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Toward the Detection and Characterization of Sub-Neptune Exoplanets Orbiting Nearby M Dwarfs
The last 20 years have seen a boom in the number of known exoplanetary systems, and many of the temperate sub-Neptune planets we study orbit low mass M stars. M dwarf stars can host temperate worlds with periods short enough for rapid followup and precise characterization but these stars are also extremely active, subjecting their planets to harsh XUV radiation and frequent flares that can sterilize and evaporate planetary atmospheres. Follow-up studies of potentially rocky planets in these systems have so far provided only tenuous evidence for their atmospheres and in some cases what are likely false positive detections caused by stellar active regions like starspots. Starspots are ubiquitous on M dwarf photospheres and introduce molecular features in the disk average stellar spectrum which, if uncorrected, can be mistaken for absorption of molecules like H2O and TiO in the planet’s atmosphere. With the successful launch of JWST and the promise of studying exoplanet atmospheres in greater detail than ever before, it is vital that we mitigate the problem of starspot contamination in exoplanet transmission spectra. How does stellar activity affect the atmospheres of short-period planets? Do temperate and warm sub-Neptune planets orbiting M dwarfs retain their atmospheres? To what extent do stellar active regions like starspots contaminate our observations and obscure the evidence of atmospheric absorption? How can we accurately measure spot characteristics and mitigate their effects on exoplanet observations? These questions have motivated the research I present in this thesis, where I describe how I used space and ground-based observations to validate new exoplanets found by TESS, search for atmospheric escape from a nearby terrestrial world, and characterize starspots on the young exoplanet host AU Microscopii.</p
Quantifying the Transit Light Source Effect: Measurements of Spot Temperature and Coverage on the Photosphere of AU Microscopii with High-Resolution Spectroscopy and Multi-Color Photometry
AU Mic is an active 24 Myr pre-main sequence M dwarf in the stellar
neighborhood (d9.7 pc) with a rotation period of 4.86 days. The two
transiting planets orbiting AU Mic, AU Mic b and c, are warm sub-Neptunes on
8.5 and 18.9 day periods and are targets of interest for atmospheric
observations of young planets. Here we study AU Mic's unocculted starspots
using ground-based photometry and spectra in order to complement current and
future transmission spectroscopy of its planets. We gathered multi-color LCO
0.4m SBIG photometry to study the star's rotational modulations and LCO NRES
high-resolution spectra to measure the different spectral components within the
integrated spectrum of the star, parameterized by 3 spectral components and
their coverage fractions. We find AU Mic's surface has at least 2 spectral
components, a K ambient photosphere with cool spots that have a
temperature of K and cover percent of the surface,
increasing and decreasing by 5 from the average throughout a rotation. We
also detect a third flux component with a filling factor less than 0.5 and
a largely uncertain temperature that we attribute to flare flux not entirely
omitted in the time-averaged spectra. We include measurements of spot
temperature and coverage fraction from both 2- and 3- temperature models, which
we find agree with each other strongly. Our expanded use of various techniques
to study starspots will help us better understand this system and may have
applications for interpreting the transmission spectra for exoplanets
transiting stars of a wide range of activity levels.Comment: 25 pages, 13 figures, Accepted to Ap
TESS Discovery of an ultra-short-period planet around the nearby M dwarf LHS 3844
Data from the newly-commissioned \textit{Transiting Exoplanet Survey
Satellite} (TESS) has revealed a "hot Earth" around LHS 3844, an M dwarf
located 15 pc away. The planet has a radius of and
orbits the star every 11 hours. Although the existence of an atmosphere around
such a strongly irradiated planet is questionable, the star is bright enough
(, ) for this possibility to be investigated with transit and
occultation spectroscopy. The star's brightness and the planet's short period
will also facilitate the measurement of the planet's mass through Doppler
spectroscopy.Comment: 10 pages, 4 figures. Submitted to ApJ Letters. This letter makes use
of the TESS Alert data, which is currently in a beta test phase, using data
from the pipelines at the TESS Science Office and at the TESS Science
Processing Operations Cente
Vetting of 384 TESS Objects of Interest with TRICERATOPS and Statistical Validation of 12 Planet Candidates
We present TRICERATOPS, a new Bayesian tool that can be used to vet and
validate TESS Objects of Interest (TOIs). We test the tool on 68 TOIs that have
been previously confirmed as planets or rejected as astrophysical false
positives. By looking in the false positive probability (FPP) -- nearby false
positive probability (NFPP) plane, we define criteria that TOIs must meet to be
classified as validated planets (FPP < 0.015 and NFPP < 10^-3), likely planets
(FPP 10^-1).
