Showing papers in "Publications of the Astronomical Society of the Pacific in 1983"
TL;DR: In this paper, a technique for imaging the surfaces of certain rapidly rotating spotted stars is discussed, which exploits the correspondence between wavelength position across a rotationally broadened spectral line and spatial position across the stellar disk.
Abstract: A technique for imaging the surfaces of certain rapidly rotating spotted stars is discussed. The method exploits the correspondence between wavelength position across a rotationally broadened spectral line and spatial position across the stellar disk. Preliminary Doppler Images of the RS CVn star HR 1099 show spots which exhibit a striking similarity in shape and location to X ray images of solar coronal holes. It is suggested that the large star spots on RS CVn's and other active late-type stars emerge at low latitude as scaled-up analogs of solar complexes, but as they migrate poleward, they more closely resemble photospheric analogs of solar coronal holes. The evolution of starspots appears remarkably similar to that of large-scale magnetic fields of the sun.
280 citations
TL;DR: In this paper, an algorithm was developed for predicting the accuracy with which the brightness of a star can be determined from its image on a digital detector array, as a function of the intensity of the background.
Abstract: Algorithms are developed for predicting the accuracy with which the brightness of a star can be determined from its image on a digital detector array, as a function of the brightness of the background. The assumption is made that a known profile is being fitted by least squares. The two profiles used correspond to ST images and to ground-based observations. The first result is an approximate rule of thumb for equivalent noise area. More rigorous results are then given in tabular form. The size of the pixels, relative to the image size, is taken into account. Astronometric accuracy is also discussed briefly; the error, relative to image size, is very similar to the photometric error relative to brightness.
99 citations
TL;DR: In this paper, a comparative study of photometric observations made using the Johnson VRI (1965) and Cousins RI (1980) color systems is presented, considering both S1-based and S20-GaAs-based bandpasses.
Abstract: A comparative study of photometric observations made using the Johnson VRI (1965) and Cousins RI (1980) color systems is presented. The R and I passbands of the different versions of the Johnson system used in published studies are compared, considering both S1-based and S20-GaAs-based bandpasses. Discrepancies are attributed to the Balmer lines, the Paschen discontinuity and reddening in hot stars, and luminosity and abundance-related metal-line and molecular-band absorption differences in late-type stars. Data obtained with the different Johnson systems are evaluated by comparing precise Cousins-system measurements of the same stars, and linear transformations are developed for converting and comparing data. The color indices of Argue (1967) and Jacobson (1970) are also treated. The results of spectral-response tests on two RCA 31034-02 GaAs tubes are presented in an appendix.
92 citations
TL;DR: In this paper, the authors investigated the VRI transformations between the photometric systems of Johnson and Kron-Cousins using the new Johnson standards now appearing in The Astronomical Almanac.
Abstract: VRI transformations between the photometric systems of Johnson and Kron-Cousins are investigated using the new Johnson standards now appearing in The Astronomical Almanac. Linear solution transformations are obtained for (V-R), (V-I), and (R-I), with transformations for very red stars being determined separately. There is no significant discontinuity in passing from the less red stars to the very red ones. No significant trend is apparent in the residuals between the observations and values predicted by the equations. The agreement with the previous work of Cousins (1980) and Bessell (1979) is good.
82 citations
TL;DR: In this article, the authors measured the 10.8 GHz flux densities of 285 radio sources at 14 epochs between January 1979 and November 1982, and found that at least 45% vary within ten years.
Abstract: The authors have measured the 10.8-GHz flux densities of 285 radio sources at 14 epochs between January 1979 and November 1982. For the subset of 258 objects constituting a complex flux-density-limited sample selected at 5 GHz, it is found that at least 45% vary within ten years. About nine-tenths of the steep spectrum (α ≤ -0.5) sources do not vary, whereas about two-thirds of the flat spectrum sources do. Quasars are much more likely to vary than either galaxies or empty field sources. Most objects from the S4 catalog identified with empty fields are galaxies beyond the plate limit.
