O. Berné

Formation of buckminsterfullerene (C60) in interstellar space

Buckminsterfullerene (C60) was recently confirmed as the largest molecule identified in space. However, it remains unclear how and where this molecule is formed. It is generally believed that C60 is formed from the buildup of small carbonaceous compounds in the hot and dense envelopes of evolved stars. Analyzing infrared observations, obtained by Spitzer and Herschel, we found that C60 is efficiently formed in the tenuous and cold environment of an interstellar cloud illuminated by strong ultraviolet (UV) radiation fields. This implies that another formation pathway, efficient at low densities, must exist. Based on recent laboratory and theoretical studies, we argue that polycyclic aromatic hydrocarbons are converted into graphene, and subsequently C60, under UV irradiation from massive stars. This shows that alternative—top-down—routes are key to understanding the organic inventory in space.

Evaporating very small grains as tracers of the UV radiation field in photo-dissociation regions

Context. In photo-dissociation regions (PDRs), polycyclic aromatic hydrocarbons (PAHs) may be produced by evaporation of very small grains (VSGs) by the impinging UV radiation field from a nearby star. Aims. We quantitatively investigate the transition zone between evaporating VSGs (eVSGs) and PAHs in several PDRs. Methods. We studied the relative contribution of PAHs and eVSGs to the mid-IR emission in a wide range of excitation conditions. We fitted the observed mid-IR emission of PDRs by using a set of template band emission spectra of PAHs, eVSGs, and gas lines. The fitting tool PAHTAT (PAH Toulouse Astronomical Templates) is made available to the community as an IDL routine. From the results of the fit, we derived the fraction of carbon feVSG locked in eVSGs and compared it to the intensity of the local UV radiation field. Results. We show a clear decrease of feVSG with increasing intensity of the local UV radiation field, which supports the scenario of photo-destruction of eVSGs. Conversely, this dependence can be used to quantify the intensity of the UV radiation field for different PDRs, including unresolved ones. Conclusions. PAHTAT can be used to trace the intensity of the local UV radiation field in regions where eVSGs evaporate, which correspond to relatively dense (nH = [100, 10 5 ]c m −3 ) and UV irradiated PDRs (G0 = [100, 5 × 10 4 ]) where H2 emits in rotational

The complete far-infrared and submillimeter spectrum of the Class 0 protostar Serpens SMM1 obtained with Herschel. Characterizing UV-irradiated shocks heating and chemistry

We present the first complete ∼55―671 μm spectral scan of a low-mass Class 0 protostar (Serpens SMM1) taken with the PACS and SPIRE spectrometers onboard Herschel. More than 145 lines have been detected, most of them rotationally excited lines of 12 CO (full ladder from J u = 4-3 to 42-41 and E u /k = 4971 K), H 2 O (up to 8 18 ―7 07 and E u /k = 1036 K), OH (up to 2 Π 1/2 J = 7/2-5/2 and E u /k = 618 K), 13 CO (up to J u = 16-15), HCN and HCO + (up to J u = 12-11). Bright [O I]63, 145 μm and weaker [C II]158 and [C I]370, 609 μm lines are also detected, but excited lines from chemically related species (NH 3 , CH + , CO + , OH + or H 2 O + ) are not. Mid-infrared spectra retrieved from the Spitzer archive are also first discussed here. The ∼10―37 μm spectrum has many fewer lines, but shows clear detections of [Ne II], [Fe II], [Si II] and [S I] fine structure lines, as well as weaker H 2 S(1) and S(2) pure rotational lines. The observed line luminosity is dominated by CO (∼54%), H 2 O (∼22%), [O I] (∼12%) and OH (∼9%) emission. A multi-component radiative transfer model allowed us to approximately quantify the contribution of the three different temperature components suggested by the 12 CO rotational ladder (T hot k ≈ 800 K, T warm k ≈ 375 K and T cool k ≈ 150 K). Gas densities n(H 2 ) ≳ 5 × 10 6 cm ―3 are needed to reproduce the observed far-IR lines arising from shocks in the inner protostellar envelope (warm and hot components) for which we derive upper limit abundances of x(CO) ≲ 10 ―4 , x(H 2 O) ≲ 0.2 x 10- 5 and x(OH) ≲ 10 ―6 with respect to H 2 . The lower energy submm 12 CO and H 2 O lines show more extended emission that we associate with the cool entrained outflow gas. Fast dissociative J-shocks (u s > 60 km s ―1 ) within an embedded atomic jet, as well as lower velocity small-scale non-dissociative shocks (υ s ≲ 20 km s ―1 ) are needed to explain both the atomic fine structure lines and the hot CO and H 2 O lines respectively. Observations also show the signature of UV radiation (weak [C II] and [C I] lines and high HCO + /HCN abundance ratios) and thus, most observed species likely arise in UV-irradiated shocks. Dissociative J-shocks produced by a jet impacting the ambient material are the most probable origin of [O I] and OH emission and of a significant fraction of the warm CO emission. In addition, H 2 O photodissociation in UV-irradiated non-dissociative shocks along the outflow cavity walls can also contribute to the [O I] and OH emission.

