The following document lists the file intro.txt
from catalogue I/203.
A plain copy of the file
(without headers/trailers) may be downloaded.
1. Introduction
The open cluster NGC 7092 (M39, C2130+482; l=92.5 deg, b=-2.3 deg)
which belongs to the group of so-called poorly-populated open
clusters has been previously studied by this author (Platais,
1984b). A new set of astrometric and photometric data have proven
the suspected anomaly of the luminosity function - genuine
deficiency of intrinsically faint cluster members. Besides the new
data for the cluster members, more than 7,500 field stars in the
area 2 x 2 deg also have homogeneous astrometric and photometric
data which were used to investigate the interstellar absorption as
a function of distance (Platais, 1988). These data might be useful
in galactic structure studies and also could provide a quick
reference for individual stars and help to make an easy star
selection. It must be noted that the Catalogue is restored from the
only existing computer-typed version deposited at the VINITI
institute in Moscow (Platais, 1983) since the tape version was
lost.
2. Data description
Although a detailed description of the data reduction can be found in
a somewhat obscure publication (Platais, 1984a) a brief summary of
basic reductions, attained accuracies and other essential information
is provided here. The most important difference between the previously
published parts of the catalogue and the present Catalogue is a
slightly changed treatment of the magnitude equation in the proper
motions. These differences are small and do not change conclusions
regarding the NGC 7092 cluster membership.
2.1 Proper motions
The relative proper motions have been determined from measurements
with the Zeiss-ASCORECORD machine. This measuring machine provides
only manual-driven human-eye centering and, taking into account all
precautions, the measuring standard error for well-exposed images is
about 2 microns. Surprisingly, it was possible to measure many very
faint and/or heavily overlapping images which may have failed using
a PDS microdensitometer. In total 7931 stars were measured on 11
Pulkovo normal astrograph plates (scale 59.57"/mm) in a
110'-diameter circle around the cluster. The Catalogue of proper
motions is complete down to B=16.0m, while the limiting magnitude is
B=16.7m. The final proper motion for each star is calculated from a
maximum of four plate pairs.
The mean standard error of the Catalogue proper motions derived
from plate pairs 1 to 4 in the central portions of the field for
stars with B<14.5m is +/-1.5 milliarcseconds per year (mas/yr).
However, for a given star the mean standard error is a function of
individual plate pair mean error, the distance from the field
center and the star's magnitude. The unit weight errors are as
follows: +/-2.8 mas/yr (pairs 1 to 4, at B=14.0m) and +/-8.0 mas/yr
(pairs 5 to 9, at B=15.8m). The plate pairs 1 to 4 all have weights
equal to 1.0. The pairs 1 to 9 have the following weights with the
pair number indicated in parentheses: 1.0 (5), 2.8 (6), 6.2 (7),
1.7 (8) and 3.0 (9). An estimate of the formal mean standard error
of proper motions is given in the Catalogue for each individual
star.
In the previous study (Platais, 1984a) the magnitude equation was
examined for each plate pair separately using the field-star
secular parallaxes. After that a small residual magnitude equation
correction was applied to the Catalogue proper motions using
provisionally identified cluster members. However, it was noticed
that the linear corrections derived from the field stars were
systematically too large at faint magnitudes, (B>14m). Over this
approximately 2-magnitude interval, the data were insufficient to
confidently derive a magnitude term and thus it was decided to
introduce no corrections for stars fainter than B=14.0m. Though,
the final proper motions are corrected using all 81 identified
probable cluster members (Platais, 1984b). After these revisions,
the mean proper motion of NGC 7092 is MUX=-5.2 and MUY=-15.4
mas/yr. In general, the extreme differences between the present
proper-motion Catalogue and its earlier version do not exceed 1
mas/yr and do not affect the conclusions about the NGC 7092
membership.
2.2 UBV photometry
The photographic UBV magnitudes were derived from plates taken with
the Schmidt telescope (80/120/240 cm) at Baldone (Latvia). The
photoelectric sequences in NGC 7092 and NGC 7067 were used for the
plate calibration. For the stars measured on plate pairs 1 to 4 the
magnitudes were derived from four Schmidt plates in each passband.
