arXiv subjects

Weidong Li

Publications and source records attributed to Weidong Li.

At least 145 records · Page 8Linked to original sources

arXiv2010

Near-Ultraviolet Properties of a Large Sample of Type Ia Supernovae as Observed with the Swift UVOT

We present ultraviolet (UV) and optical photometry of 26 Type Ia supernovae (SNe~Ia) observed from March 2005 to March 2008 with the NASA {\it Swift} Ultraviolet and Optical Telescope (UVOT). The dataset consists of 2133 individual observations, making it by far the most complete study of the UV emission from SNe~Ia to date. Grouping the SNe into three subclasses as derived from optical observations, we investigate the evolution of the colors of these SNe, finding a high degree of homogeneity within the normal subclass, but dramatic differences between that group and the subluminous and SN 2002cx-like groups. For the normal events, the redder UV filters on UVOT ($u$, $uvw1$) show more homogeneity than do the bluer UV filters ($uvm2$, $uvw2$). Searching for purely UV characteristics to determine existing optically based groupings, we find the peak width to be a poor discriminant, but we do see a variation in the time delay between peak emission and the late, flat phase of the light curves. The UV light curves peak a few days before the $B$ band for most subclasses (as was previously reported by Jha et al. 2006a), although the SN 2002cx-like objects peak at a very early epoch in the UV. That group also features the bluest emission observed among SNe~Ia. As the observational campaign is ongoing, we discuss the critical times to observe, as determined by this study, in order to maximize the scientific output of future observations.

astro-ph.CO↗

arXiv2010

The Absolute Magnitudes of Type Ia Supernovae in the Ultraviolet

We examine the absolute magnitudes and light-curve shapes of 14 nearby(redshift z = 0.004--0.027) Type Ia supernovae (SNe~Ia) observed in the ultraviolet (UV) with the Swift Ultraviolet/Optical Telescope. Colors and absolute magnitudes are calculated using both a standard Milky Way (MW) extinction law and one for the Large Magellanic Cloud that has been modified by circumstellar scattering. We find very different behavior in the near-UV filters (uvw1_rc covering ~2600-3300 A after removing optical light, and u ~3000--4000 A) compared to a mid-UV filter (uvm2 ~2000-2400 A). The uvw1_rc-b colors show a scatter of ~0.3 mag while uvm2-b scatters by nearly 0.9 mag. Similarly, while the scatter in colors between neighboring filters is small in the optical and somewhat larger in the near-UV, the large scatter in the uvm2-uvw1 colors implies significantly larger spectral variability below 2600 A. We find that in the near-UV the absolute magnitudes at peak brightness of normal SNe Ia in our sample are correlated with the optical decay rate with a scatter of 0.4 mag, comparable to that found for the optical in our sample. However, in the mid-UV the scatter is larger, ~1 mag, possibly indicating differences in metallicity. We find no strong correlation between either the UV light-curve shapes or the UV colors and the UV absolute magnitudes. With larger samples, the UV luminosity might be useful as an additional constraint to help determine distance, extinction, and metallicity in order to improve the utility of SNe Ia as standardized candles.

astro-ph.CO↗

arXiv2010

Observed Fractions of Core-Collapse Supernova Types and Initial Masses of their Single and Binary Progenitor Stars

We analyse observed fractions of core-collapse SN types from the Lick Observatory SN Search, and we discuss corresponding implications for massive star evolution. For a standard IMF, observed fractions of SN types cannot be reconciled with expectations of single-star evolution. The mass range of WR stars that shed their H envelopes via their own mass loss accounts for less than half the observed fraction of SNeIbc. Progenitors of SNeIbc must extend to a much lower range of initial masses than classical WR stars, and we argue that most SNIbc and SNIIb progenitors must arise from binary Roche-lobe overflow. SNeIc still trace higher mass and metallicity, because line-driven winds in the WR stage remove the He layer and propel the transition from SNIb to Ic. Less massive progenitors of SNeIb and IIb may not be classical WR stars; they may be underluminous with weak winds, possibly hidden by overluminous mass-gainer companions that appear as B[e] supergiants or related objects having aspherical circumstellar material. The remaining SN types (II-P, II-L, and IIn) are redistributed across the full range of initial mass. We consider direct collapse to black holes without visible SNe, but find this problematic. Major areas of remaining uncertainty are (1) the influence of binary separation, rotation, and metallicity, (2) mass differences in progenitors of SNeIIn compared to SNeII-L and II-P, and (3) SNeIc arising from single stars with eruptive mass loss, its dependence on metallicity, and how it relates to diversity within the SNIc subclass. (abridged)

astro-ph.HE↗

arXiv2010

Nearby Supernova Rates from the Lick Observatory Supernova Search. IV. A Recovery Method for the Delay Time Distribution

