Deep H α Imaging Survey of IC 348 with the Hubble Space Telescope. I. Accretion Properties of Stellar and Substellar Objects

Accretion governs the growth of young stars and the early evolution of their circumstellar disks, yet population-level measurements of accretion are often hampered by heterogeneous diagnostics and by samples preferentially selected toward disk-bearing or accreting objects. This can impact the mass accretion rate–stellar mass ( M ̇ – M ⋆ ) relation, particularly at substellar masses. We present a uniform analysis of accretion in the ∼2 Myr old star-forming cluster IC 348 based on deep Hubble Space Telescope (HST) F656N imaging. Using H α excess as a single, homogeneous accretion tracer, we derive accretion rates and robust upper limits for 200 cluster members spanning the stellar-to-substellar mass regime (3 M ⊙ to 4 M Jup ). Accretion is detected in 37% ± 3% of the sample, with fractions of 34% ± 4% among stellar members and 46% ± 6% among substellar objects. For accretors alone, the inferred M ̇ – M ⋆ relation is consistent with those measured in similarly aged regions such as Lupus. In contrast, including weak accretors and nondetections increases the scatter and flattens the slope while lowering the intercept, demonstrating the strong influence of the low-accretion tail on population-level accretion relations. For free-floating planetary-mass objects in IC 348, extrapolating the accretors-only fit overpredicts M ̇ by approximately an order of magnitude compared to the fit that includes upper limits, which instead implies mass accretion rates of <10 −12 M ⊙ yr −1 . These results show that sample selection—specifically whether the sample is restricted to disk-bearing or accreting targets or instead is drawn from a complete membership census—is a dominant factor shaping population-level accretion relations across a wide dynamic range in stellar mass.