New Massive Contacting Twin Binary in a Radio-quiet HII Region Associated with the M17 Complex

Early-B stars, much less energetic than O stars, may create an HII region that appears as radio-quiet. We report the identification of new early-B stars associated with the radio-quiet HII region G014.645--00.606 in the M17 complex. The ratio-quiet HII region G014.645--00.606 is adjacent to three radio-quiet WISE HII region candidates \citep{2014ApJS..212....1A}. The ionizing sources of the radio-quiet HII regions are expected to later than B1V, given the sensitivity about 1-2 mJy of the MAGPIS 20 cm survey. The stars were first selected if their parallaxes of GAIA EDR3 match that of the 22 GHz H2O maser source within the same region. We used the color-magnitude diagram made from the ZTF photometric catalog to select the candidates for massive stars because the intrinsic g-r colors of massive stars change little from B-type to O-type stars. Five stars lie in the areas of the color-magnitude diagram where either reddened massive stars or evolved post-main sequence stars of lower masses are commonly found. Three of the five stars, sources 1, 2, and 3, are located at the cavities of the three IR bubbles, and extended Hα emission is detected around the three IR bubbles. We suggest that sources 1, 2, and 3 are candidates for early-B stars associated with the radio-quiet region G014.645--00.606. Particularly, source 1 is an EW type eclipsing binary with a short period of 0.825 day, while source 2 is an EA type eclipsing binary with a short period of 0.919 day. The physical parameters of the two binary systems have been derived through the PHOEBE model. Source 1 is a twin binary of two stars with Teff ≈ 23,500 K, and source 2 contains a hotter component (Teff≈20,100 K) and a cooler one (Teff≈15,500 K). The O-C values of source 1 show a trend of decline, implying that the period of source is deceasing. Source 1 is likely a contacting early-B twin binary, for which mass transfer might cause its orbit to shrink.

Limits to Ionization-parameter Mapping as a Diagnostic of Hii Region Optical Depth

We employ ionization-parameter mapping (IPM) to infer the optical depth of H ii regions in the northern half of M33. We construct [O iii]λ5007/[O ii]λ3727 and [O iii]λ5007/[S ii]λ6724 ratio maps from narrowband images continuum-subtracted in this way, from which we classify the H ii regions by optical depth to ionizing radiation, based on their ionization structure. This method works relatively well in the low-metallicity regime, 12 + log ( O / H ) ≤ 8.4 , where [O iii]λ λ4959, 5007 is strong. However, at higher metallicities, the method breaks down due to the strong dependence of the [O iii]λ λ4959, 5007 emission lines on the nebular temperature. Thus, although O++ may be present in metal-rich H ii regions, these commonly used emission lines do not serve as a useful indicator of its presence, and hence the O ionization state. In addition, IPM as a diagnostic of optical depth is limited by spatial resolution. We also report a region of highly excited [O iii] extending over an area ∼1 kpc across and [O iii]λ5007 luminosity of 4.9 ± 1.5 × 1038 erg s−1, which is several times higher than the ionizing budget of any potential sources in this portion of the galaxy. Finally, this work introduces a new method for continuum subtraction of narrowband images based on the dispersion of pixels around the mode of the diffuse-light flux distribution. In addition to M33, we demonstrate the method on C iii]λ1909 imaging of Haro 11, ESO 338-IG004, and Mrk 71.

The most evolved sources in the Hi-GAL survey

Far-infrared and submillimetre surveys as the Herschel Galactic Plane Infrared Survey (Hi-GAL) represent an irreplaceable knowledge base about early phases of star formation, permitting statistical analysis based on thousands of Galaxy-wide distributed sources. Those with a regular spectral energy distribution in the Herschel wavelength range 70-500 μm span a variety of evolutionary stages, from quiescent to star forming clumps and, within the latter class, from mid-infrared dark clumps to sources appearing very bright also at shorter wavelengths (e.g. Spitzer 24 μm). A fraction of these clumps hosts the formation of high mass stars, which are expected to reach the zero-age main sequence and to develop a HII region in their surroundings while they are still embedded in their parental large-scale dusty envelope. This paper aims at selecting and studying in detail a robust sample of Hi-GAL clumps supposed to be candidate to host a HII region in their interior. They are expected to be the most evolved sources in the Hi-GAL catalogue. The Galactic locations and the physical properties (temperature, mass, bolometric luminosity and temperature, and surface density) of these sources are discussed here. The large number (1199) of selected sources constitutes an important starting point for planning further interferometric programs, aimed at resolving possible cores hosting a young high-mass star.

