arXiv subjects

Megan Delamer

Publications and source records attributed to Megan Delamer.

17 recordsLinked to original sources

arXiv2026

GEMS JWST: Hold on to your HATS(-6 b), a sub-solar metallicity giant planet with water, methane and ammonia in its atmosphere

HATS-6 b is one of several recently discovered Giant Exoplanets orbiting M-dwarf Stars (GEMS) and is part of a JWST survey that aims to compare bulk and atmospheric properties of these rare planets against their FGK star counterparts. HATS-6 b is a warm ($\mathrm{T_{eq}}\sim700$ K), Saturn-mass ($M_p\sim0.3~\mathrm{M_J}$), Jupiter-radius ($R_p\sim1~\mathrm{R_J}$) planet that transits its star every $\sim$ 3 days. In this study, we present the transmission spectrum of HATS-6 b obtained with two transits using the PRISM mode of JWST Near Infrared Spectrograph (NIRSpec), spanning a wavelength range of $0.6-5.3$ um. Analyzing these JWST observations using an iterative approach between forward modeling and free chemistry retrievals, we derive a low metallicity ($\log\mathrm{[M/H]}=-1.99^{+0.2}_{-0.2}$) sub-solar C/O ($\log\mathrm{[C/O]=-0.46^{+0.2}_{-0.2}}$) atmosphere, and find strong evidence for H$_2$O, CH$_4$, and NH$_3$ at volume mixing ratios (in $\log[X]$) of $-4.88_{-0.24}^{+0.25}$, $-5.38_{-0.19}^{+0.18}$, and $-6.03_{-0.19}^{+0.18}$, respectively. We consistently retrieve a significantly lower $\mathrm{T_{eq}}$ than predicted from the orbital configuration of HATS-6 b, which was impervious to any data reduction and retrieval choices, suggesting a non-zero bond albedo. Our planetary interior models retrieve bulk metallicities three orders of magnitude larger than our retrieved atmospheric metallicity, also suggesting that the atmosphere is not well-mixed. We find an excess feature around 3 um, and expand on possible explanations for this, such as the presence of HCN or hydrocarbons like C$_2$H$_4$. Yet, due to the degeneracies present for hydrocarbon features in this wavelength region, we do not draw any conclusions about the excess feature and instead encourage further observations and follow-up of this intriguing target.

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arXiv2026

Fabrication of focusing optics for TA-MOONS: Micro-MOONS

Understanding star and planet formation requires optical to near-infrared spectroscopic observations of a large number of young stellar objects. The TIFR-ARIES Multi-Object Optical to Near-infrared Spectrometer's (TA-MOONS) primary objective is to preform a large spectroscopic survey of young stellar objects across wavelengths of 360 nm to 2.5 microns, with the multiplexed capability of observing up to eight sources simultaneously. Multiplexity is achieved by moving pickup mirrors on robotic arms. One component of this robotic arm system is Micro-MOONS, a micro-optic array, nanofabricated to enable accurate calibration, validation, and positioning of stellar images onto the pickup mirrors. The micro-mirrors of the Micro-MOONS system are spherically concave, fabricated via two-photon polymerization(2PP) in the NanoScribe GT2 system, and are printed on plasma-cleaned silicon substrates using IP-S photoresist, followed by gold coating. 2PP allows for fine, detailed shapes on a sub-micron scale that are otherwise difficult and costly to obtain. The optimization of Micro-MOONS focuses on achieving uniformity, minimizing surface roughness, and preserving the desired radius of curvature to avoid beam obstruction near the pickup mirrors. This work demonstrates the feasibility of integrating advanced custom-printed micro-optics into multi-object spectroscopic instruments, enabling improved calibration and efficiency in large-scale stellar surveys.

