the emission line universe: galactic sources of emission lines

The Emission Line Universe: Galactic Sources of

Emission Lines

Stephen S. EikenberryUniversity of Florida

22 November 2006

OUTLINEI. Introduction

II. Infrared Emission Lines

III. Nebular Galactic Emission Line Sources

IV. Stellar Galactic Emission Line Sources

V. 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!!)

2MASS-KPOSS-R

• 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)

IR/Optical Difference: Detectors

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% (1-5.5m)

– 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)

• Vacuum systems (for thermal isolation)

• Large cryostats

• Cryogenic liquids:

•LN2 77K

•LHe 4K

•Mechanical cryocoolers:

• Ultra-pure He

• Compressors

• “Cold heads”

Implications of cryogenics

IR Obs: Atmospheric IR Transmission

IR Obs: Atmospheric IR Transmission

IR Obs: Atmospheric IR Transmission

Bandcentral (m)

min (m)

max (m)

J 1.25 1.10 1.40

H 1.65 1.50 1.80

Ks 2.15 2.00 2.30

K 2.2 1.95 2.50

L 3.8 3.5 4.1

M 4.8 4.5 5.1

IR Obs: Atmospheric IR Emission

• Dominant source of in-band background

Atmospheric IR Emission

Band Background source Comment

J OH airglow J ~15.5 mag

H OH airglow H ~14.0 mag

Ks OH airglow Ks ~13.3 mag

K OH + thermal K ~ 13.0 mag

L Thermal

M Thermal

Important IR Lines: Hydrogen

Name n1-n2

(m)

Pa 4-3 1.875

Pa 5-3 1.281

Pa 6-3 1.094

Pa 7-3 1.004

Pa(lim) -3 0.820

Paschen Series

Name n1-n2

(m)

Br 5-4 4.053

Br 6-4 2.626

Br 7-4 2.166

Br 8-4 1.945

Br(lim) -4 1.818

Brackett Series

Important IR Lines: Hydrogen

Name n1-n2

(m)

Pf 6-5 7.460

Pf 7-5 4.650

Pf 8-5 3.720

Pf 9-5 3.300

Pf(lim) -5 2.280

Pfund Series

IR Hydrogen Lines: Trouble

Name n1-n2

(m)

Pa 4-3 1.875

Pa 5-3 1.281

Pa 6-3 1.094

Pa 7-3 1.004

Pa(lim) -3 0.820

Paschen Series

Name n1-n2

(m)

Br 5-4 4.053

Br 6-4 2.626

Br 7-4 2.166

Br 8-4 1.945

Br(lim) -4 1.818

Brackett Series

IR Hydrogen Lines: Trouble

Name n1-n2

(m)

Pf 6-5 7.460

Pf 7-5 4.650

Pf 8-5 3.720

Pf 9-5 3.300

Pf(lim) -5 2.280

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)

• Galactic HII Regions

• Planetary Nebulae

• Supernova Remnants

III. Nebular Sources in the Galaxy

• 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

Galactic HII Regions: Why IR?

POSS-B POSS “IR”

2MASS-J

Example: Cepheus A

• 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: Why?

• 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

Planetary Nebulae

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

• Also [ClII] & [ArIV]

• Why?

PNe: Electron Density

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…

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 …

Planetary Nebulae

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….

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 …

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

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

Binary cores: COFs and axis-symmetry may be produced

either by : -Wind accretion from AGB onto WD or MS companion

Mastrodemos & Morris 1999

Soker & Rappaport 2000

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

Some expected implications of binary core

on COFs

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

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

• 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.)

Morpho/Kinematics: Conclusions