1 Introduction

More than 25 years ago, M. Landini and B.C. Monsignori Fossi, began to develop a numerical code to evaluate optically thin plasma emissivity, for temperature larger than 10⁴ K. The computation was performed in the so called "coronal approximation", where each level population is obtained assuming collisional and radiative coupling with the ground level only. The first result of this work was the theoretical spectrum from 1 to 100 Å both in the lines and in the free-free and free-bound continuum of a few important ions (Landini & Monsignori Fossi 1970). The spectrum was extensively used by the authors to study the solar X-ray emission measured by the SOLRAD satellites equipped with broad band detectors and for predictions of X and EUV emission from coronae of solar type stars. Since then, several upgrades have been performed, including the updating of the ionization balance (Landini & Monsignori Fossi 1991), the extension of the atomic database (Landini & Monsignori Fossi 1990) to include lines from 1 to 2000 Å, the computation of free-bound contribution from minor ions, the inclusion of two-photon continuum and the development of a library of numerical codes for the differential emission measure evaluation. The computed spectrum has been extensively applied to the study of the solar corona and to the interpretation of the X and EUV emission of solar type stars, and a version covering the spectral interval from 70 to 700 Å has been used for data reduction of the Extreme Ultraviolet Explorer (EUVE) observations. Most recently a major upgrade of the atomic database has been performed including atomic models, electron collision rates and radiative decays in order to evaluate the detailed population of each level assuming statistical balance among the excitations and decay processes. At first these procedures have been applied to several iron ions (Monsignori Fossi & Landini 1994a and 1994b) and successively extended to the most important ions of several isoelectronic sequences. Most of this work started as a collaboration with the Scientific Team of the Coronal Diagnostic Spectrometer on SOHO (Monsignori Fossi & Landini 1994c); it has now been inserted in the database of the CHIANTI project (Dere et al. 1997) developed by Brunella Monsignori Fossi, so early deceased, with Dr. H.E. Mason and Dr. K.P. Dere.

The aim of this paper is to present the latest version of the Arcetri spectral code and database which now includes the whole database already inserted in the CHIANTI project; moreover the Code allows the evaluation of free-free, free-bound and two-photons continuum (not yet included in CHIANTI) and the line intensity of most of the ions of minor elements, as described in the following sections.

The present update has proved necessary by the increasing need of the theoretical data required for the calculation of more and more accurate synthetic spectra in order to study the spectra observed by rocket and satellite-born spectrographs. This new instrumentation provides very high temporal, spatial and spectral resolution data and represents a great advance for solar and stellar spectroscopy and for the study of spectral emission from a variety of astrophysical objects. The new instrumentation which has been already launched, such as EUVE and Yohkoh, and more recently CDS, SUMER, UVCS on SOHO, and SAX, has permitted to greatly extend our knowledge of the physical processes determining the status of solar and stellar atmospheres. A much more detailed investigation of the mechanisms of solar and stellar wind formation and acceleration and of the coronal heating are expected. A large number of lines has been observed in the high resolution spectra provided by the instruments on board of SOHO (Brooks et al. 1998; Feldman et al. 1997; Raymond et al. 1997a) which could be used for temperature and density diagnostics (Landi & Landini 1997; Laming et al. 1997a and 1997b; Seely et al. 1997; Mason et al. 1997), for differential emission measure analysis (Landi & Landini 1997), and for element abundances analysis (Young & Mason 1997; Raymond et al. 1997b).

Moreover, future missions are scheduled for launch in the near future (AXAF 1998, XMM around 2000) which also will provide a unique opportunity for studying the X-ray emission from astrophysical sources, for which a large amount of theoretical data is required.

The revised version of the Arcetri Code and the updated database will be released to the scientific community at the website www.arcetri.astro.it before the end of spring 1998.