Pre-Main-Sequence of A stars

1. Pre-Main-Sequence of A stars Marcella Marconi INAF Astronomical Observatory of Capodimonte,Naples,Italy Francesco Palla INAF Astrophysical Observatory of Arcetri, Florence,Italy Poprad, July, 8, 2004

2. Outline • Initial conditions (← peculiar mass range) • Disk evolution (← IR excess, dust properties) • Environment (comparison with PMS B stars) • Variability (long term + pulsation)

3. Gravitational contraction D shell burning ZAMS Initial conditions • Zero Age Main Sequence A type stars have 1.5 < M/M< 3.5 In this mass range the PMS initial conditions, resulting from previous protostellar evolution are crucial: • For M > ~ 2M protostars radiatively stable in the inner regions thick subsurface mantle of D  thermally unrelaxed at the beginning of PMS. • Inner convection disappears for M > ~ 4M • thePMS evolutionary timeis reduced by~ 30 % for 2M and by ~ 100 % for 6M

4. Early evolution in the H-R diagram: the first 3 million years At the beginning both models are fully convective. The 2.0M model contracts for only 1.1 x 104 yr before a radiative core appears (→ the stellar radius swells) tKH = 1.2 x 107 yr →1.5M tKH = 8.0 x 106 yr →2.0M Palla & Stahler 1993

5. Early evolution of luminosity and specific entropy for a 2M star Thermal Relaxation Palla & Stahler 1993

6. Are circumstellar disks present? • Many PMS A stars show infrared excess ← active circumstellar disk ? • Residual mass accretion from a circumstellar disk is an important effect during PMS evolution. • Gas disks are found around Ae stars but not around Be stars likely because disks never formed or quickly dispersed (e.g., due to wind photoevaporation).

7. Disk mass vs stellar mass MD increases with stellar mass, but… the disk/mass ratio remains ~constant T Tauri HAe HBe Natta 2003

8. Debris disks: time evolution of the disk mass Disks around ≥75% of known Ae stars At 10 Myr, Ae stars have “normal” Vega like disks

9. Infrared images Submm images

10. IP Per (A type) BD+65° 1637 (B type) Environment • At NIR wavelenghts (2.2 μm) the low extinction allows detection of embedded young stars in the vicinity of bright Herbig stars • Herbig Ae type stars are generally found in isolation, unlike the more massive Herbig Be stars.

11. Variation of the stellar density as a function of spectral type (or mass) isolated Orion Cluster T Tauri ~few */pc3 Ae~10-100 */pc3 Be~100-1000 */pc3 Orion~104 */pc3 Be Ae T Tauri clustered Testi et al. 1999

12. Variability of PMS A stars UX Ori variability: obscuration by circumstellar dust  long term light variation

13. Variability of PMS A stars • Intrinsic Pulsation During the PMS phase intermediate mass (1.5  M/Mo  4.0) stars cross the instability strip of more evolved  Scuti pulsating stars (P ~ 0.5-8 h) Intrinsic pulsation ( Scuti) for PMS intermediate mass (Herbig) stars early suggested by Breger (1972) first two candidates in NGC2264. Subsequent observations by Kurtz & Marang (1995, for HR5999) and Donati et al. (1997, for HD104237) confirmed this occurrence. Theoretical interest  pulsation instability strip for PMS  Scuti (Marconi & Palla 1998, ApJL) in the radial nonlinear assumption.

14. V351 Ori IP Per New observations  about 20 pulsators

15. But…. • Observed frequencies are not very accurate (limitation of single ground based observations)  need for multisite campaigns and space observations (COROT, MOST, EDDINGTON) • Radial pulsation models do not always reproduce all observed frequencies  need for non radial pulsation modeling

16. Conclusions • PMS A-type stars have peculiar initial conditions as a result of their protostellar evolution • Direct evidence for circumstellar disks similar to those around T Tauri stars • Mostly found in isolation/small aggregates • These properties are not shared by the PMS B-type counterparts • Presence of both long term, high amplitude variability (UX Ori type) and short term, small amplitude (δ Scuti type) variability.