Dr. De Pree and his team at Agnes Scott College perform observations with the Expanded Very Large Array (EVLA) to study massive star formation. In particular, they investigate radiation flux variations during massive star formation in HII regions (i.e., regions in the interstellar medium in which hydrogen is mainly present as ionized atoms). Massive stars drive processes that enrich the ISM in chemical elements and significantly impact the evolution of galaxies. The Sgr B2 region is one of the most source-rich massive star forming regions in the Milky Way. It contains many morphologically diverse HII regions, and the number of unusual broad line sources make it an ideal laboratory for testing theories of ultra-compact HII region evolution. Recent high-resolution, radiation-hydrodynamic simulations indicate that dense, rotating, accretion flows that are required to form massive stars quickly become gravitationally unstable. This changes the amount of trapped ionizing radiation and the sizes of the associated ultra-compact HII regions. The orbits of dense clumps and filaments near newly born massive stars thus may irregularly trap and expose their ionizing radiation. These objects are optically thick at a wavelength of 2 cm and thus size variations are also expressed as flux changes. Hence over time, a resulting HII region "flickers" in size between being hyper-compact and ultra-compact throughout the main star accretion phase, rather than monotonically expanding. The accretion flow continues for a period ten times longer than the free expansion timescale for an HII region, and the model can address why ultra-compact HII regions do not more rapidly expand and dissipate. It could also explain the observed distribution of morphologies. The Sgr B2 region contains 49 regions that can be imaged at 1.3 cm wavelength, and 25 of these hyper-compact HII regions have physical diameters less than 5000 Astronomical Units. Dr. De Pree and his team will use the EVLA to reimage this large sample of ultra-compact HII regions to: (1) Determine the frequency and magnitude of ultra-compact HII region flux and size fluctuations over about a 20 year time baseline (since 1989) (2) Constrain and test the theoretical models, (3) Observe recombination lines with the improved spectral resolution and bandwidth of the new EVLA correlator, and characterize line profiles and velocity gradients, and (4) Examine the dynamics of sources with especially broad or multiply peaked line profiles. The EVLA has the spatial and the spectral resolution to properly examine the Broad Line Recombination Objects in the crowded Sgr B2 region. Whether or not flickering is detected, these observations will provide the highest resolution, most sensitive radio image of this prototype massive star forming region, and the reduced data will be made publicly available. The EVLA observations will provide a definitive test of the flickering UC HII region model and will be the first attempt to make time-domain (20 year baseline) observations of a large sample of ultra-compact HII regions. Agnes Scott College, with a 30% African American student body is the only women's college in the Southeastern Association for Research in Astronomy (SARA) consortium, and the Physics and Astronomy Department has a record of success in sending women on to graduate work and careers in the sciences. Dr. De Pree and his group are engaged in the well-established outreach program at Bradley Observatory at Agnes Scott College which is connected to area K12 institutions and hosting over 1500 students each year.
