Submillimeter imaging of the Galactic Center starburst Sgr B2

Santa-Maria, M. G.; Goicoechea, J. R.; Etxaluze, M.; Cernicharo, J.; Cuadrado, S.

doi:10.1051/0004-6361/202040221

Cited by 10 publications

(7 citation statements)

References 145 publications

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“…Barnes et al 2020b) because it is destroyed by CO in the gas phase, and therefore becomes more abundant only when CO has frozen out onto grains (Flower et al 2005;Bergin and Tafalla 2007). However, in the CMZ, N 2 H + is empirically widely distributed and does not select for cold gas (or dust); instead, it appears that the high cosmic ray ionisation rate produces a situation in which CO reactions are not important for regulating the N 2 H + population, making it a widespread but 'normal' dense gas tracer (Santa-Maria et al 2021). Interpretation of chemical tracers in the CMZ is complicated by both the uncertainty in abundances and by excitation; Petkova et al (2021) show, with radiative transfer modeling applied to a hydrodynamic simulation of G0.253+0.016 (Dale et al 2019;Kruijssen et al 2019), that much of the observed cloud structure can be explained by varying optical depth and excitation without considering variations in abundance.…”

Section: Observations At Cloud Scalesmentioning

confidence: 99%

Star Formation in the Central Molecular Zone of the Milky Way

Henshaw¹,

Barnes²,

Battersby³

et al. 2022

Preprint

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The Central Molecular Zone (CMZ) is a ring-like accumulation of molecular gas in the innermost few hundred parsecs of the Milky Way, generated by the inward transport of matter driven by the Galactic bar. The CMZ is the most extreme star-forming environment in the Galaxy. The unique combination of large-scale dynamics and extreme interstellar medium conditions, characterised by high densities, temperatures, pressures, turbulent motions, and strong magnetic fields, make the CMZ an ideal region for testing current star and planet formation theories. We review the recent observational and theoretical advances in the field, and combine these to draw a comprehensive, multi-scale and multi-physics picture of the cycle of matter and energy in the context of star formation in the closest galactic nucleus.

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Section: Observations At Cloud Scalesmentioning

confidence: 99%

Star Formation in the Central Molecular Zone of the Milky Way

Henshaw¹,

Barnes²,

Battersby³

et al. 2022

Preprint

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“…These regions are very important when we consider the integrated emission from spatially unresolved GMCs in distant starforming galaxies. Although HCN line intensities at any specific position of the extended cloud environment would be much fainter than at the dense star-forming cores (n(H 2 ) 10 5 cm −3 ) -where χ(e − ) is usually not high enough to collisionally excite HCN lines (e.g., Salas et al 2021) -the much larger area of the extended cloud emission, the cloud envelope, means that emission lines integrated over the entire GMC can be dominated by the lower-density extended cloud and not by the dense cores (e.g., Evans et al 2020;Santa-Maria et al 2021). This widespread (tens of pc) and more translucent GMC environment is typically illuminated by modest stellar UV fields, G 0 2-100 (e.g., Pineda et al 2013;Abdullah & Tielens 2020) that are less extreme than the incident UV field in the dense starforming clumps (∼1 pc scales) close to young massive stars (up to G 0 10 5 ; e.g., Goicoechea et al 2019;Pabst et al 2021).…”

Section: Hcn Emission From the Extended Environment Of Gmcsmentioning

confidence: 99%

Anomalous HCN emission from warm giant molecular clouds

Goicoechea

Lique

Santa-Maria

2022

A&A

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Hydrogen cyanide (HCN) is considered a good tracer of the dense molecular gas that serves as fuel for star formation. However, recent large-scale surveys of giant molecular clouds (GMCs) have detected extended HCN rotational line emission far from star-forming cores. Such observations often spectroscopically resolve the HCN J = 1–0 (partially also the J = 2–1 and 3–2) hyperfine structure (HFS). A precise determination of the physical conditions of the gas requires treating the HFS line overlap effects. Here, we study the HCN HFS excitation and line emission using nonlocal radiative transfer models that include line overlaps and new HFS-resolved collisional rate coefficients for inelastic collisions of HCN with both para-H2 and ortho-H2 (computed via the scaled-infinite order sudden approximation up to Tk = 500 K). In addition, we account for the role of electron collisions in the HFS level excitation. We find that line overlap and opacity effects frequently produce anomalous HCN J = 1–0 HFS line intensity ratios (i.e., inconsistent with the common assumption of the same Tex for all HFS lines) as well as anomalous HFS line width ratios. Line overlap and electron collisions also enhance the excitation of the higher J rotational lines. Our models explain the anomalous HCN J = 1–0 HFS spectra observed in the Orion Bar and Horsehead photodissociation regions. As shown in previous studies, electron excitation becomes important for molecular gas with H2 densities below a few 105 cm−3 and electron abundances above ~10−5. We find that when electron collisions are dominant, the relative intensities of the HCN J = 1–0 HFS lines can be anomalous too. In particular, electron excitation can produce low-surface-brightness HCN emission from very extended but low-density gas in GMCs. The existence of such a widespread HCN emission component may affect the interpretation of the extragalactic relationship HCN luminosity versus star-formation rate. Alternatively, extended HCN emission may arise from dense star-forming cores and become resonantly scattered by large envelopes of lower density gas. There are two scenarios – namely, electron-assisted (weakly) collisionally excited versus scattering – that lead to different HCN J = 1–0 HFS intensity ratios, which can be tested on the basis of observations.

