Atypical White Dwarfs in Open Clusters

arXiv:2610.10699 · astro-ph.SR, astro-ph.GA · Submitted 2026-10-07 · Read on arXiv

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Introduction to the show: ident: Astrophysics Radio. Generated commentary on the latest astrophysics papers.

Vera: I'm Vera, and with me are Jocelyn and Subrahmanyan, guest researcher.

Jocelyn: Today's paper: "Atypical White Dwarfs in Open Clusters".

Vera: The gist The study assembles the largest sample to date of atypical WDs in open clusters,

Jocelyn: First, who's behind it and why it matters.

Title and authors: Vera: So we're looking at this paper, "Atypical White Dwarfs in Open Clusters," and the authors are David R. Miller, Ilaria Caiazzo, Jeremy Heyl, and Pier-Emmanuel Tremblay.

Jocelyn: The title tells us right away that they’re focusing on white dwarfs that don't fit the typical picture of what we see in open clusters.

Vera: Exactly. They're looking at the weird stuff like magnetic WDs or WDs with spectra that aren't dominated by hydrogen, which are hard to figure out in the general field data.

Subrahmanyan: It’s interesting because open clusters give us a solid age for those white dwarfs, which helps us untangle their origins from single stars versus something more complex.

Vera: Right. What they do here is assemble what they call the largest sample to date of these atypical WDs in open clusters.

Jocelyn: And the key finding, as stated right up front on page one, is that they found no significant difference between how the mass relates to its final mass for hydrogen- and helium-atmosphere WDs, and also no robust spatial or kinematic differences compared to what we see in the field.

Subrahmanyan: That’s a bit surprising because we expect some separation when you look at different evolutionary channels.

The paper's summary: Vera: Well, the summary says they focused on isolated WDs and those in non-interacting systems where the white dwarf can be characterized independently.

Jocelyn: They built their sample starting with a compilation of non-DA cluster WDs from J. W. Barnett et al. (two thousand twenty-one), which found thirteen non-DA WDs in open-cluster fields, seven of which they assessed as likely cluster members according to Gaia DR3 astrometry where available, and they added other literature candidates <ref:2610.10699#pg1>.

Vera: They also supplemented this by using candidate atypical cluster WDs identified in the spectroscopic census from D. R. Miller et al. (2026c) <ref:2610.10699#pg1>.

Subrahmanyan: The analysis of the non-DA population shows that the cluster non-DAs are concentrated at low and intermediate masses, while the high-mass population is almost entirely DA, which is a bit different from what we see in the field.

Jocelyn: And then they found a really important deficit: at zero point eight MWD over solar mass up to one point zero MWD, the field population predicts six point zero plus zero point eight minus zero point seven non-DAs, but none are observed, which has a posterior-predictive probability of three point seven times ten to the negative three that there are none.

Vera: So they're suggesting this deficit isn't just a general reduction in the non-DA fraction across all masses, but it’s strongly dependent on mass and concentrated between zero point eight and one point zero MWD.

The paper's improvements: Vera: The paper also points out some ways they could improve things going forward. They suggest incorporating posterior-predictive distributions from temperature-dependent fractions of the DESI field sample to make the comparisons between cluster and field more rigorous, like what you see in Figure eight.

Jocelyn: That would let the AI quantify that cluster non-DA deficit more precisely, moving beyond just simple incidence comparisons with that three point seven times ten to the negative three probability.

Subrahmanyan: And they also suggest modeling and constraining binary evolution channels for magnetic WDs by analyzing how cluster ages constrain progenitor lifetimes, which is a really important way to differentiate between single-star evolution and merger remnants.

Vera: That’s a crucial point because it directly relates to separating the different formation pathways for those massive magnetic WDs they found missing in the field.

Jocelyn: On another note, they could upgrade the system to identify potential rapid rotation or merger origins in peculiar magnetic WDs by looking at their photometric properties, like that eighteen-minute periodic signal seen in Praesepe-WDJ082547+one hundred seventy-four thousand eight hundred eighteen.

Conclusion: Vera: So to wrap up this paper, the overall conclusion is that open clusters are deficient in the massive WD populations associated with merger formation and peculiar WDs like DQs.

Jocelyn: They found that the strongest discrepancy is at zero point eight MWD over solar mass up to one point zero MWD where the field predicts six point zero plus zero point eight minus zero point seven non-DAs but none are observed, with a posterior-predictive probability of three point seven times ten to the negative three that there are none.

Subrahmanyan: And massive magnetic WDs show a similar deficit; only one out of twenty cluster WDs with MWD greater than or equal to one point zero MWD and temperature greater than or equal to twenty thousand Kelvin is magnetic, compared to ten out of sixteen in the field.

Vera: So these results suggest that open clusters are deficient in the massive WD populations associated with merger formation while the incidence of DBs is similar to the field, but they also point to a difference in evolutionary timescales between clusters and the field.

Jocelyn: The simplest explanation for this lack of merger remnants is that most clusters contributing WDs to this sample are young, leaving relatively little time for delayed merger remnants to accumulate compared to the Galactic field which accumulates them over much longer timescales.

