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THCA Annual Report 2018

On 14th of March, 2019, the Tsinghua Center for Astrophysics (THCA) released its Annual Report for the year 2018, which is publicly available at the THCA website.

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Environmental dependence of alpha abundance for nearby galaxies

The alpha-to-iron ratio ([alpha/Fe]) is an important indicator for star formation histories because alpha-elements (such as O, Ne, Mg, Si, Ca, Ti) are mostly produced in core-collapse supernovae, whose progenitors are high-mass stars, while irons (Fe) are mostly produced by Type Ia supernovae, whose progenitors are low-mass compact stars. High-mass stars generally have a very short lifetime (~ 1 Million year), whilst low-mass compact stars almost all have ages larger than 1 Giga year. So a galaxy formed in a single burst or experienced fast quenching will be enhanced in [alpha/Fe] in comparison with a galaxy formed with an extended star formation history.

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PolarLight detects first light events

On December 18, PolarLight, an X-ray polarimeter onboard a CubeSat, was powered on for in-orbit test, and detected the first events triggered by cosmic X-rays and charged particles. This is the first time that the new technique for X-ray polarimetry is demonstrated in space, implying that a new window in X-ray astronomy can be opened in the future.

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X-rays Reveal Supercritically Accreting Compact Objects

Accretion onto compact objects has a critical rate when the radiation balances the gravity. The physics for supercritical accretion is still an unsolved problem. It is suggested that, because of the presence of strong radiation pressure, supercritical accretion will power a massive wind that is optically thick and Eddington-limited. Based on Chandra observations of nearby galaxies, Zhou et al. found a list of very soft X-ray sources, which are argued to be good candidates for compact objects under supercritical accretion. The results will be published in the Astrophysical Journal.

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Stellar initial mass function varies across galaxies

Stars are the building blocks of galaxies. The stellar initial mass function (IMF), which describes the mass distribution of stars at birth, has been the subject of numerous investigations in the past decades. The first estimate of the IMF was obtained by Salpeter more than half a contrary ago, described simply by a power law function with a slope of 1.3, i.e. ϕ∝m^(-1.3)) across the entire mass range of stars. Subsequent studies of resolved stellar populations in the Milky Way have revealed a more bottom-light IMF, with a shallower slope at the low-mass end (<0.5M_⊙). In most galaxies, however, directly counting the number of stars in resolved stellar populations is impossible due to the limited spatial resolution of our observational facilities. A long-standing debate on IMF is whether the IMF measured from the few very local galaxies is universal to the general population of galaxies, or it varies from galaxy to galaxy or even from region to region within a single galaxy.

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Talking about the Sun and solar activities

The 24th “New Worlds, New Horizons” Public Astronomy Lecture took place at 6C300, the No. 6 Teaching Building (the Yuyuan Building) in Tsinghua University on Dec. 1st, 2018. It was delivered by Professor WANG Jingxiu, from the University of Chinese Academy of Sciences. It attracted a large audience on site and over 1000 online viewers via the Tencent News live broadcast.

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