Internal Variations in Empirical Oxygen Abundances for Giant HII Regions in the Galaxy NGC 2403

This paper presents a spectroscopic investigation of 11 HII regions in the nearby galaxy NGC 2403. The HII regions are observed with a long-slit spectrograph mounted on the 2.16 m telescope at XingLong station of National Astronomical Observatories of China. For each of the HII regions, spectra are extracted at different nebular radii along the slit-coverage. Oxygen abundances are empirically estimated from the strong-line indices R23, N2O2, O3N2, and N2 for each spectrophotometric unit, with both observation- and model-based calibrations adopted into the derivation. Radial profiles of these diversely estimated abundances are drawn for each nebula. In the results, the oxygen abundances separately estimated with the prescriptions on the basis of observations and models, albeit from the same spectral index, systematically deviate from each other; at the same time, the spectral indices R23 and N2O2 are distributed with flat profiles, whereas N2 and O3N2 exhibit apparent gradients with the nebular radius. Because our study naturally samples various ionization levels which inherently decline at larger radii within individual HII regions, the radial distributions indicate not only the robustness of R23 and N2O2 against ionization variations but also the sensitivity of N2 and O3N2 to the ionization parameter. The results in this paper provide observational corroboration of the theoretical prediction about the deviation in the empirical abundance diagnostics. Our future work is planned to investigate metal-poor HII regions with measurable T_e, in an attempt to recalibrate the strong-line indices and consequently disclose the cause of the discrepancies between the empirical oxygen abundances.Comment: 16 pages, 10 figures, 5 tables; accepted for publication in The Astrophysical Journal; with a minor correction in the tex

Characterizing Ultraviolet and Infrared Observational Properties for Galaxies. II. Features of Attenuation Law

Variations in the attenuation law have a significant impact on observed spectral energy distributions for galaxies. As one important observational property for galaxies at ultraviolet and infrared wavelength bands, the correlation between infrared-to-ultraviolet luminosity ratio and ultraviolet color index (or ultraviolet spectral slope), i.e., the IRX-UV relation (or IRX-beta relation), offered a widely used recipe for correcting dust attenuation in galaxies, but the usability appears to be in doubt now because of considerable dispersion in this relation found by many studies. In this paper, on the basis of spectral synthesis modeling and spatially resolved measurements of four nearby spiral galaxies, we provide an interpretation of the deviation in the IRX-UV relation with variations in the attenuation law. From both theoretical and observational viewpoints, two components in the attenuation curve, the linear background and the 2175 Angstrom bump, are suggested to be the parameters in addition to the stellar population age (addressed in the first paper of this series) in the IRX-UV function; different features in the attenuation curve are diagnosed for the galaxies in our sample. Nevertheless, it is often difficult to ascertain the attenuation law for galaxies in actual observations. Possible reasons for preventing the successful detection of the parameters in the attenuation curve are also discussed in this paper, including the degeneracy of the linear background and the 2175 Angstrom bump in observational channels, the requirement for young and dust-rich systems to study, and the difficulty in accurate estimates of dust attenuations at different wavelength bands.Comment: 25 pages, 23 figures, 5 tables; accepted for publication in The Astrophysical Journa

Free-floating planets from core accretion theory: microlensing predictions

We calculate the microlensing event rate and typical time-scales for the free-floating planet (FFP) population that is predicted by the core accretion theory of planet formation. The event rate is found to be ~$1.8\times 10^{-3}$ of that for the stellar population. While the stellar microlensing event time-scale peaks at around 20 days, the median time-scale for FFP events (~0.1 day) is much shorter. Our values for the event rate and the median time-scale are significantly smaller than those required to explain the \cite{Sum+11} result, by factors of ~13 and ~16, respectively. The inclusion of planets at wide separations does not change the results significantly. This discrepancy may be too significant for standard versions of both the core accretion theory and the gravitational instability model to explain satisfactorily. Therefore, either a modification to the planet formation theory is required, or other explanations to the excess of short-time-scale microlensing events are needed. Our predictions can be tested by ongoing microlensing experiment such as KMTNet, and by future satellite missions such as WFIRST and Euclid.Comment: 6 pages, 5 figures, MNRAS in pres

