A Model For the Formation of High Density Clumps in Proto-Planetary Nebulae

The detection of NaCl at large radii in the Egg Nebula, CRL 2688, requires densities of 10^7 - 10^8 cm^-3 in a thick shell of r ~ a few X 10^17 cm. To explain these results, a mechanism is needed for producing high densities at a considerable distance from the central star. In two dimensional simulations of the interaction of the fast wind with an ambient medium, the material becomes thermally unstable. The resulting clumps can achieve the requisite conditions for NaCl excitation. We present 2D models with simple physics as proof-of-principle calculations to show that the clumping behavior is robust. Clumping is a natural outcome of cooling in the colliding wind model and comparable to that inferred from observations.Comment: 12 pages, 3 figures. Accepted for publication in the Astrophysical Journal Letter

Molecular excitation in the Interstellar Medium: recent advances in collisional, radiative and chemical processes

We review the different excitation processes in the interstellar mediumComment: Accepted in Chem. Re

Maternal restrictive feeding and eating in the absence of hunger among toddlers: a cohort study

Abstract Background Restrictive feeding by parents has been associated with greater eating in the absence of hunger (EAH) among children, a risk factor for obesity. However, few studies have examined the association between restrictive feeding and EAH longitudinally, raising questions regarding the direction of associations between restrictive feeding and child EAH. Our objective was to examine the bidirectional prospective associations between restrictive feeding and EAH among toddlers. Methods Low-income mother-child dyads (n = 229) participated when children were 21, 27, and 33 months old. Restriction with regard to food amount and food quality were measured with the Infant Feeding Styles Questionnaire. EAH was measured as kilocalories of food children consumed after a satiating meal. A cross-lagged analysis adjusting for child sex and weight-for-length z-score was used to simultaneously test cross-sectional and bidirectional prospective associations between each type of restriction and children’s EAH. Results At 21 months, mothers of children with greater EAH reported higher restriction with regard to food amount (b = 0.17, p < .05). Restriction with regard to food amount at age 21 months was inversely associated with EAH at 27 months (b = −0.20, p < .05). Restriction with regard to food amount at 27 months was not associated with EAH at 33 months and restriction with regard to food quality was not associated with EAH. EAH did not prospectively predict maternal restriction. Conclusions Neither restriction with regard to food amount nor food quality increased risk for EAH among toddlers. Current US clinical practice recommendations for parents to avoid restrictive feeding, and the potential utility of restrictive feeding with regard to food amount in early toddlerhood, deserve further consideration.https://deepblue.lib.umich.edu/bitstream/2027.42/140394/1/12966_2017_Article_630.pd

BEYOND IRC+10216: NEW TRENDS IN THE DETECTION OF METAL MOLECULES TOWARDS CIRCUMSTELLAR ENVELOPES

Author Institution: Department of Chemistry, Department of Astronomy, Steward Observatory, and Arizona Radio Observatory, University of ArizonaNew millimeter-wave observations towards the circumstellar envelopes of several late-type carbon stars have resulted in the detection of various metal-bearing species which, until now, have only been seen in IRC +10216. Measurements at 1, 2, and 3 mm with the ARO Kitt Peak 12 m telescope and the IRAM 30 m towards CRL 2688 have resulted in the identification of MgNC, NaCN, NaCl, and AlF in this source. In addition, MgNC has been detected in the circumstellar shell of CRL 618. These data indicate that metal-containing molecules are common constituents of circumstellar envelopes and may survive into the planetary nebula stage. The chemical formation processes of these molecules are quite uncertain, however. Although standard thermodynamic equilibrium chemistry has been thought to produce metal halides, for example, these new observations indicate that shock waves may be responsible for creating NaCl and NaCN in CRL 2688. ""Shock chemistry"" could be creating metal cyanides in IRC+10216 as well, although radiative association reactions may also play a role. Current theories of circumstellar metal molecule formation as well as future observational and spectroscopic challenges will be discussed