Cheyanne Shariat

The dependence of the stellar initial mass function (IMF) on star formation environment, particularly at low metallicities and high redshifts, remains poorly constrained. Ultrafaint dwarf galaxies (UFDs) are local fossils of high-redshift galaxies hosting old, metal-poor populations, and their resolved stellar populations provide unique pathways to constrain the subsolar IMF. We investigate the low-mass IMF in the Boötes I (Boo I) UFD with JWST/NIRCam, leveraging its capability to resolve over 10,000 stars reaching ≲0.15 M _⊙ , obtaining one of the largest, deepest resolved stellar samples for UFDs. We explore three different functional forms of the IMF with machine learning and statistical techniques, combining forward modeling of synthetic color–magnitude diagrams with simulation-based inference. We find that a single power-law IMF provides a poorer description of the observed luminosity function and yields a slope inconsistent with the canonical Salpeter IMF. Our best-fit broken power-law and log-normal IMF parameters are consistent with the Milky Way within 68% confidence level, providing evidence that star formation at metallicities as low as [Fe/H] ≈ −2.4 follows a similar IMF as in the Milky Way. By treating Boo I as a local relic analogous to a high-redshift galaxy with a stellar mass of ≲10 ⁵ M _⊙ at z ≳ 6, our results provide evidence for the universality of the IMF across both local and high-redshift environments.

Close white dwarf (WD) + main-sequence (MS) binaries are end products of mass transfer (MT) that occurred prior to WD formation, making their population demographics a powerful probe of binary evolution. Several recent works have constructed samples of WD+MS binaries with well-understood selection functions using data from wide-field surveys. These include (a) AU-scale astrometric binaries from Gaia that can be shown to contain a WD on dynamical grounds, (b) AU-scale astrometric binaries in which a hot WD is detected through a GALEX UV excess, and (c) close binaries discovered through eclipses. Together, these samples probe outcomes of both stable MT and common-envelope evolution, and interactions on both the red giant branch (RGB) and asymptotic giant branch (AGB). We forward model the three observed samples simultaneously. This approach produces robust constraints on uncertain binary evolution parameters because binaries removed from one population are predicted to appear in another. Our modeling includes a realistic initial binary population and treatments of the selection effects affecting all samples. We confirm that MT from AGB donors requires a critical accretor-to-donor mass ratio of ∼0.4 as found in previous work, and find that this is more stable than MT from RGB donors, for which we constrain a critical ratio ≳0.65. A common envelope efficiency of αλ ∼ 0.3 matches the relative numbers of close and wide systems and the period distribution of close systems. Most stable MT products in the sample, including those with RGB donors, retain nonzero eccentricities (≃0.1). The model does not fully reproduce the mass distribution of main-sequence stars in post-common envelope binaries, which shows a cliff below the fully convective limit, possibly pointing to missing physics that may warrant future work.

A small but growing class of binaries containing hot ( T _eff ∼ 10 ⁵ K) white dwarfs (WDs) and rapidly rotating, apparently subgiant companions—including the prototype, A35—show companions that are too large and luminous to be ordinary main-sequence stars yet too numerous to be explained as finely tuned near-twin binaries. We argue that these stars are instead main-sequence accretors temporarily inflated out of thermal equilibrium by recent mass transfer. For the subgiant of A35, a new Gaia DR3 astrometric orbit ( P _orb = 790 days) combined with updated photometric and spectroscopic constraints yield T _eff ≈ 4925 ± 75 K, R ≈ 3 ± 0.05 R _⊙ , near-solar metallicity, and rapid rotation aligned with the orbit ( v _rot ≈ 195 ± 3 km s ⁻¹ ), indicating substantial recent accretion and spin-up. Dynamical mass limits disfavor a coeval twin-binary origin, supporting the inflated-accretor interpretation. We test this scenario using self-consistent Modules for Experiments in Stellar Astrophysics binary evolution calculations with a new accretion prescription in which accreted material retains a fraction of its infall energy, against the default of setting its entropy to that of the accretor’s surface. The accretor expands to giant-like radii when M ̇ is high yet remains within its Roche lobe, allowing stable mass transfer even for mass ratios traditionally considered unstable. After mass transfer ceases, the star contracts on Myr timescales through a bloated, rapidly rotating phase whose temperatures, radii, and spins match those observed in A35–type systems. This framework naturally explains the population and unifies A35-type binaries with post-AGB binaries, blue lurkers, and wide WD+main-sequence systems as successive stages of the same post-mass-transfer evolutionary pathway.

