The topic of this thesis deals with the spiral structure in disk galaxies with a specific aim of probing the influence of the dark matter halo and the interstellar gas on the origin and longevity of the spiral arms in late-type galaxies through theoretical modeling and numerical calculations. The basic theoretical model of the galactic disk used involves gravitationally-coupled two-component system (stars and gas) embedded in a rigid and non-responsive dark matter halo, i.e., the static potential of the dark matter is used in the calculations. However, at places, depending on the nature of the problem addressed, the disk is treated as consisting of only stellar component or only gas component followed by proper justifications for the assumptions. The disk is rotationally-supported in the plane and pressure-supported perpendicular to the plane of the disk. The first part of the thesis involves searching for the dynamical effect of dark matter halo on small-scale spiral structure in dwarf low surface brightness (LSB) galaxies and also some dwarf ir-regular galaxies which host an extended H I disk. In both cases, the rotation curves are found to be dominated by the contribution of the dark matter halo over a large radial distance, starting from the inner regions of the galaxies. The next part of the thesis deals with the investigation of the possible effect of the interstellar gas on the persistence is-sue and the pattern speeds of the spiral structure in the disk galaxies. The last part of the thesis involves in studying the dynamical effect of dark matter halo on large-scale spiral structure. Following is the layout of the thesis. Chapter 1 gives a general introduction to the topic of spiral structure of late-type disk galaxies, followed by a broad overview of the theoretical development of the topic and the present status of the topic. Then the thesis starts with studying the small-scale spiral features and evolves to studying the large-scale spiral features seen in disk galaxies in the following way: Chapters 2 & 3 deal with the effect of dark matter halo on small- scale spiral structure. Chapters 4 & 5 focus on the dynamical effect of the interstellar gas on the spiral structure using the local dispersion relation. Chapters 6 & 7 discuss the possible effect of dark matter halo on large-scale spiral structure in disk galaxies. Chapter 8 contains the summary of results and future plans. Effect of dark matter halo on small-scale spiral structure The spiral arms in the disks of galaxies are often broken into several smaller parts or patches that create a messy visual impression when viewed from a ‘face-on’ configura-tion. They are generally termed as ‘small-scale’ or flocculent spiral arms. Several stud-ies showed that the small-scale spiral arms are basically material arm, i.e., they can be thought of as ‘tubes’ filled with stars and gas. Spiral arms are known to participate in the secular evolution of the disk galaxies. Since disk galaxies are believed to reside within a halo of dark matter, therefore a detailed understanding of possible effects of dark matter halo on the spiral arms is necessary. In Chapter 2, we investigate the effect of dark matter halo on small-scale spiral fea-tures in the disks of LSB galaxies. Modeling the mass distribution within a galaxy from the rotation curve of a typical small LSB galaxy reveals the generic fact that for most of the radii, dark matter halo dominates over the stellar disk. This trend is found to be true from the very inner regions of an LSB disk which in turn makes the LSBs a suitable laboratory for probing the effect of dark matter halo on the dynamics of disk galaxies. Following a semi-analytic approach, and using the observationally measured input pa-rameters for a typical superthin LSB galaxy, UGC 7321, we showed that the dominant dark matter halo suppresses the small-scale spiral structure in the disk of UGC 7321. Since UGC 7321 possesses features typical of a LSB galaxy, we argued that this finding will also hold true for other typical LSBs. The result is at par with the observational evi-dences for the lack of prominent, strong small-scale spiral structure in LSB galaxies. In Chapter 3, we employed the similar techniques for probing the effect of dark matter halo on small-scale spiral structure, except this time we took five dwarf irregular galaxies with an extended H I disk as the sample for our investigation. The main im-portant difference between these dwarf irregular galaxies with the earlier LSB galaxies is that for these dwarf irregular galaxies with extended H I disk, the largest baryonic con-tribution comes from the interstellar gas (mainly H I ), and not from the stars (as seen in LSBs). The extended H I disks of these galaxies allow one measure the rotation curve, and hence modeling the dark matter halo parameters for a large radial range from the galactic center. Here also the rotation curves are found to be dominated by dark matter halo over most of the disk, thus providing yet another ‘laboratory’ for testing the dynam-ical effect of dark matter halo on the dynamics of the disks. Using the observed input parameters for five such dwarf irregular galaxies, we showed that the dense and com-pact dark matter halo is responsible for preventing strong small-scale spiral structure in these galaxies, which is in fair agreement with the observations. Dynamical effect of interstellar gas on longevity of spiral arms Any late-type disk galaxy contains a finite amount of interstellar gas along with the stel-lar component. The atomic hydrogen (H I ) constitutes the bulk of the interstellar gas along with the molecular hydrogen (H2), ionized hydrogen (H I I ), and a trace amount of heavy elements like helium. The mass fraction present in the interstellar gas in disk galaxies is found to vary with the Hubble sequence, with the amount of interstellar gas increasing from Sa type to Scd type of galaxies. Due to the lower value of velocity disper-sion as compared to that of stars, gas is known to have a larger destabilizing effect in the disk. Therefore, the natural question arises about what possible role the interstellar gas could play in the origin and the persistence issue of spiral arms. In Chapter 4, we explored how the interstellar gas could influence the longevity of the spiral arms in late-type disk galaxies by treating the spiral structure as density waves in the disk. The disk is modeled as a gravitationally coupled stars plus gas (two-component) system, where the stars are modeled as a collisionless system and the gas treated as a fluid system. Using the appropriate local dispersion relation for the above mentioned model for the disk of galaxy, we calculated the group velocity of a wavepacket of density wave and then studied the variation of the group velocity with increasing amount of interstellar gas in the system. We showed that the group velocity of a wavepacket in a Milky Way-like disk galaxy decreases steadily with the inclusion of gas, implying that the spiral pattern will survive for a longer time-scale in a more gas-rich galaxy by a factor of few. In Chapter 5, we investigated the role of interstellar gas in obtaining a stable den-sity wave corresponding to the observed pattern speed for the spiral arms. The under-lying local dispersion relation remains same as that is in Chapter 4. Using the observa-tionally measured pattern speed and the rotation curves for three late-type disk galaxies we showed that the presence of interstellar gas in necessary in order to maintain a stable density wave corresponding to the observed values for pattern speeds. Also we proposed a method to determine a range of pattern speed values at any particular radius, corre- sponding to which the density wave can be stable. We applied this method to the same three late-type galaxies which we used in the earlier part of this chapter. We found that, for these three galaxies, the observed pattern speed values indeed fall in the predicted range. Imprint of dark matter halo on large-scale spiral structure Along with the small-scale spiral arms, there also exists another type of spiral arms – the large-scale spiral structure, like what we see M 51 or in NGC 2997, which occupy almost the entire outer optical disk in the galaxy. These spiral arms are termed as ‘grand-design’ spiral structure. One of the competing theories, namely, Density wave theory proposes that the large-scale structure is basically a density wave in the disk and the pattern ex-hibits a rigid-body rotation with a definite constant pattern speed. In the earlier part this thesis (Chapters 2 & 3), it was shown that the small-scale spiral structure gets damped by the dominant dark matter halo. Therefore, a natural question arises whether dominant dark matter plays any role on these large-scale spiral structure; and if yes, to what extent it affects the large-scale spiral structure. In Chapters 6 & 7, we investigated how the large-scale structure in disk galaxies gets affected when the disk galaxy hosts a dark matter halo that dominates over most of the disk regions. We again chose the LSB galaxies as laboratory for this study. In Chapter 6, we modeled the stellar component as a fluid system and in Chapter 7, we treated the stellar system as more realistic collisionless system. In both cases, global spiral modes are identified from the appropriate dispersion relations via a novel quantization rule, and they are used as a ‘proxy’ for the large-scale spiral structure. Using the input pa-rameters for UGC 7321, in Chapter 6 we showed that the fluid representation of stellar system failed to make an impression in suppression of the global spiral modes. However, when stellar component is treated as a more realistic collisionless system, we found that the dark matter halo suppresses the large-scale spiral features as well in the disks of LSB galaxies, in fair agreement with the observations. Finally, in Chapter 8, the thesis concludes with a summary of main results and a brief discussion of the scope for future work.

