The M-dwarf Atmosphere Problem

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Description
Most stars in our galaxy are M–dwarfs, much cooler and smaller than the sun. The ubiquitous nature of these stars is also paired with the formation of terrestrial exoplanets orbiting them. The strategic placement of M-dwarfs between main-sequence stars and…

Most stars in our galaxy are M–dwarfs, much cooler and smaller than the sun. The ubiquitous nature of these stars is also paired with the formation of terrestrial exoplanets orbiting them. The strategic placement of M-dwarfs between main-sequence stars and brown dwarfs, their uniqueness as exoplanet analogs, and their dominating presence in the galactic stellar population make them priority targets for study. This work investigates outstanding questions, including the need to acquire constraints on their chemical compositions to decode formation processes, evolution, and interaction with companion objects. Chapter 1 lays out a broad background emphasizing the importance of studying the most populous star in the galaxy, their far-reaching implications, and primarily the numerous challenges in characterizing the atmospheres and environments of these stars. Chapter 2 investigates the influence of M-dwarf star spots propagating into spectra of transiting terrestrial planets, showing that inaccurate modeling of M-dwarf photospheres leads to significant bias when inferring atmospheric properties of companion exoplanets. These biases persist despite correcting M-dwarf spot signatures imprinted onto the exoplanetary spectra, even with high-fidelity JWST observations. This result emphasizes the need for improved stellar atmosphere models as the first step to improving our understanding of the companion planets. To address this, chapter 3 introduces SPHINX—a new stellar atmosphere model grid for M-dwarfs. SPHINX provides improved constraints on fundamental properties of benchmark M-dwarf systems (e.g., temperature, surface gravity, radius, and chemistry). The improvement is significant relative to the state-of-the-art stellar model grid available today. Chapter 4 expands this model, applying it to mid-to-late type M-dwarfs, and investigating chemical trends in their atmospheric properties. Using low-resolution observations, both archival data (from SpeX Prism Library Database) and from previous empirical studies; this chapter presents constraints on fundamental atmospheric properties of 71 low-mass, late-type M-dwarfs to understand spectroscopic degeneracies arising due to stellar activity, cloud/dust condensation and convection. With SPHINX models, the chemical properties of these stars are compared against main-sequence stars to acquire a more holistic understanding of M-dwarfs as a class—in the quest to ultimately characterize their companions.
Date Created
2023
Agent

The Impacts of High Contrast Direct Imaging: A Study of Young Planetary Systems and Applications to Course-Based Undergraduate Research Experiences

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Description
Direct imaging is a powerful tool in revealing the architectures of young planetary systems, clearly showing the structure of circumstellar disks. Circumstellar disks, similar to the asteroid belt, are critical elements of any planetary system, and the study of them…

Direct imaging is a powerful tool in revealing the architectures of young planetary systems, clearly showing the structure of circumstellar disks. Circumstellar disks, similar to the asteroid belt, are critical elements of any planetary system, and the study of them is important to understanding planet formation. Disks around several main sequence stars have already been observed directly interacting with exoplanets in their respective systems. Imaging can help answer many of the key questions of how disks interact in their respective systems. The Gemini Planet Imager is a high contrast imaging instrument that has spatially resolved several circumstellar disks for the first time, many exhibiting tracers of ongoing planet formation or the presence of a perturbing exoplanet. With this new sample, population analyses of characteristics of disks can now be explored and compared to information at other wavelengths. Direct imaging is also a uniquely accessible tool in engaging students and the community in astronomy. In combination with a course-based undergraduate research experience, direct imaging has the ability to engage students in the process of doing research in a very accessible manner. In Chapter 1, I introduce the concepts related to circumstellar debris disks, further focusing on the sub-field of direct imaging and its value in understanding these systems and engaging students in astronomy. In Chapter 2, I present four images of newly-resolved debris disks in the Scorpius-Centaurus association, comparing their characteristics with many other spatially-resolved circumstellar disks within the moving group. In Chapter 3, I present a uniform analysis of debris disk structure using a consistent and empirically-informed modeling approach. In Chapter 4, I present my findings and experiences in developing and teaching a course-based undergraduate research experience for students in the country’s first online astronomy degree program centered on the direct imaging of brown dwarfs. In Chapter 5, I present my conclusions on the topics I have investigated and discuss future work within the field of direct imaging and its role in driving astronomy research and education forward.
Date Created
2023
Agent

A High Resolution Imaging Search for Brown Dwarf Companions Around High Mass Stars

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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…

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.
Date Created
2022
Agent

A Photometric and Spectroscopic Analysis of the High-Energy Evolution of K Stars

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Description
The balance between relative numbers, lifetime, and habitable zone (HZ) size of K stars (0.6 – 0.9 M⊙) in comparison with M (0.08 – 0.6 M⊙) and G (0.9 – 1.1 M⊙) stars makes them candidates to host “super-habitable” planets.…

