Characterizing the neutral and ionized ISM in a starburst cluster

Sammanfattning

Polycyclic Aromatic Hydrocarbons (PAHs) are organic molecules that are made up of multiple aromatic fused rings of carbon. These types of molecules are abundant in the interstellar medium and in star-forming gas clouds, to name a few. Emissions from these PAH molecules were discovered in the mid-infrared range after absorbing and getting excited by UV radiation from stars. With new instruments like the James Webb Space Telescope's (JWST) Mid-Infrared Instrument (MIRI), these star-forming regions can now be examined in greater detail to learn more about how the PAHs behave in different environments. This research project aimed to characterize the PAH properties across different Extreme Ultraviolet Environments (XUEs) within the NGC 6527 nebula and the three star clusters within it. The star clusters are Pismis 24, G353.2+0.7, and G353.1+0.6, and they are host to some of the most massive O-type stars, which release a large amount of UV radiation. These high-energy photons will affect the PAHs present around the XUE sources, as they absorb them, become ionized, and later emit the energy as mid-IR photons that can be observed. The high-energy photons can also photo-dissociate PAHs, destroying them. More can be learned about the PAHs by examining the specific wavelength bands emitted by the PAHs. PAH emission features, like other kinds of emission and absorption features, only occur at set wavelengths. The intensity of the features depends greatly on the ionization and the size of the PAH molecules. Regions with a lower fraction of PAHs emit more in the 11.2 micron range, while regions with a higher ionized fraction emit in the 6.2 and 7.7 micron range. Larger PAH molecules tend to emit in longer wavelengths because of their higher heat capacity. As a result, a lower 6.2/7.7 band ratio indicates larger molecules, while a lower 11.2/7.7 band ratio indicates a higher ionization. With the use of PAHFIT, the power of these features can be extracted, and then the ratio calculated. To see how diverse or homogeneous a region's PAHs are, a spatially resolved investigation is done by manually picking the edges of the PAH feature, defining a linear continuum baseline, and then taking the integral of the area between the feature and the baseline to get the flux and the ratio. The relative hardness of the regions radiation fields was determined by using the ionic line ratio [NeIII]/[NeII] to see if there is a correlation between hardness of the radiation field had an effect on the PAHs. The band ratios from the integrated spectra were obtained with both PAHFIT and manual clipping, and the results between them were not in agreement, with a few exceptions, like XUE4 being the least ionized and XUE9 being the most ionized region. The spatially resolved results showed that the previous method missed the diversity of each region. Additionally, the hardness of the global radiation field did not seem to show the expected results, where a harder region with more high-energy photons would have more ionized PAHs. All of these things can be explained by shielding effects and geometry, protecting pockets of neutral and smaller PAHs from the high-energy photons.