accretion processes in star formation lee hartmann cambridge astrophysics series, 32

Accretion Processes in Star Formation

Lee Hartmann

Cambridge Astrophysics Series, 32

Cambridge University Press

Based on:

ACCRETION SIGNATURES IN YSOACCRETION SIGNATURES IN YSO

Matter transfered from molecular cloud to YSO.

The accretion leaves distinctive signatures: . Directly, in velocity field (ex., redshifted absorption in spectra:matter is falling in. Indirectly: energy losses observed in YSO need to be compensated by external sources: Gravitational potential energy is the best candidate to supply this need

Visible YSO: (Class II ,III)Visible YSO: (Class II ,III)

The emerging flux is characterized by excess over photospheric fluxes appearing in emission lines and continua

. T Tauri Stars: 0.02 to 2-3 Msol

. Herbig Ae/Be Stars (HAeBe) : M > 1.5 Msol

Ages 1-10Myr

Protostars (Class 0,I)

Close association with molecular cloudsHeavily extincted (undetectable visible/nir wavelength range)Still actively receiving mass from the cloud

Powerfull outflows (CO, SiO, H2, [SII], H …)

Hartmann 2003

PROPERTIES OF YSO

< 1970

First spectra (photograph) of TTS (“low-mass” YSO)

“More evolved” YSO (II,III)

First spectra of Herbig Ae/Be

Herbig 1960(“Intermediate-mass YSO)

Emergin fluxEmergin flux of visible YSO is characterized by excesses over photospheric fluxes, both in emission lines and continua.

Emission lines show a wide range of conditions of formation, eg:

.Forbidden lines: nH 105 cm-3 Optical/NIR : T 104K

.Permitted lines: nH 1013 cm-3 UV lines from highly ionized species T 105-6 K

.v ~ 0 to ~ few x 100 km/s

Continuum excess appears as: flux “veiling” in photospheric absorption lines dominates the UV and IR emissions

Observational features:

Clase 1

from Hartmann 1998

Emission linesH, HeI, NaI, CaII

Late spectrum(F-M types

Teff 7-3000K) Balmer jump

Standardphotosphere

Lines with “veiling”

Absorption lines with “veiling”

T Tauri spectra

B-V no photospheric

“Modern” data

Veiling

Hartigan et al 1999

Fv

Flf

Fcl

Fl

Fc

F

FlFl

f / Fcf + r

Fc 1 + r r = Fv/ Fc

f

Veiling parameterVeiling parameter

Fv energy excess relative to the

photospheric flux (Fcf)

“Veiling”

Hartigan et al 1999

Clase 1

from Hartmann 1998

Energy excess relative to the photospheric flux in NIR and mm(Dotted curves:SED of LkCa7, classIII with no evidence for accretion)

photosphere

SEDS of TTS in Taurus MC

UV excess

Gullbring et al 2000

Colour-Colour diagrams

NIR excess

Kenyon & Hartmann 1995

Reddening lines

CTTS locus

CTTS loci(Colours corrected from reddening)

Meyer, Calvet, & Hillenbrand 1997

CTTS vs WTTSCTTS vs WTTS

from Hartmann 1998

TTS have been classified in two types:Classical (CTTS) Weak (WTTS)

Classification represented clear physical differences:WTTS :lack NIR excessno veiling excess Narrow emission linesNo forbidden linesWeaker FUV linesComparable Lx

Initially, classification based on the equivalent width: for WTTS, EW(H) < 10 A (Herbig & Bell 1988)

(K-L < 0.3 for photospheric colors)K 2.25 m; L 3.4 m

CTTS vs WTTS

Hartmann 1998

WTTS show no indication of “veiling”.

No significant NIR excess

r ratio to hot continuum to stellar continuum emission at 5700 A

CTTS vs WTTS

CTTS

Emission lines in WTTS are weaker and much narrower than CTTS