超冷矮星的首次VLBI探测：亚恒星伴星可探测性的意义
The First VLBI Detection of an Ultracool Dwarf: Implications for the Detectability of Sub-stellar Companions
摘要：
我们提出毫角秒分辨率射电甚长基线干涉（VLBI）观测的超冷矮星TVLM 513 -46546（M8.5）和2 MASS J 00361617 +1821104（L3.5），试图通过直接成像或反射运动来检测亚恒星的同伴。这两个物体都是已知的无线电发射器，有强有力的证据表明它们分别在大约2小时和3小时的时间尺度上周期性发射。我们利用内部的七个VLBA天线，在2.5质量（~50恒星半径）的尺度上探测到了来自TVLM 513 -46546的未分辨辐射，导致射电辐射亮度温度T_B > 4 × 10^5 K的直接限制。然而，由于VLBA提供了更高的空间分辨率，我们发现在低信噪比下，源似乎是边缘和不对称的，这可能表明TVLM 513 -46546是一个双星，预计分离约1质量（约20个恒星半径）。使用我们观测的7小时基线，我们发现在TVLM 513 -46546的位置上没有天体测量位移，3-sigma极限约为0.6 mas。这比预期的质量相等、轨道周期为几小时的伴星大3倍。今后在一定的时间尺度上对它的位置进行监测将提供所需的天体测量灵敏度，以探测在>15 d（>0.06 AU）轨道上质量约为10 M_J的行星伴星，或在>0.5 yr（>0.3 AU）轨道上质量约为2 M_J的行星伴星。
英文摘要：
We present milliarcsecond-resolution radio very long baseline interferometry (VLBI) observations of the ultracool dwarfs TVLM513-46546 (M8.5) and 2MASS J00361617+1821104 (L3.5) in an attempt to detect sub-stellar companions via direct imaging or reflex motion. Both objects are known radio emitters with strong evidence for periodic emission on timescales of about 2 and 3 hours, respectively. Using the inner seven VLBA antennas, we detect unresolved emission from TVLM513-46546 on a scale of 2.5 mas (~50 stellar radii), leading to a direct limit on the radio emission brightness temperature of T_B > 4x10^5 K. However, with the higher spatial resolution afforded by the full VLBA we find that the source appears to be marginally and asymmetrically resolved at a low S/N ratio, possibly indicating that TVLM513-46546 is a binary with a projected separation of ~1 mas (~20 stellar radii). Using the 7-hour baseline of our observation we find no astrometric shift in the position of TVLM513-46546, with a 3-sigma limit of about 0.6 mas. This is about 3 times larger than expected for an equal mass companion with a few-hour orbital period. Future monitoring of its position on a range of timescales will provide the required astrometric sensitivity to detect a planetary companion with a mass of ~10 M_J in a >15 d (>0.06 AU) orbit, or with a mass of ~2 M_J in an orbit of >0.5 yr (>0.3 AU).
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