For the fainter stars (pairs 5 to 9) three plates in each passband
provided the magnitudes. The standard error of the UBV magnitudes
is +/-0.05m for isolated, non-overlapping star images. As pointed
out by Turner et al. (1994) some small systematic differences
(about 0.07m or less) exist between the recent photoelectric and
photographic photometry which definitely can be attributed to the
Schmidt telescope photometric field error. On the Schmidt plates
some stars (Nos. 3368, 3740, 4036, 4292, 4716, 6140, 7013) are not
visible in the U-passband. Only for the sake of convenience a false
U-B colour has been assigned for these seven stars. In addition, a
number of stars are known or suspected variables, namely: Nos.
1083, 1357, 1588, 1821, 2513, 3368, 3456, 4624, 4638, 4892, 5382
and 7186. No attempt has been made to get mean magnitudes or
colours for these stars.
2.3 Spectral classification
Spectral types have been assigned for 511 stars based on three
spectral plates taken with the Abastumani meniscus telescope
(70/98/210 cm) in Georgia and 8-degree objective prism giving
166-angstroms/mm dispersion at H-gamma. The spectral types have
been assigned to virtually all stars brighter than B=12.0m. B- to
K-type stars can be classified as faint as B=13.3m but M-stars are
identified down to B=15.0m. The luminosity classes are reliable for
stars brighter than B=11.4m. A previous publication (Platais, 1988)
contains a more detailed description of the classification
procedure, peculiar stars, interstellar absorption and also has a
finding chart for all spectroscopically classified stars.
2.4 Cluster membership
The area covered by the Catalogue contains two open clusters: NGC
7092 and Anon (C2128+488). In order to recognize the probable
cluster members among the overwhelming majority of field stars all
membership probabilities are given, even those which have not been
included in the earlier paper (Platais, 1984b). Recently, an oddity
has been noticed by the author in the NGC 7092 membership. A small
number of supposed cluster members are grouped together at MUX=-9.0
and MUY=-20.0 mas/yr, slightly offset than the cluster. The offset,
for the same stars, is consistent in several of the proper motion
studies. However, no effort has been made to include an
interpretation of this phenomenon or to re-calculate the membership
probabilities in the current version of the Catalogue. The
membership calculation technique is presented in Platais (1984b).
Since the probabilities were calculated in three different
magnitude intervals there exists a discontinuity between the
probabilities in each of the intervals. Finally, a comment ought to
be made regarding the study by Glushkova (1991) which reports
finding 17 relatively bright, previously unknown cluster members.
None of these stars shows up as a cluster member in the present
Catalogue. Taking into consideration some irregularities and the
lower proper-motion accuracy in the Glushkova study, it is
suspected that these stars are most likely field stars.
The very loose open cluster Anon C2128+488 has been scrutinized
twice, by Platais (1986) and Turner et al. (1994). A rigorous
calculation of the cluster membership is not possible due to the
scarcity of members. All highly probable members of the Anon
C2128+488 selected by Turner et al. (1994) are marked in the
Catalogue with fake probabilities equal to "-99".
2.5 Positions
Almost all X,Y-positions in the Catalogue refer to the deepest
plate B187 taken on August 25th, 1909. However, a small fraction of
stars was not measured on that particular first-epoch plate. In
such cases, the coordinates have been transformed into the system
of plate B187 and, consequently, should be used with some caution.
For the sake of convenience, J2000 equatorial coordinates are
calculated for all Catalogue stars using 70 reference stars from
the PPM catalogue. In contrast to the X,Y-coordinates, the epoch of
the equatorial coordinates is also 2000.0. Only linear plate
constants were used in the transformation to celestial coordinates.
The r.m.s. scatter in the PPM star- position residuals is +/-0".4
in both coordinates.
3. Other proper-motion catalogues
Since NGC 7092 has been studied on several occasions, it is
worthwhile to intercompare the determined proper motions. The
differences in proper motions between a selected study and the
present Catalogue can be expressed by simple equations of condition
which include linear (or linear and quadratic)
X,Y-coordinate-dependent and magnitude terms (for reference, see
Platais (1991)). The right-hand side of the equations is the
difference between the proper motions from the present Catalogue
and the study in question. The derived solutions of the equation
system by least squares for various individual studies relative to
this Catalogue lead to the following conclusions which are
summarized in Table 1. The notation for the coordinate-dependent
errors is as follows: "-" - negligible error; "+" - notable error;
"++" - significant. The value of estimated magnitude equation is
presented in mas/yr per unit B-magnitude.
