High-Precision Differential Radial Velocities of C3PO Wide Binaries: A Test of Modified Newtonian Dynamics (MOND)

summary

The gist

Wide-binary stars, separated by thousands of AU, reside in low-acceleration regimes where Modified Newtonian Dynamics (MOND) predicts deviation from Newtonian gravity.

In short

Researchers measured differential radial velocities of 100 wide binary stars using high-resolution spectra to test Modified Newtonian Dynamics (MOND). The study achieved precision of 8–15 m s−1 per pair, significantly better than Gaia DR3. The results show tension with the standard MOND acceleration scale ($a_0$), depending on the specific MOND formulation used.

Key concepts

Differential Radial Velocity (RV): This measures the difference in how fast two stars are moving toward or away from Earth relative to each other. By comparing these velocities for wide binary stars, scientists can infer their orbital motions and test gravitational theories that predict different acceleration patterns.
Modified Newtonian Dynamics (MOND): MOND is a theory proposing that gravity behaves differently at very low accelerations, such as those found in the outer regions of galaxies. It suggests that the observed gravitational effects are not purely Newtonian but are modified by this new law, using a characteristic acceleration scale $a_0$.
Interpolating Function ($\mu(x)$): This mathematical function describes how gravity is modified in MOND. The study tests two forms: a simple one ($b=1$) and a standard one ($b=2$). This function relates the actual acceleration experienced by stars to the characteristic scale $a_0$.
$a_0$ (MOND Scale): This is the fundamental acceleration threshold in MOND. It represents the specific gravitational strength at which Newtonian gravity transitions into MOND's modified behavior. The study tests if the measured orbital parameters constrain this value, showing tension with accepted values.

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This episode discusses

The paper

High-Precision Differential Radial Velocities of C3PO Wide Binaries: A Test of Modified Newtonian Dynamics (MOND) · Read on arXiv

Department of Astronomy, The Ohio State University · Department of Astronomy and Center for Cosmology and AstroParticle Physics, The Ohio State University

Wide-binary stars, separated by thousands of AU, reside in low-acceleration regimes where Modified Newtonian Dynamics (MOND) predicts deviation from Newtonian gravity. However, Gaia radial velocities (RVs) lack the precision to resolve the small velocity differences expected in these systems, limiting previous MOND analyses to two-dimensional kinematics. In this paper, we introduce a technique to measure differential RVs of wide binary stars using high resolution, high signal-to-noise spectra. We apply this method to measure differential RVs of 85 wide-binaries from the C3PO survey and achieved precisions of about 8-15 m/s per binary pair, a about 10 - 100 times improvement (median about 24 times) over Gaia DR3. After applying a selection criterion based on the scaled velocity, we retain 57 gravitationally bound systems for further analysis. Combining these measurements with Gaia astrometry, we construct a hierarchical Bayesian model to infer the orbital elements of all wide-binary pairs and the global MOND acceleration scale (a 0). We test two commonly used interpolating functions in MOND formulation: the simple form (b=1, μ= x/(1+x)) and the standard form (b=2, μ= x/sqrt 1+x squared). Our results indicate tension with MOND at the presently accepted a 0 value: for b=1, the canonical value is excluded at 2.5σ, while for b=2, the exclusion is at 1.5σ. We discuss how systematic uncertainties in the external field effect treatment, particularly in the transition regime, may affect these conclusions.

Transcript

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

Vera: Today's paper: "High-Precision Differential Radial Velocities of C3PO Wide Binaries".

Jocelyn: Wide-binary stars, separated by thousands of AU, reside in low-acceleration regimes where Modified Newtonian Dynamics (MOND) predicts deviation from Newtonian gravity.

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

Title and authors: Vera: So we're looking at the paper titled "High-Precision Differential Radial Velocities of C3PO Wide Binaries: A Test of Modified Newtonian Dynamics (MOND)," and the authors are Serat Mahmud Saad and Yuan-Sen Ting from The Ohio State University. It sounds like they're focusing on taking wide binary stars, those separated by thousands of AU, to test if MOND makes a difference when you look at their movement.

Jocelyn: I agree, Vera; it’s interesting because this work tackles the precision issue we’ve had with Gaia data on these wide pairs. The title really emphasizes measuring differential radial velocities, which points to them moving beyond just knowing where these stars are to actually quantifying how fast they are moving relative to each other in a way that matters for MOND testing.

Subrahmanyan: From a theoretical standpoint, I find this compelling because wide binaries in those low-acceleration regimes are exactly where the predictions of MOND deviate from standard Newtonian gravity, and testing this environment in the solar neighborhood is a crucial step before we apply it to galactic rotation curves.

Vera: Exactly, Subrahmanyan; these wide binaries are ideal testbeds because their gravitational acceleration falls right into that a zero scale that MOND predicts should matter, and the paper sets up a specific test by using high-resolution spectra for this investigation <ref:2512.19652#pg0>.

