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NASA NTRS1987

First dynamic spectra of stellar microwave flares

The VLA has been used in the spectral-line mode at 1.4 GHz to obtain the first dynamic spectra of stellar sources other than the sun. Two very intense, highly circularly polarized, microwave outbursts were observed on the dMe flare star UV Cet, in addition to a slowly varying, unpolarized component. One outburst was purely left circularly polarized and showed no variations as a function of frequency across the 41 MHz band, whereas the other was as much as 70 percent right-circularly polarized and showed distinct variations with frequency. Although the slowly varying emission is probably due to incoherent gyrosynchrotron emission, the two flaring events are the result of coherent mechanisms. The coherent emission is interpreted in terms of plasma radiation and the cyclotron maser instability.

Bastian, T. S.↗

NASA NTRS1987

Dynamic spectrum of a radio flare on UV Ceti

The dMe flare star UV Ceti (L726-8B) was observed at four frequencies simultaneously in the 1385-1652 MHz band using the Very Large Array. A flare lasting 10 minutes was observed with 6.67 s time resolution 'Dynamic spectrum'-type images in the Stokes parameters I and V show considerable complexity in the frequency-time domain; some features show a positive frequency drift with time, while others are more complex, involving both positive and negative frequency drifts. The positive drift features would be consistent with disturbances traveling downward in the star's corona.

Jackson, Peter D.↗

NASA NTRS1987

Rotational modulation and flares on RS CVn and BY Dra-type stars. V - Exosat and IUE observations of a flare on EQ Pegasi

Time-trailed UV spectra and a soft X-ray lightcurve were obtained during a flare on the binary dMe star EQ Peg AB. On the basis of solar flare Mg II surface fluxes and an about 70-percent enhancement in the disk-integrated flux during the flare on EQ Peg, it is estimated that the chromospheric flare covered about 1.5 percent of the stellar surface. The size scale is estimated to be comparable to that of a solar two-ribbon flare. This unusual flare is noted to exhibit a low-energy soft X-ray rise phase which is longer than that of most flares; it is longer than the decay phase and peaks much later than the medium energy light curve. Evidence of variable Fe II emission and variable continuum emission in select UV bands during the flare is noted.

Haisch, B. M.↗

NASA NTRS1988

A VLA survey of BY Draconis variables

Observations of seven rapidly rotating BY Dra variables, obtained at 6 cm with the D-to-A hybrid array of the VLA on three consecutive days in February, 1986, are reported. The results are presented in tables and graphs (along with data from simple VLA detection observations in 1985 and published optical and X-ray data) and characterized in detail. No strong correlation is found between the radio and optical emission in the one star (BY Dra) detected at more than one session, and the observable microwave emission of the stars is tentatively attributed to flares alone. A weak correlation between the X-ray luminosity and the maximum 6-cm luminosity of dMe stars is inferred, but the evidence is considered insufficient to establish a relationship between surface signs of magnetic activity and nonthermal radio emission.

Caillault, Jean-Pierre↗

NASA NTRS1988

Flight tests of three-dimensional path-redefinition algorithms for transition from Radio Navigation (RNAV) to Microwave Landing System (MLS) navigation when flying an aircraft on autopilot

This report contains results of flight tests for three path update algorithms designed to provide smooth transition for an aircraft guidance system from DME, VORTAC, and barometric navaids to the more precise MLS by modifying the desired 3-D flight path. The first algorithm, called Zero Cross Track, eliminates the discontinuity in cross-track and altitude error at transition by designating the first valid MLS aircraft position as the desired first waypoint, while retaining all subsequent waypoints. The discontinuity in track angle is left unaltered. The second, called Tangent Path, also eliminates the discontinuity in cross-track and altitude errors and chooses a new desired heading to be tangent to the next oncoming circular arc turn. The third, called Continued Track, eliminates the discontinuity in cross-track, altitude, and track angle errors by accepting the current MLS position and track angle as the desired ones and recomputes the location of the next waypoint. The flight tests were conducted on the Transportation Systems Research Vehicle, a small twin-jet transport aircraft modified for research under the Advanced Transport Operating Systems program at Langley Research Center. The flight tests showed that the algorithms provided a smooth transition to MLS.

