arXiv subjects

Fang Huang

Publications and source records attributed to Fang Huang.

At least 37 records · Page 2Linked to original sources

arXiv2018

Efficient Continuous Top-$k$ Geo-Image Search on Road Network

With the rapid development of mobile Internet and cloud computing technology, large-scale multimedia data, e.g., texts, images, audio and videos have been generated, collected, stored and shared. In this paper, we propose a novel query problem named continuous top-$k$ geo-image query on road network which aims to search out a set of geo-visual objects based on road network distance proximity and visual content similarity. Existing approaches for spatial textual query and geo-image query cannot address this problem effectively because they do not consider both of visual content similarity and road network distance proximity on road network. In order to address this challenge effectively and efficiently, firstly we propose the definition of geo-visual objects and continuous top-$k$ geo-visual objects query on road network, then develop a score function for search. To improve the query efficiency in a large-scale road network, we propose the search algorithm named geo-visual search on road network based on a novel hybrid indexing framework called VIG-Tree, which combines G-Tree and visual inverted index technique. In addition, an important notion named safe interval and results updating rule are proposed, and based on them we develop an efficient algorithm named moving monitor algorithm to solve continuous query. Experimental evaluation on real multimedia dataset and road network dataset illustrates that our solution outperforms state-of-the-art method.

cs.MM↗

arXiv2018

Optical Observations of the 2002cx-like Supernova 2014ek, and Characterizations of SNe Iax

We present optical observations of supernova (SN) 2014ek discovered during the Tsinghua-NAOC Transient Survey (TNTS), which shows properties that are consistent with those of SN 2002cx-like events (dubbed as SNe Iax). The photometry indicates that it is underluminous compared to normal SNe Ia, with the absolute $V$-band peak magnitude being as $-17.66\pm0.20$ mag. The spectra are characterized by highly ionized Fe III and intermediate-mass elements (IMEs). The expansion velocity of the ejecta is found to be $\sim$5000 km s$^{-1}$ near the maximum light, only half of that measured for normal SNe Ia. The overall spectral evolution is quite similar to SN 2002cx and SN 2005hk, while the absorption features of the main IMEs seem to be relatively weaker. The ${}^{56}$Ni mass synthesized in the explosion is estimated to be about 0.08 M$_{\odot}$ from the pseudo bolometric light curve. Based on a large sample of SNe Iax, we examined the relations between peak luminosity, ejecta velocity, decline rate, and peak $V - R$ color but did not find noticeable correlations between these observables, in particular when a few extreme events like SN 2008ha are excluded in the analysis. For this sample, we also studied the birthplace environments and confirm that they still hold the trend of occurring preferentially in late-type spiral galaxies. Moreover, SNe Iax tend to occur in large star-forming regions of their host galaxies, more similar to SNe Ibc than SNe II, favoring that their progenitors should be associated with very young stellar populations.

astro-ph.HE↗

arXiv2018

Short-Lived Circumstellar Interaction in the Low-Luminosity Type IIP SN 2016bkv

While interaction with circumstellar material is known to play an important role in Type IIn supernovae (SNe), analyses of the more common SNe IIP and IIL have not traditionally included interaction as a significant power source. However, recent campaigns to observe SNe within days of explosion have revealed narrow emission lines of high-ionization species in the earliest spectra of luminous SNe II of all subclasses. These "flash ionization" features indicate the presence of a confined shell of material around the progenitor star. Here we present the first low-luminosity (LL) SN to show flash ionization features, SN 2016bkv. This SN peaked at $M_V = -16$ mag and has Hα expansion velocities under 1350 km/s around maximum light, placing it at the faint/slow end of the distribution of SNe IIP (similar to SN 2005cs). The light-curve shape of SN 2016bkv is also extreme among SNe IIP. A very strong initial peak could indicate additional luminosity from circumstellar interaction. A very small fall from the plateau to the nickel tail indicates unusually large production of radioactive nickel compared to other LL SNe IIP. A comparison between nebular spectra of SN 2016bkv and models raises the possibility that SN 2016bkv is an electron-capture supernova.

