M. Franchini1, C. Morossi1, P. Di Marcantonio1, M.L. Malagnini1,2,M. Chavez3

1INAF - Osservatorio Astronomico di Trieste, Via G.B. Tiepolo, 11, I-34131 Trieste, Italy franchini@oats.inaf.it,morossi@oats.inaf.it,dimarcan@oats.inaf.it

2Dipartimento di Fisica-Sezione di Astronomia, Università degli Studi di Trieste, Via G.B. Tiepolo, 11, I-34131 Trieste, Italy malagnini@oats.inaf.it

3Instituto Nacional de Astrofisica, Optica y Electronica, A.P. 51 y 216, 72000 Puebla, Mexico mchavez@inaoep.mx,

Acknowledgments:

This work received partial financial support from the Mexican CONACyT via grant 49231-E, from PRIN-INAF 2007 (CRA 1.06.10.04 P.I. M. Bellazzini). Funding for the creation and distribution of the SDSS Archive has been provided by the Alfred P. Sloan Foundation, the Participating Institutions, the National Aeronautics and Space Administration, the National Science Foundation, the U.S. Department of Energy, the Japanese Monbukagakusho, and the Max Planck Society. The SDSS Web site is http://www.sdss.org/. The SDSS is managed by the Astrophysical Research Consortium (ARC) for the Participating Institutions. The Participating Institutions are The University of Chicago, Fermilab, the Institute for Advanced Study, the Japan Participation Group, The Johns Hopkins University, the Korean Scientist Group, Los Alamos National Laboratory, the Max-Planck-Institute for Astronomy (MPIA), the Max-Planck-Institute for Astrophysics (MPA), New Mexico State University, University of Pittsburgh, University of Portsmouth, Princeton University, the United States Naval Observatory, and the University of Washington.

References: •Abazajian, K. N., et al. 2009, APJS, 182, 543 •Borkova, T.V.\& Marsakov, V.A. 2005 Astron. Zh. 82, 453 •Castelli,F. & Kurucz,R.L. 2003, IAU Symp. 210, Modelling of Stellar Atmosphere ed. N.E. Piskunov, W.W. Weiss & D.F. Gray (San Francisco: ASP), A20 •Cenarro, A.J., et al. 2007, MNRAS, 374, 664 •Franchini, M. et al. 2010, ApJ, submitted •Gray, R.O., & Corbally, D.J. L.I. 1994. AJ, 107, 742 •Moultaka, J., Ilovaisky, S. A., Prugniel, P. & Soubiran, C. 2004, “The ELODIE Archive”, PASP, 116, 693 •Newberg, H.J. et al. 2009, Bulletin of the American Astronomical Society, Vol 41, p.229 •Valdes, F., Gupta, R., Rose, J. A., Singh, H. P. & Bell, D. J. 2004, ApJS, 152, 251 (INDO-US) •Valenti, J.A. & Fischer, D.A. 2005, ApJS, 159, 141 (SPOCS) •Worthey, G., Faber, S. M., Gonzalez, J. J., & Burstein, D. 1994,ApJ, 94, 687 •York, D. G. et al. 2000, AJ, 120, 1579

The Lick/SDSS index library

Calibration and comparison with observational indices

To calibrate the Lick/SDSS library we used a sample of 333 reference stars with reliable atmospheric parameter estimates from the ELODIE (Moultaka et al. 2004), INDO--U.S (Valdes et al. 2004), and MILES (Cenarro et al., 2007) spectral libraries. Stars whose object type identification in the CDS-SIMBAD database indicates some kind of peculiarity (spectroscopic binaries, variable, etc) or with spectroscopic log g inconsistent with the value estimated from Hipparcos data were excluded.

Indications of deviations from solar partition abundances were searched in the literature and [α/Fe] estimates based on [Mg/Fe], [Si/Fe], and [Ti/Fe] were obtained, when available, from Borkova & Marsakov (2005) or from Valenti & Fischer (2005). Eventually, two lists of 154 ``SSA'' dwarf stars (i.e those with [α/Fe] values within ± 0.15 dex and log g > 3.5) and of 44 giant stars (log g < 3.5) for which no estimates of [α/Fe] were available have been obtained.

