Duane F. Carbon

Spectral irradiance calibration in the infrared. III - The influence of CO and SiO

We describe first efforts to establish a network of calibrated infrared spectra of «standard stars» suitable for calibration of at least low-resolution infrared spectrometers using ground-based, airborne, and satellite-borne broadband sensors. The focus of this paper is on the crucial 5-8 μm region, inaccessible from the ground, in K and M giants. In this region the fundamental bands of CO and SiO cause substantial departures from featureless pseudo-continua. These departures are, of course, well-known to stellar atmosphere theorists. However, they are still ignored by many astronomical infrared photometrists and spectroscopists who assume that these bright stars can be represent by blackbodies at their effective temperatures

Far-Infrared and Millimeter Continuum Studies of K Giants: α Bootis and α Tauri

We have imaged two normal, noncoronal, infrared-bright K giants, α Tau and α Boo, in the 1.4 and 2.8 mm continua using BIMA. These stars have been used as important absolute calibrators for several infrared satellites. Our goals are (1) to establish whether these stars radiate as simple photospheres or possess long-wavelength chromospheres and (2) to make a connection between millimeter-wave and far-infrared (FIR) absolute flux calibrations. To accomplish these goals we also present Infrared Space Observatory Long Wavelength Spectrometer measurements of both these K giants. The FIR and millimeter continuum radiation is produced in the vicinity of the temperature minimum in α Tau and α Boo. We find that current photospheric models predict fluxes in reasonable agreement with those observed for wavelengths that sample the upper photosphere, namely, ≤125 μm in α Tau and α Boo. We clearly detect chromospheric radiation from both stars by 2.8 mm (1.4 mm in the case of α Boo). Only additional observations can determine precisely where beyond 125 μm the purely radiative models fail. Until then, purely radiative models for these stars can only be used with confidence for calibration purposes below 125 μm.

Electron-Impact Excitation Cross Sections for Modeling Non-Equilibrium Gas

In order to provide a database for modeling hypersonic entry in a partially ionized gas under non-equilibrium, the electron-impact excitation cross sections of atoms have been calculated using perturbation theory. The energy levels covered in the calculation are retrieved from the level list in the HyperRad code. The downstream flow-field is determined by solving a set of continuity equations for each component. The individual structure of each energy level is included. These equations are then complemented by the Euler system of equations. Finally, the radiation field is modeled by solving the radiative transfer equation.

Electron Stark Broadening Database for Atomic N, O, and C Lines

A database for efficiently computing the electron Stark broadening line widths for atomic N, O, and C lines is constructed. The line width is expressed in terms of the electron number density and electronatom scattering cross sections based on the Baranger impact theory. The state-to-state cross sections are computed using the semiclassical approximation, in which the atom is treated quantum mechanically whereas the motion of the free electron follows a classical trajectory. These state-to-state cross sections are calculated based on newly compiled line lists. Each atomic line list consists of a careful merger of NIST, Vanderbilt, and TOPbase line datasets from wavelength 50 nm to 50 micrometers covering the VUV to IR spectral regions. There are over 10,000 lines in each atomic line list. The widths for each line are computed at 13 electron temperatures between 1,000 K 50,000 K. A linear least squares method using a four-term fractional power series is then employed to obtain an analytical fit for each line-width variation as a function of the electron temperature. The maximum L2 error of the analytic fits for all lines in our line lists is about 5%.

Bayesian Source Separation Applied to Identifying Complex Organic Molecules in Space

Emission from a class of benzene-based molecules known as Polycyclic Aromatic Hydrocarbons (PAHs) dominates the infrared spectrum of star-forming regions. The observed emission appears to arise from the combined emission of numerous PAH species, each with its unique spectrum. Linear superposition of the PAH spectra identifies this problem as a source separation problem. It is, however, of a formidable class of source separation problems given that different PAH sources potentially number in the hundreds, even thousands, and there is only one measured spectral signal for a given astrophysical site. Fortunately, the source spectra of the PAHs are known, but the signal is also contaminated by other spectral sources. We describe our ongoing work in developing Bayesian source separation techniques relying on nested sampling in conjunction with an ON/OFF mechanism enabling simultaneous estimation of the probability that a particular PAH species is present and its contribution to the spectrum.

