arXiv subjects

Benjamin R. Setterholm

Publications and source records attributed to Benjamin R. Setterholm.

At least 19 recordsLinked to original sources

arXiv2026

CHARA Array Observations of the Evolved Components in Symbiotic Star Systems

The nature of the mechanisms that drive mass transfer in symbiotic stars remains an area of active research in stellar astronomy. Constraining the role that both stellar winds and Roche-lobe overflow play in this process is crucial to improving our understanding of these binaries and connecting them to important transient events such as recurrent novae and Type Ia supernovae. The high-resolution capabilities of an optical interferometer can resolve the geometric structure of the red giant in symbiotic stars and help answer this question. This work presents the results of an optical interferometric study using the Center for High Angular Resolution Astronomy (CHARA) Array for the purpose of measuring the angular diameter of and imaging the cool giant in four symbiotic and related systems. Here we report \textit{H} band observations collected with MIRC-X. Model fitting and image reconstruction are used to test for Roche-lobe-filling geometries. Near-simultaneous infrared spectroscopy taken using the NASA InfraRed Telescope Facility (IRTF) is used to determine the fundamental stellar parameters of the cool giant in each system. The parametric fits reported here favor circularly symmetric disk models over elongated geometries, while imaging suggests the presence of surface features on three of these stars. We find that the three systems with constrained orbits have inferred time-averaged filling factors below unity.

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arXiv2026

Visual Orbits of Spectroscopic Binaries with the CHARA Array. V. HD 210763 and HD 221950

We present the visual orbits and dynamical masses of two longer period spectroscopic binary stars, HD 210763 and HD 221950, using long baseline interferometry with the CHARA Array and high resolution spectroscopy with the APO 3.5 m and CTIO 1.5 m telescopes. By combining the astrometric and radial velocity observations, we solve for the full, three-dimensional orbits and determine the stellar masses to within 0.5% uncertainty and the distance to within 0.2% uncertainty. For HD 210763, we found component masses of M1 = 1.748 Msun and M2 = 1.492 Msun. For HD 221950, we found masses of M1 = 1.098 Msun and M2 = 1.031 Msun. We then estimate the effective temperature and radius of each component star through disentangling and spectral energy distribution analyses. We compare the observed stellar parameters to the predictions of the stellar evolution models and estimate the system ages. The primary component of HD 210763 is at the end of the main sequence while the secondary component is on the main sequence, providing a tight age constraint for this system at 1.6 Gyr. Both components of HD 221950 are still on the main sequence with an age of 3.76 Gyr. These systems have longer orbital periods, beyond the tidal circularization limit, and therefore are better proxies for single stars and tests of stellar evolution models than short period, eclipsing systems.

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arXiv2026

Pre-nova Observations of T CrB: A view from the CHARA Array

T CrB is a symbiotic recurrent nova consisting of a red giant and white dwarf with recent eruptions in 1866 and 1946 and an anticipated eruption in the mid 2020s. We report CHARA Array observations obtained with MIRC-X (H -band) and MYSTIC (K-band) in 2022-2025. We fit limb darkened disk models constrained with literature limb darkening coefficients to the squared visibilities as only the first visibility lobe is sampled. The average limb darkened diameter of the star across these epochs is $0.70\pm0.04$ mas in H-band and $0.72\pm0.07$ mas in K-band. Adopting a distance of $914^{+24}_{-22}$ pc, the stellar radius is $69\pm5~R_{\odot}$ in H-band and $71\pm8~R_{\odot}$ in K-band. This is consistent with filling a Roche lobe volume radius of $71~R_{\odot}$ inferred from published orbital solutions. These measurements provide a pre-eruption angular diameter and support a Roche lobe filling donor.

