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- All Subjects: Astronomy
- Creators: Shkolnik, Evgenya
Description
The composition of planets and their volatile contents are intimately connected to the structure and evolution of their parent protoplanetary disks. The transport of momentum and volatiles is often parameterized by a turbulent viscosity parameter $\alpha$, which is usually assumed to be spatially and temporally uniform across the disk. I show that variable $\alpha$(r,z) (where $r$ is radius, and $z$ is height from the midplane) attributable to angular momentum transport due to MRI can yield disks with significantly different structure, as mass piles up in the 1-10 AU region resulting in steep slopes of p $>$ 2 here (where p is the power law exponent in $\Sigma \propto r^{-p}$). I also show that the transition radius (where bulk mass flow switches from inward to outward) can move as close in as 3 AU; this effect (especially prominent in externally photoevaporated disks) may significantly influence the radial water content available during planet formation.
I then investigate the transport of water in disks with different variable α profiles. While radial temperature profile sets the location of the water snowline (i.e., inside of which water is present as vapor; outside of which, as ice on solids), it is the rates of diffusion and drift of small icy solids and diffusion of vapor across the snow line that determine the radial water distribution. All of these processes are highly sensitive to local $\alpha$. I calculate the effect of radially varying α on water transport, by tracking the abundance of vapor in the inner disk, and fraction of ice in particles and larger asteroids beyond the snow line. I find one α profile attributable to winds and hydrodynamical instabilities, and motivated by meteoritic constraints, to show considerable agreement with inferred water contents observed in solar system asteroids.
Finally, I calculate the timing of gap formation due to the formation of a planet in disks around different stars. Here, I assume that pebble accretion is the dominant mechanism for planetary growth and that the core of the first protoplanet forms at the water snow line. I discuss the dependence of gap timing to various stellar and disk properties.
I then investigate the transport of water in disks with different variable α profiles. While radial temperature profile sets the location of the water snowline (i.e., inside of which water is present as vapor; outside of which, as ice on solids), it is the rates of diffusion and drift of small icy solids and diffusion of vapor across the snow line that determine the radial water distribution. All of these processes are highly sensitive to local $\alpha$. I calculate the effect of radially varying α on water transport, by tracking the abundance of vapor in the inner disk, and fraction of ice in particles and larger asteroids beyond the snow line. I find one α profile attributable to winds and hydrodynamical instabilities, and motivated by meteoritic constraints, to show considerable agreement with inferred water contents observed in solar system asteroids.
Finally, I calculate the timing of gap formation due to the formation of a planet in disks around different stars. Here, I assume that pebble accretion is the dominant mechanism for planetary growth and that the core of the first protoplanet forms at the water snow line. I discuss the dependence of gap timing to various stellar and disk properties.
ContributorsKalyaan, Anusha (Author) / Desch, Steven J (Thesis advisor) / Groppi, Christopher (Committee member) / Young, Patrick (Committee member) / Shkolnik, Evgenya (Committee member) / Bell, James (Committee member) / Arizona State University (Publisher)
Created2018
Description
The field of exoplanet science has matured over the past two decades with over 3500 confirmed exoplanets. However, many fundamental questions regarding the composition, and formation mechanism remain unanswered. Atmospheres are a window into the properties of a planet, and spectroscopic studies can help resolve many of these questions. For the first part of my dissertation, I participated in two studies of the atmospheres of brown dwarfs to search for weather variations. To understand the evolution of weather on brown dwarfs we conducted a multi-epoch study monitoring four cool brown dwarfs to search for photometric variability. These cool brown dwarfs are predicted to have salt and sulfide clouds condensing in their upper atmosphere and we detected one high amplitude variable. Combining observations for all T5 and later brown dwarfs we note a possible correlation between variability and cloud opacity.
