Oxygen-rich AGB Stars Research Articles

An empirical formula for the mass-loss rates of dust-enshrouded red supergiants and oxygen-rich Asymptotic Giant Branch stars

We present an empirical determination of the mass-loss rate as a function of stellar luminosity and effective temperature, for oxygen-rich dust-enshrouded Asymptotic Giant Branch stars and red supergiants. To this aim we obtained optical spectra of a sample of dust-enshrouded red giants in the Large Magellanic Cloud, which we complemented with spectroscopic and infrared photometric data from the literature. Two of these turned out to be hot emission-line stars, of which one is a definite B[e] star. The mass-loss rates were measured through modelling of the spectral energy distributions. We thus obtain the mass-loss rate formula log Mdot = -5.65 + 1.05 log(L / 10,000 Lsun) -6.3 log(Teff / 3500 K), valid for dust-enshrouded red supergiants and oxygen-rich AGB stars. Despite the low metallicity of the LMC, both AGB stars and red supergiants are found at late spectral types. A comparison with galactic AGB stars and red supergiants shows excellent agreement between the mass-loss rate as predicted by our formula and that derived from the 60 micron flux density for dust-enshrouded objects, but not for optically bright objects. We discuss the possible implications of this for the mass-loss mechanism.

OH spectral evolution of oxygen-rich late-type stars

We investigated the main-line spectral evolution with shell thickness of oxygen rich AGB stars. The study is based on a sample of 30 sources distributed along the IRAS colour-colour diagram. The sources were chosen to trace the Miras with thick shells and the whole range of OH/IR stars. The Miras exhibit a 1665 MHz emission strength comparable to that at 1667 MHz. Even though the Miras of the study have quite thick shells, their spectral characteristics in both main lines attest to a strong heterogeneity in their OH shell with, in particular, the presence of significant turbulence and acceleration. The expansion velocity has been found to be about the same at 1665 and 1667 MHz, taking into account a possible velocity turbulence of 1-2km/s at the location of the main-line maser emission. An increase in the intensity ratio 1667/1665 with shell thickness has been found. A plausible explanation for such a phenomenon is that competitive gain in favour of the 1667 MHz line increases when the shell is getting thicker. There is an evolution in the spectral profile shape with the appearance of a substantial inter-peak signal when the shell is getting thicker. Also, inter-peak components are found and can be as strong as the external standard peaks when the shell is very thick. This trend for an increase of the signal in between the two main peaks is thought to be the result of an increase of the saturation with shell thickness. All sources but two - a Mira and an OH/IR star from the lower part of the colour-colour diagram - are weakly polarized. The strong polarization observed for those two particular objects is thought to be the result of perturbations in their shells.

Discussion on candidates for the M stars with 11.2 μm SiC feature

The oxygen-rich stars with SiC emission features are unexpected according to the evolutionary theory in the late stage of stellar evolution. After the careful re-examination of such samples reported previously and searching for new samples, seven stars are found to be the candidates of the M stars with 11.2 μm SiC emission feature. However, with the IRAS low-resolution spectrum, the IRAS two colour diagram and other information, it is seen that no strong candidates for oxygen-rich AGB stars with carbon-rich circumstellar dust could be found so far.

Mineral formation in stellar winds

The possible formation of iron-magnesium-oxides in stellar outflows from cool stars with oxygen rich element mixture is discussed. Chemical equilibrium calculations for the non-ideal solid solution magnesiowustite with composition Mg x Fe 1 - x - Φ O show that this oxide may be formed under conditions of imperfect equilibrium condensation. A model is developed for calculating the condensation of magnesiowustite under non equilibrium conditions in stellar outflows and numerical models for multi-component dust condensation including magnesiowustite in oxygen rich stellar outflows from cool stars are calculated. It is found that magnesiowustite should be formed in small but probably detectable quantities in outflows from oxygen rich AGB-stars with mass-loss rates M ≤ 4 x 10 - 6 M O . yr - It forms a feature in the spectrum which seems to correspond to a not yet identified feature seen in some circumstellar dust shells. For higher mass-loss rates condensation of magnesiowustite is suppressed by early massive condensation of silicates.

Towards the identification of circumstellar hibonite

We have performed IR reflectance measurements on crystalline CaAl12O19 (hibonite) and derived optical constants for both principal directions of the electric field vector relative to the crystallographic c-axis. Based on the resulting dielectric functions, we calculated small particle spectra and found six maxima of the absorption cross section for spherical dust grains at the wavelengths 12.3, 15.9, 21.1, 25.1, 34.4, and 78.7m. For nonspherical particles, strong shifting and broadening of the two strongest resonances at 12.3 and 15.9m occurs. The signal-to-noise-ratio of ISO spectra of oxygen-rich AGB stars is not sucient for a definite identification of these bands, even though there is some weak indication of their presence. We propose additional observations to clarify this point.

