We search for excess in-transit absorption of neutral helium at 1.083 μm in the atmospheres of the young
(<800 Myr) sub-Jovian (0.2–0.5 RJ) planets HD 63433b, K2-100b, and V1298 Tau c using high-resolution
(R∼25,000) transit observations taken with Keck II/NIRSPEC. Our observations do not show evidence of helium
absorption for any of the planets in our sample. We calculate 3σ upper limits on the planets’ excess helium
absorption of <0.47% for HD 63433b, <0.56% for K2-100b, and <1.13% for V1298 Tau c. In terms of equivalent
width, we constrain these to <2.52, <4.44, and <8.49 mÅ for HD 63433b, K2-100b, and V1298 Tau c,
respectively. We fit our transmission spectra with one-dimensional Parker wind models to determine upper limits
on the planets’ mass-loss rates of <7.9 × 1010, <1.25 × 1011, and <7.9 × 1011g s−1
. Our nondetections align with
expectations from one-dimensional hydrodynamic escape models, magnetic fields, and stellar wind confinement.
The upper limits we measure for these planets are consistent with predicted trends in system age and He equivalent
width from 1D hydrodynamic models.