arXiv subjects

C. Ian Short

Publications and source records attributed to C. Ian Short.

At least 19 recordsLinked to original sources

arXiv2026

AutoClassMK: A public neural network for automatic 2D MK classification of normal stars in basic Python

We present AutoClassMK, a simple, fully-connected, five-layer double-headed neural network written entirely in Python and Numpy that classifies normal stellar spectra conforming to the libr18 MK atlas in the 2D MK classification system with a high degree of precision and recall. AutoClassMK has the distinction of having transparent basic code with no calls to specialized libraries. In this paper we take care to explicitly describe in detail the ideas and operations that enable the network. Training AutoClassMK required us to develop large, noisy artificial training and test sets by augmenting the libr18 and libr18_27 MK atlases and to simplify the luminosity classification so that every combination of spectral- and luminosity-class is represented in the training set. We then test the network's ability to predict the MK spectral type of noisy augmentations of spectra in the libr18_225 MK atlas. We then implemented the same architecture in PyTorch to gain further insight and to enable execution on CUDA GPU's. All codes and the training and test sets are available from the OpenStars www site: www.ap.smu.ca/OpenStars.

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arXiv2025

Chroma+ model stellar surface intensities: Spherical formal solution

We announce V. 2025-08-08 of the Chroma+ suite of stellar atmosphere and spectrum modelling codes for fast, approximate, effectively platform-independent stellar spectrum synthesis, written in a number of free well-supported programming languages. The Chroma+ suite now computes the emergent surface intensity and flux distributions and the hydrostatic pressure structure assuming a spherical atmosphere rather than local flatness by implementing the analytic formal solution of the 1D spherical radiative transfer equation of Chapman (1966} based on an integration factor. We present our adaptation and discretization of the solution and demonstrate the resulting impact of our sphericity treatment on a number of computed observables, including exo-planet transit light-curves. All codes are available from the OpenStars www site: www.ap.smu.ca/OpenStars.

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arXiv2023

The ChromaStar+ modelling suite and the VALD line list

We present Version 2023-02-04 (ISO) of the Chroma+ atmospheric, spectrum, and transit light-curve modelling suite, which incorporates the VALD atomic line list. This is a major improvement as the previous versions used the much smaller NIST line list. The NIST line list is still available in Chroma+ for those projects requiring speed over completeness of line opacity. We describe a procedure for exploiting the ''Array job'' capability of the slurm workload manager on multi-cpu machines to compute broadband high resolution spectra with the VALD line list quickly using the Java version of the code (ChromaStarServer (CSS)). The inclusion of a much larger line list more completely allows for the many weaker lines that over-blanket the blue band in late-type stars and has allowed us to reduce the amount of additional ad hoc continuous opacity needed to fit the solar spectral energy distribution (SED). The additional line opacity exposed a subtle bug in the spectrum synthesis procedure that was causing residual blue line wing opacity to accumulate at shorter wavelengths. We present our latest fits to the observed solar SED and to the observed rectified high resolution visible band spectra of the Sun and the standard stars Arcturus and Vega. We also introduce the fully automated Burke-Gaffney Observatory (BGO) at Saint Mary's University (SMU) and compare our synthetic spectra to low resolution spectra obtained with our grism spectrograph that is available to students. The fully automated BGO, the spectrograph, and the BGO spectrum reduction procedure are fully described in a companion paper. All codes are available from the OpenStars www site: www.ap.smu.ca/OpenStars.

