摘要
巨分子云的碰撞造成了大质量恒星在碰撞分子云中的形成,这些大质量恒星的形成产生了膨胀的HII区域,从而使巨分于云碎裂成小质量的分子云。这是本文提出的巨分子云碎裂机制。因此巨分子云的寿命也主要由区分子云间的碰撞几率所决定。我们的分析表明,巨分子云的寿命有赖于巨分子云所在的旋涡星系中的不同位置。寿命的最大可能存在区间为8.18×10~7yr与2.45×10~8yr。利用我们提出的机制可以在分子云研究的数值计算与数值模拟中得到应用。
In the study of the formation of giant molecular clouds the determination of lifetime of GMCs is one of the most important questions. Up to now, in the statistical steady state Kwan derived a lifetime of 2×108yr. In considering the effect of spiral structure on the formation of GMCs the lifetime of 4×107yr of GMCs was given by Casoli and Combes. From recent observational results Scoville et al. concluded that the formation of high mass star is the product of collision between GMCs. It is known that the new-born high mass stars will produce the expanding HII region which might possibly disrupt the GMCs. In this paper ti is suggested that the lifetime of GMCs is determined by this mechaniim. Because of the short pre-main sequence duration of high mass stars, the calculation of the lifetime of GMCs can be reduced to the estimation of the collision probability between GMCs.The preliminary results for the estimation of lifetime of GMCs are as follows:1. If the molecular clouds are uniformly distributed in the molecular ring 3kpc×r×7 kpc and the geometric area is taken as the collision cross-section, we haveT = 9.8 × 108yr.2. Because of the existence of spiral perturbation in the Galaxy, the density of molecular clouds in an arm region is twice higher than that in an interarm region and the collision cross-section is the same as above, thusT= 3.27×109yr.3. Clifford et al. pointed out that due to magnetic field in the Galaxy the direct cloud-cloud collision could be rare and most low-velocity clouds might interact with each other exclusively as a result of magnetic field entanglements. In considering both direct and indirect cloud collisions by field entanglement, the effective collision cross-section for a magnetic cloud will be larger than the geometric cross-section by a factor of 2 to 5, thenT=2.45×108yr in interarm region, T= 8.18×107 yr in arm region.It is shown that the minimum lifetime derived in this paper is higher than that given in Ref. [3] by a factor of 2.In order to estimate the formation rate of GMCs, the less massive clouds may be taken to have a typical mass of 5× 104M×, then each collision between this kind of clouds would form a GMC.The ratio of the collision rates between the less massive clouds to that between GMCs isvs/vg= 2.155, for a= 1/4, andvs/vg = 1.810, for a =1/3, respectively, where a is the index in the relation between the dispersion velocity and massFrom this result it is clear why the high mass contrast in arm and interarm regions is needed in Ref. [3] . This mechanism for estimating the lifetime of GMCs can be applied to both numerical integration and numerical simulation.
关键词
恒星
巨分子云
恒星形成
Star: formation of-Lifetime of GMC