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

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.

Non-stoichiometric amorphous magnesium-iron silicates in circumstellar dust shells

Aims. We study the growth of dust in oxygen-rich stellar outflows in order to find out to which extent dust growth models can quantitatively reconcile with the quantities and nature of dust as derived from observations of the infrared emission from circumstellar dust shells. Methods. We use a set of nine well-observed massive supergiants with optically thin dust shells as testbeds because of the relatively simple properties of the outflows from massive supergiants, contrary to the case of AGB stars. Models of the infrared emission from their circumstellar dust shells are compared to their observed infrared spectra to derive the essential parameters that rule dust formation in the extended envelope of these stars. The results are compared with a model for silicate dust condensation. Results. For all objects, the infrared emission in the studied wavelength range, between 6 and 25 μm, can be reproduced rather well by a mixture of non-stoichiometric iron-bearing silicates, alumina, and metallic iron dust particles. For three objects (μ Cep, RW Cyg, and RS Per), the observed spectra can be sufficiently well reproduced by a stationary and (essentially) spherically symmetric outflow in the instantaneous condensation approximation. For these objects, the temperature at the onset of massive silicate dust growth is of the order of 920 K and the corresponding outflow velocity of the order of the sound velocity. This condensation temperature is only somewhat below the vapourisation temperature of the silicate dust and suggests that the silicate dust grows on the corundum dust grains that formed well inside of the silicate dust shell at a much higher temperature. The low expansion velocity at the inner edge of the silicate dust shell further suggests that, for these supergiants, the region inside the silicate dust shell has an only subsonic average expansion velocity, though a high degree of supersonic turbulence is indicated by the widths of spectral lines. Conclusions. Our results suggest that for the two major problems of dust formation in stellar outflows, that is (i) formation of seed nuclei and (ii) their growth to macroscopic dust grains, we are gradually coming close to a quantitative understanding of the second item.

Dynamical transition from a naked singularity to a black hole

We show that a Reissner–Nordström (RN) black hole can be formed by dropping a charged thin dust shell onto a RN naked singularity. This is in contrast to the fact that a RN naked singularity is prohibited from forming by dropping a charged thin dust shell onto a RN black hole. This implies the strong tendency of the RN singularity to be covered by a horizon in favor of cosmic censorship. We show that an extreme RN black hole can also be formed from a RN naked singularity by the same process in a finite advanced time. We also discuss the evolution of the charged thin dust shells and the causal structure of the resultant space–times. a a The statements expressed in this paper are those of the authors and do not represent the views of Sumitomo Mitsui Banking Corporation or its staff.

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

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.

Mass Loss History of Evolved Stars (MLHES) Excavated by AKARI

We performed a far-IR imaging survey of the circumstellar dust shells of 144 evolved stars as a mission program of the AKARI infrared astronomical satellite. Our objectives were to characterize the far-IR surface brightness distributions of the cold dust component in the circumstellar dust shells. We found that (1) far-IR emission was detected from all but one object, (2) roughly 60–70 % of the target sources showed some extension, (3) 29 sources were newly resolved in the far-IR in the vicinity of the target sources, (4) the results of photometry measurements were reasonable with respect to the entries in the AKARI/FIS Bright Source Catalogue, and (5) an IR two-color diagram would place the target sources in a roughly linear distribution that may correlate with the age of the circumstellar dust shell.

The End: Witnessing the Death of Extreme Carbon Stars

A 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.

Dust formation and evolution around carbon stars

Evolved intermediate-mass stars with carbon-to-oxygen ratios (C/O) above unity are known as carbon stars. Carbon stars are surrounded by dust shells dominated by carbon (C) and silicon carbide (SiC) grains. These SiC grains have a diagnostic spectral feature at [about]11 [mu]m. We have selected a sample of 9 carbon stars with low mass-loss rates such that their dust shells are sufficiently optically thin to allow abundance analysis of the stars' photospheres. This allows the study of how atomic abundances affect dust formation around carbons stars. We present the result of radiative transfer modeling for these stars, and compare the resulting dust shell parameters to published abundance measurements. To constrain model parameters, we use published mass-loss rates, expansion velocities, and theoretical dust condensation models to estimate the dust condensation temperature, and spectral types to constrain stellar effective temperatures. We found significant correlations for the single-shell modeling with graphite/iron grains and amorphous carbon (AmC)/iron grains: (1) [subscript]0.55[mu]m and gas-to-dust ratio, (2) iron grains and graphite or AmC grains, (3) graphite or AmC grains and Fe/H, and (4) iron grains and Fe/H. For the collated data the significant correlations we found were: (1) for dust formation temperature and the change of temperature from the formation of graphite grains to the formation of SiC grains (2) C/O and the change of temperature from the formation of graphite grains to the formation of SiC grains. Lastly, between the abundance of SiC grains when compared to the abundances of SiC grains in graphite, AmC, graphite and iron and AmC and iron grains models. This shows that there is no sensitivity in the continuum when choosing the type of carbon to model with.