SIMBAD: The Contents of SIMBAD


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Chapter 15. The Contents of SIMBAD

[missing file: ch15.toc]

The SIMBAD data base presently contains information for about 2,750,000 astronomical objects (stars, galaxies, planetary nebulae, clusters, novae and supernovae, etc.).

The only astronomical objects specifically excluded from SIMBAD are Solar System bodies (planets, satellites, asteroids, comets)

In the following, the word object will be used to designate a star, nonstellar object, or collection of objects such as a cluster, which correspond to an individual entry in SIMBAD. For each object, the following data are included:

1  Basic data

The basic data are displayed immediately after the object has been retrieved (immediate format) or when typing basic (standard format).

They consist of:

1.1  Object type

The hierarchical classification of the objects in SIMBAD is based on the catalogue identifiers (as proposed by Ochsenbein and Dubois, 1992). From Star to Maser source, or Cluster of Galaxies, some 70 different categories, general, or very specific, are proposed (see Appendix F).

This classification aims to help in the selection of objects (through scanning of lists, or use of the filter procedure in SIMBAD). It is also a powerful tool for data cross–checking and quality control.

The classification uses a hierarchy with four levels, reflecting our knowledge of the characteristics of the astronomical object. For instance, an object can be classified as a ``Star'' (this is level 1). If photometric observations have shown variability of the object, it can be classified as a ``Variable star'' (this is level 2). Examples of level 3 and 4 are ``Pulsating variable'', and ``Cepheid''.

This hierarchy of object types (and lists of possible synonyms) is managed in the database in such a way that selecting variable stars (V*) is understood as selecting objects classified as V*, and all subdivisions (e.g. PulsV*, Mira, Cepheid, etc.). If the user is only interested in RR Lyrae type stars, he/she will use the RRLyr type, leaving aside all other variable stars for which the variability mode is different, or not known.

The classification extends to galaxies and extragalactic objects, to multiple objects, etc. and gives only wavelength information (such as IR or UV source) when no other information is known.

This classification has been designed to be practical and useful. A complete list is given in the Appendix F of this user's guide, as well as on line, by typing in SIMBAD info otype. More details are given in the above-mentioned paper.

1.2  Positional data

Coordinates, proper motions, radial velocities and parallaxes participate all in the computation of the position of an object.

They have their own characteristics, but share a general presentation and information structure:

1.3  Coordinates

The coordinates are originally stored in the database for epoch 2000.0 in the ICRS system, which is the frame defined by the Hipparcos mission. The coordinates can be computed in different frames for different epochs and equinoxes. (see chapter 6).

The available frames are :

ICRS  ICRS system.
FK5   FK5 reference frame
FK4   FK4 frame
GAL   Galactic coordinates
SGAL  Supergalactic coordinates
ECL   Ecliptic coordinates

The coordinates are of various sources. When available, the most accurate astrometric data has been selected from the HIPPARCOS (referenced as 1997A&A...323L..49P) and PPM catalogues.

Syntax of coordinates is : "ra dec (wtype) [error ellipse] quality bibcode" :

The default format gives equatorial coordinates in the FK5 system for equinox 2000 and 1950, as well as galactic coordinates. This can be modified (see chapter 19). Ecliptic and supergalactic coordinates can also be computed on request.

1.4  Proper motion

The proper motions, expressed in the ICRS system, consist in the 2 values µαcosδ and µδ, expressed in mas/yr (10–3arcsec/year), together with their error ellipse between square brackets ([ ]).

The main sources are HIPPARCOS and PPM catalogues or other astrometric catalogue (see PM measurements). The source of the proper motions is generally referenced by a bibcode (1997A&A...323L..49P for Hipparcos)

1.5  Radial Velocities

The same parameter in Simbad can accomodate the radial velocity Vr (the relative velocity between the Sun and the star, projected along the line of sight, expressed in km/s), or the redshift z used for distant galaxies and quasars (defined as the relative displacement in the wavelength of known spectrocopic lines like Hydrogen lines). For small velocities (i.e. Vr < 104km/s), the velocity is related to the redshift with the simple formula   Vr = c⋅z    if c is the speed of the light (3×105 km/s)

Radial velocities are introducted by a v, and redshift by a z; the value is followed by its wean error within square brackets [ ], by its quality from the best A for the least reliable E; its source is generally referenced by a bibcode like 1953GCRV..C......0W for Wilson's General Catalogue of stellar Radial Velocities.

