arXiv 1909.047652019-09astro-ph.GA

超宽双星在银河系中的分离分布

The Separation Distribution of Ultra-Wide Binaries across Galactic Populations

Hai-Jun Tian, Kareem El-Badry, Hans-Walter Rix, Andrew Gould

备注：: 16 pages, 9 figures, and 2 tables, Accepted to ApJS
DOI：: 10.3847/1538-4365/ab54c4

摘要：

我们提出了一个广泛的超宽双星在太阳附近的样本，专注于分离的$0.01<s/{pc}<1$。使用来自Gaia DR 2的数据，我们通过系统的切向速度定义了运动学子群，即，盘状（$v_{\perp, tot}<40$ km/s）、中间（$v_{\perp,tot}=(40-85)$ km/s）和晕状（$v_{\perp,tot}>85$ km/s）样本，假定这些速度切割代表了双星年龄和金属丰度的粗略排序。通过对天体测量精度的严格限制，我们可以获得纯二进制样本，每个样本中有数千个二进制。对于所有三个群体，二进制分离的分布平稳地扩展到1 pc，既不显示强截断，也不显示双峰。拟合一个平滑破碎的幂律的分离分布，我们发现，它的斜率在分离$s=10^{2.5-4}$ AU是相同的所有子群体，$p(s)\propto s^γ$，与$γ\sim-1.54$。然而，$p(s)$的对数斜率在$s \gtrsim 10^4$ AU处变陡。我们发现了一些证据，表明陡化程度随双星年龄的增加而增加，盘状星的陡化程度仅为$Δγ\sim0.5$，而晕状星的陡化程度为$Δγ>1$。这种趋势是相反的，如果陡峭的宽分离是由于分子云或恒星的引力扰动，这将优先破坏磁盘双星。如果我们将$s\gtrsim 10^4$ AU的变陡解释为大质量晕天体破坏的结果，我们将不得不调用一个与其他约束不一致的CANOO种群。作为一个更合理的替代方案，我们提出了一个简单的模型来预测溶解星团中形成的宽双星的分离分布。这个模型一般预测$γ\sim-1.5$观测，陡峭的较大分离由于有限大小的二进制的出生集群。

英文摘要：

We present an extensive sample of ultra-wide binary stars in the solar neighborhood, focusing on separations of $0.01<s/{pc}<1$. Using data from Gaia DR2, we define kinematic sub-populations via the systems' tangential velocities, i.e., disk-like ($v_{\perp, tot}<40$km/s), intermediate ($v_{\perp,tot}=(40-85)$km/s), and halo-like ($v_{\perp,tot}>85$km/s) samples, presuming that these velocity cuts represent a rough ordering in the binaries' age and metallicity. Through stringent cuts on astrometric precision, we can obtain pure binary samples with thousands of binaries in each sample. For all three populations, the distribution of binary separations extends smoothly to 1pc, displaying neither strong truncation nor bimodality. Fitting a smoothly-broken power law for the separation distribution, we find that its slope at separations $s=10^{2.5-4}$AU is the same for all sub-populations, $p(s)\propto s^γ$, with $γ\sim-1.54$. However, the logarithmic slope of $p(s)$ steepens at $s \gtrsim 10^4$AU. We find some evidences that the degree of steepening increases with the binaries' age, with a slope-change of only $Δγ\sim0.5$ for disk-like stars, but $Δγ>1$ for halo-like stars. This trend is contrary to what might be expected if steepening at wide separations were due to gravitational perturbations by molecular clouds or stars, which would preferentially disrupt disk binaries. If we were to interpret steepening at $s\gtrsim 10^4$AU as a consequence of disruption by massive halo objects, we would have to invoke a MACHO population inconsistent with other constraints. As a more plausible alternative, we propose a simple model to predict the separation distribution of wide binaries formed in dissolving star clusters. This model generically predicts $γ\sim-1.5$ as observed, with steepening at larger separations due to the finite size of binaries' birth clusters.

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英文摘要：

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