Ast 4 Lecture 18 Notes
Review
Stars form in large cold clouds of gas and dust called molecular clouds
• T = 10 K
• Typical sizes of 10-100 pc in diameter
• Mcl = 104 − 105 M
review
• The molecular clouds are initially in equilibrium (pressure and gravity are
balanced)
• If the cloud mass is large enought it is unstable and will begin to collapse
and fragment
• Fragments shrink into cores until they can no longer radiate away the
gravitational energy
• A core is divided into an opaque region called the protostar and the surrounding mass called the envelope
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conservation of angular momentum
Conservation Laws
• Conservation of Energy
• Conservation of momentum
Conservation of angular momentum
• The angular momentum of a core is conserved as it shrinks
• ωf Rf2 = ωi Ri2
• ω is the angular velocity - how fast it rotates
Conservation of angular momentum
• Conservation of angular momentum causes the rotation of the material to
increase as the core shrinks
• If the material rotates fast enough it will stop falling towards the center
• A disk of material forms around the protostar
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Young Stellar Objects
Young Stellar Objects (YSOs)
• envelope
A molecular core can be separated into 3 regions
• protostar
• disk
YSOs
The envelope consists of material that is
isothermally condensing and falling onto the
protostar and disk
YSOs
The protostar is the dense central region
which no longer shrinks isothermally
Some of the material accreting onto the disk is
launched into bipolar jets
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2.1
YSO classifications
Observationally we can classify the earliest stages of formation.
Class 0 objects
• Far infrared; λ > 25 µm because
it is deeply embedded
• Age: 104 yr
• Menv ∼ 0.2 − 3M high envelope
mass; main accretion phase
• Outflow activity
Main accretion phase
Class I objects
• Visible in NIR; λ > 2 µm
• Age: 105 yr
• Menv ∼ 0.02−0.3M At this stage
a large fraction of the envelope has
already accreted onto the protostar. Scattered light nebulosities
reveal outflow cavity
• Outflow activity
Optional: There are two other classifications II and III. Also known also
as CCTS and WTTS; they are optically visible, the disk makes up most of
circumstellar mass - essentially no envelope. There is still accretion and
outflow activity associated with Class II objects;
HH 30 Jets
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HH 47 Jets
HH 34 Jets
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IRAS 06047-1117
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