Outstanding talk: Roderick Chisholm
Title: Utilizing digital in-line holography to investigate polymer crystallization
Abstract:
The conventional model of polymer spherulite crystallization is the Hoffmann-Lauritzen model. With the advent of new technologies, a renewed interest in pursuing challenges to the conventional models and theories have taken place. The application of a recent microscopical technique, digital in-line holography, will be explored as it applies to polymer spherulite crystallization.

Project Title: Asteroseismic Analysis of the Pre-Main-Sequence Stars in NGC 2264

Student Coauthors: Michael P. Casey (e-mail)

Faculty: Prof. David Guenther (www)

Abstract:

NGC 2264 is a young open cluster lying above the Galactic plane in which six variable stars have previously been identified as possible pre-main-sequence (PMS) pulsators. Their oscillation spectra are relatively sparse with each star having from 2 to 12 unambiguous frequency identifications based on Microvariability and Oscillations of Stars satellite and multi-site ground-based photometry. We describe our efforts to find classical PMS stellar models (i.e., models evolved from the Hayashi track) whose oscillation spectra match the observed frequencies. We find model eigenspectra that match the observed frequencies and are consistent with the stars' locations in the HR diagram for the three faintest of the six stars. Not all the frequencies found in spectra of the three brightest stars can be matched to classical PMS model spectra possibly because of effects not included in our PMS models such as chemical and angular momentum stratification in the outer layers of the star. All the oscillation spectra contain both radial and nonradial p-modes. We argue that the PMS pulsating stars divide into two groups depending on whether or not they have undergone complete mixing (i.e., have gone through a Hayashi phase). Lower mass stars that do evolve through a Hayashi phase have oscillation spectra predicted by classical PMS models, whereas more massive stars that do not, retain mass infall effects in their surface layers and are not well modeled by classical PMS models.

Accepted for Publication (ApJ)

Project Title: MOST photometry of the enigmatic PMS pulsator HD 142666

Student Coauthors: Michael Gruberbauer (e-mail) & Michael Casey (e-mail)

Faculty: Prof. David Guenther (www)

Abstract:

Context: Modeling of pre-main sequence (PMS) stars through asteroseismology of PMS p-mode pulsators has only recently become possible, and spacebased photometry is one of the important sources of data for these efforts. We present precise photometry of the pulsating Herbig Ae star HD 142666 obtained in two consecutive years with the MOST (Microvariability & Oscilations of STars) satellite. Aims: Previously, only a single pulsation period was known for HD 142666. The MOST photometry reveals that HD 142666 is multi-periodic. However, the unique identification of pulsation frequencies is complicated by the presence of irregular variability caused by the star's circumstellar dust disk. The two light curves obtained with MOST in 2006 and 2007 provided data of unprecedented quality to study the pulsations in HD 142666 and also to monitor the circumstellar variability. Methods: Frequency analysis was performed using the routine sigspec and the results from the 2006 and 2007 campaigns were then compared to each other with the software cinderella to identify frequencies common to both light curves. The correlated frequencies were then submitted to an asteroseismic analysis. Results: We attribute 12 frequencies to pulsation. Model fits to the three frequencies with the highest amplitudes lie well outside the uncertainty box for the star's position in the HR diagram based on published values. Some of the frequencies appear to be rotationally split modes. Conclusions: The models suggest that either (1) the published estimate of the luminosity of HD 142666, based on a relation between circumstellar disk radius and stellar luminosity, is too high and/or (2) additional physics such as mass accretion may be needed in our models to accurately fit both the observed frequencies and HD 142666's position in the HR diagram. Based on data from the MOST satellite, a Canadian Space Agency mission, jointly operated by Dynacon Inc., the University of Toronto Institute for Aerospace Studies and the University of British Columbia with the assistance of the University of Vienna.

Accepted for Publication (A&A)

By: Jon Ramsey (e-mail)

Supervisor: Prof. David Clarke (www)

Date: 2009, Presented at the CASCA meeting in Toronto.

Abstract:

Jets and outflows are an integral part of the current star formation paradigm and are observed wherever star formation is ongoing. The importance of these objects lies in their ability to remove angular momentum from matter accreting onto a central protostar. While observations easily resolve these objects on scales of thousands, or even several hundred, of AU, the launching and collimating mechanism (which is < 10 AU in size) remains unresolved and is often obscured by a dusty disc or torus. In recent years, there have been many simulations of the jet launching mechanism, but these studies did not extend to spatial scales resolvable by existing observations. As such, there is a disconnect between the simulation and observation of protostellar jets and in this talk I will discuss how I am bridging this gap. I will present results of simulations with AZEuS, an adaptive mesh refinement version of the ZEUS fluid code. AZEuS allows me to simultaneously model both the small jet launching region and the large observational length scales. I will highlight new results which are only possible with adaptive mesh refinement, where this project is going in the near future, and the importance of synthetic observations.

