Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userOn the turbulence driving mode of expanding HII regions
76
0
0.0
(
0
)
Added by
Shyam Harimohan Menon
Publication date
2020
fields
Physics
and research's language is
English
Ask ChatGPT about the research
No Arabic abstract
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.
rate research
Read More
We investigate the scale dependence of fluctuations inside a realistic model of an evolving turbulent HII region and to what extent these may be studied observationally. We find that the multiple scales of energy injection from champagne flows and the photoionization of clumps and filaments leads to a flatter spectrum of fluctuations than would be expected from top-down turbulence driven at the largest scales. The traditional structure function approach to the observational study of velocity fluctuations is shown to be incapable of reliably determining the velocity power spectrum of our simulation. We find that a more promising approach is the Velocity Channel Analysis technique of Lazarian & Pogosyan (2000), which, despite being intrinsically limited by thermal broadening, can successfully recover the logarithmic slope of the velocity power spectrum to a precision of +-0.1 from high resolution optical emission line spectroscopy.
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.
The tight correlation between turbulence and luminosity in Giant HII Regions is not well understood. While the luminosity is due to the UV radiation from the massive stars in the ionizing clusters, it is not clear what powers the turbulence. Observations of the two prototypical Giant HII Regions in the local Universe, 30 Doradus and NGC604, show that part of the kinetic energy of the nebular gas comes from the combined stellar winds of the most massive stars - the cluster winds, but not all. We present a study of the kinematics of 30 Doradus based on archival VLT FLAMES/GIRAFFE data and new high resolution observations with HARPS. We find that the nebular structure and kinematics are shaped by a hot cluster wind and not by the stellar winds of individual stars. The cluster wind powers most of the turbulence of the nebular gas, with a small but significant contribution from the combined gravitational potential of stars and gas. We estimate the total mass of 30 Doradus and we argue that the region does not contain significant amounts of neutral (HI) gas, and that the giant molecular cloud 30Dor-10 that is close to the center of the nebula in projection is in fact an inflating cloud tens of parsecs away from R136, the core of the ionizing cluster. We rule out a Kolmogorov-like turbulent kinetic energy cascade as the source of supersonic turbulence in Giant HII Regions.
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.
High-mass stars form in much richer environments than those associated with isolated low-mass stars, and once they reach a certain mass, produce ionised (HII) regions. The formation of these pockets of ionised gas are unique to the formation of high-mass stars (M $>8$ M$_odot$), and present an excellent opportunity to study the final stages of accretion, which could include accretion through the HII region itself. This study of the dynamics of the gas on both sides of these ionisation boundaries in very young HII regions aims to quantify the relationship between the HII regions and their immediate environments.We present high-resolution ($sim$ 0.5$$) ALMA observations of nine HII regions selected from the Red MSX Source (RMS) survey with compact radio emission and bolometric luminosities greater than 10$^4$ L$_odot$. We focus on the initial presentation of the data, including initial results from the radio recombination line H29$alpha$, some complementary molecules, and the 256 GHz continuum emission. Of the six (out of nine) regions with H29$alpha$ detections, two appear to have cometary morphologies with velocity gradients across them, and two appear more spherical with velocity gradients suggestive of infalling ionised gas. The remaining two were either observed at low resolution or had signals that were too weak to draw robust conclusions. We also present a description of the interactions between the ionised and molecular gas (as traced by CS (J=5-4)), often (but not always) finding theHII region had cleared its immediate vicinity of molecules. Of our sample of nine, the observations of the two clusters expected to have the youngest HII regions (from previous radio observations) are suggestive of having infalling motions in the H29$alpha$ emission, which could be indicative of late stage accretion onto the stars despite the presence of an HII region.
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
