arXiv subjects

Laura Flagg

Publications and source records attributed to Laura Flagg.

At least 37 records · Page 2Linked to original sources

arXiv2024

Debris Disks can Contaminate Mid-Infrared Exoplanet Spectra: Evidence for a Circumstellar Debris Disk around Exoplanet Host WASP-39

The signal from a transiting planet can be diluted by astrophysical contamination. In the case of circumstellar debris disks, this contamination could start in the mid-infrared and vary as a function of wavelength, which would then change the observed transmission spectrum for any planet in the system. The MIRI/LRS WASP-39b transmission spectrum shows an unexplained dip starting at $\sim$10 $μ$m that could be caused by astrophysical contamination. The spectral energy distribution displays excess flux at similar levels to that which are needed to create the dip in the transmission spectrum. In this article, we show that this dip is consistent with the presence of a bright circumstellar debris disk, at a distance of $>$2 au. We discuss how a circumstellar debris disk like that could affect the atmosphere of WASP-39b. We also show that even faint debris disks can be a source of contamination in MIRI exoplanet spectra.

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arXiv2024

Muted Features in the JWST NIRISS Transmission Spectrum of Hot-Neptune LTT 9779 b

The hot-Neptune desert is one of the most sparsely populated regions of the exoplanet parameter space, and atmosphere observations of its few residents can provide insights into how such planets have managed to survive in such an inhospitable environment. Here, we present transmission observations of LTT 9779 b, the only known hot-Neptune to have retained a significant H/He-dominated atmosphere, taken with JWST NIRISS/SOSS. The 0.6-2.85$μ$m transmission spectrum shows evidence for muted spectral features, rejecting a perfectly flat line at >5$σ$. We explore water and methane-dominated atmosphere scenarios for LTT 9779 b's terminator, and retrieval analyses reveal a continuum of potential combinations of metallicity and cloudiness. Through comparisons to previous population synthesis works and our own interior structure modelling, we are able to constrain LTT 9779 b's atmosphere metallicity to 20-850x solar. Within this range of metallicity, our retrieval analyses prefer solutions with clouds at mbar pressures, regardless of whether the atmosphere is water- or methane-dominated -- though cloud-free atmospheres with metallicities >500x solar cannot be entirely ruled out. By comparing self-consistent atmosphere temperature profiles with cloud condensation curves, we find that silicate clouds can readily condense in the terminator region of LTT 9779 b. Advection of these clouds onto the day-side could explain the high day-side albedo previously inferred for this planet and be part of a feedback loop aiding the survival of LTT 9779 b's atmosphere in the hot-Neptune desert.

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arXiv2023

Near-Infrared Transmission Spectroscopy of HAT-P-18$\,$b with NIRISS: Disentangling Planetary and Stellar Features in the Era of JWST

The JWST Early Release Observations (ERO) included a NIRISS/SOSS (0.6-2.8$\,μ$m) transit of the $\sim\,$850$\,$K Saturn-mass exoplanet HAT-P-18$\,$b. Initial analysis of these data reported detections of water, escaping helium, and haze. However, active K dwarfs like HAT-P-18 possess surface heterogeneities $-$ starspots and faculae $-$ that can complicate the interpretation of transmission spectra, and indeed, a spot-crossing event is present in HAT-P-18$\,$b's NIRISS/SOSS light curves. Here, we present an extensive reanalysis and interpretation of the JWST ERO transmission spectrum of HAT-P-18$\,$b, as well as HST/WFC3 and $\textit{Spitzer}$/IRAC transit observations. We detect H$_2$O (12.5$\,σ$), CO$_2$ (7.3$\,σ$), a cloud deck (7.4$\,σ$), and unocculted starspots (5.8$\,σ$), alongside hints of Na (2.7$\,σ$). We do not detect the previously reported CH$_4$ ($\log$ CH$_4$ $<$ -6 to 2$\,σ$). We obtain excellent agreement between three independent retrieval codes, which find a sub-solar H$_2$O abundance ($\log$ H$_2$O $\approx -4.4 \pm 0.3$). However, the inferred CO$_2$ abundance ($\log$ CO$_2$ $\approx -4.8 \pm 0.4$) is significantly super-solar and requires further investigation into its origin. We also introduce new stellar heterogeneity considerations by fitting for the active regions' surface gravities $-$ a proxy for the effects of magnetic pressure. Finally, we compare our JWST inferences to those from HST/WFC3 and $\textit{Spitzer}$/IRAC. Our results highlight the exceptional promise of simultaneous planetary atmosphere and stellar heterogeneity constraints in the era of JWST and demonstrate that JWST transmission spectra may warrant more complex treatments of the transit light source effect.

