Main science objective of Small-JASMINE is "Galactic Center Archaeology", to understand the formation and evolution history of our Galaxy, the Milky Way. Small-JASMINE will measure the precise position and motion of stars in the Galactic nucleolus region, which is the core of the Milky Way. Unveiling the spatial distribution and motions for stars in the Galactic center, where the generations of stars are retained since the beginning of the Milky Way, will enable us to uncover the formation and history of the Milky Way. Understanding the formation processes of the Milky Way will also help us to understand the formation of the other galaxies.
To achieve this science objective, Small-JASMINE is designed as follows.
Keyword: Astrometry, Annual parallax, Proper motion, Galactic Nuclear Bulge (Galactic Center Region), Galactic Center Archaeology
"Galactic Center Archaeology"
The world-first near-infrared Galactic center astrometry mission, Small-JASMINE will reveal the detailed structure of the Galactic center region, so-called Galactic nuclear bulge, whose structure and the formation history are still mystery and veiled with the heavy dust extinction. One of the interesting structures in the Galactic center region is a Galactic nuclear disk, which is considered to be formed when the Galactic bar formed. Small-JASMINE will uncover the age distribution of the Mira variable stars in the Galacitc nuclear disk, and identify the formation epoch of the nuclear disk, and therefore the formation epoch of the Galactic bar.
There is a super-massive black hole (SMBH) with about 4 million times more massive than the Sun at the center of our Galaxy. However, how the SMBH is growing is not known. Small-JASMINE will reconstruct the mass distribution, non-axisymmetry and rotation properties of the Galactic nuclear bulge. From these properties we will study how and how much the gas is falling into the Galactic center and feeding the SMBH.
One of the potential formation scenarios of the SMBH is a successive mergers of massive black holes. If the SMBH of the Milky Way formed through this mechanism, such mergers must strongly impact on the kinematics of the stars in the Galactic center region. Small-JASMINE will measure the precise position and motion of stars in the Galacic center region, which would be enough to tell such impact, and therefore Small-JASMINE will tell us if or not our SMBH formed via mergers.
The accurate proper motion, tangential motion in the sky, measurements of stars in the Galactic center enable us to find hidden star clusters not possible to be identified with photometric surveys. The proper motion data enable us to identify star clusters, even the ones dissolving, as groups of stars moving coherently. The orbit and age of the star clusters will help us to uncover the formation and evolution history of the Galactic nuclear bulge, because all stars are believed to born in star clusters.
There are some stars whose velocity is too fast, hyper velocity stars, to be explained with the normal star formation process. One of promising origins of the hypervelocity stars is a population kicked out from the strong gravitational influence of the SMBH in the Galactic center. If Small-JASMINE found such hyper velocity stars to be consistent with this scenario in the Galactic center, it provides a direct evidence of the SMBH origin of hyper velocity stars.
Astrometric gravitational micro-lensing is a phenomenon that the position of a background star changes when a compact object, such as a stellar mass black hole, passed through between the background stars and us. Small-JASMINE will monitor such micro-lensing events, which will tell us the number and masses of black holes.
Small-JASMINE will make the targeted observations for the other sciences, on which unique capability of Small-JASMINE can provide strong impact, in summer and winter when the Galactic center is not observable. The selection of the target is on-going. The candidates are listed below.
Planets around a star can gravitationally attract the host star, and the position of the star in the sky slightly shifts depending on the relative positions of the planets. This can be observed as periodic positional shifts, and enables us to find new exoplanets using astrometry. Exoplanets can be found with the transits seen in the light curve of the host star, i.e. the periodic dimming of the stellar light when a planet passes the front of the star. Small-JASMINE especially targets M-type main-sequence stars, which is a low temperature low mass star, but bright in near-infrared. Because M-type main-sequence stars are intrinsically less bright, the so-called habitable zone of the planet, where the life is likely to be able to survive, is closer to the star and therefore the orbital period of habitable planets is short. Therefore, M-type main-sequence stars are the ideal target to discover a habitable planet. Small-JASMINE can provide time-series near-infrared photometry for M-type main sequence stars, and therefore Small-JASMINE is an ideal satellite to detect habitable planets around M-type main-sequence stars.
Compact objects, e.g. Cygnus X-1. Variable stars. Star forming regions.