Then, Lick/SDSS indices were computed from the spectra of the above mentioned sample after correction for radial velocities and degradation at R=1800. Eventually, by using these 198 stars, we derived a set of transformation coefficients by performing linear regressions between the observed indices and the corresponding synthetic ones at the proper atmospheric parameter values. The derived coefficients were used to convert both SSA and NSSA grids of synthetic indices into the observational Lick/SDSS system. We stress that we preserve the intrinsic differences between the SSA and the NSSA grids since the same set of transformation coefficients is used to calibrate both grids.

A direct comparison between calibrated synthetic indices and observed ones is performed by using a sample of 932 stars built from ELODIE, INDO-U.S. and MILES stellar libraries (hereafter EIM sample). It must be noticed that the actual comparison requires the knowledge of all the atmospheric parameters (i.e. Teff, log g, [Fe/H], and [α/Fe]) for each individual star in order to compare like with like. We assumed that the stars (541 “dwarfs” and 391 “giants”) and our synthetic spectra are on the same θ=5040/Teff temperature scale and we separated the stars in two groups of log g to represent “dwarf” and “giant” stars (log g=4.5±0.5 and log g=2.5±0.5, respectively). Furthermore, we performed the comparison separately in different [Fe/H] bins whose amplitude (±0.1 dex) takes into account the uncertainties affecting the estimates of stellar metallicity.

For each of the 19 indices (Fe4668 was removed from the list of indices due to calibration problems) the observational values were plotted versus θ for the two groups of stars together with SSA and NSSA iso-gravities of calibrated synthetic indices in eight different panels corresponding to intervals of ∆[Fe/H]=±0.1 dex centered at -2.5, -2.0, -1.5, -1.0, -0.5, 0.0, +0.2, and +0.5. For the Hβ index the first two columns in Fig.s 2 and 3 show the absence of any systematic offset between the observational point positions and the synthetic iso-gravities. This good agreement confirms that stars and synthetic spectra are on the same temperature scale and show the same trend versus θ since Hβ is a temperature indicator almost independent of surface gravity, metallicity and α-enhancement.

Panels in columns 3-6 of Fig.s 2 and 3 show the same kind of comparison for CaHK, and Mgb. The overall good agreement of the observational points with the isogravities for both ''dwarfs'' and ''giants'' indicates the correctness of the behaviour of our synthetic indices versus surface gravity. Moreover, Fig.s 2 and 3 show that also the dependency of observational indices on metallicity is well reproduced by our synthetic indices. As far as the [α/Fe] behaviour is concerned, the plots indicate, as expected on the basis of literature results, that the loci of the observational points get closer and closer to the prediction of NSSA (i.e. α-enhanced) indices for lower and lower metallicity.

Conclusions In conclusion, the calibrated library of Lick/SDSS synthetic indices reliably agrees with the observational picture for F,G, and K stars as checked by using EIM objects and, therefore, constitutes a valuable and powerful tool for the study of stellar populations. In particular, its internal accuracy and the fact that it was constructed starting from synthetic spectra at R=1800 makes it well suited to extend the coverage of empirical databases like SDSS and the forthcoming LAMOST in the parameter space for the analysis of old stellar systems (see more details in Franchini et al. 2010).

Moreover, the Lick/SDSS library can be used to derive atmospheric parameters (i.e. Teff, log g, [Fe/H]) (see Di Marcantonio et al., this conference) and [Ca/Fe] and [Mg/Fe] ratios (see Morossi et al., this conference) of F,G, and K stars.

