Hans Olofsson | Karlstad University (original) (raw)

Papers by Hans Olofsson

Astronomy & Astrophysics, 2008

Abstract Aims. In March 2008, the APEX facility instrument was installed on the telescope at the ... more Abstract Aims. In March 2008, the APEX facility instrument was installed on the telescope at the site of Lliano Chajnantor in northern Chile. The main objective of the paper is to introduce the new instrument to the radio astronomical community. It describes the hardware ...

Astronomy & Astrophysics, 2002

We have compared the results of this M-star sample with a similar C-star sample analysed in the s... more We have compared the results of this M-star sample with a similar C-star sample analysed in the same way. The mass loss rate characteristics are very similar for the two samples. On the contrary, the gas expansion velocity distributions are clearly different. In particular, the number of low-velocity sources is much higher in the M-star sample. We found no example of the sharply double-peaked CO line profile, which is evidence of a large, detached CO-shell, among the M-stars. About 10% of the C-stars show this phenomenon.

Astrophysical Journal Supplement Series, 1993

We have estimated and compared circumstellar and photospheric HCN, CN, and CS abundances for a sa... more We have estimated and compared circumstellar and photospheric HCN, CN, and CS abundances for a sample of bright carbon stars. The circumstellar HCN and CS abundances roughly correlate with the photospheric ones, but the former appear to be systematically overestimated by a factor of 5-10. Although we cannot exclude the presence of a circumstellar chemistry that efficiently produces HCN and CS in the inner parts of these relatively hot circumstellar envelopes, we attribute this difference between photospheric and circumstellar abundances to errors in the circumstellar envelope model used. In particular, for the low mass-loss rate objects a systematic underestimate of the mass-loss rate may be suspected. In addition to this general trend, stars that are in some sense peculiar also show circumstellar abundances that deviate significantly from the expected. There is evidence for the presence of maser features in the HCN (J = 1-0) emission toward some of the stars. The estimated circumstellar CN abundances suggest that this species is a photodissociation product of HCN. A search for circumstellar HNC, SiS, and HC3N shows that molecular emission from species other than CO, HCN, CN, and CS is very weak in these low mass-loss rate objects. An attempt has been made to determine the chemistry in the (probably detached) envelope around R Scl. The data suggest a carbon-rich chemistry. Finally, it is shown that the circumstellar 12CO/13CO and H12CN/H13CN intensity ratios roughly correlate with the photospheric 12C/13C isotope ratios for a small number of low mass-loss rate objects, although the former ratios are systematically lower than the latter by a factor of about 2.

Astronomy & Astrophysics, 2007

New SiS multi-transition (sub-)millimetre line observations of a sample of AGB stars with varying... more New SiS multi-transition (sub-)millimetre line observations of a sample of AGB stars with varying photospheric C/O-ratios and mass-loss rates are presented. A combination of low- and high-energy lines are important in constraining the circumstellar distribution of SiS molecules. A detailed radiative transfer modelling of the observed SiS line emission is performed, including the effect of thermal dust grains in the excitation analysis. We find that the circumstellar fractional abundance of SiS in these environments has a strong dependence on the photospheric C/O-ratio as expected from chemical models. The carbon stars (C/O>1) have a mean fractional abundance of 3.1E-6, about an order of magnitude higher than found for the M-type AGB stars (C/O<1) where the mean value is 2.7E-7. These numbers are in reasonable agreement with photospheric LTE chemical models. SiS appears to behave similar to SiO in terms of photodissociation in the outer part of the circumstellar envelope. In contrast to previous results for the related molecule SiO, there is no strong correlation of the fractional abundance with density in the CSE, as would be the case if freeze-out onto dust grains were important. However, possible time-variability of the line emission in the lower J transitions and the sensitivity of the line emission to abundance gradients in the inner part of the CSE may mask a correlation with the density of the wind. There are indications that the SiS fractional abundance could be significantly higher closer to the star which, at least in the case of M-type AGB stars, would require non-equilibrium chemical processes.

