arXiv subjects

Michael McElwain

Publications and source records attributed to Michael McElwain.

At least 19 recordsLinked to original sources

arXiv2025

Optical Design Pathways to Fluidic Space-Assembled Reflectors & Dual-Configuration Spectrographs for Characterizing Exo-Earths

$\textbf{Fluidic Telescopes}$ | We present a conceptual framework for optically designing space-assembled telescopes whose primary mirror is formed $\textit{in situ}$ via the enabling, scale-invariant technology of fluidic shaping. In-space assembly of optical reflectors can solve light-gathering aperture scaling, which currently limits space-borne optical telescopes. Our compass reduces the top-level optical design trade to three types of avenues---a fluidic pathway, a legacy one building upon the James Webb Space Telescope, and hybrid solutions---with a focus on exo-Earths. A primarily fluidic pathway leads, in the first place, to a post-prime-focus architecture. We apply this configuration to propose the tentative optical design for a ~1-m technology demonstrator and pathfinder for fluidic-telescope apertures scaling up to many tens of meters in diameter. $\textbf{Dual-Configuration Spectrographs}$ | The Habitable Worlds Observatory (HWO) will be the first mission equipped for the high-contrast direct imaging and remote spectral characterization, in reflected starlight, of exo-Earths in our galactic neighborhood. We present a novel concept for a compact, dual-configuration HWO spectrograph tailored for a broad wavelength range covering at least 600--1000 nm. Our design can interchange dispersive elements via a slider mechanism while preserving the rest of the optical path, enabling both a spectral resolving power $R$~140 integral-field spectrograph and a single- or multi-object spectrograph with $R$ on the order of 10$^3$. Although $R$~140 is near-optimal for the $O_2$ absorption $A$-band around 760 nm, higher values of $R$ can be utilized with spectral cross-correlation matched-filter techniques to enhance, e.g., HWO's atmospheric characterization capabilities.

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arXiv2025

X-SHYNE: X-Shooter spectra of young exoplanet analogs II. Presentation and analysis of the full library

The X-SHYNE library is a homogeneous sample of 43 medium-resolution (R=8000) infrared (0.3-2.5um) spectra of young (<500Myr), low-mass (<20Mjup), and cold (Teff=600-2000K) isolated brown dwarfs and wide-separation companions observed with the VLT/X-Shooter instrument. To characterize our targets, we performed a global comparative analysis. We first applied a semi-empirical approach. By refining their age and bolometric luminosity, we derived key atmospheric and physical properties, such as Teff, mass, surface gravity (g), and radius, using the evolutionary model COND03. These results were then compared with the results from a synthetic analysis based on three self-consistent atmospheric models. To compare our spectra with these grids we used the Bayesian inference code ForMoSA. We found similar Lbol estimates between both approaches, but an underestimated Teff from the cloudy models, likely due to a lack of absorbers that could dominate the J and H bands of early L. We also observed a discrepancy in the log(g) estimates, which are dispersed between 3.5 and 5.5 dex for mid-L objects. We interpreted this as a bias caused by a range of rotational velocities leading to cloud migration toward equatorial latitudes, combined with a variety of viewing angles that result in different observed atmospheric properties (cloud column densities, atmospheric pressures, etc.). Finally, while providing robust estimates of [M/H] and C/O for individual objects remains challenging, the X-SHYNE library globally suggests solar values, which are consistent with a formation via stellar formation mechanisms. This study highlights the strength of homogeneous datasets in performing comparative analyses, reducing the impact of systematics, and ensuring robust conclusions while avoiding over-interpretation.

