Red Giant Research Articles

An alternative formation scenario for uranium-rich giants: engulfing an Earth-like planet

ABSTRACT The actinides, such as the uranium (U) element, are typically synthesized through the rapid neutron-capture process (r-process), which can occur in core-collapse supernovae or double neutron star mergers. There exist nine r-process giant stars exhibiting conspicuous U abundances, commonly referred to as U-rich giants. However, the origins of these U-rich giants remain ambiguous. We propose an alternative formation scenario for these U-rich giants whereby a red giant (RG) engulfs an Earth-like planet. To approximate the process of an RG engulfing an Earth-like planet, we employ an accretion model wherein the RG assimilates materials from said planet. Our findings demonstrate that this engulfment event can considerably enhance the presence of heavy elements originating from Earth-like planets on the surfaces of very metal-poor stars (Z = 0.00001), while its impact on solar-metallicity stars is comparatively modest. Importantly, the structural and evolutionary properties of both very metal-poor and solar-metallicity stars remain largely unaffected. Notably, our engulfment model effectively accounts for the observed U abundances in known U-rich giants. Furthermore, the evolutionary trajectories of U abundances on the surfaces of RGs subsequent to the engulfment of Earth-like planets encompass all known U-rich giants. Therefore, it is plausible that U-rich giants are formed when an RG engulfs an Earth-like planet.

An in situ study of presolar grains and the fine‐grained matrices of the Meteorite Hills 00526 and Queen Alexandra Range 97008 unequilibrated ordinary chondrites

AbstractHere we report in situ structural and chemical analyses of four presolar grains and the matrices of the Meteorite Hills (MET) 00526 L3.05 and Queen Alexandra Range (QUE) 97008 L3.05 unequilibrated ordinary chondrites (UOCs). The presolar grains in MET 00526 include one Fe‐rich single crystal olivine, and one olivine grain that contains both amorphous and polycrystalline material. The single crystal olivine likely has origins in the circumstellar envelope (CSE) of a red giant branch (RGB) or asymptotic giant branch (AGB) star, and the amorphous/polycrystalline olivine has an O‐isotopic composition consistent with origins in a type II supernova. The presolar grains from QUE 97008 are Fe rich and include one crystalline, stoichiometric olivine that contains a Ca‐rich core and one crystalline, stoichiometric pyroxene grain, both of which have O‐isotopic compositions consistent with origins in the CSEs of low‐mass AGB/RGB stars. The matrices of both UOCs are mineralogically diverse with evidence for unaltered material in the form of amorphous silicates and a C‐rich nanoglobule and altered material in the form of Ni‐rich sulfides, abundant Fe‐rich olivine, and Fe‐Mg zoning in matrix silicates. No phyllosilicates were observed. The Fe‐rich olivine grains are the dominant alteration phase in both UOCs and likely replaced primary amorphous silicates in the presence of an Fe‐rich fluid during parent body alteration. Our work suggests that the ordinary and carbonaceous chondrites received a similar inventory of dust with comparable structures and chemistries.

Linear and non-linear properties of the Goldreich–Schubert–Fricke instability in stellar interiors with arbitrary local radial and latitudinal differential rotation

ABSTRACT We investigate the linear and non-linear properties of the Goldreich–Schubert–Fricke (GSF) instability in stellar radiative zones with arbitrary local (radial and latitudinal) differential rotation. This instability may lead to turbulence that contributes to the redistribution of angular momentum and chemical composition in stars. In our local Boussinesq model, we investigate varying the orientation of the shear with respect to the ‘effective gravity’, which we describe using the angle ϕ. We first perform an axisymmetric linear analysis to explore the effects of varying ϕ on the local stability of arbitrary differential rotations. We then explore the non-linear hydrodynamical evolution in three dimensions using a modified shearing box. The model exhibits both diffusive GSF instability and a non-diffusive instability that occurs when the Solberg-Høiland criteria are violated. We observe the non-linear development of strong zonal jets (‘layering’ in the angular momentum) with a preferred orientation in both cases, which can considerably enhance turbulent transport. By varying ϕ, we find instability with mixed radial and latitudinal shears transports angular momentum more efficiently (particularly if adiabatically unstable) than cases with purely radial shear (ϕ = 0). By exploring the dependence on box size, we find the transport properties of the GSF instability to be largely insensitive to this, implying we can meaningfully extrapolate our results to stars. However, there is no preferred length-scale for adiabatic instability, which therefore exhibits strong box-size dependence. These instabilities may contribute to the missing angular momentum transport required in red giant and subgiant stars and drive turbulence in the solar tachocline.

