arXiv subjects

A. K. Dupree

Publications and source records attributed to A. K. Dupree.

At least 19 recordsLinked to original sources

arXiv2026

New Spatially Resolved Observations of Betelgeuse from the VLA and ALMA: Evidence for Atmospheric Perturbations from a Close Companion

We present new spatially resolved observations of the red supergiant Betelgeuse ($α$ Orionis) at seven frequencies: 22 GHz ($λ$1.36cm); 44 GHz ($λ$6.8mm); 107 GHz ($λ$2.8mm); 136 GHz ($λ$2.2mm); 224 GHz ($λ$1.3mm); 338 GHz ($λ$0.89mm); and 485 GHz ($λ$0.62mm) using the Karl G. Jansky Very Large Array and the Atacama Large Millimeter/submillimeter Array. These observations (spanning 2021-2026) enable a characterization of the mean brightness temperature ($T_B$), shape, and symmetry of the star's extended atmosphere. At 22 GHz and 44 GHz $T_B$ has increased following the historic lows observed in 2019, just prior to the Great Dimming of late 2019/early 2020. We find that $T_B$ peaks at a larger projected radius ($r\sim2.7R_{\star}$) compared with measurements prior to 2016, consistent with a possible evolution in the atmospheric density profile. In addition we find a systematic decrease in $T_B$ from $\approx3500$K at $\sim2.7R_{\star}$ to $T_{B}\approx$2220K at $\sim1.2R_{\star}$, confirming a previously reported temperature minimum between the photosphere and chromosphere. At all observed frequencies $T_B$ is lower than temperatures inferred from ultraviolet measurements at comparable radii, indicating the continued presence of cooler plasma ($T\le T_{\rm eff}$) mixed with the warmer chromospheric material. The new measurements yield a global spectral index $α=1.35\pm$0.03, with no evidence for a change in $α$ over more than two decades in frequency. Recently, evidence of a close-in companion to Betelgeuse has been reported, with an orbital separation $r\sim2.3R_{\star}$. We have identified a correlation between the ellipticity of Betelgeuse, as obtained from radio measurements, and the companion's orbital phase. The results are consistent with periodic perturbations of the atmosphere by the companion.

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arXiv2023

The dusty circumstellar environment of Betelgeuse during the Great Dimming as seen by VLTI/MATISSE

The 'Great Dimming' of the prototypical red supergiant Betelgeuse, which occurred between December 2019 and April 2020, gives us unprecedented insight into the processes occurring on the stellar surface and in the inner wind of this type of star. In particular it may bring further understanding of their dust nucleation and mass loss processes. Here, we present and analyse VLTI/MATISSE observations in the N-band (8 - 13 $μ$m) taken near the brightness minimum in order to assess the status of the dusty circumstellar environment. We explore the compatibility of a dust clump obscuring the star with our mid-infrared interferometric observations using continuum 3D radiative transfer modelling, and probe the effect of adding multiple clumps close to the star on the observables. We also test the viability of a large cool spot on the stellar surface without dust present in the ambient medium. Using the visibility data, we derive a uniform disk diameter of 59.02 $\pm$ 0.64 mas in the spectral range 8 to 8.75 $μ$m. We find that both the dust clump and the cool spot models are compatible with the data. Further to this, we note that the extinction and emission of our localised dust clump in the line of sight of the star, directly compensate each other making the clump undetectable in the spectral energy distribution and visibilities. The lack of infrared brightening during the 'Great Dimming' therefore does not exclude extinction due to a dust clump as one of the possible mechanisms. The visibilities can be reproduced by a spherical wind with dust condensing at 13 stellar radii and a dust mass-loss rate of (2.1 - 4.9) $\times$ 10$^{-10}$ $\mathit{M}_{\odot} {\rm yr}^{-1}$, however, in order to reproduce the complexity of the observed closure phases, additional surface features or dust clumps would be needed.

