1. Laboratory for Astrophysics, Leiden Observatory, Leiden University, 2300 RA, Leiden, The Netherlands
2. Leiden Observatory, Leiden University, 2300 RA, Leiden, The Netherlands
3. Anton Pannekoek Institute for Astronomy, University of Amsterdam, Science Park 904, 1098 XH, Amsterdam, The Netherlands
4. Department of Astronomy, University of Maryland, College Park, MD 20742-2421, USA