Masses of the Hyades white dwarfs: A gravitational redshift measurement

Achim Weiss; A. F. Pala; H.-G. Ludwig; I.C Le\~ao; J.R. de Medeiros; L. Pasquini

arxiv: 1907.01265 · v1 · pith:BMUAZIZBnew · submitted 2019-07-02 · 🌌 astro-ph.SR

Masses of the Hyades white dwarfs: A gravitational redshift measurement

L. Pasquini , A. F. Pala , H.-G. Ludwig , I.C Le\~ao , J.R. de Medeiros , Achim Weiss This is my paper

Pith reviewed 2026-05-25 10:54 UTC · model grok-4.3

classification 🌌 astro-ph.SR

keywords white dwarfsHyades clustergravitational redshiftstellar massesGaia photometrystellar evolution modelsopen clusters

0 comments

The pith

Gravitational redshift shows Hyades white dwarf masses are systematically 0.02 to 0.05 solar masses smaller than from other methods.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The paper determines the masses of six white dwarfs in the Hyades cluster by measuring their gravitational redshift in high-resolution spectra and subtracting the known cluster velocity from astrometry. This gives the mass-to-radius ratio, which combined with radius estimates from Gaia photometry yields masses. These masses are smaller than those from evolutionary models or other techniques by 0.02 to 0.05 solar masses, though the offset is within the 5% measurement uncertainty. The authors conclude that the gravitational redshift method confirms stellar evolution models to within a few percent and provides an empirical way to measure white dwarf masses in open clusters.

Core claim

We analyse UVES-VLT spectra of six Hyades white dwarfs to measure their Doppler shift and derive M/R from the gravitational redshift after subtracting the astrometric radial velocity. Using radii from Gaia photometry and literature data, which agree to within 4%, the resulting masses are systematically smaller than those from other methods by 0.02 to 0.05 solar masses. This difference, while within uncertainty, is systematic and indicates the gravitational redshift in white dwarfs agrees with stellar evolution model predictions to within a few percent.

What carries the argument

Gravitational redshift measurement providing the mass-to-radius ratio after subtracting the cluster's astrometric radial velocity.

If this is right

The gravitational redshift technique can be extended to other open clusters for empirical mass determinations.
Dedicated spectrographs can improve the precision of these measurements.
The method may reveal interesting properties of white dwarf atmospheres.
Gravitational redshift agrees with stellar evolution models to within a few percent.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

A persistent offset could indicate adjustments needed in radius or cooling models for white dwarfs.
Extending the method to additional clusters would test dependence on age or composition.
This approach offers a way to empirically calibrate other white dwarf mass estimation techniques.

Load-bearing premise

The radii estimated from Gaia photometry and literature data are accurate to better than the 4% level that can arise from data quality differences.

What would settle it

A radius measurement for these white dwarfs that differs by more than 4% from the Gaia estimates, or higher-precision redshift data that removes the systematic mass offset.

Figures

Figures reproduced from arXiv: 1907.01265 by Achim Weiss, A. F. Pala, H.-G. Ludwig, I.C Le\~ao, J.R. de Medeiros, L. Pasquini.

**Figure 1.** Figure 1: Comparison between observations and models in an observational plane: M/R vs. Gaia G magnitudes. Each star is indicated with a different symbol and colour. For each star, its effective temperature has been computed as the average value between the measurements by Cummings et al. (2018) and Gentile Fusillo et al. (2019) and the corresponding models are over-imposed and colour-coded as the corresponding st… view at source ↗

