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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 19 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2020

Photometry as a Proxy for Stellar Activity in Radial Velocity Analyses

Stellar activity remains a limiting factor in measuring precise planet parameters from radial velocity spectroscopy, not least in the search for Earth-mass planets orbiting in the habitable zones of Sun-like stars. One approach to mitigate stellar activity is to use combined analyses of both radial velocity and time-series photometry. We present an analysis of simultaneous disk-integrated photometry and radial velocity data of the Sun in order to determine the useful limits of a combined analysis. We find that simple periodogram or autocorrelation analysis of solar photometry give the correct rotation period <50% of the time. We therefore use a Gaussian process to investigate the time variability of solar photometry and to directly compare simultaneous photometry with radial velocity data. We find that the hyperparameter posteriors are relatively stable over 70 yr of solar photometry and the amplitude tracks the solar cycle. We observe good agreement between the hyperparameter posteriors for the simultaneous photometry and radial velocity data. Our primary conclusion is a recommendation to include an additional prior in Gaussian process fits to constrain the evolutionary timescale to be greater than the recurrence timescale (i.e., the rotation period) to recover more physically plausible and useful results. Our results indicate that such simultaneous monitoring may be a useful tool in enhancing the precision of radial velocity surveys.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2021

A Stellar Activity F-statistic for Exoplanet Surveys (SAFE)

In the search for planets orbiting distant stars, the presence of stellar activity in the atmospheres of observed stars can obscure the radial velocity signal used to detect such planets. Furthermore, this stellar activity contamination is set by the star itself and cannot simply be avoided with better instrumentation. Various stellar activity indicators have been developed that may correlate with this contamination. We introduce a new stellar activity indicator called the Stellar Activity F-statistic for Exoplanet surveys (SAFE) that has higher statistical power (i.e., probability of detecting a true stellar activity signal) than many traditional stellar activity indicators in a simulation study of an active region on a Sun-like star with a moderate-to-high signal-to-noise ratio. Also through simulation, the SAFE is demonstrated to be associated with the projected area on the visible side of the star covered by active regions. We also demonstrate that the SAFE detects statistically significant stellar activity in most of the spectra for HD 22049, a star known to have high stellar variability. Additionally, the SAFE is calculated for recent observations of the three low-variability stars HD 34411, HD 10700, and HD 3651, the latter of which is known to have a planetary companion. As expected, the SAFE for these three only occasionally detects activity. Furthermore, initial exploration appears to indicate that the SAFE may be useful for disentangling stellar activity signals from planet-induced Doppler shifts.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2022

Modelling stellar activity with Gaussian process regression networks

ABSTRACT Stellar photospheric activity is known to limit the detection and characterization of extrasolar planets. In particular, the study of Earth-like planets around Sun-like stars requires data analysis methods that can accurately model the stellar activity phenomena affecting radial velocity (RV) measurements. Gaussian Process Regression Networks (GPRNs) offer a principled approach to the analysis of simultaneous time series, combining the structural properties of Bayesian neural networks with the non-parametric flexibility of Gaussian Processes. Using HARPS-N solar spectroscopic observations encompassing three years, we demonstrate that this framework is capable of jointly modelling RV data and traditional stellar activity indicators. Although we consider only the simplest GPRN configuration, we are able to describe the behaviour of solar RV data at least as accurately as previously published methods. We confirm the correlation between the RV and stellar activity time series reaches a maximum at separations of a few days, and find evidence of non-stationary behaviour in the time series, associated with an approaching solar activity minimum.

Camacho, J. D. (ORCID:0000000151215560)↗
NASA NTRS1985

Stellar activity cycles

The variety of stellar chromospheric and coronal activity provides a framework for understanding solar and stellar magnetic activity. It is pointed out that the recognition of an 11-yr periodicity within the record of sunspot numbers is a relatively recent discovery of just over a century ago. The solar activity cycle is examined, taking into account the sunspot cycle, and solar dynamo models. Time-averaged stellar chromospheric activity levels are considered along with long-term activity fluctuations of cool stars. Attention is given to the direct measurement of stellar magnetic fields, solar luminosity variations, the RS Canum Venaticorum and BY Draconis variables, continuum variations in lower main sequence stars, chromospheric variations in lower main sequence stars, and chromospheric variations in evolved stars.

