To save content items to your account,
please confirm that you agree to abide by our usage policies.
If this is the first time you use this feature, you will be asked to authorise Cambridge Core to connect with your account.
Find out more about saving content to .
To save content items to your Kindle, first ensure email@example.com
is added to your Approved Personal Document E-mail List under your Personal Document Settings
on the Manage Your Content and Devices page of your Amazon account. Then enter the ‘name’ part
of your Kindle email address below.
Find out more about saving to your Kindle.
Note you can select to save to either the @free.kindle.com or @kindle.com variations.
‘@free.kindle.com’ emails are free but can only be saved to your device when it is connected to wi-fi.
‘@kindle.com’ emails can be delivered even when you are not connected to wi-fi, but note that service fees apply.
We have obtained infrared spectra of carbon-rich AGB stars in three nearby galaxies – the Large and Small Magellanic Clouds, and the Fornax dwarf spheroidal galaxy. Our primary aim is to investigate gas compositions and mass-loss rate of these stars as a function of metallicity, by comparing AGB stars in several galaxies with different metallicities. C2H2 are detectable from AGB stars, and possibly PAHs are subsequently formed from C2H2. Thus, it is worth investigating chemical processes at low metallicity. These stars were observed using the Infrared Spectrometer (irs) onboard the Spitzer Space Telescope which covers 5–35 μm region, and the Infrared Spectrometer And Array Camera (isaac) on the Very Large Telescope which covers the 2.9–4.1 μm region. HCN, CH and C2H2 molecular bands, as well as SiC and MgS dust features are identified in the spectra. The equivalent width of C2H2 molecular bands is larger at lower metallicity, thus PAHs might be abundant in AGB stars at low metallicity. We find no evidence that mass-loss rates depend on metallicity. Chemistry of carbon stars is affected by carbon production during the AGB phase rather than the metallicities. We argue that lower detection rate of PAHs from the interstellar medium of lower metal galaxies is caused by destruction of PAHs in the ISM by stronger UV radiation field.
A brief review is given of the various types of star which are thought to be in the immediate post-AGB stage of evolution. The paper then concentrates on the properties of the RCB stars and particularly on the mass-loss process in these stars. It is suggested that grain formation takes place over the cool regions of giant convection cells in a super-Eddington outflow and in the form of small clouds or puffs. Attention is drawn to observations which suggest that grain formation in the outer atmospheres of Miras and other cool giants may also take place in puffs rather than in spherical shells. Evidence on the long-term variation of the circumstellar dust emission from RCB stars is summarized.
Near-infrared, JHKL, observations of 595 Mira variables in two fields on either side of the centre of our Galaxy, confirm that the Bulge is not spherically symmetric about its axis of rotation, but is elongated so that the part to the east of the centre is closer to us. The shape of the Bulge about its axis of rotation is not uniquely defined by these data, but the shape that deviates least from circular symmetry has an axis ratio xo/yo = 1.7, with a major axis at an angle, θ = 58° ± 7, to the plane of the sky, for a galactic centre distance, R0 = 9.4 ± 0.5 kpc. This is based on an assumed scale length in galactic coordinates of bo = 375 pc and lo/bo = 2.0.
The effect(s) of finite exposure(s) to 22.0 °C on embryo weight
gain, 3rd tarsus length, transfer of lipid from
the yolk and uptake of specific lipid components by the liver, were measured
in the chick embryo. Embryo
growth was completely suspended over the period of temperature reduction,
but resumed unimpaired when
the optimum temperature of 37.5 °C was restored. Two successive periods
of embryonic exposure to 22.0 °C
(36 h at d 3 followed by 24 at d 10) were additive in their effect on growth
retardation. Embryos subjected
to temperature reductions on d 3 of incubation displayed significant changes
in lipid concentrations and
polyunsaturated fatty acid profiles within the liver when measured at d
18 of incubation. Yolk lipid uptake
was significantly reduced but not suspended by embryonic exposure to 22.0
°C. It is suggested that the
observed suspension of embryonic development was the result of a temperature
dependent reduction in
metabolism according to a Q10∼2, which is typical of an
The large reported discrepancy between the scale height in the Galactic Bulge for objects such as RR Lyrae variables on the one hand and M-giants or IRAS sources on the other is primarily due to these scale heights being determined at different mean galactocentric distances. Mira variables apparently provide a Bulge-like population in the solar vicinity and show that the thick disc consists of a number of subsystems. There are M-giants in the extended solar neighbourhood, away from the Galactic plane, which seem similar to M-giants in the Bulge. Photometric and kinematic studies on some of these stars are in progress.
