AY201b, 4•18•13Young Stars, Disks & Outflows

http://www.cfa.harvard.edu/COMPLETE/learn/star_and_planet_formation.html

c. 2004

Sean AndrewsTuesday, 4/23!

(Interferometric Studies, Modeling)

c. 2013

1. Cloud makes core, gives it some net rotation.

2. Core undergoes “stuff” (competitive accretion, triggering, waiting?)

3. Core begins to collapse (onto disk)

4. Core Class 0 object (disk grows, outflow begins, central object?)

5. Class 0 Class 1 (~fusion begins, (episodic) accretion continues)

6. Class I Class II (diminishing accretion, core disruption, outflow widens)

7. Class II Class III (accretion ends, outflow dies)

8. Class III X-ray emitting “Weak-line T-Tauri Star”, still some disk

9. Class III “Transitional” Disk

10. “Transitional” Disk “Debris” Disk + Planets

11. Interactions Planet Migration, Remnant Disk (e.g. Kuiper Belt)

The Party Line, in Words, as of 2013

The Standard “X-Wind” Model of Shu et al.

From thesis of T. Stanke, 2001: Schematic drawing (not to scale) of the driving and collimation zone of a jet from a young stellar object. Representative magnetic field lines are drawn as blue lines, and a representative trajectory of a jet gas parcel is drawn as a black line. The central star is surrounded by an accretion disk, which is truncated at the corotation radius (the point at which the angular velocity in the disk equals the angular velocity of the star). The magnetic field within the gap between the star and the disk is the largely undisturbed field of the protostar (here assumed to be dipolar). The protostar is coupled to the inner edge of the accretion disk via the magnetic field. Matter is accreted along the field lines connecting the star and the disk in accretion funnels (indicated by the small arrows). Field lines which are anchored in the disk slightly further out first extend out radially away from the central  region and are then wound up by the inertia of the material frozen in the magnetic field, as the entire configuration rotates. Material on these field lines is flung out and centrifugally accelerated (the trajectory marks the path of a gas parcel along a field line, as the field line takes part in the overall rotation; the field line  drawn in the figure is a snapshot only). Field lines originating from the same radius of the disk form a rotation surface (flux tube, indicated by the dotted lines), along which the  (partly ionized) material from above the disk surface can flow. The toroidal field created by winding up of the field lines eventually collimates the flow in a direction parallel to the polar axis of the star/disk system.References: (Blandford & Payne 1982), Shu et al. 1994 and companion papers; see Shu et al. 1988 for an earlier version, and Shu & Shang 1997 and Shu et al. 2000 for reviews)

Disks around Young Stars: Remember, more than Gravity is at Work

The structure of a gravitationally bound disk is governed by Keplerian rotation.

However, accretion disks have a temperature structure giving rise to thermal forces (plus magnetic/turbulent forces) that can change their internal structure and dynamics.

1. Cloud makes core, gives it some net rotation.

2. Core undergoes “stuff” (competitive accretion, triggering, waiting?)

3. Core begins to collapse (onto disk)

4. Core Class 0 object (disk grows, outflow begins,

central object?)

5. Class 0 Class 1 (~fusion begins, (episodic)

accretion continues)

6. Class I Class II (diminishing accretion, core

disruption, outflow widens)

7. Class II Class III (accretion ends, outflow dies)

8. Class III X-ray emitting “Weak-line T-Tauri Star”, still some disk

9. Class III “Transitional” Disk

10. “Transitional” Disk “Debris” Disk + Planets

11. Interactions Planet Migration, Remnant Disk

(e.g. Kuiper Belt)

Q. How is this “story” used? A. Demography

e.g. Evans et al. 2009

Demography

e.g. Evans et al. 2009

Demography

e.g. Evans et al. 2009

Percentages in categories allow steps in the story to be given time scales, with assumptions about “history” (e.g. co-eval star formation, continuous star formation?)

1.Cloud makes core, gives it some net rotation.

2.Core undergoes “stuff” (competitive accretion, triggering, waiting?)

3.Core begins to collapse (onto disk)

4.Core Class 0 object (disk grows, outflow begins, central object?)

5.Class 0 Class 1 (~fusion begins, (episodic) accretion continues)

6.Class I Class II (diminishing accretion, core disruption, outflow widens)

7.Class II Class III (accretion ends, outflow dies)

8.Class III X-ray emitting “Weak-line T-Tauri Star”, still some disk

9.Class III “Transitional” Disk

10.“Transitional” Disk “Debris” Disk + Planets

11.Interactions Planet Migration, Remnant Disk (e.g. Kuiper Belt)

“A contracting mass of gas that represents an early stage in the formation of a star before nucleosynthesis has begun“

Since fusion is a negligible energy source in a protostar, its luminosity comes from gravitational contraction. From the virial theorem, half of the gravitational potential energy gain from contraction is converted into kinetic energy, while the other half is radiated away. In other words, for a homogeneous sphere, .

For a 1 solar mass star contracted to erg.

Wikipedia on “Protostar”

Wikipedia on “Pre-Main Sequence Star”

Wikipedia on “Young Stellar Object”

Evolution of the spectral energy distribution during low mass star formation. Initially the core is cold, 20-30K, peaking in the sub-millimetre (Class 0).

An infrared excess appears and first peaks in the far-IR, emission from a warm envelope heated by the accretion luminosity (Class I).

The peak shifts to the mid- and near-IR as a disk forms. (Class II). Its spectral shape depends on whether the disk is passive (merely re-processing the radiation from the central star) or active (also kept hot by ongoing accretion).

Finally, the disk dissipates (Class III).

Figure: A graphical overview of the four stages of protostar evolution are shown below (Andrea Isella’s thesis, 2006), also found in Meredith MacGregor’s WP Post on Robitaille et al. 2007. Text based on http://www.phys.unsw.edu.au/jacara/pilotscience.php, by Michael Burton.

00

I

I

II

II

Arce & Sargent 2006

What happens to the core?

C18O “core”

12COredshiftedoutflow

12COblueshifted

outflow

Arce & Sargent 2006

Explaining Disk Gaps to the Public...