The magnetic evolution of the Taurus molecular clouds. I - Large-scale properties

Heyer, M. H.; Vrba, F. J.; Snell, R. L.; Schloerb, F. P.; Strom, S. E.; Goldsmith, Paul F.; Strom, K. M.

doi:10.1086/165678

Cited by 95 publications

(88 citation statements)

References 0 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…Given the high estimated sonic Mach number in Taurus (see below) it is unlikely that large scale strong anisotropies like those seen in sub-Alfvénic conditions will be present due to magnetic fields (BFP). Gravitational collapse along magnetic field lines could in principle generate anisotropy, and indeed the small anisotropy identified in the power spectrum is oriented as expected in this case (Heyer et al 1987).…”

Section: Application To the Taurus Molecular Cloudsupporting

confidence: 69%

The density variance – Mach number relation in the Taurus molecular cloud

Brunt¹

2010

A&A

View full text Add to dashboard Cite

Supersonic turbulence in molecular clouds is a key agent in generating density enhancements that may subsequently go on to form stars. The stronger the turbulence -the higher the Mach number -the more extreme the density fluctuations are expected to be. Numerical models predict an increase in density variance, σ 2 ρ/ρ 0 , with rms Mach number, M of the form:where b is a numerically-estimated parameter, and this prediction forms the basis of a large number of analytic models of star formation. We provide an estimate of the parameter b from 13 CO J = 1−0 spectral line imaging observations and extinction mapping of the Taurus molecular cloud, using a recently developed technique that needs information contained solely in the projected column density field to calculate σ 2 ρ/ρ 0 . When this is combined with a measurement of the rms Mach number, M, we are able to estimate b. We find b = 0.48 +0.15 −0.11 , which is consistent with typical numerical estimates, and is characteristic of turbulent driving that includes a mixture of solenoidal and compressive modes. More conservatively, we constrain b to lie in the range 0.3−0.8, depending on the influence of sub-resolution structure and the role of diffuse atomic material in the column density budget (accounting for sub-resolution variance results in higher values of b, while inclusion of more low column density material results in lower values of b; the value b = 0.48 applies to material which is predominantly molecular, with no correction for sub-resolution variance). We also report a break in the Taurus column density power spectrum at a scale of ∼1 pc, and find that the break is associated with anisotropy in the power spectrum. The break is observed in both 13 CO and dust extinction power spectra, which, remarkably, are effectively identical despite detailed spatial differences between the 13 CO and dust extinction maps.

show abstract

Section: Application To the Taurus Molecular Cloudsupporting

confidence: 69%

The density variance – Mach number relation in the Taurus molecular cloud

Brunt¹

2010

A&A

View full text Add to dashboard Cite

show abstract

“…The L1517 cloud was selected because it presents a simple gas velocity field as deduced from the global study of gas kinematics inside this region (Heyer et al 1987;Ladd & Myers 1991). There, the global analysis of the C 18 O (1−0) line emission at large scales shows the overwhelming presence of spectra with a unique and narrow gas velocity component (Hacar & Tafalla 2011).…”

Section: Co Observations In the Taurus Molecular Cloudmentioning

confidence: 99%

Opacity broadening and interpretation of suprathermal CO linewidths: Macroscopic turbulence and tangled molecular clouds

