![]() ![]() With masses of more than 20 times that of the Sun and surface temperatures above 90 000 degrees, these stars are truly extreme. In three nebulae, they succeeded in identifying the sources of energetic radiation and to eludicate their exceptional properties: some of the hottest, most massive stars ever seen, some of which are double. In a dedicated search for the origin of their individual characteristics, a team of astronomers - mostly from the Institute of Astrophysics & Geophysics in Liège (Belgium) - have secured the first detailed, highly revealing images of four highly ionized nebulae in the Magellanic Clouds, two small satellite galaxies of our home galaxy, the Milky Way, only a few hundred thousand light-years away. But what are the sources of that excitation? Could it be energetic stars or perhaps some kind of exotic objects inside these nebulae? How do these peculiar objects fit into the current picture of universal evolution? New observations of a number of such unusual nebulae have recently been obtained with the Very Large Telescope (VLT) at the ESO Paranal Observatory (Chile). Depending on the atoms involved and the number of electrons lost, this process bears witness to the strength of the radiation or to the impact of energetic particles. This is the case of a small number of unusual nebulae that appear to be the subject of strong heating - in astronomical terminology, they present an amazingly "high degree of excitation." This is because they contain significant amounts of ions, i.e., atoms that have lost one or more of their electrons. Even though most of the nebulae of gas and dust in our vicinity are now rather well understood, there are some which continue to puzzle astronomers. The derived gradient of density suggests that NGC 7023 has a clumpy structure, including a high clump density of ~10^5 cm^-3 with a size smaller than ~5 x 10^-3 pc embedded in lower density regions of 10^3-10^4 cm^-3.Quite a few of the most beautiful objects in the Universe are still shrouded in mystery. To investigate the density distribution, we combine pixels in 1" x 1" areas and derive the density distribution at the 0.002 pc scale. The notable difference between PDR model predictions and the observed data, in high rotational J levels of v = 1, is that the predicted formation temperature for newly-formed H2 should be lower than that of the model predictions. In addition, we derive the column density of H2 from the observed emission lines and compare these results with PDR models in the literature covering a range of densities and incident UV field intensities. Gradients in the temperature, velocity, and OPR within the observed area imply motion of the photodissociation region (PDR) relative to the molecular cloud. In addition, the estimated ortho-to-para ratios (OPR) is 1.63-1.82, indicating that the H2 emission transitions in the observed region arises mostly from gas excited by UV fluorescence. By observing NGC 7023 in the H and K bands at R ~ 45,000 with the Immersion GRating INfrared Spectrograph (IGRINS), we detected 68 H2 emission lines within the 1" x 15" slit. Le and 19 other authors Download PDF Abstract:We have analyzed the temperature, velocity and density of H2 gas in NGC 7023 with a high-resolution near-infrared spectrum of the northwestern filament of the reflection nebula. ![]() Download a PDF of the paper titled Fluorescent H_2 Emission Lines from the Reflection Nebula NGC 7023 Observed with IGRINS, by Huynh Anh N. ![]()
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