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arXiv2026

GEMS JWST: HATS-75 b -- A giant planet with a sub-solar metallicity atmosphere orbiting an M-dwarf

HATS-75 b is one of the recently discovered Giant Exoplanets orbiting M-dwarf Stars (GEMS) with a transmission spectrum shaped by both its atmosphere and the active stellar surface it transits. As part of a JWST program studying 7 GEMS, we observed three transits of HATS-75 b with the NIRSpec PRISM instrument (0.6-5.3 um). The planet's spectra exhibit a slightly larger transit depth at shorter wavelengths, indicative of hazes or stellar contamination due to stellar heterogeneities outside the transit chord, i.e., the transit light source (TLS) effect. While both a hazy atmospheric model or TLS model can replicate the transmission spectrum, independent evidence (.e.g, stellar rotation, spot-crossing events) favors a model that includes contamination from unocculted starspots and faculae. Within this stellar heterogeneity / TLS-based framework, atmospheric retrievals yield remarkably low atmospheric metallicity (log[M/H]=-1.74^{+0.92}_{-0.76}) and super-solar carbon-to-oxygen (C/O=1.04^{+0.40}_{-0.09}), which paired with a best-fit interior model with bulk metallicity of Z_p=0.20+/-0.04, implies poor vertical mixing within the planet. Retrievals also detect robust absorption signatures of CH4, CO, and CO2. We obtain only an upper limit for H2O, consistent with its atmospheric spectral features being masked by stellar contamination. These results underscore the importance of accounting for stellar heterogeneity when interpreting exoplanet transmission spectra and highlight HATS-75 b as a significant asset to our understanding of giant exoplanets around M-dwarfs with JWST.

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arXiv2026

GEMS JWST: A sub-Solar metallicity atmosphere for giant planet TOI-5293Ab orbiting a rapidly changing M-dwarf

The growing sample of Giant Exoplanets around M-dwarf Stars (GEMS) helps probe the extremes of giant planet formation. Comparing the properties of this sample with their FGK counterparts can help us understand how planet formation and migration depend on stellar mass. We initiated a large Cycle 2 JWST transmission spectroscopy survey of seven GEMS. Here we present the atmospheric characterization using two JWST transits of TOI-5293Ab, a 0.5 $M_J$ planet orbiting an early M-dwarf with a period of $\sim$ 3 days. The two NIRSpec/PRISM transits indicate the planet is eclipsing a rapidly changing (heterogeneous) stellar photosphere. We see that Visit 1 had heterogeneity crossings across the entire transit chord, rendering inferences from it to be unreliable. The Visit 1 spectrum exhibits a downward slope ${<1}$ $\mu$m suggestive of stellar contamination from faculae. In contrast, for Visit 2 we are able to model the heterogeneity crossings and obtain a transmission spectrum free from stellar contamination. We therefore limit our conclusions to a detailed analysis of Visit 2, and using Bayesian free chemistry retrievals, we find a low atmospheric metallicity ($\log [\mathrm{M/H}] = -1.03^{+0.53}_{-0.44}$ $\times$ Solar) and slightly super-solar C/O ratio ($1.23^{+2.94}_{-0.75}$). The retrievals yield Bayes factors that indicate strong evidence for \ce{CH4} as well as low significance detections of \ce{CO2}, \ce{H2O}, \ce{NH3}. Finally, using thermal evolution models we find that the radius of TOI-5293Ab is inflated above theoretical expectations ($\sim$ 1.07 $R_J$), despite it having an temperature of $\sim$ 700 K, and hence we were unable to constrain its bulk composition.

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arXiv2026

Star Grazing with Alumina Grass: Antireflection coatings in the visible and near-infrared on IPX-Clear Microlenses assisted by Grass-like Alumina

