Despite our knowledge of stellar evolution, star birth remains a mystery. This is because protostars are obscured from view by the dense environments in which they are nurtured. It has long been known that T Tauri stars are the visible manifestations of a young star that has finally emerged. Now, however, we have been able to detect earlier stages. Infrared and submillimetre observations reveal the presence of younger objects. We are thus able to define a sequence based on prominent spectral energy distribution features: Class 0 sources (strong submillimetre), Class 1 (infrared), Class 2 (near-infrared) and Class 3 (optical).
Fundamental questions still await answers for protostellar systems. Is there a single evolutionary sequence? What physics controls the evolution? How are they distributed by mass? Can we track young stars, in order to trace their origins and final masses? There are several potential tracking parameters: bolometric luminosity and temperature, L(bol) & T(bol)$, outflow mechanical power, L(mech), millimetre luminosity L(mm), and radiative shock emission L(shock). The latter two may indicate the envelope mass, M(env) and jet power L(jet), respectively.
The key difficulty is that one object contains several components, each evolving according to its own particular physical and dynamical regulations. The components may feed each other, and correlations between properties are often found. For example, as envelopes lose mass, the outflows get weaker. The emerging picture involves five components: protostellar core, accretion disk, envelope, twin jet and bipolar outflow. The basis of unifying schemes is that the envelope supplies the disk which, in turn, supplies the jets and protostar. The jets carve a path into the environment, forming the bipolar lobes which disturb and possibly disrupt the envelope. The quest here is to understand protostellar evolution by analysing the possible relationships between these components according to the most plausible physical models.
The most recent complete version is entitled:
The Unification Scheme and some techniques for tracking the evolution of protostars
It is available in pdf, from ESO Astrophysics Symposia, eds. J. Alves & M. McCaughrean.
Aspects of the envelope and molecular jets have now been added:
Far-infrared photometry of deeply embedded outflow source
and is available in pdf, although it has not yet been accepted.
The evolution of an outbursting/erupting source...
Extra (only available on this page):
What if a star forms via a series of inbursts/outbursts (possibly producing the so-called 'FuOrs' in the low-mass case)? Here is a supplementary diagram showing the evolution of a low mass star with a 3000 year, 90 per cent oscillation in accreting mass superimposed. The star reaches a mass of 0.902 solar masses. The evolution time is expressed logarithmically - between 1000 and 100,000 years.
Also available is the evolution of a 0.5 solar mass star (on request).
Last Revised: 2010 June 30th