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\input ibvs2.sty

\begin{document}

\IBVShead{5183}{12 October 2001}

\IBVStitle{Spectroscopy and Photometry of V1137 A\lowercase{ql}}

\IBVSauth{Miroshnichenko, A. S.$^{1,2}$}

\IBVSinst{Ritter Observatory, Dept.\ of Physics and Astronomy, University of Toledo,
Toledo, OH 43606, USA}

\IBVSinst{Central Astronomical (Pulkovo) Observatory of the Russian Academy
of Sciences, Saint-Petersburg, 196140,\\ \indent Russia}

\SIMBADobj{V1137 Aql}
\IBVSkey{photometry}
\IBVSkey{spectroscopy}
\IBVSabs{New spectroscopic and multicolor photometric observations}
\IBVSabs{of the variable V1137 Aql are presented and used to derive}
\IBVSabs{parameters of the star and its dusty envelope.}

\begintext

V1137 Aql = SON 8114 was discovered by Hoffmeister (1964), who detected
its brightness variations between $m_{\rm pg} = 14\fmm5$ and 16\fmm5.
The variability type SR:{} was assigned to the object in the General
Catalogue of Variable Stars (Kholopov et al., 1985).  The star was
detected by several satellite infrared (IR) surveys (RAFGL 2413, Price
\& Murdock, 1983; IRAS 19307+1338; MSX5C--G049.8933--02.7331, Egan et
al., 1999), which revealed a strong IR flux and emission features at
9.7 and 18 $\mu$m, indicative of the circumstellar silicate dust.
Ground-based IR photometry and spectroscopy (Joyce et al., 1977;
Lebofsky et al., 1978; Eiroa et al., 1983) showed that the object's
fluxes were significantly variable.

From $BVRI$ photometry Eiroa (1981) concluded that V1137 Aql is a
heavily reddened M1-type star (see Table 1), and calculated possible
distance ($D$) and overall, inter- and circumstellar, extinction
($A_V$) toward it:  $A_V = 5\fmm05$ and $D = 313$ pc for the luminosity
type {\sc III} and $A_V = 4\fmm68$ and $D = 6.2$ kpc for {\sc I}a.
Radio observations by Josselin et al.\ (1998) resulted in a detection of
the CO line emission with a ratio $R = S_{60}/T_{\rm mb} = 293$
Jy\,K$^{-1}$ (where $S_{60}$ is the 60-$\mu$m IRAS flux and $T_{\rm mb}$ is
the brightness temperature of the CO (1--0) transition).  These authors
suggested that the latter result indicate that V1137 Aql was a
supergiant, because less luminous post AGB stars have $R \le 150$.

However despite the extensive information from the IR region, optical
observations of V1137 Aql are still represented by photographic
photometry (Gessner, 1983, 1986) and the BVRI data (Eiroa, 1981). This
allows only rough and indirect estimates of the star's physical
parameters and evolutionary state. In order to fill this gap we present
the results of our multicolor photometry and low-resolution optical
spectroscopy of V1137 Aql.

The $BVRIJHK$ observations in the Johnson photometric system were
obtained between July 1986 and August 1995 at two 1-meter telescopes
of the Fesenkov Astrophysical Institute (Kazakhstan) with a
two-channel photometer-polarimeter of the Pulkovo Observatory
(Bergner et al., 1988a). The results are presented in Table 1.  The
large difference in $R-I$ between our data and those of Eiroa (1981)
can be explained by the very red color of the object, differences in
the instrumental photometric systems, and intrinsic variability of the
star.  The detected variations are $\sim 1\mm$ in the $VRI$-bands,
while the $B$-magnitude varies between 14\fmm5 and 15\fmm9
(similar to the results of Hoffmeister, 1964).

