Engineering topics

Smith, Myron A.

Publications and source records attributed to Smith, Myron A..

13 records

NASA NTRS1999

An EUVE Study of the B Stars Epsilon and Beta Canis Majoris

This program is a study of the comparative characteristics of the temporal variations in the extreme ultraviolet (EUV), middle ultraviolet, and optical spectral regions of the pulsating star beta Canis Majoris (beta CMa) using data from the Extreme Ultraviolet Explorer satellite. (Although the title includes two stars, only beta CMa was actually observed.) This is a program Initiated at the University of Wisconsin by J. Cassinelli and D. Cohen (now at Univ. of Delaware) and subcontracted into several components. The portion delegated to CSC (Smith) had two components: a study of the stars flux variations in the middle ultraviolet wavelength region and consulting on the pulsational characteristics of this class of stars. The research for CSC's component of this work has been to identity a group of high-resolution spectra In the final archives of the International Ultraviolet Explorer satellite and to analysis the difference between selected spectra during low and high flux phases. This analysis was carried out by binning the spectral fluxes to low resolution, taking their ratio, and examining the spectrum ratio for spectrophotometric signatures sensitive to temperature difference and density. These were calibrated against spectral flux ratios obtained from model atmospheres. These results have been completed and the results communicated to the PIs of the program. We have also communicated to them the general results of the nature of the pulsational amplitude variations both from his work and from the work of other researchers in the astronomical literature. It is expected that this work will be published soon in a refereed astronomical journal.

Smith, Myron A.↗

NASA NTRS1997

X-Ray Flare Characteristics in lambda Eridani

This proposal was for a joint X-ray/ultraviolet/ground-based study of the abnormal Be star lambda Eri, which has previously shown evidence of X-ray flaring from ROSAT observations in 1991. The X-ray component consisted of observations from both the ASCA and ROSAT satellites.

Smith, Myron A.↗

NASA NTRS1997

X-Ray Flare Characteristics in the B2e Star Lambda Eridani (ROSAT)

We document the results of a simultaneous wavelength monitoring on the B2e star (lambda) Eri. This campaign was carried out from ground stations and with the ROSAT, ASCA, IUE, and Voyager 2 space platforms during a week in February-March 1995; a smaller follow-up was conducted in September 1995. During the first of these intervals (lambda) Eri exhibited extraordinary wind and disk-ejection activity. The ROSAT/HRI X-ray light curves showed no large flares such as the one the ROSAT/PSCA observed in 1991. However, possible low level fluctuations in the February-March ROSAT data occurred at the same time as unusual activity in H(alpha) He I (lambda)6678, He II (lambda)1640, and the C IV doublet. For example, the hydrogen and helium lines exhibited an emission in the blue half of their profiles, probably lasting several hours. The C IV lines showed a strong high-velocity Discrete Absorption Component (DAC) accompanied by unusually strong absorption at lower velocities. The helium line activity suggests that a mass ejection occurred at the base of the wind while the strong C III (Voyager) and C IV (IUE) lines implies that shock interactions occurred in the wind flow. It is not clear that the X-ray elevations are directly related to the strong C IV absorptions because the former changed on a much more rapid timescale than absorptions in the C IV lines. Within hours of the mild X-ray flux variations found by ROSAT on February 28, the Voyager UVS observed a "ringing" that decayed over three 3-hr. cycles. The amplitude of these fluctuations was strong (50%) at (lambda)(lambda)950-1100, decreased rapidly with wavelength, and faded to nondetection longward of (lambda)1300. Various considerations indicate that these continuum variations were not due to an instrumental pathology in the UVS. Rather, they appear to be due to a time-dependent flux deficit in the (lambda)(lambda)950-1250 region. We outline a scenario in which a dense plasma structure over the star's surface is heated and cooled quasi-periodically to produce such flux changes. Observations of new examples of this phenomenon are badly needed. Amateur astronomers can make a significant contribution to its understanding by searching for ringing in light curves of Be stars during their outburst phases. Finally we draw attention to an increase in the emission of the H(alpha) line that occurred at about the time the FUV ringing started. This increased emission hints that approximately 50,000K plasma near the star's surface can influence the circumstellar disc at approximately 12R. by its increased Lyman continuum flux.

