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NGC6231CXO - NGC 6231 Chandra X-Ray Point Source Catalog

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Overview

NGC 6231 is a massive young star cluster, near the center of the Sco OB1 association. While its OB members are well studied, its low-mass population has received little attention. In the reference paper, the authors present high-spatial resolution Chandra ACIS-I X-ray data, wherein they detect 1,613 point X-ray sources. Their main aim was to clarify the global properties of NGC 6231 down to low masses through a detailed membership assessment, and to study the cluster stars' spatial distribution, the origin of their X-ray emission, the cluster age and formation history, and its initial mass function.

The authors use X-ray data, complemented by optical and IR data, to establish cluster membership. The spatial distribution of different stellar subgroups also provides highly significant constraints on cluster membership, as does the distribution of X-ray hardness. In their study, the authors perform spectral modeling of group-stacked X-ray source spectra.

The X-ray properties of the sources detected in the Chandra observations of NGC 6231, and their cross-identifications in the catalogs of Sung, Sana, and Bessell (2013 AJ, 145, 37; hereafter SSB); VPHAS+ (Drew et al., 2014, MNRAS, 440, 2036); and 2MASS (CDS Cat. II/246), and information about membership, H-alpha or IR excess, mass and luminosity are also provided. SSB derive a distance modulus for NGC 6231 of 11.0 (1,585 pc), a reddening E(B - V) = 0.47, and a nearly normal reddening law with R = 3.2. The present authors adopt these values for this work.

NGC 6231 was observed twice in X-rays with the ACIS-I detector on-board the Chandra X-ray Observatory on 2005, July 3 to 4 (ObsId 5372) and 16 to 17 (ObsID 6291), respectively. The two pointings share the same center (aimpoint) but were performed with a different roll angle. Effective exposure times for the observations were 76.19 and 44.39 ks, respectively, making the total exposure time 120.58 ks. The data were filtered to retain the energy band 0.3 - 8.0 keV, and the full-field lightcurves were inspected to search for high-background periods, but none were found. Exposure maps were computed using standard CIAO software tasks. To these prepared datasets, the authors applied the source detection software PWDetect, a wavelet-based detection algorithm developed at INAF-Osservatorio Astronomico di Palermo. The PWDetect version used here is a modified one, able to detect sources in combined datasets, thus taking full advantage of the deep total exposure. The detection threshold was chosen such as to yield ten spurious detections in the field of view (FOV), for the given background counts. This is a more relaxed constraint than the more usual limit of one spurious detection per field, but is justified when the lowered threshold allows the detection of more than one hundred additional faint sources, as it was the case here or in the COUP Program's Orion data.

This HEASARC table contains the list of 1,613 detected X-ray point sources and information about their optical and IR counterparts, where known. It does not contain the 275 additional candidate cluster members (where their candidacy was based on their having H-alpha or IR excesses) which lack X-ray counterparts and that were also listed in Table B.2 of the reference paper.


Catalog Bibcode

2016A&A...596A..82D

References

A Chandra X-ray study of the young star cluster NGC 6231:
low-mass population and initial mass function.
    Damiani F., Micela G., Sciortino S.
    <Astron. Astrophys. 596, A82 (2016)>
    =2016A&A...596A..82D        (SIMBAD/NED BibCode

Provenance

This table was created by the HEASARC in December 2016 based on CDS Catalog J/A+A/596/A82 file tableb.dat, which is the merger of tables B.1 (the list of 1,613 X-ray sources) and B.2 (the list of 1,888 optical and near-IR identifications of X-ray sources and of IR- and H-alpha-excess stars) from the reference paper, but excluding the 275 stars listed in the latter whose candidacy was based on their having H-alpha or IR excesses and which lack X-ray counterparts. #

Parameters

Source_Number
A unique X-ray source number in order of increasing J2000.0 Right Ascension.

Name
The preferred IAU source designation for the X-ray source using the 'CXOU' prefix recommended by the CDS Dictionary of Nomenclature of Celestial Objects for unregistered Chandra X-ray sources, and the truncated J2000.0 coordinates (JHHMMSS.s+DDMMSS).

