An Atlas of high resolution synthetic spectra in the wavelength region

Astronomy and Astrophysics Supplement Series, Jul 2018

We present an Atlas of high resolution (resolving power ) synthetic spectra based on the 1993 release of atmosphere models and codes by Kurucz. The spectra are given in the wavelength interval 4850-5400 Å  for eleven values of effective temperature in the range 4000-8000 K, for nine surface gravities in the interval 1.0-5.0 dex, and for seven metallicities spanning from -1.0 to +0.5 dex. The Atlas is aimed mainly at the analysis of composite stellar populations by means of suitable spectral features extracted from the spectra and identifiable in the observed composite light of clusters and galaxies. The high resolution used in the computations makes the Atlas also suitable for chemical abundance analyses.

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An Atlas of high resolution synthetic spectra in the wavelength region

Astron. Astrophys. Suppl. Ser. An Atlas? of high resolution synthetic spectra in the wavelength region 4850 − 5400 A M. Chavez 2 M.L. Malagnini 1 C. Morossi 0 0 Osservatorio Astronomico di Trieste , Via G.B. Tiepolo, 11, I 1 Dipartimento di Astronomia, Universita degli Studi di Trieste , Via G.B. Tiepolo, 11, I 2 Instituto Nacional de Astro sica , Optica y Electronica INAOE. Apdos Postales 51 y 216, CP 72000 Puebla , Mexico 3 34131 Trieste , Italy We present an Atlas of high resolution (resolving power = = 250 000) synthetic spectra based on the 1993 release of atmosphere models and codes by Kurucz. The spectra are given in the wavelength interval 4850 − 5400 A for eleven values of e ective temperature in the range 4000 − 8000 K, for nine surface gravities in the interval 1:0 − 5:0 dex, and for seven metallicities spanning from −1:0 to +0:5 dex. The Atlas is aimed mainly at the analysis of composite stellar populations by means of suitable spectral features extracted from the spectra and identi able in the observed composite light of clusters and galaxies. The high resolution used in the computations makes the Atlas also suitable for chemical abundance analyses. stars; atmospheres | stars; late{type 1. Introduction A comprehensive spectroscopic stellar library at high resolution provides a valuable tool for facing fundamental problems in di erent branches of astronomy. For instance, it can be used for determining the atmospheric parameters of stars by comparison with observed spectra (Cayrel et al. 1991a, b) . On the other hand, synthetic spectra have proved to be an important (and sometimes the only) brick for the investigation of the behavior of prominent spectral features in terms of the stellar atmospheric parameters (e.g., Jones et al. 1996). Once this behavior has been properly modelled in stars, we can, in principle, predict the spectroscopic properties of stellar aggregates (e.g., Proc. of the conference \From Stars To Galaxies: Send o print requests to: M.L. Malagnini ? Spectra are only available in electronic form at the CDS via anonymous ftp to ( or via Out of the several aspects connected with the under standing of the evolutionary histories of stellar aggregates, we are particularly involved in the analysis and synthesis of spectral features typical of old stellar populations. Along this line, we have computed a set of synthetic spectra in the wavelength interval 4850 − 5400 A. This spectral region has been chosen because it contains conspicuous features, such as the Mgb triplet, the MgH molecular bands and FeI absorptions, which are easily observable in the integrated spectra of distant stellar systems. Moreover, the sensitivity of these lines to variations in surface gravity, e ective temperature and metal content allows the determination of these parameters for individual stars in a self-consistent way. We have not extended this region to include the blue wing of H because of the poor t between the observed and theoretical solar fluxes (Gulati 1989) : the theoretical continuum appears too high with respect to the observed one. Several reasons have been suggested to explain the dis crepancy, among them problems in the convection treatment in the models or some underestimation of the continuum opacity (see, for instance, Castelli et al. 1997) . The spectra, computed with the upgraded versions of Kurucz's models and codes (Kurucz 1993a-c) , and based on revised atomic and molecular data, cover a parameter space suitable for the study of F, G and K stars, which are the dominant components in old stellar systems. So far, these spectra have been already used for synthesizing a set of spectral features for population studies of clusters and galaxies observed at low and moderate resolution (Chavez et al. 1995, 1996) . Since this application does not exhaust the informational content of the synthetic spectra, we are making available to the community the original high resolution spectra. This paper describes the main characteristics and the and about 2000 modi cations on their parameters (log gf format of the Atlas which can be obtained from the au- values and van der Waals damping constants) were apthors. plied. The improvement achieved after the modi cations In Sect. 2 we present a short description of the line is testi ed by the decrease in the rms of the di erences bedata used in the computations. Section 3 is devoted to tween the computed and the observed spectra (both norpresent the characteristics of the spectra included in the malized to the continuum and rebinned to the constant Atlas. In Sect. 4 a subsample of the Atlas is presented in step = 0:02 A): by using the original line list, the rms pictorial form suitable to highlight low resolution features. value was 0.071 while is 0.049 after our modi cations. The distribution of the residuals (computed { observed 2. The input line data residual intensities) is represented in Figs. 1a and b: the distribution obtained after the corrections clearly shows It is well known that the completeness and the accuracy of an higher concentration in the central bin (Fig. 1a: 18 903 the atomic and molecular line data are of paramount im- instead of 16 157 data points over a total of 27 252). Figure portance for the reliability of any synthetic spectrum (see, 1b shows that the negative wing of the distribution is sigfor instance, Bell et al. 1994 and Kurucz 1995) . To improve ni cantly reduced due to the removal of strong predicted the reliability of the parameters governing the strength lines which do not correspond to observed features; unand the shape of the spectral lines, we compared the ob- fortunately, the improvement in the positive wing is less served solar spectrum (Kurucz 1991) with a synthetic one. signi cant due to the impossibility of solving the probEmpirical adjustments to line data were determined by lem of unidenti ed lines (see, for instance, the line at applying a trial and error procedure as in Peterson et al. 5170:8 A in Fig. 2). (1993). Our corrections, although small with respect to Figures 2 and 3 illustrate in more detail the kind of the uncertainties attributed to the line data parameters modi cations we performed by showing the region around (Kurucz 1995) , signi cantly improved the ability of the a strong magnesium feature at 5172.684 A and that one synthetic spectrum to mimic the observed one. containing the iron feature at 5328.038 A, respectively. In The model used for the comparison has the following both gures, the thick solid line corresponds to the obatmospheric parameters: Te = 5777 K, log g = 4:43770, served solar central intensity spectrum, the dotted line and element abundance from Anders & Grevesse (1989) . indicates the synthetic spectrum computed without modiFor the micro- and macro turbulent velocities, the values 1 fying the line parameters and the thin solid line shows the and 1.5 km s−1, respectively, were adopted from Thevenin spectrum computed after the modi cations. The better (1989). The synthetic spectrum was computed at the spec- agreement with the observations is the result of modifying tral resolution of = = 522 000, so as to match the the van der Waals damping constants and/or the log gf resolution of the observed spectrum. values in the line data of Kurucz (1993b). Moreover, for The atomic and molecular line data were extracted some lines, small adjustments in wavelength were also refrom the updated line list compiled by Kurucz. The quired (see Fig. 3). For example, in order to match the working line list includes more than 45 000 lines in the observed pro le with the theoretical one, the γVW of the 4850 − 5400 A spectral interval. Molecular features in- central MgI line in Fig. 2 was corrected by subtracting cluded in the computations are CN, C2, MgH, SiH and CH 0.18 from the original value of −7:12. For the case of the and account for the majority of the lines. Out of the work- iron feature at 5328 A (Fig. 3), its γVW in Kurucz's list ing line list, about 1 500 lines were carefully investigated (−7:87) was increased by +0:30. With the updated line list we computed the grid of high resolution synthetic spectra described in the next section. 