Two-photon polymerization (2PP) enables fabrication of high-precision micro-optics with complex freeform geometries, opening a new parameter space for custom astronomical optics. Among available resins, the newly developed IPX Clear is particularly well suited for visible applications, offering high transmission across the visible-near-IR, low surface roughness, and excellent shape fidelity. However, Fresnel reflections at the air-polymer interface introduce significant optical losses, which are detrimental in low-signal astronomy. Previous studies show grass-like alumina coatings on glass and fused silica can raise average transmission from 91.9% to approximately 99% over 400-900 nm. Here we explore the feasibility of Atomic Layer Deposition (ALD) to apply such coatings to IPX-Clear micro-optics over 400-1700 nm. Grass-like alumina anti-reflective (AR) coatings can approximate the ideal index condition by creating a gradual refractive-index transition from air to bulk IPX Clear, suppressing surface reflections. While grass-like coatings are established on bulk optics and conformal ALD films have been applied to 2PP micro-optics, we demonstrate - for the first time - alumina grass on 2PP microlenses made with the new IPX-Clear resin. We discuss key challenges and process steps, and observe that alumina-grass-coated microlenses lose only approximately 0.3% of photons to reflection in the 400-850 nm range. Future work will test performance across the full 400-1700 nm band and explore improved environmental resilience, e.g., a SiO2 overcoat. Combined with the high optical transparency of IPX Clear, these coatings enable custom-designed, highly efficient microlenses for astronomical applications.

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arXiv2025

Hipparcos, Gaia, and RVs reveal that the radio emitting F star HD 220242 has an M dwarf companion, a likely source of the radio emission

The detection of circularly polarized, low frequency radio emission offers the tantalizing possibility of the observation of interactions between stars and their possible substellar companions, as well as direct emission from exoplanets. Additional follow up of systems with radio emission is key to understanding the true origin of the emission, since multiple astrophysical mechanisms can plausibly lead to such signals. While nineteen M dwarfs were detected by LOFAR in circular polarization as part of the V-LoTSS survey, HD~220242 is the only F star to have a circularly polarized low frequency radio detection in the same survey. We conducted radial velocity follow up with the Habitable-zone Planet Finder and combined these observations with additional archival RVs and \textit{Hipparcos}-\textit{Gaia} proper motion accelerations to determine that HD~220242 has a stellar companion with P=16.79$\pm$0.04\,yrs and a mass of $0.619\pm0.014$\,M$_\odot$. We use Spectral Energy Distribution fitting and lack of any UV excess to rule out a co-evolved white dwarf companion and confirm that the companion is an M dwarf star. Given that F stars lack the coronal properties to produce such coherent emission, and the companion mass and lack of UV excess are consistent with an M dwarf, the radio emission is most plausibly associated with the companion.

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arXiv2025

Discovery of a Nearby Habitable Zone Super-Earth Candidate Amenable to Direct Imaging

We present the discovery of GJ 251 c, a candidate super-Earth orbiting in the Habitable Zone (HZ) of its M dwarf host star. Using high-precision Habitable-zone Planet Finder (HPF) and NEID RVs, in conjunction with archival RVs from the Keck I High Resolution Echelle Spectrometer (HIRES), the Calar Alto high-Resolution search for M dwarfs with Exoearths with Near-infrared and optical Echelle Spectrograph (CARMENES), and the SPectropolarim\`etre InfraROUge (SPIRou), we improve the measured parameters of the known planet, GJ 251 b ($P_{b}$ = 14.2370 days; $m \sin(i)$ = 3.85$^{+0.35}_{-0.33}$ M$_{\oplus}$), and we significantly constrain the minimum mass of GJ 251 c, placing it in a plausibly terrestrial regime (P$_{c}$ = 53.647 $\pm$ 0.044 days; $ m \sin i_{c}$ = 3.84 $\pm$ 0.75 M$_{\oplus}$). Using activity mitigation techniques that leverage chromatic information content, we perform a color-dependent analysis of the system and a detailed comparison of more than 50 models that describe the nature of the planets and stellar activity in the system. Due to GJ 251's proximity to Earth (5.5 pc), next generation, thirty meter class telescopes will likely be able to image terrestrial planets in GJ 251's HZ. In fact, GJ 251 c is currently the best candidate for terrestrial, HZ planet imaging in the Northern Sky.

