Modeling the e-APD SAPHIRA/C-RED ONE camera at low flux level: An attempt to count photons in the near-infrared with the MIRC-X interferometric combiner

This is the final version. Available on open access from EDP Sciences via the DOI in this recordContext. We implement an electron avalanche photodiode (e-APD) in the MIRC-X instrument, upgrade of the 6-telescope nearinfrared imager MIRC, at the CHARA array. This technology should improve the sensitivity of near-infrared interferometry. Aims. We characterize a near-infrared C-RED ONE camera from First Light Imaging (FLI) using an e-APD from Leonardo (previously SELEX). Methods. We first used the classical Mean-Variance analysis to measure the system gain and the amplification gain. We then developed a physical model of the statistical distribution of the camera output signal. This model is based on multiple convolutions of the Poisson statistic, the intrinsic avalanche gain distribution, and the observed distribution of the background signal. At low flux level, this model constraints independently the incident illumination level, the total gain, and the excess noise factor of the amplification. Results. We measure a total transmission of 48 ± 3% including the cold filter and the Quantum Efficiency. We measure a system gain of 0.49 ADU/e, a readout noise of 10 ADU, and amplification gains as high as 200. These results are consistent between the two methods and therefore validate our modeling approach. The measured excess noise factor based on the modeling is 1.47 ± 0.03, with no obvious dependency with flux level or amplification gain. Conclusions. The presented model allows measuring the characteristics of the e-APD array at low flux level independently of preexisting calibration. With < 0.3 electron equivalent readout noise at kilohertz frame rates, we confirm the revolutionary performances of the camera with respect to the PICNIC or HAWAII technologies. However, the measured excess noise factor is significantly higher than the one claimed in the literature (<1.25), and explains why counting multiple photons remains challenging with this camera.European Union Horizon 2020Labex OSUG@2020CNRS/INS

The MIRC-X 6-telescope imager: Key science drivers, instrument design and operation

This is the final version of the article. Available from SPIE via the DOI in this recordMIRC-X is a new beam combination instrument at the CHARA array that enables 6-telescope interferometric imaging on object classes that until now have been out of reach for milliarcsecond-resolution imaging. As part of an instrumentation effort lead by the University of Exeter and University of Michigan, we equipped the MIRC instrument with an ultra-low read-noise detector system and extended the wavelength range to the J and H-band. The first phase of the MIRC-X commissioning was successfully completed in June 2017. In 2018 we will commission polarisation control to improve the visibility calibration and implement a 'cross-talk resiliant' mode that will minimise visibility cross-talk and enable exoplanet searches using precision closure phases. Here we outline our key science drivers and give an overview about our commissioning timeline. We comment on operational aspects, such as remote observing, and the prospects of co-phased parallel operations with the upcoming MYSTIC combiner.MIRC-X is funded by a Starting Grant from the European Research Council (ERC; grant agreement No. 639889, PI: Kraus) and funds from the University of Exeter. The project builds on earlier investments from the University of Michigan and the National Science Foundation (NSF, PI: Monnier)

MIRC-X/CHARA: sensitivity improvements with an ultra-low noise SAPHIRA detector

This is the final version of the article. Available from Society of Photo Optical Instrumentation Engineers (SPIE) via the DOI in this record.MIRC-X is an upgrade of the six-telescope infrared beam combiner at the CHARA telescope array, the world's largest baseline interferometer in the optical/infrared, located at the Mount Wilson Observatory in Los Angeles. The upgraded instrument features an ultra-low noise and fast frame rate infrared camera (SAPHIRA detector) based on e-APD technology. We report the MIRC-X sensitivity upgrade work and first light results in detail focusing on the detector characteristics and software architecture.MIRC-X is funded, in parts, by a Starting Grant from the European Research Council (ERC; grant agreement No. 639889, PI: Kraus) and builds on earlier investments from the University of Michigan and the National Science Foundation (NSF, PI: Monnier). This research has made use of the Jean-Marie Mariotti Center OIFits Explorer service (http://www.jmmc.fr/oifitsexplorer)

