ASTR 511, O'CONNELL. [Fall 2003] Lecture Notes

ASTR 511 (O'Connell) Lecture Notes

3. ASTROPHYSICAL SOURCES

Sun in H-alpha showing active regions
and a flame-like "prominence."

I. INTRODUCTION

There are many distinct types of astrophysical sources: stars, AGNs, planets, H II regions, synchrotron jets, hot intracluster gas, shocked gas (SNRs), etc

Main issues:

Generation of photons
Transfer of photons to observer = radiative transfer
Deduction of physical properties from emergent spectrum

(1) Photon generation:

Many different mechanisms
Usually several types important in given source
Mechanisms usually broad-band, but often dominate distinct bands
"Thermal" vs "non-thermal"
- Thermal: emitters/absorbers close to Boltzmann distribution ("thermal equilibrium" or TEQ)
  - In many, not all, cases, radiation field is close to , the Planck function
- Non-Thermal: emitters/absorbers/radiation far from TEQ
  - Best example: synchrotron sources = continuum radiation from relativistic electrons with non-Maxwellian energy distribution moving in magnetic fields. If e-energy distribution is a power law, so is emitted radiation.
The underlying complexity of typical sources is emphasized by the image of the Sun at the top of the page. We are aware of the many different physical environments on the Sun's surface only by virtue of its proximity.

(2) Radiative Transfer:

Transfer is characterized by the optical depth,
Probability of escape from a given location in the source is
: "optically thin". See directly to sources of photons.
: "optically thick". Observed photon distribution has been transformed in , direction, and/or intensity.
- NB: "Thick" does not mean "dark." (The Sun is optically thick!) Rather, observable photons escape only from outermost ~ 1 optical depth of source.
Propagation of photon energy is described by the "equation of transfer"

(3) Deduction of Physical Properties

How to relate observables to source physics? This is one of the principal concerns of theoretical astrophysics.

II. COMPARISON OF TWO CANONICAL SOURCE TYPES

In this section, we discuss two canonical types of astrophysical sources: STARS & AGNs and compare them in the following characteristics:

Structures
Spectra: Continua
Spectra: "Lines" (fine structure)
Information carried by their spectra

STRUCTURES
STARS: Dense spheres, r ~ 10^6-13 cm Stars are dynamically stable, apart from convection and surface phenomena, throughout most of their lifetimes Isotropic radiators. Nuclear reactions maintain high-temperature (>10⁷ K) core except when masses are too small or after fuel exhaustion XR and GR radiation transferred slowly through high optical depth envelope. Degraded to UVOIR band. Observed radiation escapes from a very thin (300 km in Sun) layer = "photosphere" NB: photospheres (10¹⁷ particles/cm³ in the Sun) are DENSE by the standards of AGN emitting regions Subsidiary processes (e.g. driven by magnetic fields, accretion in binaries) can radiate in non-UVOIR bands.
AGNs ("Active Galactic Nuclei"): Supermassive black hole (M ~ 10^7-9 M_sun; R_Schw ~ 3 x 10¹³ M₈ cm) at center of an accretion disk with r 10¹⁶ cm. Accretion disk is fed by infalling material; matter is continuously transported through disk Though disks can be semi-stable, radiative properties are continuously variable; stars that approach event horizon can produce brief, luminous "tidal disruption events" Dissipation and magnetic processes near center of disk generate relativistic particles, . These can generate relativistic jets. Observed radiation emerges from a large volume with nonuniform properties Relativistic jets and disk confinement produce anisotropic radiation. Relativistic particles produce broad-band non-thermal synchrotron radiation, directly observed at radio wavelengths. Direct thermal radiation from denser, hot inner disk (UV). Photon boosting of low-energy photons by inverse Compton scattering to UV/XR bands. Compton boost: Hard radiation field produces strong ionization of gas in a large, low-density volume around disk, ===> UVOIR, XR emission lines. Strong heating of surrounding dust grains ===> IR continuum & emission lines (3-1000µ). (But grains vaporized near center.) Star-formation regions are often associated with AGN in disk galaxies (chicken or egg?).

STRUCTURES

STARS:

Dense spheres, r ~ 10^6-13 cm

Stars are dynamically stable, apart from convection and surface phenomena, throughout most of their lifetimes

Isotropic radiators.

Nuclear reactions maintain high-temperature (>10⁷ K) core except when masses are too small or after fuel exhaustion

XR and GR radiation transferred slowly through high optical depth envelope. Degraded to UVOIR band.

Observed radiation escapes from a very thin (300 km in Sun) layer = "photosphere"

NB: photospheres (10¹⁷ particles/cm³ in the Sun) are DENSE by the standards of AGN emitting regions

Subsidiary processes (e.g. driven by magnetic fields, accretion in binaries) can radiate in non-UVOIR bands.

