Coherence in Dense Cores. II. The Transition to Coherence

Goodman, Alyssa A.; Barranco, Joseph; Wilner, David J.; Heyer, M. H.

doi:10.1086/306045

Cited by 372 publications

(438 citation statements)

References 74 publications

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“…The typical density fluctuations computed by integration of S (k) over scales larger than the sonic scale are consistent with the logarithmic density dispersions derived from the probability density functions for solenoidal and compressive forcings. On the other hand, the typical density fluctuations on scales smaller than the sonic scale are significantly smaller for both forcing types, which may reflect the transition to coherent cores (e.g., Goodman et al 1998). Indeed, observations show that cores typically have transonic to subsonic internal velocity dispersions (e.g., Benson & Myers 1989;André et al 2007;Kirk et al 2007;Ward-Thompson et al 2007;Lada et al 2008;Foster et al 2009;Friesen et al 2009;).…”

Section: Discussionmentioning

confidence: 95%

“…5 shows that high-density regions exhibit lower Mach numbers on average. In real molecular clouds, the sonic scale is expected to be located on length scales λ s ≈ 0.1 pc within factors of a few (e.g., Falgarone et al 1992;Goodman et al 1998;Schnee et al 2007). For instance, Heyer et al (2006) found λ s ≈ 0.3−0.4 pc for the Rosette MC and λ s ≈ 0.1−0.2 pc for G216-2.5.…”

Section: The Sonic Scalementioning

confidence: 99%

“…Thus, dense cores might naturally exhibit transonic to subsonic velocity dispersions, because their sizes are expected to be comparable to the sonic scale. The sonic scale may be the transition scale to coherent cores (e.g., Goodman et al 1998). Although many of these "cores" here are transient, some of them are dense enough to become gravitationally bound, and accumulate enough mass to decouple from the overall supersonic turbulent flow.…”

Section: Limitationsmentioning

confidence: 99%

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Comparing the statistics of interstellar turbulence in simulations and observations

Federrath

Duval

Klessen³

et al. 2010

A&A

830

101

1,286

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Context. Density and velocity fluctuations on virtually all scales observed with modern telescopes show that molecular clouds (MCs) are turbulent. The forcing and structural characteristics of this turbulence are, however, still poorly understood. Aims. To shed light on this subject, we study two limiting cases of turbulence forcing in numerical experiments: solenoidal (divergence-free) forcing and compressive (curl-free) forcing, and compare our results to observations. Methods. We solve the equations of hydrodynamics on grids with up to 1024 3 cells for purely solenoidal and purely compressive forcing. Eleven lower-resolution models with different forcing mixtures are also analysed. Results. Using Fourier spectra and Δ-variance, we find velocity dispersion-size relations consistent with observations and independent numerical simulations, irrespective of the type of forcing. However, compressive forcing yields stronger compression at the same rms Mach number than solenoidal forcing, resulting in a three times larger standard deviation of volumetric and column density probability distributions (PDFs). We compare our results to different characterisations of several observed regions, and find evidence of different forcing functions. Column density PDFs in the Perseus MC suggest the presence of a mainly compressive forcing agent within a shell, driven by a massive star. Although the PDFs are close to log-normal, they have non-Gaussian skewness and kurtosis caused by intermittency. Centroid velocity increments measured in the Polaris Flare on intermediate scales agree with solenoidal forcing on that scale. However, Δ-variance analysis of the column density in the Polaris Flare suggests that turbulence is driven on large scales, with a significant compressive component on the forcing scale. This indicates that, although likely driven with mostly compressive modes on large scales, turbulence can behave like solenoidal turbulence on smaller scales. Principal component analysis of G216-2.5 and most of the Rosette MC agree with solenoidal forcing, but the interior of an ionised shell within the Rosette MC displays clear signatures of compressive forcing. Conclusions. The strong dependence of the density PDF on the type of forcing must be taken into account in any theory using the PDF to predict properties of star formation. We supply a quantitative description of this dependence. We find that different observed regions show evidence of different mixtures of compressive and solenoidal forcing, with more compressive forcing occurring primarily in swept-up shells. Finally, we emphasise the role of the sonic scale for protostellar core formation, because core formation close to the sonic scale would naturally explain the observed subsonic velocity dispersions of protostellar cores.

