Chemistry In Circumstellar Envelopes Research Articles

Reflections on the photodissociation of CO in circumstellar envelopes

Carbon monoxide (CO) is the most abundant molecule after molecular hydrogen and is important for the chemistry in circumstellar envelopes around evolved stars. When modelling the strength and shape of molecular lines, the size of the CO envelope is an input parameter and influences the derived mass-loss rates. In particular the low-J transition CO lines are sensitive to the CO photodissociation radius. Recently, new CO photodissociation radii have been published using different formalisms that differ considerably. One set of calculations is based on an escape-probability formalism that uses numerical approximations derived in the early 1980s. The accuracy of these approximations is investigated and it is shown that they are less accurate than claimed. Improved formalism are derived. Nevertheless, the changes in the CO envelope size are small to moderate and less than 2% for models with 10−7 < Ṁ < 10−4 M⊙ yr−1 and at most 7% for model with Ṁ = 10−8 M⊙ yr−1.

The Impact of UV Radiation on Circumstellar Chemistry

AbstractUV radiation plays a critical role in the chemistry of circumstellar envelopes (CSEs) around evolved stars on the asymptotic giant branch (AGB). However, the effects of all potential sources of UV radiation have not been included in models. We present preliminary results of adding an internal source of UV to the CSE chemistry and predict large enhancements of atomic and ionic species arising from photo-destruction of parent species. Observations of atomic carbon towards the UV-bright AGB star o Cet are consistent with the modelling. In addition, we calculate the precise depth dependence of the CO photodissociation rate in an expanding CSE. We incorporate this within a chemical network active in the outflows of AGB stars, which includes 933 species and 15182 reactions. Our results show that the CO envelope size is about 30% smaller at half abundance than the most commonly used radius reported by Mamon et al. (1988).

The photodissociation of CO in circumstellar envelopes

Carbon monoxide is the most abundant molecule after H2 and is important for chemistry in circumstellar envelopes around late-type stars. The size of the envelope is important when modelling low-J transition lines and deriving mass-loss rates from such lines. Now that ALMA is coming to full power the extent of the CO emitting region can be measured directly for nearby asymptotic giant branch (AGB) stars. In parallel, it has become obvious in the past few years that the strength of the interstellar radiation field (ISRF) can have a significant impact on the interpretation of the emission lines. In this paper an update and extension of the classical Mamon et al. (1988, ApJ, 328, 797) paper is presented; these authors provided the CO abundance profile, described by two parameters, as a function of mass-loss rate and expansion velocity. Following recent work an improved numerical method and updated H2 and CO shielding functions are used and a larger grid is calculated that covers more parameter space, including the strength of the ISRF. The effect of changing the photodissociation radius on the low-J CO line intensities is illustrated in two cases.

Chemistry and distribution of daughter species in the circumstellar envelopes of O-rich AGB stars

Context. Thanks to the advent of Herschel and ALMA, new high-quality observations of molecules present in the circumstellar envelopes of asymptotic giant branch (AGB) stars are being reported that reveal large differences from the existing chemical models. New molecular data and more comprehensive models of the chemistry in circumstellar envelopes are now available.

Organic chemistry in circumstellar envelopes: Setting the stage for prebiotic synthesis

AbstractOne of the few carbon-rich environments found in interstellar space is the ejecta of asymptotic giant branch (AGB) stars. Such material, which forms a circumstellar envelope, becomes enriched in carbon due to “dredge-up” phenomena associated with nucleosynthesis. A unique organic synthesis flourishes in the gas phase in these envelopes, and radio and millimeter observations have identified a wide range of C-bearing compounds, including long acetylenic chains such as HC5N, HC7N, C4H, C6H, C8H, C6H−, C8H−, and C3O. Oxygen-rich envelopes also have a non-negligible carbon chemistry, fostering species such as HCN and HCO+. Phosphorus chemistry appears to be active as well in circumstellar shells, as evidenced by the recent detections of HCP, CCP, and PO. Radio observations also indicate that some fraction of the circumstellar molecular material survives into the planetary nebula stage, and then becomes incorporated into diffuse, and eventually, dense clouds. The complex organic molecules found in dense clouds such as Sgr B2(N) may be the products of “seed” material that can be traced back to the carbon-enriched circumstellar gas.

