1,721,014 research outputs found
Scaling and Evolution of Stellar Magnetic Activity
No full textAbstract Magnetic activity is a ubiquitous feature of stars with convective outer layers, with implications from stellar evolution to planetary atmospheres. Investigating the mechanisms responsible for the observed stellar activity signals from days to billions of years is important in deepening our understanding of the spatial configurations and temporal patterns of stellar dynamos, including that of the Sun. In this paper, we focus on three problems and their possible solutions. We start with direct field measurements and show how they probe the dependence of magnetic flux and its density on stellar properties and activity indicators. Next, we review the current state-of-the-art in physics-based models of photospheric activity patterns and their variation from rotational to activity-cycle timescales. We then outline the current state of understanding in the long-term evolution of stellar dynamos, first by using chromospheric and coronal activity diagnostics, then with model-based implications on magnetic braking, which is the key mechanism by which stars spin down and become inactive as they age. We conclude by discussing possible directions to improve the modeling and analysis of stellar magnetic fields
Rotational Characterization of Tess Stars with Deep Learning
Get PDFRotation is a fundamental property of stars. The Kepler mission revolutionized the field of stellar rotation, delivering periods of over 50,000 stars near the plane of the Milky Way. The distribution of periods revealed unexpected gaps, dips, and edges that cannot be described by current rotational evolution models, demanding new physical explanations. To sharpen the features in the distribution and to disentangle the effects of star formation history, more measurements of rotation are needed across the entire sky. The TESS mission has the potential to probe stellar rotation in millions of stars across the entire sky, but mission systematics—instrumental noise, observing gaps, and changes in detector sensitivity—have prevented recovery of rotation periods longer than 13.7 days. We used deep learning to see through TESS systematics and recover periods from year-long light curves. Our approach uses a training set of synthesized light curves from realistic star spot evolution simulations, with real light curve systematics from quiet TESS stars. Evaluating the network on real TESS data, we estimated reliable periods for 9,837 cool dwarfs. We recovered key features of the Kepler and K2 distributions, including periods up to 60 days. We reproduced the intermediate rotation period gap for the first time using TESS, as well as a dip in photometric activity surrounding it. Combining our TESS rotation periods with spectroscopic temperatures and abundances from APOGEE, we examined the detectability of rotation across fundamental stellar parameters, finding a strong dependence on temperature and age. Using gyrochronology, we inferred masses, ages, and other fundamental properties for the 6,632 TESS stars with APOGEE spectroscopy and corroborated evolution trends of Galactic chemistry and magnetic activity seen with Kepler. Finally with new measurements of spot filling factor from APOGEE, we investigated the spottedness of stars across the period distribution. We found that stars exhibit elevated spot fractions in the same regime where magnetic braking temporarily stalls in young open cluster stars, suggesting a common cause. Now with the ability to estimate rotation periods, including long periods, across the entire sky, we can characterize stars along many more lines of sight than before, enabling detailed study of the Galaxy's stellar populations.Ph.D
Spinning Red Clocks in Crowded Fields
Get PDFTo build a clear picture of the history of our Milky Way galaxy we need to be able to reliably measure the ages of the stars within it. Unfortunately the most common stars, M dwarfs, are resistant to most of our current age measuring techniques.This leaves gyrochronology, the use of an empirical relation between rotation and age, as our method for measuring M dwarf ages. Successful gyrochronology requires the study of open clusters spanning as wide a range of ages and metallicities as possible. In this dissertation I present the work I have done on advancing our understanding of spin-down in late K and early M dwarfs through observations of the 4 Gyr old open cluster M67. Future studies that include a wider sample of open clusters, and that push the observations to lower mass stars, will require high spatial resolution wide-field imaging to remain feasible. To this end, this dissertation also presents the work I have done in developing an advanced technique for the control systems of ground layer adaptive optics. This technique, which we have dubbed “temporal tomography” is expected to facilitate the widespread adoption of ground layer adaptive optics by reducing the number of guide stars needed to obtain accurate estimates of the ground layer turbulence. These systems will be capable of imaging crowded fields, such as the open clusters needed for calibrating gyrochronology.Ph.D
Precise Demographics of Kepler Exoplanets in the Gaia Era
Get PDFA major bottleneck for transiting exoplanet demographics has been the lack of precise properties for most of the observed stars, as the transit method measures exoplanet radii relative to their host's radii. We live in a golden era of host star characterization because of access to Gaia photometry, parallaxes, and proper motions, large-scale spectroscopic surveys, and ground-based photometric and spectroscopic follow-up. I have used all of this data to sharpen our view of exoplanet demographics. First, I constrained the stellar radii of Kepler targets using Gaia DR2, which allowed the first comprehensive classification of main sequence, subgiant, and giant stars in Kepler target sample; I also identified ~4000 low-mass main sequence binary systems. With these precise stellar radii, I was the first to use Gaia to revise planet radii and incident fluxes and corroborate the existence of the planet radius gap. I discovered planets within the hot sub-Neptunian desert (2.2–3.8 Earth radii, > 650 Earth fluxes), presented an updated census of habitable zone planets, and identified a hot Jupiter inflation trend for Kepler