Hueschen, Richard M.↗

NASA NTRS1988

CH3O(A-X) fluorescence from photodissociation of dimethyl ether

The paper reports the quantitative photoabsorption and fluorescence cross sections of dimethyl ether (DME) measured with synchrotron radiation. Determinations were also made of the absolute fluorescence cross sections, the fluorescence quantum yield, and the radiative lifetime.

Suto, Masako↗

NASA NTRS1988

X-ray emission from the flare star binary Gliese 867 A

Coronal X-ray emission from the BY Draconis type dMe flare star G867 A was observed on three successive days in May 1980 with the Imaging Proportional Counter on the Einstein Observatory. A 40-percent decrease in the quiescent state X-ray intensity of the star was detected over a three-day period. It is suggested that this slow quiescent state X-ray intensity variation may be the X-ray analogue of the BY Draconis variability observed in the optical band and may be explained by the star spot model. The energy spectra of the star were obtained for the three days and are best fitted by a two-temperature Raymond-Smith thermal model. There is some indication of spectral variations in the low temperature component. Implications of these results are briefly discussed.

Agrawal, P. C.↗

NASA NTRS1988

Target acquisition and track in the laser docking sensor

A sensor designed to aid in the docking of spacecraft is under development for NASA. This sensor uses three lasers to track the prospective target and to determine the required parameters necessary to calculate the ideal approach maneuver. The system combines the inputs from several sensors, including polarization, continuous tone DME, and a CID to achieve the desired results.

Clowes, Ted J.↗

NASA NTRS1989

A study of excess H-alpha emission in chromospherically active M dwarf

Spectroscopic observations from three observatories are combined to study the properties of the excess H-alpha emission which characterizes the most chromospherically active subset of the M dwarf stars, known as the dMe stars. It is demonstrated that the excess H-alpha luminosity from these stars is a monotonically decreasing function of their (R-I) color, and evidence is presented which suggests that the product of the mean surface brightness and the mean filling factor of the emissive regions is essentially constant with color. Another significant result of the study is a linear correlation between the excess luminosity in H-alpha and the coronal X-ray luminosity.

Young, Arthur↗

NASA NTRS1989

Newborn stars and stellar winds in Barnard 228

Imaging and spectroscopic observations of premain-sequence stars in the B228 molecular cloud have identified three sites of extended, shock-excited nebulosity. A highly collimated, blueshifted jet is associated with the T Tauri star Sz68 (CoD -33:10685 deg). A candidate Herbig-Haro object is found near Sz68 and the T Tauri star Sz69. A third region of shock-excited nebulosity is identified near the embedded newborn star IRAS 15398-3359. IR excesses observed toward these stellar sources may imply a relationship between outflow activity and the presence of a circumstellar disk. Only one of the two outflow sources with well-defined orientations is aligned with the large-scale, magnetic-field geometry of the cloud in contrast to the global alignment of outflows in the Taurus cloud complex. Data for three previously unidentified emission-line stars are presented. A faint 18th magnitude dMe star located 48 arcsecs west of Sz68 is identified.

Heyer, Mark H.↗

NASA NTRS1991

Stellar Lyman-alpha emission and the local interstellar medium

The UV spectra now available in the IUE archives have been systematically searched for the presence of Lyman-alpha emission in late-type stars. The results provide the first survey of the behavior of the Ly-alpha emission line across most of the cool half of the H-R diagram. The normalized Ly-alpha emission flux is strongest in the chromospherically active RS CVn stars and in the dMe stars. The range of normalized flux values is much smaller among F-type stars than in stars of later spectral type. A dropoff appears in the flux levels of stars at B-V less than 0.30 mag. The measurements are used to search for evidence of possible high column density clouds in the local ISM. The cloud previously identified toward Alpha Oph may be seen in the reduced flux observed toward Beta Oph.

Landsman, Wayne↗

NASA NTRS1991

Peeking through the picket fence - What astrophysical surprises may be present in the 100-200 A region?

In anticipation of more sensitive EUV and FUV spectroscopic instruments, spectra are simulated including interstellar absorption of solarlike, RS CVn, and flare stars as folded through the instrument parameters of the EUVE, Lyman/FUSE Phase A, and a desirable next-generation spectrometer. Even the relatively insensitive EUVE spectrometer is able to detect sufficient spectral lines from many active binary and dMe stars to determine their coronal-emission-measure distributions. The Lyman/FUSE or next-generation spectrometers are needed to study solar-type stars or flaring stars with high time resolution. The high throughput and effective area of a nex-generation spectrometer is needed for Doppler imaging studies, stellar wind and downflow measurements, and high time and spectral resolution of stellar flares.

