Population Properties of Gravitational-Wave Neutron Star--Black Hole Mergers

Over the course of the third observing run of LIGO-Virgo-KAGRA Collaboration, several gravitational-wave (GW) neutron star--black hole (NSBH) candidates have been announced. By assuming these candidates are real signals and of astrophysical origins, we analyze the population properties of the mass and spin distributions for GW NSBH mergers. We find that the primary BH mass distribution of NSBH systems, whose shape is consistent with that inferred from the GW binary BH (BBH) primaries, can be well described as a power-law with an index of $\alpha = 4.8^{+4.5}_{-2.8}$ plus a high-mass Gaussian component peaking at $\sim33^{+14}_{-9}\,M_\odot$. The NS mass spectrum could be shaped as a near flat distribution between $\sim1.0-2.1\,M_\odot$. The constrained NS maximum mass agrees with that inferred from NSs in our Galaxy. If GW190814 and GW200210 are NSBH mergers, the posterior results of the NS maximum mass would be always larger than $\sim2.5\,M_\odot$ and significantly deviate from that inferred in the Galactic NSs. The effective inspiral spin and effective precession spin of GW NSBH mergers are measured to potentially have near-zero distributions. The negligible spins for GW NSBH mergers imply that most events in the universe should be plunging events, which supports the standard isolated formation channel of NSBH binaries. More NSBH mergers to be discovered in the fourth observing run would help to more precisely model the population properties of cosmological NSBH mergers.Comment: 14 pages, 5 figures, 3 tables, accepted for publication in Ap

Population Properties of Gravitational-wave Neutron Star-Black Hole Mergers

Over the course of the third observing run of the LIGO-Virgo-KAGRA Collaboration, several gravitational-wave (GW) neutron star-black hole (NSBH) candidates have been announced. By assuming that these candidates are real signals with astrophysical origins, we analyze the population properties of the mass and spin distributions for GW NSBH mergers. We find that the primary BH mass distribution of NSBH systems, whose shape is consistent with that inferred from the GW binary BH (BBH) primaries, can be well described as a power law with an index of α=4.8-2.8+4.5 plus a high-mass Gaussian component peaking at ∼33-9+14M⊙ . The NS mass spectrum could be shaped as a nearly flat distribution between ∼1.0 and 2.1 M ⊙. The constrained NS maximum mass agrees with that inferred from NSs in our Galaxy. If GW190814 and GW200210 are NSBH mergers, the posterior results of the NS maximum mass would be always larger than ∼2.5 M ⊙ and significantly deviate from that inferred in Galactic NSs. The effective inspiral spin and effective precession spin of GW NSBH mergers are measured to potentially have near-zero distributions. The negligible spins for GW NSBH mergers imply that most events in the universe should be plunging events, which support the standard isolated formation channel of NSBH binaries. More NSBH mergers to be discovered in the fourth observing run would help to more precisely model the population properties of cosmological NSBH mergers. © 2022. The Author(s). Published by the American Astronomical Society

Population Size, Genetic Diversity and Molecular Evidence of a Recent Population Bottleneck in Hynobius chinensis, an Endangered Salamander Species

Severe population declines can reduce species to small populations, offering permissive conditions for deleterious processes. For example, following such events, species can become prone to inbreeding and genetic drift which can lead to a loss of genetic diversity and evolutionary potentials. Hynobius chinensis is a poorly studied very rare and declining endangered amphibian species endemic to China in Changyang County. We investigated adult census population size by monitoring breeding populations from 2015 to 2018, developed microsatellite markers from the transcriptome and used them to investigate genetic diversity, and a population bottleneck in this species. We found H. chinensis in 4 different localities in a total area of 2.18 km2 and estimated the overall adult census population size at 386–404 individuals. The adult census size (mean ± SE) per breeding pond ranged from 44 ± 6 to 141 ± 8 individuals and appeared smaller than that reported in closely related species in undisturbed habitats. We developed and characterized 13 microsatellite markers in total. Analysis of data at 7 loci (N = 118) in Hardy-Weinberg equilibrium gathered from the largest population showed that genetic diversity level was low. The average number of alleles per locus was 2.14. The observed and expected heterozygosities averaged 0.38 and 0.40, respectively. The inbreeding coefficient was –0.06. All tests performed to investigate a population bottleneck, i.e. The Garza-Williamson test, Heterozygosity excess test, Mode shift test of allele frequency, and effective population size estimates detected a population bottleneck. The contemporary and the historical effective population sizes were estimated at 36 and 234 individuals, respectively. We argue that as bottleneck effects, the studied population may have become prone to genetic drift and inbreeding, losing microsatellite alleles and heterozygosity. Our results suggest that populations of H. chinensis may have been extirpated in the study area

