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The Emission Line Universe: Galactic Sources of Emission Lines. Stephen S. Eikenberry University of Florida 22 November 2006. OUTLINE. Introduction Infrared Emission Lines Nebular Galactic Emission Line Sources Stellar Galactic Emission Line Sources Summary & Future Prospects.

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The Emission Line Universe: Galactic Sources of Emission Lines

Stephen S. Eikenberry

University of Florida

22 November 2006


OUTLINE

  • Introduction

  • Infrared Emission Lines

  • Nebular Galactic Emission Line Sources

  • Stellar Galactic Emission Line Sources

  • Summary & Future Prospects


What are “Galactic” Sources?

  • Essentially all emission lines arise from discrete objects within a galaxy  almost all objects discussed in WS XVIII are fundamentally “Galactic”

  • But … others here will cover HII regions, AGN, integrated galaxy spectra, etc.

  • I’ll focus on “other” Galactic sources of emission


What are “Galactic” Sources?


II. Why Infrared Emission Lines?

  • Infrared – why bother?

  • Hydrogen has transitions in the infrared (IR), but UV (Lyman) and optical (Balmer) are stronger

  • Then again … optical – why bother? Lyman >> Balmer …

  • But, the Universe is more transparent to Balmer lines than Lyman lines

  • Even though Lyman is intrinsically brighter, Balmer is often more useful


II. Why Infrared Emission Lines?

  • The Galaxy is more transparent to IR than optical emission

  • Why? Because most dust grains are smaller than ~1 m

  • Thus, they do not absorb/scatter well at wavelengths  > 1 m

  • For instance, in the K-band (~2.2 m) AK ~ 0.1 AV (magnitudes!)

  • For many Galactic sources, IR is the ONLY waveband


Example: The Galactic Center

POSS-B

POSS-R

POSS “IR”


Example: The Galactic Center

2MASS-J

2MASS-H

2MASS-K


Example: The Galactic Center

  • AK ~ 3 mag (~6% transmission)

  • AV ~30 mag (~10-12 transmission!!)

POSS-R

2MASS-K


IR/Optical Difference:Detectors

  • CCDs do not function well @  >1.0 m

  • The “bandgap” energy of silicon (~1.0 eV) corresponds to this wavelength (the “bandgap cutoff” of silicon)

  • Instead of silicon, We use other (poorer) semiconductor materials

    • HgCdTe (0.9-2.5 m)

    • InSb (1-5.5 m)

    • Si:As BIB (~5-28 m)


Current IR Detectors

  • HgCdTe: Current state-of-the art arrays

    • QE ~70% (1-2.5 m)

    • Read noise ~10 e-

    • 2048x2048-pixel format

  • InSb: Current state-of-the-art arrays

    • QE >90% (1-5.5m)

    • Read noise ~25 e-

    • 2048x2048-pixel format


IR/Optical Difference: Cryogenics

  • For sensitive observations, we need kT  hc/ (why??)

  • (If not, thermal self-emission of the detector dominates over celestial sources)

  • 1-2.5 m  T < 70-80K (i.e. HgCdTe)

  • 1-5 m  T < 30-40K (i.e. InSb)

  • 5-30 m  T < 4-8K (i.e. Si:As BIB)


Implications of cryogenics

  • Vacuum systems (for thermal isolation)

  • Large cryostats

  • Cryogenic liquids:

    • LN2 77K

    • LHe  4K

  • Mechanical cryocoolers:

    • Ultra-pure He

    • Compressors

    • “Cold heads”


IR Obs: Atmospheric IR Transmission


IR Obs: Atmospheric IR Transmission


IR Obs: Atmospheric IR Transmission


IR Obs: Atmospheric IR Emission


Atmospheric IR Emission

  • Dominant source of in-band background


Important IR Lines: Hydrogen

Paschen Series

Brackett Series


Important IR Lines: Hydrogen

Pfund Series


IR Hydrogen Lines: Trouble

Paschen Series

Brackett Series


IR Hydrogen Lines: Trouble

Pfund Series


IR Hydrogen Lines: Implications

  • None of the “IR” hydrogen series have (ground) observable “” transitions (!)

  • From the ground, we cannot observe the equivalent of the Balmer decrement

  • We can combine Pa/Br (two strongest easily-observable transitions of each series)

    • “IR decrement” of sorts

    • But … these two have no common energy levels

    • Greater physical uncertainty in parameters


Important IR Lines: Molecules

  • Not many molecular transitions are easily observed in the optical

  • “Hard” optical/UV radiation dissocates them (!)