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arXiv2026

Interferometric Images of the Starspot Evolution of $\zeta$ Andromedae

The evolution of starspots of the giant primaries of RS CVn systems is typically detected indirectly with photometric and spectroscopic monitoring. These observations suggest slowly-evolving stellar surfaces and can constrain differential rotation as starspots move with respect to one another. However, starspot latitudes are difficult to constrain without resolved images of the stellar surfaces from which the unambiguous locations of starspots are determined. We imaged the active RS CVn primary $\zeta$ And with the 330-m-baseline Center for High Angular Resolution Astronomy Array for three epochs over approximately six rotations of the star. The resultant images show a more complicated picture of stellar activity than expected from the contemporaneous photometry and earlier Doppler images. The spot structures change on the timescale of rotation, making differential rotation difficult to study. Our observations show changes in the polar spot, growing over time. We do not detect the secondary star in the interferometric data, though the observations are sensitive to the predicted 0.75 $M_\odot$ main-sequence star, and we suggest the companion may be a white dwarf.

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arXiv2025

Multiple outflows and delayed ejections revealed by early imaging of novae

Novae are thermonuclear eruptions on accreting white dwarfs in interacting binaries. Although most of the accreted envelope is expelled, the mechanism -- impulsive ejection, multiple outflows or prolonged winds, or a common-envelope interaction -- remains uncertain. GeV $\gamma$-ray detections from $>20$ Galactic novae establish these eruptions as nearby laboratories for shock physics and particle acceleration, underscoring the need to determine how novae eject their envelopes. Here we report on near-infrared interferometry, supported with multiwavelength observations, of two $\gamma$-ray detected novae. The images of the very fast 2021 nova V1674~Her, taken just 2--3 days after discovery, reveal the presence of two perpendicular outflows. The interaction between these outflows likely drives the observed $\gamma$-ray emission. Conversely, the images of the very slow 2021 nova V1405~Cas suggest a delay in the ejection of the bulk of the accreted envelope of more than 50 days after the start of eruption, as the nova slowly rises to visible peak and during which the envelope engulfed the system in a common envelope phase. These unprecedented images offer direct observational evidence that the mechanisms driving mass ejection from the surfaces of accreting white dwarfs are not as simple as previously thought, revealing multiple outflows and delayed ejections.

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arXiv2025

Surprise non-detection of Upsilon Andromedae b with MIRC-X and MYSTIC at the CHARA Array

Ground-based long baseline interferometry is a powerful tool for characterizing exoplanets which are too close to their host star to be imaged with single-dish telescopes. The CHARA Array can resolve companions down to 0.5 milli-arcseconds, allowing us in principle to directly measure the near-infrared spectra of non-transiting "Hot Jupiter" exoplanets. We present data taken with the MIRC-X and MYSTIC instruments at the CHARA Array on the Hot Jupiter Upsilon Andromedae b. By resolving the star-planet system, we attempt to directly detect the flux from the planet. We describe our self-calibration methods for modeling systematics in the closure phase data, which allows us to reach sub-degree precision. Through combining multiple nights of data across two MIRC-X runs in 2019 and 2021, we achieved a very tentative detection of Ups And b in the H-band at a planet/star contrast of 2-3 x 10^-4. Unfortunately, we cannot confirm this detection with 2021 MYSTIC data in the K-band, or in a 2023 joint MIRC-X and MYSTIC dataset. We run updated global circulation models and create post-processed spectra for this planet, and report the resulting model spectra in H- and K-bands as a function of orbital phase. We then run planetary injection tests to explore H/K-band contrast limits, and find that we can confidently recover planets down to a planet/star contrast of 1-2 x 10^-4. We show that we are probing contrasts fainter than predicted by the model, making our non-detection surprising. We discuss prospects for the future in using this method to characterize companions with interferometry.

