NLTE AND LTE LICK INDICES FOR RED GIANTS FROM [Fe/H] 0.0 TO −6.0 AT SDSS AND IDS SPECTRAL RESOLUTION

• Published in: The Astrophysical journal Vol. 810; no. 1; pp. 1 - 18
• Main Authors: Short, C. Ian, Young, Mitchell E., Layden, Nicholas
• Format: Journal Article
• Language: English
• Published: United Kingdom The American Astronomical Society 01.09.2015
• Subjects: ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; COMPARATIVE EVALUATIONS; Extreme values; Fossils; Galactic halos; GALAXIES; Galaxy formation; LTE; LUMINOSITY; METALLICITY; POLYNOMIALS; RED GIANT STARS; RESOLUTION; SPECTRA; Spectral classification; Spectral resolution; SPECTROSCOPY; stars: atmospheres; stars: fundamental parameters; stars: late-type; STELLAR ATMOSPHERES; THERMODYNAMICS
• ISSN: 0004-637X; 1538-4357; 1538-4357
• DOI: 10.1088/0004-637X/810/1/76

ABSTRACT We investigate the dependence of the complete system of 22 Lick indices on overall metallicity scaled from solar abundances, , 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, ) of MK spectral class late-K to late-F ( 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 Local Thermodynamic Equilibrium (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 -dependent down to values of at least , and down to in five cases, while also remaining well-behaved (single-valued as a function of and positive in linear units). For these nine indices, 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, , from multi-variate linear regression for I with terms up to third order in the independent variable pairs ( , ) and ( , ), and compare them to the fitted values of Worthey et al. at IDS spectral resolution. For this fitted I data-set we present tables of LTE partial derivatives, , , , and , that can be used to infer the relation between a given difference, , and a difference or , or a difference , while the other parameters are held fixed. For Fe-dominated Lick indices, the effect of NLTE is to generally weaken the value of I at any given and values. As an example of the impact on stellar parameter estimation, for late-type giants of inferred K, an Fe-dominated I value computed in LTE that is too strong might be compensated for by inferring a value that is too large.