scholarly journals Formation of structures around HII regions: ionization feedback from massive stars

2012 ◽  
Vol 10 (H16) ◽  
pp. 590-590
Author(s):  
P. Tremblin ◽  
E. Audit ◽  
V. Minier ◽  
W. Schmidt ◽  
N. Schneider
Keyword(s):  
Uv Radiation ◽  
Molecular Cloud ◽  
Massive Stars ◽  
Hii Regions ◽  
Ionization Front ◽  
Hii Region ◽  
Data Archives ◽  
Density Enhancement ◽  
Rosette Nebula ◽  
Density Modulation

AbstractWe present a new model for the formation of dense clumps and pillars around HII regions based on shocks curvature at the interface between a HII region and a molecular cloud. UV radiation leads to the formation of an ionization front and of a shock ahead. The gas is compressed between them forming a dense shell at the interface. This shell may be curved due to initial interface or density modulation caused by the turbulence of the molecular cloud. Low curvature leads to instabilities in the shell that form dense clumps while sufficiently curved shells collapse on itself to form pillars. When turbulence is high compared to the ionized-gas pressure, bubbles of cold gas have sufficient kinetic energy to penetrate into the HII region and detach themselves from the parent cloud, forming cometary globules.Using computational simulations, we show that these new models are extremely efficient to form dense clumps and stable and growing elongated structures, pillars, in which star formation might occur (see Tremblin et al.2012a). The inclusion of turbulence in the model shows its importance in the formation of cometary globules (see Tremblin et al.2012b). Globally, the density enhancement in the simulations is of one or two orders of magnitude higher than the density enhancement of the classical “collect and collapse“ scenario. The code used for the simulation is the HERACLES code, that comprises hydrodynamics with various equation of state, radiative transfer, gravity, cooling and heating.Our recent observations with Herschel (see Schneider et al.2012a) and SOFIA (see Schneider et al.2012b) and additional Spitzer data archives revealed many more of these structures in regions where OB stars have already formed such as the Rosette Nebula, Cygnus X, M16 and Vela, suggesting that the UV radiation from massive stars plays an important role in their formation. We present a first comparison between the simulations described above and recent observations of these regions.

scholarly journals Constraining physical conditions for the PDR of Trumpler 14 in the Carina Nebula

2018 ◽  
Vol 618 ◽  
pp. A53 ◽  
Author(s):  
Ronin Wu ◽  
Emeric Bron ◽  
Takashi Onaka ◽  
Franck Le Petit ◽  
Frédéric Galliano ◽  
...  
Keyword(s):  
Uv Radiation ◽  
Massive Stars ◽  
Local Heating ◽  
Spectral Lines ◽  
Ionization Front ◽  
Thermal Pressure ◽  
Line Energy ◽  
Physical Conditions ◽  
Radiation Fields ◽  
Carina Nebula

We investigate the physical conditions of the CO gas, based on the submillimeter imaging spectroscopy from a 2′ × 7′ (1.5 × 5 pc2) area near the young star cluster, Trumpler 14 of the Carina Nebula. The observations presented in this work are taken with the Fourier Transform Spectrometer (FTS) of the Spectral and Photometric Imaging REceiver (SPIRE) onboard the Herschel Space Observatory. The newly observed spectral lines include [CI] 370 μm [CI] 609 μm, and CO transitions from J = 4−3 to J = 13−12. Our field of view covers the edge of a cavity carved by Trumpler 14 about 1 Myr ago and marks the transition from H ii regions to photo-dissociation regions. The observed CO intensities are the most prominent at the northwest region, Car I-E. With the state-of-the-art Meudon PDR code, we successfully derive the physical conditions, which include the thermal pressure (P) and the scaling factor of radiation fields (GUV), from the observed CO spectral line energy distributions (SLEDs) in the observed region. The derived GUV values generally show excellent agreement with the UV radiation fields created by nearby OB-stars and thus confirm that the main excitation source of the observed CO emission is the UV-photons provided by the massive stars. The derived thermal pressure is in the range 0.5−3 × 108 K cm-3 with the highest values found along the ionization front in Car I-E region facing Trumpler 14, hinting that the cloud structure is similar to the recent observations of the Orion Bar. We also note a discrepancy at a local position (<0.17 × 0.17 pc2) between the photo-dissociation region (PDR) modeling result and the UV radiation fields estimated from nearby massive stars, which requires further investigation on nearby objects that could contribute to local heating, including outflow. Comparing the derived thermal pressure with the radiation fields, we report the first observationally derived and spatially resolved P ~ 2 × 104 GUV relationship. As direct comparisons of the modeling results to the observed 13CO, [O I] 63 μm, and [C II] 158 μm intensities are not straightforward, we urge the reader to be cautious when constraining the physical conditions of PDRs with combinations of 12CO, 13CO, [C I], [O I] 63 μm, and [C II] 158 μm observations.

