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S254258CXO - S254-S258 Star-Forming Region Chandra X-Ray Point Source Catalog

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Overview

The aim of this study was to find an explanation for the remarkable morphology of the central part of the S254-S258 star forming complex. The authors performed a deep Chandra X-ray observation of the S254-S258 region in order to efficiently discriminate young stars (with and without circumstellar matter) from the numerous older field stars in the area. They detected 364 X-ray point sources in a 17' x 17' (~ 8 x 8 pc) field. This X-ray catalog provides, for the first time, a complete sample of all young stars in the region down to about 0.5 M_{sun}_. A clustering analysis identifies three significant clusters: the central embedded cluster S255-IR and two smaller clusterings in S256 and S258. Sixty-four X-ray sources can be classified as members in one of these clusters. After accounting for X-ray background contaminants, this implies that about 250 X-ray sources constitute a widely scattered population of young stars, distributed over the full field-of-view of the X-ray image. This distributed young stellar population is considerably larger than the previously known number of non-clustered young stars selected by infrared excesses. Comparison of the X-ray luminosity function with that of the Orion Nebula Cluster suggests a total population of ~ 2000 young stars in the observed part of the S254-S258 region.

The S254-S258 complex was observed (PI: Preibisch) in November 2009 with the Imaging Array of the Chandra Advanced CCD Imaging Spectrometer (ACIS-I). ACIS-I provides a field of view of 17' x 17' on the sky. At the 1.6 kpc distance of S254-S258 this corresponds to 7.9 x 7.9 pc. The aimpoint of the observation was RA(J2000) = 06h12m54.0s, Dec(J2000) = +17d 59' 24". The observation was performed in the standard 'Timed Event, Faint' mode (with 3 x 3 pixel event islands). The total net exposure time of 74725 s (20.76 h) was split into two parts, separated by about 4 days. The details of these two observation parts are given in Table 1 of the reference paper.

The authors first employed the wavdetect algorithm (Freeman et al. 2002, ApJS, 138, 185, a CIAO mexican-hat wavelet source detection tool) for locating X-ray sources in the merged image, and used a rather low detection threshold of 10-5. This step was performed in three different energy bands, the total band (0.5 - 8.0 keV), the soft band (0.5 - 2.0 keV), and the hard band (2.0 - 8.0) keV, and with wavelet scales between 1 and 16 pixels. They also performed a visual inspection of the images and added some 30 additional candidates to the merged catalog from the wavelet analysis, resulting in a final catalog of 511 potential X-ray sources. To clean this catalog from spurious sources, they then performed a detailed analysis of each individual candidate source with the ACIS Extract (AE hereafter) software package (Broos et al. 2010, ApJ, 714, 1582). The Poisson probability (PB) associated with the "null hypothesis", i.e. that no source exists and the extracted events are solely due to Poisson fluctuations in the local background, was computed for each source using AE. All candidate sources with PB > 0.01 were rejected as background fluctuations. After 8 iterations of this pruning procedure the final catalog consisted of 364 sources. It contains 344 primary sources with PB < 0.003, and 20 tentative sources with 0.003 < PB < 0.01.

To obtain an estimate of the intrinsic, i.e. extinction-corrected, X-ray luminosity for sources that are too weak for a detailed spectral analysis, the authors used the XPHOT software, developed by Getman et al. (2010, ApJ, 708, 1760). XPHOT is based on a non-parametric method for the calculation of fluxes and absorbing X-ray column densities of weak X-ray sources. X-ray extinction and intrinsic flux are estimated from the comparison of the apparent median energy of the source photons and apparent source flux with those of high signal-to-noise spectra that were simulated using spectral models characteristic of much brighter sources of similar class previously studied in detail. This method requires at least 4 net counts per source (in order to determine a meaningful value for the median energy) and can thus be applied to 255 of the 364 sources in this table. To calculate luminosities, a distance of 1.6 kpc was assumed. The resulting intrinsic X-ray lumonosities range from 1029.4 to 1032.3 erg s-1.


Catalog Bibcode

2011A&A...533A.121M

References

Revealing the "missing" low-mass stars in the S254-S258 star forming region by
deep X-ray imaging.
    Mucciarelli P., Preibisch T., Zinnecker H.
   <Astron. Astrophys. 533, A121 (2011)>
   =2011A&A...533A.121M

Provenance

This table was created by the HEASARC in October 2011 based on CDS Catalog J/A+A/533/A121 files table2.dat, table3.dat and table5.dat.

Parameters

Source_Number
A sequential identification number for aach X-ray source in the catalog in order of increasing J2000.0 Right Ascension.

Name
The X-ray source designation in the style recommended by the Dictionary of Nomenclature of Celestial Objects for unregistered Chandra source, viz., 'CXOU JHHMMSS.s+DDMMSS'. The HEASARC created these names from the non-standard forms, viz. 'CXOU JHHMMSS.ss+DDMMSS.s' used in the original reference table.

RA
The Right Ascension of the X-ray source in the selected equinox. This was given in J2000.0 decimal degrees to a precision of 10-6 degrees in the original reference table.

Dec
The Declination of the X-ray source in the selected equinox. This was given in J2000.0 decimal degrees to a precision of 10-6 degrees in the original reference table.

LII
The Galactic Longitude of the X-ray source.

BII
The Galactic Latitude of the X-ray source.