Animation

Project Title: Mass Function at z ~2

By: Bobby Sorba (e-mail)

Supervisor: Prof. Marcin Sawicki (www)

Date: July 2009, Presented at the VIIth Marseille International Cosmology Conference.

Abstract:

Current photometric techniques, such as Lyman Break selection, select in the rest-frame UV or optical, and thus may preferentially favour star-forming objects. However, it is possible to select galaxies based on their mass using the 1.6 $\mu$m bump caused by the presence of the H$^-$ ion in older stars. We use the 1.6 $\mu$m bump to generate photometric redshifts for galaxies in the GOODS fields using the four IRAC bands. We compare these with known spectroscopic redshifts to prove the technique is reliable. We then use our mass selected catalog of galaxies to construct a galaxy mass function at redshift 2, taking into account incompleteness effects. We compare this to mass functions made using other techniques in order to reveal any biases caused by preferentially favouring star-forming galaxies.

Poster Forthcoming

Project Title: Galaxy Formation---a Detailed Examination of Cloud-cloud Interactions

By: David Williamson (e-mail)

Supervisor: Prof. Rob Thacker (www)

Date: June 2009, Presented at the High Performance Computing Symposium, Kingston, Ontario.

Abstract:

The details of the processes involved in the formation of a galaxy from an overdensity of gas within a dark matter distribution are not well understood. The effects of phenomena such as viscosity are considered to be important but thus far have largely been treated by phenomenological models. Such models frequently invoke assumptions about the underlying geometry, something that changes rapidly during the formation of galaxies. Hence testing these predictions via full 3d hydrodynamic simulations is a key step in the validation of these models. Utilising the parallel smoothed-particle-hydrodynamics code HYDRA I have performed simulations of monolithic collapses of spherical clouds of gas with spatially coincident but more massive spheres of dark matter. This dark matter halo evolves dynamically as a system of collisionless particles, interacting with gas particles and other dark matter particles only through gravity. In these simulations, gravitational instabilities cause many clouds to be formed, which interact and take part in the transfer of angular momentum from the inner parts of the galaxy to the outer. It has been predicted using approximate analytic techniques (Bell, ApJ 581:1013-1018, 2002) that cloud-cloud collisions are not a significant source of viscosity in mature evolved galaxies. However it is as yet unclear if this is true during the early stages of galaxy formation when galaxies are particularly gas rich. I present the first results of tracking clouds and their collisions in a full 3D simulation to more fully determine their contribution to viscosity. As far as we are aware this is the first calibration of cloud-cloud effective viscosity.

Animation (mpg)

Project Title:Proton Polarimeter Calibration between 82 and 217 MeV

By: Jackie Glister (e-mail)

SMU Supervisor: Prof. Adam Sarty (www)

Date: 2009, Ph.d Research

Abstract:

The proton analyzing power in carbon has been measured for energies of 82 to 217 MeV and proton scattering angles of 5 to 41 degrees. The measurements were carried out using polarized protons from the elastic scattering H(pol. e, pol. p) reaction and the Focal Plane Polarimeter (FPP) in Hall A of Jefferson Lab. A new parameterization of the FPP p-C analyzing power was fit to the data, which is in good agreement with previous parameterizations and provides an extension to lower energies and larger angles. The main conclusions are that all polarimeters to date give consistent measurements of the carbon analyzing power, independently of the details of their construction and that measuring on a larger angular range significantly improves the polarimeter figure of merit at low energies.

Accepted for Publication (Nuclear Instruments and Methods in Physics Research)

Project Title:The Proton Elastic Form Factor Ratio at Low Momentum Transfer

By: Jackie Glister (e-mail)

SMU Supervisor: Prof. Adam Sarty (www)

Date: 2009, Ph.d Research

Abstract:

High precision measurements of the proton elastic form factor ratio have been made at four-momentum transfers, Q^2, between 0.2 and 0.5 GeV^2. The new data, while consistent with previous results, clearly show a ratio less than unity and significant differences from the central values of several recent phenomenological fits. By combining the new form-factor ratio data with an existing cross-section measurement, one finds that in this Q^2 range the deviation from unity is primarily due to GEp being smaller than the dipole parameterization.

