arXiv subjects

Todd Henry

Publications and source records attributed to Todd Henry.

At least 19 recordsLinked to original sources

arXiv2026

The Debris Disk Host $\beta$ Piscis Austrinus is a Rapidly Rotating Star Seen Nearly Pole-On

Previous studies of $\beta$ Piscis Austrinus (PsA) have speculated that the narrow and saddle-like shapes of some of its weak metallic lines are a consequence of it being a rapidly rotating star viewed nearly pole-on. Here we use the \texttt{fastrot-spec} spectral synthesis code to model high-dispersion (R = 115,000) HARPS spectra of $\beta$ PsA in order to determine its inclination and photospheric properties, with additional constraints on the surface temperature set by measures of Fe II/Fe I line ratios. The analysis confirms that $\beta$ PsA is oriented nearly pole-on ($i = 4.75^{+0.75}_{-0.50}$$^o$) and experiences substantial gravity darkening caused by its rapid rotation ($\Omega/\Omega_{crit}=0.93\pm0.17$). $\beta$ PsA has a polar temperature of $10300^{+200}_{-250}$ K that is 24% hotter than its equatorial temperature ($8275^{+317}_{-400}$ K). This results in its apparent luminosity being 48% larger than its actual luminosity of 26.2$^{+1.9}_{-2.4}$ L$_\odot$. When this methodology is applied to high-dispersion spectra of the star Vega, the analysis determines a nearly pole-on orientation that is consistent with interferometric measurements, validating the technique. Based on comparisons with PARSEC evolutionary models of stars rotating at similar velocities, $\beta$ PsA has a mass of $2.20\pm0.03$ M$_{\odot}$ and an age of $141^{+113}_{-49}$ Myr; this age is consistent with the age inferred for its G5V companion star, CD-32 17127, based on lithium depletion models. The analysis demonstrates the potential for both identifying and determining the stellar properties of rapidly rotating stars viewed nearly pole-on via spectroscopy alone.

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arXiv2024

PowerLine Unmanned Surfer (PLUS): Shape Adaptive Dynamics Development and Control Design

This paper introduces the powerline unmanned surfer (PLUS) concept to extend the limited endurance of fixed wing unmanned aerial vehicles (UAVs) via in-flight energy harvesting from overhead electrical distribution power lines, and develops the flight dynamics and control framework to support centimeter-scale longitudinal powerline frequency tracking. The dynamics framework models the UAV's shape adaptive structure, aerodynamic forces, and control inputs, and applies the coupled flight mechanics framework to low clearance tracking of powerline contours. This study develops a "trajectory to shape-adaptive UAV" controller design approach for longitudinal powerline tracking through local spatial frequency matching. The frequency-matching approach dynamically regulates the aircraft modes' frequency to the powerline catenary spatial frequency using a generalized parameter linearization approach. Performance is assessed on an example UAV implementing camber and thickness morphing by quantifying clearance distance from neighborhood to high voltage powerline environments and across span and chord combinations. This approach achieves alternating periods of low-clearance tracking and antiphase oscillation, with 34% of the powerline having tracking error less than 1m. Airfoil thickness increase has a stronger effect over Phugoid mode wavelength than thickness decrease. Wingspan, chord, and powerline parameter sensitivities are quantified, providing a foundation for near-field long-distance powerline tracking and energy harvesting.

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arXiv2020

A planet within the debris disk around the pre-main-sequence star AU Microscopii

AU Microscopii (AU Mic) is the second closest pre main sequence star, at a distance of 9.79 parsecs and with an age of 22 million years. AU Mic possesses a relatively rare and spatially resolved3 edge-on debris disk extending from about 35 to 210 astronomical units from the star, and with clumps exhibiting non-Keplerian motion. Detection of newly formed planets around such a star is challenged by the presence of spots, plage, flares and other manifestations of magnetic activity on the star. Here we report observations of a planet transiting AU Mic. The transiting planet, AU Mic b, has an orbital period of 8.46 days, an orbital distance of 0.07 astronomical units, a radius of 0.4 Jupiter radii, and a mass of less than 0.18 Jupiter masses at 3 sigma confidence. Our observations of a planet co-existing with a debris disk offer the opportunity to test the predictions of current models of planet formation and evolution.

