Engineering topics

Linsky, J. L.

Publications and source records attributed to Linsky, J. L..

At least 37 records · Page 2

NASA NTRS1988

Rotational modulation and flares on RS CVn and BY Dra stars

Evidence is presented for spots, plages, and flares on the noneclipsing RS CVn system II Peg and V 711 Tau. The large spot originally found on II Peg in 1981.8 could still be identified in 1983. Two spectroscopic flares of II Peg were detected. For the larger flare the fluxes in the strongest transition-region lines reached about six times their quiescent values. A comparison of the 1981 and 1983 flux variations show that active regions or plages are located on one hemisphere. On V 711 Tau at least two flares were observed. Outside of these flares, the strongest chromospheric and transition-region emission lines exhibited weak phase-dependent variations that persist over 173 orbital cycles (1981-1983) and are possibly associated with one of the proposed spots. For both stellar systems the fluxes from the higher temperature emission lines showed the greatest variations.

Andrews, A. D.↗

NASA NTRS1988

C 4 fluxes from the sun as a star and the correlation with magnetic flux

A total of 144 C 4 wavelength 1548 SMM-UVSP spectroheliograms of solar plages were analyzed, some of which are series of exposures of the same region on the same day. Also analyzed were C 4 wavelength 1551 rasters of plages and C 4 1548 rasters of the quiet sun. The sample contains data on 17 different plages, observed on 50 different days. The center-to-limb variations of the active regions show that the optical thickness effects in the C 4 wavelength 1548 line can be neglected in the conversion from intensity to flux density. As expected for the nearly optically thin situation, the C 4 1548 line is twice as bright as the C 4 wavelength 1551 line. The average C 4 wavelength 1548 flux density for a quiet is 2700 erg/cm/s and, with surprisingly little scatter, 18,000 erg/cm/s for plages. The intensity histograms of rasters obtained at disk centers can be separated into characteristic plage and quiet-sun contributions with variable relative filling factors. The disk-averaged flux density in the C 4 doublet and the disk-averaged magnitude of the magnetic flux density are related. The relationship between the C 4 and magnetic flux densities for spatially resolved data is inferred to be almost the same, with only an additional factor of order unity in the constant of proportionality.

Schrijver, C. J.↗

NASA NTRS1988

C IV fluxes from the Sun as a star, and the correlation with magnetic flux

A total of 144 C IV wavelength 1548 Solar Maximum Mission (SMM)-UVSP spectroheliograms of solar plages were analyzed, some of which are series of exposures of the same region on the same day. Also analyzed were the C IV wavelength 1551 rasters of plages and C IV wavelength 1548 rasters of the quiet sun. The sample contained data on 17 different plages, observed on 50 different days. The center-to-limb variations of the active regions show that the optical thickness effects in the C IV wavelength 1548 line can be neglected in the conversion from intensity to flux density. As expected for the nearly optically thin situation, the C IV wavelength 1548 line is twice as bright as the C IV 1551 line. The average C IV wavelength 1548 flux density for a quiet region is 2700 ergs/cm/s and, with surprisingly little scatter, 18,000 erg/cm/s for plages. The intensity histograms of rasters obtained at disk center can be separated into characteristic plage and quiet sun contributions with variable relative filling factors. The relationship between the C IV and magnetic flux densities for spatially resolved data is inferred to be almost the same, with only an additional factor of order unity in the constant of proportionality.

Schrijver, C. J.↗

NASA NTRS1988

IUE's legacy for the future: The final archive and goals for its implementation

Requirements for the IUE archive, and how the signal/noise (S/N) ratio in photometrically corrected images can be enhanced considerably by cross-correlating the fixed pattern in a data image with that in a suitable flat-field image are described. From these cross-correlations it is feasible to derive an accurate geometrical correction to apply to the data image before applying the intensity transfer functions. The standard IUE processing software does not generate a sufficiently accurate geometric correction so that typical spatial errors of 1 to 2 pixels conspire with the large fixed pattern in raw images to produce significant misregistration noise. Tests on flat-field images demonstrate that an explicit geometric correction procedure can avoid most of the misregistration noise and can thereby improve the S/N ratio of IUE data by factors of 1.5 to 2.4.