Baliunas, S. L.↗
NASA NTRS2002

FUV Spectra of Evolved Late-K and M Stars: Mass Loss Revisited and Stellar Activity

This is the final report for the FUSE Cycle 1 program A100: FUV Spectra of Evolved Late-K and M Stars: Mass Loss revisited and Stellar Activity. Targets alpha TrA (K3 II) and gamma Cru (M3 III) were originally assigned 25 ksec each, to be observed in the medium aperture. Once the in-flight performance and telescope alignment problems were known, the observations were reprogrammed to optimized the scientific return of the program. Alpha TrA was scheduled for 25 ksec observations in both the medium and large apertures. The principle aim of this program was to measure the stellar FUV line and continuum emission, in order to estimate the photoionization radiation field and to determine the level of stellar activity through the fluxes in the collisionally excited high temperature diagnostics: C III 977Angstroms and O VI 1032,1038Angstrom doublet. The medium aperture observations were obtained successfully while the large aperture observations were thought by Johns Hopkins University (JHU)to be lost to satellite problems. There was insufficient signal-to- noise in the medium aperture short wavelength Sic channels to do quantitative science.

Harper, Graham M.↗
NASA NTRS2005

Stellar Activity and Mass Loss from A and F Supergiants

This grant supported the observing and data analysis for FUSE Cycle 4 project DO47 to observe five late-A and F supergiants using a total observing allocation of 150 ksec. Stellar activity on A and F supergiants has been poorly studied in the past; primarily because the photospheric continuum dominates any chromospheric (~ 10 4 K) or transition region (TR; ~ 10 5 K) emission lines far into the ultraviolet. FUSE observations of A and F supergiants offer one of the best methods to study stellar activity on these stars, because many activity indicators longward of 1200 A are swamped by the photospheric continuum emission. We used FUSE FUV spectra to search for 0 VI and C I11 TR emission lines and obtained data for t Car (A8 Ib, for 60.5 ksec, on 2003 Apr 27), 8 Sco (F1 11, 22.7 ksec, 2003 Aug 2), ct Per (F5 Ib, 30.8 ksec, 2003 Oct l), a UMi (F7 Ib-11, 23.9 ksec, 2003 Oct 14), and y Cyg (F8 Ib, 25.8 ksec, 2003 Oct 18). These observations used the large LWRS aperture and collected data in time-tagged mode. The LWRS aperture is large enough that the target should remain within the aperture with the normal level of FUSE pointing jitter and target drift. We examined the stellar signal and found that the targets were well within the aperture throughout the observation. The data were split into night-time and day-time data so that the effects of airglow emission were recognizable, and combined day and night spectra were generated using CALFUSE 2.4.0 .

Brown, Alexander↗
NASA NTRS1994

Stellar activity: Astrophysics relevant to global change

FRESIP will obtain a great deal of data on stellar activity and flares on F, G and K dwarfs. Rotation periods, flare distributions and possibly stellar cycles will emerge. This apparently curiosity-driven research actually has implications for our understanding of global climate change. Significant climate change during the seventeenth-century Maunder Minimum is thought to be related to a change in the solar condition. Recently acquired data from the Greenland Ice-core Project suggest that far greater climate changes on decade time scales may have occurred during the previous interglacial. It is possible that a yet more drastic change in state of the Sun was responsible. We have no relevant solar data, but can begin to explore this possibility by observing an ensemble of solar-like stars.

Haisch, Bernhard M.↗
DOE OSTI2022

Magnitude-squared Coherence: A Powerful Tool for Disentangling Doppler Planet Discoveries from Stellar Activity

Abstract If Doppler searches for Earth-mass, habitable planets are to succeed, observers must be able to identify and model out stellar activity signals. Here we demonstrate how to diagnose activity signals by calculating the magnitude-squared coherence C ˆ xy 2 ( f ) between an activity-indicator time series x t and the radial-velocity (RV) time series y t . Since planets only cause modulation in RV, not in activity indicators, a high value of C ˆ xy 2 ( f ) indicates that the signal at frequency f has a stellar origin. We use Welch’s method to measure coherence between activity indicators and RVs in archival observations of GJ 581, α Cen B, and GJ 3998. High RV-H α coherence at the frequency of GJ 3998 b and high RV-S index coherence at the frequency of GJ 3998c, indicate that the planets may actually be stellar signals. We also replicate previous results showing that GJ 581 d and g are rotation harmonics and demonstrate that α Cen B has activity signals that are not associated with rotation. Welch’s power spectrum estimates have cleaner spectral windows than Lomb–Scargle periodograms, improving our ability to estimate rotation periods. We find that the rotation period of GJ 581 is 132 days, with no evidence of differential rotation. Welch’s method may yield unacceptably large bias for data sets with N < 75 observations and works best on data sets with N > 100. Tapering the time-domain data can reduce the bias of the Welch’s power spectrum estimator, but observers should not apply tapers to data sets with extremely uneven observing cadence. A software package for calculating magnitude-squared coherence and Welch’s power spectrum estimates is available on github.

