Silicon monoxide (SiO) maser emission has been observed towards many late-type stars. The conditions necessary for the formation of SiO masers dictate that they be produced in a region which is inside the silicate dust formation point close to the surface of the star. Very Long Baseline Interferometry (VLBI) has shown that these masers do indeed lie close to the stellar surface at a distance of a few stellar radii. This extended atmosphere is a complex region dominated by stellar pulsations and permeated by circumstellar shocks. This dissertation presents the results of a multi-epoch VLBI study of the v=1, J=1-0, 43-GHz SiO maser emission towards the symbiotic binary R Aquarii. Four epochs of full-polarization observations were recorded using the Very Long Baseline Array a facility the National Radio Astronomy Observatory. The first high-resolution images of the extended atmosphere of a Mira variable in a symbiotic binary have been produced. The SiO masers towards R Aquarii have been found to exhibit a ring-like morphology ~31 mas (~6.8 AU) in diameter. The emission changes significantly over a time period of ~1-2 months with almost no similarity in structure on timescales >6 months. An analysis of the four epochs of observations has provided the first direct evidence of SiO maser proper motions. These observations, taken as the Mira variable approached maximum light, show that over a 98-day period the masers have an average inward proper motion of ~1 mas. This contraction of the maser shell implies an infall velocity of ~4 km/s for the SiO masers during this phase of the stellar pulsation cycle. In addition to the total intensity images, maps of the linear and circular polarization morphology were also produced. These images show that the SiO masers are significantly polarized, and that the polarization structure and intensity change on timescales as short as ~1-2 months. For three of the four epochs, a mean fractional circular polarization of 4% was determined implying a magnetic field strength Bsec(theta) = 13 G. For one of the four epochs, the mean fractional circular polarization was found to be ~14% indicating a magnetic field strength Bsec(theta) = 46 G. The fractional linear polarization is fairly constant for all four epochs with mean values ranging from 20.8-25.0%, and peak values as high as ~83% for isolated maser features. Maps of the linear polarization vectors show an orderly structure over large portions of the maser shell indicating a uniform magnetic field topology in these regions of the extended atmosphere of R Aquarii. / Ph. D.

The work conducted in this thesis examines the nature of circumstellar discs by investigating irradiance and polarization of scattered light. Two circumstellar discs are investigated. Firstly, H-band high contrast imaging data on the transitional disc of the Herbig Ae/Be star HD169142 are presented. The images were obtained through the polarimetric differential imaging (PDI) technique on the Very Large Telescope (VLT) using the adaptive optics system NACO. Our observations use longer exposure times, allowing us to examine the edges of the disc. Analysis of the observations shows distinct signs of polarization due to circumstellar material, but due to excessive saturation and adaptive optics errors further information on the disc could not be inferred. The HD169142 disc is then modelled using the 3D radiative transfer code Hyperion. Initial models were constructed using a two disc structure, however recent PDI has shown the existence of an annular gap. In addition to this, the annular gap is found not to be devoid of dust. This then led to the construction of a four-component disc structure. Estimates of the mass of dust in the gap (2:10x10⁻⁶ M⊙) are made as well as for the planet (≈1:53 x 10⁻⁵ M⊙ (0.016MJupiter)) suspected to be responsible for causing the gap. The predicted polarization was also estimated for the disc, peaking at 14 percent. The use of realistic dust grains (ballistic aggregate particles) in Monte Carlo code is also examined. The fortran code DDSCAT is used to calculate the scattering properties for aggregates which are used to replace the spherical grain models used by the radiative transfer code Hyperion. Currently, Hyperion uses four independent elements to de ne the scattering matrix, therefore the use of rotational averaging and a 50/50 percent population of grains and their enantiomers were explored to reduce the number of contributing scattering elements from DDSCAT. A python script was created to extract the scattering data from the DDSCAT output les and to apply a size distribution to the data. The second circumstellar disc investigated is the debris disc of the M dwarf star AU Mic. The disc was modelled, using the radiative transfer code Hyperion, based on F606W (HST) and JHK0-band (Keck II) scattered light observations and F606Wband polarized light observations. Initially, the disc is modelled as a two component structure using two grain types: compact silicate grains and porous dirty ice water. Both models are able to reproduce the observed SED and the F606W and H-band surface brightness pro les, but are unable to t the observed F606W degree of polarization. Therefore, a more complex/realistic grain model was examined (ballistic aggregate particles). In addition, recent millimetre observations suggest the existence of a planetesimal belt < 3 AU from the central star. This belt is included in the BAM2 model and was successful in fitting the observed SED, F606W and H-band surface brightness and F606W polarization. These results demonstrate the limitations of spherical grain models and indicate the importance of modelling more realistic dust grains.

