HCN/HNC chemistry in shocks: a study of L1157-B1 with ASAI

Leflóch, B.; Busquet, G.; Viti, S.; Vastel, C.; Mendoza, Edgar; Benedettini, M.; Codella, C.; Podio, L.; Schutzer, A. de A.; Rivera-Ortiz, Pedro Ruben; Lépine, J. R. D.; Bachiller, R.

doi:10.1093/mnras/stab2134

Cited by 8 publications

(14 citation statements)

References 55 publications

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“…Previous studies (e.g., Goldsmith et al 1986;Herbst et al 2000;Meijerink et al 2011;Kazandjian et al 2012;Krieger et al 2020) have suggested that the HCN/HNC abundance ratio might probe mechanical heating processes in the ISM through its kinetic temperature sensitivity, with the ratio increasing at high kinetic temperatures. This behavior has been seen in observations of protostellar shocks (e.g., Lefloch et al 2021) and C-type shock models, where the HCN/HNC abundance ratio increases by a factor of ∼20. Hence, we would expect that mechanical heating by shocks in NGC 253 would produce high HCN/HNC abundance ratios (Viti 2017).…”

Section: Effect Of Cosmic Rays On the Hcn/hnc Ratiosupporting

confidence: 63%

Tracing Interstellar Heating: An ALCHEMI Measurement of the HCN Isomers in NGC 253

Behrens

Mangum

Holdship

et al. 2022

ApJ

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We analyze HCN and HNC emission in the nearby starburst galaxy NGC 253 to investigate its effectiveness in tracing heating processes associated with star formation. This study uses multiple HCN and HNC rotational transitions observed using the Atacama Large Millimeter/submillimeter Array via the ALCHEMI Large Program. To understand the conditions and associated heating mechanisms within NGC 253's dense gas, we employ Bayesian nested sampling techniques applied to chemical and radiative transfer models, which are constrained using our HCN and HNC measurements. We find that the volume density n H 2 and cosmic-ray ionization rate (CRIR) ζ are enhanced by about an order of magnitude in the galaxy’s central regions as compared to those further from the nucleus. In NGC 253's central giant molecular clouds (GMCs), where observed HCN/HNC abundance ratios are the lowest, n ∼ 105.5 cm−3 and ζ ∼ 10−12 s−1 (greater than 104 times the average Galactic rate). We find a positive correlation in the association of both density and CRIR with the number of star formation-related heating sources (supernova remnants, H ii regions, and super hot cores) located in each GMC, as well as a correlation between CRIRs and supernova rates. Additionally, we see an anticorrelation between the HCN/HNC ratio and CRIR, indicating that this ratio will be lower in regions where ζ is higher. Though previous studies suggested HCN and HNC may reveal strong mechanical heating processes in NGC 253's CMZ, we find cosmic-ray heating dominates the heating budget, and mechanical heating does not play a significant role in the HCN and HNC chemistry.

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Section: Effect Of Cosmic Rays On the Hcn/hnc Ratiosupporting

confidence: 63%

Tracing Interstellar Heating: An ALCHEMI Measurement of the HCN Isomers in NGC 253

Behrens

Mangum

Holdship

et al. 2022

ApJ

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“…At high temperatures, new paths to form HCN via neutral-neutral reactions produce [HCN]/[HNC] ratios larger than 1. This simple scenario was able to explain in general terms the wealth of observational data gathered thus far: the [HCN]/[HNC] ratio is observed close to 1 in cold cores (Daniel et al 2013;Lefloch et al 2021), slightly higher than 1 (between 1 and 3) in giant molecular clouds (GMCs), and increasing to values much greater than 1 in hot cores (Jin et al 2015), Young Stellar Objects (YSOs, Jørgensen et al 2004) and luminous galaxies (ULIRGs, Baan et al 2010). However, some exceptions challenge our understanding of the chemistry of nitriles.…”

