II
From cores to
stars
2.1 Phases of star formation
Star formation can be divided into five distinct stages (see last lecture):
Prestellar cores
Class 0 protostars
Class I protostars
Class II pre-MS stars (classical T Tauri stars)
Class III pre-MS stars (weak-line T Tauri stars)
Each stage lasts about 5 times longer than the previous phase.
2.1 Phases of star formation
Basic physics we think is happening is an evolutionary sequence:
Prestellar cores – collapsing bound gas core
Class 0 – mostly gas core, but with central protostar
Class I – most gas on the protostar, but still an extended envelope
Class II – pre-MS star(s) with a massive disc.
Class III - pre-MS star(s) with little (debris) disc.
(We’ll come back to the different between ‘protostar’ and ‘pre-MS
star’ later.)
2.2 Prestellar cores
Prestellar cores are dense molecular cores which are gravitationally
bound and will form stars, but have not yet done so.
They are typically 0.1pc in radius, densities of ~105 cm-3 (10-20 g cm-3),
T~10K
2.3 Stability of a core
To get a measure of the boundness of a core we need:
a) a measure of the mass (which normally comes from the
dust - we assume that 1% of the mass is in dust with possible
big errors here).
b) a measure of pressure from both thermal (random motions) and
turbulence (bulk motions) – and also possibly magnetic field
support. These come from molecular line widths (and polarisation
in the case of magnetic fields).
There are line-of-sight effects as we are projecting and getting
the properties of a column. Dense molecular tracers like ammonia
help here (see later lectures).
2.3 Stability of a core
The criteria for gravitational stability is normally given by the virial theorem
which states that:
where T is the kinetic energy, and W the potential energy. The virial
ratio of a system is:
where if Qvir=0.5 the system is in virial equilibirum. This also leads to the
virial mass – the mass an object with a given size and velocity dispersion
would have if it were in virial equilibrium.
2.3 Stability of a core
The virial mass is the mass a core would have to have to be in virial
equilibrium – compared with the true mass:
Q>0.5
(expanding/
unbound?)
Q<0.5
(collapsing)
2.3 Stability of a core
That some cores have M>>Mvir suggests that they are not bound,
and will not collapse to form stars. Therefore they are not
prestellar cores.
When we observe a core with no evidence for a protostar it can be
very difficult to determine if we are seeing a prestellar core, or a
transient density enhancement that looks like a core...
This becomes a real problem later when we examine the
origin of the IMF.
2.4 Class 0 cores
Class 0 cores have a protostar in their centres. They are distinguishable
from prestellar cores by a central IR (10s microns) point source and/or
evidence of jets and outflows.
Note that class 0s have a protostar(s)
in their centres as opposed to a
pre-MS star – we will come back
to this distinction later...
2.5 Class I cores
A class 0 turns into a class I core when more than half of the envelope
has been accreted onto the central object. Class 0 cores are defined
to have Tbol<70 K.
cold bb
star emerging from
the envelope
2.6 T Tauri stars
Once the envelope has been depleted by accretion onto the central object
all that remains is an accretion disc and often a jet. The disc is gradually
destroyed (accreted and/or blown away and/or used in planet formation).
2.7 Evolution of the SED
Classes can be distinguished from the SEDs, and from the levels of
reprocessed radiation in the IR excess.
2.7 Evolution of the SED
Observationally classes are distinguished by the SEDs Class 0 – Tbol < 70 K (peak about 100 microns – BB-like)
Class I – 70 K < Tbol < 650 K (peak at 10s microns – not a good BB)
Class II – 650 K < Tbol < 2800 K (peak at microns, BB with IR excess)
Class III – Tbol > 2800 K (BB with small IR excess)
Class II and III show other features
in their spectra as well (see later
lectures).
2.8 Lifetimes of each stage
Lifetimes of each stage can be estimated from the relative numbers of
cores in each stage – roughly a factor of 5 between each class.
Class 0 <105 yrs
Class I few x 105 yrs
Class II few x106 yrs
Class III few x107 yrs
This is very ropey as we
mostly deal with low-N
statistics.
Summary
Prestellar cores – dense cores without a central object
Class 0 – deeply embedded with a central object (IR source, or
evidence of jets/outflows). Lasts <105 yrs.
Class I – still embedded, but more material on the central object
than in the envelope. Lasts few x105 yrs.
Class II – no envelope, but a massive disc. Lasts few x106 yrs.
Class III – evidence of a low-mass disc (IR excess). Lasts few x107
yrs.
A bound core will have Q<1, and a virialised or collapsing core will
have Q<=0.5
References
A good place to find detailed reviews are ‘Protostars and Planets V’
or ‘VI’ (I prefer V even though its older, e.g. Di Francesco et al.
(2007) and Ward-Thompson et al. (2007), also White et al. (2007)).
These are rather technical references aimed at astronomers.
For more general background look at standard textbooks: especially
Whitworth & Ward-Thompson, for technical terms or specific topics
wikipedia is a pretty good source.