arXiv subjects

Dániel Apai

Publications and source records attributed to Dániel Apai.

At least 73 records · Page 4Linked to original sources

arXiv2019

Constraining Stellar Photospheres as an Essential Step for Transmission Spectroscopy of Small Exoplanets

Transmission spectra probe the atmospheres of transiting exoplanets, but these observations are also subject to signals introduced by magnetic active regions on host stars. Here we outline scientific opportunities in the next decade for providing useful constraints on stellar photospheres and inform interpretations of transmission spectra of the smallest ($R<4\,R_{\odot}$) exoplanets. We identify and discuss four primary opportunities: (1) refining stellar magnetic active region properties through exoplanet crossing events; (2) spectral decomposition of active exoplanet host stars; (3) joint retrievals of stellar photospheric and planetary atmospheric properties with studies of transmission spectra; and (4) continued visual transmission spectroscopy studies to complement longer-wavelength studies from $\textit{JWST}$. We make five recommendations to the Astro2020 Decadal Survey Committee: (1) identify the transit light source (TLS) effect as a challenge to precise exoplanet transmission spectroscopy and an opportunity ripe for scientific advancement in the coming decade; (2) include characterization of host star photospheric heterogeneity as part of a comprehensive research strategy for studying transiting exoplanets; (3) support the construction of ground-based extremely large telescopes (ELTs); (4) support multi-disciplinary research teams that bring together the heliophysics, stellar physics, and exoplanet communities to further exploit transiting exoplanets as spatial probes of stellar photospheres; and (5) support visual transmission spectroscopy efforts as complements to longer-wavelength observational campaigns with $\textit{JWST}$.

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arXiv2019

Cloud Atlas: High-Contrast Time-Resolved Observations of Planetary-Mass Companions

Directly-imaged planetary-mass companions offer unique opportunities in atmospheric studies of exoplanets. They share characteristics of both brown dwarfs and transiting exoplanets, therefore, are critical for connecting atmospheric characterizations for these objects. Rotational phase mapping is a powerful technique to constrain the condensate cloud properties in ultra-cool atmospheres. Applying this technique to directly-imaged planetary-mass companions will be extremely valuable for constraining cloud models in low mass and surface gravity atmospheres and for determining the rotation rate and angular momentum of substellar companions. Here, we present Hubble Space Telescope Wide Field Camera 3 near-infrared time-resolved photometry for three planetary-mass companions, AB Pic B, 2M0122B, and 2M1207b. Using two-roll differential imaging and hybrid point spread function modeling, we achieve sub-percent photometric precision for all three observations. We find tentative modulations ($<\!\!2σ$) for AB Pic B and 2M0122B but cannot reach conclusive results on 2M1207b due to strong systematics. The relatively low significance of the modulation measurements cannot rule out the hypothesis that these planetary-mass companions have the same vertical cloud structures as brown dwarfs. Our rotation rate measurements, combined with archival period measurements of planetary-mass companions and brown dwarfs do not support a universal mass-rotation relation. The high precision of our observations and the high occurrence rates of variable low-surface gravity objects encourage high-contrast time-resolved observations with the James Webb Space Telescope.

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arXiv2018

ACCESS: Ground-based Optical Transmission Spectroscopy of the Hot Jupiter WASP-4b

We present an optical transmission spectrum of the atmosphere of WASP-4b obtained through observations of four transits with Magellan/IMACS. Using a Bayesian approach to atmospheric retrieval, we find no evidence for scattering or absorption features in our transit spectrum. Our models include a component to model the transit light source effect (spectral contamination from unocculted spots on the stellar photosphere), which we show can have a marked impact on the observed transmission spectrum for reasonable spot covering fractions (< 5%); this is the first such analysis for WASP-4b. We are also able to fit for the size and temperature contrast of spots observed during the second and third transits, finding evidence for both small, cool and large, warm spot-like features on the photosphere. Finally, we compare our results to those published by Huitson et al. (2017) using Gemini/GMOS and May et al. (2018) using IMACS, and find that our data are in agreement.

