Exomoon search with VLTI/GRAVITY around the substellar companion HD 206893 B

TL;DR

This work pioneers exomoon searches with high-precision astrometry using VLTI/GRAVITY by monitoring the HD 206893 system, testing whether anomalies in B's motion reveal a moon. It combines astrometric modeling (including a 3-body hierarchical fit) with medium-resolution spectroscopy to characterize B's atmosphere, finding tentative moon signatures (mass ∼$0.5\,M_{\rm Jup}$, period ∼$0.76$ yr) and a robust upper limit of $0.8\,M_{\rm Jup}$. The orbital analysis refines the B and c orbits (B: $a_B\approx10.75$ au; c: $e_c\approx0.28$) and confirms B's and c's coplanarity, while the spectrum confirms water absorption with no CO detected and constrains atmospheric C/O. The study also maps out optimistic targets for GRAVITY+ and argues that high-precision astrometry is a viable path to detecting Neptune- to Earth-mass moons around directly imaged planets, motivating follow-up campaigns and future instrumentation to reach $\sim1\,\mu$as precision.

Abstract

Direct astrometric detection of exomoons remains unexplored. This study presents the first application of high-precision astrometry to search for exomoons around substellar companions. We investigate whether the orbital motion of the companion HD 206893 B exhibits astrometric residuals consistent with the gravitational influence of an exomoon or binary planet. Using the VLTI/GRAVITY instrument, we monitored the astrometric positions of HD 206893 B and c across both short (days to months) and long (yearly) timescales. This enabled us to isolate potential residual wobbles in the motion of component B attributable to an orbiting moon. Our analysis reveals tentative astrometric residuals in the HD 206893 B orbit. If interpreted as an exomoon signature, these residuals correspond to a candidate (HD 206893 B I) with an orbital period of approximately 0.76 years and a mass of $\sim$0.4 Jupiter masses. However, the origin of these residuals remains ambiguous and could be due to systematics. Complementing the astrometry, our analysis of GRAVITY $R=4000$ spectroscopy for HD 206893 B confirms a clear detection of water, but no CO is found using cross-correlation. We also find that AF Lep b, and $β$ Pic b are the best short-term candidates to look for moons with GRAVITY+. Our observations demonstrate the transformative potential of high-precision astrometry in the search for exomoons, and proves the feasibility of the technique to detect moons with masses lower than Jupiter and potentially down to less than Neptune in optimistic cases. Crucially, further high-precision astrometric observations with VLTI/GRAVITY are essential to verify the reality and nature of this signal and attempt this technique on a variety of planetary systems.

Figures (19)

• Figure 1: Schematic of the HD 206893 system.
• Figure 2: Model spectrum of HD 206893 A obtained by fitting a BT-NextGen stellar model atmosphere (black line) to archival photometry (orange points) and the Gaia XP spectrum (green points). The top panel shows the filter transmission curves and the bottom panel shows the residuals between data and model.
• Figure 3: Combined R $\sim$ 4000 spectrum obtained with GRAVITY for HD 206893 B by averaging the data obtained at different epochs (see Fig. \ref{['figspectrumapp']}).
• Figure 4: Diagram of the orbit of HD 206893 B with a potential moon around it, causing it to shift slightly due to the moon's gravitational potential. It illustrates the notation used in section \ref{['back']}.
• Figure 5: Analytical predictions of $M_{\rm moon}$ (the mass of the moon in masses of Neptune) as a function of $T_{\rm moon}$ (the period of the moon in days). The solid black lines show the model's prediction for astrometric accuracies of 10 and 100 ${\rm \mu}$as, as indicated on the graph. The values used are those of Eq. \ref{['Mmoon']}, i.e. $d=10$ pc and $M_{\rm pla}=10 \, {\rm M}_{\rm Jup}$. The dotted line shows the case of 10 $\mu$as considering $M_{\rm pla}=1 \, {\rm M}_{\rm Jup}$. If the system is instead at 100 pc, the lines are pushed up by a factor of 10. The areas beyond the Roche limit and the Hill sphere are shown in grey. The Hill sphere is shown for two values of the semi-major axis of the planet $a_{\rm pla}$ equal to 1 (closest) and 10 au (farthest). The horizontal lines indicate moon masses of 1 M$_{\rm Jup}$, 1 M$_{\rm Nep}$, 1 M$_\oplus$, and 1 M$_{\rm Ganymede}$ from top to bottom.