119187574
Gamma-Ray Burst Spectrum with Decaying Magnetic Field
In the internal shock model for gamma-ray bursts (GRBs), the synchrotron spectrum from the fast cooling electrons in a homogeneous downstream magnetic field (MF) is too soft to produce the low-energy slope of GRB spectra. However, the MF may decay downstream with distance from the shock front. Here we show that the synchrotron spectrum becomes harder if electrons undergo synchrotron and inverse-Compton cooling in a decaying MF. To reconcile this with the typical GRB spectrum with low-energy slope nu F-nu proportional to nu, the postshock MF decay time must be comparable to the cooling time of the bulk electrons (corresponding to a MF decaying length typically of similar to 10(5) skin depths); that the inverse-Compton cooling should dominate synchrotron cooling after the MF decay time; and/or that the MF decays with comoving time roughly as B proportional to t(-1.5). An internal shock synchrotron model with a decaying MF can account for the majority of GRBs with low-energy slopes not harder than nu(4/3).
118587112
Extremely metal-poor star candidates in the SDSS
For a sample of metal-poor stars ( 3.3 � (Fe/H) � 2.2) that have high- resolution spectroscopic abundance determinations, we have measured equivalent widths of the Ca II K, Mg I b and near-infrared Ca II triplet lines using low-resolution spectra from the Sloan Digital Sky Survey (SDSS), calculated effective temperatures from (g z)0 color, deduced stellar surface gravities by fitting stellar isochrones, and determined metallicities based on the aforementioned quantities. Metallicities thus derived from the Ca II K line are in much better agreement with the results deter- mined from high-resolution spectra than the values given in the SDSS Data Release 7. The metallicities derived from the Mg I b lines have a large dispersion owing to the large measurement errors, whereas those deduced from the Ca II triplet lines are too high due to both non-local thermodynamical equilibrium (NLTE) effects and mea- surement errors. Abundances after correction for the NLTE effect for the Mg I b lines and Ca II triplet lines are also presented. Following this method, we have identified six candidates of ultra-metal-poor stars with (Fe/H) � 4.0 from a sample of 166 metal-poor star candidates. One of them, SDSS J102915+172927, was recently con- firmed to be an ultra-metal-poor ((Fe/H) < 4.0) star with the lowest metallicity ever measured. Follow-up high-resolution spectroscopy for the other five ultra-metal-poor stars in our sample will therefore be of great interest.
118492896
The dark secrets of gaseous nebulae -- highlights from deep spectroscopy
In this contribution, I will briefly review the development of the theory of photoionized gaseous nebulae, highlighting some of the key events. I will then present some recent developments of deep spectroscopy of planetary nebulae (PNe) and H II regions, concentrating on observations of faint heavy element optical recombination lines (ORLs). I will show that there is strong evidence that nebulae contain another previously unknown component of cold (about 1,000 K), high-metallicity plasma, probably in the form of H-deficient inclusions embedded in the warm (about 10,000 K) diffuse nebula of "normal (i.e. near solar) composition". This cold gas emits essentially all the observed fluxes of heavy element ORLs, but is too cool to excite any significant optical or ultraviolet collisionally excited lines (CELs) and thus invisible via the latter. The existence of H-deficient gas in PNe and probably also in H II regions, not predicted by the current stellar evolution theory, provides a natural solution to the long-standing dichotomy between nebular plasma diagnostics and abundance determinations using CELs on the one hand and ORLs on the other, a discrepancy that is ubiquitously observed in Galactic and extragalactic PNe as well as H II regions.
118527168
Detections of diffuse interstellar bands in the SDSS low-resolution spectra
Diffuse interstellar bands (DIBs) have been discovered for almost a century, but their nature remains one of the most challenging problems in the astronomical spectroscopy. Most recent work to identify and investigate the properties and carriers of DIBs concentrates on highresolution spectroscopy of selected sight-lines. In this paper, we report detections of DIBs in the Sloan Digital Sky Survey (SDSS) low-resolution spectra of a large sample of Galactic stars. Using a template subtraction method, we have successfully identified the DIBs λλ5780, 6283 in the SDSS spectra of a sample of about 2000 stars and measured their strengths and radial velocities. The sample is by far the largest ever assembled. The targets span a large range of reddening, E(B − V) ∼ 0.2–1.0, and are distributed over a large sky area and involve a wide range of stellar parameters (effective temperature, surface gravity and metallicity), confirming that the carriers of DIBs are ubiquitous in the diffuse interstellar medium (ISM). The sample is used to investigate relations between strengths of DIBs and magnitudes of line-of-sight extinction, yielding results [i.e. EW(5780) = 0.61 × E(B − V) and EW(6283) = 1.26 × E(B − V)] consistent with previous studies. DIB features have also been detected in the commissioning spectra of the Guoshoujing Telescope [the Large Sky Area Multi-Object Fibre Spectroscopic Telescope (LAMOST)] of resolving power similar to that of the SDSS. Detections of DIBs of hundreds of thousands of stars are expected from the on-going and up-coming large-scale spectroscopic surveys such as Radial Velocity Experiment, SDSS III and LAMOST, particularly from the LAMOST Digital Sky Survey of the Galactic Anticentre (DSS-GAC). Such a huge data base will provide an unprecedented opportunity to study the demographical distribution and nature of DIBs as well as using DIBs to probe the distribution and properties of the ISM and the dust extinction.
