Stellar spectroscopy with XT GRID

1. Stellar spectroscopy with XTGRID Peter Nemeth Tübingen 2016

2. Motivation for developing something from scratch • Best way to know the procedure if we go along the learning curve. • We needed something flexible, grid-less, adjustable to fit Teff, logg, and individual abundances for the GALEX Survey (Vennes et al. 2011, Nemeth et al. 2012). • The script (done in Python) replaces the “user” in front of the computer and serves as an interface to Tlusty/Synspec (Ivan Hubeny and Thierry Lanz) model atmosphere code or to public libraries of synthetic spectra. • NLTE models are prone to fail, mostly due to convergence errors. XTGRID has a built in failure tolerance and recovery procedure. • Survey work requires a large parameter space (WD, sd). The atomic level structure is evaluated and adjusted accordingly by the script. Peter Nemeth

3. The fitting method • Starting out from a fully converged Tlusty model, we gradually change parameters in the direction of decreasing χ2 (or integrated residuals) until the best fit is reached (relative changes of all free parameters and χ2 are below 0.1%). • Global method. We reproduce the observation(s) with a single model. • To reliably track each fitting parameter, it is better to start from the middle of the expected temperature and gravity range, and with higher abundances than expected for the given star or type of object. • Although theoretically the starting model can be arbitrary, an educated guess can save a lot of time in fitting. For Population-II hot subdwarfs, the initial model: Teff = 40 000 K, logg = 5.6, log(nHe/nH) = -1 and log(nCNO/nH) = -2 worked well. • We assume that the model is appropriate to fully describe the observation. Peter Nemeth

4. The fitting method (demo) • Green: model starting at: • Teff = 40 000 K, log g = 5.6, log He = -1, log CNO = -2 • Red: data for GALEX J2059+4232, a BHB star with: • Teff = 20 700 K, log g = 4.5, log He = -0.4 log C = -2.8, log N = -2.9, log O < -2.6 Peter Nemeth

5. Main features • A sort of steepest-gradient method combined with the simplex method in an iterative procedure. • All features of the Tlusty code are supported. • WYSIWYG fitting: no binary files or fancy optimized algrithms are involved. We compare ASCII models to ASCII data. Although slower, the procedure is easier to develop and troubleshoot. • Full SED coverage, observations can be fitted from X-rays to the far infrared. Also simultaneously: e.g: FUSE, IUE and optical spectra can be fitted together making the most out of the combined constraints from the different spectral ranges (not for extended atmospheres!). • A model cloud (irregular grid) is automatically built for later use to accelerate the procedure. Many objects show certain groups in the Teff-logg space or abundance patterns. • Parameter relaxation is included. If relative changes are low, Teff and/or logg are fixed for a few iterations (>30% faster steps). Peter Nemeth

6. Main features • Model complexity increases with the goodness of fit. • While Teff and logg settle the atmosphere is divided to 30 depths and lines are represented with 3 frequency points. When Teff and logg have low relative changes, lines are evaluated in 5 frequencies and we use 40 depths. In the final step 7 frequencies and 50 depths are used (the default). • All control parameters (in the Tlusty flag file) can be changed on the fly to adjust the procedure. Normally the user needs only to start XTGRID and the program does everything. • Fixed or wavelength dependent spectral resolution and resolving power. • Statistical continuum normalization that improves with the goodness of fit. • A typical fit requires 40-60 iterations and: • 2-3 hours with H,He models • 1 day with H,He,C,N,O models • 4-6 weeks with the first 30 elements (14 explicit NLTE, depends on the data as well). Peter Nemeth

7. Additional features • Binary decomposition 1 • Available with the Pickles, MILES, BlueRed and Phoenix grids. Free parameters are: T, logg, metallicity, dilution and rotational velocity. Micorturbulence is included in the grid. • Teff, metallicity, rotation and dilution are degenerate parameters. • Iterative radial and rotational velocity determination for both components. • Binary decomposition 2 • For O and B type binaries both models are calculated in parallel with Tlusty. • Stellar rotation (also available in binary decomposition) • Taken from Gray (2009), no sophisticated profiles. • Vertical abundance stratification. (limited use) • External irradiation in close binaries. Peter Nemeth

8. What are missing • Wind models. • Non-static atmospheres (extended, expanding or pulsating atmospheres). • Magnetic fields (broadening and splitting). • Isotopic shifts. • ... and many others. Development is done according to demand. Peter Nemeth

9. Availability XTGRID is offered to the community through: Astroserver.org Peter Nemeth