Section: Introductionmentioning

confidence: 76%

Linking the dust and chemical evolution: Taurus and Perseus

Bop

Lique

Esplugues

et al. 2023

A&A

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Context. HCN, HNC, and their isotopologues are ubiquitous molecules that can serve as chemical thermometers and evolutionary tracers to characterize star-forming regions. Despite their importance in carrying information that is vital to studies of the chemistry and evolution of star-forming regions, the collision rates of some of these molecules have not been available for rigorous studies in the past. Aims. Our goal is to perform an up-to-date gas and dust chemical characterization of two different star-forming regions, TMC 1-C and NGC 1333-C7, using new collisional rates of HCN, HNC, and their isotopologues. We investigated the possible effects of the environment and stellar feedback in their chemistry and their evolution. Methods. We used updated collisional rates of HCN, HNC, and their isotopologues in our analysis of the chemistry of TMC 1-C (Taurus) and NGC 1333-C7 (Perseus). With millimeter observations, we derived their column densities, the C and N isotopic fractions, the isomeric ratios, and the deuterium fractionation. The continuum data at 3 mm and 850 µm allowed us to compute the emissivity spectral index and look for grain growth as an evolutionary tracer. Results. The H13CN/HN13C ratio is anticorrelated with the deuterium fraction of HCN, thus it can readily serve as a proxy for the temperature. The spectral index (β ~ 1.34–2.09) shows a tentative anticorrelation with the H13CN/HN13C ratio, suggesting grain growth in the evolved, hotter, and less deuterated sources. Unlike TMC 1-C, the south-to-north gradient in dust temperature and spectral index observed in NGC 1333-C7 suggests feedback from the main NGC 1333 cloud. Conclusions. With this up-to-date characterization of two star-forming regions, we found that the chemistry and the physical properties are tightly related. The dust temperature, deuterium fraction, and the spectral index are complementary evolutionary tracers. The large-scale environmental factors may dominate the chemistry and evolution in clustered star-forming regions.

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“…We also note that in García-Burillo et al ( 2014) and Viti et al (2014), large HCN(1-0)/HCO + (1-0) (as well as the ratio of higher J) was in fact associated with large scale shocks that were associated with the molecular outlfow, rather than XDR chemistry, a scenario also explored in García-Burillo et al (2010). Recent galactic studies, such as the study into the prototypical shock region L1157-B1 in Lefloch et al (2021), have shown a marked increase in HCN abundance relative to H 2 in these environments, relative to preshock gas. However, there are significant exceptions.…”

Section: Hcn/hco +mentioning

confidence: 87%

Understanding if molecular ratios can be used as diagnostics of AGN and starburst activity: The case of NGC 1068

Butterworth

Holdship

Viti

et al. 2022

A&A

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Context. Molecular line ratios, such as HCN(1-0)/HCO + (1-0) and HCN(4-3)/CS(7-6), are routinely used to identify active galactic nuclei (AGN) activity in galaxies. Such ratios are, however, hard to interpret as they are highly dependent on the physics and energetics of the gas, and hence can seldom be used as a unique, unambiguous diagnostic. Aims. We used the composite galaxy NGC 1068 as a 'laboratory' to investigate whether molecular line ratios between HCN, HCO + , and CS are useful tracers of AGN-dominated gas and determine the origin of the differences in such ratios across different types of gas. Such a determination will enable a more rigorous use of such ratios. Methods. First, we empirically examined the aforementioned ratios at different angular resolutions to quantify correlations. We then used local thermodynamic equilibrium (LTE) and non-LTE analyses coupled with Markov chain Monte Carlo (MCMC) sampling in order to determine the origin of the underlying differences in ratios. Results. We propose that at high spatial resolution (< 50 pc) the HCN(4-3)/CS(2-1) is a reliable tracer of AGN activity. We also find that the variations in ratios are not a consequence of different densities or temperature but of different fractional abundances, yielding to the important result that it is essential to consider the chemical processes at play when drawing conclusions from radiative transfer calculations. Conclusions. From analyses at varying spatial scales, we find that previously proposed molecular line ratios, as well as a new one, have varying levels of consistency. We also determine from an investigation of radiative transfer modelling of our data that it is essential to consider the chemistry of the species when reaching conclusions from radiative transfer calculations.

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HCN/HNC chemistry in shocks: a study of L1157-B1 with ASAI

Cited by 8 publications

References 55 publications

Tracing Interstellar Heating: An ALCHEMI Measurement of the HCN Isomers in NGC 253

Tracing Interstellar Heating: An ALCHEMI Measurement of the HCN Isomers in NGC 253

Linking the dust and chemical evolution: Taurus and Perseus

Understanding if molecular ratios can be used as diagnostics of AGN and starburst activity: The case of NGC 1068

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