Probing the Inner Disk Emission of the Herbig Ae Stars HD 163296 and HD 190073

This is the author accepted manuscript. The final version is available from American Astronomical Society / IOP Publishing via the DOI in this record.The physical processes occurring within the inner few astronomical units of proto-planetary disks surrounding Herbig Ae stars are crucial to setting the environment in which the outer planet-forming disk evolves and put critical constraints on the processes of accretion and planet migration. We present the most complete published sample of high angular resolution H- and K-band observations of the stars HD 163296 and HD 190073, including 30 previously unpublished nights of observations of the former and 45 nights of the latter with the CHARA long-baseline interferometer, in addition to archival VLTI data. We confirm previous observations suggesting significant near-infrared emission originates within the putative dust evaporation front of HD 163296 and show this is the case for HD 190073 as well. The H- and K-band sizes are the same within (3±3)% for HD 163296 and within (6±10)% for HD 190073. The radial surface brightness profiles for both disks are remarkably Gaussian-like with little or no sign of the sharp edge expected for a dust evaporation front. Coupled with spectral energy distribution analysis, our direct measurements of the stellar flux component at H and K bands suggest that HD 190073 is much younger (<400 kyr) and more massive (~5.6 M⊙) than previously thought, mainly as a consequence of the new Gaia distance (891 pc).JDM and BRS acknowledge support from NSF-AST 1506540 and AA acknowledges support from NSF-AST 1311698. CLD, AK, and SK acknowledge support from the ERC Starting Grant “ImagePlanetFormDiscs” (Grant Agreement No. 639889), STFC Rutherford fellowship/grant (ST/J004030/1, ST/K003445/1) and Philip Leverhulme Prize (PLP2013-110). FB acknowledges support from NSF-AST 1210972 and 1445935. MS acknowledges support by the NASA Origins of Solar Systems grant NAG5-9475, and NASA Astrophysics Data Program contract NNH05CD30C. The CHARA Array is supported by the National Science Foundation under Grant No. AST-1211929, AST-1636624, and AST-1715788. Institutional support has been provided from the GSU College of Arts and Sciences and the GSU Office of the Vice President for Research and Economic Development

Science cases for a visible interferometer

High spatial resolution is the key for the understanding various astrophysical phenomena. But even with the future E-ELT, single dish instruments are limited to a spatial resolution of about 4 mas in the visible. For the closest objects within our Galaxy most of the stellar photosphere remains smaller than 1 mas. With the success of long baseline interferometry these limitations were soom overcome. Today low and high resolution interferometric instruments on the VLTI and CHARA offer an immense range of astrophysical studies. Combining more telescopes and moving to visible wavelengths broadens the science cases even more. With the idea of developing strong science cases for a future visible interferometer, we organized a science group around the following topics: pre-main sequence and main sequence stars, fundamental parameters, asteroseismology and classical pulsating stars, evolved stars, massive stars, active galactic nuclei (AGNs) and imaging techniques. A meeting was organized on the 15th and 16th of January, 2015 in Nice with the support of the Action Specific in Haute Resolution Angulaire (ASHRA), the Programme National en Physique Stellaire (PNPS), the Lagrange Laboratory and the Observatoire de la Cote d'Azur, in order to present these cases and to discuss them further for future visible interferometers. This White Paper presents the outcome of the exchanges. This book is dedicated to the memory of our colleague Olivier Chesneau who passed away at the age of 41

Beam combiner for the Asgard/BIFROST instrument

This is the author accepted manuscript.BIFROST will be a short-wavelength (λ = 1.0- 1.7µm) beam combiner for the VLT Interferometer, combining both high spatial (λ/2B = 0.8 mas) and spectral (up to R = 25,000) resolution. It will be part of the Asgard Suite of visitor instruments. The new window of high spectral resolution, short wavelength observations brings with it new challenges. Here we outline the instrumental design of BIFROST, highlighting which beam combiner subsystems are required and why. This is followed by a comparison All-In-One (AIO) beam combination scheme and an Integrated Optics (IO) scheme with ABCD modulation both in terms of expected sensitivity and the practical implementation of each system.European Research Council (ERC)Science and Technology Facilities Council (STFC