Subrahmanyan: That’s a very clean way to look at it, suggesting that the high-mass WD population shows that massive WDs can form through normal single-star evolution without developing magnetism at the high rate seen in the field.

Vera: It really points to how evolutionary time plays a role here; for example, at lower masses, the lack of magnetism among young cluster WDs is consistent with fields becoming visible only later through crystallization or delayed emergence from the progenitor.

Jocelyn: So, in summary, this paper on atypical white dwarfs in open clusters shows that while they are deficient in massive WD populations linked to merger formation and peculiar WDs, the incidence of DBs is similar to the field.

Subrahmanyan: The final point is that because most clusters in this sample are too young to host comparable populations of delayed merger remnants, they're naturally suppressed in present-day cluster samples compared to the Galactic field over much longer timescales.

David R. Miller, Ilaria Caiazzo, Jeremy Heyl

Department of Physics and Astronomy, University of British Columbia · Institute of Science and Technology Austria · Department of Physics, University of Warwick

astro-ph.SR, astro-ph.GA

Submitted: 2026-10-07

Updated: 2026-10-07

Comments: 25 pages, 17 figures, 4 tables. Submitted to ApJ

License: http://arxiv.org/licenses/nonexclusive-distrib/1.0/

Importance score: 83/100

The gist: The gist The study assembles the largest sample to date of atypical WDs in open clusters, finding no significant difference between initial–final mass relations or robust spatial or kinematic

Key concepts

Atypical White Dwarfs
These are white dwarfs that do not fit the standard population of non-magnetic, hydrogen-atmosphere WDs found in open clusters. They include magnetic WDs and those with spectra dominated by helium rather than hydrogen. Investigating them helps researchers understand unusual WD origins.
Non-DA Population Deficit
The study found a strong lack of non-DA white dwarfs in the mass range of 0.8 to 1.0 solar masses within clusters compared to the general field population. This specific deficit suggests that objects in this mass range, which might be massive merger remnants, are less common or have not yet formed in young clusters.
Mass-Dependent Magnetism
The presence of magnetic fields on white dwarfs appears dependent on their mass and age within clusters. Younger low-mass WDs show no magnetism, consistent with delayed emergence. However, massive WDs can develop magnetism through single-star evolution without the high rates seen in the field, pointing to a time-dependent process.
Merger Remnants
These are white dwarfs formed from binary star mergers. The study suggests that open clusters are deficient in these remnants because young systems lack the necessary time for these delayed merger events to occur, whereas the Galactic field has accumulated them over much longer timescales.

Terminology

Summary

The gist The study assembles the largest sample to date of atypical WDs in open clusters, finding no significant difference between initial–final mass relations or robust spatial or kinematic differences compared to the field, but revealing a deficit of non-DAs between 0.8 and 1.0 M⊙ and a similar deficit in massive magnetic WDs <ref:2610.10699#pg3>.

Sample Selection

The researchers define atypical cluster WDs as those that fall outside the dominant population of non-magnetic DAs, including magnetic WDs of any atmospheric type and non-magnetic nonDA WDs <ref:2610.10699#pg4>. They focus on isolated WDs and those in non-interacting systems for which the WD can be characterized independently <ref:2610.10699#pg4>. The sample starts with the non-DA cluster WD compilation of J. W. Barnett et al. (2021), which identified 13 non-DA WDs in open-cluster fields, of which seven were assessed as likely cluster members <ref:2610.10699#pg4>. They also include additional non-magnetic, non-DA WDs with literature cluster associations <ref:2610.10699#pg4>. To supplement the literature sample, they used candidate atypical cluster WDs identified in the spectroscopic census of D. R. Miller et al. (2026c) <ref:2610.10699#pg4>.

Cluster and WD Characterization

Cluster parameters are determined using the heuristic MSTO-based PARSEC isochrone fitting procedure developed by D. R. Miller et al. (2026a) <ref:2610.10699#pg5>. For non-magnetic helium-atmosphere WDs, atmospheric parameters are derived from line-profile fitting using the model grid of E. Cukanovaite et al. (2021) <ref:2610.10699#pg6>. For DC and magnetic WDs, atmospheric parameters are determined photometrically <ref:2610.10699#pg6>. The analysis also involves photometric fitting for DC and magnetic WDs, using models from P. E. Tremblay et al. (2011) and E. Cukanovaite et al. (2021) <ref:2610.10699#pg6>.

Non-DA Population Analysis

The comparison of the cluster sample with the combined DESI WD samples shows that cluster non-DAs are concentrated at low and intermediate masses, while the high-mass population is almost entirely DA <ref:2610.10699#pg12>. A key finding is that at 0.8 ≤ MWD/M⊙ < 1.0, the field population predicts 6.0 +0.8 −0.7 non-DAs but none are observed, with a posterior-predictive probability of P(N = 0) = 3.7 × 10−3 <ref:2610.10699#pg13>. This suggests that the cluster nonDA deficit is not a general reduction in the non-DA fraction, but a strongly mass-dependent effect concentrated between 0.8 and 1.0 M⊙ <ref:2610.10699#pg13>.

