Galactic Rotation Traced By RR Lyraes And Its Chemodynamical Meaning

RR Lyraes are a special class of metal-poor,He-core-burning variable stars that are on the horizontal branch in H-R diagram,and they are generally >11 Gyr in age.In the Milky Way,it is widely distributed in the central part of the bulge,and is generally considered to be a relatively old metal-poor star in the bulge area(probably the oldest type of star).RR Lyraes’ period-luminosity relationship can be used as a standard candle to obtain distance information,so it is an excellent probe for studying the three-dimensional distribution and kinematics of bulge area.Our current understanding of the Galactic bulge is that the Milky Way is a barred spiral galaxy whose bulge exhibits a box/peanut(B/P)structure.In the high glactic latitude area,an X-shaped structure can be observed.With the large-scale survey data sets in the direction of the Galactic center in recent years,most observations believe that RR Lyraes do not go with the bar.We can not observe significant rotation,which is very different from the stars in other bulge areas.,suggesting that they may have different origins.We combined the ORL-IV RRL star with Gaia DR2’s own data and compared the results with BRAVA-RR(Kunder et al.2016).It is proved by geometric view and simulation that the rotation information obtained by proper motion and by radial velocity has almost the same indication on the rotation of the bulge region.We have found that the RRL of the bulge actually shows cylindrical rotation,but its rotation speed is lower than RGB.RRL is also a dynamically hot population.We therefore believe the bulge RRLs origins from thick disk,rather than from the the stellar halo(Minniti et al.2017).In addition,we have also noticed the difference between the metal-poor RRLs and the metal-rich ones from several aspects,which verifies the multi-origin of bulge RRLs(Pietrukowicz et al.2015).