Carbon radio recombination lines from gigahertz to megahertz frequencies towards Orion A

Salas, P.; Oonk, J. B. R.; Emig, K. L.; Pabst, C. H. M.; Toribio, M. C.; Röttgering, H. J. A.; Tielens, A. G. G. M.

doi:10.1051/0004-6361/201834532

Cited by 23 publications

(27 citation statements)

References 108 publications

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“…Indeed, hydrogen recombination lines from fully ionized H II regions display much broader profiles (∆ v >15 km s −1 for T e > 5000 K; Churchwell et al 1978). However, carbon recombination lines from the surface and edges of OMC-1, detected at radio (Natta et al 1994;Salas et al 2019) and millimeter wavelengths (Cuadrado et al 2019), show narrow profiles ∆ v = 2.5-5 km s −1 . These are typical of the neutral photodissociation region (PDR) that separates the hot H II gas from the cold molecular gas.…”

Section: Resultsmentioning

confidence: 99%

Molecular globules in the Veil bubble of Orion

Goicoechea

Pabst

Kabanovic

et al. 2020

A&A

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Strong winds and ultraviolet (UV) radiation from O-type stars disrupt and ionize their molecular core birthplaces, sweeping up material into parsec-size shells. Owing to dissociation by starlight, the thinnest shells are expected to host low molecular abundances and therefore little star formation. Here, we expand previous maps made with observations using the IRAM 30 m telescope (at 11″ ≃ 4500 AU resolution) and present square-degree 12CO and 13CO (J = 2–1) maps of the wind-driven “Veil bubble” that surrounds the Trapezium cluster and its natal Orion molecular core (OMC). Although widespread and extended CO emission is largely absent from the Veil, we show that several CO “globules” exist that are blueshifted in velocity with respect to OMC and are embedded in the [C II] 158 μm-bright shell that confines the bubble. This includes the first detection of quiescent CO at negative local standard of rest velocities in Orion. Given the harsh UV irradiation conditions in this translucent material, the detection of CO globules is surprising. These globules are small (Rg = 7100 AU), not massive (Mg = 0.3 M⊙), and are moderately dense: nH = 4 × 104 cm−3 (median values). They are confined by the external pressure of the shell, Pext∕k ≳ 107 cm−3 K, and are likely magnetically supported. They are either transient objects formed by instabilities or have detached from pre-existing molecular structures, sculpted by the passing shock associated with the expanding shell and by UV radiation from the Trapezium. Some represent the first stages in the formation of small pillars, others of isolated small globules. Although their masses (Mg <MJeans) do not suggest they will form stars, one globule matches the position of a known young stellar object. The lack of extended CO in the “Veil shell” demonstrates that feedback from massive stars expels, agitates, and reprocesses most of the disrupted molecular cloud gas, thereby limiting the star-formation rate in the region. The presence of molecular globules is a result of this feedback.

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Section: Resultsmentioning

confidence: 99%

Molecular globules in the Veil bubble of Orion

Goicoechea

Pabst

Kabanovic

et al. 2020

A&A

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“…Having spatial distributions, line widths, and radial velocities consistent with an origin in the neutral gas close to the C + /C/CO transition layer, CRRLs are particularly useful tools for probing the physical conditions and kinematics of these regions (Hoang-Binh & Walmsley 1974;Natta et al 1994;Salas et al 2019). Since the properties derived from the observed CRRL line profiles reflect the physical conditions of the PDR, coupling our observations of o-CH 2 with ancillary CRRL data will help us to constrain the origins of the observed o-CH 2 emission.…”

Section: Comparison With Carbon Radio Recombination Linesmentioning

confidence: 99%

Hunting for the elusive methylene radical

Jacob

Menten

Gong

et al. 2021

A&A

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Context. The NKaKc = 404−313 transitions of ortho-CH2 between 68 and 71 GHz were first detected toward the Orion-KL and W51 Main star-forming regions. Given their high upper level energies (225 K) above the ground state, they were naturally thought to arise in dense, hot molecular cores near newly formed stars. However, this has not been confirmed by further observations of these lines and their origin has remained unclear. Generally, there is a scarcity of observational data for CH2 and, while it is an important compound in the astrochemical context, its actual occurrence in astronomical sources is poorly constrained. Aims. In this work, we aim to investigate the nature of the elusive CH2 emission, address its association with hot cores, and examine alternative possibilities for its origin. Owing to its importance in carbon chemistry, we also extend the search for CH2 lines by observing an assortment of regions, guided by the hypothesis that the observed CH2 emission is likely to arise from the hot gas environment of photodissociation regions (PDRs). Methods. We carried out our observations first using the Kitt Peak 12 m telescope to verify the original detection of CH2 toward different positions in the central region of the Orion Molecular Cloud 1. These were followed-up by deep integrations using the higher angular resolution of the Onsala 20 m telescope. We also searched for the NKaKc = 212−303 transitions of para-CH2 between 440–445 GHz toward the Orion giant molecular cloud complex using the APEX 12 m telescope. We also obtained auxiliary data for carbon recombination lines with the Effelsberg 100 m telescope and employing archival far infrared data. Results. The present study, along with other recent observations of the Orion region reported here, rule out the possibility of an association with gas that is both hot and dense. We find that the distribution of the CH2 emission closely follows that of the [CII] 158 μm emission, while CH2 is undetected toward the hot core itself. The observations suggest, rather, that its extended emission arises from hot but dilute layers of PDRs and not from the denser parts of such regions as in the case of the Orion Bar. This hypothesis was corroborated by comparisons of the observed CH2 line profiles with those of carbon radio recombination lines (CRRLs), which are well-known PDR tracers. In addition, we report the detection of the 70 GHz fine- and hyperfine structure components of ortho-CH2 toward the W51 E, W51 M, W51 N, W49 N, W43, W75 N, DR21, and S140 star-forming regions, and three of the NKaKc = 404−313 fine- and hyperfine structure transitions between 68–71 GHz toward W3 IRS5. While we have no information on the spatial distribution of CH2 in these regions, aside from that in W51, we again see a correspondence between the profiles of CH2 lines and those of CRRLs. We see a stronger CH2 emission toward the extended HII region W51 M rather than toward the much more massive and denser W51 E and N regions, which strongly supports the origin of CH2 in extended dilute gas. We also report the non-detection of the 212−303 transitions of para-CH2 toward Orion. Furthermore, using a non-LTE radiative transfer analysis, we can constrain the gas temperatures and H2 density to (163 ± 26) K and (3.4 ± 0.3) × 103 cm−3, respectively, for the 68–71 GHz ortho-CH2 transitions toward W3 IRS5, for which we have a data set of the highest quality. This analysis confirms our hypothesis that CH2 originates inwarm and dilute PDR layers. Our analysis suggests that for the excitation conditions under the physical conditions that prevail in such an environment, these lines are masering, with weak level inversion. The resulting amplification of the lines’ spontaneousemission greatly aids in their detection.

