Massive Star Birth, Life and Death: Closing the Loop in the Local Group of Galaxies

大质量恒星是宇宙的“宇宙引擎”。这些恒星之所以发光，是因为其核心发生了核反应。这个过程也产生了今天存在的大多数氧和碳原子。当这些大质量恒星死亡时，较轻的元素会变成较重的元素，比如铁。我们实际上是由这些“恒星物质”组成的。这些恒星也释放出如此多的能量，导致新的恒星形成。尽管我们对大多数类型的恒星都很了解，但关于大质量恒星的许多事情仍然是一个谜。这个项目的目标是通过比较预测和观测来看看我们的理论是如何运作的。这个项目将让我们更清楚地了解大质量恒星衰老的过程。研究人员将利用亚利桑那州和智利的大型望远镜获得的数据，比较在不同附近星系中发现的恒星类型。该项目还将资助两名早期职业研究学生，并为两名本科生提供研究项目。研究人员将用几种方法研究大质量恒星。他们计划观察本星系群，这些星系离我们足够近，可以进行详细的研究。他们将观察这些星系中所有的大质量恒星。具体来说，他们将确定：1)双星轨道的变化如何改变大质量恒星中元素的形成；2)最大质量的恒星是如何结束生命的。研究者和他的合作者已经开始对麦哲伦星云中的沃尔夫-拉叶星（WR）进行调查。该项目已经覆盖了一半的区域，已经确认了15颗新的WR恒星的存在，其中10颗似乎是以前从未见过的新型恒星。这些发现表明麦哲伦星云中WR恒星的数量可能被低估了40%，然而几十年来，麦哲伦星云一直是评估低金属丰度大质量恒星模型的关键。完成后，麦哲伦云WR调查将允许确定WR恒星的哪些部分通过双星相互作用（罗氏叶溢出）发生了变化，哪些部分通过恒星风引起的单星物质损失显示元素丰度的变化。在M31和M33中已经确定了30个WR双星，并且正在使用6.5米的MMT望远镜进行重复的径向速度测量，同时在洛厄尔天文台进行光度监测。新的4.3米望远镜。这个项目将提供关于高金属丰度双星相互作用频率的类似数据，并提供WR恒星质量的直接测量结果，以便与模型预测进行比较。神秘的富氧WR星（WO星）的性质将通过精确测定表面碳和氧的丰度来探索。将这些观测结果与模型进行比较，将确定WO恒星是否真的是最大质量恒星的“最后一搏”。这些实验中的每一个都为改进大质量恒星核能产生的模型提供了关键信息。研究者将把本科生纳入他的研究项目，为来自小型文理学院的学生提供帮助。这种经验使他们的学生能够成功地竞争进入该国最好的研究生院。这些学生来自北亚利桑那大学管理的REU项目，以及洛厄尔天文台的麻省理工学院野外营地，现在包括来自全女性的学生？这是韦尔斯利学院。

Massive stars are the 'cosmic engines' of the Universe. These stars shine because of nuclear reactions in their cores. This process also has made most of the oxygen and carbon atoms that exist today. When these massive stars die, lighter elements are turned into heavier ones, such as iron. We are literally made up of this 'star stuff'. These stars also give off so much energy that they cause new stars to form. Although we understand most kinds of stars pretty well, there are many things about massive stars that remain a mystery. The goal of this project is to see how well our theories work by comparing predictions against observations. This project will give us a clearer picture of what happens as a massive star ages. The investigators will compare the type of stars found in different nearby galaxies, using data taken on large telescopes in Arizona and in Chile. This project will also fund two early-career research students, and provide research projects for two undergraduates.The investigators will study massive stars in several ways. They plan to observe Local Group galaxies, which are close enough to be studied in great detail. They will observe all massive stars in these galaxies. Specifically they will determine 1) how changing orbits of binary stars modify the element formation in massive stars and 2) how the most massive stars end their lives. The investigator and his collaborators have begun a survey for Wolf-Rayet (WR) stars in the Magellanic Clouds. With half of the area covered, the project has already confirmed the existence of 15 new WR stars, 10 of which appear to be a new type, never before seen. These discoveries suggest that the population of WR stars in the Magellanic Clouds may have been underestimated by as much as 40%, and yet for decades the Magellanic Clouds have served as the linchpins for evaluating massive star models at low metallicities. When complete, the Magellanic Cloud WR Survey will permit the determination of what fraction of WR stars have changed through binary interactions (Roche-lobe overflow) and fraction show changes in element abundance through single-star material losses due to stellar winds. Thirty WR binaries have been identified in M31 and M33, and repeated radial velocity measurements are being made with the 6.5-m MMT telescope, along with photometric monitoring at Lowell Observatory?s new 4.3-m telescope. This project will provide similar data on the frequency of binary interactions at higher metallicity, and provide direct measurements of the masses of WR stars for comparison with model predictions. The nature of the mysterious, oxygen-rich, WR stars (WO stars) will be explored by determining accurate surface abundances of carbon and oxygen. Comparing these observations to models will determine if the WO stars are truly the "last hurrah" of the most massive stars. Each of these experiments provides a key piece of information to improve models of massive star nuclear energy production. The investigator will bring undergraduates into his research program, providing a boost to students from small, liberal arts colleges. This experience enables their students to successfully compete for entrance to best graduate schools in the country. These students are drawn from the REU program administered through Northern Arizona University, as well as Lowell Observatory's MIT Field Camp, which now includes students from the all-women?s Wellesley College.