The OH/IR star population at the centre of the Galaxy

In the last years various OH surveys at 1612 MHz have been carried out in the galactic bulge region with high spatial resolution (Lindqvist et al. 1992; Sevenster et al. 1997; Sjouwerman et al. 1998) which have resulted in the detection of over five hundred double-peaked objects commonly associated with oxygen-rich AGB stars. More recently, the ISOGAL survey, which is part of the ISO mission, scanned several regions along the galactic plane, including the inner bulge, at 6.8 and 15 micron wavelengths. In this work we report an analysis of 104 OH/IR stars in the bulge, based on near and mid-infrared observations. Mass-loss rates, luminosities and masses are determined for a sub-sample of stars, for which near-infrared data are available in the literature.

A multi-wavelength study of the oxygen-rich AGB star CIT 3: Bispectrum speckle interferometry and dust-shell modelling

CIT 3 is an oxygen-rich long-period variable evolving along the Asymptotic Giant Branch and is one of the most extreme infrared AGB objects. Due to substantial mass loss it is surrounded by an optically thick dust shell which absorbs almost all visible light radiated by the star and nally re-emits it in the infrared regime. We present the rst near infrared bispectrum speckle-interferometry observations of CIT 3 in the J-, H-, and K 0 -band. The J-, H-, and K 0 -band resolution is 48 mas, 56 mas, and 73 mas, resp. The interferograms were obtained with the Russian 6 m telescope at the Special Astrophysical Observatory. While CIT 3 appears almost spherically symmetric in the H -a ndK 0 -band it is clearly elongated in the J-band along a symmetry axis of position angle 28. Two structures can be identied: a compact elliptical core and a fainter north-western fan- like structure. The eccentricity of the elliptical core, given by the ratio of minor to major axis, is approximately = 123 mas/154 mas = 0.8. The full opening angle of the fan amounts to approximately 40 .E xtensive radiative transfer calculations have been carried out and confronted with the observations taking into account the spectral energy distribution ranging from 1 m to 1 mm, our near-infrared visibility functions at 1.24 m, 1.65 ma nd 2.12 m, as well as 11 m ISI interferometry. The best model found to match the observations refers to a cool central star with Te = 2250 K which is surrounded by an optically thick dust shell with (0:55 m) = 30. The models give a central-star diameter of = 10:9 mas and an inner dust shell diameter of 1 =7 1: 9m as being in line with lunar occultation observations. The inner rim of the dust-shell is located at r1 =6 :6 R and has a temperature of T1 = 900 K. The grain sizes were found to comply with a grain-size distribution according to Mathis et al. (1977) with n(a) a 3:5 , and 0.005 m a 0:25 m. Uniform outflow models, i.e. density distributions with 1=r 2 , turned out to underestimate the flux beyond 20 m. A two-component model existing of an inner uniform-outflow shell region ( 1=r 2 ) and an outer region where the density declines more shallow as 1=r 1:5 proved to remove this flux deciency and to give the best overall match of the observations. The transition between both density distributions is at r2 =2 0:5 r1 = 135:7 R where the dust-shell temperature has dropped to T2 = 163 K. Provided the outflow velocity kept constant, the more shallow density distribution in the outer shell indicates that mass-loss has decreased with time in the past of CIT 3. Adopting vexp =2 0 km s 1 , the termination of that mass-loss decrease and the begin of the uniform-outflow phase took place 87 yr ago. The present-day mass-loss rate can be determined to be _ M =( 1:3 2:1) 10 5 M/yr for d = 500 800 pc.

Dust features in the 10-μm infrared spectra of oxygen-rich evolved stars

We have analyzed the m UKIRT CGS3 spectra of 142 M-type stars including 80 oxygen-rich AGB stars and 62 red supergiants, with a view to understanding the differences and similarities between the dust features of these stars. We have classified the spectra into groups according to the observed appearance of the infrared features. In each case the normalized continuum-subtracted spectrum has been compared to those of the other stars to find similarities and form groups. The dust features of the AGB stars are classified into six groups: broad AGB, where the feature extends from 8 μm to about 12.5 μm with little structure; broad+sil AGB, which consists of a broad feature with an emerging 9.7 μm silicate bump; and four silicate AGB groups in which a “classic” 9.7 μm silicate feature gets progressively narrower. Likewise, the supergiant spectra have also been classified into groups, however these do not all coincide with the AGB star groups. In the supergiant case we again have six groups: featureless, where there is little or no emission above the continuum; broad Super, where the feature extends from about 9 μm to about 13 μm; and four silicate Super groups, which again show a progression towards the narrowest “classic” 9.7 μm silicate feature. We compare the mean spectrum for each group, which yields two main results. Firstly, while the “classic” silicate feature is essentially identical for both AGB stars and red supergiants, the broad features observed for these two stellar types are quite different. We suggest that the dust in these two environments follows different evolutionary paths, with the dust around Mira stars, whose broad feature spectra can be fit by a combination of alumina (Al2O3) and magnesium silicate, progressing from this composition to dust dominated by magnesium silicate only, while the dust around supergiants, whose broad feature can be fit by a combination of Ca-Al-rich silicate and Al2O3, progresses from this initial composition to one eventually also dominated by magnesium silicate. The reason for the difference in the respective broad features is not clear as yet, but could be influenced by lower C/O ratios and chromospheric UV radiation fields in supergiant outflow environments. The second result concerns the 12.5 - 13.0 μm feature discovered in IRAS LRS spectra and widely attributed to Al2O3. This feature is seen predominantly in the spectra of semiregular variables, sometime in Miras and only once (so far) in supergiant spectra. We argue that it is unlikely that this feature is due to Al2O3 or, as has more recently been suggested, spinel (MgAl2O4), but could be associated with silicon dioxide or highly polymerized silicates (not pyroxenes or olivines).

