Understanding the Boundary Between Stars and Substellar Objects in the Solar Neighborhood

Sergio Dieterich博士被授予美国国家科学基金会天文学和天体物理学博士后奖学金，在华盛顿卡内基研究所地磁系开展研究和教育项目。褐矮星是在形成过程中从未获得足够质量的物体，无法达到点燃氢的持续核聚变所必需的内部温度和压力，而氢的持续核聚变是恒星物体的标志。了解极低质量恒星和褐矮星之间的界限对寻找宇宙生命具有深远的意义。生命很可能在围绕恒星运行的行星上进化，但由于褐矮星系统的温度不断变化，生命在褐矮星系统上进化更加困难。迪特里希博士研究恒星/亚恒星边界附近天体的温度、半径和光度趋势。他的论文根据这些趋势确定了恒星和亚恒星种群。这项工作现在将扩展到更大的恒星样本，并将探索不同恒星的化学成分如何改变这个边界。Dieterich博士将利用他作为前初中和高中物理科学教师的经验，促进天文学在高中物理课程中的整合。他将编写一本活动手册，作为物理教师的资源，这些教师希望通过用天文学的例子说明物理概念来丰富他们的课堂。Dieterich博士将在研究期间解决以下三个科学问题：(1)恒星/亚恒星边界处的光度函数是什么？(2)金属丰度如何影响质量接近氢燃烧最小质量的物体的性质？(3)我们如何使用具有已知动态质量的物体在恒星/亚恒星边界上创建更健壮和详细的质量-光度关系？智利卡耐基拉斯坎帕纳斯天文台的独特资源是回答这些问题的关键。由于精确的距离测量对于确定光度和半径至关重要，Dieterich博士和合作者将使用杜邦2.5米望远镜上的卡内基天体测量行星搜索相机（CAPSCam）来确定精确的三角视差，从而使研究样本体积完整。几架拉斯坎帕纳斯望远镜也将用于对这些非常微弱的物体进行光学光度测量，这是计算热通量所必需的。样品的金属丰度将通过麦哲伦6.5米望远镜的光谱观测来确定。麦哲伦II号望远镜上的MagAO自适应光学系统是唯一能够在光学波长中获得衍射限制图像的地面系统之一。这种独特的能力将用于在动力学质量已知的紧密双星系统中恒星的光度表征。然后将使用分辨光学光度法来确定这些恒星的光度和有效温度，从而填充质量-光度关系。

Dr. Sergio Dieterich is awarded an NSF Astronomy and Astrophysics Postdoctoral Fellowship to carry out a program of research and education at the Department of Terrestrial Magnetism of the Carnegie Institution of Washington. Brown dwarfs are objects that never acquired enough mass during their formation to attain the internal temperature and pressure necessary to ignite the sustained nuclear fusion of hydrogen, which is the hallmark of a stellar object. Understanding the boundary between very low mass stars and brown dwarfs has deep implications for the search for life in the Universe. It is likely that life may evolve on planets orbiting stars but it is more difficult for life to evolve on brown dwarf systems due to their constantly changing temperature. Dr. Dieterich studies trends in temperature, radius, and luminosity of objects near the stellar/substellar boundary. His thesis work identified a stellar and a substellar population based on these trends. This work will now be extended to a larger sample of stars, and will explore how chemical compositions for different stars change this boundary. Dr. Dieterich will use his experience as a former middle and high school physical science teacher to facilitate the integration of astronomy in the high school physics curriculum. He will compile an activity book that will serve as a resource for physics teachers who wish to enrich their classes by illustrating physical concepts with examples from astronomy. Dr. Dieterich will address the following three scientific questions during his fellowship: (1) What is the luminosity function at the stellar/substellar boundary? (2) How does metallicity affect the properties of objects with masses close to the hydrogen burning minimum mass? and (3) How can we use objects with known dynamical masses to create a more robust and detailed Mass-Luminosity Relation at the stellar/substellar boundary? The unique resources of Carnegie's Las Campanas Observatory in Chile are key to answering these questions. Because an accurate distance measurement is essential for establishing luminosity and radius, Dr. Dieterich and collaborators will use the Carnegie Astrometric Planet Search Camera (CAPSCam) on the du Pont 2.5 meter telescope to determine precise trigonometric parallaxes so as to make the study sample volume-complete. Several Las Campanas telescopes will also be used to obtain optical photometry for these very faint objects, which is necessary for calculating bolometric fluxes. Metallicities for the sample will be established through spectroscopic observations done with the Magellan 6.5 meter telescopes. The MagAO adaptive optics system on the Magellan II telescope is one of the only ground based systems capable of obtaining diffraction-limited images in optical wavelengths. This unique capability will be used for the photometric characterization of stars in close binary systems for which dynamical masses are known. The resolved optical photometry will then be used to determine luminosity and effective temperature for these stars, therefore populating the mass-luminosity relation.