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University of Heidelberg, Center for Astronomy
Giant Star-Forming Regions
Dimitrios A. Gouliermis
Lecture #2 Introduction to the Physics of the ISM
Phases of the ISM
Giant Star-Forming Regions Gouliermis & Klessen
(tentative) Schedule of the Course
WS 2012 - 2013 Lecture 2 2
Lect. 1 19-Oct-2012 Course Overview Motivation for the Course/Schedule; Overview of Physical Processes in HII Regions; Classification of HII regions
Lect. 2 26-Oct-2012 Introduction to the Physics of the ISM I Phases of the ISM; Transitions; Introduction to cooling mechanisms
Lect. 3 2-Nov-2012 Introduction to the Physics of the ISM II Gas Cooling & Heating; Collisional Excitation; Photo-ionization; Photo-electric heating
Lect. 4 9-Nov-2012 Interstellar Dust Absorption & Scattering by particles; Interstellar Extinction; Infrared Emission
Lect. 5 16-Nov-2012 Physical Processes in HII Regions I Radiative Processes; Strömgren Theory; Photo-ionization & Recombination of hydrogen; Dust
Lect. 6 23-Nov-2012 Physical Processes in HII Regions II Thermal Properties; Heating and Cooling of HII Regions; Forbidden lines and Line Diagnostics
Lect. 7 30-Nov-2012 Photodissociation regions (PDR) Ionization & Energy Balance; Dissociation of Molecular Hydrogen; Structure; Observations
Lect. 8 7-Dec-2012 Stellar Feedback Processes Dynamics of the ISM; Ionization fronts; Expansion of HII regions; Stellar Winds and Supernovas
Lect. 9 14-Dec-2012 Stellar Content of HII Regions I Massive Stellar Evolution; Mass-Loss; Rotation; Binary interaction; Spectral features of OB stars; Runaway stars - Stellar Cluster dynamics
Lect. 10 21-Dec-2012 Stellar Content of HII Regions II Pre--Main-Sequence (PMS) Stars; Young Stellar Systems; Stellar Initial Mass Function; Age determination & History
Lect. 11 11-Jan-2012 Star Formation (SF) Isothermal shperes and Jeans mass; Molecular Cores collapse; Protostars
Lect. 12 18-Jan-2012 Star Formation PMS Stellar Evolution/Contraction; Characteristics of T Tauri stars; Herbig Ae/Be Stars; Multiple SF
Lect. 13 25-Jan-2012 Summary Gas, Dust and Stars: Members of one eco-system; Up-to-date material on Giant HII Regions
Giant Star-Forming Regions Gouliermis & Klessen
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Giant Star-Forming Regions Gouliermis & Klessen
The Physics of the ISM
In this Lecture • Phases of the ISM • Examples from the Galactic “Ecosystem” • Energy balance in the ISM • Transitions on Atomic Level • A bit of “Cooling” Mechanisms in the ISM
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Giant Star-Forming Regions Gouliermis & Klessen
Introductory Notes • The ISM is *not* in thermodynamic equilibrium • The velocity distribution of the gas can be
described by one Temperature, but … • Ionization and excitation are often different from
their equilibrium values • Therefore, ISM studies are concerned with
– Identifying the processes that control ionization and energy balance
– Setting up the detailed statistical equilibrium equations – Solving them for the conditions appropriate for the
medium
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Giant Star-Forming Regions Gouliermis & Klessen
Phases of the ISM
• The gas in the ISM is organized in a variety of phases
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Giant Star-Forming Regions Gouliermis & Klessen
Neutral atomic gas
• Cold Neutral Medium (≃ 100 K); diffuse HI clouds • Typical density of 50 cm−3 and a size of 10 pc • Typical scale height (4-8 kpc from GC) ~100 pc • At high latitudes HI is in the intercloud warm medium • Warm Neutral Medium (~ 8000 K); intercloud gas • Typical density 0.5 cm−3 and scale height ~220 pc • Can be observed in optical and UV absorption • Best indicator: 21-cm line of atomic hydrogen
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Giant Star-Forming Regions Gouliermis & Klessen
Neutral atomic gas
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From Kalberla & Kerp 2009, Annu. Rev. Astro. Astrophys., Vol. 47, pp. 27-61
Giant Star-Forming Regions Gouliermis & Klessen
Neutral atomic gas
