59 research outputs found
Modification of the MOOG spectral synthesis codes to account for Zeeman broadening of spectral lines
In an attempt to widen access to the study of magnetic fields in stellar
astronomy, I present MOOGStokes, a version of the MOOG one-dimensional LTE
radiative transfer code, overhauled to incorporate a Stokes vector treatment of
polarized radiation through a magnetic medium. MOOGStokes is a suite of three
complementary programs, which together can synthesize the disk-averaged
emergent spectrum of a star with a magnetic field. The first element (a
pre-processing script called CounterPoint) calculates for a given magnetic
field strength, wavelength shifts and polarizations for the components of
Zeeman sensitive lines. The second element (a MOOG driver called SynStokes
derived from the existing MOOG driver Synth) uses the list of Zeeman shifted
absorption lines together with the existing machinery of MOOG to synthesize the
emergent spectrum at numerous locations across the stellar disk, accounting for
stellar and magnetic field geometry. The third and final element (a
post-processing script called DiskoBall) calculates the disk-averaged spectrum
by weighting the individual emergent spectra by limb darkening and projected
area, and applying the effects of Doppler broadening. All together, the
MOOGStokes package allows users to synthesize emergent spectra of stars with
magnetic fields in a familiar computational framework. MOOGStokes produces
disk-averaged spectra for all Stokes vectors (I, Q, U, and V), normalized by
the continuum. MOOGStokes agrees well with the predictions of INVERS10 a
polarized radiative transfer code with a long history of use in the study of
stellar magnetic fields. In the non-magnetic limit, MOOGStokes also agrees with
the predictions of the scalar version of MOOG.Comment: 10 pages, 4 figures Accepted for publication by the Astronomical
Journal. Code and tutorial available at http://www.mpia.de/~dee
Constructing a spectral photometer for the study of light pollution
Due to the character of the original source materials and the nature of batch digitization, quality control issues may be present in this document. Please report any quality issues you encounter to [email protected], referencing the URI of the item.Includes bibliographical references (leaf 22).In this paper, I describe a method and apparatus for carrying out a systematic spectroscopic mapping I([lambda], z, [Phi]) of the night sky, as proposed by Stefano Rosoni. Once completed, this method should prove to be simple and effective, while the SLR film camera spectral photometer constructed and described within should prove to be inexpensive and easily reproducible. The spectral photometer is simply a single slit diffraction apparatus mounted to the rear of a telescope. The resulting interference pattern is recorded as an image projected on the film inside the camera. In order to correlate the image recorded on the film to a spectrum containing information about the intensity and wavelength of the light pollution, the photographs were scanned into digital format and analyzed by a series of computer programs. While a film spectral photometer is in itself, nothing new, the computer algorithm used to extrapolate film response curves was developed by Paul Debeveck and Jitendra Malik for use in computer graphics. I apply their algorithm to the problem of calibration of a spectral photometer and bypass the myriad of tedious and time consuming calibrations which make film cameras almost more trouble than they are worth. Problems from the unsuccessful first prototype are discussed, as well as suggested improvements for further versions PACS numbers: 95.45. + i, 95.55.Qf, 95.75.P
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An investigation of the physical parameters of young stellar objects
textStudies of the temporal evolution of young stars and their associated properties rely upon the ability of astronomers to determine ages and masses of objects in different evolutionary states. The best method for determining the age and mass of a young stellar object is to place the object on the Hertzsprung-Russell (HR) diagram and to compare to theoretical evolutionary tracks. Accurate ages allow the investigation of the temporal evolution of properties associated with stellar youth (accretion rates, X-ray activity, circumstellar excess, etc...). One property intimately linked with stellar youth is the presence (or absence) of an optically thick primordial circumstellar disk. Objects in "young" star forming regions are more likely to show evidence for a disk than objects in "older" clusters. Within a single cluster, the picture is not as clear. There exist objects in very young clusters (~1 Myr) which show no evidence for circumstellar disks, and there exist objects in very old clusters (~10 Myr), which show evidence for robust disks, suggesting a variable other than stellar age is driving the evolution of the disks. To investigate whether these outliers are due to age spreads, initial conditions, or simply appear anomalous due to