Christopher M. Wright

Infrared Spectroscopy of Dust in the Diffuse Interstellar Medium toward Cygnus OB2 No. 12

Observations made with the short-wavelength spectrometer of the Infrared Space Observatory are used to investigate the composition of interstellar dust in the line of sight to Cygnus OB2 No. 12, commonly taken as representative of the diffuse (low-density) interstellar medium. Results are compared with data for the Galactic center source Sgr A*. Nondetections of the 3.0 and 4.27 μm features of H2O and CO2 ices in Cyg OB2 No. 12 confirm the absence of dense molecular material in this line of sight, whereas the presence of these features in Sgr A* indicates that molecular clouds may contribute as much as 10 mag of visual extinction toward the Galactic center. The spectrum of Cyg OB2 No. 12 is dominated by the well-known 9.7 μm silicate feature; detection of a shallow feature near 2.75 μm indicates that the silicates are at least partially hydrated, with composition possibly similar to that of terrestrial phyllosilicates such as serpentine or chlorite. However, the 2.75 μm feature is not seen in the Galactic center spectrum, suggesting that silicates in this line of sight are less hydrated or of different composition. The primary spectral signatures of C-rich dust in the diffuse ISM are weak absorptions at 3.4 μm (the aliphatic C=H stretch) and 6.2 μm (the aromatic C=C stretch). We conclude, based on infrared spectroscopy, that the most probable composition of the dust toward Cyg OB2 No. 12 is a mixture of silicates and carbonaceous solids in a volume ratio of approximately 3:2, with the carbonaceous component primarily in an aromatic form such as amorphous carbon.

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.

Investigating grain growth in disks around southern T Tauri stars at millimetre wavelengths

Received ?? ; Accepted ?? Abstract. Low-mass stars form with disks in which the coagulation of grains may eventually lead to the formation of planets. It is not known when and where grain growth occurs, as models that explain the observations are often degenerate. A way to break this degeneracy is to resolve the sources under study.

Modeling gas-phase H2O between 5 μm and 540 μm toward massive protostars

We present models and observations of gas-phase H2O lines between 5 and 540m toward deeply embedded massive protostars, involving both pure rotational and ro-vibrational transitions. The data have been obtained for 6 sources with both the Short and Long Wavelength Spectrometers (SWS and LWS) on board the Infrared Space Observatory (ISO) and with the Submillimeter Wave Astronomy Satellite (SWAS). For comparison, CO J = 7 6 spectra have been observed with the MPIfR/SRON 800 GHz heterodyne spectrometer at the James Clerk Maxwell Telescope (JCMT). A radiative transfer model in combination with dierent physical/chemical scenarios has been used to model these H2O lines for 4 sources to probe the chemical structure of these massive protostars. The results indicate that pure gas-phase production of H 2O cannot explain the observed spectra. Ice evaporation in the warm inner envelope and freeze-out in the cold outer part are important for most of our sources and occur at T 90-110 K. The ISO-SWS data are particularly sensitive to ice evaporation in the inner part whereas the ISO-LWS data are good diagnostics of freeze-out in the outer region. The modeling suggests that the 557 GHz SWAS line includes contributions from both the cold and the warm H2O gas. The SWAS line profiles indicate that for some of the sources a fraction of up to 50% of the total flux may originate in the outflow. Shocks do not seem to contribute significantly to the observed emission in other H2O lines, however, in contrast with the case for Orion. The results show that three of the observed and modeled H2O lines, the 303 212; 212 101 ,a nd 1 10 101 lines, are good candidates to observe with the Herschel Space Observatory in order to further investigate the physical and chemical conditions in massive star-forming regions.