read the original abstract

Context. It is possible to accurately measure the masses of the white dwarfs (WDs) in the Hyades cluster using gravitational redshift, because the radial velocity of the stars can be obtained independently of spectroscopy from astrometry and the cluster has a low velocity dispersion. Aims. We aim to obtain an accurate measurement of the Hyades WD masses by determining the mass-to-radius ratio (M/R) from the observed gravitational redshift, and to compare them with masses derived from other methods. Methods. We analyse archive high-resolution UVES-VLT spectra of six WDs belonging to the Hyades to measure their Doppler shift, from which M/R is determined after subtracting the astrometric radial velocity. We estimate the radii using Gaia photometry as well as literature data. Results. The M/R error associated to the gravitational redshift measurement is about 5%. The radii estimates, evaluated with different methods, are in very good agreement, though they can differ by up to 4% depending on the quality of the data. The masses based on gravitational redshift are systematically smaller than those derived from other methods, by a minimum of $\sim 0.02$ up to 0.05 solar masses. While this difference is within our measurement uncertainty, the fact that it is systematic indicates a likely real discrepancy between the different methods. Conclusions. We show that the M/R derived from gravitational redshift measurements is a powerful tool to determine the masses of the Hyades WDs and could reveal interesting properties of their atmospheres. The technique can be improved by using dedicated spectrographs, and can be extended to other clusters, making it unique in its ability to accurately and empirically determine the masses of WDs in open clusters. At the same time we prove that gravitational redshift in WDs agrees with the predictions of stellar evolution models to within a few percent.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Desk Editor's Note private letter to a colleague

This gives the first gravitational-redshift M/R values for six Hyades white dwarfs, but the claimed systematic mass offset is comparable in size to the 4% radius uncertainty from Gaia photometry.

read the letter

The paper measures gravitational redshifts from archive UVES spectra of six Hyades white dwarfs, subtracts the known astrometric velocity, and derives M/R to roughly 5% precision. Radii come from Gaia photometry plus literature values, yielding masses 0.02–0.05 solar masses below those from other techniques. The offset sits inside the stated uncertainty, yet the authors flag it as likely real because it appears in every object. They also conclude that the redshift method agrees with stellar-evolution predictions at the few-percent level. What is actually new is the direct, largely model-independent M/R route applied to a well-characterized cluster; this supplies an empirical check on the initial-final mass relation that does not recycle the same cooling tracks or atmosphere grids used in the comparison masses. The measurement itself is straightforward and the circularity burden is low. The soft spot is the radius scale. Different photometric methods already differ by up to 4%, and for typical Hyades white-dwarf masses that shift moves the final mass by an amount comparable to the reported offset. Without an external anchor on the absolute radii, it is difficult to separate a real discrepancy from a systematic in the radius step. The abstract is appropriately cautious on the offset, but the stronger claim of model agreement at a few percent inherits the same limitation. This work is for people who need independent white-dwarf masses in clusters or who want to test cooling models with new observables. It is worth sending to a serious referee because the technique is clean and extensible even if the current offset interpretation needs tighter radius control.

Referee Report

2 major / 1 minor

Summary. The manuscript reports gravitational redshift measurements for six white dwarfs in the Hyades open cluster using archival UVES spectra. After correcting for the cluster's astrometric radial velocity, the authors derive the mass-to-radius ratio (M/R) for each star with an estimated uncertainty of approximately 5%. Radii are estimated using Gaia photometry and literature values, which agree to within 4%. The resulting masses are found to be systematically lower by 0.02 to 0.05 solar masses compared to masses derived from other methods. The authors conclude that this offset is likely real and that the gravitational redshift measurements agree with stellar evolution model predictions to within a few percent. The technique is presented as a powerful tool for determining white dwarf masses in clusters.

Significance. This work offers an independent, observationally grounded method to measure white dwarf masses in a coeval population, leveraging the low velocity dispersion of the Hyades. The gravitational redshift approach directly probes M/R without relying on model atmospheres or evolutionary tracks for the mass itself. If the radius determinations are robust, the results provide a valuable empirical check on both general relativity in white dwarfs and the accuracy of standard mass determinations. The potential to extend the method to other clusters adds to its broader impact in stellar astrophysics.

major comments (2)

