Evaluating the impact of binary parameter uncertainty on stellar population properties

Binary stars have been shown to have a substantial impact on the integrated light of stellar populations, particularly at low metallicity and early ages - conditions prevalent in the distant Universe. But the fraction of stars in stellar multiples as a function of mass, their likely initial periods and distribution of mass ratios are all known empirically from observations only in the local Universe. Each has associated uncertainties. We explore the impact of these uncertainties in binary parameters on the properties of integrated stellar populations, considering which properties and timescales are most susceptible to uncertainty introduced by binary fractions and whether observations of the integrated light might be sufficient to determine binary parameters. We conclude that the effects of uncertainty in the empirical binary parameter distributions are likely smaller than those introduced by metallicity and stellar population age uncertainties for observational data. We identify emission in the He II 1640Å emission line and continuum colour in the ultraviolet-optical as potential indicators of a high mass binary presence, although poorly constrained metallicity, dust extinction and degeneracies in plausible star formation history are likely to swamp any measurable signal

Nanotechnology enabled microfluidics/Raman spectroscopy systems for bio applications

The vision for this PhD research project was born out of a desire to study the in situ behaviour of suspended nano-materials; specifically, implementing a Raman microscopy system for investigating suspended materials in the microfluidic environment. The author developed a set of innovative research goals to achieve this vision, which include: (1) forming a suitable microfluidic system which can apply controlled forces onto the suspended materials on demand, (2) implementing Raman microscopy to study the behaviour of particles under the influence of such forces while inside the microfluidic system and (3) incorporating the developed microfluidic system for investigating suspended materials of low concentration, including biological cells and surface-enhanced Raman scattering studies. The author implemented the research in three distinct stages such that the work in earlier stages could provide the platform for the future work. In the first stage, the author designed a microfluidic dielectrophoresis platform consisting of curved microelectrodes. This platform was integrated with a Raman microscopy system for creating a novel system capable of detecting suspended particles of various types and spatial concentrations. The system was benchmarked using polystyrene and tungsten trioxide suspended particles, and the outcomes of this novel integrated system showed its strong potential for the determination of suspended particles types and their direct mapping, with several unique advantages over conventional optical systems. In the second stage of this research, the author developed a novel microfluidic-DEP system that could manipulate suspended silver nanoparticles’ spacing in three dimensions. Silver nanoparticles are capable of producing strong surface enhanced Raman scattering (SERS) signals, allowing the Raman system to detect very low concentrations of suspended analytes. DEP provided facile control of the positions and spacings of the suspended silver nanoparticles, and allowed for the creation of SERS hot-spots. The system was studied to determine the optimum DEP and microfluidic flow parameters for generating SERS, and the author was able to demonstrate this as a reversible process. This stage of the research used dipicolinic acid as the target analyte, and the system was demonstrated to have detection limits as small as ~1 ppm concentration levels. In the third stage, the microfluidic-DEP platform was used for trapping and isolating yeast cells. Silver nanoparticles were again used for SERS applications. The trapped cells were interrogated by the Raman system in order to obtain deeper understandings of cells functionalities and their communications under various physical conditions: live vs. dead and isolated vs. grouped. Live vs. dead experiments were conducted as a benchmark, to observe whether SERS is capable of differentiating cells based on the life condition. The research was expanded to study cells that were isolated from one another, and compared those Raman signatures to those from cells in grouped clusters. The author was able to extract unique information from such studies, including the importance of glycine, or proteins with glycine subunits, in the proliferation of yeast cells. The developed system showed great potential as a universal platform for the in situ study of cells, their communications and functionalities

The Galactic neutron star population II -- Systemic velocities and merger locations of binary neutron stars

The merger locations of binary neutron stars (BNSs) encode their galactic kinematics and provide insights into their connection to short gamma-ray bursts (SGRBs). In this work, we use the sample of Galactic BNSs with measured proper motions to investigate their kinematics and predict their merger locations. Using a synthetic image of the Milky Way and its Galactic potential we analyse the BNS mergers as seen from an extragalactic viewpoint and compare them to the location of SGRBs on and around their host galaxies. We find that the Galactocentric transverse velocities of the BNSs are similar in magnitude and direction to those of their Local Standards of Rest, which implies that the present-day systemic velocities are not isotropically oriented and the peculiar velocities might be as low as those of BNS progenitors. Both systemic and peculiar velocities fit a lognormal distribution, with the peculiar velocities being as low as $\sim 22-157$ km s$^{-1}$. We also find that the observed BNS sample is not representative of the whole Galactic population, but rather of systems born around the Sun's location with small peculiar velocities. When comparing the predicted BNS merger locations to SGRBs, we find that they cover the same range of projected offsets, host-normalized offsets, and fractional light. Therefore, the spread in SGRB locations can be reproduced by mergers of BNSs born in the Galactic disk with small peculiar velocities, although the median offset match is likely a coincidence due to the biased BNS sample.Comment: 13 pages, 7 figures, accepted for publication in MNRA

