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Kenneth G. Carpenter

Publications and source records attributed to Kenneth G. Carpenter.

18 recordsLinked to original sources
arXiv2025

Lyman-Alpha Emission from K and M Dwarfs: Intrinsic Profiles, Variability, and Flux in the Habitable Zone

Lyman-$\alpha$ (Ly$\alpha$) is the most prominent ultraviolet emission line in low-mass stars, playing a crucial role in exoplanet atmospheric photochemistry, heating, and escape. However, interstellar medium (ISM) absorption typically obscures most of the Ly$\alpha$ profile, requiring reconstructions that introduce systematic uncertainties. We present intrinsic Ly$\alpha$ profiles for 12 high radial velocity K and M dwarfs, where Doppler shifting minimizes ISM contamination, allowing direct measurements of $\sim$50-95\% of the line flux. Our sample spans the K-to-M spectral transition, enabling us to constrain the dependence of self-reversals in Ly$\alpha$ emission profiles on effective temperature ($T_{eff}$). The depth of self-reversal, driven by non-local thermodynamic equilibrium (LTE) effects, decreases with decreasing $T_{eff}$, with M dwarfs exhibiting little to none. Two stars, Ross 1044 and Ross 451, were observed over multiple days, revealing $\sim$20\% Ly$\alpha$ variability confined to the line core - implying that studies relying on reconstructions may underestimate temporal variability. We find strong correlations between Ly$\alpha$ flux, peak-to-trough ratio, and hydrogen departure coefficients with $T_{eff}$, providing empirical constraints for stellar atmosphere models. A comparison of Ly$\alpha$ flux in the habitable zone shows measured values for high radial velocity stars less than the reconstructed values for the rest of the sample, likely due to the older ages of the high-RV stars and/or overestimated reconstructed fluxes due to model deficiency (e.g., neglecting self-reversal). Our results establish an empirical foundation for Ly$\alpha$ emission in K and M dwarfs, reducing uncertainties in reconstructions and improving models of stellar UV emission relevant to exoplanetary studies.

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arXiv2025

NASA Innovative Advanced Concepts Phase I Final Report -- A Lunar Long-Baseline UV/Optical Imaging Interferometer: Artemis-enabled Stellar Imager (AeSI)

This report presents the findings of a NIAC Phase I feasibility study for the Artemis-enabled Stellar Imager (AeSI), a proposed high-resolution, UV/Optical interferometer designed for deployment on the lunar surface. Its primary science goal is to image the surfaces and interiors of stars with unprecedented detail, revealing new details about their magnetic processes and dynamic evolution and enabling the creation of a truly predictive solar/stellar dynamo model. This capability will transform our understanding of stellar physics and has broad applicability across astrophysics, from resolving the cores of Active Galactic Nuclei (AGN) to studying supernovae, planetary nebulae, and the late stages of stellar evolution. By leveraging the stable vacuum environment of the Moon and the infrastructure being established for the Artemis Program, AeSI presents a compelling case for a lunar-based interferometer. In this study, the AeSI Team, working with the NASA Goddard Space Flight Center's Integrated Design Center (IDC), has firmly established the feasibility of building and operating a reconfigurable, dispersed aperture telescope (i.e., an interferometer) on the lunar surface. The collaboration produced a credible Baseline design featuring 15 primary mirrors arranged in an elliptical array with a 1 km major axis, with the potential to expand to 30 mirrors and larger array sizes through staged deployments. Additionally, this study identified numerous opportunities for optimization and the necessary trade studies to refine the design further. These will be pursued in follow-up investigations, such as a NIAC Phase II study, to advance the concept toward implementation.

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arXiv2024

Artemis-enabled Stellar Imager (AeSI): A Lunar Long-Baseline UV/Optical Imaging Interferometer