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arXiv2023

Characterizing the Near-infrared Spectra of Flares from TRAPPIST-1 During JWST Transit Spectroscopy Observations

We present the first analysis of JWST near-infrared spectroscopy of stellar flares from TRAPPIST-1 during transits of rocky exoplanets. Four flares were observed from 0.6--2.8 $μ$m with NIRISS and 0.6--3.5 $μ$m with NIRSpec during transits of TRAPPIST-1b, f, and g. We discover P$α$ and Br$β$ line emission and characterize flare continuum at wavelengths from 1--3.5 $μ$m for the first time. Observed lines include H$α$, P$α$-P$ε$, Br$β$, He I $λ$0.7062$μ$m, two Ca II infrared triplet (IRT) lines, and the He I IRT. We observe a reversed Paschen decrement from P$α$-P$γ$ alongside changes in the light curve shapes of these lines. The continuum of all four flares is well-described by blackbody emission with an effective temperature below 5300 K, lower than temperatures typically observed at optical wavelengths. The 0.6--1 $μ$m spectra were convolved with the TESS response, enabling us to measure the flare rate of TRAPPIST-1 in the TESS bandpass. We find flares of 10$^{30}$ erg large enough to impact transit spectra occur at a rate of 3.6$\substack{+2.1 \\ -1.3}$ flare d$^{-1}$, $\sim$10$\times$ higher than previous predictions from K2. We measure the amount of flare contamination at 2 $μ$m for the TRAPPIST-1b and f transits to be 500$\pm$450 and 2100$\pm$400 ppm, respectively. We find up to 80% of flare contamination can be removed, with mitigation most effective from 1.0--2.4 $μ$m. These results suggest transits affected by flares may still be useful for atmospheric characterization efforts.

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arXiv2023

Atmospheric Reconnaissance of TRAPPIST-1 b with JWST/NIRISS: Evidence for Strong Stellar Contamination in the Transmission Spectra

TRAPPIST-1 is a nearby system of seven Earth-sized, temperate, rocky exoplanets transiting a Jupiter-sized M8.5V star, ideally suited for in-depth atmospheric studies. Each TRAPPIST-1 planet has been observed in transmission both from space and from the ground, confidently rejecting cloud-free, hydrogen-rich atmospheres. Secondary eclipse observations of TRAPPIST-1 b with JWST/MIRI are consistent with little to no atmosphere given the lack of heat redistribution. Here we present the first transmission spectra of TRAPPIST-1 b obtained with JWST/NIRISS over two visits. The two transmission spectra show moderate to strong evidence of contamination from unocculted stellar heterogeneities, which dominates the signal in both visits. The transmission spectrum of the first visit is consistent with unocculted starspots and the second visit exhibits signatures of unocculted faculae. Fitting the stellar contamination and planetary atmosphere either sequentially or simultaneously, we confirm the absence of cloud-free hydrogen-rich atmospheres, but cannot assess the presence of secondary atmospheres. We find that the uncertainties associated with the lack of stellar model fidelity are one order of magnitude above the observation precision of 89 ppm (combining the two visits). Without affecting the conclusion regarding the atmosphere of TRAPPIST-1 b, this highlights an important caveat for future explorations, which calls for additional observations to characterize stellar heterogeneities empirically and/or theoretical works to improve model fidelity for such cool stars. This need is all the more justified as stellar contamination can affect the search for atmospheres around the outer, cooler TRAPPIST-1 planets for which transmission spectroscopy is currently the most efficient technique.