Accepted for Publication (Physical Review Letters)

Project Title:FIR Cores in NGC 2024

By: Tomomi Watanabe (e-mail)

Supervisor: Prof. George Mitchell

Date: May 2007, Presented at the AAS meeting in Honolulu Hawaii

Abstract:

The NGC 2024 region in Orion B produces intense continuum emission at mm and sub-mm wavelengths. In optical, the central region is obscured by a dark lane of dust. A ridge of mm/sub-mm emission containing compact sources named FIR 1 - FIR 7 was detected by Mezger et al. 1988 and Mezger et al. 1992. They obtained a low temperature component of 19 K from modeling the emission, and claimed that the cores were massive protostars. Studies of NGC 2024 in the following years have shown that the cores may be much warmer, and observations have suggested that they are more evolved than previously thought. We present here maps of the NGC 2024 ridge in four transitions of formaldehyde (H2CO), namely 303->202, 322->221, 505->404, and 523->422. We use H2CO line ratios in LTE and LVG models to obtain temperatures and densities for the cores and their surroundings. We find high temperatures for the cores (45 to 85 K) and similar high temperatures for the surrounding ridge gas. These results are consistent with the picture that the entire region, ridge and cores, is heated by the radiation field of the neighboring star cluster. Beam-averaged core densities are typically 2x10^6 cm^-3. Making use of 850 micrometer fluxes from Johnstone et al. 2006 and our new temperatures, we find masses of the FIR clumps to be ~2.0 solar masses lower than the original estimates. We apply the virial theorem to four of the cores, FIRs 3 - 6, and find that all four are unstable against collapse.

Poster (pdf)

Project Title:The Dense Molecular Ridge in NGC 2024

By: Tomomi Watanabe (e-mail)

Supervisor: Prof. George Mitchell

Date: 2008, Master's Research

Abstract:

NGC 2024 is a star-forming region in Orion B that contains a dense molecular ridge. A number of dense cores are seen by their dust emission. This area is investigated using observations of four formaldehyde transitions namely, 303-> 202, 322 -> 221, 505 -> 404, and 523 -> 422. Maps of the ridge are used to provide temperatures and densities, with a focus on the dense-core positions. The formaldehyde transition ratios 303 -> 202/322 -> 221 and 505 -> 404/523 -> 422 offer measurements of the kinetic temperature, while the ratio 303 -> 202/505 -> 404 allows the evaluation of number densities. The resulting temperatures are very warm (45-85 K), with no observed variation between the core centers and their envelopes. H2CO number densities of ~2x10^6 cm^3 at far-infrared (FIR) core positions and H2CO column densities in the range of 6x10^14 to 1x10^15 cm^2 are derived. Using the derived properties in combination with previous submillimeter continuum data, core masses of ~2 M sun are found within a 15'' beam size. H2CO fractional abundances are calculated and resemble previous values found in similar star-forming regions between x10^9 and x10^9. The virial theorem applied to FIR cores 3, 4, 5, and 6 suggests that all four cores are unstable against gravitational collapse.

Accepted for Publication (AJ)

Project Title: Effects of Stellar Rotation on Young Cluster HR Diagrams

By: Chris Geroux (e-mail)

Supervisor: Prof. Robert Deupree (www)

Date: Summer 2007, Master's Research

Abstract:

This thesis investigates the effects of stellar rotation on the HR diagram location of members within young clusters. The rotational effects on luminosity and temperature of a star depend on the viewing angle and the rotation rate of the star and must be included in determining the HR diagram locations. The position of members is used as an indicator of the rotational characteristics of the individual stars. The fraction of clusters with a selected member within restricted ranges of rotation rate and viewing angle has been calculated for each selected member. These selected members are: brightest cluster member, bluest cluster member, and reddest cluster member above a luminosity cutoff. The brightest and bluest members were found to be rapidly rotating and viewed pole-on in 74% and 88% of the clusters respectively. The reddest member above a luminosity cutoff was found to be rapidly rotating and viewed equator on in 94% of the clusters.

Paper (pdf)

Project Title: Determination of Effective Temperatures and Luminosities of Rotating Stars

By: Aaron Gillich (e-mail)

Supervisor: Prof. Robert Deupree (www) & Prof. Ian Short (www)

Date: Summer 2007, Master's Research

Abstract:

Spectral energy distributions for models of arbitrarily rotating stars are computed using two dimensional rotating stellar models, NLTE plane parallel model atmospheres, and a code to integrate the appropriately weighted intensities over the visible surface of the stellar disk. The spectral energy distributions depend on the inclination angle between the observer and the rotation axis of the model. We use these curves to deduce what one would infer the model's luminosity and effective temperature to be assuming the object was nonrotating.

