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velocity observations made with the CORAVEL and DAO spectrometers -including 67 new unpublished data -- we have obtained both the orbital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:
------------------------------------------------------------------------------File Name
Lrecl
Records
Explanations
------------------------------------------------------------------------------table1.dat
64
160
Radial velocity observations of DL Cas
------------------------------------------------------------------------------Byte-by-byte Description of file: table1.dat
------------------------------------------------------------------------------Bytes Format Units
Label
Explanations
------------------------------------------------------------------------------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
------------------------------------------------------------------------------Notes for file: table1.dat
------------------------------------------------------------------------------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] 16Aug-1994