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Register a new userThe hydroxyl satellite-line `flip as a tracer of expanding HII regions
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Observations of the four $^{2}Pi_{3/2},~J = 3/2$~ground state transitions of the hydroxyl radical (OH) have emerged as an informative tracer of molecular gas in the Galactic ISM. We discuss an OH spectral feature known as the `flip, in which the satellite lines at 1612 and 1720,MHz flip -- one from emission to absorption and the other the reverse -- across a closely blended double feature. We highlight 30 examples of the flip from the literature, 27 of which exhibit the same orientation with respect to velocity: the 1720,MHz line is seen in emission at more negative velocities. These same examples are also observed toward bright background continuum, many (perhaps all) show stimulated emission, and 23 of these are coincident in on-sky position and velocity with Htextsc{ii}~radio recombination lines. To explain these remarkable correlations we propose that the 1720,MHz stimulated emission originates in heated and compressed post-shock gas expanding away from a central Htextsc{ii}~region, which collides with cooler and more diffuse gas hosting the 1612,MHz stimulated emission. The foreground gas dominates the spectrum due to the bright central continuum, hence the expanding post-shock gas is blue-shifted relative to the stationary pre-shock gas. We employ non-LTE excitation modelling to examine this scenario, and find that indeed FIR emission from warm dust adjacent to the Htextsc{ii}~region radiatively pumps the 1612 MHz line in the diffuse, cool gas ahead of the expanding shock front, while collisional pumping in the warm, dense shocked gas inverts the 1720 MHz line.
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We investigate the turbulence driving mode of ionizing radiation from massive stars on the surrounding interstellar medium (ISM). We run hydrodynamical simulations of a turbulent cloud impinged by a plane-parallel ionization front. We find that the ionizing radiation forms pillars of neutral gas reminiscent of those seen in observations. We quantify the driving mode of the turbulence in the neutral gas by calculating the driving parameter $b$, which is characterised by the relation $sigma_s^2 = ln({1+b^2mathcal{M}^2})$ between the variance of the logarithmic density contrast $sigma_s^2$ (where $s = ln({rho/rho_0})$ with the gas density $rho$ and its average $rho_0$), and the turbulent Mach number $mathcal{M}$. Previous works have shown that $bsim1/3$ indicates solenoidal (divergence-free) driving and $bsim1$ indicates compressive (curl-free) driving, with $bsim1$ producing up to ten times higher star formation rates than $bsim1/3$. The time variation of $b$ in our study allows us to infer that ionizing radiation is inherently a compressive turbulence driving source, with a time-averaged $bsim 0.76 pm 0.08$. We also investigate the value of $b$ of the pillars, where star formation is expected to occur, and find that the pillars are characterised by a natural mixture of both solenoidal and compressive turbulent modes ($bsim0.4$) when they form, and later evolve into a more compressive turbulent state with $bsim0.5$--$0.6$. A virial parameter analysis of the pillar regions supports this conclusion. This indicates that ionizing radiation from massive stars may be able to trigger star formation by producing predominately compressive turbulent gas in the pillars.
The hydroxyl radical (OH) is found in various environments within the interstellar medium (ISM) of the Milky Way and external galaxies, mostly either in diffuse interstellar clouds or in the warm, dense environments of newly formed low-mass and high-mass stars, i.e, in the dense shells of compact and ultracompact HII regions (UCHIIRs). Until today, most studies of interstellar OH involved the molecules radio wavelength hyperfine structure (hfs) transitions. These lines are generally not in LTE and either masing or over-cooling complicates their interpretation. In the past, observations of transitions between different rotational levels of OH, which are at far-infrared wavelengths, have suffered from limited spectral and angular resolution. Since these lines have critical densities many orders of magnitude higher than the radio wavelength ground state hfs lines and are emitted from levels with more than 100 K above the ground state, when observed in emission, they probe very dense and warm material. We probe the warm and dense molecular material surrounding the UCHIIR/OH maser sources W3(OH), G10.62-0.39 and NGC 7538 IRS1 by studying the $^2Pi_{{1/2}}, J = {3/2} - {1/2}$ rotational transition of OH in emission and, toward the last source also the molecules $^2Pi_{3/2}, J = 5/2 - 3/2$ ground-state transition in absorption. We used the Stratospheric Observatory for Infrared Astronomy (SOFIA) to observe these OH lines, which are near 1.84 THz ($163 mu$m) and 2.51 THz ($119.3 mu$m). We clearly detect the OH lines, some of which are blended with each other. Employing non-LTE radiative transfer calculations we predict line intensities using models of a low OH abundance envelope versus a compact, high-abundance source corresponding to the origin of the radio OH lines.
