arXiv subjects

Daniel R. Hey

Publications and source records attributed to Daniel R. Hey.

At least 19 recordsLinked to original sources

arXiv2025

Unveiling the Sagittarius Dwarf Spheroidal Galaxy Core with Gaia DR3: A Red Clump Distance Precise to 2%

The Sagittarius dwarf spheroidal galaxy provides us with the unique opportunity to study an ongoing Galactic cannibalistic event between our Milky Way Galaxy and a satellite dwarf galaxy. Understanding this event crucially requires memberships and high-precision metallicities. Here, we present the first major membership star catalog of the Sagittarius dwarf core ($\approx$140,000 sources) and Messier 54 ($\approx$2000 sources) with positions, proper motions, and parallaxes from $Gaia$ DR3, supplemented with metallicities from the Apache Point Observatory Galactic Evolution Experiment. We initially isolate the Sagittarius dwarf core and Messier 54 spatially from prior literature positions. Using evolutionary sub-samples separated within a color-magnitude diagram, we analyze the substructures of the Sagittarius core and infer its positional relationship with Messier 54 within 5D phase space. A sample of Milky Way stars from a similar galactic latitude were used to identify contaminants and separate member stars from the core of the Sgr dSph and Messier 54 using a Gaussian Mixture Model. We present the derived proper motion, parallaxes, and metallicities for these evolutionary sub-samples while demonstrating the precision of our sample using red clump standard candles. We find a distance modulus for the Sagittarius core and Messier 54 of $(m-M)_{0}=16.958^{+0.044}_{-0.044}$ mag and $(m-M)_{0}=16.94^{+0.047}_{-0.056}$ mag, corresponding to a heliocentric distance of $d=24.635^{+0.49}_{-0.49}$ kpc and $d=24.452^{+0.537}_{-0.602}$ kpc respectively. With red clump distance analysis, our results imply there is no separation between the Sagittarius core and Messier 54. Finally, we describe the metallicity distributions of the evolved stars within these two systems, finding evidence for the infall scenario.

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arXiv2024

HD 60435: The star that stopped pulsating

HD 60435 is a well-known rapidly oscillating (roAp) Ap star with a series of alternating even and odd degree modes, making it a prime asteroseismic target. It is also an oblique pulsator with rotational inclination, $i$, and magnetic/pulsation obliquity, $\beta$, such that both magnetic/pulsation poles are viewed over the rotation period, $P_{\rm rot} = 7.679696$ d, determined from rotational light variations. While some roAp stars have stable pulsation mode amplitudes over decades, HD 60435 is known to have amplitude variations on time scales as short as 1 d. We show from 5 yr of {\it TESS} observations that there is strong amplitude modulation on this short time scale with possible mode interactions. Most remarkably, HD 60435 stopped pulsating during the time span of the {\it TESS} observations. This is the first time that any pulsating star has been observed to cease pulsating entirely. That has implications for mode interaction, excitation and damping, and is relevant to the problem of why only some stars in many pulsation instability strips pulsate, while others do not. During a 24.45-d time span of the {\it TESS} data when there was mode stability for a dipole mode and a quadrupole mode, the oblique pulsator model constrained $i$ and $\beta$, which we used to model those modes with a magnetic pulsation model from which we determined a polar field strength of 4 kG, in good agreement with a known magnetic measurement. We modelled the frequency separations showing that they can constrain the global metallicity, something that is not possible from spectroscopy of the highly peculiar Ap atmosphere.

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arXiv2024

Benchmarking the spectroscopic masses of 249 evolved stars using asteroseismology with TESS

