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Register a new userCompact radio sources in the vicinity of the ultracompact HII region G78.4+2.6
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Using the Very Large Array (VLA) at 3.6~cm we identify four new compact radio sources in the vicinity of the cometary HII region G78.4+2.6 (VLA~1). The four compact radio sources (named VLA~2 to VLA~5), have near-infrared counterparts, as seen in the 3.6 $mu$m Spitzer image. One of them (VLA~5) clearly shows evidence of radio variability in a timescale of hours. We explore the possibility that these radio sources are associated with pre-main sequence (PMS) stars in the vicinity of the UC HII region G78.4+2.6. Our results favor the smaller distance value of 1.7 kpc for G78.4+2.6. In addition to the detection of the radio sources in the vicinity of G78.4+2.6, we detected another group of five sources which appear located about 3 to the northwest of the HII region. Some of them exhibit extended emission.
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We present a study of the line-of-sight magnetic fields in five large-diameter Galactic HII regions. Using the Faraday rotation of background polarized radio sources, as well as dust-corrected H-alpha surface brightness as a probe of electron density, we estimated the strength and orientation of the magnetic field along 93 individual sight-lines through the HII regions. Each of the HII regions displayed a coherent magnetic field. The magnetic field strength (line-of-sight component) in the regions ranges from 2 to 6 microgauss, which is similar to the typical magnetic field strength in the diffuse interstellar medium. We investigated the relationship between magnetic field strength and electron density in the 5 HII regions. The slope of magnetic field vs. density in the low-density regime (0.8 < n_e < 30 per cubic cm) is very slightly above zero. We also calculated the ratio of thermal to magnetic pressure, beta_th, for each data point, which fell in the range 1.01 < beta_th < 25. Finally, we studied the orientation of the magnetic field in the solar neighborhood (d < 1.1 kpc) using our data from 5 HII regions along with existing measurements of the line-of-sight magnetic field strength from polarized pulsars whose distances have been determined from their annual parallax. We identify a net direction for the magnetic field in the solar neighborhood, but find no evidence for a preferred vertical direction of the magnetic field above or below the Galactic plane.
Mon R2, at a distance of 830 pc, is the only ultracompact HII region (UC HII) where the photon-dominated region (PDR) between the ionized gas and the molecular cloud can be resolved with Herschel. HIFI observations of the abundant compounds 13CO, C18O, o-H2-18O, HCO+, CS, CH, and NH have been used to derive the physical and chemical conditions in the PDR, in particular the water abundance. The 13CO, C18O, o-H2-18O, HCO+ and CS observations are well described assuming that the emission is coming from a dense (n=5E6 cm-3, N(H2)>1E22 cm-2) layer of molecular gas around the UC HII. Based on our o-H2-18O observations, we estimate an o-H2O abundance of ~2E-8. This is the average ortho-water abundance in the PDR. Additional H2-18O and/or water lines are required to derive the water abundance profile. A lower density envelope (n~1E5 cm-3, N(H2)=2-5E22 cm-2) is responsible for the absorption in the NH 1_1-0_2 line. The emission of the CH ground state triplet is coming from both regions with a complex and self-absorbed profile in the main component. The radiative transfer modeling shows that the 13CO and HCO+ line profiles are consistent with an expansion of the molecular gas with a velocity law, v_e =0.5 x (r/Rout)^{-1} km/s, although the expansion velocity is poorly constrained by the observations presented here.
The CO+ reactive ion is thought to be a tracer of the boundary between a HII region and the hot molecular gas. In this study, we present the spatial distribution of the CO+ rotational emission toward the Mon R2 star-forming region. The CO+ emission presents a clumpy ring-like morphology, arising from a narrow dense layer around the HII region. We compare the CO+ distribution with other species present in photon-dominated regions (PDR), such as [CII] 158 mm, H2 S(3) rotational line at 9.3 mm, polycyclic aromatic hydrocarbons (PAHs) and HCO+. We find that the CO+ emission is spatially coincident with the PAHs and [CII] emission. This confirms that the CO+ emission arises from a narrow dense layer of the HI/H2 interface. We have determined the CO+ fractional abundance, relative to C+ toward three positions. The abundances range from 0.1 to 1.9x10^(-10) and are in good agreement with previous chemical model, which predicts that the production of CO+ in PDRs only occurs in dense regions with high UV fields. The CO+ linewidth is larger than those found in molecular gas tracers, and their central velocity are blue-shifted with respect to the molecular gas velocity. We interpret this as a hint that the CO+ is probing photo-evaporating clump surfaces.
We present the first subarcsecond submillimeter images of the enigmatic ultracompact HII region (UCHII) G5.89-0.39. Observed with the SMA, the 875 micron continuum emission exhibits a shell-like morphology similar to longer wavelengths. By using images with comparable angular resolution at five frequencies obtained from the VLA archive and CARMA, we have removed the free-free component from the 875 micron image. We find five sources of dust emission: two compact warm objects (SMA1 and SMA2) along the periphery of the shell, and three additional regions further out. There is no dust emission inside the shell, supporting the picture of a dust-free cavity surrounded by high density gas. At subarcsecond resolution, most of the molecular gas tracers encircle the UCHII region and appear to constrain its expansion. We also find G5.89-0.39 to be almost completely lacking in organic molecular line emission. The dust cores SMA1 and SMA2 exhibit compact spatial peaks in optically-thin gas tracers (e.g. 34SO2), while SMA1 also coincides with 11.9 micron emission. In CO(3-2), we find a high-velocity north/south bipolar outflow centered on SMA1, aligned with infrared H2 knots, and responsible for much of the maser activity. We conclude that SMA1 is an embedded intermediate mass protostar with an estimated luminosity of 3000 Lsun and a circumstellar mass of ~1 Msun. Finally, we have discovered an NH3 (3,3) maser 12 arcsec northwest of the UCHII region, coincident with a 44 GHz CH3OH maser, and possibly associated with the Br gamma outflow source identified by Puga et al. (2006).
We present high angular and spectral resolution HI 21~cm line observations toward the cometary-shaped compact HII region G213.880-11.837 in the GGD~14 complex.The kinematics and morphology of the photodissociated region, traced by the HI line emission, reveal that the neutral gas is part of an expanding flow. The kinematics of the HI gas along the major axis of G213.880-11.837 shows that the emission is very extended toward the SE direction, reaching LSR radial velocities in the tail of about 14 km/s. The ambient LSR radial velocity of the molecular gas is 11.5 km/s, which suggests a champagne flow of the HI gas. This is the second (after G111.61+0.37) cometary HII/HI region known.
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