3. The synthetic spectra Kurucz (1993a) model atmospheres are available in a range of temperatures that spans from 3 500 to 50 000 K, metallicities, [M=H], from −3:5 to +1:0 and surface gravities (log g) from 0.0 to 5.0 dex. They have been computed by adopting a microturbulent velocity = 2 km s−1. For the case of solar metallicity, [M=H] = 0:0, models for di erent microturbulent velocities ranging from 0 to 8 km s−1 are also available (Kurucz 1993c) . For our purposes, models in the following ranges of parameters have been selected: { E ective temperature from 4000 to 6000 K at a step of 250 K plus the spectra for 7000 and 8000 K; { Surface gravity from 1.0 to 5.0 dex at a step of 0.5 dex; { Overall chemical composition [M=H] = −1:0, −0:5, 0.0, 0.1, 0.2, 0.3, and 0.5. The ne steps in metallicity for [M=H] 0:0 make this grid more suitable for the analysis of the so-called super-metal rich stars than our previous theoretical grids (Gulati et al. 1993; Chavez et al. 1995) . It must be noted that, in modelling stars with chemical composition di erent from solar, all heavy elements vary lock-step. Work is already planned in order to complement the Atlas by including synthetic spectra with non solar partitions in the -elements once the appropriate models are available. All the 693 spectra have been computed at the resolving power = = 250 000, rotational velocity of 0 km s−1 and microturbulent velocity = 2 km s−1. The use of a xed value for is somewhat arbitrary (see Kurucz 1996 for a detailed discussion) . Among synthetic spectra builders, di erent approaches have been followed. For instance, Barbuy (1994) considers a value of = 1:0 km s−1 for stars with gravities log g 2:0, and = 1:8 km s−1 for stars with log g < 2:0, and Milone et al. (1995) adopt = 1:0 km s−1 for log g 3:5, = 1:5 km s−1 for giant stars with 2:0 log g 3:0, and 2.0 km s−1 for supergiants (log g 1:5). Tripicco & Bell (1995) have smoothly varied the value of from 1.0 km s−1 for dwarf stars to 2.0 km s−1 for supergiants. an example, Fig. 4 shows the synthetic spectra computed with di erent microturbulent velocities ( = 1:0 km s−1 and = 2:0 km s−1) for two temperatures, namely 4000 K and 5500 K, in the wavelength region dominated by the Mg b triplet. In both panels, the di erences between the two spectra are also plotted. The average flux di erence is larger for 4000 K, while, locally, the di erences are higher for 5500 K. 4. The Atlas At present Kurucz's models for other microturbulent The Atlas consists of 711 sets of data. Each set contains velocities are available only for solar chemical abundance. a description which speci es the main spectrum parameIn order to get insight into the e ects of this parame- ters, i.e. temperature, surface gravity, chemical compositer we have computed an additional set of 18 synthetic tion and microturbulence of the parent model, wavelength spectra with the following parameters: surface gravity range, resolving power, etc. Then, for each wavelength xed at log g = 4:0, temperatures ranging from 3500 to point, both the continuum and the line blanketed absolute 8000 K with a step of 500 K, and microturbulent velocities fluxes per unit frequency (erg cm−2 s−1 Hz−1) are given = 0:0 km s−1 and = 1:0 km s−1. The e ects of di erent allowing the computation of residual intensities. The Atlas 's on spectral indices have been discussed in Chavez et al. is published electronically at the CDS. (1996): in general, they are negligible at high tempera- A subset of the spectral Atlas is displayed in Figs. 5, 6 tures, while becoming of some signi cance at low tem- and 7. Figure 5 illustrates the behaviour of the spectra in peratures. At high resolution, the microturbulent veloc- the Mg b region for di erent temperatures, metallicities, ity e ects are temperature and wavelength dependent. As and surface gravities. The general trend of the spectra in the whole wavelength range is depicted in Figs. 6 and 7. The spectra are plotted in an absolute flux scale after a degradation in resolution (F W HM = 2:5 A) to increase the readability of the gures. Figures 6 and 7 contain sequences in temperatures of spectra representative of solar chemical composition stars of intermediate and high surface gravity, 3.0 dex and 4.5 dex, respectively. Figures 5 to 7 show that our Atlas can serve as a starting point both for high and for intermediate resolution analyses. In fact, the availability of synthetic spectra at high resolution makes it possible to simulate the e ects of any instrumental pro le. Acknowledgements. This work was partially supported by the Ministero dell'Universita e della Ricerca Scienti ca e Tecnologica (60% and 40% grants), and by the Consiglio Nazionale delle Ricerche (CNR{GNA). Anders E. , Grevesse N. , 1989 , Geochim. Cosmochim. 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M. Chavez, M. L. Malagnini, C. Morossi. An Atlas of high resolution synthetic spectra in the wavelength region, Astronomy and Astrophysics Supplement Series, 267-272, DOI: 10.1051/aas:1997264