Joan Najita

Binary stars are common in the Galaxy, and understanding how stellar binarity influences the formation and evolution of planetary systems is an active area of research. In this study, we use metal-enriched white dwarfs in wide binaries as tracers of long-lived planetary systems. With Data Release 1 from the Dark Energy Spectroscopic Instrument (DESI), we find that the fraction of cool metal-enriched white dwarfs in wide binaries is 9.8% ± 2.1%, significantly lower (4.7 σ ) than the 20.5% ± 0.9% in a control sample of single systems. Furthermore, we identify a tentative dependence of metal enrichment on projected separation and white dwarf effective temperature, where enrichment fraction decreases at smaller separations and lower temperatures. These findings indicate that, compared to single stars, binary systems either start with smaller initial planetary reservoirs due to suppressed planetesimal formation or undergo more rapid depletion of planetary material during the initial part of the white dwarf stage.

DESI observational data for the GD-1 and C-19 streams are compared to stream simulations in an evolving multihalo potential of a Milky Way–like galaxy based on a cosmological Milky Way–like simulation. The number of subhalos decreases with time, and the subhalo-stream encounter velocities rise as the galaxy and its disk build up their mass. The streams develop from star clusters inserted at ≃1 Gyr after the Big Bang and evolved for 13 Gyr to their current orbital positions. The measured velocity widths of the streams are compared to the matched simulations. Streams in a cold dark matter (CDM) subhalo population provide the best match to the velocity width, on average, with considerable scatter. Streams younger than ≃12 Gyr in CDM subhalos are insufficiently hot. Streams in the same potentials but with populations of warm dark matter (WDM) 5.5 keV subhalos are not, on average, heated to the observed velocity widths, although some of the realizations do reach the observed levels. The stream density power spectrum measured along the length of the DESI GD-1 sample agrees with the CDM stream simulations, with 1.3–2.3 times more power than WDM 7 and 5.5 keV simulations. The simulations show that modeling specific streams from the time of the formation of their progenitor clusters is both feasible and necessary to reproduce their stream-averaged kinematic properties.

RR Lyrae stars (RRLs) are valuable probes of both Milky Way assembly and stellar-evolution physics. Using a sample 6240 RRLs obtained in the first year of the Dark Energy Spectroscopic Instrument (DESI) survey, we investigate the metallicity of RRLs and its correlation with their pulsation properties. We find that (1) a clear correlation between period and [Fe/H] reinforces the view that the long-standing Oosterhoff dichotomy arises from the scarcity of intermediate-metallicity Galactic globular clusters hosting sizeable RRL samples; (2) high-amplitude short-period and small-amplitude short-period variables are comparatively metal-rich, with mean [Fe/H] = −1.39 ± 0.27 and −1.30 ± 0.28, respectively; (3) in double-mode pulsators (RRd), the metallicity declines smoothly with increasing fundamental-mode period, and anomalous RRd stars occupy a remarkably narrow [Fe/H] range relative to classical RRd stars; (4) this spectroscopic sample lets us, for the first time, place empirical constraints on the metallicity-dependent topology of the instability strip using phase-corrected effective temperatures and a large number of RRLs, where we observe an instability strip that moves towards cooler T _eff with declining [Fe/H] with a width roughly consistent with stellar-evolution models; and (5) a subset of metal-rich RRLs exhibits orbits consistent with disk membership and halo kinematics. Our results confirm the tantalizing potential of DESI for Galactic and stellar astrophysics and highlight the importance of the even larger samples of RRLs and data-processing improvements forthcoming in future DESI data releases.

The evolution and dispersal of protoplanetary disks—governed by accretion, magnetically launched jets and winds, and photoevaporative winds—fundamentally shape planetary systems. Determining how these mass-loss processes coevolve is crucial for constraining planet formation pathways. We analyze archival James Webb Space Telescope (JWST) Mid-Infrared Instrument (MIRI) integral field unit data of 72 inclined ( i > 40°), mostly Class II disks to identify and characterize spatially resolved jets and winds, focusing on [Ne II ] and H ₂ lines. Extended emission in H ₂ S(1), S(3), S(5), S(7), and/or [Ne II ] is detected toward 66 disks, revealing diverse morphologies. We develop a framework to identify conical H ₂ winds and high-velocity [Ne II ] jets perpendicular to the disk, detecting them toward 46 and 40 disks, respectively. All sources with [Ne II ] jets exhibit a corresponding wind traced in either H ₂ (85%) or [O I ], establishing a connection between jets and winds. The detection fractions of [Ne II ] jets and H ₂ winds correlate positively with mass accretion rate, with no dependence on disk inclination or stellar mass. Conversely, marginally resolved low-velocity [Ne II ] winds are found preferentially toward lower accretors. Among sources with H ₂ winds, detection of hotter winds traced by S(7) and S(5) declines more rapidly with decreasing accretion rate than that of the colder S(1) component. Comparison with high-resolution [O I ] 6300Å spectroscopy reveals [O I ] low-velocity components and extended H ₂ wind detections preferentially toward moderate-to-high accretors ( ≳ 1 0 − 8.5 M ̇ ⊙ yr − 1 ), whereas lower accretors exhibit only [O I ] and [Ne II ] winds. Together, these results indicate that atomic jets and atomic+molecular winds, consistent with an MHD disk-wind origin, dominate during early, actively accreting disk phases, while at lower accretion rates, jets weaken and winds become predominantly atomic.

