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\IBVShead{4506}{11 August 1997}

\IBVStitle{A new peculiar B\lowercase{e} object MWC 657}

\IBVSauth{A.\ S.\ Miroshnichenko$^1$, K.\ S.\ Kuratov$^2$, \v{Z}.\ Ivezi\'{c}$^3$, M.\ Elitzur$^4$}

\IBVSinst{Central Astronomical Observatory of the Russian Academy of Sciences at Pulkovo,
196140 Saint--Petersburg, Russia, e-mail: anat@pulkovo.spb.su}

\IBVSinst{Fesenkov Astrophysical Institute of the Kazakhstan National Academy of Sciences, Kamenskoye plato, 480068, Almaty, Kazakhstan}
     
\IBVSinst{Princeton University Department of Astrophysical Sciences Princeton, NJ 08544, USA}

\IBVSinst{Department of Physics and Astronomy, University of Kentucky, Lexington, KY 40506-0055, USA}

\IBVSabs{Photometric and spectroscopic peculiarities of the Be star}
\IBVSabs{MWC 657 are discussed.}

\SIMBADobj{MWC657}
\IBVStyp{ GCAS }
\IBVSkey{photometry, spectroscopy, Be}


\begintext


MWC 657 was distinguished as a star of B spectral type with $m_{V} = 12^m$
and $H_\alpha$ in emission (Merill \& Burwell 1943). Dong \& Hu (1991)
identified it with an IRAS source 22407 + 6008 by coincidence of the optical
and infrared (IR) positions (R.A. $22\hr40\mm 47\fsec 6,\
Dec. +60\deg 08'
17''$, 1950.0). There is an object of $V=12\fmm5$ at this position in the
Guide Star Catalog (GSC 4265.0873). No optical and near--IR photoelectric
observations or detailed spectroscopy have been reported previously for the
object. IR--fluxes from the object measured by the IRAS and obtained by us
using the ADDSCAN procedure are 5.93, 4.22, and 0.40 $Jy$ at 12, 25, and 60
$\mu$m respectively. This means that MWC 657 has an extremely red spectral
energy distribution (SED) similar to those of the objects surrounded by
circumstellar dusty envelopes.

We obtained photoelectric $UBVRIK$ observations of MWC 657 in the Johnson
system at a 1--meter telescope of the Tien--Shan Observatory (Kazakhstan)
equipped with a two--channel photometer--polarimeter of the Pulkovo
Observatory (Bergner et al. 1988) in September and December 1996 (Table 1).
Typical errors are $0\fmm05$ in the $U$--band, $0\fmm03$ in the $BVRI$--bands,
and $0\fmm15$ in the $K$--band. HD\,214764 (F0) was used as a comparison star
and HD\,211589 as a check star (Kornilov et al. 1991). Brightness of the
latter was found to be constant with respect to the comparison star with an
accuracy of $0\fmm01$.


The photometric results lead us to the following qualitative conclusions:
\begin{enumerate}
\item The star is heavily reddened. Its $U-B$ and $B-V$ colors locate
at a reddening line for early--B stars.
\item A local peak in the SED corresponding to the $R$--band (see Fig.1)
points out on a strong emission in $H\alpha$ which is consistent with the
Merrill \& Burwell's (1943) description.
\item Brightness in the $K$--band and the IRAS colors imply presence of
the infrared excess which can be explained by circumstellar dust radiation.
A steep decrease of the observed flux towards longer wavelengths shows
that the dusty envelope should be optically thin in the optical region and
have quite a small radial extension.
\end{enumerate}


\begin{table}
\caption{Photometry of MWC 657}
\begin{center}
\begin{tabular}{ccrcccc}
\hline\noalign{\smallskip}
JD2450000+ & $V$ & $U-B$ & $B-V$ & $V-R$ & $V-I$ & $K$\\
\hline\noalign{\smallskip}
349.25 & 12.85 & $-$0.2: & 1.35 & 1.75 & 2.73 &    \\
353.29 & 12.57 &  0.01   & 1.32 & 1.51 & 2.47 &    \\
417.11 & 12.70 & $-$0.27 & 1.40 & 1.50 & 2.43 &    \\
420.08 & 12.62 &  0.00   & 1.39 & 1.55 & 2.42 &    \\
422.06 & 12.57 &  0.03   & 1.49 & 1.55 & 2.46 &6.67\\
424.10 & 12.63 & $-$0.11 & 1.48 & 1.49 & 2.47 &    \\
427.09 & 12.67 & $-$0.12 & 1.49 & 1.53 & 2.47 &6.31\\
429.09 & 12.68 &  0.02   & 1.39 & 1.55 & 2.31 &    \\
433.11 & 12.62 & $-$0.05 & 1.44 & 1.52 & 2.41 &    \\
439.13 & 12.63 & $-$0.36 & 1.80 & 1.50 & 2.25 &    \\
\hline\noalign{\smallskip}
\end{tabular}
\end{center}
\end{table}