Accepted for Publication (ApJ)

Project Title: Radial and Nonradial Oscillation Modes in Rapidly Rotating Stars

By: Catherine Lovekin (e-mail)

Supervisor: Prof. Robert Deupree (www)

Date: 2008, Ph.d. Research

Abstract:

We use two-dimensional stellar models and a two-dimensional finite difference integration of the linearized pulsation equations to calculate nonradial oscillations. This approach allows us to directly calculate the pulsation modes for a distorted rotating star without treating the rotation as a perturbation. We are also able to express the finite difference solution in the horizontal direction as a sum of multiple spherical harmonics for any given mode. Using these methods, we have investigated the effects of increasing rotation and the number of spherical harmonics on the calculated eigenfrequencies and eigenfunctions and compared the results to perturbation theory. We use 10 Msolar models with velocities ranging from 0 to 420 km s-1 (0.89 Omega_c) and examine low-order p-modes. We find that one spherical harmonic remains reasonable up to a rotation rate around 300 km s-1 (0.69 Omega_c) for the radial fundamental mode, but can fail at rotation rates as low as 90 km s-1 (0.23 Omega_c) for the l=2 p2 mode, based on the eigenfrequencies alone. Depending on the mode in question, a single spherical harmonic may fail at lower rotation rates if the shape of the eigenfunction is taken into consideration. Perturbation theory, in contrast, remains valid up to relatively high rotation rates for most modes. We find the lowest failure surface equatorial velocity is 120 km s-1 (0.30 Omega_c) for the l=2 p2 mode, but failure velocities between 240 and 300 km s-1 (0.58 Omega_c-0.69 Omega_c) are more typical.

Accepted for Publication (ApJ)

Project Title: Surface Temperature and Synthetic Spectral Energy Distributions for Rotationally Deformed Stars

By: Catherine Lovekin (e-mail)

Supervisor: Prof. Robert Deupree (www) & Prof. Ian Short (www)

Date: 2006, Ph.d. Research

Abstract:

Extreme deformation of a stellar surface, such as that produced by rapid rotation, causes the surface temperature and gravity to vary significantly with latitude. Thus, the spectral energy distribution (SED) of a nonspherical star could differ significantly from the SED of a spherical star with the same average temperature and luminosity. Calculation of the SED of a deformed star is often approximated as a composite of several spectra, each produced by a plane-parallel model of given effective temperature and gravity. The weighting of these spectra over the stellar surface, and hence the inferred effective temperature and luminosity, will be dependent on the inclination of the rotation axis of the star with respect to the observer, as well as the temperature and gravity distribution on the stellar surface. Here we calculate the surface conditions of rapidly rotating stars with a two-dimensional stellar structure and evolution code and compare the effective temperature distribution to that predicted by von Zeipel's law. We calculate the composite spectrum for a deformed star by interpolating within a grid of intensity spectra of plane-parallel model atmospheres and integrating over the surface of the star. This allows us to examine the SED for effects of inclination and degree of deformation based on the two-dimensional models. Using this method, we find that the deduced variation of effective temperature with inclination can be as much as 3000 K for an early B star, depending on the details of the underlying model. As a test case for our models, we examine the rapidly rotating star Achernar (? Eri, HD 10144). Recent interferometric observations have determined the star to be quite oblate. Combined with the ultraviolet SED measured by the OAO 2 satellite, we are able to make direct comparisons with observations.

Accepted for Publication (ApJ)

Project Title: The Role of the Equation of State in the Simulation of Astrophysical Fluids

By: Jon Ramsey (e-mail)

Supervisor: Prof. David Clarke (www)

Date: 2005, Presented at the CASCA meeting in Montreal QC

Abstract:

Recently, a number of (magneto-)hydrodynamical ([M-]HD) simulations have avoided using the internal energy equation by implementing a strict polytrope (Casse & Keppens, 2002; Fendt & Cemeljic, 2002; Godon & Livio, 1999; Kawachi & Hanawa, 1998; Kley, 1999; Laughlin et al., 1998; Li et al., 2003; Ogino et al., 1999; Tomisaka, 2002; Tsinganos & Bogovalov, 2000; Vitorino et al., 2003 to name a few). While this can be helpful numerically (with respect to stability and computational effort), and physically justified in some circumstances, the consequences of this choice are often inadequately addressed. Replacing the internal energy equation with a polytrope strictly conserves entropy, forbidding irreversible thermodynamic processes such as contact discontinuities and adiabatic shocks, changing the physics of the simulation, and affecting some of the conclusions drawn from these simulations. While we first became aware of this issue in simulations of jets launched from Keplerian discs (akin to Ouyed & Pudritz, 1997a, 1997b [OP], 1999; Ouyed, Clarke, & Pudritz, 2003), we have since discovered that this issue arises in other types of simulations. To this end, we present simulations for three different problems. First, we present axisymmetric HD jets similar to those published by Norman, Smarr, & Winkler (1985). Although, to our knowledge, no one has published results of this type of simulation using a polytropic equation of state (EOS), the differences between the internal energy equation and strict polytrope are profound and will serve to set the stage for the remainder of the discussion. Next, 2-D HD accretion disc simulations of a "simple" rotating Bondi flow are presented to demonstrate how this problem is not unique to simulations of jets. Finally, we present axisymmetric simulations based on OP. In their simulations of jets launched from Keplerian discs, OP employed a strict polytrope to assure hydrostatic balance in the initial atmosphere to within machine round-off errors. We shall demonstrate that this balance can be maintained without compromising the internal energy equation, resulting in interesting differences. We will discuss the similarities and the differences between simulations using the internal energy equation and the polytropic EOS, emphasizing the non-physical effects arising from the use of a strict polytrope.