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arXiv2023

ExoGemS High-Resolution Transmission Spectroscopy of WASP-76b with GRACES

We present high-resolution transmission spectroscopy of WASP-76b with GRACES/Gemini North obtained as part of the ExoGemS survey. With a broad spectral range of 400-1050 nm and a relatively high resolution of ~66,000, these observations are particularly well-suited to searching for atomic and molecular atmospheric species via the Doppler cross-correlation technique. We recover absorption features due to neutral iron (Fe I), sodium (Na I), and ionized calcium (Ca II) at high significance (>5$σ$), and investigate possible atmospheric temperatures and wind speeds. We also report tentative (>3$σ$) detections of Li I, K I, Cr I, and V I in the atmosphere of WASP-76b. Finally, we report non-detections of a number of other species, some of which have previously been detected with other instruments. Through model injection/recovery tests, we demonstrate that many of these species are not expected to be detected in our observations. These results allow us to place GRACES and the ExoGemS survey in context with other high-resolution optical spectrographs.

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arXiv2023

ExoGemS Detection of a Metal Hydride in an Exoplanet Atmosphere

Exoplanet atmosphere studies are often enriched by synergies with brown dwarf analogs. However, many key molecules commonly seen in brown dwarfs have yet to be confirmed in exoplanet atmospheres. An important example is chromium hydride (CrH), which is often used to probe atmospheric temperatures and classify brown dwarfs into spectral types. Recently, tentative evidence for CrH was reported in the low-resolution transmission spectrum of the hot Jupiter WASP-31b. Here, we present high spectral resolution observations of WASP-31b's transmission spectrum from GRACES/Gemini North and UVES/VLT. We detect CrH at 5.6$σ$ confidence, representing the first metal hydride detection in an exoplanet atmosphere at high spectral resolution. Our findings constitute a critical step in understanding the role of metal hydrides in exoplanet atmospheres.

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arXiv2023

Detection of carbon monoxide's 4.6 micron fundamental band structure in WASP-39b's atmosphere with JWST NIRSpec G395H

Carbon monoxide (CO) is predicted to be the dominant carbon-bearing molecule in giant planet atmospheres, and, along with water, is important for discerning the oxygen and therefore carbon-to-oxygen ratio of these planets. The fundamental absorption mode of CO has a broad double-branched structure composed of many individual absorption lines from 4.3 to 5.1 $\mathrmμ$m, which can now be spectroscopically measured with JWST. Here we present a technique for detecting the rotational sub-band structure of CO at medium resolution with the NIRSpec G395H instrument. We use a single transit observation of the hot Jupiter WASP-39b from the JWST Transiting Exoplanet Community Early Release Science (JTEC ERS) program at the native resolution of the instrument ($R \,{\sim} 2700$) to resolve the CO absorption structure. We robustly detect absorption by CO, with an increase in transit depth of 264 $\pm$ 68 ppm, in agreement with the predicted CO contribution from the best-fit model at low resolution. This detection confirms our theoretical expectations that CO is the dominant carbon-bearing molecule in WASP-39b's atmosphere, and further supports the conclusions of low C/O and super-solar metallicities presented in the JTEC ERS papers for WASP-39b.