Robust Data-driven Metallicities for 175 Million Stars from Gaia XP Spectra

We derive and publish data-driven estimates of stellar metallicity [M/H] for ∼175 million stars with low-resolution XP spectra published in Gaia DR3. The [M/H] values, along with T eff and , are derived using the XGBoost algorithm, trained on stellar parameters from APOGEE, augmented by a set of very-metal-poor stars. XGBoost draws on a number of data features: the full set of XP spectral coefficients, narrowband fluxes derived from XP spectra, and broadband magnitudes. In particular, we include CatWISE magnitudes, as they reduce the degeneracy of T eff and dust reddening. We also include the parallax as a data feature, which helps constrain and [M/H]. The resulting mean stellar parameter precision is 0.1 dex in [M/H], 50 K in T eff, and 0.08 dex in . This all-sky [M/H] sample is substantially larger than published samples of comparable fidelity across −3 ≲ [M/H] ≲ +0.5. Additionally, we provide a catalog of over 17 million bright (G < 16) red giants whose [M/H] values are vetted to be precise and pure. We present all-sky maps of the Milky Way in different [M/H] regimes that illustrate the purity of the data set, and demonstrate the power of this unprecedented sample to reveal the Milky Way’s structure from its heart to its disk.

Stellar Characterization and Radius Inflation of Hyades M-dwarf Stars from the APOGEE Survey

We present a spectroscopic analysis of a sample of 48 M-dwarf stars (0.2 M ⊙ < M < 0.6 M ⊙) from the Hyades open cluster using high-resolution H-band spectra from the Sloan Digital Sky Survey/Apache Point Observatory Galactic Evolution Experiment (APOGEE) survey. Our methodology adopts spectrum synthesis with LTE MARCS model atmospheres, along with the APOGEE Data Release 17 line list, to determine effective temperatures, surface gravities, metallicities, and projected rotational velocities. The median metallicity obtained for the Hyades M dwarfs is [M/H] = 0.09 ± 0.03 dex, indicating a small internal uncertainty and good agreement with optical results for Hyades red giants. Overall, the median radii are larger than predicted by stellar models by 1.6% ± 2.3% and 2.4% ± 2.3%, relative to a MIST and DARTMOUTH isochrone, respectively. We emphasize, however, that these isochrones are different, and the fractional radius inflation for the fully and partially convective regimes have distinct behaviors depending on the isochrone. Using a MIST isochrone there is no evidence of radius inflation for the fully convective stars, while for the partially convective M dwarfs the radii are inflated by 2.7% ± 2.1%, which is in agreement with predictions from models that include magnetic fields. For the partially convective stars, rapid rotators present on average higher inflation levels than slow rotators. The comparison with SPOTS isochrone models indicates that the derived M-dwarf radii can be explained by accounting for stellar spots in the photosphere of the stars, with 76% of the studied M dwarfs having up to 20% spot coverage, and the most inflated stars with ∼20%–40% spot coverage.

Validating Stellar Abundance Measurements from Multiresolution Spectroscopy

Large-scale surveys will provide spectroscopy for ∼50 million resolved stars in the Milky Way and Local Group. However, these data will have a high degree of heterogeneity and most will be low-resolution (R < 10,000), posing challenges to measuring consistent and reliable stellar labels. Here, we introduce a framework for identifying and remedying these issues. By simultaneously fitting the full spectrum and Gaia photometry with the Payne, we measure ∼30 abundances for eight metal-poor red giants in M15. From degraded quality Keck/HIRES spectra, we evaluate trends with resolution and signal-to-noise ratio (S/N) and find that (i) ∼20 abundances are recovered consistently within ≲0.1 dex agreement and with ≲0.05–0.15 dex systematic uncertainties from 10,000 ≲ R ≲ 80,000; (ii) for nine elements (C, Mg, Ca, Sc, Ti, Fe, Ni, Y, and Nd), this systematic precision and accuracy extends down to R ∼ 2500; and (iii) while most elements do not exhibit strong S/N-dependent systematics, there are nonnegligible biases for four elements (C, Mg, Ca, and Dy) below S/N ∼ 10 pixel−1. We compare statistical uncertainties from Markov Chain Monte Carlo sampling to the easier-to-compute Cramér–Rao bounds and find that they agree for ∼85% of elements, indicating the latter to be a reliable and faster way to estimate uncertainties. Our analysis illustrates the great promise of low-resolution spectroscopy for stellar chemical abundance work in the low-metallicity regime, and ongoing improvements to stellar models (e.g., 3D-NLTE physics) will only further extend its viability to more stars, more elements, and higher precision and accuracy.

