H<sup>12</sup>CN and H<sup>13</sup>CN excitation analysis in the circumstellar outflow of R Sculptoris

Saberi, Maryam; Maercker, M.; Beck, E. De; Vlemmings, Wouter; Olofsson, H.; Danilovich, T.

doi:10.1051/0004-6361/201629901

Cited by 9 publications

(12 citation statements)

References 47 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…Hence, the H 12 CN/H 13 CN ratio might be a more reliable tracer of the 12 C/ 13 C ratio, provided one accounts correctly for the optical thickness of the emission, which can be substantial in carbonrich CSEs formed by high mass-loss rates. In the case of R Scl, Saberi et al (2017) showed that the H 12 CN/H 13 CN ratio is in fact in agreement with the photospheric 12 C/ 13 C ratio, while the 12 CO/ 13 CO ratio varies by a factor of three through the CSE (Vlemmings et al 2013).…”

Section: Introductionmentioning

confidence: 81%

“…The photodissociation of HCN does not occur in lines. Instead photons in a broad UV range can lead to dissociation of the HCN molecule, and the photodissociation cross section is therefor continuous in wavelength (e.g., van Dishoeck & Visser 2011;Saberi et al 2017). Therefore, the H 12 CN/H 13 CN ratio is not expected to be affected by selective photodissociation.…”

Section: Hcn Isotopologuesmentioning

confidence: 99%

See 1 more Smart Citation

CO and HCN isotopologue ratios in the outflows of AGB stars

Saberi

Olofsson

Vlemmings

et al. 2020

A&A

Self Cite

View full text Add to dashboard Cite

Context. Isotopologue line intensity ratios of circumstellar molecules have been widely used to trace the photospheric elemental isotopic ratios of evolved stars. However, depending on the molecular species and the physical conditions of the environment, the isotopologue ratio in the circumstellar envelope (CSE) may deviate considerably from the stellar atmospheric value. Aims. In this paper, we aim to examine how the 12CO/13CO and H12CN/H13CN abundance ratios vary radially due to chemical reactions in the outflows of asymptotic giant branch (AGB) stars and the effect of excitation and optical depth on the resulting line intensity ratios. We study both carbon-rich (C-type) and oxygen-rich (O-type) CSEs. Methods. We performed chemical modeling to derive radial abundance distributions of our selected species in the CSEs over a wide range of mass-loss rates (10−8 < Ṁ < 10−4 M⊙ yr−1). We used these as input in a non-local thermodynamic equilibrium radiative transfer code to derive the line intensities of several ground-state rotational transitions. We also test the influence of stellar parameters, physical conditions in the outflows, the intensity of the interstellar radiation field, and the importance of considering the chemical networks in our model results. Results. We quantified deviations from the atmospheric value for typical outflows. We find that the circumstellar value of 12CO/13CO can deviate from its atmospheric value by up to 25–94% and 6–60% for C- and O-type CSEs, respectively, in radial ranges that depend on the mass-loss rate. We show that variations of the intensity of the interstellar radiation field and the gas kinetic temperature can significantly influence the CO isotopologue abundance ratio in the outer CSEs of both C-type and O-type. On the contrary, the H12CN/H13CN abundance ratio is stable throughout the CSEs for all tested mass-loss rates. The radiative transfer modeling shows that the integrated line intensity ratio I12CO/I13CO of different rotational transitions varies significantly for stars with mass-loss rates in the range from 10−7 to 10−6 M⊙ yr−1 due to combined chemical and excitation effects. In contrast, the excitation conditions for the HCN isotopologues are the same for both isotopologues. Conclusions. We demonstrate the importance of using the isotopologue abundance profiles from detailed chemical models as inputs to radiative transfer models in the interpretation of isotopologue observations. Previous studies of circumstellar CO isotopologue ratios are based on multi-transition data for individual sources and it is difficult to estimate the errors in the reported values due to assumptions that are not entirely correct according to this study. If anything, previous studies may have overestimated the circumstellar 12CO/13CO abundance ratio. The use of the HCN molecule as a tracer of C isotope ratios is affected by fewer complicating problems, but we note that the corrections for high optical depths are very large in the case of high-mass-loss-rate C-type CSEs; and in O-type CSEs the H13CN lines may be too weak to detect.

show abstract

Section: Introductionmentioning

confidence: 81%

Section: Hcn Isotopologuesmentioning

confidence: 99%

CO and HCN isotopologue ratios in the outflows of AGB stars

Saberi

Olofsson

Vlemmings

et al. 2020

A&A

Self Cite

View full text Add to dashboard Cite

show abstract

“…Hence, the H 12 CN/H 13 CN ratio might be a more reliable tracer of the 12 C/ 13 C ratio, provided one accounts correctly for the optical thickness of the emission, which can be substantial in carbon-rich CSEs formed by high mass-loss rates. In the case of R Scl, Saberi et al (2017) showed that the H 12 CN/H 13 CN ratio is in fact in agreement with the photospheric 12 C/ 13 C ratio, while the 12 CO/ 13 CO ratio varies by a factor of three through the CSE (Vlemmings et al 2013).…”