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arXiv2023

High-Resolution Transmission Spectroscopy of the Terrestrial Exoplanet GJ 486b

Terrestrial exoplanets orbiting M-dwarf stars are promising targets for transmission spectroscopy with existing or near-future instrumentation. The atmospheric composition of such rocky planets remains an open question, especially given the high X-ray and ultraviolet flux from their host M dwarfs that can drive atmospheric escape. The 1.3 $R_\oplus$ exoplanet GJ 486b ($T_{\rm{eq}} \sim$ 700 K), orbiting an M3.5 star, is expected to have one of the strongest transmission spectroscopy signals among known terrestrial exoplanets. We observed three transits of GJ 486b using three different high-resolution spectrographs: IRD on Subaru, IGRINS on Gemini-South, and SPIRou on the Canada-France-Hawai'i Telescope. We searched for atmospheric absorption from a wide variety of molecular species via the cross-correlation method, but did not detect any robust atmospheric signals. Nevertheless, our observations are sufficiently sensitive to rule out several clear atmospheric scenarios via injection and recovery tests, and extend comparative exoplanetology into the terrestrial regime. Our results suggest that GJ 486b does not possess a clear H$_2$/He-dominated atmosphere, nor a clear 100% water-vapor atmosphere. Other secondary atmospheres with high mean molecular weights or H$_2$/He-dominated atmospheres with clouds remain possible. Our findings provide further evidence suggesting that terrestrial planets orbiting M-dwarf stars may experience significant atmospheric loss.

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arXiv2022

Early Release Science of the exoplanet WASP-39b with JWST NIRCam

Measuring the metallicity and carbon-to-oxygen (C/O) ratio in exoplanet atmospheres is a fundamental step towards constraining the dominant chemical processes at work and, if in equilibrium, revealing planet formation histories. Transmission spectroscopy provides the necessary means by constraining the abundances of oxygen- and carbon-bearing species; however, this requires broad wavelength coverage, moderate spectral resolution, and high precision that, together, are not achievable with previous observatories. Now that JWST has commenced science operations, we are able to observe exoplanets at previously uncharted wavelengths and spectral resolutions. Here we report time-series observations of the transiting exoplanet WASP-39b using JWST's Near InfraRed Camera (NIRCam). The long-wavelength spectroscopic and short-wavelength photometric light curves span 2.0 - 4.0 $μ$m, exhibit minimal systematics, and reveal well-defined molecular absorption features in the planet's spectrum. Specifically, we detect gaseous H$_2$O in the atmosphere and place an upper limit on the abundance of CH$_4$. The otherwise prominent CO$_2$ feature at 2.8 $μ$m is largely masked by H$_2$O. The best-fit chemical equilibrium models favour an atmospheric metallicity of 1-100$\times$ solar (i.e., an enrichment of elements heavier than helium relative to the Sun) and a sub-stellar carbon-to-oxygen (C/O) ratio. The inferred high metallicity and low C/O ratio may indicate significant accretion of solid materials during planet formation or disequilibrium processes in the upper atmosphere.

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arXiv2022

Early Release Science of the Exoplanet WASP-39b with JWST NIRSpec G395H

Measuring the abundances of carbon and oxygen in exoplanet atmospheres is considered a crucial avenue for unlocking the formation and evolution of exoplanetary systems. Access to an exoplanet's chemical inventory requires high-precision observations, often inferred from individual molecular detections with low-resolution space-based and high-resolution ground-based facilities. Here we report the medium-resolution (R$\sim$600) transmission spectrum of an exoplanet atmosphere between 3-5 $μ$m covering multiple absorption features for the Saturn-mass exoplanet WASP-39b, obtained with JWST NIRSpec G395H. Our observations achieve 1.46x photon precision, providing an average transit depth uncertainty of 221 ppm per spectroscopic bin, and present minimal impacts from systematic effects. We detect significant absorption from CO$_2$ (28.5$σ$) and H$_2$O (21.5$σ$), and identify SO$_2$ as the source of absorption at 4.1 $μ$m (4.8$σ$). Best-fit atmospheric models range between 3 and 10x solar metallicity, with sub-solar to solar C/O ratios. These results, including the detection of SO$_2$, underscore the importance of characterising the chemistry in exoplanet atmospheres, and showcase NIRSpec G395H as an excellent mode for time series observations over this critical wavelength range.