Precise masses and ages of ~1 million RGB and RC stars observed by LAMOST

We construct a catalogue of stellar masses and ages for 696 680 red giant branch (RGB) stars, 180 436 primary red clump (RC) stars, and 120 907 secondary RC stars selected from the LAMOSTDR8. The RGBs, primary RCs, and secondary RCs are identified with the large frequency spacing (∆ν) and period spacing (∆P) estimated from the LAMOST spectra with spectral signal-to-noise ratios (S/Ns) &gt; 10 using a neural network method supervised with seismologic information from LAMOST-Kepler sample stars. The purity and completeness of both RGB and RC samples are better than 95% and 90%, respectively. The mass and age of RGBs and RCs are determined again with the neural network method by taking the LAMOST-Kepler giant stars as the training set. The typical uncertainties on stellar mass and age are 10% and 30%, respectively, for the RGB stellar sample. For RCs, the typical uncertainties on stellar mass and age are 9% and 24%, respectively. The RGB and RC stellar samples cover a large volume of the Milky Way (5 &lt; R &lt; 20 kpc and |Z| &lt; 5 kpc), which are valuable data sets for various Galactic studies.

A 3D view of dwarf galaxies with Gaia and VLT/FLAMES

We present a new homogeneous survey of VLT/FLAMES LR8 line-of-sight radial velocities (vlos) for 1604 resolved red giant branch stars in the Sculptor dwarf spheroidal galaxy. In addition, we provide reliable Ca II triplet metallicities, [Fe/H], for 1339 of these stars. From this combination of new observations (2257 individual spectra) with ESO archival data (2389 spectra), we obtain the largest and most complete sample of vlos and [Fe/H] measurements for individual stars in any dwarf galaxy. Our sample includes VLT/FLAMES LR8 spectra for ∼55% of the red giant branch stars at G &lt; 20 from Gaia DR3, and &gt; 70% of the brightest stars, G &lt; 18.75. Our spectroscopic velocities are combined with Gaia DR3 proper motions and parallax measurements for a new and more precise membership analysis. We look again at the global characteristics of Sculptor, deriving a mean metallicity of ⟨[Fe/H]⟩ = −1.82 ± 0.45 and a mean line-of-sight velocity of ⟨vlos⟩ = + 111.2 ± 0.25 km s−1. There is a clear metallicity gradient in Sculptor, −0.7deg dex−1, with the most metal-rich population being the most centrally concentrated. Furthermore, the most metal-poor population in Sculptor, [Fe/H]&lt; − 2.5, appears to show kinematic properties distinct from the rest of the stellar population. Finally, we combine our results with the exquisite Gaia DR3 multi-colour photometry to further investigate the colour-magnitude diagram of the resolved stellar population in Sculptor. Our detailed analysis shows a similar global picture as previous studies, but with much more precise detail, revealing that Sculptor has more complex properties than previously thought. This survey emphasises the role of the stellar spectroscopy technique and this galaxy as a benchmark system for modelling galaxy formation and evolution on small scales.

Elemental Abundances in M31: Individual and Coadded Spectroscopic [Fe/H] and [α/Fe] throughout the M31 Halo with SPLASH

We present spectroscopic chemical abundances of red giant branch stars in Andromeda (M31), using medium-resolution (R ∼ 6000) spectra obtained via the Spectroscopic and Photometric Landscape of Andromeda’s Stellar Halo survey. In addition to individual chemical abundances, we coadd low signal-to-noise ratio spectra of stars to obtain a high enough signal to measure average [Fe/H] and [α/Fe] abundances. We obtain individual and coadded measurements for [Fe/H] and [α/Fe] for M31 halo stars, covering a range of 9–180 kpc in projected radius from the center of M31. With these measurements, we greatly increase the number of outer halo (R proj > 50 kpc) M31 stars with spectroscopic [Fe/H] and [α/Fe], adding abundance measurements for 45 individual stars and 33 coadds from a pool of an additional 174 stars. We measure the spectroscopic metallicity ([Fe/H]) gradient, finding a negative radial gradient of −0.0084 ± 0.0008 for all stars in the halo, consistent with gradient measurements obtained using photometric metallicities. Using the first measurements of [α/Fe] for M31 halo stars covering a large range of projected radii, we find a positive gradient (+0.0027 ± 0.0005) in [α/Fe] as a function of projected radius. We also explore the distribution in [Fe/H]–[α/Fe] space as a function of projected radius for both individual and coadded measurements in the smooth halo, and compare these measurements to those stars potentially associated with substructure. These spectroscopic abundance distributions add to existing evidence that M31 has had an appreciably different formation and merger history compared to our own Galaxy.

