Random flows and diagnostics of turbulence in the high latitude cirrus

¹ Dipartmento di Fisica “Enrico Fermi”, Università di Pisa, via Buonarroti 2, Pisa 56127 and INFN – Sezione di Pisa, Italy e-mail: shore@df.unipi.it
² Department of Biological & Physical Sciences, Kennesaw State University, 1000 Chastain Road, Kennesaw, GA 30144, USA e-mail: ted@avatar.kennesaw.edu
³ Department of Physics and Astronomy, University of Georgia, Athens GA 30602, USA e-mail: loris@physast.uga.edu

Received: 1 August 2005
Accepted: 7 July 2006

Abstract

Aims.We present an analysis of the exceptionally turbulent velocity field in the high Galactic latitude cirrus cloud MBM 3. As in the other translucent clouds in our study (MBM 16 and MBM 40), there is no evidence for internal star formation. However, the large scale velocity variation in this cloud is more pronounced.

Methods.We have mapped the cloud in ¹²CO and ¹³CO (1-0) at high spatial (0.03 pc) and velocity (0.06 km s^-1) resolution. We constructed several velocity probability density functions (PDFs), estimated the turbulent transfer rate, and analyzed the linewidths as a function of the size of randomly chosen regions within the cloud.

Results.We find strong shear flows throughout the cloud that can easily power the turbulent motions. The wings of the PDFs are well approximated by a lorentzian distribution. Such distributions, related to Levy processes that are well known to be produced by correlated processes, are an unambiguous diagnostic of the turbulent intermittency.

Conclusions.We find that the linewidth-size relation frequently used to indicate the role of turbulence in molecular clouds is not an unambiguous signature of its presence.