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1. Performance of the upgraded VERITAS Stellar Intensity Interferometer (VSII)

   https://doi.org/10.1117/12.2630227  

   Kieda, David B. ; VERITAS Collaboration ( August 2022 , SPIE Astronomical Telescopes + Instrumentation, 2022, Montréal, Québec, Canada : Optical and Infrared Interferometry and Imaging VIII)

   Mérand, Antoine ; Sallum, Stephanie ; Sanchez-Bermudez, Joel (Ed.)

   The VERITAS Imaging Air Cherenkov Telescope array (IACT) was augmented in 2019 with high-speed focal plane electronics to create a new Stellar Intensity Interferometry (SII) observational capability (VERITAS-SII, or VSII). VSII operates during bright moon periods, providing high angular resolution observations ( < 1 mas) in the B photometric band using idle telescope time. VSII has already demonstrated the ability to measure the diameters of two B stars at 416 nm (Bet CMa and Eps Ori) with < 5% accuracy using relatively short (5 hours) exposures.1 The VSII instrumentation was recently improved to increase instrumental sensitivity and observational efficiency. This paper describes the upgraded VSII instrumentation and documents the ongoing improvements in VSII sensitivity. The report describes VSII’s progress in extending SII measurements to dimmer magnitude stars and improving the VSII angular diameter measurement resolution to better than 1%. 

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2. Constraints on HD 113337 fundamental parameters and planetary system: Combining long-base visible interferometry, disc imaging, and high-contrast imaging

   https://doi.org/10.1051/0004-6361/201935494  

   Borgniet, S. ; Perraut, K. ; Su, K. ; Bonnefoy, M. ; Delorme, P. ; Lagrange, A.-M. ; Bailey, V. ; Buenzli, E. ; Defrère, D. ; Henning, T. ; et al ( July 2019 , Astronomy & Astrophysics)

   Context. HD 113337 is a main-sequence F6V field star more massive than the Sun. This star hosts one confirmed giant planet and possibly a second candidate, detected by radial velocities (RVs). The star also hosts a cold debris disc detected through the presence of an infrared excess, making it an interesting system to explore. Aims. We aim to bring new constraints on the star’s fundamental parameters, debris disc properties, and planetary companion(s) by combining complementary techniques. Methods. We used the VEGA interferometer on the CHARA array to measure the angular diameter of HD 113337. We derived its linear radius using the parallax from the Gaia Second Data Release. We computed the bolometric flux to derive its effective temperature and luminosity, and we estimated its mass and age using evolutionary tracks. Then, we used Herschel images to partially resolve the outer debris disc and estimate its extension and inclination. Next, we acquired high-contrast images of HD 113337 with the LBTI to probe the ~10–80 au separation range. Finally, we combined the deduced contrast maps with previous RVs of the star using the MESS2 software to bring upper mass limits on possible companions at all separations up to 80 au. We took advantage of the constraints on the age and inclination brought by fundamental parameter analysis and disc imaging, respectively, for this analysis. Results. We derive a limb-darkened angular diameter of 0.386 ± 0.009 mas that converts into a linear radius of 1.50 ± 0.04 R ⊙ for HD 113337. The fundamental parameter analysis leads to an effective temperature of 6774 ± 125 K and to two possible age solutions: one young within 14–21 Myr and one old within 0.8–1.7 Gyr. We partially resolve the known outer debris disc and model its emission. Our best solution corresponds to a radius of 85 ± 20 au, an extension of 30 ± 20 au, and an inclination within 10–30° for the outer disc. The combination of imaging contrast limits, published RV, and age and inclination solutions allows us to derive a first possible estimation of the true masses of the planetary companions: ~7 −2 +4 M Jup for HD 113337 b (confirmed companion) and ~16 −3 +10 M Jup for HD 113337 c (candidate companion). We also constrain possible additional companions at larger separations. 

