability across epochs.
• Figure 5: Comparison of the mean $\mathrm{H}{\alpha}$ line profile of Delorme 1 (AB)b to that of two young sub-stellar objects with disks (left: CFHT BD Tau 4, center: ChaHa-1) and of a Class III 15 $\mathrm{M}_{\mathrm{Jup}}$ brown dwarf (right: KPNO Tau 4). We scaled (in red) the profile of Ca Ha 1 to the free-fall velocities expected for Delorme 1 (AB)b. The profiles are similar to that of CFHS BD Tau 4 and ChaHa-1, but not to that of KPNO Tau 4.