<p>One of the most fundamental problems in theories of planet formation in protoplanetary disks is planetary migration that arises from resonant, tidal interactions of forming planets with the natal disks. This rapid inward migration, also known as type I migration, leads to the well-known problem that its timescale is about two orders of magnitude shorter than the typical disk lifetime, so that (proto)planets plunge into the host stars within the disk lifetime. This provides a huge hurdle for understanding the statistical properties of observed extra solar planets that now amount to more than 700.</p> <p>In this thesis, we focus on one of the most general properties of protoplanetary disks - inhomogeneities. A large amount of theoretical and observational work currently suggests that protoplanetary disks are most likely to possess several kinds of inhomogeneities. Planetary migration is highly sensitive to the disk properties such as the surface density and temperature of disks, and the sensitivity leads to the formation of trapping sites for rapid type I migration at disk inhomogeneities. These local sites capturing planets undergoing migration are referred to as planet traps. We perform both analytical and numerical studies for exploring formation mechanisms of planet traps at disk inhomogeneities and their consequences for the formation and evolution of planetary systems. We focus on three kinds of the disk inhomogeneities: dead zones, ice lines, and transitions of heat sources in protoplanetary disks we refer to as heat transitions. Dead zones are an inevitable consequence of disk turbulence originating from magnetorotational instabilities (MRIs) that take place in (partially) ionized disks threaded by weak magnetic fields. One of the fundamental properties of the dead zone is a low level of turbulence there, which is the outcome of the high density, preventing the region from being ionized due to X-rays from the central stars and cosmic rays. Ice lines are formed due to low disk temperatures which lead to condensation of specific molecules there. Heat transitions arise as a consequence of the switching of the dominant heating process from viscous heating to stellar irradiation as the distance to the host stars increases.</p> <p>We summarize our major findings. 1) rapid dust settling arising in dead zones leaves a dusty wall at the outer edge of the dead zones beyond which the disks are quite turbulent, so that dust is fully mixed with the gas. Efficient heating of the wall by stellar irradiation and the subsequent backward heating of the dead zones by the wall result in a positive temperature gradient in the dead zones. This inversion in the temperature profiles leads to outward migration there. 2) Any protoplanetary disk is likely to possess up to three types of planet traps that are specified by characteristic disk radii (dead zone, ice line and heat transition traps). Disk evolution, driven by disk viscosity, lowers both the accretion rate and surface density of gas and moves traps inward at different rates. This suggests that the interactions of (proto)planets captured at different traps play the dominant role in constructing planetary system architectures. Furthermore, the distribution of planet traps depends largely on stellar masses and accretion rates, so that they are one of the principle parameters for regulating the (initial) scale of planetary systems. 3) Both multiplicity and mobility of planet traps are crucial for understanding the statistical properties of observed extra solar planets. For instance, the mass-period relation - observational manifestation that planetary mass is an increasing function of orbital periods - can be understood by constructing and following evolutionary tracks of accreting planets in planet traps. These three contribution are new results in the field.</p> / Doctor of Philosophy (PhD)

Monte Carlo radiation transfer provides an efficient modelling tool for probing the dusty local environment of young stars. Within this thesis, such theoretical models are used to study the disk structure of objects across the mass spectrum - young low mass Brown Dwarfs, solar mass T-Tauri stars, intermediate mass Herbig Ae stars, and candidate B-stars with massive disks. A Monte Carlo radiation transfer code is used to model images and photometric data in the UV - mm wavelength range. These models demonstrate how modelling techniques have been updated in an attempt to reduce the number of unknown parameters and extend the diversity of objects that can be studied.

