SCIENCE GOALS AND OBJECTIVES: NUMEROUS FORMATION THEORIES HAVE BEEN PROPOSED FOR EXOPLANETS WITH RADII BETWEEN THE 1-3 REARTH THE MOST COMMON IN THE GALAXY WITHIN P<100 DAYS. RECENT OBSERVATIONS OF A RADIUS GAP SUGGEST THESE SMALL PLANETS FALL INTO TWO GROUPS SUPER-EARTHS AND SUB-NEPTUNES BUT WHETHER THEY REPRESENT A CONTINUOUS POPULATION SHAPED BY POST-FORMATION PROCESSES OR TWO DISTINCT POPULATIONS THAT FORMED DIFFERENTLY IS STILL UNCLEAR. OUR PROGRAM ON FOLLOWING UP THE EXOPLANETS DISCOVERED BY TESS IN THE SOUTHERN HEMISPHERE WILL ADDRESS THIS ISSUE SPECIFICALLY BY ANSWERING THREE WELL-DEFINED QUESTIONS: -HOW DO PLANETARY BULK DENSITIES DEPEND ON STELLAR INSOLATION? WHILE THE PERIOD DEPENDENCE OF THE RADIUS GAP IS CONSISTENT WITH VOLATILE ENVELOPE EVAPORATION THIS PROCESS ALSO DEPENDS ON SURFACE GRAVITY. OBTAINING MASSES FOR PLANETS BETWEEN 1.5 AND 2.5 REARTH AT A RANGE OF INSOLATION FLUXES IS NECESSARY TO TEST IF PHOTOEVAPORATION HAS CAUSED THE OBSERVED GAP AND WHETHER THE BIMODALITY SEEN IN PLANET RADII EXTENDS TO THE PLANET MASS-RADIUS-INSOLATION FLUX PHASE SPACE. -HOW DO PLANETARY BULK DENSITIES DEPEND ON HOST STAR COMPOSITION? FOR THE ~100 PLANETS SMALLER THAN 3 REARTH WITH MASS AND RADIUS CONSTRAINTS THERE IS A SPREAD IN MASS AT ANY GIVEN RADIUS THAT IS NOT DUE ENTIRELY TO MEASUREMENT UNCERTAINTY. THIS MAY INDICATE A DIVERSITY IN SOLID INTERIOR COMPOSITION AND BY PROXY THE HOST STAR COMPOSITION. WE WILL MEASURE AND COMPARE THE DENSITIES OF A LARGER SAMPLE OF SMALL PLANETS TO DETAILED HOST STAR ABUNDANCES TO UNDERSTAND THE ROLE OF COMPOSITION IN THE PLANET MASS-RADIUS RELATIONSHIP. -HOW DO PLANETARY BULK DENSITIES DEPEND ON SYSTEM ARCHITECTURE? HIGH ECCENTRICITIES OF TRANSITING PLANETS AND/OR MUTUAL INCLINATIONS BETWEEN SIBLING PLANETS POINT TOWARDS DYNAMICALLY ACTIVE EVOLUTION VERSUS IN SITU FORMATION OR SMOOTH DISK MIGRATION. DETERMINING TRANSITING PLANET ECCENTRICITIES WHETHER THEY HAVE NON-TRANSITING SIBLINGS AND HOW PLANETARY SYSTEM ARCHITECTURES ARE RELATED TO PLANET BULK DENSITY WILL HELP DISTINGUISH WHETHER SUPER-EARTHS AND SUB-NEPTUNES HAVE DIFFERENT FORMATION MECHANISMS. METHODOLOGY: WE PROPOSE A COMPREHENSIVE FOLLOW-UP PROGRAM TO GO FROM PLANET CANDIDATES DETECTED IN TESS LIGHT CURVES TO ~30 CONFIRMED AND WELL-CHARACTERIZED PLANETS. OUR CHARACTERIZATION WILL INCLUDE RV-MEASURED MASSES WITH ~60 RV OBSERVATIONS PER VETTED SYSTEM HOST STAR ABUNDANCES AND CONSTRAINTS ON LONGER PERIOD NON-TRANSITING PLANETS. CRITICALLY WE WILL SELECT OUR SAMPLE WITH A TIME-INVARIANT MERIT FUNCTION FROM WHICH WE CAN ROBUSTLY INFER POPULATION-BASED TRENDS TO ANSWER OUR PROPOSED QUESTIONS. OUR TEAM MEMBERS HAVE ACCESS TO AND EXPERTISE IN USING RESOURCES FOR ALL OF THE REQUIRED STEPS. WE PLAN TO SHARE OUR RV OBSERVING PLANS ON THE TFOP PLATFORM TO FACILITATE COORDINATION AND COLLABORATION WITH OTHER TEAMS. RELEVANCE: THIS PROPOSAL DIRECTLY ADDRESSES THE KEY XRP OBJECTIVES INCLUDING DETERMINATION OF COMPOSITIONS DYNAMICS ...OF [EXO]PLANETS AND CHARACTERIZATION OF OTHER PLANETARY SYSTEMS. OUR INVESTIGATION IS CLOSELY TIED TO THE TESS MISSION AND WILL HELP MAXIMIZE ITS SCIENCE RETURN THROUGH CONFIRMING PLANETS CANDIDATES AND FURTHER CHARACTERIZING THEIR MASSES HOST STARS AND NON-TRANSITING SIBLING PLANETS. THIS WORK WILL HELP INFORM THE TARGET SELECTION FOR CURRENT (HST TEXTIT{SPITZER}) AND FUTURE (JWST LUVOIR/HABEX) NASA OBSERVATORIES.