The supernova (SN) delay-time distribution (DTD) - the SN rate versus time that would follow a brief burst of star formation - can shed light on SN progenitors, and on chemical enrichment timescales. Previous attempts to recover the DTD have used comparisons of mean SN rates vs. redshift to cosmic star-formation history (SFH), or comparison of SN rates among galaxies of different mean ages. We present an approach to recover the SN DTD that avoids such averaging. We compare the SFHs of individual galaxies to the numbers of SNe discovered in each galaxy (generally zero, sometimes one or a few SNe). We apply the method to a subsample of 3505 galaxies, hosting 82 SNe Ia and 119 core-collapse SNe (CC SNe), from the Lick Observatory SN Search (LOSS), with SFHs reconstructed from SDSS spectra. We find a >2sigma SN Ia DTD signal in our shortest-delay, "prompt", bin at <420 Myr. Despite a systematic error, due to the limited aperture of the SDSS spectroscopic fibres, which causes some of the prompt signal to leak to the later DTD bins, the data require prompt SNe Ia at the >99% confidence. We further find, at 4sigma, SNe Ia that are "delayed" by > 2.4 Gyr. Thus, the data support the existence of both prompt and delayed SNe Ia. The time integral over the CC SN DTD is 0.010+/-0.002 SNe per Msun, as expected if all stars of mass >8 Msun lead to visible SN explosions. This argues against a minimum mass for CC SNe of >10 Msun, and against a significant fraction of massive stars that collapse without exploding. For SNe Ia, the time-integrated DTD is 0.0023+/-0.0006 SNe per Msun formed, most of them with delays < 2.4 Gyr. We show, using simulations, that application of the method to the full existing LOSS sample, but with complete and unbiased SFH estimates for the survey galaxies, could provide a detailed measurement of the SN Ia DTD.

astro-ph.CO↗

arXiv2010

Large Late-time Asphericities in Three Type IIP Supernovae

Type II-plateau supernovae (SNe IIP) are the results of the explosions of red supergiants and are the most common subclass of core-collapse supernovae. Past observations have shown that the outer layers of the ejecta of SNe IIP are largely spherical, but the degree of asphericity increases toward the core. We present evidence for high degrees of asphericity in the inner cores of three recent SNe IIP (SNe 2006my, 2006ov, and 2007aa), as revealed by late-time optical spectropolarimetry. The three objects were all selected to have very low interstellar polarization (ISP), which minimizes the uncertainties in ISP removal and allows us to use the continuum polarization as a tracer of asphericity. The three objects have intrinsic continuum polarizations in the range of 0.83-1.56% in observations taken after the end of the photometric plateau, with the polarization dropping to almost zero at the wavelengths of strong emission lines. Our observations of SN 2007aa at earlier times, taken on the photometric plateau, show contrastingly smaller continuum polarizations (~0.1%). The late-time H-alpha and [O I] line profiles of SN 2006ov provide further evidence for asphericities in the inner ejecta. Such high core polarizations in very ordinary core-collapse supernovae provide further evidence that essentially all core-collapse supernova explosions are highly aspherical, even if the outer parts of the ejecta show only small deviations from spherical symmetry.

astro-ph.CO↗

arXiv2010

The Lick AGN Monitoring Project: Reverberation Mapping of Optical Hydrogen and Helium Recombination Lines

