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More massive protostars evolve faster and have outflows e.g. CO which
remain molecular. Lower-mass protostars produce ionised outflows. But
molecular v. ionised outflows is implied by the AGE Class 0 - 1
distinction also.
- From ISO colours (Pezzuto) the younger YSOs have cooler colours as expected - except bright, probably massive Class I's which have very warm colours.
- Low-mass stars show more x-ray emission.
- T Tauri stars have stronger, softer x-ray emission
- Obscured (younger) stars have fainter, harder x-ray emission
Li and Halpha emission lines are diagnostic of young low-mass YSO (and high mass?)
- Li is the first element to be burnt when a protostar gets hot enough for nulceosynthesis, so indicates start of Class I?
- Halpha indicates accretion, easier to see.
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