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Register a new userThe Mg/Fe characterization of the MILES library for stellar populations studies
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A. Milone
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We have obtained [Mg/Fe] for around 77% of the stars of the MILES library of stellar spectra in order to include this important information into simple stellar population (SSP) models. The abundance ratios, which were carefully calibrated to a single uniform scale, were obtained through a compilation from high spectral resolution works plus robust spectroscopic analysis at medium resolution. The high resolution data provided an extensive control sample. Average uncertainties (0.06 and 0.12 dex for the high and medium resolution samples respectively) and the good coverage of the stars with [Mg/Fe] over the MILESs parameter space will permit us to semi-empirically build up new SSP models with accurate alpha-enhancements for ages older than 1 Gyr. This will open new prospects for evolutionary stellar population synthesis.
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We have obtained [Mg/Fe] measurements for 76.3% of the stars in the MILES spectral library used for understanding stellar atmospheres and stellar populations in galaxies and star clusters. These abundance ratios were obtained through (1) a compilation of values from the literature using abundances from high-resolution spectroscopic studies and (2) a robust spectroscopic analysis using the MILES mid-resolution optical spectra. All the [Mg/Fe] values were carefully calibrated to a single uniform scale, by using an extensive control sample with results from high-resolution spectra. The small average uncertainties in the calibrated [Mg/Fe] values (respectively 0.09 and 0.12 dex with methods (1) and (2)) and the good coverage of the stars with [Mg/Fe] over stellar atmospheric parameter space of the library will permit the building of new simple stellar populations (SSPs) with empirical $alpha$-enhancements. These will be available for a range of [Mg/Fe], including both sub-solar and super-solar values, and for several metallicities and ages. These models will open up new prospects for testing and applications of evolutionary stellar population synthesis.
Empirical stellar libraries are extensively used to extract stellar kinematics in galaxies and to build stellar population models. An accurate knowledge of the spectral resolution of these libraries is critical to avoid propagation errors and uncertain estimates of the intrinsic stellar velocity dispersion of galaxies. In this research note we re-assess the spectral resolution of the MILES stellar library and of the stellar population models based on it. This exercise was performed, because of a recent controversy over the exact MILES resolution. We perform our test through the comparison of MILES stellar spectra with three different sets of higher-resolution templates, one fully theoretical - the MARCS library - and two empirical ones, namely the Indo-U.S. and ELODIE v3.1 libraries. The theoretical template has a well-defined very high (R=20000) resolution. Hence errors on this theoretical value do not affect our conclusions. Our approach based on the MARCS library was crucial to constrain the values of the resolution also for the other two empirical templates. We find that the MILES resolution has previously been slightly overestimated. We derive a new spectral resolution of 2.54 A FWHM, instead of the nominal 2.3 A. The reason for this difference is due to an overestimation of the resolution for the Indo-U.S. library that was previously used for estimates of the MILES resolution. For the Indo-U.S. we obtain a new value of 1.35 A FWHM. Most importantly, the results derived from the MARCS and ELODIE libraries are in very good agreement. These results are important for users of the MILES spectra library and for further development of stellar population models aimed to obtain accurate stellar kinematics in galaxies.
(Aims) We present a number of improvements to the MILES library and stellar population models. We correct some small errors in the radial velocities of the stars, measure the spectral resolution of the library and models more accurately, and give a better absolute flux calibration of the models. (Methods) We use cross-correlation techniques to correct the radial velocities of the offset stars and the penalised pixel-fitting method, together with different sets of stellar templates, to re-assess the spectral resolution of the MILES stellar library and models. We have also re-calibrated the zero-point flux level of the models using a new calibration scheme. (Results) The end result is an even more homogeneously calibrated stellar library than the originally released one, with a measured spectral resolution of ~2.5AA, almost constant with wavelength, for both the MILES stellar library and models. Furthermore, the new absolute flux calibration for the spectra excellently agrees with predictions based on independent photometric libraries. (Conclusions) This improved version of the MILES library and models (version 9.1) is available at the projects website (http://miles.iac.es).
In order to build more realistic single stellar population (SSP) models with variable alpha-enhancement, we have recently determined [Mg/Fe] in a uniform scale with a precision of about 0.1 dex for 752 stars in the MILES empirical library. The [alpha/Fe] abundance ratio is commonly used as a good temporal scale indicator of star formation, taking Mg as a template for alpha elements. Calcium is another element whose abundance is currently being investigated for the MILES stars. The MILES library is also being expanded by around 20% by including stars with known Teff, log g, [Fe/H] and [Mg/Fe]. The transformation of their photospheric parameters to the MILES system has been carried out, but the calibration of their [Mg/Fe] is still in progress. In parallel, C, N and O abundances are also being compiled from literature for the library stars because they play an important role in the photospheric opacity, particularly influencing the blue spectral region. The Galactic kinematic classification of MILES stars with compiled [Mg/Fe] has been just computed such that this information can be considered in the SSP modeling. Comparisons of theoretical stellar predictions of the Lick line-strength indices against the MILES data have revealed the good behaviour of Fe-sensitive indices predictions, while highlighting areas for improvement in some models for the higher order H-Balmer features.
We have computed low-mass stellar models and synthetic spectra for an initial chemical composition that includes the full C-N, O-Na, and Mg-Al abundance anticorrelations observed in second generation stars belonging to a number of massive Galactic globular clusters. This investigation extends a previous study that has addressed the effect of only the C-N and O-Na anticorrelations, seen in all globulars observed to date. We find that the impact of Mg-Al abundance variations at fixed [Fe/H] and Helium abundance is negligible on stellar models and isochrones (from the main sequence to the tip of the red giant branch) and bolometric corrections, when compared to the effect of C-N and O-Na variations. We identify a spectral feature at 490-520 nm, for low-mass main sequence stars, caused by MgH molecular bands. This feature has a vanishingly small effect on bolometric corrections for Johnson and Stroemgren filters that cover that spectral range. However, specific narrow-band filters able to target this wavelength region can be powerful tools to investigate the Mg-poor unevolved stars and highlight possible splittings of the MS due to variations of Mg abundances.
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