We present a study of carbon radio recombination lines towards Cassiopeia A using low frequency array (LOFAR) observations in the frequency range 10-33 MHz. Individual carbon α lines are detected... Show moreWe present a study of carbon radio recombination lines towards Cassiopeia A using low frequency array (LOFAR) observations in the frequency range 10-33 MHz. Individual carbon α lines are detected in absorption against the continuum at frequencies as low as 16 MHz. Stacking several Cα lines we obtain detections in the 11-16 MHz range. These are the highest signal-to-noise measurements at these frequencies. The peak optical depth of the Cα lines changes considerably over the 11-33 MHz range with the peak optical depth decreasing from 4 × 10-3 at 33 MHz to 2 × 10-3 at 11 MHz, while the linewidth increases from 20 km s-1 to ˜150 km s-1. The combined change in peak optical depth and linewidth results in a roughly constant integrated optical depth. We interpret this as carbon atoms close to local thermodynamic equilibrium. In this work, we focus on how the 11-33 MHz carbon radio recombination lines can be used to determine the gas physical conditions. We find that the ratio of the carbon radio recombination lines to that of the 158 μm [C II] fine-structure line is a good thermometer, while the ratio between low-frequency carbon radio recombination lines provides a good barometer. By combining the temperature and pressure constraints with those derived from the linewidth, we are able to constrain the gas properties (electron temperature and density) and radiation field intensity. Given the 1σ uncertainties in our measurements these are: Te ≈ 68-98 K, ne ≈ 0.02-0.035 cm-3 and Tr,100 ≈ 1500-1650 K. Despite challenging radio frequency interference and ionospheric conditions, our work demonstrates that observations of carbon radio recombination lines in the 10-33 MHz range can provide insight into the gas conditions. Show less
This thesis focus on the study of the Interstellar Medium (ISM) of the Milky Way and consists of two parts: in the first one we present a study of the dust properties in HII regions and their... Show moreThis thesis focus on the study of the Interstellar Medium (ISM) of the Milky Way and consists of two parts: in the first one we present a study of the dust properties in HII regions and their surrounding PDRs. We focus our studies on two compact HII regions: W3(A) and the Orion Nebula (Chapters 2 and 3, respsectively). Using SOFIA/FORCAST observations, we determine the properties of dust in the ionized gas, their surrounding PhotoDissociation Regions and their parent molecular clouds. One of the most important conclusions of this thesis is that the grain growth can affect the dust size distribution in regions of massive star formation. The large amount of archival data available for Orion allow us to study other important phenomena related to dust in this region, such as the photoelectric heating effect and Lyman alpha heating. The second part of this thesis presents theoretical studies of the properties of the cold neutral medium (CNM) using carbon radio recombination lines (CRRL). Chapters 4 and 5 consists on the complete theory of CRRLs from the level population equation to the radiative transfer equation. Using these models and observations of CRRLs the physical parameters of the CNM can be determined. Show less