Astronomy & Astrophysics, 2004

High angular resolution Australia Telescope Compact Array (ATCA) observations of SiO "thermal" mi... more High angular resolution Australia Telescope Compact Array (ATCA) observations of SiO "thermal" millimetre line emission towards the two oxygen-rich, low mass loss rate AGB stars R Dor and L 2 Pup are presented. In both cases the emission is resolved with an overall spherical symmetry. Detailed radiative transfer modelling of the SiO line emission has been performed, and the comparison between observations and models are conducted in the visibility plane, maximizing the sensitivity. The excitation analysis suggests that the abundance of SiO is as high as 4 × 10 −5 in the inner part of the wind, close to the predicted values from stellar atmosphere models. Beyond a radius of ≈1 × 10 15 cm the SiO abundance is significantly lower, about 3 × 10 −6 , until it decreases strongly at a radius of about 3 × 10 15 cm. This is consistent with a scenario where SiO first freezes out onto dust grains, and then eventually becomes photodissociated by the interstellar UV-radiation field. In these low expansion velocity sources the turbulent broadening of the lines plays an important role in the line formation. Microturbulent velocity widths in the range 1.1−1.5 km s −1 result in a very good reproduction of the observed line shapes even if the gas expansion velocity is kept constant. This, combined with the fact that the SiO and CO lines are well fitted using the same gas expansion velocity (to within 5−10%), suggest that the envelope acceleration occurs close to the stellar photosphere, within < ∼ 20−30 stellar radii.

Astrophysical Journal, 2006

New interferometric observations of SiO J=5-4 circumstellar line emission around the carbon star ... more New interferometric observations of SiO J=5-4 circumstellar line emission around the carbon star IRC+10216, using the Submillimeter Array, are presented. Complemented by multi-transition single-dish observations, including infrared observations of ro-vibrational transitions, detailed radiative transfer modelling suggests that the fractional abundance of SiO in the inner part of the envelope, between approximately 3-8 stellar radii, is as high as 1.5E-6. This is more than an order of magnitude higher than predicted by equilibrium stellar atmosphere chemistry in a carbon-rich environment and indicative of the importance of non-LTE chemical processes. In addition to the compact component, a spatially more extended (2.4E16 cm) low-fractional-abundance (1.7E-7) region is required to fit the observations. This suggests that the majority of the SiO molecules are effectively accreted onto dust grains in the inner wind while the remaining gas-phase molecules are eventually photodissociated at larger distances. Evidence of departure from a smooth wind is found in the observed visibilities, indicative of density variations of a factor 2 to 5 on an angular scale corresponding to a time scale of about 200 years. Additionally, constraints on the velocity structure of the wind are obtained.

Astronomy & Astrophysics, 2002

We present a millimetre molecular line survey of seven high mass loss rate carbon stars in both t... more We present a millimetre molecular line survey of seven high mass loss rate carbon stars in both the northern and southern skies. A total of 196 emission lines (47 transitions) from 24 molecular species were detected. The observed CO emission is used to determine mass-loss rates and the physical structure of the circumstellar envelope, such as the density and temperature structure, using a detailed radiative transfer analysis. This enables abundances for the remaining molecular species to be determined. The derived abundances generally vary between the sources by no more than a factor of five indicating that circumstellar envelopes around carbon stars with high mass-loss rates have similar chemical compositions. However, there are some notable exceptions. The most striking difference between the abundances are reflecting the spread in the 12C/13C-ratio of about an order of magnitude between the sample stars, mainly reflecting the results of nucleosynthesis. The abundance of SiO also shows a variation of more than an order of magnitude between the sources and is on the average more than an order of magnitude more abundant than predicted from photospheric chemistry in thermal equilibrium. The over abundance of SiO is consistent with dynamical modelling of the stellar atmosphere and the inner parts of the wind where a pulsation-driven shock has passed. This scenario is possibly further substantiated by the relatively low amount of CS present in the envelopes. The chemistry occurring in the outer envelope is consistent with current photochemical models.

