Paving the Road to the Habitable Worlds Observatory with High-Resolution Imaging I: New and Archival Speckle Observations of Potential HWO Target Stars

TL;DR

The paper tackles the challenge of preparing the Habitable Worlds Observatory (HWO) for direct imaging by conducting a uniform multiplicity survey of nearby bright stars from the ExEP provisional target list. It combines new and archival optical speckle imaging from 'Alopeke and Zorro with Gaia DR3 multiplicity indicators and literature data to identify close stellar companions, finding 27 multi-star systems among 80 targets, including a new companion to HD 90089. Through 100,000 simulated companions per target, the study estimates that roughly 75–85% of possible companions would be detectable with current speckle data when combined with wide-field surveys, while highlighting that faint or very close companions remain challenging and may require long-term spectroscopy or space-based/high-resolution follow-up. The results inform HWO target selection and underscore the need for a uniform, multi-technique multiplicity survey prior to launch, as well as planning for future Extremely Large Telescopes to complete the companion census and optimize direct-imaging strategies.

Abstract

One of the key goals of the Habitable Worlds Observatory (HWO) is to directly image about 25 potentially habitable exoplanets and determine their properties. This challenge will require a large survey of nearby, bright stars -- ~100 according to the Astro2020 Decadel Survey. To ensure the success of the mission and to help guide design decisions, the stellar multiplicity of the target stars must be well-understood. To this end, we present optical speckle imaging of stars in the NASA Exoplanet Exploration Program (ExEP) provisional HWO star list, which is currently the Tier 1 target list for the HWO Target Stars and Systems Sub-Working Group. We obtained new observations using `Alopeke and Zorro at Gemini Observatory and queried the Exoplanet Follow-up Observing Program Archive for archival observations, resulting in speckle imaging data for 80 of the 164 stars. We confirmed one candidate companion detected previously by Gaia (HD 90089) and obtained an ambiguous detection of a known companion (HD 212330). To examine our sensitivity to companions, we simulated stellar companions down to ~0.1 $M_{\odot}$ for each target and found that 75%-85% would be detected in our speckle images; the remaining simulated companions are either too faint or too close-in, and will require follow-up using other methods such as long-term spectroscopic measurements and space-based techniques. This work represents a first step towards surveying potential HWO targets for close-in stellar companions and helping to inform the target selection process for the HWO direct-imaging survey, bringing us closer towards the discovery of potential habitable worlds.

Figures (9)

• Figure 1: Color-magnitude diagram showing the targets in the ExEP target list for HWO. The grey points represent a sample of stars within 100 pc taken from Gaia DR3. The cyan points indicate the locations of the 2024mamajek targets that are in Gaia and have not been observed with speckle imaging. Orange squares highlight 80 ExEP list targets that have 'Alopeke, Zorro, or NESSI observations. Red stars show the 27 targets that have been identified as members of stellar multiples by this work. The top axis provides approximate $G-G_{RP}$ values for F0V, G0V, K0V, and M0V stars.
• Figure 2: The contrast curves for the two narrowband filters centered at 562 nm (blue line) and 832 nm (red line) for our observation of 71 Ori and 20 LMi using 'Alopeke at Gemini North. The insets are the reconstructed images at 832 nm.
• Figure 3: Left: Contrast curves for the 80 HWO targets surveyed in this work. The 562 nm and 832 nm contrast curves are shown as blue and red lines, respectively. The solid cyan and orange lines represent a fit to the envelope of binaries detected by 'Alopeke and Zorro and listed on ExoFOP. These contrast curves show that we detect stellar companions that are fainter than their primaries by 4-5 magnitudes at the diffraction limits of the telescopes. Right: The projected physical separation range surveyed by our speckle observations for each target, shown as magenta bars. Solid and dashed lines represent the diffraction limits for the 562 nm (blue) and 832 nm (red) filters at Gemini ('Alopeke and Zorro) and WIYN (NESSI). This figure shows that speckle imaging surveys a necessary parameter space in the search for potential companions.
• Figure 4: RUWE values of the 80 HWO targets surveyed in this work as a function of Gaia $G$ magnitude. Cyan stars indicate the 19 stars that were flagged as potential binaries by our Gaia metric criteria, while the remaining 61 stars are shown as orange squares. The red line indicates a RUWE value of 1.4, which is the value typically used to distinguish single and non-single stars. The black points represent a sample of stars within 100 pc from Gaia DR3. We label the two stars with RUWE$>7$. As one goes to brighter magnitudes, the RUWE values rise, which could indicate the presence of unresolved companions. However, it is more likely that the brightness of these targets is increasing the astrometric errors. As such, the RUWE value may not be the best indicator of stellar multiplicity for stars brighter than $G\sim4.5$ mag.
• Figure 5: Distances to the 27 multi-star systems identified in this study as a function of projected physical separation. The solid red and blue lines represent the nominal diffraction limits for the 562 nm (blue) and 832 nm (red) filters installed in 'Alopeke and Zorro. The dashed lines represent the same for NESSI. The black line is the 3$^{\prime\prime}$ cut used in the original sample selection for the ExEP provisional star list for HWO. The color of the points highlights the $G-G_{RP}$ values of the stars. If the system is triple, then it has two points representing the AB and AC components and we connect these systems by the black horizontal lines. We assume the primary's distance as the distance for the system. The majority of the stellar companions lie beyond 3$^{\prime\prime}$, which shows that the selection criteria from 2024mamajek removed most close-in multiples from the sample. However, some systems are still found with separations less than 3$^{\prime\prime}$, which highlights the need for a uniform multiplicity study of all the potential HWO target stars.