Spatial distribution of small hydrocarbons in the neighborhood of the ultra compact HII region Monoceros R2

Context. We study the chemistry of small hydrocarbons in the photon-dominated regions (PDRs) associated with the ultra-compact H ii region (UCH ii) Mon R2. Aims: Our goal is to determine the variations in the abundance of small hydrocarbons in a high-UV irradiated PDR and investigate the chemistry of these species. Methods: We present an observational study of the small hydrocarbons CH, CCH, and c-C3H2 in Mon R2 that combines spectral mapping data obtained with the IRAM-30 m telescope and the Herschel space observatory. We determine the column densities of these species, and compare their spatial distributions with that of polycyclic aromatic hydrocarbon (PAH), which trace the PDR. We compare the observational results with different chemical models to explore the relative importance of gas-phase, grain-surface, and time-dependent chemistry in these environments. Results: The emission of the small hydrocarbons show different spatial patterns. The CCH emission is extended, while CH and c-C3H2 are concentrated towards the more illuminated layers of the PDR. The ratio of the column densities of c-C3H2 and CCH shows spatial variations up to a factor of a few, increasing from N(c - C3H2)/N(CCH) ≈ 0.004 in the envelope to a maximum of ≈0.015 - 0.029 towards the 8 μm emission peak. Comparing these results with other galactic PDRs, we find that the abundance of CCH is quite constant over a wide range of G0, whereas the abundance of c-C3H2 is higher in low-UV PDRs, with the N(c - C3H2)/N(CCH) ratio ranging ≈0.008-0.08 from high to low UV PDRs. In Mon R2, the gas-phase steady-state chemistry can account relatively well for the abundances of CH and CCH in the most exposed layers of the PDR, but falls short by a factor of 10 of reproducing c-C3H2. In the low-density molecular envelope, time-dependent effects and grain surface chemistry play dominant roles in determining the hydrocarbon abundances. Conclusions: Our study shows that the small hydrocarbons CCH and c-C3H2 present a complex chemistry in which UV photons, grain-surface chemistry, and time dependent effects contribute to determining their abundances. Each of these effects may be dominant depending on the local physical conditions, and the superposition of different regions along the line of sight leads to the variety of measured abundances. Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.Based on observations carried out with the IRAM 30 m Telescope. IRAM is supported by INSU/CNRS (France), MPG (Germany) and IGN (Spain).Appendix B is available in electronic form at http://www.aanda.org

Probing the role of polycyclic aromatic hydrocarbons in the photoelectric heating within photodissociation regions

Aims. We observationally investigate the relation between the photoelectric heating efficiency in photodissociation regions (PDRs) and the charge of polycyclic aromatic hydrocarbons (PAHs), which are considered to play a key role in photoelectric heating. Methods. Using PACS onboard Herschel, we observed six PDRs spanning a wide range of far-ultraviolet radiation fields (G0 = 100−10 5 ). To measure the photoelectric heating efficiency, we obtained the intensities of the main cooling lines in these PDRs, i.e., the [O i ]6 3μm, 145 μm, and [C ii] 158 μm, as well as the far-infrared (FIR) continuum intensity. We used Spitzer/IRS spectroscopic mapping observations to investigate the mid-infrared (MIR; 5.5−14 μm) PAH features in the same regions. We decomposed the MIR PAH emission into that of neutral (PAH 0 ) and positively ionized (PAH + ) species to derive the fraction of the positively charged PAHs

