Accretion Processes in Star Formation Lee Hartmann Cambridge Astrophysics Series, 32
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Transcript of 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