The nature and kinematics of long-period variables in the Large Magellanic Cloud

Abstract

A search for long-period variables (LPVs) has been made in the bar and southern regions of the Large Magellanic Cloud (LMC) using a series of 7-band UKST plates, resulting in the discovery of 471 Mira variables and 572 SRa variables. By using mainly automated methods of determining periods and amplitudes of variability, corrections for incompleteness have been estimated. The Miras show a trend towards larger amplitudes and brighter luminosities with period, both of which should contribute to increased mass loss rates. The period distribution falls abruptly longward of ~420 days and shortward of ~140 days, whereas the corresponding limits in the solar neighbourhood are ~450 and ~220 days, suggesting different histories of star formation in the LMC and the Galaxy. In particular, there appear to be relatively more old stars in the LMC. Infrared JHK photometry and visual spectra have been obtained for a large proportion of these LPVs. Various aspects of the asymptotic giant branch (AGB) evolution of LPVs are discussed using this data. The birth/death rate of LPVs of different ages in the LMC is compared with the birth rates of appropriate samples of planetary nebula«, clump stars, Cepheids and OH/IR stars. It appears that there are many fewer large amplitude LPVs per unit galactic stellar mass in the LMC than in the Galaxy. We suggest that this may be due to the fact the evolved intermediate age AGB stars in the LMC often turn into carbon stars which tend to have smaller pulsation amplitudes than M stars. A distance modulus to the LMC of 18.6 is derived by comparing the LMC LPVs with P~200 days with the 47 Tue Mira variables in the (77, logP) plane. The (77, logP) relation is also used to examine the tilt of the LMC populations of old and intermediate age stars, with no significant tilt being found in the North-South direction. Radial velocities obtained for a sample of the LPVs showed that the shorter period LPVs have a high velocity dispersion, and low rotational velocity about the LMC, giving direct evidence tha t they are one the oldest populations yet studied in the LMC and that they belong to a bulge/halo population. This is the first evidence of the LMC possessing a spheroidal population. A kinematic analysis applied to a wide range of LMC populations (H I gas, CO molecular clouds, planetary nebulae, old clusters and CH stars) showed that the dynamics of the LMC is dominated by a single rotating disk, with all populations having a similar kinematic line of nodes We find further evidence of the older populations possessing a lower mean systemic velocity than the younger populations, in the outer parts of the LMC. Show more