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arXiv2023

The Burke-Gaffney Observatory: A fully roboticized remote-access observatory with a low resolution spectrograph

We describe the current state of the Burke-Gaffney Observatory (BGO) at Saint Mary's University - a unique fully roboticized remote-access observatory that allows students to carry out imaging, photometry, and spectroscopy projects remotely from anywhere in the world via a web browser or social media. Stellar spectroscopy is available with the ALPY 600 low resolution grism spectrograph equipped with a CCD detector. We describe our custom CCD spectroscopy reduction procedure written in the Python programming language and demonstrate the quality of fits of synthetic spectra computed with the ChromaStarServer (CSS) code to BGO spectra. The facility along with the accompanying Python BGO spectroscopy reduction package and the CSS spectrum synthesis code provide an accessible means for students anywhere to carry our projects at the undergraduate honours level. BGO web pages for potential observers are at the site: observatory.smu.ca/bgo-useme. All codes are available from the OpenStars www site: openstars.smu.ca/

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arXiv2022

Chroma+GAS: An Expedited Solution for the Chemical Equilibrium for Cool Stellar Atmospheres

We describe a unique approach to economizing the solution to the general chemical equilibrium and equation-of-state problem for late-type stars, including diatomic and polyatomic molecules, that is fast, accurate, and suitable for responsive approximate data modelling applications, and to more intensive modelling approaches in which the calculation of the gas equilibrium must be expedited to allow other aspects to be treated more realistically. The method, based on a novel economization of the Newton's method of solution of the linearized Saha and conservation equations, has been implemented in Python and made available as a stand-alone package, GASPy, and has been integrated into the interactive Python atmosphere and spectrum modelling code ChromaStarPy. As a result, ChromaStarPy now computes the state of the gas, the number density of absorbers, and the surface flux spectrum, with consistent inclusion of 105 chemical species, including 34 diatomic, and 16 polyatomic, neutral molecules, as well as H$^-$ and H$_2^+$, as well as many neutral and ionized atomic species. The economized method converges very rapidly and greatly improves the code's relevance to late-type stellar and brown dwarf spectrum modelling. We provide a brief overview of the GAS methodology, and present some illustrative results for the chemical equilibrium and spectrum for an M-type bright giant and dwarf, and a comparison to results of the PHOENIX/PPRESS package. All codes are available from the OpenStars www site: www.ap.smu.ca/OpenStars.

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arXiv2020

In situ exo-planet transit lightcurve modelling with the Chroma+ suite

We have added to the Chroma+ suite of stellar atmosphere and spectrum modelling codes the ability to synthesize the exo-planet transit lightcurve for planets of arbitrary size up to 10% of the host stellar radius, and arbitrary planetary and stellar mass and orbital radius (thus determining orbital velocity) and arbitrary orbital inclination. The lightcurves are computed in situ, integrated with the radiative transfer solution for the radiation field emerging from the stellar surface, and there is no limb-darkening parameterization. The lightcurves are computed for the Johnson-Bessel photometric system UBVRIHJK. We describe our method of computing the transit path, and the reduction in flux caused by occultation, and compare our lightcurve to an analytic solution with a four-parameter limb-darkening parameterization for the case of an edge-on transit of the Sun by Earth. This capability has been added to all ports and variations, including the Python port, ChromaStarPy, and the version that interpolates among the fully line-blanketed ATLAS9 surface intensity distributions, ChromaStarAtlas. All codes may be accessed at www.ap.smu.ca/OpenStars and at GitHub (github.com/sevenian3).

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arXiv2018

ChromaStarAtlas: Browser-based visualization of the ATLAS9 stellar structure and spectrum grid

ChromaStaraAtlas (CSA) is a web application that uses the ChromaStar (CS) user interface (UI) to allow users to navigate and display a subset of the uniformly computed comprehensive ATLAS9 grid of atmosphere and spectrum models. It provides almost the same functionality as the CS UI in its more basic display modes, but presents the user with primary and post-processed outputs, including photometric color indices, based on a properly line blanketed spectral energy distribution (SED). CSA interpolates in logarithmic quantities within the subset of the ATLAS9 grid ranging in Teff from 3500 to 25000 K, in log g from 0.0 to 5.0, and in [Fe/H] from 0.0 to -1.0 at a fixed microturbulence parameter of 2 km/s, and presents outputs derived from the monochromatic specific intensity distribution, I_lambda, in the lambda range from 250 to 2500 nm, and performs an approximate continuum rectification of the corresponding flux spectrum F_lambda based on its own internal model of the corresponding continuous extinction distribution, kappa^C_lambda, based on the procedures of CS. Optional advanced plots can be turned on that display both the primary atmospheric structure quantities from the public ATLAS9 data files, and secondary structure quantities computed from internal modeling. Unlike CS, CSA allows for activities in which students derive Teff values from fitting observed colors. The application may be found at www.ap.smu.ca/~ishort/OpenStars.