Mass-losing AGB Stars in the LMC

We show the results of an infrared study of a sample of heavily obscured AGB stars in the LMC. Both carbon-rich and oxygen-rich mass-losing AGB stars can be found at both high and low luminosities, but the percentage of carbon stars decreases with increasing luminosity. The optical depth of the circumstellar envelopes also decreases with increasing luminosity, while the mass-loss rates are (nearly) constant with luminosity. We also show tentative evidence for having found the first post-AGB stars in the LMC.

Formation of crystalline silicate around oxygen–rich AGB stars

Crystallization of silicate has been investigated within the framework of dust formation in steady state gas outflows around oxygen–rich AGB stars, where silicates are locked not only into homogeneous silicate grains but also into the mantles of heterogeneous grains. Based on the thermal history of dust grains after their formation, the crystallization calculation results in no crystalline silicate for the mass loss rate Ṁ ≤ 2 × 10−5M⊙ yr−1. Only silicate in the mantles of heterogeneous grains can be crystallized for Ṁ ≥ 3 × 10−5M⊙ yr−1, while homogeneous silicate grains remain amorphous. The mass fraction of crystalline silicate increases with increasing Ṁ. The radiation transfer calculations confirm the appearance of an emission feature around 33.5 μm, taking olivine as a representative of crystalline silicates. On the other hand, the 10μm feature appears in absorption, being dominated by homogeneous silicate grains. These trends are consistent with the observations. Thus the crystalline silicate is a diagnostics of high mass loss rate at the late stage of AGB stellar evolution, reflecting the formation process of dust grains.

ISO results on circumstellar envelopes

This review covers spectroscopic results obtained with ISO on molecular and atomic line emission from the circumstellar envelopes of oxygen-rich AGB stars and post-AGB objects and from cool supergiant stars. As well as the SWS detection of the 4.27-μm CO2 band in absorption and emission from cool star envelopes, several new emission bands which have been detected in SWS spectra between 13.9 and 16.2 μm have been identified with CO2 bands. Strong H2O line emission has been observed in the LWS and SWS spectra of high mass loss rate O-rich AGB and M supergiant stars, confirming the predicted importance of this molecule as a coolant in their outflows. The proposed infrared radiative pump mechanism for circumstellar OH masers has been directly confirmed for the first time via measurements for several stars of the OH infrared pumping lines between 34.6 μm and 163 μm. Water vapour emission lines are not present in the spectra of O-rich post-AGB objects, having been replaced by broad emission features at 43 and 62 μm due to crystalline water ice that has condensed in the very cool outflows.

Masers: Probing the mass loss process in PPN

With the advent of the IRAS All-Sky Survey a sizeable number of transition objects between the AGB and the PN-phase were found - the Proto Planetary Nebulae (PPN). Oxygen-rich AGB stars often show prominent masers of SiO, H2O, and OH, which are lost during the transition process. The heavy mass loss on the AGB however does not stop abruptly and a new axisymmetric wind develops during the PPN phase. These winds both may host new masers and they can be used to study the changes of the mass loss process after that the stars have stopped their large-amplitude variations on the AGB. Several PPN are known to have OH masers, and at least in one case, HD 101584, the presence of a bipolar outflow could be proven (te Lintel Hekkert et al. 1992). Lewis (1989) found that main-line OH masers become prominent again. I will discuss here conclusions, which can be drawn from observations of H2O masers in PPN.

ISO LWS observations of H2O from R CAS: A consistent model for its circumstellar envelope

We present an ISO LWS 43-197 µm grating spectrum of the oxygen-rich AGB star R Cas. The spectrum is rich of isolated and blended H2O lines. For their identification and in order to determine the physical parameters of the circumstellar envelope, we have constructed a model which treats radiative transfer, chemical exchange and photodissociation reactions, and various heating and cooling processes in a consistent manner. By fitting the observed line fluxes and using stellar parameters based upon the Hipparcos distance, we derived a mass-loss rate of M = 1 10-6 M⊙ yr-1 which is close to the value 6 10-7 M⊙ yr-1 previously derived for W Hya, another oxygen-rich AGB star.

Detection of detached dust envelopes around oxygen-rich AGB stars

Extended emission components are clearly found in the IRAS scan data of optically visible oxygen-rich AGB stars which show no 10µm silicate band feature in the IRAS LRS spectra but a strong infrared excess in the IRAS photometric data. It is most likely that these stars really have their circumstellar dust envelopes, which are detached from the central stars, indicating a halting of mass loss for a significant period.