This thesis presents molecular line and radio continuum observations of the giant molecular cloud (GMC) complex known as G305. The energy input from high-mass stars in the form of powerful winds and ionising radiation is one of the primary feedback mechanisms in GMCs. This feedback is thought to play a dual role both dispersing and destroying the natal environment but also sweeping up and compressing molecular gas and potentially triggering new episodes of star formation. Despite their importance to the evolution of GMCs and galaxies as a whole, the physical processes behind the formation and evolution of high-mass stars remains poorly understood. We therefore set out to obtain wide-field observations of the ionised and molecular environment to study the impact of high-mass stars on the evolution of G305. Observations conducted with the Mopra telescope of the molecular gas traced by NH3 in the (1,1), (2,2) and (3,3) transition and CO (12CO, 13CO and C18O J = 1–0) reveals the reservoir for future star formation in G305 and allows the physical properties and kinematics of the region to be studied. We identify 15 large molecular clouds and 57 smaller molecular clumps towards G305. The physical properties of the molecular gas are consistent with G305 being amongst the most massive a vigorous star forming regions in the Galaxy. We find a total molecular gas mass of 2:5–6:5 105M indicating that there is a large reservoir for future star formation. By considering virial equilibrium within the molecular clumps we discover that only 14% of the molecular clumps in G305 are gravitationally unstable, however these clumps contain > 30% of the molecular mass in G305 suggesting there is scope for considerable future star formation. To study the ionised environment towards G305 we have obtained some of the largest and most detailed wide-area mosaics with the Australia Telescope Compact Array to date. These radio continuum observations were performed simultaneously at 5.5 and 8.8 GHz and by applying two imaging techniques we are able to resolve HII regions from the ultra-compact to classical evolutionary phase. This has allowed high-mass star formation within G305 to be traced over the extent and lifetime of the complex. We discover that more than half of the observable total ionising flux in G305 is associated with embedded high-mass star formation around the periphery of a central cavity that has been driven into the molecular gas by a cluster of optically visible massive stars. By considering the contribution of embedded and visible massive stars to the observed radio continuum we suggest that more than 45 massive stars exist within G305. Combination of these two studies and recent and ongoing star formation provides the most in depth view of G305 to date and allows the star formation history and impact of high-mass stars to be investigated. We find compelling morphological evidence that suggests triggering is responsible for at least some of the observed high-mass star formation and construct a star formation history for the region.

Made available in DSpace on 2016-03-15T19:37:53Z (GMT). No. of bitstreams: 1 Marcio Ribeiro Gastaldi.pdf: 1118514 bytes, checksum: 0da632ad9d5d56b0c1ab1df75e83eee3 (MD5) Previous issue date: 2007-02-02 / We present the results of a 2-year project to monitor the spectral and temporal variability of Sagittarius A, Centaurus A, OV236 and the Einstein s Ring PKS1830-211 from 2004 to 2006. The observational data were obtained from the Itapetinga Radio Telescope at 22 and 43GHz. The data at 4,8, 8,0 and 14,5GHz for OV236 and PKS1830-211 were taken from the Michigan Radio Observatory. We also obtained previous observations of OV236 and PKS1830-211 at 100 and 230GHz from the Swedish-ESO Sub Millimeter Telescope (SEST), and VLA 23 and 43GHz data for Sgr A*. We estimated parameters such as period, angular size and brightness temperature and applied the Shock Wave Model (MARSHER; GEAR 1985) for OV236. The work we present here results from an international cooperation among Brazil (Itapetinga at 22 and 43GHz), EUA (Michigan at 4,8, 8,0 and 14,5GHz, and the VLA at 23 and 43GHz) and Finland (Metsähovi at 90GHz). / Este trabalho mostra o estudo da variabilidade temporal e espectral de Sagittarius A, Centaurus A, OV236 e do anel de Einstein PKS1830-211 em campanhas de observação mensais realizadas de janeiro de 2004 a outubro de 2006. Os dados observacionais foram obtidos através do Rádio-Observatório do Itapetinga nas freqüências de 22 e 43GHz. Os dados em 4,8, 8,0 e 14,5GHz para OV236 e PKS1830-211 foram obtidos do Rádio-Observatório de Michigan. Também foram utilizados dados do SEST para OV236 em 100 e 230GHz, e dados do VLA em 23 e 43GHz para Sgr A*. Foram feitas estimativas de período, variação angular e temperatura de brilhância. O modelo de Marscher e Gear (1985) foi aplicado a OV236. Este é um trabalho de cooperação internacional entre Brasil (Itapetinga em 22 e 43GHz), Estados Unidos (Michigan em 4,8, 8,0 e 14,5 GHz e VLA em 23 e 43GHz) e Finlândia (Metsähovi em 90GHz).