Dodson-Robinson, Sarah E. (ORCID:0000000287964974)↗
NASA NTRS1985

Stellar activity in synchronized binaries. I - Dependence on rotation

A large sample of late-type binaries with orbital periods from 1 to 100 days has been examined for relationships between stellar activity and rotation period (assumed - with a few exceptions - to be synchronized with the stellar orbital period). Most of the systems are in the RS Canum Venaticorum class of close binaries; none are contact binaries. The activity diagnostics were observed with the IUE and the Einstein Observatory and range from the mid-chromosphere to the corona. There is a dependence of activity on rotation in the synchronized binaries; surface flux is the preferable measure of activity and is best when steps are taken to remove variations in activity filling factors.

Basri, G.↗
DOE OSTI2021

Stellar Activity Manifesting at a One-year Alias Explains Barnard b as a False Positive

Barnard’s star is among the most studied stars given its proximity to the Sun. It is often considered the radial velocity (RV) standard for fully convective stars due to its RV stability and equatorial decl. Recently, an Msin i=3.3M{sub ⊕} super-Earth planet candidate with a 233 day orbital period was announced by Ribas et al. New observations from the near-infrared Habitable-zone Planet Finder (HPF) Doppler spectrometer do not show this planetary signal. We ran a suite of experiments on both the original data and a combined original + HPF data set. These experiments include model comparisons, periodogram analyses, and sampling sensitivity, all of which show the signal at the proposed period of 233 days is transitory in nature. The power in the signal is largely contained within 211 RVs that were taken within a 1000 day span of observing. Our preferred model of the system is one that features stellar activity without a planet. We propose that the candidate planetary signal is an alias of the 145 day rotation period. This result highlights the challenge of analyzing long-term, quasi-periodic activity signals over multiyear and multi-instrument observing campaigns.

47 OTHER INSTRUMENTATION↗
NASA NTRS2001

Publicly Available Numerical Codes for Modeling the X-ray and Microwave Emissions from Solar and Stellar Activity

We have posted numerical codes on the Web for modeling the bremsstrahlung x-ray emission and the a gyrosynchrotron radio emission from solar and stellar activity. In addition to radiation codes, steady-state and time-dependent Fokker-Planck codes are provided for computing the distribution and evolution of accelerated electrons. A 1-D hydrodynamics code computes the response of the stellar atmosphere (chromospheric evaporation). A code for modeling gamma-ray line spectra is also available. On-line documentation is provided for each code. These codes have been developed for modeling results from the High Energy Solar Spectroscopic Imager (HESSI) along related microwave observations of solar flares. Comprehensive codes for modeling images and spectra of solar flares are under development. The posted codes can be obtained on NASA/Goddard's HESSI Web Site at http://hesperia.gsfc.nasa.gov/hessi/modelware.htm. This work is supported in part by the NASA Sun-Earth Connection Program.

Holman, Gordon D.↗
NASA NTRS2002

NASA Fuse Cycle 1 Program A100. FUV Spectra of Evolved Late-K and M Stars: Mass Loss revisited and Stellar Activity

