arXiv subjects

Chengxing Zhai

Publications and source records attributed to Chengxing Zhai.

32 records · Page 2Linked to original sources

arXiv2013

Electric Field Conjugation with the Project 1640 coronagraph

The Project 1640 instrument on the 200-inch Hale telescope at Palomar Observatory is a coronagraphic instrument with an integral field spectrograph at the back end, designed to find young, self-luminous planets around nearby stars. To reach the necessary contrast for this, the PALM-3000 adaptive optics system corrects for fast atmospheric speckles, while CAL, a phase-shifting interferometer in a Mach-Zehnder configuration, measures the quasistatic components of the complex electric field in the pupil plane following the coronagraphic stop. Two additional sensors measure and control low-order modes. These field measurements may then be combined with a system model and data taken separately using a white-light source internal to the AO system to correct for both phase and amplitude aberrations. Here, we discuss and demonstrate the procedure to maintain a half-plane dark hole in the image plane while the spectrograph is taking data, including initial on-sky performance.

astro-ph.IM↗

arXiv2013

The Kappa Andromedae System: New Constraints on the Companion Mass, System Age & Further Multiplicity

Kappa Andromedae is a B9IVn star at 52 pc for which a faint substellar companion separated by 55 AU was recently announced. In this work, we present the first spectrum of the companion, "kappa And B," using the Project 1640 high-contrast imaging platform. Comparison of our low-resolution YJH-band spectra to empirical brown dwarf spectra suggests an early-L spectral type. Fitting synthetic spectra from PHOENIX model atmospheres to our observed spectrum allows us to constrain the effective temperature to ~2000K, as well as place constraints on the companion surface gravity. Further, we use previously reported log(g) and effective temperature measurements of the host star to argue that the kappa And system has an isochronal age of 220 +/- 100 Myr, older than the 30 Myr age reported previously. This interpretation of an older age is corroborated by the photometric properties of kappa And B, which appear to be marginally inconsistent with other 10-100 Myr low-gravity L-dwarfs for the spectral type range we derive. In addition, we use Keck aperture masking interferometry combined with published radial velocity measurements to rule out the existence of any tight stellar companions to kappa And A that might be responsible for the system's overluminosity. Further, we show that luminosity enhancements due to a nearly "pole-on" viewing angle coupled with extremely rapid rotation is unlikely. Kappa And A is thus consistent with its slightly evolved luminosity class (IV) and we propose here that kappa And, with a revised age of 220 +/- 100 Myr, is an interloper to the 30 Myr Columba association with which it was previously associated. The photometric and spectroscopic evidence for kappa And B combined with our re-assesment of the system age implies a substellar companion mass of 50^{+16}_{-13} Jupiter Masses, consistent with a brown dwarf rather than a planetary mass companion.

astro-ph.EP↗

arXiv2012

High Resolution Infrared Imaging & Spectroscopy of the Z Canis Majoris System During Quiescence & Outburst

We present adaptive optics photometry and spectra in the JHKL-bands along with high spectral resolution K-band spectroscopy for each component of the Z Canis Majoris system. Our high angular resolution photometry of this very young (<1 Myr) binary, comprised of an FU Ori object and a Herbig Ae/Be star, were gathered shortly after the 2008 outburst while our high resolution spectroscopy was gathered during a quiescent phase. Our photometry conclusively determine that the outburst was due solely to the embedded Herbig Ae/Be member, supporting results from earlier works, and that the optically visible FU Ori component decreased slightly (~30%) in luminosity during the same period, consistent with previous works on the variability of FU Ori type systems. Further, our high-resolution K-band spectra definitively demonstrate that the 2.294 micron CO absorption feature seen in composite spectra of the system is due solely to the FU Ori component, while a prominent CO emission feature at the same wavelength, long suspected to be associated with the innermost regions of a circumstellar accretion disk, can be assigned to the Herbig Ae/Be member. These findings are in contrast to previous analyses (e.g. Malbet et al 2010, Benisty et al. 2010) of this complex system which assigned the CO emission to the FU Ori component.

astro-ph.SR↗

arXiv2012

Spectral Typing of Late Type Stellar Companions to Young Stars from Low Dispersion Near-Infrared Integral Field Unit Data

