arXiv subjects

Douglas R. Gies

Publications and source records attributed to Douglas R. Gies.

At least 37 records · Page 2Linked to original sources

arXiv2022

Visual Orbits of Spectroscopic Binaries with the CHARA Array. IV. HD 61859, HD 89822, HD 109510, and HD 191692

We present the visual orbits of four spectroscopic binary stars, HD 61859, HD 89822, HD 109510, and HD 191692, using long baseline interferometry with the CHARA Array. We also obtained new radial velocities from echelle spectra using the APO 3.5 m, CTIO 1.5 m, and Fairborn Observatory 2.0 m telescopes. By combining the astrometric and spectroscopic observations, we solve for the full, three-dimensional orbits and determine the stellar masses to 1-12% uncertainty and distances to 0.4-6% uncertainty. We then estimate the effective temperature and radius of each component star through Doppler tomography and spectral energy distribution analyses. We found masses of 1.4-3.5 Msun, radii of 1.5-4.7 Rsun, and temperatures of 6400-10300K. We then compare the observed stellar parameters to the predictions of the stellar evolution models, but found that only one of our systems fits well with the evolutionary models.

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arXiv2022

ARMADA II: Further Detections of Inner Companions to Intermediate Mass Binaries with Micro-Arcsecond Astrometry at CHARA and VLTI

We started a survey with CHARA/MIRC-X and VLTI/GRAVITY to search for low mass companions orbiting individual components of intermediate mass binary systems. With the incredible precision of these instruments, we can detect astrometric "wobbles" from companions down to a few tens of micro-arcseconds. This allows us to detect any previously unseen triple systems in our list of binaries. We present the orbits of 12 companions around early F to B-type binaries, 9 of which are new detections and 3 of which are first astrometric detections of known RV companions. The masses of these newly detected components range from 0.45-1.3 solar masses. Our orbits constrain these systems to a high astrometric precision, with median residuals to the orbital fit of 20-50 micro-arcseconds in most cases. For 7 of these systems we include newly obtained radial velocity data, which help us to identify the system configuration and to solve for masses of individual components in some cases. Although additional RV measurements are needed to break degeneracy in the mutual inclination, we find that the majority of these inner triples are not well-aligned with the wide binary orbit. This hints that higher mass triples are more misaligned compared to solar and lower mass triples, though a thorough study of survey biases is needed. We show that the ARMADA survey is extremely successful at uncovering previously unseen companions in binaries. This method will be used in upcoming papers to constrain companion demographics in intermediate mass binary systems down to the planetary mass regime.

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arXiv2022

Spectroscopic Line Modeling of the Fastest Rotating O-type Stars

We present a spectroscopic analysis of the most rapidly rotating stars currently known, VFTS 102 ($v_{e} \sin i = 649 \pm 52$ km s$^{-1}$; O9: Vnnne+) and VFTS 285 ($v_{e} \sin i = 610 \pm 41$ km s$^{-1}$; O7.5: Vnnn), both members of the 30 Dor complex in the Large Magellanic Cloud. This study is based on high resolution ultraviolet spectra from HST/COS and optical spectra from VLT X-shooter plus archival VLT GIRAFFE spectra. We utilize numerical simulations of their photospheres, rotationally distorted shape, and gravity darkening to calculate model spectral line profiles and predicted monochromatic absolute fluxes. We use a guided grid search to investigate parameters that yield best fits for the observed features and fluxes. These fits produce estimates of the physical parameters for these stars (plus a Galactic counterpart, $ζ$ Oph) including the equatorial rotational velocity, inclination, radius, mass, gravity, temperature, and reddening. We find that both stars appear to be radial velocity constant. VFTS 102 is rotating at critical velocity, has a modest He enrichment, and appears to share the motion of the nearby OB association LH 99. These properties suggest that the star was spun up through a close binary merger. VFTS 285 is rotating at $95\%$ of critical velocity, has a strong He enrichment, and is moving away from the R136 cluster at the center of 30 Dor. It is mostly likely a runaway star ejected by a supernova explosion that released the components of the natal binary system.