Astronomy & Astrophysics, 2003

Imaging polarimetry has been used to study the extended, detached circumstellar shells around the... more Imaging polarimetry has been used to study the extended, detached circumstellar shells around the bright carbon stars R Scl and U Ant. The observations were done in two narrow band filters centred on the resonance lines of neutral K and Na, but much broader than the expected line widths. The polarimetric data reveal brightness distributions, in both cases, which are in perfect agreement with previous observations of scattered light obtained through direct imaging techniques. The total intensity images towards R Scl outline, in both filters, circular disk-like distributions out to a radius of ≈21 , where the intensity drops sharply. The polarised intensity images reveal, however, that the scattering occurs in a geometrically thin shell. The degree of polarisation reaches values of ≈35% in both filters. The imaging polarimetry observations of U Ant reveal a somewhat more complex structure, where the existence of several shells can be discerned. The polarised scattered light comes from a component, at a radius of ≈50 from the star, which lies outside the region where the bulk of the light is scattered. The latter comes from a dominating shell at ≈43 , which coincides spatially with the detached gas shell inferred from CO radio line data, and there may be another two shells inside this. The polarisation degree reaches ≈50% in the outer component. We model, with a code based on the Monte Carlo method, the scattered emission under the assumption of dust scattering, using the observed polarised brightness distributions as constraints. In the case of R Scl we found that the polarised, as well as the total, light distributions can be explained by scattering in a 2 wide shell of radius 20 containing a dust mass of ≈2 × 10 −6 M . This dust shell is also responsible for the thermal dust emission measured by IRAS. There is room, up to 30% of the total scattered flux, for other scattering agents. Comparison with CO radio line data shows that this dust shell probably lies outside the detached CO gas shell. In the case of U Ant the modelling explains the outer component in terms of a 5 wide shell at a radius of about 52 with a dust mass of ≈4 × 10 −6 M . This is also the dust shell responsible for the emission measured by IRAS. However, the bulk of the scattered light cannot in this case be due to scattering by dust. In accordance with a discussion in a previous paper we attribute the remaining, unpolarised, scattering to the KI and Na D resonance lines. In both cases we found evidence that a dust shell has separated from the rest of the circumstellar medium. This may be due to gas-grain drift, or to hydrodynamical effects, which may also explain the complex multiple-shell structure seen towards U Ant. The model results are very dependent on the grain size distribution, and the observational data can only be reconciled with a very steep decline in grain size.

Astronomy & Astrophysics, 2005

Two bipolar proto-planetary nebulae, IRAS 16594-4656 and IRAS 17150-3224, have been detected in v... more Two bipolar proto-planetary nebulae, IRAS 16594-4656 and IRAS 17150-3224, have been detected in various molecular lines, namely CO, 13 CO, HCN and CN, and remain undetected in several other species. CO(J = 2-1) and CO(J = 3-2) line profiles are compared to new spectra of similar PPN candidates, previously undetected in CO(J = 2-1): CPD-53 • 5736, IRAS 17106-3046, IRAS 17245-3951 and IRAS 17441-2411. CO(J = 2-1) maps of IRAS 16594-4656 and IRAS 17150-3224

Astronomy & Astrophysics, 2002

An extensive modelling of CO line emission from the circumstellar envelopes around a number of ca... more An extensive modelling of CO line emission from the circumstellar envelopes around a number of carbon stars is performed. By combining radio observations and infrared observations obtained by ISO the circumstellar envelope characteristics are probed over a large radial range. In the radiative transfer analysis the observational data are consistently reproduced assuming a spherically symmetric and smooth wind expanding at a constant velocity. The combined data set gives better determined envelope parameters, and puts constraints on the mass loss history of these carbon stars. The importance of dust in the excitation of CO is addressed using a radiative transfer analysis of the observed continuum emission, and it is found to have only minor effects on the derived line intensities. The analysis of the dust emission also puts further constraints on the mass loss rate history. The stars presented here are not likely to have experienced any drastic long-term mass loss rate modulations, at least less than a factor of about 5, over the past thousands of years. Only three, out of nine, carbon stars were observed long enough by ISO to allow a detection of CO far-infrared rotational lines.

Astronomy & Astrophysics, 2008

Conclusions: The high water abundance found for the majority of the sources suggests that either ... more Conclusions: The high water abundance found for the majority of the sources suggests that either the `normal' chemical processes are very effective in producing H2O, or else non-local thermal equilibrium atmospheric chemistry, grain surface reactions, or a release of H_2O (e.g. from icy bodies like Kuiper belt objects) play a role. We provide predictions for ortho-H2O lines in the spectral window of Herschel/HIFI.