Linsky, J. L.↗

NASA NTRS1987

Rotational modulation and flares on RS CVn and BY Dra stars. IV - The spatially resolved chromosphere of AR Lacertae

The authors observed the RS Canum Venaticorum system AR Lacertae systematically over an orbital period with the International Ultraviolet Explorer in October 1983. Contemporaneous radio observations were obtained at the VLA. The spectra of the Mg II k emission line were analyzed using a Doppler imaging technique. In this way, three discrete regions of emission were identified in the outer atmosphere of the K star-two 'plages' and a chromospheric brightening that was related to a radio flare. The widths of the plage profiles indicate that the two plages together cover about 2 percent of the visible stellar hemisphere, and their v sin i values indicate that they lie close to the equator of the K star. The Mg II k surface flux in the plages is about five times the mean Mg II k surface flux of the K star.

Walter, F. M.↗

NASA NTRS1987

Detection of X-ray emission from the young low-mass star Rossiter 137B

Rst 137B, a close M-dwarf companion to the active K-star HD 36705, has been detected in a High Resolution Image in the Einstein Observatory Archive. The X-ray surface fluxes (0.2-4 keV) from both stars are close to the empirical saturation level, F(x)/F(bol) of about 0.001, defined by rapid rotators and very young stars. This supports the earlier results of the youthfulness of the system. This young couple is an excellent subject for studies of dependence of early evolution on stellar mass. Rst 137B is one of the latest spectral types and thus lowest-mass premain-sequence stars yet detected as an X-ray source.

Vilhu, O.↗

NASA NTRS1987

VLA observations of rapid 6 cm flux variations in alpha Ori

The red supergiant star alpha Ori was monitored with the Very Large Array (VLA). Thirteen observations at 6 cm show stochastic variations, at the 30 to 40 percent level, with no long term trend. All data was clipped and tapered in AIPS to minimize differences between VLA arrays. The calibration source varied by less than 10 percent over the same interval. The VLA observations of alpha Ori were continued, as well as alpha Her and alpha Sco, at both 2 and 6 cm, to confirm this result and search for long term trends. The stochastic 6 cm flux behavior, with 30 to 40 percent changes on all timescales from the shortest interval of 10 days to the longest, seems at odds with the 400 day periodic variations in U-band photometry and Mg II UV fluxes reported by Dupree, et al. The observed 6 cm flux was 25 percent below the 6 cm flux reported earlier this decade. Several models for the outer atmosphere of alpha Ori place the 6 cm optical depth unity location at several stellar radii above the optical photosphere. The rapid, stochastic variations reported are difficult to reconcile with almost any global process, such as pulsation, Alfven waves or periastron passage.

Bookbinder, J. A.↗

NASA NTRS1987

Rotational modulation and flares on RS CVn and BY Dra stars. VI - Physical parameters of the chromospheres/transition regions of V 711 Tau (HR 1099), II Peg and AR Lac during October 1981

Ground-based optical and IUE satellite-ultraviolet observations of three RS CVn stars are combined with density sensitive line ratios and differential emission measure curves to describe the physical conditions in their outer atmospheres. Solar-like densities are found to be representative of average conditions on two of the stars, V 711 Tau and AR Lac. The total radiative losses from these two stars are estimated and found to be larger than the sun by at least two orders of magnitude. Consideration of the volume emitting in two of the principal transition region lines suggests a possible relation between the disk 'filling factor' for these two lines and the dynamo-related Rossby number. Only one hemisphere of the star II Peg, the one showing least evidence of starspots, is similar. On the opposite hemisphere the presence of a discrete emitting region is deduced which is almost coincident in phase with the passage of the dominant optical spot group across the visible disk. The dimensions of this region, and an illustrative interpretation in terms of a large emitting magnetic loop, are discussed. It is compared to large active region loops on the sun.