Linsky, Jeffrey L.↗

NASA NTRS1991

X-rays from stellar flares

A summary of X-ray observations of flares on dMe, active spectroscopic binaries and young stars is presented. Consideration is given to the energy associated with the X-ray emission and its relation to other components of the flare energy budget, the time behavior of the flaring plasma as seen by the X-ray emission, and comparisons of stellar flare parameters with solar compact and two ribbon flares. Flares are easily detected when the contrast in the emission from the flaring plasma relative to the stellar photosphere is large as in the X-ray, microwave, and UV regions of the spectrum.

Linsky, Jeffrey L.↗

NASA NTRS1994

A coronal mass ejection model for the 1992 July 15 flare on AU Microscopii observed by the extreme ultraviolet explorer

The dM1e flare star AU Microscopii (AU Mic) was observed by the EUVE Deep Survey Instrument on 1992 July 14-18. A large flare was detected in the Deep Survey Lexan/Boron (DS Lex/B)(65-190 A) band and the SW (70-190 A) and MW (140-380 A) spectrometers. The flare consisted of a sharp impulsive peak lasting approximately 2 hours followed by a decaying tail lasting about a day. We present a simple, single temperature, dynamic model for the flare decay which is consistent with the DS Lex/B light curve and reproduces the strongest, high-temperature spectral lines in the released EUVE spectra. In this model, we assume the long decay time is due to an ejected, magnetically confined, low beta plasmoid expanding self similarly in the ambient medium in a manner reminiscent of solar coronal mass ejections. We demonstrate that the long tail of the DS Lex/B light curve can be explained by rapid expansion, causing the plasma to become tenuous sufficiently quickly that it avoids catastrophic radiative cooling. From this model, we estimate the mass of the plasmoid to be approximately = 10(exp 20) g and the total energy of the event to be approximately = 10(exp 36) ergs. These values are approximately 10(exp 4) times as large as those seen during the largest solar coronal mass ejection (CME) events. We argue that the results of our model are consistent with other measurements of stellar flare parameters. We also estimate a mass-loss rate of a few times 10(exp -13) Solar mass/yr and discuss the role of mass loss from dMe stars in the mass balance of the interstellar medium. We estimate the rotational braking timescale from these events to be less than 500 million years and suggest that CME's may be an important source of angular momentum loss from late-type stars.

Cully, Scott L.↗

NASA NTRS1995

The faint X-ray sources in and out of omega Centauri: X-ray observations and optical identifications

We present the results of an observation of the globular cluster omega Cen (NGC 5139) with the Einstein high-resolution imager (HRI). Of the five low-luminosity X-ray sources toward omega Cen which were first identified with the Einstein imaging proportional counter (IPC) (Hertz and Grindlay 1983a, b), two are detected in the Einstein HRI observation: IPC sources A and D. These detections provide source positions accurate to 3 sec-4 sec; the positions are confirmed in a ROSAT HRI observation reported here. Using CCD photometry and spectroscopy, we have identified both sources as foreground dwarf M stars with emission lines (dMe). The chance projection of two Mde stars within approximately 13 min of the center of omega Cen is not extraordinary, given the space density of these stellar coronal X-ray sources. We discuss the possible nature of the three as yet unidentified IPC sources toward omega Cen, and consider the constraints that the Einstein observations place on the total population of X-ray sources in this cluster. The integrated luminosity from faint X-ray sources in omega Cen appears to be low relative to both the old open cluster M67 and the post-core-collapse globular, NGC 6397.

