IRAS sources and the nature of the Galactic Bulge

Near-infrared JHKL photometry is presented for a sample of 141 IRAS objects, selected on the basis of their IRAS colours and lying in two strips across the Galactic Bulge between 7° < |b| < 8° and 345° < l < 15°. Typically 10-12 observations of each object were obtained over a 4-5-yr time span. From the photometry the sample was shown to comprise 113 Miras, 22 M giants, a probable M dwarf, a T Tau star, three highly reddened objects of uncertain nature and one peculiar object. Of the 22 M giants, eight are in the Bulge and 13 in the foreground. Their colours, however, are consistent with them all being 'Bulge-type' M stars. Periods and bolometric magnitudes were derived for 104 of the Miras. Of the periods determined, 71 per cent lie in the range 360-560 d, with maximum and minimum values of 722 and 170 d, respectively. Thus the speculation that a large fraction of the Bulge IRAS sources would have periods in excess of 1000 d is not confirmed. It is suggested, following Renzini & Greggio, that the longest period objects (P ≧ 600 d) in the Bulge may be the progeny of binary mergers. The distances to individual objects are derived from their bolometric magnitudes and the period-luminosity relation. The spread of distances is compared to models having a minor-to-major axis ratio c/a, and a stellar number-density distribution of the form ρ ∞ a-α. Values in the range 3.0 < α < 3.5 and 0.6 < c/a < 0.8 provide a good fit to the data. The K-L and K-[12] colours are functions of the pulsation amplitude (ΔK). These colours provide a measure of the relative contributions to the total luminosity from the dust shell and the underlying star, thus the correlation supports a causal connection of pulsation and mass loss. The mass-loss rates are estimated to be between M∼4×10-7 and 3 × 10-5 M⊙ yr-1. Stars within a small range of log period show a large range of colour and amplitude which may represent an evolutionary trend to stronger pulsation and higher mass loss with time. Those objects associated with OH-maser emission have redder colours and/or longer periods than non-masers at similar distances. A period-25-μm dust-luminosity-colour relationship is deduced for Miras with optically thick shells and it is suggested as a possible method for determining the distances of these objects. These results support an evolutionary scenario in which the Mira phase is short lived and therefore involves very little change in the stellar luminosity or log period, but a large change in the pulsation amplitude and the optical depth of the circumstellar dust shell. A lower limit of about 3 Gyr is found for the ages of the longest period Miras in the Bulge. However, if they are metal-rich then they could be much older, possibly of comparable age to the globular clusters.