The distribution of molecules in the circumstellar envelope of IRC +10216

HC3N, C3N, and SiS

John H Bieging, Mario Tafalla

Research output: Contribution to journalArticle

85 Citations (Scopus)

Abstract

We present aperture synthesis images made with the BIMA mm interferometer of molecular emission lines of HC3N, C3N, and SiS at ∼109 GHz toward the carbon star IRC +10216. The images have angular resolutions of 5″-7″ and velocity resolution of ∼1 km/s. The data are compared with model calculations using a newly-developed statistical equilibrium code which predicts the brightness distribution and spectrum for linear molecules in a spherical expanding circumstellar envelope. Excitation by IR photons and collisions is included. The model calculations are used to derive the abundance of HC3N and SiS as a function of distance from the star. For C3N, an LTE calculation is used to derive the abundance distribution. The data clearly show that HC3N and C3N are distributed in concentric shells, with little or no emission from the inner envelope. The abundances derived from our model calculations are much larger than published ion-molecule chemical reaction networks predict, but are in reasonably good agreement with chemical models which include neutral molecule-radical reactions. Statistical equilibrium models for SiS indicate that the gas phase abundance of SiS must decrease substantially as the gas flows from the photosphere to a distance of ∼2×1015 cm, probably as a result of grain adsorption processes. Photodissociation appears to cut off the SiS distribution between 2-3×1016 cm. We find that IR line overlaps of SiS with HCN or C2H2 are probably not important in the excitation of SiS, but that the excitation temperatures of some rotational transitions of SiS are very sensitive to the IR flux from the central star and dust shell. This strong dependence of excitation on IR flux may account for observed spectral variability of SiS lines.

Original languageEnglish (US)
Pages (from-to)576-594
Number of pages19
JournalAstronomical Journal
Volume105
Issue number2
StatePublished - Feb 1993

Fingerprint

envelopes
molecules
excitation
shell
stars
brightness distribution
carbon stars
local thermodynamic equilibrium
image resolution
photosphere
angular resolution
interferometer
gas flow
chemical reaction
photodissociation
chemical reactions
cut-off
interferometers
collision
dust

ASJC Scopus subject areas

  • Space and Planetary Science
  • Astronomy and Astrophysics

Cite this

The distribution of molecules in the circumstellar envelope of IRC +10216 : HC3N, C3N, and SiS. / Bieging, John H; Tafalla, Mario.

In: Astronomical Journal, Vol. 105, No. 2, 02.1993, p. 576-594.

Research output: Contribution to journalArticle

@article{ff72a6cd061f4e0bb7e4e90cb3121121,
title = "The distribution of molecules in the circumstellar envelope of IRC +10216: HC3N, C3N, and SiS",
abstract = "We present aperture synthesis images made with the BIMA mm interferometer of molecular emission lines of HC3N, C3N, and SiS at ∼109 GHz toward the carbon star IRC +10216. The images have angular resolutions of 5″-7″ and velocity resolution of ∼1 km/s. The data are compared with model calculations using a newly-developed statistical equilibrium code which predicts the brightness distribution and spectrum for linear molecules in a spherical expanding circumstellar envelope. Excitation by IR photons and collisions is included. The model calculations are used to derive the abundance of HC3N and SiS as a function of distance from the star. For C3N, an LTE calculation is used to derive the abundance distribution. The data clearly show that HC3N and C3N are distributed in concentric shells, with little or no emission from the inner envelope. The abundances derived from our model calculations are much larger than published ion-molecule chemical reaction networks predict, but are in reasonably good agreement with chemical models which include neutral molecule-radical reactions. Statistical equilibrium models for SiS indicate that the gas phase abundance of SiS must decrease substantially as the gas flows from the photosphere to a distance of ∼2×1015 cm, probably as a result of grain adsorption processes. Photodissociation appears to cut off the SiS distribution between 2-3×1016 cm. We find that IR line overlaps of SiS with HCN or C2H2 are probably not important in the excitation of SiS, but that the excitation temperatures of some rotational transitions of SiS are very sensitive to the IR flux from the central star and dust shell. This strong dependence of excitation on IR flux may account for observed spectral variability of SiS lines.",
author = "Bieging, {John H} and Mario Tafalla",
year = "1993",
month = "2",
language = "English (US)",
volume = "105",
pages = "576--594",
journal = "Astronomical Journal",
issn = "0004-6256",
publisher = "IOP Publishing Ltd.",
number = "2",

