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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)

   Abstract 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. Reconstructing the near- to mid-infrared environment in the stellar merger remnant V838 Monocerotis

   https://doi.org/10.1051/0004-6361/202347322  

   Mobeen, Muhammad Zain; Kamiński, Tomasz; Matter, Alexis; Wittkowski, Markus; Monnier, John D; Kraus, Stefan; Le_Bouquin, Jean-Baptiste; Anugu, Narsireddy; Ten_Brummelaar, Theo; Davies, Claire L; et al (June 2024, Astronomy & Astrophysics)

   Context.V838 Mon is a stellar merger remnant that erupted in a luminous red nova event in 2002. Although it has been well studied in the optical, near-infrared, and submillimeter regimes, its structure in the mid-infrared wavelengths remains elusive. Over the past two decades, only a handful of infrared interferometric studies have been performed, suggesting the presence of an elongated structure at multiple wavelengths. However, given the limited nature of these observations, the true morphology of the source has not yet been conclusively determined. Aims.By performing image reconstruction using observations taken at the VLTI and CHARA, we aim to map out the circumstellar environment in V838 Mon. Methods.We observed V838 Mon with the MATISSE (LMNbands) and GRAVITY (Kband) instruments at the VLTI as well as the MIRCX/MYSTIC (HKbands) instruments at the CHARA array. We geometrically modelled the squared visibilities and the closure phases in each of the bands to obtain the constraints on the physical parameters. Furthermore, we constructed high-resolution images of V838 Mon in theHKbands using the MIRA and SQUEEZE algorithms to study the immediate surroundings of the star. Lastly, we also modelled the spectral features seen in theKandMbands at various temperatures. Results.The image reconstructions show a bipolar structure that surrounds the central star in the post-merger remnant. In theKband, the super-resolved images show an extended structure (uniform disk diameter ~1.94 mas) with a clumpy morphology that is aligned along a north-west position angle (PA) of −40°. On the other hand, in theHband, the extended structure (uniform disk diameter ~1.18 mas) lies roughly along the same PA. Yet the northern lobe is slightly misaligned with respect to the southern lobe, which results in the closure phase deviations. Conclusions.The VLTI and CHARA imaging results show that V838 Mon is surrounded by features resembling jets that are intrinsically asymmetric. This is further confirmed by the closure phase modelling. Further observations with VLTI can help to determine whether this structure shows any variations over time and also if such bi-polar structures are commonly formed in other stellar merger remnants. 

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