complex systems of stars and interstellar matter (gas and dust) displaying interesting and important phenomena [e.g., see the review by @Morris-1996]. The Galactic bulge, extending over a few kilo-parsecs, is populated predominantly by old stars, roughly 10 Gyr old [@Zoccali-2003], and possibly by a smattering of younger stars [a few Gyr old; @Bensby-2013]. In contrast, $\sim 200$-pc region around Sgr A$^*$ at the Center hosts many young stars and is the region of current star formation [@Serabyn-1996]. These young stars are distributed throughout a disk-like structure called the Nuclear Stellar Disk [hereafter NSD; @Launhardt-2002]. Interstellar gas and dust occupy the so-called the Central Molecular Zone [CMZ; @Morris-1996] found in the same region.

The most prominent stars in the NSD are young luminous stars such as O/B-type stars and Wolf-Rayet stars. These are found in large numbers in three stellar clusters; Arches, Quintuplet, and the nuclear star cluster surrounding Sgr A\*. The Arches and Quintuplet clusters are at most 6 Myr old [@Stolte-2008; @Liermann-2012; @Schneider-2014], and young massive stars also reside in the nuclear star cluster [@Krabbe-1995]. Similarly, young and massive stars are found across the NSD field [@YusefZadeh-2009; @Mauerhan-2010]. Recently @Matsunaga-2011 discovered classical Cepheids in the direction of the NSD, 