Pre-Main-Sequence of A stars

Marcella MarconiINAF Astronomical Observatory of Capodimonte,Naples,Italy

Francesco PallaINAF Astrophysical Observatory of Arcetri, Florence,Italy

Poprad, July, 8, 2004

Outline

• Initial conditions (← peculiar mass range)

• Disk evolution (← IR excess, dust properties)

• Environment (comparison with PMS B stars)

• Variability (long term + pulsation)

Initial conditions

• Zero Age Main Sequence A type stars have

1.5 < M/M < 3.5In this mass range the PMS initial conditions,resulting from previous protostellar evolutionare 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

ZAMS

Gravitational contraction

D shell burning

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 →1.5MtKH = 8.0 x 106 yr →2.0M

Palla & Stahler 1993

Early evolution of luminosity and specific entropy for a 2M star

Palla & Stahler 1993

Thermal Relaxation

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).

Disk mass vs stellar mass

MD increaseswith stellarmass, but…

the disk/massratio remains~constant

Natta 2003

T TauriHAeHBe

Debris disks: time evolution of the disk mass

Disks around ≥75% of known Ae stars

At 10 Myr, Ae stars have “normal” Vegalike disks

Infrared images

Submm images

• 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.

EnvironmentIP Per (A type)BD+65° 1637 (B type)

Variation of the stellar density as a function of spectral type (or mass)

Testi et al. 1999

Orion Cluster

T Tauri

T Tauri ~few */pc3

Ae~10-100 */pc3

Be~100-1000 */pc3

Orion~104 */pc3

isolated

clustered

Ae

Be

Variability of PMS A stars UX Ori variability: obscuration by

circumstellar dust long term light variation

Variability of PMS A stars

• Intrinsic PulsationDuring 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.

New observations about 20 pulsators

V351 Ori IP Per

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

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.