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J/MNRAS/300/665 HST VI Photometry of Six LMC Old Globular Clusters (Olsen+ 1998)
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HST colour-magnitude diagrams of six old globular clusters in the LMC
Olsen K.A.G., Hodge P.W., Mateo M., Olszewski E.W., Schommer R.A.,
Suntzeff N.B., Walker A.R.
<Mon. Not. R. Astron. Soc., 300, 665 (1998)>
=1998MNRAS.300..665O
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ADC_Keywords: Clusters, globular; Magellanic Clouds; Photometry
Mission_Name: HST
Description:
The following tables contain the results of photometry performed on
Hubble Space Telescope WFPC2 images of the Large Magellanic Cloud
globular clusters NGC 1754, 1835, 1898, 1916, 2005, and 2019. The
magnitudes reported here were measured from Planetary Camera F555W and
F814W images using DoPHOT (Schechter, Mateo, & Saha 1993) and
afterwards transformed to Johnson V/Kron-Cousins I using equation 9 of
Holtzman et al. (1995PASP..107.1065H). We carried out photometry on
both long (1500 sec combined in F555W, 1800 sec in F814W) and short
(40 sec combined in F555W, 60 sec in F814W) exposures. Where the short
exposure photometry produced smaller errors, we report those
magnitudes in place of those measured from the long exposures. For
each star, we give an integer identifier, its x and y pixel position
as measured in the F555W PC image, its V and I magnitude, the
photometric errors reported by DoPHOT, both the V and I DoPHOT object
types (multiplied by 10 if the reported magnitude was measured in the
short exposure frame), and a flag if the star was removed during our
procedure for statistical field star subtraction.
Summary of data reduction and assessment of photometric accuracy:
Cosmic ray rejection, correction for the y-dependent CTE effect
(Holtzman et al. 1995a), geometric distortion correction, and bad
pixel flagging were applied to the images before performing
photometry. For the photometry, we used version 2.5 of DoPHOT,
modified by Eric Deutsch to handle floating-point images. We found
that there were insufficient numbers of bright, isolated stars in the
PC frames for producing aperture corrections. Aperture corrections as
a function of position in the frame were instead derived using WFPC2
point spread functions kindly provided by Peter Stetson. As these
artificially generated aperture corrections agree well with ones
derived from isolated stars in the WF chips, we trust that they are
reliable to better than 0.05 mag. In agreement with the report of
Whitmore & Heyer (1997), we found an offset in mean magnitudes between
the short- and long-exposure photometry. We corrected for this effect
by adjusting the short-exposure magnitudes to match, on average, those
of the long exposures. Finally, we merged the short- and long-
exposure lists of photometry as described above and transformed the
magnitudes from the WFPC2 system to Johnson V/Kron-Cousins I, applying
the Holtzman et al. (1995PASP..107.1065H) zero points. Statistical
field star subtraction was performed using color-magnitude diagrams of
the field stars produced from the combined WF frames.
Completeness and random and systematic errors in the photometry were
extensively modelled through artificial star tests. Crowding causes
the completeness to be a strong function of position in the frame,
with detection being most difficult near the cluster centers. In
addition, we found that crowding introduces systematic errors in the
photometry, generally <0.05 mag, that depend on the V-I and V of the
star. Fortunately, these errors are well-understood. However,
unknown errors in the zero points may persist at the ~0.05 mag level.
Objects:
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RA (2000.0) DE Name(s)
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04 54 18.70 -70 26 32.1 NGC 1754
05 05 06.44 -69 24 14.5 NGC 1835
05 16 42.41 -69 39 24.6 NGC 1898
05 17 30 -69 23 NGC 1916
05 30 10.36 -69 45 09.0 NGC 2005
05 31 56.73 -70 09 33.3 NGC 2019
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File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
n1754.dat 75 7694 NGC 1754 VI photometry
n1835.dat 75 11411 NGC 1835 VI photometry
n1898.dat 75 8654 NGC 1898 VI photometry
n1916.dat 71 9211 NGC 1916 VI photometry
n2005.dat 75 9519 NGC 2005 VI photometry
n2019.dat 75 9943 NGC 2019 VI photometry
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Byte-by-byte Description of file: n1754.dat n1835.dat n1898.dat
Byte-by-byte Description of file: n2005.dat n2019.dat
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Bytes Format Units Label Explanations
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1- 5 I5 --- Star Star ID
6- 13 F8.2 pix X X pixel position from F555W PC image
14- 21 F8.2 pix Y Y pixel position from F555W PC image
22- 31 F10.4 mag Vmag V magnitude
32- 41 F10.4 mag e_Vmag Error in V magnitude (DoPHOT)
42- 51 F10.4 mag Imag I magnitude
52- 61 F10.4 mag e_Imag Error in I magnitude (DoPHOT)
62- 66 I5 --- Vtyp *DoPHOT object type for V measurement
67- 71 I5 --- Ityp *DoPHOT object type for I measurement
75 A1 --- Sub [y] indicates that star was removed
during statistical subtraction procedure
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Note on Vtyp, Ityp:
Where short exposure photometry was used, object type has been
multiplied by 10. Explanation of DoPHOT object types is given by
Schechter, Mateo, & Saha (1993PASP..105.1342S)
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Byte-by-byte Description of file: n1916.dat
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Bytes Format Units Label Explanations
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1- 5 I5 --- Star Star ID
6- 13 F8.2 pix X X pixel position from F555W PC image
14- 21 F8.2 pix Y Y pixel position from F555W PC image
22- 31 F10.4 mag Vmag V magnitude
32- 41 F10.4 mag e_Vmag Error in V magnitude (DoPHOT)
42- 51 F10.4 mag Imag I magnitude
52- 61 F10.4 mag e_Imag Error in I magnitude (DoPHOT)
62- 66 I5 --- Vtyp *DoPHOT object type for V measurement
67- 71 I5 --- Ityp *DoPHOT object type for I measurement
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Note on Vtyp, Ityp:
Where short exposure photometry was used, object type has been
multiplied by 10. Explanation of DoPHOT object types is given by
Schechter, Mateo, & Saha (1993PASP..105.1342S)
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Acknowledgments:
We thank Eric Deutsch for his modifications to DoPHOT which made it
easier to use and Peter Stetson for providing us with his WFPC2 PSFs.
This work was supported by STScI grant GO05916 to NBS.
References:
Holtzman J., et al., 1995a, Publ. Astron. Soc. Pacific, 107, 156
=1995PASP..107..156H
Holtzman J., et al., 1995b, Publ. Astron. Soc. Pacific, 107, 1065
=1995PASP..107.1065H
Schechter P.L., Mateo M., Saha A., 1993, Publ. Astron. Soc. Pacific,
105, 1342
=1993PASP..105.1342S
Whitmore B., Heyer I., 1997, WFPC2 Instrument Science Report 97-08
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(End) Knut Olsen [NOAO] and James E. Gass [ADC/SSDOO] 30-Dec-1998