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\IBVShead{4210}{27 June 1995}

\centerline{\ft YOUNG PULSATING STARS IN THE B\"OHM-VITENSE DECREMENT}

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\begintext

        A half dozen variable stars with periods near one day and
temperature near 7000\degr K have recently been reported:
\vskip 1mm
\centerline{Table 1}
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\begin{center}
\begin{tabular}{lcl}
\hline
          HD              &      $\Delta$V   &          Reference\\
\hline
        27290             &       0.10       &     Balona et al., 1994\\
        32537             &       0.10       &     Krisciunas et al., 1993\\
        164615            &       0.07       &     Abt et al., 1983\\
        218396            &       0.08       &     Rodriguez and Zerbi 1995\\
        224638            &       0.08       &     Mantegazza et al., 1994\\
        224945            &       0.06       &     Mantegazza et al., 1994\\
\hline
\end{tabular}
\end{center}
\vskip 2mm

The available Str\"omgren photometry is, listed in Table 2, together
with log T$_e$ and M$_V$ derived from Eggen (1995a):
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\centerline{log T$_e$ = 0.53 ($\beta$ $-$2.800) + 3.881}

\centerline{M$_V$ = $-$12($\beta$ $-$2.800) + 2.05 $-$F$\Delta$[c$_1$]}

\noindent   where

\centerline{$\Delta$[c$_1$] = [c$_1$] $-$2.60($\beta$ $-$2.800) + 0.792}

\centerline{ F =  $-$18.5($\beta$ $-$2.800) + 7.25}
\vskip 1mm

HD\,27290 ($\gamma$ Dor) is a member of the IC\,2391 supercluster (Eggen,
1995b) with an age near 5x10$^7$y and HD\,218396 (HR\,8799) is a
member of the Pleiades supercluster (Eggen, 1995c) with an age near
10$^8$y.  HD\,164615 (V2118 Oph) and HD\,32537 (9 Aur) are young
disk population stars with (U, V, W) = (+28, $-$13, $-$16) and (0, $-$16,
$-$16) km/sec, respectively.  HD\,224638 and HD\,224945 are almost
certainly in the young disk because the proper motions are very
small.

        The periods of HD\,27290 are 0.73 and 0.75 d with a beat period
of 23.5 d.  HD\,164615  has a period of 0.81 d and HD\,218396 has
0.51 d.  The periods of HD\,32537,  HD\,224638 and  HD\,224945 are unclear but
appear to be near 1.25 d.  The supercluster parallax of HD\,27290 and
 HD\,218396 give luminosities of +2.85 and +2.77 mag, respectively,
agreeing well with photometric values in Table 2.

        These stars are shown as open circles in the ([u$-$b], M$_V$)
plane of Figure 1 where the main sequence AF stars in the Pleiades
and $\alpha$ Persei clusters are represented by a straight line.  This
cluster main sequence has a pronounced gap between [u$-$b] = 0.97 and
1.12 mag (Eggen, 1992, 1995c, Eggen and Iben, 1988) which contains no
cluster stars.  The values of [u$-$b] = 2[m$_1$] + [c$_1$] are
reddening free.

\vfill \eject

\centerline{2}
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\epsfysize=7cm
\centerline{\epsffile{4210-f1.ps}}
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Figure 1.  The Pleiades and $\alpha$ Persei cluster main sequence in
the ([u$-$b], M$_V$) plane.  The stars in Table 2 are represented by circles.

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\begin{center}
Table 2. Pulsating Variables in the B\"ohm-Vitense Decrement
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\begin{tabular}{rlccccccll}                                                                                                                                              \hline
HD     &  Name & V & [M$_1$]&[c$_1$]&[u$-$b]&logT$_e$& M$_V$&Sp.T & vsini\\
\hline
\\
 27290& $\gamma$ Dor& 4.25&0\fm 235&0\fm 620&1\fm 090&3.848&+2\fm 81& F0V&       50km/s \\
 32537& 9 Aur    & 5.00 & 0.217&0.599 & 1.033 & 3.840  & +2.91 & F1Vp$^{\star}$&14\\
164615& V2118 Oph& 7.02 & 0.247&0.578 & 1.072 & 3.836  & +3.00 & F2V      & 60\\
218396& HR\,8799  & 5.97 & 0.193& 0.651 & 1.037&  3.850 &  +2.66&  F0V$^{\star}$& 45\\
224638&          & 7.45 & 0.205& 0.681 & 1.085&  3.850 &  +2.42&  F0       & 24\\
224945&           &6.70 & 0.213& 0.650 & 1.076&  3.842 &  +2.50&  F0        &55\\
\hline
\end{tabular}
\end{center}
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$^{\star}$       Abt and Morrell (1995). HD\,32537 has weak 4481 and HD\,218396 is of type A5 from the metal lines.