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arXiv2020

Herschel Observations of Disks around Late-type Stars

A set of twenty late-type (K5-M5) stars were observed with the Herschel Space Observatory at 100 and 160 microns with the goal of searching for far-infrared excesses indicative of the presence of circumstellar disks. Out of this sample, four stars (TYC 7443-1102-1, TYC 9340-437-1, GJ 784 and GJ 707) have infrared excesses above their stellar photospheres at either 100 or 160 micron or both. At 100 microns TYC 9340-437-1 is spatially resolved with a shape that suggests it is surrounded by a face-on disk. The 100 micron excess flux associated with GJ 707 is marginal at around 3sigma. The excess flux associated with GJ 784 is most likely due to a background galaxy as the dust radius estimated from the spectral energy fit implies that any associated dust disk should have been resolved in the Herschel images but is not. TYC 7443-1102-1 has been observed with ALMA which resolves the emission at its location into two distinct sources making the Herschel excess most likely also due to a background galaxy. It is worth noting that this star is in the 23 Myr old beta Pic association. With a disk luminosity on the order of 10^-3 L*, this system is an ideal follow-up target for high-contrast imaging and ALMA.

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arXiv2020

The First Habitable Zone Earth-sized Planet from TESS. I: Validation of the TOI-700 System

We present the discovery and validation of a three-planet system orbiting the nearby (31.1 pc) M2 dwarf star TOI-700 (TIC 150428135). TOI-700 lies in the TESS continuous viewing zone in the Southern Ecliptic Hemisphere; observations spanning 11 sectors reveal three planets with radii ranging from 1 R$_\oplus$ to 2.6 R$_\oplus$ and orbital periods ranging from 9.98 to 37.43 days. Ground-based follow-up combined with diagnostic vetting and validation tests enable us to rule out common astrophysical false-positive scenarios and validate the system of planets. The outermost planet, TOI-700 d, has a radius of $1.19\pm0.11$ R$_\oplus$ and resides in the conservative habitable zone of its host star, where it receives a flux from its star that is approximately 86% of the Earth's insolation. In contrast to some other low-mass stars that host Earth-sized planets in their habitable zones, TOI-700 exhibits low levels of stellar activity, presenting a valuable opportunity to study potentially-rocky planets over a wide range of conditions affecting atmospheric escape. While atmospheric characterization of TOI-700 d with the James Webb Space Telescope (JWST) will be challenging, the larger sub-Neptune, TOI-700 c (R = 2.63 R$_\oplus$), will be an excellent target for JWST and beyond. TESS is scheduled to return to the Southern Hemisphere and observe TOI-700 for an additional 11 sectors in its extended mission, which should provide further constraints on the known planet parameters and searches for additional planets and transit timing variations in the system.

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arXiv2019

Astro2020 APC White Paper: Durable Agency Support for Exoplanet Catalogs and Archives

Many projects in current exoplanet science make use of catalogs of known exoplanets and their host stars. These may be used for demographic, population, and statistical studies, or for identifying targets for future observations. The ability to efficiently and accurately conduct exoplanet science depends on the completeness, accuracy, and access to these catalogs. In this white paper, we argue that long-term agency support and maintenance of exoplanet archives is of crucial importance to achieving the scientific goals of the community and the strategic goals of the funding agencies. As such, it is imperative that these facilities are appropriately supported and maintained by the national funding agencies.

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arXiv2019

2020 Vision: Towards a Sustainable OIR System

Open-access telescopes of all apertures are needed to operate a competitive and efficient national science program. While larger facilities contribute light-gathering power and angular resolution, smaller ones dominate for field of view, time-resolution, and especially, total available observing time, thereby enabling our entire, diversely-expert community. Smaller aperture telescopes therefore play a critical and indispensable role in advancing science. Thus, the divestment of NSF support for modest-aperture (1 - 4 m) public telescopes poses a serious threat to U.S. scientific leadership, which is compounded by the unknown consequences of the shift from observations driven by individual investigators to survey-driven science. Given the much higher cost efficiency and dramatic science returns for investments in modest aperture telescopes, it is hard to justify funding only the most expensive facilities. We therefore urge the Astro2020 panel to explicitly make the case for modest aperture facilities, and to recommend enhancing this funding stream to support and grow this critical component of the OIR System. Further study is urgently needed to prioritize the numerous exciting potential capabilities of smaller facilities,and to establish sustainable, long-term planning for the System.