Debris disks are likely created and sculpted by planetary bodies in the orbital space they share. The properties of these disks, including mass, orbital extent, and morphology, can be indicators of their planetary shepherds. Recently, T. D. Pearce and collaborators placed limits on the masses and orbits of hypothetical planets around 178 stars with debris disks. Here, we consider 176 of these stars, all objects that have astrometric data in the Gaia Data Release 3 archive, to assess planet detection from astrometry. Our analysis begins with a set of stellar hosts of known exoplanets, selected to have parallax, apparent magnitude, and color similar to the 176 debris disk systems. We confirm that Gaia’s ruwe parameter, a measure of the quality of astrometric fitting to a linear drift model, is sensitive to the presence of massive companions, even planetary ones. We also train a machine-learning model with a range of planet host properties from Gaia, considering more detailed astrometric data beyond ruwe that might indicate orbital motion. In addition, the model incorporates Gaia photometry to inform how brightness and stellar type affect detectability, with the risk that selection bias in the training set may affect reliability. Guided by ruwe and predictions of the machine-learning model, we identify stars with debris disks that may host as-yet-undiscovered planets. Overall, the model predictions, ruwe, and the mass limits from debris disk morphology do not show strong trends. Nonetheless, our top candidates with high ruwe and high planethood probability from the machine learning model will be compelling subjects for time‑series analyses with Gaia Data Release 4.

We present the first chemical survey of Class I and flat-spectrum (I/FS) disks using the JWST Mid-Infrared Instrument/Medium-Resolution Spectrometer, targeting 16 sources in the Ophiuchus star-forming region. Through empirical line luminosity measurements and multicomponent slab modeling, we characterize the molecular reservoir of these young systems and compare them to 12 Class II disks of similar stellar mass. Water, HCN, C ₂ H ₂ , and CO ₂ are frequently detected in I/FS sources with inclinations i < 70 ^∘ , whereas edge-on systems show significantly suppressed emission. Compared to Class II disks, I/FS sources show suggestive—though not yet statistically significant—evidence for elevated cold water (∼200 K) mass and lower CO ₂ excitation temperatures. Statistical analyses identify accretion luminosity as the primary correlate of molecular mass across both evolutionary stages. Once this dependence is removed, cold water and CO ₂ masses anticorrelate with millimeter-dust disk radius, while hot water remains insensitive to disk size. These patterns are qualitatively consistent with pebble drift models that predict early water enrichment followed by delayed CO ₂ delivery, suggesting an evolutionary progression from molecular-poor Class 0 sources, through water-rich Class I/FS disks, to Class II disks with reduced cold water excess. This work provides an initial evolutionary framework for disk chemistry that requires larger, multiregion samples to confirm.

Inner-disk chemistry offers a valuable window onto disk evolution and planet formation. Key planet formation processes, including pebble drift, planetesimal and planet formation, dust traps, and snowlines, determine the delivery rates of oxygen and carbon to the inner disk. As a result, measurements of molecules sensitive to the gas-phase C/O ratio, in disks over a range of ages, can constrain the relative importance and time evolution of these processes. Here we report JWST/MIRI observations of T Tauri disks in the young cluster IC 348, extending to older ages previous studies of disks. Like younger disks, IC 348 sources show a broad range of molecular ratios, indicating diverse planet formation histories. However, the similar HCN/H ₂ O and C ₂ H ₂ /H ₂ O flux ratio distributions of IC 348 (2–5 Myr old) and Taurus (1–2 Myr old) sources imply little to no evolution in the inner-disk C/O ratio over this time interval, even as disk masses decline by a factor of 5 on average. This result contradicts the general prediction of increasing C/O ratio with time in models of disk chemical evolution, indicating the need to better understand the interplay between (and efficiencies of) planet formation and disk evolution processes. We also find that the cold-to-warm water flux ratios show no evolution and do not correlate with the hydrocarbon-to-water ratios. These results suggest that most inner T Tauri disks are not dominated by rapid icy pebble drift and avoid the “meter-size barrier problem” once thought to be an obstacle to planet formation.

