283 research outputs found
Code to reproduce Ranjan et al. (2017), ApJ, submitted
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<p>This repository holds all the code, auxiliary files, and intermediate files used in Ranjan, Wordsworth & Sasselov 2017b. Using this material, one can replicate every one of the figures used in our paper, as well as run models of one's own formulation.<br>
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The anticipated use cases are:<br>
1. Testing/validating the findings of Ranjan, Wordsworth & Sasselov (2017)<br>
2. Using the Ranjan, Wordsworth & Sasselov radiative transfer model to compute UV surface fluence for user-defined atmospheres and surfaces.<br>
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If making use of this code or the generated auxiliary files, please cite the descriptor paper:<br>
Ranjan, S, Wordsworth, Robin D. and Sasselov, Dimitar D. "The Surface UV Environment on Prebiotic Planets Orbiting M-dwarfs: Implications for Prebiotic Chemistry & Need for Experimental Follow-Up". Submitted to the Astrophysical Journal (2017). arXiv: 1705.02350<br>
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<p>This version represents the state of the code at the time of paper acceptance.</p>If making use of this code or the generated auxiliary files, please cite the descriptor paper: Ranjan, S, Wordsworth, Robin D. and Sasselov, Dimitar D. "The Surface UV Environment on Prebiotic Planets Orbiting M-dwarfs: Implications for Prebiotic Chemistry & Need for Experimental Follow-Up". Submitted to the Astrophysical Journal (2017). arXiv: 1705.0235
The mass of Kepler-93b and the composition of terrestrial planets
No full textKepler-93b is a 1.478 ± 0.019 R ⊕ planet with a 4.7 day period around a bright (V = 10.2), astroseismically characterized host star with a mass of 0.911 ± 0.033 M ☉ and a radius of 0.919 ± 0.011 R ☉. Based on 86 radial velocity observations obtained with the HARPS-N spectrograph on the Telescopio Nazionale Galileo and 32 archival Keck/HIRES observations, we present a precise mass estimate of 4.02 ± 0.68 M ⊕. The corresponding high density of 6.88 ± 1.18 g cm–3 is consistent with a rocky composition of primarily iron and magnesium silicate. We compare Kepler-93b to other dense planets with well-constrained parameters and find that between 1 and 6 M ⊕, all dense planets including the Earth and Venus are well-described by the same fixed ratio of iron to magnesium silicate. There are as of yet no examples of such planets with masses >6 M ⊕. All known planets in this mass regime have lower densities requiring significant fractions of volatiles or H/He gas. We also constrain the mass and period of the outer companion in the Kepler-93 system from the long-term radial velocity trend and archival adaptive optics images. As the sample of dense planets with well-constrained masses and radii continues to grow, we will be able to test whether the fixed compositional model found for the seven dense planets considered in this paper extends to the full population of 1-6 M ⊕ planets.Peer reviewe
The Kepler-454 system : A small, not-rocky inner planet, a Jovian world, and a distant companion
No full textKepler-454 (KOI-273) is a relatively bright (V = 11.69 mag), Sun-like starthat hosts a transiting planet candidate in a 10.6 d orbit. From spectroscopy, we estimate the stellar temperature to be 5687 +/- 50 K, its metallicity to be [m/H] = 0.32 +/- 0.08, and the projected rotational velocity to be v sin i 10 years and mass >12.1M_J . The twelve exoplanets with radii <2.7 R_Earth and precise mass measurements appear to fall into two populations, with those <1.6 R_Earth following an Earth-like composition curve and larger planets requiring a significant fraction of volatiles. With a density of 2.76 +/- 0.73 g cm-3, Kepler-454b lies near the mass transition between these two populations and requires the presence of volatiles and/or H/He gas.Peer reviewe
A Precise Estimate of the Radius of HD 149026b
No full textWe present Spitzer 8 μm transit observations of the extrasolar planet system HD 149026b. At this wavelength, transit light curves are weakly affected by stellar limb-darkening, allowing for a simpler and more accurate determination of planetary parameters. We measure a planet-star radius ratio of R_p/R∗=0.05158±0.00077, and in combination with ground-based data and independent constraints on the stellar mass and radius, we derive an orbital inclination of i = 85°.4^(+0°.9)_(−0°.8) and a planet radius of 0.755 ± 0.040 R_J. These measurements further support models in which the planet is greatly enriched in heavy elements