Program A100 was awarded 50 ksec of FUSE observing time. Targets alpha TrA (K3 II) and gamma Cru (N13 III) were originally assigned 25 ksec each, to be observed in the medium aperture. Once the in-flight performance and telescope alignment problems were known, the observations were reprogrammed to optimized the scientific return of the program. alpha TrA was scheduled for 25 ksec observations in both the medium and large apertures. The principle aim of this program was to measure the stellar FUV line and continuum emission, in order to estimate the photoionization radiation field and to determine the level of stellar activity through the fluxes in the collisionally excited high temperature diagnostics: C III 977A and O VI 1032,1038A doublet. The medium aperture observations were obtained successfully while the large aperture observations were thought by Johns Hopkins University (JHU)to be lost to satellite problems. There was insufficient signal-to noise in the medium aperture short wavelength SiC channels to do quantitative science. The long wavelength LiF spectra are of high signal-to-noise (SN) and show pronounced emission lines near lambdalambda1100 - 1140A. Emission had been observed at these wavelength in low resolution and low SN ORFEUS-SPAS 11 spectra obtained by Dupree & Brickhouse (1998 ApJ 500, L33), however, they were unable to identify these features. One of the first problems faced in the early FUSE mission was establishing a reliable wavelength scale, especially between the different channels. With the medium aperture data in hand we decided to identify the features and thereby establish an absolute wavelength scale for the channels where the emission occurred. We first established that the emission was fluorescent. and then we constructed a fluorescent line formation model for all neutrals. and singly ionized species with atomic numbers up to 28, and the H2 molecule. We used datasets of radiative rates and energy levels which were as complete as possible and available in an electronic format (millions of transitions). We found that most, but not all, emission was from Fe II pumped by H Lyalpha. This is not typical fluorescence, since the emission is shortward of the pumping transitions near 1215.6A.

Harper, Graham M.↗
NASA NTRS2020

Characterization of stellar activity using transits and its impact on habitability

Stellar magnetic field is the driver of activity in stars and cantrigger spots, energetic flares, coronal plasma ejections and ionizedwinds. These phenomena play a crucial role in understanding theinternal mechanisms of the star, but can also have potential effectsin orbiting planets. During the transit of a planet, spots can beocculted producing features imprinted in the transit light curve.Here, we modelled these features to characterize the physicalproperties of the spots (radius, intensity, and location). Inaddition, we monitor spots signatures on multiple transits to estimatemagnetic cycles length of Kepler stars. Flares have also been observedduring transits in active stars. We derive the properties of theflares and analyse their UV impact on possible living organisms inplanets orbiting in the habitable zone.

Valio, Adriana↗
NASA NTRS2020

Characterization of stellar activity using transits and its impact on habitability

Stellar magnetic field is the driver of activity in stars and can trigger spots, energetic flares, coronal plasma ejections and ionized winds. These phenomena play a crucial role in understanding the internal mechanisms of the star, but can also have potential effects in orbiting planets. During the transit of a planet, spots can be occulted producing features imprinted in the transit light curve. Here, we modelled these features to characterize the physical properties of the spots (radius, intensity, and location). In addition, we monitor spots signatures on multiple transits to estimate magnetic cycles length of Kepler stars. Flares have also been observed during transits in active stars. We derive the properties of the flares and analyse their UV impact on possible living organisms in planets orbiting in the habitable zone.

Estrela, Raissa↗
NASA NTRS2019

Stellar Activity Effects on Moist Habitable Terrestrial Atmospheres Around M Dwarfs

Transit spectroscopy of terrestrial planets around nearby M dwarfs will be a primary goal of space missions in coming decades. Three-dimensional climate modeling has shown that slow-synchronous rotating terrestrial planets may develop thick clouds at the substellar point, increasing the albedo. For M dwarfs with T(eff)>3000 K, such planets at the inner habitable zone (IHZ) have been shown to retain moist greenhouse conditions, with enhanced stratospheric water vapor ( fH2O>10(exp −3)) and low Earth-like surface temperatures. However, M dwarfs also possess strong UV activity, which may effectively photolyze stratospheric H2O. Prior modeling efforts have not included the impact of high stellar UV activity on the H2O. Here, we employ a 1D photochemical model with varied stellar UV, to assess whether H2O destruction driven by high stellar UV would affect its detectability in transmission spectroscopy. Temperature and water vapor profiles are taken from published 3D climate model simulations for an IHZ Earth-sized planet around a 3300 K M dwarf with an N2–H2O atmosphere; they serve as self-consistent input profiles for the 1D model. We explore additional chemical complexity within the 1D model by introducing other species into the atmosphere. We find that as long as the atmosphere is well-mixed up to 1 mbar, UV activity appears to not impact detectability of H2O in the transmission spectrum. The strongest H2O features occur in the James Webb Space Telescope MIRI instrument wavelength range and are comparable to the estimated systematic noise floor of ∼50 ppm.

