Electron Excitation of High Dipole Moment Molecules Re-examined

Goldsmith, Paul F.; Kauffmann, Jens

doi:10.3847/1538-4357/aa6f12

Cited by 52 publications

(26 citation statements)

References 70 publications

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“…Evans (1999) and Shirley (2015; also see Linke et al 1977) point out that HCN should become detectable at "effective" densities n eff ≈ (1 to 3) × 10 4 cm −3 for gas at 10 K and regular abundances, for which n eff n cr . Further, HCN can be excited by electrons at H 2 densities n cr if fractional electron abundances X(e − ) > 10 −5 prevail (Goldsmith & Kauffmann 2017, following a suggestion by S. Glover). Here we provide solid observational evidence supporting such work.…”

Section: Hcn As a Tracer Of Moderately Dense Gasmentioning

confidence: 79%

Molecular Line Emission as a Tool for Galaxy Observations (LEGO)

Kauffmann

Goldsmith

Melnick

et al. 2017

A&A

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132

169

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Trends observed in galaxies, such as the Gao & Solomon relation, suggest a linear relationship between the star formation rate and the mass of dense gas available for star formation. Validation of such trends requires the establishment of reliable methods to trace the dense gas in galaxies. One frequent assumption is that the HCN (J = 1-0) transition is unambiguously associated with gas at H 2 densities 10 4 cm −3 . If so, the mass of gas at densities 10 4 cm −3 could be inferred from the luminosity of this emission line, L HCN (1-0) . Here we use observations of the Orion A molecular cloud to show that the HCN (J = 1-0) line traces much lower densities ∼10 3 cm −3 in cold sections of this molecular cloud, corresponding to visual extinctions A V ≈ 6 mag. We also find that cold and dense gas in a cloud like Orion produces too little HCN emission to explain L HCN (1-0) in star forming galaxies, suggesting that galaxies might contain a hitherto unknown source of HCN emission. In our sample of molecules observed at frequencies near 100 GHz (also including 12 CO, 13 CO, C 18 O, CN, and CCH), N 2 H + is the only species clearly associated with relatively dense gas.

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Section: Hcn As a Tracer Of Moderately Dense Gasmentioning

confidence: 79%

Molecular Line Emission as a Tool for Galaxy Observations (LEGO)

Kauffmann

Goldsmith

Melnick

et al. 2017

A&A

Self Cite

132

169

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“…However, whether this can be responsible for reducing f f is unclear, as Federrath & Klessen (2012) do not find particularly low f f in their M = 3 simulations. Other alternatives would include some feedback mechanism that we have not accounted for, such as protostellar heating or outflows, or a systematic overestimation of inferred density of HCN clumps (Goldsmith & Kauffmann 2017).…”

Section: Star-forming Clouds and Clumps In The Milky Waymentioning

confidence: 99%

When feedback fails: the scaling and saturation of star formation efficiency

Grudić

Hopkins

Faucher-Giguère

et al. 2018

Monthly Notices of the Royal Astronomical Society

181

236

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We present a suite of 3D multi-physics MHD simulations following star formation in isolated turbulent molecular gas disks ranging from 5 to 500 parsecs in radius. These simulations are designed to survey the range of surface densities between those typical of Milky Way GMCs (∼ 10 2 M pc −2 ) and extreme ULIRG environments (∼ 10 4 M pc −2 ) so as to map out the scaling of the cloud-scale star formation efficiency (SFE) between these two regimes. The simulations include prescriptions for supernova, stellar wind, and radiative feedback, which we find to be essential in determining both the instantaneous per-freefall ( f f ) and integrated ( int ) star formation efficiencies. In all simulations, the gas disks form stars until a critical stellar surface density has been reached and the remaining gas is blown out by stellar feedback. We find that surface density is a good predictor of int , as suggested by analytic force balance arguments from previous works. SFE eventually saturates to ∼ 1 at high surface density. We also find a proportional relationship between f f and int , implying that star formation is feedback-moderated even over very short time-scales in isolated clouds. These results have implications for star formation in galactic disks, the nature and fate of nuclear starbursts, and the formation of bound star clusters. The scaling of f f with surface density is not consistent with the notion that f f is always ∼ 1% on the scale of GMCs, but our predictions recover the ∼ 1% value for GMC parameters similar to those found in sprial galaxies, including our own.

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“…The build-up of chemical complexity thus depends on the ionization fraction of the medium. Finally, some common molecular tracers with high dipole moments, such as HCN and HCO + , have high inelastic collision cross sections with electrons, and their excitation can be significantly affected by electron collisions for ionization fractions 10 −5 (Black & van Dishoeck 1991;Liszt 2012;Liszt & Pety 2016;Goldsmith & Kauffmann 2017). This makes the interpretation of their emission (e.g., to estimate gas density) sensitive to our knowledge of the local ionization fraction .…”

Section: Introductionmentioning

confidence: 99%

Tracers of the ionization fraction in dense and translucent gas

Bron

Roueff

Gérin

et al. 2020

A&A

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Context. The ionization fraction in the neutral interstellar medium (ISM) plays a key role in the physics and chemistry of the ISM, from controlling the coupling of the gas to the magnetic field to allowing fast ion-neutral reactions that drive interstellar chemistry. Most estimations of the ionization fraction have relied on deuterated species such as DCO + , whose detection is limited to dense cores representing an extremely small fraction of the volume of the giant molecular clouds (GMC) that they are part of. As large field-of-view hyperspectral maps become available, new tracers may be found. The growth of observational datasets is paralleled by the growth of massive modeling datasets and new methods need to be devised to exploit the wealth of information they contain. Aims. We search for the best observable tracers of the ionization fraction based on a grid of astrochemical models, with the broader aim of finding a general automated method applicable to searching for tracers of any unobservable quantity based on grids of models. Methods. We built grids of models that randomly sample a large range of physical conditions (unobservable quantities such as gas density, temperature, elemental abundances, etc.) and computed the corresponding observables (line intensities, column densities) and the ionization fraction. We estimated the predictive power of each potential tracer by training a random forest model to predict the ionization fraction from that tracer, based on these model grids. Results. In both translucent medium and cold dense medium conditions, we found several observable tracers with very good predictive power for the ionization fraction. Many tracers in cold dense medium conditions are found to be better and more widely applicable than the traditional DCO + /HCO + ratio. We also provide simpler analytical fits for estimating the ionization fraction from the best tracers, and for estimating the associated uncertainties. We discuss the limitations of the present study and select a few recommended tracers in both types of conditions. Conclusions. The method presented here is very general and can be applied to the measurement of any other quantity of interest (cosmic ray flux, elemental abundances, etc.) from any type of model (PDR models, time-dependent chemical models, etc.).

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Electron Excitation of High Dipole Moment Molecules Re-examined

Cited by 52 publications

References 70 publications

Molecular Line Emission as a Tool for Galaxy Observations (LEGO)

Molecular Line Emission as a Tool for Galaxy Observations (LEGO)

When feedback fails: the scaling and saturation of star formation efficiency

Tracers of the ionization fraction in dense and translucent gas

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