118584251
Very deep spectroscopy of the bright Saturn nebula NGC 7009 – II. Analysis of the rich optical recombination spectrum
[Abridged] We present a critical analysis of the rich optical recombination spectrum of NGC 7009, in the context of the bi-abundance nebular model proposed by Liu et al. (2000). The observed relative intensities are compared with the theoretical predictions based on high quality effective recombination coefficients, now available for the recombination line spectrum of a number of heavy element ions. The possibility of plasma diagnostics using the optical recombination lines (ORLs) of heavy element ions is discussed in detail. Plasma diagnostics based on the N II and O II recombination spectra both yield electron temperatures close to 1000 K, which is lower than those derived from the collisionally excited line (CEL) ratios by nearly one order of magnitude. The very low temperatures yielded by the O II and N II ORLs indicate that they originate from very cold regions. The C^{2+}/H^+, N^{2+}/H^+, O^{2+}/H^+ and Ne^{2+}/H^+ ionic abundance ratios derived from ORLs are consistently higher, by about a factor of 5, than the corresponding values derived from CELs. In calculating the ORL ionic abundance ratios, we have used the newly available high quality effective recombination coefficients, and adopted an electron temperature of 1000 K, as given by the ORL diagnostics and as a consequence presumably representing the physical conditions prevailing in the regions where the heavy element ORLs arise. A comparison of the results of plasma diagnostics and abundance determinations for NGC 7009 points to the existence of "cold", metal-rich (i.e. H-deficient) inclusions embedded in the hot, diffuse ionized gas, first postulated by Liu et al. (2000).
119071536
Hunting for extremely faint planetary nebulae in the SDSS spectroscopic database
Using ~1,700,000 target- and sky-fiber spectra from the SDSS, we have carried out a systematic search for Galactic planetary nebulae (PNe) via detections of the [OIII] 4959, 5007 lines. Thanks to the excellent sensitivity of the SDSS spectroscopic surveys, this is by far the deepest search for PNe ever taken, reaching a surface brightness of the [OIII] 5007 line down to about 29.0 mag./arcsec^2. The search recovers 13 previously known PNe in the Galactic Caps. In total, 44 new PN candidates are identified, including 7 candidates of multiple detections and 37 candidates of single detection. The 7 candidates of multiple detections are all extremely large (between 21' - 154') and faint, located mostly in the low Galactic latitude region and with a kinematics similar to disk stars. After checking their images in Ha and other bands, three of them are probably HII regions, one is probably associated with a new supernova remnant, another one is possibly a true PN, and the remaining two could be either PNe or supernova remnants. Based on sky positions and kinematics, 7 candidates of single detection probably belong to the halo population. If confirmed, they will increase the number of known PNe in the Galactic halo significantly. All the newly identified PN candidates are very faint, with a surface brightness of the [OIII] 5007 line between 27.0 - 30.0 mag./arcsec^2, very challenging to be discovered with previous techniques and thus may greatly increase the number of "missing" faint PNe. Our results demonstrate the power of large scale fiber spectroscopy in hunting for ultra-faint PNe and other types of emission line nebulae. Combining the large spectral databases provided by the SDSS and other on-going projects (e.g. the LAMOST Galactic surveys), it is possible to build a statistically meaningful sample of ultra-faint, large, evolved PNe, thus improving the census of Galactic PNe.