Numerical models for the dust in RCW 120

Context. The interstellar bubble RCW 120 seen around a type O runaway star is driven by the stellar wind and the ionising radiation emitted by the star. The boundary between the stellar wind and interstellar medium (ISM) is associated with the arc-shaped mid-infrared dust emission around the star within the HII region. Aims. We aim to investigate the arc-shaped bow shock in RCW 120 by means of numerical simulations, including the radiation, dust, HII region, and wind bubble. Methods. We performed 3D radiation-hydrodynamic simulations including dust using the GUACHO code. Our model includes a detailed treatment of dust grains in the ISM and takes into account the drag forces between dust and gas and the effect of radiation pressure on the gas and dust. The dust is treated as a pressureless gas component. The simulation uses typical properties of RCW 120. We analyse five simulations to deduce the effect of the ionising radiation and dust on both the emission intensity and the shape of the shock. Results. The interaction of the wind and the ionising radiation from a runaway star with the ISM forms an arc-shaped bow shock where the dust from the ISM accumulates in front of the moving star. Moreover, the dust forms a second small arc-shaped structure within the rarefied region at the back of the star inside the bubble. In order to obtain the decoupling between the gas and the dust, it is necessary to include the radiation-hydrodynamic equations together with the dust and the stellar motion. In this work all these elements are considered together, and we show that the decoupling between gas and dust obtained in the simulation is in agreement with the morphology of the infrared observations of RCW 120.

Evolutionary Population Synthesis model with binary stars – Yunnan-II model

By considering a modified version of the evolutionary population synthesis (EPS) model for stellar populations (SPs) comprising binary stars, the retrieved galaxy and HII-region parameters/properties differ from the case of neglecting binary stars. The retrieved age, stellar metallicity and mass of galaxies increase (e.g. ∼ 0.2 dex when using spectral fitting algorithm), whilst the star formation rate decreases (∼0.2 dex). The radiation fields from intermediate-age SPs with binary stars could be potentially important ionizing sources in HII regions. Under this possibility, the theoretical division between star forming galaxy and AGN on the diagnostic diagrams would move towards the up-right corner and the retrieved gaseous metallicity would decrease.Our prediction for the birth rate of binary neutron stars in SPs ranges from 10−9 to 10−6${\M {^\minus 1_\odot}} $ yr−1 when the kick velocity is from 0 to 190 km s−1.

Using CO line ratios to trace compressed areas in bubble N131

Aims. N131 is a typical infrared dust bubble showing an expanding ring-like shell. We study the CO line ratios that can be used to trace the interaction in the expanding bubble. Methods. We carried out new CO (3–2) observations toward bubble N131 using the 15 m JCMT, and derived line ratios by combining these observations with our previous CO (2–1) and CO (1–0) data from IRAM 30 m observations. To trace the interaction between the molecular gas and the ionized gas in the HII region, we used RADEX to model the dependence of the CO line ratios on kinetic temperature and H2 volume density, and examined the abnormal line ratios based on other simulations. Results. We present CO (3–2), CO (2–1), and CO (1–0) integrated intensity maps convolved to the same angular resolution (22.5″). The three different CO transition maps show a similar morphology. The line ratios of WCO (3–2)/WCO (2–1) mostly range from 0.2 to 1.2 with a median of 0.54 ± 0.12, while the line ratios of WCO (2–1)/WCO (1–0) range from 0.5 to 1.6 with a median of 0.84 ± 0.15. The high CO line ratios WCO (3–2)/WCO (2–1) ≳ 0.8 and WCO (2–1)/WCO (1–0) ≳ 1.2 are beyond the threshold predicted by numerical simulations based on the assumed density-temperature structure for the inner rims of the ring-like shell, where the compressed areas are located in bubble N131. Conclusions. These high CO integrated intensity ratios, such as WCO (3–2)/WCO (2–1) ≳ 0.8 and WCO (2–1)/WCO (1–0) ≳ 1.2, can be used as a tracer of gas-compressed regions with a relatively high temperature and density. This further suggests that the non-Gaussian part of the line-ratio distribution can be used to trace the interaction between the molecular gas and the hot gas in the bubble.

Extreme gas kinematics in an off-nuclear HII region of SDSS J143245.98+404300.3

We present and discuss the properties of an ionized gas component with extreme kinematics in a recently reported off-nuclear HII region located at ∼0.8−1.0 kpc from the nucleus of SDSS J143245.98+404300.3. The high-velocity-gas component is identified by the detection of very broad emission wings in the Hα line, with full width at half maximum (FWHM) ≥ 850−1000 km s−1. Such gas kinematics are outstandingly high compared to other HII regions in local galaxies and are similar to those reported in some star-forming clumps of galaxies at z ∼ 2. The spatially resolved analysis indicates that the high-velocity gas extends at least ∼90 pc and it could be compatible with an ionized outflow entraining gas at a rate between approximately seven and nine times faster than the rate at which gas is being converted into stars. We do not detect broad emission wings in other emission lines such as Hβ, perhaps due to moderate dust extinction, nor in [N II]λλ6548, 6584 or [S II]λλ6717, 6731, which could be due to the presence of turbulent mixing layers originated by the impact of fast-flowing winds. The lack of spectral signatures associated to the presence of Wolf–Rayet stars points towards stellar winds from a large number of massive stars and/or supernovae as the likely mechanisms driving the high-velocity gas.