LDN - Lynds Catalog of Dark Nebulae
This catalog is an updated version of the original version of the
Lynds' Catalog of Dark Nebulae that was published in 1962. The
catalog was based on a study of the red and blue prints of the
National Geographic - Palomar Observatory Sky Atlas. The catalog
contains positions for the centers of dark nebulae or clouds found
by Lynds, values for the cloud sizes in square degrees, visual
estimates of their opacity, and cross-identifications to Barnard
Objects which are associated with the tabulated clouds.
Lynds, B.T. 1962, `Catalogue of Dark Nebulae', ApJS, 7, 1. [LDN].
Marcout, J., and Ochsenbein, F. 1996, CDS revised version of LDN.
This HEASARC version of the LDN Catalog was created in June 1997.
and was derived from CDS Catalog VII/7A obtained from
ftp://cdsarc.u-strasbg.fr/cats/VII/7A. Additional information
provided in the HEASARC documentation was taken from the original
published version of this catalog.
This catalog contains dark nebulae that were identified by Lynds'
visual inspection of the Palomar-Schmidt photographs. Thus, the range
in declination is from +90 to -33 degrees. A cloud had to be visible
on both the red and the blue photographs in order to be recorded.
It is therefore very probable that the more tenuous clouds which may
be transparent in the red are not included herein. Lynds states that
it was often difficult to detect a cloud that absorbed less than
0.75 magnitudes. Many of the small dark nebulae termed `Bok Globules'
are not included in this catalog because they are apparent as dark
objects projected against the bright background of an emission
nebulosity: only objects which, on the basis of stellar density
fluctuations, indicated the presence of absorption are contained here.
This HEASARC database table is based on the updated version of the
LDN Catalog created at CDS on February 22, 1996, and contains several
changes and/or additions compared to the original published version:
(i) the galactic co-ordinates have been revised, (ii) 15 duplicate
LDN objects were removed (LDN 184, 366, 457, 465, 924, 1025, 1318,
1342, 1344, 1413, 1575, 1592, 1593, 1603, and 1792), (iii) 4 new
objects have been added (these have not been assigned LDN names so
that we have given them our default NN (no name) name for unnamed
objects), apparently by the original author, (iii) two new parameters
have been added: Run_Number, a running number in order of increasing
galactic longitude, and Lynds2_Number, a number assigned by Lynds
when the updated catalog was first created whose significance is
unknown (notice that it is not a unique identifier).
The name of the dark nebula, constructed from the LDN prefix and the
catalog number listed in the original published version. Four nebulae
added after the published version have been assigned the null name NN.
The Right Ascension of the center of the cloud.
The Declination of the center of the cloud.
The galactic longitude of the center of the cloud.
The galactic latitude of the center of the cloud.
The area, in square degrees, of the cloud.
A visual estimate of the cloud opacity on a scale of 1 (lightest) to 6
(darkest). These estimates were made by Lynds based on a comparison of
the neighboring fields for the particular Palomar photograph on which
the cloud appeared. Both the red and the blue prints were used for this
comparison. The clouds just detectable on both prints, as evidenced by
a slight decrease in the surface intensity of the general field, were
designated as having an opacity of 1. The darkest clouds, of opacity 6,
were those within which the star density, on the average, amounted to
120 stars per square degree, down to the limiting magnitude of the red
photograph. In addition to the minimum number of stars per square degree,
the opacity 6 clouds are those which appear to be the darkest - many
seem darker than the general background in the neighboring clear regions.
The areas of the sky which contain heavy obscuration usually exhibit
clouds of several degrees of opacity. In this catalog such clouds are
subdivided into areas of the same opacity: thus a single cloud may
consist of an area of opacity 3 covering several square degrees and
contain within it smaller condenstaions of opacity 6 and areas of
hundredths of a square degree. These sections are listed separately,
but have been assigned a common value of the parameter ID_Number (q.v.).
An identification Number that is used to indicate whether or not the
specific cloud is an isolated object, or is part of a larger cloud with
several sections of different opacity, or is an object lying in the
general obscuration of the Milky Way. An isolated cloud will have a
unique, non-zero value of this parameter, while an object in the general
obscuration of the Milky Way will have ID_Number = 0. A cloud with
several sections of different opacity will have a separate entry
in this catalog for each section: these entries will all have the
same non-zero value for their ID_Number.
A running number in order of increasing Galactic Longitude that was
created for this revised version of the LDN Catalog.
A number that B. Lynds assigned when the updated catalog was created.
The significance of this number is not known. Also, it is not a unique
number throughout this version of the catalog.
The numbers of the Barnard Objects that were cross-identified by Lynds
with the LDN nebulae. Some LDN objects are identified with more than
one Barnard Object. (The maximum is eight cross-identifications).
BROWSE classification type. All objects in the LDN Catalog have been
classified with the same class: Dark Nebula.
Questions regarding the LDN database table can be addressed to the
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