Late time HST UV and optical observations of AT~2018cow: extracting a cow from its background

The bright, blue, rapidly evolving AT2018cow is a well-studied peculiar extragalactic transient. Despite an abundance of multi-wavelength data, there still is no consensus on the nature of the event. We present our analysis of three epochs of Hubble Space Telescope (HST) observations spanning the period from 713-1474 days post burst, paying particular attention to uncertainties of the transient photometry introduced by the complex background in which AT2018cow resides. Photometric measurements show evident fading in the UV and more subtle but significant fading in the optical. During the last HST observation, the transient's optical/UV colours were still bluer than those of the substantial population of compact, young, star-forming regions in the host of AT2018cow, suggesting some continued transient contribution to the light. However, a compact source underlying the transient would substantially modify the resulting spectral energy distribution, depending on its contribution in the various bands. In particular, in the optical filters, the complex, diffuse background poses a problem for precise photometry. An underlying cluster is expected for a supernova occurring within a young stellar environment or a tidal-disruption event (TDE) within a dense older one. While many recent works have focused on the supernova interpretation, we note the substantial similarity in UV light-curve morphology between AT2018cow and several tidal disruption events around supermassive black holes. Assuming AT2018cow arises from a TDE-like event, we fit the late-time emission with a disc model and find $M_{BH} = 10^{3.2{\pm}0.8}$ M$_{\odot}$. Further observations are necessary to determine the late-time evolution of the transient and its immediate environment.Comment: 16 pages, 10 figures. Accepted for publication in MNRA

Searching for ejected supernova companions in the era of precise proper motion and radial velocity measurements

The majority of massive stars are born in binaries, and most unbind upon the first supernova. With precise proper motion surveys such as Gaia, it is possible to trace back the motion of stars in the vicinity of young remnants to search for ejected companions. Establishing the fraction of remnants with an ejected companion, and the photometric and kinematic properties of these stars, offers unique insight into supernova progenitor systems. In this paper, we employ binary population synthesis to produce kinematic and photometric predictions for ejected secondary stars. We demonstrate that the unbound neutron star velocity distribution from supernovae in binaries closely traces the input kicks. Therefore, the observed distribution of neutron star velocities should be representative of their natal kicks. We evaluate the probability for any given filter, magnitude limit, minimum measurable proper motion (as a function of magnitude), temporal baseline, distance and extinction that an unbound companion can be associated with a remnant. We compare our predictions with results from previous companion searches, and demonstrate that the current sample of stars ejected by the supernova of their companion can be increased by a factor of 5-10 with Gaia data release 3. Further progress in this area is achievable by leveraging the absolute astrometric precision of Gaia, and by obtaining multiple epochs of deep, high resolution near-infrared imaging with the Hubble Space Telescope, JWST and next-generation wide-field near-infrared observatories such as Euclid or the Nancy Grace Roman Space Telescope.Comment: Accepted for publication in MNRAS. 19 pages, 17 figure

AT2023fhn (the Finch): a Luminous Fast Blue Optical Transient at a large offset from its host galaxy

Luminous Fast Blue Optical Transients (LFBOTs) - the prototypical example being AT 2018cow - are a rare class of events whose origins are poorly understood. They are characterised by rapid evolution, featureless blue spectra at early times, and luminous X-ray and radio emission. LFBOTs thus far have been found exclusively at small projected offsets from star-forming host galaxies. We present Hubble Space Telescope, Gemini, Chandra and Very Large Array observations of a new LFBOT, AT2023fhn. The Hubble Space Telescope data reveal a large offset (greater than 3.5 half-light radii) from the two closest galaxies, both at a redshift of 0.24. The isolated environment of AT 2023fhn is in stark contrast with previous events, is challenging to explain with most LFBOT progenitor models, and calls into question the homogeneity of LFBOTs as a class.Comment: Submitted to MNRASL. 7 pages, 4 figures, 2 table

The Galactic neutron star population – I. An extragalactic view of the Milky Way and the implications for fast radio bursts

