M次矮星的幸运成像
Lucky Imaging of M subdwarfs
摘要：
对贫金属恒星和太阳金属恒星的双星性质的了解，可以揭示两种类型天体形成过程之间的电势差。该项目的目的是确定M亚矮星的双星性质（分离、质量比、伴星频率），即场M矮星的低金属丰度对应物，并调查两个种群之间的任何电势差。我们利用Calar Alto 2.2米望远镜上的AstraLux仪器，利用“幸运成像”技术，获得了24颗早M亚矮星和9颗极端亚矮星样本的光学高分辨率成像。我们对间隔大于0.1角秒的伴星敏感，0.1角秒时相差2等，1角秒时相差5等。我们在所研究的24颗次矮星周围没有发现伴星，在9颗极端次矮星中有一颗近距离双星。三个月后，用同样的仪器拍摄的LHS 182的第二张照片证实了双星的共同自行，间隔约为0.7角秒。此外，我们没有证实里亚兹及其合作者在LHS 1074的~2 arcsec处报告的微弱源的共同自行。我们从我们的33个天体样本中得出了M亚矮星的双星频率为3+/-3%，分离大于约5个天文单位。在我们的样本中添加了里亚兹和合作者用哈勃太空望远镜观测到的另外28颗贫金属早M矮星，我们推断出双星的比例为3.7+/-2.6%（1 sigma置信限），明显低于在相同质量和分离范围内分辨出的双星M矮星的比例（约20%）。
英文摘要：
The knowledge of the binary properties of metal-poor and solar-metallicity stars can shed light on the potential differences between the formation processes responsible for both types of objects. The aim of the project is to determine the binary properties (separation, mass ratio, frequency of companions) for M subdwarfs, the low-metallicity counterparts of field M dwarfs, and investigate any potential differences between both populations. We have obtained high-resolution imaging in the optical for a sample of 24 early-M subdwarfs and nine extreme subdwarfs with the ``Lucky Imaging'' technique using the AstraLux instrument on the Calar Alto 2.2-m telescope. We are sensitive to companions at separations larger than 0.1 arcsec and differences of ~2 magnitudes at 0.1 arcsec and ~5 mag at 1 arcsec. We have found no companion around the 24 subdwarfs under study and one close binary out of nine extreme subdwarfs. A second image of LHS 182 taken three months later with the same instrument confirms the common proper motion of the binary separated by about 0.7 arcsec. Moreover, we do not confirm the common proper motion of the faint source reported by Riaz and collaborators at ~2 arcsec from LHS 1074. We derive a binary frequency of 3+/-3% for M subdwarfs from our sample of 33 objects for separations larger than about five astronomical units. Adding to our sample the additional 28 metal-poor early-M dwarfs observed with the Hubble Space Telescope by Riaz and collaborators, we infer a binary fraction of 3.7+/-2.6% (with a 1 sigma confidence limit), significantly lower than the fraction of resolved binary M dwarfs (~20%) over the same mass and separation ranges (abridged).
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