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arXiv2022

Identification of new classical Be stars from the LAMOST MRS survey

Be stars are B-type main-sequence stars that display broad Balmer emission lines in their spectra. Identification of Be population is essential to further examine the formation and evolutionary models. We report the detection of classical Be (CBe) stars from observations with the Large sky Area Multi-Object fiber Spectroscopic Telescope Medium Resolution Survey of Date Release 7 (LAMOST MRS DR7). We used a deep convolutional neural network, the ResNet, with an 18-layer module to examine the morphology of the H alpha profile. We identified 1,162 candidate Be stars from the collection of 2,260,387 spectra for 789,918 stars in the database. The ResNet network achieves a Be star classification accuracy of 99.5%. Among the detections, 151 of these are prior known Be stars cross-matched from the literature. By applying a three-step test, we identified 183 new CBe stars. We find that 41 CBe stars are members of known open clusters. Based upon an investigation of the kinematics of the identified CBe stars from the Gaia EDR3 astrometric solutions, we identified 16 new runaways. These new identifications will provide a reference for future follow-ups to further investigate their physical properties.

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arXiv2021

The Transformative Journey of HD 93521

HD 93521 is a massive, rapidly rotating star that is located about 1 kpc above the Galactic disk, and the evolutionary age for its estimated mass is much less than the time-of-flight if it was ejected from the disk. Here we present a re-assessment of both the evolutionary and kinematical timescales for HD 93521. We calculate a time-of-flight of 39 +/- 3 Myr based upon the distance and proper motions from Gaia EDR3 and a summary of radial velocity measurements. We then determine the stellar luminosity using a rotational model combined with the observed spectral energy distribution and distance. A comparison with evolutionary tracks for rotating stars from Brott et al. yields an evolutionary age of about 5 +/- 2 Myr. We propose that the solution to the timescale discrepancy is that HD 93521 is a stellar merger product. It was probably ejected from the Galactic disk as a close binary system of lower mass stars that eventually merged to create the rapidly rotating and single massive star we observe today.

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arXiv2021

The Solar Neighborhood XLIX: Nine Giant Planets Orbiting Nearby K Dwarfs, and the CHIRON Spectrograph's Radial Velocity Performance

We report initial results of a large radial velocity survey of K dwarfs up to a distance of 50 pc from the Solar System, to look for stellar, brown dwarf, and jovian planets using radial velocities from the CHIRON spectrograph on the CTIO/SMARTS 1.5m telescope. We identify three new exoplanet candidates orbiting host stars in the K dwarf survey, and confirm a hot Jupiter from TESS orbiting TOI 129. Our techniques are confirmed via five additional known exoplanet orbiting K dwarfs, bringing the number of orbital solutions presented here to 9, each hosting an exoplanet candidate with a minimum mass of 0.5--3.0 $M_{jup}$. In addition, we provide a list of 186 nearby K dwarfs with no detected close companions that are ideal for more sensitive searches for lower mass planets. This set of stars is used to determine CHIRON's efficiency, stability, and performance for radial velocity work. For K dwarfs with V = 7--12, we reach radial velocity precisions of 5--20 ms$^{-1}$ under a wide range of observing conditions. We demonstrate the stability of CHIRON over hours, weeks, and years using radial velocity standards, and describe instrumental capabilities and operation modes available for potential users.

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arXiv2021

Outbursts and stellar properties of the classical Be star HD 6226

The bright and understudied classical Be star HD 6226 has exhibited multiple outbursts in the last several years during which the star grew a viscous decretion disk. We analyze 659 optical spectra of the system collected from 2017-2020, along with a UV spectrum from the Hubble Space Telescope and high cadence photometry from both TESS and the KELT survey. We find that the star has a spectral type of B2.5IIIe, with a rotation rate of 74% of critical. The star is nearly pole-on with an inclination of $13.4$ degree. We confirm the spectroscopic pulsational properties previously reported, and report on three photometric oscillations from KELT photometry. The outbursting behavior is studied with equivalent width measurements of H$α$ and H$β$, and the variations in both of these can be quantitatively explained with two frequencies through a Fourier analysis. One of the frequencies for the emission outbursts is equal to the difference between two photometric oscillations, linking these pulsation modes to the mass ejection mechanism for some outbursts. During the TESS observation time period of 2019 October 7 to 2019 November 2, the star was building a disk. With a large dataset of H$α$ and H$β$ spectroscopy, we are able to determine the timescales of dissipation in both of these lines, similar to past work on Be stars that has been done with optical photometry. HD 6226 is an ideal target with which to study the Be disk-evolution given its apparent periodic nature, allowing for targeted observations with other facilities in the future.