We used the Project 1640 near-infrared coronagraph and integral field spectrograph to observe 19 young solar type stars. Five of these stars are known binary stars and we detected the late-type secondaries and were able to measure their JH spectra with a resolution of R\sim30. The reduced, extracted, and calibrated spectra were compared to template spectra from the IRTF spectral library. With this comparison we test the accuracy and consistency of spectral type determination with the low-resolution near-infrared spectra from P1640. Additionally, we determine effective temperature and surface gravity of the companions by fitting synthetic spectra calculated with the PHOENIX model atmosphere code. We also present several new epochs of astrometry of each of the systems. Together these data increase our knowledge and understanding of the stellar make up of these systems. In addition to the astronomical results, the analysis presented helps validate the Project 1640 data reduction and spectral extraction processes and the utility of low-resolution, near-infrared spectra for characterizing late-type companions in multiple systems.

astro-ph.SR↗

arXiv2011

Micro-pixel accuracy centroid displacement estimation and detector calibration

Precise centroid estimation plays a critical role in accurate astrometry using telescope images. Conventional centroid estimation fits a template point spread function (PSF) to the image data. Because the PSF is typically not known to high accuracy due to wavefront aberrations and uncertainties in optical system, a simple Gaussian function is commonly used. PSF knowledge error leads to systematic errors in the conventional centroid estimation. In this paper, we present an accurate centroid estimation algorithm by reconstructing the PSF from well sampled (above Nyquist frequency) pixelated images. In the limit of an ideal focal plane array whose pixels have identical response function (no inter-pixel variation), this method can estimate centroid displacement between two 32$\times$32 images to sub-micropixel accuracy. Inter-pixel response variations exist in real detectors, {\it e.g.}~CCDs, which we can calibrate by measuring the pixel response of each pixel in Fourier space. The Fourier transforms of the inter-pixel variations of pixel response functions can be conveniently expressed in terms of powers of spatial wave numbers using their Taylor series expansions. Calibrating up to the third order terms in this expansion, we show that our centroid displacement estimation is accurate to a few micro-pixels using simulated data. This algorithm is applicable to the new proposed mission concept Nearest Earth Astrometry Telescope (NEAT) to achieve mirco-arcsecond accuracy in relative astrometry for detecting terrestrial exoplanets. This technology is also applicable to high precision photometry missions.

astro-ph.IM↗

arXiv2010

A New High Contrast Imaging Program at Palomar Observatory

We describe a new instrument that forms the core of a long-term high contrast imaging program at the 200-inch Hale Telescope at Palomar Observatory. The primary scientific thrust is to obtain images and low-resolution spectroscopy of brown dwarfs and young Jovian mass exoplanets in the vicinity of stars within 50 parsecs of the Sun. The instrument is a microlens-based integral field spectrograph integrated with a diffraction limited, apodized-pupil Lyot coronagraph, mounted behind the Palomar adaptive optics system. The spectrograph obtains imaging in 23 channels across the J and H bands (1.06 - 1.78 microns). In addition to obtaining spectra, this wavelength resolution allows suppression of the chromatically dependent speckle noise, which we describe. We have recently installed a novel internal wave front calibration system that will provide continuous updates to the AO system every 0.5 - 1.0 minutes by sensing the wave front within the coronagraph. The Palomar AO system is undergoing an upgrade to a much higher-order AO system ("PALM-3000"): a 3388-actuator tweeter deformable mirror working together with the existing 241-actuator mirror. This system will allow correction with subapertures as small as 8cm at the telescope pupil using natural guide stars. The coronagraph alone has achieved an initial dynamic range in the H-band of 2 X 10^-4 at 1 arcsecond, without speckle noise suppression. We demonstrate that spectral speckle suppression is providing a factor of 10-20 improvement over this bringing our current contrast at an arcsecond to ~2 X 10^-5. This system is the first of a new generation of apodized pupil coronagraphs combined with high-order adaptive optics and integral field spectrographs (e.g. GPI, SPHERE, HiCIAO), and we anticipate this instrument will make a lasting contribution to high contrast imaging in the Northern Hemisphere for years.

astro-ph.IM↗

arXiv1996

Transition rate for process involving particles with high momentum in a plasma and infrared physics for QED plasma

We derive a formula for computing the transition rate for a process involving particles with momentum much higher than the temperature and chemical potentials in a plasma by using an effective field theory approach. We apply it to collision of charged particles with hard momentum inside a QED plasma. The Debye screening effect and the damping of a charged particle moving in QED plasma are studied. Using the Bloch-Nordsieck resummation, the infrared divergences due to the absence of magnetic screening for QED plasma are shown not to appear in physically measurable rates. The soft plasmon absorption and emission for charged particles are discussed.

hep-ph↗

arXiv1995

Gauge-fixing parameter dependence of two-point gauge variant correlation functions

