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\IBVShead{5173}{14 September 2001}

\IBVStitle{Observations of H-alpha Emission in VV Cephei}

\IBVSauth{Pollmann, Ernst}

\IBVSinsto{http://pollmann.ernst.org}

\SIMBADobj{VV Cep}

\begintext

VV Cep is an eclipsing binary with a period of about 20.4 years that is
comprised of a M2 Iab primary star and an early B secondary star.
Goedicke (1939) was first to spectroscopically observe it. Wright
(1977) inferred the existence of intermittent mass transfer and an
H$\alpha$ emitting disk.  Kawabata et al.\ (1981) and Moellenhoff et
al.\ (1978, 1981) further described what appears to be an accretion
disk around the B star.

Appropriately equipped amateur astronomers are now able to make
scientific contributions in spectroscopy. This is largely due to the
availability of highly efficient CCD cameras. The author built a
Maksutov type mirror-prism- spectrograph with a CCD camera as the
detector. The instrument has a 100 mm aperture, 1000 mm focal length,
and a prism with breaking angle of 30 degrees. Its central wavelength
is fixed at 6563 \AA\ and its dispersion is 3 \AA/pixel. With this
equipment the author observed VV Cep from July 1996 until May 2001 and
obtained 148 spectra. This period included an eclipse of the B star
from 1997 to 1999.

With the binary at magnitude 4.9, exposure times were about 4 minutes
for each spectrum to achieve 70-80% saturation of the total dynamic
range of the sensor. 20 spectra were combined for measurement. The
integration width for computation of equivalent width ($W$) for the
H$\alpha$ emission line was 6 nm.  The formula to compute $W$ was
$$
W = \int_{\rm line}(1-I(\lambda)/I_c(\lambda))\,{\rm d}\lambda.
$$
$I_c(\lambda)$ is the continuum intensity at wavelength $\lambda$ and
$I(\lambda)$ is intensity of the emission line at the same wavelength.
A linear function was usually sufficient to fit the continuum over the
6 nm wavelength range centered on H$\alpha$. This was done in a trial
and error process. Figure~1 is a representative spectrum.

Figure~2 is a plot of $W$ for H$\alpha$ emission as a function of time.
The eclipse of the emitting disk began in March 1997 (JD 2450511) and
ended 673 days later. Ingress and egress lasted 128 and 171 days,
respectively. The B star and disk were eclipsed for 373 days. Saito et
al.\ (1980) observed the 1976--78 eclipse with $UBV$ photometry. In that case,
totality lasted about 300 days, significantly shorter than the latest
eclipse, and the entire event required about 1000 days.

\IBVSfig{10cm}{5173-f1.ps}{Standardized CCD spectrum of VV Cep}

\IBVSfig{10cm}{5173-f2.ps}{Plot of W for H$\alpha$ emission as a function of time}

\vskip-22pt

While after the ephemeris of Gaposchkin (1937) the mid-point of the
eclipse was to be expected at JD 2450790, this time can be determined
from Fig.~2 at JD 2450827, thus with a delay of 37 d (in the table the
individual values of EW with the belonging Julian Date are specified).
Graczyk et al.\ (1999) determine the mid-point of the eclipse
1997/99 from $UBV$ photometry at approximately JD 2450855, thus with 65
d delay. Leedj\"arv et al.\ (1999) obtained a similar value of 68 d
compared with the ephemeris in Gaposchkin (1937) likewise from $UBV$
photometry as well as optical spectroscopy.

Perhaps the most interesting feature of Figure~2 is the behavior of
H$\alpha$ emission outside of eclipse. Large fluctuations in W occurred
continuously over about 4.8 years. A possible explanation is variable
mass accretion from the M supergiant to the accretion disk as described
by Wright (1977) and Stencel et al.\ (1993). There may also be related
variations in the disk's temperature and density. Further, the M
supergiant has a semiregular pulsation period of 116 days (Saito et al.\
1980) that may affect the rate of accretion. Since the disk is the
apparent source of H$\alpha$ emission, it is the best candidate to
explain ongoing changes in intensity.

$V/R$ measurements of H$\alpha$ by Kawabata et al.\ (1981) during the
1976-1978 eclipse may indicate that the distribution of matter in the
disk is not homogeneous. The stronger violet emission peak may be
formed by greater density in the left side of the disk which rotates
anticlockwise.  Different strengths of the violet and red peaks during
the 1997--1999 eclipse can be inferred from the ingress and egress
branches of the plot in Figure~2.  During ingress, with the disk's left
side hidden and its right side in view, on average $W=11$ \AA\@. At
egress, with the left side emerging from eclipse, $W=17$ \AA.

\references

Gaposchkin, S., 1937, {\it Harvard Coll.\ Obs.\ Circ.}, No.\ 421

Goedicke, V., 1939, {\it Publ.\ Obs.\ Univ.\ Michigan}, {\bf 8}, 1

Graczyk D., Mikolajewski, M., Janowski, J.L., 1999, {\it IBVS}, No.\ 4679

Kawabata, S., Saijo, K., Sato, H., and Saito, M., 1981, {\it PASJ}, {\bf 33}, 177

Leedj\"arv, L., Graczyk, D., Mikolajewski, M., Puss, A., 1999, {\it A\&A}, {\bf 349}, 511

Moellenhoff, C., Schaifers, K., 1978, {\it A\&A}, {\bf 64}, 253

Moellenhoff, C., Schaifers, K., 1981, {\it A\&A}, {\bf 94}, 333

Saito, M., Sato, H., Saijo, K., Hayasaka, T., 1980, {\it PASJ}, {\bf 32}, 163

Stencel, R. E., Potter, D. E., Bauer, W. H., 1993, {\it PASP}, {\bf 105}, 45

Wright, K. O., 1977, JRASC, 71, 152

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