A key tool astronomers have to investigate the nature of extragalactic transients is their position on their host galaxies. Galactocentric offsets, enclosed fluxes, and the fraction of light statistic are widely used at different wavelengths to help infer the nature of transient progenitors. Motivated by the proposed link between magnetars and fast radio bursts (FRBs), we create a face-on image of the Milky Way using best estimates of its size, structure, and colour. We place Galactic magnetars, pulsars, low-mass, and high-mass X-ray binaries on this image, using the available distance information. Galactocentric offsets, enclosed fluxes, and fraction of light distributions for these systems are compared to extragalactic transient samples. We find that FRBs follow the distributions for Galactic neutron stars closest, with 24 (75 per cent) of the Anderson–Darling tests we perform having a p-value greater than 0.05. This suggests that FRBs are located on their hosts in a manner consistent with Galactic neutron stars on the Milky Way’s light, although we cannot determine which specific neutron star population is the best match. The Galactic distributions are consistent with other extragalactic transients much less often across the range of comparisons made, with type Ia SNe in second place, at only 33 per cent of tests exceeding 0.05. Overall, our results provide further support for FRB models invoking isolated young neutron stars, or binaries containing a neutron star

AT2023fhn (the Finch):a luminous fast blue optical transient at a large offset from its host galaxy

Luminous fast blue optical transients (LFBOTs) – the prototypical example being AT 2018cow – are a rare class of events whose origins are poorly understood. They are characterized by rapid evolution, featureless blue spectra at early times, and luminous X-ray and radio emission. LFBOTs thus far have been found exclusively at small projected offsets from star-forming host galaxies. We present Hubble Space Telescope, Gemini, Chandra, and Very Large Array observations of a new LFBOT, AT 2023fhn. The Hubble Space Telescope data reveal a large offset (>3.5 half-light radii) from the two closest galaxies, both at redshift z ∼ 0.24. The location of AT 2023fhn is in stark contrast with previous events, and demonstrates that LFBOTs can occur in a range of galactic environments

Estimating transient rates from cosmological simulations and BPASS

The detection rate of electromagnetic (EM) and gravitational wave (GW) transients is growing exponentially. As the accuracy of the transient rates will significantly improve over the coming decades, so will our understanding of their evolution through cosmic history. To this end, we present predicted rates for EM and GW transients over the age of the Universe using Binary Population and Spectral Synthesis (BPASS) results combined with four cosmic star formation histories (SFH). These include a widely used empirical SFH of Madau & Dickinson and those from three cosmological simulations: MilliMillennium, EAGLE and IllustrisTNG. We find that the choice of SFH changes our predictions: transients with short delay times are most affected by the star formation rate and change up to a factor 2, while long-delay time events tend to depend on the metallicity evolution of star formation and can change the predicted rate up to an order of magnitude. Importantly, we find that the cosmological simulations have very different metallicity evolution that cannot be reproduced by the widely used metallicity model of Langer & Norman, which impacts the binary black hole merger, stripped-envelope supernovae, and LGRBs in the local Universe most acutely. We recommend against using simple prescriptions for the metallicity evolution of the Universe when predicting the rates of events that can have long delay times and that are sensitive to metallicity evolution

Dielectrophoresis of micro/nano particles using curved microelectrodes

Dielectrophoresis, the induced motion of polarisable particles in non-homogenous electric field, has been proven as a versatile mechanism to transport, immobilise, sort and characterise micro/nano scale particle in microfluidic platforms. The performance of dielectrophoretic (DEP) systems depend on two parameters: the configuration of microelectrodes designed to produce the DEP force and the operating strategies devised to employ this force in such processes. This work summarises the unique features of curved microelectrodes for the DEP manipulation of target particles in microfluidic systems. The curved microelectrodes demonstrate exceptional capabilities including (i) creating strong electric fields over a large portion of their structure, (ii) minimising electro-thermal vortices and undesired disturbances at their tips, (iii) covering the entire width of the microchannel influencing all passing particles, and (iv) providing a large trapping area at their entrance region, as evidenced by extensive numerical and experimental analyses. These microelectrodes have been successfully applied for a variety of engineering and biomedical applications including (i) sorting and trapping model polystyrene particles based on their dimensions, (ii) patterning carbon nanotubes to trap low-conductive particles, (iii) sorting live and dead cells based on their dielectric properties, (iv) real-time analysis of drug-induced cell death, and (v) interfacing tumour cells with environmental scanning electron microscopy to study their morphological properties. The DEP systems based on curved microelectrodes have a great potential to be integrated with the future lab-on-a-chip systems.<br /