Jocelyn: And they're using data from the C3PO survey to get these measurements, which is a significant step up from what we’ve had before with Gaia's radial velocities, which the authors note lack the precision needed for these small velocity differences.

Subrahmanyan: That precision gap is exactly what opens up the possibility of testing MOND against a canonical acceleration scale a zero in a regime where dark matter effects are expected to be negligible <ref:2512.19652#pg1>.

The paper's summary: Vera: What the paper actually does is introduce a specific technique to measure these differential radial velocities between wide binary components by using high resolution, high signal-to-noise spectra, which they achieved for one hundred wide binaries from the C3PO survey <ref:2512.19652#pg0>. They managed to get precisions of about eight to fifteen m s−one per binary pair.

Jocelyn: That precision level is what really stands out because the paper states it’s about a ten to one hundred times improvement over Gaia DR3, which is a huge jump for resolving the subtle kinematic signatures they're aiming for in MOND <ref:2512.19652#pg0>.

Subrahmanyan: The core of their research involves applying this differential velocity measurement to test MOND formulations, specifically comparing the predictions of different forms of the interpolating function mu(x), like b=one and b=two <ref:2512.19652#pg0>.

Vera: And they use a hierarchical Bayesian model to simultaneously infer both the MOND parameter a zero and all six Keplerian orbital elements for each binary system, which includes stellar masses and semi-major axes <ref:2512.19652#pg2>.

Jocelyn: That modeling approach is sophisticated because it tries to fit together the observed projected separations, the differential radial velocity measurements, and even differential proper motion to constrain everything at once.

Subrahmanyan: They are also incorporating the external gravitational field from a galaxy into their acceleration equation using a modified effective acceleration equation that accounts for both internal and external accelerations based on an angle theta.

The paper's improvements: Vera: One of the main methodological improvements they introduce is this forward-modeling approach for echelle spectroscopy, which helps account for the pixel-integrated nature of the spectra to measure differential RVs and then combining data across about thirty to fifty valid echelle orders.

Jocelyn: That combination allows them to reach those impressive eight to fifteen m s−one per binary pair precisions, which is what they claim is a median improvement of about twenty-four times over Gaia DR3 <ref:2512.19652#pg0>.

Subrahmanyan: The modeling itself involves testing two specific MOND interpolating functions: the "simple" form where b=one and the "standard" form where b=two <ref:2512.19652#pg0>. These two choices give different results for how they constrain the acceleration scale a zero <ref:2512.19652#pg2>.

Vera: They also introduce a hierarchical Bayesian model that lets them infer all those orbital elements and masses at the same time, using priors specifically chosen so that the canonical MOND value of ten a zero about-nine point nine two sits near the middle of their range <ref:2512.19652#pg2>.

Jocelyn: They are also testing how robust these results are by checking prior sensitivity, showing consistent exclusion levels across different prior ranges, which adds a layer of confidence to their findings on a zero <ref:2512.19652#pg2>.

Subrahmanyan: The analysis is also careful about how they handle the external field effect; they use an effective acceleration equation that includes terms for internal and external accelerations based on the relative position of the stars.

Conclusion: Vera: So, to wrap up this paper on "High-Precision Differential Radial Velocities of C3PO Wide Binaries: A Test of Modified Newtonian Dynamics (MOND)," they found tension with MOND at the presently accepted a zero value depending on whether they used the simple b=one or standard b=two interpolating function <ref:2512.19652#pg2>.

Jocelyn: That tension is specific to each interpolation choice; for instance, when using b=one the canonical MOND value of ten a zero = -nine point nine two sits at the 99 point 8th percentile, excluded at three point one sigma.

Subrahmanyan: And when they use b=two the results are different; for that form, the canonical value lies at the 94 point 5th percentile, which is excluded at about one point nine sigma <ref:2512.19652#pg0>. This shows we can't just take one number and apply it universally across all astrophysical systems without considering this degeneracy between different MOND formulations <ref:2512.19652#pg0>.

Vera: It really highlights how much the precise kinematic data from these wide binaries is influencing our understanding of the acceleration scale a zero in a way that depends on the model we use to describe MOND itself <ref:2512.19652#pg2>.

Jocelyn: I think what’s most exciting here is that this level of precision forces us to confront whether there’s an environmental dependency for a zero or if we just need a different formulation of MOND for systems in external fields <ref:2512.19652#pg2>.

Subrahmanyan: I agree; the tension between the two forms suggests that constraints on a zero from different astrophysical systems might not be directly comparable without accounting for this degeneracy, which is a necessary caution when building our cosmic models <ref:2512.19652#pg2>.

Vera: It’s a fascinating result, and I think we're just getting started with how these high-precision kinematic tests are going to shape the next generation of cosmological tests.

Jocelyn: Definitely; we need to see what other observations can help resolve this tension, and then we can keep pushing these limits on precision.

Subrahmanyan: I look forward to seeing how this paper moves the discussion forward regarding these kinematic constraints.

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