We present a sample of 88,959 K giants from Dark Energy Spectroscopic Instrument Milky Way Survey Year 3 data, which we use to characterize the chemo-dynamical properties of the stellar halo at Galactocentric distances of 12 to ∼100 kpc. Using HDBSCAN, we identify five prominent stellar halo substructures: Aleph, the Sagittarius stream, Gaia-Sausage-Enceladus (GSE), Cetus-Palca, and the Orphan–Chenab stream. We present the properties of each of these structures as they appear in our catalog, and examine how uncertainties on distance affect the characterization of substructure with this approach. We also examine regions associated with previously reported overdensities (such as the Virgo Overdensities and the Sagittarius spur) that we do not recover with HDBSCAN. The size and distance range of our catalog allows us to explore in detail the residual stellar halo, comprising stars that we do not associate with any substructure. We find that samples of ∼2000 outer halo stars with both highly prograde and highly retrograde angular momenta have similar metallicity distribution functions (MDFs), which do not resemble the MDFs of either GSE or Sagittarius. Both the prograde and retrograde residual halo MDFs are bimodal, with a metal-poor peak at [Fe/H] ∼ −2 and a metal-rich peak at [Fe/H] ∼ −1.3 (prograde) or −1.5 (retrograde). The MDF for lower-angular-momentum residual halo K giants does not show clear evidence for a metal-poor peak, and broadly resembles the MDF of GSE, even at much lower binding energies than GSE itself. We discuss possible interpretations of these findings for GSE accretion scenarios.

Stellar streams are the result of a host galaxy’s gravitational potential tidally disrupting satellite dwarf galaxies and globular clusters (GCs), causing them to grow leading and trailing tidal tails. The C-19 stellar stream is an extremely metal-poor stellar population, showing chemical abundance patterns characteristic of a GC. However, its large velocity dispersion is difficult to reconcile with a conventional, purely baryonic, disrupting-GC progenitor. Current techniques for stream characterization are primarily applied to Gaia DR3, relying heavily on proper-motion measurements. Using the Dark Energy Spectroscopic Instrument (DESI), which provides radial velocities and metallicities for over 10 million stars and reaches significantly fainter magnitudes than comparable surveys, we employ a mixture model approach to jointly characterize stream populations in proper motions, radial velocities, and metallicities against a Milky Way halo background. By applying this framework to the C-19 stellar stream, we identify a total of 47 spectroscopically confirmed member stars, of which 41 are newly identified and only 6 were previously reported in the literature. In this work, we measure a velocity dispersion of 7 . 8 − 1.3 + 1.5 km s ⁻¹ and a mean metallicity of [Fe/H] = − 3.3 6 − 0.10 + 0.12 . We further identify a novel “spur” feature within the stream. We conclude that our measurements are in line with previous works identifying C-19 as a “hot,” metal-poor stream. In forthcoming work, we will apply this approach to many more streams in the DESI footprint, enabling population-level comparisons with predictions from simulations.

Using 64,000 halo K giants from the Dark Energy Spectroscopic Instrument second Data Release (DR2), we decompose the Milky Way (MW) stellar halo between 3 and 160 kpc into metal-rich (MR) and metal-poor (MP) components via a Gaussian mixture model. The two populations are nearly equal in number but chemically and kinematically distinct: MR stars occupy highly radial orbits with velocity anisotropy of β ≈ 0.94 and metallicity dispersion σ _[Fe/H] ≈ 0.17 dex, without obvious dependence on distance, and are mainly contributed by Gaia-Sausage/Enceladus (GSE) debris. The MR component dominates the inner 30 kpc and reemerges beyond 50 kpc, implying GSE debris can extend to ∼70–80 kpc. MP stars exhibit a weaker radial bias of β ≈ 0.46, decreasing to −0.5 beyond 80 kpc, and with a larger metallicity dispersion of σ _[Fe/H] ≈ 0.46 dex, showing signatures of multiple minor mergers. Both components exhibit net prograde rotation at ∼10–30 kpc with a stronger azimuthal signal in the MP population. The nonequilibrium motions of the outer halo (>50 kpc) are quantified with a dipole-plus-contraction velocity field. We find that the outer halo is simultaneously contracting ( μ _compr = −19 km s ⁻¹ , distance-independent) and subject to reflex motions ( μ _dipole increases from −19 to −44 km s ⁻¹ with radius), reflecting the perturbation from the Large Magellanic Cloud (LMC). We also confirm a linear dependence of mean polar velocity for the outer stellar halo on μ _dipole , a direct consequence of the LMC and MW interaction. Our results provide a quantitative distance-resolved description of the MW’s last major accretion event and its ongoing response to the first infall of the LMC.