Physics 224 The Interstellar Medium - Karin...
Transcript of Physics 224 The Interstellar Medium - Karin...
![Page 1: Physics 224 The Interstellar Medium - Karin Sandstromkarinsandstrom.github.io/sp18_phys224/l17_slides.pdf · © Karin Sandstrom, UC San Diego - Do not distribute without permission](https://reader033.fdocuments.us/reader033/viewer/2022043012/5faa2e12f26ba412dd625411/html5/thumbnails/1.jpg)
© Karin Sandstrom, UC San Diego - Do not distribute without permission
Physics 224 The Interstellar Medium
Lecture #17: Observations of Molecular Gas
![Page 2: Physics 224 The Interstellar Medium - Karin Sandstromkarinsandstrom.github.io/sp18_phys224/l17_slides.pdf · © Karin Sandstrom, UC San Diego - Do not distribute without permission](https://reader033.fdocuments.us/reader033/viewer/2022043012/5faa2e12f26ba412dd625411/html5/thumbnails/2.jpg)
© Karin Sandstrom, UC San Diego - Do not distribute without permission
Magnetic Fields in the ISM• Synchrotron emission - from charged particles interacting with the
magnetic field.
• Faraday Rotation - different phase velocities of right & left circularly polarized light in the presence of B-field leads to rotation of polarization angle
• Polarization - of starlight due to dust grains aligned along B-field or of dust emission from aligned grains
• Zeeman splitting - splitting of fine structure levels in atoms/molecules due to interaction of electron magnetic moment and B-field
Observational Tracers:
![Page 3: Physics 224 The Interstellar Medium - Karin Sandstromkarinsandstrom.github.io/sp18_phys224/l17_slides.pdf · © Karin Sandstrom, UC San Diego - Do not distribute without permission](https://reader033.fdocuments.us/reader033/viewer/2022043012/5faa2e12f26ba412dd625411/html5/thumbnails/3.jpg)
© Karin Sandstrom, UC San Diego - Do not distribute without permission
Zeeman Effect Zeeman splitting is largest when there is an unpaired electron
in outer shell: e.g. HI, OH, CN, CH,
CCS, SO, and O2
Even then, energy shift is small.
But, shifted levels produce different
circular polarizations.
Magnetic Fields in the ISM
![Page 4: Physics 224 The Interstellar Medium - Karin Sandstromkarinsandstrom.github.io/sp18_phys224/l17_slides.pdf · © Karin Sandstrom, UC San Diego - Do not distribute without permission](https://reader033.fdocuments.us/reader033/viewer/2022043012/5faa2e12f26ba412dd625411/html5/thumbnails/4.jpg)
© Karin Sandstrom, UC San Diego - Do not distribute without permission
Quick review of polarization:
Magnetic Fields in the ISM
Electric field of plane wave traveling in +z direction
with +x north and +y east.
Stokes vectors: completely quantify the
propagation of polarized radiation.
![Page 5: Physics 224 The Interstellar Medium - Karin Sandstromkarinsandstrom.github.io/sp18_phys224/l17_slides.pdf · © Karin Sandstrom, UC San Diego - Do not distribute without permission](https://reader033.fdocuments.us/reader033/viewer/2022043012/5faa2e12f26ba412dd625411/html5/thumbnails/5.jpg)
© Karin Sandstrom, UC San Diego - Do not distribute without permission
Magnetic Fields in the ISM
My favorite reference on polarization & Zeeman splitting:
T. Robishaw Ph.D. Thesis, 2008, Berkeley
![Page 6: Physics 224 The Interstellar Medium - Karin Sandstromkarinsandstrom.github.io/sp18_phys224/l17_slides.pdf · © Karin Sandstrom, UC San Diego - Do not distribute without permission](https://reader033.fdocuments.us/reader033/viewer/2022043012/5faa2e12f26ba412dd625411/html5/thumbnails/6.jpg)
© Karin Sandstrom, UC San Diego - Do not distribute without permission
Stokes Parameters for Classical Derivation of the Zeeman Effect
Magnetic Fields in the ISM
for unpolarized spectral line:
![Page 7: Physics 224 The Interstellar Medium - Karin Sandstromkarinsandstrom.github.io/sp18_phys224/l17_slides.pdf · © Karin Sandstrom, UC San Diego - Do not distribute without permission](https://reader033.fdocuments.us/reader033/viewer/2022043012/5faa2e12f26ba412dd625411/html5/thumbnails/7.jpg)
© Karin Sandstrom, UC San Diego - Do not distribute without permission
Magnetic Fields in the ISM
Even then, energy shift is small.