Smith, Myron A.↗

NASA NTRS1997

X-Ray Flare Characteristics in Lambda Eridani

Smith, Myron A.↗

NASA NTRS1995

Dynamic processes in Be star atmospheres. 3: Rapid multiwavelength variability in gamma Cassiopeiae

A review of the characteristics of X-ray emission from gamma Cas suggests various weaknesses with conventional scenarios that invoke Be wind interactions with a putative binary companion. To explore whether this emission might arise instead from sporadic high-energy events on the Be star itself, we have examined the temporal behavior of Voyager 1 FUV spectra and He I lambda 6678 line profiles. The FUV continuum light curves show a continuous rapid variability with a range of greater than or = 0.2 mag over a timescale of approximately 1 hr. All indications are that this variability is real. The difference spectrum of the inferred fluctuating component is identical to the photospheric spectrum, ruling out an origin much hotter than the star's effective temperature. A histogram of flux differences formed from consecutive observations shows a flat-topped distribution out to 0.2 mag. This result bears a striking resemblance to histograms constructed from EXOSAT and Ginga data, suggesting that the X-ray and FUV variabilities could have a common origin. Evidence is also presented of rapid variability in optical spectral lines associated with the stars photosphere. Time series of difference profiles of the lambda 6678 line confirm the existence of migrating subfeatures reported by earlier studies. The comoving average of these profiles shows that these features have a shape resembling a symmetrical 'wave-packet'. Wave packets appear to be preceeded and rejuvenated by a sharp central absorption feature. We also report a transient emission feature in lambda 6678 that is similar to brief emissions once reported by Slettebak & Snow in UV resonance line profiles of Si IV and Mg II. We argue that the FUV and optical variability may find an explanation in terms of magnetic flares. Solar white-light flares offer a possible paradigm. However, problems remain with such explanations, in particular their high mean energy, frequency, and temperature.

Smith, Myron A.↗

NASA NTRS1995

Dynamic Processes in Be Star Atmospheres.: Simultaneous X-Ray, Ultraviolet, and Optical Variations in lambda Eridani - 6

This report describes a joint X ray/ultraviolet/ground based study of the abnormal Be star lambda Eri which has previously shown evidence of X ray flaring from Rosat observations in 1991. The 1991 flare event caught the astronomical hot star community by surprise because x ray flares have not been observed from other single B-type stars, before or since. Both optical (H-alpha) and UV/Voyager observations provide evidence for transient heating events near the surface of lambda Eri.

Smith, Myron A.↗

NASA NTRS1994

Dynamic processes in Be star atmospheres. 2: He I 2P-nD line formation in lambda Eridani (outburst)

The He I lambda 6678 line of early Be stars generally shows violet (V) and red (R) emission whenever hydrogen alpha emission is present, but its use as a diagnostic has been handicapped by a poor understanding of the processes that drive it into emission. In an attempt to address this problem we obtained three series of eschelle spectra of the first two members of the singlet and triplet 2P-nD series of lambda Eri (B2e) during 1992 November 3-5 at Kitt Peak. During these observations lambda 6678 showed substantial emission variability in both the wings and central profile, providing an opportunity to compare its behavior with that of the lambda 4922, lambda 5876, and lambda 4471 lines. We found that the responses of the lines were different in several respects. Whereas the emissions in the V wings of all four lines scaled together, the R wing of the lambda 4922 line invariably responded with increased absorption whenever the R wing of lambda 6678 line showed increased emission. These same trends occurred within the central photospheric profiles. The R-wing behavior shows that much, but not all of the emission in lambda 6678 is caused by matter projected against the stellar disk. The excitation temperatures of the neighboring 2(sup 1) P transitions, lambda 6678 and lambda 4922 must be greater than and less than the photospheric continuum temperature, respectively. We have investigated departures from local thermodynamic equilibrium (LTE) for the He I spectrum in a variety of ad hoc, perturbed model atmospheres. We have found only one way to cause the source function of lambda 6678 to increase so strongly, namely, by increasing the atmospheric temperature in the line formation region to 30,000 - 40,000 K. This effect was discovered by Auer and Mihalas for O3-O4 atmospheric models, but it has not been applied to active B stars. Our models suggest that lambda 6678 emission in Be stars can be used as a sensitive monitor of localized hot spots on these stars' surfaces. The energies involved in heating the active portions of the atmosphere are too high to be produced by gravitational infall. This leaves magnetically induced flares among the few known processes on the surfaces of stars capable of sustaining this energy level.

