Mapping The Milky Way Young Disk Structure With LAMOST DR5 OB Stars

Our Galaxy,the Milky Way,is a benchmark for understanding disk galaxies.It is the only galaxy whose formation history can be studied using the full distribution of stars from faint dwarfs to supergiants.The oldest components provide us with unique insight into how galaxies form and evolve over billions of years.The Galaxy is a luminous barred spiral with a central box/peanut bulge,a dominant disk,and a diffuse stellar halo.Galactic studies will continue to play a fundamental role far into the future because there are measurements that can only be made in the near field and much of contemporary astrophysics depends on such observations.Understanding the structure of the disk is particularly important for understanding the overall structure of the Milky Way.A disk is a highly flat system that contains dust,gas and lots of baryonic matter such as stars.At the same time,the disk will be superimposed by high-light stars,galactic clusters and nebulae of the vortex structure.There are also many star-forming regions in the disk,which contains90%of the Milky Way’s stars.The stars in the disk are divided into old stellar populations,and young stellar populations in age.The old stars are widely used to study the structure of the disk,while the young stars have a short life and are limited by observation conditions,so we do not have a deep understanding of the structure of the young disk.The LAMOST spectroscopic observations provided a large sample of young stars,and we took advantage of this opportunity to answer some of the questions about young stellar disks.We present analysis of the spatial density structure for the outer disk from 8～14kpc with the LAMOST DR5 13534 OB type stars.Star counting is considered an effective way to explore the structure of disk galaxies,and it is very useful for revealing the history of the formation and evolution of structures such as flaring,warping,and spiral arms of the Milky Way.Dual exponential model is also widely used to study the spatial structure of silver disk.We propose a family of Markov chain Monte Carlo（MCMC）methods whose performance is unaffected by affine tranformations of space.These algorithms are easy to construct and require little or no additional computational overhead.They should be particulary useful for sampling badly scaled distributions.Using the above methods,we came to some interesting conclusions:（1）We discover the clear and similar flaring signatures beyond 8kpc for which the scale height is from 0.14 to 0.5 kpc in the north and south side,implying that the flaring is possibly symmetrical in the Milky Way disk.We reveal that the thickness of the OB stellar disk is similar with our previous thin disk traced by red giant branch stars,possibly implying that secular evolution is not the main contributor to the flaring but other scenarios such as interactions with passing dwarf galaxies should be more possible.For example,this may be due to the perturbation caused by the vibration of a dwarf galaxy passing through the disk between 300～900Myr.The lifetime of OB stars is mostly less than 100 Myr,and dwarf galaxies may disturb the gas disks that formed OB stars,so at least some imprint near the Sun may be inherited by OB stars.（2）When comparing the OB stellar disk with the gas disk,the former one is moderately thicker than the later one,meaning that one could use young OB stars to trace the gas properties.（3）Meanwhile,we unravel that the scale length of the young OB stellar disk is1.17±0.05 kpc,which is shorter than gas disk,suggesting that the gas disk is more extended than stellar disk.We can also infer that the disk may be more compact than the HI,CO or cold dust disk,similar to the hot dust disk.（4）What is more,by considering the mid-plane displacements（Z₀）in our density model we find that almost all are within 100 pc with the increasing trend as Galactocentric distance increases,which looks like the warp signal in young stellar population but the scenarios will need to be investigated in more detail.