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Register a new userRadio and infrared study of southern H II regions G346.056-0.021 and G346.077-0.056
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Swagat Ranjan Das
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Aims. We present a multiwavelength study of two southern Galactic H II regions G346.056-0.021 and G346.077-0.056 which are located at a distance of 10.9 kpc. The distribution of ionized gas, cold and warm dust and the stellar population associated with the two H II regions are studied in detail using measurements at near-infrared, mid-infrared, far-infrared, submillimeter and radio wavelengths. Methods. The radio continuum maps at 1280 and 610 MHz were obtained using the Giant Metrewave Radio Telescope to probe the ionized gas. The dust temperature, column density and dust emissivity maps were generated by using modified blackbody fits in the far-infrared wavelength range 160 - 500 {mu}m. Various near- and mid-infrared colour and magnitude criteria were adopted to identify candidate ionizing star(s) and the population of young stellar objects in the associated field. Results. The radio maps reveal the presence diffuse ionized emission displaying distinct cometary morphologies. The 1280 MHz flux densities translate to ZAMS spectral types in the range O7.5V - O7V and O8.5V - O8V for the ionizing stars of G346.056-0.021 and G346.077-0.056, respectively. A few promising candidate ionizing star(s) are identified using near-infrared photometric data. The column density map shows the presence of a large, dense dust clump enveloping G346.077-0.056. The dust temperature map shows peaks towards the two H II regions. The submillimetre image shows the presence of two additional clumps one being associated with G346.056-0.021. The masses of the clumps are estimated to range between {sim} 1400 to 15250 M{sun}. Based on simple analytic calculations and the correlation seen between the ionized gas distribution and the local density structure, the observed cometary morphology in the radio maps is better explained invoking the champagne-flow model.
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We present a new catalogue of radio sources in the face-on spiral galaxy M83. Radio observations taken in 2011, 2015, and 2017 with the Australia Telescope Compact Array (ATCA) at 5.5 and 9 GHz have detected 270 radio sources. Although a small number of these sources are background extragalactic sources, most are either H II regions or supernova remnants (SNRs) within M83 itself. Three of the six historical supernovae are detected, as is the very young remnant that had been identified in a recent study, which is likely the result of a supernova that exploded in the last ~100 years but was missed. All of these objects are generally fading with time. Confusion limits our ability to measure the radio emission from a number of the SNRs in M83, but 64 were detected in unconfused regions, and these have the approximate power-law luminosity function which has been observed in other galaxies. The SNRs in M83 are systematically smaller in diameter and brighter than those that have been detected at radio wavelengths in M33. A number of the radio sources are coincident with X-ray sources in M83; most of these coincident sources turn out to be supernova remnants. Our dual frequency observations are among the most sensitive to date for a spiral galaxy outside the Local Group; despite this we were not able to place realistic constraints on the spectral indices, and as a result, it was not possible to search for supernova remnants based on their radio properties alone.
We present a tutorial on the determination of the physical conditions and chemical abundances in gaseous nebulae. We also include a brief review of recent results on the study of gaseous nebulae, their relevance for the study of stellar evolution, galactic chemical evolution, and the evolution of the universe. One of the most important problems in abundance determinations is the existence of a discrepancy between the abundances determined with collisionally excited lines and those determined by recombination lines, this is called the ADF (abundance discrepancy factor) problem; we review results related to this problem. Finally, we discuss possible reasons for the large t$^2$ values observed in gaseous nebulae.