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arXiv2018

ChromaStarPy: A stellar atmosphere and spectrum modeling and visualization lab in python

We announce ChromaStarPy, an integrated general stellar atmospheric modeling and spectrum synthesis code written entirely in python V. 3. ChromaStarPy is a direct port of the ChromaStarServer (CSServ) Java modeling code described in earlier papers in this series, and many of the associated JavaScript (JS) post-processing procedures have been ported and incorporated into CSPy so that students have access to ready-made "data products". A python integrated development environment (IDE) allows a student in a more advanced course to experiment with the code and to graphically visualize intermediate and final results, ad hoc, as they are running it. CSPy allows students and researchers to compare modeled to observed spectra in the same IDE in which they are processing observational data, while having complete control over the stellar parameters affecting the synthetic spectra. We also take the opportunity to describe improvements that have been made to the related codes, ChromaStar (CS), CSServ and ChromaStarDB (CSDB) that, where relevant, have also been incorporated into CSPy. The application may be found at the home page of the OpenStars project: http://www.ap.smu.ca/~ishort/OpenStars/ .

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arXiv2017

ChromaStarDB: SQL database-driven spectrum synthesis, and more

We present an alternate deployment of the GrayStarServer (now ChromaStarServer (CSS)) pedagogical stellar atmosphere and spectrum synthesis WWW-application, namely ChromaStarDB (CSDB), in which the atomic line list used for spectrum synthesis is implemented as an SQL database table rather than as a more conventional byte-data file. This allows for very flexible selection criteria to determine which transitions are extracted from the line list for inclusion in the synthesis, and enables novel pedagogical and research experiments in spectrum synthesis. This line selection flexibility is reflected in the CSDB UI. The database extraction is very fast and would be appropriate for the larger line lists of research-grade modeling codes. We also take the opportunity to present major additions to the ChromaStar and CSS codes that are also reflected in CSDB: i) TiO band opacity in the JOLA approximation, ii) Metal b-f and Rayleigh scattering opacity, iii) 2D implementation of the flux integral, iv) Improvement of the N_e convergence, and v) Expansion of the exo-planet modeling parameters, and vi) General improvements to the UI. The applications may be found at the home page of the OpenStar project: www.ap.smu.ca/OpenStars/.

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arXiv2016

NLTE Stellar Population Synthesis of Globular Clusters using Synthetic Integrated Light Spectra I: Constructing the IL Spectra

We present an investigation of the globular cluster population synthesis method of McWilliam & Bernstein (2008), focusing on the impact of NLTE modeling effects and CMD discretization. Johnson-Cousins-Bessel U-B, B-V, V-I, and J-K colors are produced for 96 synthetic integrated light spectra with two different discretization prescriptions and three degrees of NLTE treatment. These color values are used to compare NLTE and LTE derived population ages. Relative contributions of different spectral types to the integrated light spectra for different wavebands are measured. Integrated light NLTE spectra are shown to be more luminous in the UV and optical than LTE spectra, but show stronger absorption features in the IR. The main features showing discrepancies between NLTE and LTE integrated light spectra may be attributed to light metals, primarily Fe I, Ca I, and Ti I, as well as TiO molecular bands. Main Sequence stars are shown to have negligible NLTE effects at IR wavelengths compared to more evolved stars. Photometric color values are shown to vary at the millimagnitude level as a function of CMD discretization. Finer CMD sampling for the upper main sequence and turnoff, base of the red giant branch, and the horizontal branch minimizes this variation. Differences in ages derived from LTE and NLTE IL spectra are found to range from 0.55 to 2.54 Gyr, comparable to the uncertainty in GC ages derived from color indices with observational uncertainties of 0.01 magnitudes, the limiting precision of the Harris catalog (Harris 1996).