scholarly journals Expansion of HII Regions in Density Gradients

1989 ◽  
Vol 120 ◽  
pp. 96-103
Author(s):  
José Franco ◽  
Guillermo Tenorio-Tagle ◽  
Peter Bodenheimer
Keyword(s):  
Power Law ◽  
Azimuthal Angle ◽  
Weak Shock ◽  
Hii Regions ◽  
Ionization Front ◽  
Neutral Medium ◽  
Density Gradients ◽  
Hii Region ◽  
Cylindrically Symmetric ◽  
Oriented Parallel

AbstractThe main features of HII regions expanding in spherical and disk-like clouds with density gradients are reviewed. The spherical cases assume power-law density stratifications, r~w, and the disk-like cases include exponential, gaussian, and sech2 distributions. For power-law profiles, there is a critical exponent, wcrit = 3/2, above which the ionization front cannot be “trapped” and the cloud becomes fully ionized. For clouds with w < 3/2, the radius of the ionized region grows as t4/(7-2w) and drives a shock front into the ambient neutral medium. For w = wcrit = 3/2 the shock wave cannot detach from the ionization front and the two move together with a constant speed equal to about 2ci, where ci is the sound speed in the ionized gas. For w > 3/2 the expansion corresponds to the “champagne phase”, and two regimes, fast and slow, are apparent: between 3/2 < w ≤ 3, the slow regime, the inner region drives a weak shock moving with almost constant velocity through the cloud, and for w > 3, the fast regime, the shock becomes strong and accelerates with time.For the case of disk-like clouds, which are assumed cylindrically symmetric, the dimensions of the initial HII regions along each azimuthal angle, θ, are described in terms of the Strömgren radius for the midplane density, Ro, and the disk scale height, H. For yo = Rosin(θ)/H ≤ α (where α is a constant dependent on the assumed density distribution) the whole HII region is contained within the disk, and for yo > α a conical section of the disk becomes totally ionized. The critical azimuthal angle above which the HII region becomes unbounded is defined by θcrit =sin-1(αH/Ro). The expansion of initially unbounded HII regions (i.e. with yo > α) proceeds along the z-axis and, if the disk column density remains constant during the evolution, the ionization front eventually recedes from infinity to become trapped within the expanding disk. For clouds threaded by a B-field oriented parallel to the symmetry axis, as expected in magnetically dominated clouds, this effect can be very prominent. The expanding gas overtaken by the receding ionization front maintains its linear momentum after recombination and is transformed into a high-velocity neutral outflow. In the absence of magnetic fields, the trapping has only a short duration.

Observations of the 1→0 Transition of CO Towards Southern HII Regions

1982 ◽  
Vol 4 (4) ◽  
pp. 434-440 ◽  
Author(s):  
J. B. Whiteoak ◽  
Robina E. Otrupcek ◽  
C. J. Rennie
Keyword(s):  
Radio Telescope ◽  
Molecular Cloud ◽  
Milky Way ◽  
Line Emission ◽  
Hii Regions ◽  
Hii Region ◽  
Giant Molecular Cloud ◽  
Molecular Lines ◽  
Csiro Division

The 4-m radio telescope of the CSIRO Division of Radiophysics at Epping is being used to survey the line emission associated with the 1→0 transition of CO (rest frequency 115.271 GHz) in the southern Milky Way. The programme includes mapping the CO distribution across giant molecular-cloud/HII-region complexes. As a first stage the emission has been observed towards bright southern HII regions. These results will not only serve as a basis for future extensive mapping but will also provide data which is directly comparable with observations of other molecular lines that have been made towards the HII regions.

