
"OB ASSOCIATIONS DATABASE"

M. Morel

A database of known members of 71 Galactic OB associations,
listing about 1000 stars. Contents include catalogue numbers,
spectral types, coordinates for equinox J2000, and BV data.
Based on a compilation published in 1978 by Dr. Roberta M.
Humphreys, but carefully reviewed, with reformatting, and
correction of errors.

1. INTRODUCTION.
	Many observers of the night sky have a keen interest in star clusters of the Milky Way, galactic and globular. These clusters are generally obvious targets because of their concentrated nature. However, there is another type of cluster which is hardly known to the casual observer in spite of the fact that individual member stars are sometimes brilliant naked eye stars. I am referring to the groupings known as OB associations, defined as follows,
	"An OB association is a sparsely populated grouping of very young, massive stars lying along a spiral arm of the Milky Way, whose spectral types or motions in the sky indicate a common origin. The star density is insufficient for gravitation to hold the group together against shear by differential galactic rotation but the stars have not yet had time to disperse completely. OB associations are composed of stars of spectral types O-B2."  (Hopkins 1976).
	From our viewing position within the Milky Way it is usually difficult to appreciate the fact that many naked eye stars, which may be widely separated, do in fact share membership of one, or another, OB association. The following database catalogues all of the recognized galactic OB associations, and identifies the individual members. For example look up the OB association Sco OB2 on p.26. You will find that the bright stars Alpha Sco (Antares), Delta, Beta1 and Tau Sco are all members of this association. Note that occasionally M supergiants, such as Antares, are proven members and are listed with the OB stars. Some associations are much richer, but also fainter due to great distance and/or interstellar absorption, so a good star atlas will be required to locate the fainter members.
	OB associations are usually named after the constellation in which they are found (eg Sgr OB1, Cyg OB4), but sometimes the name of an involved star cluster suffices.

2. CATALOGUE OF OB ASSOCIATIONS.
	In 1978 Dr Roberta M. Humphreys published a paper entitled "Studies of Luminous Stars in Nearby Galaxies. I. Supergiants and O Stars in the Milky Way". Among the various tables in her paper, Table 8 (Catalog of Supergiants and O stars in Associations and Clusters) is a very extensive reference work. At least, it would have been much easier to use, had equatorial coordinates been provided. Only approximate galactic coordinates were given. A set of accurate equatorial coordinates would greatly facilitate use of her table.
	Perusal of Humphreys' Table 8 will reveal some OB associations with only one entry. Undoubtedly these particular associations have many more members, but in 1978 only one star had a reliable distance modulus, a necessary parameter for inclusion in her catalogue. This no doubt reflected the state of knowledge at that time. Humphreys' paper is a mine of interesting information in other ways e.g. Table 2, which lists the visually brightest stars of our galaxy, in each of the main spectral classes.
 
3. COMPLILING MY DATABASE.
	Humphreys Table 8 became my starting point. No stars have been added, but a number of errors in her table have been found, and corrected, and I now provide equinox J2000 coordinates for all stars. To verify the Humphreys data, each individual entry was compared with corresponding entries in other catalogues, especially the Hipparcos Input Catalogue (HIC). This procedure revealed occasional errors in the HD numbers in her table. Some stars had no id. number at all, which required a bit of detective work to determine their identities. The early catalogues of MK Spectral Classifications (W. Buscombe, 1974-80) proved to be very useful, as virtually all of Humphreys' stars are in these books along with approximate coordinates for J2000.

4. EQUATORIAL COORDINATES.
	In my database, the coordinates for brighter stars (down to mag. 7.1) are taken from the Bright Star Catalogue, or Supplement. Stars from 7.1 to ~9.5 have coordinates taken from the HIC. Other stars were found in SAO, GSC, or BD catalogues, precessed to 2000. The aim is to provide coordinates which are adequate for identification purposes. Obviously coordinates from the BD will be less accurate, and users of the database should expect some slight discrepancies among the fainter stars, when their positions are compared to more modern sources.

5. FORMAT OF THE DATABASE.
	I have decided to regularize the format of the database in a number of ways, to conform to common practice in astronomical catalogues.
(a). Catalogue Numbers.
	- Henry Draper (HD) is used, when available,
	- Otherwise, a Durchmusterung number is used, as follows,
		BD numbers for stars from +89 to -22 decl. zones;
		CD numbers for stars from -23 to -51 decl. zones;
		CPD numbers for stars from -52 to -89 decl. zones.
	- Northern Luminous Star (LS) catalogue numbers are used 
	for some northern stars not listed in HD or BD catalogues.
	- Miscellaneous designations, when none of the above will 
	suffice. eg. H.241 (Hiltner OB star); Case (red stars).
(b). Sorting Records by Right Ascension.
	All stars within each OB association are arranged according
	to increasing Right Ascension. The only exception is for 
	Cyg OB2, the order being unchanged from Humphreys' table.
	The reason is that there is still a little uncertainty 
	about the correct positions of one or two stars.
(c). Variable Star Names.
	In the REMARKS column will be found the current variable 
	star name for each star, taken from the NSV or GCVS IV
	catalogues.

6. GENERAL.
	Due to shortage of space, many of the remarks in Humphreys' original table have been omitted, or abbreviated. I have retained the remarks alluding to non-members or doubtful members of associations. It has not been possible to include data such as Distance Moduli, or Absolute Magnitudes. The reader should refer to Humphreys' paper for this data.
       Hogg 22 = "NGC 6204". There is a curious tale concerning this grouping. Humphreys calls this association NGC 6204, but in fact NGC 6204 is a relatively faint and distant cluster without OB stars. The OB association is in fact     
connected with a nearby bright cluster, Hogg 22, recognized as a separate entity in 1965. Humphreys' classifications are based on earlier work, pre-1965.

