Academic literature on the topic 'Optical interferometry;Mira variable stars;AGB stars'

Context. Mass loss from long-period variable stars (LPV) is an important contributor to the evolution of galactic abundances. Dust formation is understood to play an essential role in mass loss. It has, however, proven difficult to develop measurements that strongly constrain the location and timing of dust nucleation and acceleration. Aims. Interferometric imaging has the potential to constrain the geometry and dynamics of mass loss. High angular resolution studies of various types have shown that LPVs have a distinct core-halo structure. These have also shown that LPV images commonly exhibit a non-circular shape. The nature of this shape and its implications are yet to be understood. Methods. Multi-telescope interferometric measurements taken with the Interferometric Optical Telescope Array (IOTA) provide imagery of the LPV Mira in the H-band. This wavelength region is well suited to studying mass loss given the low continuum opacity, which allows for emission to be observed over a very long path in the stellar atmosphere and envelope. Results. The observed visibilities are consistent with a simple core-halo model to represent the central object and the extended molecular layers but, in addition, they demonstrate a substantial asymmetry. An analysis with image reconstruction software shows that the asymmetry is consistent with a localized absorbing patch. The observed opacity is tentatively associated with small dust grains, which will grow substantially during a multi-year ejection process. Spatial information along with a deduced dust content of the cloud, known mass loss rates, and ejection velocities provide evidence for the pulsational pumping of the extended molecular layers. The cloud may be understood as a spatially local zone of enhanced dust formation, very near to the pulsating halo. The observed mass loss could be provided by several such active regions around the star. Conclusions. This result provides an additional clue for better understanding the clumpiness of dust production in the atmosphere of AGB stars. It is compatible with scenarios where the combination of pulsation and convection play a key role in the process of mass loss.

AbstractRadio and infrared interferometry of SiO maser stars provide complementary information on the atmosphere and circumstellar environment at comparable spatial resolution. Here, we present the latest results on the atmospheric structure and the dust condensation region of AGB stars based on our recent infrared spectro-interferometric observations, which represent the environment of SiO masers. We discuss, as an example, new results from simultaneous VLTI and VLBA observations of the Mira variable AGB star R Cnc, including VLTI near- and mid-infrared interferometry, as well as VLBA observations of the SiO maser emission toward this source. We present preliminary results from a monitoring campaign of high-frequency SiO maser emission toward evolved stars obtained with the APEX telescope, which also serves as a precursor of ALMA images of the SiO emitting region. We speculate that large-scale long-period chaotic motion in the extended molecular atmosphere may be the physical reason for observed deviations from point symmetry of atmospheric molecular layers, and for the observed erratic variability of high-frequency SiO maser emission.

We study two different samples of long-period variable Asymptotic Giant Branch (AGB) stars in a field of low and homogeneous extinction towards the Galactic bulge, the Palomar-Groningen field Nr. 3. The samples were selected to study the evolution of the late phases on the AGB. One sample consists of 486 variables (mostly Miras) optically detected and studied by Plaut (1971) and by Wesselink (1987). The other sample is selected from the IRAS Point Source Catalogue and consists of 239 sources. We made additional infrared measurements between 1.2 and 13 μm for a large fraction of both samples. This information was used to identify the IRAS sources and derive the apparent bolometric magnitudes. The samples of Miras and variable IRAS sources have a similar apparent bolometric magnitude distributions, but are displaced by an amount significantly less than expected from the Mira period-luminosity relation (Feast et al. 1989; 0.3 magnitudes as opposed to 0.6 magnitudes). The surface density distribution along the minor axis of the bulge is the same for both samples. We conclude that both samples have evolved from the same parent population and that they represent different evolutionary stages on the AGB. The IRAS sources with the longer periods (on average 450 days (Whitelock et al. 1991) versus on average 250 days for the optical sample) are the further evolved objects. As the IRAS sources have higher mass loss rates we conclude that mass loss increases during the late stages of the evolution. However, we find indications that in some stars the mass loss process has been interrupted for some time; mass loss could be an intermittent process although its overall rate increases in time.