42 citations
TL;DR: The modes of operation of photoconductive and photoemissive array detectors are described, and their performance characteristics compared and contrasted, with emphasis on their suitability for use as detectors for optical spectroscopy as discussed by the authors.
Abstract: The modes of operation of photoconductive and photoemissive array detectors are described, and their performance characteristics compared and contrasted, with emphasis on their suitability for use as detectors for optical spectroscopy. The prospects for future developments of the array detectors are discussed.
42 citations
TL;DR: In this paper, spectroscopic observations of a faint optically-selected QSO with broad absorption features in its spectrum were presented, and it was shown that the gas responsible for the broad absorption trough is moving relative to the hydrogen responsible for broad emission lines by up to 66,000 km s-1.
Abstract: The authors present spectroscopic observations of a faint optically-selected QSO with broad absorption features in its spectrum. The absorption troughs are extremely complex with both large- and small-scale structure. The low-velocity edge of the C IV trough is displaced blueward of the apparent emission line center by 13,000 km s-1. The data provide evidence that the gas responsible for the broad absorption trough is moving relative to the gas responsible for the broad emission lines by up to 66,000 km s-1.
33 citations
TL;DR: In this paper, the authors investigated the feasibility of studying time variations in line bisectors, the reality of apparent line-to-line differences within the same stellar spectrum, and bisector differences between stars of identical spectral types.
Abstract: Bisectors of spectral line profiles in cool stars indicate the strength of convection in the photospheres of these objects. The present investigation is concerned with the feasibility of studying time variations in line bisectors, the reality of apparent line-to-line differences within the same stellar spectrum, and bisector differences between stars of identical spectral types. The differences considered pertain to the shape of the bisector. The material used in the investigation was acquired at the McDonald Observatory using a 1728 diode Reticon array at the coudefocus of the 2.1-m telescope. Observed bisector errors are discussed. It is established that different lines in the same star show significantly different bisectors. The observed error bands are shown by the shaded regions. The slope and curvature are unique for each case.
33 citations
TL;DR: In this article, a color-magnitude diagram for all known UV-bright stars in 29 globular clusters was constructed for the first time, showing that only 6 of the observed stars are cluster UV-busters.
Abstract: Radial velocities of 46 stars in the fields of the globular clusters M 12 (C 1644-018) and M56 (C 1914+300) show that 23 of the stars are cluster members. When these new measurements are combined with all previous ones, the improved cluster velocities are -41.3 ± 1.5 and -134.0 ± 2.7 km s-1, respectively. Six of the observed stars are cluster UV-bright stars. A color-magnitude diagram is constructed for all known UV-bright stars in 29 globular clusters. This composite diagram suggests that UV-bright stars are present in sufficient numbers that they will contribute appreciably to the integrated ultraviolet light of old, metal-poor stellar populations. Although our figure is an improvement over earlier diagrams, it is still seriously incomplete due to selection effects.
33 citations
TL;DR: The color indices (B-V and U-B) vary in systematic fashion with time and follow closely the mean evolution found from previous Type I SN by Pskovskii (1967, 1970, 1977).
Abstract: The U, B, V, and R light curves exhibit the typical forms of a Type I SN and suggest that the maximum occurred on JD2444672 when the SN was at Bmax = 12.00. The color indices (B-V) and (U-B) vary in systematic fashion with time and follow closely the mean evolution found from previous Type I SN by Pskovskii (1967, 1970, 1977). Assuming that the absolute magnitude of SN 1981b is M0max = -18.5, based on previous Type I SN on the zero point of the de Vaucouleurs distance scale, then the distance modulus of NGC 4536 is 30.29; this is in excellent agreement with the mean modulus, 30.1 + or - 0.3, derived from eight tertiary and quaternary indicators on the same zero point. Both of these values are about 1.5 m lower than the distance modulus that would be derived from the redshift of the galaxy, even on the same zero point.
