Charting Circumstellar Chemistry of Carbon-rich AGB Stars

Stars of low to intermediate initial masses (0.8 - 8 M⊙) enter the asymptotic giant branch (AGB) phase during their late evolution. This phase is characterised by intense mass loss from the stellar surface into the interstellar medium (ISM), eventually leading to the formation of an extended circumstellar envelope (CSE) composed of the ejected gas and dust around the star. These stars are also the birthplace of many heavy elements. AGB stars thus contribute heavily to the chemical replenishment of the ISM in galaxies, by enriching it with the nuclear-processed material dredged up from the stellar interiors. This makes AGB CSEs very interesting cosmic chemical laboratories, sites of diverse, and often complex chemistry.\ua0Based on the relative abundances of carbon and oxygen in their atmospheres, AGB stars are categorised into C-rich and O-rich, the carbon stars being more chemically complex than their oxygen-rich counterparts. The study of molecular line emission from AGB CSEs is of particular interest, as it can help constrain both the physical and chemical characteristics of the envelopes. However, much of our current knowledge of AGB circumstellar chemistry, particularly of the C-rich type, is based on observations and models of a single object, IRC +10 216, which is often regarded as an archetypical carbon star. Advances in instrumentation, including the development of high angular resolution interferometers, have opened up possibilities for observational studies of additional sources in unprecedented detail.\ua0This thesis summarises our current understanding of the chemistry in C-rich CSEs, and presents the first spatially-resolved, unbiased spectral surveys of the circumstellar molecular emission from multiple carbon stars other than IRC +10216. ALMA band 3 spectral surveys of three C-rich AGB stars reveal the morphological and chemical complexity of their CSEs. We compare the results obtained from these surveys with those of IRC +10216, and discuss their implications for the generalised understanding of the chemistry in carbon star CSEs. By obtaining well-constrained estimates of the emission region sizes and circumstellar abundances of a variety of molecular species, this work aims to provide updates to existing chemical models, and put to test the archetype status attributed to IRC +10216 in the literature

The heart of Sakurai's Object revealed by ALMA

We present high angular-resolution observations of Sakurai's object using the Atacama Large Millimeter Array, shedding new light on its morpho-kinematical structure. The millimetre continuum emission, observed at an angular resolution of 20 milliarcsec (corresponding to 70 AU), reveals a bright compact central component whose spectral index indicates that it composed of amorphous carbon dust. Based on these findings, we conclude that this emission traces the previously suggested dust disc observed in mid-infrared observations. Therefore, our observations provide the first direct imaging of such a disc. The H$^{12}$CN($J$=4$\rightarrow$3) line emission, observed at an angular resolution of 300 milliarcsec (corresponding to 1000 AU), displays bipolar structure with a north-south velocity gradient. From the position-velocity diagram of this emission we identify the presence of an expanding disc and a bipolar molecular outflow. The inclination of the disc is determined to be $i$=72$^\circ$. The derived values for the de-projected expansion velocity and the radius of the disc are $v_{\rm exp}$=53 km s$^{-1}$ and $R$=277 AU, respectively. On the other hand, the de-projected expansion velocity of the bipolar outflow detected in the H$^{12}$CN($J$=4$\rightarrow$3) emission of approximately 1000 km s$^{-1}$. We propose that the molecular outflow has an hourglass morphology with an opening angle of around 60$^{\circ}$. Our observations unambiguously show that an equatorial disc and bipolar outflows formed in Sakurai's object in less than 30 years after the born-again event occurred, providing important constraints for future modelling efforts of this phenomenon.Comment: 9 pages, 5 figures. Accepted for publication as a Letter in Astronomy and Astrophysic

