16379990
A high peculiarity rate for Type Ia SNe
We have compiled a sample of 90 SNe Ia from 1997 to 1999 (up to SN 1999da) and studied the peculiarity rate of SN 1991T, SN 1991bg and SN 1986G-like objects. A Monte Carlo code was written to study the observational biases involved in the evaluation of the intrinsic peculiarity rate of SNe Ia. We find that SNe Ia have a high peculiarity rate (>30%) and a flat luminosity function.
15020605
The type Ia supernova 1997br in ESO 576-G40
The peculiar type Ia supernova SN 1997br in ESO 576-G40 was extensively observed at Beijing Astronomical Observatory and Lick Observatory. In this paper, we present and discuss the BVRI photometry and the spectra collected over 3 months, beginning 9 days before maximum brightness. The light curves of SN 1997br are similar to those of SN 1991T, with slow decline rates after the B maximum. Well-sampled data before the B maximum show unambiguously that SN 1997br rises more slowly and has a wider peak than normal type Ia supernovae. The optical color evolution of SN 1997br is also similar to that of SN 1991T. We estimate the extinction of SN 1997br to be E(B-V) = 0.35 ± 0.10 mag by comparing its BVRI light curves with those of SN 1991T and by measuring the equivalent width of interstellar Na I D absorption lines. We have conducted a thorough comparison of the spectroscopic evolution of SN 1997br, SN 1991T, and SN 1994D. Although SN 1997br is generally very similar to SN 1991T, it shows some interesting differences at various epochs. Spectra of SN 1997br seem to indicate an earlier transition to the dominant phase of Fe-peak elements after the B maximum. Si II lines in SN 1997br show a very short duration after the B maximum. We discuss the implications of our observations of SN 1997br for models of type Ia supernovae. Specifically, we suggest that some SNe Ia may result from decelerated detonations of white dwarfs.
1567381
The structure of the Galactic halo : SDSS versus SuperCOSMOS
The halo structure at high Galactic latitudes near both the north and south poles is studied using Sloan Digital Sky Survey (SDSS) and SuperCOSMOS data. For the south cap halo, the archive of the SuperCOSMOS photographic photometry sky survey is used. The coincident source rate between SuperCOSMOS data in B J band from 16.5 to 20.5 mag and SDSS data is about 92 per cent, in a common sky area in the south. While that in the R F band is about 85 per cent from 16.5 to 19.5 mag. Transformed to the SuperCOSMOS system and downgraded to the limiting magnitudes of SuperCOSMOS, the star counts in the North Galactic Cap from SDSS show up to an 16.9±6.3 per cent asymmetric ratio (defined as relative fluctuations over the rotational symmetry structure) in the B J band, and up to 13.5 ± 6.7 per cent asymmetric ratio in the R F band. From SuperCOSMOS Bj and R F bands, the structure of the Southern Galactic hemisphere does not show the same obvious asymmetric structures as the northern sky does in both the original and downgraded SDSS star counts. An axisymmetric halo model with n = 2.8 and q = 0.7 can fit the projected number density from SuperCOSMOS fairly well, with an average error of about 9.17 per cent. By careful analysis of the difference of star counts between the downgraded SDSS northern halo data and SuperCOSMOS southern halo data, it is shown that no asymmetry can be detected in the South Galactic Cap at the accuracy of SuperCOSMOS, and the Virgo overdensity is likely a foreign component in the Galactic halo.
18487685
Triggered Star Formation in the Orion Bright-rimmed Clouds
We have developed an empirical and effective set of criteria, based on the Two Micron All Sky Survey (2MASS) colors, to select candidate classical T Tauri stars (CTTSs). This provides a useful tool to study the young stellar population in star-forming regions. Here we present our analysis of the bright-rimmed clouds (BRCs) B35, B30, IC 2118, LDN 1616, LDN 1634, and Ori East to show how massive stars interact with molecular clouds to trigger star formation. Our results support the radiation-driven implosion model, in which the ionization fronts from OB stars compress a nearby cloud until the local density exceeds the critical value, thereby inducing the cloud to collapse to form stars. We find that only BRCs associated with strong IRAS 100 μm emission (a tracer of high density) and Hα emission (a tracer of ionization fronts) show signs of ongoing star formation. Relevant timescales, including the ages of O stars, expanding H II regions, and the ages of CTTSs, are consistent with sequential star formation. We also find that CTTSs are only seen between the OB stars and the BRCs, with those closer to the BRCs being progressively younger. There are no CTTSs leading the ionization fronts, i.e., within the molecular clouds. All of these findings provide strong evidence of triggered star formation and show the major roles massive stars play in sustaining the star-forming activities in the region.
