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EVIDENCE FOR A MOLECULAR CLOUD ORIGIN OF GAMMA-RAY BURSTS: IMPLICATIONS FOR THE NATURE OF STAR FORMATION IN THE UNIVERSE
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外文期刊>The Astrophysical journal
>EVIDENCE FOR A MOLECULAR CLOUD ORIGIN OF GAMMA-RAY BURSTS: IMPLICATIONS FOR THE NATURE OF STAR FORMATION IN THE UNIVERSE
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EVIDENCE FOR A MOLECULAR CLOUD ORIGIN OF GAMMA-RAY BURSTS: IMPLICATIONS FOR THE NATURE OF STAR FORMATION IN THE UNIVERSE
It appears that the majority of rapidly and well-localized gamma-ray bursts with undetected or dark optical afterglows ("dark bursts") occur in clouds of size R approx> 10L_(49)~(1/2) pc and mass M approx> 3 x 10~5L_(49) solar mass, where L is the isotropic-equivalent peak luminosity of the optical flash. We show that clouds of this size and mass cannot be modeled as a gas that is bound by pressure equilibrium with a warm or hot phase of the interstellar medium (i.e., a diffuse cloud): such a cloud would be unstable to gravitational collapse, resulting in the collapse and fragmentation of the cloud until a burst of star formation reestablishes pressure equilibrium within the fragments and the fragments are bound by self-gravity (i.e., a molecular cloud). 'Consequently, dark bursts probably occur in molecular clouds, in which case dark bursts are probably a by-product of this burst of star formation, if the molecular cloud formed recently, and/or the result of lingering or latter generation star formation if the molecular cloud formed some time ago. We then show that if bursts occur in Galactic-like molecular clouds, the column densities of which might be universal, the number of dark bursts can be comparable to the number of bursts with detected optical afterglows: this is what is observed, which suggests that the bursts with detected optical afterglows might also occur in molecular clouds. We confirm this by modeling and constraining the distribution of column densities, measured from absorption of the X-ray afterglow, of the bursts with detected optical afterglows: we find that this distribution is consistent with the expectation for bursts that occur in molecular clouds and is not consistent with the expectation for bursts that occur in diffuse clouds. Consequently, we find that all but perhaps a few bursts, dark or otherwise, probably occur in molecular clouds. Finally, we show that the limited information that is available on the column densities of the dark bursts is not consistent with the idea that the dark bursts occur in the nuclear regions of ultralu-minous infrared/submillimeter-bright galaxies, from which we draw conclusions about the nature of star formation in the universe.
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机译：看来，大多数快速且定位良好的伽马射线爆发具有未检测到的或暗的光学余辉（“暗爆发”）发生在大小R大约> 10L_（49）〜（1/2）pc和质量M大约为> 3 x 10〜5L_（49）太阳质量，其中L是闪光灯的各向同性当量峰值发光度。我们表明，这种大小和质量的云无法建模为受星际介质的热相或热相压力平衡约束的气体（即弥散云）：这样的云对于引力坍塌将是不稳定的，从而导致在云层的坍塌和破碎中，直到恒星形成爆发重新建立碎片内的压力平衡，并且碎片被自重约束（即分子云）。 ``因此，暗爆发可能发生在分子云中，在这种情况下，如果最近形成了分子云，则暗爆发可能是这种恒星形成爆发的副产品，和/或如果暗云持续存在或后代恒星形成的结果，分子云在一段时间前形成。然后，我们证明，如果爆发发生在类似银河系的分子云中，其列密度可能是通用的，那么暗爆发的数量可以与检测到的光学余辉的爆发数量相比较：这就是观察到的现象，这表明具有检测到的光学余辉的爆发也可能发生在分子云中。我们通过对通过检测到的光学余辉而爆发的X射线余辉的吸收进行建模和约束来确定并验证列密度的分布，从而证实了这一点：我们发现这种分布与分子云中发生的爆发的预期一致，并且与弥漫云中爆发的预期不一致。因此，我们发现，除了少数爆发以外，几乎所有其他爆发都可能发生在分子云中。最后，我们证明了有关暗爆发的列密度的有限信息与暗爆发发生在超发光红外/亚毫米级明亮星系的核区域的想法不符，因此我们可以得出结论关于宇宙中恒星形成的本质。
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