30 citations
TL;DR: In this article, an E(B-V) = 0.72 is derived for HDE 245770, which is the optical counterpart of A0535+26.
Abstract: Observations of A0535+26 (HDE 245770) by the ANS in September 1975 and March 1976 when it was at the X-ray low state are discussed. Also observed were 58 early-type stars lying within 10 deg of HDE 245770; these stars are used to derive a localized extinction curve. On the basis of the depth of the 2200 A interstellar absorption feature observed in the ANS flux distribution of HDE 245770 and the shape of the localized extinction curve, an E(B-V) = 0.72 is derived for HDE 245770. Observations were made of this system during its X-ray flare-up (October 1980) and quiescent (November 1981) states by the IUE. It is noted that the fine error sensor on board the IUE detected an increase in the optical brightness of HDE 245770 during the X-ray outburst. This greatly corroborates the identification of HDE 245770 as the optical counterpart of A0535+26. No significant change in the absorption-line profiles during the X-ray high and low periods is found, indicating that there was no significant increase in the stellar wind density when the X-ray flare-up occurred. The UV continuum level increased by less than 6 percent when it was near peak X-ray luminosity. The UV spectrum of HDE 245770 suggests that it is a B0 V or a B0 III star with its dereddened continuum slope slightly in favor of being a giant.
TL;DR: The use of CCD detectors for BVRI photometry is discussed in this paper, where the adopted filter system and detector linear transformations to the standard system with residuals, expressed as rms deviation per star, of only 1 or 2 percent, are determined.
Abstract: The use of CCD detectors for BVRI photometry is discussed. For the adopted filter system and detector linear transformations to the standard system with residuals, expressed as rms deviation per star, of only 1 or 2 percent, are determined. A region is defined in open cluster M67 where stars covering a (B-V) color-index range from -0.1 to +1.3 are calibrated at B, V, R, and I wavelengths. This region is useful for calibration of the system's sensitivity. Where overlap exists with previous observers, the magnitude zero points for M67 stars agree at better than the 1 percent level.
TL;DR: In the case of a few of the Wolf-Rayet stars cataloged by van der Hucht et al. (1981), certain information concerning spectral subtypes and photometry is missing.
Abstract: In the case of a few of the Wolf-Rayet stars cataloged by van der Hucht et al. (1981), certain information concerning spectral subtypes and photometry is missing. These stars are primarily the faintest and most heavily reddened of the known galactic WR stars. The stars investigated in connection with the present investigation include WR 105, WR 112, WR 118, WR 122, WR 126, and WR 149. The data were obtained with a 4-m telescope and an intensified image dissector scanner during four nights in September 1982. One of these objects, the WR 118 appears to be the lowest excitation WC star ever observed, while WR 122 is the lowest excitation WN star yet seen.
TL;DR: In this article, visual and infrared photometry of strongly polarized QSOs (BL Lacertae objects) is reported and spectral flux distributions of these objects are discussed in combination with results from earlier studies.
Abstract: Visual and infrared photometry of strongly polarized QSOs (BL Lacertae objects) is reported. Spectral flux distributions of these objects are discussed in combination with results from earlier studies. With the exception of the well-resolved galaxies Mkn 421 and Mkn 501 only one object (1218 + 304) has a change in spectral index easily explained by the presence of an underlying galaxy. Six other sources appear to have either a break in their spectra near about 1 micron or a smoother general steepening of their spectra at shorter wavelengths. A spectral break at this wavelength may be attributed to a low flux of nonthermal radiation superimposed on a stellar energy distribution or by nonthermal electron energy losses.
TL;DR: In this paper, the F3 Ia low-amplitude variable (period 44-68 days) star HR 4912 is reported using an echelle spectrograph on the 4m telescope at Cerro Tololo in April, 1981, at 5200-6900 A and nominal resolution 0.1 A.