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arXiv2025

The CHARA Array Polarization Model and Prospects for Spectropolarimetry

Polarimetric data provide key insights into infrared emission mechanisms in the inner disks of YSOs and the details of dust formation around AGB stars. While polarization measurements are well-established in radio interferometry, they remain challenging at visible and near-infrared due to the significant time-variable birefringence introduced by the complex optical beamtrain. In this study, we characterize instrumental polarization effects within the optical path of the CHARA Array, focusing on the H-band MIRC-X and K-band MYSTIC beam combiners. Using Jones matrix formalism, we developed a comprehensive model describing diattenuation and retardance across the array. By applying this model to an unpolarized calibrator, we derived the instrumental parameters for both MIRC-X and MYSTIC. Our results show differential diattenuation consistent with >= 97% reflectivity per aluminum-coated surface at 45 deg incidence. The differential retardance exhibits small wavelength-dependent variations, in some cases larger than we expected. Notably, telescope W2 exhibits a significantly larger phase shift in the Coude path, attributable to a fixed aluminum mirror (M4) used in place of deformable mirrors present on the other telescopes during the observing run. We also identify misalignments in the LiNbO_3 birefringent compensator plates on S1 (MIRC-X) and W2 (MYSTIC). After correcting for night-to-night offsets, we achieve calibration accuracies of $\pm$ 3.4% in visibility ratio and $\pm$ 1.4 deg in differential phase for MIRC-X, and $\pm$ 5.9% and $\pm$ 2.4 deg, respectively, for MYSTIC. Given that the differential intrinsic polarization of spatially resolved sources, such as AGB stars and YSOs, typically greater than these instrumental uncertainties, our results demonstrate that CHARA is now capable of achieving high-accuracy measurements of intrinsic polarization in astrophysical targets.

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arXiv2025

The Small Separation A-Star Companion Population: Tentative Signatures of Enhanced Multiplicity with Primary Mass

We present updated results from our near-infrared long-baseline interferometry (LBI) survey to constrain the multiplicity properties of intermediate-mass A-type stars within 80 pc. Previous adaptive optics surveys of A-type stars are incomplete at separations $<$ 20au. Therefore, a LBI survey allows us to explore separations previously unexplored. Our sample consists of 54 A-type primaries with estimated masses between 1.44-2.93 M$_{\odot}$ and ages 10-790 Myr, which we observed with the MIRC-X and MYSTIC instruments at the CHARA Array. We use the open source software CANDID to detect two new companions, seven in total, and we performed a Bayesian demographic analysis to characterize the companion population. We find the separation distribution consistent with being flat, and we estimate a power-law fit to the mass ratio distribution with index -0.13$^{+0.92}_{-0.95}$ and a companion frequency of 0.25$^{+0.17}_{-0.11}$ over mass ratios 0.1-1.0 and projected separations 0.01-27.54au. We find a posterior probability of 0.53 and 0.04 that our results are consistent with extrapolations based on previous models of the solar-type and B-type companion population, respectively. Our results suggest that the close companion population to A-type stars is comparable to that of solar-types and that close companions to B-type stars are potentially more frequent which may be indicative of increased disk fragmentation for stars $\gtrsim$ 3M$_{\odot}$.

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arXiv2025

Precise Age For The Binary HD 21278 In The Young Alpha Persei Cluster

We present a study of the double-lined spectroscopic binary HD 21278 that contains one of the brightest main sequence stars in the young $\alpha$ Persei open cluster. We analyzed new spectra and reanalyzed archived spectra to measure precise new radial velocity curves for the binary. We also obtained interferometric data using the CHARA Array at Mount Wilson to measure the sky positions of the two stars and the inclination of the $\sim$ 2 milliarcsecond orbit. We determine that the two stars have masses of $5.381 \pm 0.084 M_{\odot}$ and $3.353 \pm 0.064 M_{\odot}$. From isochrone fits, we find the cluster's age to be $49 \pm 7$ Myr (using PARSEC models) or $49.5 \pm 6$ Myr (MIST models). Finally, we revisit the massive white dwarfs that are candidate escapees from the $\alpha$ Persei cluster to try to better characterize the massive end of the white dwarf initial-final mass relation. The implied progenitor masses challenge the idea that Chandrasekhar-mass white dwarfs are made by single stars with masses near $8 \msun$.

