# Serendipitous observation of a white dwarf companion to a JWST/MIRI coronagraphic calibrator

Alexander Venner, Mary Anne Limbach, Mathilde Mâlin, Simon Blouin, Anthony Boccaletti, Logan A Pearce

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Alexander Venner, Mary Anne Limbach, Mathilde Mâlin, Simon Blouin, Anthony Boccaletti, et al. Serendipitous observation of a white dwarf companion to a JWST/MIRI coronagraphic calibrator. Monthly Notices of the Royal Astronomical Society: Letters, 2025, 536 (1), pp.L38 - L41. 10.1093/mn-rasl/slae106. hal-05343614

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# Serendipitous observation of a white dwarf companion to a JWST/MIRI coronagraphic calibrator

Alexander Venner<sup>1,★</sup> Mary Anne Limbach<sup>1,2</sup> Mathilde Mâlin<sup>1,3,4,5</sup> Simon Blouin<sup>1,6</sup> Anthony Boccaletti<sup>1,5</sup> and Logan A. Pearce<sup>1,2</sup>

Accepted 2024 November 14. Received 2024 November 14; in original form 2024 October 15

## ABSTRACT

We present the unplanned detection of a white dwarf companion to the star HD 218261 in mid-infrared (10–16 µm) observations with JWST/MIRI. This star was observed as a calibrator for coronagraphic observations of the exoplanet host HR 8799. HD 218261 B has only previously been detected by Gaia, and only in visible light. We confidently detect the companion in the mid-infrared, where it is less luminous than the primary by a factor of ~10⁴. The visible and mid-infrared photometry are consistent with a white dwarf of $T_{eff} \approx 10000$ K, $M \approx 0.8 M_\odot$, though observation of its optical spectrum is required to precisely constrain its physical parameters. These results demonstrate that precise mid-infrared photometry of white dwarf companions to bright stars can be obtained with MIRI, opening up new possibilities for studying white dwarfs in close binaries.

**Key words:** binaries: visual – white dwarfs – infrared: stars.

## 1 INTRODUCTION

The recently launched JWST (Gardner et al. 2006) has already revolutionized infrared (IR) astrophysics. A vast array of results have been achieved through its first years of observations. Furthermore, the unprecedented sensitivity of JWST across IR wavelengths means that there is also significant potential for serendipitous science results from observations intended for other purposes. A notable example of this phenomenon is the discovery of some of the faintest and most distant brown dwarfs ever detected in deep-field observations intended for extragalactic astrophysics (ex. Langeroodi & Hjorth 2023; Nonino et al. 2023; Hainline et al. 2024).

The Mid-InfraRed Instrument (MIRI) on JWST is the most sensitive astronomical instrument operating in mid-IR wavelengths, and has been designed to break new ground in various areas of astrophysical research (Rieke et al. 2015). MIRI is equipped four coronographs (three 4QPM coronographs between 10–16 µm, and a 23 µm Lyot mask), giving it an unparalleled capability for high-contrast imaging in the mid-IR (Boccaletti et al. 2015, 2022).

In this Letter we report the serendipitous observation of a white dwarf companion to the star HD 218261 in JWST/MIRI coronagraphic data, collected as part of the observations presented in Boccaletti et al. (2024). We highlight how this demonstrates the viability of mid-IR observations of white dwarfs in high-contrast binaries with JWST.

<table><caption>**Table 1.** Properties of HD 218261 AB from Gaia DR3.</caption><thead><tr><th>Parameter</th><th>HD 218261 A</th><th>HD 218261 B</th></tr></thead><tbody><tr><td>Source ID</td><td>2831490694929214464</td><td>2831490694928280576</td></tr><tr><td>R.A. α</td><td>23:06:31.89</td><td>23:06:31.53</td></tr><tr><td>Declination δ</td><td>+19:54:39.07</td><td>+19:54:37.95</td></tr><tr><td>Parallax (mas)</td><td>34.077 ± 0.019</td><td>33.98 ± 0.10</td></tr><tr><td>μ<sub>α</sub> (mas yr<sup>-1</sup>)</td><td>+286.890 ± 0.023</td><td>+281.90 ± 0.15</td></tr><tr><td>μ<sub>δ</sub> (mas yr<sup>-1</sup>)</td><td>+5.057 ± 0.019</td><td>-1.41 ± 0.10</td></tr><tr><td>G (mag)</td><td>6.314 ± 0.0028</td><td>15.026 ± 0.0037</td></tr><tr><td>BP (mag)</td><td>6.582 ± 0.0028</td><td>14.453 ± 0.048</td></tr><tr><td>RP (mag)</td><td>5.882 ± 0.0038</td><td>13.924 ± 0.037</td></tr></tbody></table>

