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Clasification adopted to describe the progress of a star from PMS to ZAMS to MS
Class 0
- few 10E+4 yr (timescale for each Class depends on mass)
- YSO has started to form but envelope has bulk of mass
- heating from gravitational collapse
- bulky outflows with non-dissociative shocks
- most energy output as sub-mm/far-IR, thermal
- Water and CO lines in ISO LWS spectra
- sometimes proximity to radio-bright UCHII Regions (shock triggers collapse) but don't produce them!
Class I
- 10E+5 - 10E+6 yr
- accretion disk, diffuse environment of sub-stellar mass
- ISO LWS spectra which show CO lines do not have obvious water
- IR peak, non-thermal emission also
- outflow with dissociative shocks
- x-ray flares (stellar origin? Faint, hard if obscured)
Class II - >10E+6 yr
- PMS stars with circumstellar disc
- Classical T Tauri stars
- Halpha + IR excess
Class III
- Low-mass weak-line TTs (?protoplanetary disc? gas-poor?)
- optically bright, IR excess (almost) gone
- x-ray (soft, bright)
- High mass (H) AeBe stars have less IR and X-ray but are very optically bright? Emission lines come from disc? Does this become a Vega-like disc (Scuba etc.)
JHK colours help but can be contaminated by reflection nebulae, stars etc. e.g. Haisch et al. 2000 AJ 120 1396 fig.8 Crudely, younger YSOs with discs are redder - higher values of colours.
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