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arXiv2022

Spatially Resolved Observations of Betelgeuse at 7mm and 1.3cm Just Prior to the Great Dimming

We present spatially resolved observations of Betelgeuse (Alpha Orionis) obtained with the Karl G. Jansky Very Large Array (VLA) at wavelengths of ~7mm (44~GHz) and ~1.3cm (22~GHz) on 2019 August 2, just prior to the onset of the historical optical dimming that occurred between late 2019 and early 2020. Our measurements suggest recent changes in the temperature and density structure of the atmosphere between radii r~2-3R*. At 7mm the star is ~20% dimmer than in previously published observing epochs between 1996--2004. We measure a mean gas temperature of T_B = 2270 +\- 260 K at r~2.1R*, where R* is the canonical photospheric radius. This is ~2 sigma lower than previously reported temperatures at comparable radii and >1200K lower than predicted by previous semi-empirical models of the atmosphere. The measured brightness temperature at r~2.6R* (T_B = 2580 +\- 260 K) is also cooler than expected based on trends in past measurements. The stellar brightness profile in our current measurements appears relatively smooth and symmetric, with no obvious signatures of giant convective cells or other surface features. However, the azimuthally averaged brightness profile is found to be more complex than a uniform elliptical disk. Our observations were obtained approximately six weeks before spectroscopic measurements in the ultraviolet revealed evidence of increases in the chromospheric electron density in the southern hemisphere of Betelgeuse, coupled with a large-scale outflow. We discuss possible scenarios linking these events with the observed radio properties of the star, including the passage of a strong shock wave.

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arXiv2022

A dusty veil shading Betelgeuse during its Great Dimming

Red supergiants are the most common final evolutionary stage of stars that have initial masses between 8 and 35 times that of the Sun. During this stage, which lasts roughly 100,000 years1, red supergiants experience substantial mass loss. However, the mechanism for this mass loss is unknown. Mass loss may affect the evolutionary path, collapse and future supernova light curve of a red supergiant, and its ultimate fate as either a neutron star or a black hole. From November 2019 to March 2020, Betelgeuse - the second-closest red supergiant to Earth (roughly 220 parsecs, or 724 light years, away) - experienced a historic dimming of its visible brightness. Usually having an apparent magnitude between 0.1 and 1.0, its visual brightness decreased to 1.614 +/- 0.008 magnitudes around 7-13 February 2020 - an event referred to as Betelgeuse's Great Dimming. Here we report high-angular-resolution observations showing that the southern hemisphere of Betelgeuse was ten times darker than usual in the visible spectrum during its Great Dimming. Observations and modelling support a scenario in which a dust clump formed recently in the vicinity of the star, owing to a local temperature decrease in a cool patch that appeared on the photosphere. The directly imaged brightness variations of Betelgeuse evolved on a timescale of weeks. Our findings suggest that a component of mass loss from red supergiants is inhomogeneous, linked to a very contrasted and rapidly changing photosphere

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arXiv2021

The Loudest Stellar Heartbeat: Characterizing the most extreme amplitude heartbeat star system

We characterize the extreme heartbeat star system MACHO 80.7443.1718 in the LMC using TESS photometry and spectroscopic observations from the Magellan Inamori Kyocera Echelle (MIKE) and SOAR Goodman spectographs. MACHO 80.7443.1718 was first identified as a heartbeat star system in the All-Sky Automated Survey for SuperNovae (ASAS-SN) with $P_{\rm orb}=32.836\pm0.008\,{\rm d}$. MACHO 80.7443.1718 is a young (${\sim}6$~Myr), massive binary, composed of a B0 Iae supergiant with $M_1 \simeq 35 M_\odot$ and an O9.5V secondary with $M_2 \simeq 16 M_\odot$ on an eccentric ($e=0.51\pm0.03$) orbit. In addition to having the largest variability amplitude amongst all known heartbeats stars, MACHO 80.7443.1718 is also one of the most massive heartbeat stars yet discovered. The B[e] supergiant has Balmer emission lines and permitted/forbidden metallic emission lines associated with a circumstellar disk. The disk rapidly dissipates at periastron which could indicate mass transfer to the secondary, but re-emerges immediately following periastron passage. MACHO 80.7443.1718 also shows tidally excited oscillations at the $N=25$ and $N=41$ orbital harmonics and has a rotational period of 4.4 d.

