Gregory David Martinez

We report the first proper motion and acceleration measurements of the infrared (IR) counterpart to Sagittarius A* (Sgr A*-IR), the supermassive black hole at the center of our Galaxy, in the Gaia Celestial Reference Frame 3 (Gaia-CRF3). This reference frame realizes the International Celestial Reference System, which is an absolute reference coordinate system defined by quasars. A combination of Gaia and Hubble Space Telescope data was used to transform Keck adaptive optics observations into Gaia-CRF3. We developed a method for selecting reference stars that minimizes astrometric transformation errors (statistical error = 0.10–0.63 mas) and drift of the coordinate system (systematic error ∼ 0.01 mas yr ⁻¹ ). We find the proper motion of Sgr A*-IR in Gaia-CRF3 to be μ α * = − 3.093 ± 0.085 mas yr ⁻¹ and μ _δ = −5.62 ± 0.13 mas yr ⁻¹ with the initial position at t ₀ = 2016.0 of R.A. = 266 . ° 41680848 ± 0 . ° 00000029 and decl. = −29 . ° 00783947 ± 0 . ° 00000050, which translates to a precision of 1.05 mas in R.A. and 1.79 mas in decl. This is consistent with the astrometric measurements of the radio counterpart to Sgr A* by S. Xu et al. We also place a 2 σ upper constraint of the acceleration of Sgr A*-IR on the sky at 0.061 mas yr ⁻² . This acceleration limit on Sgr A*-IR excludes any intermediate-mass black hole companion with mass ≳ 4 × 10 ⁴ M _⊙ within a distance of ∼0.01 pc, consistent with previous studies. With the release of Gaia Data Release 4, we predict these limits will be improved by at least a factor of 2.

We report the binary fraction of Wolf–Rayet (W-R) stars within 0.5 pc of the Galactic center obtained through the longest time-baseline (1994–2024) kinematic study of this population of stars. The new radial velocity (v_z) data we present are primarily from the W. M. Keck Observatory, with additional v_z measurements from Gemini North Observatory. When combining our new v_z measurements with literature measurements, we find v_z variations that suggest the presence of a companion for five out of 27 W-R stars, of which two are newly identified here (IRS 13E4, S8-181), along with three previously detected binaries (IRS 16SW, IRS 16NE, S4-258). Based on our experimental sensitivity and expected properties of the underlying population, we infer the binary fraction of the W-R stars in the Galactic center to be 0.56 ± 0.18. This is consistent with previous photometric studies of the young stars in the Galactic center and with the binary fraction of field W-R stars. When our results are combined with the results of previous photometric work, we find a binary fraction of 0.69 ± 0.17 for the W-R stars.

Measuring stellar motions at the Milky Way’s Galactic center (GC) provides unique insight into the dynamical processes within galactic nuclei. We present proper-motion measurements for 23 SiO-maser-emitting stars within 45″ of SgrA*, including four previously reported to have velocities exceeding their local escape velocities (i.e., they are “locally unbound” from the GC). Derived from 14 epochs of Hubble Space Telescope WFC3-IR observations (2010–2023), our measurements have a median precision of 0.038 mas yr ⁻¹ —up to ∼100x more precise than previous constraints for some sources. By combining these proper motions with published radial velocities, we derive updated 3D velocities for the masers and find that only one is locally unbound (IRS 9; v _3d = 370 ± 1.2 km s ⁻¹ ). Orbit integrations place the first constraints on the orbit of IRS 9, which is bound to the GC at larger radii with r _peri ≥ 0.100 ± 0.005 pc and r _apo ≥ 5.25 ± 0.18 pc. IRS 9’s high velocity relative to stars at similar radii in the Nuclear Star Cluster makes it a candidate to have experienced a strong dynamical interaction in order to place it on its orbit. We explore the Hills mechanism as a possible origin, but binary evaporation and ejection velocity limits indicate that IRS 9 is unlikely to have experienced such an event in the past 0.4 Myr (the timescale constrained by the orbit integrations). Alternative mechanisms to produce IRS 9 include binary supernova disruption, two-body interactions, and stellar collisions. Identifying additional stars like IRS 9 will be essential for understanding these various dynamical processes.

We present the first high-precision proper-motion catalog, tied to the International Celestial Reference System (ICRS), of infrared astrometric reference stars within R ≤ 25″ (1 pc) of the central supermassive black hole at the Galactic center (GC). This catalog contains ∼2900 sources in a highly extinguished region that is inaccessible via Gaia. New astrometric measurements are extracted from Hubble Space Telescope (HST) observations (14 epochs, 2010–2023) and transformed into the ICRS using 40 stars in common with Gaia-DR3. We implement a new method for modeling proper motions via Gaussian processes that accounts for systematic errors, greatly improving measurement accuracy. Proper-motion and position measurements reach precisions of ∼0.03 mas yr⁻¹ and ∼0.11 mas, respectively, representing a factor of ∼20 improvement over previous ICRS proper-motion catalogs in the region. These measurements define a novel HST–Gaia reference frame that is consistent with Gaia-CRF3 to within 0.025 mas yr⁻¹ in proper motion and 0.044 mas in position, making it the first ICRS-based reference frame precise enough to probe the distribution of extended mass within the orbits of stars near SgrA*. In addition, HST-Gaia provides an independent test of the radio measurements of stellar masers that form the basis of current GC reference frames. We find that the HST–Gaia and radio measurements are consistent to within 0.041 mas yr⁻¹ in proper motion and 0.54 mas in position at 99.7% confidence. Gaia-DR4 is expected to reduce the HST–Gaia reference-frame uncertainties by another factor of ∼2, further improving the reference frame for dynamical studies.

