The North-PHASE Legacy Survey

Using time to map space via photometric variability

North-PHASE stands for  “Periodicity, Hot spots, Accretion Stability and Early evolution in young stellar clusters in the northern hemisphere” and it is a 5-year (2023-2028) Legacy Survey at the Javalambre Observatory (Spain), led by A. Sicilia-Aguilar with an international collaboration. Using time-resolved, multi-cadence, multiwavelength, large field data, it can unveil structures and processes in young stars (YSO) at the relevant scales for inner planet formation, while also studying the connection between stars, their formation history, and their clusters, independently of astrometry. North-PHASE is unique ‘using time to map space’, covering thousands YSO for a statistical study of their variability and the physical processes to which it is linked.

In this site, we will report on the survey progress and results.

Young stars are variable due to the processes happening on the star and its planet-forming disk at the time of planet formation. The star-disk connection holds the clues to several open questions including rotational evolution, the origin of our Solar System and the feasibility of habitable planets. Early stellar evolution depends on accretion, angular momentum and mass transport between disk and star (affecting stellar activity), planet formation and migration (linked to accretion), and the cluster environment and structure. The main technical limitation for these studies is the scale: few stellar radii to sub-au are impossible to map even with interferometry. Time-resolved observations are the only way to access these tiny spatial scales.

North-PHASE “uses time to map space” in young stars via a variability survey of populous clusters in the Northern Hemisphere in an unprecedented effort to study the properties of thousands of stars aged 1-5 Myr. The JAST80 telescope allows us to cover each cluster entirely in one shot and its filters can be used to distinguish the processes happening in the star and the disk. Our program will answer questions in star formation, cluster structure, and the properties of young stars and their innermost disks. In particular, we will:

• Achieve a complete census of young stars down to 0.15-0.3 Msun via variability, including variability types and mechanisms, and stellar parameters, in six young clusters: Tr37, NGC 2264, Cep OB3, IC 5070, IC 348, and NGC 1333.
• Perform the first large-scale systematic, statistically-significant, unbiased accretion variability survey, exploring the differences in accretion mechanisms with stellar mass, the connection between accretion and the magnetic field, and the prevalence of accretion outbursts and their role in star and planet formation.
• Study the innermost disk structure and alignment by tracking extinction events and their periodicity.
• Study stellar rotation and its connection to accretion variability and to magnetic variability/cycles over 5 years.
• Study the star-disk connection, magnetosphere size, and the inner planet-forming regions, combining the information on rotation, spot-like variability, and extinction events, key to understanding planet migration.
• Provide a kinematically unbiased view of young clusters and star formation.
• Provide public Python tools for variability analysis.
• Provide a complete catalog of other variable stars in the field, including pulsating stars, eclipsing binaries, and long period variables.

By including several filters, gSDSS, rSDSS, iSDSS, zSDSS, plus the u-band filter uJAVA and the narrow band filter J0660, which covers the Halpha line, we can distinguish variability process and measure accretion. North-PHASE observations started in February 2023, and the data is already revealing important results, including unveiling different variability patterns in young stars with gSDSS magnitudes 13-19 mag, finding new cluster members that had been missed by Gaia, as well as other variables including eclipsing binaries, delta Scuti and RR Lyr variables.

See North-PHASE at the Javalambre Observatory. The data will become publicly available in releases scheduled after each publication.