But, shifted levels produce different
circular polarizations.
Crutcher 2012
Total intensity 7 hyperfine components for mm rotational lines of CN two velocity components
along line of sight.
Circularly polarized emission: 4 components with large
Zeeman splitting
Circularly polarized emission: 3 components with small
Zeeman splitting
line-of-sight B-field
Zeeman splitting is largest when there is an unpaired electron
in outer shell: e.g. HI, OH, CN, CH,
CCS, SO, and O2
![Page 8: Physics 224 The Interstellar Medium - Karin Sandstromkarinsandstrom.github.io/sp18_phys224/l17_slides.pdf · © Karin Sandstrom, UC San Diego - Do not distribute without permission](https://reader033.fdocuments.us/reader033/viewer/2022043012/5faa2e12f26ba412dd625411/html5/thumbnails/8.jpg)
© Karin Sandstrom, UC San Diego - Do not distribute without permission
Molecular CloudsObserved Characteristics
• Self-Gravity
• Turbulence
• Substructure
• Magnetic Fields
• Mass Spectrum
• Lifetimes
• Star Formation
Virial Theorem
2nd derivative of moment of Inertia of cloud
see Krumholz “Notes on Star Formation” for a very clear derivation
![Page 9: Physics 224 The Interstellar Medium - Karin Sandstromkarinsandstrom.github.io/sp18_phys224/l17_slides.pdf · © Karin Sandstrom, UC San Diego - Do not distribute without permission](https://reader033.fdocuments.us/reader033/viewer/2022043012/5faa2e12f26ba412dd625411/html5/thumbnails/9.jpg)
© Karin Sandstrom, UC San Diego - Do not distribute without permission
Molecular CloudsObserved Characteristics
• Self-Gravity
• Turbulence
• Substructure
• Magnetic Fields
• Mass Spectrum
• Lifetimes
• Star Formation
Virial Theorem
total kinetic plus thermal energy of the cloud
![Page 10: Physics 224 The Interstellar Medium - Karin Sandstromkarinsandstrom.github.io/sp18_phys224/l17_slides.pdf · © Karin Sandstrom, UC San Diego - Do not distribute without permission](https://reader033.fdocuments.us/reader033/viewer/2022043012/5faa2e12f26ba412dd625411/html5/thumbnails/10.jpg)
© Karin Sandstrom, UC San Diego - Do not distribute without permission
Molecular CloudsObserved Characteristics
• Self-Gravity
• Turbulence
• Substructure
• Magnetic Fields
• Mass Spectrum
• Lifetimes
• Star Formation
Virial Theorem
confining pressure on the cloud’s surface
fluid pressure tensor
![Page 11: Physics 224 The Interstellar Medium - Karin Sandstromkarinsandstrom.github.io/sp18_phys224/l17_slides.pdf · © Karin Sandstrom, UC San Diego - Do not distribute without permission](https://reader033.fdocuments.us/reader033/viewer/2022043012/5faa2e12f26ba412dd625411/html5/thumbnails/11.jpg)
© Karin Sandstrom, UC San Diego - Do not distribute without permission
Molecular CloudsObserved Characteristics
• Self-Gravity
• Turbulence
• Substructure
• Magnetic Fields
• Mass Spectrum
• Lifetimes
• Star Formation
Virial Theorem
difference in magnetic pressure in cloud interior vs magnetic pressure
plus tension at cloud surface
![Page 12: Physics 224 The Interstellar Medium - Karin Sandstromkarinsandstrom.github.io/sp18_phys224/l17_slides.pdf · © Karin Sandstrom, UC San Diego - Do not distribute without permission](https://reader033.fdocuments.us/reader033/viewer/2022043012/5faa2e12f26ba412dd625411/html5/thumbnails/12.jpg)