Herschel/HIFI observations of CO, H2O and NH3 in Monoceros R2

Context. Mon R2, at a distance of 830 pc, is the only ultracompact Hii region (UCHii) where the associated photon-dominated region (PDR) can be resolved with Herschel. Owing to its brightness and proximity, it is one of the best-suited sources for investigating the chemistry and physics of highly UV-irradiated PDRs. Aims. Our goal is to estimate the abundance of H2 Oa nd NH 3 in this region and investigate their origin. Methods. We present new observations ([Cii], 12 CO, 13 CO, C 18 O, o-H2O, p-H2O, o-H 18 O and o-NH3) obtained with the HIFI instrument onboard Herschel and the IRAM-30 m telescope. We investigated the physical conditions in which these lines arise by analyzing their velocity structure and spatial variations. Using a large velocity gradient approach, we modeled the line intensities and derived an average abundance of H2 Oa nd NH 3 across the region. Finally, we modeled the line profiles with a non-local radiative transfer model and compared these results with the abundance predicted by the Meudon PDR code. Results. The variations of the line profiles and intensities indicate complex geometrical and kinematical patterns. In several tracers ([Cii], CO 9 → 8a nd H 2O) the line profiles vary significantly with position and have broader line widths toward the Hii region. The H2O lines present strong self-absorption at the ambient velocity and emission in high-velocity wings toward the Hii region. The emission in the o-H 18 O ground state line reaches its maximum value around the Hii region, has smaller linewidths and peaks at the velocity of the ambient cloud. Its spatial distribution shows that the o-H 18 O emission arises in the PDR surrounding the Hii region. By modeling the o-H 18 O emission and assuming the standard [ 16 O]/[ 18 O] = 500, we derive a mean abundance of o-H2 Oo f∼10 −8 relative to H2. The ortho-H2O abundance, however, is larger (∼1 × 10 −7 ) in the high-velocity wings detected toward the Hii region. Possible explanations for this larger abundance include an expanding hot PDR and/or an outflow. Ammonia seems to be present only in the envelope of the core with an average abundance of ∼2 × 10 −9 relative to H2. Conclusions. The Meudon PDR code, which includes only gas-phase chemical networks, can account for the measured water abundance in the high velocity gas as long as we assume that it originates from a 1 mag hot expanding layer of the PDR, i.e. that the outflow has only a minor contribution to this emission. To explain the water and ammonia abundances in the rest of the cloud, the molecular freeze out and grain surface chemistry would need to be included.

CO rotational line emission from a dense knot in Cassiopeia A Evidence for active post-reverse-shock chemistry

We report a Herschel detection of high-J rotational CO lines from a dense knot in the supernova remnant Cas A. Based on a combined analysis of these rotational lines and previously observed ro-vibrational CO lines, we find the gas to be warm (two components at ~400 and 2000 K) and dense (1e6−7 cm-3), with a CO column density of ~5e17 cm-2. This, along with the broad line widths (~400 km/s), suggests that the CO emission originates in the post-shock region of the reverse shock. As the passage of the reverse shock dissociates any existing molecules, the CO has most likely reformed in the past several years in the post-shock gas. The CO cooling time is similar to the CO formation time, therefore we discuss possible heating sources (UV photons from the shock front, X-rays, electron conduction) that may maintain the high column density of warm CO.

