arXiv subjects

Douglas R. Gies

Publications and source records attributed to Douglas R. Gies.

At least 55 records · Page 3Linked to original sources

arXiv2018

Angular Sizes and Effective Temperatures of O-type Stars from Optical Interferometry with the CHARA Array

We present interferometric observations of six O-type stars that were made with the Precision Astronomical Visible Observations (PAVO) beam combiner at the Center for High Angular Resolution Astronomy (CHARA) Array. The observations include multiple brackets for three targets, $λ$~Ori~A, $ζ$~Oph, and 10~Lac, but there are only preliminary, single observations of the other three stars, $ξ$~Per, $α$~Cam, and $ζ$~Ori~A. The stellar angular diameters range from 0.55 milliarcsec for $ζ$~Ori~A down to 0.11 mas for 10~Lac, the smallest star yet resolved with the CHARA Array. The rotational oblateness of the rapidly rotating star $ζ$ Oph is directly measured for the first time. We assembled ultraviolet to infrared flux measurements for these stars, and then derived angular diameters and reddening estimates using model atmospheres and an effective temperature set by published results from analysis of the line spectrum. The model-based angular diameters are in good agreement with observed angular diameters. We also present estimates for the effective temperatures of these stars derived by setting the interferometric angular size and fitting the spectrophotometry.

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arXiv2018

A Gap in the Lower Main Sequence Revealed by Gaia Data Release 2

We present the discovery of a gap near $M_{G}\approx$10 in the main sequence on the Hertzsprung-Russell Diagram (HRD) based on measurements presented in Gaia Data Release 2 (DR2). Using an observational form of the HRD with $M_{G}$ representing luminosity and $G_{BP}-G_{RP}$ representing temperature, the gap presents a diagonal feature that dips toward lower luminosities at redder colors. The gap is seen in samples extracted from DR2 with various distances, and is not unique to the {\it Gaia} photometry --- it also appears when using near-IR photometry ($J-K_{s}$ vs $M_{K_{s}}$). The gap is very narrow ($\sim$0.05 mag) and is near the luminosity-temperature regime where M dwarf stars transition from partially to fully convective, i.e., near spectral type M3.0V. This gap provides a new feature in the H-R Diagram that hints at an underlying astrophysical cause and we propose that it is linked to the onset of full convection in M dwarfs.

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arXiv2018

A Photometric, Spectroscopic, and Apsidal Motion Analysis of the F-type Eclipsing Binary BW Aquarii from K2 Campaign 3

Eclipsing binaries are important tools for studying stellar evolution and stellar interiors. Their accurate fundamental parameters are used to test evolutionary models, and systems showing apsidal motion can also be used to test the model's internal structure predictions. For this purpose, we present a photometric and spectroscopic analysis of the eclipsing binary BW Aquarii, an evolved F-type binary with slow apsidal motion. We model the K2 C3 light curve using the Eclipsing Light Curve code to determine several orbital and stellar parameters, as well as measure the eclipse times to determine updated apsidal motion parameters for the system. Furthermore, we obtain high-resolution spectra of BW Aqr using the CHIRON echelle spectrograph on the CTIO 1.5m for radial velocity analysis. We then reconstruct the spectra of each component using Doppler tomography in order to determine the atmospheric parameters. We find that both components of BW Aqr are late F-type stars with M1 = 1.365 +/- 0.008 Msun, M2 = 1.483 +/- 0.009 Msun, and R1 = 1.782 +/- 0.021 Rsun, R2 = 2.053 +/- 0.020 Rsun. We then compare these results to the predictions of several stellar evolution models, finding that the models cannot reproduce the observed properties of both components at the same age.

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arXiv2018

Detection of additional Be+sdO systems from IUE spectroscopy

There is growing evidence that some Be stars were spun up through mass transfer in a close binary system, leaving the former mass donor star as a hot, stripped-down object. There are five known cases of Be stars with hot subdwarf (sdO) companions that were discovered through International Ultraviolet Explorer (IUE) spectroscopy. Here we expand the search for Be+sdO candidates using archival FUV spectra from IUE. We collected IUE spectra for 264 stars and formed cross-correlation functions (CCFs) with a model spectrum for a hot subdwarf. Twelve new candidate Be+sdO systems were found, and eight of these display radial velocity variations associated with orbital motion. The new plus known Be+sdO systems have Be stars with spectral subtypes of B0 to B3, and the lack of later-type systems is surprising given the large number of cooler B-stars in our sample. We discuss explanations for the observed number and spectral type distribution of the Be+sdO systems, and we argue that there are probably many Be systems with stripped companions that are too faint for detection through our analysis.