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arXiv2022

Identification of carbon dioxide in an exoplanet atmosphere

Carbon dioxide (CO2) is a key chemical species that is found in a wide range of planetary atmospheres. In the context of exoplanets, CO2 is an indicator of the metal enrichment (i.e., elements heavier than helium, also called "metallicity"), and thus formation processes of the primary atmospheres of hot gas giants. It is also one of the most promising species to detect in the secondary atmospheres of terrestrial exoplanets. Previous photometric measurements of transiting planets with the Spitzer Space Telescope have given hints of the presence of CO2 but have not yielded definitive detections due to the lack of unambiguous spectroscopic identification. Here we present the detection of CO2 in the atmosphere of the gas giant exoplanet WASP-39b from transmission spectroscopy observations obtained with JWST as part of the Early Release Science Program (ERS). The data used in this study span 3.0 to 5.5 μm in wavelength and show a prominent CO2 absorption feature at 4.3 μm (26σ significance). The overall spectrum is well matched by one-dimensional, 10x solar metallicity models that assume radiative-convective-thermochemical equilibrium and have moderate cloud opacity. These models predict that the atmosphere should have water, carbon monoxide, and hydrogen sulfide in addition to CO2, but little methane. Furthermore, we also tentatively detect a small absorption feature near 4.0 μm that is not reproduced by these models.

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arXiv2022

The Mysterious Affair of the H$_2$ in AU Mic

Molecular hydrogen is the most abundant molecule in the Galaxy and plays important roles for planets, their circumstellar environments, and many of their host stars. We have confirmed the presence of molecular hydrogen in the AU Mic system using high-resolution FUV spectra from HST-STIS during both quiescence and a flare. AU Mic is a $\sim$23 Myr M dwarf which hosts a debris disk and at least two planets. We estimate the temperature of the gas at 1000 to 2000 K, consistent with previous detections. Based on the radial velocities and widths of the H$_2$ line profiles and the response of the H$_2$ lines to a stellar flare, the H$_2$ line emission is likely produced in the star, rather than in the disk or the planet. However, the temperature of this gas is significantly below the temperature of the photosphere ($\sim$3650 K) and the predicted temperature of its star spots ($\gtrsim$2650 K). We discuss the possibility of colder star spots or a cold layer in the photosphere of a pre-main sequence M dwarf.

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arXiv2021

Characterizing the WASP-4 system with TESS and radial velocity data: Constraints on the cause of the hot Jupiter's changing orbit and evidence of an outer planet

Orbital dynamics provide valuable insights into the evolution and diversity of exoplanetary systems. Currently, only one hot Jupiter, WASP-12b, is confirmed to have a decaying orbit. Another, WASP-4b, exhibits hints of a changing orbital period that could be caused by orbital decay, apsidal precession, or the acceleration of the system towards the Earth. We have analyzed all data sectors from NASA's Transiting Exoplanet Survey Satellite together with all radial velocity (RV) and transit data in the literature to characterize WASP-4b's orbit. Our analysis shows that the full RV data set is consistent with no acceleration towards the Earth. Instead, we find evidence of a possible additional planet in the WASP-4 system, with an orbital period of ~7000 days and $M_{c}sin(i)$ of $5.47^{+0.44}_{-0.43} M_{Jup}$. Additionally, we find that the transit timing variations of all of the WASP-4b transits cannot be explained by the second planet but can be explained with either a decaying orbit or apsidal precession, with a slight preference for orbital decay. Assuming the decay model is correct, we find an updated period of 1.338231587$\pm$0.000000022 days, a decay rate of -7.33$\pm$0.71 msec/year, and an orbital decay timescale of 15.77$\pm$1.57 Myr. If the observed decay results from tidal dissipation, we derive a modified tidal quality factor of $Q^{'}_{*}$ = 5.1$\pm$0.9$\times10^4$, which is an order of magnitude lower than values derived for other hot Jupiter systems. However, more observations are needed to determine conclusively the cause of WASP-4b's changing orbit and confirm the existence of an outer companion.

