象鼻球状体IC 1396 A原恒星的性质
Properties of protostars in the Elephant Trunk globule IC 1396A
摘要：
在斯皮策太空望远镜早期对亮边球状体IC 1396 A的观测中发现了极红的天体，并根据它们的中红外颜色在光度学上被归类为I类原恒星II类T金牛星，它们在5.5至38微米（IRS）、22 GHz水脉泽频率（GBT）和光学（Palomar）光谱上被观测到。被确定为I类的源被确认为由致密包层的吸积光度占主导地位的物体，吸积率为1 e-5至1 e-6 Msun/yr。冰/硅酸盐的吸收率在信封是异常低的IC 1396 A原恒星，相比那些在附近的恒星形成区域，这表明信封化学被改变的辐射场或球压力。在IC 1396 A中只探测到一个22 GHz的水脉泽，它的红外对应物的光度<0.1 Lsun，这是第一个来自这样一个低光度天体的水脉泽。光度分类为II类的天体被确认为经典的金牛座T星，具有温暖，明亮的圆盘。盘的性质随源的光度而显著变化：低质量（G-K）恒星由于无定形硅酸盐而具有突出的9-11微米发射特征，而高质量（A-F）恒星则具有较弱的特征，需要大量的结晶硅酸盐。I类源的分布集中在分子球内，而II类源则更广泛地分散。结合光谱结果，这表明恒星形成有两个阶段，第一阶段（400万年前）导致广泛分布的II类源和IC 1396的中央O星，第二阶段（<100万年前）发生在球状体内。
英文摘要：
Extremely red objects, identified in the early Spitzer Space Telescope observations of the bright-rimmed globule IC 1396A and photometrically classified as Class I protostars Class II T Tauri stars based on their mid-infrared colors, were observed spectroscopically at 5.5 to 38 microns (IRS), at the 22 GHz water maser frequency (GBT), and in the optical (Palomar). The sources photometrically identified as Class I are confirmed as objects dominated by accretion luminosity from dense envelopes, with accretion rates 1e-5 to 1e-6 Msun/yr. The ice/silicate absorption ratio in the envelope is exceptionally low for the IC 1396A protostars, compared to those in nearby star-forming regions, suggesting the envelope chemistry is altered by the radiation field or globule pressure. Only one 22 GHz water maser was detected in IC 1396A; its infrared counterpart has luminosity <0.1 Lsun, the first H2O maser from such a low-luminosity object. The objects photometrically classified as Class II are confirmed as classical T Tauri stars with warm, luminous disks. The disk properties change significantly with source luminosity: low-mass (G-K) stars have prominent 9-11 micron emission features due to amorphous silicates while higher-mass (A-F) stars have weaker features requiring abundant crystalline silicates. The distribution of Class I sources is concentrated within the molecular globule, while the Class II sources are more widely scattered. Combined with the spectral results, this suggests two phases of star formation, the first (4 Myr ago) leading to the widespread Class II sources and the central O star of IC 1396, and the second (<1 Myr ago) occurring within the globule.
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