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arXiv2017

Gravity Modes Reveal the Internal Rotation of a Post-mass Transfer Gamma Doradus/Delta Scuti Hybrid Pulsator in Kepler Eclipsing Binary KIC 9592855

We report the discovery of a post-mass transfer Gamma Doradus/Delta Scuti hybrid pulsator in the eclipsing binary KIC~9592855. This binary has a circular orbit, an orbital period of 1.2 days, and contains two stars of almost identical masses ($M_1=1.72M_{\odot}, M_2=1.71M_{\odot}$). However, the cooler secondary star is more evolved ($R_2=1.96R_{\odot}$) while the hotter primary is still on the zero-age-main-sequence ($R_1=1.53R_{\odot}$). Coeval models from single star evolution cannot explain the observed masses and radii, and binary evolution with mass-transfer needs to be invoked. After subtracting the binary light curve, the Fourier spectrum shows low-order pressure-mode pulsations, and more dominantly, a cluster of low-frequency gravity modes at about $2$ day$^{-1}$. These g-modes are nearly equally-spaced in period, and the period spacing pattern has a negative slope. We identify these g-modes as prograde dipole modes and find that they stem from the secondary star. The frequency range of unstable p-modes also agrees with that of the secondary. We derive the internal rotation rate of the convective core and the asymptotic period spacing from the observed g-modes. The resulting values suggest that the core and envelope rotate nearly uniformly, i.e., their rotation rates are both similar to the orbital frequency of this synchronized binary.

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arXiv2017

Radial Velocities of 41 Kepler Eclipsing Binaries

Eclipsing binaries are vital for directly determining stellar parameters without reliance on models or scaling relations. Spectroscopically derived parameters of detached and semi-detached binaries allow us to determine component masses that can inform theories of stellar and binary evolution. Here we present moderate resolution ground-based spectra of stars in close binary systems with and without (detected) tertiary companions observed by NASA's Kepler mission and analyzed for eclipse timing variations. We obtain radial velocities and spectroscopic orbits for five single-lined and 35 double-lined systems, and confirm one false positive eclipsing binary. For the double-lined spectroscopic binaries we also determine individual component masses and examine the mass ratio M_2/M_1 distribution, which is dominated by binaries with like-mass pairs and semi-detached classical Algol systems that have undergone mass transfer. Finally, we constrain the mass of the tertiary component for five double-lined binaries with previously detected companions.

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arXiv2017

Detection of the Ultraviolet Spectrum of the Hot Subdwarf Companion of 60 Cygni (B1 Ve) from a Survey of IUE Spectra of Be Stars

We used archival International Ultraviolet Explorer (IUE) high-dispersion, short wavelength spectra data to search for evidence of the spectra of hot subdwarf companions of six rapidly rotating Be stars in binary systems. We searched for the signature of a hot companion through an analysis of the cross-correlation functions of observed and model spectra that were separated into primary and secondary components using a Doppler tomography algorithm and adopted spectroscopic orbital solutions. A positive detection of the flux from a hot companion was made for the reconstructed secondary cross-correlation function of just one target, 60 Cygni (B1 Ve). We estimate that the companion of the Be star in 60 Cygni has $T_{\rm eff} = 42 \pm 4$ kK, mass ratio $M_2/M_1 = 0.15 \pm 0.02$, and monochromatic flux ratio ${f_2}/{f_1} = 0.034 \pm 0.002$ in the spectral region near 1525 Å. If the companions of the other target Be stars are also hot, then they must be faint and contribute less than $\approx 1\%$ of the UV flux ($<0.6 \%$ in the case of $γ$ Cas). We also discuss in an appendix a shell episode of Pleione (28 Tau) recorded in the IUE spectra.