Exploring the SDSS Data Set with Linked Scatter Plots. I. EMP, CEMP, and CV Stars

We present the results of a search for EMP, CEMP, and cataclysmic variable stars using a new exploration tool based on linked scatter plots (LSPs). Our approach is especially designed to work with very large spectrum data sets such as the SDSS, LAMOST, RAVE, and Gaia data sets and can be applied to stellar, galaxy, and quasar spectra. As a demonstration, we conduct a search for EMP, CEMP, and cataclysmic variable stars in the SDSS DR10 data set. We first created a 3326-dimensional phase space containing nearly 2 billion measures of the strengths of over 1600 spectral features in 569,738 SDSS stars. These measures capture essentially all the stellar atomic and molecular species visible at the resolution of SDSS spectra. We show how LSPs can be used to quickly isolate and examine interesting portions of this phase space. To illustrate, we use LSPs coupled with cuts in selected portions of phase space to extract EMP stars, C-rich EMP stars, and CV stars. We present identifications for 59 previously unrecognized candidate EMP stars and 11 previously unrecognized candidate CEMP stars. We also call attention to 2 candidate He II emission CV stars found by the LSP approach that have not yet been discussed in the literature.

Characterization of Interstellar Organic Molecules

Understanding the origins of life has been one of the greatest dreams throughout history. It is now known that star‐forming regions contain complex organic molecules, known as Polycyclic Aromatic Hydrocarbons (PAHs), each of which has particular infrared spectral characteristics. By understanding which PAH species are found in specific star‐forming regions, we can better understand the biochemistry that takes place in interstellar clouds. Identifying and classifying PAHs is not an easy task: we can only observe a single superposition of PAH spectra at any given astrophysical site, with the PAH species perhaps numbering in the hundreds or even thousands. This is a challenging source separation problem since we have only one observation composed of numerous mixed sources. However, it is made easier with the help of a library of hundreds of PAH spectra. In order to separate PAH molecules from their mixture, we need to identify the specific species and their unique concentrations that would provide the given ...

A Search for Candidate Li-rich Giant Stars in SDSS DR10

We report the results of a search for candidate Li-rich giants among 569,738 stars of the SDSS DR10 dataset. With small variations, our approach is based on that taken in an earlier search for EMP/CEMP stars and uses the same dataset. As part of our investigation, we demonstrate a method for separating post-main sequence and main sequence stars cooler than Teff ~ 5800 K using our feature strength measures of the Sr II 4078, Fe I 4072, and Ca I 4227 lines. By taking carefully selected cuts in a multi-dimensional phase space, we isolate a sample of potential Li-rich giant stars. From these, using detailed comparison with dwarf and giant MILES stars, and our own individual spectral classifications, we identify a set of high likelihood candidate Li-rich giant stars. We offer these for further study to promote an understanding of these enigmatic objects.

Estimating Background Spectra

All measurements consist of a mixture of the signal of interest and additional signals called the background. Here we focus on the problem of measuring infrared spectra emitted by interstellar clouds. The signals of interest are infrared emissions from polycyclic aromatic hydrocarbons (PAHs), which are a class of complex organic molecules. The PAH emissions are characterized by emission bands near 3.3, 6.2, 7.7, 8.6, 11.2, and 15-20 microns. The background consists of a host of associated spectral signals which, in the simplest case, can include emissions from multiple Planck blackbodies as well as broadband and narrowband emissions. To analyze the PAH spectra we must accurately assess this background. To do this, we have developed a Bayesian algorithm based on nested sampling (Skilling 2005, Sivia & Skilling 2006). The spectral model consists of a mixture of Planck functions and Gaussians. We demonstrate this algorithm on both synthetic data and infrared spectra recorded from interstellar clouds. The result shows that the algorithm can accurately identify and remove simple backgrounds. In future work, we plan to incorporate mixtures of PAH spectra and more complex models for the background so that the algorithm will simultaneously estimate both the signals of interest and the background.

Absolute Spectrally Continuous Stellar Irradiance Calibration in the Infrared

We present first efforts to establish a network of absolutely calibrated continuous infrared spectra of standard stars across the l-35^m range in order to calibrate arbitrary broad and narrow passbands and low-resolution spectrometers from ground-based, airborne, balloon, and satellite-borne sensors. The value to photometry of such calibrated continuous spectra is that one can integrate arbitrary filters over the spectra and derive the stellar in-band flux, monochromatic flux density, and hence the magnitude, for any site. This work is based on new models of Sirius and Vega by Kurucz which were calculated by him, for the first time, with realistic stellar metallicities and a customized finely-gridded infrared wavelength scale. We have absolutely calibrated these two spectra and have calculated monochromatic flux densities for both stars, and isophotal wavelengths, for a number of infrared filters. Preliminarily, the current IRAS point source flux calibration is too high by 2, 6, 3, and 12% at 12, 25, 60, and 100/tm, respectively.