The Cold, Massive Molecular Cloud G216−2.5. III. Infrared Properties

Lee, Youngung; Snell, R. L.; Dickman, R. L.

doi:10.1086/178062

Cited by 16 publications

(22 citation statements)

References 3 publications

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“…At least, all those surveyed within distances less than 3 kpc form stars (Blitz 1993, Williams et al 2000, except possibly the Maddalena & Thaddeus (1985) cloud (Lee, Snell, & Dickman 1996;Williams & Blitz 1998), and this last cloud may have formed just recently.…”

Section: A Star Formation In Molecular Cloudsmentioning

confidence: 99%

Control of star formation by supersonic turbulence

2004

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Understanding the formation of stars in galaxies is central to much of modern astrophysics. However, a quantitative prediction of the star formation rate and the initial distribution of stellar masses remains elusive. For several decades it has been thought that the star formation process is primarily controlled by the interplay between gravity and magnetostatic support, modulated by neutral-ion drift (known as ambipolar diffusion in astrophysics). Recently, however, both observational and numerical work has begun to suggest that supersonic turbulent flows rather than static magnetic fields control star formation. To some extent, this represents a return to ideas popular before the importance of magnetic fields to the interstellar gas was fully appreciated. This review gives a historical overview of the successes and problems of both the classical dynamical theory, and the standard theory of magnetostatic support from both observational and theoretical perspectives. The outline of a new theory relying on control by driven supersonic turbulence is then presented. Numerical models demonstrate that although supersonic turbulence can provide global support, it nevertheless produces density enhancements that allow local collapse. Inefficient, isolated star formation is a hallmark of turbulent support, while efficient, clustered star formation occurs in its absence. The consequences of this theory are then explored for both local star formation and galactic scale star formation. It suggests that individual star-forming cores are likely not quasi-static objects, but dynamically collapsing. Accretion onto these objects varies depending on the properties of the surrounding turbulent flow; numerical models agree with observations showing decreasing rates. The initial mass distribution of stars may also be determined by the turbulent flow. Molecular clouds appear to be transient objects forming and dissolving in the larger-scale turbulent flow, or else quickly collapsing into regions of violent star formation. We suggest that global star formation in galaxies is controlled by the same balance between gravity and turbulence as small-scale star formation, although modulated by cooling and differential rotation. The dominant driving mechanism in star-forming regions of galaxies appears to be supernovae, while elsewhere coupling of rotation to the gas through magnetic fields or gravity may be important.

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Section: A Star Formation In Molecular Cloudsmentioning

confidence: 99%

Control of star formation by supersonic turbulence

2004

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“…We derive the ratio for the molecular clouds in our sample using IRAS high-resolution processing (HIRES) images. The total IR luminosity between 1 and 500 lm, L IR , can be estimated from the 60 and 100 lm flux densities in Jy, F 60 and F 100 , by the relation (Lonsdale et al 1985;Lee, Snell, & Dickman 1996) …”

