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Laboratory astrophysics: High resolution IR atomic spectroscopy and radiative lifetimes for astrophysical analysis

Final Report Summary - LAB ASTROPHYSICS (Laboratory astrophysics: High resolution IR atomic spectroscopy and radiative lifetimes for astrophysical analysis)

Astrophysical spectroscopy can be thought of as the science of blended lines. The disentangling of complex astrophysical spectra to reveal important and significant information is crucially dependent upon a detailed and precise knowledge of each molecule, atom and ion present. Infrared (IR) astrophysical spectroscopy is of particular interest to the study of dust obscured objects, such as young stars and the centre of galaxies, the composition of evolved stars and the interstellar medium, objects at high red shifts and, last but not least, cool objects such as brown dwarfs and extra-solar planets. In particular, over the past five years there have been several new ground-based and satellite-borne telescopes with medium to high resolution IR spectrometers. These included international observing platforms such as the Spitzer space telescope, which was a NASA satellite, and the European funded very large telescope (VLT), which was located at the European southern observatory (ESO), Chile. However, the observed infrared stellar and sub-stellar spectra could not be adequately analysed because the IR laboratory database lacked key parameters for non-terrestrial abundant atomic species and molecules.

The work carried out by the researcher Richard Blackwell-Whitehead combined state of the art laboratory atomic spectroscopy and laser physics to provide a vast improvement in our understanding of the IR spectra of stellar and sub-stellar objects. The research utilised wavelengths that were determined in the laboratory, line broadening parameters and branching fractions (BF) in combination with radiative lifetimes to obtain oscillator strengths for atomic transition lines. During his time as a Marie-Curie fellow at the Lund Observatory the researcher undertook research on the following subjects:
1. long radiative lifetimes and oscillator strengths. The relatively low temperature ultra cool dwarf stars and brown dwarf objects have complex atmospheres dominated by neutral atoms and molecules. These dense, low temperature plasma environments do not have enough energy to thermally excite relatively higher energy levels in the atomic species and the spectra is thus dominated by IR transitions from low-lying upper energy levels. These levels have relatively long lifetimes and only a handful have laboratory determined values due to the comparative difficulty in measuring lifetime. The researcher undertook a series of measurements and actively participated in measurement campaigns for neutral calcium (Aldenius et al. 2009), manganese (Blackwell-Whitehead et al. 2011), chromium (Gurrell et al. 2010) and yttrium to determine radiative lifetimes. The radiative lifetimes were also combined with branching fractions determined from intensity calibrated laboratory spectra to yield oscillator strengths.
2. tracer lines for sub-solar object classification. The new laboratory determined oscillator strengths targeted specific atomic tracer lines observed in the IR spectra of sub-solar objects (SSOs), such as ultra cool dwarf stars and brown dwarfs. The researcher worked closely with both observational astronomers and cool star atmospheric modellers in order to identify key atomic transitions that could be used to classify the poorly understood atmospheric parameters of cool stellar and sub-stellar objects. The work was iterative with new, more precise laboratory measurements providing more accurate stellar modelling, which in turn revealed that more accurate laboratory measurements were required for other atomic transition observed in the SSOs.