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arXiv2016

GrayStarServer: Server-side spectrum synthesis with a browser-based client-side user interface

I present GrayStarServer (GSS), a stellar atmospheric modeling and spectrum synthesis code of pedagogical accuracy that is accessible in any web browser on commonplace computational devices and that runs on a time-scale of a few seconds. The addition of spectrum synthesis annotated with line identifications extends the functionality and pedagogical applicability of GSS beyond that of its predecessor, GrayStar3 (GS3). The spectrum synthesis is based on a line list acquired from the NIST atomic spectra database, and the GSS post-processing and user interface (UI) client allows the user to inspect the plain text ASCII version of the line list, as well as to apply macroscopic broadening. Unlike GS3, GSS carries out the physical modeling on the server side in Java, and communicates with the JavaScript and HTML client via an asynchronous HTTP request. I also describe other improvements beyond GS3 such as more realistic modeling physics and use of the HTML element for higher quality plotting and rendering of results, and include a comparison to Phoenix modeling. I also present LineListServer, a Java code for converting custom ASCII line lists in NIST format to the byte data type file format required by GSS so that users can prepare their own custom line lists. I propose a standard for marking up and packaging model atmosphere and spectrum synthesis output for data transmission and storage that will facilitate a web-based approach to stellar atmospheric modeling and spectrum synthesis. I describe some pedagogical demonstrations and exercises enabled by easily accessible, on-demand, responsive spectrum synthesis. GSS may serve as a research support tool by providing quick spectroscopic reconnaissance. GSS may be found at www.ap.smu.ca/~ishort/OpenStars/.

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arXiv2015

grayStar3 - gray no more: More physical realism and a more intuitive interface - all still in a WWW browser

The goal of the openStar project is to turn any WWW browser, running on any platform, into a virtual star equipped with parameter knobs and instrumented with output displays that any user can experiment with using any device for which a browser is available. grayStar3 (gS3) is a major improvement upon GrayStar 2.0 (GS2), both in the physical realism of the modeling and the intuitiveness of the user interface. The code integrates scientific modeling in JavaScript with output visualization HTML. The user interface is adaptable so as to be appropriate for a large range of audiences from the high-school to the introductory graduate level. The modeling is physically based and all outputs are determined entirely and directly by the results of in situ modeling, giving the code significant generality and credibility for pedagogical applications. gS3 also models and displays the circumstellar habitable zone (CHZ) and allows the user to adjust the greenhouse effect and albedo of the planet. In its default mode the code is guaranteed to return a result within a few second of wall-clock time on any device. The more advanced user has the option of turning on more realistic physics modules that address more advanced topics in stellar astrophysics. gS3 is a public domain, open source project and the code is available from www.ap.smu.ca/~ishort/grayStar3/ and is on GitHub. gS3 effectively serves as a public library of generic JavaScript+HTML plotting routines that may be recycled by the community.

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arXiv2015

NLTE and LTE Lick indices for red giants from [M/H] 0.0 to -6.0 at SDSS and IDS spectral resolution

