The Interstellar Medium and Star Formation - Interstellar Dust and Gas The Formation of Protostars The Pre-Main-Sequence Evolution

The Interstellar Medium and Star Formation

•   Interstellar Dust and Gas
•   The Formation of Protostars
•   The Pre-Main-Sequence Evolution

The Interstellar Medium and Star Formation

Ashes to Ashes…Dust to
dust…

Stars are born out of gas and
dust…Stars return much of
that material in the form of
stellar winds and explosive
events…

Plus ca change …Plus c est la
meme chose...

Stars change…evolve…usually
change is SLOW…but
sometimes change can be
rapid and dramatic….

Interstellar Medium
                                                       •Three phase model
•   Interstellar Gas
      – Hydrogen
                                                            •T

Interstellar Dust/Extinction

Interstellar Dust/Extinction
•Regions of Milky way appear devoid of light….
•Not due to a lack of stars…but clouds of dust and gas blocking
Light through extinction from scattering and absorption

•Apparent magnitude relation modified to account for extinction

•Aλ > 0 represents the number of magnitudes of interstellar
extinction along the line of sight

•Relating Optical depth to change in magnitude

•The change in apparent magnitude is about equal to the optical
depth along the line of sight:

•Related to number density of dust grains and their scattering cross
section

Column density Nd is the number of scattering dust particles in a thin cylinder with a
cross-section of 1m2 stretching from the observer to the star

Mie Scattering Theory
•   Assuming that dust particles are
    spherical with radius a:

•   Extinction coefficient

•   When wavelength is about the size of
    the grains:
                                           Shorter wavelengths preferentially
                                           scattered but not as preferentially as
                                           Rayleigh scattering
•   Implies that :

•   When wavelength must larger than the

                                              Interstellar
    grains:

                                              Reddening

Interstellar Reddening

Interstellar Molecules
•   Interstellar extinction curves for
    shorter wavelengths (UV) not well
    explained by Mie Scattering
    spheres….

•   Something else besides spherical
    dust grains and atoms must be
    present…Absorption spectra give
    clues to composition

•   Graphite…
•   Hydrocarbons…
•   Silicates….

•   Nm to several microns…

Interstellar Extinction Curves
                  Can “map out” ISM by
                  studying how light
                  propagating through it is
                  attenuated…

Dominant Component of the ISM…
                Hydrogen

• How do you see it?
• Diffuse HI clouds
  – Most neutral hydrogen atoms in
    diffuse clouds are in the ground
    state
  – …need UV photon energies for
    absorption

• Molecular Tracers of H2
  – Carbon Monoxide,….

Hydrogen- Electron Spin Flip and 21 cm
     Wavelength Radio emission
             Can map presence of HI by measuring optical depth

             Great for radio astronomy!!!
             Map of Milky Way by 10 foot
             Radio Telescope…

The Classification of Interstellar Clouds

•   Diffuse Hydrogen Clouds               •Dark Cloud Complexes
                                              –AV~5
     – 1

Interstellar Chemistry
•    Chemistry can occur on the surfaces
    of dust grains…as well as in the gas
    phase

     Water production in space!!!

The Heating and Cooling of the ISM

•   Heating via cosmic-rays
     – Ionization of Hydrogen/
        molecule
     – Ejected electron carries
        kinetic energy…
        thermalizes via collisions
        with molecules
•   Heating via UV photo-
    ionization

•   Cooling Mechanisms exist to
    balance heating
     – Collisions leave molecules
       in excited state.
     – Radiate infrared photons

The Sources of Dust Grains

• Dust grains comprise
  about 1% of mass of
  molecular clouds
• Important to chemistry
• Form via
   coagulation
• Product of
  supernovae
  explosions and stellar
  winds
• ???

The Formation of Proto-stars

Collapsing Gas clouds…

    Stars

    How?

