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Register a new userPulsation-driven mass loss from massive stars behind stellar mergers in metal-poor dense clusters
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The recent discovery of high-redshift (z > 6) supermassive black holes (SMBH) favors the formation of massive seed BHs in protogalaxies. One possible scenario is formation of massive stars ~ 1e3-1e4 Msun via runaway stellar collisions in a dense cluster, leaving behind massive BHs without significant mass loss. We study the pulsational instability of massive stars with the zero-age main-sequence (ZAMS) mass Mzams/Msun = 300-3000 and metallicity Z/Zsun = 0-0.1, and discuss whether or not pulsation-driven mass loss prevents massive BH formation. In the MS phase, the pulsational instability excited by the epsilon-mechanism grows in ~ 1e3 yrs. As the stellar mass and metallicity increase, the mass-loss rate increases to < 1e-3 Msun/yr. In the red super-giant (RSG) phase, the instability is excited by the kappa-mechanism operating in the hydrogen ionization zone and grows more rapidly in ~ 10 yrs. The RSG mass-loss rate is almost independent of metallicity and distributes in the range of ~ 1e-3-1e-2 Msun/yr. Conducting the stellar structure calculations including feedback due to pulsation-driven winds, we find that the stellar models of Mzams/Msun = 300-3000 can leave behind remnant BHs more massive than ~ 200-1200 Msun. We conclude that massive merger products can seed monster SMBHs observed at z > 6.
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We report the discovery of one extremely metal-poor (EMP; [Fe/H]<-3) and one ultra metal-poor (UMP; [Fe/H]<-4) star selected from the SDSS/SEGUE survey. These stars were identified as EMP candidates based on their medium-resolution (R~2,000) spectra, and were followed-up with high-resolution (R~35,000) spectroscopy with the Magellan-Clay Telescope. Their derived chemical abundances exhibit good agreement with those of stars with similar metallicities. We also provide new insights on the formation of the UMP stars, based on comparison with a new set of theoretical models of supernovae nucleosynthesis. The models were matched with 20 UMP stars found in the literature, together with one of the program stars (SDSS J1204+1201), with [Fe/H]=-4.34. From fitting their abundances, we find that the supernovae progenitors, for stars where carbon and nitrogen are measured, had masses ranging from 20.5 M_sun to 28 M_sun and explosion energies from 0.3 to 0.9x10^51 erg. These results are highly sensitive to the carbon and nitrogen abundance determinations, which is one of the main drivers for future high-resolution follow-up of UMP candidates. In addition, we are able to reproduce the different CNO abundance patterns found in UMP stars with a single progenitor type, by varying its mass and explosion energy.
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The cosmological lithium problem, that is, the discrepancy between the lithium abundance predicted by the Big Bang nucleosynthesis and the one observed for the stars of the Spite plateau, is one of the long standing problems of modern astrophysics. Recent hints for a possible solution involve lithium burning induced by protostellar mass accretion on Spite plateau stars. The purpose of this paper is to analyze the effect of protostellar accretion on low metallicity low-mass stars with a focus on PMS lithium evolution. We computed the evolution from the protostar to the MS phase of accreting models with final masses of 0.7 and 0.8 M$_odot$, and three metallicities Z=0.0001, Z=0.0010, and Z=0.0050. The effects of changing the main parameters affecting accreting models, that is the accretion energy (cold versus hot accretion), the initial seed mass $M_{seed}$ and radius $R_{seed}$, and the mass accretion rate $dot{m}$, have been investigated in detail. As for the main stellar properties and the surface $^7 Li$ abundance, hot accretion models converge to standard non-accreting ones within 1 Myr, regardless of the actual value of $M_{seed}$, $R_{seed}$, and $dot{m}$. Also, cold accretion models with a relatively large $M_{seed}$ ($gtrsim 10~M_{jup}$) or $R_{seed}$ ($gtrsim 1~R_odot$) converge to standard non-accreting ones in less than about 10-20~Myr. A drastically different evolution occurs whenever a cold protostellar accretion process starts from small values of $M_{seed}$ and $R_{seed}$ ($M_{seed}sim 1~M_{jup}$, $R_{seed} lesssim 1~R_odot$). These models almost entirely skip the standard Hayashi track evolution and deplete Li before the end of the accretion phase. The exact amount of depletion depends on the actual combination of the accretion parameters ($dot{m}$, $M_{seed}$, and $R_{seed}$), achieving in some cases the complete exhaustion of Li in the whole star.
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