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1. Observations with the Southern Connecticut Stellar Interferometer. I. Instrument Description and First Results

   https://doi.org/10.3847/1538-3881/ac43bb  

   Horch, Elliott P. ; Weiss, Samuel A. ; Klaucke, Paul M. ; Pellegrino, Richard A. ; Rupert, Justin D. ( January 2022 , The Astronomical Journal)

   We discuss the design, construction, and operation of a new intensity interferometer, based on the campus of Southern Connecticut State University in New Haven, Connecticut. While this paper will focus on observations taken with an original two-telescope configuration, the current instrumentation consists of three portable 0.6 m Dobsonian telescopes with single-photon avalanche diode detectors located at the Newtonian focus of each telescope. Photons detected at each station are time stamped and read out with timing correlators that can give cross-correlations in timing to a precision of 48 ps. We detail our observations to date with the system, which has now been successfully used at our university in 16 nights of observing. Components of the instrument were also deployed on one occasion at Lowell Observatory, where the Perkins and Hall telescopes were made to function as an intensity interferometer. We characterize the performance of the instrument in detail. In total, the observations indicate the detection of a correlation peak at the level of 6.76σwhen observing unresolved stars, and consistency with partial or no detection when observing at a baseline sufficient to resolve the star. Using these measurements, we conclude that the angular diameter of Arcturus is larger than 15 mas and that of Vega is between 0.8 and 17 mas. While the uncertainties are large at this point, both results are consistent with measures from amplitude-based long baseline optical interferometers.

    

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2. Demonstration of stellar intensity interferometry with the four VERITAS telescopes

   https://doi.org/10.1038/s41550-020-1143-y  

   Abeysekara, A. U. ; Benbow, W. ; Brill, A. ; Buckley, J. H. ; Christiansen, J. L. ; Chromey, A. J. ; Daniel, M. K. ; Davis, J. ; Falcone, A. ; Feng, Q. ; et al ( July 2020 , Nature Astronomy)

   High angular resolution observations at optical wavelengths provide valuable insights into stellar astrophysics, and enable direct measurements of fundamental stellar parameters and the probing of stellar atmospheres, circumstellar disks, the elongation of rapidly rotating stars and the pulsations of Cepheid variable stars. The angular size of most stars is of the order of one milliarcsecond or less, and to spatially resolve stellar disks and features at this scale requires an optical interferometer using an array of telescopes with baselines on the order of hundreds of metres. We report on the implementation of a stellar intensity interferometry system developed for the four VERITAS imaging atmospheric Cherenkov telescopes. The system was used to measure the angular diameter of the two sub-milliarcsecond stars β Canis Majoris and ϵ Orionis with a precision of greater than 5%. The system uses an offline approach in which starlight intensity fluctuations that are recorded at each telescope are correlated post observation. The technique can be readily scaled onto tens to hundreds of telescopes, providing a capability that has proven technically challenging to the current generation of optical amplitude interferometry observatories. This work demonstrates the feasibility of performing astrophysical measurements using imaging atmospheric Cherenkov telescope arrays as intensity interferometers and shows the promise for integrating an intensity interferometry system within future observatories such as the Cherenkov Telescope Array. 

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3. 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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4. Delayed Photons from Binary Evolution Help Reionize the Universe

   https://doi.org/10.3847/1538-4357/abaefa  

   Secunda, Amy ; Cen, Renyue ; Kimm, Taysun ; Götberg, Ylva ; de Mink, Selma E. ( September 2020 , The Astrophysical Journal)

   High-resolution numerical simulations including feedback and aimed at calculating the escape fraction (f_esc) of hydrogen-ionizing photons often assume stellar radiation based on single-stellar population synthesis models. However, strong evidence suggests the binary fraction of massive stars is ≳70%. Moreover, simulations so far have yielded values off_escfalling only on the lower end of the ∼10%–20% range, the amount presumed necessary to reionize the universe. Analyzing a high-resolution (4 pc) cosmological radiation-hydrodynamic simulation, we study howf_escchanges when we include two different products of binary stellar evolution—stars stripped of their hydrogen envelopes and massive blue stragglers. Both produce significant amounts of ionizing photons 10–200 Myr after each starburst. We find the relative importance of these photons to be amplified with respect to escaped ionizing photons, because peaks in star formation rates (SFRs) andf_escare often out of phase by this 10–200 Myr. Additionally, low-mass, bursty galaxies emit Lyman continuum radiation primarily from binary products when SFRs are low. Observations of these galaxies by the James Webb Space Telescope could provide crucial information on the evolution of binary stars as a function of redshift. Overall, including stripped stars and massive blue stragglers increases our photon-weighted mean escape fraction () by ∼13% and ∼10%, respectively, resulting in. Our results emphasize that using updated stellar population synthesis models with binary stellar evolution provides a more sound physical basis for stellar reionization.

    

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5. The Exoplanet Transmission Spectroscopy Imager (ETSI)

   https://doi.org/10.1117/12.2562371  

   Limbach, Mary Anne ; Schmidt, Luke M. ; DePoy, Darren L. ; Mason, Jeffrey C. ; Scobey, Mike ; Brown, Pat ; Taylor, Chelsea ; Marshall, Jennifer L. ( December 2020 , Proc. SPIE 11447, Ground-based and Airborne Instrumentation for Astronomy VIII)

   Evans, Christopher J. ; Bryant, Julia J. ; Motohara, Kentaro (Ed.)

   We present the design of a novel instrument tuned to detect transiting exoplanet atmospheres. The instrument, which we call the exoplanet transmission spectroscopy imager (ETSI), makes use of a new technique called common-path multi-band imaging (CMI). ETSI uses a prism and multi-band lter to simultaneously image 15 spectral bandpasses on two detectors from 430 􀀀 975nm (with a average spectral resolution of R = =
    = 23) during exoplanet transits of a bright star. A prototype of the instrument achieved photon-noise limited results which were below the atmospheric amplitude scintillation noise limit. ETSI can detect the presence and composition of an exoplanet atmosphere in a relatively short time on a modest-size telescope. We show the optical design of the instrument. Further, we discuss design trades of the prism and multi-band lter which are driven by the science of the ETSI instrument. We describe the upcoming survey with ETSI that will measure dozens of exoplanet atmosphere spectra in  2 years on a two meter telescope. Finally, we will discuss how ETSI will be a powerful means for follow up on all gas giant exoplanets that transit bright stars, including a multitude of recently identied TESS (NASA's Transiting Exoplanet Survey Satellite) exoplanets. 

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