The physics and chemistry of circumstellar envelopes of S-stars on the AGB

AbstractPresented here are the preliminary results of a long-term study of S-stars on the AGB. S-stars are important as possible transition objects between oxygen-rich M-stars and carbon stars. The aim of the study is to compare results from our newly gathered observational database for the S-stars with those already obtained for the M- and carbon stars. We can thus follow the changes as the stars evolve along the AGB and more firmly establish the suggested M-MS-S-SC-C evolutionary sequence. It will also allow us to determine the relative importance of processes such as non-equilibrium chemistry, grain formation, and photodissociation in regulating the chemistry in circumstellar envelopes of AGB stars.

The chemistry in circumstellar envelopes of evolved stars: following the origin of the elements to the origin of life.

Mass loss from evolved stars results in the formation of unusual chemical laboratories: circumstellar envelopes. Such envelopes are found around carbon- and oxygen-rich asymptotic giant branch stars and red supergiants. As the gaseous material of the envelope flows from the star, the resulting temperature and density gradients create a complex chemical environment involving hot, thermodynamically controlled synthesis, molecule "freeze-out," shock-initiated reactions, and photochemistry governed by radical mechanisms. In the circumstellar envelope of the carbon-rich star IRC+10216, >50 different chemical compounds have been identified, including such exotic species as C(8)H, C(3)S, SiC(3), and AlNC. The chemistry here is dominated by molecules containing long carbon chains, silicon, and metals such as magnesium, sodium, and aluminum, which makes it quite distinct from that found in molecular clouds. The molecular composition of the oxygen-rich counterparts is not nearly as well explored, although recent studies of VY Canis Majoris have resulted in the identification of HCO(+), SO(2), and even NaCl in this object, suggesting chemical complexity here as well. As these envelopes evolve into planetary nebulae with a hot, exposed central star, synthesis of molecular ions becomes important, as indicated by studies of NGC 7027. Numerous species such as HCO(+), HCN, and CCH are found in old planetary nebulae such as the Helix. This "survivor" molecular material may be linked to the variety of compounds found recently in diffuse clouds. Organic molecules in dense interstellar clouds may ultimately be traced back to carbon-rich fragments originally formed in circumstellar shells.

What Species Remain to be Seen?

We review what species remain to be seen for several types of astrochemistry: Thermochemical Equilibrium (TE) in circumstellar envelopes (CSEs); photo- and ion-molecule chemistry in CSEs; ion-molecule chemistry in cold interstellar clouds; grain chemistry (passive, catalytic, disruptive); and shock chemistry. In CSEs, a rich Si gas-phase chemistry is now recognized, and two predicted species (SiN, SiH2) have been seen. Others are predicted. In the ISM, a global picture of refractory-element chemistry predicts that compounds of Mg, Na, Fe, and possibly Al occur with detectable gas-phase abundance. Predicted species require laboratory synthesis and spectroscopy. Reactions of hydrocarbon ions with neutral species dominate the formation of the families CnH, HCnN, H2Cn, and CnO in both interstellar (TMC-1) and circumstellar (IRC10216) cases, and readily explain the favored values of n in each case as well as predicting which higher-n species remain to be seen. Confirmation of H3O+ (interstellar) is discussed.

Molecules in Circumstellar Envelopes

ABSTRACTThe chemistry of circumstellar envelopes around evolved stars is extremely rich, especially for C-rich ones. 34 molecules have been identified in IRC+10216. Recent observations and modeling prove that most of the unstable species are formed by photochemistry in the outer layers of the envelope.