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\IBVShead{4574}{26 March 1998}
 
\IBVStitletl{PHOTOMETRIC INVESTIGATION OF THE NEBULA}{ IN THE AG P\lowercase{EG} SYSTEM}

\IBVSauth{N.~TOMOV, M.~TOMOVA}

\IBVSinsto{NAO Rozhen, P.O.Box 136, BG-4700 Smolyan, Bulgaria,
          e-mail: rozhen@sm.unacs.bg}

\SIMBADobj{AG Peg}
\IBVStyp{ symbiotic }
\IBVSkey{photometry}

\IBVSabs{Twenty one photoelectric UBV estimates of the symbiotic system}
\IBVSabs{AG Peg have been obtained. The continuum flux at a wavelength }
\IBVSabs{lambda 3650 A, (the U band) is supposed to be radiated mostly by the}
\IBVSabs{circumbinary nebula. Two estimates of this flux, taken in different}
\IBVSabs{photometric maxima are compared. A conclusion that its decrease is}
\IBVSabs{probably due to a decrease of the Lyman luminosity of the hot secondary}
\IBVSabs{as well as the mechanical energy of the two stellar winds}
\IBVSabs{in this system has been made.}
\IBVSabs{The contribution of the wind of the hot component in the U flux of}
\IBVSabs{the whole system was obtained to be negligible.}

\begintext  

The AG Peg (HD\,207\,257) system is the oldest known symbiotic nova.
Its hot secondary has undergone a single outburst and a high velocity 
stellar wind has appeared as a result of it. The view this system is 
at a stage of colliding winds in its evolution is now commonly accepted
(Penston \& Allen 1985, Kenyon et al. 1993, Tomov 1993, Vogel \& 
Nussbaumer 1994, Altamore \& Cassatella 1997). The UV data show that 
the luminosity and the mass-loss rate of the hot component 
gradually decrease (M\"urset et al. 1991, Kenyon et al. 1993, Vogel \& 
Nussbaumer 1994, Altamore \& Cassatella 1997). According to 
the photometric observations, the visual flux of the system gradually 
decreases too, although its orbital amplitude remains invariable 
(Belyakina 1992). In this note we compare two values of the U flux 
taken in the photometric maximum and separated by four orbital 
cycles ($\sim3290^{\rm d}$). We consider the possible relation between 
their difference and the variation of the luminosity and 
the mass-loss rate of the hot component.

Our observations (Table~1) were performed during September 1995 -- 
January 1998 with a single channel photoelectric photometer, mounted at 
the Cassegrain focus of the 0.6 m F/12.5 telescope of the National 
Astronomical Observatory Rozhen. We obtained 20 UBV estimates during 
one orbital cycle (Fig.~1). On JD 2\,450\,476.2 only B and V estimates
were taken. The star BD+11\deg4681 having 
$\rm V=8\fmm18$, ${\rm B-V}=1\fmm03$ and ${\rm U-B}=0\fmm81$
(Belyakina 1992) was used as a comparison star. The m.s. errors are 
not larger than 0\fmm01 in V and B, and 0\fmm02 in U.

The decrease of the visual light of AG Peg is caused by the decreasing 
contribution of its circumbinary nebula as the cool component of this 
system does not change and the hot one has temperature about 90\,000 K
(M\"urset et al. 1991, Kenyon et al. 1993, Altamore \& Cassatella 1997)
radiating mostly in UV. We will treat the light in the U region where
the Balmer continuum of the nebula is radiated and the cool component's
spectrum is absent.

The U magnitudes of the AG Peg system on JD~2\,446\,761.2 (26.~Nov,~1986)
and 
JD~2\,450\,049.3 (27.~Nov,~1995) are 8\fmm54 (Belyakina 1992) and 9\fmm03 
respectively.
The corresponding continuum fluxes are equal to 2.84$\times$10$^{-12}$ 
erg\,cm$^{-2}$\,s$^{-1}$\,\AA$^{-1}$ and 
1.81$\times$10$^{-12}$ erg\,cm$^{-2}$\,s$^{-1}$\,\AA$^{-1}$ 
which were corrected for interstellar reddening by 
means of the value $\rm{E(B-V)}=0.12$ (Penston \& Allen 1985) using 
the extinction law by Seaton (1979). The spectrum of the nebula is 
formed by the winds of the two stellar components and is ionized by the 
radiation of the hot secondary beyond the limit of Lyman series.
The velocity of the wind of the giant is 20 km\,s$^{-1}$, its mass-loss 
rate is 2$\times$10$^{-7}$ M$_{\odot}$\,yr$^{-1}$ (Vogel \& Nussbaumer 1994, 
M\"urset et al. 1995) and does not vary. Therefore the change 
of the nebular spectrum is probably due to a change of the hot 
stellar component and its wind. The radius of this component at the two 
times considered by us was equal to 0.11 R$_{\odot}$ and 
0.09 R$_{\odot}$ respectively. 
These values are arithmetical means of the data of Vogel \& 
Nussbaumer (1994) and Altamore \& Cassatella (1997). The second of them was 
calculated using the value of Vogel \& Nussbaumer for the year 1993 as their
observations were up to this moment. Thus the Lyman photon luminosity of 
the hot component was 1.80$\times$10$^{46}$ photons\,s$^{-1}$ and 
1.21$\times$10$^{46}$ photons\,s$^{-1}$. The intensity of
the nebular continuum is proportional to the ion density. There is, also,
linear dependence between the ion density and the density of the radiation 
field of the ionizing star. In such a case the Lyman photon luminosity will 
change in the same ratio like the continuum flux at the maximal sensitivity 
of the U photometric system. It was found that the Lyman luminosity 
decreasing of 33~\% is in agreement with the decreasing of the U flux, 
which is equal to 36 \%.

