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Bình, Nguyễn Thanh; Hoàng, Khắc Ngọc; Hoàng, Ngọc Văn

doi:10.25073/2588-1140/vnunst.4524

Cited by 5 publications

(4 citation statements)

References 2 publications

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“…To quantify the line properties we decompose the observed spectra using Gaussian line profiles. For the averaged CRRL the broadening effects that produce Lorentzian line wings are negligible, hence the line profiles should be Gaussian (e.g., Hoang-Binh & Walmsley 1974;Salgado et al 2017b). During the fitting process we do not restrict the number of velocity components, and we determine how many to include in the fit using the Akaike information criterion (AIC, Akaike 1974).…”

Section: Rrlsmentioning

confidence: 99%

The ionization fraction in OMC-2 and OMC-3

Salas¹,

Rugel²,

Emig³

et al. 2021

A&A

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Context. The electron density (ne−) plays an important role in setting the chemistry and physics of the interstellar medium. However, measurements of ne− in neutral clouds have been directly obtained only toward a few lines of sight or they rely on indirect determinations. Aims. We use carbon radio recombination lines and the far-infrared lines of C+ to directly measure ne− and the gas temperature in the envelope of the integral shaped filament (ISF) in the Orion A molecular cloud. Methods. We observed the C102α (6109.901 MHz) and C109α (5011.420 MHz) carbon radio recombination lines (CRRLs) using the Effelsberg 100 m telescope at ≈2′ resolution toward five positions in OMC-2 and OMC-3. Since the CRRLs have similar line properties, we averaged them to increase the signal-to-noise ratio of the spectra. We compared the intensities of the averaged CRRLs, and the 158 μm-[CII] and [13CII] lines to the predictions of a homogeneous model for the C+/C interface in the envelope of a molecular cloud and from this comparison we determined the electron density, temperature and C+ column density of the gas. Results. We detect the CRRLs toward four positions, where their velocity (vLSR ≈ 11 km s−1) and widths (σv ≈ 1 km s−1) confirms that they trace the envelope of the ISF. Toward two positions we detect the CRRLs, and the 158 μm-[CII] and [13CII] lines with a signal-to-noise ratio ≥5, and we find ne− = 0.65 ± 0.12 cm−3 and 0.95 ± 0.02 cm−3, which corresponds to a gas density nH ≈ 5 × 103 cm−3 and a thermal pressure of pth ≈ 4 × 105 K cm−3. We also constrained the ionization fraction in the denser portions of the molecular cloud using the HCN(1–0) and C2H(1–0) lines to x(e−) ≤ 3 × 10−6. Conclusions. The derived electron densities and ionization fraction imply that x(e−) drops by a factor ≥100 between the C+ layer and the regions probed by HCN(1–0). This suggests that electron collisional excitation does not play a significant role in setting the excitation of HCN(1–0) toward the region studied, as it is responsible for only ≈10% of the observed emission.

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Section: Rrlsmentioning

confidence: 99%

The ionization fraction in OMC-2 and OMC-3

Salas¹,

Rugel²,

Emig³

et al. 2021

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

Preprint

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Context. The N Ka Kc = 4 04 − 3 13 transitions of ortho-CH 2 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 CH 2 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 CH 2 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 CH 2 lines by observing an assortment of regions, guided by the hypothesis that the observed CH 2 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 CH 2 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 N Ka Kc = 2 12 − 3 03 transitions of para-CH 2 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 CH 2 emission closely follows that of the [CII] 158 µm emission, while CH 2 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 CH 2 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-CH 2 toward the W51 E, W51 M, W51 N, W49 N, W43, W75 N, DR21, and S140 star-forming regions, and three of the N Ka Kc = 4 04 − 3 13 fine-and hyperfine structure transitions between 68-71 GHz toward W3 IRS5. While we have no information on the spatial distribution of CH 2 in these regions, aside from that in W51, we again see a correspondence between the profiles of CH 2 lines and those of CRRLs. We see a stronger CH 2 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...

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“…Analytic expressions for the matrix M (n, l, n , l ) that contain hypergeometric functions were derived by Gordon (1929). For the Lyα transition M (n, l, n , l ) = M (2, 1, 1, 0) = √ 6(128/243) (Hoang-Binh, 1990).…”

Section: Lyαmentioning

confidence: 99%