Establishing low-lying doubly charmed baryons

We systematically study the $S$-wave doubly charmed baryons using the method of QCD sum rules. Our results suggest that the $\Xi_{cc}^{++}$ recently observed by LHCb can be well identified as the $S$-wave $\Xi_{cc}$ state of $J^P = 1/2^+$. We study its relevant $\Omega_{cc}$ state, whose mass is predicted to be around 3.7 GeV. We also systematically study the $P$-wave doubly charmed baryons, whose masses are predicted to be around 4.1 GeV. Especially, there can be several excited doubly charmed baryons in this energy region, and we suggest to search for them in order to study the fine structure of the strong interaction.Comment: 6 pages, 2 figures, 1 table; A mistake was found when evaluating decay constants of the S-wave charmed baryons. The conclusion is not change

P-wave charmed baryons from QCD sum rules

We study the P-wave charmed baryons using the method of QCD sum rule in the framework of heavy quark effective theory (HQET). We consider systematically all possible baryon currents with a derivative for internal rho- and lambda-mode excitations. We have found a good working window for the currents corresponding to the rho-mode excitations for Lambda_c(2595), Lambda_c(2625), Xi_c(2790) and Xi_c(2815) which complete two SU(3) 3F_bar multiplets of J(P)=1/2(-) and 3/2(-), while the currents corresponding to the lambda-mode excitations seem also consistent with the data. Our results also suggest that there are two Sigma_c(2800) states of J(P)=1/2(-) and 3/2(-) whose mass splitting is 14 \pm 7 MeV, and two Xi_c(2980) states whose mass splitting is 12 \pm 7 MeV. They have two Omega_c partners of J(P) = 1/2(-) and 3/2(-), whose masses are around 3.25 \pm 0.20 GeV with mass splitting 10 \pm 6 MeV. All of them together complete two SU(3) 6F multiplets of J(P)=1/2(-) and 3/2(-). They may also have J(P)=5/2(-) partners. Xi_c(3080) may be one of them, and the other two are Sigma_c(5/2(-)) and Omega_c(5/2(-)), whose masses are 85 \pm 23 and 50 \pm 27 MeV larger.Comment: 20 pages, 7 figures, accepted by PR

Decay properties of PP-wave charmed baryons from light-cone QCD sum rules

We study decay properties of the $P$-wave charmed baryons using the method of light-cone QCD sum rules, including the $S$-wave decays of the flavor $\mathbf{\bar 3}_F$ $P$-wave charmed baryons into ground-state charmed baryons accompanied by a pseudoscalar meson ($\pi$ or $K$) or a vector meson ($\rho$ or $K^*$), and the $S$-wave decays of the flavor $\mathbf{6}_F$ $P$-wave charmed baryons into ground-state charmed baryons accompanied by a pseudoscalar meson ($\pi$ or $K$). We study both two-body and three-body decays which are kinematically allowed. We find two mixing solutions from internal $\rho$- and $\lambda$-mode excitations, which can well describe both masses and decay properties of the $\Lambda_c(2595)$, $\Lambda_c(2625)$, $\Xi_c(2790)$ and $\Xi_c(2815)$. We also discuss the possible interpretations of $P$-wave charmed baryons for the $\Sigma_c(2800)$, $\Xi_c(2930)$, $\Xi_c(2980)$, and the recently observed $\Omega_c(3000)$, $\Omega_c(3050)$, $\Omega_c(3066)$, $\Omega_c(3090)$, and $\Omega_c(3119)$.Comment: 44 pages, 9 figures. Version appears in Phys. Rev.