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Stella Offner edited section_Introduction_Turbulence_in_the__.tex
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One fundamental puzzle in star formation is why the efficiency at which dense gas forms stars is only a few percent per free fall time \citep[][]{krumholz14review}. Early three-dimensional hydrodynamic simulations discovered that supersonic turbulence decays rapidly and predicted that without additional energy input turbulence should decay significantly within a dynamical time \citep{stone98,maclow99}. This implies that gravity should be able to efficiently form stars after a dynamical time. However, turbulence observed within molecular clouds does not appear to weaken and star formation efficiencies are small after several dynamical times \citep{KandT07} One explanation for the longevity of observed turbulence is that motions are driven internally via feedback from forming or evolved stars \citep[][and references therein]{krumholz14ppvi}. In principle this should introduce a characteristic energy input scale \citep{carroll09,hansen12,Offner_2015}, which should impact turbulent statistics. However, from an observational prospective, stellar feedback is {\it messy} and identifying clear feedback signatures is complex for the reasons mentioned above. %Analysis is typically restricted by the assumption that the velocities and extent along perpendicular directions are s
Disentangling feedback signatures from the turbulent background and assessing their role is challenging since any low-velocity motions excited by feedback are lost in the general cloud turbulence \citep{swift08,arce10,arce11}.
Few prior numerical or observational studies have examined the response of
a turbulent
statistic statistics to
stellar feedback. The most commonly computed turbulent statistic,
the velocity power
spectra, spectrum, may reflect the characteristic scale of energy input from feedback. Turbulence with both isolated and clusters of outflow exhibit a steepened velocity power spectrum \citep{nakamura07,cunningham09,carroll09}. While \citet{swift08} identified a break in the power spectrum of the $^{13}$CO intensity moment map, which they attribute to a characteristic scale associated with the embedded protostellar outflows (the break is absent in the $^{12}$CO data). Numerical simulations of point-source (supernovae) driving also discovered changes in the
power spectrum spectral slope but
found no
obvious critical injection scale
is obvious \citep{joung06}. \citet{beaumont13} find that observed CO velocity distributions extend to higher velocities than simulations
with pure
turbulence predict; large-scale turbulence; they attribute this
to expanding shells associated with stellar winds. \citet{Offner15} confirm that when winds are included
the a high-velocity tail
is reproduced. appears. In contrast, the intensity probability distribution does not appear
noticably different. sensitive to the inclusion of stellar feedback \citep{beaumont13}.
\citet{burkhart10}, in analyzing HI maps of the Small Magellenic Cloud, noted the possible signature of supernovae on the bispectrum (three-point correlation function), which appears as break around $\sim$160 pc.
%Jung & Mac Low 2006 -pdf, velocity and ke power spectrum, 2-pt correlation function
%hansen 2-point correlation function - outflows+compare to rho Oph, little dependence on feedback