天琴座RR型变星的测距应用

Distance measurement based on RR Lyrae variable stars

从2000年至今,随着大天区时域巡天设备的发展,天琴座RR型变星的数目已由千余颗增加至30万颗.其测距方法也得到了优化,从依赖可见光金属丰度光度关系逐渐转变为红外的周期金属丰度光度关系.得益于大麦哲伦云的几何距离和Gaia视差,天琴座RR型变星的周期金属丰度光度关系的零点得到显著优化.在大样本天琴座RR型变星的基础上,银河系晕、核球、麦哲伦云的结构得到了细化,球状星团和矮星系的距离也得到了准确测量.更重要的是,研究发现了双周期的天琴座RR型变星,这把唯一不受金属丰度影响的量天尺为批量高精度星系测距开辟了道路.鉴于我国在未来5年有多项投入使用的时域巡天设备,例如司天工程、中国空间站巡天望远镜等,本文将总结过去20年中天琴座RR型变星测距应用的进展.

This paper summarizes the applications of RR Lyrae variable stars for distance measurements in the last 20 years.RR Lyrae stars are low-mass,metal-poor stars located in the overlap region of the pulsation instability strip and the horizontal branch.Due to the presence of radial pulsations,there are periodic brightness variations in RR Lyrae stars.RR Lyrae stars can be easily identified because of their sawtooth-shaped light curves.Based on the shape of the light curve,RR Lyrae stars can be mainly classified into fundamental mode(RRab),first-overtone mode(RRc)and double-mode(RRd)sub-types.With the development of large-scale time-domain telescopes,the current sample of RR Lyrae stars based on 1-meter-class telescopes is almost complete,with a number of about 300000 stars.RR Lyrae stars are good standard candles due to the tight relationship between luminosity and period and metallicity.The distance measurements based on RR Lyrae stars have also been optimized,gradually shifting from relying on visible metallicity-luminosity relations to infrared period-metallicityluminosity relations.The zero points of the period-metallicity-luminosity relations for the RR Lyrae stars have been significantly optimized thanks to the improvement of the geometric distance of the Large Magellanic Cloud and the Gaia parallax.RRab stars and RRc stars were used to measure distances with an accuracy of about 5%for a single object and up to 2%for multiple objects.RRab stars have been the best distance tracer of old population for the last decades.On the basis of a large sample of RRab stars,the structure of the Galactic halo,bulge,and the Magellanic Clouds is refined,and the distances to globular clusters and dwarf galaxies are more accurately measured.In 2023,based on LAMOSTand SDSS low resolution spectra,RRd stars were found to have period-luminosity relations that do not depend on metallicities.It is the only valuable object in standard candles that is independent of metallicities,and can break the bottleneck of the lack of metallicity in photometry-based distance measurements.RRd stars open the way to bulk measurements of high-precision distances to galaxies.Based on multiple RRd stars,the distance accuracy can be up to 1%which is better than distance accuracy based on RRab stars or tip of red giant branch stars.At the same time,RRd stars can also provide the metallicity of the objects to help optimize the accuracy of RRab star-based distances.With the future Chinese Space Station Telescope and other instruments,the number of RRd stars will increase to tens of thousands.RRd stars will be used to determine highprecision distances to hundreds of galaxies or dwarf galaxies with an error of about 1%,increasing that sample by a factor of 20,and will then be able to optimize the Hubble constant from multiple distance ladders.