We explore variations of the dust extinction law of the Milky Way by selecting stars from the Swift/UVOT Serendipitous Source Catalogue, cross-matched with Gaia DR2 and 2MASS to produce a sample of 10 452 stars out to ∼4 kpc with photometry covering a wide spectral window. The near ultraviolet passbands optimally encompass the 2175 Å bump, so that we can simultaneously fit the net extinction, quoted in the V band (AV), the steepness of the wavelength dependence (δ), and the bump strength (Eb). The methodology compares the observed magnitudes with theoretical stellar atmospheres from the models of Coelho. Significant correlations are found between these parameters, related to variations in dust composition that are complementary to similar scaling relations found in the more complex dust attenuation law of galaxies – that also depend on the distribution of dust among the stellar populations within the galaxy. We recover the strong anticorrelation between AV and Galactic latitude, as well as a weaker bump strength at higher extinction. δ is also found to correlate with latitude, with steeper laws towards the Galactic plane. Our results suggest that variations in the attenuation law of galaxies cannot be fully explained by dust geometry.

中文翻译：

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使用 Swift/UVOT、Gaia 和 2MASS 对银河系尘埃消光定律的约束

我们通过从 Swift/UVOT 偶然源目录中选择恒星，与 Gaia DR2 和 2MASS 交叉匹配，以产生 10 452 颗恒星的样本，以 ∼4 kpc 的光度覆盖广泛光谱窗口。近紫外通带最佳地包含 2175 Å 凸块，因此我们可以同时拟合净消光，在 V 波段 (AV)、波长依赖性的陡度 (δ) 和凸块强度 (Eb) 中引用。该方法将观测到的星等与来自 Coelho 模型的理论恒星大气进行了比较。在这些参数之间发现了显着的相关性，与尘埃成分的变化有关，这些变化与更复杂的星系尘埃衰减定律中发现的类似比例关系相辅相成——这也取决于星系内恒星群之间尘埃的分布。我们恢复了 AV 和银河纬度之间的强反相关，以及在较高消光时较弱的碰撞强度。还发现 δ 与纬度相关，朝向银河平面的规律更陡峭。我们的研究结果表明，尘埃几何不能完全解释星系衰减规律的变化。对银河平面有更陡峭的定律。我们的研究结果表明，尘埃几何不能完全解释星系衰减规律的变化。对银河平面有更陡峭的定律。我们的研究结果表明，尘埃几何不能完全解释星系衰减规律的变化。