From Stellar Progenitors to Multimessenger Signals: Subsolar Compact Objects, Gas-Modified Gravitational Waves and Electromagnetic Counterparts

Fragmentation of collapsar accretion disks offers a potential stellar origin for subsolar compact objects, with gravitational-wave (GW) signals modified by gas and possible electromagnetic (EM) counterparts from stellar collapse and jet activity. We connect rotating MESA progenitors to disk formation, fragmentation, fragment growth and migration, and jet propagation, exploring potential supernova and kilonova associations. Of 884 evaluated progenitors, 564 form circularized disks and 67 satisfy our fragmentation and validation criteria. Fragmenting progenitors span zero-age main-sequence masses of 27--64 Msun and initial rotation speeds of 150--600 km/s, at metallicities ~0.02Zsun or 0.2Zsun. We compare four initial fragment-mass prescriptions: the Toomre mass and 0.1M_J, M_J, and 10M_J, where M_J is our adopted Jeans mass scale. Toomre-mass seeds grow by factors of 55--176, reaching 0.085--0.132 Msun, whereas 10M_J seeds gain only 1.6\%--4.8\%, reaching 0.536--1.171 Msun. Accretion increases the GW amplitude, while accretion torques and Type~I/II migration accelerate inspiral and suppress low-frequency characteristic strain relative to fixed-mass vacuum comparisons. The resulting changes in spectral shape and duration motivate waveform models incorporating gas effects. At D_L=100 Mpc aLIGO signal-to-noise ratios span 1.44--4.69 for Toomre-mass seeds, 1.71--5.12 for 0.1M_J, 2.16--6.29 for M_J, and 8.80--20.66 for 10M_J. Successful jet breakout occurs in 501 models, including 190 without eligible disks. Among fragmenting progenitors, 41 permit breakout and 26 have choked jets; breakout can precede disk formation or follow inspiral. These results motivate searches for gas-modified subsolar inspirals with diverse EM associations and further investigation of possible connections to candidates such as S251112cm and S250818k