planets. I also performed isochrone modeling for the entire Kepler target sample and produced the Gaia-Kepler Stellar Properties Catalog, the first homogeneous catalog to include stellar ages, in addition to precise radii, masses, and mean stellar densities for Kepler target stars. Using these homogeneously derived properties, I found the first observational evidence of a stellar age dependence of the planet radius gap, where sub-Neptunes (1.8–3.5 Earth radii) become super-Earths (1.0–1.8 Earth radii) on roughly Gyr timescales. This result built upon my previous work, where I measured lithium abundances to separate old and young Kepler stars (using the Hyades's empirical A(Li)-effective temperature isochrone at ~650 Myr) and discovered that the young planets were statistically larger than the old planets. In addition, I investigated the stellar mass dependence of the planet radius valley and provided stringent constraints that will be required to discern between the theories of core-powered mass-loss and photoevaporation. I also confirmed the existence of planets within the hot sub-Neptunian desert, discovered that most desert planets entered recently because of their host's evolution, investigated Jupiters at low incident fluxes with radii larger than the theoretical maximum, and demonstrated that planets in single and multiple transiting systems share the same age distribution.Ph.D
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Next Generation Stellar Models with Rotation
No full textThis thesis details a line of work, carried out over several years, that has seen the development of a next generation, self-consistently evolved, rotating stellar evolution model grid. The forthcoming chapters describe their application in studying various observed phenomena. Stellar rotation is the third fundamental parameter of stellar evolution, alongside mass and metallicity, that sets the evolutionary track of a star. Thus, accurately modeling stellar rotation is essential in accurately predicting stellar behavior, and interpreting stellar data. The model grid developed in this thesis is one of only several constructed thus far in order to incorporate, and study the effects of stellar rotation on evolution.
In Chapter 2, we describe the initial grid that was created to benchmark stellar behavior, as modeled in the 1D stellar evolution code MESA using well-studied, nearby stellar populations. We applied our models to the Pleiades, the Hyades, and the Praesepe galactic open clusters to derive cluster ages and metalicities. We found, contrary to a contemporaneous study carried out with an alternative model set, that stellar rotation did not appear to greatly affect the derived cluster properties in comparison to previous studies that used non-rotating stellar models. Thus, our results highlighted the uncertainty present due to unconstrained physics in stellar models, primarily the assumptions made in modeling stellar rotation, and convective mixing. We also demonstrated that the dominant effect of stellar rotation appears to be gravity darkening in our MESA models, versus main sequence lifetime extension (due to rotation-enhanced mixing) in the alternative model set. These model discrepancies necessitate further studies aimed at constraining the effects of stellar rotation (in addition to other uncertain aspects of stellar evolution).
In Chapter 3 we extended the application of our models to study the extended main sequence turn off phenomenon, observed in all open clusters younger than about 2 Gyr. Our results confirm previous findings that it appears stellar rotation distributions are responsible for this phenomenon, but can not fully rule out an age spread in our modeling. While an combination of an age spread and rotation rate distribution often is the most statistically favorable model in replicating observations on color-magnitude diagrams, age spreads also introduce features that are not observed. In addition, rotation rate distribution appear to be able to account for the majority of the extended main sequence turn off morphology, even in absence of an age spread. The primary means by which stellar rotation creates an extended main sequence turn off is by gravity darkening according to our models. Our models also manage to simultaneously replicate the observed split main sequence (a feature common to open clusters younger than about 400 Myr) in the cluster NGC 1866. This provides strong evidence for stellar rotation being a primary factor in explaining these stellar population phenomena.
In Chapter 4 we describe improvements made to our models in modeling the rotation behavior of stellar masses . In our previous studies, modeling stars in this mass range was not done well, meaning that our grid had been limited in not being able to model rotation effects, e.g., in clusters aged greater than about 1.7 Gyr, which is roughly when stars of mass enter the main sequence turn off in our modeling. The work presented in Chapter 4 implemented two options for modeling the angular momentum evolution of stars : those of \cite{matt.etal:2015} and \cite{garraffo.etal:2018}. These stars require special treatment because stars in this mass range begin to develop surface convection zones, and surface magnetic fields. These magnetic fields couple to outgoing stellar winds and gradually slow the star in a process known as magnetic braking. In this work, we tested and compared our newly implemented magnetic braking models, finding that both fail to completely reproduce observed rotation periods in open cluster, but do provide some successes as well. Our results are, broadly speaking, in line with other stellar evolution models that employ the same magnetic braking formalisms. Thus, both of the implemented braking models serve as incomplete, but promising means of modeling the rotation rate evolution of stars with convective envelopes. We also check for our models ability to replicate the observed lithium depletion of solar-like stars, and are not able to reproduce it, indicating additional, missing physical processes are likely required
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Using Modern Stellar Observables to Constrain Stellar Parameters and the Physics of the Stellar Interior
No full textVariations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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