erroneous age determinations, we must determine better placements in the HR diagram by carefully transforming observable quantities (spectral type and apparent magnitude) into the quantities necessary for comparison evolutionary models (effective temperature and luminosity). In the Ophiuchus star forming region, I investigate whether or not objects with disks are younger than disk-less objects. I find no difference in the ages of the two populations, but the systematic and random uncertainties are large enough to mask all but the largest age differences. In the hope of better determining the physical parameters of young stellar objects, I embark on a spectral synthesis campaign to produce comparison synthetic spectra which account for the effects of magnetic fields. This requires the modification of the MOOG spectral synthesis program to handle the full Stokes vector treatment for polarized radiation through a magnetized medium. I create a grid of synthetic spectra covering ranges in effective temperature, surface gravity, and average magnetic field strength relevant for studies of young stellar objects, and develop a Chi-squared minimization routine to determine the best fit synthetic spectrum for a given observed spectrum at an arbitrary resolving power. This grid of synthetic spectra will be an invaluable complement to future near infrared, large band-pass, high-resolving power spectrographs (i.e. IGRINS). In addition to these observational and theoretical attempts to reduce systematic errors, I also helped to develop a suite of silicon and KRS-5 grisms for use in the FORCAST instrument, a mid infrared camera on the SOFIA telescope. These grisms will afford the imaging instrument a mid infrared spectroscopic capability at wavelengths normally inaccessible from the ground. I also report on my work to help write FG Widget, the quick-look reduction software package developed to support grism observations.Astronom
A Grism Design Review and the as-built performance of the silicon grisms for JWST-NIRCAM
Grisms are dispersive transmission optics that find their most frequent use
in instruments that combine imaging and spectroscopy. This application is
particularly popular in the infrared where imagers frequently have a cold pupil
in their optical path that is a suitable location for a dispersive element. In
particular, several recent and planned space experiments make use of grisms in
slit-less spectrographs capable of multi-object spectroscopy. We present an
astronomer-oriented general purpose introduction to grisms and their use in
current and future astronomical instruments. We present a simple, step-by-step
procedure for adding a grism spectroscopy capability to an existing imager
design. This procedure serves as an introduction to a discussion of the device
performance requirements for grisms, focusing in particular on the problems of
lithographically patterned silicon devices, the most effective grism technology
for the 1.1-8 micron range. We begin by summarizing the manufacturing process
of monolithic silicon gratings. We follow this with a report in detail on the
as-built performance of parts constructed for a significant new space
application, the NIRCam instrument on JWST and compare these measurements to
the requirements.Comment: Accepted for publication in PAS
Infrared wavefront sensing for adaptive optics assisted Galactic Center observations with the VLT interferometer and GRAVITY: operation and results
This article describes the operation of the near-infrared wavefront sensing
based Adaptive Optics (AO) system CIAO. The Coud\'e Infrared Adaptive Optics
(CIAO) system is a central auxiliary component of the Very Large Telescope
(VLT) interferometer (VLTI). It enables in particular the observations of the
Galactic Center (GC) using the GRAVITY instrument. GRAVITY is a highly
specialized beam combiner, a device that coherently combines the light of the
four 8-m telescopes and finally records interferometric measurements in the
K-band on 6 baselines simultaneously. CIAO compensates for phase disturbances
caused by atmospheric turbulence, which all four 8 m Unit Telescopes (UT)
experience during observation. Each of the four CIAO units generates an almost
diffraction-limited image quality at its UT, which ensures that maximum flux of
the observed stellar object enters the fibers of the GRAVITY beam combiner. We
present CIAO performance data obtained in the first 3 years of operation as a
function of weather conditions. We describe how CIAO is configured and used for
observations with GRAVITY. In addition, we focus on the outstanding features of
the near-infrared sensitive Saphira detector, which is used for the first time
on Paranal, and show how it works as a wavefront sensor detector.Comment: 12 pages, 8 figures, accepted for publication in Instruments (open
access journal from mdpi
The GRAVITY Coud\'e Infrared Adaptive Optics (CIAO) system for the VLT Interferometer
GRAVITY is a second generation instrument for the VLT Interferometer,
designed to enhance the near-infrared astrometric and spectro-imaging
