Dax L. Feliz, PhD

NEMESIS: Exoplanet Transit Survey of Nearby M-dwarfs in TESS FFIs Transit Survey Open-source Python

NEMESIS is an open-source Python framework for extracting, detrending, searching, and vetting time-series photometry from NASA's Transiting Exoplanet Survey Satellite (TESS) Full Frame Images (FFIs), optimized for nearby M-dwarf stars. Unlike searches restricted to preselected short-cadence targets, NEMESIS uses the FFIs to investigate M dwarfs that may have been observed only at the 30-minute cadence of the TESS Primary Mission.

The workflow starts with TESSCut image cutouts and constructs target-centered light curves through background estimation, aperture photometry, neighbor-contamination correction, quality masking, pixel-level decorrelation (PLD), and variability detrending. Each light curve is searched using both Box Least Squares (BLS) and Transit Least Squares (TLS), with automated diagnostics and LEO-Vetter tests used to characterize possible transits. Separate injection–recovery and orbit-scrambling experiments measure detection sensitivity and the rate of noise-generated false alarms. For candidates, follow-up modeling in the NEMESIS II analysis uses jaxoplanet transit fits and TRICERATOPS false-positive probabilities.

NEMESIS I (Feliz et al. 2021, The Astronomical Journal) introduced the survey and searched nearby M dwarfs observed in TESS Sectors 1–5, reporting 29 planet candidates, including 24 new detections. One candidate from this survey, TOI-7384 b, has since been confirmed in follow-up work.

NEMESIS II (Feliz et al., in preparation; November 2026 draft) expands the search to the complete TESS Primary Mission (Sectors 1–26). We analyzed 213,658 single-sector light curves from 113,047 M dwarfs within 100 pc. After automated vetting and cross-sector reconciliation, the catalog contains 525 distinct transit-like signals with converged transit fits. TRICERATOPS favors a planetary origin for 262 of these, including 232 on stars without previously cataloged TESS candidates or transiting planets. These remain planet candidates, not validated planets; the survey's injection–recovery and scrambling controls quantify the selection effects and noise contamination needed to interpret the population.

Selected figures from NEMESIS II

Figures are from the manuscript in preparation; click any image to open the full-resolution version.

Stellar Flare Activity & Evolution Flares Time-Domain

Stellar flares are not just nuisances for transit searches — they are the primary record of a star's historical XUV output and thus the dominant driver of atmospheric escape. I am building multi-epoch flare frequency distributions (FFDs) for nearby M-dwarfs using TESS, K2, and ground-based photometry, with the goal of reconstructing each star's cumulative high-energy irradiation history.

Related work includes a NASA TESS Guest Investigator Cycle 9 proposal (PI: D. L. Feliz, under review): Long-Term Evolution of M-dwarf Flare Activity with K2, TESS, and Ground-Based Photometry. I am also a member of the ESA PLATO Flare Removal Working Group (2020–present), developing algorithms to detect and remove flare signatures from PLATO photometric time series.

Collaborative contributions: Paudel et al. 2024 (multiwavelength TESS/K2/Swift/HST M-dwarf flare survey); Cool Stars 22 poster on photometric activity cycles in M-dwarfs (Zenodo).

Future Directions: From Detections to Atmosphere Forecasts TESS / SDSS-V / JWST Proposed Research

Which rocky planets around nearby M dwarfs retain atmospheres, and how does that depend on their host stars' activity, age, and chemistry? My next research direction is to move beyond finding candidate planets toward predictions that can be tested—and potentially falsified—with atmospheric observations. M dwarfs offer some of the most accessible rocky planets for JWST, but their long-lived magnetic activity can expose those planets to intense X-ray and ultraviolet (XUV) irradiation. Stellar luminosity alone may not capture the irradiation histories that determine atmospheric survival.

The radius valley distinguishes likely rocky and gas-rich planet populations statistically; it does not establish whether an individual rocky planet has an atmosphere today. The cosmic shoreline (Zahnle & Catling 2017) provides a complementary, empirical hypothesis: atmospheric survival depends on the competition between a planet's escape velocity and its historical, time-integrated XUV exposure. I use the Atmosphere Retention Metric (ARM) as a proposed measure of a planet's position relative to that boundary. The ARM is a predictor to be calibrated, not currently a measured probability of atmospheric retention.

First, characterize the planets and their stars. I will extend NEMESIS to longer multi-sector TESS light curves while measuring which signals the survey misses through injection–recovery and false-alarm controls. Rotation and flare measurements from TESS, K2, and ground-based data, combined with Gaia-based age constraints and SDSS-V chemical abundances, will inform ranges of plausible XUV histories. I will test differences between partially and fully convective M-dwarf hosts, whose magnetic evolution need not follow the same history.

Second, obtain masses where they have the greatest leverage. Planet masses and radii set escape velocities, but many potentially rocky M-dwarf planets lack useful mass constraints. I will prioritize precision radial-velocity observations (including facilities such as NEID and KPF), or transit-timing constraints where feasible, by asking which achievable mass measurement would most clearly separate competing atmospheric-retention predictions. This work will support an open catalog of planet masses, escape velocities, and a target-ranking tool.

Third, confront the forecasts with observations. Published JWST and other atmosphere-sensitive measurements will be treated as graded constraints—not simply as binary atmosphere detections—and compared with activity-informed shoreline predictions and the three-dimensional luminosity-based shoreline proposed by Berta-Thompson et al. (2026). If the available sample does not constrain both sides of the boundary, the scientifically meaningful result will be retention limits and prioritized follow-up targets, rather than an unsupported probability calibration. As the evidence grows, the framework can test whether atmospheric survival changes with stellar convective structure, age, or chemistry.

A Multi-Year Transit Search of Proxima Centauri Ground-Based Photometry Published 2018 / 2019

Proxima Centauri, the nearest star to the Sun, hosts the radial-velocity-discovered planet Proxima Centauri b, with an orbital period of approximately 11.186 days. Finding transits would provide its radius and help determine its bulk composition, but Proxima is a magnetically active M dwarf whose flares, spot variability, and observational systematics can mimic or conceal transit-like signals. To investigate this, our collaboration assembled 329 epochs of ground-based photometry from 2006–2017, obtained with a worldwide network of small telescopes. Following quality cuts, 262 light curves formed the principal search sample.

Paper I — Blank et al. (2018), for which I served as corresponding author: We compared observations with published radial-velocity ephemerides and tentative transit claims. The 96 light curves overlapping those predictions did not independently corroborate the proposed transits, while demonstrating the star's persistent and complex photometric variability.

Paper II — Feliz et al. (2019): We carried out a blind Box Least Squares (BLS) search of the 262 high-quality light curves for periodic signals over approximately 1–30 days. False-alarm tests and 550 synthetic transit injections characterized what our data could detect. We found no convincing periodic transit signal. The observations strongly constrain a transit of the expected ~5 mmag depth at Proxima b's period in the tested configurations, and largely exclude transits deeper than ~3 mmag at periods shorter than five days. However, incomplete phase coverage and reduced sensitivity to shallower transits mean that all possible Proxima b transits cannot be ruled out.

This work helped establish a principle that now underpins my NEMESIS survey: a transit non-detection becomes scientifically informative only when its sensitivity is measured.

Selected figures from the Proxima Centauri studies

Figures from the published papers; click for a larger view.