The Formation of Protostars
                    The Jeans Criterion
•   What Conditions must exist for        •Condition for collapse
    collapse to occur?
•   Start with Virial Theorem

                                          •Radius from initial density and Mass
•   Gravitational potential energy of a
    spherical cloud of constant density

                                          •Minimum mass necessary to initiate
•   Cloud s internal kinetic energy       spontaneous collapse of cloud

                                          •Jeans Mass (Eq’n 12.14)

•   Total number of particles

                                          •Jeans length

Formation of Protostars

•   The previous Jeans’ Mass derivation neglected the external
    pressure due to the surrounding interstellar medium…Accounting
    for this external pressure P0 , the mass required for star formation
    becomes…

•   Bonner-Ebert mass

                                           Reduces required mass
•   Where Isothermal sound speed
                                           for collapse compared to
                                           simplified Jean’s Mass
                                           analysis…

Homologous Collapse

•   Assume that pressure gradients are            •At the beginning of the collapse dr/
    too small to influence the collapse           dt=0 and r=r0

                                                  •Solving for velocity at the surface
•   Also assume that througout the free
    fall phase the tempertature of the gas
    is constant (isothermal). We can then
    write                                         •Working towards solving for the
                                                  position as a function of time

•   To describe the surface of a sphere of
    radius r within the collapsing cloud as
    a function of time. Interested in the
    surface that contains mass Mr. Then
    Mr is constant…we can then write

•   Which yields                              Also Assume:
                                              • Cloud remains optically thin to radiate away
                                                 released gravitational potential energy!!!

Homologous collapse

Homologous collapse
•   Equation of motion for the gravitational collapse of the cloud

•   Free-fall timescale. When radius of the collapsing sphere=0!!!

                                                 Eq’n 12.26

•   If initial density uniform, all parts of the cloud will take the
    same amount of time to collapse. The density will increase at
    the same rate everywhere!!!!…Homologous Collapse

Homologous collapse

Fragmentation of Collapsing Clouds

The Fragmentation of Collapsing Clouds
                                    What stops the fragmentation process?
                                    minimum size segment depends on the point
• Clouds usually do not             where the gas goes from isothermal to
  form single stars                 adiabatic….

• Preference to form in
  groups…binary systems
  to clusters…
• Initial inhomogeneities

       Fragmentation
Fragmentation ceases when the
segments of the original cloud       Minimum obtainable Jean’s Mass
begin to reach the range of solar
mass objects….

Additional Processes

 Ambipolar Diffusion

Pre-Main_sequence Evolution

•   Collapse of Gas clouds can happen
    spontaneosly
•   Timescale chararcterized by free-fall
    timescale equation 12.26
•   Quasi-static protostar forms. Rate of
    evolution controlled by rate at which
    star can thermally adjust to collapse.
•   Gravitational potential energy liberated
    by the collapse is released over time      •The Hayashi track:
    and is the source of the object s               •Large opacity (H-) causes outer
    luminosity                                      shell of contracting protostar to
•   Kelvin Helmholtz timescale >> free-fall         become convective
    timescale                                       •Constraints of convection limits
                                                    its quasi-static evolutionay track
                                                    to a line that is nearly vertical in
                                                    the H-R diagram
•   Protostellar evolution proceeds at a            •Consequently as the protostar
    much slower rate than free fall                 collapse slows, its luminosity
    collapse. A 1 solar mass star takes             decreases while its effective
    approximately 40 million years to               temperature increases slightly..
    contract quasi-statically to its main           •Forbidden region to the right of
    sequence structure                              Hayashi track…

Numerical Simulations of Proto-stellar Evolution

•   Need numerical solutions also involving
    magnetohydrodynamics
•   Early stages of collapse: free-fall, isothermal
•   Collapse rate increases toward center with higher density
•   Center becomes optically thick  more adiabatic

•   Central region nearly in hydrostatic equilibrium with R~5 Rsun…
    Protostar!!
•   Photosphere develops…object now can be placed on H-R
    diagram and evolutionary track calculated…
•   Infalling material hits forming core in hydrostatic equilibrium 
    shock front causes heating  provides Luminosity…
•   Opacity drops when dust is vaporized at T~1000K radius of
    “core” in hydrostatic equilibrium shrinks….

