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ONCCXOOPT - Orion Nebula Cluster Chandra HRC Optical Sample


In the first of two companion papers on the Orion Nebula Cluster (ONC), the authors presented their analysis of a 63 ks Chandra HRC-I observation that yielded 742 X-ray detections within the 30' x 30' field of view. To facilitate their interpretation of the X-ray image, they compiled a multiwavelength catalog of nearly 2900 known objects in the region by combining 17 different catalogs from the recent literature. They defined two reference groups: an infrared sample, containing all objects detected in the K band, and an optical sample comprising low-extinction, well-characterized ONC members. They showed for both samples that field object contamination is generally low. Their X-ray sources are primarily low-mass ONC members. The detection rate for optical sample stars increases monotonically with stellar mass from zero at the brown dwarf limit to ~100% for the most massive stars but shows a pronounced dip between 2 and 10 M_sun. They determined L_X and L_X/L_bol or all stars in their optical sample and utilized this information in their companion paper to study correlations between X-ray activity and other stellar parameters.

In particular, the authors assembled an extensive catalog of known X-ray/optical/IR and radio objects that fell within the HRC FOV. In addition to their list of HRC sources and the Chandra source lists of Garmire et al. (2000, CDS Cat. <J/AJ/120/1426>) and Schulz et al. (2001, CDS Cat. <J/ApJ/549/441>), they considered 14 catalogs from recent publications, producing a database of nearly 2900 distinct objects reported in at least one of the studies considered. A full list of references is given in the first column of Table 2 of the reference paper, along with a concise classification of the work and the referenced table number(s) from the original work. The authors' ONC optical sample is comprised of stars in the HRC FOV for which they have a mass estimate, whose values of the visual absorption A_V are less than 3.0, and which are either confirmed proper motion members or have unknown proper motion (see Section 3.4.1 of the reference paper). For the 696 stars of this optical sample, this HEASARC table (the full version of Table 4 of the reference paper) lists sky position, mass, age, rotational period, Ca II line equivalent width, HRC basal count rate (see Section 5 of the reference paper), X-ray luminosity, and L_X/L_bol (see Sections 4 and 5 of the reference paper).

Catalog Bibcode



Chandra X-ray observation of the Orion Nebula Cluster.
I. Detection, identification, and determination of X-ray luminosities.
    Flaccomio E., Damiani F., Micela G., Sciortino S., Harnden F.R.Jr,
    Murray S.S., Wolk S.J.
   <Astrophys. J. 582, 382 (2003)>


This table was created by the HEASARC in March 2007 based on the CDS table J/ApJ/582/382, file table4.dat.

HEASARC Implementation

The table of Chandra HRC X-ray sources detected in this observation (Table 1 of the reference paper) is available as the HEASARC Browse table ONCCXOXRAY. The HEASARC has created links connecting entries in the present table to those in this related table, based on having a positional coincidence within 5 arcseconds. This compromise value may be too large for positions near the center of the field (and hence produce multiple matches) but too small for positions near the edge of the field (and hence miss some matches): users can use the cross-correlation functionality in Browse to vary the matching radius between the two tables.


A running number for the optical star in order of increasing J2000.0 Right Ascension within the field. Note that this is not the same as the source numbering scheme used for the X-ray sources.

The name of the star in the ONC Optical Sample using the '[FDM2003] Opt' prefix recommended by the CDS Dictionary of Nomenclature of Celestial Objects for these objects combined with the optical star source number.

The Right Ascension of the star in the selected equinox. This was given in J2000.0 coordinates to a precision of 0.01 seconds of time in the original table.

The Declination of the star in the selected equinox. This was given in J2000.0 coordinates to a precision of 0.01 arcseconds in the original table.

The Galactic Longitude of the star.

The Galactic Latitude of the star.

The stellar mass, in solar masses. See Section 3.3 of the reference paper for details on how this was estimated.

The logarithm of the stellar age, in years. See Section 3.3 of the reference paper for details on how this was estimated.

The stellar rotation period, in days.See Table 2 and Section 3.3 of the reference paper for the sources of this information.

The optical star's Ca II 8542 Angstrom line equivalent width, in Angstrom. See Table 2 of the reference paper for the source of this information.

This parameter is set to '<' if the corresponding HRC count rate is an upper limit rather than a detection.

The basal HRC count rate, in ct/s, of the X-ray source with which the optical star has been associated. Because the intensity of many of the HRC sources varied during the observation, the authors did not use mean count rates, but instead defined a basal count rate in order to remove the effects of short-term (<~ 63 ks) variability. Their basal rate algorithm searched for that count rate which was compatible with the largest possible portion of the HRC light curve.

This parameter is set to '<' if the corresponding logarithm of the X-ray luminosity is an upper limit rather than an actual value.

The logarithm of the stellar X-ray luminosity in the spectral band between 0.1 and 4.0 keV, in erg/s, based on the basal X-ray count rate of the source, an assumed distance to the ONC of 470 pc, and an HRC count rate to X-ray flux conversion factor calculated as discussed in Section 5.1 of the reference paper, i.e., assuming an isothermal (kT = 2.16 keV), optically thin Raymond & Smith plasma, absorbed by a gas column density proportional to the optical extinction: N_H = 2 x 10^21 x A_V.

This parameter is set to '<' if the corresponding logarithm of the ratio of the X-ray to bolometric luminosity is an upper limit rather than an actual value.

The logarithm of the stellar X-ray to bolometric luminosity ratio, using the estimated X-ray luminosity as discussed above, and the bolometric luminosity obtained from the source given in Table 2 of the reference paper.

Contact Person

Questions regarding the ONCCXOOPT database table can be addressed to the HEASARC User Hotline.
If you have any problems, please consult the help page or mail ledas-help@star.le.ac.uk