Close-in faint companions mimicking interferometric hot exozodiacal dust observations

TL;DR

We address whether faint, unresolved companions can reproduce infrared visibility deficits attributed to hot exozodiacal dust in nearby main-sequence stars. We develop an analytic framework for a star+companion within a 2 au × 2 au field, deriving expressions for visibilities and closure phases as functions of the companion-to-star flux ratio $f$ and baseline geometry, and show that $|\Delta V|$ can scale as $2f$ while $|\Phi|$ scales as $3f/\min\{V_{s,ij}\}$ for $f \ll 1$. We demonstrate that close-in companions can produce measurable visibility deficits with near-zero closure phases, yielding non-detection probabilities that challenge past companion-rejection criteria, and apply this to κ Tuc A to identify a possible faint companion with $f \approx 0.7\%$ and $p_{\rm non-detect} \approx 0.21$. The work provides a practical diagnostic to distinguish hot exozodiacal dust from faint companions and highlights the need for multi-wavelength, multi-configuration interferometry to robustly interpret inner-system signatures, supported by a publicly available Jupyter notebook.

Abstract

Context: Interferometric observations of various nearby main-sequence stars show an unexpected infrared excess, raising the question of its origin. The two dominant interpretations favor hot exozodiacal dust or a faint companion, as both can produce similar signatures. Method: We modeled a system consisting of a star and a faint companion within a field of view of 2au x 2au. We calculated the visibility and closure phases for three VLTI instruments (PIONIER, GRAVITY, and MATISSE) and four telescope configurations. Aim: We aim to investigate the interferometric signatures of faint companions and assess their detectability. We explore limitations of current detection methods and evaluate the challenges in distinguishing between hot exozodiacal dust and a faint companion as the source of the observed excess. Results: We derived an upper limit for the companion-induced visibility deficit and closure phase. Contrary to the common interpretation that near-zero closure phases rule out the presence of a companion, we show that companions can remain undetected in closure phase data, as indicated by significant non-detection probabilities, yet, these companions can still produce measurable visibility deficits. We confirmed our results by reevaluating an L-band observation of kappa Tuc A. We found indications for a faint companion with a flux ratio of 0.7% and an estimated non-detection probability of around 21%, which could explain the variability of the previously observed visibility deficit. Conclusions: Previous companion rejection criteria, such as near-zero closure phases and flux estimates based on Gaussian-distributed dust densities, are not universally valid. This highlights the need for a reevaluation of companion rejections in former studies of the hot exozodiacal dust phenomenon. In addition, we propose a method for distinguishing both sources of visibility deficit.

Figures (13)

• Figure 1: Expected visibility amplitudes, $V$, and closure phases, $\Phi$, of a binary system composed of a central star and a companion (dashed lines, denoted by the index s+c), exemplarily modeled for $\kappa$ Tuc A and a faint companion at the projected distance of $\approx 1.4\,$au, assuming a companion-to-star flux ratio of $f=1\%$ and a fixed telescope triplet VLTI_Manual_2024. In comparison to that, the plot shows the modeled visibilities and closure phases of an edge-on (solid lines, denoted by the index s+er) and face-on (dash-dot lines, denoted by the index s+fr) hot dust ring with the inner radius $R_{\rm in}=0.1\,$au Kirchschlager2020 and a flux ratio of $f\approx 1\%$.
• Figure 2: Maximum visibility deficits (left) and closure phases (right) of the system in dependence on the position of the assumed companion for all simulated MATISSE wavelengths and medium configuration. Field of view of $2 \, \mathrm{au} \times 2 \, \mathrm{au}$, or $0.09 \, \mathrm{as} \times 0.09 \, \mathrm{as}$, (top) and $0.3 \, \mathrm{au} \times 0.3 \, \mathrm{au}$, or $0.014 \, \mathrm{as} \times 0.014 \, \mathrm{as}$, (bottom). The arrows show the direction of the orbiting companion. The blue and red solid lines show the theoretical orbits of a companion, explained in Sect. \ref{['sec:blind_spots']}.
• Figure 3: Theoretical variation of the maximum visibility deficit $\max\left\lvert\Delta V \right\rvert$ and the maximum closure phase $\max\left\lvert\Phi \right\rvert$ along the trajectory of the face-on (blue) orbit (left) and inclined (red) orbit (right) in the direction shown by the arrows displayed in Fig. \ref{['fig:max_dvs']}. The dashed lines indicate the adopted generic detection limits: a visibility deficit of $\Delta V = 0.01$ and a closure phase of $\Phi = 1^{\circ}$. Their colors match those of the corresponding observables. Detection limits are discussed in detail in Sect. \ref{['sec:non_detection']}.
• Figure 4: Non-detection probabilities in closure phases as a function of the threshold for the face-on (blue) and inclined (red) orbits shown in Fig. \ref{['fig:max_dvs']}. For details, see Sect. \ref{['sec:theoretical_limits']}.
• Figure 5: Non-detection probabilities in closure phases for a companion at (top) and within (bottom) a given projected distance to the star in dependency of an observational accuracy (threshold).