The Galactic transient V1309 Sco was the result of a merger in a low-mass star system, while V838 Mon was thought to be a similar merger event from a more massive B-type progenitor. In this paper, we study a recent optical and infrared (IR) transient discovered in the nearby galaxy NGC4490 named NGC4490-OT2011 (NGC 4490-OT hereafter), which appeared similar to these merger events (unobscured progenitor, irregular multi-peaked light curve, increasingly red colour, similar optical spectrum, IR excess at late times), but which had a higher peak luminosity and longer duration in outburst. NGC4490-OT has less in common with the class of SN 2008S-like transients. A progenitor detected in pre-eruption Hubble Space Telescope (HST) images, combined with upper limits in the IR, requires a luminous and blue progenitor that has faded in late-time HST images. The same source was detected by Spitzer and ground-based data as a luminous IR (2-5 mu m) transient, indicating a transition to a self-obscured state qualitatively similar to the evolution seen in other stellar mergers and in luminous blue variables. The post-outburst dust-obscured source is too luminous and too warm at late times to be explained with an IR echo, suggesting that the object survived the event. The luminosity of the enshrouded IR source is similar to that of the progenitor. Compared to proposed merger events, the more massive progenitor of NGC4490-OT seems to extend a correlation between stellar mass and peak luminosity, and may suggest that both of these correlate with duration. We show that spectra of NGC4490-OT and V838 Mon also resemble light-echo spectra of eta Car, prompting us to speculate that eta Car may be an extreme extension of this phenomenon.

I have developed a new general-purpose deterministic 2D radiative transfer code for astrophysical dusty environments named LELUYA (www.leluya.org). It can provide the solution to an arbitrary axially symmetric multi-grain dust distribution around an arbitrary heating source. By employing a new numerical method, the implemented algorithm automatically traces the dust density and optical depth gradients, creating the optimal unstructured triangular grid. The radiative transfer equation includes dust scattering, absorption and emission. Unique to LELUYA is also its ability to self-consistently reshape the sublimation/condensation dust cavity around the source to accommodate for the anisotropic diffuse radiation. LELUYAs capabilities are demonstrated in the study of the asymptotic giant branch (AGB) star IRC+10011. The stellar winds emanating from AGB stars are mostly spherically symmetric, but they evolve into largely asymmetric planetary nebulae during later evolutionary phases. The initiation of this symmetry breaking process is still unexplained. IRC+10011 represents a rare example of a clearly visible asymmetry in high-resolution near-infrared images of the circumstellar dusty AGB wind. LELUYA shows that this asymmetry is produced by two bipolar cones with 1/r0.5 density profile, imbedded in the standard 1/r2 dusty wind profile. The cones are still breaking though the 1/r2 wind, suggesting they are driven by bipolar jets. They are about 200 years old, thus a very recent episode in the final phase of AGB evolution before turning into a proto-planetary nebula, where the jets finally break out from the confining spherical wind. IRC+10011 provides the earliest example of this symmetry breaking thus far.