Abstract: Spectrographic observations of the F3 Ia low-amplitude variable (period 44-68 days) star HR 4912 are reported. Spectra obtained with an echelle spectrograph on the 4-m telescope at Cerro Tololo in April, 1981, at 5200-6900 A and nominal resolution 0.1 A were digitized using the KPNO microdensitometer, reconstructed, and smoothed; digital spectra obtained with a Reticon-equipped coudespectrometer on the 2.7-m reflector at McDonald Observatory in June, 1982, in 100-A exposures at 5380, 6300, 6385, 6470, 6560, and 8710 A were FFT smoothed; the continuum was set by hand from both data sets, and usable equivalent widths were obtained for about 300 lines. The analytical model and its basic assumptions are discussed. Tables of calculated metallic and CNO abundances are presented. From the relatively low Fe/H ratio (-1.2) it is inferred that HR 4912 is a representative of the old-disk population and not a classical cepheid. The absolute magnitude estimation (about -4) places the star to the left of the asymptotic giant branch on the HR-diagram evolutionary track for a 0.6-solar-mass-function CO-core model, consistent with an SRd-variable classification.
TL;DR: In this article, a model based solely on the phase-dependent behavior of the hydrogen shell lines in the 80.860 binary was developed by Peters and Polidan (1973) and by Peters (1976), with a refinement taking into account the utilization of an orbital solution obtained from measurements of the wings of the broad photospheric features observed in the rapidly rotating primary.
Abstract: The discovery of an unusual, periodic, two-component shell phase of short duration in the 'classical' Be star HR2142 (HD41335, MWC133) offered convincing evidence that this object is a mass-transfer binary system. A model based solely on the phase-dependent behavior of the hydrogen shell lines in this 80(d).860 binary was developed by Peters and Polidan (1973) and by Peters (1976). The present investigation is concerned with a refinement to the earlier model, taking into account the utilization of an orbital solution obtained from measurements of the wings of the broad photospheric features observed in the rapidly rotating primary. Velocities and equivalent widths from the sharp 'shell' lines, presumably formed in or near the gas stream, provide additional information on the mass flow in the Balmer-line-formation region.
TL;DR: In this paper, photometric observations of 16 early-type (O9 to A3) supergiants have been obtained and the light curves are irregular, but characteristic time scales or "quasi-periods" can be identified.
Abstract: Photometric observations of 16 early-type (O9 to A3) supergiants have been obtained. All but two of the stars are variable, with a range of about 0.05 mag. The light curves are irregular, but characteristic time scales or 'quasi-periods' can be identified. The ratio of the quasi-period to the theoretical period (for pulsation in the fundamental radial mode) varies from 2-10 in the earliest-type supergiants to 0.2-2 in the later-type supergiants. The variability is most likely due to nonradial pulsation, at least in the O-B-type supergiants.
TL;DR: In this article, six cataclysmic variable stars were searched for 6-cm emission using the NRAO Very Large Array, and none were detected; the flux density range between 0.1 and 0.4 mJy.
Abstract: Six cataclysmic variable stars were searched for 6-cm emission using the NRAO Very Large Array. The candidate stars were SS Cyg, U Gem, TT Ari, RW Sex, EX Hya, and A0 Psc (H2252-035). None were detected; the flux density range between 0.1 and 0.4 mJy. These limits are, respectively, between a factor of 6 and 1.5 lower than the 6-cm emission detected from the magnetic binary AM Her by Chanmugam and Dulk.
TL;DR: A photometric survey of main-sequence stars in the Pleiades, Hyades, and Malmquist Field, to investigate variability among solar-type stars, had been continued at Cloudcroft Observatory for a second season as discussed by the authors.
