scholarly journals Identifying circumstellar dust around oxygen-rich mira variables with maser emission via continuum elimination

2021 ◽  
Author(s):  
◽  
Lisa Shepard
Keyword(s):  
Major Effect ◽  
Peak Position ◽  
Asymptotic Giant Branch ◽  
Correct Identification ◽  
Spectral Features ◽  
Agb Stars ◽  
Dust Grains ◽  
Maser Emission ◽  
Dust Shells ◽  
Mira Variables

Stars between about 0.8 and 8 times the mass of the Sun will eventually evolve, becoming asymptotic giant branch (AGB) stars, where they pulsate and eject mass from their atmospheres, forming dust shells in the space around them. Evolved low- and intermediate-mass stars with carbon-to-oxygen ratios (C/O) below unity are known as oxygen-rich stars. O-rich stars are surrounded by dust shells containing mineral species dominated by silicate dust grains. In this dissertation, I examine whether dust grains around evolved, oxygen-rich AGB stars have any correlation with maser emission, and to understand the connection, if any, between specific types of maser emission and dust spectral features. I have investigated several methods of continuum elimination using spectroscopy data for the archetypal dusty AGB star, Mira. I have investigated the ~10[mu]m and ~18[mu]m spectral features in the continuum-eliminated spectrum including peak position, barycenter, and full width half maxima (FWHM). The positions and widthved spectral features were compared with those seen in laboratory spectra. I then looked for a correlation between maser emission and dust spectral features in a sample of Mira variables. The types of masers have been identified, and peak positions, barycenter positions, and FWHM have been measured for the sample spectra. The results show that the method of continuum elimination matters for correct identification of dust minerals, while varying the temperature and precise continuum shapes do not have a major effect on the positions of spectral features. Observed astronomical silicate features are complex and indicate the need for different compositions of minerals. Finally, there does not appear to be a correlation between the presence of a maser and dust spectral features based on the information available for analysis.

scholarly journals From molecules to dust grains: The role of alumina cluster seeds

2019 ◽  
Vol 15 (S350) ◽  
pp. 245-248
Author(s):  
David Gobrecht ◽  
John M.C. Plane ◽  
Stefan T. Bromley ◽  
Leen Decin ◽  
Sergio Cristallo
Keyword(s):  
Bulk Material ◽  
Spectral Feature ◽  
Chemical Kinetic ◽  
Asymptotic Giant Branch ◽  
Nucleation Theory ◽  
Classical Nucleation Theory ◽  
Circumstellar Envelope ◽  
Agb Stars ◽  
Dust Grains ◽  
Alumina Cluster

AbstractAsymptotic Giant Branch (AGB) stars contribute a major part to the global dust budget in galaxies. Owing to their refractory nature alumina (stoichiometric formula AlO) is a promising candidate to be the first condensate emerging in the atmospheres of oxygen-rich AGB stars. Strong evidence for that is supplied by the presence of alumina in pristine meteorites and a broad spectral feature observed around ∼ 13 μm. The emergence of a specific condensate depends on the thermal stability of the solid, the gas density and its composition. The evaluation of the condensates is based on macroscopic bulk properties. The growth and size distribution of dust grains is commonly described by Classical Nucleation Theory (CNT). We question the applicability of CNT in an expanding circumstellar envelope as CNT presumes thermodynamic equilibrium and requires, in practise, seed nuclei on which material can condense. However, nano-sized molecular clusters differ significantly from bulk analogues. Quantum effects of the clusters lead to non-crystalline structures, whose characteristics (energy, geometry) differ substantially, compared to the bulk material. Hence, a kinetic quantum-chemical treatment involving various transition states describes dust nucleation most accurately. However, such a treatment is prohibitive for systems with more than 10 atoms. We discuss the viability of chemical-kinetic routes towards the formation of the monomer (Al2O3) and the dimer (Al4O6) of alumina.

scholarly journals VLBI monitoring of Mira variables with VERA

2007 ◽  
Vol 3 (S242) ◽  
pp. 326-327
Author(s):  
A. Nakagawa ◽  
T. Omodaka ◽  
K. M. Shibata ◽  
T. Kurayama ◽  
H. Imai ◽  
...  
Keyword(s):  
Monitoring Program ◽  
Asymptotic Giant Branch ◽  
Agb Stars ◽  
Current Analysis ◽  
Mira Variables ◽  
Vlbi Observations ◽  
The Galaxy ◽  
Absolute Magnitudes ◽  
Infrared Monitoring ◽  
Good Coverage