Byrne, P. B.↗

NASA NTRS1987

The chromospheres and coronae of five G-K main-sequence stars

Previous data and high and low dispersion IUE observations of Chi 1 Ori, Alpha Cen A, Xi Boo A, Alpha Cen B, and Epsilon Eri are used to model the atmospheres of these stars from the high chromosphere to the corona. The energy lost by radiation and transferred by thermal conduction is investigated to establish the heating requirements, and results similar to those found for the solar atmosphere are found. The observed emission measurement distribution below 100,000 K can be described by a model in which Alfven wave energy input, observed through nonthermal line broadening, is balanced by radiation losses. Comparison of the coronal, transition region, and chromospheric pressures shows scaling relations which are compatible with previous flux correlations.

Jordan, C.↗

NASA NTRS1987

IUE observations of hydrogen and deuterium in the local interstellar medium

High-resolution Ly-alpha spectra of the late-type stars Epsilon Eri, Procyon, Altair, Capella, and HR 1099 taken with the short-wavelength camera on IUE are presented. The density, velocity dispersion, and bulk velocity of the interstellar H I toward each of the stars is derived from the spectra. Lower limits on the deuterium-to-hydrogen ratio toward these stars are obtained.

Murthy, J.↗

NASA NTRS1987

Rotational modulation and flares on RS CVn and BY Dra stars. III - IUE observations of V711 Tau (= HR 1099), II Peg, and AR Lac

Observations of three RS CVn stars, which were obtained over the stellar rotation cycles with the IUE satellite, are presented. Emission lines from high-temperature transition regions and chromospheres analogous to those observed in the solar spectrum were observed. The only visible component of II Peg and both components of V711 Tau and AR Lac appear to be chromospherically active. For the latter systems, the Mg II line surface flux from the G-type star is higher than that from the K subgiant, which dominates the observed UV line flux. Moreover, evidence of long-term ultraviolet variability is presented for AR Lac. The emission line fluxes for II Peg and, marginally, for the other two systems were observed to vary in antiphase with the optical variations at the time of the IUE observations. By comparing the results of Rodono et al. (1986) for two-spot models with the variation of UV line flux, evidence of a close spatial correlation between spot and plagelike features is found.

Rodono, M.↗

NASA NTRS1987

X-ray sources in regions of star formation. II - The pre-main-sequence G star HDE 283572

This paper reports the detection of HDE 283572, a ninth-magnitude G star 8 arcmin south of RY Tau, as a bright X-ray source. The observations reveal this object to be a fairly massive (about 2 solar masses) pre-main-sequence star associated with the Taurus-Auriga star formation complex. It exhibits few of the characteristics of the classical T Tauri stars and is a good example of a 'naked' T Tauri star. The star is a mid-G subgiant, of about three solar radii and rotates with a period of 1.5 d. The coronal and chromospheric surface fluxes are similar to those of the most active late type stars (excluding T Tauri stars). The X-ray and UV lines most likely arise in different atmospheric structures. Radiative losses are some 1000 times the quiet solar value and compare favorably with those of T Tauri stars.

Walter, F. M.↗

NASA NTRS1987

Rotational modulation and flares on RS CVn and BY Dra systems. II - IUE observations of BY Draconis and AU Microscopii

The modulation of the strong chromospheric and transition region lines over one cycle of the optical light curves of BY Dra and AU Mic is studied using IUE observations. The IUE observations were made from October 2-5, 1981 and include 7 short wavelength (SW) spectra and 13 long wavelength (LW) spectra. The integrated line flux data reveal that for BY Dra in the SW spectra there is modulation in the C IV, C II, O I, and He II lines and nonflare surface fluxes; and in the LW spectra, flares are detected in the Mg II and Fe II bands. It is observed that there is no modulation in AU Mic, and the optical photometry data reveal a poor correlation between optical flare strength and UV emission line enhancements. The differential emission measure curves for BY Dra and AU Mic are compared with solar curves. It is noted that both stars show the presence of hot material throughout their rotation period.

Butler, C. J.↗

NASA NTRS1986

High-dispersion observations of alpha Bootis (K1 III) with the International Ultraviolet Explorer

Echelle spectrograms were obtained of the early-K red giant Arcturus in the far UV 1150-2000 A and middle-UV 2000-3200 A regions. Faint emission from forbidden Si II at 1892.0 A was detected. A surface flux of C IV was measured that was less than 1 percent of the solar surface flux. An observed faint spectral structure near 1380 A is believed due to the bands of the A-X fourth positive system of CO fluoresced by strong chromospheric emissions of the O I triplet. Constraints on the chromospheric structure, as evidenced by the fluorescent emissions and density-sensitive line ratios from multiplet UV lines of forbidden C II, are discussed.