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arXiv2018

The Transit Light Source Effect II: The Impact of Stellar Heterogeneity on Transmission Spectra of Planets Orbiting Broadly Sun-like Stars

Transmission spectra probe exoplanetary atmospheres, but they can also be strongly affected by heterogeneities in host star photospheres through the transit light source effect. Here we build upon our recent study of the effects of unocculted spots and faculae on M-dwarf transmission spectra, extending the analysis to FGK dwarfs. Using a suite of rotating model photospheres, we explore spot and facula covering fractions for varying activity levels and the associated stellar contamination spectra. Relative to M dwarfs, we find that the typical variabilities of FGK dwarfs imply lower spot covering fractions, though they generally increase with later spectral types, from $\sim 0.1\%$ for F dwarfs to 2-4$\%$ for late-K dwarfs. While the stellar contamination spectra are considerably weaker than those for typical M dwarfs, we find that typically active G and K dwarfs produce visual slopes that are detectable in high-precision transmission spectra. We examine line offsets at H$α$ and the Na and K doublets and find that unocculted faculae in K dwarfs can appreciably alter transit depths around the Na D doublet. We find that band-averaged transit depth offsets at molecular bands for CH$_{4}$, CO, CO$_{2}$, H$_{2}$O, N$_{2}$O, O$_{2}$, and O$_{3}$ are not detectable for typically active FGK dwarfs, though stellar TiO/VO features are potentially detectable for typically active late-K dwarfs. Generally, this analysis shows that inactive FGK dwarfs do not produce detectable stellar contamination features in transmission spectra, though active FGK host stars can produce such features and care is warranted in interpreting transmission spectra from these systems.

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arXiv2018

The LEECH Exoplanet Imaging Survey: Limits on Planet Occurrence Rates Under Conservative Assumptions

We present the results of the largest $L^{\prime}$ ($3.8~μ$m) direct imaging survey for exoplanets to date, the Large Binocular Telescope Interferometer (LBTI) Exozodi Exoplanet Common Hunt (LEECH). We observed 98 stars with spectral types from B to M. Cool planets emit a larger share of their flux in $L^{\prime}$ compared to shorter wavelengths, affording LEECH an advantage in detecting low-mass, old, and cold-start giant planets. We emphasize proximity over youth in our target selection, probing physical separations smaller than other direct imaging surveys. For FGK stars, LEECH outperforms many previous studies, placing tighter constraints on the hot-start planet occurrence frequency interior to $\sim20$ au. For less luminous, cold-start planets, LEECH provides the best constraints on giant-planet frequency interior to $\sim20$ au around FGK stars. Direct imaging survey results depend sensitively on both the choice of evolutionary model (e.g., hot- or cold-start) and assumptions (explicit or implicit) about the shape of the underlying planet distribution, in particular its radial extent. Artificially low limits on the planet occurrence frequency can be derived when the shape of the planet distribution is assumed to extend to very large separations, well beyond typical protoplanetary dust-disk radii ($\lesssim50$ au), and when hot-start models are used exclusively. We place a conservative upper limit on the planet occurrence frequency using cold-start models and planetary population distributions that do not extend beyond typical protoplanetary dust-disk radii. We find that $\lesssim90\%$ of FGK systems can host a 7 to 10 $M_{\mathrm{Jup}}$ planet from 5 to 50 au. This limit leaves open the possibility that planets in this range are common.

