We present the first ~7.5'×11.5' velocity-resolved (~0.2 km s) map of the [C ii] 158 m line toward the Orion molecular cloud 1 (OMC 1) taken with the/HIFI instrument. In combination with far-infrared (FIR) photometric images and velocity-resolved maps of the H41 hydrogen recombination and CO =2-1 lines, this data set provides an unprecedented view of the intricate small-scale kinematics of the ionized/PDR/molecular gas interfaces and of the radiative feedback from massive stars. The main contribution to the [C ii] luminosity (~85 %) is from the extended, FUV-illuminated face of the cloud (>500, >5×10 cm) and from dense PDRs (≳10, ≳10 cm) at the interface between OMC 1 and the H ii region surrounding the Trapezium cluster. Around ~15 % of the [C ii] emission arises from a different gas component without CO counterpart. The [C ii] excitation, PDR gas turbulence, line opacity (from [C ii]) and role of the geometry of the illuminating stars with respect to the cloud are investigated. We construct maps of the [C ii]/ and / ratios and show that [C ii]/ decreases from the extended cloud component (~10-10) to the more opaque star-forming cores (~10-10). The lowest values are reminiscent of the "[C ii] deficit" seen in local ultra-luminous IR galaxies hosting vigorous star formation. Spatial correlation analysis shows that the decreasing [C ii]/ ratio correlates better with the column density of dust through the molecular cloud than with /. We conclude that the [C ii] emitting column relative to the total dust column along each line of sight is responsible for the observed [C ii]/ variations through the cloud.

The Orion Bar is the archetypal edge-on molecular cloud surface illuminated by strong ultraviolet radiation from nearby massive stars. Owing to the close distance to Orion (about 1,350 light-year), the effects of stellar feedback on the parental cloud can be studied in detail. Visible-light observations of the Bar1 show that the transition between the hot ionised gas and the warm neutral atomic gas (the ionisation front) is spatially well separated from the transition from atomic to molecular gas (the dissociation front): about 15 arcseconds or 6,200 astronomical units. (One astronomical unit is the Earth-Sun distance.) Static equilibrium models2,3 used to interpret previous far-infrared and radio observations of the neutral gas in the Bar4,5,6 (typically at 10-20 arcsecond resolution) predict an inhomogeneous cloud structure consisting of dense clumps embedded in a lower density extended gas component. Here we report 1 arcsecond resolution millimetre-wave images that allow us to resolve the molecular cloud surface and constrain the gas density and temperature structures at small spatial scales. In contrast to stationary model predictions7,8,9, there is no appreciable offset between the peak of the H2 vibrational emission (delineating the H/H2 transition) and the edge of the observed CO and HCO+ emission. This implies that the H/H2 and C+/C/CO transition zones are very close. These observations reveal a fragmented ridge of high-density substructures, photo-ablative gas flows and instabilities at the molecular cloud surface. They suggest that the cloud edge has been compressed by a high-pressure wave that currently moves into the molecular cloud. The images demonstrate that dynamical and nonequilibrium effects are important. Thus, they should be included in any realistic description of irradiated interstellar matter.

Context. Carbon chemistry plays a pivotal role in the interstellar medium (ISM) but even the synthesis of the simplest hydrocarbons and how they relate to polycyclic aromatic hydrocarbons (PAHs) and grains is not well understood. Aims. We study the spatial distribution and chemistry of small hydrocarbons in the Orion Bar photodissociation region (PDR), a prototypical environment in which to investigate molecular gas irradiated by strong UV fields. Methods. We used the IRAM 30 m telescope to carry out a millimetre line survey towards the Orion Bar edge, complemented with ∼2 × 2 maps of the C 2 H and c-C 3 H 2 emission. We analyse the excitation of the detected hydrocarbons and constrain the physical conditions of the emitting regions with non-LTE radiative transfer models. We compare the inferred column densities with updated gas-phase photochemical models including 13 CCH and C 13 CH isotopomer fractionation.Results. Approximately 40% of the lines in the survey arise from hydrocarbons (C 2 H, C 4 H, c-C 3 H 2 , c-C 3 H, C 13 CH, 13 CCH, l-C 3 H, and l-H 2 C 3 in decreasing order of abundance). We detect new lines from l-C 3 H + and improve its rotational spectroscopic constants. Anions or deuterated hydrocarbons are not detected, but we provide accurate upper limit abundances:Conclusions. Our models can reasonably match the observed column densities of most hydrocarbons (within factors of <3). Since the observed spatial distribution of the C 2 H and c-C 3 H 2 emission is similar but does not follow the PAH emission, we conclude that, in high UV-flux PDRs, photodestruction of PAHs is not a necessary requirement to explain the observed abundances of the smallest hydrocarbons. Instead, gas-phase endothermic reactions (or with barriers) between C + , radicals, and H 2 enhance the formation of simple hydrocarbons. Observations and models suggest that the [C 2 H]/[c-C 3 H 2 ] ratio (∼32 at the PDR edge) decreases with the UV field attenuation. The observed low cyclic-to-linear C 3 H column density ratio (≤3) is consistent with a high electron abundance (x e ) PDR environment. In fact, the poorly constrained x e gradient influences much of the hydrocarbon chemistry in the more UV-shielded gas. The inferred hot rotational temperatures for C 4 H and l-C 3 H + also suggest that radiative IR pumping affects their excitation. We propose that reactions of C 2 H isotopologues with 13 C + and H atoms can explain the observed [C 13 CH]/[ 13 CCH] = 1.4 ± 0.1 fractionation level.

