[论文解读] FUV and X-ray irradiated protoplanetary disks: a grid of models II - Gas diagnostic line emission

G. Aresu, R. Meijerink+4 more|arXiv (Cornell University)|Sep 4, 2012

Astrophysics and Star Formation Studies被引用 26

一句话总结

本研究构建了240个原行星盘模型的网格，以探究中心恒星的远紫外（FUV）和X射线辐射如何影响气体示踪剂（如[O i] 63 μm、[C ii] 157 μm、[Ne ii] 12.8 μm、[Ar ii]以及H₂O转动能级跃迁线）的禁线发射。关键发现为：当X射线 luminosity（L_X）低于10³⁰ erg s⁻¹时，[O i] 63 μm发射主要受FUV主导；当L_X > 10³⁰ erg s⁻¹时则转为X射线主导；[C ii]主要由FUV通过C⁺产生驱动；而[Ne ii]与X射线强相关，其双峰线轮廓表明发射源同时来自盘大气层和内边缘区域。

ABSTRACT

Most of the mass in protoplanetary disks is in the form of gas. The study of the gas and its diagnostics is of fundamental importance in order to achieve a detailed description of the thermal and chemical structure of the disk. The radiation from the central star (from optical to X-ray wavelengths) and viscous accretion are the main source of energy and dominates the disk physics and chemistry in its early stages. This is the environment in which the first phases of planet formation will proceed. We investigate how stellar and disk parameters impact the fine-structure cooling lines [NeII], [ArII], [OI], [CII] and H2O rotational lines in the disk. These lines are potentially powerful diagnostics of the disk structure and their modelling permits a thorough interpretation of the observations carried out with instrumental facilities such as Spitzer and Herschel. Following Aresu et al. (2011), we computed a grid of 240 disk models, in which the X-ray luminosity, UV-excess luminosity, minimum dust grain size, dust size distribution power law and surface density distribution power law, are systematically varied. We solve self-consistently for the disk vertical hydrostatic structure in every model and apply detailed line radiative transfer to calculate line fluxes and profiles for a series of well known mid- and far-infrared cooling lines. The [OI] 63 micron line flux increases with increasing FUV luminosity when Lx < 1e30 erg/s, and with increasing X-ray luminosity when LX > 1e30 erg/s. [CII] 157 micron is mainly driven by FUV luminosity via C+ production, X-rays affect the line flux to a lesser extent. [NeII] 12.8 micron correlates with X-rays; the line profile emitted from the disk atmosphere shows a double-peaked component, caused by emission in the static disk atmosphere, next to a high velocity double-peaked component, caused by emission in the very inner rim. (abridged)

研究动机与目标

• 理解恒星和盘参数如何影响原行星盘中关键气体示踪线的发射。
• 确定FUV和X射线辐射在驱动[O i]、[C ii]、[Ne ii]、[Ar ii]及H₂O等物种发射中的相对作用。
• 为解释斯皮策（Spitzer）和赫歇尔（Herschel）望远镜的观测结果，提供一个预测性框架，涵盖广泛盘和恒星参数下的谱线流量与轮廓。
• 使谱线发射成为探测盘热结构与化学结构的工具，尤其适用于尘埃光学厚或空间上未分辨的区域。

提出的方法

• 计算了240个盘模型的网格，系统性地变化X射线 luminosity、FUV超量 luminosity、最小尘埃颗粒尺寸、尘埃尺寸分布幂律指数以及表面密度分布幂律指数。
• 对每个模型自洽求解盘的垂直静力平衡结构，以确定温度和密度分布。
• 应用详细的谱线辐射转移方法，计算中远红外禁线的流量与轮廓。
• 使用热化学代码ProDiMo模拟盘的热化学结构，结果输入至辐射转移代码MCFOST。
• 模型考虑了X射线和FUV加热、电离及冷却过程，包括盘大气层中的库仑加热和光致脱附。
• 计算了关键跃迁的谱线轮廓，包括[O i] 63 μm、[C ii] 157 μm、[Ne ii] 12.8 μm、[Ar ii]以及多个H₂O转动能级跃迁线。

实验结果

研究问题

• RQ1 当FUV和X射线 luminosity 变化时，[O i] 63 μm谱线流量如何响应？
• RQ2 [C ii] 157 μm谱线发射主要由FUV还是X射线驱动？光谱细节如何影响流量？
• RQ3 [Ne ii] 12.8 μm谱线轮廓能否独立于尘埃观测揭示内盘边缘的位置？
• RQ4 不同区域的水谱线与[O i] 63 μm如何相关？这揭示了盘结构的哪些信息？
• RQ5 X射线与FUV对盘大气中关键气体示踪剂的激发与发射的相对贡献是什么？

主要发现

• [O i] 63 μm谱线流量在X射线 luminosity低于10³⁰ erg s⁻¹时随FUV luminosity增加而增强，当L_X超过10³⁰ erg s⁻¹时则随X射线 luminosity增加而增强，表明主导加热机制发生转变。
• [C ii] 157 μm谱线主要由FUV辐射通过C⁺产生驱动，X射线诱导的库仑加热仅起次要贡献。
• [Ne ii] 12.8 μm谱线轮廓呈现双峰结构，源于盘大气层的静态发射与内边缘附近高速气体的发射，可通过高分辨率光谱观测进行检验。
• [Ar ii]谱线与[Ne ii]探测到相同的盘区域，其在少数源中的检测结果与模型预测一致。
• 水谱线流量与[O i] 63 μm的相关性因激发能级和起源区域而异，低激发谱线表现出次线性斜率，证实了模型的化学与热结构。
• 该模型网格可为赫歇尔与斯皮策观测提供稳健解释，预测的[O i]流量与GASPS巡天观测结果在预期不确定度范围内一致。