(Abridged) The chemical behaviour of an ample sample of PNe in NGC6822 is analyzed. Spectrophotometric data of 11 PNe and two H II regions were obtained with the OSIRIS spectrograph attached to the Gran Telescopio Canarias. Data for other 13 PNe and three H II regions were retrieved from the literature. Physical conditions and chemical abundances of O, N, Ne, Ar and S were derived for 19 PNe and 4 H II regions. Abundances in the PNe sample are widely distributed showing 12+log(O/H) from 7.4 to 8.2 and 12+log(Ar/H) from 4.97 to 5.80. Two groups of PNe can be differentiated: one old, with low metallicity (12+log(O/H)<8.0 and 12+log(Ar/H)<5.7) and another younger with metallicities similar to the values of H II regions. The old objects are distributed in a larger volume than the young ones. An important fraction of PNe (>30%) was found to be highly N-rich (Type I PNe). Such PNe occur at any metallicity. In addition, about 60% of the sample presents high ionization (He++/He >= 0.1), possessing a central star with effective temperature larger than 10^6 K. Possible biases in the sample are discussed. From comparison with stellar evolution models by A. Karakass group of the observed N/O abundance ratios, our PNe should have had initial masses lower than 4 M_sun, although if the comparison is made with Ne vs. O abundances, the initial masses should have been lower than 2 M_sun. It appears that these models of stars of 2-3 M_sun are producing too much 22Ne in the stellar surface at the end of the AGB. On the other hand, the comparison with another set of stellar evolution models by P. Venturas group with a different treatment of convection and on the assumptions concerning the overshoot of the convective core during the core H-burning phase, provided a reasonable agreement between N/O and Ne/H observed and predicted ratios if initial masses of more massive stars are of about 4 M_sun.
Interstellar bubbles around O stars are driven by a combination of the stars wind and ionizing radiation output. The wind contribution is uncertain because the boundary between the wind and interstellar medium is difficult to observe. Mid-infrared observations (e.g., of the H II region RCW 120) show arcs of dust emission around O stars, contained well within the H II region bubble. These arcs could indicate the edge of an asymmetric stellar wind bubble, distorted by density gradients and/or stellar motion. We present two-dimensional, radiation-hydrodynamics simulations investigating the evolution of wind bubbles and H II regions around massive stars moving through a dense (n=3000 cm^{-3}), uniform medium with velocities ranging from 4 to 16 km/s. The H II region morphology is strongly affected by stellar motion, as expected, but the wind bubble is also very aspherical from birth, even for the lowest space velocity considered. Wind bubbles do not fill their H II regions (we find filling factors of 10-20%), at least for a main sequence star with mass M~30 Msun. Furthermore, even for supersonic velocities the wind bow shock does not significantly trap the ionization front. X-ray emission from the wind bubble is soft, faint, and comes mainly from the turbulent mixing layer between the wind bubble and the H II region. The wind bubble radiates <1 per cent of its energy in X-rays; it loses most of its energy by turbulent mixing with cooler photoionized gas. Comparison of the simulations with the H II region RCW 120 shows that its dynamical age is <=0.4 Myr and that stellar motion <=4 km/s is allowed, implying that the ionizing source is unlikely to be a runaway star but more likely formed in situ. The regions youth, and apparent isolation from other O or B stars, makes it very interesting for studies of massive star formation and of initial mass functions.
Sgr B1 is a luminous H II region in the Galactic Center immediately next to the massive star-forming giant molecular cloud Sgr B2 and apparently connected to it from their similar radial velocities. In 2018 we showed from SOFIA FIFI-LS observations of the [O III] 52 and 88 micron lines that there is no central exciting star cluster and that the ionizing stars must be widely spread throughout the region. Here we present SOFIA FIFI-LS observations of the [O I] 146 and [C II] 158 micron lines formed in the surrounding photodissociation regions (PDRs). We find that these lines correlate neither with each other nor with the [O III] lines although together they correlate better with the 70 micron Herschel PACS images from Hi-GAL. We infer from this that Sgr B1 consists of a number of smaller H II regions plus their associated PDRs, some seen face-on and the others seen more or less edge-on. We used the PDR Toolbox to estimate densities and the far-ultraviolet intensities exciting the PDRs. Using models computed with Cloudy, we demonstrate possible appearances of edge-on PDRs and show that the density difference between the PDR densities and the electron densities estimated from the [O III] line ratios is incompatible with pressure equilibrium unless there is a substantial pressure contribution from either turbulence or magnetic field or both. We likewise conclude that the hot stars exciting Sgr B1 are widely spaced throughout the region at substantial distances from the gas with no evidence of current massive star formation.
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