Characterizing the Atmospheres of Hot Jupiters: From Spectra to Multi-Color Maps
No full textWe present new observations of the emission spectrum of the hot Jupiter TrES-4 designed
to test the theory that the presence of temperature inversions in the atmospheres of these
planets are correlated with the amount of radiation received by the planet. Our observations
reveal that TrES-4 has an emission spectrum similar to that of HD 209458b, which requires
the presence of an inversion layer high in the atmosphere and water emission bands in order to
explain the observed features, providing additional support for that theory. We also present new
observations of the thermal phase curve of HD 189733b at 24 μm, which we combine with our
previous observations at 8 μm to examine how circulation in this planet’s atmosphere varies as a
function of depth. We discuss the relationship between the strength of the day-night circulation
on both planets and their other observable properties, in particular their emission spectra
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Theoretical Investigations of the Water Cycle on Earth and Other Planets
No full textFrom exoplanets to Solar System bodies to modern Earth, the multi-scale and interconnected processes of the water cycle are fundamental drivers of planetary climate, evolution, and habitability. This thesis confronts problems stemming from the water cycle’s complexity across diverse planetary environments from a theoretical perspective. I construct simplified representations of more complicated systems within planetary water cycles to infer observable consequences of system behavior, to elucidate fundamental controls on system behavior, and to parameterize system behavior. I begin by using the coupling between the water cycle and the sulfur cycle to propose two new observational diagnostics for the absence of an exoplanet ocean—a challenging but highly desirable observable for constraining the prevalence of Earth-like worlds. Next, from a generalized planetary perspective, I use the simplicity of how raindrops fall and evaporate to place constraints on aspects of cloud evolution independent of the complex processes governing raindrop growth. I demonstrate across broad planetary conditions that raindrop size is the predominant determiner of raindrop ability to vertically transport condensed mass (i.e., precipitate) and that a new non-dimensional number can capture the fundamental behavior of falling raindrops. Finally, from a modern-Earth perspective, I consider the initiation of rain via liquid drop coagulation (i.e., collision and subsequent coalescence). I document the parameterizations of coagulation in global climate models participating in the most recent phase of the Coupled Model Intercomparison Project (CMIP6)—representing the world’s most comprehensive attempts to model modern-Earth climate. These coagulation parameterizations share five conceptual assumptions that I demonstrate lead to too rapid rain initiation in a manner consistent with a widespread and longstanding global model bias predicting too frequent precipitation relative to observations. To address the deficient conceptual assumptions underlying the CMIP6 coagulation parameterizations, I design and implement three approaches (two novel) for parameterizing coagulation that show improved rain initiation timing relative to the CMIP6-based approaches in an idealized test. Overall, the work of this thesis highlights the productivity of a comparative planetology approach for studying the water cycle
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The Tierras Observatory: An Ultra-precise Time-series Photometer to Characterize Nearby Low-mass Stars and Their Terrestrial Exoplanets
No full textAlthough the study of exoplanets has seen dramatic advances in the past decade, the analogs of many of the denizens of our own Solar system remain beyond the grasp of our current observatories. Most of the known terrestrial exoplanets are significantly larger than the Earth, and it remains an open question whether Mars and Mercury-sized worlds are common. No exo-satellites or exo-rings have been discovered to-date. And, we've only begun to characterize the long term variability of stars and its impact on their attendant worlds. Time-series photometry is the path to significant progress on all these questions, but we are limited by the photometric precision of our observatories.