Poster

Project Title:Asteroseismology of Intermediate Mass Stars: Pre-main Sequence Evolution

By: Joel Tanner (e-mail)

Supervisor: Prof. David Guenther (www)

Date: Summer 2007, Master's Research

Abstract:

We investigate in detail the potential of asteroseismology in exploring the structure of intermediate mass pre-main sequence stars. It is expected that pre- and post-main sequence stars will produce differing oscillation spectra due to the dependance of oscillation frequencies on the internal structure of that star. We compute densely populated grids of pre- and post-main sequence stellar models, which are then used as a tool to explore the oscillation frequencies of pre-main sequence stars. We examine the cause of the oscillation spectra by correlating the frequencies with changes in stellar structure and comparing fundamental properties of pre-main sequence stars to their post-main sequence counterparts. By fashioning a set of oscillation frequencies designed to mimic observed oscillation spectra, we determine the conditions under which we can distinguish the evolutionary state of a star through its oscillation spectra alone, and explore our ability to constrain stellar parameters as a function of the quality of the observed frequency spectrum. Using the dense grids to construct precise pre-main sequence isochrones, we fit them to two young open clusters. Finally, we present efforts in constraining the stellar parameters of two pre-main sequence stars by matching the observed frequencies to the calculated frequency spectra of all grid models.

Available Through Proquest Dissertations (free preview)

Project Title:The Cepheid Impostor HD 18391 and its Anonymous Parent Cluster

Student Coauthors: Daniel Majaess (www) & Kathleen Moncrieff (e-mail)

Faculty: Prof. David Turner (www) & Dave Lane (www)

Date: 2009,

Abstract:

New and existing photometry for the G0 Ia supergiant HD 18391 is analyzed in order to confirm the nature of the variablity previously detected in the star, which lies off the hot edge of the Cepheid instability strip. Small-amplitude variability at a level of \Delta V = 0.016+-0.002 is indicated, with a period of P=123.04+-0.06 d. A weaker second signal may be present at P=177.84+-0.18 with \Delta V = 0.007+-0.002, likely corresponding to fundamental mode pulsation if the primary signal represents overtone pulsation (123.04/177.84 = 0.69). The star, with a spectroscopic reddening of E(B-V) = 1.02, is associated with heavily-reddened B-type stars in its immediate vicinity that appear to be outlying members of an anonymous young cluster centered ~10 arcmin to the west and 1661+-73 pc distant. The cluster has nuclear and coronal radii of r_n=3.5 arcmin and R_c=14 arcmin, respectively, while the parameters for HD 18391 derived from membership in the cluster with its outlying B stars are consistent with those implied by its Cepheid-like pulsation, provided that it follows the semi-period-luminosity relation expected of such objects. Its inferred luminosity as a cluster member is M_V=-7.76+-0.10, its age (9+-1)x10^6 years, and its evolutionary mass ~19 M_{\sun}. HD 18391 is not a classical Cepheid, yet it follows the Cepheid period-luminosity relation closely, much like another Cepheid impostor, V810 Cen.

Accepted for Publication (AN)

Project Title: A study of the brightness variations in the red supergiant BC Cygni

By: Mina Rohanizadegan

Supervisor: Prof. David Turner (www)

Date: Summer 2006, Master's Research

Abstract:

The variability of the type C semi-regular (SRC) M3.5 Ia supergiant variable BC Cyg is examined with reference to measurements of its photographic B magnitude derived from 866 archival plates in the Harvard and Sternberg collections as well as eye estimates of its visual V magnitude made by members of the AAVSO. BC Cyg is the brightest member of the young open cluster Berkeley 87, so it has a well established reddening, distance, and age. A discrete Fourier analysis was performed on the BC Cygni light curve, as well as on individual subsets of the data. The analysis was made to study the fundamental periods of variability in this red supergiant variable. BC Cyg exhibits interesting features in its century-long baseline of brightness variations that relate to fundamental mode envelope pulsation as well as evolution: an 0.5 magnitude increase in < B > over the past century in conjunction with a steady decrease in pulsation period from ~697 d in 1900 to 688 d in 2000. Despite the increase in < B >, the star's luminosity appears to have decreased over the past century, presumably as a result of stellar evolutionary effects. A detailed examination of a well sampled portion of the star's light curve, data spanning the interval HJD 2442000 to 2449000, by means of non-linear least squares analysis indicates that only one periodicity (686 days) exists in the observations. No secondary peridicity can be detected, to within the constraints of observational uncertainty.