Multiplicity of massive O stars and evolutionary implications

Nearby companions alter the evolution of massive stars in binary systems. Using a sample of Galactic massive stars in nearby young clusters, we simultaneously measure all intrinsic binary characteristics relevant to quantify the frequency and nature of binary interactions. We find a large intrinsic binary fraction, a strong preference for short orbital periods and a flat distribution for the mass-ratios. Our results do not support the presence of a significant peak of equal-mass `twin' binaries. As a result of the measured distributions, we find that over seventy per cent of all massive stars exchange mass with a companion. Such a rate greatly exceeds previous estimates and implies that the majority of massive stars have their evolution strongly affected by interaction with a nearby companion

High spectral-resolution interferometry down to 1 micron with Asgard/BIFROST at VLTI: Science drivers and project overview

This is the author accepted manuscript.We present science cases and instrument design considerations for the BIFROST instrument that will open the short-wavelength (Y/J/H-band), high spectral dispersion (up to R=25,000) window for the VLT Interferometer. BIFROST will be part of the Asgard Suite of instruments and unlock powerful venues for studying accretion & mass-loss processes at the early/late stages of stellar evolution, for detecting accreting protoplanets around young stars, and for probing the spin-orbit alignment in directly-imaged planetary systems and multiple star systems. Our survey on GAIA binaries aims to provide masses and precision ages for a thousand stars, providing a legacy data set for improving stellar evolutionary models as well as for Galactic Archaeology. BIFROST will enable off-axis spectroscopy of exoplanets in the 0.025-1" separation range, enabling high-SNR, high spectral resolution follow-up of exoplanets detected with ELT and JWST. We give an update on the status of the project, outline our key technology choices, and discuss synergies with other instruments in the proposed Asgard Suite of instruments.European Research Council (ERC)Science and Technology Facilities Council (STFC)European Union Horizon 202

Viscous heating and boundary layer accretion in the disk of outbursting star FU Orionis

This is the final version. Available from EDP Sciences via the DOI in this recordContext. FU Orionis is the archetypal FUor star, a subclass of young stellar object (YSO) that undergo rapid brightening events, often gaining 4-6 magnitudes on timescales of days. This brightening is often associated with a massive increase in accretion; one of the most ubiquitous processes in astrophysics from planets and stars to super-massive black holes. We present multi-band interferometric observations of the FU Ori circumstellar environment, including the first J-band interferometric observations of a Y SO. Aims. We investigate the morphology and temperature gradient of the inner-most regions of the accretion disk around FU Orionis. We aim to characterise the heating mechanisms of the disk and comment on potential outburst triggering processes. Methods. Recent upgrades to the MIRC-X instrument at the CHARA array allowed the first dual-band J and H observations of YSOs. Using baselines up to 331 m, we present high angular resolution data of a YSO covering the near-infrared bands J, H, and K. The unprecedented spectral range of the data allows us to apply temperature gradient models to the innermost regions of FU Ori. Results. We spatially resolve the innermost astronomical unit of the disk and determine the exponent of the temperature gradient of the inner disk to T ∝ r −0.74±0.02. This agrees with theoretical work that predicts T ∝ r −0.75 for actively accreting, steady state disks, a value only obtainable through viscous heating within the disk. We find a disk which extends down to the stellar surface at 0.015 ± 0.007 au where the temperature is found to be 5800 ± 700 K indicating boundary layer accretion. We find a disk inclined at 32 ± 4 ◦ with a minor-axis position angle of 34 ± 11◦ . Conclusions. We demonstrate that J-band interferometric observations of YSOs are feasible with the MIRC-X instrument at CHARA. The temperature gradient power-law derived for the inner disk is consistent with theoretical predictions for steady-state, optically thick, viciously heated accretion disks.European Research Council (ERC