For the second half of my thesis, I focused on characterizing the atmospheres of directly imaged exoplanets. In the first study Hubble Space Telescope data on HR8799, in wavelengths unobservable from the ground, provide constraints on the presence of clouds in the outer planets. Next, I present research done in collaboration with the Gemini Planet Imager Exoplanet Survey (GPIES) team including an exploration of the instrument contrast against environmental parameters, and an examination of the environment of the planet in the HD 106906 system. By analyzing archival HST data and examining the near-infrared colors of HD 106906b, we conclude that the companion shows weak evidence of a circumplanetary dust disk or cloud. Finally, I measure the properties of the low mass directly imaged planet 51 Eridani b. We combined published J, H spectra with updated LP photometry, new K1, K2 spectra, and MS photometry. The new data confirms that the planet has redder than similar spectral type objects, which might be due to the planet still transitioning from to L-to-T. Model atmospheres indicate a cooler effective temperature best fit by a patchy cloud atmosphere making 51 Eri b an excellent candidate for future variability studies with the James Webb Space Telescope.
For the second half of my thesis, I focused on characterizing the atmospheres of directly imaged exoplanets. In the first study Hubble Space Telescope data on HR8799, in wavelengths unobservable from the ground, provide constraints on the presence of clouds in the outer planets. Next, I present research done in collaboration with the Gemini Planet Imager Exoplanet Survey (GPIES) team including an exploration of the instrument contrast against environmental parameters, and an examination of the environment of the planet in the HD 106906 system. By analyzing archival HST data and examining the near-infrared colors of HD 106906b, we conclude that the companion shows weak evidence of a circumplanetary dust disk or cloud. Finally, I measure the properties of the low mass directly imaged planet 51 Eridani b. We combined published J, H spectra with updated LP photometry, new K1, K2 spectra, and MS photometry. The new data confirms that the planet has redder than similar spectral type objects, which might be due to the planet still transitioning from to L-to-T. Model atmospheres indicate a cooler effective temperature best fit by a patchy cloud atmosphere making 51 Eri b an excellent candidate for future variability studies with the James Webb Space Telescope.
ContributorsRajan, Abhijith (Author) / Patience, Jennifer (Thesis advisor) / Young, Patrick (Thesis advisor) / Scowen, Paul (Committee member) / Butler, Nathaniel (Committee member) / Shkolnik, Evgenya (Committee member) / Arizona State University (Publisher)
Created2017
Description
The Star Planet Activity Research CubeSat (SPARCS) will be a 6U CubeSat devoted to photometric monitoring of M dwarfs in the far-ultraviolet (FUV) and near-ultraviolet (NUV) (160 and 280 nm respectively), measuring the time-dependent spectral slope, intensity and evolution of M dwarf stellar UV radiation. The delta-doped detectors baselined for SPARCS have demonstrated more than five times the in-band quantum efficiency of the detectors of GALEX. Given that red:UV photon emission from cool, low-mass stars can be million:one, UV observation of thes stars are susceptible to red light contamination. In addition to the high efficiency delta-doped detectors, SPARCS will include red-rejection filters to help minimize red leak. Even so, careful red-rejection and photometric calibration is needed. As was done for GALEX, white dwarfs are used for photometric calibration in the UV. We find that the use of white dwarfs to calibrate the observations of red stars leads to significant errors in the reported flux, due to the differences in white dwarf and red dwarf spectra. Here we discuss the planned SPARCS calibration model and the color correction, and demonstrate the importance of this correction when recording UV measurements of M stars taken by SPARCS.
ContributorsOsby, Ella (Author) / Shkolnik, Evgenya (Thesis director) / Ardila, David (Committee member) / School of Earth and Space Exploration (Contributor) / Barrett, The Honors College (Contributor)
Created2021-05
Description
Standard cosmological models predict that the first astrophysical sources formed from a Universe filled with neutral hydrogen (HI) around one hundred million years after the Big Bang. The transition into Cosmic Dawn (CD) that seeded all the structures seen today can only be probed directly by the 21-cm line of neutral hydrogen. Redshifted by the Hubble expansion, HI signal during CD is expected to be visible in radio frequencies. Precisely characterized and carefully calibrated low-frequency instruments are necessary to measure the predicted ~10-200 mK brightness temperature of this cosmological signal against foregrounds. This dissertation focuses on improving the existing instrumental and analysis techniques for the Experiment to Detect the Global EoR Signature (EDGES) and building capabilities for future space-based 21-cm instruments, including the Farside Array for Radio Science Investigations of the Dark ages and Exoplanets (FARSIDE) concept.Frequency-dependent antenna beams of 21-cm instruments limit the removal of bright galactic foreground emission (~10^3 - 10^4K) from observations. Using three electromagnetic simulation packages, I modeled the EDGES low-band antenna, including the ground plane and soil, and quantified its variations as a function of frequency. I compared simulated observations to sky data and obtained absolute agreement within 4% and qualitatively similar spectral structures.
