ALMA submm measurements of the trans-Neptunian binary system satellites Ilmarë, Actaea, Hi’iaka and Namaka

While a significant fraction of large Trans-Neptunian Objects (TNOs) possess satellites, our understanding of the mechanisms that could lead to their formation of these systems remains limited. The prevailing view is that giant impacts are primarily responsible for creating these satellites; however, most simulations have focused on individual systems such as Pluto or Haumea [1,2]. Arakawa et al. [3] conducted simulations of satellite formation that revealed a wide range of possible outcomes—such as varying satellite-to-system mass ratios—depending on impact parameters (e.g., velocity and angle) as well as the internal structure and composition of the colliding bodies. Nonetheless, these findings are insufficient to robustly constrain the general formation conditions of large TNOs, primarily due to limited observational data. Moreover, they do not fully account for the high frequency of satellites among large TNOs, often relying on assumptions about conditions in the protoplanetary disk during the impact era. Barr and Schwamb [4] proposed a broader framework in which collisions are classified as either Charon-forming or as producing small icy fragments, yet it remains unclear how most known systems fit into this paradigm. In addition to the well-studied Pluto–Charon system, the satellite-to-primary mass ratios of the dwarf planet systems Eris–Dysnomia and Orcus–Vanth have also been determined through ALMA observations [5], yielding values of q≤0.085 and q=0.16, respectively. These measurements provide important constraints on their possible formation scenarios.

In this study, we present new ALMA Band 7 (~870 µm or 340 GHz) spatially resolved observations of two additional systems: Varda–Ilmarë and Salacia–Actaea. In each case, the satellites were successfully detected, with positions closely matching those predicted by the latest available orbital solutions (see Fig. 1). To further characterize these systems, we conducted detailed thermal emission modeling using supplementary archival data from the Spitzer Space Telescope and the Herschel Space Observatory, applying the Near-Earth Asteroid Thermal Model (NEATM).

Figure 1: ALMA band-7 (344 GHz) intensity contour maps of Varda (left), observed at a single epoch, and Salacia (middle and right), observed at two epochs. V1/S1 mark the primaries, and V2/S2 mark the satellites, in all cases very close to the expected position at the specific epoch.

For Ilmarë, we derive an effective diameter of D=403±40 km and a geometric albedo of p_V=0.068±0.011, indicating that Ilmarë is approximately half the size of Varda (D=740 km) and notably darker (with pV=0.099 for Varda). Assuming equal densities (ρ=1.15 g/cm³) for both bodies, the resulting satellite-to-primary mass ratio is q=0.16, closely matching that of the Orcus–Vanth system. Even under the assumption of a lower density of ρ=0.7 g/cm³—typical for trans-Neptunian objects (TNOs) in the ~400 km size range—the mass ratio remains high at q=0.11. This places Varda–Ilmarë among the systems with the largest known mass ratios, comparable to that of Pluto–Charon.

Using the same approach, we obtain D=393±33 km and p_V=0.021±0.004 for Actaea and  D=838±44 km and p_V=0.041±0.004 for Salacia. Again, assuming equal densities (ρ≈1.45 g/cm³) the satellite-to-primary mass ratio is q=0.10, however, this would be an exceptionally high density for the satellite. Assuming a more typical satellite density of 0.7 g/cm³ we obtain q = 0.044.

We also observed the Haumea system with ALMA in band-7 high spatial resolution measurements. For Hi’iaka, we dervied D=349±9 km and p_V=0.88±0.09, considering both occultation (Fernandez-Valenzuela, priv. comm.) and thermal emission data. This suggests that Hi’iaka has the brightest surface among small and mid-sized TNOs and Centaurs, and only the largest trans-Neptunian belt objects – Eris, Makemake and Triton – and the giant planet satellites have comparably high geometric albedos. We also derive a very high albedo surface, pV≥0.6, for Namaka from the same set of measurements

When supplemented with data from other dwarf planet systems, our results clearly indicate that the transition from smaller, nearly equal-sized binaries to systems with small satellites is not abrupt. Instead, there is a gradual decrease in the satellite-to-primary mass ratio with increasing system mass—a trend that binary formation models should be able to account for.

References:

[1] Canup, R. M. 2005, Science, 307, 546

[2] Canup, R. M. 2011, AJ, 141, 35

[3] Arakawa, S., Hyodo, R., & Genda, H. 2019, Nature Astronomy, 3, 802

[4] Barr, A. C. & Schwamb, M. E. 2016, MNRAS, 460, 1542

[5] Brown M.,E., Butler B.J., 2023, PSJ, 4, 193.