Using the Starlight Polarization Efficiency Integral to Constrain Shapes and Porosities of Interstellar Grains
Using the Starlight Polarization Efficiency Integral to Constrain Shapes and Porosities of Interstellar Grains
复制标题
DOI：
10.3847/1538-4357/ac0050
复制
发表时间：
2021-09-01
影响因子：
4.9
通讯作者：
Hensley, Brandon S.
中科院分区：
文献类型：
--
作者：
Draine, B. T.;Hensley, Brandon S.
我们发展了一种新的方法，利用观测到的星光偏振和偏振亚毫米辐射来约束星际颗粒的形状和孔隙度。我们首先提出了修改后的栅栏近似，并验证它是足够准确的星光偏振建模。然后，我们引入所观察到的星光偏振积分Pi（obs）作为所观察到的星光偏振的总体强度的度量，和星光偏振效率积分Phi来表征不同颗粒类型产生星光偏振的有效性。比率Pi（obs）/Phi确定颗粒的质量加权排列< f（align）>。近似的ISM中的对齐的晶粒作为球体，我们使用Pi（obs）/Φ表明，所观察到的星光偏振约束的晶粒具有最小程度的非球面性。对于孔隙率P = 0，最小轴比对于扁球体类似于1.4，或者对于长球体类似于1.8。如果颗粒是多孔的，则需要更极端的轴比。产生星光偏振的相同颗粒能够提供在亚毫米波长处观察到的偏振发射，但在形状和孔隙度上具有进一步的限制。排除大于或类似于0.75的孔隙率P。如果星际颗粒可以近似为“星尘”球状体，我们预测10 μ m偏振的星光偏振p（V）的比率：p（10 μ m）/p（V）= 0.219 +/- 0.029。对于Cyg OB 2 -12，我们预测p（10 μ m）=（2.1 +/- 0.3）%，这应该是可观察的。
We have developed a new method for using the observed starlight polarization and polarized submillimeter emission to constrain the shapes and porosities of interstellar grains. We first present the modified picket-fence approximation and verify that it is sufficiently accurate for modeling starlight polarization. We then introduce the observed starlight polarization integral Pi(obs) as a measure of the overall strength of the observed polarization of starlight, and the starlight polarization efficiency integral Phi to characterize the effectiveness of different grain types for producing polarization of starlight. The ratio Pi(obs)/Phi determines the mass-weighted alignment < f(align)> of the grains. Approximating the aligned grains in the ISM as spheroids, we use Pi(obs)/Phi to show that the observed starlight polarization constrains the grains to have a minimum degree of asphericity. For porosity P = 0, the minimum axial ratio is similar to 1.4 for oblate spheroids, or similar to 1.8 for prolate spheroids. If the grains are porous, more extreme axial ratios are required. The same grains that produce the starlight polarization are able to provide the observed polarized emission at submillimeter wavelengths but with further limits on shape and porosity. Porosities P greater than or similar to 0.75 are ruled out. If interstellar grains can be approximated by "astrodust" spheroids, we predict the ratio of 10 mu m polarization to starlight polarization p(V) : p(10 mu m)/p(V) = 0.219 +/- 0.029. For Cyg OB2-12, we predict p(10 mu m) = (2.1 +/- 0.3)%, which should be observable.