The balance between relative numbers, lifetime, and habitable zone (HZ) size of K stars (0.6 – 0.9 M⊙) in comparison with M (0.08 – 0.6 M⊙) and G (0.9 – 1.1 M⊙) stars makes them candidates to host “super-habitable” planets. Understanding the high- energy radiation environment of planets around these stars is crucial, since ultraviolet (UV) and X-ray radiation may cause severe photodissociation and ionization of the atmosphere, with the potential for complete erosion. In this thesis, I present the first broad study of the UV and X-ray evolution of K stars. I first focused on Galaxy Evolution Explorer (GALEX) and Ro ̈ntgen Satellit (ROSAT) photometric UV and X-ray evolutions of K stars and compared this with the age evolution of both early- (0.35 – 0.6 M⊙) and late-M (0.08 – 0.35 M⊙) stars. I found that the fractional UV and X-ray flux from M and K stars is similar; however, the wider and farther HZs of K stars mean that there is less incident UV radiation on HZ planets. Next, I led a spectroscopic study of 41 K stars using Hubble Space Telescope Cosmic Origins Spectrograph (HST/COS) data to show that the UV line and continua emission show no decrease in flux beyond 650 Myr whereas early-M star flux declines by 150 Myr; therefore, the K star intrinsic UV flux is greater than early-M stars after this time. I suggest that this phenomenon is related to K star rotational spin-down stalling. Lastly, I revisited the GALEX and ROSAT data with newly-available distances from the Gaia mission for both K and M stars. I find that the UV flux for K stars is an order of magnitude higher for M stars at all ages and the flux in their respective HZs is similar. However, K star X-ray flux is an order of magnitude less in the HZ than for M stars. The age of decline shows a dependency on wavelength, a phenomenon which is not seen in either the early- or late-M star data. These results suggest thatK stars may not exhibit quite the advantage as HZ planet host stars as the scientific community originally thought.
Date Created
2022
Agent

A Uniform Atmospheric Retrieval Analysis of Ultra-Cool Brown Dwarfs

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Description
Brown dwarfs are a unique class of object which span the range between the lowest mass stars, and highest mass planets. New insights into the physics and chemistry of brown dwarfs comes from the comparison between spectroscopic observations, and theoretical…

Brown dwarfs are a unique class of object which span the range between the lowest mass stars, and highest mass planets. New insights into the physics and chemistry of brown dwarfs comes from the comparison between spectroscopic observations, and theoretical atmospheric models. In this thesis, I present a uniform atmospheric retrieval analysis of the coolest Y, and late-T spectral type brown dwarfs using the CaltecH Inverse ModEling and Retrieval Algorithms (CHIMERA). In doing so, I develop a foundational dataset of retrieved atmospheric parameters including: molecular abundances, thermal structures, evolutionary parameters, and cloud properties for 61 different brown dwarfs. Comparisons to other modeling techniques and theoretical expectations from the James Webb Space Telescope (JWST) are made. Finally, I describe the techniques used to improve CHIMERA to run on Graphical Processing Units (GPUs), which directly enabled the creation of this large dataset.
Date Created
2022
Agent

Composition of Potential Planet Forming Material in Supernova Fallback Disks

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Description

The first extrasolar planet discovered orbited the millisecond pulsar PSR B1257+12. These so-called "pulsar planets" have proved to be more uncommon than their early discovery might have suggested. The proximity of many known pulsar planets to their host neutron stars…

The first extrasolar planet discovered orbited the millisecond pulsar PSR B1257+12. These so-called "pulsar planets" have proved to be more uncommon than their early discovery might have suggested. The proximity of many known pulsar planets to their host neutron stars indicates that they formed post-supernova, possibly from material produced in the supernova. Any pre-existing planets that close would have been obliterated in the supernova. Material from the supernova falls back to an accretion disk around the neutron star analogous to a protoplanetary disk around a protostar. The composition of the supernova thus determines the composition of the planet-forming material. The pulsar planet then forms from collisions between particles within the disk. This research examines the composition of supernova remnants to explore this formation process. Chemical abundances of supernova ejecta were obtained from 3D supernova simulations. The velocities of particles containing silicate-mineral forming elements were filtered to determine what might stay in the system and thus be available for the formation of a fallback disk. The abundances of the remaining particles were compared to characterize the potential composition of such a fallback disk. Overall, the composition was roughly silicate-like, but the rates of mixing versus dust formation could lead to the production of highly exotic minerals.

Date Created
2022-05
Agent

Directly Imaging Circumstellar Debris Disks

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Description

Debris disks are a collection of dust grains and planetesimals around a star and are thought to contain the remnants of planet formation. Directly imaging debris disks and studying their morphologies is valuable for studying the planet formation process. In…

