Champagne Phase Research Articles

NGC 3503 and its molecular environment

We present a study of the molecular gas and interstellar dust distribution in the environs of the HII region NGC 3503 associated with the open cluster Pis 17 with the aim of investigating the spatial distribution of the molecular gas linked to the nebula and achieving a better understanding of the interaction of the nebula and Pis 17 with their molecular environment. We based our study in ^{12}CO(1-0) observations of a region of ~0.6 deg. in size obtained with the 4-m NANTEN telescope, unpublished radio continuum data at 4800 and 8640 MHz obtained with the ATCA telescope, radio continuum data at 843 MHz obtained from SUMSS, and available IRAS, MSX, IRAC-GLIMPSE, and MIPSGAL images. We found a molecular cloud (Component 1) having a mean velocity of -24.7 km s^{-1}, compatible with the velocity of the ionized gas, which is associated with the nebula and its surroundings. Adopting a distance of 2.9 +/- 0.4 kpc the total molecular mass and density yield (7.6 +/- 2.1) x 10^3 Msun and 400 +/- 240 cm^{-3}, respectively. The radio continuum data confirm the existence of an electron density gradient in NGC 3503. The IR emission shows the presence of a PDR bordering the higher density regions of the nebula. The spatial distribution of the CO emission shows that the nebula coincides with a molecular clump, with the strongest CO emission peak located close to the higher electron density region. The more negative velocities of the molecular gas (about -27 km s^{-1}), is coincident with NGC 3503. Candidate YSOs were detected towards the HII region, suggesting that embedded star formation may be occurring in the neighbourhood of the nebula. The presence of a clear electron density gradient, along with the spatial distribution of the molecular gas and PAHs in the region indicates that NGC 3503 is a blister-type HII region that probably has undergone a champagne phase.

Self-similar polytropic champagne flows in H ii regions

We explore large-scale hydrodynamics of H II regions for various self-similar shock flows of a polytropic gas cloud under self-gravity and with quasi-spherical symmetry. We formulate cloud dynamics by invoking specific entropy conservation along streamlines and obtain global self-similar ‘champagne flows’ for a conventional polytropic gas with shocks as a subclass. Molecular cloud cores are ionized and heated to high temperatures after the onset of nuclear burning of a central protostar. We model subsequent evolutionary processes in several ways and construct possible self-similar shock flow solutions. We may neglect the mass and gravity of the central protostar. The ionization and heating of the surrounding medium drive outflows in the inner cloud core and a shock travels outwards, leading to the so-called ‘champagne phase’ with an expanding outer cloud envelope. Complementarily, we also consider the expansion of a central cavity around the centre. As the inner cloud expands plausibly due to powerful stellar winds, a cavity (i.e. ‘void’ or ‘bubble’) can be created around the centre, and when the cavity becomes sufficiently large, one may neglect the gravity of the central protostar. We thus present self-similar shock solutions for ‘champagne flows’ with an expanding central void. We compare our solutions with isothermal solutions and find that the generalization to the polytropic regime brings about significant differences of the gas dynamics, especially for cases of n < 1, where n is a key scaling index in the self-similar transformation. We also compare our global polytropic self-similar solutions with numerical simulations on the expansion of H II regions. We further explore other possible dynamic evolutions of H II regions after the initiation of nuclear burning of the central protostar, for example asymptotic inflows or contractions far from the cloud centre and the ongoing infall around a central protostar. In particular, it is possible to use the downstream free-fall solution with shocks to describe the dynamic evolution of H II regions shortly after the nascence of the central protostar. We also give an analysis on the invariant form of self-similar polytropic flows by ignoring self-gravity.

