When do Type II Cepheids form?

1. Type II Cepheids:
BL Her, UY Eri, W Vir
George Wallerstein
University of Washington Department of Astronomy

2. History
• 1888: M3 #154
• 1937: W Virginis found to show hydrogen in emission
• 1900 - 1950: Cepheids in Globular Clusters
• 1956: Metal-rich clusters, none have Cepheids
• 1966: High latitude field Cepheids found to belong to
a Thick Disk population by Wooley, not a metal-poor
Halo population

3. Cepheids
• P = 1 - 4 days, BL Herculis and UY Eri stars
• P = 5 - 20 days, W Virginis stars
• P > 20 days, RV Tauri stars

4. Metallicities
• Only Halo Globular Clusters have metal-poor
Cepheids.
• Metal-rich Globular Clusters are mostly in the
Bulge, except for 47 Tucanae and NGC 6838.
They have no Cepheids.

5. BL Herculis Stars
• Metal-rich group, 0 ≥ [Fe/H] ≥ -1.0
• Metal-poor group, -1.5 ≥ [Fe/H] ≥ -2.5

6. Field Type II Cepheids

7. Globular Clusters with Type II Cepheids