Spitzer Observations of Large Amplitude Variables in the LMC and IC 1613

The 3.6 and 4.5 micron characteristics of AGB variables in the LMC and IC1613 are discussed. For C-rich Mira variables there is a very clear period-luminosity-colour relation, where the [3.6]-[4.5] colour is associated with the amount of circumstellar material and correlated with the pulsation amplitude. The [4.5] period-luminosity relation for dusty stars is approximately one mag brighter than for their naked counterparts with comparable periods.Comment: 5 pages, 4 figures, to appear in Proceedings of "Wide-Field Variability Surveys: A 21st Century Perspective

A Remarkable Oxygen-rich Asymptotic Giant Branch Variable in the Sagittarius Dwarf Irregular Galaxy

We report and discuss JHKs photometry for Sgr dIG, a very metal-deficient galaxy in the Local Group, obtained over 3.5 years with the Infrared Survey Facility in South Africa. Three large amplitude asymptotic giant branch variables are identified. One is an oxygen-rich star that has a pulsation period of 950 days, that was until recently undergoing hot bottom burning, with Mbol~-6.7. It is surprising to find a variable of this sort in Sgr dIG, given their rarity in other dwarf irregulars. Despite its long period the star is relatively blue and is fainter, at all wavelengths shorter than 4.5microns, than anticipated from period-luminosity relations that describe hot bottom burning stars. A comparison with models suggests it had a main sequence mass Mi~5 times solar and that it is now near the end of its AGB evolution. The other two periodic variables are carbon stars with periods of 670 and 503 days (Mbol~-5.7 and -5.3). They are very similar to other such stars found on the AGB of metal deficient Local Group Galaxies and a comparison with models suggests Mi~3 times solar. We compare the number of AGB variables in Sgr dIG to those in NGC6822 and IC1613, and suggest that the differences may be due to the high specific star formation rate and low metallicity of Sgr dIG.Comment: 12 pages, 10 figures, accepted for MNRA

Obscured Asymptotic Giant Branch Variables in the Large Magellanic Cloud and the Period-Luminosity Relation

The characteristics of oxygen-rich and carbon-rich, large amplitude (dK>0.4 mag), asymptotic giant branch variables in the Large Magellanic Clouds are discussed, with an emphasis on those obscured by dust. Near-infrared photometry, obtained over about 8 years, is combined with published mid-infrared observations from IRAS and ISO to determine bolometric magnitudes for 42 stars. Pulsation periods of the O-rich stars are in the range 116<P<1393 days, while those for C-rich stars have 298<P<939 days. In addition to the regular pulsations, one O-rich star and four C-rich stars show large amplitude, dK> 0.6 mag, secular or very long period variations which may be associated with changes in their mass-loss rates. We discuss and compare various methods of determining the bolometric magnitudes and show, perhaps surprisingly, that most of the very long period stars seem to follow an extrapolation of the period-luminosity relation determined for stars with shorter periods - although the details do depend on how the bolometric magnitudes are calculated. Three stars with thin shells, which are clearly more luminous than the obscured AGB stars, are undergoing hot bottom burning, while other stars with similar luminosities have yet to be investigated in sufficient detail to determine their status in this regard. We suggest that an apparent change in slope of the period luminosity relation around 400-420 days is caused by variables with luminosities brighter than the predictions of the core-mass luminosity relation, due to excess flux from hot bottom burning.Comment: 20 pages, 20 figures, accepted for MNRA

Cepheid Parallaxes and the Hubble Constant

Revised Hipparcos parallaxes for classical Cepheids are analysed together with 10 HST-based parallaxes (Benedict et al.). In a reddening-free V,I relation we find that the coefficient of logP is the same within the uncertainties in our Galaxy as in the LMC, contrary to some previous suggestions. Cepheids in the inner region of NGC4258 with near solar metallicities (Macri et al.) confirm this result. We obtain a zero-point for the reddening-free relation and apply it to Cepheids in galaxies used by Sandage et al. to calibrate the absolute magnitudes of SNIa and to derive the Hubble constant. We revise their result from 62 to 70+/-5 km/s/Mpc. The Freedman et al. 2001 value is revised from 72 to 76+/-8 km/s/Mpc. These results are insensitive to Cepheid metallicity corrections. The Cepheids in the inner region of NGC4258 yield a modulus of 29.22+/-0.03(int) compared with a maser-based modulus of 29.29+/-0.15. Distance moduli for the LMC, uncorrected for any metallicity effects, are; 18.52+/-0.03 from a reddening-free relation in V,I; 18.47+/-0.03 from a period-luminosity relation at K; 18.45+/-0.04 from a period-luminosity-colour relation in J,K. Adopting a metallicity correction in V,I from Marci et al. leads to a true LMC modulus of 18.39+/-0.05.Comment: 9 pages, 1 figure, on-line material from [email protected]. Accepted for MNRA