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We report high angular resolution (4.9 × 3.0 ) images of reactive ions SH + , HOC + , and SO + toward the Orion Bar photodissociation region (PDR). We used ALMA-ACA to map several rotational lines at 0.8 mm, complemented with multi-line observations obtained with the IRAM 30 m telescope. The SH + and HOC + emission is restricted to a narrow layer of 2 -to 10 -width (≈800 to 4000 AU depending on the assumed PDR geometry) that follows the vibrationally excited H * 2 emission. Both ions efficiently form very close to the H/H 2 transition zone, at a depth of A V 1 mag into the neutral cloud, where abundant C + , S + , and H * 2 coexist. SO + peaks slightly deeper into the cloud. The observed ions have low rotational temperatures (T rot ≈ 10−30 K T k ) and narrow line-widths (∼2−3 km s −1 ), a factor of 2 narrower that those of the lighter reactive ion CH + . This is consistent with the higher reactivity and faster radiative pumping rates of CH + compared to the heavier ions, which are driven relatively more quickly toward smaller velocity dispersion by elastic collisions and toward lower T rot by inelastic collisions. We estimate column densities and average physical conditions from an excitation model (n(H 2 ) ≈ 10 5 −10 6 cm −3 , n(e − ) ≈ 10 cm −3 , and T k ≈ 200 K). Regardless of the excitation details, SH + and HOC + clearly trace the most exposed layers of the UV-irradiated molecular cloud surface, whereas SO + arises from slightly more shielded layers.

We present ∼2 ×2 spectral-maps of Orion BN/KL outflows taken with Herschel at ∼12 resolution. For the first time in the far-IR domain, we spatially resolve the emission associated with the bright H 2 shocked regions "Peak 1" and "Peak 2" from that of the Hot Core and ambient cloud. We analyze the ∼54-310 µm spectra taken with the PACS and SPIRE spectrometers. More than 100 lines are detected, most of them rotationally excited lines of 12 CO (up to J=48-47), H 2 O, OH, 13 CO, and HCN. Peaks 1/2 are characterized by a very high L(CO)/L F IR ≈5×10 −3 ratio and a plethora of far-IR H 2 O emission lines. The high-J CO and OH lines are a factor ≈2 brighter toward Peak 1 whereas several excited H 2 O lines are 50% brighter toward Peak 2. Most of the CO column density arises from T k ∼200-500 K gas that we associate with low-velocity shocks that fail to sputter grain ice mantles and show a maximum gas-phase H 2 O/CO 10 −2 abundance ratio. In addition, the very excited CO (J>35) and H 2 O lines reveal a hotter gas component (T k ∼2500 K) from faster (v S >25 km s −1 ) shocks that are able to sputter the frozen-out H 2 O and lead to high H 2 O/CO 1 abundance ratios. The H 2 O and OH luminosities cannot be reproduced by shock models that assume high (undepleted) abundances of atomic oxygen in the preshock gas and/or neglect the presence of UV radiation in the postshock gas. Although massive outflows are a common feature in other massive star-forming cores, Orion BN/KL seems more peculiar because of its higher molecular luminosities and strong outflows caused by a recent explosive event. Subject headings: stars: protostars ISM: jets and outflows infrared: ISM shock waves jr.goicoechea@icmm.csic.es * Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.

We investigate the chemical segregation of complex O-bearing species (including the largest and most complex ones detected to date in space) towards Orion KL, the closest high-mass star-forming region. The molecular line images obtained using the ALMA science verification data reveal a clear segregation of chemically related species depending on their different functional groups. We map the emission of 13CH3OH, HCOOCH3, CH3OCH3, CH2OCH2, CH3COOCH3, HCOOCH2CH3, CH3CH2OCH3, HCOOH, OHCH2CH2OH, CH3COOH, CH3CH2OH, CH3OCH2OH, OHCH2CHO, and CH3COCH3 with ~1.5″ angular resolution and provide molecular abundances of these species toward different gas components of this region. We disentangle the emission of these species in the different Orion components by carefully selecting lines free of blending and opacity effects. Possible effects in the molecular spatial distribution due to residual blendings and different excitation conditions are also addressed. We find that while species containing the C–O–C group, i.e. an ether group, exhibit their peak emission and higher abundance towards the compact ridge, the hot core south is the component where species containing a hydroxyl group (–OH) bound to a carbon atom (C–O–H) present their emission peak and higher abundance. This finding allows us to propose methoxy (CH3O–) and hydroxymethyl (−CH2OH) radicals as the major drivers of the chemistry in the compact ridge and the hot core south, respectively, as well as different evolutionary stages and prevailing physical processes in the different Orion components.

As many organic molecules, formic acid (HCOOH) has two conformers (trans and cis). The energy barrier to internal conversion from trans to cis is much higher than the thermal energy available in molecular clouds. Thus, only the most stable conformer (trans) is expected to exist in detectable amounts. We report the first interstellar detection of cis-HCOOH. Its presence in ultraviolet (UV) irradiated gas exclusively (the Orion Bar photodissociation region), with a low trans-to-cis abundance ratio of 2.8 ± 1.0, supports a photoswitching mechanism: a given conformer absorbs a stellar photon that radiatively excites the molecule to electronic states above the interconversion barrier. Subsequent fluorescent decay leaves the molecule in a different conformer form. This mechanism, which we specifically study with ab initio quantum calculations, was not considered in Space before but likely induces structural changes of a variety of interstellar molecules submitted to UV radiation.