M82-F: a doomed super star cluster?
M82-F: a doomed super star cluster?
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DOI：
10.1046/j.1365-8711.2001.04627.x
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发表时间：
2001-04
影响因子：
4.8
通讯作者：
Linda J. Smith;J. Gallagher
中科院分区：
文献类型：
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作者：
Linda J. Smith;J. Gallagher
我们展示了M82中非常明亮的年轻超级星团（SSC） ‘ F ’的高色散梯队光谱，这是用4.2 m威廉赫歇尔望远镜（WHT）获得的，目的是得出它的动态质量，并评估它是否会存活下来成为一个古老的球状星团。除了恒星谱线外，光谱还包含复杂的Na - I吸收和电离气体的宽发射谱线。我们从HST/WFPC2档案图像中测量到恒星速度色散为13.4±0.7 km−1，投影半光半径为2.8±0.3 pc，并得出动态质量为1.2±0.1×10 6 M⊙，证明M82-F是一个非常大质量的致密星团。我们确定M82-F的当前光度质量比（LV /M）为45±13。与光谱综合模型的比较表明，(LV /M)⊙比标准Kroupa（2001）初始质量函数（IMF）在确定年龄的M82-F为60±20 Myr时预测的(LV /M)⊙高出5倍。（LV /M）⊙的高值表明M82-F中缺少低质量恒星；当前质量函数（MF）明显是“头重脚轻”的。我们发现，对于斜率为2.3的MF，需要较低的2-3M⊙的质量截止值来匹配观测值。由于该星团显然缺乏寿命较长的低质量恒星，因此它不会成为一个古老的球状星团，而可能会在62 Gyr时解散。我们还得到了较年轻的ssc NGC 1569A和NGC 1705-1的最新光度质量比。对于第一个天体，观测结果与稍微陡峭的MF （α = 2.5）相一致，而对于NGC 1705-1，观测到的比值要求MF在2 M⊙附近被截断，斜率为α = 2.3。我们讨论了我们的研究结果在大规模IMF变化的背景下的影响；根据目前的数据，头重MF可以反映星团诞生时的局部质量分离效应。M82-F可能形成于密集的分子云；然而，它相对于M82中心的高径向速度（175 km s - 1）表明它在一个偏心轨道上，现在离它的诞生地很远，所以它形成的环境是未知的。
We present high dispersion echelle spectroscopy of the very luminous, young super star cluster (SSC) ‘F’ in M82, obtained with the 4.2-m William Herschel Telescope (WHT), for the purpose of deriving its dynamical mass and assessing whether it will survive to become an old globular cluster. In addition to the stellar lines, the spectrum contains complex Na I absorption and broad emission lines from the ionized gas. We measure a stellar velocity dispersion of 13.4±0.7 kms −1 , a projected half-light radius of 2.8 ± 0.3 pc from archival HST/WFPC2 images, and derive a dynamical mass of 1.2±0.1×10 6 M⊙, demonstrating that M82-F is a very massive, compact cluster. We determine that the current luminosity-to-mass ratio (LV /M)⊙ for M82-F is 45 ± 13. Comparison with spectral synthesis models shows that (LV /M)⊙ is a factor of � 5 higher than that predicted for a standard Kroupa (2001) initial mass function (IMF) at the well-determined age for M82-F of 60 ± 20 Myr. This high value of (LV /M)⊙ indicates a deficit of low mass stars in M82-F; the current mass function (MF) evidently is ‘top-heavy’. We find that a lower mass cutoff of 2–3M⊙ is required to match the observations for a MF with a slope � = 2.3. Since the cluster apparently lacks longlived low mass stars, it will not become an old globular cluster, but probably will dissolve at an age of 62 Gyr. We also derive up-dated luminosity-to-mass ratios for the younger SSCs NGC 1569A and NGC 1705-1. For the first object, the observations are consistent with a slightly steeper MF (� = 2.5) whereas for NGC 1705-1, the observed ratio requires the MF to be truncated near 2 M⊙ for a slope of � = 2.3. We discuss the implications of our findings in the context of large scale IMF variations; with the present data the top-heavy MF could reflect a local mass segregation effect during the birth of the cluster. M82-F likely formed in a dense molecular cloud; however, its high radial velocity with respect to the centre of M82 (� 175 km s −1 ) suggests it is on an eccentric orbit and now far from its birthplace, so the environment of its formation is unknown.