CDROM/AJ/V107/P2093     DL Cas (Gieren+ 1994)
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Independent Distance Determinations to Milky Way Cepheids in Open Clusters and 
Associations. I. The Binary Cepheid DL Cas in NGC 129
      Wolfgang P. Gieren, Douglas L. Welch, Jean-Claude Mermilliod, Jaymie
      M. Matthews, & Gisela Hertling
      <1994, AJ, 107, 2093>
      =1994AJ....107.2093G
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Abstract:
  As an eight-day Cepheid which is both a component of a spectroscopic
  binary and a member of the open cluster NGC 129, DL Cas is potentially
  a very accurate calibrator of the period-luminosity (PL) relation and
  Cepheid mass.  From 160 high-precision (sigma < 1.5 km/s) radial
  velocity observations made with the CORAVEL and DAO spectrometers --
  including 67 new unpublished data -- we have obtained both the orbital
  and pulsational velocity curves of this binary Cepheid.  This body of
  RV data makes DL Cas one of the best observed Cepheids in our galaxy.
  Our analysis yields an orbital period of 684.4 +/- 0.4 days which
  confirms DL Cas as one of the shortest-period binaries containing a
  Cepheid.  We derive new precise orbital elements which replace earlier
  preliminary values found by Harris et al.  Isochrone fitting to the
  V,B-V data points of Turner et al. yields an age of NGC 129 of (7.6 +/-
  0.4) x 10^7 yr and a Cepheid mass of 5.6 Msun.  Evidence from age, a
  possible period change, and strip crossing times suggest that DL Cas is
  a solar-abundance star making its third (redward) crossing through the
  Cepheid instability strip.  Existing observational constraints from our
  mass function of the DL Cas system and an IUE spectrum suggest that the
  companion is a main sequence star in the mass range from 2.6 to 5.6
  Msun.  We use the pulsational velocity curve and published photometry
  to derive the distance and mean radius of DL Cas with the surface
  brightness method, finding values of 2034 +/- 110 pc and 66.0 +/- 3.5
  Rsun, respectively.  The radius we derive indicates that DL Cas is a
  fundamental-mode pulsator, removing any possible ambiguity in mode
  identification.  The distance corresponds to a mean absolute visual
  magnitude of <Mv> = -4.2 +/- 0.3 mag whose error is dominated by the
  uncertainty of the absorption correction.  Since our very precise
  distance contributes only 0.12 mag to the error in <Mv>, improved
  reddening studies of NGC 129 would make DL Cas a very tight calibrator
  of the PL relation.  Our value of the gamma velocity of the DL Cas
  system is identical to the mean radial velocity of the stars in NGC
  129, strengthening the case for cluster membership.  However, our
  distance for DL Cas, and thus for NGC 129, is significantly larger than
  the 1670 +/- 13 pc obtained by Turner et al. from ZAMS fitting of the
  cluster.  Possible causes for this discrepancy, and their implications
  for Cepheid distance scale calibrations, are discussed.


File Summary:
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File Name    Lrecl    Records    Explanations
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table1.dat      64        160    Radial velocity observations of DL Cas
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Byte-by-byte Description of file: table1.dat
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   Bytes Format  Units      Label    Explanations
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   1- 9   F9.3    day        HJD     Heliocentric Julian Date - 2400000
  10-17   F8.2    km/s       RV     *Radial velocity
  18-21   4X      ---        ---     Blank
  22-24   A3      ---        Instr   Instrument, 'DAO' or 'COR'
  25-33   F9.3    ---        OPh     Orbital phase
  34-41   F8.2    km/s       OVel    Orbital velocity
  42-48   F7.2    km/s       O-C     Orbital O-C
  49-56   F8.3    ---        PPh     Pulsational phase
  57-64   F8.2    km/s       PVel    Pulsational velocity
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Notes for file: table1.dat
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RV:
  The standard deviations of the individual radial velocity observations are 
  between 0.4 and 1.5 km/s, and 0.8 km/s on average for both the DAO and 
  CORAVEL instruments.
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(End)                                             Lee Brotzman [ADS] 16-Aug-1994