Vortex fiber nulling for exoplanet observations: implementation and first light

Abstract

Vortex fiber nulling (VFN) is a single-aperture interferometric technique for detecting and characterizing exoplanets separated from their host star by less than a diffracted beam width. VFN uses a vortex mask and single-mode fiber to selectively reject starlight while coupling off-axis planet light with a simple optical design that can be readily implemented on existing direct imaging instruments that can feed light to an optical fiber. With its axially symmetric coupling region peaking within the inner working angle of conventional coronagraphs, VFN is more efficient at detecting new companions at small separations than conventional direct imaging, thereby increasing the yield of on-going exoplanet search campaigns. We deployed a VFN mode operating in K band (2.0 to 2.5 μm) on the Keck Planet Imager and Characterizer (KPIC) instrument at the Keck II Telescope. We present the instrument design of this first on-sky demonstration of VFN and the results from on-sky commissioning, including planet and star throughput measurements and predicted flux-ratio detection limits for close-in companions. The instrument performance is shown to be sufficient for detecting a companion 10 3 times fainter than a fifth magnitude host star in 1 h at a separation of 50 mas (1.1 λ∕D). This makes the instrument capable of efficiently detecting substellar companions around young stars. We also discuss several routes for improvement that will reduce the required integration time for a detection by a factor >3.

Vortex Fiber Nulling for Exoplanet Observations: First Direct Detection of M Dwarf Companions around HIP 21543, HIP 94666, and HIP 50319

“…CHARA had confident detections on two of the targets, including the most challenging one that was tentative for VFN. This allowed us to verify that we are close to our VFN contrast predictions from Echeverri et al (2023). Moreover, our combined results highlight the complementary nature between long-baseline interferometry and cross-aperture nulling techniques.…”

“…A prior cross-aperture fiber nuller detected η Peg B with a flux ratio of 100 and measured the stellar diameter of the primary at a flux ratio of ∼2000 (Serabyn et al 2019). Our previous VFN paper predicted contrast limits of ∼1000 at ∼1 λ/D (Echeverri et al 2023), and this Letter now adds companion detections with flux ratios around 100 and a tentative detection at ∼430. These VFN results also represent the first companion detection at these contrast levels with high (R > 10,000) spectral-resolution nulling on-sky, showcasing the power of combining nulling interferometry with highresolution spectroscopy and complementing the capabilities of previous instruments.…”

“…We only use KPIC echelle order 6 (∼2.29-2.34 μm-correspondingly NIRSPEC order 33) in this Letter since it covers the CO bandhead where we expect many strong absorption lines from the M dwarf companions. Furthermore, this echelle order is close to the central wavelength of 2.225 μm where the vortex provides the deepest nulls (Echeverri et al 2023).…”

“…VFN's simple optical design makes it easy to implement on existing and upcoming high-contrast imaging instruments with a fiber injection unit, thereby providing access to companions at 1 λ/D (5 au at 100 pc for λ = 2.3 μm and D = 10 m). An on-sky VFN demonstrator is now operational (Echeverri et al 2023) as a new mode in the Keck Planet Imager and Characterizer (KPIC; Mawet 2021; Delorme et al 2021;Echeverri et al 2022;N. Jovanovic et al 2024, in preparation) instrument at the Keck II Telescope.…”

“…Additionally, since the VFN mode does not require prior knowledge of the exact position of the companion, it can be used to detect new companions. Previous commissioning results showed that ignoring systematics such as fringing, the KPIC VFN mode's on-sky performance is sufficient for detecting companions 1000 times fainter than their host in the K band (2.0-2.4 μm) in 1 hr at separations of 30-80 mas (Echeverri et al 2023).…”