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arXiv2018

ACCESS: A featureless optical transmission spectrum for WASP-19b from Magellan/IMACS

The short period ($0.94$-day) transiting exoplanet WASP-19b is an exceptional target for transmission spectroscopy studies, due to its relatively large atmospheric scale-height ($\sim 500$ km) and equilibrium temperature ($\sim 2100$ K). Here we report on six precise spectroscopic Magellan/IMACS observations, five of which target the full optical window from $0.45-0.9μ$m and one targeting the $0.4-0.55μ$m blue-optical range. Five of these datasets are consistent with a transmission spectrum without any significant spectral features, while one shows a significant slope as a function of wavelength, which we interpret as arising from photospheric heterogeneities in the star. Coupled with HST/WFC3 infrared observations, our optical/near-infrared measurements point to the presence of high altitude clouds in WASP-19b's atmosphere in agreement with previous studies. Using a semi-analytical retrieval approach, considering both planetary and stellar spectral features, we find a water abundance consistent with solar for WASP-19b and strong evidence for sub-solar abundances for optical absorbers such as TiO and Na; no strong optical slope is detected, which suggests that if hazes are present, they are much weaker than previously suggested. In addition, two spot-crossing events are observed in our datasets and analyzed, including one of the first unambiguously detected bright spot-crossing events on an exoplanet host star.

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arXiv2018

Retrieval of planetary and stellar properties in transmission spectroscopy with Aura

Transmission spectroscopy provides a powerful probe of the atmospheric properties of transiting exoplanets. To date, studies of exoplanets in transit have focused on inferring their atmospheric properties such as chemical compositions, cloud/haze properties, and temperature structures. However, surface inhomogeneities in the host stars of exoplanets in the form of cool spots and hot faculae can in principle imprint signatures on the observed planetary transit spectrum. Here we present Aura, a new retrieval paradigm for inferring both planetary and stellar properties from a transmission spectrum. We apply our retrieval framework to a sample of hot giant exoplanets to determine the significance of stellar heterogeneity and clouds/hazes in their spectra. The retrieval analyses distinguish four groups of planets. First, the spectra of WASP-6b and WASP-39b are best characterised by imprints of stellar heterogeneity and hazes and/or clouds. HD 209458b and HAT-P-12b comprise the second group for which there is weak evidence for stellar heterogeneity and a high significance of hazes and/or clouds. The third group constitutes HAT-P-1b and WASP-31b and shows weak evidence against stellar heterogeneity but weak to substantial indications of clouds/hazes. The fourth group -- WASP-19b, WASP-17b, and WASP-12b -- is fit best by molecular and alkali absorbers with H$_2$ scattering without evidence for stellar heterogeneity and weak to no evidence for clouds/hazes. Our retrieval methodology paves the way to simultaneous information on the star and planet from higher resolution spectra using future facilities such as the James Webb Space Telescope and large ground-based facilities.

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arXiv2018

A comprehensive understanding of planet formation is required for assessing planetary habitability and for the search for life

Dozens of habitable zone, approximately earth-sized exoplanets are known today. An emerging frontier of exoplanet studies is identifying which of these habitable zone, small planets are actually habitable (have all necessary conditions for life) and, of those, which are earth-like. Many parameters and processes influence habitability, ranging from the orbit through detailed composition including volatiles and organics, to the presence of geological activity and plate tectonics. While some properties will soon be directly observable, others cannot be probed by remote sensing for the foreseeable future. Thus, statistical understanding of planetary systems' formation and evolution is a key supplement to the direct measurements of planet properties. Probabilistically assessing parameters we cannot directly measure is essential to reliably assess habitability, to prioritizing habitable-zone planets for follow-up, and for interpreting possible biosignatures.

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arXiv2018

Understanding Stellar Contamination in Exoplanet Transmission Spectra as an Essential Step in Small Planet Characterization

Transmission spectroscopy during planetary transits is expected to be a major source of information on the atmospheres of small (approximately Earth-sized) exoplanets in the next two decades. This technique, however, is intrinsically affected by stellar spectral contamination caused by the fact that stellar photo- and chromospheres are not perfectly homogeneous. Such stellar contamination will often reach or exceed the signal introduced by the planetary spectral features. Finding effective methods to correct stellar contamination -- or at least to quantify its possible range -- for the most important exoplanets is a necessary step for our understanding of exoplanet atmospheres. This will require significantly deepening our understanding of stellar heterogeneity, which is currently limited by the available data.

