Das and colleagues studied the effect of mass loss driven by stellar winds on the formation and evolution of Supermassive Stars (SMSs) in dense Nuclear Star Clusters (NSCs) using idealised N-body simulations. They found that the interaction of stellar wind and the gas inside the cluster play an important role in the evolution of SMS. Their study recently appeared in Arxiv.
So far, we have discovered more than two hundred supermassive black holes (SMBHs) with masses ≳ 109 M within the first ∼ Gyr after the Big Bang, which have challenged our general understanding of black hole growth and formation. How these massive objects actually formed and grew over cosmic time is one of the biggest puzzles to solve in astrophysics. These SMBHs are created from ‘seed’ black holes that grow via gas accretion and mergers. The ‘seed’ black holes are categorized into two categories: (i) low mass seeds (≲ 10² M) and (ii) high mass seeds (∼ 104¯6 M). These seeds were formed at redshift, 𝑧 ∼ 20 − 30, and then they rapidly grew to their final masses by gas accretion and mergers.
Low mass seeds are believed to be formed from Pop III stellar remnants. While, high mass seeds are believed to be formed from massive black holes via direct collapse. A key requirement for this scenario is large inflow rate of ∼ 0.1 Myr¯1 which can be obtained easily in metal free halos. In this scenario supermassive stars (SMSs) of masses ∼ 104¯5 M are formed, which are massive enough to grow to 109 M by 𝑧 ∼ 7. These SMSs collapse into seed BHs with minimal mass loss at the end of their lifetime. A possible formation channel of these SMSs is the interplay of gas accretion and runaway stellar collisions inside dense nuclear star clusters (NSCs). However, mass loss due to stellar winds could be an important limitation for the formation of the SMSs and affect the final mass. So, Das and colleagues now explored the effect of mass loss due to stellar winds on the final mass of SMSs produced in Nuclear Star Clusters (NSCs) via gas accretion and runaway collisions, using idealised N-body simulations.
Considering different accretion scenarios, they have studied the effect of the mass loss rates over a wide range of metallicities 𝑍∗ = [.001 − 1]Z and Eddington factors 𝑓Edd = 𝐿∗/𝐿Edd = 0.5, 0.7, & 0.9.
They found that,
- For a high accretion rate of 10¯4 Myr¯1, SMSs with masses ≳ 10³ M could be formed even in a high metallicity environment.
- For a lower accretion rate of 10¯5 Myr¯1, SMSs of masses ∼ 104 M can be formed for all adopted values of 𝑍∗ and 𝑓Edd, except for 𝑍∗ = Z and 𝑓Edd = 0.7 or 0.9.
- For Eddington accretion, SMSs of masses ∼ 103 M can be formed in low metallicity environments with 𝑍∗ ≲ 0.01Z. While, it will not be formed in regime 𝑍∗ ≳ 0.01Z
- The most massive SMSs of masses ∼ 105 M can be formed for Bondi-Hoyle accretion in environments with 𝑍∗ ≲ 0.5Z. While, the formation of SMS will not be possible in the high metallicity regime of 𝑍∗ ≳ 0.5Z.
Finally, authors mentioned that the interaction of the stellar wind and the gas inside the cluster play an important role in the evolution of the SMSs.
Interestingly, previous study have found that for a Salpeter type mass function stellar wind cannot remove the gas inside the cluster. Hence, we do not expect the stellar wind to remove gas from the cluster.— wrote authors of the study
They suggested, an intermediate regime is likely to exist where the mass loss from the winds might no longer be relevant, while the kinetic energy deposition from the wind could still inhibit the formation of a very massive object.
“In future work, it will be important to study detailed gas dynamics where the kinetic energy deposition of winds as well as the supernova feedback is taken into account.“— concluded authors of the study
Reference: Arpan Das, Dominik R. G. Schleicher, Shantanu Basu, Tjarda C. N. Boekholt, “Effect of mass loss due to stellar winds on the formation of supermassive black hole seeds in dense nuclear star clusters”, pp. 1-9, 2021. https://arxiv.org/abs/2105.03450
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