Distant Echoes of the Milky Way’s Last Major Merger

The majority of the Milky Way’s stellar halo consists of debris from our galaxy’s last major merger, the Gaia-Sausage-Enceladus (GSE). In the past few years, stars from the GSE have been kinematically and chemically studied in the inner 30 kpc of our galaxy. However, simulations predict that accreted debris could lie at greater distances, forming substructures in the outer halo. Here we derive metallicities and distances using Gaia DR3 XP spectra for an all-sky sample of luminous red giant stars, and map the outer halo with kinematics and metallicities out to 100 kpc. We obtain follow-up spectra of stars in two strong overdensities—including the previously identified outer Virgo Overdensity—and find them to be relatively metal rich and on predominantly retrograde orbits, matching predictions from simulations of the GSE merger. We argue that these are apocentric shells of GSE debris, forming 60–90 kpc counterparts to the 15–20 kpc shells that are known to dominate the inner stellar halo. Extending our search across the sky with literature radial velocities, we find evidence for a coherent stream of retrograde stars encircling the Milky Way from 50 to 100 kpc, in the same plane as the Sagittarius Stream but moving in the opposite direction. These are the first discoveries of distant and structured imprints from the GSE merger, cementing the picture of an inclined and retrograde collision that built up our galaxy’s stellar halo.

A close-in giant planet escapes engulfment by its star.

When main-sequence stars expand into red giants, they are expected to engulf close-in planets1-5. Until now, the absence of planets with short orbital periods around post-expansion, core-helium-burning red giants6-8 has been interpreted as evidence that short-period planets around Sun-like stars do not survive the giant expansion phase of their host stars9. Here we present the discovery that the giant planet 8 Ursae Minoris b10 orbits a core-helium-burning red giant. At a distance of only 0.5 AU from its host star, the planet would have been engulfed by its host star, which is predicted by standard single-star evolution to have previously expanded to a radius of 0.7 AU. Given the brief lifetime of helium-burning giants, the nearly circular orbit of the planet is challenging to reconcile with scenarios in which the planet survives by having a distant orbit initially. Instead, the planet may have avoided engulfment through a stellar merger that either altered the evolution of the host star or produced 8 Ursae Minoris b as a second-generation planet11. This system shows that core-helium-burning red giants can harbour close planets and provides evidence for the role of non-canonical stellar evolution in the extended survival of late-stage exoplanetary systems.

A unified exploration of the chronology of the Galaxy

ABSTRACT The Milky Way has distinct structural stellar components linked to its formation and subsequent evolution, but disentangling them is non-trivial. With the recent availability of high-quality data for a large numbers of stars in the Milky Way, it is a natural next step for research in the evolution of the Galaxy to perform automated explorations with unsupervised methods of the structures hidden in the combination of large-scale spectroscopic, astrometric, and asteroseismic data sets. We determine precise stellar properties for 21 076 red giants, mainly spanning 2–15 kpc in Galactocentric radii, making it the largest sample of red giants with measured asteroseismic ages available to date. We explore the nature of different stellar structures in the Galactic disc by using Gaussian mixture models as an unsupervised clustering method to find substructure in the combined chemical, kinematic, and age subspace. The best-fitting mixture model yields four distinct physical Galactic components in the stellar disc: the thin disc, the kinematically heated thin disc, the thick disc, and the stellar halo. We find hints of an age asymmetry between the Northern and Southern hemisphere, and we measure the vertical and radial age gradient of the Galactic disc using the asteroseismic ages extended to further distances than previous studies.

