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\IBVShead{4579}{6 April 1998}

\IBVStitletl{T\lowercase{i}O- and V-Band Photometry of}{ the Pulsating Red Giant V CV\lowercase{n}}

\IBVSauth{ Rick Wasatonic, Edward F. Guinan}

\IBVSinsto{Department of Astronomy and Astrophysics, Villanova University, Villanova, PA. 19085, USA}
\vskip -6mm

\SIMBADobj{V CVn}

\begintext

   V CVn (HD\,115898, HIP 65006) is a bright semi-regular red SRa 
variable star, located on the asymptotic red giant branch.  V CVn is 
interesting because it has properties intermediate between classical Mira
variables and Small Amplitude Red Variables (SARVs) as defined by Percy et al. (1996).  The study of V CVn by Vetesnik and Papousek (1986) yields a mean
photometric period near 192 days, with an average light amplitude 
of $\approx$ 1.5 to 2.0 magnitudes.  From chiefly AAVSO visual estimates, the
visual magnitude of V CVn typically varies from about +6.8 mag to +8.8 mag.
The spectral type varies from M4 IIIe to M6 IIIe, and its spectrum is 
dominated by strong TiO bands (Boyle et al., 1986).  Recently the parallax
of V CVn has been improved by the Hipparcos satellite to yield a distance
as 375 $\pm$ 125 pc.  Because of the interesting characteristics of V CVn
and the lack of detailed multi-wavelength photometry, the star was added
to the ongoing program of photometry of cool giants and supergiants at
Wasatonic Observatory and Villanova University Observatory.  The results
presented in this paper represent the first time the radius, luminosity 
and effective temperature of V CVn have been estimated using narrow to 
intermediate-band IR photometric techniques.  Additionally, observations
were made with a filter matched to the Johnson V-band filter at 550 nm.

   Photometry was carried out using the Wing near-IR three filter system
with filters centered on 719, 754, and 1024 nm, designated as
A, B, and C, respectively (Wing, 1992; Morgan et al., 1997).  TiO indices,
(B$-$C)-IR color indices, and IR apparent magnitudes are calculated from the 
observations.  The TiO index is calculated from the standardized A, B, and
C magnitudes using the following equation adapted from Wing (1992):
\vskip 2mm
\hspace*{4cm}TiO Index = A$-$B $-$ 0.13(B$-$C) \hfill (1)          
\vskip 2mm
\noindent where B$-$C is the IR color index.  As defined, a numerical increase 
in the TiO index corresponds to an increase in the TiO absorption at 719nm. 
In our calibrations of TiO index with spectral type, we find a linear 
correlation between the index and spectral type up to M8.  For stars with
spectral types later than M8, the A and B bandpasses become contaminated  
by molecular absorption bands such as VO and water vapor.  

Using a large number of Wing standard stars (Wing, 1978), 
calibrations were done to establish the difference 
between observed C-band (1024 nm) magnitudes and calculated bolometric 
magnitudes using bolometric corrections from Novotny (1973).  The
observed 1024 nm magnitudes were then transformed to bolometric magnitudes 
(m$_{bol}$).  Using the Hipparcos parallax, absolute bolometric
magnitudes, and hence luminosities, were determined.  Also, from Wing 
(1979) standard star data, IR color index - temperature calibrations
were determined, enabling temperatures to be estimated from the observed 
IR color and TiO indices data. Knowing both luminosities and temperatures,
radii were calculated throughout the pulsational cycle by using the 
equation:

\vskip 2mm
\hspace*{4cm}(R$_\star$/R$_\odot$) = [(L$_\star$/L$_\odot$)/(T$_\star$/T$_\odot$)$^{4}$]$^{0.5}$ \hfill (2)
\vskip 2mm

\noindent  Mahler et al. (1997) and Morgan et al. (1997) provide additional details
of the Wing 3-color near-IR system, which they used to estimate changing 
effective temperatures, luminosities, and radii of Mira and Betelgeuse, 
respectively. These same techniques were used to calculate T$_{eff}$, 
$L_\star$/$L_\odot$ 
and $R_\star$/$R_\odot$ for V CVn over its 191.8 day pulsational cycle.


\IBVSfig{8.5cm}{4579-f1.ps}{IR (1024 nm) magnitude, (B$-$C) color index,
visual magnitude (550 nm), and TiO index of V CVn during the 1997 observing
season}
\vskip -.8cm


   Differential photometry of V CVn was conducted during 1997 on 27 nights
from JD 2450462 to JD 2450655 at Wasatonic Observatory.  An uncooled SSP-3   
Optec photometer, attached to a 20-cm Schmidt-Cassegrain telescope, was
used to carry out the photometry.  The detector was an uncooled silicon 
PIN-photodiode.  The comparison star was HR\,5067 (V = +5.88; B$-$V = +0.97;
K0 III),and the check star was HD\,120315 ($\eta$ UMa, V = +1.86; B$-$V =
$-$0.19; B3 V).  The check star also served as the primary Wing IR standard  
star, from which all subsequent standardized comparison and variable star
IR magnitudes were calculated.  All data reductions accounted for 
atmospheric extinction effects and conversions of UT to HJD.  The phases 
for V CVn were computed using the epoch given in the 1997 Astronomical 
Almanac, as follows:

