The WEAVE-TwiLight-Survey: Expanding WEAVE's Reach to Bright and Low-Surface-Density Targets with a Novel Observing Mode

TL;DR

The paper tackles the inefficiency of observing bright, low-density targets with multi-object spectroscopy by introducing WEAVE-Tumble-Less, a mode that merges multiple fields around a central guide star to reduce calibration overhead. Building on PLATO LOP fields and Gaia DR3, the WTLS defines field-batches through a greedy geometric set-cover with circles of radius $r$ and an overlap constraint $\alpha$, allocated by a custom guide-fibre algorithm to maximize science targets while avoiding fibre crossings. Initial twilight tests demonstrate feasible, high-quality spectroscopy for $6 \leq V \leq 11.5$ targets, achieving ~$(1.6)$× S/N improvements and confirming twilight sky subtraction works with the new mode. WTLS aims to yield approximately $6{,}300$ bright stars, including $\approx 62$ known exoplanet hosts, enabling homogeneous chemical-abundance studies relevant to planetary formation and Galactic context, and establishing WEAVE-TL as a practical, efficient observing option for bright, low-density fields.

Abstract

Current-day multi-object spectroscopic surveys are often limited in their ability to observe bright stars due to their low surface densities, resulting in increased observational overheads and reduced efficiency. Addressing this, we have developed a novel observing mode for WEAVE (William Herschel Telescope Enhanced Area Velocity Explorer) that enables efficient observations of low-surface-density target fields without incurring additional overheads from calibration exposures. As a pilot for the new mode, we introduce the WEAVE-TwiLight-Survey (WTLS), focusing on bright exoplanet-host stars and their immediate surroundings on the sky. High observational efficiency is achieved by superimposing multiple low-target-density fields and allocating the optical fibres in this configuration. We use a heuristic method to define fields relative to a central guide star, which serves as a reference for their superposition. Suitable guide fibres for each merged configuration are selected using a custom algorithm. Test observations have been carried out, demonstrating the feasibility of the new observing mode. We show that merged field configurations can be observed with WEAVE using the proposed method. The approach minimizes calibration times and opens twilight hours to WEAVE's operational schedule. WTLS is built upon the new observing mode and sourced from the ESA PLATO long-duration-phase fields. This survey will result in a homogeneous catalogue of approximately 6300 bright stars, including 62 known planet hosts, laying the groundwork for future elemental abundance studies tracing chemical patterns of planetary formation. This new observing mode (WEAVE-Tumble-Less) expands WEAVE's capabilities to rarely used on-sky time and low-density field configurations without sacrificing efficiency.

Figures (9)

• Figure 1: Fraction of currently known host stars (NASA Exoplanet Archive, August 2024) that have been observed by APOGEE DR17 abdurroufSeventeenthDataRelease2022, grouped by magnitude bin. The total number of known exoplanet hosts is shown in bold above each bin. The number below indicates the number of observed hosts in APOGEE DR17 in that bin. Note the sharp decline in observations for magnitudes brighter than 11 (to the left of the dashed line). This reflects the low sky densities of bright stars, which result in higher observational overheads.
• Figure 2: Sky projection of the proto catalogues after applied magnitude cuts (see Section \ref{['s:defining_fields_for_WTL']}) in Equatorial coordinates, constituting the superset from which the northern and southern WEAVE-TwiLight fields are derived. Red stars are part of the PLATO Long-duration Observation Phase (LOP) fields (nascimbeniPLATOFieldSelection2022, nascimbeniPLATOFieldSelection2025). Black circles and diamonds signify known exoplanet hosts and candidates, respectively. For orientation the Kepler FOV is overlaid in dark blue.
• Figure 3: Regions accessible by guide fibres are indicated by dashed lines. Note that the allocation process used for WEAVE-TwiLight restricts the fibre reach to the end of each dashed cone to prevent fibre crossings. The numbers along the fibre gates (black dots) represent the fibre IDs for individual guide fibres on plate B. For plate A, the geometry remains the same. At the time the WTLS fields were defined, the following guide fibres were not in operation, plate A: 879; plate B: 249.
• Figure 4: Merging of two fields on the sky onto the WEAVE field plate. Filled and empty diamonds (black & red) depict central and off-centre (secondary) guide stars, respectively. Left and centre panels: on-sky fields from the WEAVE-TwiLight-Survey input catalogue. Right: the mirrored projection onto the field plate.
• Figure 5: Panel a: guide star number density per $\pi$ deg$^2$ ($G_{\mathit{Gaia}} \sim 14.5$ mag) in the northern WEAVE-TwiLight proto field (see Fig. \ref{['im:initial_fields']}, left panel). Panels b, c and d:green circles show WEAVE pointings for which guide fibres could be successfully assigned (one central and two off-centre guide stars), allowing for autoguiding. Empty circles depict fields where the algorithm was not able to determine a viable allocation. In the bottom left corner of each panel, the number of fields with sufficient guide coverage ($N_{\mathrm{sufficient}}$) is given. Note that the above represents allocation results under twilight conditions. At nighttime, the number of viable guide stars increases significantly, allowing for field coverage of up to $\sim95$% with 8 guide fibres.