It has recently been discovered that some, if not all, classical novae emit GeV gamma-rays during outburst, but the mechanisms involved in the production ofgamma-rays are still not well understood. We present here a comprehensive multiwavelength data set-from radio to X-rays-for the most gamma-ray-luminous classical nova to date, V1324 Sco. Using this data set, we show that V1324 Sco is a canonical dusty Fe II-type nova, with a maximum ejecta velocity of 2600 km s(-1) and an ejecta mass of a few x 10(-5) M-circle dot. There is also evidence for complex shock interactions, including a double-peaked radio light curve which shows high brightness temperatures at early times. To explore why V1324. Sco was so gamma-ray luminous, we present a model of the nova ejecta featuring strong internal shocks and find that higher gamma-ray luminosities result from higher ejecta velocities and/or mass-loss rates. Comparison of V1324. Sco with other gamma-ray-detected novae does not show clear signatures of either, and we conclude that a larger sample of similarly well-observed novae is needed to understand the origin and variation of gamma-rays in novae.

Aims. The aim of this work is to constrain properties of the disk around the archetype FU Orionis object, FU Ori, with as good as similar to 25 au resolution. Methods. We resolved FU Ori at 29-37 GHz using the Karl G. Jansky Very Large Array (JVLA) in the A-array configuration, which provided the highest possible angular resolution to date at this frequency band (similar to 0 ''.07). We also performed complementary JVLA 8-10 GHz observations, Submillimeter Array (SMA) 224 GHz and 272 GHz observations, and compared these with archival Atacama Large Millimeter Array (ALMA) 346 GHz observations to obtain the spectral energy distributions (SEDs). Results. Our 8-10 GHz observations do not find evidence for the presence of thermal radio jets, and constrain the radio jet/wind flux to at least 90 times lower than the expected value from the previously reported bolometric luminosity-radio luminosity correlation. The emission at frequencies higher than 29 GHz may be dominated by the two spatially unresolved sources, which are located immediately around FU Ori and its companion FU Ori S, respectively. Their deconvolved radii at 33 GHz are only a few au, which is two orders of magnitude smaller in linear scale than the gaseous disk revealed by the previous Subaru-HiCIAO 1.6 mu m coronagraphic polarization imaging observations. We are struck by the fact that these two spatially compact sources contribute to over 50% of the observed fluxes at 224 GHz, 272 GHz, and 346 GHz. The 8-346 GHz SEDs of FU Ori and FU Ori S cannot be fit by constant spectral indices (over frequency), although we cannot rule out that it is due to the time variability of their (sub)millimeter fluxes. Conclusions. The more sophisticated models for SEDs considering the details of the observed spectral indices in the millimeter bands suggest that the >29 GHz emission is contributed by a combination of free-free emission from ionized gas and thermal emission from optically thick and optically thin dust components. We hypothesize that dust in the innermost parts of the disks (less than or similar to 0.1 au) has been sublimated, and thus the disks are no longer well shielded against the ionizing photons. The estimated overall gas and dust mass based on SED modeling, can be as high as a fraction of a solar mass, which is adequate for developing disk gravitational instability. Our present explanation for the observational data is that the massive inflow of gas and dust due to disk gravitational instability or interaction with a companion/intruder, was piled up at the few-au scale due to the development of a deadzone with negligible ionization. The piled up material subsequently triggered the thermal instability and the magnetorotational instability when the ionization fraction in the inner sub-au scale region exceeded a threshold value, leading to the high protostellar accretion rate.