In this work, we measure the spectral index of Sagittarius A* (Sgr A*) between the H (1.6 μm) and K ′ (2.2 μm) broadband filters in the near-infrared (NIR), sampling over a factor ∼40 in brightness, the largest range probed to date by a factor ∼3. Sgr A*-NIR is highly variable, and studying the spectral index α (with F _ν ∝ ν ^α ) is essential to determine the underlying emission mechanism. For example, variations in α with flux may arise from shifts in the synchrotron cutoff frequency, changes in the distribution of electrons, or multiple concurrent emission mechanisms. We investigate potential variations of α H − K ′ with flux by analyzing seven epochs (2005–2022) of Keck Observatory imaging observations from the Galactic Center Orbits Initiative. We remove the flux contribution of known sources confused with SgrA*-NIR, which can significantly impact color at faint flux levels. We interpolate between the interleaved H and K ′ observations using multi-output Gaussian processes. We introduce a flexible empirical model to quantify α variations and probe different scenarios. The observations are best fit by an α H − K ′ = − 0.50 ± 0.08 stat ± 0.17 sys that is constant from ∼1 mJy to ∼40 mJy (dereddened 2 μm flux). We find no evidence for a flux dependence of Sgr A*'s intrinsic spectral index. In particular, we rule out a model explaining NIR variability purely by shifts in the synchrotron cutoff frequency. We also constrain the presence of redder, quiescent emission from the black hole, concluding that the dereddened 2 μm flux contribution must be ≤0.3 mJy at 95% confidence level.

The 1RXS J034231.8+121622 system consists of an M dwarf primary and a directly imaged low-mass stellar companion. We use high-resolution spectroscopic data from Keck/KPIC to estimate the objects' atmospheric parameters and radial velocities (RVs). Using PHOENIX stellar models, we find that the primary has a temperature of 3460 ± 50 K and a metallicity of 0.16 ± 0.04, while the secondary has a temperature of 2510 ± 50 K and a metallicity of 0.13 − 0.11 + 0.12 . Recent work suggests this system is associated with the Hyades, giving it an older age than previous estimates. Both metallicities agree with current Hyades [Fe/H] measurements (0.11–0.21). Using stellar evolutionary models, we obtain significantly higher masses for the objects, 0.30 ± 0.15 M _⊙ and 0.08 ± 0.01 M _⊙ (84 ± 11 M _Jup), respectively. Using the RVs and a new astrometry point from Keck/NIRC2, we find that the system is likely an edge-on, moderately eccentric ( 0.41 − 0.08 + 0.27 ) configuration. We also estimate the C/O ratio of both objects using custom grid models, obtaining 0.42 ± 0.10 (primary) and 0.55 ± 0.10 (companion). From these results, we confirm that this system most likely went through a binary star formation process in the Hyades. The significant changes in this system's parameters since its discovery highlight the importance of high-resolution spectroscopy for both orbital and atmospheric characterization of directly imaged companions.

The eccentricity of a substellar companion is an important tracer of its formation history. Directly imaged companions often present poorly constrained eccentricities. A recently developed prior framework for orbit fitting called “observable-based priors” has the advantage of improving biases in derived orbit parameters for objects with minimal phase coverage, which is the case for the majority of directly imaged companions. We use observable-based priors to fit the orbits of 21 exoplanets and brown dwarfs in an effort to obtain the eccentricity distributions with minimized biases. We present the objects’ individual posteriors compared to their previously derived distributions, showing in many cases a shift toward lower eccentricities. We analyze the companions’ eccentricity distribution at a population level, and compare this to the distributions obtained with the traditional uniform priors. We fit a Beta distribution to our posteriors using observable-based priors, obtaining shape parameters α = 1.09 − 0.22 + 0.30 and β = 1.42 − 0.25 + 0.33 . This represents an approximately flat distribution of eccentricities. The derived α and β parameters are consistent with the values obtained using uniform priors, though uniform priors lead to a tail at high eccentricities. We find that separating the population into high- and low-mass companions yields different distributions depending on the classification of intermediate-mass objects. We also determine via simulation that the minimal orbit coverage needed to give meaningful posteriors under the assumptions made for directly imaged planets is ≈15% of the inferred period of the orbit.

Sagittarius A* (Sgr A*) is the variable radio, near-infrared (NIR), and X-ray source associated with accretion onto the Galactic center black hole. We have analyzed a comprehensive submillimeter (including new observations simultaneous with NIR monitoring), NIR, and 2–8 keV data set. Submillimeter variations tend to lag those in the NIR by ∼30 minutes. An approximate Bayesian computation fit to the X-ray first-order structure function shows significantly less power at short timescales in the X-rays than in the NIR. Less X-ray variability at short timescales, combined with the observed NIR–X-ray correlations, means the variability can be described as the result of two strictly correlated stochastic processes, the X-ray process being the low-pass-filtered version of the NIR process. The NIR–X-ray linkage suggests a simple radiative model: a compact, self-absorbed synchrotron sphere with high-frequency cutoff close to NIR frequencies plus a synchrotron self-Compton scattering component at higher frequencies. This model, with parameters fit to the submillimeter, NIR, and X-ray structure functions, reproduces the observed flux densities at all wavelengths, the statistical properties of all light curves, and the time lags between bands. The fit also gives reasonable values for physical parameters such as magnetic flux density B ≈ 13 G, source size L ≈ 2.2R _S , and high-energy electron density n _e ≈ 4 × 10⁷ cm⁻³. An animation illustrates typical light curves, and we make public the parameter chain of our Bayesian analysis, the model implementation, and the visualization code.