Project Title:Bridging the Gap: Synthetic Radio Observations of Numerical Simulations of Extragalactic Jets

By: Nick MacDonald (e-mail)

Supervisor: Prof. David Clarke (www), Jon Ramsey (e-mail)

Date: May 2008, Presented at the CASCA meeting in Victoria BC

Abstract:

The standard paradigm for radio galaxies is based on high-speed plasma jets, formed in active galactic nuclei, which then penetrate into the surrounding intergalactic medium creating giant lobes of luminous material. These lobes then emit in the radio due to synchrotron radiation from high-energy electrons immersed in weak magnetic fields. Modern computational resources have allowed increasingly sophisticated magnetohydrodynamical (MHD) simulations of these plasma flows; however, simulating the emission from these jet models, and thus bridging the gap between theory and observation, remains a difficult task. I will present a semi-empirical model of synchrotron emission that I have incorporated into full three-dimensional MHD jet simulations. From these models I generate synthetic radio maps that can then be compared to actual observations. My presentation will include results from this radio mapping procedure. By synthetically observing a source whose structure is known beforehand, one can hope to gain insights into what real observations are telling us about these types of jets.

Poster (pdf)

Project Title: Rapid differential rotation in intermediate mass stars

Student Coauthors: Aaron Gillich (e-mail) & Catherine Lovekin (e-mail)

Faculty: Prof. Natalie Toque & Prof. Robert Deupree (www)

Abstract:

ZAMS models with internal differential rotation are computed for 6.5 Mo. and 10 Mo. models with the 2D evolution code ROTORC. Two applications of these models, their pulsational frequencies and deduced locations in the HR diagram, are discussed in order to show how the internal differential rotation could be investigated.

Accepted for Publication (AN)

Project Title: Seismology of Pre-Main-Sequence Stars in NGC 6530

Student Coauthor: Joel Tanner (e-mail)

Faculty: Prof. David Guenther (www)

Abstract:

Nonradial p-mode oscillation spectra, computed from a dense grid of pre-main-sequence models, are fit to observed oscillation spectra of several stars in the young cluster NGC 6530. The five stars we consider, all previously identified as pulsating pre-main-sequence stars, each have from two to nine observed oscillation frequencies. For those stars with a more complete set of frequencies we are able to constrain the models using the oscillation spectra alone and confirm that the stars are in their pre-main-sequence and not post-main-sequence phase of evolution. For the stars with only two observed frequencies we are able to reduce the solution space of possible models. Comparing our model fits to the surface temperatures and luminosities derived from the observed colors and parallaxes, we find that the model fits are consistent with the cluster's distance, i.e., the luminosities agree, but we discover that all of our models are systematically too cool. We attribute some of the discrepancy in the surface temperature to uncertainties in the surface boundary conditions of our models, but argue that most of the difference is a direct consequence of applying a single average color-dependent dereddening correction to all the stars when, in fact, it appears that the stars we selected are embedded in varying degrees of gas and dust. For one of the stars we identify a rotationally split l=1 p-mode from which we derive a rotation period of 18 days. Based on observations with the 0.9 m telescope at the Cerro Tololo Inter-American Observatory, La Serena, Chile.

Accepted for Publication (ApJ)

Project Title: The Period Changes of Polaris

Student Coauthors: Jon Savoy (e-mail) & Jayme Derrah (e-mail)

Faculty: Prof. David Turner (www)

Abstract:

The evolutionary changes in pulsation period for the Cepheid Polaris are reinvestigated using archival observational material (radial velocities, photometry, and eye observations) over the interval 1844 to the present, including new photometry for the star obtained in 2003-2004. The star's pulsation period increased at a rate of 4.5 s yr-1 during that interval, with the exception of a brief hiatus between 1963 and 1966, when it suddenly decreased, possibly as a result of a brief reduction in average stellar radius amounting to -0.055%. At roughly the same time, the pulsation amplitude of Polaris underwent a marked change. Prior to 1963 the V amplitude was in excess of about 0.1 mag, possibly decreasing at a rate of 0.019 mag century-1. Following the hiatus of 1963-1966, the pulsation amplitude underwent a sharp decline and now appears to be erratic on a cycle-to-cycle basis, always smaller than 0.05 mag. The rapid rate of period increase for Polaris is consistent with a first crossing of the Cepheid instability strip, while the hiatus of 1963-1966 and sudden decrease in pulsation amplitude thereafter suggest that the star may have left the instability strip for first crossers at that time, leaving it near the center of the instability strip for Cepheids in higher crossing modes.