The gauge-fixing parameter $ξ$ dependence of two-point gauge variant correlation functions is studied for QED and QCD. We show that, in three Euclidean dimensions, or for four-dimensional thermal gauge theories, the usual procedure of getting a general covariant gauge-fixing term by averaging over a class of covariant gauge-fixing conditions leads to a nontrivial gauge-fixing parameter dependence in gauge variant two-point correlation functions (e.g. fermion propagators). This nontrivial gauge-fixing parameter dependence modifies the large distance behavior of the two-point correlation functions by introducing additional exponentially decaying factors. These factors are the origin of the gauge dependence encountered in some perturbative evaluations of the damping rates and the static chromoelectric screening length in a general covariant gauge. To avoid this modification of the long distance behavior introduced by performing the average over a class of covariant gauge-fixing conditions, one can either choose a vanishing gauge-fixing parameter or apply an unphysical infrared cutoff.

hep-ph↗

arXiv1995

The Free Energy of Hot Gauge Theories with Fermions Through g^5

We compute the free energy density $F$ for gauge theories, with fermions, at high temperature and zero chemical potential. In the expansion $F=T^4 [c_0+c_2 g^2+c_3 g^3+(c'_4\ln g+c_4)g^4+ (c'_5\ln g+c_5)g^5+O(g^6)]$, we determine $c'_5$ and $c_5$ analytically by calculating two- and three-loop diagrams. The $g^5$ term constitutes the first correction to the $g^3$ term and is for the non-Abelian case the last power of $g$ that can be computed within perturbation theory. We find that the $g^5$ term receives no contributions from overlapping double-frequency sums and that $c'_5$ vanishes.

hep-ph↗

arXiv1995

Atomic effects in tritium beta decay

The electron neutrino mass has been measured in several tritium beta decay experiments. These experiments are sensitive to a small neutrino mass because the energy release of the decay is small. But the very smallness of the energy release implies that the Coulomb interactions of the slowly moving emitted beta electron are relatively large. Using field theoretic techniques, we derive a systematic and controlled expansion which accounts for the Coulomb effects, including the mutual interaction of the beta ray electron and the electron in the final $^3{\rm He}^+$ ion. In our formulation, an effective potential which describes the long range Coulomb force experienced by the beta ray is introduced to ensure that our expansion is free of infrared divergences. Both the exclusive differential decay rate to a specific final $^3{\rm He}^+$ state and the inclusive differential decay rate are calculated to order $η^2$, where $η$ is the usual Coulomb parameter. We analyze the order $η^2$ correction to the beta ray spectrum and estimate how it may affect the neutrino mass squared parameter and the endpoint energy when this corrected spectrum is used to compare with the experiments. We find that the effect is small.

hep-ph↗

arXiv1994

The Three-Loop Free Energy for High-Temperature QED and QCD with Fermions

We compute the free energy density for gauge theories, with fermions, at high temperature and zero chemical potential. Specifically, we analytically compute the free energy through $O(g^4)$, which requires the evaluation of three-loop diagrams. This computation extends our previous result for pure gauge QCD.

hep-ph↗

arXiv1994

The Three-Loop Free Energy for Pure Gauge QCD

We compute the free energy density for pure non-Abelian gauge theory at high temperature and zero chemical potential. The three-loop result to $O(g^4)$ is [a lovely formula. Get the paper and print it out.] We examine the sensitivity of this result to the choice of renormalization scale. We also give a result for the free energy of scalar $ϕ^4$ theory, correcting a result previously given in the literature.

hep-ph↗

arXiv1992

2 \to n threshold production at tree level

The threshold behavior of a theory with two coupled scalar fields $χ$ and $ϕ$ is investigated. We compute the amplitude for two on-mass-shell $χ$-particles to produce an arbitrary number of $ϕ$-particles at rest in the tree approximation.

hep-ph↗

arXiv1992

Large-order perturbation theory for the electromagnetic current-current correlation function

The constraints imposed by asymptotic freedom and analyticity on the large-order behavior of perturbation theory for the electromagnetic current-current correlation function are examined. By suitably applying the renormalization group, the coefficients of the asymptotic expansion in the deep Euclidean region may be expressed explicitly in terms of the perturbative coefficients of the Minkowski space discontinuity (the $R$-ratio in $e^+ e^-$ scattering). This relation yields a ``generic'' prediction for the large-order behavior of the Euclidean perturbation series and suggests the presence of non-perturbative $1/q^2$ correction in the Euclidian correlation function. No such ``generic'' prediction can be made for the physically measurable $R$-ratio. A novel functional method is developed to obtain these results.

hep-ph↗