Molecular cloud evolution: from formation to cluster formation through filaments

Special Colloquium
Dr. Enrique Vázquez-Semadeni
Institute of Radioastronomy and Astrophysics UNAM Mexico

I will discuss the process of molecular cloud (MC) evolution, from their formation trhough their destruction by stellar feedback. For Solar Neighborhood conditions, MCs originate from compressive motions in the warm medium, which produce cold, dense, and turbulent atomic clouds. These clouds can quickly become strongly Jeans unstable as they incorporate material by the compression, because of their low temperature and high density. Global collapse sets in, and proceeds hierarchically: the small-scale, large-amplitude (SS) density fluctuations have shorter free-fall times than the large-scale, small amplitude (LS) ones. The LS collapses culminate a few Myr later than the SSLA ones and consist of filamentary flows that accrete onto massive central clumps. The filaments produced by this process are consistent with the observed properties of observed MC filaments. The SS collapses consist of clumps that are embedded in the filaments and are falling onto the large-scale collapse centers. The stars formed in the early, small-scale collapses share the infall motion of their parent clumps. Thus, the filaments feed both gaseous and stellar material to the massive central clump. The hierarchical collapse of the cloud leads to a hierarchical structure of the forming cluster, which naturally reproduces the observed age and mass segregation of young clusters. The global collapse of the clouds implies that their star formation rate (SFR) increases with time. Eventually, massive stars begin to form, which then begin to erode the cloud and reduce its SFR. A simple model for the collapse of MCs describes the evolution of the clouds' SFR, and explains the average cloud star formation efficiency and its observed scatter, as well as the observed age histograms of YSOs in young stellar clusters.

Scattering variability of the Crab Pulsar

Special Colloquium
Laura Driessen
University of Amsterdam

The Crab pulsar is a bright pulsar with interesting features such as giant pulses, glitches and anomalous scattering. We observed the Crab at 350MHz over two years with the Westerbork Synthesis Radio Telescope to invesigate a period of anomalous scattering that occured from late 2012 to early 2013. During this period an extra feature on the Crab, called an echo, was observed. The echo varies on short timescales and is thought to be caused by scattering close to the pulsar. I will discuss modelling the Crab pulse profile and our investigation of the echo.

Diffuse Neutral Clouds in the Milky Way, a High-Resolution Study

Lunch Colloquium
Dr. Yurii Pidopryhora

A set of diffuse interstellar clouds in the inner Galaxy has been observed at an angular resolution of ~1 arcmin combining data from the NRAO Very Large Array and the Green Bank Telescope. These are the diffuse neutral HI clouds that may constitute a considerable fraction of the interstellar medium (ISM). At the estimated distance of the clouds, the linear resolution ranges from ~1.9 to ~2.8 pc. These clouds have been chosen to be outside of the Galactic plane in order not to be confused with unrelated emission, and to lie near the tangent points in the inner Galaxy so that their distances can be quantified. But in general they belong to a widespread and ubiquitous Galactic population. Their locations are at 2.3 < R < 6.0 kpc from the Galactic Center and -1000 < z < +610 pc from the Galactic plane. Peak HI column densities lie in the range N_HI = 0.8--2.9 10^20 cm^-2. Cloud diameters vary between about 10 and 100 pc, and their HI mass spans the range from less than a hundred to a few thousands solar masses. The clouds hardly show any morphological consistency, except that their shapes are highly irregular. One cloud may lie within the hot wind bubble around the nucleus of the Galaxy, and some clouds show evidence of two distinct thermal phases as would be expected from equilibrium models of the ISM. High-resolution study of these clouds finally allows us to uncover interesting hints about their origin and dynamics.