ULLYSES—A DD Spectroscopic Legacy Project

N. Reid (inr[at]stsci.edu)

As the Allocating Official for Hubble, STScI Directors have the authority to allocate up to 10% of the observing time at their discretion. This Director’s Discretionary (DD) time is used for a number of purposes. The most frequent use is for small-scale, community-requested, peer-reviewed programs that are designed to follow up on transient events that occur during a given cycle. However, DD time is also used for monitoring programs (e.g., the Outer Planet Atmosphere Legacy program), for public outreach (e.g., anniversary images); to subsidize GO programs (e.g., the Multi-Cycle Treasury programs and the recent Fundamental Physics initiative), and for larger-scale programs that tackle questions that are difficult, or even impossible, to address through the standard TAC process (e.g., the original Hubble Deep Fields, the Hubble Ultra-Deep Field, and the Frontier Fields). The ULLYSES program is the latest addition to the last category, but with a focus on spectroscopy of (relatively) nearby stars rather than deep imaging for galaxy evolution.

ULLYSES was developed over the past year. Following discussions with the Space Telescope Users Committee (STUC), the current STScI Director, Ken Sembach, constituted a Working Group (WG), chaired by Professor Sally Oey (U. Michigan), to advise him on how best Hubble could use its unique UV capabilities to probe star formation and the associated stellar physics. As a first step, the WG polled the general community for their input on outstanding "must do" science questions in these science areas that remain outstanding for Hubble, which programs fell beyond the purview of the standard GO Call, and how those programs could be conducted most effectively. The WG also invited the submission of short white papers to expand on particular themes. Some 196 survey responses were received by the deadline of December 7,  together with 34 white papers.

Two science themes emerged as predominant favorites in the survey feedback: UV spectroscopy of young OB stars, particularly at low metallicities, as an aid to stellar population modelling; and UV spectroscopy of young low-mass stars to probe accretion processes. The WG recruited additional members to supplement its expertise and developed a comprehensive science case through a series of discussions. The final report was submitted to the STScI Director on February 5. Once received, the report was distributed for high-level review by four experienced researchers who were asked to assess the likely scientific impact, the potential for complementary and supplementary observations, and the potential for related theoretical investigations. The reviews were overwhelming positive, and the Director has informed the STUC that he intends to proceed with the program.

The primary recommendation of the report is "…that the HST UV Initiative be devoted to obtaining a Hubble UV Legacy Library of Young Stars as Essential Standards (ULLYSES) to serve as a UV spectroscopic reference sample of high- and low-mass young stars. The recommended library will provide observations that uniformly sample the fundamental astrophysical parameter space for each class of stars, i.e., spectral type, luminosity class, and metallicity for massive stars; and spectral type, age, and disk accretion rate in low-mass stars…" The program envisages obtaining UV spectroscopy of 70 LMC and 70 SMC OB stars, as well as a sprinkling of lower-metallicity high-mass stars, together with UV spectra of 40 K- and M-type T Tauri stars and brown dwarfs in Galactic star-forming regions. The full WG report will be released to the community in mid-April.

ULLYSES will be implemented following the model of the successful Frontier Fields. All of the data obtained will be non-proprietary. STScI is currently constituting an in-house implementation team to construct the detailed observing program and produce an appropriate set of high-level data products. The in-house team will continue to work with the community to ensure that the program is implemented in an effective manner. The final target list will not be constructed until after the Cycle 27 TAC results have been finalized and will be adjusted to avoid pre-empting any GO programs selected through that process. The Cycle 27 TAC will be instructed to focus on the science inherent in each GO program without speculation on ULLYSES.