Spatial distribution of organics in the Horsehead nebula: signposts of chemistry driven by atomic carbon

TL;DR

This study maps simple and complex organic molecules at the UV-illuminated edge of the Horsehead nebula using ALMA-7m+30m and IRAM 30m data, deriving detailed gas-density, temperature, and column-density profiles. Radiative transfer modeling and a C^{17}O-based density calibration reveal a steep density gradient and high thermal pressure consistent with a compressed, isobaric PDR. The observed abundance patterns—O- and N-bearing COMs peaking at the PDR, with CH3OH and HC3N peaking deeper—support a scenario in which atomic C diffusion on grain surfaces under FUV irradiation drives key grain-surface formation pathways for CH2CO, CH3CHO, HNCO, and CH3CN, while CH3OH and HC3N are less enhanced. These results have broader implications for UV-irradiated environments, including protoplanetary disks, and motivate refined chemical modeling and higher-resolution observations to assess the generality of these pathways.

Abstract

(Abridged) Complex organic molecules (COMs) are considered essential precursors to prebiotic species. While COMs were once expected to be efficiently destroyed under UV-irradiated conditions, detections in photodissociation regions (PDRs) have challenged this view. However, the mechanisms by which UV radiation contributes to their formation are still uncertain. Here, we present moderately resolved maps of simple and complex organic molecules at the UV-illuminated edge of the Horsehead nebula, obtained by combining ALMA and IRAM 30m single-dish observations at $\sim 15^{\prime\prime}$ resolution. We analyze the spatial distribution of species such as C$^{17}$O, CH$_2$CO, CH$_3$CHO, HNCO, CH$_3$CN, and HC$_3$N. By incorporating previous C$^{17}$O and C$^{18}$O single-dish data as well as PdBI maps of H$_2$CO and CH$_3$OH, we derive profiles of gas density, temperature, thermal pressure, and column densities of the organic species as a function of distance from the UV source. Our results show that most organic species$-$particularly H$_2$CO, CH$_2$CO, CH$_3$CHO, HNCO, and CH$_3$CN$-$exhibit enhanced column densities at the UV-illuminated edge compared to cloud interiors, possibly indicating efficient dust-grain surface chemistry driven by the diffusion of atomic C and radicals produced via photodissociation of CO and CH$_3$OH, as supported by recent laboratory experiments. The exceptions, HC$_3$N and CH$_3$OH, can be attributed to inefficient formation on dust grains and ineffective non-thermal desorption into the gas phase, respectively. Additionally, contributions from gas-phase hydrocarbon photochemistry$-$possibly seeded by grain-surface products$-$cannot be ruled out. Further chemical modeling is needed to confirm the efficiency of these pathways for the studied species, which could have important implications for other cold, UV-irradiated environments such as protoplanetary disks.

Figures (10)

• Figure 1: Composite image of the field-of-view mapped in the Horsehead nebula (Barnard 33). Left: The Horsehead nebula and the adjacent H II region IC 434 imaged by Euclid Early Release Observations (ESA/Euclid/Euclid Consortium/NASA). Right: Zoom of the edge of Horsehead nebula imaged with data combined from the ALMA-7m and the IRAM 30m telescopes. The emission of different N-bearing molecules, such as CH$_3$CN ($J_{K} = 6_{0}-5_{0}$, in green) and HC$_3$N ($J = 11-10$, in blue), are represented by colors. The emission from hot ionized gas is also shown, traced by the H$\alpha$ line (in red) observed with the $0.9$ m KPNO telescope Pound2003. The average beam size of the ALMA-7m+30m observations and a physical scale reference are shown in the bottom left and bottom right corner, respectively.
• Figure 2: Zeroth-moment maps gallery for different molecular species moderately resolved at the Horsehead edge. Maps have been rotated $14^\circ$ counterclockwise to bring the illuminating star direction in the horizontal direction. Contours are $[3, 5, 10, 15, 20, 25, 30]\times\sigma$, where $\sigma$ is the zeroth-moment rms listed in Table \ref{['table:obs-params']}. The red vertical line represents the horizontal zero, delineating the PDR edge Pety2005, whereas the cyan and green crosses show the dense core Pety2007 and PDR Gerin2009 positions, respectively. The beam size and a scale bar indicating $4000$ au are shown in the bottom left and bottom right corner, respectively, of each panel. Transitions corresponding to the same molecular species are shown using the same color-scale range.
• Figure 3: Left column: Integrated line intensity profiles along the direction of the exciting star extracted from the maps shown in Fig. \ref{['fig:hh-coms-grid']} and the convolved maps obtained from Guzman2013. For molecules with more than one transition, the brightest is displayed. The profiles were extracted at the $\delta y$ position of the PDR (top panel) and dense core (bottom panel), and averaged over $10^{\prime\prime}$ in the $\delta y$ direction. The dotted vertical lines represent the $\delta x$ position of the PDR and dense core, and the vertical red line represents the PDR edge. The average beam size is represented by the horizontal gray bar. The colored area of each profile displays the $\pm \sigma$ significance levels, taking into account the standard deviation of the average and the zeroth-moment rms. Right column: Regions used to extract the profiles, indicated by white rectangles overlaid on the HNCO zeroth-moment map. The two crosses, the red vertical line, and the contours are the same as in Fig. \ref{['fig:hh-coms-grid']}.
• Figure 4: Empirical gas density (top panel), kinetic temperature (middle panel), and thermal pressure (bottom panel) profiles determined from C$^{17}$O and CH$_3$CN observations of the Horsehead edge, extracted at the $\delta y$ position of the PDR (blue circles) and dense core (orange circles). The beam sizes of the respective molecular tracers from which the values were derived are represented by the horizontal gray bar in the top left corner of each panel. The red vertical line and the dotted vertical lines are the same as in the left panels of Fig. \ref{['fig:hh-coms-profiles']}. For comparison, the gas density profile modeled by Habart2005 convolved with a Gaussian of 15$^{\prime\prime}$ FWHM (green line) and the thermal pressure range from Hernandez-Vera2023 (purple-shaded region) are also shown.
• Figure 5: Column density profiles derived at the $\delta y$ position of the PDR from the radiative transfer modeling of the O-bearing (left panel) and N-bearing (right panel) molecules analyzed in this work. For each color, the solid line depicts the best-fit values, whose uncertainties are represented by the shaded areas of the same color. The red vertical line, the dotted vertical lines, and the horizontal gray bar are the same as in the left panels of Fig. \ref{['fig:hh-coms-profiles']}. The same profiles but at the $\delta y$ position of the dense core are shown in Fig. \ref{['fig:hh-logN-profiles-CORE']}.