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3. Resolving the Binary System Spica using the VERITAS Stellar Intensity Interferometer

   Breeland-Newcomb, Katie ; Kieda, David ; Davis, Jonathan ( January 2023 , American Astronomical Society meeting)

   AB In 2019, the Very Energetic Radiation Imaging Telescope Array System (VERITAS) was augmented with high-speed optical electronics in order to allow for Stellar Intensity Interferometry (SII) observational capabilities. This research shows how VERITAS-SII (VSII), which measures correlations of starlight intensity fluctuations across spatially separated telescopes, can enable the characterization of binary stellar systems. We first use VSII data collected on the binary star Spica to develop a dynamic analysis technique. We then simulate the squared visibility curve given a particular orientation of Spica's components. Because of Spica's 4.0145-day period, the binary separation and orientation, and therefore the simulated squared visibility, should vary greatly from night to night. These variations are consistent with measured variations in the observed squared visibility curves. The initial results indicate that VSII observations potentially demonstrate good sensitivity to the evolution of the Spica binary system. With further development, it may be possible to fit a multi-dimensional image to the system, opening the door to model-dependent VSII imaging. 

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4. Refining the Stellar Parameters of τ Ceti: a Pole-on Solar Analog

   https://doi.org/10.3847/1538-3881/ace906  

   Korolik, Maria ; Roettenbacher, Rachael M. ; Fischer, Debra A. ; Kane, Stephen R. ; Perkins, Jean M. ; Monnier, John D. ; Davies, Claire L. ; Kraus, Stefan ; Le Bouquin, Jean-Baptiste ; Anugu, Narsireddy ; et al ( August 2023 , The Astronomical Journal)

   To accurately characterize the planets a star may be hosting, stellar parameters must first be well determined.τCeti is a nearby solar analog and often a target for exoplanet searches. Uncertainties in the observed rotational velocities have made constrainingτCeti’s inclination difficult. For planet candidates from radial velocity (RV) observations, this leads to substantial uncertainties in the planetary masses, as only the minimum mass ($m\sin i$) can be constrained with RV. In this paper, we used new long-baseline optical interferometric data from the CHARA Array with the MIRC-X beam combiner and extreme precision spectroscopic data from the Lowell Discovery Telescope with EXPRES to improve constraints on the stellar parameters ofτCeti. Additional archival data were obtained from a Tennessee State University Automatic Photometric Telescope and the Mount Wilson Observatory HK project. These new and archival data sets led to improved stellar parameter determinations, including a limb-darkened angular diameter of 2.019 ± 0.012 mas and rotation period of 46 ± 4 days. By combining parameters from our data sets, we obtained an estimate for the stellar inclination of 7° ± 7°. This nearly pole-on orientation has implications for the previously reported exoplanets. An analysis of the system dynamics suggests that the planetary architecture described by Feng et al. may not retain long-term stability for low orbital inclinations. Additionally, the inclination ofτCeti reveals a misalignment between the inclinations of the stellar rotation axis and the previously measured debris disk rotation axis (i_disk= 35° ± 10°).

    

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5. The subgiant HR 7322 as an asteroseismic benchmark star

   https://doi.org/10.1093/mnras/stz2222  

   Stokholm, Amalie ; Nissen, Poul Erik ; Silva Aguirre, Víctor ; White, Timothy R ; Lund, Mikkel N ; Mosumgaard, Jakob Rørsted ; Huber, Daniel ; Jessen-Hansen, Jens ( October 2019 , Monthly Notices of the Royal Astronomical Society)

   Abstract We present an in-depth analysis of the bright subgiant HR 7322 (KIC 10005473) using Kepler short-cadence photometry, optical interferometry from CHARA, high-resolution spectra from SONG, and stellar modelling using garstec grids, and the Bayesian grid-fitting algorithm basta. HR 7322 is only the second subgiant with high-quality Kepler asteroseismology for which we also have interferometric data. We find a limb-darkened angular diameter of 0.443 ± 0.007 mas, which, combined with a distance derived using the parallax from Gaia DR2 and a bolometric flux, yields a linear radius of 2.00 ± 0.03 R⊙ and an effective temperature of 6350 ± 90 K. HR 7322 exhibits solar-like oscillations, and using the asteroseismic scaling relations and revisions thereof, we find good agreement between asteroseismic and interferometric stellar radius. The level of precision reached by the careful modelling is to a great extent due to the presence of an avoided crossing in the dipole oscillation mode pattern of HR 7322. We find that the standard models predict a stellar radius systematically smaller than the observed interferometric one and that a sub-solar mixing length parameter is needed to achieve a good fit to individual oscillation frequencies, interferometric temperature, and spectroscopic metallicity. 

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