197473308
Spectrum Analysis of the Type Ib Supernova 1999dn: Probable Identifications of C II and H-alpha
Low resolution spectra of SN 1999dn at early times are presented and compared with synthetic spectra generated with the parameterized supernova synthetic-spectrum code SYNOW. We find that the spectra of SN 1999dn strongly resemble those of SN 1997X and SN 1984L, and hence we classify it as a Type Ib event. Line-identifications are established through spectrum synthesis. Strong evidence of both H-alpha and C II 6580 is found. We infer that H-alpha appears first, before the time of maximum brightness, and then is blended with and finally overwhelmed by the C II line after maximum; this favors a thin high-velocity hydrogen skin in this Type Ib supernova.
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.
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.
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).
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.
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.
119246785
Properties of resonant states in 18Ne relevant to key 14O(alpha,p)17F breakout reaction in type I x-ray bursts
The $^{14}$O($\alpha$,$p$)$^{17}$F reaction is one of the key reactions involved in the breakout from the hot-CNO cycle to the rp-process in type I x-ray bursts. The resonant properties in the compound nucleus $^{18}$Ne have been investigated through resonant elastic scattering of $^{17}$F+$p$. The radioactive $^{17}$F beam was separated by the CNS Radioactive Ion Beam separator (CRIB) and bombarded a thick H$_2$ gas target at 3.6 MeV/nucleon. The recoiling light particles were measured by using three ${\Delta}$E-E silicon telescopes at laboratory angles of $\theta$$_{lab}$$\approx$3$^\circ$, 10$^\circ$ and 18$^\circ$, respectively. Five resonances at $E_{x}$=6.15, 6.28, 6.35, 6.85, and 7.05 MeV were observed in the excitation functions. Based on an $R$-matrix analysis, $J^{\pi}$=1$^-$ was firmly assigned to the 6.15-MeV state. This state dominates the thermonuclear $^{14}$O($\alpha$,$p$)$^{17}$F rate below 1 GK. We have also confirmed the existence and spin-parities of three states between 6.1 and 6.4 MeV. As well, a possible new excited state in $^{18}$Ne was observed at $E_{x}$=6.85$\pm$0.11 MeV and tentatively assigned as $J$=0. This state could be the analog state of the 6.880 MeV (0$^{-}$) level in the mirror nucleus $^{18}$O, or a bandhead state (0$^+$) of the six-particle four-hole (6$p$-4$h$) band. A new thermonuclear rate of the $^{14}$O($\alpha$,$p$)$^{17}$F reaction has been determined, and its astrophysical impact has been examined within the framework of one-zone x-ray burst postprocessing calculations.
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.
95137172
Soft x-ray imaging and spectroscopy of single nanocrystals
Resonant photoemission electron microscopy (PEEM) at the Fe L3,2 absorption edge was utilized to image single γ-Fe2O3 nanocrystals of 10 nm average diameter (∼20 000 Fe atoms) and to record soft x-ray absorption spectra of individual particles. Within the spectral resolution of the experiment, no damage to the individual nanoparticles occurs during repeated, prolonged exposure to the intense x-ray beam. Furthermore, no differences in the position or shape of the soft x-ray absorption spectrum of a single nanocrystal and the ensemble are observed within the experimental resolution. PEEM contrast images and soft x-ray absorption spectra, however, show strong intensity variations between different particles reflecting the size distribution of the sample. This proof-of-principle experiment successfully demonstrates the applicability of x-ray spectromicroscopy to the study of nanoscale systems on a hitherto unachieved length scale.
121460689
Relativistic Hartree-Fock model with bare nucleon-nucleon interaction for neutron-rich matter
Abstract We study neutron-rich matter in the relativistic Hartree–Fock model (RHF) with the bare nucleon–nucleon interaction. As an important ingredient, we take into account the short range correlation in terms of the unitary correlation operator method (UCOM). We calculate the equations of state (EOS) of neutron-rich matter with Bonn potential. Our results on neutron-rich matter compare very nicely with those in the Relativistic Brueckner–Hartree–Fock (RBHF) theory. On the other hand, when we apply the RHF model with the UCOM on symmetric nuclear matter, the EOS is largely different from the RBHF result particularly in the low density region. This is because the other important ingredient as the tensor interaction plays important role in symmetric nuclear matter. We also discuss the proton and neutron Dirac masses in neutron-rich matter and find that the proton Dirac mass is larger than the neutron mass in agreement with the RBHF theory using projection techniques.
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.