Our sensitive (sigma(n) approximate to 572 nJy beam(-1)), high-resolution (FWHM theta(1/2) = 0"22 approximate to 2 kpc at z greater than or similar to 1), 10 GHz image covering a single Karl G.. Jansky Very Large Array (VLA) primary beam (FWHM circle minus(1/2) approximate to 4.'25) in the GOODS-N field contains 32 sources with S-p greater than or similar to 2 mu Jy beam(-1) and optical and/or near-infrared (OIR) counterparts. Most are about as large as the star-forming regions that power them. Their median FWHM major axis is <theta(M)>= 167 +/- 32 mas approximate to 1.2 +/- 0.28 kpc, with rms scatter approximate to 91 mas approximate to 0.79 kpc. In units of the effective radius re that encloses half their flux, these radio sizes are re approximate to 69 +/- 13 mas approximate to 509 +/- 114 pc, with rms scatter approximate to 38 mas approximate to 324 pc. These sizes are smaller than those measured at lower radio frequencies, but agree with dust emission sizes measured at mm/sub-mm wavelengths and extinction-corrected H alpha sizes. We made a lowresolution (theta(1/2) = 1."0) image with approximate to 10x better brightness sensitivity, in order to detect extended sources and measure matched-resolution spectral indices alpha(10 GHz)(1.4 GHz) 10 GHz. It contains six new sources with Sp. 3.9 mJy beam-1 and OIR counterparts. The median redshift of all 38 sources is similar to z similar to = 1.24 +/- 0.15. The 19 sources with 1.4 GHz counterparts have a median spectral index of <alpha(1.4 GHz) (10 GHz)> = -0.74 +/- 0.10 10 GHz, with rms scatter approximate to 0.35. Including upper limits on a for sources not detected at 1.4 GHz flattens the median to <alpha(1.4 GHz) (10 GHz)> greater than or similar to -0.61 10 GHz, suggesting that the mu Jy radio sources at higher redshifts-and hence those selected at higher rest-frame frequencies-may have flatter spectra. If the non-thermal spectral index is alpha(NT) approximate to -0.85, the median thermal fraction of sources selected at median rest-frame frequency approximate to 20 GHz is greater than or similar to 48%.