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arXiv2019

First evidence of enhanced recombination in astrophysical environments and the implications for plasma diagnostics

We report the first unambiguous observational evidence of Rydberg Enhanced Recombination (RER), a potentially important recombination mechanism that has hitherto been unexplored in low-temperature photoionized plasmas. RER shares similarities with dielectronic recombination, with the difference that the electron is captured into a highly excited state below the ionization threshold -- rather than above the threshold -- of the recombining ion. We predict transitions of carbon and oxygen ions that are formed via the RER process, and their relative strengths with collisional-radiative spectral models. Optical C II RER features are detected in published high-resolution spectra of eight planetary nebulae, and a C III transition has been found in the ultraviolet spectrum in a symbiotic star system. The relative intensities of these lines are consistent with their production by this recombination mechanism. Because RER has not previously been accounted for in photoionized plasmas, its inclusion in models can significantly impact the predicted ionization balance and hence abundance calculations of important astrophysical species. Calculations for C+ suggest that the enhancement in the total recombination rate can amount to a factor of 2.2 at 8100 K, increasing to 7.5 at T = 3500 K. These results demonstrate the importance of including RER in models of photoionized astrophysical plasmas and in elemental abundance determinations.

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arXiv2017

NGC 1866: First Spectroscopic Detection of Fast Rotating Stars in a Young LMC Cluster

High-resolution spectroscopic observations were taken of 29 extended main sequence turn-off (eMSTO) stars in the young ($\sim$200 Myr) LMC cluster, NGC 1866 using the Michigan/Magellan Fiber System and MSpec spectrograph on the Magellan-Clay 6.5-m telescope. These spectra reveal the first direct detection of rapidly rotating stars whose presence has only been inferred from photometric studies. The eMSTO stars exhibit H-alpha emission (indicative of Be-star decretion disks), others have shallow broad H-alpha absorption (consistent with rotation $\gtrsim $150 km s$^{-1}$), or deep H-alpha core absorption signaling lower rotation velocities ($ \lesssim $150 km s$^{-1}$ ). The spectra appear consistent with two populations of stars - one rapidly rotating, and the other, younger and slowly rotating.

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arXiv2017

The Kepler Follow-Up Observation Program. I. A Catalog of Companions to Kepler Stars from High-Resolution Imaging

We present results from high-resolution, optical to near-IR imaging of host stars of Kepler Objects of Interest (KOIs), identified in the original Kepler field. Part of the data were obtained under the Kepler imaging follow-up observation program over seven years (2009 - 2015). Almost 90% of stars that are hosts to planet candidates or confirmed planets were observed. We combine measurements of companions to KOI host stars from different bands to create a comprehensive catalog of projected separations, position angles, and magnitude differences for all detected companion stars (some of which may not be bound). Our compilation includes 2297 companions around 1903 primary stars. From high-resolution imaging, we find that ~10% (~30%) of the observed stars have at least one companion detected within 1" (4"). The true fraction of systems with close (< ~4") companions is larger than the observed one due to the limited sensitivities of the imaging data. We derive correction factors for planet radii caused by the dilution of the transit depth: assuming that planets orbit the primary stars or the brightest companion stars, the average correction factors are 1.06 and 3.09, respectively. The true effect of transit dilution lies in between these two cases and varies with each system. Applying these factors to planet radii decreases the number of KOI planets with radii smaller than 2 R_Earth by ~2-23% and thus affects planet occurrence rates. This effect will also be important for the yield of small planets from future transit missions such as TESS.

