A Catalogue of HII Regions

Stewart Sharpless

U.S. Naval Observatory

Received September 10, 1959

Abstract

A catalogue is given of 313 H II regions north of declination −27°. Known early-type stars associated with them are listed. The apparent distribution of H II regions is discussed, and the galactic pole with respect to ionized hydrogen is determined.

I. Introduction

In an earlier paper (Paper I, Sharpless 1953) a catalogue was given of H II regions between galactic longitudes 315° and 105° and extending several degrees on either side of the galactic circle. That survey was based on a series of 48-inch Schmidt plates. As the National Geographic-Palomar Sky Atlas became available, the catalogue was continued on the basis of the Sky Atlas prints, and it now covers the entire sky north of declination −27°. These results, incorporating those of Paper I, are presented here. This catalogue is designed to serve as a finding list for radio and optical astronomers as well as a source of diameters and brief descriptions of H II regions.

The term "H II region" (Stromgren 1948) is used here instead of the term "emission nebula". An H II region is an entity defined not only in terms of the ionized gas but also in terms of the hot stars which are responsible for the ionization. It was in accordance with this principle that the present catalogue was assembled. This method of cataloguing requires judgment as to whether several isolated patches of nebulosity belong to the same H II region. Knowledge of the positions of hot stars responsible for the ionization is usually sufficient to determine this. The distribution of field stars can often reveal the existence of foreground dark lanes which divide an H II region into several apparently detached portions. Those few cases in which an ambiguity still exists are noted as such in the catalogue. An attempt was made to exclude reflection nebulae on the basis of a comparison of the red and blue photographs.

The following sections contain a description of the catalogue, a comparison with previous catalogues, a discussion of the apparent distribution of H II regions, and a determination of the galactic pole with respect to ionized hydrogen.

II. The catalogue

The catalogue is given in Table 1, the columns of which contain the following information:

Column 1.–A running number. An asterisk here indicates a remark at the end of the table.

Columns 2, 3.–Galactic co-ordinates based on the Lund pole.

Columns 4, 5.–Equatorial co-ordinates for 1900. The position, measured with respect to a nearby BD star, generally refers to the geometric center of the nebula. If the nebula, however, contains large variations in brightness, an attempt was made to give a position more nearly coinciding with the center of light.

Column 6.–Maximum angular diameter. Owing to the diffuse nature of many of these objects, the diameters given here may have an uncertainty of the order of 20 per cent.

Column 7.–Classification as to form (1 = circular, 2 = elliptical, 3 = irregular).

[continued on page 266 ]

Column 8.–Classification as to structure, ranging from 1 = amorphous to 3 = filamentary.

Column 9.–Classification as to brightness on an arbitrary scale, from 1 = faintest to 3 = brightest.

Column 10.–The number of associated stars as identified in Table 2.

Stars which are associated with these regions and are responsible for their ionization are listed in Table 2. Magnitudes and spectral types are taken from the Henry Draper Catalogue or its extensions. Some stars are listed with no spectral type. Most of these were taken from the finding list of OB stars of Nassau and Morgan (1951), while a few of them are central stars in circular H II regions and are probably the exciting stars although no spectra are available. In the case of regions associated with large aggregates of early-type stars, e.g., Orion, only a partial list of associated stars is given.

Table 1
A Catalogue of H II regions

Notes to Table 1

(The notes, given below the table in the article, have here been incorporated into Table 1.)

III. PREVIOUS CATALOGUES

Table 3 contains descriptions of previous Hα surveys. The first column gives the reference to the investigation. The next four columns describe the equipment and the spectral region employed for the comparison with the Hα plates. Succeeding columns give the interval of galactic longitudes covered by the investigation, the number of entries contained there-in, and the symbol used here to designate each catalogue in Table 4. Each of the entries of these catalogues, in the respective region of overlap, has been accounted for in terms of the present catalogue numbers in Table 4.

Most of the previous surveys were compiled on the basis of principles similar to those used here. The close agreement of these catalogues with the one given here attests the validity of the cataloguing procedure, especially in view of the large differences in aperture involved. Many of the H II regions listed in Table 1 were not found in the earlier surveys on account of the smaller apertures employed. In general, those regions missed by the earlier survey are those having diameters less than 15′ and low surface brightness. A few discrepancies between the present catalogue and earlier ones result from the fact that some of the previous investigations listed occasional planetary nebulae, reddened extragalactic nebulae, or clusters, since the true nature of these objects was not recognizable on the small-scale plates on which they were found. All such objects are properly identified in Table 4 as follows: 1 = reflection nebula; 2 = galactic cluster; 3 = planetary nebula; 4 = globular cluster; 5 = extragalactic nebula; 6 = not visible on Palomar Atlas.

Several regions listed in Paper I are not included in Table 1. These were visible on original 48-inch Schmidt plates but were not found on the prints. These were omitted for the sake of homogeneity of the catalogue.

IV. THE APPARENT DISTRIBUTION OF H II REGIONS

The apparent distribution of H II regions is shown in Figures 1 through 5. The Network Nebula, the Crab Nebula, and IC 443 have been included, although these are not typical H II regions. The larger regions are represented by hatched areas, while the smaller regions, regardless of shape, are represented by circles.

The features of the apparent distribution discussed in Paper I are even better defined here. The decrease in average angular diameter from l = 315° to l = 5° is in keeping with an inner spiral arm (Sagittarius arm) going off in perspective in the direction of Scutum (Morgan, Whitford, and Code 1953); and the increase in angular sizes from l = 15° to l = 55° agrees qualitatively with the curvature of our own spiral arm (Westerhout 1957). The relative scarcity of nebulae between l = 92° and 102°, and between l = 56° and 66°, is indicative of a rather spotty distribution of H II regions in the Perseus arm. The second gap is apparent also in the distribution of OB stars (Nassau and Morgan 1951) and red supergiants (Nassau and Blanco 1954).

Table 2. A catalogue of associated early-type stars (not reproduced here)

The H II regions between l = 45° and 180° follow the bifurcation of the Orion arm (Morgan, Whitford, and Code 1953; Helfer and Tatel 1955), the nearer branch extending into negative latitudes.

V. THE GALACTIC POLE WITH RESPECT TO IONIZED HYDROGEN

(not reproduced here)

Table 3
PREVIOUS Hα SURVEYS