Direct N-body simulations of globular clusters -- III. Palomar\,4 on an eccentric orbit
Direct N-body simulations of globular clusters -- III. Palomar\,4 on an eccentric orbit
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球状星团的直接 N 体模拟 - III Palomar,4 在偏心轨道上
DOI：
10.1093/mnras/stx130
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发表时间：
2017
期刊：
影响因子：
--
通讯作者：
Baumgardt
中科院分区：
文献类型：
--
作者：
Zonoozi;Kroupa;Küpper;A. H. W;Baumgardt
帕洛玛4（Palomar 4）是一个位于银河系外晕的低密度球状星团（GC），目前质量约为30 000 M <$，两体弛豫时间约为哈勃时间。然而，它是强烈的质量分离，并包含一个恒星质量函数耗尽低质量恒星。2004年要么是以这种方式诞生的，要么是非凡的动态进化的结果。由于两体松弛不能单独解释这些特征，通过潮汐冲击增强的质量损失可能对1944年产生了强烈的影响。在这里，我们计算一个网格的直接N体模拟模型在银河系内的各种偏心轨道上找到可能的初始条件，重现其观测到的质量，半光半径，恒星斜率的质量函数和视线速度色散。结果表明，双星4极有可能是在一个偏心轨道上运行，偏心率为e = 0.9，近心距为Rp = 5 kpc。在这种情况下，出生时所需的三维半质量半径与典型GC的平均大小（Rh = 4-5 pc）相似，而其出生质量约为M0 × 105 M Ω。我们还发现星团恒星之间存在高度的原始质量分离，这似乎在我们考虑的每一种情况下都是必要的。因此，利用潮汐效应来限制双星4轨道的近银河距离，我们预测双星4的自行应该在−0.52 ≤ μδ≤ −0.38 mas yr− 1和−0.30 ≤ μαcos δ≤ − 0.15 mas yr−1之间。
Palomar 4 (Pal 4) is a low-density globular cluster (GC) with a current mass ≈30 000 M⊙in the outer halo of the Milky Way with a two-body relaxation time of the order of a Hubble time. Yet, it is strongly mass segregated and contains a stellar mass function depleted of low-mass stars. Pal 4 was either born this way or it is a result of extraordinary dynamical evolution. Since two-body relaxation cannot explain these signatures alone, enhanced mass-loss through tidal shocking may have had a strong influence on Pal 4. Here, we compute a grid of directN-body simulations to model Pal 4 on various eccentric orbits within the Milky Way potential to find likely initial conditions that reproduce its observed mass, half-light radius, stellar slope of the mass function and line-of-sight velocity dispersion. We find that Pal 4 is most likely orbiting on an eccentric orbit with an eccentricity ofe≈ 0.9 and pericentric distance ofRp≈ 5 kpc. In this scenario, the required 3D half-mass radius at birth is similar to the average sizes of typical GCs (Rh≈ 4–5 pc), while its birth mass is aboutM0≈ 105M⊙. We also find a high degree of primordial mass segregation among the cluster stars, which seems to be necessary in every scenario we considered. Thus, using the tidal effect to constrain the perigalactic distance of the orbit of Pal 4, we predict that the proper motion of Pal 4 should be in the range −0.52 ≤ μδ≤ −0.38 mas yr−1and −0.30 ≤ μαcos δ≤ − 0.15 mas yr−1.