Approximately one-third of luminous pulsating red giant stars exhibit long secondary periods (LSPs): stable photometric variability with periods of several months to years in addition to their much shorter primary pulsation cycles. Now nearly a century after their discovery, the physical origin of LSPs remains unresolved. A leading explanation invokes binarity, in which the LSP corresponds to the orbital period of a low-mass companion responsible for both the photometric variability and the radial-velocity (RV) modulation. We test this hypothesis using a nearby sample of LSP stars from the Gaia Focused Product Release, which provides multi-epoch RVs and contemporaneous optical photometry. We find that interpreting the observed RV variability as orbital motion implies companion masses narrowly distributed around M ₂ ≈ 0.1 M _⊙ with separations of 1–3 au, placing them squarely in the brown dwarf desert observed around their solar-type progenitors. Assuming such companions exist, we then forward-model the astrometric signature expected in Gaia DR3 and predict systematically elevated astrometric wobble for nearby LSPs. In contrast, the observed astrometric wobble of nearby LSP stars is systematically lower than these predictions and consistent with most systems exhibiting LSPs being single. This discrepancy disfavors low-mass stellar or substellar companions as the dominant origin of LSPs in evolved stars, motivating a further exploration of alternative stellar mechanisms.

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The detection of fast radio bursts (FRBs) in both young and old stellar populations suggests multiple formation pathways, beyond just young magnetars from core-collapse supernovae (SNe). A promising delayed channel involves the formation of FRB-emitting neutron stars through the merger- or accretion-induced collapse of a massive white dwarf (WD). By simulating a realistic stellar population with both binaries and triples, we identify pathways to WD collapse that could produce FRB candidates. We find that: (i) triple dynamics open new merger channels inaccessible to isolated binaries, significantly enhancing the overall merger rate; (ii) triple-induced mergers broaden the delay-time distribution, producing long-delay (≳1–8 Gyr) events largely independent of metallicity, alongside a shorter-delay population (≲100 Myr) of rapid mergers; (iii) these long delays naturally yield FRBs in older environments, such as quiescent host galaxies and galactic halos; (iv) when convolved with the cosmic star formation history, binary channels track the star formation rate ( z _peak ∼ 2), while triple channels peak later ( z _peak ∼ 1), giving a combined local source rate of R ₀ ≈ 2 × 10 ⁴ Gpc ⁻³ yr ⁻¹ , consistent with observations; and (v) applying the same framework to SNe Ia, we find that triples extend the delay-time tail and roughly double the SNe Ia efficiency relative to binaries, yielding rates and redshift evolutions in good agreement with observations. If FRBs originate from the collapse of WDs, our results establish triples, alongside binaries, as a crucial and previously overlooked formation pathway whose predicted rates, host demographics, and redshift evolution offer clear tests for upcoming surveys.

Common-envelope evolution (CEE) is among the most uncertain phases in binary evolution. To empirically constrain CEE, we construct a uniformly selected sample of eclipsing post–common-envelope binaries (PCEBs). Starting from an unresolved white dwarf–main-sequence (WDMS) candidate sample within 200 pc selected from the Gaia color-magnitude diagram, we identify 39 detached eclipsing WDMS binaries using Zwicky Transient Facility light curves. The binaries contain cool M dwarfs orbiting warm white dwarfs with orbital periods ( P _orb ) of 0.1–2 days. The sample’s simple selection function allows us to model observational incompleteness and infer intrinsic properties of the PCEB population. We find an orbital-period distribution consistent with being log-uniform over 0.1–2 days, contrary to recent reports of a bimodal distribution. The intrinsic companion-mass distribution peaks around 0.25 M _⊙ and declines steeply toward larger masses. The estimated local space density is 7.2 × 10 ⁻⁵ pc ⁻³ , corresponding to a Galaxy-wide birth rate of 0.01 yr ⁻¹ . Combining our results with recent Gaia-based constraints on wider WDMS binaries, we construct an empirical period distribution of post-interaction WDMS binaries spanning 0.1–1000 days. The emerging period distribution is roughly log-flat ( d N / d log P orb ∝ P orb 0 ) at P _orb < 2 days and log-increasing ( d N / d log P orb ∝ P orb 1 ) at P _orb = 100–1000 days. The 10–100 days regime is poorly constrained by existing surveys, but a few nearby systems suggest that it is also well-populated. Short-period PCEBs ( P _orb ≲ 2 days) with M dwarf companions are roughly 2–3 times more common than wide ( P _orb = 100–1000 days) WDMS binaries with FGK companions, which likely formed through stable mass transfer. These results provide direct observational constraints on CEE and an empirical benchmark for binary-population models.