I used the new open-source edges-analysis pipeline to carry out rigorous fits of the absorption feature on the same low-band data and lab calibration measurements as (Bowman et. al. 2018). Using a Bayesian framework, I tested a few calibration choices and found posteriors of the best-fit 21-cm model parameters well within the 1σ values reported in B18. To test for the ``global'' nature of the reported cosmic absorption feature, I performed a time-dependent analysis. Initial results from this analysis successfully retrieved physical estimates for the foregrounds and estimates of the cosmic signal consistent with previous findings.
The array layout of FARSIDE, a NASA probe-class concept to place a radio interferometer on the lunar farside, is a four-arm spiral configuration consisting of 128 dual-polarized antennas with a spatial offset between the phase centers of its orthogonal polarizations. I modeled the impact of direction-dependent beams and phase offsets on simulated observations of all four Stokes parameter images of a model and quantified its effects on the two primary science cases: 21-cm cosmology and exoplanet studies.
ContributorsMahesh, Nivedita (Author) / Bowman, Judd D (Thesis advisor) / Jacobs, Daniel C (Committee member) / Groppi, Christopher (Committee member) / Shkolnik, Evgenya (Committee member) / Windhorst, Rogier (Committee member) / Arizona State University (Publisher)
Created2022
Description
This thesis presents the results of a brown dwarf companion direct imaging survey. Over a total of 4 nights, 200 B and A stars were imaged using the Keck telescope and the Near Infrared Camera 2 (NIRC2). Presented here are preliminary results from the nights of 04 June 2014 and 17 December 2013. Brown dwarfs are partially degenerate objects that have masses between approximately 13 MJup and 75 MJup. Currently, the number of brown dwarf companions found around high mass stars is small. Finding brown dwarfs as companions to B and A stars will allow astronomers to study these objects when they are young and bright, giving key insights into their formation and evolution.
\par A pipeline was written specifically for these data sets that includes dark subtraction, flat field correction, bad pixel correction, distortion correction, centering, filtering, and point spread function (PSF) subtraction. This subtraction was accomplished using the Karhunen-Loeve Image Processing (KLIP) algorithm which employs principal component analysis and Karhunen-Loeve (KL) transforms to subtract out starlight and artifacts from the images and allow for easier detection of a candidate companion.
\par Only candidate companions from the night of 04 June 2014 were analyzed, with 95 candidate companions found around 22 stars. Due to a lack of some necessary images, 91 companions around 20 stars were analyzed and their masses were found to be approximately 6 MJup to 150 MJup with projected separations from the host star of approximately 100AU to 900AU. An upper limit of 6.6% was placed on stellar companion frequency and an upper limit of 93% was placed on brown dwarf companion frequency. This survey achieved a median sensitivity of ΔK of 12.6 at 1" and a ΔK of 15.1 at 3.6". Further observations will be required to determine whether the candidates found are true co-moving companions or background stars not bound to the host star.
