AC03
Stellar spectra from dark lines to abundance models
One dark line carries several kinds of information. Start by separating motion from width, then follow the light through an emitting and absorbing layer. Fit a saved spectrum only after the instrument and continuum have joined the calculation.
Start here
What dark lines in starlight tell us
Follow light through a stellar layer to separate the clues left by temperature, absorbing atoms, motion and a blurred spectrograph.
L1
Separate line position from line width
Define vacuum wavelength, radial velocity and intensity, then derive a first Doppler estimate and calculate thermal and instrumental widths.
L2
Put absorption and emission in one equation
Derive the uniform LTE slab solution, check transparent and isothermal limits, and connect optical depth to the curve of growth.
L3
From emergent intensity to a fitted spectrum
Combine continuum placement, instrumental convolution and errors in one observing model, then test wavelength calibration and model mismatch.
Tutorial
Fit a frozen calibrated stellar spectrum
Rerun pinned CALSPEC Vega data and a synthetic control with standard-library Python. Fit continuum, line centers and widths, then inspect failures.
L4 · Paper guide
Read a 3D non-LTE abundance paper
Read a pinned iron-abundance paper figure by figure, checking model comparisons, correction signs, interpolation errors and stellar tests.
The browser uses a stated teaching slab. The tutorial keeps observed CALSPEC fluxes, synthetic parameter-recovery data and fitted empirical profiles distinct. The final guide reads a published 3D non-LTE calculation.
Sources and calculation scope
- Condon & Ransom, Essential Radio Astronomy, chapter 7.3
Equations 7.38–7.64: transition energy, Einstein detailed balance, stimulated emission as negative absorption, LTE populations and Planck source. Used to define the consistent artificial finite-level atom.
- STIS Data Handbook, section 4.3, Factors Limiting Flux and Wavelength Accuracy
Wavelength zero point, dispersion and slit centering; used to distinguish an apparent profile displacement from a precision radial velocity.
- Krumholz, ASTR 4003/8003 (2019), Class 1, Radiative transfer I
Equations 24–28: integrating factor, formal solution and constant-source slab. Independently derived and checked by numerical transport here.
- Condon & Ransom, Essential Radio Astronomy, Appendix E
Transfer equation 2.27, LTE Kirchhoff relation 2.30 and Planck law 2.86; used at optical wavelengths with the full Planck function.
- STScI CALSPEC, alpha_lyr_stis_011.fits (25 October 2022)
Pinned FITS header and all 82 selected observed STIS rows from 4750 to 4975 vacuum angstroms. File checksum and exact source rows accompany the tutorial.
- MAST reference atlases and CALSPEC releases
Data provenance and release context. The exercise deliberately pins version eleven rather than the changing current directory.
- Amarsi, Liljegren & Nissen (2022), 3D non-LTE iron abundances in FG-type dwarfs, arXiv:2209.13449v3
Version three, 9 December 2022: sections 2.1–2.4 and 3.1, figures 2, 3, 8 and 10. The paper-reading guide checks conventions and sensitivity; it does not rerun the 3D solver.
- Amarsi et al., public non-LTE grids
Author-linked model-data entry for later study; files are not loaded or reproduced by the browser Lab.