Astronomy & Astrophysics, 2010

Context: Geometrically thin, detached shells of gas have been found around a handful of carbon st... more Context: Geometrically thin, detached shells of gas have been found around a handful of carbon stars. --Aims: Previous observations of scattered stellar light in the circumstellar medium around the carbon star U Ant were taken through filters centred on the resonance lines of K and Na. These observations could not separate the scattering by dust and atoms. The aim of this paper is to remedy this situation. --Methods: We have obtained polarization data on stellar light scattered in the circumstellar medium around U Ant through filters which contain no strong lines, making it possible to differentiate between the two scattering agents. Kinematic, as well as spatial, information on the gas shells were obtained through high-resolution echelle spectrograph observations of the KI and NaD lines. --Results: We confirm the existence of two detached shells around U Ant. The inner shell (at a radius of approx 43" and a width of approx 2") consists mainly of gas, while the outer shell (at a radius of approx 50" and a width of approx 7") appears to consist exclusively of dust. Both shells appear to have an over-all spherical geometry. The gas shell mass is estimated to be 2x10^-3 M(Sun), while the mass of the dust shell is estimated to be 5x10^-5 M(Sun). The derived expansion velocity, from the KI and NaD lines, of the gas shell, 19.5 km/s, agrees with that obtained from CO radio line data. The inferred shell age is 2700 years. There is structure, e.g. in the form of arcs, inside the gas shell, but it is not clear whether these are due to additional shells. --Conclusions: Our results support the hypothesis that the observed geometrically thin, detached shells around carbon stars are the results of brief periods of intense mass loss, probably associated with thermal pulses, and subsequent wind-wind interactions.

Astrophysical Journal Supplement Series, 1993

Results are presented of a survey of circumstellar CO emission on a sample of bright carbon stars... more Results are presented of a survey of circumstellar CO emission on a sample of bright carbon stars, which is relatively complete out to about 900 pc from the sun. A total of 68 detections were made. All objects within 600 pc of the sun were detected. It is suggested that the large majority of all carbon stars have circumstellar envelopes.

Astronomy & Astrophysics, 2008

Abstract Aims. In March 2008, the APEX facility instrument was installed on the telescope at the ... more Abstract Aims. In March 2008, the APEX facility instrument was installed on the telescope at the site of Lliano Chajnantor in northern Chile. The main objective of the paper is to introduce the new instrument to the radio astronomical community. It describes the hardware ...

Astronomy & Astrophysics, 2002

We have compared the results of this M-star sample with a similar C-star sample analysed in the s... more We have compared the results of this M-star sample with a similar C-star sample analysed in the same way. The mass loss rate characteristics are very similar for the two samples. On the contrary, the gas expansion velocity distributions are clearly different. In particular, the number of low-velocity sources is much higher in the M-star sample. We found no example of the sharply double-peaked CO line profile, which is evidence of a large, detached CO-shell, among the M-stars. About 10% of the C-stars show this phenomenon.

Astrophysical Journal Supplement Series, 1993

We have estimated and compared circumstellar and photospheric HCN, CN, and CS abundances for a sa... more We have estimated and compared circumstellar and photospheric HCN, CN, and CS abundances for a sample of bright carbon stars. The circumstellar HCN and CS abundances roughly correlate with the photospheric ones, but the former appear to be systematically overestimated by a factor of 5-10. Although we cannot exclude the presence of a circumstellar chemistry that efficiently produces HCN and CS in the inner parts of these relatively hot circumstellar envelopes, we attribute this difference between photospheric and circumstellar abundances to errors in the circumstellar envelope model used. In particular, for the low mass-loss rate objects a systematic underestimate of the mass-loss rate may be suspected. In addition to this general trend, stars that are in some sense peculiar also show circumstellar abundances that deviate significantly from the expected. There is evidence for the presence of maser features in the HCN (J = 1-0) emission toward some of the stars. The estimated circumstellar CN abundances suggest that this species is a photodissociation product of HCN. A search for circumstellar HNC, SiS, and HC3N shows that molecular emission from species other than CO, HCN, CN, and CS is very weak in these low mass-loss rate objects. An attempt has been made to determine the chemistry in the (probably detached) envelope around R Scl. The data suggest a carbon-rich chemistry. Finally, it is shown that the circumstellar 12CO/13CO and H12CN/H13CN intensity ratios roughly correlate with the photospheric 12C/13C isotope ratios for a small number of low mass-loss rate objects, although the former ratios are systematically lower than the latter by a factor of about 2.