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B\"ohm-Vitense (1970) has suggested that an abrupt onset of
convection in a stellar atmosphere will cause a gap in the
distribution of stellar temperatures (B$-$V).  B\"ohm-Vitense and
Canterna (1974) found such a gap in the distribution of B$-$V values
for field stars but failed to find it in the Pleiades and $\alpha$
Persei clusters.  This failure is partly due to a deviant reddening
in a small area of the Pleiades near Merope (23 Tau) and
uncertainties in the observed values of (B$-$V) for the cluster
stars.  The gap is obvious in the ([u$-$b], M$_V$) plane and from
stellar parameters for zero age main sequence stars with (X, Y) =
(0.70, 0.28) (Maeder and Meynet, 1991) we find log g = 4.3, M$_V$
= +2.9 and a mass of 1.65 M$_{\odot}$ for the mean temperature (7000\degr
K) of the stars in Table 2.  The mean luminosity in the
table, +2.7 mag, is in close agreement.  The stellar atmospheres by
Lester et al. (1986) give T$_e$ $\sim$ 7000 to 7500\degr K for the
gap of [u$-$b] = 0.97 to 1.12 mag.  B\"ohm-Vitense and Canterna found
that the conversion to convection in the stellar atmosphere occurs
at T$_e$ $\sim$ 7700\degr K, in close agreement.  We will therefore
refer to this gap in Figure 1 as the B\"ohm-Vitense decrement, BVD,
and the variables in Table 2, that lie in this gap, as BVDS.  These
are main sequence variables cooler than the red edge of the
instability strip and with periods considerably longer than the
USPC ($\delta$ Scuti) variables.
\vfill \eject

\centerline{3}
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        It should be noted that a mass of 1.65 M$_{\odot}$ is very close to
that which divides the old disk and young disk populations (Eggen,
1995a), with the former forming Hyades-like giant sequences and the
later M67-like subgiant and giant sequences.  We should therefore
expect the BVDS to be young disk stars.  Candidates that should be
monitored for light variation include the following stars in Table 3:
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\centerline{Table 3}
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\begin{center}
\begin{tabular}{rlclcccclc}
\hline

 HR  & [u$-$b]&  M(sub)V & Sp.T & vsini &  HR &  [u$-$b]& M(sub)V&  Sp.T &vsini\\
\hline
\\
 571&  1\fm 23  & +2\fm 95 &   F0V  & ...  &  4616 & 1\fm 046 & +2\fm 56 &  F0IV  & 65\\
1981&  1.017 & +2.80 &   F3V  & ...  &  4623 & 0.987 & +2.79 &  F2V   & 16\\
3649&  1.047 & +2.96 &   F0IV & 21   &  4971 & 1.076 & +2.70 &  F0V   & 70\\
3874&  1.056 & +2.75 &   F2V  & ...  &  5817 & 1.086 & +2.53 &  F0V   & 60\\
4324&  0.991 & +2.93 &   F2IV & 11   &  6449 & 1.060 & +2.78 &  F3V   & 18\\
     &       &       &        &      &  7877 & 1.040 & +2.92 &  F2V   & 10\\
\hline
\end{tabular}
\end{center}

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\beginauthors
\authorsrightcol
Olin J. EGGEN
NOAO (CTIO)$^1$ 
Casilla 603, La Serena
Chile 225415
\endauthors
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$^1$ Cerro Tololo Inter-American Observatory, National Optical
Observatories, operated by the Association of Universities for
Research in Astronomy, Inc. (AURA), under cooperative agreement
with the National Science Foundation.
\vskip 6mm

\references

Abt, H., Bollinger, G., and Burke, Jr., E., 1983, {\it ApJ}, {\bf 272}, 196

Abt, H., and Morrell, N., 1995, {\it ApJS},  (in press) \BIBCODE{1995ApJS...99..135A}

Balona, L., Krisciunas, K., and Cousins, A., 1994, {\it MNRAS}, {\bf 270}, 905

B\"ohm-Vitense, E., 1970, {\it A\&A}, {\bf 8}, 283

B\"ohm-Vitense, E., and Canterna, R., 1974, {\it ApJ}, {\bf 194}, 629

Cousins, A.W., 1992, {\it Obs.}, {\bf 112}, 53

Eggen, O.J., 1992, {\it AJ}, {\bf 103}, 1382

Eggen, O.J., 1995a, {\it AJ} (in press) \BIBCODE{1995AJ....109.2229E}

Eggen, O.J., 1995b, {\it AJ} (in press) \BIBCODE{1995AJ....110.2862E}

Eggen, O.J., 1995c, {\it AJ} (in press) \BIBCODE{1995AJ....110.1749E}

Eggen, O.J., and Iben, Jr., I., 1988, {\it AJ}, {\bf 96}, 635

Krisciunas, K., et al., 1993, {\it MNRAS}, {\bf 263}, 781

Lester, J., Gray, R., and Kurucz, R., 1986, {\it ApJS}, {\bf 61}, 509

Maeder, A., and Meynet, G., 1991, {\it A\&A}, {\bf 99}, 451

Mantegazza, E., Poretti, E., and Zerbi, F., 1994, {\it MNRAS}, {\bf 270}, 439

Rodriguez, E., and Zerbi, F., 1995, {\it IBVS}, No.\,4170

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