We apply this procedure on 384 unclassified TOIs and statistically validate 12,
classify 125 as likely planets, and classify 52 as likely nearby false
positives. Of the 12 statistically validated planets, 9 are newly validated.
TRICERATOPS is currently the only TESS vetting and validation tool that models
transits from nearby contaminant stars in addition to the target star. We
therefore encourage use of this tool to prioritize follow-up observations that
confirm bona fide planets and identify false positives originating from nearby
stars.Comment: Accepted to A
Early Release Science of the exoplanet WASP-39b with JWST NIRISS
Transmission spectroscopy provides insight into the atmospheric properties
and consequently the formation history, physics, and chemistry of transiting
exoplanets. However, obtaining precise inferences of atmospheric properties
from transmission spectra requires simultaneously measuring the strength and
shape of multiple spectral absorption features from a wide range of chemical
species. This has been challenging given the precision and wavelength coverage
of previous observatories. Here, we present the transmission spectrum of the
Saturn-mass exoplanet WASP-39b obtained using the SOSS mode of the NIRISS
instrument on the JWST. This spectrum spans m in wavelength and
reveals multiple water absorption bands, the potassium resonance doublet, as
well as signatures of clouds. The precision and broad wavelength coverage of
NIRISS-SOSS allows us to break model degeneracies between cloud properties and
the atmospheric composition of WASP-39b, favoring a heavy element enhancement
("metallicity") of the solar value, a sub-solar
carbon-to-oxygen (C/O) ratio, and a solar-to-super-solar potassium-to-oxygen
(K/O) ratio. The observations are best explained by wavelength-dependent,
non-gray clouds with inhomogeneous coverage of the planet's terminator.Comment: 48 pages, 12 figures, 2 tables. Under review at Natur
TOI 122b and TOI 237b: Two Small Warm Planets Orbiting Inactive M Dwarfs Found by TESS
peer reviewedWe report the discovery and validation of TOI 122b and TOI 237b, two warm planets transiting inactive M dwarfs observed by the Transiting Exoplanet Survey Satellite (TESS). Our analysis shows that TOI 122b has a radius of 2.72 ± 0.18 R[SUB]⊕[/SUB] and receives 8.8 ± 1.0 times Earth's bolometric insolation, and TOI 237b has a radius of 1.44±0.12 R⊕ and receives 3.7 ± 0.5 times Earth's insolation, straddling the 6.7 × Earth insolation that Mercury receives from the Sun. This makes these two of the cooler planets yet discovered by TESS, even on their 5.08 and 5.43 day orbits. Together, they span the small-planet radius valley, providing useful laboratories for exploring volatile evolution around M dwarfs. Their relatively nearby distances (62.23 ± 0.21 pc and 38.11 ± 0.23 pc, respectively) make them potentially feasible targets for future radial velocity follow-up and atmospheric characterization, although such observations may require substantial investments of time on large telescopes
A Super-Earth and Sub-Neptune Transiting the Late-type M Dwarf LP 791-18
Planets occur most frequently around cool dwarfs, but only a handful of specific examples are known to orbit the latest-type M stars. Using TESS photometry, we report the discovery of two planets transiting the low-mass star called LP 791-18 (identified by TESS as TOI 736). This star has spectral type M6V, effective temperature 2960 K, and radius 0.17 R o, making it the third-coolest star known to host planets. The two planets straddle the radius gap seen for smaller exoplanets; they include a 1.1R ⊕ planet on a 0.95 day orbit and a 2.3R ⊕ planet on a 5 day orbit. Because the host star is small the decrease in light during these planets' transits is fairly large (0.4% and 1.7%). This has allowed us to detect both planets' transits from ground-based photometry, refining their radii and orbital ephemerides. In the future, radial velocity observations and transmission spectroscopy can both probe these planets' bulk interior and atmospheric compositions, and additional photometric monitoring would be sensitive to even smaller transiting planets
TESS Discovery of an Ultra-short-period Planet around the Nearby M Dwarf LHS 3844
Data from the newly commissioned Transiting Exoplanet Survey Satellite has revealed a 'hot Earth' around LHS 3844, an M dwarf located 15 pc away. The planet has a radius of R ⊕ and orbits the star every 11 hr. Although the existence of an atmosphere around such a strongly irradiated planet is questionable, the star is bright enough (I = 11.9, K = 9.1) for this possibility to be investigated with transit and occultation spectroscopy. The star's brightness and the planet's short period will also facilitate the measurement of the planet's mass through Doppler spectroscopy