We have recently completed a 64-night spectroscopic monitoring campaign at the Lick Observatory 3-m Shane telescope with the aim of measuring the masses of the black holes in 12 nearby (z < 0.05) Seyfert 1 galaxies with expected masses in the range ~10^6-10^7M_sun and also the well-studied nearby active galactic nucleus (AGN) NGC 5548. Nine of the objects in the sample (including NGC 5548) showed optical variability of sufficient strength during the monitoring campaign to allow for a time lag to be measured between the continuum fluctuations and the response to these fluctuations in the broad Hbeta emission, which we have previously reported. We present here the light curves for the Halpha, Hgamma, HeII 4686, and HeI 5876 emission lines and the time lags for the emission-line responses relative to changes in the continuum flux. Combining each emission-line time lag with the measured width of the line in the variable part of the spectrum, we determine a virial mass of the central supermassive black hole from several independent emission lines. We find that the masses are generally consistent within the uncertainties. The time-lag response as a function of velocity across the Balmer line profiles is examined for six of the AGNs. Finally we compare several trends seen in the dataset against the predictions from photoionization calculations as presented by Korista & Goad. We confirm several of their predictions, including an increase in responsivity and a decrease in the mean time lag as the excitation and ionization level for the species increases. Further confirmation of photoionization predictions for broad-line gas behavior will require additional monitoring programs for these AGNs while they are in different luminosity states. [abridged]

astro-ph.CO↗

arXiv2010

The Lick AGN Monitoring Project: The MBH - sigma Relation For Reverberation-Mapped Active Galaxies

(Abridged) To investigate the black hole mass (MBH) vs. stellar velocity dispersion relation of active galaxies, we measured the velocity dispersions of a sample of local Seyfert 1 galaxies, for which we have recently determined MBH using reverberation mapping. For most objects, velocity dispersions were measured from high S/N ratio optical spectra centered on the Ca II triplet region (~8500 A), obtained at the Keck, Palomar, and Lick Observatories. For two objects, in which the Ca II triplet region was contaminated by nuclear emission, the measurement was based on high-quality H-band spectra obtained with the OSIRIS at the Keck-II Telescope. Combining our new measurements with data from the literature, we assemble a sample of 24 active galaxies with stellar velocity dispersions and reverberation-based MBH measurements in the range of black hole mass 10^6<MBH/M_sun<10^9. We use this sample to obtain reverberation mapping constraints on the slope and intrinsic scatter of the M-sigma relation of active galaxies. Assuming a constant virial coefficient f for the reverberation mapping black hole masses, we find a slope b=3.55+-0.60 and the intrinsic scatter sigma_int=0.43+-0.08 dex in the relation log (MBH/M_sun) = a + b log (sigma/200 km/s), which are consistent with those found for quiescent galaxies. We derive an updated value of the virial coefficient f by finding the value which places the reverberation masses in best agreement with the M-sigma relation of quiescent galaxies; using the quiescent M-sigma relation determined by Gultekin et al. we find log f=0.72^{+0.09-0.10} with an intrinsic scatter of 0.44+-0.07 dex. The uncertainty of the virial coefficient remains one of the main sources of the uncertainty in black hole mass determinations using reverberation mapping, and therefore also in single-epoch spectroscopic estimates of MBH in active galaxies.

astro-ph.CO↗

arXiv2010

Discovery of Precursor LBV Outbursts in Two Recent Optical Transients: The Fitfully Variable Missing Links UGC 2773-OT and SN 2009ip

We present progenitor-star detections, light curves, and optical spectra of SN2009ip and the 2009 optical transient in UGC2773 (U2773-OT), which were not genuine SNe. Precursor variability in the decade before outburst indicates that both of the progenitor stars were LBVs. Their pre-outburst light curves resemble the S Doradus phases that preceded giant eruptions of eta Carinae and SN1954J (V12 in NGC2403), with intermediate progenitor luminosities. HST detections a decade before discovery indicate that the SN2009ip and U2773-OT progenitors were supergiants with likely initial masses of 50-80 Msun and $\ga$20 Msun, respectively. Both outbursts had spectra befitting known LBVs, although in different physical states. SN 2009ip exhibited a hot LBV spectrum with characteristic speeds of 550 km/s, plus faster material up to 5000 km/s, resembling the slow Homunculus and fast blast wave of eta Carinae. U2773-OT shows a forest of narrow absorption and emission lines comparable to that of S Dor in its cool state, plus [CaII] emission and an IR excess indicative of dust, similar to SN2008S and N300-OT. [CaII] emission is probably tied to a dusty pre-outburst environment, and not the outburst mechanism. SN2009ip and U2773-OT may provide a critical link between historical LBV eruptions, while U2773-OT may provide a link between LBVs and SN2008S and N300-OT. Future searches will uncover more examples of precursor LBV variability of this kind, providing key clues that may help unravel the instability driving LBVs.