\IBVSfig{12cm}{4506-f1.eps}{Spectral energy distribution of MWC 657 (circles) and the
modeling results. The model with $\alpha$=1.5 is shown by solid line,
with $\alpha$=2.0 by dashed line. Other parameters are described in text.
The SED is expressed in units of the bolometric flux ($F$).}

\vskip8mm




Taking into consideration these items we tried to model the SED of MWC 657
constructed from averaged photoelectric data and the IRAS photometry
using a radiative transfer code DUSTY by Ivezi\'{c}, Nenkova, \& Elitzur
(1997) for spherical dusty envelopes. The dust temperature distribution
is calculated self--consistently including dust scattering, absoprtion, and
emission. We used the Kurucz (1979) models to describe radiation of the
central star and optical properties of the interstellar dust (Mathis,
Rumpl, \& Nordsieck 1977) for the dust particles in the envelope. The dust
sublimation temperature was fixed at 1500 K. A grid of models with different
stellar temperatures ($T_*$), the dust density distributions ($\propto
r^{-\alpha}$, where $r$ is the distance from the star), the envelope optical
depths ($\tau_V$), and ratios of its outer and inner radii ($Y_{out}$)
has been calculated. An additional free parameter of the fitting was
interstellar extinction ($A_V$).

The modeling showed that the SED in the IR region could be fitted well with
different parameters of the envelope while $T_*$ and $A_V$ defining the SED
in the optical regime remained the same: 20000 K and $4\fmm9$ respectively
(Fig.1). The best fit was found for $\alpha$ between 1.5 and 2.0. Other
envelope parameters are different at these boundaries: $\tau_V$ = 0.03,
$Y_{out}$ = 100 for $\alpha$ =1.5 and $\tau_V$ = 0.05, $Y_{out}$ = 700 for
$\alpha$ =2.0. The former density law is expected for free--fall matter while
the other one for constant mass loss from the central object.

The bolometric flux calculated ($F_{bol}$) from the theoretical SED scaled
with the dereddened observations is $1.2\times10^{-4}\, {\rm W}{\rm m}^{-2}$. Since stellar
radiation dominates in the optical region and scattering is negligible, one
can estimate the star's angular diameter or ratio of its radius ($R_*$) to
distance ($D$) towards the object using a formula
$ F_{bol} = \sigma\,T_{*}^{4} ({R_* \over D})^{2}$.
This value turned out to be $1.2\times10^{-10}$ which corresponds to $R_{*} = 5.9
R\odot D_{kpc}$. Interstellar extinction in the object's direction smoothly
increases with distance from the Sun up to 3--4$^{m}$ at $D$ of about 2--3 kpc
with almost no further increase beyond this region (Bergner et al. 1986).
Thus, even if we take 2 kpc as a distance toward the star it will have a
bolometric luminosity $\log {L_{bol} \over L\odot}$ = 3.67 and locate above the
main sequence.

Some suggestions on the object's evolutionary state can be presented.
Usually such optically and geometrically thin envelopes are not observed in
pre--main--sequence objects. However, there are several peculiar B--type
stars with a similar far--IR brightness decrease which have been suspected as
possible Herbig Ae/Be stars (HD 45677, HD 50138).
Other interpretations cannot be excluded, i.e. a binary system or a very
young planetary nebula, but they require additional observational evidences.
It is certainly needed further study of this interesting object. This should
include at least optical spectroscopy, near--IR photometry, and optical
photometric monitoring.


\references

Bergner, Yu.K., Miroshnichenko, A.S., Yudin, R.V., et al.: 1986,
Astron. Tsirk. No. 1459, 5

Bergner, Yu.K., Bondarenko, S.L., Miroshnichenko, A.S., et al.: 
1988, Izvestia Glavn. Astron. Obs. v Pulkove, 205, 142

Dong, Y.-S., Hu, J.-Y.: 1991, Chin. A\&A, 15, 275

Ivezi\'{c}, \v{Z}., Nenkova, M., \& Elitzur, M.: 1997,
Univ. of Kentucky, Internal Report

Kornilov, V.G., et al.: 1991, Sternberg Astron. Inst. Proc., v.63

Kurucz, R.L.: 1979, ApJS, 40, 1

Mathis, J.S., Rumpl, W., \& Nordsieck, K.H.: 1977, ApJ, 217, 425

Merrill, P.M., \& Burwell, C.G.: 1943, ApJ, 98, 153

\endreferences

\end{document}