Table 1 lists galaxies in which Cepheids are known. The early work on the detection and period determination for these stars forms the basis of subsequent studies. These later studies include the infrared photometry of Cepheids pioneered by the Toronto group and the efforts by various workers to improve the optical photometry. An example of the importance of this latter work is the recent study of M33 by Sandage (1983) in which a revised magnitude scale leads to a distance modulus 0.67 greater than that previously adopted.
Magellanic Cloud cepheids are of special importance for studies of stellar pulsation, of stellar evolution, of the nature of the Clouds themselves and of the extragalactic distance scale. It is not possible to cover all aspects of this subject in a short review and fortunately this is probably unnecessary since a whole symposium on cepheids is planned for Toronto next year. This paper attempts the much more modest task of assessing the current status on three main topics; chemical abundances, reddenings and the P-L and P-L-C relations (including recent infrared work). Conflicting views on some of these topics have recently appeared in the literature and a survey of the situation seems rather desirable.
In 1948 the South African amateur astronomers de Kock and Kirchhoff were the first to notice that RR Tel had brightened to 7th magnitude. Subsequent examination of the Harvard Patrol plates showed that it had in fact reached a maximum in 1944 after rapidly brightening 7 magnitudes (Mayall, 1949). Prior to 1944 the object showed variations with a period of 387 days and an amplitude of up to 2 magnitudes in the blue. After the outburst the spectrum evolved in a manner characteristic of very slow novae: an F5 supergiant absorption spectrum gave way to strong permitted and forbidden emission lines. The ionization levels characterising the emission increased with time. The spectral development has been extensively studied and has been well summarised for the period up to 1973 in the Thackeray's (1977) monograph on the subject.
Radial velocities based on 81 image tube spectra at 30 Å mm−1 are given for 25 stars (17 early type supergiants or upper main sequence stars and 8 late type stars) in the young SMC cluster NGC 330. An upper (one standard deviation) limit to the velocity dispersion is found to be 2 km s−1. The mass-to-light ratio is likely to be less than 0.1 in solar units. Results for other young Magellanic Cloud clusters derived on the assumption that they are tidally limited are consistent with this result. Spectroscopic binaries with semi-amplitudes, K, greater than about 10 km s−1 appear to be rare or absent amongst the stars studied.
Recently derived bolometric absolute magnitudes are used to investigate the pulsational properties of some classes of red variables. The Mira variables with periods over 200 days have pulsation constants appropriate to overtone pulsatore but the shorter period 135 day Miras seem to be hotter and less luminous and may be fundamental pulsators. The large amplitude Shapley-Nail variables in the SMC are probably fundamental pulsators with masses in the 5-10 solar range. Bright supergiant red variables in the LMC show evidence of the period-luminosity relation expected if they are in the early stages of core helium burning.
Mira variables are attractive for galactic centre studies because (1) they are numerous there; (2) they have high luminosities (MK ∼ −7.5) in the infrared where they can be seen through heavy absorption; (3) good radial velocities can be obtained from emission lines even for very faint objects; (4) at least in the solar neighbourhood and in globular clusters the period seems a good indicator of age and/or chemical composition.
A general review of the RCB stars is given. The available observations (spectra, photometry (including infrared) and polarization) seem most easily explained by a model in which a cloud of dust is ejected towards the observer at times of deep minima. The large radius changes in the pulsation cycle of RY Sgr suggest a high luminosity. This is also indicated by the RCB stars in the LMC. Theoretical work on RCB stars is briefly reviewed. The possible relation of the variables to the planetary nebulae and the novae is mentioned.
This paper discusses the following topics: (1) Mira Variables in Globular Clusters; (2) infrared observations of Globular Cluster Variables; (3) carbon Stars in Magellanic Cloud Clusters; (4) the Se Variables and the superlithium rich Variables.