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arXiv2017

KIC 8262223: A Post-Mass Transfer Eclipsing Binary Consisting of a Delta Scuti Pulsator and a Helium White Dwarf Precursor

KIC~8262223 is an eclipsing binary with a short orbital period ($P=1.61$ d). The {\it Kepler} light curves are of Algol-type and display deep and partial eclipses, ellipsoidal variations, and pulsations of Delta Scuti type. We analyzed the {\it Kepler} photometric data, complemented by phase-resolved spectra from the R-C Spectrograph on the 4-meter Mayall telescope at Kitt Peak National Observatory and determined the fundamental parameters of this system. The low mass and oversized secondary ($M_2=0.20M_{\odot}$, $R_2=1.31R_{\odot}$) is the remnant of the donor star that transferred most of its mass to the gainer, and now the primary star. The current primary star is thus not a normal $δ$ Scuti star but the result of mass accretion from a lower mass progenitor. We discuss the possible evolutionary history and demonstrate with the MESA evolution code that the system can be understood as the result of non-conservative binary evolution similar to that for the formation of EL CVn type binaries. The pulsations of the primary star can be explained as radial and non-radial pressure modes. The equilibrium models from single star evolutionary tracks can match the observed mass and radius ($M_1=1.94M_{\odot}$, $R_1=1.67R_{\odot}$) but the predicted unstable modes associated with these models differ somewhat from those observed. This work presents a preliminary asteroseismic analysis of the abnormal Delta Scuti pulsators, and we discuss the need for better theoretical understanding of such pulsating mass gaining stars.

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arXiv2016

Tidally Induced Pulsations in Kepler Eclipsing Binary KIC 3230227

KIC 3230227 is a short period ($P\approx 7.0$ days) eclipsing binary with a very eccentric orbit ($e=0.6$). From combined analysis of radial velocities and {\it Kepler} light curves, this system is found to be composed of two A-type stars, with masses of $M_1=1.84\pm 0.18M_{\odot}$, $M_2=1.73\pm 0.17M_{\odot}$ and radii of $R_1=2.01\pm 0.09R_{\odot}$, $R_2=1.68\pm 0.08 R_{\odot}$ for the primary and secondary, respectively. In addition to an eclipse, the binary light curve shows a brightening and dimming near periastron, making this a somewhat rare eclipsing heartbeat star system. After removing the binary light curve model, more than ten pulsational frequencies are present in the Fourier spectrum of the residuals, and most of them are integer multiples of the orbital frequency. These pulsations are tidally driven, and both the amplitudes and phases are in agreement with predictions from linear tidal theory for $l=2, m=-2$ prograde modes.

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arXiv2016

Distance Dependent Offsets between Parallaxes for Nearby Stars and Gaia DR1 Parallaxes

We use 612 single stars with previously published trigonometric parallaxes placing them within 25 pc to evaluate parallaxes released in Gaia's first data release (DR1). We find that the Gaia parallaxes are, on average, $0.24 \pm 0.02$ mas smaller than the weighted mean trigonometric parallax values for these stars in the solar neighborhood. We also find that the offset changes with distance out to 100 pc, in the sense that the closer the star, the larger the offset. We find no systematic trends in the parallax offsets with stellar $V$ magnitude, $V-K$ color, or proper motion. We do find that the offset is roughly twice as large for stars south of the ecliptic compared to those that are north.

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arXiv2016

A Young Eclipsing Binary and its Luminous Neighbors in the Embedded Star Cluster Sh 2-252E

We present a photometric and light curve analysis of an eccentric eclipsing binary in the K2 Campaign 0 field that resides in Sh 2-252E, a young star cluster embedded in an H II region. We describe a spectroscopic investigation of the three brightest stars in the crowded aperture to identify which is the binary system. We find that none of these stars are components of the eclipsing binary system, which must be one of the fainter nearby stars. These bright cluster members all have remarkable spectra: Sh 2-252a (EPIC 202062176) is a B0.5 V star with razor sharp absorption lines, Sh 2-252b is a Herbig A0 star with disk-like emission lines, and Sh 2-252c is a pre-main sequence star with very red color.

