Sarah Tucker Abstract
Sarah Tucker, Finding Extended Red Emission in the Horsehead Nebula (Adolf Witt)
Extended Red Emission (ERE) is a generic term for the energy that the smallest particles in interstellar space with low-lying electronic states give off when they release vibrational energy as red photons. While thought to have been a unique phenomenon when first observed in 1975 in the Red Rectangle nebula, ERE has since been found throughout our galaxy in reflection nebulae and emission nebulae alike as well as in diffuse interstellar medium. Of special interest are cases where ERE is found on the surfaces of dark nebulae that are illuminated by nearby hot O/B stars, known as photo-dissociation regions (PDR). Investigating a dark nebula in the Orion Constellation, the Horsehead nebula, I was able to confirm the occurrence of ERE on the surface of this dark nebula that is illuminated by the close-by star Sigma Orionis. By using an image taken with a red filter (620 nm – 700 nm) and a narrow-band H-alpha filter (FWHM = 5 nm), I was able to create a program in Python that aligned the two images, scaled the H-alpha image to the desired ratio, and then subtracted the H-alpha emission present in the red-filter image. The resulting image was expected to retain structures due to ERE as well as a dust-scattered light continuum. To eliminate confusion due to the dust-scattered light, I divided the difference image by an image taken in the nearby continuum around 525 nm (green), with the result revealing the location and morphology of the ERE in the Horsehead Nebula PDR. Using the imaging software SAO/ds9 I was able to analyze the final image further and create contour maps that outline the position of the ERE relative to the emission from ionized polycyclic aromatic hydrocarbons (PAH) within the dust cloud, obtained from maps produced by the Spitzer Infrared Space Telescope, as well as the H-alpha emission surrounding the Horsehead. The resulting image shows that the ERE present in the Horsehead nebula rests in a narrow zone between the ionized atomic hydrogen and the neutral molecular hydrogen within the dust cloud. This leads to the conclusion that the excitation of the ERE process requires photons with less than 13.6 eV, the ionization potential of atomic hydrogen, but more than 7 – 8 eV, the energy required to ionize PAH molecules. This identifies the most likely process involved in producing the ERE as recurrent fluorescence by isolated particles consisting of about 20 to 35 atoms, most likely carbon, heated by far-ultraviolet photons from a nearby O/B star. This research will allow the James Webb Space Telescope to further examine the ERE in the Horsehead nebula in greater detail.