Browsing by Author "MENNICKENT, RE"
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- ItemH-BETA LINE-PROFILE VARIABILITY OF 7 SOUTHERN BE STARS(1991) MENNICKENT, REThe results of spectroscopic observations of six B0-B4e and one B7e stars at the Manuel Foster Observatory (Chile) during 1984-87 are reported. By means of photographic spectroscopy (20 angstrom/mm), equivalent widths, peak separations and V/R ratios were obtained for a total of 106 H-beta and 7 H-gamma line profiles. Emission episodes in H-beta were detected in 27 CMa and lambda Pav, lasting at least 5 months and 18 days respectively. Significant variations in the equivalent width of the H-beta emission line, W-beta, in time scales of months were found in kappa CMa, PP Car and QY Car. On the other hand alpha Ara and member-of PsA showed a relatively constant strength of emission. The variations in W-beta were interpreted as the cumulative effect of several individual ejections from the stellar surface with mass loss rates of the order of 10(-10) M./yr. A variable, centrally detached, disk-type H-beta emitting region with extensions r-beta less-than-or-equal-to 5r* and rotation law v(r) approximately 1/r, which implies outflow or inflow of matter rather than a stable Keplerian disk, was derived by comparing emission line profile data of all stars.
- ItemLONG-TERM PHOTOMETRY OF BE-STARS .1. FADING EVENTS AND VARIATIONS ON TIME SCALES OF YEARS(1994) MENNICKENT, RE; VOGT, N; STERKEN, CStromgren 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.
- ItemON THE ROTATION PROPERTIES OF BE STARS AND THEIR ENVELOPES(1994) MENNICKENT, RE; VOGT, N; BARRERA, LH; COVARUBIAS, R; RAMIREZ, AWe 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.
- ItemSPECTROSCOPY OF SOUTHERN BE STARS 1984-1987(1988) MENNICKENT, RE; VOGT, N
- ItemTIME-RESOLVED H-ALPHA SPECTROSCOPY OF THE SU UMAJORIS STAR TU MENSAE IN QUIESCENCE(1995) MENNICKENT, REWe have analyzed 67 spectra of TU Men in the lambda 5800-7000 Angstrom spectral region with 2 Angstrom resolution covering a time range of approximate to 14 hours. An orbital period of 2.(h)813 +/- 0.(h)005 was derived from the radial velocity variations of the H alpha double emission line. This implies a not-evolved secondary mass M(2) = 0.35 +/- 0.03 M.. Unfortunately, our K semiampiitude diagnostic method gives two possible solutions: the most probable K is 108 +/- 10 km s(-1). From that value and using measures on the emission profile, a system mass ratio q (M(2)/M(1)) = 0.455 +/- 0.045, a primary mass M(1) = 0.785 +/- 0.145M. and an inclination angle i = 52 +/- 5 degrees were calculated. However, a second K value arising from the analysis of the central parts of the H alpha profile is K = 87 +/- 2 km s(-1). This solution yields to a mass ratio more compatible with the tidal disk instability model (q approximate to 0.33, M(1) approximate to 1.06M. and i approximate to 44 degrees). Periodic variations up to 100% of the H alpha equivalent width characterized by two minima centered at phases 0.8 and 0.3 were observed. The FWHM of the Ha red and violet peak are anticorrelated and show a double wave behaviour during the orbital cycle. The peak separation and the V/R ratio between the intensities of the violet and the red peak and also the FWHM of the whole emission profile, oscillate with the orbital period. Two bright disk inhomogeneities could explain these results.
- ItemV/R VARIATIONS IN H-BETA EMISSION PROFILES OF BE STARS(1991) MENNICKENT, RE; VOGT, NThe 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.