astro-ph.HE↗

arXiv2018

The First Release of the AST3-1 Point Source Catalogue from Dome A, Antarctica

The three Antarctic Survey Telescopes (AST3) aim to carry out time domain imaging survey at Dome A, Antarctica. The first of the three telescopes (AST3-1) was successfully deployed on January 2012. AST3-1 is a 500\,mm aperture modified Schmidt telescope with a 680\,mm diameter primary mirror. AST3-1 is equipped with a SDSS $i$ filter and a 10k $\times$ 10k frame transfer CCD camera, reduced to 5k $\times$ 10k by electronic shuttering, resulting in a 4.3 deg$^2$ field-of-view. To verify the capability of AST3-1 for a variety of science goals, extensive commissioning was carried out between March and May 2012. The commissioning included a survey covering 2000 deg$^2$ as well as the entire Large and Small Magellanic Clouds. Frequent repeated images were made of the center of the Large Magellanic Cloud, a selected exoplanet transit field, and fields including some Wolf-Rayet stars. Here we present the data reduction and photometric measurements of the point sources observed by AST3-1. We have achieved a survey depth of 19.3\,mag in 60 s exposures with 5\,mmag precision in the light curves of bright stars. The facility achieves sub-mmag photometric precision under stable survey conditions, approaching its photon noise limit. These results demonstrate that AST3-1 at Dome A is extraordinarily competitive in time-domain astronomy, including both quick searches for faint transients and the detection of tiny transit signals.

astro-ph.IM↗

arXiv2018

Efficient Top K Temporal Spatial Keyword Search

Massive amount of data that are geo-tagged and associated with text information are being generated at an unprecedented scale in many emerging applications such as location based services and social networks. Due to their importance, a large body of work has focused on efficiently computing various spatial keyword queries. In this paper,we study the top-$k$ temporal spatial keyword query which considers three important constraints during the search including time, spatial proximity and textual relevance. A novel index structure, namely SSG-tree, to efficiently insert/delete spatio-temporal web objects with high rates. Base on SSG-tree an efficient algorithm is developed to support top-k temporal spatial keyword query. We show via extensive experimentation with real spatial databases that our method has increased performance over alternate techniques

cs.DB↗

arXiv2017

Energetic eruptions leading to a peculiar hydrogen-rich explosion of a massive star

Every supernova hitherto observed has been considered to be the terminal explosion of a star. Moreover, all supernovae with absorption lines in their spectra show those lines decreasing in velocity over time, as the ejecta expand and thin, revealing slower moving material that was previously hidden. In addition, every supernova that exhibits the absorption lines of hydrogen has one main light-curve peak, or a plateau in luminosity, lasting approximately 100 days before declining. Here we report observations of iPTF14hls, an event that has spectra identical to a hydrogen-rich core-collapse supernova, but characteristics that differ extensively from those of known supernovae. The light curve has at least five peaks and remains bright for more than 600 days; the absorption lines show little to no decrease in velocity; and the radius of the line-forming region is more than an order of magnitude bigger than the radius of the photosphere derived from the continuum emission. These characteristics are consistent with a shell of several tens of solar masses ejected by the star at supernova-level energies a few hundred days before a terminal explosion. Another possible eruption was recorded at the same position in 1954. Multiple energetic pre-supernova eruptions are expected to occur in stars of 95-130 solar masses, which experience the pulsational pair instability. That model, however, does not account for the continued presence of hydrogen, or the energetics observed here. Another mechanism for the violent ejection of mass in massive stars may be required.

astro-ph.HE↗

arXiv2016

Optical and Ultraviolet Observations of the Very Young Type IIP SN 2014cx in NGC 337

