Ralf Timmermann

The Aromatic Infrared Bands as seen by ISO-SWS: Probing the PAH model ?

We discuss the Aromatic Infrared Band (AIB) proles observed by ISO-SWS towards a number of bright interstellar regions where dense molecular gas is illuminated by stellar radiation. Our sample spans a broad range of excitation conditions (exciting radiation elds with eective temperature, Te, ranging from 23 000 to 45 000 K). The SWS spectra are decomposed coherently in our sample into Lorentz proles and a broadband continuum. We nd that the individual proles of the main AIBs at 3.3, 6.2, 8.6 and 11.3 m are well represented with at most two Lorentzians. The 7.7 m-AIB has a more complex shape and requires at least three Lorentz proles. Furthermore, we show that the positions and widths of these AIBs are remarkably stable (within a few cm 1 ) conrming, at higher spectral resolution, the results from ISOCAM-CVF and ISOPHOT-S. This spectral decomposition with a small number of Lorentz proles implicitly assumes that most of the observed bandwidth arises from a few, large carriers. Boulanger et al. (1998b) recently proposed that the AIBs are the intrinsic proles of resonances in small carbon clusters. This interpretation can be tested by comparing the AIB prole parameters (band position and width) given in this work to laboratory data on relevant species when it becomes available. Taking advantage of our decomposition, we extract the proles of individual AIBs from the data and compare them to a state-of-the-art model of Polycyclic Aromatic Hydrocarbon (PAH) cation emission. In this model, the position and width of the AIBs are rather explained by a redshift and a broadening of the PAH vibrational bands as the temperature of the molecule increases (Joblin et al. 1995). In this context, the present similarity of the AIB proles requires that the PAH temperature distribution remains roughly the same whatever the radiation eld hardness. Deriving the temperature distribution of interstellar PAHs, we show that its hot tail, which controls the AIB spectrum, sensitively depends on Nmin (the number of C-atoms in the smallest PAH) and Te. Comparing the observed proles of the individual AIBs to our model results, we can match all the AIB proles (except the 8.6 m-AIB prole) if Nmin is increased with Te. This increase is naturally explained in a picture where small PAHs are more eciently photodissociated in harsher radiation elds. The observed 8.6 m-prole, both intensity and width, is not explained by our model. We then discuss our results in the broader context of ISO observations of fainter interstellar regions where PAHs are expected to be in neutral form.

Atomic Oxygen in Molecular Clouds? High-Resolution Spectroscopy of the [O I] 63 Micron Line toward DR 21

We report the first high-resolution spectra of the [O I] 63 μm fine-structure line toward DR 21. The observations were made from NASAs Kuiper Airborne Observatory using the MPE/UCB imaging spectrometer FIFI. The spectra show a pronounced dip, which may be due to absorption by a foreground molecular cloud, seen against the broad [O I] emission from DR 21. In this case we derive a column density of cold, atomic oxygen of 5 × 1018 cm-2, corresponding to a relative abundance of atomic oxygen of [O]/[H] 6 × 10-4. Therefore, most of the gas-phase oxygen in this cloud would be in atomic form. This result is in contrast to predictions by common chemistry models for a steady state cloud and may support models which predict a high abundance of atomic oxygen in dark cores of molecular clouds.

Airborne Observations of the 184 Micron H218O 221 ? 212 Transition in the Shocked Region of Orion BN-IRc2

We report the first far-IR detection of isotopic water in the Orion BN-IRc2 region from aboard the Kuiper Airborne Observatory. The observation of the 183.5290 μm H218O 221 → 212 emission line indicates that it is likely to originate from shocked gas in the outflow around Orion BN-IRc2, rather than from warm gas of the Orion plateau, where the H2O abundance is low ([H2O]/[H] 2.5 × 10-5). Our observation infers a ratio of [H2O]/[H] 1.7 × 10-3, which indicates that O is efficiently converted to H2O in shocked gas.

Excitation of H2 and HD in Shocks and PDRs

Photodissociation regions (PDRs) and shocks give rise to conspicuous emission from rotationally and vibrationally excited molecular hydrogen. This line emission has now been studied with ISO and from the ground in great detail. A remarkable discovery has been that toward the Orion outflow and other shock-excited regions, the H_2 level populations show a very high excitation component. We suggest that these high-excitation populations may arise from non-thermal pumping processes, such as H_2 formation and high-velocity ion-molecule collision in partially dissociative shocks. In PDRs such as NGC 7023 however, formation pumping is always less important than fluorescent pumping. We furthermore present two HD emission line detections toward Orion Peak 1. This enables the first comparison of the H_2 and the HD excitation, which surprisingly turn out to be identical.