1.6  Parallaxes

The parallax p represents the angle at which the Earth-Sun distance (one astronomical unit, close to 150×106km) is seen at the star's distance. It is expressed in mas (10–3arcsec), and is related to the distance d of the star by

         d=1000/p when d is expressed in pc
	(parsecs)
         d=3260/p when d is expressed in al
   (année-lumière  or light-year)

As for the other positional parameters, the parallax is followed by its mean error within square brackets [ ], its quality from the best A for the least reliable E, and its source, which is mostly Hipparcos (1997A&A...323L..49P)

1.7  Magnitudes

Magnitudes are given, when possible, in the Johnson's UBV system. Otherwise, the (AB) flag is given if it cames from a AB system.

The data could come from any published papers associated to the object.

Syntax of fluxes (or magnitudes) is : "filter-name (System) flux-value [error] quality MultVarFlags bibcode"

The magnitude value may be followed by a semicolon meaning they cannot be made homogeneous to the UBV system. B and V magnitudes are separated by two columns which may contain the following flags:

1.8  Spectral type

The spectral types of stars have been selected in the Michigan Catalogues of Two-Dimensional Spectral Types for the HD stars (Houk N., 1975, and seq.), or in the bibliographical surveys of MK classifications (Jaschek M., 1978: CDS Inf. Bull. 15, 121). In the absence of a full MK classification, the HD spectral type is recorded.

Most of the spectral types need less than 5 characters, but this field can be as long as 36 characters. Complete syntax is given in Appendix D.

1.9  Morphological type

The morphological types of galaxies have been selected from the Uppsala General Catalogue of Galaxies (UGC, Nilson 1973), the Morphological Catalogue of Galaxies (MCG, Vorontsov-Vel'jaminov 1962-68), and other catalogues (see Dubois, Ochsenbein & Paturel, 1983).

Complete syntax is given in Appendix E.

1.10  Dimension of galaxy

The following data, when available, are given for galaxies (for more details, see Dubois, Ochsenbein & Paturel, 1983):

 logD25      is the logarithm of the major axis a
expressed in tenths of arc minutes 
 logR25      is the logarithm of the ratio a/b
where a and b are the major and minor axis 
orientation         orientation angle (in degrees) 
(inclination)       inclination (in units of 15° from 0 to 7) 

2  Cross-identifications

2.1  Aliases

Cross–identifications of stars and galaxies have been searched for SIMBAD entries from (currently) about 3000 source catalogues and published tables, included, either completely or partially, in the data base.

The list of aliases, thus constituted, is one of the unique features of the SIMBAD database.

The info cat command (described in chapter 12) gives access to an on-line database of catalogue descriptions, which is maintained up-to-date in coordination with the Dictionary of Nomenclature for Celestial Objects produced at Observatoire de Paris and Strasbourg by M.-C. Lortet et al. (1994).

2.2  Multiple systems

It is to be noted that for a double system in which the components can be observed separately, SIMBAD will frequently include three entries: A and B components, and an additional entry for AB carrying the observational data and references related to the system as a whole.

3  Observational data

Observational data (also called measurements) are presently given for the following measurement types:

|CEL   |Ultraviolet photometry from Celescope  |
|Cl.G  |Cluster of Galaxies  (Abell & Corwin)|
|Einstein |The Einstein Observatory Soft X-ray Source List|
|Fe    |Compilation of Stellar Metallicity Values (Cayrel & al.)|
|GCRV  |Stellar Radial velocities|
|GEN   |U B V B1B2V1G photometry in Geneva photometric system|
|GJ    |Absolute magnitudes and spatial velocities of nearby stars.  |
|Hbet  |Hβ index (used in connection with Strömgren's uvby
                 photometry)| 
|HGAM  |Hγ equivalent width |
|IRAS  |IRAS Point Source Catalog |
|IRC   |KI photometry from Two Micron Sky Survey|
|IUE   |International Ultraviolet Explorer (Merged Observation Log) |
|JP11  |UBVRIJKLMNH 11-colour Johnson's photometry|
|MK    |Stellar spectral classification in Morgan-Keenan system  |
|PLX   |Trigonometric parallaxes |
|pm    |Proper motions (in 1950 equatorial frame)|
|pos   |Equatorial positions |
|ROT   |Rotational velocities (V . sini) |
|oRV   |Stellar radial velocities (will replace GCRV)|
|RVEL  |Radial velocities of extragalactic objects (quasars and galaxies)|
|SAO   |Positions and proper motions from SAO catalogue. |
|TD1   |Ultraviolet magnitudes from TD1 experiment|
|UBV    |UBV photometry in Johnson's photometric system|
|uvby  |Photometry in Strömgren's photometric system|
|V*    |Data related to variable stars|
|z     |Redshifts (of distant galaxies and quasars|
|ze    |Redshifts with error information (will replace z)|
|

For each type, one can retrieve individual data with their bibliographical references, and, when available, weighted means computed from existing observed values by specialists in each data type.

