极低质量恒星的空间分辨H_2辐射
Spatially resolved H_2 emission from a very low-mass star
摘要：
来自极低质量恒星（VLMS）和褐矮星的分子外流研究很少。到目前为止，只有少数CO流出已被观察到，使我们能够映射直接的拱星环境。我们提出了第一个空间分辨的H2发射IRS 54（YLW 52），~0.1-0.2 Msun的I类源。通过VLT SINFONI K波段的观测，我们探测了H2的发射，从源的第一个~50 AU。分子发射显示出一个复杂的结构，描绘了一个大的外流腔和不对称的分子射流。由于探测到了几次H2跃迁，我们能够估计出沿喷流状结构（从源位置到节点D）的平均值Av约为28等，T约为2000-3000 K，H2柱密度N（H2）约为1.7 × 10^17 cm^-2。这使我们能够估算出热H2组分的质量损失率约为2 × 10 ^-10 Msun/yr。此外，根据Br-γ线的总通量，我们推断吸积光度和质量吸积速率分别为0.64 Lsun/yr和~ 3 × 10 ^-7 Msun/yr。外流结构与低质量I类和CTTS中发现的结构相似。然而，Lacc/Lbol比率非常高（约80%），与质量大致相同的物体相比，质量吸积率高出约一个数量级，这表明所研究的源的年轻性质。
英文摘要：
Molecular outflows from very low-mass stars (VLMSs) and brown dwarfs have been studied very little. So far, only a few CO outflows have been observed, allowing us to map the immediate circumstellar environment. We present the first spatially resolved H2 emission around IRS54 (YLW52), a ~0.1-0.2 Msun Class I source. By means of VLT SINFONI K-band observations, we probed the H2 emission down to the first ~50 AU from the source. The molecular emission shows a complex structure delineating a large outflow cavity and an asymmetric molecular jet. Thanks to the detection of several H2 transitions, we are able to estimate average values along the jet-like structure (from source position to knot D) of Av~28 mag, T~2000-3000 K, and H2 column density N(H2)~1.7x10^17 cm^-2. This allows us to estimate a mass loss rate of ~2x10^-10 Msun/yr for the warm H2 component . In addition, from the total flux of the Br Gamma line, we infer an accretion luminosity and mass accretion rate of 0.64 Lsun and ~3x10^-7 Msun/yr, respectively. The outflow structure is similar to those found in low-mass Class I and CTTS. However, the Lacc/Lbol ratio is very high (~80%), and the mass accretion rate is about one order of magnitude higher when compared to objects of roughly the same mass, pointing to the young nature of the investigated source.
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