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“…where χ n =15.79•10 4 n −2 T −1 e and EM S + = n e n S + L (in pc cm −6 ) is the S + emission measure along a slab of thickness L (along the line-of-sight; L should not be confused with the cloud depth in the UV-illumination direction). The factor 15 is specific to 54α RRLs and includes the oscillator strength ∆nM(∆n) 0.1908 for ∆n = 1 (Brocklehurst & Seaton 1971;Dupree 1974;Salas et al 2019). When excitation conditions are close to LTE and as the principal quantum number n increases, the departure coefficients approach b n 1.…”

Section: Appendix A: Absolute Intensities Of Sulfur Rrlsmentioning

confidence: 99%

“…In the first layers of a PDR, the dominant state of elements with ionization potential (IP) below H is singly ionized: C + (11.3 eV), S + (10.4 eV), Si + (8.2 eV), Fe + (7.9 eV), or Mg + (7.6 eV). Indeed, the narrower carbon RRLs probe these neutral PDRs adjacent to H ii regions (Ball et al 1970;Natta et al 1994;Wyrowski et al 1997;Salas et al 2019;Cuadrado et al 2019). Carbon RRLs have historically been detected in the centimeter (cm) domain: ∼8.6 GHz for the C91α line (where α stands for ∆n = 1 transitions).…”

Section: Introductionmentioning

confidence: 99%

“…Compared to the fine-structure 2 P 3/2 -2 P 1/2 line of C + , the very important far-infrared [C ii] 158 µm cooling line (Hollenbach & Tielens 1999), carbon RRLs are optically thin and their intensity is proportional to n 2 e T −1.5 e . That is, they have different excitation properties than the [C ii] 158 µm line (e.g., Natta et al 1994;Salas et al 2019).…”

Section: Introductionmentioning

confidence: 99%

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The initial gas-phase sulfur abundance in the Orion Molecular Cloud from sulfur radio recombination lines

Goicoechea

Cuadrado

2021

A&A

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The abundances of chemical elements and their depletion factors are essential parameters for understanding the composition of the gas and dust that are ultimately incorporated into stars and planets. Sulfur is an abundant but peculiar element in the sense that, despite being less volatile than other elements (e.g., carbon), it is not a major constituent of dust grains in diffuse interstellar clouds. Here, we determine the gas-phase carbon-to-sulfur abundance ratio, [C] / [S], and the [S] in a dense star-forming cloud from new radio recombination lines (RRLs) detected with the Yebes 40m telescope-at relatively high frequencies (∼ 40 GHz 7 mm) and angular resolutions (down to 36)-in the Orion Bar, a rim of the Orion Molecular Cloud (OMC). We detect nine Cnα RRLs (with n = 51 to 59) as well as nine narrow line features separated from the Cnα lines by δv = −8.4 ± 0.3 km s −1. Based on this velocity separation, we assign these features to sulfur RRLs, with little contribution of RRLs from the more condensable elements Mg, Si, or Fe. Sulfur RRLs lines trace the photodissociation region (PDR) of the OMC. In these neutral gas layers, up to A V 4, the ions C + and S + lock in most of the C and S gas-phase reservoir. We determine a relative abundance of [C] Ori / [S] Ori = 10.4 ± 0.6 and, adopting the same [C] Ori measured in the translucent gas toward star θ 1 Ori B, an absolute abundance of [S] Ori = (1.4 ± 0.4)•10 −5. This value is consistent with emission models of the observed sulfur RRLs if N(S +) 7•10 17 cm −2 (beam-averaged). The [S] Ori is the "initial" sulfur abundance in the OMC, before an undetermined fraction of the [S] Ori goes into molecules and ice mantles in the cloud interior. The inferred abundance [S] Ori matches the solar abundance, thus implying that there is little depletion of sulfur onto rocky dust grains, with D(S) = 0.0 ± 0.2 dex.

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Carbon radio recombination lines from gigahertz to megahertz frequencies towards Orion A

Cited by 23 publications

References 108 publications

Molecular globules in the Veil bubble of Orion

Molecular globules in the Veil bubble of Orion

Hunting for the elusive methylene radical

The initial gas-phase sulfur abundance in the Orion Molecular Cloud from sulfur radio recombination lines

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