Ultracompact X-ray binaries (UCXBs) are a subclass of low-mass X-ray binaries characterized by tight orbits and degenerate donors, which pose significant challenges to our understanding of their formation. Recent discoveries of black hole (BH) candidates with main-sequence or red giant companions suggest that BH–white dwarf (BH–WD) binaries are common in the Galactic field. Motivated by these observations and the fact that most massive stars are born in triples, we show that wide BH–WD systems can naturally form UCXBs through the eccentric Kozai–Lidov (EKL) mechanism. Notably, EKL-driven eccentricity excitations combined with gravitational wave (GW) emission and WD dynamical tides can effectively shrink and circularize the orbit, leading to mass-transferring BH–WD binaries. These systems represent promising multimessenger sources in both X-ray and GW observations. Specifically, we predict that the wide triple channel can produce ∼3–27 (∼1–5) detectable UCXBs in the Milky Way (Andromeda galaxy), including ∼1 system observable by the mHz GW detection of LISA. If the final WD mass can reach sufficiently small values, this channel could contribute up to ∼10 ³ UCXBs in the Galaxy. Furthermore, the identification of tertiary companions in observed UCXBs would provide direct evidence for this formation pathway and yield unique insights into their dynamical origins.

Precise stellar ages are fundamental to Galactic archaeology. However, obtaining reliable age estimates and uncertainties for field stars has been a long-standing challenge. We test the fidelity of ages from recent catalogs of giants and subgiants using wide binaries, whose components formed at the same time and thus should have consistent inferred ages. We find that subgiant ages based on spectroscopic metallicities from Xiang & Rix (2022) are generally consistent within their reported uncertainties, implying that fractional uncertainties of 5-10% are realistically achievable. In contrast, we find that published photometric subgiant ages underestimate true uncertainties by factors of 2-3. Spectroscopic age estimates for red giant and red clump stars also show reliable uncertainties, but are generally less precise (25-30%). These results demonstrate that accurate metallicity and $α$-element abundances are essential for precise subgiant ages and establish wide binaries as a powerful, model-independent benchmark for calibrating stellar age measurements in the era of large spectroscopic surveys.

We report the discovery and characterization of a wide binary population in the ultrafaint dwarf galaxy Boötes I using deep JWST/NIRCam imaging. Our sample consists of 52 candidate binaries with projected separations of 7000–16,000 au and stellar masses from near the hydrogen-burning limit to the main-sequence turnoff (∼0.1–0.8 M _⊙ ). By forward-modeling selection biases and chance alignments, we find that 1.25% ± 0.25% of Boötes I stars are members of wide binaries with separations beyond 5000 au. This fraction, along with the distributions of separations and mass ratios, matches that in the Solar neighborhood, suggesting that wide binary formation is largely insensitive to metallicity, even down to [Fe/H] ≈ −2.5. The observed truncation in the separation distribution near 16,000 au is well explained by stellar flyby disruptions. We also discuss how the binaries can be used to constrain the galaxy’s dark matter properties. We show that our detection places new limits on primordial black hole dark matter, finding that compact objects with M ≳ 5 M _⊙ cannot constitute more than ∼1% of the dark matter content. In contrast to previous work, we find that wide binaries are unlikely to provide robust constraints on the dark matter profile of ultrafaint galaxies given the uncertainties in the initial binary population, flyby disruptions, and contamination from chance alignments. These findings represent the most robust detection of wide binaries in an external galaxy to date, opening a new avenue for studying binary star formation and survival in extreme environments.