分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： As is well known, there are various mass limits for compact stars. For example, the maximum mass for non-rotating white dwarfs is given by the famous Chandrasekhar limit about $1.4 M_\odot$ (solar masses). Although the mass limit for neutron stars is not so clear to date, one of the widely accepted values is about $2.1 M_\odot\,$. Recently, challenges to these mass limits appeared. Motivated by the super-Chandrasekhar mass white dwarfs with masses up to $2.4 \sim 2.8 M_\odot\,$, and compact objects (probably neutron stars) in the mass gap (from $2.5 M_\odot$ or $3 M_\odot$ to $5 M_\odot$) inferred from gravitational waves detected by LIGO/Virgo in the third observing run (O3), we reconsider the mass limits for compact stars in the present work. Without invoking strong magnetic field and/or exotic equation of state (EOS), we try to increase the mass limits for compact stars in modified gravity theory. In this work, we propose an inverse chameleon mechanism, and show that the fifth-force mediated by the scalar field can evade the severe tests on earth, in solar system and universe, but manifest itself in compact stars such as white dwarfs and neutron stars. The mass limits for compact stars in the inverse chameleon mechanism can be easily increased to $3 M_\odot\,$, $5 M_\odot$ or even larger. We argue that the inverse chameleon mechanism might be constrained by the observations of exoplanets orbiting compact stars (such as white dwarfs and neutron stars), and gravitational waves from the last stage of binary compact star coalescence.
分类： 地球科学 >> 空间物理学 提交时间： 2016-05-03
摘要： GNSS buoys are innovative ocean instruments, and can provide calibrations of the sea surface height and significant wave height (SWH) products of satellite altimeters. This paper investigated the principles of the wave retrieval by GNSS buoys, and proposed a method of calibration of altimeter SWH product by GNSS buoys. The error sources in the calibration procedure were analyzed. Ocean calibration campaigns were implemented to measure the SWH at the HY-2 altimeter nadir points, and acquired a SWH bias of +7.92 +/- 5.04 cm using four-parameters retracking algorithm. The SWH bias was +20.37 +/- 12.79 cm base on the HY-2 operational SWH products. The calibration campaigns showed that, GNSS buoys could serve for the satellite altimeter SWH calibration under low sea state condition, and the four-parameters retracking algorithm outperformed the conventional three-parameters retracking algorithm in accuracy and consistency, at least for SWH retrieving.
分类： 地球科学 >> 空间物理学 提交时间： 2016-05-03
摘要： An airborne delay Doppler altimeter was flown in autumn 2014. A calibration suite including a GNSS buoy, two tide gauges and two GPS stations were established. An experiment was carried out to test the GNSS buoy as a whole under dynamic ocean condition, to validate its performance and accuracy via the inter-comparison of tide gauges, and to demonstrate the altimetry calibration methods and to estimate the altimeter system bias. This paper presented some methods and results. The GNSS buoy system was proven to have the ability of measuring the sea surface height within 1cm, and the altimeter system bias had a consistency of centimeter level.
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