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We present the results of a medium resolution optical spectroscopic survey of 92 cool ($3,000 \lesssim T_{\rm eff} \lesssim 4,500\,$K) southern TESS candidate planet hosts, and describe our spectral fitting methodology used to recover stellar parameters. We quantify model deficiencies at predicting optical fluxes, and while our technique works well for $T_{\rm eff}$, further improvements are needed for [Fe/H]. To this end, we developed an updated photometric [Fe/H] calibration for isolated main sequence stars built upon a calibration sample of 69 cool dwarfs in binary systems, precise to $\pm0.19\,$dex, from super-solar to metal poor, over $1.51 < {\rm Gaia}~(B_P-R_P) < 3.3$. Our fitted $T_{\rm eff}$ and $R_\star$ have median precisions of 0.8% and 1.7%, respectively and are consistent with our sample of standard stars. We use these to model the transit light curves and determine exoplanet radii for 100 candidate planets to 3.5% precision and see evidence that the planet-radius gap is also present for cool dwarfs. Our results are consistent with the sample of confirmed TESS planets, with this survey representing one of the largest uniform analyses of cool TESS candidate planet hosts to date., Accepted version. 33 Pages, 12 Figures, 9 Tables

A wide-field zenith-looking telescope operating in a mode similar to Time-Delay-Integration (TDI) or drift scan imaging can perform an infrared sky survey without active pointing control but it requires a high-speed, low-noise infrared detector. Operating from a hosted payload platform on the International Space Station (ISS), the Emu space telescope employs the paradigm-changing properties of the Leonardo SAPHIRA electron avalanche photodiode array to provide powerful new observations of cool stars at the critical water absorption wavelength (1.4 $\mu$m) largely inaccessible to ground-based telescopes due to the Earth's own atmosphere. Cool stars, especially those of spectral-type M, are important probes across contemporary astrophysics, from the formation history of the Galaxy to the formation of rocky exoplanets. Main sequence M-dwarf stars are the most abundant stars in the Galaxy and evolved M-giant stars are some of the most distant stars that can be individually observed. The Emu sky survey will deliver critical stellar properties of these cool stars by inferring oxygen abundances via measurement of the water absorption band strength at 1.4 $\mu$m. Here we present the TDI-like imaging capability of Emu mission, its science objectives, instrument details and simulation results., Comment: Accepted for publication in Journal of Astronomical Telescopes, Instruments, and Systems (JATIS)

In the current era of Gaia and large, high signal to noise stellar spectroscopic surveys, there is an unmet need for a reliable library of fundamentally calibrated stellar effective temperatures based on accurate stellar diameters. Here we present a set of precision diameters and temperatures for a sample of 6 dwarf, 5 sub-giant, and 5 giant stars observed with the PIONIER beam combiner at the VLTI. Science targets were observed in at least two sequences with five unique calibration stars each for accurate visibility calibration and to reduce the impact of bad calibrators. We use the standard PIONIER data reduction pipeline, but bootstrap over interferograms, in addition to employing a Monte-Carlo approach to account for correlated errors by sampling stellar parameters, limb darkening coefficients, and fluxes, as well as predicted calibrator angular diameters. The resulting diameters were then combined with bolometric fluxes derived from broadband Hipparcos-Tycho photometry and MARCS model bolometric corrections, plus parallaxes from Gaia to produce effective temperatures, physical radii, and luminosities for each star observed. Our stars have mean angular diameter and temperatures uncertainties of 0.8% and 0.9% respectively, with our sample including diameters for 10 stars with no pre-existing interferometric measurements. The remaining stars are consistent with previous measurements, with the exception of a single star which we observe here with PIONIER at both higher resolution and greater sensitivity than was achieved in earlier work., Comment: Published in MNRAS, 22 pages, 7 figures

In order to accurately determine stellar properties, knowledge of the effective temperature of stars is vital. We implement Gaia and 2MASS photometry in the InfraRed Flux Method and apply it to over 360,000 stars across different evolutionary stages in the GALAH DR3 survey. We derive colour-effective temperature relations that take into account the effect of metallicity and surface gravity over the range 4000 to 8000 kelvin, from very metal-poor stars to super solar metallicities. The internal uncertainty of these calibrations is of order 40-80 kelvin depending on the colour combination used. Comparison against solar-twins, Gaia benchmark stars and the latest interferometric measurements validates the precision and accuracy of these calibrations from F to early M spectral types. We assess the impact of various sources of uncertainties, including the assumed extinction law, and provide guidelines to use our relations. Robust solar colours are also derived., MNRAS accepted. Include Gaia solar colours. Colour-effective temperature routines for Gaia DR2 and DR3 system available at https://github.com/casaluca/colte

In the new era of Extremely Large Telescopes (ELTs) currently under construction, challenging requirements drive spectrograph designs towards techniques that efficiently use a facility's light collection power. Operating in the single-mode (SM) regime, close to the diffraction limit, reduces the footprint of the instrument compared to a conventional high-resolving power spectrograph. The custom built injection fiber system with 3D-printed microlenses on top of it for the replicable high-resolution exoplanet and asteroseismology spectrograph (RHEA) at Subaru in combination with extreme adaptive optics of SCExAO, proved its high efficiency in a lab environment, manifesting up to ∼77% of the theoretical predicted performance.

