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[vsnet-chart 230] Photometry of stars near galaxies



     Since there has been a perennial problem obtaining photometric sequences
near galaxies hosting supernovae (once and future), I have started determining
coordinates and IDs for stars measured by those who have done multi-aperture
photometry of galaxies.  In most such publications, stars superposed on
galaxies are either ignored (leading to bogus magnitudes for the galaxies
themselves) or are taken out via separate measurement (either using a single-
channel photometer or CCD) but not reported in publication.
     The McDonald observers (now all formerly of Univ. of Texas - Austin) have
been very thorough not only in measuring superposed stars, but also reporting
them in publication.  Much of their work is covered in the three Univ. of Texas
monographs collecting galaxy photometry from the literature.  Those
publications have a large number of galaxy field stars measured, and I've just
begun to get those identified.
     As a start, below I show data for a few dozen galaxies in five galaxy
clusters observed by Ron Buta and published in 1996.  He used mostly the
McDonald 75cm and 90cm telescopes.  Details of the observations can be found
in the source paper.  The basic procedure was to measure the galaxy in
successively larger photometer apertures (this is all single-channel stuff)
in order to obtain a total magnitude and colors for the galaxy.  Superposed
stars were measured separately using a tiny aperture along with 'sky'
measurements of the immediate galaxy background along the same isophote as the
star.  The results vary in accuracy of course depending on brightness and
location within the galaxy.  Stars fainter than about mag. 15 will have
external uncertainties in the +/- 0.1 mag. range, but brighter stars should be
fairly reliable (say 0.03-0.05 mag. errors).  It is worth noting that all but
one or two of the stars were measured on only a single night.
     The table below is arranged similarly to my large photometric reference
file.  It shows the host galaxy name in the first column.  Next comes the
equinox 2000 position for the star.  These are mostly taken from either
GSC-ACT ("GSC v1.3") or USNO-A2.0; one was estimated using a SkyView DSS image.
The source is indicated in column 's':  A = USNO-A2.0, G = GSC-ACT, S = DSS.
The GSC number is shown if available.  I have copied the V and B-V data from
the source paper.  I omit the U-B colors since these are for the most part not
very reliable simply because the stars are so faint and the galaxy background
problematic.  The 'Location' column gives a crytic description of where the
star is relative to the galaxy nucleus.  Some examples:

sup SW side     superposed on the southwest side of galaxy
NW tip          near the northwest tip of a spindle-shaped galaxy
gx sup SW side  measurement is of a galaxy (not a star) superposed on the
                  target object
off SE side     outside the main body of the galaxy to the southeast

In any case the accurate coordinates allow unambiguous identification using
digital images.
     Some isolated stars are marked with 'i' directly before the coordinates.
These are far enough from the galaxy that they can be used for generic
photometric calibration of photographic or CCD images.  I have added these
few stars to my generic reference file since they form a useful supplement to
existing sequences (http://ftp.lowell.edu/pub/bas/starcats/loneos.phot.gz).
This file contains more complete sequences in the fields of over 100 other
galaxies.
     The identifications can be made in the absence of charts based on which
aperture the star was included in for the galaxy photometry.  This allows one
to determine the approximate radius of the star from the galaxy nucleus and
the identification then made from a DSS image.  There were several cases,
however, where this method was either uncertain or fails completely usually
because there are several candidate stars.  I have asked Dr. Buta if he still
had working sketches, but since the observing was done more than 10 years ago
they were not easy to retrieve.  These uncertain/impossible stars are given in
two lists at the bottom.  Most of the "uncertain" stars are probably correct,
since one can make a fair guess of the identification using relative magnitudes
and colors from the GSC and USNO-A.  Locations and coords are noted for each
candidate so they can be checked with new observations.  (N.B. two candidates
near NGC 7750 shown for the same star.)  Finally, the two unidentifiable fields
are roughly described.
     Besides direct calibration of CCD images from the isolated field stars
mentioned previously, the stars can be used to make local adjustments to the
GSC or USNO series magnitudes to produce temporary rough sequences for visual
observers.

All for now; more to follow.

\Brian

============================

source:  1996AJ....111..591B
         BUTA R.
         Astron. J., 111, 591-602 (1996)
         Photoelectric UBV photometry of galaxies in the clusters Pegasus I,
           Pegasus II, Abell 262, Abell 1367, and Abell 2197-9.


