Browsing by Author "VOGT, N"

Now showing 1 - 17 of 17
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    A SPECTROSCOPIC STUDY OF THE DWARF NOVA CN ORIONIS
    (1989) BARRERA, LH; VOGT, N
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    AN ASTROMETRIC CATALOG OF SOUTHERN AND EQUATORIAL DWARF NOVAE
    (1992) BRUCH, A; MEIJER, J; NAUMANN, M; SCHIMPKE, T; UNGRUHE, R; VOGT, N
    An astrometric catalogue of 117 southern and equatorial dwarf novae is presented, complementing earlier work for the northern sky. The mean internal accuracy of the coordinates if of the order of +/- 0.025s in alpha cos-delta and +/- 0.3" in delta.
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    EVIDENCE FOR ELLIPSOIDAL VARIATIONS OF THE BE-STAR BINARY HD-50123
    (1994) STERKEN, C; VOGT, N; MENNICKENT, R
    On the basis of 8 years of photoelectric monitoring in the Stromgren system we were able to establish that HD 50123 is an interacting binary consisting of a B6 Ve primary and an early K giant secondary which fills its Roche lobe. The orbital period is 28.601 days. The secondary contributes considerably to the flux in the y and b bands, in which the double-sinusoidal light curve typical for ellipsoidal variables is seen. A few published radial velocities of the primary are consistent with a semiamplitude of K-1 = 25 +/- 6 km s(-1), and a mass function f(M) = 0.046 +/- 0.026 M.. Preliminary estimates of the stellar masses give 5 and 1.5 M., respectively, for primary and secondary. Some implications for the binary interpretation of Be stars, as well as for the period distribution of ellipsoidal variables, are discussed.
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    LONG-TERM PHOTOMETRY OF BE-STARS .1. FADING EVENTS AND VARIATIONS ON TIME SCALES OF YEARS
    (1994) MENNICKENT, RE; VOGT, N; STERKEN, C
    Stromgren differential uvby photometry of 7 Be stars was analyzed. The data were taken during 1983-1991 by the Long-Term Photometry of Variables team at La Silla. Two low v sin i stars (27 CMa and 28 CMa) showed fading events-that is, sudden fading of a few weeks duration-that were preceded by quasi-periodic oscillations (QPO) on time scales T similar to 10-20 days and amplitudes up to 0.(m)2. In both cases the color variations indicated occurrence of dust ejections. QPO were observed also in 48 Lib and in epsilon Cap, but these were not related to any fading event. Long-term photometric variations up to 0.(m)6 amplitude were seen for all stars. In most cases, these variations were larger in the u band, the stars being bluest when brightest. The Be stars seems to follow a fixed track in the c(1)-(b-y) color plane. The b-y color is not very sensitive to changes in brightness, whereas the c(1) color is strongly correlated with the u magnitude. Moreover, the largest cl changes, compared with u changes, occurred in high v sin i Be stars with shell characteristics. Additional variations on time scales of days were always present; these were larger in the u band. HR 3237 shows peculiar photometric variations, being the exception to the behaviour above described. The amplitudes of short-term and long-term variations of 13 Be stars are correlated; the tightest correlation was found in the y band. We discuss long-term photometric variations and quasi-periodic oscillations as due to enhanced mass loss episodes preceded by global one-armed thin-disk oscillations, which could be excited by some type of photospheric activity.
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    LONG-TERM PHOTOMETRY OF VARIABLES AT ESO .2. THE 2ND DATA CATALOG (1986-1990)
    (1993) STERKEN, C; MANFROID, J; ANTON, K; BARZEWSKI, A; BIBO, E; BRUCH, A; BURGER, M; DUERBECK, HW; DUEMMLER, R; HECK, A; HENSBERGE, H; HIESGEN, M; INKLAAR, F; JORISSEN, A; JUETTNER, A; KINKEL, U; LIU, ZL; MEKKADEN, MV; NG, YK; NIARCHOS, P; PUTTMANN, M; SZEIFERT, T; SPILLER, F; VANDIJK, R; VOGT, N; WANDERS, I
    In this paper we present the second catalogue of photometric data in the Stromgren system obtained during the period October 1986 - September 1990 in the framework of the Long-Term Photometry of Variables (LTPV) program at the European Southern Observatory. The catalogue is available in computer readable form at the Centre de Donnees de Strasbourg.
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    LONG-TERM PHOTOMETRY OF VARIABLES AT ESO .3. THE 3RD DATA CATALOG (1990-1992)
    (1995) MANFROID, J; STERKEN, C; CUNOW, B; DEGROOT, M; JORISSEN, A; KNEER, R; KRENZIN, R; KRUIJSWIJK, M; NAUMANN, M; NIEHUES, M; SCHONEICH, W; SEVENSTER, M; VOS, N; VOGT, N
    In this paper we present the third catalogue of photometric data in the Stromgren system obtained during the period October 1990 - January 1992 in the framework of the Long-Term Photometry of Variables (LTPV) program at the European Southern Observatory. The catalogue is available in computer readable form at the Centre de Donnees de Strasbourg.