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arXiv2016

Chromospheric Models and the Oxygen Abundance in Giant Stars

Realistic stellar atmospheric models of two typical metal-poor giant stars in Omega Centauri that include a chromosphere influence the formation of optical lines of Oxygen I: the forbidden lines (630nm, 636nm) and the infrared triplet (777.1-777.5 nm). One-dimensional semi-empirical non-LTE models are constructed based on observed Balmer lines. A full non-LTE formulation is applied in evaluating line strengths of O I including photoionization by the Lyman continuum and photoexcitation by Ly-alpha and Ly-beta. Chromospheric models (CHR) yield forbidden oxygen transitions that are stronger than in radiative/convective equilibrium (RCE) models. The triplet oxygen lines from high levels also appear stronger than produced in an RCE model. The inferred oxygen abundance from realistic CHR models for these two stars is decreased by factors ~3 as compared to values derived from RCE models. A lower oxygen abundance suggests that intermediate mass AGB stars contribute to the observed abundance pattern in globular clusters. A change in the oxygen abundance of metal-poor field giants could affect models of deep mixing episodes on the red giant branch. Changes in the oxygen abundance can impact other abundance determinations critical to astrophysics including chemical tagging techniques and galactic chemical evolution.

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arXiv2015

The 10830 Angstrom Helium Line Among Evolved Stars in the Globular Cluster M4

Helium is a pivotal element in understanding multiple main sequences and extended horizontal branches observed in some globular clusters. Here we present a spectroscopic study of helium in the nearby globular cluster M4. We have obtained spectra of the chromospheric He I 10830 A line in 16 red horizontal branch, red giant branch, and asymptotic giant branch stars. Clear He I absorption or emission is present in most of the stars. Effective temperature is the principal parameter that correlates with 10830 A line strength. Stars with T_eff < 4450 K do not exhibit the helium line. Red horizontal branch stars, which are the hottest stars in our sample, all have strong He I line absorption. A number of these stars show very broad 10830 A lines with shortward extensions indicating outflows as high as 80-100 km/s and the possibility of mass loss. We have also derived [Na/Fe] and [Al/Fe] abundances to see whether these standard tracers of "second generation" cluster stars are correlated with He I line strength. Unlike the case for our previous study of Omega Cen, no clear correlation is observed. This may be because the sample does not cover the full range of abundance variations found in M4, or simply because the physical conditions in the chromosphere, rather than the helium abundance, primarily determine the He I 10830 A line strength. A larger sample of high-quality He I spectra of both "first" and "second" generation red giants within a narrow range of T_eff and luminosity is needed to test for the subtle spectroscopic variations in He I expected in M4.

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arXiv2014

HST FUV monitoring of TW Hya

Classical T Tauri stars (CTTS) are young (< 10 Myr), cool stars that actively accrete matter from a disk. They show strong, broad and asymmetric, atomic FUV emission lines. Neither the width, nor the line profile is understood. Likely, different mechanisms influence the line profile; the best candidates are accretion, winds and stellar activity. We monitored the C IV 1548/1550 Ang doublet in the nearby, bright CTTS TW Hya with the Hubble Space Telescope Cosmic Origin Spectrograph (HST/COS) to correlate it with i) the cool wind, as seen in COS NUV Mg II line profiles, ii) the photometric period from joint ground-based monitoring, iii) the accretion rate as determined from the UV continuum, and iv) the Ha line profile from independent ground-based observations. The observations span 10 orbits distributed over a few weeks to cover the typical time scales of stellar rotation, accretion and winds. Here we describe a model with intrinsically asymmetric C IV lines.