Stellar cooling limits on light scalar boson revisited

We revisit the stellar cooling limits on the light scalar boson whose coupling to the Standard Model particles is described by its mixing with the Higgs boson. Strong constraints have been obtained from the electron-nucleus bremsstrahlung process (Ne→NeS) and the resonant plasma effect in the medium. We find that the bremsstrahlung contribution from the electron and nucleus scattering is of similar magnitude to the plasma mixing effect including the off-resonant mixing. The constraints on the scalar coupling are found to be about three orders of magnitude weaker than the previous evaluations because of the missing factor of the electron-nucleon mass ratio. For white dwarfs, the stellar cooling constraint is even more suppressed due to the Pauli blocking effect. We obtain limits on the Higgs-scalar mixing angle of 10−10–10−9, which are given by white dwarfs or red giants up to the luminosity limits, in the region where the scalar mass is lighter than about 10 keV.

Improved S factor of the C12(p,γ)N13 reaction at E=320–620 keV and the 422 keV resonance

The 12C(p,{\gamma})13N reaction is the onset process of both the CNO and Hot CNO cycles that drive massive star, Red and Asymptotic Giant Branch star and novae nucleosynthesis. The 12C(p,{\gamma})13N rate affects the final abundances of the stable 12,13C nuclides, with ramifications for meteoritic carbon isotopic abundances and the s-process neutron source strength. Here, a new underground measurement of the 12C(p,{\gamma})13N cross-section is reported. The present data, obtained at the Felsenkeller shallow-underground laboratory in Dresden (Germany), encompass the 320-620 keV center of mass energy range to include the wide and poorly constrained E = 422 keV resonance that dominates the rate at high temperatures. This work S-factor results, lower than literature by 25%, are included in a new comprehensive R-matrix fit, and the energy of the 1+ first excited state of 13N is found to be 2369.6(4) keV, with radiative and proton width of 0.49(3) eV and 34.9(2) keV respectively. A new reaction rate, based on present R-matrix fit and extrapolation, is suggested.

Precise resonance parameter measurement in the 12C(p,γ)13N astrophysically important reaction

Precise knowledge of the 12C(p,γ)13N reaction cross section is crucial to understand the evolution of various types of red giant stars and to explain their astronomically observed surface 12C/13C abundance ratio. Experimental data show discordance near resonances below Ep=2000 keV. Dedicated measurements using in-beam γ-spectroscopy have been carried out in the vicinity of Ep=460 and 1700 keV to obtain resonance parameters, such as the resonance energy and width. Using a simplified R-matrix model, the following values of resonance energy and width were obtained: Er=459.8±0.8; Γtotal= 38.2±0.5 keV and Er=1685.1±0.7; Γtotal=57.6±0.8 keV given in laboratory system. These values with reduced uncertainties provide new input parameters for future compilations of 12C(p,γ)13N reaction and thus for astrophysics cross-section extrapolations.

New insights into the limit of the magnetic monopole flux and the heating source in white dwarfs* *Supported in part by the National Natural Science Foundation of China (11965010, 11565020), the Natural Science Foundation of Hainan Province of China (118MS071, 2019RC239), the Counterpart Foundation of Sanya (2016PT43, 2019PT76), the Special Foundation of Science and Technology Cooperation for Advanced Academy and

Based on the magnetic monopole (MM) catalytic nuclear decay (Rubakov-Callan (RC) effect), we propose five new models to discuss the limit of the MM flux and the heating energy resources of white dwarfs (WDs) based on observations of 13 red giant branch (RGB) stars. We find that the number of MMs captured can reach a maximum value of when GeV, , . The good agreement of our calculated luminosities for WDs with observation provides support for our model based on the RC effect by MMs. We obtain a new limit of the MM flux of , and at when , , and , respectively. Our results show that the RC effect could cause heating that prevents white dwarfs from cooling down into a stellar graveyard. Our results will also provide a new idea for further research on the upper limit of MM flow (note: are the baryon number density, reaction cross section, mass, MM flux, and the new limit of the MM flux, respectively, and is the ratio of the speed of MMs to that of light).

Quantifying Uncertainties on the Tip of the Red Giant Branch Method

We present an extensive grid of numerical simulations quantifying the uncertainties in measurements of the tip of the red giant branch (TRGB). These simulations incorporate a luminosity function composed of 2 mag of red giant branch (RGB) stars leading up to the tip, with asymptotic giant branch (AGB) stars contributing exclusively to the luminosity function for at least a magnitude above the RGB tip. We quantify the sensitivity of the TRGB detection and measurement to three important error sources: (1) the sample size of stars near the tip, (2) the photometric measurement uncertainties at the tip, and (3) the degree of self-crowding of the RGB population. The self-crowding creates a population of supra-TRGB stars due to the blending of one or more RGB stars just below the tip. This last population is ultimately difficult, although still possible, to disentangle from true AGB stars. In the analysis given here, the precepts and general methodology as used in the Chicago-Carnegie Hubble Program (CCHP) have been followed. However, in the appendix, we introduce and test a set of new tip detection kernels, which internally incorporate self-consistent smoothing. These are generalizations of the two-step model used by the CCHP (smoothing followed by Sobel-filter tip detection), where the new kernels are based on successive binomial-coefficient approximations to the derivative-of-a-Gaussian edge-detector, as is commonly used in modern digital image processing.