Stars form inside clouds of molecular gas and dust. In the early stages of stellar evolution the remainders of the initial cloud form a circumstellar disk. For the next few million years the disk will slowly dissipate via accretion, outflows, photoevaporation and planet growth while the star makes its way onto the Main Sequence. This stage of a star's life is referred to as the T Tauri phase and is characterised by high-level spectrophotometric variability. This thesis aims to study and map out the environments of T Tauri stars down to the very low mass regime by the means of time-domain monitoring. Different physical processes in the system manifest themselves as variability on different time- scales as well as produce characteristic spectroscopic and photometric features at various wave- lengths. In order to study young stellar objects in depth, the observing campaigns presented in this work were designed to cover a large range of time-scales - minutes, hours, days and months. Combining all the data, this thesis establishes a baseline of over a decade for some objects. The observations also cover a wide range of wavelengths from the optical to the mid-infrared part of the spectrum. The star RW Aur experienced two long-lasting dimming events in 2010 and 2014. This thesis presents a large collection of spectral and photometric measurements carried out just before and during the 2014 event. Spectral accretion signatures indicate no change in the accretion activity of the system. Photometry indicates that parallel to the dimming in the optical the star becomes brighter in the mid-infrared. The observations in this work combined with literature data suggest that the origin of the 2014 event is most likely obscuration of the star by hot dust from the disk being lifted into the disk wind. Very low mass stars (< 0.4 M⊙) are the most common type of star in the Galaxy. In order to understand the early stages of stellar evolution we must study young very low mass stars. This work investigates the photometric and spectroscopic variability of seven brown dwarfs in star forming regions near σ Ori and ε Ori. All targets exhibit optical photometric variability between from 0.1 to over 1.0 magnitude that persists on a time-scale of at least one decade. Despite the photometric variability no change in the spectral type is measured. In the cases where the stars are accreting, modelling of the spectral changes suggest the accretion flow is more homogeneous and less funnelled compared to Sun-like T Tauri stars. The non-accreting variables are more plausibly explained by obscuration by circumstellar material, possibly a ring made out of multiple clouds of dust grains and pebbles with varying optical depths. The star-disk systems studied in this thesis have some broader implications for star and planet formation theory. The case-study of RW Aur has unambiguously demonstrated that the planet- forming environment is very dynamic and can change dramatically on short time-scales, which in turn would have implications for the diversity of planetary systems found in the Galaxy. The Orion stars have shown that the current theory for the T Tauri stage of stellar evolution is valid down to the very low mass regime. The seven dwarfs are a good example for the evolutionary path of circumstellar disks, showing the transition from gas-high, flared accretion disks (σ Ori) to dust-dominated, depleted, structured debris disks (ε Ori).

The dependence of the mass accretion rate on the stellar properties is a key constraint for star formation and disk evolution studies. Here we present a study of a sample of stars in the Chamaeleon I star-forming region carried out using spectra taken with the ESO VLT/X-shooter spectrograph. The sample is nearly complete down to stellar masses (M-star) similar to 0.1 M-circle dot for the young stars still harboring a disk in this region. We derive the stellar and accretion parameters using a self-consistent method to fit the broadband flux-calibrated medium resolution spectrum. The correlation between accretion luminosity to stellar luminosity, and of mass accretion rate to stellar mass in the logarithmic plane yields slopes of 1.9 +/- 0.1 and 2.3 +/- 0.3, respectively. These slopes and the accretion rates are consistent with previous results in various star-forming regions and with different theoretical frameworks. However, we find that a broken power-law fit, with a steeper slope for stellar luminosity lower than similar to 0.45 L-circle dot and for stellar masses lower than similar to 0.3 M-circle dot is slightly preferred according to different statistical tests, but the single power-law model is not excluded. The steeper relation for lower mass stars can be interpreted as a faster evolution in the past for accretion in disks around these objects, or as different accretion regimes in different stellar mass ranges. Finally, we find two regions on the mass accretion versus stellar mass plane that are empty of objects: one region at high mass accretion rates and low stellar masses, which is related to the steeper dependence of the two parameters we derived. The second region is located just above the observational limits imposed by chromospheric emission, at M-star similar to 0.3-0.4 M-circle dot. These are typical masses where photoevaporation is known to be effective. The mass accretion rates of this region are similar to 10(-10) M-circle dot/yr, which is compatible with the value expected for photoevaporation to rapidly dissipate the inner disk.

