giant star-forming regions · gouliermis & klessen giant star-forming regions (tentative)...

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

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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


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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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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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Phases of the ISM

•  The gas in the ISM is organized in a variety of phases

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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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Neutral atomic gas

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From Kalberla & Kerp 2009, Annu. Rev. Astro. Astrophys., Vol. 47, pp. 27-61


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


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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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


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


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°

+20°

+10°

−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

c L

atit

ude

Orion Complex

Ursa Major

0°60°120°180° 180°240°300°

−20°

0°

+20°

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.


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


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


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


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


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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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


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


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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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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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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