Using multi-wavelength and multi-epoch observations we examine the evolution of circumstellar disks around pre-main sequence stars from massive, optically thick flared disks to wispy debris disks. We examine a young cluster of nearby stars, identifying likely members and studying dust properties using 3-24μm photometry and accretion rates using optical spectroscopy. We find that 79% of the stars have disks and that almost all of the stars with disks are actively accreting. The stars that show evidence for evolution in their dust properties also exhibit a decrease in the accretion activity suggesting that the evolution of the dust and gas is closely connected. Focusing on a sub-sample of transition disks we study the source of recently discovered infrared variability and whether it can be used to further our understanding of disk structure. We are particularly interested in sources that show a ’seesaw’ behavior in their SED in which the short wavelength infrared flux increases while the long wavelength flux decreases causing the SED to pivot about one wavelength. We develop simple geometric models of disks with nonaxisymmetric structure and find that the precession of this structure is not able to reproduce the strength or the wavelength dependence of observed infrared variability while a model with an inner warp whose scale height rapidly varies is much more successful. We follow this up with detailed observations covering a wide range of wavelengths from optical to mid-infrared of six transition disks in order to better understand the physical source of the variability. We find that the variability is consistent with a variable scale height of the inner disk, finding direct evidence for this effect in two transition disks. Contemporaneous measures of the infrared flux and the accretion rate find in some cases a correlation between these two properties, although in none of our stars is it likely that the accretion rate variability is the source of the infrared variability. The most likely cause is either a companion embedded in the disk or a dynamic interface between the stellar magnetic field and the disk.

It was once thought that molecules in the interstellar medium (ISM) would be destroyed in the harsh surroundings and conditions of space, and therefore unobservable by radio techniques. However, it is now understood that the chemistry of the ISM is vast and complex. The question still remains as to just how complex is this chemistry. Much is clearly still not understood. This dissertation presents work on the study of metal compounds and cations in the circumstellar envelopes of oxygen- and carbon-rich asymptotic giant branch (AGB) and supergiant stars. Laboratory studies were also conducted on several transition metal compounds of interstellar interest, some with high spin and orbital angular momentum states. Work has been completed to confirm the detection of the debated metal cyanide KCN in the carbon-rich AGB star IRC+10216. KCN joins the list as the fifth interstellar metal cyanide/isocyanide detected in this source. In addition, preliminary results on the search for TiO are presented towards the oxygen-rich supergiant star, VY CMa. To further understand the evolutionary processes of carbon- and oxygen-rich stars, a survey of HCO⁺ was taken towards the carbon star IRC+10216, the oxygen-rich AGBs TX Cam, IK Tau, and W Hya and the oxygen-rich supergiant NML Cyg. While HCO⁺ was detected towards all of these sources, the results vary. The outflow of NML Cyg proves to be asymmetric and further study is necessary. The emission from W Hya is significantly narrower than the other sources. The abundances of HCO⁺ in circumstellar gas increases inversely with mass-loss rate and ion-molecule chemistry appears to influence the chemistry of evolved circumstellar envelopes. To understand species in space with more confidence, a laboratory search for several 3d transition metal species of astrochemical interest was conducted in the laboratory: HZnCl (X¹∑⁺), ZnO (X¹∑⁺ and a³Πᵢ), ZnCl (X²∑⁺), TiS (X³Δᵣ) and CrS (X⁵Πᵣ). All of the molecules have been observed for the first time with high resolution gas phase rotational spectroscopy and the work on ZnO was the first gas-phase study of this molecule. Synthesis of the species required exotic production methods, including use of a DC discharge to produce all zinc species. By studying the rotational spectra, rest frequencies were determined that will be beneficial for future astronomical searches.

In this thesis, images of a debris disc are used to examine the evidence for the presence of a Neptune-like planet around ε Eridani and detections of protoplanetary discs are used to investigate the evidence for star and future planet formation. A χ² analysis of the movement of clumps in the ε Eridani debris disc is presented using 850 μm SCUBA data taken over a 4 year period and compared with results from simulated data. A rotation is detected at the 2σ level and is faster than the Keplerian rate, consistent with theoretical models in which dust trapped in mean motion resonances tracks a planet orbiting the star at ≈26 AU. Future observations that could be taken with SCUBA-2 are also simulated and demonstrate that the true rotation rate cannot be recovered without the identification of the background sources aligned with the clumpy debris disc. Near and mid infrared observations are used to perform a survey of YSOs in the Rosette Molecular Cloud. Although triggering by compression of the molecular cloud by the expanding HII region at the centre of the Rosette Nebula is a possible origin for some of the recent star formation, the majority of the active star formation is occurring in already dense regions of the cloud not compressed by the expansion of the HII region. Mid-infrared data for W4 and SCUBA data for the star forming region AFGL 333 are also presented. A survey of YSOs reveals that whilst some young sources are coincident with the W4 loop, consistent with a scenario of triggered star formation in a swept-up shell, several young sources are found to be forming outside of this ring. The dust temperature and mass of AFGL 333 are estimated and the result implies a star formation efficiency of ~4% in the W4 loop.