7. CONCLUDING REMARKS.
	I call this compilation a database, but on reflection perhaps I should have called it a "finding list" or similar. There exists much additional astrophysical data that a full-blown database would have included. This is beyond the scope of my compilation, which is intended to provide the observer with the most basic data on stars in OB associations.
	No doubt much more information on OB associations has been published since 1978, and many more stars may now be assigned membership. I make no apology for the incompleteness of my data-base. Should anyone have more up-to-data information, I would be pleased to receive it.
	I would be pleased to hear from any reader who detects a mistake.

8. REFERENCES.
Buscombe, W. & Kennedy, P.M. 1974. MK Spectral Classifications,
2nd General Catalogue, Evanston.
Buscombe, W. 1977. MK Spectral Classifications, 3rd General Catalogue, Evanston.
Buscombe, W. 1980. MK Spectral Classifications, 4th General Catalogue, Evanston.
Hipparcos Input Catalogue. 1992. European Space Agency.
Hoffleit, D. 1982. The Bright Star Catalogue, 4th ed. Yale Univ. Observatory.
Hoffleit, D., Saladyga, M. and Wlasuk, P. 1983. A Supplement to the Bright Star Catalogue. Yale. Univ. Observatory, New Haven.   
Hopkins, Jean. 1976. Glossary of Astronomy and Astrophysics, University of Chicago Press.
Humphreys, R. M. 1978. Astrophys. J. Suppl.38: 309-350.
Whiteoak, J. B. 1963. Mon. Not. R. Ast. Soc. 125,105.
NASA-ADC. 1992. Selected Astronomical Catalogues, Vol. 1.                  



Issued by MOREL ASTROGRAPHICS
c/o M. Morel,
6 Blakewell Road, Thornton NSW 2322 AUSTRALIA
Tel./fax: 61 49 662078.      E-mail: mmorel7@bigpond.com

(C) 1997, 2012  M. Morel.







 


























































                        ways e.g. Table 2, which lists the visually brightest stars of our galaxy, in each of the main spectral classes. 
Compiling my Database:
	Humphrey's Table 8 became my starting point. No stars have been added, but a number of errors in her table have been found, and corrected, and I now provide equinox 2000 coordinates for all stars. To verify the Humphreys' data, each individual entry was compared with corresponding entries in other catalogues, especially the Hipparcos Input Catalogue (HIC). This procedure revealed occasional errors in the HD numbers in her table. Some stars had no id. number at all, which required a bit of detective work to determine their identities. The early catalogues of MK Spectral Classifications (W. Buscombe, 1974-80) proved to be very useful, as virtually all of Humphreys' stars are in these books along with approximate coordinates for 2000.
Equatorial Coordinates:
	In my database, the coordinates for brighter stars (down to mag. 7.1) are taken from the Bright Star Catalogue, or Supplement. Stars from 7.1 to ~9.5 have coordinates taken from the HIC. Other stars were found in SAO, GSC, or BD catalogues, precessed to 2000. The aim is to provide coordinates which are adequate for identification purposes. Obviously coordinates from the BD will be less accurate, and users of the database should expect some slight discrepancies among the fainter stars, when their positions are compared to more modern sources.
The Format of the Database:
	I have decided to regularize the format of the database in a number of ways, to conform to common practice in astronomical catalogues.
(a). Catalogue Numbers.
	- Henry Draper (HD) is used, when available,
	- Otherwise, a Durchmusterung number is used, as follows,
		BD numbers for stars from +89 to -22 decl. zones;
		CD numbers for stars from -23 to -51 decl. zones;
		CPD numbers for stars from -52 to -89 decl. zones.
	- Northern Luminous Star (LS) catalogue numbers are used 
	for some northern stars not listed in HD or BD catalogues.
	- Miscellaneous designations, when none of the above will 
	suffice. eg. H.241 (Hiltner OB star); Case (red stars).
(b). Sorting Records by Right Ascension.
	All stars within each OB association are arranged according
	to increasing Right Ascension. The only exception is for 
	Cyg OB2, the order being unchanged from Humphreys' table.
	The reason is that there is still a little uncertainty 
	about the correct positions of one or two stars.
(c). Variable Star Names.
	In the REMARKS column will be found the current variable 
	star name for each star, taken from the NSV or GCVS IV
	catalogues.
General:
	Due to shortage of space, many of the remarks in Humphreys' original table have been omitted, or abbreviated. I have retained the remarks alluding to non-members or doubtful members of associations. It has not been possible to include data such as Distance Moduli, or Absolute Magnitudes. The reader should refer to Humphreys' paper for this data.
Hogg 22 = "NGC 6204". There is a curious tale concerning this grouping. Humphreys calls this association NGC 6204, but in fact this is a relatively faint and distant cluster without OB stars.
The OB association in fact is connected with a nearby bright cluster, Hogg 22, recognized as a separate entity in 1965. Humphreys' classifications are based on earlier work, pre-1965.
Concluding Remarks:
	I call this compilation a database, but on reflection perhaps I should have called it a "finding list" or similar. There exists much additional astrophysical data that a full-blown database would have included. This is beyond the scope of my compilation, which is intended to provide the observer with the most basic data on stars in OB associations.
	No doubt much more information on OB associations has been published since 1978, and many more stars may now be assigned membership. I make no apology for the incompleteness of my data-base. Should anyone have more up-to-data information, I would be pleased to receive it.
	I would be pleased to hear from any reader who detects a mistake.
References:
Buscombe, Wܥe #	                  H+  K                  