In this thesis, I describe the design, construction, commissioning, on-sky performance, and first science results of the \textit{Tierras} Observatory, a refurbished 1.3-m ultra-precise fully-automated photometer designed to further our understanding of terrestrial exoplanets, exo-satellites, and the variability of mid-to-late M dwarf stars. The observatory re-utilizes the 28-year-old 2MASS North telescope, located at the Fred Lawrence Whipple Observatory atop Mount Hopkins, Arizona. \textit{Tierras} is designed to regularly achieve a photometric precision of 250 ppm on a time scale of both 10 minutes and across an observing season. I begin this thesis by detailing the five design choices that allow \textit{Tierras} to achieve 250 ppm precision per 10-min bins, and recounting their execution. \textit{First} is the optical and opto-mechanical design, alignment, bonding, and deployment of a four-lens focal reducer and field-flattener, which increases the field-of-view of the 1.3-m telescope from 0.19 degrees to 0.48 degrees on a side and allows us to accommodate an optimal number of comparison stars in the field and correct for atmospheric variation effects. \textit{Second} is a custom narrow bandpass filter (FWHM 40 nm) centered around 863.5 nm to minimize precipitable water vapor errors known to limit state-of-the-art ground-based photometers targeting M dwarf stars. \textit{Third} is a deep-depletion CCD with a quantum efficiency of 90\% in the \textit{Tierras} bandpass, operating in shutter-less, frame-transfer mode to minimize readout time and readout noise, while maximizing our time gathering science data. The CCD is housed inside a custom dewar, and cooled to C via a cryo-cooler requiring a single annual refill. \textit{Fourth}, is a fully automated mode of operation, enabled by a fully refurbished telescope and dome that we retrofitted with absolute position encoders, a modern telescope control software, a new integrated (telescope and camera) control interface, and a fully operational observing robot. \textit{Fifth} is a custom set of baffles, designed and built in-house, which reduce sky background by two orders of magnitude.
In the second part of this thesis, I summarize the \textit{Tierras} Observatory commissioning effort, present the results of our first year of science observations, and quantify our photometric precision as a function of bin size, and across the 2022-2023 observing season. I show that \textit{Tierras} is already delivering 250-ppm precision light curves (in 10 min bins) for M dwarf stars of varied spectral types, and outline current and future efforts to improve the instrument's long-term photometric stability further. I conclude with a description of the observational efforts we plan to undertake during the next 3-5 years. We will survey stars known to host a transiting planet to search for additional terrestrial planets too small or too cool to be detected by TESS or other ground-based facilities; monitor known exoplanets (both terrestrials and jovians) at longer orbital periods to search for satellites or systems of circumstellar rings; and undertake a long term monitoring campaign to determine hitherto unmeasured rotation periods of M dwarf stars in a volume-complete sample
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A New Spin on the Origin of Biological Homochirality
No full textEssential molecules of life—amino acids, nucleic acids, and sugars—are chiral; they exist in mirror-symmetrical pairs. However, biological systems exclusively use only one form of these pairs: right-handed sugars and nucleic acids, along with left-handed amino acids. This phenomenon characterizes life as homochiral. However, the origins of this asymmetry remain elusive, and it is this long-standing mystery that we address in this thesis.
The chiral-induced spin selectivity (CISS) effect has established a strong coupling between electron spin and molecular chirality and this coupling paves the way for breaking the chiral molecular symmetry by spin-selective processes. Achiral magnetic surfaces, when spin-polarized, can function as chiral agents due to the CISS effect, serving as templates for the asymmetric crystallization of chiral molecules.
In this thesis, we studied the spin-selective crystallization of racemic ribo-aminooxazoline (RAO), a central precursor of RNA, on magnetite surfaces—achieving homochirality in two crystallization steps. Moreover, we have shown the chirality-induced avalanche magnetization of magnetite by RAO molecules, which verifies the reciprocal nature of the effect and allows for a cooperative feedback between chiral molecules and magnetic surfaces. Finally, based on empirical evidence, we propose a pathway through which the achieved homochirality in a single chiral compound, RAO, can efficiently propagate throughout the entire prebiotic network, starting from D-nucleic acids, to L-peptides, and then to homochiral metabolites.
Our results demonstrate a prebiotically plausible way of achieving systems-level homochirality from completely racemic starting materials through a process initiated by the physical environment
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Towards Building a Self-replicating Protocell: Nonenzymatic RNA Copying Driven by Potentially Prebiotic Activation and Higher-order Behaviors in Prebiotic Vesicles
No full textThe emergence of a protocell capable of Darwinian evolution would require replicating compartments and genetic materials. Compartmentalization keeps informational polymers localized so that the functions they encode lead to an advantage in terms of replication or survival. Meanwhile, a mechanism for the inheritance of useful functions must exist. RNA and its close relatives are attractive candidate prebiotic molecules because they harbor the capacity for both inheritance and function in a single class of molecules. In this RNA world model, RNA polymers assemble from activated nucleotides, and when RNA oligomers of adequate lengths lead to ribozymes, RNA can begin to facilitate its own reproduction (Gilbert 1986). The propagation of functional RNAs within reproducing compartments leads to replicating protocells, ultimately giving rise to modern cellular life. My dissertation work explores both chemical and physical processes relevant to the transition from chemical to biological evolution on the early earth.