\begin{table}[htb]
\caption{Photometric data on V1137 Aql = CRL 2413$^{\rm a}$}
\begin{center}
\begin{tabular}{rcccccccc}
\hline
\lower6pt\hbox{Date}     & JD     & \lower6pt\hbox{$B-V$} & \lower6pt\hbox{$V$}   & \lower6pt\hbox{$V-R$} & \lower6pt\hbox{$V-I$} & \lower6pt\hbox{$V-J$} & \lower6pt\hbox{$V-H$} & \lower6pt\hbox{$V-K$} \\[-4pt]
         &2440000\,+&       &       &       &       &       &       &       \\
\hline
/07/78 &$^{\rm b}$& 3.21  & 12.03 & 2.59  & 4.83  &  --   &  --   &   --  \\
02/07/86 &6614.23 & 3.16  & 11.33 & 2.59  & 4.26  & 5.74  &  --   &   --  \\
13/09/89 &7783.19 & 3.29  & 11.57 & 2.61  & 4.32  & 5.76  & 6.75  & 7.33  \\
16/09/89 &7786.18 & 3.45  & 11.55 & 2.55  & 4.30  & 5.74  & 6.69  & 7.34  \\
27/08/91 &8496.16 &  --   & 11.95 & 2.61  & 4.29  &  --   &  --   &  --   \\
07/11/92 &8934.05 & 3.57  & 12.28 & 2.77  & 4.36  &  --   &  --   &  --   \\
12/08/95 &9942.31 & 3.16  & 11.83 & 2.44  & 3.99  & 5.76  & 6.77  & 7.47  \\
\hline
\end{tabular}
\normalsize
\begin{list}{}
\item $^{\rm a}$ The mean errors (including those of translation from the
instrumental to the standard photometric system) are as follows: 0\fmm02 in
$R-I$, 0\fmm03 in $V-R$, $V-K$, and the $V$-band, 0\fmm05 in $B-V$,
$V-J$, and $V-H$.
\item $^{\rm b}$ The errors are 0\fmm01 in the
$V$-band and 0\fmm02 in the color-indices (Eiroa, 1981).
\end{list}
\end{center}

\caption{Intensities of the TiO bands in the spectrum of V1137 Aql}
\begin{center}
\begin{tabular}{ccccccccc}
\hline
Band                & 4626&4669&4761&4804&4893&4955&5167&6158\\
\hline
$I_{\rm min}/I_{\rm max}$ & 0.67&0.62&0.58&0.81&0.69&0.66&0.44&0.75\\
\hline
\end{tabular}
\end{center}
\end{table}

Two spectra of V1137 Aql (reciprocal dispersion 50 \AA\,mm$^{-1}$,
resolution 2.1 \AA) were obtained on 1991 July 20 (4235--5245 \AA) and
July 21 (5981--7013 \AA) at the 6-meter telescope of the Russian
Academy of Sciences with a TV-scanner mounted in the Nasmyth focus.
Its most prominent features are TiO bands (see Table 2), whose
intensities we measured using the technique by Boyarchuk (1969). We
also detected Balmer lines in absorption, many strong metallic lines,
and no obvious emission lines.

The TiO band strengths were compared with those of M stars with known
spectral types from Boyarchuk (1969), who used dispersions of 61 and 80
\AA\,mm$^{-1}$.  The resulting spectral type is M2-3. The spectrum of
$\mu$ Cep (M2 {\sc i}), obtained at the Ritter Observatory with a 0.25
\AA\ resolution, turned out to be similar to our red spectrum of V1137
Aql, except for the H$\alpha$ line which can be partly filled in by an
emission component (Fig.~1, left panel). We also compared the blue part of the
object's spectrum with that of AS 501 (M4-5 {\sc i-ii}, Bergner et al.,
1988b), which we obtained on 1991 July 20 at the 6-meter telescope with
the same equipment.  The luminosity dependent intensity ratios of the
Fe {\sc i} lines at 4376, 4383, and 4389 \AA\ and at 4427 and 4431
\AA\ indicate that V1137 Aql is less luminous than AS 501.  Thus, our
spectroscopic data suggest an MK type of M2/3 {\sc ii-iii} for V1137
Aql.

\IBVSfig{6cm}{5183-f1.ps}{{\bf Left panel.} A part of the spectrum of
V1137 Aql near the H$\alpha$ line (solid line). The dashed line
represents the spectrum of $\mu$ Cep obtained at the 1-meter telescope
of the Ritter Observatory of the University of Toledo with a fiber-fed
\'echelle spectrograph and a Wright Instruments Ltd.\ CCD camera (the
resolution is 0.2 \AA) and re-binned to a constant wavelength
increment of 2 \AA\@. Both spectra are normalized to the continuum level
near the H$\alpha$ line. The wavelengths are in \AA\@.  {\bf Right panel.} The averaged
observed and dereddened SED of V1137 Aql and a theoretical model (solid
line) calculated with the parameters described in text.  Our optical
and near-IR data are shown by filled circles, the MSX fluxes by filled
triangles, the IRAS fluxes by filled squares, and the IRAS
low-resolution spectrum by pluses}

