Radio continuum monitoring of the extreme carbon star IRC+10216

We describe Very Large Array observations of the extreme carbon star IRC+10216 at 8.4, 14.9, and 22.5 GHz made over a two year period. We find possible variability correlated with the infrared phase and a cm- to sub-millimeter wavelength spectral index very close to 2. The variability, observed flux densities, and upper limit on the size are consistent with the emission arising from the stellar photosphere or a slightly larger radio photosphere.Comment: 9 pages, incl. 4 figures. To appear in Astronomy & Astrophysic

Evidence for orbital motion of CW Leonis from ground-based astrometry

© 2017 The Authors.Recent Atacama Large Millimeter/submillimeter Array (ALMA) observations indicate that CW Leo, the closest carbon-rich asymptotic giant branch star to Sun, might have a low-mass stellar companion. We present archival ground-based astrometric measurements of CW Leo obtained within the context of the Torino Parallax Program and with > 6 yr (1995-2001) of time baseline. The residuals to a single-star solution show significant curvature, and they are strongly correlatedwith thewell-known I-band photometric variations due to stellar pulsations. We describe successfully the astrometry of CW Leo with a variability-induced motion (VIM) + acceleration model. We obtain proper motion and parallax of the centre-of-mass of the binary, the former in fair agreement with recent estimates, the latter at the near end of the range of inferred distances based on indirect methods. The VIM + acceleration model results allow us to derive a companion mass in agreement with that inferred by ALMA, they point towards a somewhat longer period than implied by ALMA, but are not compatible with much longer period estimates. These data will constitute a fundamental contribution towards the full understanding of the orbital architecture of the system when combined with Gaia astrometry, providing an ~25 yr time baseline.Peer reviewe

Spatial distributions of 13CO and CS in a carbon-rich AGB star IRC+10216

IRC+10216 is the typical carbon-rich asymptotic giant branch star, and more than 100 species have been observed in its circumstellar envelope so far. The use of interferometric arrays to map molecular emission in this source has been widely reported, but to study the angular extent of molecular emission, single-dish mapping must be employed. We report here the mapping of the 13CO J = 1–0 and CS J = 2–1 lines towards IRC+10216 using the 13.7 m mm-wave radio telescope at Purple Mountain Observatory. As far as we know, these maps are the largest published 13CO J = 2–1 and CS J = 2–1 images to date. Both molecules have roughly spherically symmetric distribution, with 13CO having an emission radius of over 80″and CS extending up to ∼50″. Assuming that the two molecules are in local thermodynamic equilibrium, the column density and fractional abundance relative to H2 for 13CO and the lower limits for CS are obtained. They are 4.35 × 1016 cm−2 and > 4.72 × 1014 cm−2, and 5.25 × 10−5 and > 2.56 × 10−7 for 13CO and CS, respectively

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

Molecules in the circumstellar envelope of the evolved carbon-rich star IRC+10216

Modern radio interferometers allow the imaging of spectral line emission over a wide range of frequencies with high angular resolution. In this thesis we present a detailed empirical examination and description of the molecules in the circumstellar envelope (CSE) of the evolved carbon-rich star IRC+10216 between 18 and 40 GHz obtained with the Karl G. Jansky Very Large Array (VLA). The interferometric mapping of the molecular emission in the radio regime has been fairly unexplored for this star. Therefore, this spectral line and imaging survey is an important addition to the studies of IRC+10216. By constraining the molecular spatial distribution, the understanding of the formation and shaping of the observed CSE morphology and the inherent chemistry is greatly improved in the cool (10-500 K) outer CSE. Most detected species are carbon-bearing molecules and are distributed in hollow spheres around the star. We focus on the cyanopolyynes and carbon chains that are the brightest transitions and trace the UV-photon induced chemistry. The substructure of these hollow spheres are shells, arcs, and clumps. With a new automatized procedure, we perform a detailed spatio-kinematical study of the images of the molecular line transitions. This work quantifies the three dimensional physical and chemical structure and substructure of IRC+10216 in unique detail. We find a main molecular shell component at an angular distance of about 15 arcsec (corresponding to ~500 stellar radii) and for the first time constrain a second shell component at 22-23 arcsec from the star. Both components have a thickness of about 4-8 arcsec, so they are mostly resolved. The morphology is non-concentric, non-regular, and has asymmetries with clear differences in the detailed substructure. We qualitatively compare the VLA observations to state-of-the-art chemical models. Overall, we find a good correspondence between the azimuthally averaged data and the models. There are deviations in the substructure within the different image quarters, especially for the carbon chains C4H and C6H. The models do not completely reproduce the observations in a consistent way. Therefore, our VLA data provide excellent material to improve future chemical and radiative transfer models of the CSE of IRC+10216 and of AGB stars in general. With the extracted information on the shell geometry from the VLA data, we are able to perform an approximate physical analysis of the CSE. We estimate molecular column densities and abundances of HC3N and HC5N. Especially for HC5N the results compare well with previous studies. Furthermore, we discuss the effect of a more realistic density structure which includes density-enhanced shells in the circumstellar medium