One way to understand planet formation is through studying the correlations between planet occurrence rates and stellar mass. However, measuring stellar mass in the red giant regime is very difficult. In particular, the spectroscopic masses of certain evolved stars, often referred to as "retired A-stars", have been questioned in the literature. Efforts to resolve this mass controversy using spectroscopy, interferometry and asteroseismology have so far been inconclusive. A recent ensemble study found a mass-dependent mass offset, but the result was based on only 16 stars. With NASA's Transiting Exoplanet Survey Satellite (TESS), we expand the investigation of the mass discrepancy to a total of 92 low-luminosity stars, synonymous with the retired A-stars. We measure their characteristic oscillation frequency, $\mathrm{\nu}_{\mathrm{max}}$, and the large frequency separation, $\mathrm{\Delta\nu}$, from their TESS photometric time series. Using these measurements and asteroseismic scaling relations, we derive asteroseismic masses and compare them with spectroscopic masses from five surveys, to comprehensively study the alleged mass-dependent mass offset. We find a mass offset between spectroscopy and seismology that increases with stellar mass. However, we note that adopting the seismic mass scale does not have a significant effect on the planet occurrence-mass-metallicity correlation for the so-called retired A-stars. We also report seismic measurements and masses for 157 higher luminosity giants (mostly helium-core-burning) from the spectroscopic surveys.

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arXiv2024

Asteroseismology of the Nearby K-Dwarf $\sigma$ Draconis using the Keck Planet Finder and TESS

Asteroseismology of dwarf stars cooler than the Sun is very challenging due to the low amplitudes and rapid timescales of oscillations. Here, we present the asteroseismic detection of solar-like oscillations at 4-minute timescales ($\nu_{\mathrm{max}}\sim4300\mu$Hz) in the nearby K-dwarf $\sigma$ Draconis using extreme precision Doppler velocity observations from the Keck Planet Finder and 20-second cadence photometry from NASA's Transiting Exoplanet Survey Satellite. The star is the coolest dwarf star to date with both velocity and luminosity observations of solar-like oscillations, having amplitudes of $5.9\pm0.8\,$cm$\,\text{s}^{-1}$ and $0.8\pm0.2$ ppm, respectively. These measured values are in excellent agreement with established luminosity-velocity amplitude relations for oscillations and provide further evidence that mode amplitudes for stars with $T_{\mathrm{eff}}<\,5500\,$K diminish in scale following a $(L/M)^{1.5}$ relation. By modeling the star's oscillation frequencies from photometric data, we measure an asteroseismic age of $4.5\pm0.9\,\rm{(ran)} \pm 1.2\,\rm{(sys)}$ Gyr. The observations demonstrate the capability of next-generation spectrographs and precise space-based photometry to extend observational asteroseismology to nearby cool dwarfs, which are benchmarks for stellar astrophysics and prime targets for directly imaging planets using future space-based telescopes.

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arXiv2024

A New Catalog of 100,000 Variable \emph{TESS} A-F Stars Reveals a Correlation Between $\delta$ Scuti Pulsator Fraction and Stellar Rotation

{\delta} Scuti variables are found at the intersection of the classical instability strip and the main sequence on the Hertzsprung-Russell diagram. With space-based photometry providing millions of light-curves of A-F type stars, we can now probe the occurrence rate of {\delta} Scuti pulsations in detail. Using 30-min cadence light-curves from NASA's Transiting Exoplanet Survey Satellite's (TESS) first 26 sectors, we identify variability in 103,810 stars within 5-24 cycles per day down to a magnitude of $T=11.25$. We fit the period-luminosity relation of the fundamental radial mode for {\delta} Scuti stars in the Gaia $G$-band, allowing us to distinguish classical pulsators from contaminants for a subset of 39,367 stars. Out of this subset, over 15,918 are found on or above the expected period-luminosity relation. We derive an empirical red edge to the classical instability strip using Gaia photometry. The center where pulsator fraction peaks at 50-70%, combined with the red edge, agree well with previous work in the Kepler field. While many variable sources are found below the period-luminosity relation, over 85% of sources inside of the classical instability strip derived in this work are consistent with being {\delta} Scuti stars. The remaining 15% of variables within the instability strip are likely hybrid or {\gamma} Doradus pulsators. Finally, we discover strong evidence for a correlation between pulsator fraction and spectral line broadening from the Radial Velocity Spectrometer (RVS) aboard the Gaia spacecraft, confirming that rotation has a role in driving pulsations in {\delta} Scuti stars.

