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Results for “STELLAR ACTIVITY”

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
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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.↗
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.↗
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↗
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.↗
NASA NTRS1995

Stellar Activity at the End of the Main Sequence: GHRS Observations of the M8 Ve Star VB 10

We present Goddard High Resolution Spectrograph observations of the M8 Ve star VB 10 (equal to G1 752B), located very near the end of the stellar main sequence, and its dM3.5 binary companion G1 752A. These coeval stars provide a test bed for studying whether the outer atmospheres of stars respond to changes in internal structure as stars become fully convective near mass 0.3 solar mass (about spectral type M5), where the nature of the stellar magnetic dynamo presumably changes, and near the transition from red to brown dwarfs near mass 0.08 solar mass (about spectral type M9), when hydrogen burning ceases at the end of the main sequence. We obtain upper limits for the quiescent emission of VB 10 but observe a transition region spectrum during a large flare, which indicates that some type of magnetic dynamo must be present. Two indirect lines of evidence-scaling from the observed X-ray emission and scaling from a time-resolved flare on AD Leo suggest that the fraction of the stellar bolometric luminosity that heats the transition region of VB 10 outside of obvious flares is comparable to, or larger than, that for G1 752A. This suggests an increase in the magnetic heating rates, as measured by L(sub line)/L(sub bol) ratios, across the radiative/convective core boundary and as stars approach the red/brown dwarf boundary. These results provide new constraints for dynamo models and models of coronal and transition-region heating in late-type stars.

Linsky, Jeffrey L.↗
NASA NTRS1984

Intense X-ray flares from active stellar systems - EV Lacertae and HD 8357

The HEAO A-1 Sky Survey Experiment included X-ray data used to define light curves for the flare star EV Lac and for X-ray flares observed in the binary system HD 8357. The data were taken during flare events and were detailed enough to calculate the flare rates and flaring luminosities. The peak luminosities during flares were several times the luminosities in normal X-ray flares emitted by the objects. Peak luminosities reached 30-50 times the normal variations and were associated with an order of magnitude increase in energy output. EV Lac was sufficiently active to be recommended for inclusion in future X-ray monitoring programs.

Ambruster, C.↗
NASA NTRS1975

Further observations of the lambda 10830 He line in stars and their significance as a measure of stellar activity

Measurements of the lambda 1030 He line in 198 stars are given along with data on other features in that spectral range. Nearly 80% of all G and K stars show some lambda 10830; of these, half are variable and 1/4 show emission. It was confirmed that lambda 10830 is not found in M stars, is weak in F stars, and is particularly strong in close binaries. The line is found in emission in extremely late M and S stars, along with P gamma, but P gamma is not in emission in G and K stars with lambda 10830 emissions. Variable He emission and Ti I emission are found in the RV Tauri variables R Scuti and U Mon. In R Aqr the Fe XIII coronal line lambda 10747 and a line at lambda 11012 which may be singlet He or La II are found, as well as lambda 10830 and P gamma. The nature of coronas or hot chromospheres in the various stars is discussed. It was concluded that the lambda 10830 intensity must be more or less proportional to the energy deposited in the chromosphere corona by non-thermal processes.

Zirin, H.↗
NASA NTRS1976

Further observations of the 10830-A helium line in stars and their significance as a measure of stellar activity

Measurements of the 10830-A He line in 198 stars are given along with data on other features in that spectral range. Nearly 80 per cent of all G and K stars show some 10830-A absorption or emission; of these, half are variable and one-quarter show emission. The results of Vaughan and Zirin (1968) are confirmed that the 10830-A line is not found in M stars, is weak in F stars, and is particularly strong in close binaries. The line is found in emission in extremely late M and S stars along with P-gamma, but the latter is not in emission in the G and K stars with 10830-A emissions. Variable He emission along with Ti I emission is found in the RV Tauri variables R Sct and U Mon. In R Aqr, the Fe XIII coronal line at 10747 A is found in emission along with a line at 11012 A, which may be singlet He or La II, as well as the 10830-A and P-gamma lines. There is possibly a five-year period in the appearance of 10830-A emission in theta Her. The nature of coronae or hot chromospheres in the various stars is extensively discussed without definite conclusion, except that the 10830-A intensity must be more or less proportional to the energy deposited in the chromosphere-corona by nonthermal processes.

Zirin, H.↗
NASA NTRS1983

Rapid rotation and stellar activity in the triple system HD 165590

X-ray and ultraviolet observations of the spectroscopic and visual binary HD 165590 are presented. Results show that the spectroscopic binary component has a period of 0.88 day and a near-solar spectral type. A substantial soft X-ray flux for HD 165590 is also found with an X-ray luminosity of 10 to the 30.6th ergs/sec for the system. No obvious periodicities of less than 0.5 days are detected in the X-ray data, although stochastic variability is present. Observations of the transition region emission lines using the International Ultraviolet Explorer satellites are shown to be consistent with the X-ray data and reveal an approximately 50% variability on the time scale of days. It is concluded that comparisons of the results for HD 165590 with X-ray observations of other late F and G stars demonstrate that rotation-activity scaling relations are useful as gross predictors of X-ray emission.

Stern, R. A.↗
NASA NTRS1986

Stellar activity at radio wavelengths

Hundreds of stars have now been detected at radio wavelengths. At least four processes are believed to cause the radio emission: thermal bremsstrahlung, gyrosynchrotron, cyclotron maser, and plasma radiation. Here the characteristics of the various classes of detected stars are reviewed, along with the putative reasons for the radio emission. The classes of stars described include: single stars, binaries containing two main sequences and/or giant stars, binaries containing one white dwarf, and binaries containing one neutron star or black hole.

Dulk, George A.↗