ContributorsGarani, Jasmine (Author) / Patience, Jennifer (Thesis advisor) / Simon, Molly (Committee member) / Line, Michael (Committee member) / Nielsen, Eric (Committee member) / Arizona State University (Publisher)
Created2022
Description
As the search for life in our universe grows, it is important to not only locate planets outside of our solar system, but also to work towards the ability to understand and characterize their nature. Many current research endeavors focus on the discovery of exoplanets throughout the surrounding universe; however, we still know very little about the characteristics of these exoplanets themselves, particularly their atmospheres. Observatories, such as the Hubble Space Telescope and the Spitzer Space Telescope, have made some of the first observations which revealed information about the atmospheres of exoplanets but have yet to acquire complete and detailed characterizations of exoplanet atmospheres. The EXoplanet Climate Infrared TElescope (EXCITE) is a mission specifically designed to target key information about the atmospheres of exoplanets - including the global and spatially resolved energy budget, chemical bulk-compositions, vertical temperature profiles and circulation patterns across the surface, energy distribution efficiency as a function of equilibrium temperatures, and cloud formation and distribution - in order to generate dynamic and detailed atmospheric characterizations. EXCITE will use phase-resolved transit spectroscopy in the 1-4 micron wavelength range to accomplish these science goals, so it is important that the EXCITE spectrograph system is designed and tested to meet these observational requirements. For my thesis, I present my research on the EXCITE mission science goals and the design of the EXCITE spectrograph system to meet these goals, along with the work I have done in the beginning stages of testing the EXCITE spectrograph system in the lab. The primary result of my research work is the preparation of a simple optics setup in the lab to prepare a laser light source for use in the EXCITE spectrograph system - comparable to the preparation of incoming light by the EXCITE telescope system - which successfully yields an F# = 12.9 and a spot size of s = 39 ± 7 microns. These results meet the expectations of the system and convey appropriate preparation of a light source to begin the assembly and testing of the EXCITE spectrograph optics in the lab.
ContributorsHorvath, Zoe (Author) / Butler, Nathaniel (Thesis director) / Line, Michael (Committee member) / Scowen, Paul (Committee member) / Barrett, The Honors College (Contributor) / School of Earth and Space Exploration (Contributor) / Department of Physics (Contributor)
Created2022-05
Description
I present a catalog of 1,794 stellar evolution models for solar-type and low-mass stars, which is intended to help characterize real host-stars of interest during the ongoing search for potentially habitable exoplanets. The main grid is composed of 904 tracks, for 0.5-1.2 M_sol at scaled metallicity values of 0.1-1.5 Z_sol and specific elemental abundance ratio values of 0.44-2.28 O/Fe_sol, 0.58-1.72 C/Fe_sol, 0.54-1.84 Mg/Fe_sol, and 0.5-2.0 Ne/Fe_sol. The catalog includes a small grid of late stage evolutionary tracks (25 models), as well as a grid of M-dwarf stars for 0.1-0.45 M_sol (856 models). The time-dependent habitable zone evolution is calculated for each track, and is strongly dependent on stellar mass, effective temperature, and luminosity parameterizations. I have also developed a subroutine for the stellar evolution code TYCHO that implements a minimalist coupled model for estimating changes in the stellar X-ray luminosity, mass loss, rotational velocity, and magnetic activity over time; to test the utility of the updated code, I created a small grid (9 models) for solar-mass stars, with variations in rotational velocity and scaled metallicity. Including this kind of information in the catalog will ultimately allow for a more robust consideration of the long-term conditions that orbiting planets may experience.
In order to gauge the true habitability potential of a given planetary system, it is extremely important to characterize the host-star's mass, specific chemical composition, and thus the timescale over which the star will evolve. It is also necessary to assess the likelihood that a planet found in the "instantaneous" habitable zone has actually had sufficient time to become "detectably" habitable. This catalog provides accurate stellar evolution predictions for a large collection of theoretical host-stars; the models are of particular utility in that they represent the real variation in stellar parameters that have been observed in nearby stars.
In order to gauge the true habitability potential of a given planetary system, it is extremely important to characterize the host-star's mass, specific chemical composition, and thus the timescale over which the star will evolve. It is also necessary to assess the likelihood that a planet found in the "instantaneous" habitable zone has actually had sufficient time to become "detectably" habitable. This catalog provides accurate stellar evolution predictions for a large collection of theoretical host-stars; the models are of particular utility in that they represent the real variation in stellar parameters that have been observed in nearby stars.
