Research Highlights

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极光计划X射线偏振探测器在轨工作两周年

2020年10月29日,由清华大学主导的空间天文项目“极光计划”成功实现了在轨工作两周年。2018年10月29日,装载“极光计划”的“铜川1号”立方星在酒泉卫星发射中心发射入轨;同年12月18日,“极光计划”探测到首光事例;2019年3月,在完成了卫星调试之后,“极光计划”全面进入科学观测阶段,先后对蟹状星云和天蝎座X-1开展了X射线偏振测量,同时对在轨本底进行观测研究。探测器至今工作正常,没有发现性能衰减。

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通过自行推断银河系核球中RR Lyrae变星的旋转特性

RR Lyrae变星是一类贫金属、氦核燃烧的水平支巨星,通常年龄在10 Gyr以上。它们的周光关系使得我们能更精确地确定它们的距离,从而使我们研究它们的空间分布和运动学特征成为可能。

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“极光计划”成果登上《自然·天文》封面

5月11日,清华大学天文系教授冯骅课题组与合作者在《自然·天文》杂志发表封面文章,报道了清华大学主导的空间天文项目“极光计划”的最新成果:“极光计划”配备的X射线偏振探测器在卫星上经过1年的观测,探测到来自蟹状星云及脉冲星的软X射线偏振信号,并首次发现了脉冲星自转突变和恢复过程中X射线偏振信号的变化,说明在此过程中脉冲星磁场发生了变化。这一探测结果也标志着,由于技术困难停滞了40多年的天文软X射线偏振探测窗口重新开启。

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Estimating stellar dust attenuation from galactic spectra

The observed spectrum of a galaxy is a combination of several components: a continuum, absorption and emission lines. The continuum and absorption lines are both dominated by starlight, thus usually referred to as the stellar component of the spectrum. The emission-line component is produced in ionized Hydrogen (HII) regions around hot stars, or emission-line regions of active nuclei, or both. All these components, however, are modified by the attenuation of dust grains distributed in the inter-stellar space. Dust attenuation can affect galaxy spectra over a wide range of wavelengths, from ultraviolet (UV), optical to infrared, by absorbing short-wavelength photons in UV/optical and re-emitting photons in the infrared, and the absorption is stronger in shorter wavelength. Consequently, dust attenuation can cause changes in the overall shape of a galaxy spectrum. Such attenuation has to be taken into account before one can measure the different components of an observed spectrum reliably. Traditionally, dust attenuation is treated as a free parameter when fitting the spectrum with a stellar population synthesis model, and so it is hard to measure the dust attenuation given the well-known dust–age–metallicity degeneracy.

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Recovering star formation histories of low-mass galaxies

Characterizing the star formation history (SFH) of galaxies is an important step toward a full picture of galaxy formation and evolution. Despite being the most abundant galaxy population in the universe, dwarf (low-mass) galaxies remain elusive as far as their formation and evolution path is concerned. The left panel of Figure 1 shows the optical image of a typical low-mass galaxy observed by the SDSS-IV MaNGA survey. Impressed by their blue color, people have long believed dwarf galaxies are exclusively composed of massive,young stars that are bright, blue, and formed very recently. Recent investigations about low-mass galaxies challenge this stereotype. From ‘archaeological’ age reconstruction of local group dwarf galaxies in which individual stars can be resolved, most stars in these galaxies are found to be older than 5 Gyr, with an age similar to or even older than the Sun. However, the number of dwarf galaxies for which stars can be resolved is very limited, and so it is difficult to draw reliable statistical conclusions.

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Searching for Wolf-Rayet Regions of Massive Stars in Galaxies

Wolf-Rayet (WR) galaxies are a rare population of galaxies that host living high-mass stars during their WR phase (called WR stars). These galaxies can be used to study a variety of important astrophysical questions including constraints on the stellar initial mass function, stellar evolution models, the relation between supernovae and gamma-ray bursts, etc. The first WR galaxy was identified in 1976 and a total of about 130 WR galaxies had been reported by the end of last century. Thanks to the SDSS I & II survey of nearby galaxies (targeting galaxy centers), many more WR galaxies were found, but the number was still limited to a few hundred [1]. Integral field spectroscopy (IFS) surveys have become available recently and provides a more efficient way of identifying WR galaxies, as WR stars are expected to be more preferentially found in discs than central regions that earlier phases of the SDSS probed.

