arXiv subjects

Harvey Liszt

Publications and source records attributed to Harvey Liszt.

25 records · Page 2Linked to original sources

arXiv2017

Turbulence and star formation efficiency in molecular clouds: solenoidal versus compressive motions in Orion B

The nature of turbulence in molecular clouds is one of the key parameters that control star formation efficiency: compressive motions, as opposed to solenoidal motions, can trigger the collapse of cores, or mark the expansion of Hii regions. We try to observationally derive the fractions of momentum density ($ρv$) contained in the solenoidal and compressive modes of turbulence in the Orion B molecular cloud and relate these fractions to the star formation efficiency in the cloud. The implementation of a statistical method developed by Brunt & Federrath (2014), applied to a $^{13}$CO(J=1-0) datacube obtained with the IRAM-30m telescope, allows us to retrieve 3-dimensional quantities from the projected quantities provided by the observations, yielding an estimate of the compressive versus solenoidal ratio in various regions of the cloud. Despite the Orion B molecular cloud being highly supersonic (mean Mach number $\sim$ 6), the fractions of motion in each mode diverge significantly from equipartition. The cloud's motions are on average mostly solenoidal (excess > 8 % with respect to equipartition), which is consistent with its low star formation rate. On the other hand, the motions around the main star-forming regions (NGC 2023 and NGC 2024) prove to be strongly compressive. We have successfully applied to observational data a method that was so far only tested on simulations, and have shown that there can be a strong intra-cloud variability of the compressive and solenoidal fractions, these fractions being in turn related to the star formation efficiency. This opens a new possibility for star-formation diagnostics in galactic molecular clouds.

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arXiv2016

The detectability of mm-wave molecular rotational transitions

Elaborating on a formalism that was first expressed some 40 years ago, we consider the brightness of low-lying mm-wave rotational lines of strongly polar molecules at the threshold of detectability. We derive a simple expression relating the brightness to the line of sight integral of the product of the total gas and molecular number densities and a suitably-defined temperature-dependent excitation rate into the upper level of the transition. Detectability of a line is contingent only on the ability of a molecule to channel enough of the ambient thermal energy into the line and the excitation can be computed in bulk by summing over rates without solving the multi-level rate equations or computing optical depths and excitation temperatures. Results for \hcop, HNC and CS are compared with escape-probability solutions of the rate equations using closed-form expressions for the expected range of validity of our {\it ansatz}, with the result that gas number densities as high as $10^4 \pccc$ or optical depths as high as 100 can be accommodated in some cases. For densities below a well-defined upper bound, the range of validity of the discussion can be cast as an upper bound on the line brightness which is 0.3 K for the J=1-0 lines and 0.8 - 1.7 K for the J=2-1 lines of these species. The discussion casts new light on interpretation of line brightnesses under conditions of weak excitation, simplifies derivation of physical parameters and eliminates the need to construct grids of numerical solutions of the rate equations.

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arXiv2015

Molecular gas in absorption and emission along the line of sight to W31C G10.62-0.38

We used the ARO 12m antenna to observe emission from the J=1-0 lines of carbon monoxide, \hcop\ and HNC and the J=2-1 line of CS toward and around the continuum peak used for absorption studies and we compare them with CH, HNC, C\p\ and other absorption spectra from PRISMAS. We develop a kinematic analysis that allows a continuous description of the spectral properties and relates them to viewing geometry in the Galaxy. As for CH, HF, C\p, \hcop\ and other species observed in absorption, mm-wave emission in CO, \hcop, HNC and CS is continuous over the full velocity range expected for material between the Sun and W31 4.95 kpc away. CO emission is much stronger than average in the Galactic molecular ring and the mean \HH\ density derived from CH, $4 \pccc \la$ 2$<$n(\HH)$>$ $\la 10 \pccc$ at 4 $\la$ R $\la$ 6.4 kpc, is similarly elevated. The CO-\HH\ conversion factor falls in a narrow range \XCO\ $= 1-2\times10^{20}~\HH\ \pcc~({\rm K}-\kms)^{-1}$ if the emitting gas is mostly on the near side of the sub-central point, as we suggest. The brightnesses of \hcop, HNC, and CS are comparable (0.83\%, 0.51\% and 1.1\% respectively relative to CO) and have no variation in galactocentric radius with respect to CO. Comparison of the profile-averaged \hcop\ emission brightness and optical depth implies local densities n(H) $\approx 135\pm25\pccc$ with most of excitation of \hcop\ from electrons. At such density, a consistent picture of the \HH-bearing gas, accounting also for the CO emission, has a volume filling factor 3\% and a 5 pc clump or cloud size.

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arXiv2014

What microwave astronomical spectroscopy can tell you about the carriers of the DIBS

Astronomical microwave spectroscopy has greatly broadened the inventory of identifiable chemical species in diffuse molecular gas and is an increasingly effective way to measure the abundances of polar molecules that may be candidate diffuse interstellar band carriers. Here we review some recent developments that hold new promise for chemical abundance determinations. We summarize and categorize the molecular inventory that has accrued in the past twenty years from microwave observations of diffuse clouds and we present summary tables of the molecular abundances within various chemical families in both dark and diffuse molecular gas.