Accepted for Publication (PASP)

Project Title: Effects of Uniform and Differential Rotation on Stellar Pulsations

Student: Catherine Lovekin (e-mail)

Supervisor: Prof. Roberter Deupree (www)

Date: 2008, Ph.d Research

We have investigated the effects of uniform rotation and a specific model for differential rotation on the pulsation frequencies of 10 \Msun\ stellar models. Uniform rotation decreases the frequencies for all modes. Differential rotation does not appear to have a significant effect on the frequencies, except for the most extreme differentially rotating models. In all cases, the large and small separations show the effects of rotation at lower velocities than do the individual frequencies. Unfortunately, to a certain extent, differential rotation mimics the effects o f more rapid rotation, and only the presence of some specific observed frequencies with well identified modes will be able to uniquely constrain the internal rotation of pulsating stars.

Accepted for Publication (ApJ)

Project Title: Exploring a scale-free cosmology with spectral index n=-2.25

By: Mark Richardson (email, www)

Supervisor: Prof. Rob Thacker (e-mail)

Date: Spring 2009, 4th Year Undergrad

Abstract:

Structure in today's Universe is comprised of galaxies, groups, clusters and super clusters. This hierarchy is believed to have developed out of the primordial density inhomogeneities in the very early Universe. Development of efficient parallel algorithms and access to high performance computing resources have allowed for the computational analysis of the evolution of these inhomogeneities.
The largest scale-free cosmological simulation was performed to investigate the dynamics of the power spectrum, P(k), where initially P(k) prop k^-2.25. A scale-free simulation is one with no characteristic scales. The cosmological model used was an Einstein-de Sitter Universe where the Universe is flat and only has regular matter contributing to a critical density value. This means the scale factor, a, will evolve as t^2/3. An Einstein-de Sitter Universe satisfies one of two conditions required for a simulation to be scale-free (Smith et al., 2003), while the other is that perturbations satisfy a power spectrum that can be scaled to other wave-numbers.
Structure formation is increasingly difficult to model accurately for n~-3. These difficulties are exhibited by a deviation in the power spectrum from a simple powerlaw. Our simulation is the first of such low n to maintain the scale-free power spectrum. Preliminary comparisons between the simulation's mass function and that predicted by Press and Schechter (1974) are attained, while significant work remains. Difficulties arose in data processing and memory managing, however these issues will be solved in the near future.

Paper (pdf)

Project Title: Calculating Asymmetries In Electron-Proton Scattering

By: Nathan Deg

Supervisor: Prof. Aleksandrs Aleksejevs (e-mail)

Date: Spring 2007, 4th Year Undergrad

Abstract:

In this project we developed a method for calculating asymmetries in electron-proton scattering using Pauli and Dirac Form factors with a monopole form factor approximation. This method was used to calculate contributions to the asymmetry from the leading order and next-to-leading order radiative effects without resorting to one quark radiative corrections. By comparing our calculation to the results of the G0 experiment we were able to extract that the anomalous strange magnetic moment has a positive bias towards $\mu_{S}\sim0.06$.

Paper (pdf)

Project Title:Analysis of Space Charge and Dead Zone Effects in the TWIST Spectrometer

By: Brynle Barrett (e-mail)

Supervisor: Prof. Adam Sarty (www)

Date: Spring 2005, 4th Year Undergrad

Abstract:

Multi-wire chamber ionization detectors experience effects from space charges due to high energy loss of the radiative particles that penetrate them. One of these effects is that due to the dead zoneóthe region near a primary ionization path that becomes non-responsive to a subsequent particle traversal. This thesis reports a thorough study of the dead zone produced by μ⁺ in the proportional counters closest to the target stop in the TWIST spectrometer (PC5 and PC6), with emphasis placed on the method of analysis and the use of raw data to measure dead zone effects.

Two parameters that describe the properties of a dead zone are the dead length, L_dead, which is the average length along a wire in the proportional counter that becomes unresponsive, and the healing time, τh, which is the average time it takes for a dead zone to become responsive again. The dead lengths and healing times for the dead zones in PC5 and PC6 were computed from raw data and are listed in Table 1. Brief descriptions are also given of the TWIST experiment and the physics of dead zones. 

Paper (pdf)

Project Title: Assessing the Yarkovsky Effect in the Kuiper Belt

By: Daniel Majaess (e-mail)

Supervisor: Prof. Joseph Hahn (www)

Date: Spring 2005, 4th Year Undergrad

Abstract:

The Yarkovsky Effect (YE) is a radiation force that while minute, has been shown to help deliver asteroids into near earth crossing orbits (NEOs). The force is divided into a seasonal variant due to an objectís yearly revolution and a diurnal component due to its daily rotation. We show that the seasonal YE causes an orbital decay of <Â>~ −1AU/Gyr for a small subset of Kuiper Belt Objects (KBOs) whose diameters range from 5−100m, while the diurnal effect is of little consequence in the Kuiper Belt. Moreover, compared to a controlled run, the seasonal YE augments the number of KBOs that diffuse into the inner Solar System by a factor of ~ 100. It was thought that the YE could enhance the inward transport of KBOs where they may be detected as Jupiter Family Comets (JFCs), but the bodies considered here are 1-2 orders of magnitude too small.