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arXiv2021

Long Term Evolution of Surface Features on the Red Supergiant AZ Cyg

We present H-band interferometric observations of the red supergiant (RSG) AZ Cyg made with the Michigan Infra-Red Combiner (MIRC) at the six-telescope Center for High Angular Resolution Astronomy (CHARA) Array. The observations span 5 years (2011-2016), offering insight into the short and long-term evolution of surface features on RSGs. Using a spectrum of AZ Cyg obtained with SpeX on the NASA InfraRed Telescope Facility (IRTF) and synthetic spectra calculated from spherical MARCS, spherical PHOENIX, and SAtlas model atmospheres, we derive $T_{\text{eff}}$ is between $3972 K$ and $4000 K$ and $\log~g$ between $-0.50$ and $0.00$, depending on the stellar model used. Using fits to the squared visibility and Gaia parallaxes we measure its average radius $R=911^{+57}_{-50}~R_{\odot}$. Reconstructions of the stellar surface using our model-independent imaging codes SQUEEZE and OITOOLS.jl show a complex surface with small bright features that appear to vary on a timescale of less than one year and larger features that persist for more than one year. 1D power spectra of these images suggest a characteristic size of $0.52-0.69~R_{\star}$ for the larger, long lived features. This is close to the values of $0.51-0.53~R_{\star}$ derived from 3D RHD models of stellar surfaces. We conclude that interferometric imaging of this star is in line with predictions of 3D RHD models but that short-term imaging is needed to more stringently test predictions of convection in RSGs.

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arXiv2021

$ν$ Gem: a hierarchical triple system with an outer Be star

Time series of spectroscopic, speckle-interferometric, and optical long-baseline-interferometric observations confirm that $ν$ Gem is a hierarchical triple system. It consists of an inner binary composed of two B-type stars and an outer classical Be star. Several photospheric spectral lines of the inner components were disentangled, revealing two stars with very different rotational broadening ($\sim$260 and $\sim$140 kms$^{-1}$, respectively), while the photospheric lines of the Be star remain undetected. From the combined spectroscopic and astrometric orbital solution it is not possible to unambiguously cross-identify the inner astrometric components with the spectroscopic components. In the preferred solution based on modeling of the disentangled line profiles, the inner binary is composed of two stars with nearly identical masses of 3.3 M$_\odot$ and the more rapidly rotating star is the fainter one. These two stars are in a marginally elliptical orbit ($e$ = 0.06) about each other with a period of 53.8 d. The third star also has a mass of 3.3 M$_\odot$ and follows a more eccentric ($e$ = 0.24) orbit with a period of 19.1 yr. The two orbits are co-directional and, at inclinations of 79$^{\circ}$ and 76$^{\circ}$ of the inner and the outer orbit, respectively, about coplanar. No astrometric or spectroscopic evidence could be found that the Be star itself is double. The system appears dynamically stable and not subject to eccentric Lidov-Kozai oscillations. After disentangling, the spectra of the components of the inner binary do not exhibit peculiarities that would be indicative of past interactions. Motivations for a wide range of follow-up studies are suggested.

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arXiv2021

The Detection and Characterization of Be+sdO Binaries from HST/STIS FUV Spectroscopy

The B-emission line stars are rapid rotators that were probably spun up by mass and angular momentum accretion through mass transfer in an interacting binary. Mass transfer will strip the donor star of its envelope to create a small and hot subdwarf remnant. Here we report on Hubble Space Telescope/STIS far-ultraviolet spectroscopy of a sample of Be stars that reveals the presence of the hot sdO companion through the calculation of cross-correlation functions of the observed and model spectra. We clearly detect the spectral signature of the sdO star in 10 of the 13 stars in the sample, and the spectral signals indicate that the sdO stars are hot, relatively faint, and slowly rotating as predicted by models. A comparison of their temperatures and radii with evolutionary tracks indicates that the sdO stars occupy the relatively long-lived, He-core burning stage. Only one of the ten detections was a known binary prior to this investigation, which emphasizes the difficulty of finding such Be+sdO binaries through optical spectroscopy. However, these results and others indicate that many Be stars probably host hot subdwarf companions.

