Infrared colours for Mira-like long-period variables found in the Hipparcos catalogue

Abstract

Near-infrared, JHKL, photometry is presented for 193 Mira and semi-regular variables that were observed by Hipparcos; periods, bolometric magnitudes and amplitudes are derived for 92 of them. Because of the way in which the Hipparcos targets were selected, this group of stars provides a useful data base of Miras with low mass-loss rates (Ṁ ≲ 10-7 M⊙ yr-1). Various period-colour relationships are discussed in detail. The colour, particularly K - L, at a given period is found to depend on the pulsation amplitude of the star. A comparison with models suggests that this is a consequence of atmospheric extension, in the sense that large-amplitude pulsators have very extended atmospheres and redder K - L and H - K but bluer J - H than their lower amplitude counterparts. The stars with very extended atmospheres also have higher values of K - [12] and hence higher mass-loss rates. This finding provides further evidence for the causal connection between pulsation and mass loss Two sequences are identified in the Hp - K versus log P diagram (where Hp is the Hipparcos broad-band magnitude) at short periods (log P < 2.35). At a given period these two groups have, on average, the same pulsation amplitude, but different JHKL colours and spectral types. The short-period stars in the bluer sequence have similar near-infrared colours to the Miras found in globular clusters. Long-term trends in the infrared light curves are discussed for stars that have sufficient data.