Abstract
ABSTRACT Only a small number of exoplanets have been identified in stellar cluster environments. We initiated a high angular resolution direct imaging search using the Hubble Space Telescope (HST) and its Near-Infrared Camera and Multi-Object Spectrometer (NICMOS) instrument for self-luminous giant planets in orbit around seven white dwarfs in the 625 Myr old nearby (≈45 pc) Hyades cluster. The observations were obtained with Near-Infrared Camera 1 (NIC1) in the F110W and F160W filters, and encompass two HST roll angles to facilitate angular differential imaging. The difference images were searched for companion candidates, and radially averaged contrast curves were computed. Though we achieve the lowest mass detection limits yet for angular separations ≥0.5 arcsec, no planetary mass companion to any of the seven white dwarfs, whose initial main-sequence masses were >2.8 M⊙, was found. Comparison with evolutionary models yields detection limits of ≈5–7 Jupiter masses (MJup) according to one model, and between 9 and ≈12 MJup according to another model, at physical separations corresponding to initial semimajor axis of ≥5–8 au (i.e. before the mass-loss events associated with the red and asymptotic giant branch phase of the host star). The study provides further evidence that initially dense cluster environments, which included O- and B-type stars, might not be highly conducive to the formation of massive circumstellar discs, and their transformation into giant planets (with m ≥ 6 MJup and a ≥6 au). This is in agreement with radial velocity surveys for exoplanets around G- and K-type giants, which did not find any planets around stars more massive than ≈3 M⊙.
Highlights
Several 1000 exoplanets and exoplanet candidates have been identified (Perryman 2018; Shabram et al 2020), less than 1 per cent of these reside in stellar clusters (Kovacs et al 2014)
Comparison with evolutionary models yields detection limits of ≈5–7 Jupiter masses (MJup) according to one model, and between 9 and ≈12 MJup according to another model, at physical separations corresponding to initial semimajor axis of ≥5–8 au
The data were obtained with Near-Infrared Camera 1 (NIC1) in MULTIACCUM mode (NSAMP=18, STEP32), applying a twopoint dither pattern with the science targets centred in the top-right or bottom-right quadrant, respectively, and two telescope roll angles differing by 20◦
Summary
Several 1000 exoplanets and exoplanet candidates have been identified (Perryman 2018; Shabram et al 2020), less than 1 per cent of these reside in stellar clusters (Kovacs et al 2014). With the exception of WD 0806−661 B (Luhman, Burgasser & Bochanski 2011), direct detection spectroscopic and imaging searches for planetary mass companions to white dwarfs were unsuccessful far (e.g. Chu et al 2001; Hogan, Burleigh & Clarke 2011; Xu et al 2015) Their proximity with distances between 35 and 50 pc (Bailer-Jones et al 2018), relatively young age, and main-sequence progenitor masses of ≈3 M make the Hyades cluster white dwarfs promising targets to search for substellar companions at orbital separations of several tens of au
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