## 2 OBSERVATIONS

### 2.1 Gaia

HD 218261 (HR 8792, HIP 114096) is a V = 6.4 F8.5V star at a distance of 29.31 ± 0.02 pc (Gray, Napier & Winkler 2001; Gaia Collaboration 2023). The star was not known to be in any way unusual until a comoving stellar companion was discovered by Gaia (Gaia Collaboration 2016) at a projected separation of 5.24 arcsec (~150 au; El-Badry, Rix & Heintz 2021). In Table 1, we summarize the properties of the two stars reported in Gaia DR3 (Gaia Collaboration 2023).

The companion star, HD 218261 B, has an estimated absolute magnitude $M_G = 12.69$ mag and BP-RP colour ≈0.53, properties consistent with a white dwarf (as previously identified by Golovin et al. 2024). However, this star does not appear in existing Gaia

© 2024 The Author(s).

Published by Oxford University Press on behalf of Royal Astronomical Society. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse, distribution, and reproduction in any medium, provided the original work is properly cited.

<footnote n="1"><sup>1</sup>Centre for Astrophysics, University of Southern Queensland, Toowoomba, QLD 4350, Australia</footnote>

<footnote n="2"><sup>2</sup>Department of Astronomy, University of Michigan, Ann Arbor MI 48109, USA</footnote>

<footnote n="3"><sup>3</sup>Space Telescope Science Institute, Baltimore MD 21218, USA</footnote>

<footnote n="4"><sup>4</sup>Department of Physics & Astronomy, Johns Hopkins University, Baltimore MD 21218, USA</footnote>

<footnote n="5"><sup>5</sup>LESIA, Observatoire de Paris, Université PSL, CNRS, Sorbonne Université, Univ. Paris Diderot, Sorbonne Paris Cité, F-92195 Meudon, France</footnote>

<footnote n="6"><sup>6</sup>Department of Physics and Astronomy, University of Victoria, Victoria BC V8W 2Y2, Canada</footnote>

<footnote>*E-mail: alexandervenner@gmail.com</footnote>
white dwarf catalogues (Gentile Fusillo et al. 2019, 2021). The star is brighter in both BP/RP compared to the G band, which is incongruous since these photometric passbands are approximately complementary (Riello et al. 2021). Pursuing this, there is evidence that the Gaia BP/RP photometry for this star is of poor quality; the Gaia DR3 phot_bp RP excess_factor is 2.39, and following Riello et al. (2021, equation 6) this results in a normalized flux excess factor C* = 1.21, which is significantly exceeds C* = 0 expected for well-behaved sources. This also fails the C* < 0.6 quality cut employed by Gentile Fusillo et al. (2021), which explains the absence from their white dwarf catalogue.

We interpret the high value of C* for HD 218261 B as evidence for flux contamination from the much brighter primary. The BP and RP photometry is extracted from a 3.5×2.1 arcsec² window (Riello et al. 2021, section 9.3), which is comparable to the angular separation of the binary; considering that HD 218261 A is brighter than B by a factor of 3 × 10⁴ in G, even a small fraction of stray light could easily swamp the BP/RP photometry of its companion. We conclude that the BP and RP photometry for HD 218261 B is unreliable.

## 2.2 JWST

HD 218261 was selected for observation with MIRI coronagraphic imaging as part of JWST GTO programme 1194 (PI: Beichman), as a proximate reference star used to calibrate the point spread function (PSF) of the exoplanet host HR 8799 (~1.24° separation, Boccaletti et al. 2024). HD 218261 was originally selected for this purpose prior to the 2018 release of Gaia DR2, so the existence of HD 218261 B was not known when the observation plan was first prepared.

