V649 CASSIOPEIAE: A MULTIPLE STAR SYSTEM?van Hamme, W.
doi: 10.1086/132984pmid: N/A
Previously published light and radial-velocity curves of the eclipsing and spectroscopic binary V649 Cas are reexamined using the Wilson-Devinney program. Two solutions are presented, both leading to typical B-type masses and radii for the two components. One of the solutions calls for large amounts of third light (42% of the total light in V, 46% in B). We speculate about the possible nature of this source of third light, and we plead for new photometric and spectroscopic observations of this bright (V = 6.m53) system.
ON THE USE OF SUPERNOVAE AS RADIO-OPTICAL ASTROMETRIC FIDUCIAL POINTSWeiler, Kurt W.; Johnston, K. J.; van Dyk, Schuyler D.
doi: 10.1086/132985pmid: N/A
The high accuracy radio astrometric reference frame is based on extragalactic radio sources which are optically faint (mV > 15), while the most accurate optical astrometric reference frame is made up of bright galactic stars which are almost all radio quiet to current sensitivity limits. The HIPPARCOS mission is establishing positional accuracies for these bright stars at the several millarcsecond level of precision, and advancements in space-based observations should eventually allow precisions at the submilliarcsecond level. However, the relationship between the two reference frames is difficult to directly establish to a level consistent with the internal precision of the two independent frames. The compact extragalactic radio sources are too faint for most optical astrometric techniques, and the bright stars which are active radio sources are often in binary or extended envelope systems where the radio and optical photocenters may be related at accuracies of only a few millarcseconds. This difficulty may be alleviated by using the occasional optically and radio bright supernovae as point-like fiducial marks for establishing relationships between the two reference frames. These independent reference frame relationships can be accomplished from currently available ground-based platforms.
THE ULTRAVIOLET SPECTRUM OF HH 24A AND ITS RELATION TO OPTICAL SPECTRABoehm, K. H.; Noriega-Crespo, A.; Solf, J.; Brugel, E. W.
doi: 10.1086/132986pmid: N/A
We have studied the spectrum of the brightest part (HH 24A) of the complex Herbig-Haro object HH 24 in the short wavelength (1300 A The ultraviolet spectrum is definitely detected in three observations with exposure times of 560 to 680 min. It is, however, sufficiently faint that corrections for the IUE "artifact spectrum" (as described by Crenshaw et al.) have to be applied. The spectrum shows only a continuum or a quasi-continuum and is not comparable not to that of the typical high excitation object like HH1 or HH2 nor to that of a low excitation object like HH 43 or HH 47. The HH 24A spectrum does not resemble the spectra of the T Tauri stars observed with the IUE. It is therefore not probable that, as in the optical range, we are seeing dust scattered light from a T Tauri or similar star. Two of the three spectra show a suprisingly similar detailed wavelength structure. If we assume that this means that these structures are real then no convincing explanation of the spectrum seems yet to be available. If, on the other hand, we are satisfied with a crude interpretation of the average wavelength dependence (ignoring detailed structures of the spectrum) the interpretation as a collisionally enhanced two-photon continuum may be acceptable. A study of the spatial_ distribution of the ultraviolet continuum emission shows that this distribution is considerably wider than that of the optical forbidden line emission, e.g., the [S II]6716/6731 lines.
X-RAY CONTINUUM WITH AN IRON EMISSION LINE FROM THE RADIO-QUIET QUASAR PHL 909Ghosh, K. K.; Soundararajaperumal, S.
doi: 10.1086/132987pmid: N/A
Results of the spectral analysis of the X-ray (0.1 - 10 keV) spectrum of the radio-quiet quasar, PHL 909, are presented. A spectrum of this quasar which was observed on 1984/349, was obtained from the EXOSAT archives. Different models were used to fit the ME and LE+ME data. Fit parameters to the ME data show a steep spectral index for this quasar typical of radio-quiet quasars. A weak 'soft excess' was detected in PHL 909. Two-power-law and broken-power-law models fitted well with the LE+ME data. A highly significant (> 99.9%) emission line was detected with line center energy being in the range 5.9-6.6 keV and the measured equivalent width between 300 and 100 eV. A thermal bremsstrahlung model with a gaussian line component also fits well with the data set (kT 1.60.4-0.3 keV), suggesting a cluster origin from the known cluster around this quasar. Alternatively the detected line emission may be due to the fluorescence of cold matter around the central continuum source outside our line-of-sight.
MILLISECOND TIME RESOLUTION WITH THE KITT PEAK PHOTON COUNTING ARRAYSharp, N. A.
doi: 10.1086/132988pmid: N/A
The Kitt Peak Photon-Counting Array (KPCA) has been modified to provide access to individual frames, with the accompanying short frame time per exposure of between two and five milliseconds. These changes are described and the instrument's performance is illustrated. Imaging of the Crab pulsar with millisecond time resolution is presented as an astronomical example. Poor observing conditions precluded the photometric quality which would normally be achieved with a photon counter.