Extensive photometric and spectroscopic observations are presented for SN 2014cx, a type IIP supernova (SN) exploding in the nearby galaxy NGC 337. The observations are performed in optical and ultraviolet bands, covering from -20 to +400 days from the peak light. The stringent detection limit from prediscovery images suggests that this supernova was actually detected within about 1 day after explosion. Evolution of the very early-time light curve of SN 2014cx is similar to that predicted from a shock breakout and post-shock cooling decline before reaching the optical peak. Our photometric observations show that SN 2014cx has a plateau duration of ~ 100 days, an absolute V-band magnitude of ~ -16.5 mag at t~50 days, and a nickel mass of 0.056+-0.008 Msun. The spectral evolution of SN 2014cx resembles that of normal SNe IIP like SN 1999em and SN 2004et, except that it has a slightly higher expansion velocity (~ 4200 km/s at 50 days). From the cooling curve of photospheric temperature, we derive that the progenitor has a pre-explosion radius of ~ 640 Rsun, consistent with those obtained from SNEC modeling (~ 620 Rsun) and hydrodynamical modeling of the observables (~ 570 Rsun). Moreover, the hydrodynamical simulations yield a total explosion energy of ~ 0.4*10e51 erg, and an ejected mass of ~ 8 Msun. These results indicate that the immediate progenitor of SN 2014cx is likely a red supergiant star with a mass of ~ 10 Msun.

astro-ph.SR↗

arXiv2016

The Oxygen Features in Type Ia Supernovae and the Implications for the Nature of Thermonuclear Explosions

The absorption feature O I 7773 is an important spectral indicator for type Ia supernovae (SNe~Ia) that can be used to trace the unburned material at outer layers of the exploding white dwarf. In this work, we use a large sample of SNe~Ia to examine this absorption at early phases (i.e., -13 days <t <-7 days), and make comparisons with the absorption features of Si~II 6355 and Ca~II near-infrared (NIR) triplet. We show that for a subgroup of spectroscopically normal SNe with normal photospheric velocities (i.e., v_si < 12,500 km s^{-1} at optical maximum), the line strength of high velocity feature (HVF) of O~I is inversely correlated with that of Si~II (or Ca~II), and this feature also shows a negative correlation with the luminosity of SNe Ia. This finding, together with other features we find for the O~I HVF, reveal that for this subgroup of SNe~Ia explosive oxygen burning occurs at the outermost layer of supernova and difference in burning there could lead to the observed diversity, which are in remarkable agreement with the popular delayed-detonation model of Chandrasekhar mass WD.

astro-ph.HE↗

arXiv2016

UV-Optical Observation of Type Ia Supernova SN 2013dy in NGC 7250

Extensive and independent observations of Type Ia supernova (SN Ia) SN 2013dy are presented, including a larger set of $UBVRI$ photometry and optical spectra from a few days before the peak brightness to $\sim$ 200 days after explosion, and ultraviolet (UV) photometry spanning from $t \approx -10$ days to $t \approx\ +15$ days referring to the $B$ band maximum. The peak brightness (i.e., $M_{\rm B} = -19.65 \pm 0.40$ mag, $L_{\rm max} = [1.95 \pm 0.55] \times 10^{43}$ erg s$^{-1}$) and the mass of synthesised $^{56}$Ni (i.e., $M$($^{56}$Ni) = 0.90 $\pm$ 0.26 M$_{\odot}$) are calculated, and they conform to the expectation for a SN Ia with a slow decline rate (i.e., $Δm_{15}(B)$ = 0.90 $\pm$ 0.03 mag, Phillips 1993). However, the near infrared (NIR) brightness of this SN (i.e., $M_{\rm H} = -17.33 \pm 0.30$ mag) is at least 1.0 mag fainter than usual. Besides, spectroscopy classification reveals that SN 2013dy resides on the border of "core normal" and "shallow silicon" subclasses in the Branch et al. (2009) classification scheme, or on the border of the "normal velocity" SNe Ia and 91T/99aa-like events in the Wang et al. (2009a) system. These suggest that SN 2013dy is a slow-declining SN Ia located on the transitional region of nominal spectroscopic subclasses and might not be a typical normal sample of SNe Ia.

