Observations of CO/J = 2-1/ emission from molecular clouds

Phillips, T. G.; Huggins, P. J.; Wannier, P. G.; Scoville, N. Z.

doi:10.1086/157237

Cited by 74 publications

(57 citation statements)

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“…HCN linewidths are fairly similar to DCN linewidths, however, the agreement between ∆v for H 2 CO and HDCO is not so good. This broadening is likely to be due to the optical depth in the H 2 CO lines, since, for an opacity of 5, the line width is expected to increase by a factor of 1.7 (Phillips et al 1979), which is close to the observed H 2 CO vs. HDCO linewidth ratio for most sources. …”

Section: Our Values For [Hdco]/[h 2 Co] and [Dcn]/[hcn] Are Summarisesupporting

confidence: 71%

A survey of [HDCO]/[ H$_\mathsf{2}$CO] and [DCN]/[HCN] ratios towards low-mass protostellar cores

Roberts

Fuller

Millar

et al. 2002

A&A

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. We compare our results with predictions from detailed, chemical models, and to other observations made in these sources. We find best agreement between models and observations by assuming that interaction between gas phase molecules and dust grains has impacted on the chemistry during the cold pre-collapse phase of the cloud's history. The abundance of deuterated species indicates that the dense gas out of which a low mass protostar forms, evolves and collapses on a timescale of ≤50 000 years. We find no marked difference between molecular D/H ratios towards different regions, or between Class 0 and Class I protostars. However, the striking difference between the [DCN]/[HCN] ratios we have measured and those previously observed towards Hot Molecular Cores leads us to suggest that there are significant evolutionary differences between high and low mass star forming regions.

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Section: Our Values For [Hdco]/[h 2 Co] and [Dcn]/[hcn] Are Summarisesupporting

confidence: 71%

A survey of [HDCO]/[ H$_\mathsf{2}$CO] and [DCN]/[HCN] ratios towards low-mass protostellar cores

Roberts

Fuller

Millar

et al. 2002

A&A

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“…While suitable for optically thin lines, the above assumption of ∆V int = ∆V is no longer valid in the case of flat-topped and (Phillips et al 1979); (orange) values derived from the observed ∆V directly measured from the line emission; (blue) values derived ∆V from Gaussian fits. Lower panel: contribution of both opacity (red solid line) and intrinsic, Doppler broadening (thermal + nonthermal motions; black dashed line) to the total observed linewidth for different line opacities.…”

Section: Curve Of Growthmentioning

confidence: 99%

“…saturated spectra. In analogy to the optical absorption lines, the so-called curve of growth of molecular lines describes the line broadening produced by the increasing line opacity and can be defined analytically as (e.g., Phillips et al 1979),…”

Section: Curve Of Growthmentioning

confidence: 99%

“…Characteristic values of τ(C 18 O) between 0.2 and 0.6 based on the study of the C 18 O (1−0) transition are found in different studies of dark clouds (Myers et al 1983;Vilas-Boas et al 1994Onishi et al 1996). Assuming typical relative abundances for the main three CO isotopologues in the local ISM (i.e., X(C 18 O):X( 13 CO):X( 12 CO) = 1:7.3:560 Wilson & Rood 1994) these opacities translate into characteristic line opacities ranging between τ( 13 CO) ∼ 1.5-4.4 and τ( 12 CO) ∼ 110-340 for the J = 1−0 transitions of the most abundant CO isotopologue (e.g., Phillips et al 1979;Wong et al 2008). Since ∆V = β τ ∆V int , it is then clear that while the effects of opacity broadening can be neglected in the case of the optically thin C 18 O (1−0) lines their contribution can severely affect the observed FWHM in both 13 CO (1−0) and, in particular, the 12 CO (1−0) optically thick line profiles.…”

Section: Co Linewidths: Differential Opacity Broadeningmentioning

confidence: 99%

“…(4) and in the absence of additional kinematic effects, a line opacity broadening is expected to occur for lines with similar emission properties and increasing isotopic abundances. By comparing point-like observations of multiple CO transitions in different molecular clouds, Phillips et al (1979) suggested that the differences in linewidth observed between the distinct CO isotopologues follow the expected curve of growth of an emission line with increasing opacity. Continuing with this line of argument we compared the observed line properties of the ground state J = (1−0) transition for the three main CO isotopologues in the L1517 region in Taurus (Lynds 1962).…”

Section: Co Observations In the Taurus Molecular Cloudmentioning

confidence: 99%

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

Hacar

Alves

Burkert

et al. 2016

A&A

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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.

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