Self‐similar Champagne Flows in HiiRegions

We consider the idealized expansion of an initially self-gravitating, static, singular, isothermal cloud core. For t ≥ 0, the gas is ionized and heated to a higher uniform temperature by the formation of a luminous but massless star in its center. The approximation that the mass and gravity of the central star are negligible for the subsequent motion of the H II region holds for distances r much greater than ~100 AU and for the massive cloud cores that give rise to high-mass stars. If the initial ionization and heating are approximated to occur instantaneously at t = 0, then the subsequent flow (for r ≫ 100 AU) caused by the resulting imbalance between self-gravity and thermal pressure is self-similar. Because of the steep density profile (ρ ∝ r-2), pressure gradients produce a shock front that travels into the cloud, accelerating the gas to supersonic velocities in what has been called the "champagne phase." The expansion of the inner region at t > 0 is connected to the outer envelope of the now ionized cloud core through this shock, whose strength depends on the temperature of the H II gas. In particular, we find a modified Larson-Penston (L-P) type of solution as part of the linear sequence of self-similar champagne outflows. The modification involves the proper insertion of a shock and produces the right behavior at infinity (v → 0) for an outflow of finite duration, reconciling the long-standing conflict on the correct (inflow or outflow) interpretation for the original L-P solution. For realistic heating due to a massive young central star that ionizes and heats the gas to ~104 K, we show that even the self-gravity of the ionized gas of the massive molecular cloud core can be neglected. We then study the self-similar solutions of the expansion of H II regions embedded in molecular clouds characterized by more general power-law density distributions: ρ ∝ r-n with 3/2 < n < 3. In these cases, the shock velocity is an increasing function of the exponent n and diverges as n → 3. We show that this happens because the model includes an origin where the pressure driving the shock diverges because the enclosed heated mass is infinite. Our results imply that the continued photoevaporation of massive reservoirs of neutral gas (e.g., surrounding disks and/or globules) near the embedded ionizing source is required in order to maintain over a significant timescale the emission measure observed in champagne flows.

VLA Observations of Carbon Radio Recombination Lines toward the HiiRegion Complex S88B

We present high angular resolution VLA observations of the C92α, C110α, and C166α radio recombination lines of carbon from the region of massive star formation known as S88B. The observations reveal that the carbon emission arises from two distinct components that are intimately associated with the compact (S88B2) and cometary (S88B1) regions of ionized gas within the complex. The brighter carbon component has an angular size of ~66, an average line-center velocity of 21.0 ± 0.5 km s-1, and an average line width of 5.1 ± 1.0 km s-1; it is associated with the compact H II region. The second component has an angular size of ~16'' and is found projected toward the head of the cometary-like H II region. The average center velocity and width of the carbon line emission are 21.1 ± 0.7 km s-1 and 5.1 ± 1.7 km s-1, respectively. The spatial location and velocity of both carbon regions suggest that the emission arises in layers of photodissociated gas at the interface between the molecular cloud and the regions of ionized gas that are undergoing a champagne phase. From a model analysis of the dependence of the recombination line intensity with principal quantum number, we conclude that the carbon emission originates in warm photodissociated regions. The electron temperatures and electron densities of the photodissociated gas range between 400 and 600 K and between 40 and 80 cm-3, respectively. Further, we find that stimulated amplification of the background H II region continuum radiation contributes significantly to the carbon emission in both components. We also detected emission in sulfur radio recombination lines from both components. We find that the ratios of sulfur to carbon line intensities are considerable larger than the [S/C] cosmic abundance ratio and that they vary with principal quantum number, with values in the range between 0.3 and 0.6. We attribute the large values of the intensity ratios to depletion of carbon in the gas phase by a factor of ~5 and the variations with principal quantum number to stimulated emission effects in a region of low electron density (ne ~ 3 cm-3) and low temperature (Te ~ 50 K) that surrounds the C+ region.