\vskip-11pt
Our photometry supported by the longer-wavelength data indicate that
V1137 Aql is surrounded by a large amount of circumstellar dust, whose
characteristics can be derived by modelling the observed spectral
energy distribution (SED)\@.  The IR data obtained by different authors
show that the object's flux at 11 $\mu$m varies from 46 Jy (Joyce et
al., 1977) to 195 Jy (Price \& Murdock, 1983). This is comparable with
the amplitude of the optical variations. The IRAS and MSX data, which
represent an intermediate brightness level, were used along with the
averaged optical data to construct the SED. Despite the uncertainty in
the IR fluxes, its shape is better determined, which is seen from our
photometric data. To calculate theoretical SEDs, we used a radiative
transfer code DUSTY by Ivezi\'{c}, Nenkova, \& Elitzur (1999) for
spherical dusty envelopes. The dust temperature distribution is
calculated self-consistently including dust scattering, absorption, and
emission.  A Kurucz (1994) model for $T_{\rm eff} = 3750$ K and $\log g
= 1.0$, roughly corresponding to an M2/3 {\sc iii} star, was used to
describe radiation of the star and optical properties of the
interstellar dust (Mathis, Rumpl, \& Nordsieck, 1977) to model dust
particles in the envelope.  Models with different dust sublimation
temperatures ($T_{\rm sub}$), the envelope optical depths at 0.55
$\mu$m ($\tau_V$), and ratios of its outer and inner radii ($Y_{\rm out}$)
were calculated. The dust density distribution $\propto r^{-2}$ (where
$r$ is the distance from the star) was fixed.  We compared the observed
and theoretical SEDs adjusting the interstellar extinction $A_{V}^{\rm IS}$
with the best fit shown in the right panel of Fig.~1.

The modelling shows that the strengths of the silicate features are
well reproduced by the interstellar dust with $\tau_V = 5.2$, while
$A_{V}^{\rm IS} \simeq 0\fmm1$--0\fmm2. $T_{\rm sub}$ of 500--600 K is
required to match the near-IR part of the SED and $Y_{out} \sim 100$
to match its slope at $\lambda \ge 25~\mu$m.  However, the combination
of the satellite IR and our data, obtained non--simultaneously, make
the relative contribution of the circum- and interstellar extinction
uncertain.  Since the observed near-IR color-indices are not
consistent with a large $A_{V}^{\rm IS}$, we do not expect it to be $\ge
1\mm$.  The observed $J-H=1\fmm02\pm0\fmm03$, which is lightly
affected by the thermal radiation, and intrinsic $(J-H)_{0}=0\fmm88$
(Bessell \& Brett, 1988) suggest $A_{V}^{\rm IS} \le 1\fmm2$.

The results of our calculations suggest that the dusty envelope around
V1137 Aql is optically thin in the IR but optically thick in the
optical domain. Using the bolometric flux ($F_{\rm bol}$) and a
relation of $A_{V}^{\rm IS}$ versus $D$ in the object's direction, we
can estimate its luminosity. $F_{\rm bol}$, calculated from the
theoretical SED scaled with the observed fluxes, is $5\times 10^{-5}$
W\,m$^{-2}$ and is uncertain within at least a factor of 2.  Eiroa
(1981) estimated $A_{V}^{\rm IS} \sim 3\mm$ at $D \ge 1$ kpc in this
direction. Miroshnichenko (1996) studied the interstellar extinction
law in a region of $\sim 2\deg$ around MWC 314, located in $\sim 3\deg$
from V1137 Aql, and showed that $A_{V}^{\rm IS}$ reaches $\sim 1\mm$ at
$D \sim 1$ kpc.  Since $\displaystyle F_{\rm bol} = \sigma
T_{*}^{4}\left({R_{*} \over D}\right)^{2}$, where $R_*$ is the star's
radius, at $D = 1$ kpc $M_{\rm bol} = -3\fmm2$ and is close to that of
the luminosity type {\sc iii} (Strai\v{z}ys \& Kurilene, 1981).  It
corresponds to $R_* \sim 90~R_\odot$ and is consistent with recent
estimates for normal M3-type giants by Dumm \& Schild (1998).

Thus, we conclude that V1137 Aql is an intermediate-luminosity
oxygen-rich early M-type star showing brightness variations similar to
those of the Mira stars. This is consistent with its
location in a region of optical Mira variables in the IRAS
color--color diagram (Olnon et al., 1984). Our luminosity
estimate implies a main sequence mass of $\sim 1~M_\odot$
and a possible period of the variations of $\sim 100\day$--150\day
(Wood et al., 1983). The results of our study can be verified by follow up
optical photometric monitoring as well as by simultaneous
photometric observations in a spectral range from 0.4 to $\sim 10~\mu$m.

I thank N.V. Borisov for his help with obtaining spectroscopy
and D.B. Mukanov, K.S. Kuratov, and T.A. Sheikina
for their assistance with obtaining photometry.
This research has made use of the SIMBAD database, operated at CDS,
Strasbourg, France, and of the spectral archive of the Ritter Observatory
of the University of Toledo, Ohio, USA.

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\end{document}