Hacar

Alves

Burkert

et al. 2016

A&A

View full text Add to dashboard Cite

Context. Since their first detection in the interestellar medium, (sub-)millimeter line observations of different CO isotopic variants have routinely been employed to characterize the kinematic properties of the gas in molecular clouds. Many of these lines exhibit broad linewidths that greatly exceed the thermal broadening expected for the low temperatures found within these objects. These observed suprathermal CO linewidths are assumed to originate from unresolved supersonic motions inside clouds. Aims. The lowest rotational J transitions of some of the most abundant CO isotopologues, 12 CO and 13 CO, are found to present large optical depths. In addition to well-known line saturation effects, these large opacities present a non-negligible contribution to their observed linewidths. Typically overlooked in the literature, in this paper we aim to quantify the impact of these opacity broadening effects on the current interpretation of the CO suprathermal line profiles. Methods. Combining large-scale observations and LTE modeling of the ground J = 1−0 transitions of the main 12 CO, 13 CO, C 18 O isotopologues, we have investigated the correlation of the observed linewidths as a function of the line opacity in different regions of the Taurus molecular cloud. Results. Without any additional contributions to the gas velocity field, a large fraction of the apparently supersonic (M ∼ 2-3) linewidths measured in both 12 CO and 13 CO (J = 1−0) lines can be explained by the saturation of their corresponding sonic-like, optically thin C 18 O counterparts assuming standard isotopic fractionation. Combined with the presence of multiple components detected in some of our C 18 O spectra, these opacity effects also seem to be responsible for most of the highly supersonic linewidths (M > 8-10) detected in some of the broadest 12 CO and 13 CO spectra in Taurus.Conclusions. Our results demonstrate that most of the suprathermal 12 CO and 13 CO linewidths reported in nearby clouds like Taurus could be primarily created by a combination of opacity broadening effects and multiple gas velocity components blended in these saturated emission lines. Once corrected by their corresponding optical depth, each of these gas components present transonic intrinsic linewidths consistently traced by the three isotopologues, 12 CO, 13 CO, and C 18 O, with differences within a factor of 2. Highly correlated and velocity-coherent at large scales, the largest and highly supersonic velocity differences inside clouds are generated by the relative motions between individual gas components. In contrast to the classical interpretation within the framework of microscopic turbulence, this highly discretized structure of the molecular gas traced in CO suggest that the gas dynamics inside molecular clouds could be better described by the properties of a fully resolved macroscopic turbulence.

show abstract

“…comm. ), 13 CO (Heyer et al 1987), and SO emission (Sect. 4), so they must reflect the true distribution of gas in the L1517 cloud, and are not mere artifacts of the C 18 O chemistry or excitation.…”

Section: Filament Identificationmentioning

confidence: 99%

“…L1517 appears in optical images as a region of enhanced obscuration associated with the reflection nebulosity from the young stars AB Aur and SU Aur (Lynds 1962;Strom et al 1976;Schneider & Elmegreen 1979). The CO observations of the cloud have shown that the extended gas consists of several filamentary components that extend to the northwest of the nebulosity and occupy a region of about 20 × 10 coincident with the optical obscuration (Heyer et al 1987). Embedded in these components lies a collection of several dense cores that are easily distinguished in the optical images thanks to the contrast provided by the bright nebulosity from AB Aur and SU Aur, which lie at least 0.3 pc in projection from the cores (Schneider & Elmegreen 1979).…”

Section: Introductionmentioning

confidence: 99%

Dense core formation by fragmentation of velocity-coherent filaments in L1517

Hacar

Tafalla

2011

A&A

199

237

View full text Add to dashboard Cite

Context. Low-mass star-forming cores differ from their surrounding molecular cloud in turbulence, shape, and density structure. Aims. We aim to understand how dense cores form out of the less dense cloud material by studying the connection between these two regimes. Methods. We observed the L1517 dark cloud in C 18 O(1-0), N 2 H + (J = 1−0), and SO(J N = 3 2 −2 1 ) with the FCRAO 14 m telescope, and in the 1.2 mm dust continuum with the IRAM 30 m telescope. Results. Most of the gas in the cloud lies in four filaments that have typical lengths of 0.5 pc. Five starless cores are embedded in these filaments and have chemical compositions indicative of different evolutionary stages. The filaments have radial profiles of C 18 O(1−0) emission with a central flattened region and a power-law tail, and can be fitted approximately as isothermal, pressure-supported cylinders. The filaments, in addition, are extremely quiescent. They have subsonic internal motions and are coherent in velocity over their whole length. The large-scale motions in the filaments can be used to predict the velocity inside the cores, indicating that core formation has not decoupled the dense gas kinematically from its parental material. In two filaments, these large-scale motions consist of oscillations in the velocity centroid, and a simple kinematic model suggests that they may be related to core-forming flows. Conclusions. Core formation in L1517 seems to have occurred in two steps. First, the subsonic, velocity-coherent filaments have condensed out of the more turbulent ambient cloud. Then, the cores fragmented quasi-statically and inherited the kinematics of the filaments. Turbulence dissipation has therefore occurred mostly on scales on the order of 0.5 pc or larger, and seems to have played a small role in the formation of the individual cores.

show abstract

The magnetic evolution of the Taurus molecular clouds. I - Large-scale properties

Cited by 95 publications

References 0 publications

The density variance – Mach number relation in the Taurus molecular cloud

The density variance – Mach number relation in the Taurus molecular cloud

Opacity broadening and interpretation of suprathermal CO linewidths: Macroscopic turbulence and tangled molecular clouds

Dense core formation by fragmentation of velocity-coherent filaments in L1517

Contact Info

Product

Resources

About