astro-ph.SR↗

arXiv2016

Optical Observations of the Type Ia Supernova 2011fe in M101 for Nearly 500 Days

We present well-sampled optical observations of the bright Type Ia supernova (SN~Ia) SN 2011fe in M101. Our data, starting from $\sim16$ days before maximum light and extending to $\sim463$ days after maximum, provide an unprecedented time series of spectra and photometry for a normal SN~Ia. Fitting the early-time rising light curve, we find that the luminosity evolution of SN 2011fe follows a $t^n$ law, with the index $n$ being close to 2.0 in the $VRI$ bands but slightly larger in the $U$ and $B$ bands. Combining the published ultraviolet (UV) and near-infrared (NIR) photometry, we derive the contribution of UV/NIR emission relative to the optical. SN 2011fe is found to have stronger UV emission and reaches its UV peak a few days earlier than other SNe~Ia with similar $Δm_{15}(B)$, suggestive of less trapping of high-energy photons in the ejecta. Moreover, the $U$-band light curve shows a notably faster decline at late phases ($t\approx 100$--300 days), which also suggests that the ejecta may be relatively transparent to UV photons. These results favor the notion that SN 2011fe might have a progenitor system with relatively lower metallicity. On the other hand, the early-phase spectra exhibit prominent high-velocity features (HVFs) of O~I $λ$7773 and the Ca~II~NIR triplet, but only barely detectable in Si~II~6355. This difference can be caused either by an ionization/temperature effect or an abundance enhancement scenario for the formation of HVFs; it suggests that the photospheric temperature of SN 2011fe is intrinsically low, perhaps owing to incomplete burning during the explosion of the white dwarf.

astro-ph.HE↗

arXiv2016

A Luminous Peculiar Type Ia Supernova SN 2011hr: More Like SN 1991T or SN 2007if?

Photometric and spectroscopic observations of a slowly declining, luminous Type Ia supernova (SN Ia) SN 2011hr in the starburst galaxy NGC 2691 are presented. SN 2011hr is found to peak at $M_{B}=-19.84 \pm 0.40\,\rm{mag}$, with a post-maximum decline rate $Δ$m$_{15}$(B) = 0.92 $\pm$ 0.03\,$\rm{mag}$. From the maximum-light bolometric luminosity, $L=(2.30 \pm 0.90) \times 10^{43}\,\rm{erg\,s^{-1}}$, we estimate the mass of synthesized \Nifs\ in SN 2011hr to be $M(\rm{^{56}Ni})=1.11 \pm 0.43\,M_{\sun}$. SN 2011hr appears more luminous than SN 1991T at around maximum light, and the absorption features from its intermediate-mass elements (IMEs) are noticeably weaker than the latter at similar phases. Spectral modeling suggests that SN 2011hr has the IMEs of $\sim$\,0.07 M$_{\sun}$ in the outer ejecta, which is much lower than the typical value of normal SNe Ia (i.e., 0.3 -- 0.4 M$_{\sun}$) and is also lower than the value of SN 1991T (i.e., $\sim$\,0.18 M$_{\sun}$). These results indicate that SN 2011hr may arise from a Chandrasekhar-mass white dwarf progenitor that experienced a more efficient burning process in the explosion. Nevertheless, it is still possible that SN 2011hr may serve as a transitional object connecting the SN 1991T-like SNe Ia with the superluminous subclass like SN 2007if given that the latter also shows very weak IMEs at all phases.

astro-ph.SR↗

arXiv2015

The Silicon and Calcium High-Velocity Features in Type Ia Supernovae from Early to Maximum Phases