A Wide View of the Galactic Globular Cluster NGC 2808: Red Giant and Horizontal Branch Star Spatial Distributions

Wide-field and deep DECam multiband photometry, combined with HST data for the core of the Galactic globular cluster NGC 2808, allowed us to study the distribution of various stellar subpopulations and stars in different evolutionary phases out to the cluster tidal radius. We used the C ugi = (u − g) − (g − i) index to identify three chemically distinct subpopulations along the red giant branch and compared their spatial distributions. The most light-element-enriched subpopulation (P3) is more centrally concentrated; however, it shows a more extended distribution in the external regions of the cluster compared to the primordial (P1) and intermediate (P2) composition populations. Furthermore, the P3 subpopulation centroid is off-center relative to those of the P1 and P2 groups. We also analyzed the spatial distribution of horizontal branch stars and found that the relative fraction of red horizontal branch stars increases for radial distances larger than ≈1.′5, while that of the blue and hotter stars decreases. These new observations, combined with literature spectroscopic measurements, suggest that the red horizontal branch stars are the progeny of all the stellar subpopulations in NGC 2808, i.e., primordial and light-element enhanced, while the blue stars are possibly the result of a combination of the “hot-flasher” and the “helium-enhanced” scenarios. A similar distribution of different red giant branch subpopulations and horizontal branch stars was also found for the most massive Galactic globular cluster, ω Cen, based on combined DECam and HST data, which suggests that the two may share a similar origin.

Carbon Abundance of Globular Cluster M22 (NGC 6656) and the Surface Carbon Depletion Rates of the Milky Way Globular Clusters

It is well known that metal-poor red giant branch (RGB) stars show variations in some elemental abundances, including carbon, due to the internal mixing accompanied by their own in situ CN cycle in the hydrogen burning shell. With our new photometric carbon abundance measurements of RGB stars in M22 and other globular clusters (GCs) in our previous studies, M5, M3, and M92, we derive the carbon depletion rates against the V magnitude, d[C/Fe]/M V , for individual populations in each GC. We find the metallicity dependence of the carbon depletion rates, d[C/Fe]/M V ∝ −0.25[Fe/H]. Our results also suggest that the carbon depletion rates of the second generation (SG) of stars are larger than those of the first generation (FG) of stars in our sample GCs, most likely due to different internal temperature profiles with different initial helium abundances between the FG and SG. Our results can provide critical constraints both on understanding the mixing efficiency in the theoretical models, which is largely unknown, and on interpretation of the observational carbon abundance evolution of the bright halo RGB stars.

An Eclipsing Binary Comprising Two Active Red Stragglers of Identical Mass and Synchronized Rotation: A Post-mass-transfer System or Just Born That Way?

We report the discovery of 2M0056–08 as an equal-mass eclipsing binary (EB), comprising two red straggler stars (RSSs) with an orbital period of 33.9 days. Both stars have masses of ≈1.419 M ⊙, identical to within 0.2%. Both stars appear to be in the early red-giant phase of evolution; however, they are far displaced to cooler temperatures and lower luminosities compared to standard stellar models. The broadband spectral energy distribution shows NUV excess and X-ray emission, which is consistent with chromospheric and coronal emission from magnetically active stars. Indeed, the stars rotate more rapidly than typical red giants and they evince light-curve modulations due to spots. These modulations also reveal the stars to be rotating synchronously with one another. There is evidence for excess FUV emission and long-term modulations in radial velocities, although it is not clear if they are also attributable to magnetic activity or if they reveal a tertiary companion. Stellar evolution models that are modified to account for the effects of spots can reproduce the observed radii and temperatures of the RSSs. If the system possesses a white dwarf tertiary, then mass-transfer scenarios could explain the manner by which the stars came to possess such remarkably identical masses and by which they came to be synchronized. However, if the stars are presumed to have been formed as identical twins and they then managed to become tidally synchronized as they evolved toward the red-giant branch, then all of the features of the system can be explained via activity effects without requiring a complex dynamical history.