低金属丰度原恒星和辐射反馈导致的最大恒星质量
Low-Metallicity Protostars and the Maximum Stellar Mass Resulting from Radiative Feedback
摘要：
新生恒星的最终质量被设定在恒星上的质量吸积终止的时期。我们研究了吸积原恒星的演化和低金属丰度环境中吸积的极限。通过星前核不同金属丰度的温度演化，估算了原恒星的吸积速率。导出的速率随金属丰度的降低而增加，从Z = Z_sun时的10^-6 M_sun/yr增加到Z = 0时的10^-3 M_sun/yr。利用导出的吸积率，数值计算了原恒星的演化。我们发现，在较低的金属丰度，原恒星有一个较大的半径，并达到零年龄主序（ZAMS）在较高的恒星质量。利用这一原恒星的演化，我们评估了辐射反馈阻碍质量吸积的恒星质量上限。我们考虑了辐射压力对吸积包络的影响，以及HII区的膨胀。当Z > 10^-3 Z_sun时，物质吸积最终被作用在包层中尘埃颗粒上的辐射压力所终止，而当金属量较低时，物质吸积最终被HII区的膨胀所终止。这些效应的质量极限随着金属丰度的降低而增加，从Z = Z_sun时的10 M_sun增加到Z = 10^-6 Z_sun时的300 M_sun。吸积的终止发生在所有金属丰度的中心恒星到达ZAMS之后，这使得我们在讨论恒星反馈的质量上限时可以忽略原恒星演化的影响。在低金属丰度的云中，由线冷却引起的碎裂产生质量足够大的星前核，最终的恒星质量由反馈效应决定。
英文摘要：
The final mass of a newborn star is set at the epoch when the mass accretion onto the star is terminated. We study the evolution of accreting protostars and the limits of accretion in low metallicity environments. Accretion rates onto protostars are estimated via the temperature evolution of prestellar cores with different metallicities. The derived rates increase with decreasing metallicity, from 10^-6 M_sun/yr at Z = Z_sun to 10^-3 M_sun/yr at Z = 0. With the derived accretion rates, the protostellar evolution is numerically calculated. We find that, at lower metallicity, the protostar has a larger radius and reaches the zero-age main-sequence (ZAMS) at higher stellar mass. Using this protostellar evolution, we evaluate the upper stellar mass limit where the mass accretion is hindered by radiative feedback. We consider the effects of radiation pressure exerted on the accreting envelope, and expansion of the HII region. The mass accretion is finally terminated by radiation pressure on dust grains in the envelope for Z > 10^-3 Z_sun and by the expanding HII region for lower metallicty. The mass limit from these effects increases with decreasing metallicity from 10 M_sun at Z = Z_sun to about 300 M_sun at Z = 10^-6 Z_sun. The termination of accretion occurs after the central star arrives at the ZAMS at all metallicities, which allows us to neglect protostellar evolution effects in discussing the upper mass limit by stellar feedback. The fragmentation induced by line cooling in low-metallicity clouds yields prestellar cores with masses large enough that the final stellar mass is set by the feedback effects.
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