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arXiv2025

An Earth-Sized Planet in a 5.4h Orbit Around a Nearby K dwarf

We present the discovery and confirmation of the ultra-short period (USP) planet TOI-2431 b orbiting a nearby ($d\sim36$ pc) late K star ($T_{\mathrm{eff}}$ = $4109 \pm 28 \, {\rm K}$) using observations from the Transiting Exoplanet Survey Satellite (TESS), precise radial velocities with the NEID and the Habitable-zone Planet Finder (HPF) spectrographs, as well as ground-based high contrast imaging from NESSI. TOI-2431 b has a period of 5 hours and 22 minutes, making it one of the shortest-period exoplanets known to date. TOI-2431 b has a radius of $1.536 \pm 0.033\, \rm{R_\oplus}$, and a mass of $6.2 \pm 1.2\, \rm{M_\oplus}$, suggesting it has a density compatible with an Earth-like composition and, due to its high irradiation, is likely a 'lava-world' with a $T_{\mathrm{eq}}$ = $2063 \pm 30 \, {\rm K}$. We estimate that the current orbital period is only 30% larger than the Roche-limit orbital period, and that it has an expected orbital decay timescale of only $\sim$31 Myr. Finally, due to the brightness of the host star ($V = 10.9$, $K = 7.6$), TOI-2431 b has a high Emission Spectroscopy Metric of 27, making it one of the best USP systems for atmospheric phase-curve analysis.

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arXiv2024

Searching for GEMS: TOI-5688 A b, a low-density giant orbiting a high-metallicity early M-dwarf

We present the discovery of a low-density planet orbiting the high-metallicity early M-dwarf TOI-5688 A b. This planet was characterized as part of the search for transiting giant planets ($R \gtrsim8$ M${}_\oplus$) through the Searching for GEMS (Giant Exoplanets around M-dwarf Stars) survey. The planet was discovered with the Transiting Exoplanet Survey Satellite (TESS), and characterized with ground-based transits from Red Buttes Observatory (RBO), the Table Mountain Observatory of Pomona College, and radial velocity (RV) measurements with the Habitable-Zone Planet Finder (HPF) on the 10 m Hobby Eberly Telescope (HET) and NEID on the WIYN 3.5 m telescope. From the joint fit of transit and RV data, we measure a planetary mass and radius of $124\pm24$ M$_\oplus$ ($0.39\pm0.07$ M${}_J$) and $10.4\pm0.7$ R$_\oplus$ ($0.92\pm0.06$ R${}_J$) respectively. The spectroscopic and photometric analysis of the host star TOI-5688 A shows that it is a metal-rich ([Fe/H] $ = 0.47\pm0.16$ dex) M2V star, favoring the core-accretion formation pathway as the likely formation scenario for this planet. Additionally, Gaia astrometry suggests the presence of a wide-separation binary companion, TOI-5688 B, which has a projected separation of $\sim5"$ (1110 AU) and is an M4V, making TOI-5688 A b part of the growing number of GEMS in wide-separation binary systems.

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arXiv2024

Gaia-4b and 5b: Radial Velocity Confirmation of Gaia Astrometric Orbital Solutions Reveal a Massive Planet and a Brown Dwarf Orbiting Low-mass Stars

Gaia astrometry of nearby stars is precise enough to detect the tiny displacements induced by substellar companions, but radial velocity data are needed for definitive confirmation. Here we present radial velocity follow-up observations of 28 M and K stars with candidate astrometric substellar companions, which led to the confirmation of two systems, Gaia-4b and Gaia-5b, and the refutation of 21 systems as stellar binaries. Gaia-4b is a massive planet ($M = 11.8 \pm 0.7 \:\mathrm{M_J}$) in a $P = 571.3 \pm 1.4\:\mathrm{day}$ orbit with a projected semi-major axis $a_0=0.312 \pm 0.040\:\mathrm{mas}$ orbiting a $0.644 \pm 0.02 \:\mathrm{M_\odot}$ star. Gaia-5b is a brown dwarf ($M = 20.9 \pm 0.5\:\mathrm{M_J}$) in a $P = 358.58 \pm 0.19\:\mathrm{days}$ eccentric $e=0.6412 \pm 0.0027$ orbit with a projected angular semi-major axis of $a_0 = 0.947 \pm 0.038\:\mathrm{mas}$ around a $0.34 \pm 0.03 \mathrm{M_\odot}$ star. Gaia-4b is one of the first exoplanets discovered via the astrometric technique, and is one of the most massive planets known to orbit a low-mass star.

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arXiv2023

Direct Imaging and Astrometric Detection of a Gas Giant Planet Orbiting an Accelerating Star

Direct imaging of gas giant exoplanets provides key information on planetary atmospheres and the architectures of planetary systems. However, few planets have been detected in blind surveys used to achieve imaging detections. Using Gaia and Hipparcos astrometry we identified dynamical evidence for a gas giant planet around the nearby star HIP 99770 and then confirmed this planet by direct imaging with the Subaru Coronagraphic Extreme Adaptive Optics Project. HIP 99770 b orbits 17 astronomical units from its host star, with an insolation comparable to Jupiter's and a dynamical mass of 13.9--16.1 Jupiter masses. Its planet-to-star mass ratio (7--8$\times$10$^{-3}$) is comparable to that other directly-imaged planets. The planet's atmosphere resembles an older, less-cloudy analogue of the atmospheres of previously-imaged exoplanets around HR 8799.

