Spatially resolved images of reactive ions in the Orion Bar

Abstract

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.

Constraining physical conditions for the PDR of Trumpler 14 in the Carina Nebula

“…The thermal pressure derived in this work is of the order of magnitude 10 8 K cm −3 across our FOV, with the highest values found at the dissociation front in the Car I-E region. Although the spatial resolution of our observations prevents us from obtaining a detailed cloud structure at the dissociation front, our results are in general agreement with the recent ALMA observations of the Orion Bar that the observed cloud edge is compressed by a highpressure (2 × 10 8 K cm −3 ) wave propagating into the molecular cloud (Goicoechea et al 2016;Goicoechea et al 2017). 5.…”

“…Based on the non spatially-resolved CO observations from J = 4 − 3 up to J = 19 − 18 in NGC 7023 and up to J = 23 − 22 in the Orion Bar, as well as other atomic and molecular lines, and their comparisons with the Meudon PDR code (Le Petit et al 2006), it is suggested that the FUV radiation field produced by the Trapezium stars cluster and HD 200775 are responsible for a high pressure layer (P ∼10 8 K cm −3 ) at the very edge of the two PDRs and is sufficient to support the high-J CO excitation observed in the two objects (Joblin et al 2018). These results are in resonance with the recent ALMA observations which confirm the presence of a thin layer with high pressure at the edge of the PDR in the Orion Bar (Goicoechea et al 2016;Goicoechea et al 2017). More studies as such with detailed PDR modeling are necessary for understanding the excitation of CO in the interstellar environment.…”

Formation of interstellar SH+from vibrationally excited H2: Quantum study of S++ H2⇄ SH++ H reaction and inelastic collision

“…The new models yield column densities that are a factor of four higher than those obtained with the previous formation and destruction rate constants (Zanchet et al 2013a). The new fractional abundances of SH + are within the uncertainties of the SH + column densities inferred from observations (Goicoechea et al 2017).…”

Radiative and mechanical feedback into the molecular gas in the Large Magellanic Cloud

“…(e.g., Goicoechea et al 2016Goicoechea et al , 2017. Photoevaporation by strong UV radiation fields from young stars is considered to play a critical role in maintaining such high pressure at the edge of PDRs (e.g., Bron et al 2018).…”