Asteroseismology is a powerful tool for probing the internal structures of stars by using their natural pulsation frequencies. It relies on identifying sequences of pulsation modes that can be compared with theoretical models, which has been done successfully for many classes of pulsators, including low-mass solar-type stars, red giants, high-mass stars and white dwarfs. However, a large group of pulsating stars of intermediate mass--the so-called delta Scuti stars--have rich pulsation spectra for which systematic mode identification has not hitherto been possible. This arises because only a seemingly random subset of possible modes are excited, and because rapid rotation tends to spoil the regular patterns. Here we report the detection of remarkably regular sequences of high-frequency pulsation modes in 60 intermediate-mass main-sequence stars, allowing definitive mode identification. Some of these stars have space motions that indicate they are members of known associations of young stars, and modelling of their pulsation spectra confirms that these stars are indeed young., published in Nature https://www.nature.com/articles/s41586-020-2226-8

In the Gaia era, the majority of stars in the Solar neighbourhood have parallaxes and proper motions precisely determined while spectroscopic age indicators are still missing for a large fraction of low-mass young stars. In this work we select 756 overluminous late K and early M young star candidates in the southern sky and observe them over 64 nights with the ANU 2.3m Telescope at Siding Spring Observatory using the Echelle (R=24,000) and Wide Field spectrographs (WiFeS, R=3000-7000). Our selection is kinematically unbiased to minimize the preference against low-mass members of stellar associations that dissipate first, and to include potential members of diffuse components. We provide measurements of H$\alpha$ and calcium H&K emission, as well as lithium absorption line, that enable identification of stars as young as $\sim$10-30 Myr which is a typical age of a stellar association. We report on 346 stars showing a detectable lithium line, 318 of which are not found in the known catalogs of young stars. We also report 126 additional stars in our sample which have no detectable lithium but signs of stellar activity indicating youth. Radial velocities are determined for WiFeS spectra with a precision of 3.2 $\mathrm{km\;s^{-1}}$ and 1.5 $\mathrm{km\;s^{-1}}$ for the Echelle sample., Comment: Submitted to MNRAS. 15 pages, 12 figures

RHEA is a compact high-resolution single-mode spectrograph that can easily be produced in larger quantities as budgets allow. The instrument will be housed in a temperature-stabilized vacuum chamber which is surrounded by several layers of thermal shielding. The optical design employs cost-effective commercially available compo- nents, a cooled CMOS detector, and a double-fiber input which permits simultaneous wavelength calibration.

RHEA is a single-mode échelle spectrograph designed to be a replicable and cost effective method of undertaking precision radial velocity measurements. The instrument has a novel fiber feed with an integral field unit injecting into a grid of single-mode fibers reformatted to form a pseudo-slit, increasing throughput and enabling highspatial resolution observations when operating behind Subaru and the SCExAO adaptive optics system. The past 18 months have seen a replacement cable constructed for the instrument to address modal noise caused by closely packed fibers with similar path lengths. Here we detail the cable fabrication procedure, design improvements, increased precision in meeting the required sub-micron optical tolerances, throughput gains, and known remaining issues.

Australian Research Council [DP120103751]; Research School of Astronomy and Astrophysics; Australian National University

The Replicable High-resolution Exoplanet and Asteroseismology (RHEA) spectrograph is being developed to serve as a basis for multiple copies across a network of small robotic telescopes. The spectrograph operates at the diffraction-limit by using a single-mode fiber input, resulting in a compact and modal-noise-free unit. The optical design is mainly based on off-the-shelf available components and comprises a near-Littrow configuration with prism cross-disperser. The echelle format covers a wavelength range of 430-650 nm at R=75,000 resolving power. In this paper we briefly summarize the current status of the instrument and present preliminary results from the first on-sky demonstration of the prototype using a fully automated 16" telescope, where we observe stable and semi-variable stars up to V=3.5 magnitude. Future steps to enhance the efficiency and passive stability of RHEA are discussed in detail. For example, we show the concept of using a multi-fiber injection unit, akin to a photonic lantern, which not only enables increased throughput but also offers simultaneous wavelength calibration.

SCExAO is the premier high-contrast imaging platform for the Subaru Telescope. It offers high Strehl ratios at near-IR wavelengths (y-K band) with stable pointing and coronagraphs with extremely small inner working angles, optimized for imaging faint companions very close to the host. In the visible, it has several interferometric imagers which offer polarimetric and spectroscopic capabilities. A recent addition is the RHEA spectrograph enabling spatially resolved high resolution spectroscopy of the surfaces of giant stars, for example. New capabilities on the horizon include post-coronagraphic spectroscopy, spectral differential imaging, nulling interferometry as well as an integral field spectrograph and an MKID array. Here we present the new modules of SCExAO, give an overview of the current commissioning status of each of the modules and present preliminary results.