Astronomy & Astrophysics, 2007

New SiS multi-transition (sub-)millimetre line observations of a sample of AGB stars with varying... more New SiS multi-transition (sub-)millimetre line observations of a sample of AGB stars with varying photospheric C/O-ratios and mass-loss rates are presented. A combination of low- and high-energy lines are important in constraining the circumstellar distribution of SiS molecules. A detailed radiative transfer modelling of the observed SiS line emission is performed, including the effect of thermal dust grains in the excitation analysis. We find that the circumstellar fractional abundance of SiS in these environments has a strong dependence on the photospheric C/O-ratio as expected from chemical models. The carbon stars (C/O>1) have a mean fractional abundance of 3.1E-6, about an order of magnitude higher than found for the M-type AGB stars (C/O<1) where the mean value is 2.7E-7. These numbers are in reasonable agreement with photospheric LTE chemical models. SiS appears to behave similar to SiO in terms of photodissociation in the outer part of the circumstellar envelope. In contrast to previous results for the related molecule SiO, there is no strong correlation of the fractional abundance with density in the CSE, as would be the case if freeze-out onto dust grains were important. However, possible time-variability of the line emission in the lower J transitions and the sensitivity of the line emission to abundance gradients in the inner part of the CSE may mask a correlation with the density of the wind. There are indications that the SiS fractional abundance could be significantly higher closer to the star which, at least in the case of M-type AGB stars, would require non-equilibrium chemical processes.

Astronomy & Astrophysics, 2004

High angular resolution Australia Telescope Compact Array (ATCA) observations of SiO "thermal" mi... more High angular resolution Australia Telescope Compact Array (ATCA) observations of SiO "thermal" millimetre line emission towards the two oxygen-rich, low mass loss rate AGB stars R Dor and L 2 Pup are presented. In both cases the emission is resolved with an overall spherical symmetry. Detailed radiative transfer modelling of the SiO line emission has been performed, and the comparison between observations and models are conducted in the visibility plane, maximizing the sensitivity. The excitation analysis suggests that the abundance of SiO is as high as 4 × 10 −5 in the inner part of the wind, close to the predicted values from stellar atmosphere models. Beyond a radius of ≈1 × 10 15 cm the SiO abundance is significantly lower, about 3 × 10 −6 , until it decreases strongly at a radius of about 3 × 10 15 cm. This is consistent with a scenario where SiO first freezes out onto dust grains, and then eventually becomes photodissociated by the interstellar UV-radiation field. In these low expansion velocity sources the turbulent broadening of the lines plays an important role in the line formation. Microturbulent velocity widths in the range 1.1−1.5 km s −1 result in a very good reproduction of the observed line shapes even if the gas expansion velocity is kept constant. This, combined with the fact that the SiO and CO lines are well fitted using the same gas expansion velocity (to within 5−10%), suggest that the envelope acceleration occurs close to the stellar photosphere, within < ∼ 20−30 stellar radii.

Astrophysical Journal, 2006

New interferometric observations of SiO J=5-4 circumstellar line emission around the carbon star ... more New interferometric observations of SiO J=5-4 circumstellar line emission around the carbon star IRC+10216, using the Submillimeter Array, are presented. Complemented by multi-transition single-dish observations, including infrared observations of ro-vibrational transitions, detailed radiative transfer modelling suggests that the fractional abundance of SiO in the inner part of the envelope, between approximately 3-8 stellar radii, is as high as 1.5E-6. This is more than an order of magnitude higher than predicted by equilibrium stellar atmosphere chemistry in a carbon-rich environment and indicative of the importance of non-LTE chemical processes. In addition to the compact component, a spatially more extended (2.4E16 cm) low-fractional-abundance (1.7E-7) region is required to fit the observations. This suggests that the majority of the SiO molecules are effectively accreted onto dust grains in the inner wind while the remaining gas-phase molecules are eventually photodissociated at larger distances. Evidence of departure from a smooth wind is found in the observed visibilities, indicative of density variations of a factor 2 to 5 on an angular scale corresponding to a time scale of about 200 years. Additionally, constraints on the velocity structure of the wind are obtained.

