http://arxiv.org/abs/2208.04337
Optically luminous quasars are metal rich across all redshifts. Surprisingly, there is no significant trend in the broad-line region (BLR) metallicity with different star formation rates (SFR). The average N V/ C IV metallicity does not exceed $9.5~Z_\odot$ and the average Si IV/ C IV metallicity is similarly $\sim10~Z_\odot$. Combined, these observations are indicative of a metallicity ceiling. Here, we study whether a metallicity ceiling can exist in quasar disks due to the evolution of embedded stars. We develop a simple model that starts with gas in a closed box, which is enriched by cycles of stellar evolution until eventually newly formed stars undergo significant mass loss before they reach the supernovae stage and no further enrichment is possible. Using the MESA code, we create a grid over a parameter space of masses ($>8~M_\odot$) and metallicities ($1-10~Z_\odot$), and locate portions of the parameter space where mass loss via winds occurs on a timescale shorter than the lifetime of the stars. We find that for reasonable assumptions about stellar winds, sufficiently massive ($8-22~M_\odot$) and metal-rich ($\sim9~Z_\odot$) stars lose significant mass via winds and fail to evolve to the supernovae stage, thereby failing to enrich and increase the metallicity of their surroundings. This suggests that a metallicity ceiling is the final state of a closed-box system of gas and stars.
S. Cheng and A. Loeb
Wed, 10 Aug 22
58/66
Comments: 8 pages, 6 figures, submitted to ApJ
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