Jinzhong Liu

Using pulsar timing arrays and the quantum normalization condition to constrain relic gravitational waves

In the non-standard model of relic gravitational waves (RGWs) generated in the early universe, the theoretical spectrum is mainly described by an amplitude r and a spectral index beta, the latter usually being determined by the slope of the inflaton potential. Pulsar timing arrays (PTAs) data have imposed constraints on the amplitude of strain spectrum for a power-law form as a phenomenological model. Applying these constraints to a generic, theoretical spectrum with r and beta as independent parameters, we convert the PTAs constraint into an upper bound on the index beta, which turns out to be less stringent than those upper bounds from the Big Bang nucleosynthesis, cosmic microwave background and LIGO/VIRGO, respectively. Moreover, it is found that PTAs constrain the nonstandard RGWs more stringently than the standard RGWs. If the condition of the quantum normalization is imposed upon a theoretical spectrum of RGWs, r and beta become related. With this condition, a minimum requirement of the horizon size during inflation is greater than the Planck length that results in an upper bound on beta, which is comparable in magnitude to that by PTAs. When both PTAs and the quantum normalization are applied to a theoretical spectrum of RGWs, constraints can be obtained for other cosmic processes of the early universe, such as the reheating, a process less understood observationally so far. The resulting constraint is consistent with the slow-roll, massive scalar inflation model. The future square kilometer array will be able to constrain RGWs further and might even detect RGWs, rendering an important probe to the very early universe.

Testing three derivative methods of stellar population synthesis models

The number of methods used to study the properties of galaxies is increasing, and it is very important to test these methods. Galactic globular clusters (GCs) provide an excellent medium for such tests, because they can be considered as simple stellar populations. We present ages and metallicities for 40 Galactic GCs as determined from three publicly available techniques, namely the colour, Lick-index and spectrum-fitting methods, based on Bruzual & Charlot evolutionary population synthesis (EPS) models. By comparisons with the ages obtained from colour-magnitude diagrams (CMDs) and metallicities obtained from the spectra of stars, we are able to estimate the ability of these methods to determine GC parameters, which is essential. We find that: (i) for the metallicity, our derived metallicities agree with those derived from the spectra of stars - the Lick-index method is suitable for studying metallicity for stellar population systems in the range - 1.5 less than or similar to [Fe/H] less than or similar to -0.7 and the spectrum-fitting method is suitable for studying metallicity for stellar population systems in the range -2.3 less than or similar to [Fe/H] less than or similar to -1.5; (ii) for the age, these three methods have difficulties in age determination - our derived ages are smaller (by about 2.0 Gyr, on average) than the results of CMDs for all three methods. We use Vazdekis and Maraston models to analyse whether our results are dependent on EPS models, and find that the tendency of these two models is the same as that of Bruzual & Charlot models. Our results are independent of the EPS models. In addition, our test is based on old GCs and our conclusions may hold for old stellar population systems. Besides the age-metallicity degeneracy, we examine the possible effects of other factors (horizontal branch morphology, blue straggler stars, binary interactions and alpha-enhancement) and give a quantitative analysis of the influences of these factors on age determinations (except for alpha-enhancement). For the colour and spectrum-fitting methods, the age can be underestimated by about 0.0-3.0, 0.0-2.0 and 0.0-3.0Gyr owing to the influence of horizontal branch, blue straggler and binary stars, respectively. For the Lick-index method, the lower limit of maximal change of age is 6.0, 5.0 and 3.0 Gyr owing to the influence of horizontal branch, blue straggler and binary stars, respectively.

A Comprehensive Study of Close Double White Dwarfs as Gravitational Wave Sources: Evolutionary Channels, Birth Rates, and Physical Properties

Close double white dwarfs (CDWDs) are believed to dominate the Galactic gravitational wave (GW) radiation in the frequency range 10(-4) to 0.1 Hz, which will be detected by the Laser Interferometer Space Antenna (LISA) detector. The aim of this detector is to detect GW radiation from astrophysical sources in the universe and to help improve our understanding of the origin of the sources and their physical properties (masses and orbital periods). In this paper, we study the probable candidate sources in the Galaxy for the LISA detector: CDWDs. We use the binary population synthesis approach of CDWDs together with the latest findings of the synthesis models from Han, who proposed three evolutionary channels: (1) stable Roche lobe overflow plus common envelope (RLOF+CE), (2) CE+CE, and (3) exposed core plus CE. As a result, we systematically investigate the detailed physical properties (the distributions of masses, orbital periods, and chirp masses) of the CDWD sources for the LISA detector, examine the importance of the three evolutionary channels for the formation of CDWDs, and carry out Monte Carlo simulations. Our results show that RLOF+CE and CE+CE are the main evolutionary scenarios leading to the formation of CDWDs. For the LISA detectable sources, we also explore and discuss the importance of these three evolutionary channels. Using the calculated birth rate, we compare our results to the LISA sensitivity curve and the foreground noise floor of CDWDs. We find that our estimate for the number of CDWD sources that can be detected by the LISA detector is greater than 10,000. We also find that the detectable CDWDs are produced via the CE+CE channel and we analyze the fraction of the detectable CDWDs that are double helium (He+He), or carbon-oxygen plus helium (CO+He) WD binary systems.