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Section: Discussionmentioning

confidence: 95%

Section: The Sonic Scalementioning

confidence: 99%

Section: Limitationsmentioning

confidence: 99%

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Comparing the statistics of interstellar turbulence in simulations and observations

Federrath

Duval

Klessen³

et al. 2010

A&A

830

101

1,286

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“…Transitions from supersonic turbulence at low densities to coherence at high densities have been observed in dense cores both in molecular clouds (e.g. Goodman et al 1998;Caselli et al 2002;Pineda et al 2010) and in isolation (Quinn 2013). This behaviour is consistent with models of turbulent dissipation (e.g.…”

Section: Gravitational and Internal Energymentioning

confidence: 99%

The JCMT Gould Belt Survey: first results from the SCUBA-2 observations of the Ophiuchus molecular cloud and a virial analysis of its prestellar core population

Pattle¹,

Ward-Thompson²,

Kirk³

et al. 2015

Monthly Notices of the Royal Astronomical Society

139

146

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In this paper we present the first observations of the Ophiuchus molecular cloud performed as part of the James Clerk Maxwell Telescope (JCMT) Gould Belt Survey (GBS) with the SCUBA-2 instrument. We demonstrate methods for combining these data with previous HARP CO, Herschel, and IRAM N 2 H + observations in order to accurately quantify the properties of the SCUBA-2 sources in Ophiuchus. We produce a catalogue of all of the sources found by SCUBA-2. We separate these into protostars and starless cores. We list all of the starless cores and perform a full virial analysis, including external pressure. This is the first time that external pressure has been included in this level of detail. We find that the majority of our cores are either bound or virialised. Gravitational energy and external pressure are on average of a similar order of magnitude, but with some variation from region to region. We find that cores in the Oph A region are gravitationally bound prestellar cores, while cores in the Oph C and E regions are pressure-confined. We determine that N 2 H + is a good tracer of the bound material of prestellar cores, although we find some evidence for N 2 H + freezeout at the very highest core densities. We find that non-thermal linewidths decrease substantially between the gas traced by C 18 O and that traced by N 2 H + , indicating the dissipation of turbulence at higher densities. We find that the critical Bonnor-Ebert stability criterion is not a good indicator of the boundedness of our cores. We detect the pre-brown dwarf candidate Oph B-11 and find a flux density and mass consistent with previous work. We discuss regional variations in the nature of the cores and find further support for our previous hypothesis of a global evolutionary gradient across the cloud from southwest to northeast, indicating sequential star formation across the region.

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“…Direct evidence for a transition between different physical regimes on different scales has been provided by studies of the internal kinematics of star-forming cloud cores by Goodman et al (1998). On scales larger than about 0.1 pc, the non-thermal component of the velocity dispersion in these cores increases with region size following the general linewidth-size relation for molecular clouds, while on smaller scales the non-thermal velocity dispersion becomes smaller than the sound speed and almost independent of region size.…”

Section: Evidence and Theory Concerning The Present-day Imfmentioning

confidence: 99%

Early star formation and the evolution of the stellar initial mass function in galaxies

Larson¹

1998

Monthly Notices of the Royal Astronomical Society

410

433

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It has frequently been suggested in the literature that the stellar IMF in galaxies was top-heavy at early times. This would be plausible physically if the IMF depends on a mass scale such as the Jeans mass that was higher at earlier times because of the generally higher temperatures that were present then. In this paper it is suggested, on the basis of current evidence and theory, that the IMF has a universal Salpeter-like form at the upper end but flattens below a characteristic stellar mass that may vary with time. Much of the evidence that has been attributed to a top-heavy early IMF, including the ubiquitous G-dwarf problem, the high abundance of heavy elements in clusters of galaxies, and the high rate of formation of massive stars in high-redshift galaxies, can be accounted for with such an IMF if the characteristic stellar mass was several times higher during the early stages of galaxy evolution. However, significant variations in the mass-to-light ratios of galaxies and large amounts of dark matter in stellar remnants are not as easily explained in this way because they require more extreme and less plausible assumptions about the form and variability of the IMF. Metal-free 'population III' stars are predicted to have an IMF that consists exclusively of massive stars, and they could help to account for some of the evidence that has been attributed to a top-heavy early IMF, as well as contributing importantly to the energetics and chemical enrichment of the early universe.

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Coherence in Dense Cores. II. The Transition to Coherence

Cited by 372 publications

References 74 publications

Comparing the statistics of interstellar turbulence in simulations and observations

Comparing the statistics of interstellar turbulence in simulations and observations

The JCMT Gould Belt Survey: first results from the SCUBA-2 observations of the Ophiuchus molecular cloud and a virial analysis of its prestellar core population

Early star formation and the evolution of the stellar initial mass function in galaxies

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