The search for eclipsing binaries and rotation rates for Pre Main Sequence (PMS) Stars
Pre-Main-Sequence of A stars
description
Transcript of Pre-Main-Sequence of A stars
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.