© Karin Sandstrom, UC San Diego - Do not distribute without permission
Molecular CloudsObserved Characteristics
• Self-Gravity
• Turbulence
• Substructure
• Magnetic Fields
• Mass Spectrum
• Lifetimes
• Star Formation
Virial Theorem
gravitational energy of the cloud
![Page 13: Physics 224 The Interstellar Medium - Karin Sandstromkarinsandstrom.github.io/sp18_phys224/l17_slides.pdf · © Karin Sandstrom, UC San Diego - Do not distribute without permission](https://reader033.fdocuments.us/reader033/viewer/2022043012/5faa2e12f26ba412dd625411/html5/thumbnails/13.jpg)
© Karin Sandstrom, UC San Diego - Do not distribute without permission
Molecular CloudsObserved Characteristics
• Self-Gravity
• Turbulence
• Substructure
• Magnetic Fields
• Mass Spectrum
• Lifetimes
• Star Formation
Virial Theorem
rate of change of momentum flux across cloud surface
![Page 14: Physics 224 The Interstellar Medium - Karin Sandstromkarinsandstrom.github.io/sp18_phys224/l17_slides.pdf · © Karin Sandstrom, UC San Diego - Do not distribute without permission](https://reader033.fdocuments.us/reader033/viewer/2022043012/5faa2e12f26ba412dd625411/html5/thumbnails/14.jpg)
© Karin Sandstrom, UC San Diego - Do not distribute without permission
Molecular CloudsObserved Characteristics
• Self-Gravity
• Turbulence
• Substructure
• Magnetic Fields
• Mass Spectrum
• Lifetimes
• Star Formation
Virial Theorem
for cloud in equilibrium between gravitational force and magnetic field
wheremagnetic flux through cloud
![Page 15: Physics 224 The Interstellar Medium - Karin Sandstromkarinsandstrom.github.io/sp18_phys224/l17_slides.pdf · © Karin Sandstrom, UC San Diego - Do not distribute without permission](https://reader033.fdocuments.us/reader033/viewer/2022043012/5faa2e12f26ba412dd625411/html5/thumbnails/15.jpg)
© Karin Sandstrom, UC San Diego - Do not distribute without permission
Molecular CloudsObserved Characteristics
• Self-Gravity
• Turbulence
• Substructure
• Magnetic Fields
• Mass Spectrum
• Lifetimes
• Star Formation
Virial Theorem
for cloud in equilibrium between gravitational force and magnetic field
and“magnetic
critical mass”
![Page 16: Physics 224 The Interstellar Medium - Karin Sandstromkarinsandstrom.github.io/sp18_phys224/l17_slides.pdf · © Karin Sandstrom, UC San Diego - Do not distribute without permission](https://reader033.fdocuments.us/reader033/viewer/2022043012/5faa2e12f26ba412dd625411/html5/thumbnails/16.jpg)
© Karin Sandstrom, UC San Diego - Do not distribute without permission
Molecular CloudsObserved Characteristics
• Self-Gravity
• Turbulence
• Substructure
• Magnetic Fields
• Mass Spectrum
• Lifetimes
• Star Formation
“magnetic critical mass”
if
then
and the cloud will collapse
“magnetically super-critical” means B-field is not strong enough
to support cloud against gravitational collapse
![Page 17: Physics 224 The Interstellar Medium - Karin Sandstromkarinsandstrom.github.io/sp18_phys224/l17_slides.pdf · © Karin Sandstrom, UC San Diego - Do not distribute without permission](https://reader033.fdocuments.us/reader033/viewer/2022043012/5faa2e12f26ba412dd625411/html5/thumbnails/17.jpg)
© Karin Sandstrom, UC San Diego - Do not distribute without permission
Molecular CloudsObserved Characteristics