Debris disks are a collection of dust grains and planetesimals around a star and are thought to contain the remnants of planet formation. Directly imaging debris disks and studying their morphologies is valuable for studying the planet formation process. In some stellar systems that have a directly imaged debris disk, there are also directly imaged planets. Debris disk structures like gaps and asymmetries can show the gravitational e↵ects of planets that are below the brightness threshold for being detected via direct imaging. We investigate a sample of debris disks in Scorpius-Centaurus (Sco-Cen) that were imaged with the Gemini Planet Imager (GPI), which is an adaptive optics system with a coronagraph to block starlight. We look at two GPI data sets, the GPIES campaign Sco-Cen targets, and a follow-up observing program for Sco-Cen targets. We resolve 5 debris disks in the follow-up program and 13 from the GPIES campaign. By calculating contrast curves, we determine the planet detection limit in each of the GPI images. We find that we could have detected 5 Jupiter mass planets at angular separations greater than about 0.6 arcseconds in our GPIES campaign images. In three of our images we could have detected 2 Jupiter mass planets in wide orbits, but 2 Jupiter masses below the detection limit in our other images. We identify one point source around HD 108904 as a sub-stellar companion candidate. To further check for evidence of planets that are below the detection limit, we measure the surface brightness profile of the disks to check for asymmetries in brightness. We find that one of the edge-on disks has an asymmetric surface brightness profile, HD 106906, and three other edge-on disks have symmetric surface brightness profiles. We also find that two disks, HD 106906 and HD 111520, are asymmetric in radial extent, which is possibly evidence for gravitational interactions with planets.

Date Created
2021-05
Agent

A Uniform Retrieval Analysis of Ultra-cool Dwarfs. IV. A Statistical Census from 50 Late T-dwarfs

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Description
The spectra of brown dwarfs are key to exploring the chemistry and physics thattake place in their atmospheres. Late T dwarf (950 - 500 K) spectra are particularly
diagnostic due to their relatively cloud free atmospheres and deep molecular
bands. With the…

The spectra of brown dwarfs are key to exploring the chemistry and physics thattake place in their atmospheres. Late T dwarf (950 - 500 K) spectra are particularly
diagnostic due to their relatively cloud free atmospheres and deep molecular
bands. With the use of powerful atmospheric retrieval tools, these properties permit
constraints on molecular/atomic abundances and temperature profiles. Building
upon previous analyses on T and Y dwarfs (Line et al. 2017; Zalesky et al. 2019),
I present a uniform retrieval analysis of 50 T dwarfs via their low-resolution near infrared
spectra. This analysis more than doubles the sample of T dwarfs with retrieved
properties. I present updates on current compositional trends and thermal
profile constraints amongst the T dwarf population. My analysis shows that my collection
of objects form trends that are consistent with solar grid model expectations
for water, ammonia, methane, and potassium. I also establish a consistency between
the thermal structures of my objects with those of grid models. Moreover, I explore
the origin of gravity-metallicity discrepancies that are observed in some of my brown
dwarf candidates.
Date Created
2020
Agent

Following the cosmic evolution of the pristine gas: Pop III star formation and the first galaxies

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Description
The formation of the firsts stars some 100-300 Myr after the Big Bang marked the end of the cosmic darks ages and created the elemental building blocks of not only rocky planets but eventually us. Understanding their formation, lifetimes, and…

The formation of the firsts stars some 100-300 Myr after the Big Bang marked the end of the cosmic darks ages and created the elemental building blocks of not only rocky planets but eventually us. Understanding their formation, lifetimes, and contributions to the evolution of our universe is one of the current frontiers in astronomy and astrophysics.

In this work I present an improved model for following the formation of Pop III stars, their effects on early galaxy evolution, and how we might search for them. I make use of a new subgrid model of turbulent mixing to accurately follow the time scales required to mix supernova (SN) ejecta -- enriched with heavy elements -- into the pristine gas. I implement this model within a large-scale cosmological simulation and follow the fraction of gas with metallicity below a critical value marking the boundary between Pop III and metal enriched Population II (Pop II) star formation. I demonstrate that accounting for subgrid mixing results in a Pop III stars formation rate that is 2-3 times higher than standard models with the same physical resolution.

I also implement and track a new "Primordial metals" (PM) scalar that tracks the metals generated by Pop III SNe. These metals are taken up by second generation stars and likely result in a subclass of carbon-enhanced, metal-poor (CEMP) stars. By tracking both regular metals and PM, I can model, in post-processing, the elemental abundances of simulation stars. I find good agreement between observations of CEMP-no Milky Way halo stars and second generation stars within the simulation when assuming the first stars had a typical mass of 60 M☉, providing clues as to the Pop III initial mass function.
Date Created
2018
Agent

Interstellar Landscapes: Landscapes of Carbon and Magnesium Planets

Description
How do we visualize environments outside our solar system? I have researched two very alien planets and their compositions with the goal of finding out how those differences would affect the way a planet appears on its surface. The first…

How do we visualize environments outside our solar system? I have researched two very alien planets and their compositions with the goal of finding out how those differences would affect the way a planet appears on its surface. The first is a planet orbiting the nearby G type star Tau Ceti. This star has Mg/Si ratio of 1.78, compared to 1.2 found on the Earth. A planet formed around this star could have a very active surface, covered in volcanoes. The other planet is a hypothetical carbon planet that could orbit the star HD 144899. This star has a C/O ratio of 0.8, compared to 0.5 in the Sun. A planet formed here might be comprised mostly of carbides, with a hydrocarbon atmosphere. It would likely be geologically dead, the main forces shaping its surface being meteorites. Both planets, due to their extremes, would likely be barren and lifeless. The results of this project are two digital paintings showcasing my vision of these planets.
Date Created
2015-05
Agent
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