4MOST - 4-metre Multi-Object Spectroscopic Telescope
4-m Multi-Object Spectroscopic Telescope
4MOST

Galactic Community Surveys

The six Galactic Community Surveys tackle a broad range of science goals in stellar and Galactic astrophysics. Their design is highly complementary to the Consortium Surveys, whose main focus is on Milky Way field stars and the Magellanic Clouds. The Community Surveys will add several systems to the observing programme that are critical to understanding the build-up of the Milky Way; namely the Sagittarius stream, three of the largest satellite galaxies, and hundreds of stellar clusters in all Galactic components. The unique properties of RR Lyraes will help us form a clearer view of structure in the Galaxy out to large distances. Another key focus of the Community Surveys is to determine accurate stellar ages, by tracing stellar evolution all the way from newborn stars to stellar remnants, thereby being able to address fundamental questions about how stars form, live their lives and die in violent explosions.

Below we provide more details on each of the six surveys (click on the survey titles).

This survey will target resolved Stellar Clusters in the Milky Way (MW) and the Magellanic Clouds (MCs): 151 Globulars in the MW and the MCs and essentially all visible MW Open Clusters and Star Forming Regions, filling the metallicity/age distribution from the [Fe/H] = −2.5 dex of GCs to super-solar OCs, from a few Myr to 13.5 Gyr. The clusters will be studied both with LR and HR. We will ultimately be able to: understand how clusters form, evolve, dissolve, and populate the MW; calibrate complex physics that affect stellar evolution, on which our ability to measure ages ultimately stands; measure the contribution of star clusters to the formation and evolution of the individual Galactic components with unparalleled precision, accuracy and statistics. The survey, targeting ~120,000 stars at low resolution and ~90,000 at high resolution, complements the planned Galactic and MC Consortium Surveys, allowing us to derive a thorough and homogeneous chemo-dynamical picture of unprecedented accuracy and size, thereby setting precious constraints on models of Galaxy formation.

The White Dwarf Binary Survey will make use of binary stars containing a white dwarf to tackle a wide diversity of open problems, which have been difficult to address due to the small number of systems at the different stages of the binary evolution. We will study close (and interacting) as well as wide binary systems. The analysis of the close binary population will allow us to constrain current theories of common envelope evolution and angular momentum loss mechanisms as well as to provide observational constraints to test different channels for type Ia supernovae. The white dwarfs in wide binaries will be used as clocks to constrain the age-metallicity relation and the age-velocity dispersion relation of the Galactic disc and the age-activity-rotation relation of low-mass main sequence stars. We will do this by modelling high- and low-resolution spectra of more than 150,000 binaries for which we will measure their effective temperatures, masses, radii and chemical abundances.

Most young stars (<100 Myr) in the Galactic disc no longer reside in their dense, clustered birthplaces; they are found all around us. 4SYS will identify a representative sample of 100,000 young stars within 500 pc of the Sun, a scale that increases the number of identified young field stars and the volume explored by two orders of magnitude. We will measure stellar chemistry, 3D kinematics and ages, and use these to: trace the spatial and dynamical evolution of star forming structures as they disperse; quantify the local disk star forming rate and chemical inhomogeneity at a range of spatial scales; vastly expand the numbers of identified young stars for exoplanetary studies; and provide huge coeval samples to improve young stellar evolutionary models. This is of fundamental importance in understanding the physical processes that drive star formation, the origins of the Galactic field population and the early evolution of stars, their discs and planetary systems.

The 4GRoundS project will measure the radial velocities and metallicities of southern RR Lyrae stars detected by Gaia and published in their DR3 data release. The most accurate distances to old, distant stellar populations are those to RR Lyrae, making them invaluable dynamical tracers of the Galaxy. Their excellent photometric distances enable the exquisite Gaia proper motions to be converted into physically-useful transverse velocities. Armed with the missing radial velocity, 4GRoundS will provide the community an exquisite sample that will enable studies of the orbital structure of the Galaxy, and allow realistic modelling of its major sub-components. It will also enable the identification of coherent dynamically-cold streams. Together these analyses will map the mass of the Milky Way out to 100 kpc and test models of the dark sector.

The goal of this survey is to spectroscopically follow up stars that might have compact companions – black holes or neutron components, in dormant, non-interacting binaries. Those stars were identified as ellipsoidal binaries by the Optical Gravitational Lensing Experiment (OGLE) in the Magellanic Clouds. A sample of 678 ellipsoidals with periods shorter than 2.5 days will be observed to obtain multi-epoch radial-velocity measurements. The 4MOST radial velocities combined with the OGLE photometric data will allow for determination of the secondary component mass and therefore identification of the ones with compact companions.

The 4DWARFS survey will target all the dwarf galaxies in the 4MOST footprint, in particular the main bodies of three large dwarf spheroidal galaxies, Sagittarius, Fornax and Sculptor. In addition, the survey will focus on the Sagittarius stream, which is a major constituent of the Galactic halo, and presents our best opportunity to observe a galaxy being currently disrupted. This survey aims at providing new and valuable insights into: I) the properties of the first stars; II) the origin of the chemical elements through various nucleosynthetic channels (e.g. SN type Ia, AGB stars and the r-process); III) The hierarchical galaxy formation, e.g. by quantifying the number of previous mergers in the Milky Way, and within the dwarf galaxies themselves. Using both low and high-resolution spectra, 4DWARFS will provide stellar ages, radial velocities and chemical abundances for 130,000 stars, and will thus increase the number of stars in dwarf galaxies and streams with detailed abundance information by several orders of magnitude.