斯皮策c2d对附近致密岩芯的调查。五、在“无星”致密地核L328中发现一个VeLLO
The Spitzer c2d Survey of Nearby Dense Cores. V. Discovery of a VeLLO in the "Starless" Dense Core L328
摘要：
本文报道了利用斯皮策太空望远镜和近红外到毫米波段的地面观测在“无星”致密核L328中发现的一个极低光度天体（VeLLO）。斯皮策8微米的图像显示，L328由三个子核心组成，其中最小的一个可能有一个源，L328-IRS，而另外两个子核心仍然没有恒星。L328-IRS是一颗0级原恒星，根据它的热测量温度（44 K）和它在亚毫米波长下发射的高部分~ 72%的光度。它的“内部光度”（0.04 - 0.06 Lsun）使用辐射转移模型，在最合理的假设下，它的距离为200 pc，比典型的原恒星要暗得多，甚至比以前研究的其他VeLLO更暗。然而，请注意，如果我们考虑L328-IRS距离的两个极端值（125或310 pc），则其推断的光度可能有2-3倍的不确定性。CO的低角分辨率观测没有显示出分子外流活动的任何明确证据。但是，L328的宽线宽，以及斯皮策和近红外图像显示的星云可能追踪外流腔，强烈表明外流活动的存在。如果L328-IRS的吸积包线不超过0.1Msun，且恒星形成效率接近标准值~ 30%，则L328-IRS的吸积量不超过0.05Msun。在假设的200 pc的距离，L328-IRS注定是一颗褐矮星。
英文摘要：
This paper reports the discovery of a Very Low Luminosity Object (VeLLO) in the "starless" dense core L328, using the Spitzer Space Telescope and ground based observations from near-infrared to millimeter wavelengths. The Spitzer 8 micron image indicates that L328 consists of three subcores of which the smallest one may harbor a source, L328-IRS while two other subcores remain starless. L328-IRS is a Class 0 protostar according to its bolometric temperature (44 K) and the high fraction ~72 % of its luminosity emitted at sub-millimeter wavelengths. Its inferred "internal luminosity" (0.04 - 0.06 Lsun) using a radiative transfer model under the most plausible assumption of its distance as 200 pc is much fainter than for a typical protostar, and even fainter than other VeLLOs studied previously. Note, however, that its inferred luminosity may be uncertain by a factor of 2-3 if we consider two extreme values of the distance of L328-IRS (125 or 310 pc). Low angular resolution observations of CO do not show any clear evidence of a molecular outflow activity. But broad line widths toward L328, and Spitzer and near-infrared images showing nebulosity possibly tracing an outflow cavity, strongly suggest the existence of outflow activity. Provided that an envelope of at most ~0.1 Msunis the only mass accretion reservoir for L328-IRS, and the star formation efficiency is close to the canonical value ~30%, L328-IRS has not yet accreted more than 0.05 Msun. At the assumed distance of 200 pc, L328-IRS is destined to be a brown dwarf.
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