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arXiv2020

CHARA Array adaptive optics: complex operational software and performance

The CHARA Array is the longest baseline optical interferometer in the world. Operated with natural seeing, it has delivered landmark sub-milliarcsecond results in the areas of stellar imaging, binaries, and stellar diameters. However, to achieve ambitious observations of faint targets such as young stellar objects and active galactic nuclei, higher sensitivity is required. For that purpose, adaptive optics are developed to correct atmospheric turbulence and non-common path aberrations between each telescope and the beam combiner lab. This paper describes the AO software and its integration into the CHARA system. We also report initial on-sky tests that demonstrate an increase of scientific throughput by sensitivity gain and by extending useful observing time in worse seeing conditions. Our 6 telescopes and 12 AO systems with tens of critical alignments and control loops pose challenges in operation. We describe our methods enabling a single scientist to operate the entire system.

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arXiv2020

ARMADA I: Triple Companions Detected in B-Type Binaries alpha Del and nu Gem

Ground-based optical long-baseline interferometry has the power to measure the orbits of close binary systems at ~10 micro-arcsecond precision. This precision makes it possible to detect "wobbles" in the binary motion due to the gravitational pull from additional short period companions. We started the ARrangement for Micro-Arcsecond Differential Astrometry (ARMADA) survey with the MIRC-X instrument at the CHARA array for the purpose of detecting giant planets and stellar companions orbiting individual stars in binary systems. We describe our observations for the survey, and introduce the wavelength calibration scheme that delivers precision at the tens of micro-arcseconds level for <0.2 arcsecond binaries. We test our instrument performance on a known triple system kappa Peg, and show that our survey is delivering a factor of 10 better precision than previous similar surveys. We present astrometric detections of tertiary components to two B-type binaries: a 30-day companion to alpha Del, and a 50-day companion to nu Gem. We also collected radial velocity data for alpha Del with the Tennessee State University Automated Spectroscopic Telescope at Fairborn Observatory. We are able to measure the orbits and masses of all three components in these systems. We find that the previously published RV orbit for the inner pair of nu Gem is not consistent with our visual orbit. The precision achieved for these orbits suggests that our ARMADA survey will be successful at discovering new compact triple systems to A/B-type binary systems, leading to better statistics of hierarchical system architectures and formation history.

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arXiv2020

Spectroscopic Detection of the Pre-White Dwarf Companion of Regulus

Mass transfer in an interacting binary will often strip the mass donor of its entire envelope and spin up the mass gainer to near critical rotation. The nearby B-type star Regulus represents a binary in the post-mass transfer stage: it is a rapid rotator with a very faint companion in a 40 d orbit. Here we present the results of a search for the spectral features of the stripped-down star in an extensive set of high S/N and high resolution spectra obtained with the CFHT/ESPaDOnS and TBL/NARVAL spectrographs. We first determine revised orbital elements in order to set accurate estimates of the orbital Doppler shifts at the times of observation. We then calculate cross-correlation functions of the observed and model spectra, and we search for evidence of the companion signal in the residuals after removal of the strong primary component. We detect a weak peak in the co-added residuals that has the properties expected for a faint pre-white dwarf. We use the dependence of the peak height and width on assumed secondary velocity semiamplitude to derive the semiamplitude, which yields masses of $M_1/M_\odot = 3.7 \pm 1.4$ and $M_2/M_\odot = 0.31 \pm 0.10$ (assuming orbital inclination equals the spin inclination of Regulus). We estimate the pre-white dwarf temperature $T_{\rm eff} = (20 \pm 4)$~kK through tests with differing temperature model spectra, and we find the radius $R_2/R_\odot = 0.061 \pm 0.011$ from the component temperatures and the flux ratio associated with the amplitude of the signal in the cross-correlation residuals.

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arXiv2020

Visual Orbits of Spectroscopic Binaries with the CHARA Array. III. HD 8374 and HD 24546

We present the visual orbits of two long period spectroscopic binary stars, HD 8374 and HD 24546, using interferometric observations acquired with the CHARA Array and the Palomar Testbed Interferometer. We also obtained new radial velocities from echelle spectra using the APO 3.5 m and Fairborn 2.0 m telescopes. By combining the visual and spectroscopic observations, we solve for the full, three-dimensional orbits and determine the stellar masses and distances to within 3% uncertainty. We then estimate the effective temperature and radius of each component star through Doppler tomography and spectral energy distribution analyses, in order to compare the observed stellar parameters to the predictions of stellar evolution models. For HD 8374, we find masses of M1 = 1.636 +/- 0.050 Msun and M2 = 1.587 +/- 0.049 Msun, radii of R1 = 1.84 +/- 0.05 Rsun and R2 = 1.66 +/- 0.12 Rsun, temperatures of Teff1 = 7280 +/- 110 K and Teff2 = 7280 +/- 120 K, and an estimated age of 1.0 Gyr. For HD 24546, we find masses of M1 = 1.434 +/- 0.014 Msun and M2 = 1.409 +/- 0.014 Msun, radii of R1 = 1.67 +/- 0.06 Rsun and R2 = 1.60 +/- 0.10 Rsun, temperatures of Teff1 = 6790 +/- 120 K and Teff2 = 6770 +/- 90 K, and an estimated age of 1.4 Gyr. HD 24546 is therefore too old to be a member of the Hyades cluster, despite its physical proximity to the group.