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arXiv2023

The Science Performance of JWST as Characterized in Commissioning

This paper characterizes the actual science performance of the James Webb Space Telescope (JWST), as determined from the six month commissioning period. We summarize the performance of the spacecraft, telescope, science instruments, and ground system, with an emphasis on differences from pre-launch expectations. Commissioning has made clear that JWST is fully capable of achieving the discoveries for which it was built. Moreover, almost across the board, the science performance of JWST is better than expected; in most cases, JWST will go deeper faster than expected. The telescope and instrument suite have demonstrated the sensitivity, stability, image quality, and spectral range that are necessary to transform our understanding of the cosmos through observations spanning from near-earth asteroids to the most distant galaxies.

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arXiv2023

NEID Reveals that The Young Warm Neptune TOI-2076 b Has a Low Obliquity

TOI-2076 b is a sub-Neptune-sized planet ($R= 2.39 \pm 0.10 {R_\oplus}$) that transits a young ($204 \pm 50 {MYr}$) bright ($V = 9.2$) K-dwarf hosting a system of three transiting planets. Using spectroscopic observations with the NEID spectrograph on the WIYN 3.5 m Telescope, we model the Rossiter-McLaughlin effect of TOI-2076 b, and derive a sky-projected obliquity of $λ=-3_{-15}^{+16\:\circ}$. Using the size of the star ($R=0.775 \pm0.015 {R_\odot}$), and the stellar rotation period ($P_{\mathrm{rot}}=7.27\pm0.23$ days), we estimate an obliquity of $ψ=18_{-9}^{+10\:\circ}$ ($ψ< 34^\circ$ at 95\% confidence), demonstrating that TOI-2076 b is on a well-aligned orbit. Simultaneous diffuser-assisted photometry from the 3.5 m Telescope at Apache Point Observatory rules out flares during the transit. TOI-2076 b joins a small but growing sample of young planets in compact multi-planet systems with well-aligned orbits, and is the fourth planet with an age $\lesssim 300$ Myr in a multi-transiting system with an obliquity measurement. The low obliquity of TOI-2076 b and the presence of transit timing variations in the system suggest the TOI-2076 system likely formed via convergent disk migration in an initially well-aligned disk.

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arXiv2022

The JWST Early Release Science Program for the Direct Imaging & Spectroscopy of Exoplanetary Systems

The direct characterization of exoplanetary systems with high contrast imaging is among the highest priorities for the broader exoplanet community. As large space missions will be necessary for detecting and characterizing exo-Earth twins, developing the techniques and technology for direct imaging of exoplanets is a driving focus for the community. For the first time, JWST will directly observe extrasolar planets at mid-infrared wavelengths beyond 5$μ$m, deliver detailed spectroscopy revealing much more precise chemical abundances and atmospheric conditions, and provide sensitivity to analogs of our solar system ice-giant planets at wide orbital separations, an entirely new class of exoplanet. However, in order to maximise the scientific output over the lifetime of the mission, an exquisite understanding of the instrumental performance of JWST is needed as early in the mission as possible. In this paper, we describe our 55-hour Early Release Science Program that will utilize all four JWST instruments to extend the characterisation of planetary mass companions to $\sim$15$μ$m as well as image a circumstellar disk in the mid-infrared with unprecedented sensitivity. Our program will also assess the performance of the observatory in the key modes expected to be commonly used for exoplanet direct imaging and spectroscopy, optimize data calibration and processing, and generate representative datasets that will enable a broad user base to effectively plan for general observing programs in future cycles.

