We have constructed a detailed radiative transfer disk model which reproduces the main features of the spectrum of the outbursting young stellar object FU Orionis from ~ 4000 angstrom, to ~ 8 micron. Using an estimated visual extinction Av~1.5, a steady disk model with a central star mass ~0.3 Msun and a mass accretion rate ~ 2e-4 Msun/yr, we can reproduce the spectral energy distribution of FU Ori quite well. With the mid-infrared spectrum obtained by the Infrared Spectrograph (IRS) on board the Spitzer Space Telescope, we estimate that the outer radius of the hot, rapidly accreting inner disk is ~ 1 AU using disk models truncated at this outer radius. Inclusion of radiation from a cooler irradiated outer disk might reduce the outer limit of the hot inner disk to ~ 0.5 AU. In either case, the radius is inconsistent with a pure thermal instability model for the outburst. Our radiative transfer model implies that the central disk temperature Tc > 1000 K out to ~ 0.5 - 1 AU, suggesting that the magnetorotational instability (MRI) can be supported out to that distance. Assuming that the ~ 100 yr decay timescale in brightness of FU Ori represents the viscous timescale of the hot inner disk, we estimate the viscosity parameter (alpha) to be ~ 0.2 - 0.02 in the outburst state, consistent with numerical simulations of MRI in disks. The radial extent of the high mass accretion region is inconsistent with the model of Bell & Lin, but may be consistent with theories incorporating both gravitational instability and MRI.
High redshift radio quiet quasars - exploring the parameter space of accretion models. Part I: hot semi-spherical flow
目前有两类模型被用来描述黑洞上的吸积流和观测到的X射线辐射的产生：（1）一个标准的冷吸积盘，其上有一个热日冕;（2）一个外部截断的吸积盘，其内有一个热的半球形流。我们计算的方案与热的内流包围的截断吸积盘覆盖的热日冕和测试的结果的高红移（z > 4）类星体的样品上观察到的钱德拉。我们发现，为了再现光通量与X射线通量的比值（$α_{\rm ox}$参数），康普顿化等离子体的光学深度必须相当低（在热的内流中，$τ= 0.02 - 0.15 in the corona above the disk, and $ τ= 0.15 - 0.70）。这与观测到的X射线光子指数一起，意味着热等离子体中的高温（$kT_{\rm e} = 90 - 500 keV), or a nonthermal electron distribution in the plasma. We put an upper limit on the disk truncation radius, $ r_{\rm tr} \leq 40 R_{\rm S} $. The modeled accretion rate is high, $ \dot{m} > 0.2爱丁顿速率），这可能表明高z射电宁静类星体是高或非常高状态X射线双星的类似物。
XMM-Newton observations of the sigma Ori cluster. I. The complex RGS spectrum of the hot star sigma Ori AB
我们提出了XMM-牛顿观测的年轻（~2-5百万年）集群周围的热（O9.5V）恒星猎户座西格玛AB，旨在获得高分辨率的RGS光谱的热恒星以及EPIC成像数据的整个领域。我们发现，RGS光谱的σ Ori AB可能被污染的较弱的附近的来源，这需要开发一个合适的程序来提取一个干净的RGS光谱，并确定热的恒星的热结构和风的属性。我们还报告检测到的耀斑从B2 Vp星西格玛Ori E，我们讨论了耀斑是否起源于热恒星本身或更确切地说，从一个看不见的晚型同伴。这次观测的其他结果包括：在猎户座σ区域探测到174个X射线源，其中75个被确定为星团成员，包括非常低质量的恒星，直到亚恒星极限;在猎户座σ AB附近的一颗晚型恒星中发现旋转调制;未检测到谱线增宽和偏移（<800 km s^-1）在σ Ori AB的光谱中，O VII的禁止线与互组合线的比值显著较低，日冕丰度异常高CNO元素