Table 1. Comparison of the proper-motion catalogues
------------------------------------------------------------------------
Source Coord.-dep. error Magnitude equation
in X in Y X (mas/yr/magn) Y
------------------------------------------------------------------------
Ebbighausen (1940) + ++ - -0.6
van Schewick (1957) + + - -
Lavdovsky (1961) - - 0.5 0.4
Artiukhina, Kalinina (1970) + + -1.9 0.8
McNamara and Sanders (1977) - - - 0.7
Glushkova (1991) - + -0.5 0.5
------------------------------------------------------------------------
As can be seen from Table 1, almost every catalogue is affected by
magnitude equation. The discovery of lower main sequence members in
NGC 7092 is the principal reason these magnitude effects can now be
detected.
All cross-identifications of common stars in different catalogues
were found by coordinate coincidence. Multiple "hits" were resolved
by examining the magnitude differences as well as by checking the
finding chart when available. It must be noted that the finding
chart provided by McNamara and Sanders (1977) has a number of
mis-labeled stars. In Table 2 is given a list of incorrect or
dubious star labelings in the finding chart. Also, in the case of
an unresolved double star in the McNamara and Sanders catalogue, a
colon (:) is added after the star number. A few stars which have
correct labeling but show unusually large proper-motion
discrepancies with respect to the Catalogue are marked with "pm?".
In total, out of 1710 McNamara and Sanders catalogue stars, 22
cannot be identified in the Catalogue. Obviously, some of them are
intrinsically fainter than the limiting magnitude of this
Catalogue, although a few others (e.g. Nos. 47, 402, 798, 919,
1198) may not have the reported large proper motions.
Table 2. A list of McNamara and Sanders (1977) stars
----------------------------------------------------
1 187pm? 258 446 641 1113 1510
2 195: 270 448 652 1209 1607
9 202 277 459 712 1259 1720
19 207 291 464 868: 1298 1733
35 210: 320 527 877 1339
36: 213 413pm? 561 880 1361
109 218 421 570 976 1483
138 226 428 589: 1019 1484
155 235 429 614: 1072 1485
184 246: 435 641 1086: 1491
------------------------------------------------------
Acknowledgments
I am grateful to Drs. Zdenka Kadla and Vladimir Frolov from Pulkovo
Observatory for invaluable help they provided when I was just entering
the thorny realm of open clusters.
References
[1] Artiukhina, N.M., Kalinina, E.P.: 1970, Trudy Sternberg Astron. Inst.,
vol. 40, 3.
[2] Ebbighausen, E.G.: 1940, Astrophys. J., vol. 92, 434.
[3] Glushkova, E.V.: 1991, Soviet Astron., vol 35, 466.
[4] Lavdovsky, V.V.: 1961, Trudy Pulkovo Obs., Ser. 2, vol. 73, 5.
[5] Maevers, F.-W.: 1940, Astron. Nachr., vol. 270, 201.
[6] McNamara, B.J, Sanders, W.L.: 1977, Astron. Astrophys. Suppl.,
vol. 30, 45.
[7] Platais, I.: 1983, "Catalogue of relative proper motions and
photographic UBV magnitudes in the field of NGC 7092 (M39)".
VINITI manuscript depository in Moscow No. 3126-83, 229 pp.
[8] Platais, I.: 1984a, "On the Catalogue of proper motions and UBV
magnitudes in the field of NGC 7092 (M39)". VINITI manuscript
depository in Moscow No. 1318-84, 75 pp. (in Russian)
[9] Platais, I.: 1984b, Soviet Astron. Lett, vol. 10, 84.
[10] Platais, I.: 1986, Nauchnye Inform., vol. 61, 89. (in Russian)
[11] Platais, I.: 1988, Nauchnye Inform., vol. 65, 119. (in Russian)
[12] Platais, I.: 1991, Astron. Astrophys. Suppl., vol. 87, 69.
[13] Turner, D.G., Mandushev, G.I., Forbes, D.: 1994, Astron. J. (in press)
[14] van Schewick, H.: 1957, Veroeff. Univ.-Sternw. Bonn, No. 47, 1.
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