AbstractWe have started a VLBI monitoring program for Asymptotic Giant Branch (AGB) stars at 22 and 43 GHz as part of a project of the VLBI Exploration of Radio Astrometry (VERA) for precisely obtaining the period-luminosity (PL) relation of Galactic Mira variables. Using accurate distances measured with VERA, we reveal PL relation in the Galaxy based on the absolute magnitudes of the sources. We selected the sources for VLBI monitoring so that they have a good coverage of various pulsation periods. Photometry in the infrared J, H, and K bands for over 600 AGB stars has also started since 2003 with the 1m telescope of Kagoshima University to obtain the pulsation periods and magnitudes. Current analysis of the phase referencing VLBI observations of S Crt shows that the parallax of 2.3±0.2 milliarcsec (mas) corresponds to a distance of 435+41−35 pc. From the infrared monitoring data, pulsation periods and magnitudes in K band for 248 sources were obtained.

scholarly journals Maser emission from the CO envelope of the asymptotic giant branch star W Hydrae

2021 ◽  
Vol 654 ◽  
pp. A18
Author(s):  
W. H. T. Vlemmings ◽  
T. Khouri ◽  
D. Tafoya
Keyword(s):  
Mass Loss ◽  
Radiation Field ◽  
Asymptotic Giant Branch ◽  
Small Scale ◽  
High Angular Resolution ◽  
Circumstellar Envelope ◽  
Agb Stars ◽  
Maser Emission ◽  
Co Emission ◽  
Loss Rates

Context. Observation of CO emission around asymptotic giant branch (AGB) stars is the primary method to determine gas mass-loss rates. While radiative transfer models have shown that molecular levels of CO can become mildly inverted, causing maser emission, CO maser emission has yet to be confirmed observationally. Aims. High-resolution observations of the CO emission around AGB stars now have the brightness temperature sensitivity to detect possible weak CO maser emission. Methods. We used high angular resolution observations taken with the Atacama Large Millimeter/submillimeter Array (ALMA) to observe the small-scale structure of CO J = 3−2 emission around the oxygen-rich AGB star W Hya. Results. We find CO maser emission amplifying the stellar continuum with an optical depth τ ≈−0.55. The maser predominantly amplifies the limb of the star because CO J = 3−2 absorption from the extended stellar atmosphere is strongest towards the centre of the star. Conclusions. The CO maser velocity corresponds to a previously observed variable component of high-frequency H2O masers and with the OH maser that was identified as the amplified stellar image. This implies that the maser originates beyond the acceleration region and constrains the velocity profile since we find the population inversion primarily in the inner circumstellar envelope. We find that inversion can be explained by the radiation field at 4.6 μm and that the existence of CO maser emission is consistent with the estimated mass-loss rates for W Hya. However, the pumping mechanism requires a complex interplay between absorption and emission lines in the extended atmosphere. Excess from dust in the circumstellar envelope of W Hya is not sufficient to contribute significantly to the required radiation field at 4.6 μm. The interplay between molecular lines that cause the pumping can be constrained by future multi-level CO observations.

scholarly journals Properties of dust in the detached shells around U Antilae, DR Serpentis, and V644 Scorpii

2018 ◽  
Vol 620 ◽  
pp. A106 ◽  
Author(s):  
M. Maercker ◽  
T. Khouri ◽  
E. De Beck ◽  
M. Brunner ◽  
M. Mecina ◽  
...  
Keyword(s):  
Mass Loss ◽  
Dust Emission ◽  
Far Infrared ◽  
Asymptotic Giant Branch ◽  
Dust Particles ◽  
Spectral Energy ◽  
Agb Stars ◽  
Dust Grains ◽  
Grain Sizes ◽  
Dust Mass

Context. Asymptotic giant branch (AGB) stars experience strong mass loss driven by dust particles formed in the upper atmospheres. The dust is released into the interstellar medium, and replenishes galaxies with synthesised material from the star. The dust grains further act as seeds for continued dust growth in the diffuse medium of galaxies. As such, understanding the properties of dust produced during the asymptotic giant branch phase of stellar evolution is important for understanding the evolution of stars and galaxies. Recent observations of the carbon AGB star R Scl have shown that observations at far-infrared and submillimetre wavelengths can effectively constrain the grain sizes in the shell, while the total mass depends on the structure of the grains (solid vs. hollow or fluffy). Aims. We aim to constrain the properties of the dust observed in the submillimetre in the detached shells around the three carbon AGB stars U Ant, DR Ser, and V644 Sco, and to investigate the constraints on the dust masses and grain sizes provided by far-infrared and submm observations. Methods. We observed the carbon AGB stars U Ant, DR Ser, and V644 Sco at 870 μm using LABOCA on APEX. Combined with observations from the optical to far-infrared, we produced dust radiative transfer models of the spectral energy distributions (SEDs) with contributions from the stars, present-day mass-loss and detached shells. We assume spherical, solid dust grains, and test the effect of different total dust masses and grain sizes on the SED, and attempted to consistently reproduce the SEDs from the optical to the submm. Results. We derive dust masses in the shells of a few 10−5 M ⊙. The best-fit grain radii are comparatively large, and indicate the presence of grains between 0.1 μm and 2 μm. The LABOCA observations suffer from contamination from 12CO (3 − 2), and hence gives fluxes that are higher than the predicted dust emission at submm wavelengths. We investigate the effect on the best-fitting models by assuming different degrees of contamination and show that far-infrared and submillimetre observations are important to constrain the dust mass and grain sizes in the shells. Conclusions. Spatially resolved observations of the detached shells in the far-infrared and submillimetre effectively constrain the temperatures in the shells, and hence the grain sizes. The dust mass is also constrained by the observations, but additional observations are needed to constrain the structure of the grains.

