Title: The Density and Mass of Unshocked Ejecta in Cassiopeia A through Low Frequency Radio Absorption Show Chinese title

Title: 仙后座A中未冲击抛射物的密度和质量通过低频无线电吸收测定

Abstract: Characterizing the ejecta in young supernova remnants is a requisite step towards a better understanding of stellar evolution. In Cassiopeia A the density and total mass remaining in the unshocked ejecta are important parameters for modeling its explosion and subsequent evolution. Low frequency (<100 MHz) radio observations of sufficient angular resolution offer a unique probe of unshocked ejecta revealed via free-free absorption against the synchrotron emitting shell. We have used the Very Large Array plus Pie Town Link extension to probe this cool, ionized absorber at 9 arcseconds and 18.5 arcseconds resolution at 74 MHz. Together with higher frequency data we estimate an electron density of 4.2 electrons per cubic centimeters and a total mass of 0.39 Solar masses with uncertainties of a factor of about 2. This is a significant improvement over the 100 electrons per cubic centimeter upper limit offered by infrared [S III] line ratios from the Spitzer Space Telescope. Our estimates are sensitive to a number of factors including temperature and geometry. However using reasonable values for each, our unshocked mass estimate agrees with predictions from dynamical models. We also consider the presence, or absence, of cold iron- and carbon-rich ejecta and how these affect our calculations. Finally we reconcile the intrinsic absorption from unshocked ejecta with the turnover in Cas A's integrated spectrum documented decades ago at much lower frequencies. These and other recent observations below 100 MHz confirm that spatially resolved thermal absorption, when extended to lower frequencies and higher resolution, will offer a powerful new tool for low frequency astrophysics. Show Chinese abstract

Abstract: 表征年轻超新星遗迹中的喷出物是更好地理解恒星演化的重要步骤。在仙后座A（Cassiopeia A）中，未冲击的喷出物中剩余的密度和总质量是建模其爆炸及其随后演化的关键参数。低频（<100 MHz）具有足够角分辨率的射电观测提供了通过自由-自由吸收揭示未冲击喷出物的独特探针。我们使用甚大阵列（VLA）加上Pie Town Link扩展，在74 MHz处以9弧秒和18.5弧秒的分辨率探测了这一冷却的离子吸收剂。结合更高频率的数据，我们估计电子密度为每立方厘米4.2个电子，总质量为0.39太阳质量，不确定性约为2倍。这比斯皮策太空望远镜提供的红外[S III]线比值得出的每立方厘米100个电子的上限有了显著改进。我们的估算对温度和几何形状等多种因素敏感。然而，使用合理的值，我们的未冲击质量估算与动力学模型的预测一致。我们还考虑了冷铁和富含碳的喷出物的存在与否以及它们如何影响我们的计算。最后，我们将未冲击喷出物的固有吸收与几十年前在低得多的频率下记录的仙后座A整体谱的转折点相协调。这些以及其他最近在100 MHz以下的观测证实，当空间分辨的热吸收扩展到更低的频率和更高的分辨率时，将为低频天文学提供强大的新工具。