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. Initial conditions Zero Age Main Sequence A type stars have
ZAMS
Gravitational contraction
D shell burning
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
the PMS evolutionary time is 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. Environment
BD+65° 1637 (B type)
IP Per (A type)
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~ */pc3
Be~ */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 ~ 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. New observations  about 20 pulsators
V351 Ori
IP Per

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.