[Results section] Results section: The assertion that the systematic mass offset of 0.02–0.05 M⊙ is 'likely real' despite lying within the measurement uncertainty relies on the assumption that radius estimates from Gaia photometry are accurate to better than the 4% level at which they can differ. However, a 4% uncertainty in radius directly propagates to a comparable uncertainty in mass (∼0.03 M⊙ for a typical 0.7 M⊙ white dwarf), which is similar in magnitude to the reported offset. No quantitative assessment is provided to show that radius systematics do not contribute to or correlate with the observed difference.
[Conclusions section] Conclusions section: The claim that gravitational redshift in WDs agrees with the predictions of stellar evolution models to within a few percent requires a direct comparison between the measured M/R values and model-predicted M/R, including full error propagation from both the 5% gravitational redshift uncertainty and the radius uncertainty. The abstract does not detail this comparison or the resulting agreement metric.

minor comments (1)

[Abstract] Abstract: The statement that radii 'are in very good agreement' is immediately followed by the note that they 'can differ by up to 4%'; a specific quantitative metric (e.g., mean fractional difference or reduced chi-squared) for the agreement across the six stars would improve clarity.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for the constructive report and positive assessment of the work's significance. We address the two major comments below and will revise the manuscript accordingly to strengthen the presentation of our results.

read point-by-point responses

Referee: [Results section] Results section: The assertion that the systematic mass offset of 0.02–0.05 M⊙ is 'likely real' despite lying within the measurement uncertainty relies on the assumption that radius estimates from Gaia photometry are accurate to better than the 4% level at which they can differ. However, a 4% uncertainty in radius directly propagates to a comparable uncertainty in mass (∼0.03 M⊙ for a typical 0.7 M⊙ white dwarf), which is similar in magnitude to the reported offset. No quantitative assessment is provided to show that radius systematics do not contribute to or correlate with the observed difference.

Authors: We agree that the manuscript lacks a quantitative assessment of how radius systematics could contribute to or correlate with the observed mass offset. Although the offset appears systematic across the sample and the radii from independent methods agree to within 4%, a 4% radius uncertainty can indeed produce a mass difference of similar size. In the revised manuscript we will add an explicit error-propagation analysis and a sensitivity test to evaluate whether radius systematics could fully account for the offset. revision: yes
Referee: [Conclusions section] Conclusions section: The claim that gravitational redshift in WDs agrees with the predictions of stellar evolution models to within a few percent requires a direct comparison between the measured M/R values and model-predicted M/R, including full error propagation from both the 5% gravitational redshift uncertainty and the radius uncertainty. The abstract does not detail this comparison or the resulting agreement metric.

Authors: The referee correctly notes that the stated agreement with stellar-evolution models is not supported by a direct M/R comparison that includes full error propagation. The present text asserts agreement to within a few percent but does not show the metric or propagate the combined 5% redshift and radius uncertainties. We will revise both the conclusions section and the abstract to include this direct comparison and the resulting quantitative agreement level. revision: yes

Circularity Check

0 steps flagged

No significant circularity in the derivation chain

full rationale

The paper measures gravitational redshift directly from UVES spectra, subtracts independent astrometric radial velocities to isolate the M/R ratio, then multiplies by radii obtained from Gaia photometry and literature values to derive masses. These masses are compared to results from other methods and to stellar evolution model predictions. No parameters are fitted to the target masses or M/R values, no equations reduce to their inputs by construction, and no self-citations or ansatzes are invoked to justify the central measurement or the agreement claim. The derivation is an observational chain that remains independent of the reported outcomes.

Axiom & Free-Parameter Ledger

0 free parameters · 2 axioms · 0 invented entities

The measurement rests on two standard domain assumptions about cluster kinematics and photometric radius determination; no free parameters are introduced and no new physical entities are postulated.

axioms (2)

domain assumption The Hyades cluster has a low velocity dispersion that permits precise subtraction of the astrometric radial velocity from the spectroscopic shift.
Explicitly invoked in the context paragraph to justify isolation of the gravitational redshift.
domain assumption White dwarf radii can be reliably recovered from Gaia photometry to within a few percent.
Required to convert measured M/R into mass and to compare with other mass determinations.

pith-pipeline@v0.9.0 · 5889 in / 1446 out tokens · 50747 ms · 2026-05-25T10:54:15.918736+00:00 · methodology

discussion (0)

Reference graph

Works this paper leans on

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