Error_Radius
The estimated standard deviation of the random component of the positional error, in arcseconds, sqrt({sigma}x2+{sigma}y2). The single-axis positional errors, {sigma}x and {sigma}y, are estimated from the single-axis standard deviations of the PSF inside the extraction region and the number of counts extracted.

Off_Axis
The off-axis angle of the X-ray source from the aimpoint of the observation, in arcminutes.

Counts
The net (background-subtracted) counts extracted in the total energy band (0.5 - 8 keV) for the X-ray source.

Counts_Error
The average of the upper and lower 1-sigma errors in the net (background-subtracted) counts extracted in the total energy band (0.5 - 8 keV) for the X-ray source.

Bck_Counts
The number of (total-band) background counts expected in the source extraction region (defined as the 90% contours of the local PSF, or smaller in the case of other nearby sources) of the X-ray source.

HB_Counts
The net (background-subtracted) counts extracted in the hard energy band (2 - 8 keV) for the X-ray source.

PSF_Fraction
The fraction of the PSF (at 1.497 keV) enclosed within the extraction region: this is usually 0.9 (90%), but could be smaller in the case of other nearby encroaching sources. Thus, a reduced PSF fraction significantly below 90% usually indicates that the source is in a crowded region.

Significance
The photometric significance of the X-ray source computed as the net counts divided by its upper uncertainty.

Log_Bck_Prob_Limit
This limit parameter is set to '<' if the corresponding parameter value is an upper limit.

Log_Bck_Prob
The logarithmic probability that the extracted total counts are solely from a Poisson background fluctuation. Some sources have PB values above the 1% threshold that defines the catalog because local background estimates can rise during the final extraction iteration after sources are removed from the catalog.

Source_Anomaly_Flag
This flag parameter is set to 'g' to indicate that the fractional time that the X-ray source was on a detector was <0.9.

Var_Flag
This flag parameter contains coded information on the X-ray source variability using a characterization based on the 1-sided Kolmogorov-Smirnov (K-S) statistic, as follows:

     a = no evidence for variability (PKS > 0.05)
     b = possibly variable (0.005 < PKS <0.05)
     c = definitely variable (PKS < 0.005)
     - = No value is reported for sources with fewer than four counts or
         for sources in chip gaps or on field edges.
  
where PKS is the probability that the source is constant.

Exposure
The effective exposure time for the X-ray source. This is the approximate time the source would have to be observed at the aimpoint of the ACIS-I detector in Cycle 11 in order to obtain the reported number of net counts.

Median_Energy
The background-corrected median photon energy in the total band for the X-ray source, in keV.

Source_Flag
This flag parameter is set to 'B' for a subset of the 25 brightest sources with more than 80 net counts for which a more sophisticated spectral analysis was performed using AE and XSPEC (see Section 3.1.3 of the reference paper). The X-ray fluxes and luminosities for these 25 sources calculated in this way are available in Table 4 of the reference paper.

Photon_Flux
The incident full-band (0.5 - 8.0 keV) flux from the AE software, in photons cm-2 s-1, calculated from the number of detected photons and using the scalar mean of the ancillary response function (ARF). This is an estimate of the observed, i.e., not the intrinsic, X-ray flux, and is called FLUX2 in AE.

Log_HB_Lx
The logarithm of the intrinsic (absorption-corrected) hard-band (2.0 - 8.0 keV) luminosity of the source, in erg s-1, calculated using XPHOT (see Overview above and/or the reference paper for more details).

Log_HB_Lx_Obs
The logarithm of the apparent (observed) hard-band (2.0 - 8.0 keV) luminosity of the source, in erg s-1, calculated using XPHOT (see Overview above and/or the reference paper for more details).

Log_Lx
The logarithm of the intrinsic (absorption-corrected) total-band (0.5 - 8.0 keV) luminosity of the source, in erg s-1, calculated using XPHOT (see Overview above and/or the reference paper for more details).

Log_Lx_Obs
The logarithm of the apparent (observed) total-band (0.5 - 8.0 keV) luminosity of the source, in erg s-1, calculated using XPHOT (see Overview above and/or the reference paper for more details).

DSS_Flag
This flag parameter is set to 'Y' to indicate that there is an optical counterpart to the X-ray source in at least the Digitized Sky Survey (DSS) red image.

Jmag_Flag
This flag parameter is set to 'J' to indicate that there is an infrared counterpart to the X-ray source in the J-band in the Two Micron All Sky Survey (2MASS) point source catalog.

Hmag_Flag
This flag parameter is set to 'H' to indicate that there is an infrared counterpart to the X-ray source in the J-band in the Two Micron All Sky Survey (2MASS) point source catalog.

Kmag_Flag
This flag parameter is set to 'K' to indicate that there is an infrared counterpart to the X-ray source in the J-band in the Two Micron All Sky Survey (2MASS) point source catalog.

Cha08_Flag
This flag parameter is set to 'Y' to indicate that there is an infrared counterpart to the X-ray source in the Spitzer-IRAC catalog from Chavarria et al. (2008, ApJ, 682, 445).

Source_Type
The IR classification of the X-ray source counterpart, when available. Infrared classes are only reported for those sources where the closest match in the Chavarria et al. (2008, ApJ, 682, 445) catalog has a known infrared class.

Class
The HEASARC Browse object classification of the source, based on the value of the source_type parameter.


Contact Person

Questions regarding the S254258CXO database table can be addressed to the HEASARC User Hotline.

Page Author: Browse Software Development Team
Last Modified: 7-Oct-2011