capabilities of VLTI. Combining beams from four telescopes, GRAVITY will
provide an astrometric precision of order 10 micro-arcseconds, imaging
resolution of 4 milli-arcseconds, and low and medium resolution
spectro-interferometry, pushing its performance far beyond current infrared
interfero- metric capabilities. To maximise the performance of GRAVITY,
adaptive optics correction will be implemented at each of the VLT Unit
Telescopes to correct for the effects of atmospheric turbulence. To achieve
this, the GRAVITY project includes a development programme for four new
wavefront sensors (WFS) and NIR-optimized real time control system. These
devices will enable closed-loop adaptive correction at the four Unit Telescopes
in the range 1.4-2.4 {\mu}m. This is crucially important for an efficient
adaptive optics implementation in regions where optically bright references
sources are scarce, such as the Galactic Centre. We present here the design of
the GRAVITY wavefront sensors and give an overview of the expected adaptive
optics performance under typical observing conditions. Benefiting from newly
developed SELEX/ESO SAPHIRA electron avalanche photodiode (eAPD) detectors
providing fast readout with low noise in the near-infrared, the AO systems are
expected to achieve residual wavefront errors of \leq400 nm at an operating
frequency of 500 Hz.Comment: to be published in Proc. SPIE vol. 8446 (2012
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Hyperpolarized<sup>13</sup>C-Pyruvate Metabolism as a Surrogate for Tumor Grade and Poor Outcome in Renal Cell Carcinoma—A Proof of Principle Study
Differentiating aggressive clear cell renal cell carcinoma (ccRCC) from indolent lesions is challenging using conventional imaging. This work prospectively compared the metabolic imaging phenotype of renal tumors using carbon-13 MRI following injection of hyperpolarized [1-13C]pyruvate (HP-13C-MRI) and validated these findings with histopathology. Nine patients with treatment-naïve renal tumors (6 ccRCCs, 1 liposarcoma, 1 pheochromocytoma, 1 oncocytoma) underwent pre-operative HP-13C-MRI and conventional proton (1H) MRI. Multi-regional tissue samples were collected using patient-specific 3D-printed tumor molds for spatial registration between imaging and molecular analysis. The apparent exchange rate constant (kPL) between 13C-pyruvate and 13C-lactate was calculated. Immunohistochemistry for the pyruvate transporter (MCT1) from 44 multi-regional samples, as well as associations between MCT1 expression and outcome in the TCGA-KIRC dataset, were investigated. Increasing kPL in ccRCC was correlated with increasing overall tumor grade (ρ = 0.92, p = 0.009) and MCT1 expression (r = 0.89, p = 0.016), with similar results acquired from the multi-regional analysis. Conventional 1H-MRI parameters did not discriminate tumor grades. The correlation between MCT1 and ccRCC grade was confirmed within a TCGA dataset (p < 0.001), where MCT1 expression was a predictor of overall and disease-free survival. In conclusion, metabolic imaging using HP-13C-MRI differentiates tumor aggressiveness in ccRCC and correlates with the expression of MCT1, a predictor of survival. HP-13C-MRI may non-invasively characterize metabolic phenotypes within renal cancer.</jats:p
Hyperpolarized 13C-Pyruvate Metabolism as a Surrogate for Tumor Grade and Poor Outcome in Renal Cell Carcinoma-A Proof of Principle Study.
Differentiating aggressive clear cell renal cell carcinoma (ccRCC) from indolent lesions is challenging using conventional imaging. This work prospectively compared the metabolic imaging phenotype of renal tumors using carbon-13 MRI following injection of hyperpolarized [1-13C]pyruvate (HP-13C-MRI) and validated these findings with histopathology. Nine patients with treatment-naïve renal tumors (6 ccRCCs, 1 liposarcoma, 1 pheochromocytoma, 1 oncocytoma) underwent pre-operative HP-13C-MRI and conventional proton (1H) MRI. Multi-regional tissue samples were collected using patient-specific 3D-printed tumor molds for spatial registration between imaging and molecular analysis. The apparent exchange rate constant (kPL) between 13C-pyruvate and 13C-lactate was calculated. Immunohistochemistry for the pyruvate transporter (MCT1) from 44 multi-regional samples, as well as associations between MCT1 expression and outcome in the TCGA-KIRC dataset, were investigated. Increasing kPL in ccRCC was correlated with increasing overall tumor grade (ρ = 0.92, p = 0.009) and MCT1 expression (r = 0.89, p = 0.016), with similar results acquired from the multi-regional analysis. Conventional 1H-MRI parameters did not discriminate tumor grades. The correlation between MCT1 and ccRCC grade was confirmed within a TCGA dataset (p < 0.001), where MCT1 expression was a predictor of overall and disease-free survival. In conclusion, metabolic imaging using HP-13C-MRI differentiates tumor aggressiveness in ccRCC and correlates with the expression of MCT1, a predictor of survival. HP-13C-MRI may non-invasively characterize metabolic phenotypes within renal cancer
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