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arXiv2024

Rotation at the Fully Convective Boundary: Insights from Wide WD + MS Binary Systems

Gyrochronology, a valuable tool for determining ages of low-mass stars where other techniques fail, relies on accurate calibration. We present a sample of 185 wide ($>$$100$ au) white dwarf + main sequence (WD + MS) binaries. Total ages of WDs are computed using all-sky survey photometry, Gaia parallaxes, and WD atmosphere models. Using a magnetic braking law calibrated against open clusters, along with assumptions about initial conditions and angular momentum transport, we construct gyrochrones to predict the rotation periods of MS stars. Both data and models show that, at the fully convective boundary (FCB), MS stars with WD ages up to 7.5 Gyr and within a $<50$ K effective temperature range experience up to a threefold increase in rotation period relative to stars slightly cooler than the FCB. We suggest that rapid braking at this boundary is driven by a sharp rise in the convective overturn timescale ($\tau_{\mathrm{cz}}$) caused by structural changes between partially and fully convective stars and the $^3 \textrm{He}$ instability occurring at this boundary. While the specific location in mass (or temperature) of this feature varies with model physics, we argue that its existence remains consistent. Stars along this feature exhibit rotation periods that can be mapped, within 1$\sigma$, to a range of gyrochrones spanning $\approx 6$ Gyr. Due to current temperature errors ($\simeq$$50$ K), this implies that a measured rotation period cannot be uniquely associated to a single gyrochrone, implying that gyrochronology may not be feasible for M dwarfs very close to the FCB.

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arXiv2024

The Gasing Pangkah Collaboration: I. Asteroseismic Identification and Characterisation of a Rapidly-Rotating Engulfment Candidate

We report the discovery and characterisation of TIC 350842552 ("Zvrk"), an apparently isolated, rapidly-rotating ($P_\text{rot} \sim 99\ \mathrm{d}$) red giant observed by TESS in its Southern Continuous Viewing Zone. The star's fast surface rotation is independently verified by the use of p-mode asteroseismology, strong periodicity in TESS and ASAS-SN photometry, and measurements of spectroscopic rotational broadening. A two-component fit to APOGEE spectra indicates a coverage fraction of its surface features consistent with the amplitude of the photometric rotational signal. Variations in the amplitude of its photometric modulations over time suggest the evolution of its surface morphology, and therefore enhanced magnetic activity. We further develop and deploy new asteroseismic techniques to characterise radial differential rotation, and find weak evidence for rotational shear within Zvrk's convective envelope. This feature, in combination with such a high surface rotation rate, is incompatible with models of angular-momentum transport in single-star evolution. Spectroscopic abundance estimates also indicate a high lithium abundance, among other chemical anomalies. Taken together, all of these suggest a planet-ingestion scenario for the formation of this rotational configuration, various models for which we examine in detail.

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arXiv2024

Identifying 850 delta Scuti pulsators in a narrow Gaia colour range with TESS 10-minute full-frame images

We use TESS 10-minute Full Frame Images (Sectors 27-55) to study a sample of 1708 stars within 500 pc of the Sun that lie in a narrow colour range in the centre of the delta Scuti instability strip (0.29 < BP-RP < 0.31). Based on the Fourier amplitude spectra, we identify 848 delta Scuti stars, as well as 47 eclipsing or contact binaries. The strongest pulsation modes of some delta Scuti stars fall on the period-luminosity relation of the fundamental radial mode but many correspond to overtones that are approximately a factor of two higher in frequency. Many of the low-luminosity delta Scuti stars show a series of high-frequency modes with very regular spacings. The fraction of stars in our sample that show delta Scuti pulsations is about 70% for the brightest stars (G<8), consistent with results from Kepler. However, the fraction drops to about 45% for fainter stars and we find that a single sector of TESS data only detects the lowest-amplitude delta Scuti pulsations (around 50 ppm) in stars down to about G=9. Finally, we have found four new high-frequency delta Scuti stars with very regular mode patterns, and have detected pulsations in lambda Mus that make it the fourth-brightest delta Scuti in the sky (G=3.63). Overall, these results confirm the power of TESS and Gaia for studying pulsating stars.

