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Dense molecular cloud cores as a source of micrometer-sized grains in galaxies

Coreshine in dense molecular cloud cores (dense cores) is interpreted as evidence for micrometer-sized grains (referred to as very large grains, VLGs). VLGs may have a significant influence on the total dust amount and the extinction curve. We estimate the total abundance of VLGs in the Galaxy, assuming that dense cores are the site of VLG formation. We find that the VLG abundance relative to the total dust mass is roughly $ϕ_\mathrm{VLG}\sim 0.01(1-ε)/ε(τ_\mathrm{SF}/5\times 10^9~\mathrm{yr})^{-1} (f_\mathrm{VLG}/0.5)(t_\mathrm{shat}/10^8~\mathrm{yr})$, where $ε$ is the star formation efficiency in dense cores, $τ_\mathrm{SF}$ the timescale of gas consumption by star formation, $f_\mathrm{VLG}$ the fraction of dust mass eventually coagulated into VLGs in dense cores, and $t_\mathrm{shat}$ the lifetime of VLGs (determined by shattering). Adopting their typical values for the Galaxy, we obtain $ϕ_\mathrm{VLG}\sim 0.02$--0.09. This abundance is well below the value detected in the heliosphere by Ulysses and Galileo, which means that local enhancement of VLG abundance in the solar neighborhood is required if the VLGs originate from dense cores. We also show that the effects of VLGs on the extinction curve are negligible even with the upper value of the above range, $ϕ_\mathrm{VLG}\sim 0.09$. If we adopt an extreme value, $ϕ_\mathrm{VLG}\sim 0.5$, close to that inferred from the above spacecraft data, the extinction curve is still in the range of the variation in Galactic extinction curves, but is not typical of the diffuse ISM.