Numerical Simulations of Proto-stellar Evolution

•   When Temperature rises to ~2000K molecular hydrogen
    dissociates  absorbs energy that would otherwise provide a
    pressure gradient second collapse occurs until R~1.3 Rsun
•   Accretion still ongoing second shock front
•   Temperature in deep core enough that Deuterium begins to
    burn in the protostar. Not PP-I yet…and Deuterium burns
    out….

Numerical Simulations of Proto-stellar Evolution

Search for protostars

                    Study details of IR
                    Spectra of Bok
                    globules…

                    Difficult due to
                    shielding of external
                    cloud…

Pre-Main_sequence Evolution

The Formation of Protostars

     http://www.ukaff.ac.uk/starcluster/

The Formation of Protostars

     http://www.ukaff.ac.uk/starcluster/

Formation of Brown Dwarfs
• 0.013 M (13 Mjupiter)-
  0.072 M
• Fragmentation models
  indicate that protostars
  will not form below this
  mass
• Core temperature hot
  enough to burn
  deuterium but nuclear
  burning not at a
  sufficient rate to form a
  main-sequence star
• Spectral Types L and T

Brown Dwarfs

•   http://www.science.psu.edu/alert/Luhman11-2005.htm

Massive Star Formation
•   Central Temperature becomes quickly high enough to burn
    Carbon-12 as well as hydrogen.
•   CNO cycle dominates.
•   Core remains convective even after main sequence is reached

Possible Modifications to the Classical Models

•    Many approximations to classical
     models of star formation
•    Rotation plays an important role
•    Turbulence
•    Magnetic fields
•    Inhomogeneities
•    Strong stellar winds
•    Ionizing radiation from massive nearby
     stars

•    …Life is complicated…

•    Accretion disks…Stars interact with
     infalling matter…

Zero Age Main Sequence (ZAMS)
•   Diagonal line on H-R diagram
    where stars of various masses
    first reach the main sequence
    and begin equilibrium helium
    burning is known as the Zero-
    Age-Main-Sequence (ZAMS)
•   Inverse relation between star
    formation time and stellar
    mass
•   How do lower mass stars form
    in the presence of quickly
    forming high-mass stars that
    can disperse the surrounding
    cloud by its intense
    radiation????

Initial Mass Function

•Observations
indicate that more low
mass stars are
formed than high
mass stars
•This functional
dependence is known
as the Initial Mass
Function

•Depends on local
environment

HII Regions

http://sparky.rice.edu/~hartigan/images/orion.gif     http://abyss.uoregon.edu/~js/images/hii_region.gif

Stroemgren Radius
                              1/3
                         " 3N % (2/3
                    rS ! $     ' nH
                         # 4!" &

The effect of massive stars on Gas Clouds
                  OB Associations

•   http://www.hopkins.k12.mn.us/pages/high/courses/online/astro/course_documents/stars/brightest/brightest.htm

Star forming regions

T-Tauri Stars
•   T Tauri stars are a class of low
    mass pre-main sequence
    objects that represent a
    transition between stars that
    are still shrouded in dust and
    main-sequence stars…
•   Large, fairly rapid irregular
    variations in luminosity
•   Strong emission lines
•   May be experiencing
    significant mass loss as
    indicated by doppler shifts of
    spectral lines
•   Sometimes mass accretion…

T-Tauri Stars

•   http://abyss.uoregon.edu/~js/ast122/
    lectures/lec13.html

T-Tauri Stars

T-Tauri Stars

T-Tauri Stars

Young Stars

•   FU Orionis Stars
•   Herbig Ae/Be Stars
•   Herbig-Haro Objects
•   Young Stars with
    Circumstellar Disks

                          http://www.noao.edu/outreach/press/pr03/sb0308.html

Herbig Haro Object 30

Beta Pictoris

       Artist’s conception of Beta Pictoris

        http://www.nasa.gov/vision/universe/
        starsgalaxies/betapic.html

Early Model of a T-Tauri Star with an accretion disk

Carrol & Ostlie’s Artist’s depiction of Beta Pictoris