Atomic and molecular data for the transitions of a number of astrophysically interesting species are summarized, in- cluding energy levels, statistical weights, Einstein A-coefficients and collisional rate coefficients. Available collisional data from quantum chemical calculations and experiments are extrapolated to higher energies (up to E/k ∼ 1000 K). These data, which are made publically available through the WWW at http://www.strw.leidenuniv.nl/∼moldata, are essential input for non-LTE line radiative transfer programs. An online version of a computer program for performing statistical equilibrium calcu- lations is also made available as part of the database. Comparisons of calculated emission lines using different sets of collisional rate coefficients are presented. This database should form an important tool in analyzing observations from current and future (sub)millimetre and infrared telescopes.

/pdf/an-atomic-and-molecular-database-for-analysis-of-4xud6qp5de.pdf

An atomic and molecular database for analysis of submillimetre line observations

We present the revised "Meudon" model of photon-dominated region (PDR) code, available on the Web under the GNU Public License. General organization of the code is described down to a level that should allow most observers to use it as an interpretation tool with minimal help from our part. Two grids of models, one for low-excitation diffuse clouds and one for dense highly illuminated clouds, are discussed, and some new results on PDR modelization highlighted.

https://hal.archives-ouvertes.fr/hal-00018656/document

A model for atomic and molecular interstellar gas: the meudon pdr code

We analyze the effects of molecular depletion on the thermal balance of well-shielded, quiescent dark cloud cores. Recent observations of the significant depletion of molecules from the gas phase onto grain surfaces in dark clouds suggest the possibility that the gas-phase cooling in these regions is greatly reduced and consequently that gas kinetic temperatures might be increased. We reexamine cooling and heating processes in light of possible molecular depletion, including the effect of coupling between the gas and the grains. At densities ≤103.5 cm-3, the gas temperature can be significantly increased by the depletion of coolant species without significantly affecting the dust temperature because of the relatively weak gas-dust coupling. At higher densities, this coupling becomes sufficiently rapid to overwhelm the effect of the reduced gas-phase cooling, and depletion has little effect on the gas temperature while raising the dust temperature 1 K. The result is that depletion at densities ≥104.5 cm-3 can proceed without being evident as an enhanced gas temperature or without self-limiting due to an increase in the dust temperature increasing the desorption rate. This is consistent with observations of depletion in cold, dense regions of quiescent molecular clouds. It also suggests that depletion in moderate density regions can increase the thermal gas pressure, effectively enhancing the confinement of denser portions of molecular clouds and possibly accelerating the collapse of cloud cores.

Molecular Depletion and Thermal Balance in Dark Cloud Cores

The physical structure of the envelopes around a sample of 14 massive young stars is investigated using maps and spectra in submillimeter continuum and lines of C^(17)O, CS, C^(34)S, and H_2CO. Nine of the sources are highly embedded luminous (10^3-10^5 L_☉) young stellar objects that are bright near-infrared sources but weak in radio continuum; the other objects are similar but not bright in the near-infrared and contain "hot-core"-type objects and/or ultracompact H II regions. The data are used to constrain the temperature and density structure of the circumstellar envelopes on 10^2-10^5 AU scales, to investigate the relation between the different objects, and to search for evolutionary effects. The total column densities and the temperature profiles are obtained by fitting self-consistent dust models to submillimeter photometry. The calculated temperatures range from 300 to 1000 K at ~10^2 AU and from 10 to 30 K at ~10^5 AU from the star. Visual extinctions are a few hundred to a few thousand magnitudes, assuming a grain opacity at 1300 μm of ≈1 cm^(-2) g^(-1) of dust, as derived earlier for one of our sources. The mid-infrared data are consistent with a 30% decrease of the opacity at higher temperatures, caused by the evaporation of the ice mantles. The CS, C^(34)S, and H_2CO data as well as the submillimeter dust emission maps indicate density gradients n ∝ r^(-α). Assuming a constant CS abundance throughout the envelope, values of α = 1.0-1.5 are found, which is significantly flatter than the α = 2.0 ± 0.3 generally found for low-mass objects. This flattening may indicate that in massive young stellar objects, nonthermal pressure is more important for the support against gravitational collapse, while thermal pressure dominates for low-mass sources. We find α = 2 for two hot-core-type sources but regard this as an upper limit since, in these objects, the CS abundance may be enhanced in the warm gas close to the star. The assumption of spherical symmetry is tested by modeling infrared absorption line data of ^(13)CO, CS emission-line profiles and near-infrared continuum. There is a distinct, but small deviation from spherical symmetry: the data are consistent with a decrease of the optical depth by a factor of ≈3 in the central ≾10". The homogeneity of the envelopes is verified by the good agreement of the total masses in the power-law models with the virial masses. Modeling of C^(17)O emission shows that ≈40%-90% of the CO is frozen out onto the dust. The CO abundances show a clear correlation with temperature, as expected if the abundance is controlled by freeze-out and thermal desorption. The CS abundance is 3 × 10^(-9) on average, ranging from (4-8) × 10^(-10) in the cold source GL 7009S to (1-2) × 10^(-8) in the two hot-core-type sources. Dense outflowing gas is seen in the CS and H_2CO line wings; the predominance of blueshifted emission suggests the presence of dense, optically thick material within 10" of the center. Interferometric continuum observations at 1300-3500 μm show compact emission, probably from a 0".3 diameter, optically thick dust component, such as a dense shell or a disk. The emission is a factor of 10-100 stronger than expected for the envelopes seen in the single-dish data, so that this component may be opaque enough to explain the asymmetric CS and H^2CO line profiles. The evolution of the sources is traced by the overall temperature (measured by the far-infrared color), which increases systematically with the decreasing ratio of envelope mass to stellar mass. The observed anticorrelation of near-infrared and radio continuum emission suggests that the erosion of the envelope proceeds from the inside out. Conventional tracers of the evolution of low-mass objects do not change much over this narrow age range.

