The Equilibrium State of Molecular Regions in the Outer Galaxy

Heyer, M. H.; Carpenter, John M.; Snell, R. L.

doi:10.1086/320218

Cited by 302 publications

(480 citation statements)

References 39 publications

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“…For example, T f = T * = 10 Myr corresponds to α ≈ 1.7, which agrees well with observations (Solomon et al 1987;Kramer et al 1998;Heyer et al 2001;Roman Duval et al 2010).…”

supporting

confidence: 89%

The formation and destruction of molecular clouds and galactic star formation

Inutsuka

Inoue

Iwasaki

et al. 2015

A&A

216

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We describe an overall picture of galactic-scale star formation. Recent high-resolution magneto-hydrodynamical simulations of twofluid dynamics with cooling, heating, and thermal conduction have shown that the formation of molecular clouds requires multiple episodes of supersonic compression. This finding enables us to create a scenario in which molecular clouds form in interacting shells or bubbles on a galactic scale. First, we estimated the ensemble-averaged growth rate of molecular clouds on a timescale longer than a million years. Next, we performed radiation hydrodynamics simulations to evaluate the destruction rate of magnetized molecular clouds by the stellar far-ultraviolet radiation. We also investigated the resulting star formation efficiency within a cloud, which amounts to a low value (a few percent) if we adopt the power-law exponent ∼− 2.5 for the mass distribution of stars in the cloud. We finally describe the time evolution of the mass function of molecular clouds on a long timescale (>1 Myr) and discuss the steady state exponent of the power-law slope in various environments.

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“…For example, T f = T * = 10 Myr corresponds to α ≈ 1.7, which agrees well with observations (Solomon et al 1987;Kramer et al 1998;Heyer et al 2001;Roman Duval et al 2010).…”

supporting

confidence: 89%

The formation and destruction of molecular clouds and galactic star formation

Inutsuka

Inoue

Iwasaki

et al. 2015

A&A

216

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“…The value of α found for Draco is significantly different of that found for giant molecular clouds in general (α ∼ 0.8 for M > 10 4 M , Solomon et al 1987;Kramer et al 1998;Heyer et al 2001;Marshall et al 2009). On the other hand, a mass distribution with a similar shape (log-normal plus powerlaw tail) and a similar range in mass was found in a study of the core mass function (CMF) in Aquila by Könyves et al (2010).…”

Section: Masscontrasting

confidence: 90%

Structure formation in a colliding flow: The Herschel view of the Draco nebula

Miville-Deschênes

Salomé

Martín

et al. 2017

A&A

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Context. The Draco nebula is a high Galactic latitude interstellar cloud observed at velocities corresponding to the intermediate velocity cloud regime. This nebula shows unusually strong CO emission and remarkably high-contrast small-scale structures for such a diffuse high Galactic latitude cloud. The 21 cm emission of the Draco nebula reveals that it is likely to have been formed by the collision of a cloud entering the disk of the Milky Way. Such physical conditions are ideal to study the formation of cold and dense gas in colliding flows of diffuse and warm gas. Aims. The objective of this study is to better understand the process of structure formation in a colliding flow and to describe the effects of matter entering the disk on the interstellar medium. Methods. We conducted Herschel-SPIRE observations of the Draco nebula. The clumpfind algorithm was used to identify and characterize the small-scale structures of the cloud. Results. The high-resolution SPIRE map reveals the fragmented structure of the interface between the infalling cloud and the Galactic layer. This front is characterized by a Rayleigh-Taylor (RT) instability structure. From the determination of the typical length of the periodic structure (2.2 pc) we estimated the gas kinematic viscosity. This allowed us to estimate the dissipation scale of the warm neutral medium (0.1 pc), which was found to be compatible with that expected if ambipolar diffusion were the main mechanism of turbulent energy dissipation. The statistical properties of the small-scale structures identified with clumpfind are found to be typical of that seen in molecular clouds and hydrodynamical turbulence in general. The density of the gas has a log-normal distribution with an average value of 10 3 cm −3 . The typical size of the structures is 0.1−0.2 pc, but this estimate is limited by the resolution of the observations. The mass of these structures ranges from 0.2 to 20 M and the distribution of the more massive structures follows a power-law dN/dlog(M) ∼ M −1.4 . We identify a mass-size relation with the same exponent as that found in molecular clouds (M ∼ L 2.3 ). On the other hand, we found that only 15% of the mass of the cloud is in gravitationally bound structures. Conclusions. We conclude that the collision of diffuse gas from the Galactic halo with the diffuse interstellar medium of the outer layer of the disk is an efficient mechanism for producing dense structures. The increase of pressure induced by the collision is strong enough to trigger the formation of cold neutral medium out of the warm gas. It is likely that ambipolar diffusion is the mechanism dominating the turbulent energy dissipation. In that case the cold structures are a few times larger than the energy dissipation scale. The dense structures of Draco are the result of the interplay between magnetohydrodynamical turbulence and thermal instability as self-gravity is not dominating the dynamics. Interestingly they have properties typical of those found in more classical molecular clouds.

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“…Molecular clouds indeed respond to large scale environmental variations. In the outer Galaxy molecular clouds have smaller masses than in the inner regions, and it is still a debated question whether clouds of small mass are pressure-bounded, or in virial equilibrium (Heyer et al 2001;Blitz & Rosolowsky 2005;Heyer et al 2009). …”

Section: Comparison With Spatially Resolved Molecular Cloudsmentioning

confidence: 99%

On the nature of faint mid-infrared sources in M 33

Corbelli

Giovanardi

Palla

et al. 2011

A&A

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Aims. We investigate the nature of 24 μm sources in M 33 that have weak or no associated Hα emission. Both bright evolved stars and embedded star-forming regions are visible as compact infrared sources in the 8 and 24 μm Spitzer maps of M 33 and contribute to the more diffuse and faint emission in these bands. Can we distinguish the two populations? Methods. We carry out deep CO J = 2-1 and J = 1-0 line searches at the location of 18 compact mid-IR sources and two optically selected ones to unveil an ongoing star formation process throughout the disk of M 33. In the absence of high-resolution CO maps we use different assumptions to estimate cloud masses from pointed observations. We also analyze if the spectral energy distribution and mid-IR colors can be used to discriminate between evolved stars and star-forming regions.Results. Molecular emission is detected at the location of 17 sources at the level of 0.3 K km s −1 or higher in at least one of the CO rotational lines. Even though the number of giant molecular clouds drops beyond 4 kpc in M 33, our deep observations reveal that clouds of smaller mass are common out to 6.8 kpc. Estimated cloud masses range between 10 4 and 10 5 M , assuming likely values of the CO-to-H 2 conversion factor and virial equilibrium. Sources that are known to be evolved variable stars show weaker or undetectable CO lines. Evolved stars occupy a well defined region of the IRAC color-color diagrams. Star-forming regions are scattered throughout a larger area, even though the bulk of the distribution has different IRAC colors than evolved variable stars. We estimate that about half of the 24 μm sources without an Hα counterpart are genuine embedded star-forming regions. Sources with faint but compact Hα emission have an incomplete Initial Mass Function (IMF) at the high-mass end and are compatible with a population of young clusters with a stochastically sampled, universal IMF.

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The Equilibrium State of Molecular Regions in the Outer Galaxy

Cited by 302 publications

References 39 publications

The formation and destruction of molecular clouds and galactic star formation

The formation and destruction of molecular clouds and galactic star formation

Structure formation in a colliding flow: The Herschel view of the Draco nebula

On the nature of faint mid-infrared sources in M 33

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