Context. Variability is a key property of stars on the asymptotic giant branch (AGB). Their pulsation period is related to the luminosity and mass-loss rate (MLR) of the star. Long-period variables (LPVs) and Mira variables are the most prominent of all types of variability of evolved stars. However, the reddest, most obscured AGB stars are too faint in the optical and have eluded large variability surveys. Aims. Our goal is to obtain a sample of LPVs with large MLRs by analysing WISE W1 and W2 light curves (LCs) for about 2000 sources, photometrically selected to include known C-stars with the 11.3 μm silicon carbide dust feature in absorption, and Galactic O-stars with periods longer than 1000 days. Methods. Epoch photometry was retrieved from the AllWISE and NEOWISE database and fitted with a sinus curve. Photometry from other variability surveys was also downloaded and fitted. For a subset of 316 of the reddest stars, spectral energy distributions (SEDs) were constructed, and, together with mid-infrared (MIR) spectra when available, fitted with a dust radiative transfer programme in order to derive MLRs. Results. WISE based LCs and fits to the data are presented for all stars. Periods from the literature and periods from refitting other literature data are presented. The results of the spatial correlation with several (IR) databases is presented. About one-third of the sources are found to be not real, but it appears that these cannot be easily filtered out by using WISE flags. Some are clones of extremely bright sources, and in some cases the LCs show the known pulsation period. Inspired by a recent paper, a number of non-variable OH/IRs are identified. Based on a selection on amplitude, a sample of about 750 (candidate) LPVs is selected of which 145 have periods > 1000 days, many of them being new. For the subset of the stars with the colours of C-rich extremely red objects (EROs) the fitting of the SEDs (and available MIR spectra) separates them into C- and O-rich objects. Interestingly, the fitting of MIR spectra of mass-losing C-stars is shown to be a powerful tracer of interstellar reddening when AV ≳ 2 mag. The number of Galactic EROs appears to be complete up to about 5 kpc and a total dust return rate in the solar neighbourhood for this class is determined. In the LMC 12 additional EROs are identified. Although this represents only about 0.15% of the total known LMC C-star population adding their MLRs increases the previously estimated dust return by 8%. Based on the EROs in the Magellanic Clouds, a bolometric period luminosity is derived. It is pointed out that due to their faintness, EROs and similar O-rich objects are ideal targets for a NIR version of Gaia to obtain distances, observing in the K-band or, even more efficiently, in the L-band.

Abstract OZ Geminorum (OZ Gem) is a galactic Mira variable in the Milky Way (MW). We measured its annual parallax with VLBI Exploration of Radio Astrometry to be π = 0.806 ± 0.039 mas, corresponding to a distance of D = 1.24 ± 0.06 kpc. Based on multi-epoch infrared observations with the Kagoshima University 1 m telescope, we also derived the mean J-, H-, and K′-band magnitudes of OZ Gem to be 5.75 ± 0.47 mag, 4.00 ± 0.16 mag, and 2.65 ± 0.16 mag, respectively. We derived a pulsation period of OZ Gem as 592 ± 1 d from the K′-band lightcurve. From the period–luminosity (P–L) relation and two-color diagram of the Large Magellanic Cloud (LMC), the property of OZ Gem suggests that OZ Gem is assigned among the carbon-rich Mira variables. However, our optical spectroscopic observational results (with the 1.5 m Kanata telescope) confirmed OZ Gem to be an oxygen-rich Mira star with the detection of multiple titanium oxide transition absorption lines. We suggest that OZ Gem is a low-mass star evolving to an OH/IR star with large mass loss and dust formation. It is predicted that the lower limit to the initial mass of AGB stars for developing the C-rich surface chemistry is larger in the MW than in the LMC because of larger metallicity, and OZ Gem is likely to be the first example to prove this. Our results highlight the necessity of deriving the PL relation of the Milky Way with high accuracy.

This thesis describes the development of a red tip/tilt and fringe detection system at the Sydney University Stellar Interferometer (SUSI), modelling the instrumental performance and effects of seeing at SUSI, making observations of Mira variable stars and finally modelling the atmospheres of Mira variables with physically self-consistent models. The new SUSI tip/tilt system is based around a CCD detector and has been successfully used to both track the majority of tip/tilt power in median seeing at an R magnitude of 4.5, and to provide seeing measures for post processing. The new fringe-detection system rapidly scans 33 to 140 $\mu$m in delay and detects the fringes using two avalanche-photodiodes. It has been used to acquire fringe data, provide user feedback and to track the fringe group-delay position. The system visibility (fringe visibility for a point source) and throughput were found to be consistent with models of the SUSI optical beam train. Observations were made of a variety of sources, including the Mira variables R Car and RR Sco, which were observed in two orthogonal polarization states. These measurements were the first successful use of Optical Interferometric Polarimetry (OIP), and enabled scattered light to be separated from bright photospheric flux. Dust scattering was found to originate from a thin shell 2-3 continuum radii from these stars, with an optical depth of 0.1 to 0.2 at 900 nm. Physical models of Mira variables including dust formation were developed, providing consistent explanations for these results as well as many other photometric and interferometric observations.