HD 218261 was observed by JWST on 2022-11-08. Images were acquired with the F1065C, F1140C, and F1550C coronagraphic filters. At a projected separation of 5.2 arcsec, HD 218261 B is sufficiently faint and widely separated from HD 218261 A to be spatially resolved with MIRI and does not affect its use as a reference star for removing the stellar diffraction pattern in observations of HR 8799, which are mainly found below <1 arcsec. Indeed, HD 218261 B is fainter than several background sources in the MIRI fields. However, these observations do allow for serendipitous detection of HD 218261 B in the mid-IR.

As a first step, we take the inverse of the PSF-subtracted images of HR 8799 to inspect the detected sources in the HD 218261 field. We show the resulting images in Fig. 1. We clearly observe a source at the companion position expected from Gaia ($\rho$ = 5.24 arcsec, PA = 256°) in the F1065C and F1140C filters, and also weakly detect the same source in F1550C. We also note that the corresponding negative source is quite visible in Boccaletti et al. (2024, fig. 1.1, 1.2). This suffices to demonstrate that HD 218261 B has been detected in the MIRI observations.

Next, we performed a more involved analysis of the MIRI data to extract precise photometry for HD 218261 B. We started from the calibrated images used in Boccaletti et al. (2024). We performed a PSF subtraction to eliminate the stellar diffraction pattern of HD 218261 A, using the commissioning data from JWST GTO 1037 (Boccaletti et al. 2022) for the reference star and applying a PCA-based method as in Boccaletti et al. (2024). The inner region of the image still shows residual starlight because the reference star was not optimized for the observations of HD 218261. However, the processing performs well enough for larger separations.

Following the method described in Mâlin et al. (2024), we modelled the PSF of HD 218261 B with WEBBPSF (Perrin et al. 2014). We simulate a PSF for each filter taking into account the combination of filter, coronagraphic mask and pupil mask, as well as

the position on the detector. We minimize the residuals between the simulated PSF and the data in a 2λ/D region centred on the object. Minimization is performed using the Nelder-Mead algorithm (Gao & Han 2012) with the optimize.minimize function from SCIPY (Virtanen et al. 2020). To initialize the minimizer, we chose the position of the brightest pixel and an arbitrary flux of 1 DN/s. A possible ±1-pixel offset is allowed in the minimization process to account for sub-pixel offsets between the model and the simulated PSF.

The coronagraphic transmission is ≈1 at the separation of HD 218261 B, so this has a negligible impact on the measured flux. As a result, we measured the photometry by simply calculating the flux of the best-fitting PSF model for each filter. HD 218261 A was observed using the 9-point dither pattern (as the observing strategy was designed to catch the PSF diversity for the HR 8799 observations). We therefore repeated the PSF model at all dither positions to obtain the uncertainty on the flux measurement.

## 3 RESULTS

In Table 2, we report our extracted MIRI photometry for HD 218261 B along with existing measurements from Gaia DR3. In the three MIRI coronagraphic filters we retrieve fluxes between 18–40μJy, two orders of magnitude lower than observed in the optical. Qualitatively this can only be explained by a small and hot star, confirming the white dwarf identification of Golovin et al. (2024) based on the poor-quality Gaia photometry. The mid-IR flux of HD 218261 A has been measured at 11.6 µm by WISE (Wright et al. 2010) as 0.250 ± 0.003 Jy. Hence, the flux contrast with the companion is about 10⁴ (~10 mag) at these wavelengths. The high significance of this detection exemplifies the exceptional imaging capabilities of JWST and its potential for serendipitous science.

Our constraints on the spectral energy distribution of HD 218261 B are highly incomplete, being that we have only a limited set of visible and mid-IR photometry. Nonetheless, we attempt to use white dwarf spectral models to make a preliminary exploration of its properties. We used the models of Blouin, Dufour & Allard (2018a) and Blouin et al. (2018b) to estimate the atmospheric parameters of HD 218261 B. We make the assumption that HD 218261 B has a hydrogen-dominated (DA) spectrum, as this is the most common atmospheric composition among white dwarfs; however, this cannot be proven or falsified with the existing data. We adjusted the effective temperature to match the observed slope between the GaiaG band and MIRI F1065C, F1140C, and F1550C filters. We then scaled the model flux to match the observed fluxes, given the distance implied by the Gaia parallax, to determine the stellar radius. The surface gravity was self-consistently adjusted using the evolutionary models of Bédard et al. (2020) to relate the mass and radius of the white dwarf.