No description available.

Polyethylene glycol-stabilized lipodisks have emerged as a novel type of lipid-based nanoparticles with high potential as both drug carriers and biomimetic membranes. In this thesis we assess both of these applications, and show how the properties of the lipodisks can be further developed and optimized. Initially, we show that the antimicrobial peptides melittin, alamethicin and magainin 2, in spite of their very different physico-chemical properties and suggested modes of action on membranes, all have high affinity to lipodisks. Using melittin as a model peptide, we confirm a maintained antimicrobial effect of disk-formulated peptides. We also show that melittin dissociates slowly from the disks, resulting in extended drug release and prolonged antibacterial effect. Additionally, we present evidence that the peptide is protected against enzymatic degradation when formulated in the disks. Further, we develop a stable HPLC-MS system with immobilized lipodisks as model membranes. The stability of the system is confirmed by drug partitioning analysis using 15 different drug compounds. We also show how the lipodisk column can be supplemented with cyclooxygenase by in situ incorporation of the protein in the lipodisks. The specific binding of the protein to the disks is confirmed using QCM-D. Finally, by changing the polymer length and applying a new preparation protocol, we have optimized the lipodisks for use as drug carriers and biomimetic membranes. Previous lipodisk studies have been conducted on systems containing PEG-lipids with polymer molecular weights of 2000 or 5000 Da. Also, conventional protocols for the preparation of lipodisks typically require a PEG-lipid concentration of 15 mol% or more. Here we show that stable lipodisks can also be produced using PEG-lipids with a 1000 Da molecular weight polymer and that the use of shorter PEG-lipids dramatically improve the amount of lipodisks that can be immobilized on silica surfaces. Moreover, through the development of a method in which lipid mixtures are sonicated at low temperatures, we produce lipodisks containing as little as 2 mol% PEG-lipid. We present data verifying that these disks are superior to disks with higher PEG-lipid content in terms of their ability to incorporate externally added PEG-lipids functionalized with targeting agents.

We present axisymmetric hydrodynamical simulations of the long-term accretion of a rotating gamma-ray burst progenitor star, a "collapsar," onto the central compact object, which we take to be a black hole. The simulations were carried out with the adaptive mesh refinement code FLASH in two spatial dimensions and with an explicit shear viscosity. The evolution of the central accretion rate exhibits phases reminiscent of the long GRB [gamma]-ray and X-ray light curve, which lends support to the proposal by Kumar et al. (2008a,b) that the luminosity is modulated by the central accretion rate. In the first "prompt" phase, the black hole acquires most of its final mass through supersonic quasiradial accretion occurring at a steady rate of [scientific symbols]. After a few tens of seconds, an accretion shock sweeps outward through the star. The formation and outward expansion of the accretion shock is accompanied with a sudden and rapid power-law decline in the central accretion rate Ṁ [proportional to] t⁻²̇⁸, which resembles the L[subscript x] [proportional to] t⁻³ decline observed in the X-ray light curves. The collapsed, shock-heated stellar envelope settles into a thick, low-mass equatorial disk embedded within a massive, pressure-supported atmosphere. After a few hundred seconds, the inflow of low-angular-momentum material in the axial funnel reverses into an outflow from the thick disk. Meanwhile, the rapid decline of the accretion rate slows down, which is potentially suggestive of the "plateau" phase in the X-ray light curve. We complement our adiabatic simulations with an analytical model that takes into account the cooling by neutrino emission and estimate that the duration of the prompt phase can be ~ 20 s. The model suggests that the steep decline in GRB X-ray light curves is triggered by the circularization of the infalling stellar envelope at radii where the virial temperature is below 10¹⁰ K, such that neutrino cooling is inefficient and an outward expansion of the accretion shock becomes imminent; GRBs with longer prompt [gamma]-ray emission should have more slowly rotating envelopes. / text

