Radio & Infrared Observations of the W3/W4/W5 Star-forming Regions

Natt, Kiz (2015). Radio & Infrared Observations of the W3/W4/W5 Star-forming Regions. PhD thesis The Open University.



As part of the Low Frequency Array (LOFAR) Surveys KSP, targeted observations of H II regions close to high mass star-forming regions in the Galactic plane have been conducted, as part of the telescope’s commissioning. This thesis details the calibration and analysis of one particular commissioning field, the prominent Galactic star-forming W3/W4/W5 complex and 2 supernova remnants (SNRs) HB3 and 3C 58, at unprecedented low radio frequencies ~ 30–76 MHz (§ 1). This part of the radio spectrum contains large-scale filaments, arcs, bubbles and shells which would be interesting to detail in new parameter space, both spatially and spectrally. A LOFAR calibration pipeline was developed as part of this work (§ 3) resulting in unique continuum maps ~10 deg2 that detail extended diffuse emission of W3/W4/W5 at unprecedented sensitivity and angular resolution at low frequencies. Our final full bandwidth image (at ~ 53.5 MHz) reaches a final resolution of between ~ 62–85" and noise level of 4.78 mJy/beam, making it the deepest image obtained in this frequency range. Spectral index maps (§ 4) made from 35.2 MHz, 54.7 MHz and 69.9 MHz continuum images give strong evidence of radio emission by thermal Bremsstrahlung with some localised regions demonstrating free-free self-absorption (§ 2), although we suspect a complete turnover of the H II regions outside of our observed range < 30 MHz. We obtain a spectral index of ɑ ~ -0.03±0.02 for 3C 58 and observe a possible plateau in the emission of HB3 near ~ 55 MHz. High spectral resolution data was also analysed for the possibility of a carbon RRL detection in absorption < 100 MHz in the direction of W3 (§ 5). An initial all-sky image was also made during the commissioning of the Chilbolton LOFAR single station (§ 3); the wide FoV gave an overview of the large-scale diffuse Galactic synchrotron emissivity with the intention of measuring the spectral index < 100 MHz.

High mass star formation is an important area of research in modern astrophysics and triggers of this process along with many of the aspects of early stellar evolution including the onset of maser emission being poorly understood. Stars form out of large gas and dust clouds, detected through: (i) thermal dust continuum emission and (ii) line emission of organic molecules traced through masers. Hence, ancillary observations of W3 were made using the Onsala 20m radio viii telescope for maser source detection and a point source catalogue of ~ 1300 sources at far-IR wavelengths of predominantly cold proto-stars was made from the newest AKARI maps (§ 6).

Finally, in addition to the Galactic diffuse radio emission, ~ 300 extragalactic point sources were captured in the background of the FoV with LOFAR. Their extracted fluxes were catalogued and over ~ 50 % provided a rich sample of spectral index information to constrain turnover of synchrotron < 100 MHz. We observed strong and moderate SED flattening attributed to free-free absorption processes at the source for ~ 3.2 % and ~ 19.1 % of sources respectively which is consistent with Rafferty et al. (2013).

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