Context. The derived physical parameters for young HII regions are normally determined assuming the emission region to be optically thin. However, this assumption is unlikely to hold for young HII regions such as hyper-compact HII(HCHII) and ultra-compact HII(UCHII) regions and leads to the underestimation of their properties. This can be overcome by fitting the SEDs over a wide range of radio frequencies. Aims. The two primary goals of this study are (1) to determine the physical properties of young HII regions from radio SEDs in the search for potential HCHII regions, and (2) to use these physical properties to investigate their evolution. Method. We used the Karl G. Jansky Very Large Array (VLA) to observe the X-band and K-band with angular resolutions of ~1.7 and ~0.7, respectively, toward 114 HII regions with rising-spectra between 1-5 GHz. We complement our observations with VLA archival data and construct SEDs in the range of 1-26 GHz and model them assuming an ionization-bounded HII region with uniform density. Results. Our sample has a mean electron density of ne=1.6E4cm^{-3}, diameter diam=0.14pc, and emission measure EM = 1.9E7pc*cm^{-6}. We identify 16 HCHII region candidates and 8 intermediate objects between the classes of HCHII and UCHII regions. The ne, diam, and EM change as expected, but the Lyman continuum flux is relatively constant over time. We find that about 67% of Lyman-continuum photons are absorbed by dust within these HII regions and the dust absorption fraction tends to be more significant for more compact and younger HII regions. Conclusion. Young HII regions are commonly located in dusty clumps; HCHII regions and intermediate objects are often associated with various masers, outflows, broad radio recombination lines, and extended green objects, and the accretion at the two stages tends to be quickly reduced or halted.
Magnetic fields play an important role during star formation. Direct magnetic field strength observations have proven specifically challenging in the extremely dynamic protostellar phase. Because of their occurrence in the densest parts of star forming regions, masers, through polarization observations, are the main source of magnetic field strength and morphology measurements around protostars. Of all maser species, methanol is one of the strongest and most abundant tracers of gas around high-mass protostellar disks and in outflows. However, as experimental determination of the magnetic characteristics of methanol has remained largely unsuccessful, a robust magnetic field strength analysis of these regions could hitherto not be performed. Here we report a quantitative theoretical model of the magnetic properties of methanol, including the complicated hyperfine structure that results from its internal rotation. We show that the large range in values of the Land{e} g-factors of the hyperfine components of each maser line lead to conclusions which differ substantially from the current interpretation based on a single effective g-factor. These conclusions are more consistent with other observations and confirm the presence of dynamically important magnetic fields around protostars. Additionally, our calculations show that (non-linear) Zeeman effects must be taken into account to further enhance the accuracy of cosmological electron-to-proton mass ratio determinations using methanol.
Because of their relatively simple morphology, bubble HII regions have been instrumental to our understanding of star formation triggered by HII regions. With the far-infrared (FIR) spectral coverage of the Herschel satellite, we can access the wavelengths where these regions emit the majority of their energy through their dust emission. At Herschel wavelengths 70 micron to 500 micron, the emission associated with HII regions is dominated by the cool dust in their photodissociation regions (PDRs). We find average dust temperatures of 26K along the PDRs, with little variation between the HII regions in the sample, while local filaments and infrared dark clouds average 19K and 15K respectively. Higher temperatures lead to higher values of the Jeans mass, which may affect future star formation. The mass of the material in the PDR, collected through the expansion of the HII region, is between ~300 and ~10,000 Solar masses for the HII regions studied here. These masses are in rough agreement with the expected masses swept up during the expansion of the hii regions. Approximately 20% of the total FIR emission is from the direction of the bubble central regions. This suggests that we are detecting emission from the near-side and far-side PDRs along the line of sight and that bubbles are three-dimensional structures. We find only weak support for a relationship between dust temperature and beta, of a form similar to that caused by noise and calibration uncertainties alone.
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