Abstract: A photometric survey of main-sequence stars in the Pleiades, Hyades, and Malmquist Field, to investigate variability among solar-type stars, had been continued at Cloudcroft Observatory for a second season. Possibly six of 40 Pleiades stars, two (possibly six) of 49 Hyades stars and possibly four of 42 stars in the Malmquist Field are found to be variable. Parallel observations of Hyades stars were obtained also at Lowell Observatory. These measurements, which achieved better precision, reveal significiant variations in eleven (possibly 16) of 36 Hyades stars, all of spectral type F7 V-K2 V. Agreement between the two surveys is acceptable. The two seasons of the Cloudcroft survey show that changes in the mean annual brightness of the Hyades stars accompany changes in the amplitude of their short-term variability, in the sense that a star tends to become fainter as its variability increases and vice versa. Color effects are absent.
TL;DR: In this article, the authors report observational evidence for a predominance of blueshifts among the radial velocities of the forbidden lines in the spectra of T Tauri stars.
Abstract: The authors report observational evidence for a predominance of blueshifts among the radial velocities of the forbidden lines in the spectra of T Tauri stars. The similarities of the velocity distributions and spectra suggests a related physical nature of the H-H objects and the unresolved forbidden-line-producing gas volumes around T Tauri stars.
TL;DR: In this paper, the authors selected 22 outstanding American astronomers whose careers terminated before 1970 and counted the citations in 197() 79 to the papers published throughout their careers and found that most (84%) of the citations are to papers published between the ages of 40 and 75 years.
Abstract: We selected 22 outstanding American astronomers whose careers terminated before 1970 and we counted the citations in 197() 79 to the papers published throughout their careers. It turns out that most (84%) of the citations are to papers published between the ages of 40 and 75 years. If we overcorrect for the fact that citations to outstanding papers decreased exponentially after a peak, the change is small: the years account for 77% of the citations and the years account for 94%. These results are not sensitive to the types of research done by these astronomers. Two likely reasons for the high citation rates in the later years are that astronomers tend to write reviews and do major studies in later years, both of which are highly cited. These results apply to outstanding normal astronomers and not necessarily to the rare revolutionary scientist.
TL;DR: A number of techniques for determining stellar radial velocities with a precision better than 100 m s-1 are now becoming available as discussed by the authors, using for wavelength reference either telluric lines, Fabry-Perot fringes, or lines generated in an absorption cell.
Abstract: A number of techniques for determining stellar radial velocities with a precision better than 100 m s-1 are now becoming available. Three types of precision velocity techniques have been proposed, using for wavelength reference either telluric lines, Fabry-Perot fringes, or lines generated in an absorption cell. The author has used the latter technique, with a hydrogen fluoride cell, to obtain time series observations of Procyon on three nights. The internal error in these velocities is 10 m s-1, and the mean night-to-night difference is about 6 m s-1. These results suggest that the HF technique has good long-term stability, and therefore can be applied to the search for sub-stellar mass companions to solar-type stars.
TL;DR: In this article, the authors have observed eleven solar-type main-sequence stars to search for continuum variability and find evidence that two of these eleven stars are variable and that one other may be.
Abstract: The authors have observed eleven solar-type main-sequence stars to search for continuum variability. Stars were selected from Wilson's chromospheric activity survey, and were observed for periods of three to six months with intermediate-band Stroemgren uvby photometry. The authors find evidence that two of these eleven stars are variable and that one other may be. Comparison with contemporaneous Ca II H + K emission flux measurements shows that the amplitude of photometric variability correlates strongly with both the mean emission flux and the amplitude of its variation. Variations in continuum light and emission flux tend to correlate in time, as well, with continuum minima coinciding with emission maxima and vice-versa. The authors infer that the surface activity of these stars tends to be confined to localized activity centers that include both emission plages and dark spots, similar to the active regions observed on the sun.
TL;DR: The basic phenomenon characterizing a QSO is believed to be accretion of material by a supermassive (10 to the 8th solar-mass) black hole (SBH) at the nucleus of a galaxy as mentioned in this paper.