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arXiv2021

Detection of Ionized Calcium in the Atmosphere of the Ultra-Hot Jupiter WASP-76b

Recent observations of the ultra-hot Jupiter WASP-76b have revealed a diversity of atmospheric species. Here we present new high-resolution transit spectroscopy of WASP-76b with GRACES at the Gemini North Observatory, serving as a baseline for the Large and Long Program "Exploring the Diversity of Exoplanet Atmospheres at High Spectral Resolution" (Exoplanets with Gemini Spectroscopy, or ExoGemS for short). With a broad spectral range of $400 - 1050$ nm, these observations allow us to search for a suite of atomic species. We recover absorption features due to neutral sodium (Na I), and report a new detection of the ionized calcium (Ca II) triplet at ~ $850$ nm in the atmosphere of WASP-76b, complementing a previous detection of the Ca II H & K lines. The triplet has line depths of $0.295 \pm 0.034$% at ~ $849.2$ nm, $0.574 \pm 0.041$% at ~ $854.2$ nm, and $0.454 \pm 0.024$% at ~ $866.2$ nm, corresponding to effective radii close to (but within) the planet's Roche radius. These measured line depths are significantly larger than those predicted by model LTE and NLTE spectra obtained on the basis of a pressure-temperature profile computed assuming radiative equilibrium. The discrepancy suggests that the layers probed by our observations are either significantly hotter than predicted by radiative equilibrium and/or in a hydrodynamic state. Our results shed light on the exotic atmosphere of this ultra-hot world, and will inform future analyses from the ExoGemS survey.

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arXiv2021

Detection of H$_2$ in the TWA 7 System: A Probable Circumstellar Origin

Using HST-COS FUV spectra, we have discovered warm molecular hydrogen in the TWA 7 system. TWA 7, a $\sim$9 Myr old M2.5 star, has a cold debris disk and has previously shown no signs of accretion. Molecular hydrogen is expected to be extremely rare in a debris disk. While molecular hydrogen can be produced in star spots or the lower chromospheres of cool stars such as TWA 7, fluxes from progressions that get pumped by the wings of Ly$α$ indicate that this molecular hydrogen could be circumstellar and thus that TWA 7 is accreting at very low levels and may retain a reservoir of gas in the near circumstellar environment.

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arXiv2020

ACRONYM IV: Three New, Young, Low-mass Spectroscopic Binaries

As part of our search for new low-mass members of nearby young moving groups (YMG), we discovered three low-mass, spectroscopic binaries, two of which are not kinematically associated with any known YMG. Using high-resolution optical spectroscopy, we measure the component and systemic radial velocities of the systems, as well as their lithium absorption and H$α$ emission, both spectroscopic indicators of youth. One system (2MASS J02543316-5108313, M2.0+M3.0) we confirm as a member of the 40 Myr old Tuc-Hor moving group, but whose binarity was previously undetected. The second young binary (2MASS J08355977-3042306, K5.5+M1.5) is not a kinematic match to any known YMG, but each component exhibits lithium absorption and strong and wide H$α$ emission indicative of active accretion, setting an upper age limit of 15 Myr. The third system (2MASS J10260210-4105537, M1.0+M3.0) has been hypothesized in the literature to be a member of the 10 Myr old TW Hya Association (TWA), but with our measured systemic velocity, shows the binary is in fact not part of any known YMG. This last system also has lithium absorption in each component, and has strong and variable H$α$ emission, setting an upper age limit of 15 Myr based on the lithium detection.

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arXiv2019

CO Detected in CI Tau b: Hot Start Implied by Planet Mass and M$_K$

We acquired high resolution IR spectra of CI Tau, the host star of one of the few young planet candidates amenable to direct spectroscopic detection. We confirm the planet's existence with a direct detection of CO in the planet's atmosphere. We also calculate a mass of 11.6 M$_J$ based on the amplitude of its radial velocity variations. We estimate its flux contrast with its host star to get an absolute magnitude estimate for the planet of 8.17 in the K band. This magnitude implies the planet formed via a "hot start" formation mechanism. This makes CI Tau b the youngest confirmed exoplanet as well as the first exoplanet around a T Tauri star with a directly determined, model-independent, dynamical mass.

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