• Self-Gravity
• Turbulence
• Substructure
• Magnetic Fields
• Mass Spectrum
• Lifetimes
• Star Formation
Crutcher 2012
Molecular clouds fall in magnetically “supercritical” range
i.e. B-field alone cannot prevent collapse
![Page 18: Physics 224 The Interstellar Medium - Karin Sandstromkarinsandstrom.github.io/sp18_phys224/l17_slides.pdf · © Karin Sandstrom, UC San Diego - Do not distribute without permission](https://reader033.fdocuments.us/reader033/viewer/2022043012/5faa2e12f26ba412dd625411/html5/thumbnails/18.jpg)
© Karin Sandstrom, UC San Diego - Do not distribute without permission
Molecular CloudsObserved Characteristics
• Self-Gravity
• Turbulence
• Substructure
• Magnetic Fields
• Mass Spectrum
• Lifetimes
• Star Formation
γMW = -1.5 ± 0.1
Rosolowsky 2005
![Page 19: Physics 224 The Interstellar Medium - Karin Sandstromkarinsandstrom.github.io/sp18_phys224/l17_slides.pdf · © Karin Sandstrom, UC San Diego - Do not distribute without permission](https://reader033.fdocuments.us/reader033/viewer/2022043012/5faa2e12f26ba412dd625411/html5/thumbnails/19.jpg)
© Karin Sandstrom, UC San Diego - Do not distribute without permission
Molecular CloudsColombo et al. 2014 - PAWS survey of M51
![Page 20: Physics 224 The Interstellar Medium - Karin Sandstromkarinsandstrom.github.io/sp18_phys224/l17_slides.pdf · © Karin Sandstrom, UC San Diego - Do not distribute without permission](https://reader033.fdocuments.us/reader033/viewer/2022043012/5faa2e12f26ba412dd625411/html5/thumbnails/20.jpg)
© Karin Sandstrom, UC San Diego - Do not distribute without permission
Molecular CloudsColombo et al. 2014 - PAWS survey of M51
![Page 21: Physics 224 The Interstellar Medium - Karin Sandstromkarinsandstrom.github.io/sp18_phys224/l17_slides.pdf · © Karin Sandstrom, UC San Diego - Do not distribute without permission](https://reader033.fdocuments.us/reader033/viewer/2022043012/5faa2e12f26ba412dd625411/html5/thumbnails/21.jpg)
© Karin Sandstrom, UC San Diego - Do not distribute without permission
Molecular CloudsObserved Characteristics
• Self-Gravity
• Turbulence
• Substructure
• Magnetic Fields
• Mass Spectrum
• Lifetimes
• Star Formation
Kawamura et al. 2009 If star formation rate is constant,
relative numbers of clouds in each
evolutionary state, plus ages of
clusters when no molecular gas is around gives you cloud lifetimes.
~20-30 Myr
![Page 22: Physics 224 The Interstellar Medium - Karin Sandstromkarinsandstrom.github.io/sp18_phys224/l17_slides.pdf · © Karin Sandstrom, UC San Diego - Do not distribute without permission](https://reader033.fdocuments.us/reader033/viewer/2022043012/5faa2e12f26ba412dd625411/html5/thumbnails/22.jpg)
© Karin Sandstrom, UC San Diego - Do not distribute without permission
Star Formation
Bergin & Tafalla 2007
![Page 23: Physics 224 The Interstellar Medium - Karin Sandstromkarinsandstrom.github.io/sp18_phys224/l17_slides.pdf · © Karin Sandstrom, UC San Diego - Do not distribute without permission](https://reader033.fdocuments.us/reader033/viewer/2022043012/5faa2e12f26ba412dd625411/html5/thumbnails/23.jpg)
© Karin Sandstrom, UC San Diego - Do not distribute without permission
Larson 1981
At small scales in clouds, thermal pressure support takes over from turbulence.