The high-velocity features (HVFs) in optical spectra of type Ia supernovae (SNe Ia) are examined with a large sample including very early-time spectra (e.g., t < -7 days). Multiple Gaussian fits are applied to examine the HVFs and their evolutions, using constraints on expansion velocities for the same species (i.e., SiII 5972 and SiII 6355). We find that strong HVFs tend to appear in SNe Ia with smaller decline rates (e.g., dm15(B)<1.4 mag), clarifying that the finding by Childress et al. (2014) for the Ca-HVFs in near-maximum-light spectra applies both to the Si-HVFs and Ca-HVFs in the earlier phase. The Si-HVFs seem to be more common in fast-expanding SNe Ia, which is different from the earlier result that the Ca-HVFs are associated with SNe Ia having slower SiII 6355 velocities at maximum light (i.e., Vsi). This difference can be due to that the HVFs in fast-expanding SNe Ia usually disappear more rapidly and are easily blended with the photospheric components when approaching the maximum light. Moreover, SNe Ia with both stronger HVFs at early phases and larger Vsi are found to have noticeably redder B-V colors and occur preferentially in the inner regions of their host galaxies, while those with stronger HVFs but smaller Vsi show opposite tendencies, suggesting that these two subclasses have different explosion environments and their HVFs may have different origins. We further examine the relationships between the absorption features of SiII 6355 and CaII IR lines, and find that their photospheric components are well correlated in velocity and strength but the corresponding HVFs show larger scatter. These results cannot be explained with ionization and/or thermal processes alone, and different mechanisms are required for the creation of HVF-forming region in SNe Ia.

astro-ph.HE↗

arXiv2015

SN 2013ej in M74: A Luminous and Fast-declining Type II-P Supernova

We present extensive ultraviolet, optical, and near-infrared observations of the type IIP supernova (SN IIP) 2013ej in the nearby spiral galaxy M74. The multicolor light curves, spanning from $\sim$ 8--185 days after explosion, show that it has a higher peak luminosity (i.e., M$_{V}$ $\sim$$-$17.83 mag at maximum light), a faster post-peak decline, and a shorter plateau phase (i.e., $\sim$ 50 days) compared to the normal type IIP SN 1999em. The mass of $^{56}$Ni is estimated as 0.02$\pm$0.01 M$_{\odot}$ from the radioactive tail of the bolometric light curve. The spectral evolution of SN 2013ej is similar to that of SN 2004et and SN 2007od, but shows a larger expansion velocity (i.e., $v_{Fe II} \sim$ 4600 km s$^{-1}$ at t $\sim$ 50 days) and broader line profiles. In the nebular phase, the emission of H$α$ line displays a double-peak structure, perhaps due to the asymmetric distribution of $^{56}$Ni produced in the explosion. With the constraints from the main observables such as bolometric light curve, expansion velocity and photospheric temperature of SN 2013ej, we performed hydrodynamical simulations of the explosion parameters, yielding the total explosion energy as $\sim$0.7$\times$ 10$^{51}$ erg, the radius of the progenitor as $\sim$600 R$_{\odot}$, and the ejected mass as $\sim$10.6 M$_{\odot}$. These results suggest that SN 2013ej likely arose from a red supergiant with a mass of 12--13 M$_{\odot}$ immediately before the explosion.

astro-ph.SR↗

arXiv2014

The THU-NAOC Transient Survey: the Performance and the First-year Result

The Tsinghua University-National Astronomical Observatories of China (NAOC) Transient Survey (TNTS) is an automatic survey for a systematic exploration of optical transients (OTs), conducted with a 60/90 cm Schmidt telescope at Xinglong station of NAOC. This survey repeatedly covers ~ 1000 square degrees of the north sky with a cadence of 3-4 days. With an exposure of 60 s, the survey reaches a limited unfiltered magnitude of about 19.5 mag. This enables us to discover supernovae at their relatively young stages. In this paper, we describe the overall performance of our survey during the first year and present some preliminary results.

astro-ph.IM↗

arXiv2014

Optical and Ultraviolet Observations of A Low-Velocity Type II-Plateau Supernova 2013am in M65

