As so often, this code started out as a small project in 1987. Back then, Claus Leitherer wanted to have a little computer program to calculate ionizing fluxes for individual O stars. The program evolved from handling a single star to a whole population. At the same time, there was the desire to assign positions in the HRD for comparison with observations. Results from this rather simple code were reported in ApJS, 73, 1 (1990). By 1990 the program had grown to compute the output of the mechanical luminosity of winds and of supernovae. A paper on that is in ApJ, 401, 498 (1992). The "real" spectral synthesis capability was implemented in 1991, when we added Kurucz and Schmutz model atmospheres from the far-UV to the near-IR. This allowed us to calculate spectra and colors for young populations. Carmelle Robert included an IUE spectral library in the code in order to compute synthetic lines at 0.75 A resolution (ApJ, 418, 749 , ApJS, 99, 173 ).
By 1994, we had computed so many models, synthesizing all kinds of galaxy parameters that the suggestion was made to combine everything, plot it in a homogeneous way, and publish it (ApJS, 96, 9 ). Who would have predicted that quite a few astronomers found this useful? Later we began to add a few more specialized routines, like the computation of O VI line profiles by Rosa Gonzalez-Delgado (ApJ, 489, 601 ). The most recent subroutine was written by Jeff Goldader. This part calculates the strengths of selected stellar features in the near-IR.
In 1997 it become clear that the code needed a major upgrade. Daniel Schaerer replaced the ancient Maeder (1990) tracks by the most recent (1992-94) model series of Geneva. He also implemented the capability to perform isochrone synthesis in addition to classical evolutionary synthesis. One of the subroutines used for this was written by Georges Meynet. We also decided to take advantage of the homogeneous atmosphere grid compiled by Lejeune et al. (1997) and replaced the original Kurucz models by the new set. The updated code was made available to the community via a spiffy website called Starburst99 (Leitherer et al. 1999, ApJ, 123, 3).
Since then, four important improvements were made. First, Duilia de Mello added a high-resolution spectral library of B stars in fall 1999. A description is in ApJ, 530, 251 (2000). This library replaced the low-resolution B-star library we used before. (O stars have always been from a high-dispersion library.)
Second, Alessandra Aloisi added supernova yields to the code. Before that, only yields from stellar winds were taken into account, and the supernova yields of those elements which undergo no nuclear processing during a supernova explosion. Now the models are up-to-date with respect to the nucleosynthesis in type II supernovae (and only those -- we don't do type I's yet). This change was made in August 2000.
Third, Claus Leitherer and summer student Joao Leao Souza added a spectral library of LMC and SMC stars in August 2000 (Leitherer et al. 2001, ApJ, 550, in press). This allows calculations of models for UV spectra with sub-solar metallicities. Only O-stars are used for SMC/LMC metallicity, everything else is solar. This is not perfect -- but it is a starting point.
Fourth, Miguel Cervino, in collaboration with Daniel Schaerer greatly improved several numerical aspects of the code. In particular he wrote new code to handle the calculation of the supernova rate in isochrone synthesis. Previously, all SN related parameters showed rather ugly discontinuities due to numerical instabilities (see Leitherer et al. 1999). This was fixed in October 2000.
More to come in the future --- stay tuned!