Section: Physical Characteristics and Star Formationmentioning

confidence: 99%

Molecular Counterparts of Ultracompact HiiRegions with Extended Envelopes

Kim

Koo

2003

ApJ

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We have carried out 13 CO J ¼ 1-0, CS, and C 34 S J ¼ 2-1 and J ¼ 3-2 line observations of molecular clouds associated with 16 ultracompact (UC) H ii regions with extended envelopes. The molecular clouds are the ones that give birth to rich stellar clusters and/or very massive (O7-O4) stars. Our data show that the clouds are very clumpy and of irregular morphology. They usually have much larger masses, velocity dispersions, and fractions of dense gas than molecular clouds that form early B or late O stars. This is compatible with earlier findings that more massive stars form in more massive cores. The IR luminosity-to-mass ratio has a mean value of 9 L /M and is little correlated with the cloud mass. Most molecular clouds have star formation efficiencies of 1%-2%. We find size-line width and size-density relations in the forms of Dv / D 0:4 and nðH 2 Þ / D À1:2 . 13 CO cores are in general associated with compact H ii regions regardless of the presence of UC H ii regions therein. In contrast, CS cores are preferentially associated with compact H ii regions that contain UC H ii regions. As with the fact that the compact H ii regions containing UC H ii regions are more compact than those not associated with UC H ii regions, these indicate that the former may be in an earlier evolutionary phase than the latter. The diffuse extended envelopes of H ii regions often develop in the direction of decreasing molecular gas density. Based on detailed comparison of molecular line data with radio continuum and recombination line data, the extended ionized envelopes are likely the results of champagne flows in at least 10 sources in our sample. Together these results appear to support a published suggestion that the extended emission around UC H ii regions can be naturally understood by combining the champagne flow model with the hierarchical structure of molecular clouds, taking into account various inclinations and low resolutions of our data. In addition, the blister model seems to be still applicable to most H ii regions, even though massive stars usually form in the interiors rather than on the surfaces of molecular clouds. This is possible because massive star-forming clouds have hierarchical structure and irregular morphology.

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“…Here we took average intensities on the cloud boundary as the background emission levels. The total IR luminosity in the wavelength range 1-500 lm, L IR , can be calculated from F 60 and F 100 in Jy using the formula (Lonsdale et al 1985;Lee, Snell, & Dickman 1996) …”

Section: Declination (B1950)mentioning

confidence: 99%

Interaction between Ionized and Molecular Gas in the Active Star‐forming Region W31

Kim

Koo

2002

ApJ

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We have carried out 21 cm radio continuum, H76 radio recombination line, and various ( 12 CO, 13 CO, CS, and C 34 S) molecular line observations of the W31 complex. Our radio continuum data show that W31 is composed of two extended H ii regions, G10.2À0.3 and G10.3À0.1, each of which comprises an ultracompact H ii region, two or more compact components, and a diffuse envelope. The W31 cloud appears as an incomplete shell on the whole and consists of southern spherical and northern flat components, which are associated with G10.2À0.3 and G10.3À0.1, respectively. For an assumed distance of 6 kpc, the molecular cloud has a size of 48 pc and a mass of 6:2 Â 10 5 M . The IR luminosity-to-mass ratio and the star formation efficiency are derived to be 9 L /M and 3%, respectively. These estimates are greater than average values of the inner Galactic plane. We detect two large (16 and 11 pc) and massive (2:1 Â 10 5 and 8:2 Â 10 4 M ) CSemitting regions in the northern and southern cloud components. The large amount (48% in mass and 16% in area) of dense gas may suggest that the W31 cloud has the ability to form rich stellar clusters and that star formation has only recently begun. The extended envelopes of both G10.2À0.3 and G10.3À0.1 are likely to be results of the champagne flows, based on the distributions of ionized and molecular gas and the velocity gradient of H76 line emission. According to the champagne model, the dynamical ages of the two H ii regions would be ð4 12Þ Â 10 5 yr. We find strong evidence of bipolar molecular outflows associated with the two ultracompact H ii regions. In the vicinity of the ultracompact and compact H ii regions in G10.3À0.1, the 12 CO J ¼ 2 1=J ¼ 1 0 intensity ratio is high (1.4), and a small but prominent molecular gas hollow exists. Together, these observations strongly indicate that the H ii regions and their ionizing stars are interacting with the molecular cloud. Therefore, it is most likely that recently formed massive stars are actively disrupting their parental molecular cloud in the W31 complex.

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The Cold, Massive Molecular Cloud G216−2.5. III. Infrared Properties

Cited by 16 publications

References 3 publications

Control of star formation by supersonic turbulence

Control of star formation by supersonic turbulence

Molecular Counterparts of Ultracompact HiiRegions with Extended Envelopes

Interaction between Ionized and Molecular Gas in the Active Star‐forming Region W31

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