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arXiv2025

POKEMON: print optimization for kilo-fiber experiments using micro-optics and nanostructures

The most pressing problems in modern astrophysics have often required the largest telescopes. With the cost scaling of mirror diameters, the field as a whole is faced with a challenge -- how to replicate or improve on the collecting area and sensitivity of the current generation of ELTs, which already boast 30 m class apertures and are multi-billion dollar facilities. One such approach is being pursued by the Large Fiber Array Spectroscopic Telescope (LFAST) -- a scalable array telescope. Each element of the array will consist of multiple mirrors each feeding to an individual fiber; with those fiber feeds feeding optical and infrared spectrometers. Coupling fiber bundle to spectrometer slit input must be optimized to take full advantage of the photon collecting ability of the telescope array, requiring precise alignment of microlenses to each fiber. Advances in two photon polymerization processes (2PP) now allow for optical quality microlenses with wavefront aberrations as small as $\lambda/20 to be created, opening up the design parameter of bespoke optical design and custom fabricated lenses. We present our approach to tackling these coupling problems with rapid prototyping and detailed quantification of the tolerances of the lenses. Our approach leverages our access to the Nanoscribe GT2 system at Penn State, enabling tests of new optically transparent resins like IPX-Clear to explore multiple design approaches. Our goal is to share our results and enable wider use of these techniques for astronomical applications.

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arXiv2025

Searching for GEMS: Confirmation of TOI-5573b, a Cool, Saturn-like Planet Orbiting An M-dwarf

We present the confirmation of TOI-5573b, a Saturn-sized exoplanet on an 8.79-day orbit around an early M-dwarf (3790 K, 0.59 R$\odot$, 0.61 M$\odot$, 12.30 J mag). TOI-5573b has a mass of $112^{+18}_{-19}$ M$\oplus$ (0.35$\pm$0.06 M$\mathrm{Jup}$) and a radius of $9.75\pm0.47$ R$\oplus$ (0.87$\pm$0.04 R$\mathrm{Jup}$), resulting in a density of $0.66^{+0.16}_{-0.13}$ g cm$^{-3}$, akin to that of Saturn. The planet was initially discovered by TESS and confirmed using a combination of 11 transits from four TESS sectors (20, 21, 47 and 74), ground-based photometry from the Red Buttes Observatory, and high-precision radial velocity data from the Habitable-zone Planet Finder (HPF) and NEID spectrographs, achieving a 5$\sigma$ precision on the planet's mass. TOI-5573b is one of the coolest Saturn-like exoplanets discovered around an M-dwarf, with an equilibrium temperature of $528\pm10$ K, making it a valuable target for atmospheric characterization. Saturn-like exoplanets around M-dwarfs likely form through core accretion, with increased disk opacity slowing gas accretion and limiting their mass. The host star's super-solar metallicity supports core accretion, but uncertainties in M-dwarf metallicity estimates complicate definitive conclusions. Compared to other GEMS (Giant Exoplanets around M-dwarf Stars) orbiting metal-rich stars, TOI-5573b aligns with the observed pattern that giant planets preferentially form around M-dwarfs with super-solar metallicity. Further high-resolution spectroscopic observations are needed to explore the role of stellar metallicity in shaping the formation and properties of giant exoplanets like TOI-5573b.

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arXiv2025

GEMS JWST: Transmission spectroscopy of TOI-5205b reveals significant stellar contamination and a metal-poor atmosphere

