Near-IR variability properties of a selected sample of AGB stars

¹ Observatorio Astronómico Nacional, Apartado 112, 28803 Alcalá de Henares, Spain e-mail: f.jimenez-esteban@oan.es
² FRACTAL SLNE, Avda San Sebastián 5 3-D, 38003 Santa Cruz de Tenerife, Spain
³ Hamburger Sternwarte, Gojenbergsweg 112, 21029 Hamburg, Germany
⁴ Research and Scientific Support Department of ESA, European Space Astronomy Centre, Villafranca del Castillo, Apartado de Correos 50727, 28080 Madrid, Spain
⁵ Instituto de Astrofísica de Canarias, Via Lactea s/n, 38200 La Laguna, Tenerife, Spain
⁶ Consejo Superior de Investigaciones Científicas, Spain

Received: 4 August 2005
Accepted: 8 June 2006

Abstract

We present the results of a near-infrared monitoring programme of a selected sample of stars, initially suspected to be Mira variables and OH/IR stars, covering more than a decade of observations. The objects monitored cover the typical range of IRAS colours shown by O-rich stars on the asymptotic giant branch and show a surprisingly large diversity of variability properties. Sixteen objects are confirmed as large-amplitude variables. Periods between 360 and 1800^d  and typical amplitudes 1^m    2^m  could be determined for nine of them. In three light curves, we find a systematic decrease in the mean brightness, and two light curves show pronounced asymmetry. One source, IRAS 07222–2005, shows infrared colours typical of Mira variables, but it pulsates with a much longer period (≈1200^d) than a normal Mira. Two objects are either close to (IRAS 03293+6010) or probably in (IRAS 18299–1705) the post-AGB phase. In IRAS 16029–3041 we found a systematic increase in the colour of ≈1^m, which we interpret as evidence of a recent episode of enhanced mass loss. IRAS 18576+0341, a heavily obscured luminous blue variable was also monitored. The star showed a continued decrease in brightness over a period of 7 years (1995–2002).