Deuteration around the ultracompact HII region Monoceros R2

Context. The massive star-forming region Monoceros R2 (Mon R2) hosts the closest ultra-compact Hii region, where the photondominated region (PDR) between the ionized and molecular gas can be spatially resolved with current single-dish telescopes. Aims. We aim at studying the chemistry of deuterated molecules toward Mon R2 to determine the deuterium fractions around a highUV irradiated PDR and investigate the chemistry of these species. Methods. We used the IRAM-30 m telescope to carry out an unbiased spectral survey toward two important positions (namely IF and MP2) in Mon R2 at 1, 2, and 3 mm. This spectral survey is the observational basis of our study of the deuteration in this massive starforming region. Our high spectral resolution observations (∼0.25–0.65 km s −1 ) allowed us to resolve the line profiles of the different species detected. Results. We found a rich chemistry of deuterated species at both positions of Mon R2, with detections of C2D, DCN, DNC, DCO + , D2CO, HDCO, NH2D, and N2D + and their corresponding hydrogenated species and rarer isotopologs. The high spectral resolution of our observations allowed us to resolve three velocity components: the component at 10 km s −1 is detected at both positions and seems associated with the layer most exposed to the UV radiation from IRS 1; the component at 12 km s −1 is found toward the IF position and seems related to the foreground molecular gas; finally, a component at 8.5 km s −1 is only detected toward the MP2 position, most likely related to a low-UV irradiated PDR. We derived the column density of the deuterated species (together with their hydrogenated counterparts), and determined the deuterium fractions as Dfrac = [XD]/[XH]. The values of Dfrac are around 0.01 for all the observed species, except for HCO + and N2H + , which have values 10 times lower. The values found in Mon R2 are similar to those measured in the Orion Bar, and are well explained with a pseudo-time-dependent gas-phase model in which deuteration occurs mainly via ion-molecule reactions with H2D + ,C H 2D + and C2HD + . Finally, the [H 13 CN]/[HN 13 C] ratio is very high (∼11) for the 10 km s −1 component, which also agree with our model predictions for an age of ∼0.01 to a few 0.1 Myr. Conclusions. The deuterium chemistry is a good tool for studying the low-mass and high-mass star-forming regions. However, while low-mass star-forming regions seem well characterized with Dfrac(N2H + )o rDfrac(HCO + ), a more complete chemical modeling is required to date massive star-forming regions. This is due to the higher gas temperature together with the rapid evolution of massive protostars.

Physical structure of the photodissociation regions in NGC 7023 - Observations of gas and dust emission with Herschel

Context. The determination of the physical conditions in molecular clouds is a key step towards our understanding of their formation and evolution of associated star formation. We investigate the density, temperature, and column density of both dust and gas in the photodissociation regions (PDRs) located at the interface between the atomic and cold molecular gas of the NGC 7023 reflection nebula. We study how young stars affect the gas and dust in their environment. Aims. Several Herschel Space Telescope programs provide a wealth of spatial and spectral information of dust and gas in the heart of PDRs. We focus our study on Spectral and Photometric Image Receiver (SPIRE) Fourier-Transform Spectrometer (FTS) fully sampled maps that allow us for the first time to study the bulk of cool/warm dust and warm molecular gas (CO) together. In particular, we investigate if these populations spatially coincide, if and how the medium is structured, and if strong density and temperature gradients occur, within the limits of the spatial resolution obtained with Herschel. Methods. The SPIRE FTS fully sampled maps at different wavelengths are analysed towards the northwest (NW) and the east (E) PDRs in NGC 7023. We study the spatial and spectral energy distribution of a wealth of intermediate rotational (CO)-C-12 4 \textless= J(u) \textless= 13 and (CO)-C-13 5 \textless= J(u) \textless= 10 lines. A radiative transfer code is used to assess the gas kinetic temperature, density, and column density at different positions in the cloud. The dust continuum emission including Spitzer, the Photoconductor Array Camera and Spectrometer (PACS), and SPIRE photometric and the Institute for Radio Astronomy in the Millimeter Range (IRAM) telescope data is also analysed. Using a single modified black body and a radiative transfer model, we derive the dust temperature, density, and column density. Results. The cloud is highly inhomogeneous, containing several irradiated dense structures. Excited (CO)-C-12 and (CO)-C-13 lines and warm dust grains localised at the edge of the dense structures reveal high column densities of warm/cool dense matter. Both tracers give a good agreement in the local density, column density, and physical extent, leading to the conclusion that they trace the same regions. The derived density profiles show a steep gradient at the cloud edge reaching a maximum gas density of 10(5) -10(6) cm(-3) in the PDR NGC 7023 NW and 10(4)-10(5) cm(-3) in the PDR NGC 7023 E and a subsequent decrease inside the cloud. Close to the PDR edges, the dust temperature (30 K and 20 K for the NW and E PDRs, respectively) is lower than the gas temperature derived from CO lines (65-130 K and 45-55 K, respectively). Further inside the cloud, the dust and gas temperatures are similar. The derived thermal pressure is about 10 times higher in NGC 7023 NW than in NGC 7023 E. Comparing the physical conditions to the positions of known young stellar object candidates in NGC 7023 NW, we find that protostars seem to be spatially correlated with the dense structures. Conclusions. Our approach combining both dust and gas delivers strong constraints on the physical conditions of the PDRs. We find dense and warm molecular gas of high column density in the PDRs.

C60 in Photodissociation Regions

Recent studies have confirmed the presence of buckminsterfullerene (C