David Robertson, a 45-year-old American who has been among the rising stars of the conducting world in recent years, has been appointed the new music director of the St. Louis Symphony Orchestra, beginning in fall 2005. The SLSO has been without a chief conductor since last April, when Hans Vonk was forced to step down because of severe health problems. The appointment is something of a public relations coup for the orchestra: the SLSO came close to bankruptcy last year before making a good recovery, and Robertson had been on the reported shortlist of nearly every major orchestra searching for a music director over the last few years.