Stellar Astrophysics With Cassini: Syzygies, Stardust, and the Sizes Of Stars

The Cassini spacecraft has been exploring the complex and fascinating Saturnian system for over a decade. This thesis presents Cassini observations employed for the study of evolved stars. Utilising the on-board near-infrared spectrometer, we demonstrate the recovery of flux calibrated stellar spectra. Data were taken from a publicly-accessible archive, and the overwhelming majority were obtained for various spacecraft engineering and calibration purposes; their application to stellar astrophysics being an opportunistic extension to the mission outcomes. An atlas of stellar spectra has been compiled utilising existing observations acquired to monitor the performance of the instrument. Exploiting archival observations of stars as they are occulted by edges within Saturn’s rings, we demonstrate the recovery of stellar spatial information, specifically angular diameters, and compare these to measurements from ground-based interferometry. High-resolution two-dimensional images of stellar environments are reconstructed by tomographically combining these occultation observations from different edges within the planetary rings. An extensive astrophysical study of the evolved star Mira employing all of these techniques over multiple epochs reveals spectrally dependant molecular shells in its extended atmosphere, and allows for the appraisal of state-of-the-art models which aim to describe the atmospheres of such stars. Finally, the carbon star, IRC+10216 is shown to be embedded in a dynamic shroud of thick dusty circumstellar clouds, challenging existing models of the inner structure of the stellar environment

(Sub)stellar companions shape the winds of evolved stars

Binary interactions dominate the evolution of massive stars, but their role is less clear for low- and intermediate-mass stars. The evolution of a spherical wind from an asymptotic giant branch (AGB) star into a nonspherical planetary nebula (PN) could be due to binary interactions. We observed a sample of AGB stars with the Atacama Large Millimeter/submillimeter Array (ALMA) and found that their winds exhibit distinct nonspherical geometries with morphological similarities to planetary nebulae (PNe). We infer that the same physics shapes both AGB winds and PNe; additionally, the morphology and AGB mass-loss rate are correlated. These characteristics can be explained by binary interaction. We propose an evolutionary scenario for AGB morphologies that is consistent with observed phenomena in AGB stars and PNe

(Sub)stellar companions shape the winds of evolved stars

Binary interactions dominate the evolution of massive stars, but their role is less clear for low- and intermediate-mass stars. The evolution of a spherical wind from an asymptotic giant branch (AGB) star into a nonspherical planetary nebula (PN) could be due to binary interactions. We observed a sample of AGB stars with the Atacama Large Millimeter/submillimeter Array (ALMA) and found that their winds exhibit distinct nonspherical geometries with morphological similarities to planetary nebulae (PNe). We infer that the same physics shapes both AGB winds and PNe; additionally, the morphology and AGB mass-loss rate are correlated. These characteristics can be explained by binary interaction. We propose an evolutionary scenario for AGB morphologies that is consistent with observed phenomena in AGB stars and PNe