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arXiv2022

Constrained Reference Star Differential Imaging: Enabling High-Fidelity Imagery of Highly Structured Circumstellar Disks

High-contrast imaging presents us with the opportunity to study circumstellar disks and the planets still embedded within them -- providing key insights into the formation and evolution of planetary systems. However, the post-processing techniques that are often needed to suppress stellar halo light typically result in significant and variable loss of circumstellar light, even when using relatively conservative approaches like reference star differential imaging (RDI). We introduce ``constrained reference star differential imaging" (constrained RDI), a new class of RDI point spread function (PSF)-subtraction techniques for systems with circumstellar disks. Constrained RDI utilizes either high-resolution polarized intensity (PI) images or disk models to severely limit or even eliminate the signal loss due to oversubtraction that is common to RDI. We demonstrate the ability of constrained RDI utilizing polarimetric data to yield an oversubtraction-free detection of the AB Aurigae protoplanetary disk in total intensity. PI-constrained RDI allows us to decisively recover the spectral signature of the confirmed, recently-discovered protoplanet, AB Aurigae b (Currie et al. 2022). We further demonstrate that constrained RDI can be a powerful analysis tool for soon-to-be-acquired James Webb Space Telescope coronagraphic imaging of disks. In both cases, constrained RDI provides analysis-ready products that enable more detailed studies of disks and more robust verification of embedded exoplanets.

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arXiv2020

SCExAO/CHARIS Direct Imaging Discovery of a 20 au Separation, Low-Mass Ratio Brown Dwarf Companion to an Accelerating Sun-like Star

We present the direct imaging discovery of a substellar companion to the nearby Sun-like star, HD 33632 Aa, at a projected separation of $\sim$ 20 au, obtained with SCExAO/CHARIS integral field spectroscopy complemented by Keck/NIRC2 thermal infrared imaging. The companion, HD 33632 Ab, induces a 10.5$σ$ astrometric acceleration on the star as detected with the $Gaia$ and $Hipparcos$ satellites. SCExAO/CHARIS $JHK$ (1.1--2.4 $μ$m) spectra and Keck/NIRC2 $L_{\rm p}$ (3.78 $μ$m) photometry are best matched by a field L/T transition object: an older, higher gravity, and less dusty counterpart to HR 8799 cde. Combining our astrometry with $Gaia/Hipparcos$ data and archival Lick Observatory radial-velocities, we measure a dynamical mass of 46.4 $\pm$ 8 $M_{\rm J}$ and an eccentricity of $e$ $<$0.46 at 95\% confidence. HD 33632 Ab's mass and mass ratio (4.0\% $\pm$ 0.7\%) are comparable to the low-mass brown dwarf GJ 758 B and intermediate between the more massive brown dwarf HD 19467 B and the (near-)planet mass companions to HR 2562 and GJ 504. Using $Gaia$ to select for direct imaging observations with the newest extreme adaptive optics systems can reveal substellar or even planet-mass companions on solar system-like scales at an increased frequency compared to blind surveys.

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arXiv2019

No Clear, Direct Evidence for Multiple Protoplanets Orbiting LkCa 15: LkCa 15 bcd are Likely Inner Disk Signals

Two studies utilizing sparse aperture masking (SAM) interferometry and $H_{\rm α}$ differential imaging have reported multiple jovian companions around the young solar-mass star, LkCa 15 (LkCa 15 bcd): the first claimed direct detection of infant, newly-formed planets ("protoplanets"). We present new near-infrared direct imaging/spectroscopy from the SCExAO system coupled with the CHARIS integral field spectrograph and multi-epoch thermal infrared imaging from Keck/NIRC2 of LkCa 15 at high Strehl ratios. These data provide the first direct imaging look at the same wavelengths and in the same locations where previous studies identified the LkCa 15 protoplanets and thus offer the first decisive test of their existence. The data do not reveal these planets. Instead, we resolve extended emission tracing a dust disk with a brightness and location comparable to that claimed for LkCa 15 bcd. Forward-models attributing this signal to orbiting planets are inconsistent with the combined SCExAO/CHARIS and Keck/NIRC2 data. An inner disk provides a more compelling explanation for the SAM detections and perhaps also the claimed $H_α$ detection of LkCa 15 b. We conclude that there is currently no clear, direct evidence for multiple protoplanets orbiting LkCa 15, although the system likely contains at least one unseen jovian companion. To identify jovian companions around LkCa 15 from future observations, the inner disk should be detected and its effect modeled, removed, and shown to be distinguishable from planets. Protoplanet candidates identified from similar systems should likewise be clearly distinguished from disk emission through modeling.