We present the first scattered-light images of the debris disk around 49 Ceti, a similar to 40 Myr A1 main-sequence star at 59 pc, famous for hosting two massive dust belts as well as large quantities of atomic and molecular gas. The outer disk is revealed in reprocessed archival Hubble Space Telescope NICMOS-F110W images, as well as new coronagraphic H-band images from the Very Large Telescope SPHERE instrument. The disk extends from 1 1 (65 au) to 4.'' 6 (250 au) and is seen at an inclination of 73 degrees, which refines previous measurements at lower angular resolution. We also report no companion detection larger than 3 M-Jup at projected separations beyond 20 au from the star (0.'' 34). Comparison between the F110W and H-band images is consistent with a gray color of 49 Ceti's dust, indicating grains larger than greater than or similar to 2 mu m. Our photometric measurements indicate a scattering efficiency/infrared excess ratio of 0.2-0.4, relatively low compared to other characterized debris disks. We find that 49 Ceti presents morphological and scattering properties very similar to the gas-rich HD 131835 system. From our constraint on the disk inclination we find that the atomic gas previously detected in absorption must extend to the inner disk, and that the latter must be depleted of CO gas. Building on previous studies, we propose a schematic view of the system describing the dust and gas structure around 49 Ceti and hypothetical scenarios for the gas nature and origin.

Context. Extended circumstellar emission has been detected within a few 100 milli-arcsec around >= 10% of nearby main sequence stars using near-infrared interferometry. Follow-up observations using other techniques, should they yield similar results or non-detections, can provide strong constraints on the origin of the emission. They can also reveal the variability of the phenomenon. Aims. We aim to demonstrate the persistence of the phenomenon over the timescale of a few years and to search for variability of our previously detected excesses. Methods. Using Very Large Telescope Interferometer (VLTI)/Precision Integrated Optics Near Infrared ExpeRiment (PIONIER) in H band we have carried out multi-epoch observations of the stars for which a near-infrared excess was previously detected using the same observation technique and instrument. The detection rates and distribution of the excesses from our original survey and the follow-up observations are compared statistically. A search for variability of the excesses in our time series is carried out based on the level of the broadband excesses. Results. In 12 of 16 follow-up observations, an excess is re-detected with a significance of >2 sigma, and in 7 of 16 follow-up observations significant excess (>3 sigma) is re-detected. We statistically demonstrate with very high confidence that the phenomenon persists for the majority of the systems. We also present the first detection of potential variability in two sources. Conclusions. We conclude that the phenomenon responsible for the excesses persists over the timescale of a few years for the majority of the systems. However, we also find that variability intrinsic to a target can cause it to have no significant excess at the time of a specific observation.

We present new Atacama Large Millimeter/sub-millimeter Array (ALMA) 1.3 mm continuum observations of the SR 24S transition disk with an angular resolution less than or similar to 0'.18 (12 au radius). We perform a multi-wavelength investigation by combining new data with previous ALMA data at 0.45 mm. The visibilities and images of the continuum emission at the two wavelengths are well characterized by a ring-like emission. Visibility modeling finds that the ring-like emission is narrower at longer wavelengths, in good agreement with models of dust-trapping in pressure bumps, although there are complex residuals that suggest potentially asymmetric structures. The 0.45 mm emission has a shallower profile inside the central cavity than the 1.3 mm emission. In addition, we find that the (CO)-C-13 and (CO)-O-18 (J = 2-1) emission peaks at the center of the continuum cavity. We do not detect either continuum or gas emission from the northern companion to this system (SR 24N), which is itself a binary system. The upper limit for the dust disk mass of SR 24N is less than or similar to 0.12 M-circle plus, which gives a disk mass ratio in dust between the two components of M-dust,M-SR 24S/M-dust,M-SR 24N greater than or similar to 840. The current ALMA observations may imply that either planets have already formed in the SR 24N disk or that dust growth to millimeter sizes is inhibited there and that only warm gas, as seen by rovibrational CO emission inside the truncation radii of the binary, is present.