Effects of radiation feedback on Star and Planet Formation

辐射反馈对恒星和行星形成的影响

• 批准号: ST/F008260/2
• 负责人: Barbara Ercolano
• 金额: $ 41.47万
• 依托单位: UNIVERSITY OF EXETER
• 依托单位国家: 英国
• 项目类别: Fellowship
• 财政年份: 2010
• 资助国家: 英国
• 起止时间: 2010 至 无数据
• 项目状态: 已结题
• 来源: https://gtr.ukri.org/projects?ref=ST%2FF008260%2F2
• 关键词: Effects; radiation; feedback; Star; Planet

Star formation plays a major role in the evolution of our Universe and understanding the circumstellar (CS) environment of protostars and young stars can also provide crucial information on how planets form. It is believed that low mass stars may form by piling up (accretion) of material onto a circumstellar disc, the material finally spirals into the protostar, increasing its mass. However the formation process of high mass stars is still not well understood. When the central engine of the newly born star turns on, the radiation produced impinges on the circumstellar dust and gas. The higher the mass of the star, the stronger the radiation emitted by it. When the photons hit the dust and gas two important effects may occur: (1) At the first encounter with the dust, the radiation pressure generated pushes the material away from the star. If the momentum transferred from the photons to the dust/gas mixture is high enough the star may lose its ability to accrete material and become a more massive star. Some studies predict that the limit for the formation of stars by accretion is in fact 10 solar masses. However stars more massive than 10 solar masses exist and have been observed! If the theory that radiation pressure on dust grain inhibits accretion is true, it means that higher mass stars must form in a different way (maybe by merging of multiple, lower mass stars). It is therefore crucial to establish a more robust limit for stars that can form by accretion through a disk. Although some studies have been carried out in the past, the calculations are very complex and so far unrealistic approximations have had to be adopted, meaning that the final word on this subject is yet to be said. I have developed new tools and the expertise to study this process in detail, by constructing realistic models for many different environments in which stars form. The models will simulate the photon interactions with the dust grains, allowing a definite answer to this fundamental question to be finally obtained. (2) High mass stars are very hot and emit high energy radiation. The high energy photons heat the cooler gas in the circumstellar region of hot stars. The temperature increase makes the gas expand creating a compression at the boundary of the heated region, which may encourage the formation of new stars there. However another effect that must be considered is that stars form from the collapse of very large dust and gas clouds, which fragment as they collapse and may therefore form multiple stars at the same time. However if massive stars form first and start sweeping away all the gas from the centre of the cloud, this could prevent the formation of other stars. The question of whether high energy radiation has a net positive or net negative effect on the rate of star formation is still open. The models I will construct will allows us for the first time to peek into the interaction regions and determine if the formation of stars is being helped or harmed by the radiation from massive stars. Radiation pressure in low mass stars is too weak to impede accretion or to significantly affect cluster environments and the formation process in this case is better understood. Nevertheless, special attention is due to the circumstellar environment of young low-mass stars because of their potential of hosting a planetary system like our own. Protoplanetary discs are deeply affected by the radiation from the newly born star. For example, emission from young solar-mass stars can heat the outer layers of their discs, making them disperse (the timescale of dispersion is crucial to planet formation). X-ray photons can penetrate to larger depths, heating the gas there and providing the mechanism thought to be responsible for disc accretion. I will study the structure of irradiated protoplanetary disks and create templates that may be used to decipher the wealth of observational data already available to us and in the future with new facilities.

恒星形成在我们的宇宙演化中起着重要作用，了解原恒星和年轻恒星的星周(CS)环境也可以提供关于行星如何形成的关键信息。人们认为，低质量恒星可能是通过将物质堆积(吸积)到恒星周围的圆盘上而形成的，这些物质最终螺旋进入原恒星，增加了它的质量。然而，大质量恒星的形成过程仍然没有被很好地理解。当这颗新诞生的恒星的中央引擎打开时，产生的辐射会撞击到恒星周围的尘埃和气体。恒星的质量越大，它发出的辐射就越强。当光子撞击尘埃和气体时，可能会产生两个重要的影响：(1)在第一次与尘埃相遇时，产生的辐射压力会将物质推离恒星。如果从光子转移到尘埃/气体混合物的动量足够高，恒星可能会失去吸收物质的能力，成为一颗质量更大的恒星。一些研究预测，恒星吸积形成的极限实际上是10个太阳质量。然而，质量超过10个太阳质量的恒星仍然存在，并且已经被观测到了！如果尘埃颗粒上的辐射压力抑制吸积的理论是正确的，这意味着高质量恒星必须以不同的方式形成(可能是通过多颗低质量恒星的合并)。因此，为可以通过盘状吸积形成的恒星建立一个更可靠的极限是至关重要的。虽然过去已经进行了一些研究，但计算非常复杂，到目前为止不得不采用不切实际的近似，这意味着这个问题还没有定论。我已经开发了新的工具和专业知识来详细研究这一过程，通过为恒星形成的许多不同环境构建现实模型。这些模型将模拟光子与尘埃颗粒的相互作用，从而最终得到这个基本问题的确切答案。(2)大质量恒星非常热，并发出高能辐射。高能光子加热炽热恒星周围较冷的气体。温度的升高使气体膨胀，在受热区域的边界产生压缩，这可能会鼓励在那里形成新的恒星。然而，必须考虑的另一个影响是，恒星是由非常大的尘埃和气体云坍塌形成的，这些尘埃和气体云在坍塌时破碎，因此可能同时形成多颗恒星。然而，如果大质量恒星首先形成，并开始扫除星云中心的所有气体，这可能会阻止其他恒星的形成。高能辐射对恒星形成率的净正面影响还是净负面影响的问题仍然悬而未决。我将构建的模型将使我们第一次能够窥探相互作用区域，并确定恒星的形成是受到大质量恒星辐射的帮助还是损害。低质量恒星中的辐射压力太弱，不能阻止吸积，也不能显著影响星团环境，这种情况下的形成过程更好地被理解。尽管如此，年轻的低质量恒星的星周环境受到了特别的关注，因为它们有可能拥有像我们自己的行星系统。原行星盘深受这颗新生恒星辐射的影响。例如，来自年轻的太阳质量恒星的辐射可以加热其圆盘的外层，使它们分散(分散的时间尺度对行星的形成至关重要)。X射线光子可以穿透到更深的地方，加热那里的气体，并提供被认为是导致盘状吸积的机制。我将研究辐照的原行星盘的结构，并创建模板，这些模板可以用来破译我们已经获得的丰富的观测数据，并在未来使用新的设施。