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arXiv2019

The Critical, Strategic Importance of Adaptive Optics-Assisted Ground-Based Telescopes for the Success of Future NASA Exoplanet Direct Imaging Missions

Ground-based telescopes coupled with adaptive optics (AO) have been playing a leading role in exoplanet direct imaging science and technological development for the past two decades and will continue to have an indispensable role for the next decade and beyond. Over the next decade, extreme AO systems on 8-10m telescopes will 1) mitigate risk for WFIRST-CGI by identifying numerous planets the mission can spectrally characterize, 2) validate performance requirements and motivate improvements to atmosphere models needed to unambiguously characterize solar system-analogues from space, and 3) mature novel technological innovations useful for space. Extremely Large Telescopes can deliver the first thermal infrared (10 $μm$) images of rocky planets around Sun-like stars and identify biomarkers. These data provide a future NASA direct imaging flagship mission (i.e. HabEx, LUVOIR) with numerous exo-Earth candidates and critical ancillary information to help clarify whether these planets are habitable.

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arXiv2019

Key Technologies for the Wide Field Infrared Survey Telescope Coronagraph Instrument

The Wide Field Infrared Survey Telescope (WFIRST) Coronagraph Instrument (CGI) is a high-contrast imager and integral field spectrograph that will enable the study of exoplanets and circumstellar disks at visible wavelengths. Ground-based high-contrast instrumentation has fundamentally limited performance at small working angles, even under optimistic assumptions for 30m-class telescopes. There is a strong scientific driver for better performance, particularly at visible wavelengths. Future flagship mission concepts aim to image Earth analogues with visible light flux ratios of more than 10^10. CGI is a critical intermediate step toward that goal, with a predicted 10^8-9 flux ratio capability in the visible. CGI achieves this through improvements over current ground and space systems in several areas: (i) Hardware: space-qualified (TRL9) deformable mirrors, detectors, and coronagraphs, (ii) Algorithms: wavefront sensing and control; post-processing of integral field spectrograph, polarimetric, and extended object data, and (iii) Validation of telescope and instrument models at high accuracy and precision. This white paper, submitted to the 2018 NAS Exoplanet Science Strategy call, describes the status of key CGI technologies and presents ways in which performance is likely to evolve as the CGI design matures.

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arXiv2018

Isochronal age-mass discrepancy of young stars: SCExAO/CHARIS integral field spectroscopy of the HIP 79124 triple system

We present SCExAO/CHARIS 1.1--2.4 micron integral field direct spectroscopy of the young HIP 79124 triple system. HIP 79124 is a member of the Scorpius-Centaurus association, consisting of an A0V primary with two low-mass companions at a projected separation of <1 arcsecond. Thanks to the high quality wavefront corrections provided by SCExAO, both companions are decisively detected without the employment of any PSF-subtraction algorithm to eliminate quasi-static noise. The spectrum of the outer C object is very well matched by Upper Scorpius M4 pm 0.5 standard spectra, with a Teff = 2945 pm 100 and a mass of 350 MJup. HIP 79124 B is detected at a separation of only 180 mas in a highly-correlated noise regime, and it falls in the spectral range M6 pm 0.5 with Teff = 2840 pm 190 and 100 MJup. Previous studies of stellar populations in Sco-Cen have highlighted a discrepancy in isochronal ages between the lower-mass and higher-mass populations. This could be explained either by an age spread in the region, or by conventional isochronal models failing to reproduce the evolution of low-mass stars. The HIP 79124 system should be coeval, and therefore it provides an ideal laboratory to test these scenarios. We place the three components in a color-magnitude diagram and find that the models predict a younger age for the two low-mass companions (3 Myr) than for the primary star (6 Myr). These results imply that the omission of magnetic effects in conventional isochronal models inhibit them from reproducing early low-mass stellar evolution, which is further supported by the fact that new models that include such effects provide more consistent ages in the HIP 79124 system.