Introduction This poster presents Lick/SDSS, a library of Lick-like synthetic spectral indices designed to be used in the analysis of SDSS and LAMOST databases. The synthetic spectral indices are derived from a library of high resolution synthetic stellar spectra computed considering both solar scaled abundances and [α/Fe]=+0.4 in the temperature range 3500-7000 K. The proposed Lick/SDSS system differs from the original Lick/IDS one since it is ad-hoc designed to be used on current observational databases which are at higher resolving power and with more homogeneous spectral response and wavelength calibration than the IDS spectra. An additional UV spectral index, CaHK, in the region of CaII H and K lines, is introduced as well suited to study Calcium abundance in F,G, and K stars. Transformation of synthetic indices into the observational framework is accomplished by means of a calibration set of Solar Neighborhood (SN) stars and its accuracy is checked by comparing observed and synthetic index trends versus effective temperature for a sample of more than 1000 SN stars. The calibrated synthetic indices accurately reproduce those of FGK stars and thus provide a valuable tool for connecting our understanding of star and galaxy formation.

Table 1: LICK/SDSS spectral indices belonging to: [α/Fe] sensitive group (red), intermediate group (green), and quasi-indipendent group (blue).

The synthetic Lick/SDSS indices A set of 2550 synthetic spectra has been computed at wavelength sampling ∆λ=0.05 Å, rotational velocity of 0 km s -1 and microturbulent velocity ξ=2 km s -1 in the wavelength range 3000-7000Å. The spectra were computed by means of the stellar spectral synthesis program SPECTRUM v2.75 (Gray & Corbally, 1994) and ODFNEW models by Castelli & Kurucz (2003) in the following ranges of parameters:

Teff : 3500÷7000 K at a step of 250 K log g : 0.5 ÷ 5.0 dex at a step of 0.5 dex. Two different assumptions on chemical composition lead to two different grids of models and synthetic spectra. The first one, SSA grid, refers to Solar Scaled Abundances at overall metallicities [M/H]=-2.5, -2.0, -1.5, -1.0, -0.5, 0.0, 0.2, 0.5. The second one, NSSA grid, refers to Non-Solar Scaled Abundances with enhanced α-over-iron ratios [α/Fe]=+0.4, and [Fe/H]=-4.0, -2.5, -2.0, -1.5, -1.0, -0.5, 0.0, 0.2, 0.5. The α-elements considered are O, Ne, Mg, Si, S, Ar, Ca, and Ti. Out of the 25 Lick/IDS indices given in Table1 of Worthey et al. (1994) 19 indices were included in the Lick/SDSS system and computed from our synthetic spectra by using the central and pseudo-continuum passbands given in Table 1. An additional index, namely CaHK, was also introduced to better analyze the [Ca/Fe] abundance. Since we want to use our synthetic indices to analyze Sloan Digital Sky Survey (York et al. 2000; SDSS-DR7, Abazajian 2009) spectra each synthetic spectrum was degraded to the SDSS resolution (R=1800) before the actual computation of the indices. This resolution is also close to that one of LAMOST low resolution-half slit width mode, thus making it possible to use the same synthetic indices also in the LEGUE (Newberg et al. 2009) framework.

Fig. 1: Spectral index response to [α/Fe]: Teff≥4250 blue points, Teff<4250 red points

Name Feature Bandpass Pseudocontinua Units IDS Error Measures ------------------------------------------------------------------------------------ 01 Cahk 3900.000-4000.000* 3837.000-3877.000 Ang 0.64 CaII 4040.000-4080.000 02 CN 1 4143.375-4178.375 4081.375-4118.875 mag 0.021 CN, FeI 4245.375-4285.375 03 CN 2 4143.375-4178.375 4085.125-4097.625 mag 0.023 CN, FeI 4245.375-4285.375 04 Ca4227 4223.500-4236.000 4212.250-4221.000 Ang 0.27 CaI, FeI, FeII 4242.250-4252.250 05 G4300 4282.625-4317.625 4267.625-4283.875 Ang 0.39 CH, FeI 4320.125-4336.375 06 Fe4383 4370.375-4421.625 4360.375-4371.625 Ang 0.53 FeI, TiII 4444.125-4456.625 07 Ca4455 4453.375-4475.875 4447.125-4455.875 Ang 0.25 CaI, FeI, NiI, 4478.375-4493.375 TiII, MnI, VI 08 Fe4531 4515.500-4560.500 4505.500-4515.500 Ang 0.42 FeI, TiI, FeII, 4561.750-4580.500 TiII 09 FE4668 4634.000-4720.250 4611.500-4630.250 Ang 0.64 FeI, TiI, CrI, 4742.750-4756.500 MgI,NiI,C2