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arXiv2016

The Hot Companion and Circumbinary Disk of the Be Star HR 2142

We present a spectroscopic investigation of the Be+sdO binary system HR 2142 that is based upon large sets of ultraviolet observations from the International Ultraviolet Explorer and ground-based H-alpha observations. We measured radial velocities for the Be star component from these spectra, and computed a revised orbit. In order to search for the spectral signature of the hot subdwarf, we cross-correlated the short wavelength end of each IUE spectrum with a model hot star spectrum, and then we used the predicted Doppler shifts of the subdwarf to shift-and-add all the cross-correlation functions to the frame of the subdwarf. This merged function shows the weak signal from the spectral lines of the hot star, and a best fit is obtained with a mass ratio M_2/M_1 = 0.07 +/- 0.02, companion temperature T_{eff} > 43 +/- 5 kK, projected rotational velocity V sin i < 30 km/s, and a monochromatic flux ratio near 1170 Angstroms of f_2/f_1 > 0.009 +/- 0.001. This hot subdwarf creates a one-armed spiral, tidal wake in the disk of the Be star, and we present a circumbinary disk model that can explain the occurrence of shell absorption lines by gas enhancements that occur where gas crossing the gap created by the subdwarf strikes the disk boundaries. The faint companion of HR 2142 may be representative of a significant fraction of Be stars with undetected former mass donor companion stars.

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arXiv2016

The CHARA Array resolves the long-period Wolf-Rayet binaries WR 137 and WR 138

We report on interferometric observations with the CHARA Array of two classical Wolf-Rayet stars in suspected binary systems, namely WR 137 and WR 138. In both cases, we resolve the component stars to be separated by a few milliarcseconds. The data were collected in the H-band, and provide a measure of the fractional flux for both stars in each system. We find that the WR star is the dominant H-band light source in both systems ($f_{\rm WR, 137} = 0.59\pm0.04$; $f_{\rm WR, 138} = 0.67\pm0.01$), which is confirmed through both comparisons with estimated fundamental parameters for WR stars and O dwarfs, as well as through spectral modeling of each system. Our spectral modeling also provides fundamental parameters for the stars and winds in these systems. The results on WR 138 provide evidence that it is a binary system which may have gone through a previous mass-transfer episode to create the WR star. The separation and position of the stars in the WR 137 system together with previous results from the IOTA interferometer provides evidence that the binary is seen nearly edge-on. The possible edge-on orbit of WR 137 aligns well with the dust production site imaged by the Hubble Space Telescope during a previous periastron passage, showing that the dust production may be concentrated in the orbital plane.

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arXiv2016

Kepler Eclipsing Binaries with $δ$ Scuti/$γ$ Doradus Pulsating Components 1: KIC 9851944

KIC 9851944 is a short period ($P=2.16$ days) eclipsing binary in the {\it Kepler} field of view. By combining the analysis of {\it Kepler} photometry and phase resolved spectra from Kitt Peak National Observatory and Lowell Observatory, we determine the atmospheric and physical parameters of both stars. The two components have very different radii ($2.27R_{\odot}$, $3.19R_{\odot}$) but close masses ($1.76 M_{\odot}$, $1.79M_{\odot}$) and effective temperatures ($7026$K, $6902$K), indicating different evolutionary stages. The hotter primary is still on the main sequence (MS), while the cooler and larger secondary star has evolved to post-MS, burning hydrogen in a shell. A comparison with coeval evolutionary models shows that it requires solar metallicity and a higher mass ratio to fit the radii and temperatures of both stars simultaneously. Both components show $δ$ Scuti type pulsations which we interpret as p-modes and p and g mixed modes. After a close examination of the evolution of $δ$ Scuti pulsational frequencies, we make a comparison of the observed frequencies with those calculated from MESA/GYRE.

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arXiv2016

Fundamental Parameters of Kepler Eclipsing Binaries. I. KIC 5738698

Eclipsing binaries serve as a valuable source of stellar masses and radii that inform stellar evolutionary models and provide insight into additional astrophysical processes. The exquisite light curves generated by space-based missions such as Kepler offer the most stringent tests to date. We use the Kepler light curve of the 4.8-day eclipsing binary KIC 5739896 with ground based optical spectra to derive fundamental parameters for the system. We reconstruct the component spectra to determine the individual atmospheric parameters, and model the Kepler photometry with the binary synthesis code ELC to obtain accurate masses and radii. The two components of KIC 5738698 are F-type stars with M1 = 1.39+/-0.04M, M2 = 1.34+/-0.06M, and R1 = 1.84+/-0.03R, R2 = 1.72+/-0.03R. We also report a small eccentricity (e < 0.0017) and unusual albedo values that are required to match the detailed shape of the Kepler light curve. Comparisons with evolutionary models indicate an approximate age of 2.3 Gyr for the system.