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arXiv2018

Cloud Atlas: Rotational Modulations in the L/T Transition Brown Dwarf Companion HN Peg B

Time-resolved observations of brown dwarfs' rotational modulations provide powerful insights into the properties of condensate clouds in ultra-cool atmospheres. Multi-wavelength light curves reveal cloud vertical structures, condensate particle sizes, and cloud morphology, which directly constrain condensate cloud and atmospheric circulation models. We report results from Hubble Space Telescope/Wide Field Camera 3 near-infrared G141 taken in six consecutive orbits observations of HN Peg B, an L/T transition brown dwarf companion to a G0V type star. The best-fit sine wave to the $1.1-1.7μ$m broadband light curve has the amplitude of $1.206\pm0.025\%$ and period of $15.4\pm0.5$ hr. The modulation amplitude has no detectable wavelength dependence except in the 1.4 $μ$m water absorption band, indicating that the characteristic condensate particle sizes are large ($>1μ$m). We detect significantly ($4.4σ$) lower modulation amplitude in the 1.4$μ$m water absorption band, and find that HN Peg B's spectral modulation resembles those of early T type brown dwarfs. We also describe a new empirical interpolation method to remove spectral contamination from the bright host star. This method may be applied in other high-contrast time-resolved observations with WFC3.

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arXiv2018

The Transit Light Source Effect: False Spectral Features and Incorrect Densities for M-dwarf Transiting Planets

Transmission spectra are differential measurements that utilize stellar illumination to probe transiting exoplanet atmospheres. Any spectral difference between the illuminating light source and the disk-integrated stellar spectrum due to starspots and faculae will be imprinted in the observed transmission spectrum. However, few constraints exist for the extent of photospheric heterogeneities in M dwarfs. Here, we model spot and faculae covering fractions consistent with observed photometric variabilities for M dwarfs and the associated 0.3-5.5 $μ$m stellar contamination spectra. We find that large ranges of spot and faculae covering fractions are consistent with observations and corrections assuming a linear relation between variability amplitude and covering fractions generally underestimate the stellar contamination. Using realistic estimates for spot and faculae covering fractions, we find stellar contamination can be more than $10 \times$ larger than transit depth changes expected for atmospheric features in rocky exoplanets. We also find that stellar spectral contamination can lead to systematic errors in radius and therefore the derived density of small planets. In the case of the TRAPPIST-1 system, we show that TRAPPIST-1's rotational variability is consistent with spot covering fractions $f_{spot} = 8^{+18}_{-7}\%$ and faculae covering fractions $f_{fac} = 54^{+16}_{-46}\%$. The associated stellar contamination signals alter transit depths of the TRAPPIST-1 planets at wavelengths of interest for planetary atmospheric species by roughly 1-15 $\times$ the strength of planetary features, significantly complicating $JWST$ follow-up observations of this system. Similarly, we find stellar contamination can lead to underestimates of bulk densities of the TRAPPIST-1 planets of $Δ(ρ) = -3^{+3}_{-8} \%$, thus leading to overestimates of their volatile contents.

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arXiv2017

Parallaxes and Infrared Photometry of three Y0 dwarfs

We have followed up the three Y0 dwarfs WISEPA J041022.71+150248.5, WISEPA J173835.53+273258.9 and WISEPC J205628.90+145953.3 using the UKIRT/WFCAM telescope/instruments. We find parallaxes that are more consistent and accurate than previously published values. We estimate absolute magnitudes in photometric pass-bands from $Y$ to $W3$ and find them to be consistent between the three Y0 dwarfs indicating the inherent cosmic absolute magnitude spread of these objects is small. We examine the MKO $J$ magnitudes over the four year time line and find small but significant monotonic variations. Finally we estimate physical parameters from a comparison of spectra and parallax to equilibrium and non-equilibrium models finding values consistent with solar metallicity, an effective temperature of 450-475\,K and log~g of 4.0-4.5.