  • Many molecular transitions observable in the IR from “cool” objects

  • Particularly strong are H2 ro-vibrational transitions (many from 1-3 m; strongest at 2.12 m)

  • Also, CO bandheads at 2.3-2.5 m

    • Mostly seen in absorption in cool giant stars

    • Also seen in emission occasionally (more later)


III. Nebular Sources in the Galaxy

  • Galactic HII Regions

  • Planetary Nebulae

  • Supernova Remnants


Galactic HII Regions

  • These are generally covered elsewhere in the Winter School lectures

  • Important point: hydrogen is dominant (why?)

  • One Milky Way –centric point: while most past work has been done in the optical/UV (even in our Galaxy), IR is still important for current/future work


Galactic HII Regions: Why IR?

Example: Cepheus A

POSS “IR”

POSS-B

2MASS-J


Planetary Nebulae: Why?

  • PNe are the (near-)final evolutionary phase for most stars in the Universe

  • The PNe phase is responsible for the return of chemically-enriched material to the ISM

  • They exhibit very interesting outflow physics

  • They are PRETTY!


Planetary Nebulae: Why?


Planetary Nebulae

  • What can PNe emission lines tell us?

    • Electron density

    • Electron temperature

    • Ionic abundance

    • H2 shows shock vs radiative excitation

    • [FeII] shows shocks

    • Kinematics of Outflows & Morphology


PNe: Electron Density

  • Key transitions: 4S3/2-2D5/2 and 4S3/2-2D3/2 for [OII] and [SII]

  • Also [ClII] & [ArIV]

  • Why?

From Stanghellini & Kaler


PNe: Other basics

  • Similar diagnostics for electron temperatures

  • Combine temperatures & densities with models  ionic abundances

  • Major sources of uncertainty for Planetary Nebulae diagnostics:

    • distance

    • internal extinction (throws off line ratios; less so in the IR)


PNe: IR Spectra


PNe: H2 Diagnostics

  • H2 lines can be excited by both fluorescence and by thermal (collisional/shock) mechanisms

  • At low densities, with UV excitation of cool (T ~100K) material have 2.12/2.25-micron ratio of ~1.7

  • These are 1-0 S(1) and 2-1 S(1) transitions

  • At higher densities (>104 cm-3), this ratio increases and becomes a good probe of temperature (up to ~1000K)


PNe: [FeII] Diagnostics

  • Fe usually “depletes” onto dust grains in ISM

  • shocks break up dust  greatly increase Fe abundance in ISM (temporarily)

  • Thus, [FeII] provides excellent shock diagnostic (kinematics, density) for PNe

  • Typically only seen in the fastest-moving PNe shocks


PNe: Outflows & Morphology

  • Contrary to simple expectations, most PNe seem to be VERY non-spherical (!)

  • Most show very eye-catching aspheric symmetry

  • Strong indications of collimated outflows in some


Collimation :

“mild” “high”


Point-symmetry is pervasive…


Planetary Nebulae

Point-symmetry is usually associated with:

  • Bipolarity

    - A progressive variation in the direction of the outflows

  • episodic events of (collimated) mass-loss.

    Thus, point-symmetry indicates the presence of a

    Bipolar, Rotating, Episodic Jet or Collimated Outflow ( BRET).

    A few representative examples next …


Point-symmetry  morphology--BRET  kinematicsIn a true BRET morphology is reflected in its kinematics


Possible Models for Morphologies


There is a wide range of speeds in the COFsfrom a few tens to several hundred km/s….

MyCn18, first PN to break the ~500 km/s barrier…now other examples such as He 3-1475 and Mz 3…

However, their masses (~1028-29 g), kinetic energy (~1043-44 ergs) and mechanical power (~1033-34 ergs/s) still are poorly determined in most cases …


MHD models with magnetic axis tilted with respect to bipolar wind axis…


Mastrodemos & Morris 1999

Binary cores: COFs and axis-symmetry may be produced either by : -Wind accretion from AGB onto WD or MS companion

Wind accretion may produce bipolar COFs that explain plane – symmetry, such as in the case of M2-9 (Soker & Livio 2001)

Soker & Rappaport 2000


…or via RLOF after a CE phase where low mass secondary is destroyed during an unstable mass transfer process, forming an accretion disk…

Some expected implications of binary core on COFs

Accretion through RLOF is short-lived at end of AGB .


Morpho/Kinematics: Conclusions

  • COFs as BRETs (Poly-polar or P-S) are ubiquitous in PNe.

  • COFs develop since the very early stages of formation of the proto-PN.

  • Although their velocities are now well characterized, their masses, kinetic energy and luminosities need better determination to confront ionized, atomic and molecular parameters with stellar power input (radiative, gravitational, etc.)


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