astro-ph.SR↗

arXiv2009

An Unusually Fast-Evolving Supernova

Analyses of supernovae (SNe) have revealed two main types of progenitors: exploding white dwarfs and collapsing massive stars. We present SN2002bj, which stands out as different from any SN reported to date. Its light curve rises and declines very rapidly, yet reaches a peak intrinsic brightness greater than -18 mag. A spectrum obtained 7 days after discovery shows the presence of helium and intermediate-mass elements, yet no clear hydrogen or iron-peak elements. The spectrum only barely resembles that of a Type Ia supernova, with added carbon and helium. Its properties suggest that SN2002bj may be representative of a class of progenitors that previously has been only hypothesized: a helium detonation on a white dwarf, ejecting a small envelope of material. New surveys should find many such objects, despite their scarcity.

astro-ph.SR↗

arXiv2009

First-principles LDA+U and GGA+U study of neptunium dioxide

We have performed a systematic first-principles investigation to calculate the electronic structures, mechanical properties, and phonon dispersion curves of NpO$_{2}$. The local density approximation$+U$ and the generalized gradient approximation$+U$ formalisms have been used to account for the strong on-site Coulomb repulsion among the localized Np $5f$ electrons. By choosing the Hubbard \emph{U} parameter around 4 eV, the orbital occupancy characters of Np 5\emph{f} and O 2\emph{p} are in good agreement with recent experiments [J. Nucl. Mater. \textbf{389}, 470 (2009)]. Comparing with our previous study of ThO$_{2}$, we note that stronger covalency exists in NpO$_{2}$ due to the more localization behavior of 5\emph{f} electrons of Np in line with the localization-delocalization trend exhibited by the actinides series.

cond-mat.mtrl-sci↗

arXiv2009

The Carnegie Supernova Project: Analysis of the First Sample of Low-Redshift Type-Ia Supernovae

We present the analysis of the first set of low-redshift Type Ia supernovae (SNe Ia) by the Carnegie Supernova Project. Well-sampled, high-precision optical (ugriBV) and near-infrared (NIR; YJHKs) light curves obtained in a well-understood photometric system are used to provide light-curve parameters, and ugriBVYJH template light curves. The intrinsic colors at maximum light are calibrated to compute optical--NIR color excesses for the full sample, thus allowing the properties of the reddening law in the host galaxies to be studied. A low value of Rv~1.7, is derived when using the entire sample of SNe. However, when the two highly reddened SNe in the sample are excluded, a value Galactic standard of Rv~3.2 is obtained. The colors of these two events are well matched by a reddening model due to circumstellar dust. The peak luminosities are calibrated using a two-parameter linear fit to the decline rates and the colors, or alternatively, the color excesses. In both cases, dispersions in absolute magnitude of 0.12--0.16 mag are obtained, depending on the filter-color combination. In contrast to the results obtained from color excesses, these fits give Rv~1--2, even when the two highly reddened SNe are excluded. This discrepancy suggests that, beyond the "normal" interstellar reddening produced in the host galaxies, there is an intrinsic dispersion in the colors of SNe Ia which is correlated with luminosity but independent of the decline rate. Finally, a Hubble diagram is produced by combining the results of the fits for each filter. The resulting scatter of 0.12 mag appears to be limited by peculiar velocities as evidenced by the strong correlation between the distance-modulus residuals among the different filters. The implication is that the actual precision of SN Ia distances is 3--4%.

astro-ph.CO↗

arXiv2009

The Carnegie Supernova Project: First Photometry Data Release of Low-Redshift Type Ia Supernovae

The Carnegie Supernova Project (CSP) is a five-year survey being carried out at the Las Campanas Observatory to obtain high-quality light curves of ~100 low-redshift Type Ia supernovae in a well-defined photometric system. Here we present the first release of photometric data that contains the optical light curves of 35 Type Ia supernovae, and near-infrared light curves for a subset of 25 events. The data comprise 5559 optical (ugriBV) and 1043 near-infrared (YJHKs) data points in the natural system of the Swope telescope. Twenty-eight supernovae have pre-maximum data, and for 15 of these, the observations begin at least 5 days before B maximum. This is one of the most accurate datasets of low-redshift Type Ia supernovae published to date. When completed, the CSP dataset will constitute a fundamental reference for precise determinations of cosmological parameters, and serve as a rich resource for comparison with models of Type Ia supernovae.

