Time-Series Photometry of Main-Belt Binary Asteroids: A Case Study of (7344) Summerfield

Binary asteroids provide a unique opportunity to study the key physical and dynamic properties of small solar system bodies. Unlike single bodies, binaries allow us to estimate component masses and orbital parameters based on mutual orbital dynamics. This provides constraints on system architecture and formation mechanisms. These systems are particularly valuable for testing evolutionary scenarios in which a rapidly rotating progenitor undergoes rotational fission, followed by binary reaccumulation of the fragments into a bound system.
Among the known formation pathways, YORP-induced rotational fission is expected to operate more efficiently for small asteroids near the Sun, whereas collisional formation dominates larger bodies. Theoretical time-scale comparisons suggest that these two mechanisms become equally effective at around 6 km in diameter (Jacobson et al. 2014), which closely matches the size of (7344) Summerfield (D = 6.25 km; NEOWISE). The asteroid’s rotation period of ~2.6 hours (Pray et al., 2017, CBET 4412) is slightly above the spin barrier (~2.2 hours), which is consistent with a history of rotational acceleration. Although a collisional origin cannot be ruled out, the mass ratio of ~0.18 is consistent with the observed limit for binaries formed by rotational fission (q < 0.2; Pravec et al. 2010). Near this threshold, fission events may be especially violent, allowing for slightly slower rotation than the critical value.
(7344) Summerfield lies near the critical boundaries where rotational fission might just be able to form a binary system, making a good candidate to evaluate this mechanism. To investigate this possibility, we conducted a photometric study of (7344) Summerfield, a confirmed main-belt binary chosen for its previously reported periodic brightness variations and potential mutual events, high visibility, and favorable photometric amplitude under our observing conditions.
We conducted observations using the 1.8 m Bohyunsan Optical Astronomy Observatory (BOAO), the 0.6 m Sobaeksan Optical Astronomy Observatory (SOAO), and the 1.0 m Lemmonsan Optical Astronomy Observatory (LOAO) from February to April 2025. We employed R-band time-series photometry for this study. This multi-site approach improved the system’s temporal resolution and phase coverage across its rotation.
Initial analysis reveals a rotation period of approximately 2.59 hours, with mutual events manifesting as characteristic brightness dips. We measured an eclipse depth of 0.04 magnitudes (~3.7% brightness decrease), corresponding to a radius ratio of ~0.19 between the two components. These values are consistent with those reported independently by Pray et al. (2017, CBET 4412), which further supports the binary nature of the system.
Assuming that (7344) Summerfield is an S-type asteroid, we use a bulk density of 2700 kg/m^3, which is typical for this class (Carry 2012), and we assume that both components are spherical. We then estimate the component masses as ~3.5 × 10¹⁴ kg for the primary and ~2.4 × 10¹² kg for the secondary. Using the orbital period of P=17.41 hr from CBET 4412 and applying Kepler’s third law, we estimate the system’s semi-major axis to be approximately 13 km. This value is consistent with the ~11 km estimate reported in the Asteroids with Satellites Database (Johnston’s Archive), considering large uncertainties in shape and bulk density. We measured a lightcurve amplitude of 0.177 magnitudes for the primary. Assuming a prolate shape (a ≥ b ≃ c), and applying the relation a/b = 10^(0.4A) (Binzel et al. 1989), we calculate an elongation ratio of a/b ≈ 1.18, which indicates that the primary has a low degree of elongation. This shape is characteristic of primaries formed via rotational fission, which evolve toward a top-shape due to reaccumulation and spin-driven relaxation.
Studying additional binary systems near the critical thresholds for rotational fission, such as (7344) Summerfield, through systematic photometric surveys would enable a statistical characterization of their properties. This approach could significantly advance our understanding of how rotational fission contributes to the formation and dynamical evolution of binary asteroids in the main belt.