Optical and ultraviolet observations for the nearby type II-plateau supernova (SN IIP) 2013am in the nearby spiral galaxy M65 are presented in this paper. The early spectra are characterized by relatively narrow P-Cygni features, with ejecta velocities much lower than observed in normal SNe IIP (i.e., $\sim$2000 km s$^{-1}$ vs. $\sim$5000 km $^{-1}$ in the middle of the plateau phase). Moreover, prominent Ca II absorptions are also detected in SN 2013am at relatively early phases. These spectral features are reminiscent of those seen in the low-velocity and low-luminosity SN IIP 2005cs. However, SN 2013am exhibits different photometric properties, having shorter plateau phases and brighter light-curve tails if compared to SN 2005cs. Adopting $R_{V}$=3.1 and a mean value of total reddening derived from the photometric and spectroscopic methods(i.e., $E(B-V)=0.55\pm$0.19 mag), we find that SN 2013am may have reached an absolute $V$-band peak magnitude of $-15.83\pm0.71$ mag, and produced a $^{56}$Ni mass of $0.016^{+0.010}_{-0.006} solar masses in the explosion. These parameters are close to those derived for SN 2008in and SN 2009N which have been regarded as "gap-filler" objects linking the faint SNe IIP to the normal ones. This indicates that some low-velocity SNe IIP may not necessarily result from the low-energetic explosions, and the low expansion velocities could be due to a lower metallicity of the progenitor stars, a larger envelope mass ejected in the explosion, or that is was observed at an angle that is away from the polar direction.

astro-ph.SR↗

arXiv2012

Type IIn Supernova SN 2010jl: Optical Observations for Over 500 Days After Explosion

We present extensive optical observations of a Type IIn supernova (SN) 2010jl for the first 1.5 years after the discovery. The UBVRI light curves demonstrated an interesting two-stage evolution during the nebular phase, which almost flatten out after about 90 days from the optical maximum. SN 2010jl has one of the highest intrinsic H_alpha luminosity ever recorded for a SN IIn, especially at late phase, suggesting a strong interaction of SN ejecta with the dense circumstellar material (CSM) ejected by the progenitor. This is also indicated by the remarkably strong Balmer lines persisting in the optical spectra. One interesting spectral evolution about SN 2010jl is the appearance of asymmetry of the Balmer lines. These lines can be well decomposed into a narrow component and an intermediate-width component. The intermediate-width component showed a steady increase in both strength and blueshift with time until t ~ 400 days after maximum, but it became less blueshifted at t ~ 500 days when the line profile appeared relatively symmetric again. Owing to that a pure reddening effect will lead to a sudden decline of the light curves and a progressive blueshift of the spectral lines, we therefore propose that the asymmetric profiles of H lines seen in SN 2010jl is unlikely due to the extinction by newly formed dust inside the ejecta, contrary to the explanation by some early studies. Based on a simple CSM-interaction model, we speculate that the progenitor of SN 2010jl may suffer a gigantic mass loss (~ 30-50 M_sun) in a few decades before explosion. Considering a slow moving stellar wind (e.g., ~ 28 km/s) inferred for the preexisting, dense CSM shell and the extremely high mass-loss rate (1-2 M_sun per yr), we suggest that the progenitor of SN 2010jl might have experienced a red supergiant stage and explode finally as a post-red supergiant star with an initial mass above 30-40 M_sun.

astro-ph.SR↗

arXiv2012

The Photometric System of Tsinghua-NAOC 80-cm Telescope at NAOC Xinglong Observatory

Tsinghua-NAOC (National Astronomical Observatories of China) Telescope (hereafter, TNT) is an 80-cm Cassegrain reflecting telescope located at Xinglong bservatory of NAOC, with 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 in this paper a systematic test and analysis of the photometric performance of this telescope. Based on the calibration observations on twelve photometric nights, spanning the period from year 2004 to year 2012, we derived an accurate transformation relationship 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 Xinglong Observatory may become gradually worse over the period from year 2005 to year 2012 (e.g., $\sim$21.4 mag vs. $\sim$20.1 mag in the V band).

astro-ph.IM↗

arXiv2010

2-Modules and the Representation of 2-Rings

In this paper, we develop 2-dimensional algebraic theory which closely follows the classical theory of modules. The main results are giving definitions of 2-module and the representation of 2-ring. Moreover, for a 2-ring $\cR$, we prove that its modules form a 2-Abelian category.

math.CT↗