The Marseille Observatory Hα Survey: Comparisons by CO, 6 CM and IRAS Data

1998 ◽  
Vol 179 ◽  
pp. 186-188
Author(s):  
D. Russeil ◽  
P. Amram ◽  
Y.P. Georgelin ◽  
Y.M. Georgelin ◽  
M. Marcelin ◽  
...  
Keyword(s):  
Molecular Cloud ◽  
Large Scale ◽  
Galactic Plane ◽  
Photon Counting ◽  
Hii Regions ◽  
Radio Continuum ◽  
Line Of Sight ◽  
Ionized Gas ◽  
Hii Region ◽  
Pattern Determination

The Marseille Observatory Hα survey supplies Hα velocities of the ionized hydrogen over large zones of the sky towards the galactic plane. This survey, led at the ESO La Silla, uses a 36 cm telescope equiped with a scanning Fabry-Perot interferometer and a photon counting camera (Le Coarer et al. 1992). About 250 fields (39′×39′) toward the galactic plane have already been covered (see Figure 1) with a spatial resolution of 9″×9″ and a spectral resolution of 5 km s–1. This allows us to observe the discrete HII regions and the diffuse ionized gas widely distributed between them and to separate the distinct layers found along the line of sight. HII regions are often grouped on the molecular cloud surface, then CO, radio continuum and recombination lines surveys of the galactic plane are also essential to distinguish the HII region-molecular cloud complexes met on the line of sight, and in order to take dynamical effects into account, such as the champagne effect, for the kinematic distance determination. Indeed, the spiral structure pattern determination requires avoiding any artificial spread by clearly identifying the giant complexes composed of molecular clouds, HII regions, diffuse ionized hydrogen widely surrounding them, and exciting stars. On the other hand the ionized gas data (Hα and recombination lines) associated with IRAS data help us to study the nature of the young objects constituent of these complexes and to assess their detectability. We present two fields from the Hα survey and parallel large scale investigations.

scholarly journals Fine Structure of Shocked Photodissociation Regions in the Orion Bright Bar

1994 ◽  
Vol 140 ◽  
pp. 249-250
Author(s):  
Toshihiro Omodaka ◽  
Yasuhiro Murata ◽  
Masahiko Hayashi ◽  
Yoshimi Kitamura ◽  
Masatoshi Ohishi
Keyword(s):  
Shock Wave ◽  
Fine Structure ◽  
Molecular Cloud ◽  
Column Density ◽  
Ionization Front ◽  
Molecular Gas ◽  
Hii Region ◽  
Fine Structures ◽  
Photodissociation Regions ◽  
Integrated Intensities

The Orion bright bar is a prominent ionization front located approximately 2’ southeast of the Trapezium stars. Because this ionization front is seen almost edge-on, it provides an opportunity to study the interaction between the HII region and the adjacent molecular cloud. The molecular bar has been thought to be a narrow layer of ~ 50” (0.1 pc) in width parallel to the ionization front with enhanced temperature, density and column density. The molecular gas outside the ionization front was redshifted with respect to the ambient molecular cloud by 1-2 kms−1 (Omodaka et al. 1984, 1986, 1992), suggesting that the expanding HII region generated by the Trapezium stars had driven a shock wave into the molecular cloud at the southeast of the bar. This layer is exposed to intense UV radiation from the Trapezium stars, resulting in the formation of photodissociated regions.We have made aperture synthesis observations of CS(J=1-0) line and 49 GHz continuum in the Orion bright bar with the Nobeyama Millimeter Array. Figure 1, a map of integrated intensities of CS, clearly revealed fine structures of the molecular bar and more than six prominent features are confirmed. It is noted that these features are lined up at 30” from the ionization front inside the molecular cloud.

scholarly journals Magnetic Fields in Molecular Cloud Cores

1990 ◽  
Vol 140 ◽  
pp. 291-292
Author(s):  
Z.P. Zhou ◽  
X.W. Zheng
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
Circular Polarization ◽  
Molecular Cloud ◽  
Zeeman Splitting ◽  
Hii Regions ◽  
Observational Evidence ◽  
Hii Region ◽  
Vlbi Observations ◽  
Cloud Cores

Strong circular polarization of OH masers at 1665 and 1667 MHz lines has been observed towards the molecular cloud cores associated with HII regions. Magnetic field strengths of a few mG are derived from the Zeeman splitting of OH lines. For instance, a magnetic field of about 4 mG in the masing region of W3(OH) has been estimated by OH-line Zeeman splitting (Davies, 1974). VLBI observations show that the OH maser spots project onto or very close to the surface of associated compact HII regions (Reid et al., 1986). The observational evidence demonstrates that the scales of OH maser components surrounding a compact HII region (R ~ 1016 cm) are about 1014 cm in diameter with an amplification pathlength of ~1015 cm. Hence the magnetic fields determined by the Zeeman splitting of OH maser lines appear partly very close to the associated HII region. Elitzur (1979) has theoretically obtained similar results as above.