NASA's return to the Moon presents unparalleled opportunities to advance high-impact scientific capabilities. At the cutting edge of these possibilities are extremely high-resolution interferometric observations at visible and ultraviolet wavelengths. Such technology can resolve the surfaces of stars, explore the inner accretion disks of nascent stars and black holes, and eventually enable us to observe surface features and weather patterns on nearby exoplanets. We have been awarded Phase 1 support from NASA's Innovative Advanced Concepts (NIAC) program to explore the feasibility of constructing a high-resolution, long-baseline UV/optical imaging interferometer on the lunar surface, in conjunction with the Artemis Program. A 1996 study comparing interferometers on the Moon versus free-flyers in space concluded that, without pre-existing lunar infrastructure, free-flyers were preferable. However, with the advent of the Artemis Program, it is now crucial to revisit the potential of building lunar interferometers. Our objective is to conduct a study with the same level of rigor applied to large baseline, free-flying interferometers during the 2003-2005 NASA Vision Missions Studies. This preparation is essential for timely and effective utilization of the forthcoming lunar infrastructure. In this paper, we highlight the groundbreaking potential of a lunar surface-based interferometer. This concept study will be a huge step forward to larger arrays on both the moon and free-flying in space, over a wide variety of wavelengths and science topics. Our Phase 1 study began in April 2024, and here we present a concise overview of our vision and the progress made so far.

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arXiv2019

Stars at High Spatial Resolution

We summarize some of the compelling new scientific opportunities for understanding stars and stellar systems that can be enabled by sub-milliarcsec (sub-mas) angular resolution, UV-Optical spectral imaging observations, which can reveal the details of the many dynamic processes (e.g., evolving magnetic fields, accretion, convection, shocks, pulsations, winds, and jets) that affect stellar formation, structure, and evolution. These observations can only be provided by long-baseline interferometers or sparse aperture telescopes in space, since the aperture diameters required are in excess of 500 m (a regime in which monolithic or segmented designs are not and will not be feasible) and since they require observations at wavelengths (UV) not accessible from the ground. Such observational capabilities would enable tremendous gains in our understanding of the individual stars and stellar systems that are the building blocks of our Universe and which serve as the hosts for life throughout the Cosmos.

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arXiv2019

Constraining stellar parameters and atmospheric dynamics of the carbon AGB star V Oph

Molecules and dust produced by the atmospheres of cool evolved stars contribute to a significant amount of the total material found in the interstellar medium. To understand the mechanism behind the mass loss of these stars, it is of pivotal importance to investigate the structure and dynamics of their atmospheres. Our goal is to verify if the extended molecular and dust layers of the carbon-rich asymptotic giant branch (AGB) star V Oph, and their time variations, can be explained by dust-driven winds triggered by stellar pulsation alone, or if other mechanisms are operating. We model V Oph mid-infrared interferometric VLTI-MIDI data ($8$-$13~μ$m), at phases $0.18$, $0.49$, $0.65$, together with literature photometric data, using the latest-generation self-consistent dynamic atmosphere models for carbon-rich stars: DARWIN. We determine the fundamental stellar parameters: $T_\text{eff} = 2600~$K, $L_\text{bol} = 3585~$L$_{\odot}$, $M = 1.5~$M$_{\odot}$, $C/O = 1.35$, $\dot{M} = 2.50\cdot10^{-6}$M$_{\odot}$/yr. We calculate the stellar photospheric radii at the three phases: $479$, $494$, $448$ R$_{\odot}$; and the dust radii: $780$, $853$, $787$ R$_{\odot}$. The dynamic models can fairly explain the observed $N$-band visibility and spectra, although there is some discrepancy between the data and the models, which is discussed in the text. We discuss the possible causes of the temporal variations of the outer atmosphere, deriving an estimate of the magnetic field strength, and computing upper limits for the Alfvén waves velocity. In addition, using period-luminosity sequences, and interferometric modeling, we suggest V Oph as a candidate to be reclassified as a semi-regular star.

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arXiv2019

Cool, evolved stars: results, challenges, and promises for the next decade

Cool, evolved stars are the main source of chemical enrichment of the interstellar medium (ISM), and understanding their mass loss and structure offers a unique opportunity to study the cycle of matter in the Universe. Pulsation, convection, and other dynamic processes in cool evolved stars create an atmosphere where molecules and dust can form, including those necessary to the formation of life (e.g.~Carbon-bearing molecules). Understanding the structure and composition of these stars is thus vital to several aspects of stellar astrophysics, ranging from ISM studies to modeling young galaxies and exoplanet research. Recent modeling efforts and increasingly precise observations now reveal that our understanding of cool stars photospheric, chromospheric, and atmospheric structures is limited by inadequate knowledge of the dynamic and chemical processes at work. Here we outline promising scientific opportunities for the next decade. We identify and discuss the following main opportunities: (1) identify and model the physical processes that must be included in current 1D and 3D atmosphere models of cool, evolved stars; (2) refine our understanding of photospheric, chromospheric, and outer atmospheric regions of cool evolved stars, their properties and parameters, through high-resolution spectroscopic observations, and interferometric observations at high angular resolution; (3) include the neglected role of chromospheric activity in the mass loss process of red giant branch and red super giant stars, and understand the role played by their magnetic fields; (4) identify the important shaping mechanisms for planetary nebulae and their relation with the parent asymptotic giant branch stars.