THE PHYSICAL DIAGNOSTICS AND IONIZATION STRUCTURE OF THE ORION NEBULA

ABSTRACT The most detailed study to date of the physical diagnostics and ionization structure of the core of M42 (the Orion Nebula) is presented using new long slit spectra and existing CCD imagery. Cospatial temperature and density diagnostics are presented for distances out to ~5 arcminutes from the Trapezium and clearly show gradients in Te and Ne. Temperatures are derived using S+, O+, N+, O++ and the Balmer Discontinuity, TBac. Values for T(S+) in the range 11,000-14,500 K are presented and suggest shock heating at the ionization front as predicted by some models. A gradient in TBac is derived with temperatures as low as 3000 K and is left open to interpretation. Density gradients for S+, O+ and Cl++ are also presented and are consistent with model predictions for an H II region in the champagne phase of evolution. Using imagery we have constructed a picture of the core of Orion, including previously unknown features. Ratio maps show a high ionization cavity centered on the Trapezium and a low ionization region to the southwest. Ripples and bends in the background molecular cloud are detected. Further to the southwest the ionization begins to increase with distance, which is interpreted as the molecular cloud curving back toward the observer. Additional features in the imagery include a bar-like structure to the northeast of the Trapezium and a high density knot and ellipse of material believed to be a Herbig Haro object. Our diagnostics provide unprecedented detail as to the variation of temperature fluctuations with position. The mean square temperature fluctuation, t2, for the ions O++ and N+ is similar on a global scale, with a value in the range 0.03-0.05 for an assumed constant recombination temperature. Using a gradient in TBac predicts t2 values of 0.01-0.15 with values increasing outward. The ion S+ gives unusually high values of t2 and is not a reliable indicator of fluctuations in the H+ zone since some of the S II emission arises from beyond the ionization front.

Further studies on the champagne phase of GM 24 (IRAS 17136-3617)

view Abstract Citations (4) References (18) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Further Studies on the Champagne Phase of GM 24 (IRAS 17136-3617) Gomez, J. F. ; Torrelles, J. M. ; Tapia, M. ; Gomez, Y. ; Rodriguez, L. F. ; Roth, M. ; Ho, P. T. P. Abstract The H II region associated with GM 24 has been observed with the VLA at the 3.6 and 6 cm continuum, as well as at the H92-alpha recombination line and the H2O maser line; these observations have suggested a 'champagne' flow of ionized gas. Spectral data indicate two velocity components toward GM 24 that are separated by about 10 km/sec. The weaker, extended velocity component is redshifted by about 10 km/sec relative to the velocity of the ambient molecular cloud. It is hypothesized that this velocity component constitutes emission from the ionized gas, which is breaking out of the cloud and expanding away from the direction of observation. Publication: The Astrophysical Journal Pub Date: May 1993 DOI: 10.1086/172661 Bibcode: 1993ApJ...409..269G Keywords: Continuous Radiation; H Ii Regions; Infrared Astronomy Satellite; Interstellar Matter; Ionized Gases; Very Large Array (Vla); Gas Flow; H Alpha Line; Interstellar Masers; Recombination Reactions; Water Masers; Astrophysics; ISM: H II REGIONS; ISM: INDIVIDUAL ALPHANUMERIC: GM 24; RADIO CONTINUUM: INTERSTELLAR full text sources ADS | data products SIMBAD (1)

On the formation and expansion of H II regions

The evolution of H II regions in spherical clouds with small, constant-density cores and power-law density distributions r exp -w outside the core is described analytically. It is found that there is a critical exponent above which the cloud becomes completely ionized. Its value in the formation phase depends on the initial conditions, but it has a well-defined value w(crit) = 3/2 during the expansion phase. For w less than w(crit), the radius of the H II region grows at a given rate, while neutral mass accumulates in the interphase between the ionization and shock fronts. For w = w(crit), the fronts move together without mass accumulation. Cases with w greater than w(crit) lead to the champagne phase: once the cloud is fully ionized, the expansion becomes supersonic. For self-gravitating disks without magnetic fields, the main features include a new 'variable-size' stage. The initial shape of the H II region has a critical point beyond which the disk becomes completely ionized.

High Rate of Destruction of Molecular Clouds by Hot Stars

Tenorio-Tagle (1979) first proposed the idea of a third dynamical phase, the champagne phase, following the formation and expansion phases of an HII region; the idea was further explored by Bodenheimer et al. (1979) and Tenorio-Tagle et al. (1979). The champagne phase begins when the high pressure gas of an HII region formed inside a molecular cloud reaches the edge of the cloud and bursts into the lower pressure, low density, intercloud medium. One important implication of the model is the prediction of an enormous enhancement of the rate of erosion of the molecular cloud by the ionising radiation of hot stars, which begins as soon as the process of the decrease of the gas density between the star and the cloud is started. The proportion of hydrogen molecules eroded by ionising photons may reach about 10-2. The mass eroded may exceed the mass of the ionised gas in the case where the ionisation front reaching the edge of the cloud is of D-type. Additional mechanisms (for instance stellar winds), if at work, may even increase the efficiency of the mechanism.