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

Abstract

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 < T eff < 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 (single-valued as a function of [ M H ] and positive in linear units). 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 (T eff , [ 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. For this fitted I data-set we present tables of LTE partial derivatives, ∂I ∂T eff | [M/H] , ∂I ∂[M/H] | T eff , ∂I ∂(V −K) | [M/H] , and ∂I ∂[M/H] | (V −K) , that can be used to infer the relation between a given difference, ∆I, and a difference ∆T eff or ∆(V − K), or a difference ∆[ M H ], 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 give T eff and [ M H ] values.As an example of the impact on stellar parameter estimation, for late-type giants of inferred T eff 4200 K, an Fe-dominated I value computed in LTE that is too strong might be compensated for by inferring a T eff value that is too large.

Metal-rich, Metal-poor: Updated Stellar Population Models for Old Stellar Systems

“…It is well-known that departures from LTE can substantially alter the depths and shapes of absorption lines, especially at low surface gravity and/or low metallicity (e.g., Asplund 2005). The effect of NLTE can be important even at low spectral resolution at solar metallicity (Short et al 2015). However, in our models, like many others (Trager et al 2000a;Thomas et al 2003;Schiavon 2007;Thomas et al 2011b), the theoretical spectra are only used differentially via the response functions, and so it unclear to what extent NLTE may play a role.…”

Multiple populations within globular clusters in early-type galaxies exploring their effect on stellar initial mass function estimates

“…While the effects of non-local thermal equilibrium can be significant for some spectra features (e.g. Asplund 2005;Short et al 2015), the effects of nonlocal thermal equilibrium on the response to abundance changes are not significant (Conroy et al 2018). In addition, the solar abundance patterns adopted by the isochrones are slightly different from the ones we use for our stellar atmosphere calculations.…”