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arXiv2014

Structure and Dynamics of the Accretion Process and Wind in TW Hya

Time-domain spectroscopy of the classical accreting T Tauri star, TW Hya, covering a decade and spanning the far UV to the near-infrared spectral regions can identify the radiation sources, the atmospheric structure produced by accretion,and properties of the stellar wind. On time scales from days to years, substantial changes occur in emission line profiles and line strengths. Our extensive time-domain spectroscopy suggests that the broad near-IR, optical, and far-uv emission lines, centered on the star, originate in a turbulent post-shock region and can undergo scattering by the overlying stellar wind as well as some absorption from infalling material. Stable absorption features appear in H-alpha, apparently caused by an accreting column silhouetted in the stellar wind. Inflow of material onto the star is revealed by the near-IR He I 10830A line, and its free-fall velocity correlates inversely with the strength of the post-shock emission, consistent with a dipole accretion model. However, the predictions of hydrogen line profiles based on accretion stream models are not well-matched by these observations. Evidence of an accelerating warm to hot stellar wind is shown by the near-IR He I line, and emission profiles of C II, C III, C IV, N V, and O VI. The outflow of material changes substantially in both speed and opacity in the yearly sampling of the near-IR He I line over a decade. Terminal outflow velocities that range from 200 km/s to almost 400 km/s in He I appear to be directly related to the amount of post- shock emission, giving evidence for an accretion-driven stellar wind. Calculations of the emission from realistic post- shock regions are needed.

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arXiv2013

On the Galactic chemical evolution of sulphur. Sulphur abundances from the [S i] 1082 nm line in giants

Context. The Galactic chemical evolution of sulphur is still under debate. At low metallicities some studies find no correlation between [S/Fe] and [Fe/H], others find [S/Fe] increasing towards lower metallicities, and still others find a combination of the two. Each scenario has different implications for the Galactic chemical evolution of sulphur. Aims. To contribute to the discussion on the Galactic chemical evolution of sulphur by deriving sulphur abundances from non-LTE insensitive spectral diagnostics in Disk and Halo stars with homogeneously determined stellar parameters. Methods. We derive Teff from photometric colours, logg from stellar isochrones and Bayesian estimation, and [Fe/H] and [S/Fe] from spectrum synthesis. We derive [S/Fe] from the [S i] 1082 nm line in 39 mostly cool and metal-poor giants, using 1D LTE MARCS model atmospheres to model our high-resolution NIR spectra obtained with the VLT, NOT and Gemini South telescopes. Results. We derive homogeneous stellar parameters for 29 stars. Our results argue for a chemical evolution of sulphur that is typical for alpha-elements, contrary to some previous studies. Our abundances are systematically higher by about 0.1 dex in comparison to other studies that arrived at similar conclusions using other sulphur diagnostics. Conclusions. We find the [S i] line to be a valuable diagnostic of sulphur abundances in cool giants down to [Fe/H] ~ -2.3. We argue that a homogeneous determination of stellar parameters is necessary, since the derived abundances are sensitive to them. Our results ([S/Fe]) show reasonable agreement with predictions of contemporary models of Galactic chemical evolution. In these models sulphur is predominantly created in massive stars by oxygen burning, and ejected in the ISM during Type II SNe explosions. Systematic differences with previous studies likely fall within modelling uncertainties.

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arXiv2013

Direct Evaluation of the Helium Abundances in Omega Centauri

A direct measure of the helium abundances from the near-infrared transition of He I at 1.08 micron is obtained for two nearly identical red giant stars in the globular cluster Omega Centauri. One star exhibits the He I line; the line is weak or absent in the other star. Detailed non-LTE semi-empirical models including expansion in spherical geometry are developed to match the chromospheric H-alpha, H-beta, and Ca II K lines, in order to predict the helium profile and derive a helium abundance. The red giant spectra suggest a helium abundance of Y less than or equal 0.22 (LEID 54064) and Y=0.39-0.44 (LEID 54084) corresponding to a difference in the abundance Delta Y greater or equal than 0.17.Helium is enhanced in the giant star (LEID 54084) that also contains enhanced aluminum and magnesium. This direct evaluation of the helium abundances gives observational support to the theoretical conjecture that multiple populations harbor enhanced helium in addition to light elements that are products of high-temperature hydrogen burning. We demonstrate that the 1.08 micron He I line can yield a helium abundance in cool stars when constraints on the semi-empirical chromospheric model are provided by other spectroscopic features.

