IR Spectroscopy

6. Interstellar Medium  Hot ISM : hot ionized matter at very low density ( n ~ 5x10 -3 cm -3 ) and heated to very high temperature (~ 5x10 5 K)  Warm ISM : n around 3 x cm -3 and T of order 10,000K   Cool ISM (diffuse) : n= cm -3, T=80 K  Refractory survive =silicates (9.7  m 18.8  m  Weaker feature at 3.0 and 3.4  m due to H 2 O or CH- containing compounds  Cold ISM (dense) : n > 10 2 cm -3, T ~ 15 K :refractory features of the cool clouds as well as water at 3.05  m and other bands from CO, CO 2, and CH 3 OH ices

7. Photodissociation regions (PDR)  Interface zones between molecular clouds and ionized or HII regions  n > 10 3 cm -3, cm -3, 100 < T gas < 1500 K  forbidden fine-structure lines of  [OI] 63 and 146  m.  [CII] 158  m and  [SiII] 35  m  from magnetic dipole radiation  called fine-structure cooling

7. 1. H 2 in PDR  Cold clouds (T<100 K) : not radiate in the rot-vib of H 2, but two electronic states above the ground state can be excited by photons of less than 13.6 ev. They are C 1  u and B 1  + u  Then, 10% end up in the vib continuum of the ground state to be dissociated, remainder end up as bound vib states and eventually decay (fluoresce) as rot-vib transitions Characterize by having the intensity ratio of 2.12  m 1-0 S(1) to 2.25  m 2-1 S(1) about 2

7. 1. H 2 in PDR –con  In warm high-density regions (n > 10 4 cm -3 ), excitation by UV become insignificant compared to thermal collisional excitation. T ~ 10 3 K  lowest excited level is populated much more than higher levels  ratio of the 1-0 S(1) and 2-1 S(1) lines is close to the kinetic T of the gas  diagnostics : the ratio of the 2-1 S(1) to the 6-4 Q(1) lines ( =  m), the ortho:para ratio and rotational temperatures : Draine and Bertoldi, 1996)

7. 2. Pure rot lines of H 2  Pure rot lines generated in PDR and shocks  Burton, Hollenbach and Tielens (1992) : predict  Observed in or near YSO (Wesselius et al (1996) and Timmermann et al (1996) consistent with thermal excitation by gas at K

8. HII regions  H II region is dense enough so that no ionizing photons escape from it. Their energy is degraded into more numerous lower-energy photons.  Menzel’s case B  IR spectrum of H II region : continuum and the recombination lines  F-f and f-b continua :  Line emission and continuum : depend on n e 2 V ( n e =electron density, V = volume ) and T  The ratio of line strength to continuum strength is only a function of T

8. 1. H I continuous emission  Ferland (1980) : calculated the H I and He II continuous emission coeficients for T from 500 K to 2X10 6 K.  HI emission coeficients averaged over commonly used filter bands  color of the continuous emission component :Table

8.2. Recombination spectrum  No significant collisional process except ultra- dense conditions in the broad-line region of the nuclei of active galaxies.  Ratios of the H Balmer line strengths to H  : Brocklehurst (1971) with Paschen-to-Balmer intensity ratios and corresponding ratios for the He+ lines  Extend to Brackett-to-Balmer line ratios : Giles (1977)  Further extensions to cover a greater range of physical conditions : Hummer and Story (1987 )

8. 3 H I Recombination Iine ratios  H I Recombination Iine ratios (case B)

8.4. Ratio of line to continuum flux  Emission In Br  obtained  Table. Wavelengths of some hydrogen lines Series Line Wavelength (μm) Lyman (n=l) Ly α Ly β Ly γ Ly Limit B aimer (n = 2) H α H β H γ H limit Paschen (n = 3) P α P β P γ P limit Brackett (n = 4) Br α Br β Br γ Br limit Pfund (n = 5) Pf α Pf β Pf γ Pf limit 2.279

8.4.

8.5. Recombination spectrum  ZAMS star surrounded by dust in a compact HII region can be estimated roughly from its 5 GHz continuum flux and bolometric mag. Bolometric flux == mainly emerging in IR, Bolometric flux == mainly emerging in IR, 5 GHz continuum == indicator of the number of Lymann continuum photons 5 GHz continuum == indicator of the number of Lymann continuum photons  Steepness of the luminosity dependence constrains the spectral type

8.6 Fine-structure forbidden lines in H II, PN, and Active galaxies  In addition to the H lines,

8.6. Fine-structure forbidden lines  Critical density n e of electrons for each transition above which collision de-excitation predominates over radiation  When radiative de-excitation dominates,  the line flux ~ n i n e dV  the line flux ~ n i n e dV

8.6. Fine-structure forbidden lines  P 1 & p 5 : doublet  only one fine-structure transition in ground states ([SIV] at 10.5  m, [NeII] at 12.8  m)  P 2 & P 4 ; triplet ground  pairs of IR lines ([OIII] at 52 & 88  m)  For certain ranges of n e, the critical densities of one line may be proportional to n e, and the other to n i * n e  so that n i may be determined  Te (electron) determined from IR fine-structure lines and visible forbidden lines from the same ion. == require detail information of collision strengths (Barlow 1989, Herter 1989)