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arXiv2018

HD 202772A B: A Transiting Hot Jupiter Around A Bright, Mildly Evolved Star In A Visual Binary Discovered By Tess

We report the first confirmation of a hot Jupiter discovered by the Transiting Exoplanet Survey Satellite (TESS) mission: HD 202772A b. The transit signal was detected in the data from TESS Sector 1, and was confirmed to be of planetary origin through radial-velocity measurements. HD 202772A b is orbiting a mildly evolved star with a period of 3.3 days. With an apparent magnitude of V = 8.3, the star is among the brightest known to host a hot Jupiter. Based on the 27days of TESS photometry, and radial velocity data from the CHIRON and HARPS spectrographs, the planet has a mass of 1.008+/-0.074 M_J and radius of 1.562+/-0.053 R_J , making it an inflated gas giant. HD 202772A b is a rare example of a transiting hot Jupiter around a quickly evolving star. It is also one of the most strongly irradiated hot Jupiters currently known.

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arXiv2018

TESS Discovery of an ultra-short-period planet around the nearby M dwarf LHS 3844

Data from the newly-commissioned \textit{Transiting Exoplanet Survey Satellite} (TESS) has revealed a "hot Earth" around LHS 3844, an M dwarf located 15 pc away. The planet has a radius of $1.32\pm 0.02$ $R_\oplus$ and orbits the star every 11 hours. Although the existence of an atmosphere around such a strongly irradiated planet is questionable, the star is bright enough ($I=11.9$, $K=9.1$) for this possibility to be investigated with transit and occultation spectroscopy. The star's brightness and the planet's short period will also facilitate the measurement of the planet's mass through Doppler spectroscopy.

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arXiv2017

The Solar Neighborhood XLII. Parallax Results from the CTIOPI 0.9-m Program --- Identifying New Nearby Subdwarfs Using Tangential Velocities and Locations on the H-R Diagram

Parallaxes, proper motions, and optical photometry are presented for 51 systems made up 37 cool subdwarf and 14 additional high proper motion systems. Thirty-seven systems have parallaxes reported for the first time, 15 of which have proper motions of at least 1"/yr. The sample includes 22 newly identified cool subdwarfs within 100 pc, of which three are within 25 pc, and an additional five subdwarfs from 100-160 pc. Two systems --- LSR 1610-0040 AB and LHS 440 AB --- are close binaries exhibiting clear astrometric perturbations that will ultimately provide important masses for cool subdwarfs. We use the accurate parallaxes and proper motions provided here, combined with additional data from our program and others to determine that effectively all nearby stars with tangential velocities greater than 200 km s$^{-1}$ are subdwarfs. We compare a sample of 167 confirmed cool subdwarfs to nearby main sequence dwarfs and Pleiades members on an observational Hertzsprung-Russell diagram using $M_V$ vs.~$(V-K_{s})$ to map trends of age and metallicity. We find that subdwarfs are clearly separated for spectral types K5--M5, indicating that the low metallicities of subdwarfs set them apart in the H-R diagram for $(V-K_{s})$ = 3--6. We then apply the tangential velocity cutoff and the subdwarf region of the H-R diagram to stars with parallaxes from {\it Gaia} Data Release 1 and the MEarth Project to identify a total of 29 new nearby subdwarf candidates that fall clearly below the main sequence.

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arXiv2016

Cool Subdwarf Investigations III: Dynamical Masses of Low Metallicity Subdwarfs

We report dynamical mass measurements for the components of the previously known double-lined spectroscopic subdwarfs G 006-026 B and C using the Fine Guidance Sensors (FGS) on the Hubble Space Telescope. To build the empirical mass-luminosity relation for low metallicity subdwarfs, we collect four other subdwarf systems with dynamical masses that we compare to theoretical models for various metallicities on the mass-luminosity relation. For most stars, they fall in the regions where the models predict to be low metallicity. This effort highlights the scarcity of dynamical masses for subdwarfs and that much work remains to be done to improve the mass errors and metallicity measurements of low mass subdwarfs in our Galaxy.

