The Interstellar

Medium

Lecturer: Dr. Paul van der Werf

Oortgebouw 565, ext 5883

pvdwerf@strw.leidenuniv.nl

Assistant: Kirstin Doney

Huygenslaboratorium 528

doney@strw.leidenuniv.nl

http://www.strw.leidenuniv.nl/~pvdwerf/teaching/ISM/

Fall 2014

Physical basis (microphysics)

Discussion of the composition and physical processes of the ISM in

order of increasing complexity

Develop physical understanding of physics, diagnostics, and life

cycle of ISM

Very active field of research with key links to stellar evolution,

galactic structure, & galaxy evolution

Aims

Overview of the structure and constituents of the ISM

Understanding of the physical processes shaping the ISM

Appreciation of the role of the ISM in the evolution of galaxies

Methods

Required background

Radiative processes (Planck function, Einstein coefficients, ...)

Statistical physics (Boltzmann and Maxwell distributions)

Quantum physics (H-atom)

Required literature

Draine, Physics of the Interstellar and Intergalactic Medium

(Princeton University Press) - REQUIRED

Book emphasizes theoretical foundations observational aspects and

practical cases will be given in class

Other (non-required) literature listed on class website

Slides will be available from course website after the lecture

Class OrganizationLectures most Fridays (see class website) 11:1513:00, HL414

Problem classes on 5 Friday afternoons (see class website)

Problem sets must be handed in. Average of problem sets counts for

25% of final grade. They must be done in groups of 2 people (one

group of 3 if necessary) because of their challenging nature and in

order to enforce discussion.

Exam: oral, by appointment, in December 2014 or January 2015

Study tipsRead the relevant book sections before the lecture

During the lecture, make notes

After the lecture, study the relevant book sections; see class website

for details

Problem sets are (very) challenging and time-consuming! Start on

time!

Class Schedule (full details on website)

1. Introduction. Basic physical processes Draine Ch 1

2. Emission and absorption processes. Radiative transfer Draine Ch 6 & 7

3. The HI 21cm line Draine Ch 8 & 9

4. Ionization and recombination Draine Ch 12, 13 & parts of Ch 14

5. Photoionization and HII regions Draine Ch 15 (parts)

6. Collisional excitation. Nebular diagnostics Draine Ch 17 & parts of Ch 18

7. Molecular energy levels and excitation. Radiative trapping Draine Ch 5 (parts) & 19

8. Interstellar dust Draine Ch 21 & parts of 23 & 24

9. Thermal balance and the two-phase model of the ISM Draine Ch 27 (parts) & 28, 29, 30

10. Molecular clouds Draine Ch 31 (parts) & 32

11. Shocks, supernova remnants and the 3-phase ISM model Parts of Draine Ch 35, 36 & 39

Material (almost) not covered

Astrochemistry (see lectures Ewine van Dishoeck)

Star formation (see lectures Ewine van Dishoeck)

Collisionally ionized nebulae (novae, supernovae)

Optical properties of dust grains

Astrophysical gas dynamics

Radio continuum emission (see Radiative Processes course)

Masers

Magnetic fields

Todays Lecture

Class introduction

ISM: history of discovery and research

Constituents of the ISM

Basic physical conditions

In red: essential exam material

Corresponding textbook material: Draine Ch. 1

History of ISM research

1656: Huygens describes the Orion Nebula

~1800: W. Herschel, catalog of bright patches called nebulae

1864: Huggins, spectra of Andromeda (Sun-like) and Orion (gaseous emission) nebulae

1904: Hartmann, stationary Ca II lines in spectrum of spectroscopic binary Ori

Discovery of ISM

1919: Barnard, catalog of dark nebulae holes in stellar distribution or obscuring matter?