scholarly journals Dust, HII and molecules towards OH and H2O masers

2002 ◽  
Vol 206 ◽  
pp. 14-17
Author(s):  
James R. Forster
Keyword(s):  
Molecular Cloud ◽  
Hii Regions ◽  
Hii Region ◽  
Star Forming ◽  
The Core ◽  
Star Forming Regions ◽  
Early Formation ◽  
Young Object ◽  
Compact Hii Regions

Radio synthesis observations made with the BIMA interferometer at 29, 86 and 216 GHz are presented for twelve galactic fields containing multiple interstellar OH and H2O maser sites. A dusty molecular cloud was found at 20 of the 23 maser sites in the fields studied. The clouds have masses in the range 50 to 800 M⊙ and diameters between 0.1 and 0.5 pc.The data show that most masers are located near the centers of massive, dusty molecular cores. The cores appear to be centrally condensed and internally excited. These results suggest that most masers found in star-forming regions are associated with a massive young object at the center of a collapsing molecular cloud. The kinematics of the core gas, and association with ultra-compact HII regions, implies that the duration of the maser phase includes collapse, expansion and early formation of an HII region.

scholarly journals Supernova Remnants in Giant HII Regions

1988 ◽  
Vol 101 ◽  
pp. 201-204
Author(s):  
You-Hua Chu ◽  
Robert C. Kennicutt
Keyword(s):  
Supernova Remnants ◽  
Detection Rate ◽  
Massive Stars ◽  
Line Emission ◽  
Hii Regions ◽  
Radio Continuum ◽  
Hii Region ◽  
The Core ◽  
Large Numbers ◽  
Optical Line

Giant HII regions contain large numbers of massive stars, and hence are expected to contain large numbers of SNRs. Until recently, however, only a few SNRs have been identified in extragalactic giant HII regions. Moreover, most of these SNRs are located at the outskirts of HII regions, instead of the core where most of the stars are located. The low detection rate and the outlying locations of the SNRs may be due to: 1) observational difficulties - the background HII regions are much more luminous than the SNRs in both optical line emission and radio continuum; 2) intrinsic invisibility of SNRs - stellar wind and SNRs may have created a supershell (Mac Low and McCray 1987), and the core of a giant HII region is filled with hot tenuous coronal gas; or 3) a genuine deficiency of supernovae and SNRs in the HII regions (Sramek and Weedman 1986).

The Wolf-Rayet Stars in 30 Doradus

1982 ◽  
Vol 99 ◽  
pp. 545-549 ◽  
Author(s):  
Jorge Melnick
Keyword(s):  
Star Formation ◽  
Chemical Composition ◽  
Massive Star ◽  
Massive Stars ◽  
Star Clusters ◽  
Hii Regions ◽  
Young Star ◽  
Hii Region ◽  
Formation And Evolution ◽  
Young Star Clusters

Giant HII regions as sites of massive star formation.Giant HII regions are the brightest extragalactic emission line objects that can be studied in detail. With diameters of several hundreds of parsecs, these nebulae can be easily resolved out to distances of a few Mpc. Typically 100 or more 0 stars are required to account for the observed ionization of the nebular gas and this implies that the cores of giant HII regions contain populous young star clusters. The stars in these clusters have essentially the same age and chemical composition. Thus, giant HII region cores provide excellent sites where theories of the formation and evolution of massive stars and, in particular, of Wolf-Rayet (WR) stars can be tested.

scholarly journals An Investigation of the Molecular Clouds of the Carina HII Region/Molecular Cloud Complex—First Results

1998 ◽  
Vol 15 (2) ◽  
pp. 202-207 ◽  
Author(s):  
K. J. Brooks ◽  
J. B. Whiteoak ◽  
J. W. V. Storey
Keyword(s):  
Molecular Cloud ◽  
Molecular Clouds ◽  
Massive Stars ◽  
Molecular Gas ◽  
Hii Region ◽  
Carina Nebula ◽  
First Results ◽  
Multi Wavelength ◽  
Cloud Complex

AbstractThe Carina Nebula is an extremely bright southern HII region embedded in a giant molecular cloud and contains some of the most massive stars known in our Galaxy. We are undertaking a multi-wavelength study of the Carina Nebula in order to examine the detailed kinematics and distribution of the molecular and ionised gas, and to look for further evidence of ongoing star formation. Here we present the results of the initial molecular cloud observations which were made by observing the 12CO(1−0) emission with the Mopra antenna. The observations reveal the clumpy morphology of the molecular gas, and allow us to identify many interesting regions for follow-up observations.

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