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arXiv2019

High Angular Resolution Astrophysics: Resolving Stellar Surface Features

We are now in an era where we can image details on the surfaces of stars. When resolving stellar surfaces, we see that every surface is uniquely complicated. Each imaged star provides insight into not only the stellar surface structures, but also the stellar interiors suggesting constraints on evolution and dynamo models. As more resources become operational in the coming years, imaging stellar surfaces should become commonplace for revealing the true nature of stars. Here, we discuss the main types of stars for which imaging surface features is currently useful and what improved observing techniques would provide for imaging stellar surface features.

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arXiv2018

The Advanced Spectral Library (ASTRAL): Reference Spectra for Evolved M-Stars

The HST Treasury Program "Advanced Spectral Library Project: Cool Stars" was designed to collect representative, high quality ultraviolet spectra of eight evolved F-M type cool stars. The Space Telescope Imaging Spectrograph (STIS) echelle spectra of these objects enable investigations of a broad range of topics including stellar and interstellar astrophysics. This paper provides a guide to the spectra of the two evolved M-stars, the M2Iab supergiant Alpha Ori and the M3.4 giant Gamma Cru, with comparisons to the prototypical K1.5 giant Alpha Boo. It includes identifications of the significant atomic and molecular emission and absorption features and discusses the character of the photospheric and chromospheric continua and line spectra. The fluorescent processes responsible for a large portion of the emission line spectrum, the characteristics of the stellar winds, and the available diagnostics for hot and cool plasmas are also summarized. This analysis will facilitate the future study of the spectra, outer atmospheres, and winds, not only of these objects, but for numerous other cool, low-gravity stars for years to come.

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arXiv2018

GHRS Observations of Cool, Low-Gravity Stars. VI Mass-Loss Rates and Wind Parameters for M Giants

The photon-scattering winds of M-giants absorb parts of the chromospheric emission lines and produce self-reversed spectral features in high resolution {\it HST}/GHRS spectra. These spectra provide an opportunity to assess fundamental parameters of the wind, including flow and turbulent velocities, the optical depth of the wind above the region of photon creation, and the star's mass-loss rate. This paper is the last paper in the series `GHRS Observations of Cool, Low-Gravity Stars'; the last several have compared empirical measurements of spectral emission lines with models of the winds and mass-loss of K-giant and supergiants. We have used the Sobolev with Exact Integration (SEI) radiative transfer code, along with simple models of the outer atmosphere and wind, to determine and compare the wind characteristics of the two M-giant stars, $γ$~Cru (M3.5III) and $μ$~Gem (M3IIIab), with previously derived values for low-gravity K-stars. The analysis specifies the wind parameters and calculates line profiles for the \ion{Mg}{2} resonance lines, in addition to a range of unblended \ion{Fe}{2} lines. Our line sample covers a large range of wind opacities and, therefore, probes a range of heights in the atmosphere. Our results show that $μ$~Gem has a slower and more turbulent wind then $γ$~Cru. Also, $μ$~Gem has weaker chromosphere, in terms of surface flux, with respect to $γ$~Cru. This suggests that $μ$~Gem is more evolved than $γ$~Cru. Comparing the two M-giants in this work with previously studied K-giant and supergiant stars ($α$~Tau, $γ$~Dra, $λ$~Vel) reveals that the M-giants have slower winds than the earlier giants, but exhibit higher mass-loss rates. Our results are interpreted in the context of the winds being driven by Alfvén waves.

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arXiv2018

The Mass of the Cepheid V350 Sgr

V350 Sgr is a classical Cepheid suitable for mass determination. It has a hot companion which is prominent in the ultraviolet and which is not itself a binary. We have obtained two high resolution echelle spectra of the companion at orbital velocity maximum and minimum with the Hubble Space Telescope (HST) Space Telescope Imaging Spectrograph (STIS) in the 1320 to 1510 Å\/ region. By cross-correlating these spectra we obtained the orbital velocity amplitude of the companion with an uncertainty in the companion amplitude of 1.9 km sec$^{-1}$. This provides a mass ratio of the Cepheid to the companion of 2.1. The ultraviolet energy distribution of the companion provides the mass of the companion, yielding a Cepheid mass of 5.2 $\pm$ 0.3 M$_\odot$. This mass requires some combination of moderate main sequence core convective overshoot and rotation to match evolutionary tracks.