10 H beta 4847.875-4876.625 4827.875-4847.875 Ang 0.22 H, FeI 4876.625-4891.625 11 Fe5015 4977.750-5054.000 4946.500-4977.750 Ang 0.46 FeI, NiI, TiI 5054.000-5065.250 12 Mg 1 5069.125-5134.125 4895.125-4957.625 mag 0.007 MgH, FeI, NiI 5301.125-5366.125 13 Mg 2 5154.125-5196.625 4895.125-4957.625 mag 0.008 MgH, MgI, FeI 5301.125-5366.125 14 Mg b 5160.125-5192.625 5142.625-5161.375 Ang 0.23 MgI 5191.375-5206.375 15 Fe5270 5245.650-5285.650 5233.150-5248.150 Ang 0.28 FeI, CaI 5285.650-5318.150 16 Fe5335 5312.125-5352.125 5304.625-5315.875 Ang 0.26 FeI 5353.375-5363.375 17 Fe5406 5387.500-5415.000 5376.250-5387.500 Ang 0.20 FeI, CrI 5415.000-5425.000 18 Fe5709 5698.375-5722.125 5674.625-5698.375 Ang 0.18 FeI, NiI, MgI, 5724.625-5738.375 CrI, VI 19 Fe5782 5778.375-5798.375 5767.125-5777.125 Ang 0.20 FeI, CrI, CuI, 5799.625-5813.375 MgI 20 Na D 5878.625-5911.125 5862.375-5877.375 Ang 0.24 NaI 5923.875-5949.875 * 3931.5-3935.5, 3965.5-3970.5 excluded

The effects of α-element enhancement on the 20 indices are investigated by comparing the predictions of the NSSA grid with those from the subset of the SSA grid with the same [Fe/H] values. The direct comparison of NSSA and SSA individual indices at the same (Teff, log g, [Fe/H]) values is shown in Fig.1. We can cluster the indices into three groups:

•The [α/Fe] sensitive group: CaHK, CN1, CN2, Ca4227, Fe4668, Mg1, Mg2, and Mgb. In all the cases but Mg1 the NSSA index values are larger (CaHK, Ca4227, Mg2, and Mgb) or smaller (CN1, CN2, and Fe4668) than the SSA ones. In general the differences vanish with the vanishing of the indices themselves, i.e. at high temperatures. •The intermediate group: Fe4383, Fe5270, Fe5335, Fe5406, Fe5709, and NaD which show only marginal differences between NSSA and SSA values, mostly confined at the lowest temperatures. •The [α/Fe] quasi-independent group: G4300, Ca4455, Fe4531, Hβ, Fe5015, and Fe5782 which do not show significant differences between NSSA and SSA values.

Fig. 2: Spectral indices Hβ, CaHK, and Mgb of EIM “dwarfs” vs θ=5040/Teff in different metallicity groups. Synthetic SSA (solid lines) and NSSA (dotted lines) isogravities are overplotted: log g=4.0 (green), log g=4.5 (yellow), and log g=5.0 (red). Vertical bar in the left-upper corner represents observational uncertainty as derived by comparing index values for stars with multiple spectra.

Fig. 3: Spectral indices Hβ, CaHK, and Mgb of EIM “giants” vs θ=5040/Teff in different metallicity groups. Synthetic SSA (solid lines) and NSSA (dotted lines) isogravities are overplotted: log g=4.0 (green), log g=4.5 (yellow), and log g=5.0 (red). Vertical bar in the left-lower corner represents observational uncertainty as derived by comparing index values for stars with multiple spectra.