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arXiv2016

Precise Near-Infrared Radial Velocities

We present the results of two 2.3 micron near-infrared radial velocity surveys to detect exoplanets around 36 nearby and young M dwarfs. We use the CSHELL spectrograph (R ~46,000) at the NASA InfraRed Telescope Facility, combined with an isotopic methane absorption gas cell for common optical path relative wavelength calibration. We have developed a sophisticated RV forward modeling code that accounts for fringing and other instrumental artifacts present in the spectra. With a spectral grasp of only 5 nm, we are able to reach long-term radial velocity dispersions of ~20-30 m/s on our survey targets.

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arXiv2014

UCAC4 Nearby Star Survey: A Search for Our Stellar Neighbors

We use data from the U.S. Naval Observatory fourth CCD Astrograph Catalog (UCAC4) in combination with photometry from the AAVSO Photometric All-Sky Survey (APASS) and Two Micron All-Sky Survey (2MASS) to identify stars within 25 pc of the Sun. A sample of nearby stars with accurate trigonometric parallaxes from the Research Consortium On Nearby Stars (RECONS) is used to generate a set of 16 new photometric color-$M_{K{_s}}$ relations that provide distance estimates with uncertainties of 15\%. This work expands the available suites of well-calibrated photometric distance relations that can be used to identify nearby stellar systems. The distance relations are used with quality cuts to extract an initial sample of stars from the UCAC4 estimated to be within 25 pc. Color, proper motion and existing literature sources are then used to obtain a clean sample of red dwarfs, while limiting the amount of contamination from background giants, resulting in a sample of 1761 candidate nearby stars within 25 pc. Of these, 339 are new discoveries with no previously known published parallax or distance estimate, primarily with proper motions less than 0.2 arcsec/year. Five stars are estimated to be within 10 pc, with the nearest, TYC 3980 1081 1 with V$=$ 10.50, estimated to be at 5.93 pc. That several hundred new stars have been revealed so close to the Sun illustrates once again that there is considerable work yet to be done to map the solar neighborhood, and that additional nearby stars are likely still to be discovered.

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arXiv2012

Stellar Diameters and Temperatures II. Main Sequence K & M Stars

We present interferometric diameter measurements of 21 K- and M- dwarfs made with the CHARA Array. This sample is enhanced by literature radii measurements to form a data set of 33 K-M dwarfs with diameters measured to better than 5%. For all 33 stars, we compute absolute luminosities, linear radii, and effective temperatures (Teff). We develop empirical relations for \simK0 to M4 main- sequence stars between the stellar Teff, radius, and luminosity to broad-band color indices and metallicity. These relations are valid for metallicities between [Fe/H] = -0.5 and +0.1 dex, and are accurate to ~2%, ~5%, and ~4% for Teff, radius, and luminosity, respectively. Our results show that it is necessary to use metallicity dependent transformations to convert colors into stellar Teffs, radii, and luminosities. We find no sensitivity to metallicity on relations between global stellar properties, e.g., Teff-radius and Teff-luminosity. Robust examinations of single star Teffs and radii compared to evolutionary model predictions on the luminosity-Teff and luminosity-radius planes reveals that models overestimate the Teffs of stars with Teff < 5000 K by ~3%, and underestimate the radii of stars with radii < 0.7 R\odot by ~5%. These conclusions additionally suggest that the models overestimate the effects that the stellar metallicity may have on the astrophysical properties of an object. By comparing the interferometrically measured radii for single stars to those of eclipsing binaries, we find that single and binary star radii are consistent. However, the literature Teffs for binary stars are systematically lower compared to Teffs of single stars by ~ 200 to 300 K. Lastly, we present a empirically determined HR diagram for a total of 74 nearby, main-sequence, A- to M-type stars, and define regions of habitability for the potential existence of sub-stellar mass companions in each system. [abridged]

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arXiv2008

The Luminosity and Mass Functions of Low-Mass Stars in the Galactic Disk: I. The Calibration Region

