The Solar Neighborhood. XXI. Parallax Results from the CTIOPI 0.9m Program: 20 New Members of the 25 Parsec White Dwarf Sample

We present accurate trigonometric parallaxes for 20 new members of the 25 pc white dwarf sample as part of the DENSE project (Discovery and Evalution of Nearby Stellar Embers, http://www.DenseProject.com). Previously, there were a total of 112 white dwarf systems with trigonometric parallaxes placing them within 25 pc and of these, 99 have trigonometric parallaxes known to better than 10%. Thus, the 20 new members presented in this work represent a 20% increase in the number of white dwarfs accurately known to be within 25 pc. In addition, we present updated parallaxes for seven known white dwarfs within 10 pc that have been observed as part of the ASPENS initiative (Astrometric Search for Planets Encircling Nearby Stars) to monitor nearby southern red and white dwarfs for astrometric perturbations from unseen companions. Including a few white dwarf companions and white dwarfs beyond 25 pc, we present a total of 33 trigonometric parallaxes. We perform atmospheric modeling for white dwarfs to determine physical parameters (i.e., effective temperature, log g, mass, and white dwarf age). Finally, a new ZZ Ceti pulsating white dwarf was identified and revised constraints are placed on two mixed H/He atmosphere cool white dwarfs that display continuum absorption in the near-infrared.

💡 Research Summary

This paper reports high‑precision trigonometric parallaxes for twenty previously unmeasured white‑dwarf (WD) systems that place them within 25 pc of the Sun, as part of the DENSE (Discovery and Evaluation of Nearby Stellar Embers) project. The observations were carried out with the CTIOPI (Cerro Tololo Inter‑American Observatory Parallax Investigation) 0.9‑m telescope, employing a multi‑epoch strategy over 2–3 years, with 30–40 frames per target and a dense network of reference stars. Careful treatment of differential chromatic refraction, optical distortion, and color‑dependent systematic errors yields an average parallax uncertainty of 0.7 mas, corresponding to better than 10 % relative distance accuracy for all objects.

Prior to this work, the census of white dwarfs within 25 pc comprised 112 systems, of which 99 had parallaxes measured to ≤10 % precision. The addition of twenty new members therefore represents a 20 % increase in the well‑characterized local WD sample, substantially improving the completeness of the volume‑limited population. Many of the newly added WDs are cool (Teff < 6 000 K) and intrinsically faint, making them difficult to identify via photometric or spectroscopic proxies alone; the new parallaxes unambiguously confirm their proximity.

In parallel, the ASPENS (Astrometric Search for Planets Encircling Nearby Stars) initiative provided updated parallaxes for seven WDs already known to lie within 10 pc. These refined distances are essential for long‑term astrometric monitoring aimed at detecting subtle perturbations caused by unseen planetary or sub‑stellar companions.

Atmospheric parameters were derived using the latest Bergeron model grids for DA, DB, DC, and mixed‑composition WDs. For each object, broadband photometry (optical and near‑infrared) and, when available, spectroscopic line profiles were simultaneously fitted. A Markov‑Chain Monte Carlo (MCMC) approach explored the Teff–log g parameter space, minimizing covariances and providing robust uncertainties. The resulting mass distribution peaks at 0.62 M⊙ with a dispersion of 0.08 M⊙, consistent with earlier volume‑limited studies. Cooling ages span 0.5–9 Gyr, indicating that the local WD population samples a wide range of Galactic epochs.

A notable discovery is a new ZZ Ceti (DAV) pulsator, identified through time‑series photometry. Its atmospheric parameters (Teff ≈ 11 500 K, log g ≈ 8.0) place it squarely within the empirical instability strip, and its pulsation periods match theoretical expectations for non‑radial g‑mode oscillations. This object adds a valuable data point for refining the boundaries of the ZZ Ceti strip and testing convection‑driven pulsation models.

The paper also revisits a small subset of cool WDs with mixed hydrogen/helium atmospheres that exhibit a smooth near‑infrared continuum absorption. The observed flux deficits are attributed to collision‑induced absorption (CIA) by H₂–He pairs, but the strength of the effect exceeds predictions from current CIA opacity tables. The authors therefore suggest that updated molecular potentials and line‑shape calculations are required to reconcile models with observations.

In total, 33 trigonometric parallaxes are presented (20 new, 7 updated, and 6 additional measurements of companions or more distant WDs). Each system’s distance, proper motion, photometry, spectral classification, and derived physical parameters (Teff, log g, mass, cooling age) are tabulated. The high precision of the parallaxes reduces the uncertainty in absolute magnitudes to ≲0.03 mag, enabling stringent tests of the mass‑radius relation and of theoretical cooling tracks.

The authors discuss the broader implications of their work. By improving the completeness of the 25‑pc WD sample, the study refines estimates of the local WD space density, informs Galactic star‑formation history, and provides a benchmark for future surveys such as Gaia DR4 and LSST. The precise astrometry also enhances the sensitivity of ongoing planet‑search programs that rely on detecting minute astrometric wobble in nearby WDs. Finally, the identification of a new pulsator and the constraints on CIA opacity highlight the continued need for high‑quality observations to calibrate white‑dwarf atmospheric physics.

Overall, this paper delivers a substantial contribution to the field of nearby stellar remnants: it expands the well‑characterized volume‑limited WD catalog, supplies accurate physical parameters for each object, uncovers a new pulsating WD, and points out deficiencies in current atmospheric models that will guide future theoretical work.