Recent discoveries of transiting giant exoplanets ($R_p\gtrsim8\mathrm{~R_\oplus}$) around M dwarfs (GEMS) present an opportunity to investigate their atmospheric compositions and explore how such massive planets form around low-mass stars contrary to the prediction from formation models. We present the first transmission spectra of TOI-5205b, a short-period ($P=1.63~\mathrm{days}$) Jupiter-like planet ($M_p=1.08~\mathrm{M_J}$ and $R_p=0.94~\mathrm{R_J}$) orbiting an M4 dwarf ($M_\star=0.392~\mathrm{M_\odot}$, $R_\star=0.394~\mathrm{R_\odot}$). We obtained three transits using the PRISM mode of the JWST Near Infrared Spectrograph (NIRSpec) spanning $0.6-5.3$ um. The data reveal significant stellar contamination that is evident in the light curves as spot-crossing events and in the transmission spectra as a larger transit depth at bluer wavelengths. Atmospheric retrievals demonstrate that stellar contamination from unocculted starspots and faculae is the dominant component of the transmission spectrum at wavelengths $\lambda\lesssim3.0$ um, reducing the sensitivity to the presence of clouds or hazes in our models and preventing detection of $\mathrm{H_2O}$. The wavelength coverage enabled a robust detection of $\mathrm{CH_4}$ and $\mathrm{H_2S}$, which have detectable molecular features between $3.0-5.0$ um. For both clear or cloudy atmospheres, Bayesian retrievals consistently favored an atmosphere with sub-solar metallicity ($3\sigma$ upper limit of $\log\mathrm{[M/H]}\lesssim-1.24$) and super-solar C/O ratio ($3\sigma$ lower limit of $\log\mathrm{[C/O]}\gtrsim0.09$), although this may partly be driven by the non-detection of water due to stellar contamination. Planetary interior models predict a bulk metallicity of 10--20\%, which is larger than the atmospheric metallicity and suggests that the interior of TOI-5205b is decoupled from its atmosphere.

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arXiv2025

Million Points of Light (MPoL): a PyTorch library for radio interferometric imaging and inference

Astronomical radio interferometers achieve exquisite angular resolution by cross-correlating signal from a cosmic source simultaneously observed by distant pairs of radio telescopes to produce a Fourier-type measurement called a visibility. Million Points of Light (MPoL) is a Python library supporting feed-forward modeling of interferometric visibility datasets for synthesis imaging and parametric Bayesian inference, built using the autodifferentiable machine learning framework PyTorch. Neural network components provide a rich set of modular and composable building blocks that can be used to express the physical relationships between latent model parameters and observed data following the radio interferometric measurement equation. Industry-grade optimizers make it straightforward to simultaneously solve for the synthesized image and calibration parameters using stochastic gradient descent.

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arXiv2024

Searching for GEMS: Two Super-Jupiters around M-dwarfs -- Signatures of Instability or Accretion?

We present the discovery of TOI-6303b and TOI-6330b, two massive transiting super-Jupiters orbiting a M0 and a M2 star respectively, as part of the Searching for GEMS survey. These were detected by TESS and then confirmed via ground-based photometry and radial velocity observations with the Habitable-zone Planet Finder (HPF). TOI-6303b has a mass of 7.84 +/- 0.31 MJ, a radius of 1.03 +/- 0.06 RJ , and an orbital period of 9.485 days. TOI-6330b has a mass of 10.00 +/- 0.31 MJ , a radius of 0.97 +/- 0.03 RJ , and an orbital period of 6.850 days. We put these planets in context of super-Jupiters around M-dwarfs discovered from radial-velocity surveys, as well as recent discoveries from astrometry. These planets have masses that can be attributed to two dominant planet formation mechanisms - gravitational instability and core-accretion. Their masses necessitate massive protoplanetary disks that should either be gravitationally unstable, i.e. forming through gravitational instability, or be amongst some of the most massive protoplanetary disks to form objects through core-accretion. We also discuss the eccentricity distribution of these objects, as a potential indicator of their formation and evolutionary mechanisms.

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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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arXiv2024

Searching for Giant Exoplanets around M-dwarf Stars (GEMS) I: Survey Motivation

Recent discoveries of transiting giant exoplanets around M-dwarf stars (GEMS), aided by the all-sky coverage of TESS, are starting to stretch theories of planet formation through the core-accretion scenario. Recent upper limits on their occurrence suggest that they decrease with lower stellar masses, with fewer GEMS around lower-mass stars compared to solar-type. In this paper, we discuss existing GEMS both through confirmed planets, as well as protoplanetary disk observations, and a combination of tests to reconcile these with theoretical predictions. We then introduce the \textit{Searching for GEMS} survey, where we utilize multi-dimensional nonparameteric statistics to simulate hypothetical survey scenarios to predict the required sample size of transiting GEMS with mass measurements to robustly compare their bulk-density with canonical hot-Jupiters orbiting FGK stars. Our Monte-Carlo simulations predict that a robust comparison requires about 40 transiting GEMS (compared to the existing sample of $\sim$ 15) with 5-$\sigma$ mass measurements. Furthermore, we discuss the limitations of existing occurrence estimates for GEMS, and provide a brief description of our planned systematic search to improve the occurrence rate estimates for GEMS.

