Engineering topics

Bookbinder, J. A.

Publications and source records attributed to Bookbinder, J. A..

8 records

NASA NTRS1994

A search for radio emission from the 'nonmagnetic' chemically peculiar stars

We have observed 23 members of the Am and HgMn subclasses of chemically peculiar (CP) stars with the Very Large Array (VLA) to search for nonthermal radio emission at levels comparable to those found for the Si and He peculiar subclasses of the CP stars by Linsky et al. (1992). This study was motivated by recent claims that magnetic fields of kilogauss strength are present in at least some of the Am and HgMn stars, contrary to previous beliefs, which would indicate that radio-emitting magnetospheres could be present in these stars. We detected none of the Am and HgMn stars as radio emitters with upper limits typically less than 0.20 mJy. Applying a correlation between radio luminosity, surface magnetic field, and effective temperature derived from previous radio studies of the Si and He peculiar CP stars, we find that the predicted radio luminosities of alpha And (an HgMn star) and Sirius (a hot Am star) are more than an order of magnitude larger than the observed upper limits, indicating that these stars lack magnetospheres, and, by inference, surface magnetic fields.

Drake, S. A.↗

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.↗

NASA NTRS1994

A far-ultraviolet flare on a Pleiades G dwarf

The Hubble Space Telescope/Faint Object Spectrograph (HST/FOS) recorded a remarkable transient brightening in the C IV lambda lambda 1548,50 emissions of the rapidly rotating Pleiades G dwarf H II 314. On the one hand the 'flare' might be a rare event luckily observed; on the other hand it might be a bellwether of the coronal heating in very young solar-mass stars. If the latter, flaring provides a natural spin-down mechanism through associated sporadic magnetospheric mass loss.

Ayres, T. R.↗

NASA NTRS1993

Stellar coronae at the end of the main sequence: A Rosat survey of the late M dwarfs

X-ray data, both detections and upper limits, from the Rosat all sky survey for most known M dwarfs later than type M5 are presented. Selected Rosat pointed observations of some of these stars are included. These data are compared to similar data for early M dwarfs in an attempt to probe the nature of the magnetic dynamo and coronal heating mechanism for the very late M dwarfs, which are presumably totally convective. The results indicate that late M dwarfs can have coronae which are just as active as those for the early M dwarfs and that coronal heating efficiency for 'saturated' stars does not drop at spectral type M6.

Fleming, T. A.↗

NASA NTRS1990

The intrinsic H I Lyman-alpha line profiles of late-type stars

The Lyman-alpha line of neutral hydrogen is probably the most important cooling channel for chromospheric plasma in late-type stars, yet it is also the least studied major line in the far ultraviolet. The scattering of much of the stellar Lyman-alpha flux by interstellar hydrogen, coupled with the geocoronal emission foreground, seriously complicates the analysis of the Lyman-alpha spectra. The influence of the local interstellar medium on the observed profiles was circumvented by observing stars with radial velocities sufficiently high to Doppler shift the center of the stellar emission line out of the interstellar absorption core. There are several stars that have high radial velocities by virtue of their presence in close binary systems. High resolution IUE (International Ultraviolet Explorer) spectra of Ly alpha line of two such eclipsing binary stars, AR Lac and TY Pyx, are obtained, at each orbital quadrature phase, when the projected orbital velocity is a maximum. By combining the spectra from opposite quadratures it is possible to piece together the entire stellar emission profiles. The third star in this study, delta Lep, is a single star with a high space velocity.

Neff, J. E.↗

NASA NTRS1987

Discovery of radio emission from AE Aquarii

VLA 1.4-GHz and 4.9-GHz observations of six DQ Her cataclysmic variables, obtained in the C/D hybrid configuration with 50-MHz bandwidth, 7-sec time resolution, and limiting flux density about 200 microJy on July 21, 1984, are reported. Variable radio emission with time scale less than 5 min, circular polarization less than 15 percent, and flux density 3-5 mJy at 1.4 GHz and 8-16 mJy at 4.9 GHz is detected from AE Aqr. This emission is tentatively attributed to synchrotron emission from mildly relativistic electrons, powered by the MHD torque coupling the magnetic white dwarf to either (1) a secondary with a strong magnetic field or (2) an accretion disk.

Bookbinder, J. A.↗

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

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.↗