Badhan, Mahmuda Afrin↗
NASA NTRS2019

Stellar Activity Effects on Moist Habitable Terrestrial Atmospheres Around M Dwarfs

Transit spectroscopy of terrestrial planets around nearby M dwarfs is a primary goal of space missions in coming decades. 3-D climate modeling has shown that slow-synchronous rotating terrestrial planets may develop thick clouds at the substellar point, increasing the albedo. For M dwarfs with T(eff) > 3000 K, such planets at the inner habitable zone (IHZ) have been shown to retain moist greenhouse conditions, with enhanced stratospheric water vapor (fH2O > 10(exp -3)) and low Earth-like surface temperatures. However, M dwarfs also possess strong UV activity, which may effectively photolyze stratospheric H2O. Prior modeling efforts have not included the impact of high stellar UV activity on the H2O. Here, we employ a 1-D photochemical model with varied stellar UV, to assess whether H2O destruction driven by high stellar UV would affect its detectability in transmission spectroscopy. Temperature and water vapor profiles are taken from published 3-D climate model simulations for an IHZ Earth-sized planet around a 3300 K M dwarf with an N2-H2O atmosphere; they serve as self-consistent input profiles for the 1-D model. We explore additional chemical complexity within the 1-D model by introducing other species into the atmosphere. We find that as long as the atmosphere is well-mixed up to 1 mbar, UV activity appears to not impact detectability of H2O in the transmission spectrum. The strongest H2O features occur in the JWST MIRI instrument wavelength range and are comparable to the estimated systematic noise floor of ~50 ppm.

Mahmuda Afrin Badhan↗
DOE OSTI2020

Dynamical Mass Estimates of the β Pictoris Planetary System through Gaussian Process Stellar Activity Modeling

Nearly 15 yr of radial velocity (RV) monitoring and direct imaging enabled the detection of two giant planets orbiting the young, nearby star β Pictoris. The δ Scuti pulsations of the star, which overwhelm planetary signals, need to be carefully suppressed. In this work, we independently revisit the analysis of the RV data following a different approach than available in the literature to model the activity of the star. We show that a Gaussian process (GP) with a stochastically driven damped harmonic oscillator kernel can model the δ Scuti pulsations. It provides similar results to parametric models but with a simpler framework, using only three hyperparameters. It also enables us to model poorly sampled RV data that were excluded from previous analyses, hence extending the RV baseline by nearly five years. Altogether, the orbit and mass of both planets can be constrained from RV only, which was not possible with the parametric modeling. To characterize the system more accurately, we also perform a joint fit of all available relative astrometry and RV data. Our orbital solutions for β Pic b favor a low eccentricity of 0.029$_{−0.024}^{+0.061}$ and a relatively short period of 21.1$_{−0.8}^{+2.0}$ yr. The orbit of β Pic c is eccentric with 0.206$_{−0.063}^{+0.074}$ with a period of 3.36 ± 0.03 yr. We find model-independent masses of 11.7 ± 1.4 and 8.5 ± 0.5 M Jup for β Pic b and c, respectively, assuming coplanarity. The mass of β Pic b is consistent with the hottest start evolutionary models, at an age of 25 ± 3 Myr. A direct detection of β Pic c would provide a second calibration measurement in a coeval system.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1983

The nature of stellar active regions

Observational estimates of chromospheric radiative loss rates in Ca II H and K in main-sequence F, G, and K dwarfs are used to compare stars that span a range of activity levels with each other and with the sun. For stars with spectral types similar to that of the sun, it is found that chromospheric radiative losses in stars with active chromospheres exceed the flux emitted by plages on the sun at solar maximum by about a factor of 6. If this difference is due primarily to a difference in the fraction of the stellar surface covered by plage-like material, and not to a change in the character of the emission-line regions, then more than half of the stellar surface in stars with large chromospheric radiative loss rates must be covered by active regions. It is also possible to show that active regions in young solar-type stars extend at least to latitudes + or - 25 deg from the rotational equator and that a typical individual region contributes no more than 2 percent - 3 percent of the total H and K emission. In Kappa Cet, observations show that the rotational modulation persisted in both phase and amplitude for at least 100 days (12 rotation cycles).

Wolff, S. C.↗