Gas-phase H2O and CO2 toward massive protostars

We present a study of gas-phase H2O and CO2 toward a sample of 14 massive protostars with the Short Wavelength Spectrometer (SWS) on board the Infrared Space Observatory (ISO). Modeling of the H2O spectra using a homogeneous model with a constant excitation temperature T_ex shows that the H2O abundances increase with temperature, up to a few times 10^-5 with respect to H2 for the hottest sources (T_ex ~500 K). This is still a factor of 10 lower than the H2O ice abundances observed toward cold sources in which evaporation is not significant (Keane et al. 2001). Gas-phase CO2 is not abundant in our sources. The abundances are nearly constant for T_ex>~100 K at a value of a few times 10^-7, much lower than the solid-state abundances of ~1--3 times 10^-6 (Gerakines et al. 1999). For both H2O and CO2 the gas/solid ratio increases with temperature, but the increase is much stronger for H2O than for CO2, suggesting a different type of chemistry. In addition to the homogeneous models, a power law model has been developed for one of our sources, based on the physical structure of this region as determined from submillimeter data by van der Tak et al. (1999). The resulting H2O model spectrum gives a good fit to the data.

The ISO-SWS 2.4-45.2 micron spectrum toward Orion IRc2

The complete infrared spectrum from 2.4 to 45.2 μm toward the prototypical massive star-forming region Orion IRc2 is presented, obtained with the Short Wavelength Spectrometer (SWS) on board the Infrared Space Observatory (ISO) at a resolving power λ/Δλ ≈ 1300-2500. A wealth of emission and absorption features is found, including H2 vibration-rotation lines, the full set of H2 pure rotational lines (0,0) S(1)-S(17), H recombination lines, ionic fine-structure lines, PAH emission features, and absorption and emission bands by interstellar ices and gas-phase molecules, including CO2, CH4, and SO2. Particularly interesting is the detection of strong emission and absorption lines in the H2O ν2 bending mode at 6.2 μm and the observation of highly excited pure rotational lines of H2O in absorption at 25-45 μm. The origin of these lines in each of the physical components included in the ISO-SWS beam (H II region, PDR, quiescent ridge, shocked low-velocity plateau) is briefly discussed.

Grain growth signatures in the protoplanetary discs of Chamaeleon and Lupus

We present Australia Telescope Compact Array results of a 3 and 7 mm continuum survey of 20 T Tauri stars in the Chamaeleon and Lupus star-forming regions. This survey aims to identify protoplanetary discs with signs of grain growth. We detected 90 per cent of the sources at 3 and 7 mm, and determined the spectral slopes, dust opacity indices and dust disc masses. We also present temporal monitoring results of a small subset of sources at 7, 15 mm and 3+6 cm to investigate grain growth to centimetre (cm) sizes and constrain emission mechanisms in these sources. Additionally, we investigated the potential correlation between grain growth signatures in the infrared (10 μm silicate feature) and millimetre (1–3 mm spectral slope, α). Eleven sources at 3 and 7 mm have dominant thermal dust emission up to 7 mm, with seven of these having a 1–3 mm dust opacity index less than unity, suggesting grain growth up to at least mm sizes. The Chamaeleon sources observed at 15 mm and beyond show the presence of excess emission from an ionized wind and/or chromospheric emission. Long-time-scale monitoring at 7 mm indicated that cm-sized pebbles are present in at least four sources. Short-time-scale monitoring at 15 mm suggests that the excess emission is from thermal free–free emission. Finally, a weak correlation was found between the strength of the 10 μm feature and α, suggesting simultaneous dust evolution of the inner and outer parts of the disc. This survey shows that grain growth up to cm-sized pebbles and the presence of excess emission at 15 mm and beyond are common in these systems, and that temporal monitoring is required to disentangle these emission mechanisms.