There is also in the AG Peg system a wind shed by its hot component and its
mechanical energy must be considered too. The two winds collide head on,
creating
a collision region with a small thickness, as described in the very
simplified 
approximation by Girard \& Willson (1987). Let us consider every one of
these
parts of the circumbinary nebula. The hot wind has had a constant velocity
of 1000 km\,s$^{-1}$ (Tomov et al. 1998) during the period treated, but the
mass-loss 
rate of the hot secondary has decreased from 1.48$\times$10$^{-7}$
M$_{\odot}$\,yr$^{-1}$ 
to 7.98$\times$10$^{-8}$ M$_{\odot}$\,yr$^{-1}$.
These values are arithmetical means of the estimates of Vogel \& Nussbaumer 
(1994) and Altamore \& Cassatella (1997). We calculated the continuum flux 
at the wavelength $\lambda$ 3650 \AA\, determined by recombinations and
free-free
transitions in the hot wind for the two moments, considered by us, allowing 
helium abundance 0.1 and a distance to the system equal to 650 pc 
(M\"urset et al. 1991, Vogel \& Nussbaumer 1994, M\"urset et al. 1995,
Altamore \& Cassatella 1997). It turned out that the contribution of this 
radiation in the U flux of the whole system is negligible and is equal to 
about 4 \% and 2 \% respectively. That is why the observed variation 
of the U flux is not due to the decreasing contribution of the hot wind. 
In such a case the U light of the AG Peg system probably appears mostly 
in the collision region of the winds.\\

\vskip .5cm

\centerline{Table 1. Photometric observations of AG Peg}
\vskip 3mm
\begin{center}
\begin{tabular}{ccrrrccrrr}
\hline
JD--2449000&~n~&~~~V~~~&~~~B~~~&~~~U~~~&
JD--2449000&~n~&~~~V~~~&~~~B~~~&~~~U~~~\\
\hline
~974.3 & 3 & 8.56 &  9.76 &  9.16 & 1398.2 & 3 & 8.85 & 10.22 & 10.32\\
~988.4 & 2 & 8.55 &  9.73 &  9.13 & 1434.2 & 1 & 8.88 & 10.19 & 10.21\\
~993.4 & 2 & 8.55 &  9.71 &  9.09 & 1476.2 & 1 & 8.86 & 10.18 &  ---~\\
~996.4 & 2 & 8.58 &  9.76 &  9.10 & 1628.5 & 3 & 8.80 & 10.03 &  9.75\\
1012.3 & 2 & 8.56 &  9.75 &  8.97 & 1651.5 & 2 & 8.74 &  9.98 &  9.61\\
1024.3 & 2 & 8.56 &  9.69 &  9.08 & 1698.4 & 3 & 8.70 &  9.89 &  9.41\\
1049.3 & 2 & 8.56 &  9.71 &  9.02 & 1701.4 & 2 & 8.69 &  9.90 &  9.42\\
1092.2 & 3 & 8.61 &  9.76 &  9.12 & 1741.2 & 3 & 8.69 &  9.85 &  9.26\\
1267.4 & 2 & 8.77 & 10.07 &  9.95 & 1742.2 & 3 & 8.68 &  9.84 &  9.22\\
1295.4 & 3 & 8.75 & 10.08 & 10.03 & 1828.2 & 2 & 8.64 &  9.78 &  9.13\\
1297.4 & 2 & 8.78 & 10.08 & 10.08 & & & & & \\
\hline
\end{tabular}
\end{center}
\newpage

According to the theoretical approximation of Girard \& Willson (1987)
there are two mechanisms of ionization in this region: radiative one and
shock
one. The latter of them is determined by the sum of the kinetic energies of 
the winds. This sum in the two moments under consideration is calculated 
to be 4.69$\times$10$^{34}$ erg\,s$^{-1}$ and 2.53$\times$10$^{34}$ erg\,s$^{-1}$.
It has thus decreased with 46 \%, which is also
in agreement with the observed decreasing of the U flux.


\IBVSfig{15cm}{4574-f1.ps}{The UBV light curves of AG Peg}


\references

Altamore, A., Cassatella, A., 1997,
	{\it A\&A}, {\bf 317}, 712

Belyakina, T.S., 1992,
	{\it Izv. Krym. Astrofiz. Obs.}, {\bf 84}, 49

Girard, T., Willson, L.A., 1987,
	{\it A\&A}, {\bf 183}, 247   

Kenyon, S.J., Mikolajewska, J., Mikolajewski, M.,
   	Polidan R.S., Slovak M.H., 1993,
        {\it AJ}, {\bf 106}, 1573

M\"urset, U., Nussbaumer, H., Schmid, H.M., Vogel, M., 1991,
	{\it A\&A}, {\bf 248}, 458
	
M\"urset, U., Jordan, S., Walder, R., 1995,
      	{\it A\&A}, {\bf 297}, L87

Penston, M.V., Allen, D.A., 1985,
      	{\it MNRAS}, {\bf 212}, 939

Seaton, M.J., 1979,
	{\it MNRAS}, {\bf 187}, 73p

Tomov, N.A., 1993,
	{\it Izv. Krym. Astrofiz. Obs.}, {\bf 88}, 22

Tomov, N.A., Tomova, M.T., Raikova, D.V., 1998,
	{\it A\&AS}, {\bf 129}, 1

Vogel, M., Nussbaumer, H., 1994,
   	{\it A\&A}, {\bf 284}, 145

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