119270494
Multiwavelength study of nearly face-on low surface brightness disk galaxies
We study the ages of a large sample (1802) of nearly face-on disk low surface brightness galaxies (LSBGs) using the evolutionary population synthesis (EPS) model PEGASE with an exponentially decreasing star formation rate to fit their multiwavelength spectral energy distributions (SEDs) from far-ultraviolet (FUV) to near-infrared (NIR). The derived ages of LSBGs are 1–5 Gyr for most of the sample no matter if constant or varying dust extinction is adopted, which are similar to most of the previous studies on smaller samples. This means that these LSBGs formed the majority of their stars quite recently. However, a small part of the sample (~2%–3%) has larger ages of 5–8 Gyr, meaning their major star forming process may have occurred earlier. At the same time, a large sample (5886) of high surface brightness galaxies (HSBGs) are selected and studied using the same method for comparisons. The derived ages are 1–5 Gyr for most of the sample (97%) as well. These results probably mean that these LSBGs have not much different star formation histories from their HSBGs counterparts. However, we should notice that the HSBGs are generally about 0.2 Gyr younger, which could mean that the HSBGs may have undergone more recent star forming activities than the LSBGs.
118411365
The photometric system of the Tsinghua-NAOC 80-cm telescope at NAOC Xinglong Observatory
The Tsinghua-NAOC (National Astronomical Observatories of China) Telescope (hereafter, TNT) is an 80-cm Cassegrain reflecting telescope located at the Xinglong Observatory of NAOC, with the main scientific goals of monitoring various transients in the universe such as supernovae, gamma-ray bursts, novae, variable stars, and active galactic nuclei. We present a systematic test and analysis of the photometric performance of this telescope. Based on the calibration observations taken on 12 photometric nights, spanning the period from 2004 to 2012, we derived an accurate transformation equation between the instrumental ubvri magnitudes and standard Johnson UBV and Cousins RI magnitudes. In particular, the color terms and the extinction coefficients of different passbands are well determined. With these data, we also obtained the limiting magnitudes and the photometric precision of TNT. It is worthwhile to point out that the sky background at the Xinglong Observatory became gradually worse over the period from 2005 to 2012 (e.g., ~21.4 mag vs. ~20.1 mag in the V band).
119772144
Microscopic study of the ground state properties of light nuclei
Ground state properties of light isotopes from helium to oxygen, such as binding energies, spin parities, rms radii, and halo phenomena are studied systematically by the Skyrme Hartree-Fock model with fit E{sub {sigma} } as the effective interaction. Since the particle number of the studied system is rather small, pairing interactions between identical nucleons are treated with the particle number conservation method. The results of the calculation show that most of the ground state properties can be well described. Some unsuccessful points of the calculation are pointed out. {copyright} {ital 1997} {ital The American Physical Society}
121881392
Extended chiral Hartree-Fock model for nuclear matter
We study the properties of nuclear matter with extended chiral Hartree‐Fock model. The two‐particle‐two‐hole (2p‐2h) states are introduced in the nucleon ground wave function to take the tensor components of pion into account. We also consider the form factor effect of pion to regularize its high momentum behavior. We find that the pion contribution from 2p‐2h states plays a very important role in the saturation mechanism of nuclear matter. Furthermore, the saturation properties are also influenced by the cut‐off moment of form factor.
14520300
Distance Determination of Variable Galactic Sources
We have developed a timing analysis method to determine the distances of variable galactic X-ray sources based on the method advanced by Trumper and Schonfelder in 1973. The light-curve of the halo produced by the scattering of X-rays off the interstellar dust is delayed and smeared by the dust grains. This method utilizes the differences between the power density spectra of the point source and the halo. We present the details of this method and our first applications of this method to the Chandra data of X-ray binary Cyg X-3.
17626951
Time expansion chamber system for characterization of TWIST low energy muon beams
A low-mass time expansion chamber (TEC) has been developed to measure distributions of position and angle of the TRIUMF low-energy surface muon beam used for the TRIUMF Weak Interaction Symmetry Test (TWIST) experiment. The experiment is a high-precision measurement of muon decay and is dominated by systematic uncertainties, including the stability, reproducibility, and characterization of the beam. The distributions measured by two TEC modules are one essential ingredient of an accurate simulation of TWIST. The uncertainties, which are extracted through comparisons of data and simulation, must be known to assess potential systematic uncertainties of the TWIST results. The design criteria, construction, alignment, calibration, and operation of the TEC system are discussed, including experiences from initial beam studies. A brief description of the use of TEC data in the TWIST simulation is also included.
119402342
ROSAC: Studying the clustering properties of X-ray selected AGNs
We present the first ROSAC results of an AGN clustering analysis. This study comprises a sample of 200 AGNs, 75% of which being at low redshifts z<0.5, in the Ursa Major constellation. The spatial 2-point-correlation function (SCF) as well as the minimal spanning tree (MST) technique were applied. Some evidence for clustering is found in the SCF, although with low significance. Using the MST technique, we could find two AGN groups. This result is preliminary and the exact significance will be tested with careful simulations.