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arXiv2023

HIP 65426 is a High-Frequency Delta Scuti Pulsator in Plausible Spin-Orbit Alignment with its Directly Imaged Exoplanet

HIP 65426 hosts a young giant planet that has become the first exoplanet directly imaged with JWST. Using time-series photometry from the Transiting Exoplanet Survey Satellite (TESS), we classify HIP 65426 as a high-frequency $\delta$ Scuti pulsator with a possible large frequency separation of $\Delta \nu =$7.23$\pm$0.02 cycles day$^{-1}$. We check the TESS data for pulsation timing variations and use the nondetection to estimate a 95% dynamical mass upper limit of 12.8 Mjup for HIP 65426 b. We also identify a low-frequency region of signal that we interpret as stellar latitudinal differential rotation with two rapid periods of 7.85$\pm$0.08 hr and 6.67$\pm$0.04 hr. We use our TESS rotation periods together with published values of radius and $v \sin{i}$ to jointly measure the inclination of HIP 65426 to $i_{\star}=107_{-11}^{+12}$$^\circ$. Our stellar inclination is consistent with the orbital inclination of HIP 65426 b ($108_{-3}^{+6}$$^{\circ}$) at the $68\%$ percent level based on our orbit fit using published relative astrometry. The lack of significant evidence for spin-orbit misalignment in the HIP 65426 system supports an emerging trend consistent with preferential alignment between imaged long-period giant planets and their host stars.

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arXiv2023

The far side of the Galactic bar/bulge revealed through semi-regular variables

The Galactic bulge and bar are critical to our understanding of the Milky Way. However, due to the lack of reliable stellar distances, the structure and kinematics of the bulge/bar beyond the Galactic center have remained largely unexplored. Here, we present a method to measure distances of luminous red giants using a period-amplitude-luminosity relation anchored to the Large Magellanic Cloud, with random uncertainties of 10-15% and systematic errors below 1-2%. We apply this method to data from the Optical Gravitational Lensing Experiment (OGLE) to measure distances to $190,302$ stars in the Galactic bulge and beyond out to 20 kpc. Using this sample we measure a distance to the Galactic center of $R_0$ = $8108\pm106_{\rm stat}\pm93_{\rm sys}$ pc, consistent with astrometric monitoring of stars orbiting Sgr A*. We cross-match our distance catalog with Gaia DR3 and use the subset of $39,566$ overlapping stars to provide the first constraints on the Milky Way's velocity field ($V_R,V_ϕ,V_z$) beyond the Galactic center. We show that the $V_R$ quadrupole from the bar's near side is reflected with respect to the Galactic center, indicating that the bar is both bi-symmetric and aligned with the inner disk, and therefore dynamically settled along its full extent. We also find that the vertical height $V_Z$ map has no major structure in the region of the Galactic bulge, which is inconsistent with a current episode of bar buckling. Finally, we demonstrate with N-body simulations that distance uncertainty plays a major factor in the alignment of the major and kinematic axes of the bar and distribution of velocities, necessitating caution when interpreting results for distant stars.

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arXiv2023

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 planets. Until now, the absence of planets with short orbital periods around post-expansion, core-helium-burning red giants has been interpreted as evidence that short-period planets around Sun-like stars do not survive the giant expansion phase of their host stars. Here we present the discovery that the giant planet 8 Ursae Minoris b 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 planet. 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.

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arXiv2023

A non-interacting Galactic black hole candidate in a binary system with a main-sequence star