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arXiv2018

SCExAO/CHARIS Near-Infrared Direct Imaging, Spectroscopy, and Forward-Modeling of kappa And b: A Likely Young, Low-Gravity Superjovian Companion

We present SCExAO/CHARIS high-contrast imaging/$JHK$ integral field spectroscopy of $κ$ And b, a directly-imaged low-mass companion orbiting a nearby B9V star. We detect $κ$ And b at a high signal-to-noise and extract high precision spectrophotometry using a new forward-modeling algorithm for (A-)LOCI complementary to KLIP-FM developed by Pueyo (2016). $κ$ And b's spectrum best resembles that of a low-gravity L0--L1 dwarf (L0--L1$γ$). Its spectrum and luminosity are very well matched by 2MASSJ0141-4633 and several other 12.5--15 $M_{\rm J}$ free floating members of the 40 $Myr$-old Tuc-Hor Association, consistent with a system age derived from recent interferometric results for the primary, a companion mass at/near the deuterium-burning limit (13$^{+12}_{-2}$ M$_{\rm J}$), and a companion-to-primary mass ratio characteristic of other directly-imaged planets ($q$ $\sim$ 0.005$^{+0.005}_{-0.001}$). We did not unambiguously identify additional, more closely-orbiting companions brighter and more massive than $κ$ And b down to $ρ$ $\sim$ 0.3" (15 au). SCExAO/CHARIS and complementary Keck/NIRC2 astrometric points reveal clockwise orbital motion. Modeling points towards a likely eccentric orbit: a subset of acceptable orbits include those that are aligned with the star's rotation axis. However, $κ$ And b's semimajor axis is plausibly larger than 75 au and in a region where disk instability could form massive companions. Deeper $κ$ And high-contrast imaging and low-resolution spectroscopy from extreme AO systems like SCExAO/CHARIS and higher resolution spectroscopy from Keck/OSIRIS or, later, IRIS on the Thirty Meter Telescope could help clarify $κ$ And b's chemistry and whether its spectrum provides an insight into its formation environment.

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arXiv2018

SCExAO/CHARIS Near-IR High-Contrast Imaging and Integral Field Spectroscopy of the HIP 79977 Debris Disk

We present new, near-infrared (1.1--2.4 $μm$) high-contrast imaging of the bright debris disk surrounding HIP 79977 with the Subaru Coronagraphic Extreme Adaptive Optics system (SCExAO) coupled with the CHARIS integral field spectrograph. SCExAO/CHARIS resolves the disk down to smaller angular separations of (0.11"; $r \sim 14$ au) and at a higher significance than previously achieved at the same wavelengths. The disk exhibits a marginally significant east-west brightness asymmetry in $H$ band that requires confirmation. Geometrical modeling suggests a nearly edge-on disk viewed at a position angle of $\sim$ 114.6$^{o}$ east of north. The disk is best-fit by scattered-light models assuming strongly forward-scattering grains ($g$ $\sim$ 0.5--0.65) confined to a torus with a peak density at $r_{0}$ $\sim$ 53--75 au. We find that a shallow outer density power law of $α_{out}=$-1-- -3 and flare index of $β= 1$ are preferred. Other disk parameters (e.g.~inner density power law and vertical scale height) are more poorly constrained. The disk has a slightly blue intrinsic color and its profile is broadly consistent with predictions from birth ring models applied to other debris disks. While HIP 79977's disk appears to be more strongly forward-scattering than most resolved disks surrounding 5--30 Myr-old stars, this difference may be due to observational biases favoring forward-scattering models for inclined disks vs. lower inclination, ostensibly neutral-scattering disks like HR 4796A's. Deeper, higher signal-to-noise SCExAO/CHARIS data can better constrain the disk's dust composition.

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arXiv2018

The WFIRST Exoplanet Microlensing Survey

The Wide Field Infrared Survey Telescope (WFIRST) was the top ranked large space mission in the 2010 New Worlds, New Horizons decadal survey, and it was formed by merging the science programs of 3 different mission concepts, including the Microlensing Planet Finder (MPF) concept (Bennett \etal\ 2010). The WFIRST science program (Spergel \etal\ 2015) consists of a general observer program, a wavefront controlled technology program, and two targeted science programs: a program to study dark energy, and a statistical census of exoplanets with a microlensing survey, which uses nearly one quarter of WFIRST's observing time in the current design reference mission. The New Worlds, New Horizons (decadal survey) midterm assessment summarizes the science case for the WFIRST exoplanet microlensing survey with this statement: "WFIRST's microlensing census of planets beyond 1 AU will perfectly complement Kepler's census of compact systems, and WFIRST will also be able to detect free-floating planets unbound from their parent stars\rlap."

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