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\IBVShead{4185}{19 April 1995}

\title{NARROWING THE MAIN SEQUENCE MASS GAP}

\begintext

   Andersen's (1991) compilation of stellar masses contains 3 stars 
with masses between 1.10 solar masses and the long-known masses of the 
M1 dwarf, YY Gem, 0.59 m$_{\odot}$, namely the secondary of FL Lyr (G8, 0.96 m$_{\odot}$) 
and the components of HS Aur (G8, 0.90; K0, 0.88).  The discussion of 
visual binaries by Henry and McCarthy (1993) contains only one star 
with a mass in this range with comparable accuracy, Alpha Cen B (K0V, 
0.90).

   In recent years a program has been undertaken at the Lick Observatory 
with the primary aim of helping to obtain masses of additional 
main-sequence stars in the mass range 1.0 to 0.6 m$_{\odot}$.  The observations 
consist of spectra of eclipsing binaries which, on the basis of 
existing information, may contain at least one detached main-sequence 
component of spectral type G or K.  To date, spectra have been obtained 
of approximately 60 eclipsing binaries, and the most promising ones are 
given high priority on the observing program.  A progress report 
giving preliminary results as of about 2 1/2 years ago has been 
published (Popper 1993), as have definitive analyses of two of the 
systems, RT And and CG Cyg (Popper 1994).  These two papers discuss the 
nature of the program in some detail.

   An up-dated progress report is given in Table 1.  It contains results 
based on limited material for those systems for which a provisional 
value of m$\times$sin$^3$i of at least one component is equal to or less than  
1.10 solar masses. Alpha CrB and V818 Tau (see Popper 1993) are binaries 
containing a star having less than one solar mass not observed in this  
program.  The table includes five systems (ZZ UMa, HP Aur, 1E1919+0427,    
HD\,197010, and BH Vir) for which this is the first announcement of new 
mass determinations.  The very provisional nature of the results is to 
be emphasized.  The systems are listed in order of decreasing period.
Several of the provisional results in Table 1 are for systems (UV Psc,
BH Vir, CG Cyg, RT And, UV Leo) that have been previously observed 
with much lower resolution than that employed in this program.    
References are in Batten et al. (1989).

   The spectral types listed in Table 1 are rough values based on the 
strengths of the Na\,D lines, which are sensitive to temperature.  The 
types are mean values weighted according to the luminosities of the 
components and to the intrinsic strengths of the D lines.  The apparent 
magnitudes, presumably in the V band, are from a variety of sources.  
References to binaries not having constellation names are as follows: 
HD\,192825, Kissling et al. 1993; HD\,197010, Marschall et al. 1991; 
1E1919+0427, Summers et al. 1992.  The ephemerides of these recently 
discovered eclipsing binaries are not firmly established.  I have found 
it necessary to subtract 0.05 from phases of HD\,197010 computed with the 
ephemeris given in IBVS 3633.  The ephemeris given in IBVS 3708 for 
1E1919+0427 does not fit my velocities.  An improved ephemeris is
JD(min)=2449260.644 + 0.873713E.  
\vfill \eject

\centerline{2}
\vskip 1mm
\centerline{Table 1. Detached eclipsing binaries with provisional values of m$\times$sin$^{-3}$i}
\centerline{1.10 solar masses or less.}
\vskip 1mm
\begin{center}
\begin{tabular}{lcrlll}
\hline
      Binary    &  Period (d)& V (mag)&Type$^{\star}$& m$_1$$\times$sin$^3$i& m$_2$$\times$sin$^3$i\\
\hline
      ZZ UMa      &  2.30   &    10.0 &  G2  &    1.18  &     0.96\\
      HP Aur      &  1.42   &    11.3 &  G4  &    0.90  &    0.75\\
      DU Leo      &  1.37   &     9.5 &  F8  &    0.97  &     0.95\\
      HD\,192825    &  1.18   &     8.9 &  G0  &    1.08: &     1.06:\\
      1E1919+0427 &  0.87   &    10   &  K1  &    0.94  &     0.85\\
      UV Psc      &  0.86   &     9.4 &  G8-K0&   0.99  &     0.76\\
      CV Boo      &  0.85   &    10.2 &  G2   &   1.00  &     0.94\\
      BH Vir      &  0.82   &     9.6 &  F8  &    1.13  &     1.01\\
      HD\,197010    &  0.71   &     8.9 &  F9  &    1.08  &     0.69\\
      CG Cyg      &  0.63    &    9.7 &  K1  &    0.92  &     0.79\\
      RT And      &  0.63   &     9.0 &  F9  &   1.24   &    0.91\\
      UV Leo      &  0.60   &     8.9 &  G0  &    1.10  &     1.07 \\
\hline
\end{tabular}
\end{center}

\begin{center}
$^{\star}$ Types are means weighted according to the relative luminosities of  
the components and to the intrinsic strengths of their Na\,D lines. 
\end{center}
\vskip 5mm

   The spectroscopic material on which the preliminary results 
summarized in Table 1 have been based is obtained with the Hamilton 
echelle-CCD spectrometer on the 3 m Shane telescope of the Lick 
Observatory (Vogt 1987).  This instrument is proving to be ideal for the 
purpose: resolution approximately 60,000, scale 3 \AA mm$^{-1}$  in the visual 
region, S/N in the continuum of roughly 100 per resolution element for an 
exposure of about 20 minutes on a star of 10th magnitude.  Details of 
the cross-correlation procedure employed for the velocities (IRAF 
program) are given in Popper (1994) and Popper and Jeong (1994).