“Larson’s Laws”
Star Formation
![Page 24: Physics 224 The Interstellar Medium - Karin Sandstromkarinsandstrom.github.io/sp18_phys224/l17_slides.pdf · © Karin Sandstrom, UC San Diego - Do not distribute without permission](https://reader033.fdocuments.us/reader033/viewer/2022043012/5faa2e12f26ba412dd625411/html5/thumbnails/24.jpg)
© Karin Sandstrom, UC San Diego - Do not distribute without permission
Cores in Molecular Clouds
Bergin & Tafalla 2007
Column density profiles of dense
cores are similar to Bonnor-Ebert
profile (isothermal, marginally stable spherical cloud,
supported against collapse by pressure)
![Page 25: Physics 224 The Interstellar Medium - Karin Sandstromkarinsandstrom.github.io/sp18_phys224/l17_slides.pdf · © Karin Sandstrom, UC San Diego - Do not distribute without permission](https://reader033.fdocuments.us/reader033/viewer/2022043012/5faa2e12f26ba412dd625411/html5/thumbnails/25.jpg)
© Karin Sandstrom, UC San Diego - Do not distribute without permission
![Page 26: Physics 224 The Interstellar Medium - Karin Sandstromkarinsandstrom.github.io/sp18_phys224/l17_slides.pdf · © Karin Sandstrom, UC San Diego - Do not distribute without permission](https://reader033.fdocuments.us/reader033/viewer/2022043012/5faa2e12f26ba412dd625411/html5/thumbnails/26.jpg)
© Karin Sandstrom, UC San Diego - Do not distribute without permission
![Page 27: Physics 224 The Interstellar Medium - Karin Sandstromkarinsandstrom.github.io/sp18_phys224/l17_slides.pdf · © Karin Sandstrom, UC San Diego - Do not distribute without permission](https://reader033.fdocuments.us/reader033/viewer/2022043012/5faa2e12f26ba412dd625411/html5/thumbnails/27.jpg)
© Karin Sandstrom, UC San Diego - Do not distribute without permission
Alves et al. 2007
![Page 28: Physics 224 The Interstellar Medium - Karin Sandstromkarinsandstrom.github.io/sp18_phys224/l17_slides.pdf · © Karin Sandstrom, UC San Diego - Do not distribute without permission](https://reader033.fdocuments.us/reader033/viewer/2022043012/5faa2e12f26ba412dd625411/html5/thumbnails/28.jpg)
© Karin Sandstrom, UC San Diego - Do not distribute without permission
Alves et al. 2007
x 1/3
![Page 29: Physics 224 The Interstellar Medium - Karin Sandstromkarinsandstrom.github.io/sp18_phys224/l17_slides.pdf · © Karin Sandstrom, UC San Diego - Do not distribute without permission](https://reader033.fdocuments.us/reader033/viewer/2022043012/5faa2e12f26ba412dd625411/html5/thumbnails/29.jpg)
© Karin Sandstrom, UC San Diego - Do not distribute without permission
The Initial Mass Function
Offner et al. 2014
Number of stars per unit log(M)
that are formed.
Controversy persists over whether it is the
same everywhere.
![Page 30: Physics 224 The Interstellar Medium - Karin Sandstromkarinsandstrom.github.io/sp18_phys224/l17_slides.pdf · © Karin Sandstrom, UC San Diego - Do not distribute without permission](https://reader033.fdocuments.us/reader033/viewer/2022043012/5faa2e12f26ba412dd625411/html5/thumbnails/30.jpg)
© Karin Sandstrom, UC San Diego - Do not distribute without permissionBastian et al. 2010 ARA&A
![Page 31: Physics 224 The Interstellar Medium - Karin Sandstromkarinsandstrom.github.io/sp18_phys224/l17_slides.pdf · © Karin Sandstrom, UC San Diego - Do not distribute without permission](https://reader033.fdocuments.us/reader033/viewer/2022043012/5faa2e12f26ba412dd625411/html5/thumbnails/31.jpg)
© Karin Sandstrom, UC San Diego - Do not distribute without permission
Offner et al. 2014
When does the CMF map to the IMF?