We investigate the dependence of the complete system of 22 Lick indices on overall metallicity scaled from solar abundances, [M/H], from the solar value, 0.0, down to the extremely-metal-poor (XMP) value of -6.0, for late-type giant stars (MK luminosity class III, log(g)=2.0) of MK spectral class late-K to late-F (3750 < Teff < 6500 K) of the type that are detected as "fossils" of early galaxy formation in the Galactic halo and in extra-galactic structures. Our investigation is based on synthetic index values, I, derived from atmospheric models and synthetic spectra computed with PHOENIX in LTE and Non-LTE (NLTE), where the synthetic spectra have been convolved to the spectral resolution, R, of both IDS and SDSS (and LAMOST) spectroscopy. We identify nine indices, that we designate "Lick-XMP", that remain both detectable and significantly [M/H]-dependent down to [M/H] values of at least ~-5.0, and down to [M/H] ~ -6.0 in five cases, while also remaining well-behaved . For these nine, we study the dependence of I on NLTE effects, and on spectral resolution. For our LTE I values for spectra of SDSS resolution, we present the fitted polynomial coefficients, C_n, from multi-variate linear regression for I with terms up to third order in the independent variable pairs (Teff, [M/H]), and (V-K, [M/H]), and compare them to the fitted C_n values of Worthey et al. (1994) at IDS spectral resolution.

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arXiv2014

GrayStar: A Web application for pedagogical stellar atmosphere and spectral line modelling and visualisation

GrayStar is a stellar atmospheric and spectral line modelling, post-processing, and visualisation code, suitable for classroom demonstrations and laboratory-style assignments, that has been developed in Java and deployed in JavaScript and HTML. The only software needed to compute models and post-processed observables, and to visualise the resulting atmospheric structure and observables, is a common Web browser. Therefore, the code will run on any common PC or related X86 (-64) computer of the type that typically serves classroom data projectors, is found in undergraduate computer laboratories, or that students themselves own, including those with highly portable form-factors such as net-books and tablets. The user requires no experience with compiling source code, reading data files, or using plotting packages. More advanced students can view the JavaScript source code using the developer tools provided by common Web browsers. The code is based on the approximate gray atmospheric solution and runs quickly enough on current common PCs to provide near-instantaneous results, allowing for real time exploration of parameter space. I describe the user interface and its inputs and outputs and suggest specific pedagogical applications and projects. Therefore, this paper may serve as a GrayStar user manual for both instructors and students. In an accompanying paper, I describe the computational strategy and methodology as necessitated by Java and JavaScript. I have made the application itself, and the HTML, CSS, JavaScript, and Java source files available to the community. The Web application and source files may be found at www.ap.smu.ca/~ishort/GrayStar.

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arXiv2014

GrayStar: A Web application for pedagogical stellar atmosphere and spectral line modelling and visualisation II: Methods

GrayStar is a stellar atmospheric and spectral line modelling, post-processing, and visualisation code, suitable for classroom demonstrations and laboratory-style assignments, that has been developed in Java and deployed in JavaScript and HTML. The only software needed to compute models and post-processed observables, and to visualise the resulting atmospheric structure and observables, is a common Web browser. Therefore, the code will run on any common PC or related X86 (-64) computer of the type that typically serves classroom data projectors, is found in undergraduate computer laboratories, or that students themselves own, including those with highly portable form-factors such as net-books and tablets. The user requires no experience with compiling source code, reading data files, or using plotting packages. More advanced students can view the JavaScript source code using the developer tools provided by common Web browsers. The code is based on the approximate gray atmospheric solution and runs quickly enough on current common PCs to provide near-instantaneous results, allowing for real time exploration of parameter space. I describe the computational strategy and methodology as necessitated by Java and JavaScript. In an accompanying paper, I describe the user interface and its inputs and outputs and suggest specific pedagogical applications and projects. I have made the application itself, and the HTML, CSS, JavaScript, and Java source files available to the community. The Web application and source files may be found at www.ap.smu.ca/~ishort/GrayStar.