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arXiv2025

The Dynamic Inner Disk of a Planet Forming Star

Planets are a natural byproduct of the stellar formation process, resulting from local aggregations of material within the disks surrounding young stars. Whereas signatures of gas-giant planets at large orbital separations have been observed and successfully modeled within protoplanetary disks, the formation pathways of planets within their host star's future habitable zones remain poorly understood. Analyzing multiple nights of observations conducted over a short, two-month span with the MIRC-X and PIONIER instruments at the CHARA Array and VLTI, respectively, we uncover a highly active environment at the inner-edge of the planet formation region in the disk of HD 163296. In particular, we localize and track the motion of a disk feature near the dust-sublimation radius with a pattern speed of less than half the local Keplerian velocity, providing a potential glimpse at the planet formation process in action within the inner astronomical unit. We emphasize that this result is at the edge of what is currently possible with available optical interferometric techniques and behooves confirmation with a temporally dense followup observing campaign.

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arXiv2024

Visual Orbits of Wolf-Rayet Stars II: The Orbit of the Nitrogen-Rich WR Binary WR 138 measured with the CHARA Array

Classical Wolf-Rayet stars are descendants of massive OB-type stars that have lost their hydrogen-rich envelopes, and are in the final stages of stellar evolution, possibly exploding as type Ib/c supernovae. It is understood that the mechanisms driving this mass-loss are either strong stellar winds and or binary interactions, so intense studies of these binaries including their evolution can tell us about the importance of the two pathways in WR formation. WR 138 (HD 193077) has a period of just over 4 years and was previously reported to be resolved through interferometry. We report on new interferometric data combined with spectroscopic radial velocities in order to provide a three-dimensional orbit of the system. The precision on our parameters tend to be about an order of magnitude better than previous spectroscopic techniques. These measurements provide masses of the stars, namely $M_{\rm WR} = 13.93\pm1.49M_{\odot}$ and $M_{\rm O} = 26.28\pm1.71M_{\odot}$. The derived orbital parallax agrees with the parallax from \textit{Gaia}, namely with a distance of 2.13 kpc. We compare the system's orbit to models from BPASS, showing that the system likely may have been formed with little interaction but could have formed through some binary interactions either following or at the start of a red supergiant phase, but with the most likely scenario occurring as the red supergiant phase starts for a $\sim 40M_\odot$ star.

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arXiv2024

Visual Orbits of Wolf-Rayet Stars I: The Orbit of the dust-producing Wolf-Rayet binary WR\,137 measured with the CHARA Array

Classical Wolf-Rayet stars are the descendants of massive OB stars that have lost their hydrogen envelopes and are burning helium in their cores prior to exploding as type Ib/c supernovae. The mechanisms for losing their hydrogen envelopes are either through binary interactions or through strong stellar winds potentially coupled with episodic mass-loss. Amongst the bright classical WR stars, the binary system WR\,137 (HD\,192641; WC7d + O9e) is the subject of this paper. This binary is known to have a 13-year period and produces dust near periastron. Here we report on interferometry with the CHARA Array collected over a decade of time and providing the first visual orbit for the system. We combine these astrometric measurements with archival radial velocities to measure masses of the stars of $M_{\rm WR} = 9.5\pm3.4 M_\odot$ and $M_{\rm O} = 17.3\pm 1.9 M_\odot$ when we use the most recent \textit{Gaia} distance. These results are then compared to predicted dust distribution using these orbital elements, which match the observed imaging from \textit{JWST} as discussed recently by Lau et al. Furthermore, we compare the system to the BPASS models, finding that the WR star likely formed through stellar winds and not through binary interactions. However, the companion O star did likely accrete some material from the WR's mass-loss to provide the rotation seen today that drives its status as an Oe star.

