Sergey E. Koposov

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.

We present the catalog of RR Lyrae stars (RRLs) observed in the first year of operations of the Dark Energy Spectroscopic Instrument (DESI) survey. This catalog contains 6240 RRLs out to ∼120 kpc from the Galactic center and over 12,000 individual epochs with homogeneously derived stellar atmospheric parameters. We introduce a novel methodology to model the cyclical variation of the spectroscopic properties of RRLs from single-epoch measurements. We employ this method to infer the systemic velocities and mean temperatures of fundamental- and first-overtone-mode RRLs in our sample (without distinguishing between individual spectral lines). For fundamental-mode pulsators, we obtain radial velocity curves with amplitudes of ∼30–80 km s ⁻¹ and effective temperature curves with 300–1000 K variations, whereas for first-overtone pulsators these amplitudes are ∼20 km s ⁻¹ and ∼600 K, respectively. We use our sample to study the metallicity distribution of the halo and its dependence on Galactocentric distance ( R _GC ). Using a radius-dependent mixture model, we split the data into chemodynamically distinct components and find that our inner halo sample ( R _GC ≲ 50 kpc) is predominantly composed of stars with [Fe/H] ∼ −1.5 and largely radial orbits (with an anisotropy parameter β ∼ 0.94), which we associate with the Gaia–Sausage–Enceladus merger. Stars in the halo field exhibit a broader and more metal-poor [Fe/H] distribution with more circular orbits ( β ∼ 0.39). The metallicity gradient of the metal-rich and metal-poor components is found to be 0.005 and 0.010 dex kpc ⁻¹ , respectively. Our catalog highlights DESI’s tantalizing potential for studying the Milky Way and the pulsation properties of RRLs in the era of large spectroscopic surveys.

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.

We present results of a spectroscopic campaign around the diffuse dwarf galaxy Crater 2 (Cra2) and its tidal tails as part of the Southern Stellar Stream Spectroscopic Survey ( S ⁵ ). Cra2 is a Milky Way dwarf spheroidal satellite with extremely cold kinematics, but a huge size similar to the Small Magellanic Cloud, which may be difficult to explain within collisionless cold dark matter. We identify 143 Cra2 members, of which 114 belong to the galaxy’s main body, and 29 are deemed part of its stellar stream. We confirm that Cra2 is dynamically cold (central velocity dispersion 2.5 1 − 0.30 + 0.33 km s − 1 ) and also discover a ≈7 σ velocity gradient consistent with its tidal debris track. We separately estimate the stream’s internal velocity dispersion to be 5.7 4 − 0.83 + 0.98 km s − 1 . We develop a suite of N -body simulations with both cuspy and cored density profiles on a realistic Cra2 orbit to compare with S ⁵ observations. We find that the velocity dispersion ratio between the Cra2 stream and galaxy ( 2.3 0 − 0.35 + 0.41 ) is difficult to reconcile with a cuspy halo with fiducial concentration and an initial mass predicted by standard stellar mass–halo mass relationships. Instead, either a cored halo with a relatively small core radius or a low-concentration cuspy model can reproduce this ratio. Despite tidal mass loss, Cra2 is metal-poor (〈[Fe/H]〉 = −2.16 ± 0.04) compared to the stellar mass–metallicity relation for its luminosity. Other diffuse dwarf galaxies similar to Cra2 in the Local Group (Antlia 2 and Andromeda 19) also challenge galaxy formation models. Finally, we discuss possible formation scenarios for Cra2, including ram-pressure stripping of a gas-rich progenitor combined with tides.

Context . The formation, extension, and morphology of extra-tidal stars around globular clusters depend on the internal kinematics of the host cluster and the influence of the Galactic potential. Tracing the kinematics of faint tidal tails sheds light on their formation and contribution to the Milky Way halo assembly. Aims . NGC 7492 is an outer halo globular cluster with conflicting evidence regarding the presence of tidal tails. If present, the tails are expected to be faint and embedded in the Sagittarius tidal stream, located at a similar heliocentric distance but having different kinematics. Methods . We carried out a GIRAFFE/FLAMES spectroscopic follow-up of ten fields at the expected location of tidal tails associated with NGC 7492 to obtain the radial velocity component. Gaia parallaxes were used to remove foreground contaminants, while only loose constraints on Gaia proper motion were applied to select the targets. From the high-resolution spectra, radial velocities and metallicities were derived for more than 700 stars, from the red giant branch down to the upper main sequence. Results . Cluster and extra-tidal stars were identified based on their proper motions, radial velocities, and metallicities. This population extends over at least ∼1.8 deg from the cluster center, confirming the tidal tails previously detected only photometrically. The extra-tidal stars are located at positions consistent with spray-particle models for the cluster’s disruption. The Sagittarius stream is also clearly identified through its distinct proper motion distribution, radial velocities and more metal-rich population. Conclusions . Despite the low spatial density of extra-tidal stars, the kinematic signature of the cluster is clearly detected, confirming the presence of tidal tails overlapping on the sky with the Sagittarius stream, although the two structures are physically unrelated. The present spectroscopic dataset provides a robust basis for future studies aimed at extending the characterization of the tails, both in extension and limiting magnitude.

The paper DESI Data Release 1: Stellar Catalogue (https://doi.org/10.33232/001c.155260) by Koposov et al, published on January 12 2026, contained an error in Figure 15 concerning the DR2 results. The figure has been corrected in the version linked to on arXiv by this post and by the original article. The main conclusions of the paper are unchanged.

Mass functions are used in all areas of astrophysics. The stellar initial mass function (IMF), in particular, plays a central role in modeling stellar populations in galaxies. However, few dedicated tools for working directly with the IMF and its precursor functions are widely available. We present the imf package, a Python library integrated into the wider scientific Python ecosystem that implements common and variant forms of mass functions, especially the IMF and its pre- and protostellar equivalents, as probability distribution functions based on SciPy’s statistics architecture. This package enables the performance of operations such as sampling and integration on a wide array of highly customizable mass functions. imf is publicly available on the Python package index pypi under the project name initial_mass_function .

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.