ContributorsTruitt, Amanda Rosendall (Author) / Young, Patrick (Thesis advisor) / Anbar, Ariel (Committee member) / Desch, Steven (Committee member) / Patience, Jennifer (Committee member) / Shkolnik, Evgenya (Committee member) / Arizona State University (Publisher)
Created2017
Description
The 78 secondary eclipse depths for a sample of 36 transiting hot Jupiters observed at 3.6- and 4.5 μm using the Spitzer Space Telescope is here reported. Eclipse results for 27 of these planets are new and include highly irradiated worlds such as KELT-7b (Kilodegree Extremely Little Telescope), WASP-87b (Wide Angle Search for Planets), WASP-76b, and WASP-64b, and important targets for the James Webb Space Telescope (JWST) such as WASP-62b. WASP-62b is found to have a slightly eccentric orbit (ecosω=0.00614±0.00058), and the eccentricities of HAT-P-13b (Hungarian Automated Telescope Project) and WASP-14b are confirmed. The remainder are individually consistent with circular orbits, but there is statistical evidence for eccentricity increasing with orbital period in this range from 1 to 5 days. Day-side brightness temperatures (Tb) for the planets yield information on albedo and heat redistribution, following Cowan and Agol (2011). Planets having maximum day side temperatures exceeding ∼2200 K are consistent with zero albedo and distribution of stellar irradiance uniformly over the day-side hemisphere. The most intriguing result is a detection of a systematic difference between the emergent spectra of these hot Jupiters as compared to blackbodies. The ratio of observed brightness temperatures, Tb(4.5)/Tb(3.6), increases with equilibrium temperature by 98±26 parts-per-million per Kelvin, over the entire temperature range in the sample (800K to 2500K). No existing model predicts this trend over such a large range of temperature. This may be due to a structural difference in the atmospheric temperature profile between the real planetary atmospheres as compared to models.
ContributorsGarhart, Emily (Author) / Christensen, Phil (Thesis advisor) / Line, Michael (Committee member) / Shim, Dan (Committee member) / Arizona State University (Publisher)
Created2019
Description
With the ability to observe the atmospheres of terrestrial exoplanets via transit spectroscopy on the near-term horizon, the possibility of atmospheric biosignatures has received considerable attention in astrobiology. While traditionally exoplanet scientists looking for life focused on biologically relevant trace gases such as O2 and CH4, this approach has raised the spectre of false positives. Therefore, to address these shortcomings, a new set of methods is required to provide higher confidence in life detection. One possible approach is measuring the topology of atmospheric chemical reaction networks (CRNs). To investigate and assess this approach, the ability of network-theoretic metrics to distinguish the distance from thermochemical equilibrium in the atmosphere of hot jupiters was tested. After modeling the atmospheres of hot jupiters over a range of initial conditions using the VULCAN modeling package, atmospheric CRNs were constructed from the converged models and their topology measured using the Python package NetworkX. I found that network metrics were able to predict the distance from thermochemical equilibrium better than atmospheric species abundances alone. Building on this success, I modeled 30,000 terrestrial worlds. These models divided into two categories: Anoxic Archean Earth-like planets that varied in terms of CH4 surface flux (modeled as either biotic or abiotic in origin), and modern Earth-like planets with and without a surface flux of CCl2F2 (to represent the presence of industrial civilizations). I constructed atmospheric CRNs from the converged models, and analyzed their topology. I found that network metrics could distinguish between atmospheres with and without the presence of life or technology. In particular, mean degree and average shortest path length consistently performed better at distinguishing between abiotic and biotic Archean-like atmospheres than CH4 abundance.
ContributorsFisher, Theresa Mason (Author) / Walker, Sara I (Thesis advisor) / Hartnett, Hilairy (Committee member) / Line, Michael (Committee member) / Shkolnik, Evgenya (Committee member) / Okie, Jordan (Committee member) / Arizona State University (Publisher)
Created2023