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arXiv2014

E(B-V), N(H I) and N(H_2)

We consider the structure of the N(H I) - E(B-V) relationship when H I is measured in the 21 cm radio line and \EBV\ is defined by far-IR dust-derived measures. We derive reddening-dependent corrections to N(H I) based on interferometric absorption measurements over the past 30 years that follow a single power-law relationship $\int τ(H I) dv = 14.07 ~\kms\ $\EBV$^{1.074}$ at 0.02 $\la$ \EBV\ $\la$ 3 mag. Corrections to 21cm line-derived H I column densities are too small to have had any effect on the ratio N(H I)/\EBV\ $= 8.3 \times 10^{21}\pcc$ mag$^{-1}$ we derived at 0.015 $\la$ \EBV\ $\la$ 0.075 mag and \absb\ $\ge$ 20\degr; they are also too small to explain the break in the slope of the N(H I) - \EBV\ relation at \EBV\ $\ga$ 0.1 mag that we demonstrated around the Galaxy at \absb $\ge 20$\degr. The latter must therefore be attributed to the onset of \HH-formation and we show that models of \HH\ formation in a low density diffuse molecular gas can readily explain the inflected N(H I)- \EBV\ relationship. Below \absb\ = 20\degr\ N(H I)/\EBV\ measured at 0.015 $\la$ \EBV\ $\la$ 0.075 mag increases steadily down to \absb\ = 8\degr\ where sightlines with small \EBV\ no longer occur. By contrast, the ratio N(H I)/\EBV\ measured over all \EBV\ declines to N(H I)/\EBV\ $= 5-6 \times 10^{21}\pcc$ mag$^{-1}$ at \absb\ $\la 30$\degr, perhaps providing an explanation of the difference between our results and the gas/reddening ratios measured previously using stellar spectra.

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arXiv2012

The abundance of C3H2 and other small hydrocarbons in the diffuse interstellar medium

Hydrocarbons are ubiquitous in the interstellar medium, observed in diverse environments ranging from diffuse to molecular dark clouds and strong photon-dominated regions near HII regions. Recently, two broad diffuse interstellar bands (DIBs) at 4881Å and 5450Å were attributed to the linear version of propynylidene l-C3H2, a species whose more stable cyclic conformer c-C3H2 has been widely observed in the diffuse interstellar medium at radio wavelengths. This attribution has already been criticized on the basis of indirect plausibility arguments because the required column densities are quite large, N(l-C3H2)/EB-V = 4 \times 1014 cm-2 mag-1. Here we present new measurements of N(l-C3H2) based on simultaneous 18-21 GHz VLA absorption profiles of cyclic and linear C3H2 taken along sightlines toward extragalactic radiocontinuum background sources with foreground Galactic reddening EB-V = 0.1 - 1.6 mag. We find that N(l-C3H2)/N(c-C3H2) ? 1/15 - 1/40 and N(l-C3H2)/EB-V ? 2 \pm 1 \times 1011 cm-2 mag-1, so that the column densities of l-C3H2 needed to explain the diffuse interstellar bands are some three orders of magnitude higher than what is observed. We also find N(C4H)/EB-V < 1.3 \times 1013 cm-2 mag-1 and N(C4H-)/EB-V < 1 \times 1011 cm-2 mag-1 (3?). Using available data for CH and C2H we compare the abundances of small hydrocarbons in diffuse and dark clouds as a guide to their ability to contribute as DIB carriers over a wide range of conditions in the interstellar medium.

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arXiv2010

The CO luminosity and CO-H2 conversion factor of diffuse ISM: does CO emission trace dense molecular gas?

Aims: We wish to separate and quantify the CO luminosity and CO-H2 conversion factor applicable to diffuse but partially-molecular ISM when H2 and CO are present but C+ is the dominant form of gas-phase carbon. Methods: We discuss galactic lines of sight observed in \HI, HCO+ and CO where CO emission is present but the intervening clouds are diffuse (locally \AV\ $\la 1$ mag) with relatively small CO column densities $\NCO \la 2\times10^{16}\pcc$. We separate the atomic and molecular fractions statistically using \EBV\ as a gauge of the total gas column density and compare NH2 to the observed CO brightness. Results: Although there are H2-bearing regions where CO emission is too faint to be detected, the mean ratio of integrated CO brightness to NH2 for diffuse ISM does not differ from the usual value of 1\K km/s of integrated CO brightness per $2\times10^{20}$ H2 $\pcc$ . Moreover, the luminosity of diffuse CO viewed perpendicular to the galactic plane is 2/3 that seen at the Solar galactic radius in surveys of CO emission near the galactic plane. Conclusions: Commonality of the CO-H2 conversion factors in diffuse and dark clouds can be understood from considerations of radiative transfer and CO chemistry. There is unavoidable confusion between CO emission from diffuse and dark gas and misattribution of CO emission from diffuse to dark or giant molecular clouds. The character of the ISM is different from what has been believed if CO and H2 that have been attributed to molecular clouds on the verge of star formation are actually in more tenuous, gravitationally-unbound diffuse gas.

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