A description of measurement types presently included in SIMBAD is given in Appendix G.

4  Bibliographical references

Bibliography includes references to all published papers from the journals regularly scanned (currently about 100 titles; the list is given in Appendix H).

Articles are scanned in their entirety, and references to all objects mentioned are included in the bibliography.

4.1  Bibliography

One of the key features of the SIMBAD astronomical database is the unique coverage of bibliographical references to objects. The bibliographic index contains references to stars from 1950 onwards, and to galaxies and all other objects outside the solar system from 1983 onwards. Presently there are more than 3,000,000 references taken from 110,000 papers published in the 100 most important astronomical periodical publications. The scanning of the literature is made possible through a collaboration between the Institut d'Astrophysique de Paris, and Paris, Bordeaux and Strasbourg Observatories, which has been described by Laloë (1995).

A code, called bibcode, is assigned to each considered paper: this 19-digit bibcode contains enough information to locate the article (including year of publication, journal, volume, page, etc.). The structure of this code has been defined in close collaboration with the NED group at NASA/IPAC so that both databases apply the same coding system (Schmitz et al., 1995). It is also used by ADS and the main journals like A&A, ApJ, AJ.

When one retrieves the bibliography of a SIMBAD object, a list of codes is given, and (according to the options used) these codes are automatically matched against a bibliographic file which provides for each citation the full reference, title and list of authors.

4.2  The abstracts

SIMBAD does not include the abstracts of referenced papers. However abstract services are available, and progress are made to allow easy retrieval of the abstracts for SIMBAD users (see chapter 07.2).

An important step, for the CDS, was the creation of an abstract service for Astronomy & Astrophysics (main journal and Supplement Series) announced in A&A, 284, E1 (April 1994). This service is made available as part of the CDS World-Wide Web server (Egret and Ochsenbein, 1994), and complements the service already existing for on-line tables of recent papers (Ochsenbein, 1993).

A similar agreement has been implemented with the Publications of the Astronomical Society of the Pacific (PASP) for which the abstracts since January 1994 are available.

4.3  Bibliographic reference coding convention

The bibliographic reference coding convention has been defined in cooperation with the staff of NASA/IPAC Extragalactic Database (NED), at IPAC (Schmitz et al., 1995).

Reference codes have the following structure:

YYYYJJJJJVVVVMPPPPA

where:

YYYY   Year of the publication

JJJJJ A standard abbreviation for the periodical

VVVV The volume number (for a journal) or one of the following abbreviations for another publication:

         B     book  
 C     catalogue  
 P     preprint  
 R     report  
 S     symposium 
 T     thesis  
 U    unpublished 

M Specific qualifier for a paper:

  L       letter  
  p       pink page  
  a-z     issue number within a volume   
  A-K     fascicule number  
  Q-Z     to distinguish articles on the same page. 

PPPP The page number

A The first letter of the name of the first author (or ':' if uncorrect or incomplete coding).

There are some exceptions to these rules, but they are of minor importance. For a more complete description see Schmitz et al. (1995), or on the WWW server: http://cdsweb.u-strasbg.fr/simbad/refcode.html.

Example:     1991A&A...246L..24M
for   Astron. Astrophys., 246, 24, 1991 (L), a Letter to the Editor of A&A, by Motch et al.

4.4  Updating

The bibliography is reasonably complete back to 1950 for stars, but only to 1983 for nonstellar objects (although many earlier papers are included for certain objects).

The updating of SIMBAD is a continuous daily process performed through a collaboration with the Institut d'Astrophysique de Paris and the Paris, Bordeaux and Strasbourg observatories.

The bibliographers in Paris, Strasbourg, and Bordeaux input directly to SIMBAD from journals as they are received in the libraries. It is therefore highly probable that the references are completely up to date one or two weeks after publication.

4.5  Comments in the references

Several types of comments are associated with the references in SIMBAD and normally displayed after the reference with the following headings:

4.6  Resolving a bibliographical reference code

It is possible to obtain the complete bibliographical reference, from the reference code, by typing it in answer to the simbad search -> prompt.

A reference code can be supplied without indicating all the fields: the first reference corresponding to the truncated code will be displayed. However, at least three digits are needed. Am ampersand ('&') should be added to the truncated bibcode (see chapter 7).

5  References


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