Section: Introductionmentioning

confidence: 81%

CO and HCN isotopologue ratios in the outflows of AGB stars

Saberi,

Olofsson,

Vlemmings

et al. 2020

Preprint

Self Cite

View full text Add to dashboard Cite

Context. Isotopologue line intensity ratios of circumstellar molecules have been widely used to trace the photospheric elemental isotopic ratios of evolved stars. However, depending on the molecular species and the physical conditions of the environment, the isotopologue ratio in the circumstellar envelope (CSE) may deviate considerably from the stellar atmospheric value. Aims. In this paper, we aim to examine how the 12 CO/ 13 CO and H 12 CN/H 13 CN abundance ratios vary radially due to chemical reactions in the outflows of asymptotic giant branch (AGB) stars and the effect of excitation and optical depth on the resulting line intensity ratios. We study both carbon-rich (C-type) and oxygen-rich (O-type) CSEs. Methods. We performed chemical modeling to derive radial abundance distributions of our selected species in the CSEs over a wide range of mass-loss rates (10 −8 < Ṁ < 10 −4 M yr −1 ). We used these as input in a non-local thermodynamic equilibrium (non-LTE) radiative transfer code to derive the line intensities of several ground-state rotational transitions. We also test the influence of stellar parameters, physical conditions in the outflows, the intensity of the interstellar radiation field, and the importance of considering the chemical networks in our model results.Results. We quantified deviations from the atmospheric value for typical outflows. We find that the circumstellar value of 12 CO/ 13 CO can deviate from its atmospheric value by up to 25-94 % and 6-60 % for C-and O-type CSEs, respectively, in radial ranges that depend on the mass-loss rate. We show that variations of the intensity of the interstellar radiation field and the gas kinetic temperature can significantly influence the CO isotopologue abundance ratio in the outer CSEs of both C-type and O-type. On the contrary, the H 12 CN/H 13 CN abundance ratio is stable throughout the CSEs for all tested mass-loss rates. The radiative transfer modeling shows that the integrated line intensity ratio I12 CO /I13 CO of different rotational transitions varies significantly for stars with mass-loss rates in the range from 10 −7 to 10 −6 M yr −1 due to combined chemical and excitation effects. In contrast, the excitation conditions for the HCN isotopologues are the same for both isotopologues. Conclusions. We demonstrate the importance of using the isotopologue abundance profiles from detailed chemical models as inputs to radiative transfer models in the interpretation of isotopologue observations. Previous studies of circumstellar CO isotopologue ratios are based on multi-transition data for individual sources and it is difficult to estimate the errors in the reported values due to assumptions that are not entirely correct according to this study. If anything, previous studies may have overestimated the circumstellar 12 CO/ 13 CO abundance ratio. The use of the HCN molecule as a tracer of C isotope ratios is affected by fewer complicating problems, but we note that the corrections for high optical depths are very large in the case of high-mass-loss-ra...

show abstract

“…Chemical models which consider an internal UV field can explain the variation of the 12 CO/ 13 CO throughout the CSE. On the other hand, from a detailed excitation analysis of H 12 CN and H 13 CN, Saberi et al (2017) found a ratio of H 12 CN/H 13 CN ∼ 26 ± 12 close to the star, in agreement with the photospheric estimate of 12 C/ 13 C ∼ 19 ± 6 reported by Lambert et al (1986). Since CO isotopologues are photodissociated by the UV radiation in well-defined bands and HCN is dissociated in the continuum, we suggested that isotopeselective photodissociation of 12 CO and 13 CO causes this discrepancy (Saberi et al 2017).…”

Section: The Isotopologue Ratiomentioning

confidence: 99%

“…To constrain the effects of both internal and external UV radiation in the chemical modelling of CSEs, observations of main photodissociation/photoionization products are required. Moreover, comparison of isotopologue ratios of molecular species such as 12 CO/ 13 CO and H 12 CN/H 13 CN, which dissociate through different mechanisms, can be used to trace the UV-chemistry (Saberi et al 2017).…”

Section: Introductionmentioning

confidence: 99%

The Impact of UV Radiation on Circumstellar Chemistry

Saberi¹,

Vlemmings²,

Millar

et al. 2018

Proc. IAU

View full text Add to dashboard Cite

UV 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).

show abstract

H¹²CN and H¹³CN excitation analysis in the circumstellar outflow of R Sculptoris

Cited by 9 publications

References 47 publications

CO and HCN isotopologue ratios in the outflows of AGB stars

CO and HCN isotopologue ratios in the outflows of AGB stars

CO and HCN isotopologue ratios in the outflows of AGB stars

The Impact of UV Radiation on Circumstellar Chemistry

Contact Info

Product

Resources

About

H12CN and H13CN excitation analysis in the circumstellar outflow of R Sculptoris

Cited by 9 publications

References 47 publications

CO and HCN isotopologue ratios in the outflows of AGB stars

CO and HCN isotopologue ratios in the outflows of AGB stars

CO and HCN isotopologue ratios in the outflows of AGB stars

The Impact of UV Radiation on Circumstellar Chemistry

Contact Info

Product

Resources

About

H¹²CN and H¹³CN excitation analysis in the circumstellar outflow of R Sculptoris