The interstellar medium
Stars & Planets (week 1)
Literature: Schulz, chapter 3 + 4
(see also: slides and videos by Troels Haugbølle from last year)
Michael Küffmeier
kueffmeier@nbi.ku.dk
Niels Bohr Institute
You find the original slides with videos included here: https://slides.com/kuffmeier/m
The interstellar medium
1500 \mathrm{pc} \approx 1500 \times 2.06\times 10^5 \mathrm{au} \approx 3 \times 10^8 \mathrm{au}
"parsec"
"astronomical unit"
(distance between Sun and Earth)
Hi-GAL: part of Galactic plane of the Milky Way
Interstellar medium is dynamic!
Brief overview of phases
- gas and dust between the stars
- building blocks of new stars
Gas appears in different phases (Schulz p. 36)
Q: Why is denser gas colder?
Interstellar Medium:
P= nk_{\rm B}T\approx \mathrm{const}
ISM
components & processes
- gas: mostly H and He
- dust: up to 75 % of heavier elements ("metals") are part of dust, carbon- and silicate-based, can be ice-covered, ~1% of gas mass
dust is important!
...and beautiful
ISM
components & processes
- gas: mostly H and He
-
dust: up to 75 % of heavier elements ("metals") are part of dust, carbon- and silicate-based,
in cold phase covered by ice
- radiation: stars, cosmic microwave background, thermal emission of dust, molecular and atomic lines
- cosmic-rays: high-energy particles produced in supernova shocks and of extra-galactic origin
- magnetic fields: dynamically important for all phases
- turbulence: gas moves subsonic in HIM, transsonic in WIM, supersonic in molecular clouds
ISM
energy density
- thermal:
- turbulence:
- magnetic:
- cosmic-rays:
- stellar irradiation:
- dust emission:
- CMB:
3/2\, P = 0.4 \left(\frac{P}{3000 \, \mathrm{K}\, \mathrm{cm}^{-3}}\right) \mathrm{eV}\, \mathrm{cm}^{-3}
1/2\, \rho v^2= 2.1 \left(\frac{n_{\mathrm{H}}} {100 \mathrm{cm}^{-3}} \right) \left(\frac{v}{2\, \mathrm{km}\, \mathrm{s}^{-1}}\right)^2 \mathrm{eV}\, \mathrm{cm}^{-3}
1/2\, B^2 = 0.6 \left(\frac{B}{5\, \mu \mathrm{G}} \right)^2 \mathrm{eV}\, \mathrm{cm}^{-3}
\approx 1 \mathrm{eV}\, \mathrm{cm}^{-3}
0.5 \mathrm{eV}\, \mathrm{cm}^{-3}
0.3 \mathrm{eV}\, \mathrm{cm}^{-3}
0.25 \mathrm{eV}\, \mathrm{cm}^{-3}
all components have about the same energy density!
...but there are huge fluctuations in contribution to various phases
ISM
energy density
- non-thermal pressure contributes significantly
- disk is in hydrostatic equlibrium
- disk in Milky Way is thin because
v_{\mathrm{thermal}} \ll v_{\mathrm{rot}}
Element abundance
(in the Sun)*
- Hydrogen (X=0.7381) and Helium (Y=0.2485) are produced in Big Bang
- Other elements ("metals") (Z=0.0134) are produced by stars
- Most abundant (in per mille for the Sun): O (5.74), C (2.37), Fe (1.29), Ne (1.26), Mg (0.71), N (0.69), Si (0.67), S (0.31)
Note! abundances by mass (e.g., used to estimate mean molecular mass)
Alternative: abundance by number (important for radiation transfer & collisions)
N_{\mathrm{H}}=0.921
N_{\mathrm{He}}=0.078
N_{\mathrm{Z}}=0.0008
To compute mean molecular mass:
\mu = \frac{\mu N_{\rm tot}}{N_{\rm tot}} = \frac{\Sigma \mu_i N_i + \mu_e N_e } {\Sigma N_i + N_e} = \frac{\Sigma \mu_i N_i } {\Sigma N_i + N_e} = \frac{\Sigma X_i } {\Sigma X_i/\mu_i + N_e/(\mu N_{\rm tot})}=\frac{1}{\Sigma X_i/\mu_i+N_e/(\mu N_{\rm tot})}
\mu = \frac{1}{0.7381/1 + 0.2485/4 + 0.0134/20 + 0.7381} = 0.65
Example: gas consisting of ionized hydrogen, helium and metals
*Asplund 2009
see also Schulz table 3.3, page 60
Hot ionized medium
T \approx 10^6 \, \mathrm{K}
Totally ionized gas
Tycho's SN in X-ray
- supernovae heat up ISM to millions of K.
Groups can produce super bubbles
- cosmic rays in SNe shock waves provide extra pressure
- plasma rises from disk in galactic fountains
- above the plane, gas expands and cools,
the cold gas falls back on the disk
Milky Way: Filling factor: 20 to 70 % (?), mass (?)