/pdf/structure-and-evolution-of-the-envelopes-of-deeply-embedded-1vtqn9wthr.pdf

Structure and evolution of the envelopes of deeply embedded massive young stars

We present a multiline study of the dense core L1544 in the Taurus molecular complex. Although L1544 does not harbor an embedded star, it presents several characteristics of cores that have already undergone star formation, suggesting that it may be rather advanced in its evolution toward becoming a star-forming core. The spectral lines from L1544 present an interesting dichotomy, with the thick dense gas tracers su†ering very strong self absorption while CO and its isotopes are not being absorbed at all. The presence of the self absorptions allows us to study both the density structure and kinematics of the gas in detail. A simple analysis shows that the core is almost isothermal and that the self absorptions are due to very subthermal excitation of the dense gas tracers in the outer layers. The density has to decrease outward rapidly, and a detailed radiative transfer calculation that simultaneously -ts three iso- topes of CO and two of CS shows that the density approximately follows a r~1.5 power law. The self absorptions, in addition, allow us to measure the relative velocity between the inner and outer layers of the core, and we -nd that there is a global pattern of inward motions (background and foreground approaching each other). The relative speed between the foreground and background changes with posi- tion, and we use a simple two-layer model to deduce that while the foreground gas has a constant veloc- ity, the background material presents systematic velocity changes that we interpret as arising from two velocity components. We explore the origin of the inward motions by comparing our observations with models of gravitational collapse. A model in which the infall starts at the center and propagates outward (as in the inside-out collapse of Shu) is inconsistent with the large extension of the absorption (that sug- gests an advanced age) and the lack of a star at the core center (that suggests extreme youth). Ambipolar di†usion seems also ruled out because of the large amount of the inward speed (up to 0.1 km s~1) and the fact that ionized species move with speeds similar to those of the neutrals. Other infall models seem also to have problems -tting the data, so if L1544 is infalling, it seems to be doing so in a manner not contemplated by the standard theories of star formation. Our study of L1544 illustrates how little is still known about the physical conditions that precede star formation and how detailed studies of starless cores are urgently needed. Subject headings: ISM: individual (L1544) E ISM: kinematics and dynamics E stars: formation

https://iopscience.iop.org/article/10.1086/306115/pdf

L1544: A Starless dense core with extended inward motions

Eight molecular species, in the hot dense clump IC 443G, believed to be impacted by the shock wave from the SNR IC 443, are investigated. The clump consists of two distinct regions, one relatively cool, and one hotter and denser. Region 1 contains CO, HCO(+), HCN, and CN, whose abundances may be explained either by ion-molecule chemistry, or by a D shock of 60-90 km/s, passing through a clump of about 100,000/cu cm. Region 2 gives rise to SiO, CS, SO, and H2CO, and requires an ND shock of 5-15 km/s passing through a region of about 1,000,000/cu cm. Observed fractional abundances fit ND shock models if L is about 6.6 x 10 exp 15 cm. In general, observed line widths vary inversely with derived excitation density, while centroid velocities of all species are essentially identical.

Tests of shock chemistry in IC 443G

The abundances of lithium and boron provide important information about big bang nucleosynthesis, Galactic chemical evolution, stellar evolution, and cosmic-ray spallation reactions. We conducted the -rst search for the ground-state hyper-ne-structure transitions of Li I F \ 2E1; 803 MHz) and B I (2S 1@2 ; F \ 2E1; 732 MHz). We used the 43 m NRAO radio telescope to search for enhanced Galactic (2P 1@2 ; center (GC) Li and B expected from models of Galactic activity. We did not detect Li I or B I and obtained upper limits of N(Li I) \ 1.9 ) 1016 cm~2, (Li/H) \ 3.9 ) 10~8, N(B I) \ 2.2 ) 1018 cm~2, and (B/H) \ 9.2 ) 10~6 for the dense 20 km s~1 Sgr A molecular cloud where our largest sources of uncer- tainties are Li I/Li, B I/B, and N(H). Our observations imply (Li/H) GC \ 22 (Li/H) disk , (Li/H) GC \ 39 For a simple (Li/H) diskvspallation , (B/H) GC \ 1.2 ) 104 (B/H) disk , (B/H) GC \ 1.5 ) 104 (B/H) diskvspallation . model combining mass loss from AGB stars (only for Li), spallation reactions, and SN l-nucleosynthesis, we estimate (13 times enhancement) and (10 times (Li/H) GC \ 1.3 ) 10~8 (B/H) GC \ 7.4 ) 10~9 enhancement). If Li is primarily produced via spallation reactions from a cosmic-ray proton Nux / p (t) with the same energy and trapping as in the disk, then (/ Comparing our / p (t)dt) GC \ 13(/ / p (t)dt) disk . results to AGN models, we conclude that the GC has not had an extended period of AGN activity containing a large cosmic-ray Nux ergs s~1 for 108 yr), a large low-energy cosmic-ray Nux (L CR " 1044 (less than 100 times the disk Nux), or a large c-ray Nux ergs s~1 for 109 yr). Furthermore, (L c \ 1042 since any Galactic deuterium production will signi-cantly enhance the abundances of Li and B, our results imply that there are no sources of D in the GC or Galaxy. Therefore, all the Galactic D orig- inated from the infall of primordial matter with the current D/H reduced by astration and mixing. Subject headings: Galaxy: center E Galaxy: evolution E ISM: abundances E radio lines: general E radio lines: ISM

D. A. Lubowich

Papers

Tests of shock chemistry in IC 443G

Constraints on Galactic Center Activity: A Search for Enhanced Galactic Center Lithium and Boron