The End: Witnessing the Death of Extreme Carbon Stars

2018 ◽  
Vol 14 (S343) ◽  
pp. 305-308
Author(s):  
G. C. Sloan ◽  
K. E. Kraemer ◽  
I. McDonald ◽  
A. A. Zijlstra
Keyword(s):  
Spherical Symmetry ◽  
Magellanic Clouds ◽  
Asymptotic Giant Branch ◽  
Circumstellar Dust ◽  
Agb Stars ◽  
Carbon Stars ◽  
Dust Shells

AbstractA Last affiliation changed 3 to 4 against MS. Please check and confirm if it is fine. small number of the sample of 184 carbon stars in the Magellanic Clouds show signs that they are in the act of evolving off of the asymptotic giant branch. Most carbon stars grow progressively redder in all infrared colors and develop stronger pulsation amplitudes as their circumstellar dust shells become optically thicker. The reddest sources, however, have unexpectedly low pulsation amplitudes, and some even show blue excesses that could point to deviations from spherical symmetry as they eject the last of their envelopes. Previously, all dusty carbon-rich AGB stars have been labeled “extreme,” but that term should be reserved for the truly extreme carbon stars. These objects may well hold the clues needed to disentangle what actually happens when a star ejects the last of its envelope and evolves off of the AGB.

scholarly journals Interferometric observations of SiO thermal emission in the inner wind of M-type AGB stars IK Tauri and IRC+10011

2019 ◽  
Vol 624 ◽  
pp. A107 ◽  
Author(s):  
J. L. Verbena ◽  
V. Bujarrabal ◽  
J. Alcolea ◽  
M. Gómez-Garrido ◽  
A. Castro-Carrizo
Keyword(s):  
Mass Loss ◽  
Thermal Emission ◽  
Mass Loss Rate ◽  
Asymptotic Giant Branch ◽  
Dust Formation ◽  
Circumstellar Envelope ◽  
Agb Stars ◽  
Dust Grains ◽  
Gas Drive ◽  
Massive Outflow

Context. Asymptotic giant branch (AGB) stars go through a process of strong mass loss that involves pulsations of the atmosphere, which extends to a region in which the conditions are adequate for dust grains to form. Radiation pressure acts on these grains which, coupled to the gas, drive a massive outflow. The details of this process are not clear, including which molecules are involved in the condensation of dust grains. Aims. We seek to study the role of the SiO molecule in the process of dust formation and mass loss in M-type AGB stars. Methods. Using the IRAM NOEMA interferometer we observed the 28SiO and 29SiO J = 3−2, v = 0 emission from the inner circumstellar envelope of the evolved stars IK Tau and IRC+10011. We computed azimuthally averaged emission profiles to compare the observations to models using a molecular excitation and ray-tracing code for SiO thermal emission. Results. We observe circular symmetry in the emission distribution. We also find that the source diameter varies only marginally with radial velocity, which is not the expected behaviour for envelopes expanding at an almost constant velocity. The adopted density, velocity, and abundance laws, together with the mass-loss rate, which best fit the observations, give us information concerning the chemical behaviour of the SiO molecule and its role in the dust formation process. Conclusions. The results indicate that there is a strong coupling between the depletion of gas-phase SiO and gas acceleration in the inner envelope. This could be explained by the condensation of SiO into dust grains.

scholarly journals Improving the parallaxes of OH bearing Miras

2007 ◽  
Vol 3 (S242) ◽  
pp. 342-343
Author(s):  
W. H. T. Vlemmings ◽  
H. J. van Langevelde
Keyword(s):  
Physical Properties ◽  
Mass Loss ◽  
Solar Minimum ◽  
Asymptotic Giant Branch ◽  
High Mass ◽  
Agb Stars ◽  
Long Baseline ◽  
Mira Variables ◽  
High Mass Loss ◽  
Very Long Baseline Array