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arXiv2017

NGC 1980 is not a foreground population of Orion: Spectroscopic survey of young stars with low extinction in Orion A

We perform a spectroscopic survey of the foreground population in Orion A with MMT/Hectospec. We use these data, along with archival spectroscopic data and photometric data, to derive spectral types, extinction values, and masses for 691 stars. Using the Spitzer Space Telescope data, we characterize the disk properties of these sources. We identify 37 new transition disk (TD) objects, one globally depleted disk candidate, and 7 probable young debris disks. We discover an object with a mass less than 0.018-0.030 M$_{\odot}$, which harbors a flaring disk. Using the H$α$ emission line, we characterize the accretion activity of the sources with disks, and confirm that fraction of accreting TDs is lower than that of optically thick disks (46$\pm$7% versus 73$\pm$9%, respectively). Using kinematic data from the Sloan Digital Sky Survey and APOGEE INfrared Spectroscopy of Young Nebulous Clusters program (IN-SYNC), we confirm that the foreground population shows similar kinematics to their local molecular clouds and other young stars in the same regions. Using the isochronal ages, we find that the foreground population has a median age around 1-2 Myr, which is similar to the one of other young stars in Orion A. Therefore, our results argue against the presence of a large and old foreground cluster in front of Orion A.

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arXiv2017

A Physical Model-based Correction for Charge Traps in the Hubble Space Telescope's Wide Field Camera 3 Near-IR Detector and Applications to Transiting Exoplanets and Brown Dwarfs

The Hubble Space Telescope (HST) Wide Field Camera 3 (WFC3) near-IR channel is extensively used in time-resolved observations, especially for transiting exoplanet spectroscopy and brown dwarf and directly imaged exoplanet rotational phase mapping. The ramp effect is the dominant source of systematics in the WFC3 for time-resolved observations, which limits its photometric precision. Current mitigation strategies are based on empirical fits and require additional orbits "to help the telescope reach a thermal equilibrium". We show that the ramp effect profiles can be explained and corrected with high fidelity using charge trapping theories. We also present a model for this process that can be used to predict and to correct charge trap systematics. Our model is based on a very small number of parameters that are intrinsic to the detector. We find that these parameters are very stable between the different datasets, and we provide best-fit values. Our model is tested with more than 120 orbits ($\sim40$ visits) of WFC3 observations and is proved to be able to provide near photon noise limited corrections for observations made with both staring and scanning modes of transiting exoplanets as well as for starting-mode observations of brown dwarfs. After our model correction, the light curve of the first orbit in each visit has the same photometric precision as subsequent orbits, so data from the first orbit need no longer be discarded. Near IR arrays with the same physical characteristics (e.g., JWST/NIRCam) may also benefit from the extension of this model, if similar systematic profiles are observed.

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arXiv2017

Probabilistic Constraints on the Mass and Composition of Proxima b

Recent studies regarding the habitability, observability, and possible orbital evolution of the indirectly detected exoplanet Proxima b have mostly assumed a planet with $M \sim 1.3$ $M_\oplus$, a rocky composition, and an Earth-like atmosphere or none at all. In order to assess these assumptions, we use previous studies of the radii, masses, and compositions of super-Earth exoplanets to probabilistically constrain the mass and radius of Proxima b, assuming an isotropic inclination probability distribution. We find it is ~90% likely that the planet's density is consistent with a rocky composition; conversely, it is at least 10% likely that the planet has a significant amount of ice or an H/He envelope. If the planet does have a rocky composition, then we find expectation values and 95% confidence intervals of $\left _\text{rocky} = 1.63_{-0.72}^{+1.66}$ $M_\oplus$ for its mass and $\left _\text{rocky} = 1.07_{-0.31}^{+0.38}$ $R_\oplus$ for its radius.