8.6.1 [Fe II] and [Fe III] lines  [Fe II] : diagnostics of electron density, n e and temperature T e (Pradhan and Zhang 1993) Young PN Hubble 12 ; [Fe II] and [Fe III] : Luhman and Rieke (1996) shows fluorescent excitation of H 2. Young PN Hubble 12 ; [Fe II] and [Fe III] : Luhman and Rieke (1996) shows fluorescent excitation of H 2.  Greenhouse et al (1991) : [Fe II] emission of galaxies traces SN activity and is independent of content of HII and PDR. It originates in shocked material. Shocked-excited [Fe II] lines at 1.26 & 1.64  m arise from the same level and can be used for determining extinction.  [Fe III] emission at  m from the cavity at the Galactic center ; Lutz et al (1993) ;its excitation comes from photoionized material with T e ~ 7000 K

8.7. modeling of H II  CLOUDY code (Ferland 1996):  deals with material around a hot source  ; Incorporates the numerous lines of atomic and molecular data with parameters, luminosity, spectral shape of the central source, the density, chemical composition and filling factor of the clouds.  spectrum output  ;applicable from the intergalactic medium to the broad-line region of active galaxies

9. Shocks  Arise when material moving faster than the speed of sound encounters a slower-moving medium.  “speed of sound” : the speed at which a compressional disturbance is propagated with gas pressure as the restoring force, typical 1 km s -1 in dense molecular cloud (Chernoff and McKee 1990)  Star-forming regions : jet-like outflow from a protostar encounters a dense molecular cloud, giving rise to a Herbig-Haro object.  The expansion of a compact H II region, forming around a young massive hot star, caused a rapid outflow of material == > rise a shocked zone at the boundary of the surrounding cooler medium  Expansion of a SNR into the ISM

9. Two Shocks  C-shocks (continous) : occur when the difference in velocity is relatively low ( ~ 50 Km s -1 ) and the kinetic energy of the individual ions is insufficient to dissociate the neutral medium. The cooling of the shock zone is primarily in the IR, eg in H 2 emission  J-shocks (jump) occur at higher velocities, when the kinetic energy is sufficient to dissociate the H 2. the shock energy dissipate mainly in UV. The line ratios observed similar to those observed from fluorescent emission in PDR but their widths greater The line ratios observed similar to those observed from fluorescent emission in PDR but their widths greater Geballe (1990) : summary for H 2 lines Geballe (1990) : summary for H 2 lines

9. Shock  MAPPINGS : numerical radiative transfer codes (Sutherland et al 1999)  Fine-structure lines with IP > 50 ev ; seen in Seyfort, not in starbursts  Collisionally excited in shocks outside the broad-line region of AGN : modelled by Ferguson et al (1997)

10. Solid-state features  In sloid state, molecules no longer free to rotate,  observed bands are of vibrational origin with broadening and wavelengths shifting from those in gaseous state due to close packing and interference by other substances of grain materials and also the shapes of the particles.

10.1. Silicon compounds  Silicate features at 9.7  m (Si-O stretching) and 18.5  m (O-Si-O bending) : in emission (optically thin) in M stars and some PN, also in absorption (optically thick) in ISM  Silicon carbide (SiC) at 11.2  m : emission in C-rich red giants and PN, not in ISM  Silicate and SiC correlate with optical evidence for O-richness and C- richness.  CO forms easily and at high T at stellar Atm. Remain in C- rich or O-rich medium  silicates and SiC  A. Amorphous silicates : IS absorption at 9.7  m accompany by anther band at 18  m  9.7  m Strength proportional to A v (1987, Whittet)     = _1  But Galaxtic center : A v /   7 = 9 +_ 1 ( Roche and Aitken, 1985)  B. Crystalline silicates, olivine and pyroxenes ; features in  m in dust shells of O-rich objects (Tielens et al 1988)

Ices  Various types of ices form on the outer layers of grains in the dense regions of molecular clouds. : CO, CO2, CH3OH, CH4  Review :Whittet et al 1996

CO Ice, CO 2 Ice, XCN  CO ice : Cold (<17 K) ; CO gas condense as a frost onto grains  cause significant depletion of CO gas  CO ice width related to the dipole moment of the predominant host susbstance. CO frosts annealed at ~ 100 K, but survive due to impurity in H 2 O ice. (Whittet an Duley 1991)  CO 2 Ice ; very widely observed in solid form, not found as a gas in ISM (van Dishoeck et al, 1996), shape of feature depends on whether the matrix of ices is polar or non- polar, ie whether it is dominated by H 2 O or not. (de Graauw et al 1996, Ehrenfreund et al, 1996)  XCN : feature at 4.62  m due to CΞN bonds

10.3. PAH bands  Unidentified IR bands (UIBs) at 3.3, 6.2, 7.7, 8.6, and 11.3  m ; observed in PN, H II region, reflection nebulae around early type stars and in some galaxies. They contribute to the diffuse emission from the disk of the galaxy (Mattila et al, 1996)  Due to C-C and C-H stretching and bending vibrations in polycyclic aromatic hydrocarbons (PAHs) (Puget and Leger 1989)  Or due to hydrogenated amorphous graphitic particles (Duley and Williams 1988)  PAH compounds : platelets of hexagonal carbon rings of various sizes ; eg. C 24 H 12  Seen only in emission Horsehead Nebular