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From Kalberla et al. 2005, Astronomy & Astrophysics, Vol. 440, pp. 775-782
The Leiden/Argentine/Bonn (LAB) Survey of Galactic HI
Giant Star-Forming Regions Gouliermis & Klessen
Ionized gas
• Distinct HII Regions (≃ 10000 K); most Hα emission • WIM: Warm Ionized Medium (≃ 8000 K); almost all
of the mass of ionized gas (109 M⊙) resides in this diffuse component
• Typical density of 0.1 cm−3 and a scale of ≃1 kpc • Strong filamentary structure • WIM ionizing source unclear; O-stars’ photons? • Can be observed in optical and UV absorption • Best indicator: Hα recombination hydrogen line
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Giant Star-Forming Regions Gouliermis & Klessen
Ionized gas
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From Hafner et al. 2005, Astrophysical Journal Supplement Series, Vol. 149, p. 405
The Wisconsin H-Alpha Mapper (WHAM) Survey of Galactic HII
Giant Star-Forming Regions Gouliermis & Klessen
Molecular gas • Localized in discrete Giant Molecular Clouds • Typical GMC parameters:
– Mass: 4×105 M⊙
– Size: 40 pc – Density: ≃ 200 cm−3
– Temperature: ~10 K • GMCs have spatial structure on all scales. • SF: cores with sizes ≃ 1 pc, densities ≥ 104 cm−3,
and masses in the range 10-103 M⊙ • Best tracer: CO J=1−0 transition at 2.6 mm • Dominant species is H2; H2/CO ratio of 104-105 WS 2012 - 2013 Lecture 2 12
Giant Star-Forming Regions Gouliermis & Klessen
Molecular gas
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From Dame et al. 1987, Astrophysical Journal, Vol. 322, pp. 706-720
A Composite CO Survey of the Entire Milky Way
+30°
−30°
−20°
0°
+20°
+10°
−10°
+30°
−30°
−20°
0°
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−10°
Galactic Longitude
Gal
acti
c L
atit
ude
180° 160° 140° 120° 100° 80° 60° 40° 20° 0° 340° 320° 300° 280° 260° 240° 220° 200° 180°170° 150° 130° 110° 90° 70° 50° 30° 10° 350° 330° 310° 290° 270° 250° 230° 210° 190°
Beam
S235
Per OB2
Polaris Flare
Cam CepheusFlare
W3
Gr e a t R i f t
NGC7538Cas A Cyg
OB7 Cyg XW51 W44
AquilaRift
R CrA
Ophiuchus
Lupus
GalacticCenter G317−4
Chamaeleon
CoalSack
CarinaNebula
Vela
Ori A & B
Mon R2
Maddalena’sCloud
CMaOB1
MonOB1
Rosette
GemOB1
S147S147CTA-1
S212
λ O r i
R i n g
Tau-Per-Aur Complex
AquilaSouth
Pegasus
Lacerta GumNebula S. Ori
Filament
Hercules
Galactic Longitude
Gal
acti
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atit
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Orion Complex
Ursa Major
0°60°120°180° 180°240°300°
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0.0 0.5 1.0 1.5 2.0
log Tmb dv (K km s−1)∫
FIG. 2.–Velocity-integrated CO map of the Milky Way. The angular resolution is 9´ over mostof the map, including the entire Galactic plane, but is lower (15´ or 30´) in some regions outof the plane (see Fig. 1 & Table 1). The sensitivity varies somewhat from region to region,since each component survey was integrated individually using moment masking or clippingin order to display all statistically significant emission but little noise (see §2.2). A dotted linemarks the sampling boundaries, given in more detail in Fig. 1.
Giant Star-Forming Regions Gouliermis & Klessen
Hot Intercloud Medium (HIM)
• Hot (~105-106 K) Coronal Gas • It is traced through UV absorption lines of highly
ionized species (e.g., CIV, SVI, NV, OVI) • Typical densities of ≃ 10−3 cm−3 • Fills most of halo volume (scale height ≃ 3 kpc) • Gas is heated and ionized through shocks driven
by stellar winds and supernova explosions • High-latitude gas may have been vented by
super-bubbles created by OB associations
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Global Galactic ISM
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From Cox 2005, Annu. Rev. Astron. Astrophys. 2005, Vol. 43, pp. 337-385
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Heart & Soul nebula (NASA/WISE)
100 pc 1.56o
W5
W4
W3
W3 (Spitzer/IRAC)
10 pc
The Hierarchical Strcuture of the ISM
W3 Main (LBT/LUCI)
1 pc
Giant Star-Forming Regions Gouliermis & Klessen
Energy Balance of the ISM
• Gas Cooling Processes – Particles in the gas
emitting line- and continuum-radiation, escape the region and carry off energy.