Astronomy & Astrophysics, 2002

We present a millimetre molecular line survey of seven high mass loss rate carbon stars in both t... more We present a millimetre molecular line survey of seven high mass loss rate carbon stars in both the northern and southern skies. A total of 196 emission lines (47 transitions) from 24 molecular species were detected. The observed CO emission is used to determine mass-loss rates and the physical structure of the circumstellar envelope, such as the density and temperature structure, using a detailed radiative transfer analysis. This enables abundances for the remaining molecular species to be determined. The derived abundances generally vary between the sources by no more than a factor of five indicating that circumstellar envelopes around carbon stars with high mass-loss rates have similar chemical compositions. However, there are some notable exceptions. The most striking difference between the abundances are reflecting the spread in the 12C/13C-ratio of about an order of magnitude between the sample stars, mainly reflecting the results of nucleosynthesis. The abundance of SiO also shows a variation of more than an order of magnitude between the sources and is on the average more than an order of magnitude more abundant than predicted from photospheric chemistry in thermal equilibrium. The over abundance of SiO is consistent with dynamical modelling of the stellar atmosphere and the inner parts of the wind where a pulsation-driven shock has passed. This scenario is possibly further substantiated by the relatively low amount of CS present in the envelopes. The chemistry occurring in the outer envelope is consistent with current photochemical models.

Astronomy & Astrophysics, 2003

Imaging polarimetry has been used to study the extended, detached circumstellar shells around the... more Imaging polarimetry has been used to study the extended, detached circumstellar shells around the bright carbon stars R Scl and U Ant. The observations were done in two narrow band filters centred on the resonance lines of neutral K and Na, but much broader than the expected line widths. The polarimetric data reveal brightness distributions, in both cases, which are in perfect agreement with previous observations of scattered light obtained through direct imaging techniques. The total intensity images towards R Scl outline, in both filters, circular disk-like distributions out to a radius of ≈21 , where the intensity drops sharply. The polarised intensity images reveal, however, that the scattering occurs in a geometrically thin shell. The degree of polarisation reaches values of ≈35% in both filters. The imaging polarimetry observations of U Ant reveal a somewhat more complex structure, where the existence of several shells can be discerned. The polarised scattered light comes from a component, at a radius of ≈50 from the star, which lies outside the region where the bulk of the light is scattered. The latter comes from a dominating shell at ≈43 , which coincides spatially with the detached gas shell inferred from CO radio line data, and there may be another two shells inside this. The polarisation degree reaches ≈50% in the outer component. We model, with a code based on the Monte Carlo method, the scattered emission under the assumption of dust scattering, using the observed polarised brightness distributions as constraints. In the case of R Scl we found that the polarised, as well as the total, light distributions can be explained by scattering in a 2 wide shell of radius 20 containing a dust mass of ≈2 × 10 −6 M . This dust shell is also responsible for the thermal dust emission measured by IRAS. There is room, up to 30% of the total scattered flux, for other scattering agents. Comparison with CO radio line data shows that this dust shell probably lies outside the detached CO gas shell. In the case of U Ant the modelling explains the outer component in terms of a 5 wide shell at a radius of about 52 with a dust mass of ≈4 × 10 −6 M . This is also the dust shell responsible for the emission measured by IRAS. However, the bulk of the scattered light cannot in this case be due to scattering by dust. In accordance with a discussion in a previous paper we attribute the remaining, unpolarised, scattering to the KI and Na D resonance lines. In both cases we found evidence that a dust shell has separated from the rest of the circumstellar medium. This may be due to gas-grain drift, or to hydrodynamical effects, which may also explain the complex multiple-shell structure seen towards U Ant. The model results are very dependent on the grain size distribution, and the observational data can only be reconciled with a very steep decline in grain size.