We began iterating through possible white dwarf parameters using log g = 8.0 (i.e. 10⁸ cm s⁻²), equivalent to a M = 0.6 M⊙ white dwarf lying close to the peak of the mass distribution (Tremblay et al. 2016). However, we found that the resulting model spectra overpredict the mid-IR flux. Exploring other models with lower luminosities, we find that a T eff = 10000 K, log g = 8.3 (M = 0.8 M⊙) model spectrum provides a satisfactory fit to the photometry (Fig. 2). No model can reproduce the large difference between the G and BP/RP fluxes, consistent with our interpretation that this measurements are contaminated by the primary.

As a check for consistency, we can compare the implied age of HD 218261 B with that of A as both stars are expected to be coeval.
<figure><img src="image_2.png" /><figcaption>**Figure 1.** Reference star subtracted images of HD 218261 in the three MIRI coronagraphic filters (left to right: F1065C, F1140C, F1550C). The full MIRI 4QFM coronagraphic field of view (24 × 24 arcsec) is displayed. The white dwarf companion HD 218261 B is labelled as 'WD'. Negative sources from the HR 8799 field are apparent at small separations.</figcaption></figure>

<table><caption>**Table 2.** Photometry of HD 218261 B.</caption><thead><tr><th>Band/Filter</th><th>λ (μm)</th><th>Flux (μJy)</th></tr></thead><tbody><tr><td>BP</td><td>0.52</td><td>5880 ± 260<sup>a</sup></td></tr><tr><td>G</td><td>0.64</td><td>3150 ± 10</td></tr><tr><td>RP</td><td>0.78</td><td>6880 ± 240<sup>a</sup></td></tr><tr><td>F1065C</td><td>10.58</td><td>40.0 ± 3.2</td></tr><tr><td>F1140C</td><td>11.3</td><td>26.3 ± 2.3</td></tr><tr><td>F1550C</td><td>15.5</td><td>17.7 ± 2.0</td></tr></tbody></table>

<figure><img src="image_3.png" /><figcaption>**Figure 2.** Spectral energy distribution of HD 218261 B. A model spectrum with $T_{\text{eff}} = 10000$ K, log $g = 8.3$ adequately reproduces the measured photometry, except for the Gaia BP/RP bands (unfilled points).</figcaption></figure>

Using the Bédard et al. (2020) models, a $T_{\text{eff}} = 10000$ K, log $g = 8.3$ white dwarf entails a cooling age of ≈1.0 Gyr. The main-sequence progenitor of a $0.8\ M_{\odot}$ white dwarf has a mass of ~3.5 $M_{\odot}$, implying an additional pre-white dwarf lifetime of ≈0.3 Gyr (Cummings et al. 2018). Combined, this implies a total age of approximately ≈1.3 Gyr. The isochrone age of HD 218261 A has previously been estimated as $1.6_{-0.8}^{+1.0}$ Gyr (Valenti & Fischer 2005). Though the uncertainties are relatively large, the consistency of the stellar ages suggests our parameter estimates for HD 218261 B are reasonable.

Though these parameters provide an adequate model for the observed photometry, the existing data is insufficient to fully characterise HD 218261 B. In particular, we cannot test our assumption

of a hydrogen-dominated atmospheric composition. Precise characterisation of HD 218261 B will require observation of its optical spectrum.

## 4 DISCUSSION AND CONCLUSIONS

In this work we have presented the serendipitous detection of a white dwarf companion to the star HD 218261 in JWST/MIRI coronagraphic observations. To our knowledge, this is the first observation of a white dwarf with MIRI using a coronagraphic observing mode; the closest parallel that we know of is the detection of HD 114174 B (Crepp et al. 2013; Zhang et al. 2023) with NIRCam coronagraphy in JWST commissioning (Girard et al. 2022; Kammerer et al. 2022).