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arXiv2013

Kepler-62: A five-planet system with planets of 1.4 and 1.6 Earth radii in the Habitable Zone

We present the detection of five planets -- Kepler-62b, c, d, e, and f -- of size 1.31, 0.54, 1.95, 1.61 and 1.41 Earth radii, orbiting a K2V star at periods of 5.7, 12.4, 18.2, 122.4 and 267.3 days, respectively. The outermost planets (Kepler-62e & -62f) are super-Earth-size (1.25 < planet radius/earth radius < 2.0) planets in the habitable zone (HZ) of their host star, receiving 1.2 +- 0.2 and 0.41 +- 0.05 times the solar flux at Earth's orbit. Theoretical models of Kepler-62e and -62f for a stellar age of ~7 Gyr suggest that both planets could be solid: either with a rocky composition or composed of mostly solid water in their bulk.

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arXiv2013

Direct Ultraviolet Imaging and Spectroscopy of Betelgeuse

Direct images of Betelgeuse were obtained over a span of 4 years with the Faint Object Camera on the Hubble Space Telescope. These images reveal the extended ultraviolet continuum emission (about 2 times the optical diameter), the varying overall ultraviolet flux levels and a pattern of bright surface continuum features that change in position and appearance over several months or less. Concurrent photometry and radial velocity measures support the model of a pulsating star, first discovered in the ultraviolet from IUE. Spatially resolved HST spectroscopy reveals a larger extention in chromospheric emissions of Mg II as well as the rotation of the supergiant. Changing localized subsonic flows occur in the low chromosphere that can cover a substantial fraction of the stellar disk and may initiate the mass outflow.

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arXiv2013

New Insights: the Accretion Process and Variable Wind from TW Hya

For the first time in a classical T Tauri star, we are able to trace an accretion event signaled by an hour-long enhancement of X-rays from the accretion shock and revealed through substantial sequential changes in optical emission line profiles. Downflowing turbulent material appears in H-alpha and H-beta emission. He D3 (5876 Angstrom) broadens, coupled with an increase in flux. Two hours after the X-ray accretion event, the optical veiling increases due to continuum emission from the hot splashdown region. The response of the stellar coronal emission to the heated photosphere follows about 2.4 hours later, giving direct evidence that the stellar corona is heated in part by accretion. Then, the stellar wind becomes re-established. A model that incorporates the dynamics of this sequential series of events includes: an accretion shock, a cooling downflow in a supersonically turbulent region, followed by photospheric and later, coronal heating. This model naturally explains the presence of broad optical and ultraviolet lines, and affects the mass accretion rates currently determined from emission line profiles. These results, coupled with the large heated coronal region revealed from X-ray diagnostics, suggest that current models are not adequate to explain the accretion process in young stars.

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arXiv2012

X-Ray Determination of the Variable Rate of Mass Accretion onto TW Hydrae

Diagnostics of electron temperature (T_e), electron density (n_e), and hydrogen column density (N_H) from the Chandra High Energy Transmission Grating spectrum of He-like Ne IX in TW Hydrae (TW Hya), in conjunction with a classical accretion model, allow us to infer the accretion rate onto the star directly from measurements of the accreting material. The new method introduces the use of the absorption of Ne IX lines as a measure of the column density of the intervening, accreting material. On average, the derived mass accretion rate for TW Hya is 1.5 x 10^{-9} M_{\odot} yr^{-1}, for a stellar magnetic field strength of 600 Gauss and a filling factor of 3.5%. Three individual Chandra exposures show statistically significant differences in the Ne IX line ratios, indicating changes in N_H, T_e, and n_e by factors of 0.28, 1.6, and 1.3, respectively. In exposures separated by 2.7 days, the observations reported here suggest a five-fold reduction in the accretion rate. This powerful new technique promises to substantially improve our understanding of the accretion process in young stars.

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