Astronomy & Astrophysics, 2005

Two bipolar proto-planetary nebulae, IRAS 16594-4656 and IRAS 17150-3224, have been detected in v... more Two bipolar proto-planetary nebulae, IRAS 16594-4656 and IRAS 17150-3224, have been detected in various molecular lines, namely CO, 13 CO, HCN and CN, and remain undetected in several other species. CO(J = 2-1) and CO(J = 3-2) line profiles are compared to new spectra of similar PPN candidates, previously undetected in CO(J = 2-1): CPD-53 • 5736, IRAS 17106-3046, IRAS 17245-3951 and IRAS 17441-2411. CO(J = 2-1) maps of IRAS 16594-4656 and IRAS 17150-3224

Astronomy & Astrophysics, 2002

An extensive modelling of CO line emission from the circumstellar envelopes around a number of ca... more An extensive modelling of CO line emission from the circumstellar envelopes around a number of carbon stars is performed. By combining radio observations and infrared observations obtained by ISO the circumstellar envelope characteristics are probed over a large radial range. In the radiative transfer analysis the observational data are consistently reproduced assuming a spherically symmetric and smooth wind expanding at a constant velocity. The combined data set gives better determined envelope parameters, and puts constraints on the mass loss history of these carbon stars. The importance of dust in the excitation of CO is addressed using a radiative transfer analysis of the observed continuum emission, and it is found to have only minor effects on the derived line intensities. The analysis of the dust emission also puts further constraints on the mass loss rate history. The stars presented here are not likely to have experienced any drastic long-term mass loss rate modulations, at least less than a factor of about 5, over the past thousands of years. Only three, out of nine, carbon stars were observed long enough by ISO to allow a detection of CO far-infrared rotational lines.

Astronomy & Astrophysics, 2008

Conclusions: The high water abundance found for the majority of the sources suggests that either ... more Conclusions: The high water abundance found for the majority of the sources suggests that either the `normal' chemical processes are very effective in producing H2O, or else non-local thermal equilibrium atmospheric chemistry, grain surface reactions, or a release of H_2O (e.g. from icy bodies like Kuiper belt objects) play a role. We provide predictions for ortho-H2O lines in the spectral window of Herschel/HIFI.

Astronomy & Astrophysics, 2010

Context: Geometrically thin, detached shells of gas have been found around a handful of carbon st... more Context: Geometrically thin, detached shells of gas have been found around a handful of carbon stars. --Aims: Previous observations of scattered stellar light in the circumstellar medium around the carbon star U Ant were taken through filters centred on the resonance lines of K and Na. These observations could not separate the scattering by dust and atoms. The aim of this paper is to remedy this situation. --Methods: We have obtained polarization data on stellar light scattered in the circumstellar medium around U Ant through filters which contain no strong lines, making it possible to differentiate between the two scattering agents. Kinematic, as well as spatial, information on the gas shells were obtained through high-resolution echelle spectrograph observations of the KI and NaD lines. --Results: We confirm the existence of two detached shells around U Ant. The inner shell (at a radius of approx 43" and a width of approx 2") consists mainly of gas, while the outer shell (at a radius of approx 50" and a width of approx 7") appears to consist exclusively of dust. Both shells appear to have an over-all spherical geometry. The gas shell mass is estimated to be 2x10^-3 M(Sun), while the mass of the dust shell is estimated to be 5x10^-5 M(Sun). The derived expansion velocity, from the KI and NaD lines, of the gas shell, 19.5 km/s, agrees with that obtained from CO radio line data. The inferred shell age is 2700 years. There is structure, e.g. in the form of arcs, inside the gas shell, but it is not clear whether these are due to additional shells. --Conclusions: Our results support the hypothesis that the observed geometrically thin, detached shells around carbon stars are the results of brief periods of intense mass loss, probably associated with thermal pulses, and subsequent wind-wind interactions.

Astrophysical Journal Supplement Series, 1993

Results are presented of a survey of circumstellar CO emission on a sample of bright carbon stars... more Results are presented of a survey of circumstellar CO emission on a sample of bright carbon stars, which is relatively complete out to about 900 pc from the sun. A total of 68 detections were made. All objects within 600 pc of the sun were detected. It is suggested that the large majority of all carbon stars have circumstellar envelopes.