In my thesis I investigate the occurrence of mid-infrared excess around stars and their evolution. Since the launch of the first infrared satellite, IRAS, we have known that a large fraction of stars exhibit significant levels of infrared emission above their predicted photospheric level. Resolved optical and infrared images have revealed the majority of these excesses to arise from circumstellar disk structures, made up of distributions of planetesimals, rocks, and dust. These structures are descriptively called debris disks. The first part of my thesis analyzes the Spitzer Space Telescope Observations of δ Velorum. The 24 μm Spitzer images revealed a bow shock structure in front of the star. My analysis showed that this is a result of the star’s high speed interaction with the surrounding interstellar medium. We place this observation and model in context of debris disk detections and the origin of λ Boötis stars. The second part of my thesis summarizes our observational results on the open cluster Praesepe. Using 24 μm data, I investigated the fraction of stars with mid-infrared excess, likely to have debris disks. I also assembled all results from previous debris disk studies and followed the evolution of the fraction of stars with debris disks. The majority of debris disks systems are evolved, few hundred million or a Gyr old. Since the dissipation timescale for the emitting dust particles is less than the age of these systems, they have to be constantly replenished through collisional grinding of the larger bodies. The last two chapters of my thesis is a theoretical analysis of the collisional cascade in debris disks, the process that produces the constant level of dust particles detected. I introduce a numerical model that takes into account all types of destructive collisions in the systems and solves the full scattering equation. I show results of comparisons between my and other published models and extensive verification tests of my model. I also analyze the evolution of the particle size distribution as a function of the variables in my model and show that the model itself is quite robust against most variations.

General relativity has been tested by many experiments, which, however, almost exclusively probe weak spacetime curvatures. In this thesis, I create two frameworks for testing general relativity in the strong-field regime with observations of black holes in the electromagnetic spectrum using current or near-future instruments. In the first part, I design tests of the no-hair theorem, which uniquely characterizes the nature of black holes in terms of their masses and spins in general relativity and which states that these compact objects are described by the Kerr metric. I investigate a quasi-Kerr metric and construct a Kerr-like spacetime, both of which contain an independent parameter in addition to mass and spin. If the no-hair theorem is correct, then any deviation from the Kerr metric has to be zero. I show that already moderate changes of the deviation parameters in either metric lead to significant modifications of the observed signals. First, I apply this framework to the imaging of supermassive black holes using very-long baseline interferometry. I show that the shadow of a black hole as well as the shape of a bright and narrow ring surrounding the shadow depend uniquely on its mass, spin, inclination, and the deviation parameter. I argue that the no-hair theorem can be tested with observations of the supermassive black hole Sgr A*. Second, I investigate the potential of quasi-periodic variability observed in both galactic black holes and active galactic nuclei to test the no-hair theorem in two different scenarios. Third, I show that the profiles of relativistically broadened iron lines emitted from the accretion disks of black holes imprint the signatures of deviations from the Kerr metric. In the second part, I devise a method to test the predicted evaporation of black holes in the Randall-Sundrum model of string theory-inspired braneworld gravity through the orbital evolution of black-hole X-ray binaries and obtain constraints on the size of the extra dimension from A0620-00 and XTE J1118+480. I predict the first detection of orbital evolution in a black-hole binary.