A Submillimetre Study of Nearby Star Formation using Molecular Line Data
Drabek-Maunder, Emily R.
Thesis or dissertation
University of Exeter
This thesis primarily uses submillimetre molecular line data from HARP, a heterodyne array on the James Clerk Maxwell Telescope (JCMT), to further investigate star formation in the Ophiuchus L1688 cloud. HARP was used to observe CO J = 3-2 isotopologues: 12CO, 13CO and C18O; and the dense gas tracer HCO+ J = 4-3. A method for calculating molecular line contamination in the SCUBA-2 450 and 850 μm dust continuum data was developed, which can be used to convert 12CO J =6-5and J =3-2 maps of integrated intensity (K km s−1) to molecular line flux (mJy beam−1) contaminating the continuum emission. Using HARP maps of 12CO J = 3-2, I quantified the amount of molecular line contamination found in the SCUBA-2 850 μm maps of three different regions, including NGC 1333 of Perseus and NGC 2071 and NGC 2024 of Orion B. Regions with ‘significant’ (i.e. > 20%) molecular line contamination correspond to molecular outflows. This method is now being used to remove molecular line contamination from regions with both SCUBA-2 dust continuum and HARP 12CO map coverage in the Gould Belt Legacy Survey (GBS). The Ophiuchus L1688 cloud was observed in all three CO J = 3-2 isotopologues. I carried out a molecular outflow analysis in the region on a list of 30 sources from the Spitzer ‘c2d’ survey [Evans et al., 2009]. Out of the 30 sources, 8 had confirmed bipolar outflows, 20 sources had ‘confused’ outflow detections and 2 sources did not have outflow detections. The Ophiuchus cloud was found to be gravitationally bound with the turbulent kinetic energy a factor of 7 lower than the gravitational binding energy. The high-velocity outflowing gas was found to be only 21% of the turbulence in the cloud, suggesting outflows are significant but not the dominant source of turbulence in the region. Other factors were found to influence the global high-velocity outflowing gas in addition to molecular outflows, including hot dust from nearby B-type stars, outflow remnants from less embedded sources and stellar winds from the Upper Scorpius OB association. To trace high density gas in the Ophiuchus L1688 cloud, HCO+ J = 4-3 was observed to further investigate the relationship between high column density and high density in the molecular cloud. Non-LTE codes RADEX and TORUS were used to develop density models corresponding to the HCO+ emission. The models involved both constant density and peaked density profiles. RADEX [van der Tak et al., 2007] models used a constant density model along the line-of-sight and indicated the HCO+ traced densities that were predominantly subthermally excited with den- sities ranging from 10^3–10^5 cm^−3. Line-of-sight estimates ranged from several parsecs to 90 pc, which was unrealistic for the Ophiuchus cloud. This lead to the implementation of peaked density profiles using the TORUS non-LTE radiative transfer code. Initial models used a ‘triangle’ density profile and a more complicated log-normal density probability density function (PDF) profile was subsequently implemented. Peaked density models were relatively successful at fitting the HCO+ data. Triangle models had density fits ranging from 0.2–2.0×10^6 cm^−3 and 0.1–0.3×10^6 cm^−3 for the 0.2 and 0.3 pc cloud length models re- spectively. Log-normal density models with constant-σ had peak density ranges from 0.2–1.0 ×10^5 cm^−3 and 0.6–2.0×10^5 cm^−3 for 0.2 and 0.3 pc models respectively. Similarly, log-normal models with varying-σ had lower and upper density limits corresponding to the range of FWHM velocities. Densities (lower and upper limits) ranged from 0.1–1.0 ×10^6 and 0.5–3.0 ×10^5 cm^-3 for the 0.2 and 0.3 pc models respectively. The result of the HCO+ density modelling indicated the distributions of starless, prestellar and protostellar cores do not have a preference for higher densities with respect to the rest of the cloud. This is contrary to past research suggesting the probability of finding a submillimetre core steeply rises as a function of column density (i.e. density; Belloche et al. 2011; Hatchell et al. 2005). Since the majority of sources are less embedded (i.e Class II/III), it is possible the evolutionary state of Ophiuchus is the main reason the small sample of Class 0/I protostars do not appear to have a preference for higher densities in the cloud.
University of Exeter studentship
E. Drabek, J. Hatchell, P. Friberg, J. Richer, S. Graves, J. V. Buckle, D. Nutter, D. Johnstone, and J. Di Francesco. Molecular line contamination in the SCUBA- 2 450 and 850 μm continuum data. MNRAS, 426:23–39, October 2012. doi: 10.1111/j.1365-2966.2012.21140.x. 33, 69, 71, 250
J. Hatchell, T. Wilson, E. Drabek, E. Curtis, J. Richer, D. Nutter, J. Di Francesco, D. Ward-Thompson, and JCMT GBS Consortium. The JCMT Gould Belt Survey: SCUBA-2 observations of radiative feedback in NGC 1333. MNRAS, 429:L10–L14, February 2013. doi: 10.1093/mnrasl/sls015. 33, 70, 71, 72, 75, 250, 251
S. I. Sadavoy, J. Di Francesco, D. Johnstone, M. J. Currie, E. Drabek, J. Hatchell, D. Nutter, P. Andr ́e, D. Arzoumanian, M. Benedettini, J.-P. Bernard, A. Duarte- Cabral, C. Fallscheer, R. Friesen, J. Greaves, M. Hennemann, T. Hill, T. Jen- ness, V. Ko ̈nyves, B. Matthews, J. C. Mottram, S. Pezzuto, A. Roy, K. Rygl, N. Schneider-Bontemps, L. Spinoglio, L. Testi, N. Tothill, D. Ward-Thompson, G. White, t. JCMT, and Herschel Gould Belt Survey Teams. The Herschel and JCMT Gould Belt Surveys: Constraining Dust Properties in the Perseus B1 Clump with PACS, SPIRE, and SCUBA-2. ApJ, 767:126, April 2013. doi: 10.1088/0004-637X/767/2/126. 33, 70, 71, 72, 73, 75, 251
PhD in Physics