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arXiv2019

HST/COS Spectra of the Wind Lines of VFTS 102 and 285

Rapid rotation in massive stars imposes a latitudinal variation in the mass loss from radiatively driven winds that can lead to enhanced mass loss at the poles (with little angular momentum loss) and/or equator (with maximal angular momentum loss). Here we present an examination of the stellar wind lines of the two O-type stars with the fastest known equatorial velocities, VFTS 102 ($V\sin i = 610 \pm 30$ km/s; O9:Vnnne+) and VFTS 285 ($V\sin i = 609 \pm 29$ km/s; O7.5 Vnnn) in the Large Magellanic Cloud. Ultraviolet spectra of both stars were obtained with the Hubble Space Telescope Cosmic Origins Spectrograph. The spectrum of VFTS 285 displays a fast outflow in N V and a much slower wind in Si IV, and we argue that there is a two-wind regime in which mass loss is strong at the poles (fast and tenuous wind) but dominant at the equator (slow and dense winds). These ions and wind lines are not present in the spectrum of the cooler star VFTS 102, but the double-peaked H$α$ emission in its spectrum implies equatorial mass loss into a circumstellar disk. The results suggest that in the fastest rotating O-stars, most mass is lost as an equatorial outflow, promoting angular momentum loss that contributes to a spin down over time.

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arXiv2019

Visual Orbits of Spectroscopic Binaries with the CHARA Array. II. The eclipsing binary HD 185912

We present the visual orbit of the double-lined eclipsing binary, HD 185912, from long baseline interferometry with the CHARA Array. We also obtain echelle spectra from the Apache Point observatory to update the spectroscopic orbital solution and analyze new photometry from Burggraaff et al. to model the eclipses. By combining the spectroscopic and visual orbital solutions, we find component masses of M1 = 1.361 +/- 0.004 Msun and M2 = 1.331 +/- 0.004 Msun, and a distance of d = 40.75 +/- 0.30 pc from orbital parallax. From the light curve solution, we find component radii of R1 = 1.348 +/- 0.016 Rsun and R2 = 1.322 +/- 0.016 Rsun. By comparing these observed parameters to stellar evolution models, we find that HD 185912 is a young system near the zero age main sequence with an estimated age of 500 Myr.

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arXiv2019

Astro2020 Science White Paper: Stellar Physics and Galactic Archeology using Asteroseismology in the 2020's

Asteroseismology is the only observational tool in astronomy that can probe the interiors of stars, and is a benchmark method for deriving fundamental properties of stars and exoplanets. Over the coming decade, space-based and ground-based observations will provide a several order of magnitude increase of solar-like oscillators, as well as a dramatic increase in the number and quality of classical pulsator observations, providing unprecedented possibilities to study stellar physics and galactic stellar populations. In this white paper, we describe key science questions and necessary facilities to continue the asteroseismology revolution into the 2020's.

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arXiv2019

Visual Orbits of Spectroscopic Binaries with the CHARA Array. I. HD 224355

We present the visual orbit of the double-lined spectroscopic binary HD 224355 from interferometric observations with the CHARA Array, as well as an updated spectroscopic analysis using echelle spectra from the Apache Point Observatory 3.5m telescope. By combining the visual and spectroscopic orbital solutions, we find the binary components to have masses of M1 = 1.626 +/- 0.005 Msun and M2 = 1.608 +/- 0.005 Msun, and a distance of d = 63.98 +/- 0.26 pc. Using the distance and the component angular diameters found by fitting spectrophotometry from the literature to spectral energy distribution models, we estimate the stellar radii to be R1 = 2.65 +/- 0.21 Rsun and R2 = 2.47 +/- 0.23 Rsun. We then compare these observed fundamental parameters to the predictions of stellar evolution models, finding that both components are evolved towards the end of the main sequence with an estimated age of 1.9 Gyr.

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