Long-term Near-infrared Brightening of Nonvariable OH/IR Stars
Long-term Near-infrared Brightening of Nonvariable OH/IR Stars
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DOI：
10.3847/1538-4357/ab9829
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发表时间：
2020-07
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通讯作者：
T. Kamizuka;Y. Nakada;K. Yanagisawa;R. Ohsawa;Y. Ita;H. Izumiura;H. Mito;H. Onozato;K. Asano-K.-Asa
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作者：
T. Kamizuka;Y. Nakada;K. Yanagisawa;R. Ohsawa;Y. Ita;H. Izumiura;H. Mito;H. Onozato;K. Asano-K.-Asa
不变的 OH/IR 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，它们仍然必须表现出由AGB阶段喷射的尘埃引起的强烈星周消光，并且由于恒星质量损失停止后星周尘埃的扩散，消光预计会随着时间的推移而减少。随着时间的推移，消光的减少使得恒星明显变得更亮、更蓝，尤其是在近红外（NIR）范围内。我们通过使用 2MASS、UKIDSS 和 OAOWFC 调查数据来寻找不变 OH/IR 恒星的长期增亮现象。因此，我们获得了 16 颗不变 OH/IR 恒星中的 6 颗在 20 年期间（1997-2017 年）采集的多历元 NIR 数据，并且发现所有 6 个物体都在变亮。五个天体的K波段增亮率范围为0.010到0.130 mag yr−1，这用常规图片可以合理解释。然而，一颗 OH/IR 星 OH 31.0−0.2 显示出快速变亮，这不能仅用尘埃壳的扩散来解释。获得三个物体的多色（J、H 和 K 波段）数据：OH 25.1−0.3、OH 53.6−0.2 和 OH 77.9+0.2。令人惊讶的是，它们似乎都没有变蓝，并且发现 OH 53.6−0.2 在 (J – K) 中以 0.013 mag yr−1 的速度变红。我们的研究结果表明了其他机制，例如恒星特性（温度或光度）的快速变化或新一批尘埃颗粒的产生。
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.