To eliminate uncertainties introduced by extinction by dust in the optical, we examine to what extent the radio continuum (RC) can probe star formation in dwarf galaxies. Star formation (SF) drives galaxy formation and evolution; acquiring accurate measurements of SF thus becomes crucial in order to understand galaxies. As radio technology improves further, RC surveys will probe the fainter, more quiescent regime of the radio sky. Having a robust manner by which to convert RC luminosities to star formation rate (SFR) has the potential to provide millions of independent SFR measurements out to intermediate redshifts. In order to calibrate the RC to infer SFR, the 40 dwarf galaxies that make up the LITTLE THINGS sample were chosen as the bedrock of the thesis due to the large range of galactic parameters that they cover. RC observations of these galaxies were taken with the VLA between L- and Ka-band (1-33GHz) using the B-, C-, and Darrays, yielding images with 3-10" resolution and rms noise levels between 3 and 15 μJy beam⁻¹. On a global scale, 27 out of the 40 dwarf galaxies exhibited RC emission above the detection threshold, 17 of which were new RC detections. The general picture is an interstellar medium (ISM) largely void of RC emission, interspersed by isolated pockets of RC associated with SF regions; this general picture agreed with what was expected given current models of dwarf galaxies - weaker magnetic fields in the ISM leading to a higher escape of CRe (and resulting reduction in RCNTh emission). This was also backed-up by the relatively low RCNTh fraction - 61 ± 7% at C-band. The observed RC-SFR relation was calibrated to allow the observed RC luminosity of a gas rich dwarf galaxy to be used to infer the SFR; the calibration takes the form SFR [M⊙ yr⁻¹] = 5 x 10⁻¹⁸(RC [WHz⁻¹])0.85. On a resolved basis, only the RCNTh was examined - this is because whether scales of 1 pc, or 1 kpc are investigated, the relationship between the Hα (current SF) and RCTh was not expected to change. Calibrating the resolved RCNTh-SFR relation was best done when using discrete SF regions which varied from 10s up to 100s of pc in size. On these scales, the calibration allows the SFR to be inferred from an observed RCNTh luminosity, and takes the form SFR [M⊙ yr⁻¹] = 1.36 x 10⁻²³(LNTh [WHz⁻¹])1.15. This calibration, however, is only valid for resolved regions forming stars at a rate ≳ 2 x 10⁻⁴M⊙ yr⁻¹. Despite the low flux densities of RCNTh measured from these discrete SF regions, the RCNTh still works well as a SFR tracer whereas Hα, which is largely dependent on stars with mass ≳ 18M⊙, and is thus dependent on the high mass tail of the stellar IMF, will suffer from stochasticity. In a few dwarfs, the equipartition magnetic field strength reaches as high as 30 μG in multiple 100 pc regions, and in one case, 70 μG. However, generally, the weaker magnetic fields in the ISM give the CRe longer lifetimes, and thus more time to be advected out of the galaxy with the magnetic fields frozen into the gas in outflows, or diffuse. This explains in part the lack of RCNTh emission observed in the ISM of dwarf galaxies. Through implementing a simple galactic CRemodel, itwas found that the RCNTh emission associated with the CRe can be used as a SF tracer from approximately 5 up to 70Myr following a burst of SF, while RCTh can be used in its absence prior to 5Myr. The RCNTh luminosity reaches its peak approximately 55Myr after the SF episode, but actually remains nearly constant over the 60Myr following the SF episode, highlighting its potential to be used to infer SFR. The CRemodel also tracked the evolution of the RCNTh spectral index with time. Between values of about -0.4 and -0.7, the RCNTh spectral index can be calibrated to infer the time elapsed since a burst of SF through t[Myr] = -25αNTh. RCNTh spectral indices of -0.8 are consistent with ages between 20 and 55Myr, suggesting that the oft observed spectral index of -0.8 in galaxies may come from the fact that C-band RCNTh emission is dominated by the steep spectral indices of -0.8 from these older SF regions (20-55Myr). For the galaxies that displayed RCNTh emission that was bright enough and sufficiently well resolved, a spectral decomposition of the RC spectrum was performed to infer Hα-independent RCTh, RCNTh, and RCNTh spectral index maps. The spectral decomposition showed DDO50 and NGC1569 to have a low thermal fraction of 23% and 10%, respectively, at C-band, while NGC2366 and NGC4214 were shown to have higher thermal fractions of 48% and 66%, respectively. In summary, dwarf galaxies are not only faint in the RC due to their lower SF activity, but they are also fainter than expected due to CRe escape. Nonetheless, the RC can be used to probe SF in dwarf galaxies not only on a global scale, but also within discrete SF complexes 10s to 100s of pc in size. Theoretically, the RC can be used right from the onset of a burst of SF, where RCTh will dominate, up to ~ 70Myr, at which point RCNTh will dominate. Calibrated by the RC observations in this thesis, both resolved and global SFRs of gas rich, low mass galaxies can be inferred with an uncertainty of ±0.2 dex; the relations allow SFRs of between approximately 2x10⁻⁴ and 0.1M⊙ yr⁻¹ to be inferred.

