2,406 research outputs found
Spectropolarimetric multi line analysis of stellar magnetic fields
In this paper we study the feasibility of inferring the magnetic field from
polarized multi-line spectra using two methods: The pseudo line approach and
The PCA-ZDI approach. We use multi-line techniques, meaning that all the lines
of a stellar spectrum contribute to obtain a polarization signature. The use of
multiple lines dramatically increases the signal to noise ratio of these
polarizations signatures. Using one technique, the pseudo-line approach, we
construct the pseudo-line as the mean profile of all the individual lines. The
other technique, the PCA-ZDI approach proposed recently by Semel et al. (2006)
for the detection of polarized signals, combines Principle Components Analysis
(PCA) and the Zeeman Do ppler Imaging technique (ZDI). This new method has a
main advantage: the polarized signature is extracted using cross correlations
between the stellar spectra nd functions containing the polarization properties
of each line. These functions are the principal components of a database of
synthetic spectra. The synthesis of the spectra of the database are obtained
using the radiative transfer equations in LTE. The profiles built with the
PCA-ZDI technique are denominated Multi-Zeeman-Signatures. The construction of
the pseudo line as well as the Multi-Zeeman-Signatures is a powerful tool in
the study of stellar and solar magnetic fields. The information of the physical
parameters that governs the line formation is contained in the final polarized
profiles. In particular, using inversion codes, we have shown that the magnetic
field vector can be properly inferred with both approaches despite the magnetic
field regime.Comment: Accepted for publication in Astronomy and Astrophysic
Zeeman-Doppler Imaging of Late-Type Stars -- The Surface Magnetic Field of II Peg
Late-type stars in general possess complicated magnetic surface fields which
makes their detection and in particular their modeling and reconstruction
challenging. In this work we present a new Zeeman-Doppler imaging code which is
especially designed for the application to late-type stars. This code uses a
new multi-line cross-correlation technique by means of a principal component
analysis to extract and enhance the quality of individual polarized line
profiles. It implements the full polarized radiative transfer equation and uses
an inversion strategy that can incorporate prior knowledge based on solar
analogies. Moreover, our code utilizes a new regularization scheme which is
based on local maximum entropy to allow a more appropriate reproduction of
complex surface fields as those expected for late-type stars. In a first
application we present Zeeman-Doppler images of II Pegasi which reveal a
surprisingly large scale surface structure with one predominant (unipolar)
magnetic longitude which is mainly radially oriented.Comment: Astronomische Nachrichten / Astronomical Notes Vol. 328, Issue 10, p.
104
PCA detection and denoising of Zeeman signatures in stellar polarised spectra
Our main objective is to develop a denoising strategy to increase the signal
to noise ratio of individual spectral lines of stellar spectropolarimetric
observations.
We use a multivariate statistics technique called Principal Component
Analysis. The cross-product matrix of the observations is diagonalized to
obtain the eigenvectors in which the original observations can be developed.
This basis is such that the first eigenvectors contain the greatest variance.
Assuming that the noise is uncorrelated a denoising is possible by
reconstructing the data with a truncated basis. We propose a method to identify
the number of eigenvectors for an efficient noise filtering.
Numerical simulations are used to demonstrate that an important increase of
the signal to noise ratio per spectral line is possible using PCA denoising
techniques. It can be also applied for detection of magnetic fields in stellar
atmospheres. We analyze the relation between PCA and commonly used well-known
techniques like line addition and least-squares deconvolution. Moreover, PCA is
very robust and easy to compute.Comment: accepted to be published in A&
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Understanding Turnover in Employees of Color in STEM Fields: The Role of Identity, Fit, Microaggressions, and Racial Climate
The purpose of this study was to expand on previous literature by examining the role of racial identity attitudes in the workplace, which have been infrequently studied. The current study tested the relationships between workplace variables and racial identity attitude statuses, specifically in STEM fields. This study, using a national sample of 485 STEM employees, examined associations using bivariate correlations between two predictor variables (racial climate and racial/ethnic microaggressions) on three outcome variables (job satisfaction, turnover intentions, and person-organization fit). Moderation relationships of four variables (racial identity attitude statuses: Conformity, Dissonance, Immersion, Internalization) on the paths between the predictor and outcome variables were also examined using structural equation modeling. Results provide mixed support for hypothesized associations. Racial climate and microaggressions attitudes yielded significant direct associations with the workplace outcome variables and Dissonance and Immersion attitudes yielded moderation effects on the paths between predictor and outcome variables. Implications of these findings are discussed, as well as implications for future research, clinical interventions, and workplace recommendations
Radio wars in PNG: NauFM's tough struggle
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A Challenging Process Outlined
The development process is defined by variables. A successful developer identifies, monitors, understands, and attempts to control these variables. Most development projects require the developer to be prepared to negotiate with any number of parties, and much of the time, the tie-up or acquisition of the property is only the first step in a long process. For many projects, the discretionary approval process at the local level, commonly called entitlement, is the major consumer of time and resources. It is through this very process, however, that value is created. Gaining an understanding of the legal aspects of land-use regulation in general— and entitlement in particular—can help developers effectively identify and manage risk. This paper provides an overview of the procedural and legal underpinnings of the entitlement process in order to illustrate the risks involved. Several techniques for creating value and mitigating risk, as applied through the approvals process, are also discussed
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