Host galaxy      RA  (2000)  Dec    s     GSC       V    B-V   Location
NGC 687        1 50 30.9  +36 23 06 A             15.68  0.61  first star NW
NGC 703        1 52 38.6  +36 09 53 A             15.81  0.49  sup SW side
NGC 708        1 52 46.1  +36 09 20 G  2319-0855  14.76  0.87  sup N side
NGC 714        1 53 26.3  +36 13 38 G  2319-0461  15.03  0.91  NW tip
NGC 714        1 53 26.9  +36 13 26 A             17.83 -0.10  due W of nuc
NGC 717      i 1 53 57.3  +36 13 57 A             15.91  0.80  NE side
NGC 735        1 56 36.7  +34 10 48 G  2315-0156  14.13  0.79  sup NW side
NGC 735      i 1 56 37.9  +34 09 45 A             16.06  0.54  due S
NGC 735        1 56 39.6  +34 09 45 A             15.96  0.84  S of SE tip
NGC 797        2 03 28.2  +38 07 21 A             16.88  0.69  sup NNE
NGC 797        2 03 24.6  +38 06 44 G  2829-1403  15.76  0.82  gx sup SW side
NGC 797        2 03 24.2  +38 07 20 G  2829-1696  13.85  1.11  * NW
NGC 818        2 08 47.5  +38 46 49 A             16.06  0.84  ENE edge
NGC 834      i 2 11 05.5  +37 40 09 G  2830-0978  13.40  1.42  off E side
NGC 845        2 12 21.0  +37 27 43 A             15.49  0.66  S of SE tip
NGC 3701     i11 29 29.9  +24 06 27 G  1982-0818  14.90  0.86  off N side
NGC 3802     i11 40 20.9  +17 45 06 G  1438-0463  15.43  0.53  off SE
NGC 3802     i11 40 23.2  +17 45 54 G  1438-0450  13.86  0.80  off E tip
NGC 3842      11 44 00.3  +19 56 29 A             17.77  0.56  sup SW side
NGC 3842      11 44 04.7  +19 56 37 A             15.17  1.03  sup SE side
NGC 3947      11 53 23.8  +20 45 08 G  1443-2559  15.05  1.14  E edge
NGC 3987      11 57 24.3  +25 11 46 A             16.29  0.80  on SE flank
NGC 3987     i11 57 26.0  +25 11 18 A             16.48  1.07  off SE side
NGC 7495     i23 08 59.0  +12 04 07 G  1164-0931  12.88  0.65  br* NE
NGC 7535      23 14 11.7  +13 34 47 G  1167-0500  15.10  0.74  n=2;sup SW side
NGC 7535      23 14 16.3  +13 34 32 A             16.22  0.69  off ESE
NGC 7536      23 14 11.5  +13 24 55 A             16.56  0.62  SW edge
NGC 7563      23 15 53.3  +13 12 00 G  1167-1567  15.42  1.33  NW edge
NGC 7570      23 16 43.5  +13 28 41 A             16.46  0.75  sup SW side
NGC 7591      23 18 17.9  +06 34 28 A             16.14  1.63  sup SE side
NGC 7593      23 17 58.8  +11 21 11 A             15.51  0.36  NE edge
NGC 7626      23 20 39.0  +08 13 07 G  1162-0514  14.51  0.76  W edge
NGC 7634      23 21 42.2  +08 52 53 G  1162-1248  14.12  0.53  sup SSE side
NGC 7648      23 23 56.2  +09 40 01 A  1162-0197  15.31  0.71  E edge
NGC 7648      23 23 51.2  +09 40 00 A             16.01  0.81  W edge
IC 5309       23 19 11.6  +08 06 12 G  1162-1766  14.37  0.75  sup S side
IC 5309      i23 19 15.0  +08 06 22 A             15.54  0.88  off E side
UGC 1344       1 52 34.7  +36 29 38 G  2319-0343  10.78  0.62  sup S side
UGC 1347       1 52 45.5  +36 37 02 S             15.11  0.52  sup SW of nuc
UGC 6670     i11 42 27.4  +18 19 33 A             16.60  0.37  off SW flank
UGC 10436     16 31 01.2  +41 09 54 A             17.04  1.77  NW edge
UGC 10488    i16 38 06.3  +39 54 35 A             17.10  1.36  off SW side
UGC 12361     23 06 24.8  +11 17 16 A             17.49  0.73  E tip
UGC 12370    i23 07 06.6  +09 58 24 G  1160-0162  10.68  0.49  N side
UGC 12423     23 13 13.9  +06 25 00 G  0583-0651  14.98  0.96  SSE of center
UGC 12497    i23 19 14.1  +07 42 50 A             17.12  0.73  off NE
UGC 12544     23 21 47.9  +09 04 01 G  1162-0570  14.94  0.64  just off SE edge
UGC 12571    i23 23 21.9  +13 19 49 A             15.70  0.78  off N side
MCG +7-34-35 i16 26 49.6  +41 15 54 G  3066-0702  12.96  0.80  off NW
CGCG 406-82   23 21 17.9  +07 28 22 A             15.30  0.71  close dbl



uncertain identifications

NGC 753      ? 1 57 42.1  +35 55 42 A             16.07  0.88  due N
NGC 753      ? 1 57 38.1  +35 54 47 A             16.09  1.18  due W
NGC 7495     ?23 08 57.8  +12 03 33 A             15.70  0.95  closer on N
NGC 7495     ?23 08 57.4  +12 03 50 A             16.01  0.95  farther N
NGC 7643    i?23 22 54.0  +11 58 14 A             16.95  0.63  off SE
NGC 7750     ?23 46 36.1  +03 48 14 A             16.56  0.79  NW edge
NGC 7750     ?23 46 38.0  +03 49 11 A             16.56  0.79  off N end
IC 1474      ?23 12 50.6  +05 48 54 A             17.34  0.73  gx N end
UGC 12304   i?23 01 14.7  +05 39 00 A             17.45  0.74  off E



unidentifiable fields

NGC 801   five similarly-bright stars
UGC 1350  three stars

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