Distance and age of galactic open cluster M44
as determined by 12.401, 1998 Spring

Stephen M. Slivan, Joseph Curran, Alfred Faber, Stefan Hurwitz, Eric Nielsen, Geoffrey Reber, Sean Robinson, James Tanabe, Virginia Toren, and Gabe Weinberg
Department of Earth, Atmospheric, and Planetary Sciences
Massachusetts Institute of Technology, Room 54-410
77 Massachusetts Ave., Cambridge, MA 02139
USA

I. Color-Magnitude diagram of program stars

Error bars represent one-sigma uncertainty with respect to four cluster members listed in the SAO catalog, which were used as local secondary standard stars. These secondary standards themselves, whose V magnitudes fall between 6.5 and 7.0, are therefore plotted without error bars. The overall uncertainty of the transformations to standard V is 0.04 magnitude. The relative uncertainties in R are also 0.04 magnitude, but the overall R calibration could be systematically in error by up to perhaps 0.1 or 0.2 magnitude. Two more stars from Subset I are plotted without error bars (box symbols at V=8.4 and V=10.7) because only a single successful brightness measurement in R was made of each.

• Subsequent comparison with published V magnitudes of stars in M44 by Johnson reveals that the V calibration of our secondary standards is systematically faint by 0.04 mag., which is just equal to our formal uncertainty.

• The accuracy of our R calibration can be gauged only indirectly by transforming our V-R colors to B-V colors and then comparing with Johnson's published B-V results. This transformation was approximated by fitting the B-V vs. V-R magnitudes of all the main-sequence stars listed in the ``UBVRI Standard Stars'' list published in the Astronomical Almanac, Section H, and yielded the following transformation: (B-V) = -0.104(V-R)^2 + 1.355(V-R) - 0.080 with a fit error of 0.04 mag.

  Our R magnitudes seem to be systematically bright by about 0.11 mag., consistent with the expected possible error based on the conditions during that night's observing session.

II. Reduction for distance

• According to the ``Lund Catalogue of Open Cluster Data, Fifth Ed.'' (Lynga, 1987) the color excess of M44 E(B-V)=0.00, i.e., no correction needs to be made for interstellar reddening.

• From our data it appears that the program stars with V > 9 are those still on the zero-age main sequence (ZAMS). The fitted reference ZAMS line plotted in Fig. 2 of ``Investigation of the Praesepe Cluster'' (Mermilliod et al., 1990) was un-shifted back to the reference distance of 10 pc and then used to apply ``spectroscopic parallax'' to our photometry of the ZAMS program stars to determine the following distances:

          no. of 
          ZAMS    derived
  Subset  stars   distance (pc)
  ------  ------  -------------
      I      1    155 +- 40        weighted average all
     II      4    165 +- 20           subsets (8 stars) = 160 +- 16
    III      2    184 +- 80        
     VI      1    115 +- 55        
  

  Our results are in excellent agreement with the accepted value of 158 pc for the distance to M44 (and there was much rejoicing!)

III. Reduction for age

The reference isochrones plotted in Fig. 2 of ``New Dating of Galactic Open Clusters'' (Meynet et al., 1993) were overlaid on our data to crudely estimate ``by eye'' an approximate age range for the cluster of 500 million to 800 million yr. The approximate isochrone for an age of about 630 million years is plotted as a meandering solid line on the accompanying HR diagram. The accepted age for M44 is about 830 million yr (Lynga, 1987).

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Last modified 1998 Aug 10