Abstract: The basic phenomenon characterizing a QSO is believed to be accretion of material by a supermassive (10 to the 8th solar-mass) black hole (SBH) at the nucleus of a galaxy. Arguments and recent evidence are presented which strongly support this view. Continuum radiation from QSOs can be modeled by an accretion disk, a hot blackbody, and line blends. Optical imaging reveals underlying galaxies, frequently disturbed by an encounter which may be the fueling process. Radio imaging reveals the sometimes complex history of the SBH spin axis, and together with radio VLBI, optical, and X-ray variability, indicates relativistic ejection velocities. Optical and UV line profiles yield clues as to the central geometry and possible binary nature of some galactic nuclei. Various current difficulties are described, and the place of BL Lacertae objects and Seyfert galaxies in the overall scenario are discussed.
TL;DR: In this article, the authors describe the survey in which the pulsating DB white dwarf GD 358 was found, and they present a list of the DB white dwarfs observed in the survey that did not have detectable pulsations.
Abstract: The authors describe the survey in which the pulsating DB white dwarf GD 358 was found, and they present a list of the DB white dwarfs observed in the survey that did not have detectable pulsations. The authors show that there are probably no pulsational instability strips for the DB white dwarfs at temperatures less than 20,000K.
TL;DR: In this paper, the 180° ambiguity in the longitude of the ascending node of Capella has been resolved by means of an analysis of photographic speckle observations and the results indicate that the correct nodal quadrant is that given by Anderson in 1920 and 180° from the currently assumed value.
Abstract: The 180° ambiguity in the longitude of the ascending node of Capella has been resolved by means of an analysis of photographic speckle observations The results indicate that the correct nodal quadrant is that given by Anderson in 1920 and 180° from the currently assumed value
TL;DR: Using the established relationship between the Hubble types and the absolute magnitudes of galaxies and the scale length of the radial distribution of H II regions in galaxies, it is seen that the Hubble type of the Milky Way Galaxy is Sc or SBc.
Abstract: Using the established relationship between the Hubble types and the absolute magnitudes of galaxies and the scale length of the radial distribution of H II regions in galaxies, it is seen that the Hubble type of the Milky Way Galaxy is Sc or SBc. Consideration is given to the uncertainties in this conclusion.
TL;DR: In this paper, observations are reported for 17 objects that are listed in the Catalogue of Galactic Planetary Nebulae (CGPN) that have either been misclassified as planetary nebulae, have been identified incorrectly by their observers or in the CGPN, or have had peculiar central stars erroneously assigned to them.
Abstract: Observations are reported for 17 objects that are listed in the Catalogue of Galactic Planetary Nebulae (CGPN). These have either been misclassified as planetary nebulae, have been identified incorrectly by their observers or in the CGPN, or have had peculiar central stars erroneously assigned to them. In about one-half of the cases, we confirm previous reclassifications. The general characteristics of each object are described, and we provide relative emission-line fluxes for two that are already known to be symbiotic stars.
TL;DR: In this paper, the spectral type of the secondary is estimated to be G8 V and the secondary velocity curve as well as that of the primary, together with the orbits and the minimum masses of the two components are determined, despite the relatively long, 10.2-day period.
Abstract: Reticon observations of the spectroscopic binary Iota Peg at 6430 A show the secondary star's weak, but well defined lines. Determinations have accordingly been made of the secondary velocity curve as well as that of the primary, together with the orbits and the minimum masses of the two components. The 1.31 + or - 0.02 and 0.81 + or - 0.01 solar mass minimum masses are sufficiently close to the expected actual masses to suggest eclipses, despite the relatively long, 10.2-day period. The spectral type of the secondary is estimated to be G8 V.
TL;DR: In this paper, a relation is derived which predicts the effective temperatures of such stars from their dereddened (V-R) color indices, from a subset of these, plus objects from additional sources.
Abstract: (V-R) photometry is presented for 90 of the extremely metal-deficient red giants listed by Bond (1980). From a subset of these, plus objects from additional sources, a relation is derived which predicts the effective temperatures of such stars from their dereddened (V-R) color indices.