We present measurements of the luminosity and mass functions of low-mass stars constructed from a catalog of matched Sloan Digital Sky Survey (SDSS) and 2 Micron All Sky Survey (2MASS) detections. This photometric catalog contains more than 25,000 matched SDSS and 2MASS point sources spanning ~30 square degrees on the sky. We have obtained follow-up spectroscopy, complete to J=16, of more than 500 low mass dwarf candidates within a 1 square degree sub-sample, and thousands of additional dwarf candidates in the remaining 29 square degrees. This spectroscopic sample verifies that the photometric sample is complete, uncontaminated, and unbiased at the 99% level globally, and at the 95% level in each color range. We use this sample to derive the luminosity and mass functions of low-mass stars over nearly a decade in mass (0.7 M_sun > M_* > 0.1 M_sun). We find that the logarithmically binned mass function is best fit with an M_c=0.29 log-normal distribution, with a 90% confidence interval of M_c=0.20--0.50. These 90% confidence intervals correspond to linearly binned mass functions peaking between 0.27 M_sun and 0.12 M_sun, where the best fit MF turns over at 0.17 M_sun. A power law fit to the entire mass range sampled here, however, returns a best fit of alpha=1.1 (where the Salpeter slope is alpha = 2.35). These results agree well with most previous investigations, though differences in the analytic formalisms adopted to describe those mass functions can give the false impression of disagreement. Given the richness of modern-day astronomical datasets, we are entering the regime whereby stronger conclusions can be drawn by comparing the actual datapoints measured in different mass functions, rather than the results of analytic analyses that impose structure on the data a priori. (abridged)

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arXiv2008

Angular Diameters of the G Subdwarf $μ$ Cassiopeiae A and the K Dwarfs $σ$ Draconis and HR 511 from Interferometric Measurements with the CHARA Array

Using the longest baselines of the CHARA Array, we have measured the angular diameter of the G5 V subdwarf $μ$ Cas A, the first such determination for a halo population star. We compare this result to new diameters for the higher metallicity K0 V stars, $σ$ Dra and HR 511, and find that the metal-poor star, $μ$ Cas A, has an effective temperature ($T_{\rm eff}=5297\pm32$ K), radius ($R=0.791\pm0.008 R_{\rm \odot}$), and absolute luminosity ($L=0.442\pm0.014 L_{\rm \odot}$) comparable to the other two stars with later spectral types. We show that stellar models show a discrepancy in the predicted temperature and radius for $μ$ Cas A, and we discuss these results and how they provide a key to understanding the fundamental relationships for stars with low metallicity.

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arXiv2005

Ages of White Dwarf-Red Subdwarf Systems

We provide the first age estimates for two recently discovered white dwarf-red subdwarf systems, LHS 193AB and LHS 300AB. These unusual systems provide a new opportunity for linking the reliable age estimates for the white dwarfs to the (measurable) metallicities of the red subdwarfs. We have obtained precise photometry in the $V_{J}R_{KC}I_{KC}JH$ bands and spectroscopy covering from 6000Åto 9000Åfor the two new systems, as well as for a comparison white dwarf-main sequence red dwarf system, GJ 283 AB. Using model grids available in the literature, we estimate the cooling age as well as temperature, surface gravity, mass, progenitor mass and {\it total} lifetimes of the white dwarfs. The results indicate that the two new systems are probably ancient thick disk objects with ages of at least 6-9 Gyr. We also conduct searches of red dwarf and white dwarf compendia from SDSS data and the L{é}pine Shara Proper Motion (LSPM) catalog for additional common proper motion white dwarf-red subdwarf systems. Only seven new candidate systems are found, which indicates the rarity of these systems.

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arXiv2004

Astrometric Discovery of GJ 164B

We discovered a low-mass companion to the M-dwarf GJ 164 with the CCD-based imaging system of the Stellar Planet Survey (STEPS) astrometric program. The existence of GJ 164B was confirmed with Hubble Space Telescope NICMOS imaging observations. A high-dispersion spectral observation in V sets a lower limit of delta m> 2.2 mag between the two components of the system. Based upon our parallax value of 0.082 +/- 0.008, we derive the following orbital parameters: P = 2.04 +/- 0.03 y, a = 1.03 +/- 0.03 AU, and Mtotal = 0.265 +/- 0.020 MSun. The component masses are MA = 0.170 +/- 0.015 MSun and MB = 0.095 +/- 0.015 MSun. Based on its mass, colors, and spectral properties, GJ 164B has spectral type M6-8 V.

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