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arXiv2015

Spectroscopy, MOST Photometry, and Interferometry of MWC 314: Is it an LBV or an interacting binary?

MWC 314 is a bright candidate luminous blue variable that resides in a fairly close binary system, with an orbital period of 60.753$\pm$0.003 d. We observed MWC 314 with a combination of optical spectroscopy, broad-band ground- and space-based photometry, as well as with long baseline, near-infrared interferometry. We have revised the single-lined spectroscopic orbit and explored the photometric variability. The orbital light curve displays two minima each orbit that can be partially explained in terms of the tidal distortion of the primary that occurs around the time of periastron. The emission lines in the system are often double-peaked and stationary in their kinematics, indicative of a circumbinary disc. We find that the stellar wind or circumbinary disc is partially resolved in the K\prime-band with the longest baselines of the CHARA Array. From this analysis, we provide a simple, qualitative model in an attempt to explain the observations. From the assumption of Roche Lobe overflow and tidal synchronisation at periastron, we estimate the component masses to be M1 $\approx 5$ M$_\odot$ and M2$\approx 15$ M$_\odot$, which indicates a mass of the LBV that is extremely low. In addition to the orbital modulation, we discovered two pulsational modes with the MOST satellite. These modes are easily supported by a low-mass hydrogen-poor star, but cannot be easily supported by a star with the parameters of an LBV. The combination of these results provides evidence that the primary star was likely never a normal LBV, but rather is the product of binary interactions. As such, this system presents opportunities for studying mass-transfer and binary evolution with many observational techniques.

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arXiv2015

The Optical Wind Line Variability of $η$ Carinae During the 2009.0 Event

We report on high-resolution spectroscopy of the 2009.0 spectroscopic event of $η$ Carinae collected via SMARTS observations using the CTIO 1.5 m telescope and echelle spectrograph. Our observations were made almost every night over a two-month interval around the photometric minimum of $η$ Car associated with the periastron passage of a hot companion. The photoionizing flux of the companion and heating related to colliding winds causes large changes in the wind properties of the massive primary star. Here we present an analysis of temporal variations in a sample of spectral lines that are clearly formed in the wind of the primary star. These lines are affected by a changing illumination of the flux of the secondary star during the periastron passage. We document the sudden onset of blue-shifted absorption that occurred in most of the lines near or slightly after periastron, and we argue that these absorption components are seen when we view the relatively undisturbed wind of the foreground primary star. We present time series measurements of the net equivalent width of the wind lines and of the radial velocities of the absorption trough minima and the emission peak midpoints. Most lines decrease in emission strength around periastron, and those high excitation lines formed close to the primary exhibit a red-ward velocity excursion. We show how these trends can be explained using an illuminated hemisphere model that is based on the idea that the emission originates primarily from the side of the primary facing the hot companion.

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arXiv2015

HST/STIS Ultraviolet Spectroscopy of the Components of the Massive Triple Star delta Ori A

The multiple star system of delta Orionis is one of the closest examples of a system containing a luminous O-type, bright giant star (component Aa1). It is often used as a spectral-type standard and has the highest observed X-ray flux of any hot-star binary. The main component Aa1 is orbited by two lower mass stars, faint Aa2 in a 5.7 day eclipsing binary, and Ab, an astrometric companion with an estimated period of 346 years. Generally the flux from all three stars is recorded in ground-based spectroscopy, and the spectral decomposition of the components has proved difficult. Here we present HST/STIS ultraviolet spectroscopy of delta Ori A that provides us with spatially separated spectra of Aa and Ab for the first time. We measured radial velocities for Aa1 and Ab in two observations made near the velocity extrema of Aa1. We show tentative evidence for the detection of the Aa2 component in cross-correlation functions of the observed and model spectra. We discuss the appearance of the UV spectra of Aa1 and Ab with reference to model spectra. Both stars have similar effective temperatures, but Ab is fainter and is a rapid rotator. The results will help in the interpretation of ground-based spectroscopy and in understanding the physical and evolutionary parameters of these massive stars.

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