astro-ph.CO↗

arXiv2009

The Lick AGN Monitoring Project: Photometric Light Curves and Optical Variability Characteristics

The Lick AGN Monitoring Project targeted 13 nearby Seyfert 1 galaxies with the intent of measuring the masses of their central black holes using reverberation mapping. The sample includes 12 galaxies selected to have black holes with masses roughly in the range 10^6-10^7 solar masses, as well as the well-studied AGN NGC 5548. In conjunction with a spectroscopic monitoring campaign, we obtained broad-band B and V images on most nights from 2008 February through 2008 May. The imaging observations were carried out by four telescopes: the 0.76-m Katzman Automatic Imaging Telescope (KAIT), the 2-m Multicolor Active Galactic Nuclei Monitoring (MAGNUM) telescope, the Palomar 60-in (1.5-m) telescope, and the 0.80-m Tenagra II telescope. Having well-sampled light curves over the course of a few months is useful for obtaining the broad-line reverberation lag and black hole mass, and also allows us to examine the characteristics of the continuum variability. In this paper, we discuss the observational methods and the photometric measurements, and present the AGN continuum light curves. We measure various variability characteristics of each of the light curves. We do not detect any evidence for a time lag between the B- and V-band variations, and we do not find significant color variations for the AGNs in our sample.

astro-ph.CO↗

arXiv2009

Molecular dynamics study of hcp/fcc nucleation and growth in bcc iron driven by uniaxial strain

Molecular dynamics simulations are performed to investigate the structural phase transition in body-centered cubic (bcc) single crystal iron under high strain rate loading. We study the nucleation and growth of the hexagonal-close-packed (hcp) and face-centered-cubic (fcc) phases, and their crystal orientation dependence. Results reveal that the transition pressures are less dependent on the crystal orientations ($\mathtt{\sim}$14 GPa for loading along [001], [011], and [111] directions). However, the pressure interval of mixed phase for [011] loading is much shorter than loading along other orientations. And the temperature increased amplitude for [001] loading is evidently lower than other orientations. The hcp/fcc nucleation process is presented by the topological medium-range-order analysis. For loading along [001] direction, we find that the hcp structure occurs firstly and grows into laminar morphology in the (011)$_{\text{bcc}}$ planes with a little fcc atoms as intermediate structure. For loading along [011] and [111] directions, both the hcp and fcc structures nucleation and growth along the \{110\}$_{\text{bcc}}$ planes are observed, whose morphology is also discussed.

cond-mat.mtrl-sci↗

arXiv2009

First-principles study of ground state properties and high pressure behavior of ThO2

The mechanical properties, electronic structure and phonon dispersion of ground state ThO$_{2}$ as well as the structure behavior up to 240 GPa are studied by using first-principles density-functional theory. Our calculated elastic constants indicate that both the ground state fluorite structure and high pressure cotunnite structure of ThO$_{2}$ are mechanically stable. The bulk modulus, shear modulus, and Young's modulus of cotunnite ThO$_{2}$ are all smaller by approximately 25% compared with those of fluorite ThO$_{2}$. The Poisson's ratios of both structures are approximately equal to 0.3 and the hardness of fluorite ThO$_{2}$ is 27.33 GPa. The electronic structure and bonding nature of fluorite ThO$_{2}$ are fully analyzed, which show that the Th-O bond displays a mixed ionic/covalent character. The valence of Th and O ions in fluorite ThO$_{2}$ can be represented as Th$^{3.834+}$ and O$^{0.452-}$. The phase transition from the fluorite to cotunnite structure is calculated to be at the pressure of 26.5 GPa, consistent with recent experimental measurement by Idiri \emph{et al}. \cite{Idiri}. For the cotunnite phase it is further predicted that an isostructural transition takes place in the pressure region of 80 to 130 GPa.

cond-mat.mtrl-sci↗

arXiv2009

The Lick AGN Monitoring Project: Broad-Line Region Radii and Black Hole Masses from Reverberation Mapping of Hbeta

