E. Habart

Accretion Rates in Herbig Ae stars

Aims. Accretion rates from disks around pre-main sequence stars are of importance for our understanding of planetary formation and disk evolution. We provide in this paper estimates of the mass accretion rates in the disks around a large sample of Herbig Ae stars. Methods. We obtained medium resolution 2 μm spectra and used the results to compute values of M acc from the measured luminosity of the Br γ emission line, using a well established correlation between L(Br γ ) and the accretion luminosity L acc . Results. We find that 80% of the stars, all of which have evidence of an associated circumstellar disk, are accreting matter, with rates 3 x 10 -9 ≤ M acc ≤ 10 -6 M ⊙ /yr; for 7 objects, 6 of which are located on the ZAMS in the HR diagram, we do not detect any line emission. Few HAe stars (25%) have M acc > 10 -7 M ⊙ /yr. Conclusions. In most HAe stars the accretion rate is sufficiently low that the gas in the inner disk, inside the dust evaporation radius, is optically thin and does not prevent the formation of a puffed-up rim, where dust is directly exposed to the stellar radiation. When compared to the M acc values found for lower-mass stars in the star forming regions Taurus and Ophiuchus, HAe stars have on average higher accretion rates than solar-mass stars; however, there is a lack of very strong accretors among them, probably due to the fact that they are on average older.

H2 formation and excitation in the diffuse interstellar medium

We use far-UV absorption spectra obtained with FUSE towards three late B stars to study the formation and ex- citation of H2 in the diuse ISM. The data interpretation relies on a model of the chemical and thermal balance in photon- illuminated gas. The data constrain well the nRproduct between gas density and H2 formation rate on dust grains: nR= 1t o 2:2 10 15 s 1 . For each line of sight the mean eective H2 density n, assumed uniform, is obtained by the best fit of the model to the observed N(J= 1)=N(J= 0) ratio, since the radiation field is known. Combining n with the nRvalues, we find similar H2 formation rates for the three stars of about R= 4 10 17 cm 3 s 1 . Because the target stars do not interact with the absorbing matter we can show that the H2 excitation in the J> 2 levels cannot be accounted for by the UV pumping of the cold H2 but implies collisional excitation in regions where the gas is much warmer. The existence of warm H2 is corroborated by the fact that the star with the largest column density of CH + has the largest amount of warm H2.

H2 infrared line emission across the bright side of the ρ Ophiuchi main cloud

We present imaging and spectroscopic observations of dust and gas (H2) emission, obtained with ISO, from the western edge of the ρ Ophiuchi molecular cloud illuminated by the B2 star HD147889 (χ ∼ 400). This photodissociation region (PDR) is one of the nearest PDRs to the Sun (d = 135 ± 15 pc from the stellar parallax) and the layer of UV light penetration and of H2 emission is spatially resolved. It is therefore an ideal target to test the prediction of models on the integrated fluxes but also on the spatial distribution. The emission from dust heated by the external UV radiation, from collisionally excited and fluorescent H2 are observed to coincide spatially. The spectroscopic data, obtained with ISO-SWS, allows us to estimate the gas temperature to be 300-345 K in the H2 emitting layer, in which the ortho-to-para H2 ratio is about 1 or significantly smaller than the equilibrium ratio (∼3 at that temperature). We interpret this data with an equilibrium PDR model. In this low excitation PDR, the gas heat budget is dominated by the contribution of the photoelectric heating from very small grains and polycyclic aromatic hydrocarbons (PAHs). With the standard PAH abundance ((C/H)PAH � 5 × 10 −5 ), we find that the H2 formation rate Rf must be high in warm gas (∼6 times the rate derived by Jura, 1975), in order to account for the observed H2 emission. This result and the spatial coincidence between the PAHs and H2 emission suggest that H2 forms efficiently by chemisorption on the PAHs surface. If the latter interpretation is correct, the enhancement in Rf may also result from an increased PAH abundance: assuming that Rf scales with the PAH abundance, the observed H2 excitation is well explained with Rf � 1 × 10 −16 cm 3 s −1 at Tgas = 330 K (∼3 times the rate derived by Jura 1975) and (C/H)PAH � 7.5 × 10 −5 .