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arXiv2015

Lithium in Open Cluster Red Giants Hosting Substellar Companions

We have measured stellar parameters, [Fe/H], lithium abundances, rotation, and 12C/13C in a small sample of red giants in three open clusters that are each home to a red giant star that hosts a substellar companion (NGC2423 3, NGC4349 127, and BD+12 1917 in M67). Our goal is to explore whether the presence of substellar companions influences the Li content. Both 12C/13C and stellar rotation are measured as additional tracers of stellar mixing. One of the companion hosts, NGC2423 3, is found to be Li-rich with A(Li)_NLTE=1.56 dex, and this abundance is significantly higher than the A(Li) of the two comparison stars in NGC2423. All three substellar companion hosts have the highest A(Li) and 12C/13C when compared to the control red giants in their respective clusters; however, except for NGC2423 3, at least one control star has similarly high abundances within the uncertainties. Higher A(Li) could suggest that the formation or presence of planets plays a role in the degree of internal mixing on or before the red giant branch. However, a multitude of factors affect A(Li) during the red giant phase, and when the abundances of our sample are compared to abundances of red giants in other open clusters available in the literature, we find that they all fall well within a much larger distribution of A(Li) and 12C/13C. Thus, even the high Li in NGC2423 3 cannot be concretely tied to the presence of the substellar companion.

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arXiv2014

The HST Treasury "Advanced Spectral Library" (ASTRAL) Programs

The "Advanced Spectral Library (ASTRAL)" Project (PI = T. Ayres) consists of two Treasury Programs: the Cycle 18 "Cool Stars" (GO-12278) Program and the Cycle 21 "Hot Stars" (GO-13346) Program. The primary goal of these programs is to collect, for the use of the astronomical community over the coming decades, a definitive set of representative, high-resolution (R~30,000-100,000), high signal/noise (S/N>100) spectra, with full UV coverage (~1150 - 3100 A) of prototypical stars across the HR diagram, utilizing the high-performance Space Telescope Imaging Spectrograph (STIS). The Cycle 18 program obtained spectra of 8 F-M evolved late-type stars, while the Cycle 21 program is in the process of observing 21 early-type stars, which span a broad range of spectral types between early-O and early-A. All of these data will be available from the HST archive and, in post-processed and merged form, at http://casa.colorado.edu/~ayres/ASTRAL/. These data will enable investigations of a broad range of problems -- stellar, interstellar, and beyond -- for many years into the future. We describe here the details of the observing programs, including the program targets and the observing strategies utilized to optimize the quality of the spectra, and present some illustrative examples of the on-going scientific analyses, including a study of the outer atmospheres and winds of the two evolved M stars in the sample and a first look at a "high definition" UV spectrum of a magnetic chemically peculiar "Ap" star.

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arXiv2010

Winds from Luminous Late-Type Stars: II. Broadband Frequency Distribution of Alfvén Waves

We present the numerical simulations of winds from evolved giant stars using a fully non-linear, time dependent 2.5-dimensional magnetohydrodynamic (MHD) code. This study extends our previous fully non-linear MHD wind simulations to include a broadband frequency spectrum of Alfvén waves that drive winds from red giant stars. We calculated four Alfvén wind models that cover the whole range of Alfvén wave frequency spectrum to characterize the role of freely propagated and reflected Alfvén waves in the gravitationally stratified atmosphere of a late-type giant star. Our simulations demonstrate that, unlike linear Alfvén wave-driven wind models, a stellar wind model based on plasma acceleration due to broadband non-linear Alfvén waves, can consistently reproduce the wide range of observed radial velocity profiles of the winds, their terminal velocities and the observed mass loss rates. Comparison of the calculated mass loss rates with the empirically determined mass loss rate for alpha Tau suggests an anisotropic and time-dependent nature of stellar winds from evolved giants.