Smith, Myron A.↗

NASA NTRS1994

Photospheric Activity in Selected Be STARS: lambda Eri and gamma Cas

Recent observations of rapid variations in optical He I lines, X-rays, and FUV wavelengths in the prototypical classical Be stars lambda Eri and star gamma Cas hint that the violent processes occur on the surfaces of these stars almost all the time. We suggest that of these phenomena show greater similarities with magnetic flaring than any other process through to occur on stars.

Smith, Myron A.↗

NASA NTRS1994

Puzzling Problems of He I Line Formation in Early B Stars

Although NLTE model atmospheres have been shown to resolve most of the equivalent width (EW) discrepancies for blue He I lines (Auer and Mihalas 1972, 1973) Wolff and Heasley 1984, 1985) have demonstrated that discrepancies remain for the leading members of the singlet/triplet 2P - nD series viz. lambda 6678 and lambda 5876. These two lines are the strongest nonresonance He I transitions and are important because they respond to thermal changes in the superficial atmosphere (approximately 10(exp -3)) of early B stars. In order to understand the observed rapid variations of the lambda 6678 line in mild Be stars, we undertook a survey of EW's of lambda 6678 and lambda 4388, namely the first and third member of the same series. These two lines have a log gf ratio of 15 but have similar EW's in B star spectra. Our new observations confirm the red line discrepancy noted by WH85 and point to additional EW differences among various groups of B stars not noted hitherto.

Smith, Myron A.↗

NASA NTRS1993

A giant X-ray flare on Lambda Eridani (B2e)

A 30 ks observation with the ROSAT PSPC distributed over 39 hr shows that the putatively single, mild B2e star Lambda Eri emits at most times a soft X-ray flux at a rate and temperature consistent with other B stars. However, during the middle of our observations, this star's X-ray flux increased by a factor of 6 before returning to the basal level. This brightening, due entirely to photon energies of 0.7 keV or greater, can be fitted well to a Raymond-Smith temperature parameter of 14 MK and luminosity 4 x 10 exp 31 ergs/s; these are characteristics of giant stellar flares. With an estimated duration of about 50,000 s, this event is arguably the strongest X-ray flare yet observed. We consider several possible scenarios for the site of the flare, including several with an active cool secondary or degenerate companion. We find that IUE and optical spectra do not support a binary picture and that it is most probable that the flare site is on or related to Lambda Eri itself. This supports other evidence for violent magnetic activity on some B-type stars.

Smith, Myron A.↗

NASA NTRS1993

Dynamic processes in Be star atmospheres. I - 'Dimple' formation in the He I lambda 6678 line of lambda Eridani

Several examples of weakenings of the C IV and N V resonance lines are found to coincide with the appearance of lambda 6678 dimples. The absence of variations in other UV lines and in the UV continuum at the same time or nearly the same time argues against dimples being caused by thermal variations from the underlying star. It is instead suggested that the resonance line weakenings are caused by non-LTE effects associated with the condensation of high density structures at some elevation over the star. A simple model of an opaque, essentially stationary slab which backscatters lambda 6678 line radiation into a surrounding 'penumbral' region is presented. Lambda 6678 photons are scattered a second time in this region back into the observer's line of sight and in the process acquire the local projected Doppler shift from rotation. Slabs would probably produce too little emission to be easily detected in the H alpha profile. Their detection in strong He I lines seems the best strategy among early Be stars.

Smith, Myron A.↗

NASA NTRS1990

A revised parametrization of the dispersion constants for high dispersion IUE images

An economical system of parameterizing dispersion constants spectra is derived and applied to International Ultraviolet Explorer (IUE) spectra by equating specific terms to geometric coordinates of an echellogram. Because of the virtual property of the IUE camera pixels, several geometrical rectifications can be performed analytically before the dispersion relations are solved for line and sample position. This permits the relations to be decoupled and reduced to two terms apiece, the simplest representation possible. A generalization of these principles allows the dispersion relations to be determined neatly in other echelle systems having certain well understood distortions. Prismatic cross dispersion, S wave distortion, and optical aberrations are some areas in which this system can be used.

Smith, Myron A.↗

NASA NTRS1990

A new approach to remove interorder background in high dispersion IUE images

A two dimensional Chebyshev interpolating scheme, followed by modeling of point spread functions as Voigt profiles, is proposed for final archiving IUESIPS to remove the interorder overlap from the smoothed background fluxes of high dispersion International Ultraviolet Explorer (IUE) images. Tests of the first and last stages of the algorithm suggest that it can give a considerably better zero point than the old IUESIPS algorithm which samples local interorder background.

Smith, Myron A.↗