The Stellar Halo in the Large Magellanic Cloud: Mass, Luminosity, and Microlensing Predictions

Recently obtained kinematic data has shown that the Large Magellanic Cloud (LMC) possesses an old stellar halo. In order to further characterize the properties of this halo, parametric King models are fit to the surface density of RR Lyrae stars. Using data from both the MACHO and OGLE II microlensing surveys, the model fits yield the center of their distribution at RA = 05:21.1+-0.8, Dec = -69:45+-6 (J2000) and a core radius of 1.42+-0.12 kpc. As a check the halo model is compared with RR Lyrae star counts in fields near the LMC's periphery previously surveyed with photographic plates. These data, however, require a cautious interpretation. Several topics regarding the LMC stellar halo are discussed. First, the properties of the halo imply a global mass-to-light ratio of M/L_V = 5.3+-2.1 and a total mass of 1.6+-0.6 10^10 M_sun for the LMC in good agreement with estimates based on the rotation curve. Second, although the LMC's disk and halo are kinematically distinct, the shape of the surface density profile of the halo is remarkably similar to that of the young disk. For example, the best-fit exponential scale length for the RR Lyrae stars is 1.47+-0.08 kpc, which compares to 1.46 kpc for the LMC's blue light. In the Galaxy, the halo and disk do not resemble each other like this. Finally, a local maximum in the LMC's microlensing optical depth due to halo-on-disk stellar self-lensing is predicted. For the parameters of the stellar halo obtained, this maximum is located near MACHO events LMC-4 and LMC-23, and is large enough to possibly account for these two events, but not for all of the observed microlensing.Comment: 11 pages, 1 figure, accepted to ApJ Letter

The Mass of the MACHO-LMC-5 Lens Star

We combine the available astrometric and photometric data for the 1993 microlensing event MACHO-LMC-5 to measure the mass of the lens, M=0.097 +/- 0.016 Msun. This is the most precise direct mass measurement of a single star other than the Sun. In principle, the measurement error could be reduced as low as 10% by improving the trig parallax measurement using, for example, the Space Interferometry Mission. Further improvements might be possible by rereducing the original photometric lightcurve using image subtraction or by obtaining new, higher-precision baseline photometry of the source. We show that the current data strongly limit scenarios in which the lens is a dark (i.e., brown-dwarf) companion to the observed M dwarf rather than being the M dwarf itself. These results set the stage for a confrontation between mass estimates of the M dwarf obtained from spectroscopic and photometric measurements and a mass measurement derived directly from the star's gravitational influence. This would be the first such confrontation for any isolated star other than the Sun

New Understanding of Large Magellanic Cloud Structure, Dynamics and Orbit from Carbon Star Kinematics

We derive general expressions for the LMC velocity field which we fit to kinematical data for 1041 carbon stars. We demonstrate that all previous studies of LMC kinematics have made unnecessary over-simplifications that have led to incorrect estimates of important structural parameters. We compile and improve LMC proper motion estimates to support our analysis. We find that the kinematically determined position angle of the line of nodes is 129.9 +/- 6.0 deg. The LMC inclination changes at a rate di/dt = -103 +/- 61 deg/Gyr, a result of precession and nutation induced by Milky Way tidal torques. The LMC rotation curve V(R) has amplitude 49.8 +/- 15.9 km/s, 40% lower than what has previously (and incorrectly) been inferred from e.g. HI. The dynamical center of the carbon stars is consistent with the center of the bar and the center of the outer isophotes, but not with the HI kinematical center. The enclosed mass inside 8.9 kpc is (8.7 +/- 4.3) x 10^9 M_sun, more than half of which is due to a dark halo. The LMC has a larger vertical thickness than has traditionally been believed. Its V/sigma is less than the value for the Milky Way thick disk. We discuss the implications for the LMC self-lensing optical depth. We determine the LMC velocity and orbit in the Galactocentric rest frame and find it to be consistent with the range of velocities that has been predicted by models for the Magellanic Stream. The Milky Way dark halo must have mass >4.3 x 10^{11} M_sun and extent >39 kpc for the LMC to be bound. We predict the LMC proper motion velocity field, and discuss techniques for kinematical distance estimation. [ABRIDGED]Comment: 57 pages, LaTeX, with 11 PostScript figures. Submitted to the Astronomical Journa

Feynman path-sum quantum computer simulator

Dissertação de mestrado em Physics EngineeringClassical quantum simulators are essential tools for studying quantum systems and simulating quantum algorithms. The hardware limitations of NISQ (Noisy Intermediate Scale Quantum) devices make being able to build, test and run a quantum circuit/algorithm many times, and even test it under various noise scenarios, in a classical computer, extremely useful. Currently, the most prominent technique for classically simulating quantum circuits is known as Schrodinger type simulation. The memory usage of simulations using this technique increases exponentially with the number of qubits in a circuit, reaching prohibitive memory values relatively fast. This serves as motivation to investigate complementary classical quantum simulation techniques. The present work offers an investigation on how to improve the runtime of the Feynman path-sum approach for classical simulation of quantum circuits, taking into account the computational basis input and output states given and the branching structure generated by branching gates in a quantum circuit. The main contributions of this dissertation are two Feynman path-sum based simulation algorithms. These algorithms were able to successfully simulate quantum circuits with a large number of qubits (> 30) using polynomial space and it was demonstrated that the time complexity of these algorithms is more strongly influenced by the circuit structure rather than the circuit size.Os simuladores quânticos clássicos são ferramentas essenciais para o estudo de sistemas quânticos e simulação de algoritmos quânticos. As limitações de hardware dos dispositivos NISQ (Noisy Intermediate-Scale Quantum) tornam extremamente útil a possibilidade de construir, testar e executar um circuito/algoritmo quântico muitas vezes, e mesmo testá-lo sob vários cenários de ruído, num computador clássico. Actualmente, a técnica mais proeminente de simulação clássica de circuitos quânticos e conhecida como simulação do tipo Schrodinger. A utilização de memoria das simulações que utilizam esta técnica aumenta exponencialmente com o número de qubits num circuito, atingindo valores de memória proibitivos com relativa rapidez. Este facto serve de motivação para investigar técnicas complementares de simulação quântica clássica. O presente trabalho oferece uma investigação sobre a forma de melhorar o tempo de execução do método de soma de caminhos de Feynman para a simulação clássica de circuitos quânticos, tendo em conta os estados, na base computacional, de entrada e saída e a estrutura de ramificação gerada pelas portas de ramificação num circuito quântico. As principais contribuições desta dissertação˜ são dois algoritmos de simulação baseados na soma de caminhos de Feynman. Estes algoritmos foram capazes de simular com sucesso circuitos quânticos com um grande número de qubits (> 30) usando espaço polinomial e foi demonstrado que a complexidade temporal destes algoritmos e mais fortemente influenciada pela estrutura do circuito do que pelo tamanho do circuito