On V/R Variability of H
Emission in  Tauri
Ernst Pollmann
Charlottenburgerstrasse 26c, 51377 Leverkusen, Germany
email:
ErnstPollmann@aol.com
Received: 2002 January 24; Revised: 2002 February 4
In many Be-stars the H
emission line profiles occasionally
have the form of asymmetrical double peaks. For the characterization
of asymmetry the maximum intensity of the shortest wavelength peak of
the line (in units of the intensity of the neighboring continuum) with
V (violet) is designated, and those of the longest wavelength peak
with R (red). The direction of asymmetry is described by the quotient
of the intensities V/R. This paper reports the results of
spectroscopic monitoring of the Be star
 Tau. Specifically, it
describes periodic changes in the V/R relationship and the reversal of
that relationship from >1 to <1.
Tau is one of the best known Be
stars in the northern sky. The
literature covers all aspects of the object including its spectral
characteristics. Most publications address questions about the
equatorial gas disk which the central star hurls into orbit due to its
high rotation speed. The kinetics of gas motion in the disk and the
disk's density structure and stability are subjects of ongoing
research. Okazaki [1] addressed these questions with a new model based
on global oscillations in cool, non-self-gravitating, geometrical
disks that are susceptible to radial disturbances. His approach
depended on variations in the V/R relationship. A so-called
"one-armed oscillation" in such a disk can extend throughout its
structure as wavy density disturbances.
Adding to the difficulty of understanding
Tau is irregular
mass transfer from the star's photosphere into the surrounding gas
envelope. This is inferred from emission lines in the disk that
sometimes have relatively constant intensities and radial velocities
and at other times show cyclical change. For example, Mon et al. [2] found
quasi-periodic behavior in the V/R relationship during
1970-1986. However, the observations have gaps ranging from months to
four or five years. Guo [3] observed during 1990-1994 but the rate of
observation was not great enough to prove short-term periodicities in
V/R. Guo et al. [4], who observed during 1974-1995, and Hanuschik et
al. [5], who covered 1982-1993, provided the first evidence for
intensity changes on a timescale of days and weeks. However,
Miroshnichenko et al. [6] found no periodic change over weeks.
Figure 1 shows my observations from November 2000 to April 2001. This
regrettably short coverage resulted because my spectrograph was not
available before November and
Tau's apparition ended in
April. A sine function fit to the points indicates that the V/R
relationship changed with a period of about 71 days. Miroshnichenko
[7] assumed that orbital movements by interactive tidal disturbances
in the disk exert an influence on V/R. Figure 1 may be observational
evidence of additional oscillation modes that could exist if this
assumption is correct.
Fig. 1
Delplace [8] observed from March 1960 to September 1967 at the rate of
a few observations per year. Based on these results, the time of V/R
reversal is uncertain by several months ranging from December 1963 to
August 1964. By sampling the object 18 times between November 2000 and
November 2001, as shown in Figure 2, I observed reversal in October
2001. By April recovery of the V peak had begun. By October 16, my
first observation of the new season, the V peak's progress is clear
and by October 23, just seven days later, the reversal to V/R < 1
had occurred. Figure 2 demonstrates how a high rate of observation
can be helpful in determining an accurate V/R period.
Fig. 2
In the season that ran from October 2001 to January 2002, bad weather
created large gaps in the coverage. It was not possible to confirm the
V/R periodicity shown in Figure 1. Finally, Mon et al. [2] found that
the cycle of V/R variations changed about 1982. Guo [3] observed the
beginning of a new cycle that lasted about 3.5 years. It appears that
V/R variations are very complicated. Durations and amplitudes of
cycles change and sometimes cycles disappear only to become
unexpectedly active again. Such instability makes it important to
monitor Tau as often as possible.
REFERENCES
[1] Okazaki, A. T. 1991, "Long-Term V/R-Variations of Be Stars Due to
Global One-Armed Oscillations of Equatorial Disks", PASJ, 43, 75
[2] Mon, M., Kogure, T., Suzuki, M., & Singh, M. 1992,
"Spectroscopic State of the Be Star Tau in 1976-1986", PASJ, 44, 73
[3] Guo, Y. 1994, "A New V/R Cyclic Change of H in Tau",
IBVS, Nr. 4112
[4] Guo, Y., Huang, L., Hao, J., Coa, H., Guo, Z., & Guo, X. 1995,
"A Spectroscopic and Photometric Study of the Be/shell Star Tau", A&AS, 112, 201
[5] Hanuschik, R. W., Hummel, W., Satorius, E., Dietle, O., &
Thimm, G. 1996, "Atlas of High-Resolution Emission and Shell Lines in
Be Stars. Line Profiles and Short-term Variability", A&AS, 116,
309
[6] Miroshnichenko, A., Bjorkman, K., & Krugov, V. D. 2000,
"Long-term High-resolution Spectroscopy of  Cas, Tau,
and  Aqr", BAAS, 32, 681
[7] A. Miroshnichenko, A. 2001, private communication
[8] Delplace, A. M. 1970, "Instabilities in the Envelope of the Be
Star Tau", A&A, 7, 68
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