Non-variable 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 the stellar mass loss. The reduction of the 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 non-variable OH/IR stars by using 2MASS, UKIDSS, and OAOWFC survey data. As a result, we get multi-epoch NIR data taken over a 20-year period (1997-2017) for 6 of 16 non-variable OH/IR stars, and all six 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, OH31.0-0.2, shows a rapid brightening, which cannot be explained only by the dispersal of the dust shell. Multi-color (J-, H-, and K-band) data are obtained for three objects, OH25.1-0.3, OH53.6-0.2, and OH77.9+0.2. Surprisingly, none of them appears to have become bluer, and OH53.6-0.2 is found to have been reddened with 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.

中文翻译：

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非可变 OH/IR 恒星的长期近红外增亮

非变 OH/IR 恒星被认为刚刚离开渐近巨星分支 (AGB) 阶段。在这张常规图片中，它们一定仍然显示出由 AGB 阶段喷出的尘埃引起的强烈星周灭绝，并且由于恒星质量损失停止后星周尘埃的扩散，预计灭绝会随着时间的推移而减少。消光的减少使恒星随着时间的推移变得明显更亮和更蓝，尤其是在近红外 (NIR) 范围内。我们通过使用 2MASS、UKIDSS 和 OAOWFC 调查数据寻找非可变 OH/IR 恒星的这种长期增亮。结果，我们获得了 20 年期间（1997-2017 年）为 16 颗非可变 OH/IR 恒星中的 6 颗采集的多时代 NIR 数据，并且发现所有 6 个天体都在变亮。五个物体的 K 波段增亮率范围为 0。010 到 0.130 mag yr$^{-1}$，用常规图片合理解释。然而，一颗 OH/IR 星 OH31.0-0.2 显示出快速增亮，这不能仅用尘埃壳的扩散来解释。获得了三个物体的多色（J-、H- 和 K-波段）数据，OH25.1-0.3、OH53.6-0.2 和 OH77.9+0.2。令人惊讶的是，它们似乎都没有变得更蓝，并且发现 OH53.6-0.2 以 0.013 mag yr$^{-1}$ in (JK) 的速率变红。我们的研究结果表明存在其他机制，例如恒星属性（温度或光度）的快速变化或新一批尘埃颗粒的产生。获得了三个物体的多色（J-、H- 和 K-波段）数据，OH25.1-0.3、OH53.6-0.2 和 OH77.9+0.2。令人惊讶的是，它们似乎都没有变得更蓝，并且发现 OH53.6-0.2 以 0.013 mag yr$^{-1}$ in (JK) 的速率变红。我们的研究结果表明存在其他机制，例如恒星属性（温度或光度）的快速变化或新一批尘埃颗粒的产生。获得了三个物体的多色（J-、H- 和 K-波段）数据，OH25.1-0.3、OH53.6-0.2 和 OH77.9+0.2。令人惊讶的是，它们似乎都没有变得更蓝，并且发现 OH53.6-0.2 以 0.013 mag yr$^{-1}$ in (JK) 的速率变红。我们的研究结果表明存在其他机制，例如恒星属性（温度或光度）的快速变化或新一批尘埃颗粒的产生。