Cool, Adrienne M.↗

NASA NTRS1994

Light echo detection of circumstellar disks around flaring stars

Light echoes can be used to detect and characterize disks around flaring stars. Such disks are thought to be a hallmark of planet formation but are very difficult to detect by ordinary means. Dwarf emission-line M stars experience flares with luminosities comparable to their quiescent photospheres on time scales of minutes, less than the light travel time across a disk many astronomical units in extent; they are thus ideal candidates for such a search. Bromley (1992) calculated that the detection of Jupiter-sized companions using light echoes requires photometric accuracies better than 1 part in 10(exp 6). However, a disk consisting of grains or small bodies will scatter a much larger fraction of the light than a planet of similar mass. I estimate the light echo amplitutdes from plausible geometries of circumstellar material and present simulation light curves. The light echo amplitudes are typically 1% of the flare and I conclude that such events will be detected best in cases where the flare is eclipsed by the star. An examination of the time scales associated with internal processes in a protoplanetary disks around dM stars indicates that any primordial disks may become undetectable in 10(exp 4) years and will have completely disappeared by 10(exp 8) years, the estimated age of dMe stars in the solar neighborhood. However, searches for light echoes might constrain the amount of material continuing to fall into these young stellar systems in the form of comet-like objects.

Gaidos, Eric J.↗

NASA NTRS1994

Rotational modulation and flares on RS Canum Venaticorum and BY Draconis stars. 18: Coordinated VLA, ROSAT, and IUE observations of RS CVn binaries

As part of a coordinated program of multi-wavelength observations of RS CVn close binary systems, we observed 15 systems with the Very Large Array (VLA) and 10 systems with IUE, simultaneously or nearly simultaneously with the ROSAT All Sky Survey observations of these stars. Of the 22 systems observed with ROSAT, three were observed both by IUE and the VLA. Radio observations were made at 3.6, 6 and 20 cm. Of the 15 observed RS CVn systems, we detected 11 with greater than 4 sigma confidence at one or more wavelengths. The IUE observations were made within the RIASS (ROSAT-IUE All Sky Survey) program. We present the results of the VLA observations, along with the corresponding subsets of the ROSAT Position Sensitive Proportional Counter (PSPC) X-ray and Wide Field Camera (WFC) XUV survey, and RIASS IUE observations. We obtained an extended VLA/IUE/ROSAT simultaneous coverage of one system, TY Pyx, covering more than one orbital period. These observations reveal that the quiescent radio flux of TY Pyx is relatively constant over time scales of up to 7 hours, but that it did change by a factor of 3 over 24 hours, probably due to a flare on 1990 Nov. 12. The UV, XUV and X-ray fluxes do not show large day-to-day or phase-related variability. The observation of the decay phase of a radio flare on EI Eri, with no accompanying X-ray or XUV flare, suggests that the lack of a strong correlation between X-ray and radio flares previously noted for dMe flare stars holds for RS CVn systems as well. We suggest that the radio flare may have been due to a coherent emission process such as electron cyclotron emission. The simultaneous measurements presented here provide a unique test of the general correlation between radio and soft X-ray luminosities, L(sub radio) approximately L(sub x exp m) (Drake et al. 1989) with a power-law slope close to unity, which was previously derived using data obtained years apart. Our derived slopes are consistent with and thus support the general correlations between coronal and chromospheric/transition region emissions previously derived from nonsimultaneous measurements of a much larger sample of these variable sources. However, the importance of simultaneous measurements for accurate energy balance calculations is stressed.

Fox, David C.↗

NASA NTRS1994

High sensitivity dynamic spectral search for flare star radio

We observed ten well-known flare stars with the Arcibo radio telescope at 1.4 GHz and 5 GHz, using a special observing technique to discriminate between real flares and radio freqeuncy interference. With a high sensitivity of 5.5 K/Jy at 1.4 GHz when averaged over a 50 MHz band, we are able to recognize flux enhancements as weak as approximately 6 mJy above the sky background variations. In about 85 hours of observation, about a dozen bursts were detected, only from AD Leo. All had flux densities lower than 70 mJy, which probably explains their lack of fine structures (except for the strongest one), such as were reported in the literature for stronger flares. Half of the bursts that we recorded are 100% circularly polarized, and half are not circularly polarized. Our results are a first attempt of reliable statistics on dMe flare rates at 1.4 GHz. The high brightness temperatures we infer for the observed bursts are interpreted in terms of coherent emission processes, either the cyclotron maser instability or plasma radiation. Efficiencies are comparable to those of solar or planetary radio emissions in the case of the cyclotron maser, and higher than the solar efficiency in the case of plasma radiation, with the caveat that there are great uncertainties in the coronal model and the source size.

Abada-Simon, M.↗