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LHS 1815b: The First Thick-Disk Planet Detected By TESS

To date, more than 4000 exoplanets have been detected, but few of them have been claimed to be in the thick disk of the Milky Way. A common way to separate different components of the Milky Way (for example, thin and thick disks) relies on the three-dimensinal (3D) spatial motions of stars. This has become possible with GAIA, a space telescope which gives distance, proper motions for relatively bright stars. Furthermore, the recent launched Transiting Exoplanet Survey Satellite (TESS) aims to discover a large sample (10,000) of planets around bright stars in the solar neighborhood across the whole sky. Combined with GAIA, TESS offers an exciting opportunity to study the difference in the planet formation efficiency between stars in the thin and thick disks, which have different age and metallicity distributions.

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Relating structure of dark halos to their assembly and environment

Dark matter halos are the building blocks of the cosmic large-scale structures and the bridges between the dark and luminous sector of the Universe. They are diverse in internal structure, mass assembly history and interactions with environment. The figure at the top illustrates various quantities that are commonly adopted in literature to describe the structural properties of a typical dark matter halo, as well as its formation history and environment. A crucial step toward a full picture of dark matter halo formation is to understand the intrinsic relationship between the structure and the assembly history and environment of dark halos.

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Machine learning recovers H II morphology during reionization

The epoch of reionization (EoR) is a unique period of time in cosmic evolution, during which ultraviolet (UV) and X-ray photons emitted from the first luminous objects (e.g. first stars and galaxies) ionize hydrogen atoms first in the surrounding intergalactic medium (IGM) and form bubbles of H II regions, and eventually these H II bubbles fill the whole Universe. The bubble size distribution of ionized hydrogen regions probes the information about the morphology of H II bubbles during the reionization, and it can be derived from the tomographic imaging data of the redshifted 21cm signal.

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Gas Evolution in the Early Universe Revealed with Keck

Over the past few decades, astronomers have studied the process of gas accretion that drives the formation of stars and galaxies within dark matter halos. People have established a theoretical paradigm, which envisions galaxies fed by cool ‘streams’ of gas, linked to the surrounding circumgalactic medium (CGM) and intergalactic medium (IGM) by a web of cosmic filaments. Despite the general agreement among theorists, fundamental questions are yet to be solved and tested empirically. Due to their extremely low densities, the emission of CGM and IGM are very faint with very low surface brightness. Fortunately, recent development of more advanced instrumentations allows astronomers to see the low-surface brightness IGM emission for the first time.

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Constraining nature of ultra-light dark matter particles with 21cm forest

The ultra-light scalar fields can arise ubiquitously, for instance, as a result of the spontaneous breaking of an approximate symmetry such as the axion and more generally the ultra-light particles (ULPs). In addition to the particle physics motivations, these particles can also play a major role in cosmology by contributing to dark matter abundance and affecting the structure formation at sub-Mpc scales.

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New insight into Jupiter’s diluted core

Jupiter was smacked head-on by a massive planetary embryo about 4.5 billion years ago in the early solar system, according to a new study published in the journal Nature on August 15, 2019. An international collaboration team including two astronomers from Tsinghua University, Dr. Xiaochen Zheng from Department of Astronomy and Department of Physics and Prof. Doug Lin from Institute for Advanced Study, demonstrate their giant impact scenario can explain Jupiter’s large diluted core inferred from Jupiter’s gravity field measurement by the Juno mission. The study was led by Dr. Shangfei Liu from Sun Yat-Sen University. The paper can be found at https://www.nature.com/articles/s41586-019-1470-2

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