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arXiv2024

CHARA Near-Infrared Imaging of the Yellow Hypergiant Star $ρ$ Cassiopeiae: Convection Cells and Circumstellar Envelope

Massive evolved stars such as red supergiants and hypergiants are potential progenitors of Type II supernovae, and they are known for ejecting substantial amounts of matter, up to half their initial mass, during their final evolutionary phases. The rate and mechanism of this mass loss play a crucial role in determining their ultimate fate and the likelihood of their progression to supernovae. However, the exact mechanisms driving this mass ejection have long been a subject of research. Recent observations, such as the Great Dimming of Betelgeuse, have suggested that the activity of large convective cells, combined with pulsation, could be a plausible explanation for such mass loss events. In this context, we conducted interferometric observations of the famous yellow hypergiant, $ρ$ Cassiopeiae using the CHARA Array in H and K-band wavelengths. $ρ$ Cas is well known for its recurrent eruptions, characterized by periods of visual dimming ($\sim$1.5-2 mag) followed by recovery. From our observations, we derived the diameter of the limb-darkened disk and found that this star has a radius of $1.04\pm0.01$ milliarcseconds (mas), or $564 - 700 R_\odot$. We performed image reconstructions with three different image reconstruction software packages, and they unveiled the presence of giant hot and cold spots on the stellar surface. We interpret these prominent hot spots as giant convection cells, suggesting a possible connection to mass ejections from the star's envelope. Furthermore, we detected spectral CO emission lines in the K-band ($λ=2.31-2.38 μ$m), and the image reconstructions in these spectral lines revealed an extended circumstellar envelope with a radius of $1.45\pm0.10$ mas.

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arXiv2024

The Orbit and Mass of the Cepheid AW Per

The Cepheid AW Per is a component in a multiple system with a long period orbit. The radial velocities of Griffin (2016) cover the 38 year orbit well. An extensive program of interferometry with the CHARA array is reported here, from which the long period orbit is determined. In addition, a {\it Hubble Space Telescope} high resolution spectrum in the ultraviolet demonstrates that the companion is itself a binary with nearly equal mass components. These data combined with a distance from {\it Gaia} provide a mass of the Cepheid (primary) of M$_1$ = 6.79 $\pm$ 0.85 $M_\odot$. The combined mass of the secondary is M$_S$ = 8.79 $\pm$ 0.50 $M_\odot$. The accuracy of the mass will be improved after the fourth Gaia data release expected in approximately two years.

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arXiv2023

Orbits and Dynamical Masses for the Active Hyades Multiple System HD 284163

We report near-infrared long-baseline interferometric observations of the Hyades multiple system HD 284163, made with the CHARA array, as well as almost 43 yr of high-resolution spectroscopic monitoring at the CfA. Both types of observations resolve the 2.39 d inner binary, and also an outer companion in a 43.1 yr orbit. Our observations, combined with others from the literature, allow us to solve for the 3D inner and outer orbits, which are found to be at nearly right angles to each other. We determine the dynamical masses of the three stars (good to better than 1.4% for the inner pair), as well as the orbital parallax. The secondary component (0.5245 +/- 0.0047 MSun) is now the lowest mass star with a dynamical mass measurement in the cluster. A comparison of these measurements with current stellar evolution models for the age and metallicity of the Hyades shows good agreement. All three stars display significant levels of chromospheric activity, consistent with the classification of HD 284163 as an RS CVn object. We present evidence that a more distant fourth star is physically associated, making this a hierarchical quadruple system.

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arXiv2023

Reconstructing the mid-infrared environment in the stellar merger remnant V838 Monocerotis