The hypothesized central role of RNA in the origin of life suggests that RNA propagation predated the advent of complex protein enzymes. Such a process in turn requires a source of chemical energy for in situ nucleotide activation. Previously identified activation chemistries lead to damaging side reactions that destroy both templates and substrates (Biron and Pascal 2004, Fahrenbach, Giurgiu et al. 2017). A chemical process that can continuously reactivate hydrolyzed substrates and is compatible with chemical RNA copying is needed. A potentially prebiotic nucleotide activation that might be compatible with RNA copying remained elusive for nearly 60 years. I have demonstrated a potentially prebiotic pathway that chemically activates RNA nucleotides in a manner compatible with RNA copying (Zhang, Duzdevich et al. 2020). Following that, I have developed and characterized a prebiotically plausible scenario under which previously isolated key steps of nucleotide activation to nonenzymatic RNA copying can happen together under mutually compatible conditions (Zhang, Duzdevich et al. 2022). This pathway enables more realistic models of RNA propagation.
Early cell membranes are thought to have been composed of fatty acids and related single-chain amphiphiles. Their probable involvement in the origin of life has been further demonstrated by their capability to grow and divide without complex biochemical machinery and encapsulate RNA templates that are being nonenzymatically copied (Hanczyc and Szostak 2004, Budin, Debnath et al. 2012, Adamala and Szostak 2013). However, without complex protein machinery, protocells would have had to rely on passive diffusion for internalizing nutrients. I have shown how primitive cells composed of fatty acids endocytose via a purely physicochemical process upon encountering nutrients in conjunction with extra membrane material. I then found that such inward vesiculation events could lead to internalization of nutrient solutes including mononucleotides and oligonucleotides, drawing further parallels to endocytosis. Such processes could have helped primitive cells capture nutrients that are otherwise impermeable.
Taken together, these results demonstrate prebiotically plausible pathways from nucleotides to RNA copying as well as a scenario in which protocells internalize useful nutrients bypassing permeability limits, bringing us closer to developing primitive cells capable of Darwinian evolution
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All Features Great and Small: Distinguishing the effects of specific magnetically active features on radial-velocity exoplanet detections
No full textState of the art radial velocity (RV) exoplanet searches are limited by the effects of stellar magnetic activity. Magnetically active features, such as spots, plage, and network regions, each contribute to the observed RV shift through a variety of mechanisms, including via the suppression of convective blueshift and by creating rotational imbalance due to brightness inhomogeneities. However, the extent to which these RV contributions depend on the specific properties of individual active regions remains unknown. In this work, we investigate the effects of active region size on activity-driven RV variations, and develop tools for modelling these RV variations on Sun-like stars. We analyze solar observations acquired over Carrington Cycle 24 to test models of stellar magnetic activity and the resulting RV variations of Sun-like stars: we compare direct measurements of solar plage, spots, and network using the Solar Dynamics Observatory (SDO) to measurements of the solar RV and S-index from the solar telescope at the High Accuracy Radial velocity Planet Searcher for the Northern hemisphere (HARPS-N), solar photometry from the Solar Radiation and Climate Experiment (SORCE), and variations in the Sun's acoustic oscillation frequencies from the Birmingham Solar-Oscillations Network (BiSON). By comparing estimates of the contributions of the suppression of convective blueshift and the rotational flux imbalance derived from SDO images to the HARPS-N solar RVs, we find that that magnetic active regions smaller than 60 Mm^2 do not significantly suppress convective blueshift. Differentiating the relative coverage, or filling factors, of these large plage regions from small network regions is thus necessary to differentiate between activity-driven RV signatures and Doppler shifts due to planetary orbits.
We then investigate several methods for extracting the relative coverage, or filling factors, of spots, plage, and network features. We demonstrate that variations in the solar p-mode frequency are highly sensitive to the presence of large plage regions, and are significantly less sensitive to smaller network regions, making the p-mode frequency a possible — though observsationally intensive — avenue for differentiating the contributions of different classes of active regions. We then develop a technique to estimate feature-specific magnetic filling factors on stellar targets using only spectroscopic and photometric observations. Linear and machine learning implementations of this technique both yield filling factor estimates that are highly correlated with the observed values. Modeling the solar RVs using these filling factors reproduces the expected contributions of the suppression of convective blueshift and rotational imbalance due to brightness inhomogeneities, providing an avenue for estimating these RV contributions on well-observed, Sun-like stars
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