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arXiv2014

NLTE 1.5D Modeling of Red Giant Stars

Spectra for 2D stars in the 1.5D approximation are created from synthetic spectra of 1D non-local thermodynamic equilibrium (NLTE) spherical model atmospheres produced by the PHOENIX code. The 1.5D stars have the spatially averaged Rayleigh-Jeans flux of a K3-4 III star, while varying the temperature difference between the two 1D component models ($ΔT_{\mathrm{1.5D}}$), and the relative surface area covered. Synthetic observable quantities from the 1.5D stars are fitted with quantities from NLTE and local thermodynamic equilibrium (LTE) 1D models to assess the errors in inferred $T_{\mathrm{eff}}$ values from assuming horizontal homogeneity and LTE. Five different quantities are fit to determine the $T_{\mathrm{eff}}$ of the 1.5D stars: UBVRI photometric colors, absolute surface flux SEDs, relative SEDs, continuum normalized spectra, and TiO band profiles. In all cases except the TiO band profiles, the inferred $T_{\mathrm{eff}}$ value increases with increasing $ΔT_{\mathrm{1.5D}}$. In all cases, the inferred $T_{\mathrm{eff}}$ value from fitting 1D LTE quantities is higher than from fitting 1D NLTE quantities and is approximately constant as a function of $ΔT_{\mathrm{1.5D}}$ within each case. The difference between LTE and NLTE for the TiO bands is caused indirectly by the NLTE temperature structure of the upper atmosphere, as the bands are computed in LTE. We conclude that the difference between $T_{\mathrm{eff}}$ values derived from NLTE and LTE modelling is relatively insensitive to the degree of the horizontal inhomogeneity of the star being modeled, and largely depends on the observable quantity being fit.

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arXiv2013

Modeling the near-UV band of GK stars, Paper III: Dependence on abundance pattern

We extend the grid of NLTE models presented in Paper II to explore variations in abundance pattern in two ways: 1) The adoption of the Asplund et al. (2009) (GASS10) abundances, 2) For stars of metallicity, [M/H], of -0.5, the adoption of a non-solar enhancement of alpha-elements by +0.3 dex. Moreover, our grid of synthetic spectral energy distributions (SEDs) is interpolated to a finer numerical resolution in both T_eff (Delta T_eff = 25 K) and log g (Delta log g = 0.25). We compare the values of T_eff and log g inferred from fitting LTE and Non-LTE SEDs to observed SEDs throughout the entire visible band, and in an ad hoc "blue" band. We compare our spectrophotometrically derived T_eff values to a variety of T_eff calibrations, including more empirical ones, drawn from the literature. For stars of solar metallicity, we find that the adoption of the GASS10 abundances lowers the inferred T_eff value by 25 - 50 K for late-type giants, and NLTE models computed with the GASS10 abundances give T_eff results that are marginally in better agreement with other T_eff calibrations. For stars of [M/H]=-0.5 there is marginal evidence that adoption of alpha-enhancement further lowers the derived T_eff value by 50 K. Stellar parameters inferred from fitting NLTE models to SEDs are more dependent than LTE models on the wavelength region being fitted, and we find that the effect depends on how heavily line blanketed the fitting region is, whether the fitting region is to the blue of the Wien peak of the star's SED, or both.

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arXiv2012

Matching the Spectral Energy Distribution and p Mode Oscillation Frequencies of the Rapidly Rotating Delta Scuti Star ? Ophiuchi with a 2D Rotating Stellar Model

Spectral energy distributions are computed using 2D rotating stellar models and NLTE plane parallel model atmospheres. A rotating, 2D stellar model has been found which matches the observed ultraviolet and visible spectrum of ? Oph. The SED match occurs for the interferometrically deduced surface shape and inclination, and is different from the SED produced by spherical models. The p mode oscillation frequencies in which the latitudinal variation is modelled by a linear combination of eight Legendre polynomials were computed for this model. The five highest and seven of the nine highest amplitude modes show agreement between computed axisymmetric, equatorially symmetric mode frequencies and the mode frequencies observed by MOST to within the observational error. Including nonaxisymmetric modes up through |m| = 2 and allowing the possibility that the eight lowest amplitude modes could be produced by modes which are not equatorially symmetric produces matches for 24 out of the 35 MOST modes to within the observational error and another eight modes to within twice the observational error. The remaining three observed modes can be fit within 4.2 times the observational error, but even these may be fit to within the observational error if the criteria for computed modes are expanded.

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