V838 Mon is a stellar merger remnant that erupted in 2002 in a luminous red novae event. Although it is well studied in the optical, near infrared and submillimeter regimes, its structure in the mid-infrared wavelengths remains elusive. We observed V838 Mon with the MATISSE (LMN bands) and GRAVITY (K band) instruments at the VLTI and also the MIRCX/MYSTIC (HK bands) instruments at the CHARA array. We geometrically modelled the squared visibilities and the closure phases in each of the bands to obtain constraints on physical parameters. Furthermore, we constructed high resolution images of V838 Mon in the HK bands, using the MIRA and SQUEEZE algorithms to study the immediate surroundings of the star. Lastly, we also modelled the spectral features seen in the K and M bands at various temperatures. The image reconstructions show a bipolar structure that surrounds the central star in the post merger remnant. In the K band, the super resolved images show an extended structure (uniform disk diameter $\sim 1.94$ mas) with a clumpy morphology that is aligned along a north-west position angle (PA) of $-40^\circ$. Whereas in the H band, the extended structure (uniform disk diameter $\sim 1.18$ mas) lies roughly along the same PA. However, the northern lobe is slightly misaligned with respect to the southern lobe, which results in the closure phase deviations. The VLTI and CHARA imaging results show that V838 Mon is surrounded by features that resemble jets that are intrinsically asymmetric. This is also confirmed by the closure phase modelling. Further observations with VLTI can help to determine whether this structure shows any variation over time, and also if such bipolar structures are commonly formed in other stellar merger remnants.

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arXiv2023

Three-dimensional orbit of AC Her determined: Binary-induced truncation cannot explain the large cavity in this post-AGB transition disk

Some evolved binaries, namely post-asymptotic giant branch binaries, are surrounded by stable and massive circumbinary disks similar to protoplanetary disks found around young stars. Around 10% of these disks are transition disks: they have a large inner cavity in the dust. Previous interferometric measurements and modeling have ruled out the cavity being formed by dust sublimation and suggested that the cavity is due to a massive circumbinary planet that traps the dust in the disk and produces the observed depletion of refractory elements on the surface of the post-AGB star. In this study, we test alternative scenario in which the large cavity could be due to dynamical truncation from the inner binary. We performed near-infrared interferometric observations with the CHARA Array on the archetype of such a transition disk around a post-AGB binary: AC Her. We detect the companion at ten epochs over 4 years and determine the 3-dimensional orbit using these astrometric measurements in combination with the radial velocity time series. This is the first astrometric orbit constructed for a post-AGB binary system. We derive the best-fit orbit with a semi-major axis $2.01 \pm 0.01$ mas ($2.83\pm0.08$ au), inclination $(142.9 \pm 1.1)^\circ$ and longitude of the ascending node $(155.1 \pm 1.8)^\circ$. We find that the theoretical dynamical truncation and dust sublimation radius are at least $\sim3\times$ smaller than the observed inner disk radius ($\sim21.5$ mas or 30 au). This strengthens the hypothesis that the origin of such a cavity is due to the presence of a circumbinary planet.

astro-ph.EP↗

arXiv2023

Precise Age for the Binary Star System 12 Com in the Coma Berenices Cluster

We present measurements of the interferometrically-resolved binary star system 12 Com and the single giant star 31 Com in the cluster Coma Berenices. 12 Com is a double-lined spectroscopic binary system consisting of a G7 giant and an A3 dwarf at the cluster turnoff. Using an extensive radial velocity dataset and interferometric measurements from PTI and the CHARA array, we measured masses $M_1 =2.64 \pm 0.07 M_\odot$ and $M_2 =2.10 \pm 0.03 M_\odot$. Interferometry also allows us to resolve the giant, and measure its size as $R_1 = 9.12 \pm 0.12 \pm 0.01 R_\odot$. With the measured masses and radii, we find an age of $533 \pm 41 \pm 42$ Myr. For comparison, we measure the radius of 31 Com to be $8.36 \pm 0.15 R_\odot$. Based on the photometry and radius measurements, 12 Com A is likely the most evolved bright star in the cluster, large enough to be in the red giant phase, but too small to have core helium burning. Simultaneous knowledge of 12 Com A's mass and photometry puts strong constraints on convective core overshooting during the main sequence phase, which in turn reduces systematic uncertainties in the age. Increased precision in measuring this system also improves our knowledge of the progenitor of the cluster white dwarf WD1216+260.

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