Integrated spectral energy distributions of binary star composite stellar populations

This paper presents the theoretically integrated spectral energy distributions (SEDs) of binary star composite stellar populations (bsCSPs) in early-type galaxies and how the bsCSP model can be used for the spectral studies of galaxies. All bsCSPs are built based on three adjustable inputs (metallicity, ages of old and young components). The effects of binary interactions and stellar population mixture are taken into account. The results show some ultraviolet (UV) upturn SEDs naturally for bsCSPs. The SEDs of bsCSPs are affected obviously by all of the three stellar population parameters, and the effects of three parameters are degenerate. This suggests that the effects of metallicity, and the ages of the old (major in stellar mass) and young (minor) components of stellar populations should be taken into account in the SED studies of early-type galaxies. The sensitivities of SEDs at different wavelengths to the inputs of a stellar population model are also investigated. It is shown that UV SEDs are sensitive to all of the three stellar population parameters, rather than to only stellar age. Special wavelength ranges according to some SED features that are relatively sensitive to the stellar metallicity, young-component age and old-component age of bsCSPs are found by this work. For example, the shapes of SEDs with the wavelength ranges of 51105250, 52505310, 53105350, 58305970 and 20 95023 550 angstrom are relatively sensitive to the stellar metallicity of bsCSPs. The shapes of SEDs within 965985, 10051055 and 12051245 angstrom are sensitive to the old-component age, while SED features within the wavelength ranges of 21852245, 24552505, 25052555, 27752825 and 28252875 angstrom are sensitive to the young-component age. The results suggest that some line indices within these special wavelength ranges are possibly better for stellar population studies compared to the others, and greater weights may be given to these special SED parts in the determination of the stellar population parameters of early-type galaxies from fitting SEDs via bsCSPs.

Determining gravitational wave radiation from close galaxy pairs using a binary population synthesis approach

Context. The early phase of the coalescence of supermassive black hole (SMBH) binaries from their host galaxies provides a guaranteed source of low-frequency (nHz-mu Hz) gravitational wave (GW) radiation by pulsar timing observations. These types of GW sources would survive the coalescing and be potentially identifiable. Aims. We aim to provide an outline of a new method for detecting GW radiation from individual SMBH systems based on the Sloan Digital Sky Survey (SDSS) observational results, which can be verified by future observations. Methods. Combining the sensitivity of the international Pulsar Timing Array (PTA) and the Square Kilometer Array (SKA) detectors, we used a binary population synthesis (BPS) approach to determine GW radiation from close galaxy pairs under the assumption that SMBHs formed at the core of merged galaxies. We also performed second post-Newtonian approximation methods to estimate the variation of the strain amplitude with time. Results. We find that the value of the strain amplitude h varies from about 10(-14) to 10(-17) using the observations of 20 years, and we estimate that about 100 SMBH sources can be detected with the SKA detector.

A systematic observational study of radio properties of H2O megamaser Seyfert-2 galaxies

A systematic study is performed on radio properties of H

The impact of binary stars on the colors of high-redshift galaxies

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A study of variable stars in the open cluster NGC 1582 and its surrounding field

O megamaser host Seyfert 2 galaxies, through multi-band radio continuum observations (at 11cm, 6.0 cm, 3.6 cm, 2.0 cm and 1.3 cm) with the Effelsberg 100-m radio telescope within a total time duration of four days. For comparison, a control Seyfert 2 galaxy sample without detected maser emission was also observed. Spectral indices were determined for those sources for which measurements exist at two adjacent bands assuming a power-law dependence S

Star–Disk Interactions in Multiband Photometric Monitoring of the Classical T Tauri Star GI Tau

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Transiting Exoplanet Monitoring Project (TEMP). I. Refined System Parameters and Transit Timing Variations of HAT-P-29b

, where S is the flux density and