}

TY - JOUR

T1 - The distribution of molecules in the circumstellar envelope of IRC +10216

T2 - HC3N, C3N, and SiS

AU - Bieging, John H

AU - Tafalla, Mario

PY - 1993/2

Y1 - 1993/2

N2 - We present aperture synthesis images made with the BIMA mm interferometer of molecular emission lines of HC3N, C3N, and SiS at ∼109 GHz toward the carbon star IRC +10216. The images have angular resolutions of 5″-7″ and velocity resolution of ∼1 km/s. The data are compared with model calculations using a newly-developed statistical equilibrium code which predicts the brightness distribution and spectrum for linear molecules in a spherical expanding circumstellar envelope. Excitation by IR photons and collisions is included. The model calculations are used to derive the abundance of HC3N and SiS as a function of distance from the star. For C3N, an LTE calculation is used to derive the abundance distribution. The data clearly show that HC3N and C3N are distributed in concentric shells, with little or no emission from the inner envelope. The abundances derived from our model calculations are much larger than published ion-molecule chemical reaction networks predict, but are in reasonably good agreement with chemical models which include neutral molecule-radical reactions. Statistical equilibrium models for SiS indicate that the gas phase abundance of SiS must decrease substantially as the gas flows from the photosphere to a distance of ∼2×1015 cm, probably as a result of grain adsorption processes. Photodissociation appears to cut off the SiS distribution between 2-3×1016 cm. We find that IR line overlaps of SiS with HCN or C2H2 are probably not important in the excitation of SiS, but that the excitation temperatures of some rotational transitions of SiS are very sensitive to the IR flux from the central star and dust shell. This strong dependence of excitation on IR flux may account for observed spectral variability of SiS lines.

AB - We present aperture synthesis images made with the BIMA mm interferometer of molecular emission lines of HC3N, C3N, and SiS at ∼109 GHz toward the carbon star IRC +10216. The images have angular resolutions of 5″-7″ and velocity resolution of ∼1 km/s. The data are compared with model calculations using a newly-developed statistical equilibrium code which predicts the brightness distribution and spectrum for linear molecules in a spherical expanding circumstellar envelope. Excitation by IR photons and collisions is included. The model calculations are used to derive the abundance of HC3N and SiS as a function of distance from the star. For C3N, an LTE calculation is used to derive the abundance distribution. The data clearly show that HC3N and C3N are distributed in concentric shells, with little or no emission from the inner envelope. The abundances derived from our model calculations are much larger than published ion-molecule chemical reaction networks predict, but are in reasonably good agreement with chemical models which include neutral molecule-radical reactions. Statistical equilibrium models for SiS indicate that the gas phase abundance of SiS must decrease substantially as the gas flows from the photosphere to a distance of ∼2×1015 cm, probably as a result of grain adsorption processes. Photodissociation appears to cut off the SiS distribution between 2-3×1016 cm. We find that IR line overlaps of SiS with HCN or C2H2 are probably not important in the excitation of SiS, but that the excitation temperatures of some rotational transitions of SiS are very sensitive to the IR flux from the central star and dust shell. This strong dependence of excitation on IR flux may account for observed spectral variability of SiS lines.

UR - http://www.scopus.com/inward/record.url?scp=0011347069&partnerID=8YFLogxK

UR - http://www.scopus.com/inward/citedby.url?scp=0011347069&partnerID=8YFLogxK

M3 - Article

VL - 105

SP - 576

EP - 594

JO - Astronomical Journal

JF - Astronomical Journal

SN - 0004-6256

IS - 2

ER -