Ayres, T. R.↗

NASA NTRS1986

Definition and empirical structure of the range of stellar chromospheres-coronae across the H-R diagram: Cool stars

Major advances in our understanding of non-radiative heating and other activity in stars cooler than T sub eff = 10,000K has occured in the last few years. This observational evidence is reviewed and the trends that are now becoming apparent are discussed. The evidence for non-radiatively heated outer atmospheric layers (chromospheres, transition regions, and coronae) in dwarf stars cooler than spectral type A7, in F and G giants, pre-main sequence stars, and close bindary systems is unambiguous, as is the evidence for chromospheres in the K and M giants and supergiants. The existence of non-radiative heating in the outer layers of the A stars remains undetermined despite repeated searches at all wavelengths. Two important trends in the data are the decrease in plasma emission measure with age on the main sequence and decreasing rotational velocity. Variability and atmospheric inhomogeneity are commonly seen, and there is considerable evidence that magnetic fields define the geometry and control the energy balance in the outer atmospheric layers. In addition, the microwave observations imply that non-thermal electrons are confined in coronal magnetic flux tubes in at least the cool dwarfs and RS CVn systems. The chromospheres in the K and M giants and supergiants are geometrically extended, as are the coronae in the RS CVn systems and probably also in other stars.

Linsky, J. L.↗

NASA NTRS1986

Where do flares occur in RS CVn systems? Analysis of the October 3, 1981 flare on V711 Tau = HR 1099 observed by IUE

A set of IUE observations of V711 Tau = HR 1099 (K1 IV and G2 V) in which high-resolution spectra with the SWP and LWR cameras were obtained during the luminous flare of October 3, 1981 is discussed. Multigaussian fits to the Mg II k and C IV 1548 A lines are consistent with the flare radial velocity being identical to that of the K star. Thus, the flare probably occurred on the K star. There is no evidence for flows as seen during a flare on UX Ari. The strongest emitters of UV radiation during the flare were the L alpha and Mg II lines, which emitted seven times as much energy as all the transition region lines combined. The peak luminosity of the flare in the UV emission lines is 1.5 times 10 to the 31st power ergs/sec, and the total emission in these lines during the flare was about 4 times 10 to the 35th power ergs.

Linsky, J. L.↗

NASA NTRS1986

High dispersion IUE observations of hybrid-chromosphere stars

High dispersion, wavelength calibrated IUE spectra of the hybrid-chromosphere stars alpha TrA (K2 IIb-IIa) and upsilon Aql (K3 II) are presented. These observations allow accurate measurement of the wind terminal velocities as seen in the Mg II emission line profiles. The terminal velocities deduced for upsilon Aql and alpha TrA are 110 and 120 km/sec respectively. The terminal velocity for alpha TrA is significantly reduced from the 180 km/sec seen in spectra obtained 2 yr earlier. The intersystem emission lines of C I (1993 A), C II (2325 A), C III (1908 A), O I (1641 A; fluoresced) and Si III (1892 A) are at essentially their rest wavelengths. The upsilon Aql C II intersystem multiplet line fluxes indicate an electron density near 1 billion/cc in their line formation region. Results suggest that the transition region intersystem lines are unlikely to be formed in the high velocity wind seen in the Mg II lines but are certainly formed in a region with significant turbulence.

Brown, A.↗

NASA NTRS1986

Ultraviolet and optical variability of RY Tauri

Ultraviolet and optical observations of the T Tauri star RY Tau were analyzed. They show evidence for strong variability over 1985 to 1986. High dispersion IUE observations of the Mg II emission line profiles show significant changes in the wind of RY Tau, with the wind absorption almost disappearing on 1986 March 20. The behavior of the emission line flux variability when compared to the optical light curve shows evidence for changes in circumstellar extinction and changes in starspot/plage distribution on the visible hemisphere of the star.

Brown, A.↗