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arXiv2009

Active Galactic Nuclei and their role in Galaxy Formation and Evolution

There are several key open questions as to the nature and origin of AGN including: 1) what initiates the active phase, 2) the duration of the active phase, and 3) the effect of the AGN on the host galaxy. Critical new insights to these can be achieved by probing the central regions of AGN with sub-mas angular resolution at UV/optical wavelengths. In particular, such observations would enable us to constrain the energetics of the AGN "feedback" mechanism, which is critical for understanding the role of AGN in galaxy formation and evolution. These observations can only be obtained by long-baseline interferometers or sparse aperture telescopes in space, since the aperture diameters required are in excess of 500 m - a regime in which monolithic or segmented designs are not and will not be feasible and because these observations require the detection of faint emission near the bright unresolved continuum source, which is impossible from the ground, even with adaptive optics. Two mission concepts which could provide these invaluable observations are NASA's Stellar Imager (SI; Carpenter et al. 2008 & http://hires.gsfc.nasa.gov/si/) interferometer and ESA's Luciola (Labeyrie 2008) sparse aperture hypertelescope.

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arXiv2009

Mass Transport Processes and their Roles in the Formation, Structure, and Evolution of Stars and Stellar Systems

We summarize some of the compelling new scientific opportunities for understanding stars and stellar systems that can be enabled by sub-mas angular resolution, UV/Optical spectral imaging observations, which can reveal the details of the many dynamic processes (e.g., variable magnetic fields, accretion, convection, shocks, pulsations, winds, and jets) that affect their formation, structure, and evolution. These observations can only be provided by long-baseline interferometers or sparse aperture telescopes in space, since the aperture diameters required are in excess of 500 m - a regime in which monolithic or segmented designs are not and will not be feasible - and since they require observations at wavelengths (UV) not accessible from the ground. Two mission concepts which could provide these invaluable observations are NASA's Stellar Imager (SI; http://hires.gsfc.nasa.gov/si/) interferometer and ESA's Luciola sparse aperture hypertelescope, which each could resolve hundreds of stars and stellar systems. These observatories will also open an immense new discovery space for astrophysical research in general and, in particular, for Active Galactic Nuclei (Kraemer et al. Decadal Survey Science Whitepaper). The technology developments needed for these missions are challenging, but eminently feasible (Carpenter et al. Decadal Survey Technology Whitepaper) with a reasonable investment over the next decade to enable flight in the 2025+ timeframe. That investment would enable tremendous gains in our understanding of the individual stars and stellar systems that are the building blocks of our Universe and which serve as the hosts for life throughout the Cosmos.

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arXiv2006

The Stellar Imager (SI) Vision Mission

The Stellar Imager (SI) is a UV-Optical, Space-Based Interferometer designed to enable 0.1 milli-arcsecond (mas) spectral imaging of stellar surfaces and of the Universe in general and asteroseismic imaging of stellar interiors. SI is identified as a "Flagship and Landmark Discovery Mission" in the 2005 Sun Solar System Connection (SSSC) Roadmap and as a candidate for a "Pathways to Life Observatory" in the Exploration of the Universe Division (EUD) Roadmap (May, 2005). SI will revolutionize our view of many dynamic astrophysical processes: its resolution will transform point sources into extended sources, and snapshots into evolving views. SI's science focuses on the role of magnetism in the Universe, particularly on magnetic activity on the surfaces of stars like the Sun. SI's prime goal is to enable long-term forecasting of solar activity and the space weather that it drives. SI will also revolutionize our understanding of the formation of planetary systems, of the habitability and climatology of distant planets, and of many magneto-hydrodynamically controlled processes in the Universe. The results of the SI "Vision Mission" Study are presented in this paper. Additional information on the SI mission concept and related technology development can be found at URL: http://hires.gsfc.nasa.gov/si/.

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arXiv2002

The Fizeau Interferometer Testbed

The Fizeau Interferometer Testbed (FIT) is a collaborative effort between NASA's Goddard Space Flight Center, the Naval Research Laboratory, Sigma Space Corporation, and the University of Maryland. The testbed will be used to explore the principles of and the requirements for the full, as well as the pathfinder, Stellar Imager mission concept. It has a long term goal of demonstrating closed-loop control of a sparse array of numerous articulated mirrors to keep optical beams in phase and optimize interferometric synthesis imaging. In this paper we present the optical and data acquisition system design of the testbed, and discuss the wavefront sensing and control algorithms to be used. Currently we have completed the initial design and hardware procurement for the FIT. The assembly and testing of the Testbed will be underway at Goddard's Instrument Development Lab in the coming months.

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