10^{-4} \mathrm{cm}^{-3} < n < 10^{-2} \mathrm{cm}^{-3}
10^{5} \mathrm{K} < T < 10^{7} \mathrm{K}
Warm ionized medium
T \approx 8000 \, \mathrm{K}
HII - ionized gas
massive O and B stars emit plenty UV
- origin of Strömgren spheres & large regions of ionized H
- hydrogen recombines to higher electron energy levels and emits blue H-alpha emission
- large cross-section for ionising photons causes sharp transition to WNM in HII regions
Milky Way: Filling factor: 20 to 50 % (?), mass >
0.2 \, \mathrm{cm}^{-3} < n < 0.5 \, \mathrm{cm}^{-3}
T \approx 8000 K
1.6 \times 10^9 \, M_{\odot}
Warm ionized medium
T \approx 8000 \, \mathrm{K}
HII - ionized gas
Milky Way: Filling factor: 20 to 50 % (?), mass >
1.6 \times 10^9 \, M_{\odot}
seen via H-alpha line
Finkbeiner 2003
0.2 \, \mathrm{cm}^{-3} < n < 0.5 \, \mathrm{cm}^{-3}
T \approx 8000 K
Neutral medium
T \sim 80 \, \mathrm{K}
HI - neutral gas
Milky Way: Filling factor: 10 to 20 %, mass >
6 \times 10^9 \, M_{\odot}
seen via 21 cm line
Finkbeiner 2003
0.2 \, \mathrm{cm}^{-3} < n < 100 \, \mathrm{cm}^{-3}
8000 \, \mathrm{K}
-
50 \, \mathrm{K} < T < 8000 \, \mathrm{K}
ISM
structure & thermodynamics
Tielens 2013
- absorption and shock wave -> heating
- emission due to collisions -> cooling
- chemical energy (e.g., recombination of hydrogen) -> thermostat!
- almost pressure equilibrium between the phases as long as gravity is negligible
ISM
structure & thermodynamics
Schulz, page 37
in reality:
structure and environment are more complex
ISM
dynamic and very active
gas density
gas temperature
ISM
Molecular gas and dust
T \sim 10 \, \mathrm{K}
- stars form in such molecular clouds
- molecular hydrogen is the densest component of the ISM
- difficult to observe cold molecular hydrogen directly, but possible to trace dust (see JWST image in near-IR on the left) and/or CO molecules in sub-mm to radio
Milky Way: Filling factor: <1 %, mass ~
2.5 \times 10^9 \, M_{\odot}
10 \, \mathrm{K} < T < 20 \, \mathrm{K}
10^{2} \mathrm{cm}^{-3} < n < 10^{6} \mathrm{cm}^{-3}
ISM
Molecular gas and dust
T \sim 10 \, \mathrm{K}
Dust
- responsible for reddening; depending on dust particle size
- emits and/or absorbs light, reemission in IR to sub-mm
- can cool or heat gas through collisions
- catalyst for chemistry in molecular clouds
- dust-to-gas ratio is approximately 1:100
ISM
Molecular gas and dust
T \sim 10 \, \mathrm{K}
Dust
- responsible for reddening; depending on dust particle size
- emits and/or absorbs light, reemission in IR to sub-mm
- can cool or heat gas through collisions
- catalyst for chemistry in molecular clouds
- dust-to-gas ratio is approximately 1:100
Reddening by dust
reddening happens throughout all observational wavelengths
- reddening depends on dust size distribution & chemical composition and it can vary significantly
- measurable by comparing observed magnitude with expected magnitude in two filters:
E(B-V) = (B-V)_{\rm obs} - (B-V)_{\rm expected} = A_{\rm B} - A_{\rm V}
- visual extinction can be directly linked to difference in V (
R_{\rm V}=3.1
)
A_V=(3.1\pm0.1)E(B-V)
Reddening by dust
computing E(B-V)
Depletion of metals in ISM
Reddening by dust
Dust and gas are generally distributed similarly
correlation between reddening by dust and gas (Schulz eq. 3.25)
A_{\rm V}=5.6 \left( \frac{N_{\rm H}}{10^{22} \mathrm{cm}^{-2}} \right) + 0.23
As hydrogen is everywhere in the Milky Way, and extragalactic observations require to "look" through it, reddening is essential for almost all observations!
Accounting for reddening the magnitude-distance formula is:
m_{\rm v} - M_{\rm V} = 5 \mathrm{log}_{10} \left( \frac{d}{10 \, \mathrm{pc}} \right) + A_{\rm V}
GAIA 3D extinction map
GAIA DR3: Delchambre 2022
GAIA 3D distances
By measuring reddening and distance to stars, we can also determine distribution of dust and distances to molecular clouds
Zucker 2019
Take-away
- Interstellar medium (ISM) consists of multiple phases:
hot ionized medium (HIM), warm ionized medium (WIM), warm neutral medium (WNM), cold neutral medium (CNM), molecular clouds (MCs)
- Approximately pressure equilibrium on large scales, but ISM is very dynamic with variations (thermal, chemical, radiative)
- Pressure depends on mass of gas in Milky Way's disk: other galaxies have varying pressures!
- The Milky Way is an open system; warm gas can be ejected while new gas can fall in
- Presence of dust requires us to go to large wavelengths to observe molecular clouds
- Dust occurs in all phases, but is most relevant in the cold phase and for reddened light
ISM_lecture_020924
By kuffmeier