We describe the discovery of a solar neighborhood (d=468 pc) binary system with a main-sequence sunlike star and a massive non-interacting black hole candidate. The spectral energy distribution (SED) of the visible star is described by a single stellar model. We derive stellar parameters from a high signal-to-noise Magellan/MIKE spectrum, classifying the star as a main-sequence star with $T_{\rm eff} = 5972 \rm K$, $\log{g} = 4.54$, and $M = 0.91$ \msun. The spectrum shows no indication of a second luminous component. To determine the spectroscopic orbit of the binary, we measured radial velocities of this system with the Automated Planet Finder, Magellan, and Keck over four months. We show that the velocity data are consistent with the \textit{Gaia} astrometric orbit and provide independent evidence for a massive dark companion. From a combined fit of our spectroscopic data and the astrometry, we derive a companion mass of $11.39^{+1.51}_{-1.31}$\msun. We conclude that this binary system harbors a massive black hole on an eccentric $(e =0.46 \pm 0.02)$, $185.4 \pm 0.1$ d orbit. These conclusions are independent of \cite{ElBadry2022Disc}, who recently reported the discovery of the same system. A joint fit to all available data (including \cite{ElBadry2022Disc}'s) yields a comparable period solution, but a lower companion mass of $9.32^{+0.22}_{-0.21} M_{\odot}$. Radial velocity fits to all available data produce a unimodal solution for the period that is not possible with either data set alone. The combination of both data sets yields the most accurate orbit currently available.

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arXiv2023

A prescription for the asteroseismic surface correction

In asteroseismology, the surface effect refers to a disparity between the observed and the modelled frequencies in stars with solar-like oscillations. It originates from improper modelling of the surface layers. Correcting the surface effect usually requires using functions with free parameters, which are conventionally fitted to the observed frequencies. On the basis that the correction should vary smoothly across the H--R diagram, we parameterize it as a simple function of surface gravity, effective temperature, and metallicity. We determine this function by fitting a wide range of stars. The absolute amount of the surface correction decreases with luminosity, but the ratio between it and $ν_{\rm max}$ increases, suggesting the surface effect is more important for red giants than dwarfs. Applying the prescription can eliminate unrealistic surface correction, which improves parameter estimations with stellar modelling. Using two open clusters, we found a reduction of scatter in the model-derived ages for each star in the same cluster. As an important application, we provide a new revision for the $Δν$ scaling relation that, for the first time, accounts for the surface correction. The values of the correction factor, $f_{Δν}$, are up to 2\% smaller than those determined without the surface effect considered, suggesting decreases of up to 4\% in radii and up to 8\% in masses when using the asteroseismic scaling relations. This revision brings the asteroseismic properties into an agreement with those determined from eclipsing binaries. The new correction factor and the stellar models with the corrected frequencies are available at {https://www.github.com/parallelpro/surface}.

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arXiv2023

First Observations of the Brown Dwarf HD 19467 B with JWST

We observed HD 19467 B with JWST's NIRCam in six filters spanning 2.5-4.6 $μm$ with the Long Wavelength Bar coronagraph. The brown dwarf HD 19467 B was initially identified through a long-period trend in the radial velocity of G3V star HD 19467. HD 19467 B was subsequently detected via coronagraphic imaging and spectroscopy, and characterized as a late-T type brown dwarf with approximate temperature $\sim1000$K. We observed HD 19467 B as a part of the NIRCam GTO science program, demonstrating the first use of the NIRCam Long Wavelength Bar coronagraphic mask. The object was detected in all 6 filters (contrast levels of $2\times10^{-4}$ to $2\times10^{-5}$) at a separation of 1.6 arcsec using Angular Differential Imaging (ADI) and Synthetic Reference Differential Imaging (SynRDI). Due to a guidestar failure during acquisition of a pre-selected reference star, no reference star data was available for post-processing. However, RDI was successfully applied using synthetic Point Spread Functions (PSFs) developed from contemporaneous maps of the telescope's optical configuration. Additional radial velocity data (from Keck/HIRES) are used to constrain the orbit of HD 19467 B. Photometric data from TESS are used to constrain the properties of the host star, particularly its age. NIRCam photometry, spectra and photometry from literature, and improved stellar parameters are used in conjunction with recent spectral and evolutionary substellar models to derive physical properties for HD 19467 B. Using an age of 9.4$\pm$0.9 Gyr inferred from spectroscopy, Gaia astrometry, and TESS asteroseismology, we obtain a model-derived mass of 62$\pm 1M_{J}$, which is consistent within 2-$σ$ with the dynamically derived mass of 81$^{+14}_{-12}M_{J}$.

