In this paper we explore the relationship between protostellar outflows and turbulence in molecular clouds. Using 3-D numerical simulations we focus on the hydrodynamics of multiple outflows interacting within a parsec scale volume. We explore the extent to which transient outflows injecting directed energy and momentum into a sub-volume of a molecular cloud can be converted into random turbulent motions. We show that turbulence can readily be sustained by these interactions and show that it is possible to broadly characterize an effective driving scale of the outflows. We compare the velocity spectrum obtained in our studies to that of isotropically forced hydrodynamic turbulence finding that in outflow driven turbulence a power law of the form E(k) ∝ k −β is indeed achieved. However we find a steeper spectrum β ∼ 3 is obtained in outflow driven turbulence models than in isotropically forced simulations β ∼ 2.0. We discuss possible physical mechanisms responsible for these results as well and their implications for turbulence in molecular clouds where outflows will act in concert with other processes such as gravitational collapse.
Feedback from protostellar outflows can influence the nature of turbulence in star forming regions even if they are not the primary source of velocity dispersion for all scales of molecular clouds. For the rate and power expected in star forming regions, we previously ) demonstrated that outflows could drive supersonic turbulence at levels consistent with the scaling relations from Matzner (2007) although with a steeper velocity power spectrum than expected for an isotropically driven supersonic turbulent cascade. Here we perform higher resolution simulations and combine simulations of outflow driven turbulence with those of isotropically forced turbulence. We find that the presence of outflows within an ambient isotropically driven turbulent environment produces a knee in the velocity power spectrum at the outflow scale and a steeper slope at sub-outflow scales than for a purely isotropically forced case. We also find that the presence of outflows flattens the density spectrum at large scales effectively reducing the formation of large scale turbulent density structures. These effects are qualitatively independent of resolution. We have also carried out Principal Component Analysis (PCA) for synthetic data from our simulations. We find that PCA as a tool for identifying the driving scale of turbulence has a misleading bias toward low amplitude large scale velocity structures even when they are not necessarily the dominant energy containing scales. This bias is absent for isotropically forced turbulence but manifests strongly for collimated outflow driven turbulence.
The link between turbulence in star formatting environments and protostellar jets remains controversial. To explore issues of turbulence and fossil cavities driven by young stellar outflows we present a series of numerical simulations tracking the evolution of transient protostellar jets driven into a turbulent medium. Our simulations show both the effect of turbulence on outflow structures and, conversely, the effect of outflows on the ambient turbulence. We demonstrate how turbulence will lead to strong modifications in jet morphology.More importantly, we demonstrate that individual transient outflows have the capacity to re-energize decaying turbulence. Our simulations support a scenario in which the directed energy/momentum associated with cavities is randomized as the cavities are disrupted by dynamical instabilities seeded by the ambient turbulence. Consideration of the energy power spectra of the simulations reveals that the disruption of the cavities powers an energy cascade consistent with Burgers'-type turbulence and produces a driving scale-length associated with the cavity propagation length. We conclude that fossil cavities interacting either with a turbulent medium or with other cavities have the capacity to sustain or create turbulent flows in star forming environments. In the last section we contrast our work and its conclusions with previous studies which claim that jets can not be the source of turbulence.
dall. Heart rate-arterial blood pressure relationship in conscious rat before vs. after spinal cord transection. Am J Physiol Regul Integr Comp Physiol 283: R748-R756, 2002. First published April 18, 2002 10.1152/ajpregu.00003. 2002This experiment quantified the initial disruption and subsequent adaptation of the blood pressure (BP)-heart rate (HR) relationship after spinal cord transection (SCT). BP and HR were recorded for 4 h via an implanted catheter in neurally intact, unanesthetized rats. The animals were then anesthetized, and their spinal cords were severed at T1-T2 (n ϭ 5) or T4-T5 (n ϭ 6) or sham lesioned (n ϭ 4). BP was recorded for 4 h daily over the ensuing 6 days. The neurally intact rat showed a positive cross correlation, with HR leading BP at the peak by 1.8 Ϯ 0.8 (SD) s. The cross correlation in unanesthetized rats (n ϭ 2) under neuromuscular blockade was also positive, with HR leading. After SCT at T1-T2, the cross correlation became negative, with BP leading HR, and did not change during the next 6 days. The cross correlation also became negative 1-3 days after SCT at T4-T5, but in four rats by day 6 and thereafter the cross correlation progressively reverted to a positive value. We propose that the positive cross correlation with HR leading BP in the intact rat results from an open-loop control that depends on intact supraspinal input to sympathetic preganglionic neurons in the spinal cord. After descending sympathetic pathways were severed at T1-T2, the intact vagal pathway to the sinoatrial node dominated BP regulation via the baroreflex. We suggest that reestablishment of the positive correlation after SCT at T4-T5 was attributable to the surviving sympathetic outflow to the heart and upper vasculature reasserting some effective function, perhaps in association with decreased spinal sympathetic hyperreflexia. The HR-BP cross correlation may index progression of sympathetic dysfunction in pathological processes. sympathetic; parasympathetic; dysautonomia; cross correlation; baroreflex SHORT-TERM STABILITY of arterial blood pressure (BP) is achieved in large part by appropriate adjustments in sympathetic and parasympathetic outflow from the central nervous system to the cardiovascular effector mechanisms. Many of these neuroregulatory mechanisms, including the baroreflex, are integrated within the medulla oblongata or within more rostral regions of the brain. Spinal cord transection (SCT), depending on the level, severs the descending sympathetic pathways targeted to the heart and arterial resistance vessels. Such injury compromises BP stability (for review, see Ref. 15) and would be expected to manifest itself in alterations in the relationship between changes in BP and heart rate (HR).The precise nature and degree of the changes in the integrity of BP stability after SCT depend on the site and severity of the injury (9). For instance, complete SCT at T 1 in rat virtually disconnects descending supraspinal sympathetic outflow from all effectors, whereas SCT at T 5 preserves most of the ...
We consider the interaction between a marginally stable Bonnor-Ebert (BE) sphere and the surrounding ambient medium. In particular, we explore how the infall from an evolving ambient medium can trigger the collapse of the sphere using three-dimensional adaptive mesh refinement simulations. We find the resulting collapse dynamics to vary considerably with ambient density. In the highest ambient density cases, infalling material drives a strong compression wave into the cloud. It is the propagation of this wave through the cloud interior that triggers the subsequent collapse. For lower ambient densities, we find the main trigger of collapse to be a quasistatic adjustment of the BE sphere to gravitational settling of the ambient gas. In all cases, we find that the classic "outside-in" collapse mode for super-critical BE spheres is recovered before a protostar (i.e., sink particle) forms. Our work supports scenarios in which BE dynamics naturally begins with either a compression wave or infall dominated phase, and only later assumes the usual outside-in collapse behavior.
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