SPIRE spectroscopy of the prototypical Orion Bar photodissociation region

Aims: We present observations of the Orion Bar photodissociation region (PDR) obtained with the SPIRE instrument on-board Herschel. Methods: We obtained SPIRE Fourier-transform spectrometer (FTS) sparse sampled maps of the Orion bar. Results: The FTS wavelength coverage and sensitivity allow us to detect a wealth of rotational lines of CO (and its isotopologues), fine structure lines of C and N+, and emission lines from radicals and molecules such as CH+, CH, H2O or H2S. For species detected from the ground, our estimates of the column densities agree with previously published values. The comparison between 12CO and 13CO maps shows particularly the effects of optical depth and excitation in the molecular cloud. The distribution of the 12CO and 13CO lines with upper energy levels indicates the presence of warm (~100-150 K) CO. This warm CO component is a significant fraction of the total molecular gas, confirming previous ground based studies. Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.

Evolution of dust in the Orion Bar with Herschel - I. Radiative transfer modelling

Context . Interstellar dust is a key element in our understanding of the interstellar medium and star formation. The manner in which dust populations evolve with the excitation and the physical conditions is a first step in comprehending the evolution of interstellar dust. Aims . Within the framework of the Evolution of interstellar dust Herschel key programme, we have acquired PACS and SPIRE spectrophotometric observations of various photodissociation regions, to characterise this evolution. The aim of this paper is to trace the evolution of dust grains in the Orion Bar photodissociation region. Methods . We used Herschel /PACS (70 and 160 μm) and SPIRE (250, 350 and 500 μm) together with Spitzer /IRAC observations to map the spatial distribution of the dust populations across the Bar. Brightness profiles were modelled using the DustEM model coupled with a radiative transfer code. Results . Thanks to Herschel , we are able to probe in great detail the dust emission of the densest parts of the Orion Bar with a resolution from 5.6″ to 35.1″. These new observations allow us to infer the temperature of the biggest grains at different positions in the Bar, which reveals a gradient from ~70 K to 35 K coupled with an increase of the spectral emissivity index from the ionization front to the densest regions. Combining Spitzer /IRAC observations, which are sensitive to the dust emission from the surface, with Herschel maps, we were able to measure the Orion Bar emission from 3.6 to 500 μm. We find a stratification in the different dust components that can be quantitatively reproduced by a simple radiative transfer model without dust evolution (diffuse interstellar medium abundances and optical properties). However, including dust evolution is needed to explain the brightness in each band. Polycyclic aromatic hydrocarbon (PAH) abundance variations, or a combination of PAH abundance variations with an enhancement of the biggest grain emissivity caused by coagulation give good results. Another hypothesis is to consider a length of the Bar along the line of sight different at the ionization front than in the densest parts.

Excitation of H2 in photodissociation regions as seen by Spitzer

Aims. We present spectroscopic observations obtained with the infrared Spitzer Space Telescope, which provide insight into the H 2 physics and gas energetics in photodissociation regions (PDRs) of low to moderate far-ultraviolet (FUV) fields and densities. Methods. We analyze data on six well known Galactic PDRs (L1721, California, N7023E, Horsehead, rho Oph, N2023N), sampling a poorly explored range of excitation conditions (χ ∼ 5-10 3 ), relevant to the bulk of molecular clouds in galaxies. Spitzer observations of H 2 rotational lines are complemented with H 2 data, including ro-vibrational line measurements, obtained with ground-based telescopes and ISO, to constrain the relative contributions of ultraviolet pumping and collisions to the H 2 excitation. The data analysis is supported by model calculations with the Meudon PDR code. Results. The observed column densities of rotationally excited H 2 are observed to be much higher than PDR model predictions. In the lowest excitation PDRs, the discrepancy between the model and the data is about one order of magnitude for rotational levels J ≥ 3. We discuss whether an enhancement in the H 2 formation rate or a local increase in photoelectric heating, as proposed for brighter PDRs in former ISO studies, may improve the data-model comparison. We find that an enhancement in the H 2 formation rates reduces the discrepancy, but the models still fall short of the data. Conclusions. This large disagreement suggests that our understanding of the formation and excitation of H 2 and/or of PDRs energetics is still incomplete. We discuss several explanations, which could be further tested using the Herschel Space Telescope.