118440713
The open-charm radiative and pionic decays of molecular charmonium Y(4274)
In this work, we investigate the decay widths and the line shapes of the open-charm radiative and pionic decays of Y(4274) with the $D_{s}\bar{D}_{s0}(2317)$ molecular charmonium assignment. Our calculation indicates that the decay widths of $Y(4274)\to D^{+}_{s}D^{*-}_{s}\gamma$ and $Y(4274)\to D^{+}_{s}D^{-}_{s}\pi^{0}$ can reach up to 0.05 keV and 0.75 keV, respectively. In addition, the result of the line shape of the photon spectrum of $Y(4274)\to D_{s}^{+} {D}_{s}^{*-} \gamma$ shows that there exists a very sharp peak near the large end point of photon energy. The line shape of the pion spectrum of $Y(4274)\to D_{s}^{+} {D}_{s}^{*-} \pi^{0}$ is similar to that of the pion spectrum of $Y(4274)\to D_{s}^{+} {D}_{s}^{*-} \gamma$, where we also find a very sharp peak near the large end point of pion energy. According to our calculation, we suggest further experiments to carry out the search for the open-charm radiative and pionic decays of Y(4274).
118534156
Two-year monitoring of intra-day variability of quasar 1156+295 at 4.8 GHz
Aims. The quasar 1156+295 (4C +29.45) is one of the targets in the Urumqi monitoring program which aimed to search for evidence of annual modulation in the timescales of intra-day variable (IDV) sources.
119155043
Effects of hadronic loops on the direct CP violation of $B_c$
It is well-known that the final state interaction plays an important role in the decays of B meson. The contribution of the final state interaction, which is supposed to be long-distance effects, to the concerned processes can interfere with that of the short-distance effects produced via the tree and/or loop diagrams at quark-gluon level. The interference may provide a source for the direct CP violation A{sub CP} in the process B{sub c}{sup +}{yields}D{sup 0}{pi}{sup +}. We find that a typical value of A{sub CP} when the final state interaction effect is taken into account can be about -22% which is different from that without the final state interaction effect. Therefore, when we extract information on CP violation from the data which will be available at the Large Hadron Collider beauty experiment (LHCb) and the new experiments in B factories, the contribution from the final state interaction must be included. This study may be crucial for searching new physics in the future.
119289466
Anomalous dipion invariant mass distribution of the $\Upsilon(4S)$ decays into $\Upsilon(1S) \pi^{+} \pi^{-}$ and $\Upsilon(2S) \pi^{+} \pi^{-}$
To solve the discrepancy between the experimental data on the partial widths and lineshapes of the dipion emission of $\Upsilon(4S)$ and the theoretical predictions, we suggest that there is an additional contribution which was not taken into account in previous calculations. Noticing that the mass of $\Upsilon(4S)$ is above the production threshold of $B\bar B$, the contribution of the sequential process $\Upsilon(4S)\to B\bar B\to \Upsilon(nS)+S\to \Upsilon(nS)+\pi^+\pi^-$ ($n=1,2$) may be sizable, and its interference with that from the direct production would be important. The goal of this work is to investigate if a sum of the two contributions with a relative phase indeed reproduces the data. Our numerical results on the partial widths and the lineshapes $d\Gamma(\Upsilon(4S)\to \Upsilon(2S,1S)\pi^+\pi^-)/d(m_{\pi^+\pi^-})$ are satisfactorily consistent with the measurements, thus the role of this mechanism is confirmed. Moreover, with the parameters obtained by fitting the data of the Belle and Babar collaborations, we predict the distributions $d\Gamma(\Upsilon(4S)\to \Upsilon(2S,1S)\pi^+\pi^-)/d\cos\theta$ which have not been measured yet.
118578199
$Z_b(10610)$ and $Z_b(10650)$ structures produced by the initial single pion emission in the $\Upsilon(5S)$ decays
We propose an unique mechanism called {\it Initial Single Pion Emission} existing in the $\Upsilon(5S)$ decays, and further study the line shapes of $d\Gamma(\Upsilon(5S\to \Upsilon(nS)\pi^+\pi^-))/dm_{\Upsilon(nS)\pi^+}$ ($n=1,2,3$) and $d\Gamma(\Upsilon(5S\to h_b(mP)\pi^+\pi^-))/dm_{h_b(mP)\pi^+}$ ($m=1,2$). We find sharp structures around 10610 MeV and 10650 MeV in the obtained theoretical line shapes of $d\Gamma(\Upsilon(5S\to \Upsilon(nS)\pi^+\pi^-))/dm_{\Upsilon(nS)\pi^+}$ and $d\Gamma(\Upsilon(5S\to h_b(mP)\pi^+\pi^-))/dm_{h_b(mP)\pi^+}$ distributions, which could naturally correspond to the $Z_b(10610)$ and $Z_b(10650)$ structures newly observed by Belle.