First Images of the Molecular Gas around a Born-again Star Revealed by ALMA

Born-again stars allow probing stellar evolution in human timescales and provide the most promising path for the formation of hydrogen-deficient post-asymptotic giant branch objects, but their cold and molecular components remain poorly explored. Here we present ALMA observations of V 605 Aql that unveil for the first time the spatio-kinematic distribution of the molecular material associated with a born-again star. Both the continuum and molecular line emission exhibit a clumpy ring-like structure with a total extent of approximate to 1 \u27\u27 in diameter. The bulk of the molecular emission is interpreted as being produced in a radially expanding disk-like structure with an expansion velocity v(exp) similar to 90 km s(-1) and an inclination i approximate to 60 degrees with respect to the line of sight. The observations also reveal a compact high-velocity component, v(exp) similar to 280 km s(-1), that is aligned perpendicularly to the expanding disk. This component is interpreted as a bipolar outflow with a kinematical age tau less than or similar to 20 yr, which could either be material that is currently being ejected from V 605 Aql, or is being dragged from the inner parts of the disk by a stellar wind. The dust mass of the disk is in the range M-dust similar to 0.2-8 x 10(-3) M-circle dot, depending on the dust absorption coefficient. The mass of the CO is MCO approximate to 1.1 x 10(-5) M-circle dot, which is more than three orders of magnitude larger than the mass of the other detected molecules. We estimate a C-12/C-13 ratio of 5.6 +/- 0.6, which is consistent with the single stellar evolution scenario in which the star experienced a very late thermal pulse instead of a nova-like event as previously suggested

The heart of Sakurai's Object revealed by ALMA

We present high-angular-resolution observations of Sakurai’s object using the Atacama Large Millimeter Array, shedding new light on its morpho-kinematical structure. The millimetre continuum emission observed at an angular resolution of 20 milliarcsec (corresponding to 70 AU) reveals a bright compact central component whose spectral index indicates that it is composed of amorphous carbon dust. Based on these findings, we conclude that this emission traces the previously suggested dust disc observed in mid-infrared observations, and therefore our observations provide the first direct imaging of this disc. The H12CN(J = 4 → 3) line emission observed at an angular resolution of 300 milliarcsec (corresponding to 1000 AU) displays a bipolar structure with a north–south velocity gradient. From the position–velocity diagram of this emission, we identify the presence of an expanding disc and a bipolar molecular outflow. The inclination of the disc is determined to be i = 72°. The derived values for the de-projected expansion velocity and the radius of the disc are vexp = 53 km s−1 and R = 277 AU, respectively. On the other hand, the de-projected expansion velocity of the bipolar outflow detected in the H12CN(J = 4 → 3) emission is of approximately 1000 km s−1. We propose that the molecular outflow has an hourglass morphology with an opening angle of around 60°. Our observations unambiguously show that an equatorial disc and bipolar outflows formed in Sakurai’s object during the 30 years following the occurrence of the born-again event, providing important constraints for future modelling efforts of this phenomenon

The heart of Sakurai’s object revealed by ALMA

We present high-angular-resolution observations of Sakurai’s object using the Atacama Large Millimeter Array, shedding new light on its morpho-kinematical structure. The millimetre continuum emission observed at an angular resolution of 20 milliarcsec (corresponding to 70 AU) reveals a bright compact central component whose spectral index indicates that it is composed of amorphous carbon dust. Based on these findings, we conclude that this emission traces the previously suggested dust disc observed in mid-infrared observations, and therefore our observations provide the first direct imaging of this disc. The H12CN(J = 4 → 3) line emission observed at an angular resolution of 300 milliarcsec (corresponding to 1000 AU) displays a bipolar structure with a north–south velocity gradient. From the position–velocity diagram of this emission, we identify the presence of an expanding disc and a bipolar molecular outflow. The inclination of the disc is determined to be i = 72°. The derived values for the de-projected expansion velocity and the radius of the disc are vexp = 53 km s−1 and R = 277 AU, respectively. On the other hand, the de-projected expansion velocity of the bipolar outflow detected in the H12CN(J = 4 → 3) emission is of approximately 1000 km s−1. We propose that the molecular outflow has an hourglass morphology with an opening angle of around 60°. Our observations unambiguously show that an equatorial disc and bipolar outflows formed in Sakurai’s object during the 30 years following the occurrence of the born-again event, providing important constraints for future modelling efforts of this phenomenon