PRECISION RADIAL VELOCITIES WITH AN IODINE ABSORPTION CELLMarcy, Geoffrey W.; Butler, R. P.
doi: 10.1086/132989pmid: N/A
We have used gaseous iodine for generating reference absorption lines in stellar spectra taken at high resolution. A major advance involves the use of a fast echelle spectrograph and a 2048X2048 CCD which acquires the near ultraviolet, the entire visible, and the near infrared spectrum in a single exposure. The superimposed iodine lines provide both a highly precise wavelength scale (calibrated with a Fourier transform spectrum) and a specification of the spectrograph PSF in situ_ over the entire echelle format. Test observations of three solar-type stars exhibit a velocity scatter of less than 25 m s-1 over a one-year duration, and only 1/5 of the available spectrum has been employed in the analysis to date. Velocity precision of 50 m-1 can be achieved for magnitude V=12 in one hour exposures on a 3-m telescope. We discuss an on-going project to detect brown dwarf and planetary companions to F, G, K, and M-type main sequence stars, designed to complement other efforts. The current velocity precision permits detection of companions with masses as low as 3 MJup located up to 5 AU from the star. We also discuss the use of precision velocities in revising cepheid distances.
A SIMPLE VISUAL CASSEGRAIN CCD CAMERA FOR THE WYOMING INFRARED OBSERVATORYGillam, S. D.; Johnson, P. E.; Smith, M.
doi: 10.1086/132990pmid: N/A
A visual Cassegrain CCD camera system, called the C3, has been constructed for use at the 2.3-m telescope at the Wyoming Infrared Observatory. It is optically, mechanically and cyrogenically simple. The instrument is based upon a Texas instruments TI-4849584x390 virtual-phase CCD and an Hitachi 63701 microprocessor. It is controlled remotely from an IBM PC XT microcomputer. The system will include an Ethernet connection to another computer which will be used as an in-situ image reduction facility. The design and photometric performance of the instrument are described, and an image taken using the C3 is presented. Further developments of the system are also discussed.
THE NYQUIST CRITERION IN CCD PHOTOMETRY FOR SURFACE BRIGHTNESSWildey, Robert L.
doi: 10.1086/132991pmid: N/A
Astronomers routinely violate the directive to sample surface brightness with at least twice the frequency of the highest spatial frequency of the Fourier transform of the continuous image, when doing direct CCD imaging. It is reasonably speculated that this practice is rationalized on the basis that the CCD does not actually sample_ the surface brightness at periodic intervals, but instead integrates the surface brightness over contiguous regions (the CCD pixels). It is herein derived that this mode of sampling changes the form of aliasing error, but the aliasing error is nevertheless present when undersampling occurs. The very nature of the error betrays the possibility of detecting its presence, a priori. It would be of value to develop an active optical apodizer to accommodate a given CCD, in terms of the Nyquist criterion, without the need to abandon either the full light-gathering power of the telescope or the plate-scale at the chosen observing station.
ON BLACKBODY BEHAVIOR AND THE TRANSFORMATION FROM INSTRUMENTAL TO STANDARD MAGNITUDES AND COLOR-INDICESWildey, Robert L.
doi: 10.1086/132992pmid: N/A
A first-order error analysis in the application of standard transformation techniques connecting instrumental to standard magnitude/color-index systems has been performed in which terms for departures from blackbody behavior and departures of the latter from the Wien approximation are separated. A best case is taken in the sense that confoundment by interstellar reddening or reflection from planetary surfaces is omitted, as well as the effects of finite spatial bandwidth. The results are presented in a form easily delineable with respect to departures from unity in the coefficients of the color equations. What has been determined can therefore be summarized as the error resulting from a mismatch of effective wavelength, at a spectral resolution of nominally 200A. Although the Wien approximation is not good to 10% over the range of wavelengths and temperatures found in published stellar photometry, it is reasonably concluded that ordinary care makes it a source of error well under 1% (0.01 mag.), except in the far infrared. This is not true for other terms. Errors of close to 0.1 mag. are possible in the ultraviolet and intermediate infrared. Ordinary techniques rarely lead to millimagnitude accuracy anywhere and can easily run a few hundredths. High precision is always possible; the problem is that it is not ubiquitous. The problem is further analyzed in terms of sampling theory, using the results of a companion paper, this issue. It is shown that aliasing errors can be approximately but not rigorously avoided. The justification of the efficacy of using a filter pair of slightly displaced bandpasses, one pair for each standard magnitude, as advocated by Manfroid based on suggestions by Young, is investigated. The result is advocacy of the only existing technique that can reliably transform instrumental values to existing multi-color photometry systems with proper accuracy.