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arXiv2023

Astrometry and Precise Radial Velocities Yield a Complete Orbital Solution for the Nearby Eccentric Brown Dwarf LHS 1610 b

The LHS 1610 system consists of a nearby ($d=9.7$ pc) M5 dwarf hosting a candidate brown dwarf companion in a $10.6$ day, eccentric ($e \sim 0.37$) orbit. We confirm this brown dwarf designation and estimate its mass ($ 49.5_{-3.5}^{+4.3}$ $M_{\text{Jup}} $) and inclination ($ 114.5^\circ$ $_{-10.0}^{+7.4}$) by combining discovery radial velocities (RVs) from TRES and new RVs from the Habitable-zone Planet Finder with the available Gaia astrometric two-body solution. We highlight a discrepancy between the measurement of the eccentricity from the Gaia two-body solution ($e=0.52 \pm 0.03$) and the RV-only solution ($e=0.3702\pm0.0003$). We discuss possible reasons for this discrepancy, which can be further probed when the Gaia astrometric time series becomes available as part of Gaia DR4. As a nearby mid-M star hosting a massive short-period companion with a well-characterized orbit, LHS 1610 b is a promising target to look for evidence of sub-Alfv\'enic interactions and/or auroral emission at optical and radio wavelengths. LHS 1610 has a flare rate ($0.28\pm 0.07$ flares/day) on the higher-end for its rotation period ($84 \pm 8$ days), similar to other mid-M dwarf systems such as Proxima Cen and YZ Ceti that have recent radio detections compatible with star-planet interactions. While available TESS photometry is insufficient to determine an orbital phase-dependence of the flares, our complete orbital characterization of this system makes it attractive to probe star-companion interactions with additional photometric and radio observations.

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arXiv2023

TOI-4201: An Early M-dwarf Hosting a Massive Transiting Jupiter Stretching Theories of Core-Accretion

We confirm TOI-4201 b as a transiting Jovian mass planet orbiting an early M dwarf discovered by the Transiting Exoplanet Survey Satellite. Using ground based photometry and precise radial velocities from NEID and the Planet Finder Spectrograph, we measure a planet mass of 2.59$^{+0.07}_{-0.06}$ M$_{J}$, making this one of the most massive planets transiting an M-dwarf. The planet is $\sim$0.4\% the mass of its 0.63 M$_{\odot}$ host and may have a heavy element mass comparable to the total dust mass contained in a typical Class II disk. TOI-4201 b stretches our understanding of core-accretion during the protoplanetary phase, and the disk mass budget, necessitating giant planet formation to either take place much earlier in the disk lifetime, or perhaps through alternative mechanisms like gravitational instability.

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arXiv2023

An extreme test case for planet formation: a close-in Neptune orbiting an ultracool star

In current theories of planet formation, close-orbiting planets as massive as Neptune are expected to be very rare around low-mass stars. We report the discovery of a Neptune-mass planet orbiting the `ultracool' star LHS 3154, which is nine times less massive than the Sun. The planet's orbital period is 3.7 days and its minimum mass is 13.2 Earth masses, giving it the largest known planet-to-star mass ratio among short-period planets ($<$\,100 days) orbiting ultracool stars. Both the core accretion and gravitational instability theories for planet formation struggle to account for this system. In the core-accretion scenario, in particular, the dust mass of the protoplanetary disk would need to be an order of magnitude higher than typically seen in protoplanetary disk observations of ultracool stars.

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