Huygens describes the

Orion Nebula (1656)

Huygens, Systema Saturnium

(1656)

NGC 1976: the Orion Nebula

HST image

Dark cloud B68

ESO-VLT

Alves et al. 2001

History (contd)

1937 40: Swings & Rosenfeld, McKellar, Adams, first

small interstellar molecules (CH, CH+, CN)

Spectrum toward by Adams (1941), showing the sharp

interstellar CH and CN lines superposed on the broad stellar He line

CN CH

History (contd)

1945: prediction of HI 21cm line by Van de Hulst

1951: Ewen & Purcell, Oort & Muller, detection of 21 cm

line

1950s 60s: 21 cm maps galactic disk contains 5x109

M of gas (10% of disk mass)

=1 cm-3

1968: NH3 (first polyatomic molecule)

History (contd)

1970: CO J = 10 emission at 2.6 mm

1970s-1980s: Galactic distribution of CO

Distribution molecular vs atomic gas

1970s - now: Many new interstellar molecules found (>100); some very exotic

1973: Carruthers, UV lines of H2 from rocket

1970s 80s: Infrared astronomy (H2 infrared lines, small dust particles, very large molecules)

1980s 90s: Submillimeter astronomy (warm interfaces of molecular clouds, cold protostellar regions)

The Impact of Space Astronomy

1973 80: Copernicus UV satellite

Very highly ionized atoms (e.g., O VI) very hot , tenuous component of ISM

Leads to 3-phase model of ISM

1983: IRAS: Full-sky survey at 12, 25, 60 and 100 m

NL participation

Presence of very small dust particles (10-100 ) and/or large molecules (PAHs?)

Ultra-luminous infrared galaxies

IRAS All-Sky Image

Blue: 12 m Green: 60 m Red: 100m

The Milky Way is largely empty

distance between stars 2 pc

heliosphere 235 AU

stars occupy 3x10-10 fraction of MW volume

The remaining 0.9999999997 filled by the ISM

hydrogen, helium, + traces of metals

ionized (H II, ...), neutral (H I, ...), molecular (H2, ...)

gas phase or solid state (dust, ice, ...)

What is the ISM?

(Tielens, Ch 1)

ElementAbundance

(by number)Element

Abundance

(by number)

H 1 Mg 4.3710-5

He 0.095 Al 2.9510-6

C 2.9510-4 Si 3.5510-5

N 7.4110-5 S 1.4510-5

O 5.3710-4 Ca 2.1410-6

Na 2.0410-6 Fe 3.4710-5

Protosolar abundances (Asplund 2009)

based on photospheric and meteoritic measurements;

ISM abundances in solar neighbourhood thought to be similar

Classical approach: Observationally distinct objects

H II regions

reflection nebulae

dark clouds

supernova remnants

molecular clouds

More physical classification in different phases:

cool molecular clouds

cool H I clouds

warm intercloud gas

hot coronal gas

What are the basic properties of the phases and how are they related?

The Galactic ecosystem

Objects: HII Regions

H II regions surrounding early-type (25,000 K)

stars, emitting lots of photons beyond Lyman limit (13.6

eV)

ionized gas, bright visible nebulous objects

Associated with massive star-forming regions

optical spectra dominated by H and He recombination lines;

collisionally excited, (forbidden) optical lines from ions

like [O II], [O III], and [N II]

strong sources of thermal radio emission (free-free) +

infrared emission from warm dust

Objects: Reflection Nebulae

Nebulae reflecting light from nearby stars

e.g., NGC 2023 in Orion; emission around the

Pleiades

No radio emission, but infrared emission from

warm dust present

Illuminated by stars later than B1 (no ionizing

radation)

Either cloud material from which star was

formed; or chance encounter (Pleiades!);

sometimes ejecta of late-type stars (e.g., Red

Rectangle)

Objects: Dark nebulae

Dark bands straddle the Milky Way

Dark clouds range from tiny (0.01 pc) so-

called Bok globules, to tens of pc for

large clouds; large range in AV

Sometimes very faint reflected light +

often bright in mid- and far IR

Some even dark at mid-IR : Infrared Dark

Clouds (IRDCs)

Objects: Supernova remnants (SNR)

Left over ejecta from SN explosion

About 100 SNRs visible in MW

Filamentary and shell-like structures (but some

compact, e.g., Crab), emitting line radiation

Strong in radio due to synchrotron emission;

and bright in X-rays because of hot (106 K gas)

Phase: Neutral atomic gas

Traced by H I 21 cm line or optical/UV

absorption lines of a variety of elements

against background stars

Consists of

cold, diffuse H I clouds (100 K): CNM

warm intercloud gas (5000 K): WNM

Galactic distribution

its everywhere!