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arXiv2022

TESS observations of the Pleiades cluster: a nursery for delta Scuti stars

We studied 89 A- and F-type members of the Pleiades open cluster, including five escaped members. We measured projected rotational velocities (v sin i) for 49 stars and confirmed that stellar rotation causes a broadening of the main sequence in the color-magnitude diagram. Using time-series photometry from NASA's TESS Mission (plus one star observed by Kepler/K2), we detected delta Scuti pulsations in 36 stars. The fraction of Pleiades stars in the middle of the instability strip that pulsate is unusually high (over 80%), and their range of effective temperatures agrees well with theoretical models. On the other hand, the characteristics of the pulsation spectra are varied and do not correlate with stellar temperature, calling into question the existence of a useful nu_max relation for delta Scutis, at least for young stars. By including delta Scuti stars observed in the Kepler field, we show that the instability strip is shifted to the red with increasing distance by interstellar reddening. Overall, this work demonstrates the power of combining observations with Gaia and TESS for studying pulsating stars in open clusters.

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arXiv2022

Revisiting bright delta Scuti stars and their period-luminosity relation with TESS and Gaia DR3

We have used NASA's TESS mission to study catalogued delta Scuti stars. We examined TESS light curves for 434 stars, including many for which few previous observations exist. We found that 62 are not delta Scuti pulsators, with most instead showing variability from binarity. For the 372 delta Scuti stars, we provide a catalogue of the period and amplitude of the dominant pulsation mode. Using Gaia DR3 parallaxes, we place the stars in the period-luminosity diagram and confirm previous findings that most stars lie on a ridge that corresponds to pulsation in the fundamental radial mode, and that many others fall on a second ridge that is a factor two shorter in period. This second ridge is seen more clearly than before, thanks to the revised periods and distances. We demonstrate the value of the period-luminosity diagram in distinguishing delta Scuti stars from short-period RR Lyrae stars, and we find several new examples of high-frequency delta Scuti stars with regular sequences of overtone modes, including XX Pyx and 29 Cyg. Finally, we revisit the sample of delta Scuti stars observed by Kepler and show that they follow a tight period-density relation, with a pulsation constant for the fundamental mode of Q = 0.0315 d.

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arXiv2022

Discovery of post-mass-transfer helium-burning red giants using asteroseismology

A star expands to become a red giant when it has fused all the hydrogen in its core into helium. If the star is in a binary system, its envelope can overflow onto its companion or be ejected into space, leaving a hot core and potentially forming a subdwarf-B star. However, most red giants that have partially transferred envelopes in this way remain cool on the surface and are almost indistinguishable from those that have not. Among $\sim$7000 helium-burning red giants observed by NASA's Kepler mission, we use asteroseismology to identify two classes of stars that must have undergone dramatic mass loss, presumably due to stripping in binary interactions. The first class comprises about 7 underluminous stars with smaller helium-burning cores than their single-star counterparts. Theoretical models show that these small cores imply the stars had much larger masses when ascending the red giant branch. The second class consists of 32 red giants with masses down to 0.5 M$_\odot$, whose implied ages would exceed the age of the universe had no mass loss occurred. The numbers are consistent with binary statistics, and our results open up new possibilities to study the evolution of post-mass-transfer binary systems.

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arXiv2021

TESS Eclipsing Binary Stars. I. Short cadence observations of 4584 eclipsing binaries in Sectors 1-26

In this paper we present a catalog of 4584 eclipsing binaries observed during the first two years (26 sectors) of the TESS survey. We discuss selection criteria for eclipsing binary candidates, detection of hither-to unknown eclipsing systems, determination of the ephemerides, the validation and triage process, and the derivation of heuristic estimates for the ephemerides. Instead of keeping to the widely used discrete classes, we propose a binary star morphology classification based on a dimensionality reduction algorithm. Finally, we present statistical properties of the sample, we qualitatively estimate completeness, and discuss the results. The work presented here is organized and performed within the TESS Eclipsing Binary Working Group, an open group of professional and citizen scientists; we conclude by describing ongoing work and future goals for the group. The catalog is available from http://tessEBs.villanova.edu and from MAST.

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