\vskip 2mm
\hspace*{4cm}T$_{max}$ = HJD 2443929.00 + 191\fday 89 $\times$ E\hfill (3)
\vskip 2mm

    Figure 1 shows the various photometric magnitudes and indices found
for V CVn, plotted against pulsation phase.  The C-band (1024 nm)
magnitudes are plotted in the top panel. These data show a nearly smooth
quasi-sinusoidal variation with a full amplitude of about $\delta$ C =
0.30 mag.  Maximum and minimum brightness occur at phase 0.00 and 0.75,
respectively. As discussed previously, the behavior of the
C-band (1024 nm) magnitude is very similar to the apparent bolometric
magnitude (m$_{bol}$) or the absolute bolometric magnitude (M$_{bol}$) when scaled with the Hipparcos parallax.  
  
   The IR (B$-$C) index is shown in the second panel in Figure 1.  Note
that this index varies in a similar manner to the C-band (1024 nm) 
magnitude. The B$-$C values vary from about +0.05 mag at IR maximum 
brightness to about +0.70 mag during IR light minimum.  However, the most 
positive values of the (B$-$C) index occur over a broad, relatively flat
minimum that extends from about phase 0.50 to 0.75.  From previous 
(B$-$C)-T$_{eff}$ calibrations, V CVn attains its maximum temperature of
T$_{max}$ $\approx$ 3200 K near phase 0.00 and reaches a minimum value of 
T$_{min}$ $\approx$ 2200 K during the 0.50 to 0.75 phase interval.  The absolute
radius of the star can be found for each ($L_\star$/$L_\odot$;T$_{eff}$) 
data set from equation (2).

In this case, the star attains a maximum radius of R$_{max}$ $\approx$ 400
$R_\odot$  near light minimum and shrinks to R$_{min}$ $\approx$ 200
$R_\odot$ at maximum luminosity, when the star is hottest. 
Table 1 lists the estimated effective temperatures and, in solar units, 
radii and luminosities of V CVn 
at phases 0.00 and 0.75, corresponding to the C-band (1024 nm)           
maximum and minimum magnitudes, respectively.

\begin{center}
Table 1. Temperatures, Radii, and Luminosities of V CVn at C-Band (1024nm) \\
                 Maximum and Minimum Brightness\\
\end{center}

\begin{center}
\small
\begin{tabular}{lccccc}\hline
 Phase&C-band mag&Temp.(K)&Radius ($R_\odot$)&Luminosity ($L_\odot$)\\ \hline
\hline

 0.00 & +2.65 & 3200 & 200 & 4600\\
 0.75 & +3.10 & 2200 & 400 & 3100\\ \hline
\end{tabular}
\end{center}
\large

   The observed V-band curve is shown in the third panel of Figure 1.
It is similar to the light curves of V CVn by Magalhaes et al. (1986) and
Kruszewski et al. (1968).  Note the secondary maximum or ``hump" centered 
at phase $\approx$ 0.65, which is also a feature in the Kruszewski light 
curve.  As pointed 
out by Wing (1986), visual light curves for cool stars are strongly 
dependent on TiO blanketing.  The hump in the V-band curve occurs at the 
same phase where the TiO index decreases, indicating a significant weakening
of the TiO absorption. The calculated TiO index is shown in the bottom panel
of Figure 1. From TiO - temperature and B$-$C color index calibrations, it 
was found that the numerical value of the TiO index gives a reliable 
measure of the TiO line strength out to values of approximately TiO   
$\approx$ 1.8.

Therefore, the nearly constant value of the TiO index of about +1.7 
from phase 0.00 to phase 0.50 is near the limit where the TiO index is 
useful as a TiO band measure. However, the observed decrease of the TiO index
to about +1.35 mag at phase 0.65 is very reliable and  
indicates TiO dissociation.  This normally occurs with an increase in
temperature.  Note, however, that the temperature, as indicated by the
(B$-$C) IR color-index curve (second panel, Figure 1) is relatively constant
during this drop in the TiO index.  Hence the TiO dissociation at this phase
is not due to an increase in temperature, but probably results from other
mechanisms.  A possible explanation is that a shock wave propagated through
the atmosphere, causing the TiO dissociation without producing a significant
change in temperature.
This could have led to the increase in brightness in the visual magnitude
(the hump) at phase 0.65.  As the shock dissipated, the TiO index increased
from phase 0.65 to 0.80, indicating TiO recombination.  This was accompanied
by the decrease in the V-band light curve due to the TiO blanketing effect.
As shown in the figure, from phase 0.85 to phase 0.00 the TiO index 
again drops as the star brightens in both the V-band and the C-band (1024 nm).
These changes are accompanied by a decrease in the (B$-$C) index, indicative
of increasing effective temperature and a dissociation of TiO.  The decrease
in the TiO line strength causes the drop in the TiO index itself.  Data for 
all four panels of Figure 1 is tabulated in Table 2.