With an F-type main-sequence primary and a white dwarf secondary, HD 218261 is an example of a 'Sirius-like system' as defined by Holberg et al. (2013). Though in this case HD 218261 B had already been discovered by Gaia, Sirius-like white dwarfs have been quite frequently discovered as a result of coronagraphic imaging observations (e.g. Crepp et al. 2013, 2018; Zurlo et al. 2013; Hirsch et al. 2019; Bonavita et al. 2020; Bowler et al. 2021). In the near-IR, white dwarfs are as luminous as brown dwarfs, and can easily be confused with them without information from other sources (Mugrauer & Neuhäuser 2005; Crepp et al. 2013; Zurlo et al. 2013). In the mid-IR, we find that HD 218261 B is even fainter than the HR 8799 planets (Boccaletti et al. 2024), highlighting the potential for confusion between white dwarfs and planets. We encourage observers to take heed of the possibility that faint co-moving sources detected in MIRI observations could be white dwarfs instead of planets or brown dwarfs. For newly detected objects, care should be taken to distinguish between these possibilities. In particular, as white dwarfs and planets have very different temperatures, broadband photometry at shorter wavelengths can be an efficient way to break this potential degeneracy.

Holberg et al. (2013) found a significant decline in the number of known Sirius-like systems known beyond 20 pc; with a distance of 29 pc, HD 218261 adds to this undersampled area of the local Sirius-like population. These systems can act as important benchmarks for aspects of white dwarf physics (e.g. Venner et al. 2023). To improve on our understanding of this system, we advocate for future observation of the optical spectrum of HD 218261 B to precisely determine the physical parameters of this white dwarf.

<footnote><sup>a</sup>Note. Contaminated by HD 218261 A.</footnote>
HD 218261 B joins the relatively small list of WDs detected beyond 10 µm, highlighting the exceptional capabilities of MIRI in this area. Observations of white dwarfs with MIRI are testing the limits of physical models for these stars (e.g. Blouin et al. 2024). Further afield, MIRI provides an unprecedented sensitivity to exoplanets orbiting white dwarfs (Limbach et al. 2022) and has been used to discover a small number of candidate planets thus far (Limbach et al. 2024; Mullally et al. 2024; Poulsen et al. 2024). These observations have previously been conducted with MIRI wide-field imaging and have hence been excluded from observing white dwarfs in close binaries. However, the successful detection of HD 218261 B in MIRI coronagraphic imaging presented here raises the possibility that the same methods could be applied to planet detection for white dwarfs with IR-bright stellar companions. Of the six white dwarfs within 6 parsecs of the Sun, four are in close binaries (≤10 arcsec) with main-sequence stars (Sirius B; Procyon B; 40 Eri B; Gliese 169.1 B). By using the coronagraphs to block out the bright companions, MIRI could conceivably be used to search for white dwarf planets in these and other systems.

## ACKNOWLEDGEMENTS

We acknowledge and pay respect to Australia's Aboriginal and Torres Strait Islander peoples, who are the traditional custodians of the lands, waterways and skies all across Australia. We thank the anonymous referee for their comments that have helped to improve this work. AV would like to acknowledge the 'JWST Master Class 2022' workshop, hosted at Swinburne University, for bringing the observations of HD 218261 to their attention. AV is supported by ARC DECRA project DE200101840. LAP acknowledges support from the University of Michigan through the ELT Fellowship Program.

This work is based on observations made with the NASA/ESA/CSA JWST. Data was obtained from the Mikulski Archive for Space Telescopes at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-03127 for JWST. These observations are associated with JWST programme #1037 and #1194. This work has made use of data from the European Space Agency (ESA) mission Gaia (https://www.cosmos.esa.int/gaia), processed by the Gaia Data Processing and Analysis Consortium (DPAC, https://www.cosmos.esa.int/web/gaia/dpac/consortium). Funding for the DPAC has been provided by national institutions, in particular the institutions participating in the Gaia Multilateral Agreement. This publication makes use of data products from the Wide-field Infrared Survey Explorer, which is a joint project of the University of California, Los Angeles, and the Jet Propulsion Laboratory/California Institute of Technology, funded by the National Aeronautics and Space Administration. This research has made use of the SIMBAD data base and VizieR catalogue access tool, operated at CDS, Strasbourg, France. This research has made use of NASA's Astrophysics Data System.

## DATA AVAILABILITY

This paper makes use of observations from the JWST that are publicly accessible from the Mikulski Archive for Space Telescopes (MAST; https://archive.stsci.edu/missions-and-data/jwst).

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