RA
The Right Ascension of the X-ray source in the selected equinox. This was given in J2000.0 decimal degree coordinates to a precision of 10-5 degrees (0.036 arcseconds) in the original table. The authors found and corrected a systematic shift between X-ray and 2MASS positions, of 0.08 arcseconds in RA, and 0.12 arcseconds in Declination.

Dec
The Declination of the X-ray source in the selected equinox. This was given in J2000.0 decimal degree coordinates to a precision of 10-5 degrees (0.036 arcseconds) in the original table. The authors found and corrected a systematic shift between X-ray and 2MASS positions, of 0.08 arcseconds in RA, and 0.12 arcseconds in Declination.

LII
The Galactic Longitude of the X-ray source.

BII
The Galactic Latitude of the X-ray source.

Error_Radius
The positional error of the X-ray source, in arcseconds

Counts
The estimated net counts from the extraction area for the X-ray source in the total energy band (0.3 - 8.0 keV).

Count_Rate
The count rate of the X-ray source in the total energy band (0.3 - 8.0 keV), in counts per second. This was given in counts per kilosecond in the original reference.

Count_Rate_Error
The uncertainty in the count rate of the X-ray source in the total energy band (0.3 - 8.0 keV), in counts per second. This was given in counts per kilosecond in the original reference.

Ks_Prob_Not_Constant
The Kolmogorov-Smirnov test probability PKS that the X-ray source emission is not compatible with being constant.

Hardness_Ratio_1
The hardness ratio HR1. The authors define two hardness ratios, HR1 = (M - S)/(M + S), and HR2 = (H - M)/(H + M), where S , M, and H are the source X-ray counts in the soft (0.3 - 1.2 keV), medium (1.2 - 2.2 keV), and hard (2.2 - 8.0 keV) bands, respectively. Relatively high values of HR1 and HR2 are indicative of harder (hotter) X-ray emission, while a high HR1 is also indicative of high absorption, the latter hiding selectively the softer emission.

Hardness_Ratio_2
The hardness ratio HR2. The authors define two hardness ratios, HR1 = (M - S)/(M + S), and HR2 = (H - M)/(H + M), where S , M, and H are the source X-ray counts in the soft (0.3 - 1.2 keV), medium (1.2 - 2.2 keV), and hard (2.2 - 8.0 keV) bands, respectively. Relatively high values of HR1 and HR2 are indicative of harder (hotter) X-ray emission, while a high HR1 is also indicative of high absorption, the latter hiding selectively the softer emission.

Log_Lx
The logarithm of the average X-ray (0.3 - 8.0 keV) luminosity of the source, in erg/s. This was calculated based on the average count rate, assuming that the X-ray source is at the distance of the cluster (1,585 pc), and using a count rate to flux conversion factor that depends on the stellar group (given in Table 2 of the reference paper) to which the authors have assigned the particular source.

Log_Lx_Quiescent
The logarithm of the quiescent X-ray (0.3 - 8.0 keV) luminosity of the source, in erg/s, calculated for those sources showing flares in their X-ray lightcurves. This was calculated based on the interpolated or extrapolated count rate excluding the flare segments, assuming that the X-ray source is at the distance of the cluster (1,585 pc), and using a count rate to flux conversion factor that depends on the stellar group (given in Table 2 of the reference paper) to which the authors have assigned the particular source.

SBB_Source_Number
The source identifier of the counterpart to the X-ray source that is given in SBB (Sung et al., 2013, AJ, 145, 37).

Vmag
The V magnitude of the counterpart to the X-ray source that is given in SBB (Sung et al., 2013, AJ, 145, 37).

VI_Color
The V-I color of the counterpart to the X-ray source that is given in SBB (Sung et al., 2013, AJ, 145, 37).

SBB_Halpha_Index
The H-alpha index of the counterpart to the X-ray source that is given in SBB (Sung et al., 2013, AJ, 145, 37).