Evolution of interstellar dust with Herschel. First results in the photodissociation regions of NGC 7023

Context. In photodissociation regions (PDRs), the physical conditions and the excitation evolve on short spatial scales as a function of depth within the cloud, providing a unique opportunity to study how the dust and gas populations evolve with the excitation and physical conditions. The mapping of the PDRs in NGC 7023 performed during the science demonstration phase of Herschel is part of the “Evolution of interstellar dust” key program. The goal of this project is to build a coherent database on interstellar dust emission from diffuse clouds to the sites of star formation. Aims: We study the far-infrared/submillimeter emission of the PDRs and their fainter surrounding regions. We combine the Herschel and Spitzer maps to derive at each position the full emission spectrum of all dust components, which we compare to dust and radiative transfer models in order to learn about the spatial variations in both the excitation conditions and the dust properties. Methods: We adjust the emission spectra derived from PACS and SPIRE maps using modified black bodies to derive the temperature and the emissivity index β of the dust in thermal equilibrium with the radiation field. We present a first modeling of the NGC 7023-E PDR with standard dust properties and abundances. Results: At the peak positions, a value of β equal to 2 is compatible with the data. The detected spectra and the spatial structures are strongly influenced by radiative transfer effects. We are able to reproduce the spectra at the peak positions deduced from Herschel maps and emitted by dust particles at thermal equilibrium, and also the evolution of the spatial structures observed from the near infrared to the submillimeter. On the other hand, the emission of the stochastically heated smaller particles is overestimated by a factor ~2. Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.

First detection of the methylidyne cation (CH+) fundamental rotational line with the Herschel/SPIRE FTS

Aims: To follow the species chemistry arising in diverse sources of the Galaxy with Herschel. Methods: SPIRE FTS sparse sampled maps of the Orion bar & compact HII regions G29.96-0.02 and G32.80+0.19 have been analyzed. Results: Beyond the wealth of atomic and molecular lines detected in the high-resolution spectra obtained with the FTS of SPIRE in the Orion Bar, one emission line is found to lie at the position of the fundamental rotational transition of CH+ as measured precisely in the laboratory by Pearson and Drouion. This coincidence suggests that it is the first detection of the fundamental rotational transition of CH+. This claim is strengthened by the observation of the lambda doublet transitions arising from its relative, CH, which are also observed in the same spectrum. The broad spectral coverage of the SPIRE FTS allows for the simultaneous measurement of these closely related chemically species, under the same observing conditions. The importance of these lines are discussed and a comparison with results obtained from models of the photon dominated region (PDR) of Orion are presented. The CH+ line also appears in absorption in the spectra of the two galactic compact HII regions G29.96-0.02 and G32.80+0.19, which is likely due to the presence of CH+ in the the cold neutral medium of the galactic plane. These detections will shed light on the formation processes and on the existence of CH+, which are still outstanding questions in astrophysics. Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.

Spatial variation of the cooling lines in the Orion Bar from Herschel/PACS

Sherpa Romeo green journal. Reproduced with permission from Astronomy & Astrophysics, ©ESO.

Photoelectric effect on dust grains across the L1721 cloud in the

We present ISO-LWS measurements of the main gas cooling lines, (C + ) 158 ma nd (O 0 )6 3m towards a moderate opacity molecular cloud (Av 3), L1721, illuminated by the B2 star Sco ( =5 10). These data are combined with an extinction map and IRAS dust emission images to test our understanding of gas heating and cooling in photodissociation regions (PDRs). This nearby PDR is spatially resolved in the IRAS images; variations in the IRAS colors across the cloud indicate an enhanced abundance of small dust grains within the PDR. A spatial correlation between the gas cooling lines and the infrared emission from small dust grains illustrates the dominant role of small dust grains in the gas heating through the photoelectric eect. The photoelectric eciency, determined from the observations by ratioing the power radiated by gas and small dust grains, is in the range 2 to 3%, in close agreement with recent theoretical estimates (Bakes & Tielens 1994; Weingartner & Draine 2001). The brightness proles across the PDR in the (C + ) 158 ma nd (O 0 )6 3 ml ines are compared with model calculations where the density prole is constrained by the extinction data and where the gas chemical and thermal balances are solved at each position. We show that abundance variations of small dust grains across the PDR must be considered to account for the LWS observations.