Infrared Space Observatory-Long Wavelength Spectrometer Detection of the 112 Micron HD J = 1 ? 0 Line toward the Orion Bar

We report the first detection outside of the solar system of the lowest pure rotational J=1→0 transition of the HD molecule at 112 μm. The detection was made toward the Orion Bar using the Fabry-Perot interferometer of the Long Wavelength Spectrometer (LWS) on board the Infrared Space Observatory. The line appears in emission with an integrated flux of (0.93±0.17)×10−19 W cm−2 in the LWS beam, implying a beam-averaged column density in the v=0, J=1 state of (1.2±0.2)×1017 cm−2. Assuming LTE excitation, the total HD column density is (2.9±0.8)×1017 cm−2 for temperatures between 85 and 300 K. Combined with the total, warm H2 column density of ~(1.5-3.0)×1022 cm−2 derived from either the H2 pure rotational lines, the C18O observations, or the dust continuum emission, the implied HD abundance, HD/H2, ranges from 0.7×10−5 to 2.6×10−5, with a preferred value of (2.0±0.6)×10−5. The corresponding deuterium abundance of [D]/[H] = (1.0±0.3)×10−5 is compared with recent values derived from ultraviolet absorption-line observations of atomic H I and D I in interstellar clouds in the solar neighborhood and in Orion.

A COMPACT CONCENTRATION OF LARGE GRAINS IN THE HD 142527 PROTOPLANETARY DUST TRAP

A pathway to the formation of planetesimals, and eventually giant planets, may occur in concentrations of dust grains trapped in pressure maxima. Dramatic crescent-shaped dust concentrations have been seen in recent radio images at sub-mm wavelengths. These disk asymmetries could represent the initial phases of planet formation in the dust trap scenario, provided that grain sizes are spatially segregated. A testable prediction of azimuthal dust trapping is that progressively larger grains should be more sharply conned and furthermore the trapped grains should follow a distribution that is markedly dierent from the gas. However, gas tracers such as CO and the infrared emission from small grains are both very optically thick where the submm continuum originates, so observations have been unable to test the trapping predictions or to identify compact concentrations of larger grains required for planet formation by core-accretion. Here we report multifrequency observations of HD 142527, from 34 GHz to 700 GHz, that reveal a compact concentration of cm-sized grains, with a few Earth masses, embedded in a large-scale crescent of mm-sized particles. The emission peaks at wavelengths shorter than 1 mm are optically thick and trace the temperature structure resulting from shadows cast by the inner regions. Given this temperature structure, we infer that the largest dust grains are concentrated in the 34 GHz clump. We conclude that dust trapping is ecient for approximately cm-sized grains and leads to enhanced concentrations, while the smaller grains largely reect the gas distribution. Subject headings: Protoplanetary disks | Planet-disk interactions | Stars: individual: (HD 142527)

Cavity and other radial substructures in the disk around HD 97048

Context. Gaps, cavities, and rings in circumstellar disks are signposts of disk evolution and planet-disk interactions. We follow the recent suggestion that Herbig Ae/Be disks with a flared disk harbor a cavity, and investigate the disk around HD 97048. Aims. We aim to resolve the 34 ± 4 AU central cavity that has been predicted and to investigate the structure of the disk. Methods. We imaged the disk around HD 97048 using ALMA at 0.85 mm and 2.94 mm and ATCA (multiple frequency) observations. Our observations also include the 12 CO J = 1−0, 12 CO J = 3−2 and HCO + J = 4−3 emission lines. Results. A central cavity in the disk around HD 97048 is resolved with a 40−46 AU radius. Additional radial structure present in the surface brightness profile can be accounted for either by an opacity gap at 90 AU or by an additional emitting ring at 150 AU. The continuum emission tracing the dust in the disk is detected out to 355 AU. The 12 CO J = 3−2 disk is detected 2.4 times farther out. The 12 CO emission can be traced down to ≈10 AU scales. Apparent non-Keplerian kinematics are detected inside the cavity on the HCO + J = 4−3 velocity map. The mm spectral index measured from ATCA observations suggests that grain growth has occurred in the HD 97048 disk. Finally, we resolve a highly inclined disk out to 150 AU around the nearby 0.5 M ⊙ binary ISO-ChaI 126. Conclusions. The data presented here reveal a cavity in the disk of HD 97048, and prominent radial structure in the surface brightness. The cavity size varies for different continuum frequencies and gas tracers. The gas inside the cavity follows non-Keplerian kinematics seen in HCO + emission. The variable cavity size along with the kinematical signature suggests the presence of a substellar companion or a massive planet inside the cavity.