AGNs ("Active Galactic Nuclei"):

Supermassive black hole (M ~ 10^7-9 M_sun; R_Schw ~ 3 x 10¹³ M₈ cm) at center of an accretion disk with r 10¹⁶ cm.

Accretion disk is fed by infalling material; matter is continuously transported through disk

Though disks can be semi-stable, radiative properties are continuously variable; stars that approach event horizon can produce brief, luminous "tidal disruption events"

Dissipation and magnetic processes near center of disk generate relativistic particles, . These can generate relativistic jets.

Observed radiation emerges from a large volume with nonuniform properties

Relativistic jets and disk confinement produce anisotropic radiation.

Relativistic particles produce broad-band non-thermal synchrotron radiation, directly observed at radio wavelengths.

Direct thermal radiation from denser, hot inner disk (UV).

Photon boosting of low-energy photons by inverse Compton scattering to UV/XR bands.

Compton boost:

Hard radiation field produces strong ionization of gas in a large, low-density volume around disk, ===> UVOIR, XR emission lines.

Strong heating of surrounding dust grains ===> IR continuum & emission lines (3-1000µ). (But grains vaporized near center.)

Star-formation regions are often associated with AGN in disk galaxies (chicken or egg?).

CONTINUUM SPECTRA
STARS Primary component is UVOIR radiation from photosphere. Thermal source; emergent spectrum Planck function Small spread of temperature around characteristic effective T_e of photosphere, approximately depth where at any wavelength. peak at Å Photospheric temperatures 1000-100000 K. Strong time variation only in minority of cases Strong concentration to UVOIR, with rise to peak, then dropoff. Spectral slope at higher allows estimate of mean T_e. Major absorption discontinuities from ionization edges of abundant ions (e.g. H: Lyman edge 912 Å, Balmer edge 3646 Å). In normal stars, there is only low level radiation in other bands from high temperature corona, synchrotron radiation, etc.
AGNs: Primary component is very broad-band, nonthermal radiation extending from radio to XR. Shape: no simple parameterization. Thermal components in IR (dust grains) & UV ("UV bump" from inner accretion disk). Continuum spectrum depends on viewing angle because of thick obscuring tori Strong time variation common

CONTINUUM SPECTRA

STARS

Primary component is UVOIR radiation from photosphere.

Thermal source; emergent spectrum Planck function

Small spread of temperature around characteristic effective T_e of photosphere, approximately depth where at any wavelength.

peak at Å

Photospheric temperatures 1000-100000 K.

Strong time variation only in minority of cases

Strong concentration to UVOIR, with rise to peak, then dropoff.

Spectral slope at higher allows estimate of mean T_e.

Major absorption discontinuities from ionization edges of abundant ions (e.g. H: Lyman edge 912 Å, Balmer edge 3646 Å).

In normal stars, there is only low level radiation in other bands from high temperature corona, synchrotron radiation, etc.

AGNs:

Primary component is very broad-band, nonthermal radiation extending from radio to XR.

Shape: no simple parameterization.

Thermal components in IR (dust grains) & UV ("UV bump" from inner accretion disk).

Continuum spectrum depends on viewing angle because of thick obscuring tori

Strong time variation common

LINE SPECTRA
STARS: Complex, narrow, absorption lines Produced by transitions in those atoms, ions, and molecules which are prevalent at characteristic T_e and pressure. From thin layers of cooler gas projected against higher T continuum of inner photosphere. Doppler widths small (thermal), ~ few km s^-1 Line spectrum reflects physical state (composition, temperature, pressure) of photosphere Lines and local continuum usually coupled (imply ~ same T)
AGNs: Emission lines since generally not viewed against continuum source. Wide range of ionizations (e.g. neutral to Fe XIV) Wide range of Doppler widths in different galaxies, to >10⁴ km s^-1 Lines and local continuum decoupled since originate in different volumes Doppler widths reflect kinematic motions of gas clouds "Broad" lines from 1 pc "Narrow" lines to 100 pc from BH Line spectrum depends on viewing angle (inner regions can be concealed by thick tori, dust clouds if viewed at large angle to axis). Polarization.

LINE SPECTRA

STARS:

Complex, narrow, absorption lines

Produced by transitions in those atoms, ions, and molecules which are prevalent at characteristic T_e and pressure.

From thin layers of cooler gas projected against higher T continuum of inner photosphere.

Doppler widths small (thermal), ~ few km s^-1

Line spectrum reflects physical state (composition, temperature, pressure) of photosphere

Lines and local continuum usually coupled (imply ~ same T)

AGNs:

Emission lines since generally not viewed against continuum source.