Paper (pdf)

Project Title: A Numerical Examination of Spiral Density Waves

By: Adam Chaffey (e-mail)

Supervisor: Prof. Joseph Hahn (www)

Date: Spring 2005, 4th Year Undergrad

Abstract:

Spiral wave phenomena have long been observed to exist in discreet gravitating particle-disks. Observations of galactic disks are perhaps the most notable example. The existence of these waves in planetary-ring systems has also been confirmed by direct observation: most prominently in the rings of Saturn by both the Voyager missions, and the more recent Cassini observations. In spite of the phenomena being apparently well-understood, to date no n-body simulations of forced spiral density waves, launched by a perturbing satellite in a gravitating particle-disk, have been
successful.
 

My research consisted of utilizing a modern n-body integration scheme to resolve spiral waves in a planetary-ring system. The integrator utilizes a very fast momentum-conserving tree code to perform force calculations, and a Wisdom-Holman-type map (Wisdom and Holman, 1991) to evolve the system based upon the forces calculated. Many simulations of appropriate candidate systems for the formation of spiral waves were carried out. However, none of these simulations were successful in resolving spiral density waves in a gravitating n-body disk. It is suspected that issues related to the background granularity that is inherent in any randomly generated particle disk was sufficient to mask out any forced wave response actually occurring. I shall attempt to quantify such granularity, and indicate how its effects might be mitigated so that one can observe a true forced spiral density wave response.

Paper (pdf)

Project Title: The Effect of Varying Metal Abundances on the Solar Age
By: Chris Geroux (e-mail)

Supervisor: Prof. David Guenther (www)

Date: Spring 2005, 4th Year Undergrad

Abstract:

After the review of a recent paper by Asplund et al. (2004), which gives a new metal ratio of 0.0165, the following research was inspired.  The effects of varying the metal ratio on the helioseismic age are explored for metal ratios of 0.0165, 0.0244, and 0.275.  In addition three different reaction cross sections were considered (BP92, BP95, and BP97.9) to determine any effects on the helioseismic age.
 

Paper (pdf)

Project Title: Determining the Physical Properties of Molecular Gas in S0 Galaxies
By: Jon Savoy (e-mail)

Supervisor: Prof. Gary Welch (www)

Date: Spring 2005, 4th Year Undergrad

Abstract:

The goal of this study is to use CO line ratios to examine the physical conditions in the molecular gas of several S0 galaxies. These galaxies are disk galaxies that have an extremely small gaseous component. The paucity of gas makes S0s interesting because it implies that their evolution must be significantly different from that of normal spiral galaxies. The molecular gas is studied by determining the intensities of two or more rotational transitions of CO, a molecule that serves as a tracer for molecular hydrogen. Observed intensity ratios are compared with predictions of a Large Velocity Gradient (LVG) model to establish the temperature and density of the gas. The ability to accurately determine these properties is investigated by examining the limitations of the LVG procedure due to uncertainty in both the observations and in the necessary input parameters. The present study is unusual in that data has been collected from two different telescopes (the 30m IRAM radio telescope and the 15m JCMT) that have nearly identical beamwidths for the ¹²CO(1-0) and ¹²CO(2-1) transitions, respectively. This allows the important 2-1/1-0 ratio to be determined without having to make very uncertain corrections for differences in beam sizes. Results for several galaxies are presented with particular attention given to NGC 3607.
 

Paper (pdf)

Project Title: Non-LTE Analysis of Carbon and Oxygen Abundances in Arcturus

By: Jayme Derrah (e-mail)

Supervisor: Prof. Ian Short (www)

Date: Spring 2007, 5th Year Undergrad

Abstract:

Inspired by recent revisions in the derived Carbon (C) and Oxygen (O) abundances in the Sun, we compared LTE and Non-LTE synthetic spectra for models with scaled solar and alpha-enhanced abundances computed with the atmospheric modelling code PHOENIX to the observed spectrum of the standard red giant star Arcturus. We paid special attention to the fit to select C and O lines that were used in the recent re-determination of the Sun's abundances. An equivalent width analysis was performed to determine the error in the previously adopted C and O abundances of Arcturus for the case of LTE and Non-LTE modelling, and scaled solar and alpha-enhanced models. We find that the previously adopted abundances of C and O give rise to discrepant equivalent widths for all modelling treatments, with alpha-enhanced models providing a less discrepant fit than scaled solar models. However, we find that different lines of the same element give inconsistent results for the inferred abundance, which may indicate other inadequacies in the modelling.

Project Title: Determining Planck's constant using the Photoelectric effect.