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LDN - Lynds Catalog of Dark Nebulae

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Overview

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.

Catalog Bibcode

1962ApJS....7....1L

References

Lynds, B.T. 1962, `Catalogue of Dark Nebulae', ApJS, 7, 1. [LDN]. Marcout, J., and Ochsenbein, F. 1996, CDS revised version of LDN.

Provenance

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.

Description

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).


Parameters

Name
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.

RA
The Right Ascension of the center of the cloud.

Dec
The Declination of the center of the cloud.

LII
The galactic longitude of the center of the cloud.

BII
The galactic latitude of the center of the cloud.

Cloud_Area
The area, in square degrees, of the cloud.

Opacity
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.).

ID_Number
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.

Run_Number
A running number in order of increasing Galactic Longitude that was created for this revised version of the LDN Catalog.

Lynds2_Number
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.

Barnard_Ids
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).

Class
BROWSE classification type. All objects in the LDN Catalog have been classified with the same class: Dark Nebula.


Contact Person

Questions regarding the LDN database table can be addressed to the HEASARC User Hotline.

Page Author: Browse Software Development Team
Last Modified: 9-Nov-2004