Wide range of ionizations (e.g. neutral to Fe XIV)

Wide range of Doppler widths in different galaxies, to >10⁴ km s^-1

Lines and local continuum decoupled since originate in different volumes

Doppler widths reflect kinematic motions of gas clouds

"Broad" lines from 1 pc

"Narrow" lines to 100 pc from BH

Line spectrum depends on viewing angle (inner regions can be concealed by thick tori, dust clouds if viewed at large angle to axis). Polarization.

DEDUCTIONS FROM OBSERVATIONS
STARS: Continuum slope & structure: Temperature Ionization edge discontinuities: pressure/gravity Line widths: pressure/gravity, rotation, outflows Line strengths: T, pressure/gravity, chemical abundances. E.g.: Classic "spectral-type" sequence is a temperature-sequence Abundances: selected species easy to measure: e.g. Ca/H, Mg/H, Fe/H. He/H (hot stars only) Light element (e.g. C,N,O) abundances more difficult: C IV (UV) in hot stars; various atomic lines in cool stars require high spec resol; molecules in cool stars (CH, CN, NH, etc) Ionization decreases with increasing gas pressure: e.g. use Mg I 5175 Å strength as dwarf/giant discriminant for Galactic structure studies NB: Derived abundances are sensitive to proper T,P estimation Integrated light of stars allows inferences concerning ages, abundances of distant stellar systems. "Population synthesis": Fit full energy distribution with combinations of single generation models to determine star formation history & abundances Use selected lines for age-abundance separation, e.g. Balmer lines, Mg, Fe for old pops
AGNs: Slope & structure of continuum related to energy distribution of electrons, importance of Compton scattering, accretion disk structure, dust emission/absorption, etc. Less definitive interpretation than stellar continua because of multiple components, complex generation mechanisms, absence of near-TEQ. One test for a nonthermal source: polarization. Another: compare its mean surface brightness to the Planck function and derive corresponding T: where f is the flux, B is Planck function and is the angular area (or upper limit) of the source. Is T "unphysically" high? (Emission) line strengths yield electron temperature & gas density; Line ionization level constrains far-UV continuum ("hardness" of ionizing radiation). Line widths, positions probe kinematics of turbulent gas near BH, outflows, etc. Line strengths yield abundances ...although of different species than in stars: e.g. O, N, He, S, Fe (uncommon)...but not Ca, Mg (unless UV access), etc. ID's, Surveys: strong emission line sources (photons concentrated to narrow bands) easier to detect than pure continuum sources

DEDUCTIONS FROM OBSERVATIONS

STARS:

Continuum slope & structure: Temperature

Ionization edge discontinuities: pressure/gravity

Line widths: pressure/gravity, rotation, outflows

Line strengths: T, pressure/gravity, chemical abundances. E.g.:

Classic "spectral-type" sequence is a temperature-sequence

Abundances: selected species easy to measure: e.g. Ca/H, Mg/H, Fe/H. He/H (hot stars only)

Light element (e.g. C,N,O) abundances more difficult: C IV (UV) in hot stars; various atomic lines in cool stars require high spec resol; molecules in cool stars (CH, CN, NH, etc)

Ionization decreases with increasing gas pressure: e.g. use Mg I 5175 Å strength as dwarf/giant discriminant for Galactic structure studies

NB: Derived abundances are sensitive to proper T,P estimation

Integrated light of stars allows inferences concerning ages, abundances of distant stellar systems. "Population synthesis":

Fit full energy distribution with combinations of single generation models to determine star formation history & abundances

Use selected lines for age-abundance separation, e.g. Balmer lines, Mg, Fe for old pops

AGNs:

Slope & structure of continuum related to energy distribution of electrons, importance of Compton scattering, accretion disk structure, dust emission/absorption, etc.

Less definitive interpretation than stellar continua because of multiple components, complex generation mechanisms, absence of near-TEQ.

One test for a nonthermal source: polarization. Another: compare its mean surface brightness to the Planck function and derive corresponding T:

where f is the flux, B is Planck function and is the angular area (or upper limit) of the source. Is T "unphysically" high?

(Emission) line strengths yield electron temperature & gas density;

Line ionization level constrains far-UV continuum ("hardness" of ionizing radiation).

Line widths, positions probe kinematics of turbulent gas near BH, outflows, etc.

Line strengths yield abundances

...although of different species than in stars: e.g. O, N, He, S, Fe (uncommon)...but not Ca, Mg (unless UV access), etc.

ID's, Surveys: strong emission line sources (photons concentrated to narrow bands) easier to detect than pure continuum sources

References:

Properties of stars: D. F. Gray (1992) "Observation & Analysis of Stellar Photospheres"
Properties of AGNs: B. M. Peterson (1997) "An Introduction to Active Galactic Nuclei"
Spectroscopy: LLM, Sec. 5.1

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