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    ON THE ROTATION PROPERTIES OF BE STARS AND THEIR ENVELOPES
    (1994) MENNICKENT, RE; VOGT, N; BARRERA, LH; COVARUBIAS, R; RAMIREZ, A
    We present the results of low resolution spectroscopy (4 angstrom) obtained at CTIO for a sample of 42 Be, 4 B and 3 O stars covering the lambda 3700-7000 angstrom spectral range. Equivalent widths of the Balmer lines were measured up to H10. We also give the V/R state of the Halpha emission line. In addition, the peak separation and the width at the base of the Hbeta and Hgamma emission lines are tabulated for a total of 243 medium resolution (1 angstrom) spectra of 51 southern Be stars taken at the Manuel Foster Observatory, Chile. We analyzed our Halpha equivalent widths (Walpha) together with published data for a total of 122 Be and 2 Oe stars. An upper limit of Walpha incrasing with the projected rotational velocity v sin i is shown by the data. The Walpha values depend strongly on spectral type, being lower for later-type Be stars. This could indicate that the maximum strength of emission depends mainly on the spectral type and rotation velocity. The relation between Walpha (max) and v sin i, together with the observed excess in number of low v sin i Be stars and the reported anti-correlation between photometric period and v sin i give evidence for a considerable range of the true rotation velocities of Be stars: definitely there are intrinsically slow rotators among them. However, our results could also be interpreted in terms of anisotropic Halpha emission coming from the disk. The analysis of the net equivalent widths and peak separations of Halpha double emission line profiles strongly favor a disk type emitting envelope with a r(-j) rotation law with j = 1.4 +/- 0.2 which do not depend on the spectral subtype. This result is interpreted as evidence of radial motions in the envelopes of Be stars. The mean radial extensions of the regions which emit Balmer lines were determined to range between 30% (H10) and 60% (Hbeta) of the Halpha emitting envelope in the case j = 1. The mean electron density within the envelope has been found to vary only by a factor of 4. We compare our results with earlier investigations.
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    OY CARINAE REVISITED - DEVELOPMENT OF THE ACCRETION DISK DURING A NORMAL OUTBURST
    (1992) RUTTEN, RGM; KUULKERS, E; VOGT, N; VANPARADIJS, J
    We re-analyse Vogt's (1983) unique set of white-light eclipse light curves of the dwarf nova OY Carinae during a normal outburst, employing the maximum-entropy eclipse-mapping technique. These eclipse observations provide snapshots of the accretion disk during quiescence, during the early rising phase of an outburst, and at maximum outburst. The basic evolutionary aspects of the disk structure already presented by Vogt (1983) are placed on a more quantitative basis. In particular, the radial brightness profile and brightness temperature profile of the disk during the rise to outburst maximum are presented and discussed in the light of theoretical outburst models; from the observed stability of the disk radius and from the absence of an increase in the luminosity of the bright spot we conclude that the current observations do not offer support for the mass-transfer instability model. Furthermore, our analysis suggests the development of an extra, uneclipsed component during outburst rise, which contributes up to approximately 15% of the total light at outburst maximum. We argue that this extra component most likely represents luminous material well above the orbital plane, which may indicate the development of a wind.
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    SPECTROSCOPIC MONITORING OF SOUTHERN BE-STARS
    (1987) BARRERA, LH; VOGT, N
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    SPECTROSCOPY OF SOUTHERN BE STARS 1984-1987
    (1988) MENNICKENT, RE; VOGT, N
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    THE PHOTOMETRIC PERIOD OF THE RECURRENT NOVA T-PYXIDIS
    (1992) SCHAEFER, BE; LANDOLT, AU; VOGT, N; BUCKLEY, D; WARNER, B; WALKER, AR; BOND, HE
    T Pyx is one of four known recurrent novae suspected to be powered by thermonuclear runaways. If the orbital period can be measured with high accuracy before the next eruption, then it may be possible to detect its period change caused by the eruption, which can then be directly related to the mass ejected from the system. This dynamical measurement of an important model parameter is independent of the various uncertainties that plague spectroscopic determinations of the mass loss. This paper presents 1713 photometric measurements from 1966 until 1990. The light curve of T Pyx shows roughly sinusoidal variations with a typical amplitude of 0.09 mag and time scale of 2 hr. Discrete Fourier transforms of data from individual nights and runs reveal a highly significant periodic modulation. We prove this modulation to be coherent only on time scales shorter than from 6 days to under 1 day. The variable period of T Pyx is disappointing since the modulation cannot be accurately tied to the orbital period, so that no pre-eruption orbital period will be available. We measure the modulation period with sufficient accuracy that the daily alias problem is probably solved. Our period for the modulations is 0.07616+/-0.00017 days. We note that other stars have variable photometric periods (possibly related to the superhump phenomenon) which are slightly different from the orbital period. so that we suggest an orbital period of near 0.073 days for T Pyx.
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    V/R VARIATIONS IN H-BETA EMISSION PROFILES OF BE STARS
    (1991) MENNICKENT, RE; VOGT, N
    The V/R ratio between the violet and red intensity peaks of the H/beta emission line of 33 southern Be stars was examined, covering periods from several years to several decades. 76% of our sample stars were found to be V/R variables. Long-term variables are characterized by quasiperiods mainly in the 2-13 years range with a mean T = 7 yr. Six short-term variables were found which are best characterized by V > R to V < R changes on time-scales of 6- 100 d. A statistical analysis of these data, including also data for 10 northern V/R variables, did not reveal any correlation of the quasiperiod T with the spectral type and projected rotational velocity (v sin i) of the central star. Long-term quasiperiods do not depend on the envelope size. However, only stars with small envelopes (r-beta less-than-or-similar-to 4r*) show short-term variations. We discuss several possible mechanisms for these variations and have concluded that the most satisfactory general interpretation for the long-term variations seems to be expansion and contraction of the envelope, triggered by a variable stellar wind. On the other hand, the short-term V/R variations could be caused by the rotation of inhomogeneities in the circumstellar envelope (blobs), possibly combined with atmospheric phenomena in the central star such as non-radial pulsations and/or anisotropic radiation fields.