Magnetic WD Population Analysis

Massive magnetic WDs show a similar deficit: only one of 20 cluster WDs with MWD ≥ 1.0 M⊙ and Teff ≥ 20,000 K is magnetic, compared with 10 of 16 in the field <ref:2610.10699#pg9>. This deficit is consistent with an additional field population of massive merger remnants, particularly high-mass magnetic WDs and warm/hot DQs <ref:2610.10699#pg9>. The cluster sample helps constrain single-star channels, as all five magnetic WDs with viable singlestar progenitor solutions lie at cooling ages younger than even the lower uncertainty bound on the predicted mainsequence dynamo breakout times of M. Camisassa et al. (2024) <ref:2610.10699#pg19>.

Summary and Conclusions

Overall, the expanded sample of atypical cluster WDs reveals significant differences from the field in both the high-mass non-DA and magnetic populations <ref:2610.10699#pg22>. The most striking result is that clusters lack the massive non-DA population seen in the field, particularly DQs and other peculiar WDs <ref:2610.10699#pg22>. The strongest discrepancy occurs at 0.8 ≤ MWD/M⊙ < 1.0, where the field population predicts 6.0 +0.8 −0.7 non-DAs but none are observed, with a posterior-predictive probability of P(N = 0) = 3.7 × 10−3 <ref:2610.10699#pg22>. Second, massive magnetic WDs are similarly rare: only one of 20 cluster WDs with MWD ≥ 1.0 M⊙ and Teff ≥ 20,000 K is magnetic, compared with 10 of 16 field WDs, with P(N ≤ 1) = 1 × 10−4 <ref:2610.10699#pg24>. Together, these results suggest that open clusters are deficient in the massive WD populations associated with merger formation, while the incidence of DBs is similar to the field <ref:2610.10699#pg24>. The simplest explanation for the apparent lack of merger remnants is the different evolutionary timescales sampled by clusters and the field <ref:2610.10699#pg25>. Most clusters contributing WDs to the present sample are young, leaving relatively little time for delayed merger remnants to accumulate <ref:2610.10699#pg25>. The Galactic field, in contrast, accumulates merger remnants over much longer timescales <ref:2610.10699#pg25>. Open clusters old enough to produce substantial delayed merger populations are also increasingly likely to lose members or dissolve, further suppressing these objects in present-day cluster samples <ref:2610.10699#pg25>. The high-mass WD population shows that massive WDs can form through normal single-star evolution without developing magnetism at the high rate seen in the field <ref:2610.10699#pg25>. This difference may reflect the time available for merger channels <ref:2610.10699#pg25>. The cluster sample points to the strong role evolutionary time plays in the appearance of WD magnetism <ref:2610.10699#pg25>. At lower masses, the lack of magnetism among young cluster WDs is consistent with fields becoming visible only later through crystallization or delayed emergence from the progenitor <ref:2610.10699#pg25>. In contrast, the five magnetic WDs with viable single-star progenitor solutions show that some fields can be visible much earlier than the breakout times estimated by M. Camisassa et al. (2024), including essentially from the start of WD cooling in the hottest object <ref:2610.10699#pg25>. At high mass, the cluster sample shows that massive WDs can form through normal single-star evolution without developing magnetism at the high rate seen in the field <ref:2610.10699#pg25>. This difference may reflect the field accumulating magnetic merger remnants over longer evolutionary timescales, while most clusters in our sample are too young to host comparable populations <ref:2610.10699#pg25>. The study concludes that open clusters are deficient in the massive WD populations associated with merger formation, while the incidence of DBs is similar to the field <ref:2610.10699#pg25>. The final sample remains limited by small numbers and uneven coverage in mass and temperature, but larger spectroscopic samples will strengthen these comparisons and enable increasingly stringent tests of WD evolution <ref:2610.10699#pg25>.

Improvements for AI systems

  1. The AI system can be improved to perform rigorous, probabilistic comparisons between cluster WD populations and field WD populations by incorporating posterior-predictive distributions from temperature-dependent fractions of the DESI field sample, as shown in Figure 8. This allows the AI to quantify the cluster nonDA deficit with a posterior-predictive probability of P(N = 0) = 3.7 × 10−3, moving beyond simple incidence comparisons.

  2. The system can be enhanced to model and constrain binary evolution channels for magnetic WDs by analyzing the constraints imposed by cluster ages on progenitor lifetimes, as suggested by the text stating, The cluster sample provides an important constraint on binary formation channels. This allows the AI to differentiate between single-star evolution and merger remnants based on evolutionary timescales.

  3. The AI system can be upgraded to identify potential rapid rotation or merger origins in peculiar magnetic WDs by analyzing their photometric properties, such as the 18-minute periodic signal observed in Praesepe-WDJ082547+174818, which may indicate a rapidly rotating magnetic WD.

  4. The system can be refined to assess the role of evolutionary timescales in explaining population differences by comparing the cluster sample with field samples across different mass and cooling age bins, as indicated by the conclusion that the Galactic field, in contrast, accumulates merger remnants over much longer timescales.

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