Continued photometry of V CVn is planned.  In obtaining more data we 
hope to ascertain whether the phenomena observed during the 1996/97
observing season reoccurs.  The authors wish to thank Robert Wing for his
assistance and advice. We also acknowledge the use of the SIMBAD database 
and the Hipparcos Main Catalogue.  This research is supported by NSF grants
AST-9315365 to Villanova University and AST-9528506 to the Four College
Consortium.

\begin{center}
Table 2. Plotted V CVn Data
\end{center}
\begin{center}
\small
\begin{tabular}{lccccc}
\hline
 HJD 2450+&Phase&Visual mag&C (1024 nm) mag&Color Index&TiO Index \\ \hline \hline

 462.766 & 0.046 & +7.045 & +2.649 & +0.250 & +1.645 \\
 469.775 & 0.082 & +7.076 & +2.650 & +0.219 & +1.717 \\
 489.626 & 0.186 & +7.266 & +2.719 & +0.271 & +1.707 \\
 499.926 & 0.238 & +7.330 & +2.723 & +0.291 & +1.699 \\
 508.593 & 0.285 & +7.283 & +2.725 & +0.319 & +1.627 \\
 518.604 & 0.337 & +7.433 & +2.802 & +0.300 & +1.748 \\
 524.647 & 0.369 & +7.595 & +2.783 & +0.415 & +1.723 \\
 535.643 & 0.426 & +7.949 & +2.874 & +0.507 & +1.760 \\
 543.553 & 0.468 & +8.001 & +2.915 & +0.600 & +1.714 \\
 550.577 & 0.504 & +8.036 & +2.951 & +0.671 & +1.647 \\
 554.575 & 0.525 & +8.029 & +2.965 & +0.662 & +1.663 \\
 565.576 & 0.582 & +7.725 & +3.019 & +0.629 & +1.418 \\
 575.694 & 0.634 & +7.632 & +3.018 & +0.639 & +1.339 \\
 585.647 & 0.686 & +7.782 & +3.074 & +0.629 & +1.343 \\
 590.590 & 0.713 & +7.880 & +3.062 & +0.658 & +1.433 \\
 595.634 & 0.739 & +8.015 & +3.097 & +0.671 & +1.570 \\
 600.606 & 0.765 & +8.109 & +3.076 & +0.638 & +1.750 \\
 604.607 & 0.786 & +8.192 & +3.057 & +0.682 & +1.714 \\
 614.598 & 0.838 & +7.975 & +2.908 & +0.535 & +1.835 \\
 615.600 & 0.843 & +7.890 & +2.905 & +0.514 & +1.771 \\
 619.610 & 0.864 & +7.668 & +2.833 & +0.386 & +1.805 \\
 622.609 & 0.879 & +7.458 & +2.811 & +0.357 & +1.661 \\
 627.608 & 0.905 & +7.180 & +2.715 & +0.254 & +1.747 \\
 633.602 & 0.937 & +6.838 & +2.667 & +0.169 & +1.567 \\
 640.598 & 0.974 & +6.587 & +2.670 & +0.051 & +1.495 \\ 
 650.586 & 0.025 & +6.688 & +2.664 & +0.091 & +1.470 \\
 655.573 & 0.051 & +6.882 & +2.674 & +0.160 & +1.556 \\ \hline
\end{tabular}
\end{center}
\large
\vskip  -.7cm

\references
  
Nautical Almanac Office, USNO., H.M. Nautical Almanac 
           Office, Royal Greenwich Observatory, Astronomical 
           Almanac, 1997, U.S. Government Printing Office, p. H71

Boyle, R.P., Aspin, C., Coyne,G.V., McLean, I.S., 1986,
           {\it Astron. Astrophys.}, {\bf 164}, 310

Kruszewski, A., Gehrels, T., Serkowski, K., 1968,
           {\it AJ.}, {\bf 73}, 677

Magalhaes, A.M., Coyne, G.V., Benedetti, E.K., 1986,
           {\it AJ.}, {\bf 91}, 919

 Mahler, T., Wasatonic, R.P., Guinan, E.F. 1997, 
           {\it IBVS}, No.\,4500

Morgan, N., Wasatonic, R.P., Guinan, E.F. 1997, 
           {\it IBVS}, No.\,4499

Percy, J.R., Desjardins, A., Yu, L., Landis, H.J., 1996,
           {\it PASP}, {\bf 108}, 139

Novotny, E., 1973, Introduction to Stellar Interiors,
           (Oxford University Press), p.11

Vetesnik, M., Papousek, J., 1986, Studies on the Light Changes of
           Semiregular Red Variable Stars - 1. University of J.E. Purkyne,
           Czechoslovakia 

Wing, R.F., 1978 in Spectral Classification and Color Temperatures,
           for 280 Bright Stars in the Range K4 - M8, Astronomy Department,
           Ohio State University

Wing, R.F., 1979, Dudley Obs. Report 14, A.G. Davis Philip, ed.

Wing, R.F., 1986 in The Study of Variable Stars Using Small
Telescopes, ed. J.R. Percy, Cambridge University Press,
Cambridge, England, p. 131

Wing, R.F., 1992, {\it JAAVSO}, {\bf 21}, 42

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