We have recently completed a 64-night spectroscopic monitoring campaign at the Lick Observatory 3-m Shane telescope with the aim of measuring the masses of the black holes in 12 nearby (z < 0.05) Seyfert 1 galaxies with expected masses in the range ~10^6-10^7 M_sun and also the well-studied nearby active galactic nucleus (AGN) NGC 5548. Nine of the objects in the sample (including NGC 5548) showed optical variability of sufficient strength during the monitoring campaign to allow for a time lag to be measured between the continuum fluctuations and the response to these fluctuations in the broad Hbeta emission. We present here the light curves for the objects in this sample and the subsequent Hbeta time lags for the nine objects where these measurements were possible. The Hbeta lag time is directly related to the size of the broad-line region, and by combining the lag time with the measured width of the Hbeta emission line in the variable part of the spectrum, we determine the virial mass of the central supermassive black hole in these nine AGNs. The absolute calibration of the black hole masses is based on the normalization derived by Onken et al. We also examine the time lag response as a function of velocity across the Hbeta line profile for six of the AGNs. The analysis of four leads to ambiguous results with relatively flat time lags as a function of velocity. However, SBS 1116+583A exhibits a symmetric time lag response around the line center reminiscent of simple models for circularly orbiting broad-line region (BLR) clouds, and Arp 151 shows an asymmetric profile that is most easily explained by a simple gravitational infall model. Further investigation will be necessary to fully understand the constraints placed on physical models of the BLR by the velocity-resolved response in these objects.

astro-ph.CO↗

arXiv2009

Variable Sodium Absorption in a Low-Extinction Type Ia Supernova

Recent observations have revealed that some Type Ia supernovae exhibit narrow, time-variable Na I D absorption features. The origin of the absorbing material is controversial, but it may suggest the presence of circumstellar gas in the progenitor system prior to the explosion, with significant implications for the nature of the supernova progenitors. We present the third detection of such variable absorption, based on six epochs of high-resolution spectroscopy of the Type Ia supernova SN 2007le from Keck and the HET. The data span ~3 months, from 5 days before maximum light to 90 days after maximum. We find that one component of the Na D absorption lines strengthened significantly with time, indicating a total column density increase of ~2.5 x 10^12 cm^-2. The changes are most prominent after maximum light rather than at earlier times when the UV flux from the SN peaks. As with SN 2006X, we detect no change in the Ca II H&K lines over the same time period, rendering line-of-sight effects improbable and suggesting a circumstellar origin for the absorbing material. Unlike the previous two SNe exhibiting variable absorption, SN 2007le is not highly reddened (E_B-V = 0.27 mag), also pointing toward circumstellar rather than interstellar absorption. Photoionization models show that the data are consistent with a dense (10^7 cm^-3) cloud or clouds of gas located ~0.1 pc from the explosion. These results broadly support the single-degenerate scenario previously proposed to explain the variable absorption, with mass loss from a nondegenerate companion star responsible for providing the circumstellar gas. We also present tentative evidence for narrow Halpha emission associated with the SN, which will require followup observations at late times to confirm. [abridged]

astro-ph.HE↗

arXiv2009

Mechanical and chemical bonding properties of ground state BeH$_2$

The crystal structure, mechanical properties and electronic structure of ground state BeH$_{2}$ are calculated employing the first-principles methods based on the density functional theory. Our calculated structural parameters at equilibrium volume are well consistent with experimental results. Elastic constants, which well obey the mechanical stability criteria, are firstly theoretically acquired. The bulk modulus \emph{B}, Shear modulus \emph{G}, Young's modulus \emph{E} and Poisson's ratio $\upsilon$ are deduced from the elastic constants. The bonding nature in BeH$_{2}$ is fully interpreted by combining characteristics in band structure, density of state, and charge distribution. The ionicity in the Be$-$H bond is mainly featured by charge transfer from Be 2\emph{s} to H 1\emph{s} atomic orbitals while its covalency is dominated by the hybridization of H 1\emph{s} and Be 2\emph{p} states. The valency in BeH$_{2}$ can be represented as Be$^{1.99+}$H$^{0.63-}$, which suggests that significant charge transfer process exists.

cond-mat.mtrl-sci↗