This thesis describes the development of a red tip/tilt and fringe detection system at the Sydney University Stellar Interferometer (SUSI), modelling the instrumental performance and effects of seeing at SUSI, making observations of Mira variable stars and finally modelling the atmospheres of Mira variables with physically self-consistent models. The new SUSI tip/tilt system is based around a CCD detector and has been successfully used to both track the majority of tip/tilt power in median seeing at an R magnitude of 4.5, and to provide seeing measures for post processing. The new fringe-detection system rapidly scans 33 to 140 $\mu$m in delay and detects the fringes using two avalanche-photodiodes. It has been used to acquire fringe data, provide user feedback and to track the fringe group-delay position. The system visibility (fringe visibility for a point source) and throughput were found to be consistent with models of the SUSI optical beam train. Observations were made of a variety of sources, including the Mira variables R Car and RR Sco, which were observed in two orthogonal polarization states. These measurements were the first successful use of Optical Interferometric Polarimetry (OIP), and enabled scattered light to be separated from bright photospheric flux. Dust scattering was found to originate from a thin shell 2-3 continuum radii from these stars, with an optical depth of 0.1 to 0.2 at 900 nm. Physical models of Mira variables including dust formation were developed, providing consistent explanations for these results as well as many other photometric and interferometric observations.

1.1 A Brief History of Interferometry The theoretical angular resolution of a telescope with perfect optics is proportional to the diameter of the aperture. A 10 m telescope (e.g. the Keck I telescope on Mauna Kea, Hawaii) in the visible range of light (x1 z 500 nm) could theoretically achieve resolutions of 12 milliarcseconds (mas). It might seem that in order to increase angular resolution, building bigger telescopes would be the only reasonable thing to do! In reality, the comfortable blanket of the Earth’s atmosphere has proven to be quite a show stopper for high-resolution observations. As telescopes with greater and greater diameters were built, astronomers realised that the angular resolution of any given telescope was limited to ~l” by atmospheric turbulence, corresponding to the resolution obtainable from a 10-20 cm aperture. In view of this restriction, Simon Newcomb, founder and first president of the American Astronomical Society, reportedly said in 1888 “We are probably nearing the limit of all we can know about astronomy” (Green & Lomask 1970). Nonetheless, it is clearly necessary to obtain better angular resolution to study the universe. All stars (apart from the Sun) have angular diameters that cannot be resolved with a 20 cm tele— scope. It was some time after Thomas Young’s double-slit experiment in 1803 proved the wavelike nature of light that a physicist had the idea to use this property for the advancement of astronomy. Fizeau (1868) was the first to suggest that light from astronomical sources could also be made to form interference patterns. As in Young’s double-slit experiment, the size of the source of light affects the contrast of the interference fringes. The double slit became a mask with two sub-apertures in front of the telescope’s main aperture, and astronomical interferometry was born. Forty years after the foundations for stellar interferometry had been laid, Michelson & Pease (1921) successfully measured the angular diameter of a Ori (Betelgeuse), using interference fringes from a 6m baseline two-aperture interferometer mounted on the 100 inch telescope on Mt. Wilson, California, USA. The angular resolution of interferometric observations increases with increasing baseline lengths and decreasing wavelengths, but so does the demand on the precision of the instrumental set-up. Atmospheric effects (“seeing”) which degraded fringe contrast, also dictated short expo— sure times and the need for active electronic control systems. So optical and near—infrared (NIR) interferometry had to bear the limits imposed by technology and was limited to bright, nearby stars. In the meantime, radio astronomy made advances in interferometry and aperture synthesis. Groups led by Martin Ryle at Cambridge and Bernard Mills at the University of Sydney were the first to apply interferometric techniques to radio astronomy. As instrumentation advanced (CCD technology, laser metrology, infrared detectors, Adaptive Optics, etc.), and the angular resolution of stellar observations increased, so did the demands on theoretical models. These are hard-pressed to cope with the vast amount of constraints that observations in multiple wavelengths deliver today. In this thesis I apply the latest developments in astronomical Fizeau interferometry, namely non-redundant aperture masking. Results from observations of Mira stars with the aperture masking experiment at the Keck I telescope are reported in Chapters 2 and 3. I also describe the ZORAO and LAMP experiments, both of which use Low Light Level CCDs, Adaptive Optics systems and non-redundant masking (Chapters 4, 5 and 6). Experimental results from LAMP are presented which resolve the Mira variable R Cas.