Long-term Near-infrared Brightening of Nonvariable OH/IR Stars

Abstract

Nonvariable OH/IR stars are thought to have just left the asymptotic giant branch (AGB) phase. In this conventional picture, they must still show strong circumstellar extinction caused by the dust ejected during the AGB phase, and the extinction is expected to decrease over time because of the dispersal of the circumstellar dust after the cessation of stellar mass loss. The reduction of extinction makes the stars become apparently brighter and bluer with time, especially in the near-infrared (NIR) range. We look for such long-term brightening of nonvariable OH/IR stars by using 2MASS, UKIDSS, and OAOWFC survey data. As such, we obtain multiepoch NIR data taken over a 20 yr period (1997–2017) for 6 of 16 nonvariable OH/IR stars, and all 6 objects are found to be brightening. The K-band brightening rate of five objects ranges from 0.010 to 0.130 mag yr−1, which is reasonably explained with the conventional picture. However, one OH/IR star, OH 31.0−0.2, shows a rapid brightening, which cannot be explained only by the dispersal of the dust shell. Multicolor (J-, H-, and K-band) data are obtained for three objects, OH 25.1−0.3, OH 53.6−0.2, and OH 77.9+0.2. Surprisingly, none of them appears to have become bluer, and OH 53.6−0.2 is found to have reddened at a rate of 0.013 mag yr−1 in (J – K). Our findings suggest other mechanisms such as rapid changes in stellar properties (temperature or luminosity) or a generation of a new batch of dust grains.

An infrared study of Galactic OH/IR stars. III. Variability properties of the Arecibo sample

“…This increase could be due to the dilution of its CSE on a timescale of few decades after the source had left the AGB and it is rapidly decreasing its mass-loss rate. However, the rate of brightening surpasses the upper limit determined by Kamizuka et al (2020) in their models for the K-band brightening rate. Because of its high brightness (K ∼ 11 mag in 2005), IRAS 19566+3423 is an ideal target for follow-up observations to study the short AGB to post-AGB transition phase.…”

“…14). Kamizuka et al (2020) studied the K band light curves of a sample of non-variable OH/IR stars finding for several of them a typical brightening rates from 0.01 to 0.13 mag yr −1 . They modelled the increase in the NIR emission as the result of the dilution of the CSE into space and the reappearance of the central star.…”

A WISE view on extreme AGB stars

“…This relation is used to obtain the distances to the galactic objects, and when no period is available the median of 7100 L is used. nonvariable OH/IR stars In a recent paper (Kamizuka et al 2020, hereafter K20) investigated the NIR brightening of nonvariable OH/IR stars. The OH maser emission of OH/IR stars on the AGB is expected to follow the pulsation period of the underlying star.…”

A WISE view of extreme AGB stars

“…In a recent paper, Kamizuka et al (2020) investigated the NIR brightening of non-variable OH/IR stars. The OH maser emission of OH/IR stars on the AGB is expected to follow the pulsation period of the underlying star, see Sect.…”

“…However, non-variable OH/IR stars are known to exist (Herman & Habing 1985) and this is expected to happen in the transition from the AGB to the P-AGB phase when large-amplitude pulsations stop. Kamizuka et al (2020) selected 16 stars from the sample in Herman & Habing (1985), which had the smallest variability amplitudes in their OH/IR maser emission. They established NIR multi-epoch data for six objects, based on archival data from 2MASS (Cutri et al 2003), UKIDSS (Lucas et al 2008), and data taken with the Okayama Astrophysical Observatory Wide Field Camera (OAOWFC; Yanagisawa et al 2019).…”