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arXiv2016

Atmospheric Compositions of Three Brown Dwarfs and Implications for their Formation Conditions

The formation mechanisms and chemical compositions of brown dwarfs likely span a diverse range. If they are formed predominantly in isolation by gravitational collapse like dwarf stars then their compositions might follow those of dwarf stars, spanning a relatively narrow range in metallicities (~ -0.4 to +0.4 dex) and predominantly oxygen-rich (C/O <~ 0.7). On the other hand, giant planets in the solar system are all super-solar in metallicity ([C/H] > 0.5~dex), which is thought to be a consequence of formation by core-accretion in a circumstellar disk. In this study we determine the atmospheric chemical compositions of three brown dwarfs and find them to be neither characteristic of dwarf stars nor giant planets. We derive high-precision atmospheric chemical abundances using high-S/N HST near-infrared spectra of three mid T Dwarfs which, together with two previously studied objects, display distinctly sub-solar metallicities and span C/O ratios of 0.4-1.0. These abundance patterns indicate either an old sub-stellar population and a multitude of formation environments or objects formed in circumstellar disks followed by ejection later. We show that for a simple synthetic model population of brown dwarfs with the same spectral-type as our sample the predicted age distribution is predominantly older than the Sun and, hence, more metal poor, consistent with our derived abundances. The diverse C/O ratios we find are indicative of either different chemical reservoirs in their formation environments or different formation pathways. Our results open the possibility that cool brown dwarfs may provide important probes of early galactic chemical evolution and inhomogeneity.

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arXiv2016

ACCESS I: An Optical Transmission Spectrum of GJ 1214b Reveals a Heterogeneous Stellar Photosphere

GJ 1214b is the most studied sub-Neptune exoplanet to date. Recent measurements have shown its near-infrared transmission spectrum to be flat, pointing to a high-altitude opacity source in the exoplanet's atmosphere, either equilibrium condensate clouds or photochemical hazes. Many photometric observations have been reported in the optical by different groups, though simultaneous measurements spanning the entire optical regime are lacking. We present an optical transmission spectrum ($4,500-9,260$Å) of GJ 1214b in 14 bins measured with Magellan/IMACS repeatedly over three transits. We measure a mean planet-to-star radius ratio of ${R_{p}/R_{s} = 0.1146\pm{2\times10^{-4}}}$ and mean uncertainty of $σ(R_{p}/R_{s})=8.7\times10^{-4}$ in the spectral bins. The optical transit depths are shallower on average than observed in the near-infrared. We present a model for jointly incorporating the effects of a composite photosphere and atmospheric transmission (CPAT) through the exoplanet's limb, and use it to examine the cases of absorber and temperature heterogeneities in the stellar photosphere. We find the optical and near-infrared measurements are best explained by the combination of (1) photochemical haze in the exoplanetary atmosphere with a mode particle size $r=0.1~μ$m and haze-forming efficiency $f_{haze}=10 \%$ and (2) faculae in the unocculted stellar disk with a temperature contrast $ΔT=354^{+46}_{-46}$ K, assuming 3.2% surface coverage. The CPAT model can be used to assess potential contributions of heterogeneous stellar photospheres to observations of exoplanet transmission spectra, which will be important for searches for spectral features in the optical.

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arXiv2016

A candidate planetary-mass object with a photoevaporating disk in Orion

In this work, we report the discovery of a candidate planetary-mass object with a photoevaporating protoplanetary disk, Proplyd 133-353, which is near the massive star $θ^{1}$ Ori C at the center of the Orion Nebula Cluster (ONC). The object was known to have extended emission pointing away from $θ^{1}$ Ori C, indicating ongoing external photoevaporation. Our near-infrared spectroscopic data suggests that the central source of Proplyd 133-353 is substellar ($\sim$M9.5), might have a mass probably less than 13 Jupiter mass and an age younger than 0.5 Myr. Proplyd 133-353 shows a similar ratio of X-ray luminosity to stellar luminosity to other young stars in the ONC with a similar stellar luminosity, and has a similar proper motion to the mean one of confirmed ONC members. We propose that Proplyd 133-353 was formed in a very low-mass dusty cloud near $θ^{1}$ Ori C as a second-generation of star formation, which can explain both its young age and the presence of its disk.

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