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“…These regions are very important when we consider the integrated emission from spatially unresolved GMCs in distant star-forming galaxies. Although HCN line intensities at any specific position of the extended cloud environment would be much fainter than at the dense star-forming cores (n(H 2 ) 10 5 cm −3 ) -where χ(e − ) is usually not high enough to collisionally excite HCN lines (e.g., Salas et al 2021) -the much larger area of the extended cloud emission, the cloud envelope, means that emission lines integrated over the entire GMC can be dominated by the lower-density extended cloud and not by the dense cores (e.g., Evans et al 2020;Santa-Maria et al 2021). This widespread (tens of pc) and more translucent GMC environment is typically illuminated by modest stellar UV fields, G 0 2-100 (e.g., Pineda et al 2013;Abdullah & Tielens 2020) that are less extreme than the incident UV field in the dense starforming clumps (∼1 pc scales) close to young massive stars (up to G 0 10 5 ; e.g., Goicoechea et al 2019;Pabst et al 2021).…”

Section: Hcn Emission From the Extended Environment Of Gmcsmentioning

confidence: 99%

Anomalous HCN emission from warm giant molecular clouds

Goicoechea,

Lique,

Santa-Maria

2021

Preprint

Self Cite

View full text Add to dashboard Cite

Hydrogen cyanide (HCN) is considered a good tracer of the dense molecular gas that serves as fuel for star formation. However, recent large-scale surveys of giant molecular clouds (GMCs) have detected extended HCN rotational line emission far from starforming cores. Such observations often spectroscopically resolve the HCN J = 1-0 (partially also the J = 2-1 and 3-2) hyperfine structure (HFS). A precise determination of the physical conditions of the gas requires treating the HFS line overlap effects.Here, we study the HCN HFS excitation and line emission using nonlocal radiative transfer models that include line overlaps and new HFS-resolved collisional rate coefficients for inelastic collisions of HCN with both para-H 2 and ortho-H 2 (computed via the scaled-infinite order sudden approximation up to T k = 500 K). In addition, we account for the role of electron collisions in the HFS level excitation. We find that line overlap and opacity effects frequently produce anomalous HCN J = 1-0 HFS line intensity ratios (i.e., inconsistent with the common assumption of the same T ex for all HFS lines) as well as anomalous HFS line width ratios. Line overlap and electron collisions also enhance the excitation of the higher J rotational lines. Our models explain the anomalous HCN J = 1-0 HFS spectra observed in the Orion Bar and Horsehead photodissociation regions. As shown in previous studies, electron excitation becomes important for molecular gas with H 2 densities below a few 10 5 cm −3 and electron abundances above ∼ 10 −5 . We find that when electron collisions are dominant, the relative intensities of the HCN J = 1-0 HFS lines can be anomalous too. In particular, electron excitation can produce low-surface-brightness HCN emission from very extended but low-density gas in GMCs. The existence of such a widespread HCN emission component may affect the interpretation of the extragalactic relationship HCN luminosity versus star-formation rate. Alternatively, extended HCN emission may arise from dense star-forming cores and become resonantly scattered by large envelopes of lower density gas. There are two scenarios -namely, electron-assisted (weakly) collisionally excited versus scattering -that lead to different HCN J = 1-0 HFS intensity ratios, which can be tested on the basis of observations.

show abstract

Submillimeter imaging of the Galactic Center starburst Sgr B2

Cited by 10 publications

References 145 publications

Star Formation in the Central Molecular Zone of the Milky Way

Star Formation in the Central Molecular Zone of the Milky Way

Anomalous HCN emission from warm giant molecular clouds

Anomalous HCN emission from warm giant molecular clouds

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