Sources and distribution of particulate organic carbon in Great Wall Cove and Ardley Cove, King George Island, West Antarctica

Concentrations of chlorophyll-a (Chl-a), particulate organic carbon (POC) and its stable carbon isotope composition (δ13C) were analyzed to investigate the biogeochemical characteristics and sources of POC in Great Wall Cove (GWC) and Ardley Cove (AC) during the austral summer. POC concentrations ranged from 50.51 to 115.41 μg·L−1 (mean±1 standard deviation: 77.69±17.27 μg·L−1) in GWC and from 63.42 to 101.79 μg·L−1 (82.67±11.83 μg·L−1) in AC. The POC δ13C ranged from −30.83‰ to −26.12‰ (−27.40‰±0.96‰) in GWC and from −28.21‰ to −26.65‰ (−27.45‰±0.47‰) in AC. The temperature and salinity results showed distinct runoff signals in both GWC and AC, although the δ13C data and POC distribution indicate a negligible influence of land sources upon POC. The δ13C values suggest that POC is of predominantly marine origin. The POC/Chl-a ratio and the relationship between POC and Chl-a indicate that phytoplankton, organic detritus and heterotrophic organisms are significant contributors to POC in GWC and AC

Seawater nutrient and chlorophyll α distributions near the Great Wall Station, Antarctica

We examined the influences upon nutrient, temperature, salinity and chlorophyll a distributions in Great Wall Cove (GWC) and Ardley Cove (AC), near the Chinese Antarctic Great Wall Station, using measurements taken in January 2013 and other recent data. Nutrient concentrations were high, with phosphate concentrations of 1.94 (GWC) and 1.96 (AC) μmol·L−1, DIN(dissolved inorganic nitrogen) concentrations of 26.36 (GWC) and 25.94 (AC) μmol·L−1 and silicate concentrations of 78.6 (GWC) and 79.3 (AC) μmol·L−1. However, average concentrations of chlorophyll a were low (1.29 μg·L−1, GWC and 1.08 μg·L−1, AC), indicating that this region is a high-nutrient and low-chlorophyll (HNLC) area. Nutrient concentrations of freshwater (stream and snowmelt) discharge into GWC and AC in the austral summer are low, meaning freshwater discharge dilutes the nutrient concentrations in the two coves. Strong intrusion of nutrient-rich water from the Bransfield Current in the south was the main source of nutrients in GWC and AC. Low water temperature and strong wind-induced turbulence and instability in the upper layers of the water column were the two main factors that caused the low phytoplankton biomass during the austral summer

Kilonovae and Optical Afterglows from Binary Neutron Star Mergers. II. Optimal Search Strategy for Serendipitous Observations and Target-of-opportunity Observations of Gravitational Wave Triggers

In the second work of this series, we explore the optimal search strategy for serendipitous and gravitational-wave-triggered target-of-opportunity (ToO) observations of kilonovae and optical short-duration gamma-ray burst (sGRB) afterglows from binary neutron star (BNS) mergers, assuming that cosmological kilonovae are AT2017gfo-like (but with viewing-angle dependence) and that the properties of afterglows are consistent with those of cosmological sGRB afterglows. A one-day cadence serendipitous search strategy with an exposure time of ∼30 s can always achieve an optimal search strategy of kilonovae and afterglows for various survey projects. We show that the optimal detection rates of the kilonovae (afterglows) are ∼0.3/0.6/1/20 yr−1 (∼50/60/100/800 yr−1) for Zwicky the Transient Facility (ZTF)/Multi-channel Photometric Survey Telescope (Mephisto)/Wide Field Survey Telescope (WFST)/Large Synoptic Survey Telescope (LSST), respectively. A better search strategy for SiTian than the current design is to increase the exposure time. In principle, a fully built SiTian can detect ∼7(2000) yr−1 kilonovae (afterglows). Population properties of electromagnetic (EM) signals detected by serendipitous observations are studied in detail. For ToO observations, we predict that one can detect ∼11 yr−1 BNS gravitational wave (GW) events during the fourth observing run (O4) by considering an exact duty cycle of the third observing run. The median GW sky localization area is expected to be ∼10 deg2 for detectable BNS GW events. For O4, we predict that ZTF/Mephisto/WFST/LSST can detect ∼5/4/3/3 kilonovae (∼1/1/1/1 afterglows) per year, respectively. The GW detection rates, GW population properties, GW sky localizations, and optimistic ToO detection rates of detectable EM counterparts for BNS GW events at the Advanced Plus, LIGO Voyager, and ET&CE eras are detailedly simulated in this paper