AbstractWe have carried out observations with the Very Long Baseline Array (VLBA) to measure the parallaxes of Mira variables and are able to improve the distance estimates significantly for a fraction of our sample (U Her, S CrB and RR Aql). This is predominantly because we have enhanced our technique by making use of nearby, in-beam calibrators. Additionally, the observing conditions have improved during the current solar minimum. The distances of these stars are of fundamental importance for studying the physical properties of Asymptotic Giant Branch (AGB) stars with high mass loss.

scholarly journals Results and Future Uses of Heterodyne Spatial Interferometry at 11 Microns

1979 ◽  
Vol 50 ◽  
pp. 16-1-16-14 ◽  
Author(s):  
Edmund C. Sutton
Keyword(s):  
Spatial Distribution ◽  
Circumstellar Dust ◽  
Dust Grains ◽  
Heterodyne Interferometry ◽  
Dust Shells ◽  
Future Developments ◽  
Mira Variables ◽  
Infrared Stars

AbstractHeterodyne spatial interferometry at a wavelength of 11 microns has been used to examine properties of circumstellar dust shells. Among the objects which have been observed are several M-type supergiants and Mira variables as well as several peculiar infrared stars. These measurements provide information on the temperature and spatial distribution of dust grains. Possible future developments in heterodyne interferometry include longer baselines for higher resolution and the use of larger telescopes for greater sensitivity.

scholarly journals Gaia DR2 data and the evolutionary status of eight high-velocity hot post-AGB candidates

2020 ◽  
Vol 72 (6) ◽  
Author(s):  
Mudumba Parthasarathy ◽  
Tadafumi Matsuno ◽  
Wako Aoki
Keyword(s):  
Effective Temperature ◽  
High Velocity ◽  
Circumstellar Dust ◽  
Evolutionary Status ◽  
Agb Stars ◽  
Horizontal Branch ◽  
Dust Shells ◽  
The Past ◽  
Gaia Dr2 ◽  
Branch Star

Abstract From Gaia DR2 data of eight high-velocity hot post-AGB candidates, LS 3593, LSE 148, LS 5107, HD 172324, HD 214539, LS IV −12 111, LS III +52 24, and LS 3099, we found that six of them have accurate parallaxes which made it possible to derive their distances, absolute visual magnitudes (MV) and luminosity (log L/L⊙). All the stars except LS 5107 have an accurate effective temperature (Teff) in the literature. Some of these stars are metal poor, and some of them do not have circumstellar dust shells. In the past, the distances of some stars were estimated to be 6 kpc, which we find to be incorrect. The accurate Gaia DR2 parallaxes show that they are relatively nearby, post-AGB stars. When compared with post-AGB evolutionary tracks we find their initial masses to be in the range 1 M⊙ to 2 M⊙. We find the luminosity of LSE 148 to be significantly lower than that of post-AGB stars, suggesting that this is a post-horizontal-branch star or post-early-AGB star. LS 3593 and LS 5107 are new high-velocity hot post-AGB stars from Gaia DR2.

scholarly journals Discovery of Infrared Stars in Globular Clusters in the Magellanic Clouds and Their Light Variations

1998 ◽  
Vol 11 (1) ◽  
pp. 395-395
Author(s):  
S. Nishida ◽  
T. Tanabé ◽  
S. Matsumoto ◽  
T. Onaka ◽  
Y. Nakada ◽  
...  
Keyword(s):  
Mass Loss ◽  
Globular Clusters ◽  
Main Sequence ◽  
Near Infrared ◽  
Magellanic Clouds ◽  
Infrared Light ◽  
Agb Stars ◽  
Dust Shells ◽  
Infrared Stars ◽  
Infrared Survey

A systematic near-infrared survey was made for globular clusters in the Magellanic Clouds. Two infrared stars were discovered in NGC419 (SMC) and NGC1783 (LMC). NGC419 and NGC1783 are well-studied rich globular clusters whose turn-off masses and ages are estimated MTO ~ 2.0 Mʘ and т ~1.2 Gyr for NGC419, and MT0 ~ 2.0 Mʘ and т ʘ 0.9 Gyr for NGC1783, respectively. The periods of the infrared light variations were determined to be 540 dfor NGC419IR1 and to be 480 d for NGC1783IR1, respectively. Comparison of the measurements with the period—if magnitude relation for carbon Miras in the LMC by Groenewegen and Whitelock(1996) revealed that the Kmagnitudes of the infrared stars were fainter by about 0.3 — 0.8 magnitude than those predicted by the P — K relation. This deviation can be explained if the infrared stars are surrounded by thick dust shells and are obscured even in the K band. The positions of NGC419IR1and NGC1783IR1 on the P — K diagram suggest that AGB stars with the main sequence masses of about 2 Mʘ start their heavy mass-loss when P ʘ 500 d.

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