   The principal purpose of presenting these preliminary results at  
this time is to direct the attention of photometric observers to these 
important systems.  Without reliable photometric observations and 
analysis, the spectroscopic results will have limited value.  The short 
periods of most of these detached systems are noteworthy--the shorter 
the period, the greater the likelihood that eclipses will occur.  This 
effect also permits coverage of the light curve in a relatively short 
interval of time, a favorable situation for observers.  On the other 
hand, short periods are associated with larger rotational velocities, 
which, in turn, favor intrinsic variations associated with star spots and 
perhaps other manifestation of stellar activity.  For this reason, it is 
essential that a complete light curve be observed in a relatively short 
interval and that the coverage be repeated several times over a period 
of a few years.  RT And (Zeilik et al. 1989) and CG Cyg (Zeilik et al. 
1994) are examples of systems for which repeated photometric coverage by 
a variety of observers has led to consistent basic solutions of the 
light curve (as well as to evaluations of star-spot parameters).  While 
such numerous observations as in the cases of RT And and CG Cyg are ideal, 
considerably less frequent coverage of the light curves of the systems 
of Table 1 should lead to results of adequate reliability.  


   Photometry should be carried out in a photometric system carefully 
tied to a standard system.  The Str\"omgren intermediate-band v,b,y system 
has the advantage of giving information on metallicity, which is of 
considerable importance in interpreting the stellar properties in terms 
of evolutionary models, as discussed by Andersen (1991), for example.  \vfill\eject

\centerline{3}
\vskip 1mm
It is not possible to evaluate metallicity directly from spectra of 
rapidly rotating stars.  None of the binaries is particularly faint, and 
a number of them can be observed with adequate precision (not worse than 
0.01 mag) with modest equipment.

   Pending more nearly definitive spectroscopic orbits and photometric solutions for most of the systems of Table 1, it appears that we may 
indeed be able to narrow the gap in the masses of detached main-sequence 
stars to the as yet unfilled interval 0.7 to 0.59 solar masses.  One     
may hope that further discoveries--some of those represented in Table
1 are quite recent--will add significantly to our store of stellar  
masses in this mass range.  Concentrated photometry on any or all   
of these binaries would provide a major contribution to this endeavor.
\vskip 1cm
\beginauthors
\authorsrightcol
 Daniel M. POPPER
 Department of Astronomy
 University of California
 Los Angeles, CA 90095-1562 U.S.A. 
 internet:
 popper@bonnie.astro.ucla.edu                         %email

\endauthors


\references

Andersen, J., 1991, {\it Astron. Astrophys. Rev.}, {\bf 3}, 91

Batten, A. H., Fletcher, J. M., and MacCarthy, D. G., 1989, {\it Publ. Dom.
   Astrophys. Obs.}, {\bf 17}, 1

Henry, T. J., and McCarthy, D. W., 1993, {\it Astron. J.}, {\bf 106}, 773

Kissling, W. M., Blow, G. L., and Gilmore, A. C., 1993, {\it IBVS}, No.\,3872

Marschall, L. A., Gauthier, C. P., Nations, H. L., Taylor, B. W., and
   Huard, T. L., 1991, {\it IBVS}, No.\,3633

Popper, D. M., 1993, {\it Astrophys. J. Lett.}, {\bf 404}, L67

Popper, D. M., 1994, {\it Astron. J.}, {\bf 108}, 1091

Popper, D. M., and Jeong, Y.-C., 1994, {\it Publ. Astron. Soc. Pacific}, {\bf 106}, 109

Summers, D. L., Zeilik, M., Jaderlund, E., Hoeppe, G., and Collins, A.,
   1992, {\it IBVS}, No.\,3708

Vogt, S. S., 1987, {\it Publ. Astron. Soc. Pacific}, {\bf 99}, 1214

Zeilik, M., Cox, D. A., DeBlasi, C., Rhodes, M., and Budding, E.,
   1989, {\it Astrophys. J.}, {\bf 345}, 991

Zeilik, M., Gordon, S., Jaderlund, E., Ladlow, M., Summers, D. L.,
   Heckert, P. A., Budding, E., and Banks, T. S., 1994, {\it Astrophys. J.},
   {\bf 421}, 303



 
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