Phase: Ionized gas

traced through H emission, optical/UV ionic absorption lines,

pulsar dispersion

H emission dominated by H II regions, but most ionized gas

resides in a huge, diffuse reservoir (109 M)

Warm Ionized Medium (WIM), 0.2 cm-3, 8000 K

Ionized by photons escaped from HII regions

Phase: Molecular gas

Traced through CO lines (2nd most abundant molecule,

CO/H2104) at millimeter wavelengths, since H2 difficult to

observe

Concentrated in Giant Molecular Clouds

40 pc, 4x105 M, 200 cm-3, 10 K

But: large range in properties, and complex substructure

Self-gravitating

Pressure from turbulence and magnetic fields important

Sites of Star Formation

200+ molecular species detected

Phase: Coronal gas

Coronal gas: very hot, tenuous gas pervading the ISM

T ~ 105.5 106 K

n 0.004 cm-3

traced through highly ionized species, C IV, S VI, N V, O VI

in absorption against background stars; also: free-free

emission, radiative recombination, UV, X-ray lines

Fills most of the halo; disk less clear

heated, ionized by (SN) shocks; Sun in Local Bubble;

Galactic fountain fills the MW halo

Example: FUSE Spectrum of CSPN K1-16

Absorption lines due to intervening gas marked below

Overview ISM phases in Milky Way

phase n (cm-3) T (K) fV

coronal (HIM) ~0.004 >105.5 ~0.5?

warm, neutral

(WNM)~0.6 ~5000 ~0.4

warm, ionized

(WIM)~0.2 ~8000 ~0.1

cold, neutral

(CNM)~30 ~100 ~0.01

molecular

clouds~103-6 ~10-50 ~0.0001

HII regions ~1-105 ~104 very small

Cycle of material between phases

Added ingredient: Interstellar dust

absorption, scattering, reddening, extinction, polarization, infrared

emission

~1% of gas mass

Much C, Si, Mg, Fe, Al, Ti, Ca (=refractory elements) locked up in dust:

depletion

ISM mass budget

MW: 1.8x1011 M stars; 6.7x109 M gas

Energy sources and densities

Radiation

Magnetic fields

Cosmic rays

Mechanical energy

Energy Densities in Local ISM

Energy densities

All six energy densities are of comparable magnitude

uthermal , uhydro , umagnetic are coupled (magneto-)

hydrodynamically

uthermal is (weakly) coupled to ustarlight

u3K CBR is not coupled to anything else

In thermodynamic equilibrium at temperature T, the

Maxwell, Boltzmann, and Planck distributions apply

Maxwell distribution of velocities

Boltzmann distribution of population of energy levels

gu and gl are statistical weights of upper and lower levels

ISM: basic physical conditions

Thermal equilibrium requires detailed balance, i.e., each process

occurs as often as the inverse process

This is frequently not true in ISM, e.g., collisional excitation is

followed by radiative decay (because of low density)

Example: O2+=O III in H II region

Collisions

5007

4363 2321

1S

3P

1D

Physics and chemistry: out of thermal

equilibrium

Energy flow between states, phases

Maxwell: yes

Elastic collisions are sufficiently frequent to thermalize velocity distribution

Usually Tkin Te = Ti = Tn (NB: exceptions exist)

Boltzmann: no

Define excitation temperature Tex by

In general Tex Tkin

Which Distributions are Valid in ISM?

Statistical Equilibrium

Because radiation field cannot be described by Planck function, thermal equilibrium does not hold if both radiative processes and collisions are important

Proceed by assuming statistical equilibrium:

Sum of rates of all processes populating level i = sum of rates of all processes depopulating level i

Q: what is difference with detailed balance?

Next lecture: Review of Radiative Processes

Radiation definitions and quantities

Einstein coefficients

Radiative transfer