O abundance ratios toward the GC region relative to molecular clouds in the Galactic disk. Furthermore, even inside the GC region, ratios appear not to be uniform. The low GC values are consistent with an inside-out formation scenario for our Galaxy.

The GW Ori system is a pre-main sequence triple system (GW Ori A/B/C) with companions (GW Ori B/C) at ∼1 AU and ∼8 AU, respectively, from the primary (GW Ori A). The primary of the system has a mass of 3.9 M ⊙ , but shows a spectral type of G8. Thus, GW Ori A could be a precursor of a B star, but it is still at an earlier evolutionary stage than Herbig Be stars. GW Ori provides us an ideal target for experiments and observations (being a "blown-up" upscaled Solar System with a very massive "sun" and at least two "upscaled planets"). We present the first spatially-resolved millimeter interferometric observations of the disk around the triple pre-main-sequence system GW Ori, obtained with the the Submillimeter Array, both in continuum and in the 12 CO J = 2 − 1, 13 CO J = 2 − 1, and C 18 O J = 2 − 1 lines. These new data reveal a huge, massive, and bright disk in the GW Ori system. The dust continuum emission suggests a disk radius around 400 AU. But, the 12 CO J = 2 − 1 emission shows much more extended disk with a size around 1300 AU. Due to the spatial resolution (∼1 ′′ ), we cannot detect the gap in the disk which is inferred from spectral energy distribution (SED) modeling. We characterize the dust and gas properties in the disk by comparing the observations with the predictions from the disk models with various parameters calculated with a Monte Carlo radiative transfer code RADMC-3D. The disk mass is around 0.12 M ⊙ , and the disk inclination with respect to the line of sight is around ∼ 35• . The kinematics in the disk traced by the CO line emission strongly suggest that the circumstellar material in the disk is in Keplerian rotation around GW Ori. Tentatively substantial C 18 O depletion in gas phase is required to explain the characteristics of the line emission from the disk.

We compiled all 119 OH maser galaxies (110 out of them are megamasers, i.e., L OH > 10 L ) published so far and cross-identified these OH masers with the Wide-Field Infrared Survey Explorer (WISE) catalog, to investigate the middle infrared (MIR) properties of OH maser galaxies. The WISE magnitude data at the 3.4, 4.6, 12 and 22 μm (W1 to W4) are collected for the OH maser sample and one control sample, which are non-detection sources. The color-color diagrams show that both OH megamaser (OHM) and non-OHM (ultra)luminous infrared galaxies ((U)LIRGs) are far away from the single blackbody model line and many of them can follow the path described by the power-law model. The active galaxy nuclei (AGN) fraction is about ∼40% for both OHM and non-OHM (U)LIRGs, according to the AGN criteria W1−W2 ≥ 0.8. Among the Arecibo survey sample, OHM sources tend to have a lower luminosity at short MIR wavelengths (e.g., 3.4 μm and 4.6 μm) than that of non-OHM sources, which should come from the low OHM fraction among the survey sample with large 3.4 μm and 4.6 μm luminosity. The OHM fraction tends to increase with cooler MIR colors (larger F 22 μm /F 3.4 μm ). These may be good for sample selection when searching OH megamasers, such as excluding extreme luminous sources at short MIR wavelengths, choosing sources with cooler MIR colors. In the case of the power-law model, we derived the spectral indices for our samples. For the Arecibo survey sample, OHM (U)LIRGs tend to have larger spectral index α 22−12 than non-OHM sources, which agrees with previous results. One significant correlation exists between the WISE infrared luminosity at 22 μm and the color [W1]-[W4] for the Arecibo OHM hosts. These clues should provide suitable constraints on the sample selection for OH megamaser surveys by future advanced telescopes (e.g., FAST). In addition, the correlation of maser luminosity and the MIR luminosity of maser hosts tends to be non-significant, which may indirectly support the pumping of OHM emission that is dominated by the far infrared radiation, instead of the MIR radiation.

Context. CN, HCN, and HNC have been used to discuss the star formation sequence in galaxies, but recent studies of large samples involving these lines did not yet provide convincing results. Aims. We intend to determine whether the global line ratios CN/HCN, CN/HNC, and HCN/HNC can be used to trace the high-mass star formation sequence in the Milky Way. Methods. We performed map observations of CN(1-0), HCN(1-0), and HNC(1-0) toward 38 high-mass star-forming regions, which includes high-mass starless cores (HMSC), high-mass protostars (HMPO), UCHII (ultra-compact HII) and normal HII regions. We identified the molecular clumps associated with them, and removed the clumps that were affected by environment. We averaged all the detected emission from each clump to obtain global line ratios and investigated their variations with the evolutionary stages of high-mass star-forming clumps. Results. The global line ratios of I CN /I HNC and I HCN /I HNC for HMSC clumps (HMSCCs), HMPO clumps (HMPOCs), UCHII region clumps (UCHIICs), and HII region clumps display an increasing trend. We tentatively divide the star-forming regions into two types. Type 1 sources include HMSCCs and HMPOCs that are not associated with external 20 cm continuum emission. Type 2 sources include all UCHIICs and HIICs. Our analysis shows that the I CN /I HNC and I HCN /I HNC line ratios can trace the evolution from type 1 to type 2 well. The same method may be used to study the evolution of external galaxies. Conclusions. I CN /I HNC and I HCN /I HNC appear to be good tracers for the evolution of high-mass star-forming regions in the Milky Way.

Context. Level population inversion of hydrogen atoms in ionized gas may lead to stimulated emission of hydrogen recombination lines, and the level populations can in turn be affected by powerful stimulated emissions. Aims. In this work the interaction of the radiation fields and the level population inversion of hydrogen atoms is studied. The effect of the stimulated emissions on the line profiles is also investigated. Methods. Our previous nl-model for calculating level populations of hydrogen atoms and hydrogen recombination lines is improved. The effects of line and continuum radiation fields on the level populations are considered in the improved model. By using this method the properties of simulated hydrogen recombination lines and level populations are used in analyses.Results. The simulations show that hydrogen radio recombination lines are often emitted from the energy level with an inverted population. The widths of Hnα lines can be significantly narrowed by strong stimulated emissions to be even less than 10 km s −1 . The amplification of hydrogen recombination lines is more affected by the line optical depth than by the total optical depth. The influence of stimulated emission on the estimates of electron temperature and density of ionized gas is evaluated. We find that comparing multiple line-to-continuum ratios is a reliable method for estimating the electron temperature, while the effectiveness of the estimation of electron density is determined by the relative significance of stimulated emission.