1. The Group of Bootis Stars Dr. Ernst Paunzen Institute for Astronomy University of Vienna

2. Abundance Pattern C, N, O und S solar abundant C, N, O und S solar abundant Heavier elements underabundant, (Na) Heavier elements underabundant, (Na) Large Scatter Large Scatter Paunzen et al. (1999) Paunzen et al. (2002a) Heiter (2002)

3. Evolutionary Status Paunzen et al. (2002b)

4. Group properties About 50 members About 50 members Typical abundance pattern Typical abundance pattern Temperature range: 11000 to 6500K Temperature range: 11000 to 6500K From ZAMS to TAMS From ZAMS to TAMS No magnetic fields detected No magnetic fields detected Not found in open clusters Not found in open clusters Only spectroscopic binaries of the type „ Bootis“ Only spectroscopic binaries of the type „ Bootis“

5. Diffusion/Mass-loss Mass-loss „conserves“ He convection zone Mass-loss „conserves“ He convection zone BUT: Rotation induces mixing BUT: Rotation induces mixing Time scale: ca. 1 Gyr Time scale: ca. 1 Gyr Charbonneau (1993)

6. Diffusion/Accretion Star accretes gas component of the interstellar environment Star accretes gas component of the interstellar environment Diffusion mixes atmosphere Diffusion mixes atmosphere Number of measurements Heiter et al. (2002) Turcotte & Charbonneau (1993)

7. Diffusion/Accretion Source of material only from star formation Source of material only from star formation Time scale: max. 100 Myr Time scale: max. 100 Myr

8. Diffusion/Accretion Disk visible in IR Disk visible in IR Measurements for 26  Bootis Stars Measurements for 26  Bootis Stars Six with IR Excess, upper limits otherwise Six with IR Excess, upper limits otherwise Paunzen et al. (2003)

9. Theories versus Observations

10. A New Accretion Model What happens if a star crosses denser ISM? What happens if a star crosses denser ISM? Questions: Time scale Needed densities of ISM Relative velocity range Accretion of gas realistic Range of spectral types Statistical frequency

11. A New Accretion Model ISM model: „standard“ abundances, density about 10 ISM model: „standard“ abundances, density about 10 cm -3 Velocity range: 10 to 20 kms - 1 Time Scale: 1 pc, Time Scale: 1 pc, 17 kms - 1, ca. 60000 yr Hot end: Stellar Winds, 12000K Hot end: Stellar Winds, 12000K Cool end: Convection, 6500K Cool end: Convection, 6500K Dust is blown away Open Clusters: no gas to accrete Statistics: about 15 objects predicted within 60 pc, 9 known Kamp & Paunzen (2002)

12. Theories versus Observations

13. Pulsating  Bootis Stars Koen et al. (2003) Paunzen et al. (2002b)

14.  Bootis versus  Scuti Instability strip shifted Pulsational constants smaller, Overtones excited Paunzen et al. (2002b)

15. Which Mechanism can be studied? Diffusion Diffusion Accretion Accretion Rotation Rotation Pulsation Pulsation depending on: Metallicity Age Stellar Environment

16. References Charbonneau, 1993, ApJ, 405, 720 Charbonneau, 1993, ApJ, 405, 720 Heiter, 2002, A&A, 381, 959 Heiter, 2002, A&A, 381, 959 Heiter, Weiss, Paunzen, 2002, A&A, 381, 971 Heiter, Weiss, Paunzen, 2002, A&A, 381, 971 Kamp, Paunzen, 2002, MNRAS, 335, L45 Kamp, Paunzen, 2002, MNRAS, 335, L45 Koen, Paunzen, van Wyk, et al., 2003, MNRAS, 338, 931 Koen, Paunzen, van Wyk, et al., 2003, MNRAS, 338, 931 Paunzen, Kamp, Iliev, et al., 1999, A&A, 345, 597 Paunzen, Kamp, Iliev, et al., 1999, A&A, 345, 597 Paunzen, Iliev, Kamp, Barzova, 2002a, MNRAS, 336, 1030 Paunzen, Iliev, Kamp, Barzova, 2002a, MNRAS, 336, 1030 Paunzen, Handler, Weiss, et al., 2002b, A&A, 392, 515 Paunzen, Handler, Weiss, et al., 2002b, A&A, 392, 515 Paunzen, Kamp, Weiss, Wiesemeyer, 2003, A&A, 404, 579 Paunzen, Kamp, Weiss, Wiesemeyer, 2003, A&A, 404, 579 Turcotte, Charbonneau, 1993, ApJ, 413, 376 Turcotte, Charbonneau, 1993, ApJ, 413, 376