• Gas Heating Processes – Different mechanisms
depending on the conditions characteristic for every phase.
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Energy Balance of the ISM • the thermal balance between heating and cooling
is expressed in terms of the Loss Function, L: L(n,T) = n2Λ(T) – nG
n2Λ(T) is the cooling rate per unit volume, nG is the heating rate per unit volume, n is the gas density
• L > 0 ⇒ Net Cooling • L < 0 ⇒ Net Heating • L = 0 ⇒ Equilibrium WS 2012 - 2013 Lecture 2 18
Giant Star-Forming Regions Gouliermis & Klessen
Radiative Loss Function
• Forbidden transitions dominate for T ≤ 104 K. • Permitted transitions dominate for 104 K ≤ T ≤ 105 K. • Free-free emission is most important at high T ≥ 107 K. • Gas at T ~ 105 K cools quickly – many atomic losses at
these temperatures. WS 2012 - 2013 Lecture 2 19
Plot shows results from different processes for ne = nH = 1 cm–3
From Cox & Daltabuit 1971, Astrophysical Journal, Vol. 167, pp.113-117
Giant Star-Forming Regions Gouliermis & Klessen
Continuum & Line Emission
400 600 800! /nm
relati
veF !
400 600 800! /nm
relati
veF !
black-bodysource
AB
C
absorption spectrum
relati
veF !
400 600 800! /nm
continuous spectrum emission spectrum
thin gas
AB
C
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Transitions on Atomic Level
• All atoms thrive to be in the lowest energetic state.
• Atoms in an excited state will normally quickly decay via the permitted transitions to states with lower total energy.
• Transitions are directly related to the binding energies of the species involved.
• Electronic binding energies for atoms increase from left to right in the Periodic Table from ~ 5 – 20 eV.
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The Periodic Table of Elements
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Giant Star-Forming Regions Gouliermis & Klessen
Transitions on Atomic Level
• Transitions between levels obey certain selection rules.
• These rules depend on whether they are electric dipole, magnetic dipole, electric quadrupole, etc.
• This directly influences the strength of the transitions.
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Transition Rules – Selection Rules of Quantum Mechanics
• Forbidden emission lines have only been observed in extremely low-density gases & plasmas
• Common lines: [N II] at 654.8 and 658.4 nm, [S II] at 671.6 and 673.1 nm, [O II] at 372.7 nm, and [O III] at 495.9 and 500.7 nm.
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Energy Levels nomenclature
• The energy states in the atom are defined by the various quantum numbers in units of h/2π.
• For hydrogen atom: – n: Principal quantum number (1, 2, 3,…) – l: Orbital angular momentum (0,…,n – 1)
transitions: s, p, d, f, g, h,… – s: Spin angular momentum (± 1/2) – j: Total angular momentum = s+l
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• For heavier atoms: L, S, J=L+S • For molecules: Λ, Σ, Ω=Λ+Σ
Giant Star-Forming Regions Gouliermis & Klessen
Atomic hydrogen emission spectrum
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103 n
m
n = 1
n = 2
n = 3
n = 4n = 6n = 5
434 nm
122 n
m
Lyman series
Balmer series
Paschen series
94 nm
410 nm
486 nm656 nm
1875 nm1282 nm
1094 nm
97 nm
95 nm
Giant Star-Forming Regions Gouliermis & Klessen
Cooling mechanisms
1. Free-free (Thermal bremsstrahlung) – for T>106 K (cooling ∝ T1/2) – Least important – usually negligible
2. Recombination emission. – Electrons recombine with ions to form excited atoms. – Atom cascades to lower levels. – Emitted photons that escape serve to cool the nebula. – Calculated from kinetic energy-weighted
recombination coefficients to each level of atom.
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Giant Star-Forming Regions Gouliermis & Klessen
Cooling mechanisms
3. Collisional cooling – Most important mode of nebular cooling. – Metals have many low-lying energy states that
can be populated by collisions with electrons having Ee ~ 1 eV. (kT = 0.86 eV at T = 10000 K)
– If spontaneous de-excitation takes place before another collision de-excites the atom, photon escapes and cools nebula.
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Important Interstellar Cooling Lines
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Giant Star-Forming Regions Gouliermis & Klessen
Collisionally excited line emission
• Collisional excitation gives rise to spectral lines. • Photoelectrons collide with atoms or ions within
the gas, and excite them. • When excited atoms or ions revert to their
ground state they emit a photon. • The emitted lines are called collisionally excited
lines (CELs). • CELs are only seen in gases at very low
densities (typically less than a few thousand particles per cm³).
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