The First Complete Census of Stars in Key Globular Clusters
| Program ID | 2000 |
|---|---|
| Science Category | Stellar Populations |
| Program Type | GAS |
| Hours | 167.2 |
| Category | Medium |
| Principal Investigator | Luigi R. BEDIN |
| PI Institution | Istituto Nazionale di Astrofisica, Osservatorio Astronomico di Padova, Italy |
| Co-Investigators |
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| Abstract | We propose a 160-hour Roman Legacy Survey delivering the first complete, homogeneous census of stellar populations across entire globular cluster environments. By exploiting the unique combination of high resolution, depth, wide-field coverage, and astrometric stability of the Nancy Grace Roman Space Telescope, this program will transform globular clusters from partially sampled laboratories into globally mapped fossil records of early Galactic evolution. The survey targets a sample of close benchmark systems that collectively span the full diversity of metallicity, total mass, structural concentration, dynamical state, and assembly history represented among Milky Way clusters. Roman’s HST-class image quality, coupled with a field of view nearly two orders of magnitude larger, enables contiguous coverage from dense cluster cores to tidal outskirts, overcoming the small-number statistics that have historically limited studies of the faintest populations. As the definitive wide-field astrometric second epoch to decades of archival HST imaging, Roman will provide high-precision proper motions (sub-milli-arcsecond) over unprecedented time baselines. This enables the secure disentanglement of cluster members from foreground and background contaminants down to the faintest detectable homogeneous magnitudes. With contamination removed and ultra-deep imaging extending beyond V>30, the survey will reveal complete white dwarf cooling sequences, trace the main sequence below the hydrogen-burning limit, and deliver the first statistically robust samples of cluster brown dwarfs. These data will place decisive constraints on stellar evolution at low metallicity, the initial–final mass relation, compact-remnant retention, dynamical mass segregation, and the universality of the low-mass initial mass function. The resulting dataset will constitute a cornerstone legacy resource for stellar and substellar evolution, cluster dynamics, and the early assembly history of the Milky Way. |
| Summary of Observations | The proposed observational program requests 167.2 hours (including overheads) using the Roman's Wide Field Instrument (WFI). The survey targets four nearby benchmark globular clusters: M4, NGC6397, NGC104, and NGC5139, which cover a wide range of masses, metallicities, and dynamical states. For each globular, we map a wide 2degx2deg field of view to capture the full environment from the dense core to the extended tidal boundaries. To cover this area, the team will execute a 2x4 mosaic of WFI-tiles for each system. The observations will utilize the F087 & F158 filters, a combination specifically optimized to distinguish mPOPs, the faintest stellar remnants, and to separate hydrogen- and helium-atmosphere white dwarfs along the cooling track. At each pointing, we use the BOXGAP9_2 dither pattern, capturing 9 dithered exposures per filter to seamlessly bridge detector gaps. Each individual exposure will last approximately 1000 seconds, amounting to ~2.5 hours of total integration time per filter per pointing. In total, the program will yield 576 images (72 images per filter, per cluster), translating to ~160 hours of pure science exposure time. Keeping the same observational pattern and exposure times for all clusters (which have comparable distances and spatial extensions on the sky) IT IS STRATEGIC for data reduction, as this homogeneity helps to better track systematic errors and ensure the temporal stability of photometric & astrometric calibrations. This ultra-deep imaging strategy is designed to reach magnitudes of V~30 with a signal-to-noise ratio of at least ~5, and m_F087 between 27.3 and 28.5, which is deep enough to reach the bottom peak of the white dwarf cooling sequence across all targeted clusters. Ultimately, these observations will serve as a definitive astrometric second epoch meant to be combined with legacy HST, JWST, Euclid, and Gaia data to derive precise proper motions and cleanly isolate faint cluster members from field contaminants. |
| Status of Observations | https://www.stsci.edu/roman-program-info/program/?program=2000 |