O’Fallon-based SunEdison Inc. launched its long-awaited initial public offering of shares in its SunEdison Semiconductor Ltd. unit, a move that formally separates its solar business from its semiconducter unit. The shares debuted Thursday on the Nasdaq at $13 a share under the symbol SEMI and closed at $15. The company said it offered 7.2 million shares for sale, or about 18 percent of the stock. Underwriters also have a 30-day option to buy an additional 1.08 million shares at the IPO price. The new semiconductor company will be based in Singapore, while SunEdison will remain its largest shareholder, with a majority stake in the business. It also will maintain U.S. offices in O’Fallon.