The present International Celestial Reference Frame (ICRF), the ICRF-3 is based on a catalogue of 4536 quasar positions obtained from Very Long Baseline Interferometry (VLBI) radio measurements. This radio frame is crucial for many applications, from measurements of Earth’s orientation in space to spacecraft navigation and measurements of sea-level rise. However, the deficit in ICRF source density in the South and lack of dedicated imaging campaigns in the South, to monitor structural changes, remain a big concern. These ICRF sources can exhibit spatially extended emission structures that can have a significant effect on astrometric VLBI measurements. The Celestial Reference Frame Deep South (CRDS) is a dedicated astrometric VLBI programme to observe Southern ICRF sources on a regular basis. In an effort to improve the situation in the South, these CRDS sessions have recently been optimized for VLBI imaging. In this thesis, I present VLBI images and source structure analysis results for southern ICRF sources observed in four of these CRDS sessions. For some of these sources, I present the very first high-resolution radio images. I also present results from source structure analysis and a corresponding assessment of astrometric quality, and I also present results from efforts to increase the ICRF source density in the South. / Mathematical Sciences / Ph. D. (Astronomy)

In an effort to understand unusual power-law spectral slopes observed in some hypercompact H II regions, we consider the radio continuum energy distribution from an ensemble of spherical clumps. An analytic expression for the free-free emission from a single spherical clump is derived. The radio continuum slope (with Fv ∝ vα) is governed by the population of clump optical depths N(τ) such that (1) at frequencies where all clumps are thick, a continuum slope of +2 is found, (2) at frequencies where all clumps are optically thin, a flattened slope of -0.11 is found, and (3) at intermediate frequencies, a power-law segment of significant bandwidth with slopes between these two limiting values can result. For the ensemble distribution, we adopt a power-law distribution N(T) ∝-γ and find that significant power-law segments in the spectral energy distribution with slopes from +2 to -0.11 result only for a relatively restricted range of 7 values from 1 to 2. Furthermore, a greater range of clump optical depths for this distribution leads to a wider bandwidth over which the intermediate power-law segment exists. The model is applied to the source W49N B2 with an observed slope of α ≈ +0.9, but that may be turning over to become optically thin around 2 mm. An adequate fit is found in which most clumps are optically thin and there is little "shadowing" of rearward clumps by foreground clumps (i.e., the geometrical covering factor C ≪ 1). The primary insight gained from our study is that in the Rayleigh-Jeans limit for the Planck function that applies for the radio band, it is the distribution in optical depth of the clump population that is solely responsible for setting the continuum shape, with variations in the size and temperature of clumps serving to modulate the level of free-free emission.

The structured winds of single massive stars can be classified into two broad groups: stochastic structure and organized structure. While the former is typically identified with clumping, the latter is typically associated with rotational modulations, particularly the paradigm of corotating interaction regions (CIRs). While CIRs have been explored extensively in the ultraviolet band, and moderately in the X-ray and optical, here we evaluate radio variability from CIR structures assuming free-free opacity in a dense wind. Our goal is to conduct a broad parameter study to assess the observational feasibility, and to this end, we adopt a phenomenological model for a CIR that threads an otherwise spherical wind. We find that under reasonable assumptions, it is possible to obtain radio variability at the 10 per cent level. The detailed structure of the folded light curve depends not only on the curvature of the CIR, the density contrast of the CIR relative to the wind, and viewing inclination, but also on wavelength. Comparing light curves at different wavelengths, we find that the amplitude can change, that there can be phase shifts in the waveform, and the entire waveform itself can change. These characterstics could be exploited to detect the presence of CIRs in dense, hot winds.

To this date ψ Per is the only classical Be star that was angularly resolved in radio (by the VLA at λ = 2 cm). Gaussian fit to the azimuthally averaged visibility data indicates a disk size (FWHM) of ~500 stellar radii (Dougherty & Taylor 1992). Recently, we obtained new multi-band cm flux density measurements of ψ Per from the enhanced VLA. We modeled the observed spectral energy distribution (SED) covering the interval from ultraviolet to radio using the Monte Carlo radiative transfer code HDUST (Carciofi & Bjorkman 2006). An SED turndown, that occurs between far-IR and radio wavelengths, is explained by a truncated viscous decretion disk (VDD), although the shallow slope of the radio SED suggests that the disk is not simply cut off, as is assumed in our model. The best-fit size of a truncated disk derived from the modeling of the radio SED is 100+5 −15 stellar radii, which is in striking contrast with the result of Dougherty & Taylor (1992). The reasons for this discrepancy are under investigation.