VPHAS_Name
The source identifier of the counterpart to the X-ray source that is given in VPHAS+, e.g., VPHAS JHHMMSS.s+DDMMSS.s, (Drew et al., DR2, 2016, CDS Cat. II/341).

Rmag
The r magnitude of the counterpart to the X-ray source that is given in VPHAS+.

Imag
The i magnitude of the counterpart to the X-ray source that is given in VPHAS+.

VPHAS_Halpha_Mag
The H-alpha magnitude of the counterpart to the X-ray source that is given in VPHAS+.

TwoMASS_Name
The source identifier of the counterpart to the X-ray source that is given in 2MASS, e.g., 2MASS JHHMMSSss+DDMMSSs, (Cutri et al. 2003, CDS Cat. II/246).

Jmag
The J magnitude of the 2MASS counterpart to the X-ray source.

Hmag
The H magnitude of the 2MASS counterpart to the X-ray source.

Kmag
The K magnitude of the 2MASS counterpart to the X-ray source.

Stellar_Group
The grouping in the V-I vs. V color-magnitude diagram (CMD) to which the counterpart to the X-ray source belongs. These correspond to the following physical stellar or other types:

  Group 1:  O-B (main-sequence or evolved) spectral type stars;
  Group 2:  A-F spectral type stars which have just arrived on the main sequence
            (or are about to arrive);
  Group 3:  Pre-main sequence (PMS) stars (the bulk of the X-ray detections);
  Group 4:  Field main-sequence stars, typically X-ray detections that lie
            distinctly below the PMS band in the V-I vs. V plane;
  Group 5:  Foreground stars, typically X-ray detections that lie
            distinctly above the PMS band in the V-I vs. V plane;
  Group 6:  Unidentified sources, i.e., X-ray sources with no optical IDs.
  

IR_Excess_Flag
This flag parameter is set to 'Y' to indicates that the authors consider that the counterpart shows a significant (> 3 sigma) IR excess based on its position in V-I vs. H-K and r-i vs. H-K color-color diagrams (see Section 5.4 of the reference paper for more details)..

Halpha_Excess_Flag
This flag parameter is set to 'Y' to indicates that the authors consider that the counterpart shows a significant H-alpha emission excess, which is regarded as a complementary cluster membership indicator to X-ray detection (see Section 5.3 of the reference paper for more details).

Mass
The derived stellar mass of the counterpart to the X-ray source, in solar masses. Apart from massive stars, very little detail is known for the other cluster members. For these latter, the authors use the available optical photometry to estimate stellar masses and bolometric luminosities. The mass was calculated by the authors assuming for all stars the same average cluster reddening and distance (see Overview). In particular, masses and bolometric luminosities were computed for PMS stars using the Baraffe et al. (2015, A&A, 577, A42) or Siess et al. (2000, A&A, 358, 593) 5-Myr isochrone in the (V, V - I) CMD (as discussed in Sect. 5 of the reference paper), and the masses of higher-mass MS stars (above ~7 solar masses) using the 3-Myr non-rotating solar-metallicity isochrone from Ekstrom et al. (2012, A&A, 537, A146). The B0.7Ia supergiant HD 152235 was assigned a mass of 30 solar masses (Fraser et al. 2010, MNRAS, 404, 1306).

Log_Lbol
The logarithm of the bolometric luminosity of the counterpart to the X-ray source, in solar luminosities. Apart from massive stars, very little detail is known for the other cluster members. For these latter, the authors use the available optical photometry to estimate stellar masses and bolometric luminosities. The stellar luminosity was calculated by the authors assuming for all stars the same average cluster reddening and distance (see Overview). In particular, masses and bolometric luminosities were computed for PMS stars using the Baraffe et al. (2015, A&A, 577, A42) or Siess et al. (2000, A&A, 358, 593) 5-Myr isochrone in the (V, V - I) CMD (as discussed in Sect. 5 of the reference paper).


Contact Person

Questions regarding the NGC6231CXO database table can be addressed to the HEASARC User Hotline.

Page Author: Browse Software Development Team
Last Modified: 16-Dec-2016