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CG12CXO - CG 12 Chandra X-Ray Point Source Catalog

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Overview

The mysterious high Galactic latitude cometary globule CG 12 has been observed with the ACIS detector on board the Chandra X-Ray Observatory. 128 X-ray sources are detected, of which half are likely young stars formed within the globule's head. This new population of >~ 50 T Tauri stars and one new embedded protostar is far larger than the previously reported few intermediate-mass and two protostellar members of the cloud. Most of the newly discovered stars have masses of 0.2-0.7 Msolar, and 9% - 15% have K-band excesses from inner protoplanetary disks. X-ray properties provide an independent distance estimate consistent with the unusual location of CG 12 >~200 pc above the Galactic plane. The star formation efficiency in CG 12 appears to be 15% - 35%, far above that seen in other triggered molecular globules. The median photometric age found for the T Tauri population, assuming Siess et al. (2000, A&A, 358, 593) isochrones, is ~4 Myr with a large spread of <1 - 20 Myr and ongoing star formation in the molecular cores. The stellar age and spatial distributions are inconsistent with a simple radiation-driven implosion (RDI) model and suggest either that CG 12 is an atypically large shocked globule or that it has been subject to several distinct episodes of triggering and ablation. In their paper the authors report a previously unnoticed group of B-type stars northwest of CG 12 that may be the remnants of an OB association that produced multiple supernova explosions that could have shocked and ablated the cloud over a 15 - 30 Myr period. HD 120958 (B3e), the most luminous member of the group, may be currently driving an RDI shock into the CG 12 cloud.

The current project combines four X-ray observations of the globule:

Field   ObsID   Start Time             Expo.       R.A.     Decl.    Roll Angle
                (UT)                   (ks)          (J2000.0)         (deg)
I....   6423    2006 Apr 15 16:19:17   30.8    13 57 44.52  39 58 48.31  11.5
II...   6424    2006 Jun 02 07:25:09    3.1    13 57 42.87  39 43 01.76 285.0
III..   6425    2006 Apr 13 08:44:08    3.1    13 56 19.40  39 42 47.94  14.7
IV...   6426    2006 Apr 15 12:54:20    3.1    13 56 19.40  39 58 48.09  11.1
where the units of right ascension are hours, minutes, and seconds, and the units of declination are degrees, arcminutes, and arcseconds, ObsID values are from the Chandra Observation Catalog, exposure times are the sum of Good Time Intervals (GTIs) for the CCD at the telescope aim point (CCD3) minus 1.3% to account for CCD readouts, and the aim points and roll angles are obtained from the satellite aspect solution before astrometric correction was applied.

There is one primary field (I in Fig. 1 of the reference paper) with ~31 ks exposure directed at the globule's core and three secondary fields (II, III, and IV in Fig. 1) with ~3 ks exposures positioned contiguously to the north and west of the core. The primary pointing is intended to detect the population of pre-main sequence (PMS) stars forming in the molecular head of the globule. The secondary pointings are designed to locate an older population of stars expected if the present cloud is only the ablated remnant of a larger cloud that experienced sequential star formation triggering events, similar to the sequence of stars found in the authors' Chandra study of IC 1396N (Getman et al. 2007, ApJ, 654, 316, available in Browse as the IC1396NCXO table).

Source searching was performed with data images and exposure maps constructed at three spatial resolutions (0.5", 1.0", and 1.4" pixel-1) using the CIAO wavdetect tool. The authors ran wavdetect with a low threshold P = 10-5, which is highly sensitive but permits false detections at this point in the analysis. This was followed by visual examination to locate other candidate sources, mainly close doubles and candidate sources near the detection threshold. Using ACIS Extract, photons were extracted within polygonal contours of ~90% encircled energy using position-dependent models of the PSF. The background was measured locally in source-free regions. Due to the very low, spatially invariant ACIS-I background in the Chandra observations of CG 12, there is a one-to-one correspondence between a source's significance and net counts. Following the procedure of Getman et al. (2007, ApJ, 654, 316), the list of candidate sources ws trimmed to omit sources with fewer than ~5 estimated source net counts, net full-band counts/PSF fraction <~ 4.5. In the case of the CG 12 observations, the above criterion is equivalent to accepting sources with a source significance of >~ 1.1. Thus, most of the statistically insignificant source candidates found during the wavdetect step were eliminated by the application of these source existence criteria.

For Chandra sources with > 20 net counts, the authors performed spectral analysis with the XSPEC spectral fitting package version 12.2. The unbinned source and background spectra were fitted with one-temperature APEC plasma emission models using the maximum likelihood method. They assumed 0.3 times solar elemental abundances previously suggested as typical for young stellar objects (YSOs) in other star-forming regions. Solar abundances were taken from Anders & Grevesse (1989, Geochim. Cosmochim. Acta, 53, 197). X-ray absorption was modeled using the atomic cross sections of Morrison & McCammon (1983, ApJ, 270, 119). For absorbed thermal spectra characteristics of PMS stars, the absorption NH can be estimated to roughly a factor of 2 precision for 20 count sources.


Catalog Bibcode

2008ApJ...673..331G

References

The stellar population and origin of the mysterious high-latitude star-forming
cloud CG 12.
    Getman K.V., Feigelson E.D., Lawson W.A., Broos P.S., Garmire G.P.
   <Astrophys. J., 673, 331-353 (2008)>
   =2008ApJ...673..331G

Provenance

This table was created by the HEASARC in October 2010 based on the electronic versions of Tables 2, 3 and 4 from the paper which were obtained from the CDS (their catalog J/ApJ/673/331).

Parameters

Field_Code
The code for the Chandra ACIS-I field in which the X-ray source was found, 'I', 'II', 'III', or 'IV'. The locations of these fields and details of these pointings are discussed in the Overview (see above).

Source_Number
A running X-ray source number in order of increasing J2000.0 Right Ascension that uniquely identifies a source in a given field. To uniquely identify a source in the survey as a whole, both the field code and the source number are required, e.g., source 'I 1'.

Alt_Name
An alternative name for the X-ray source using the '[GFL2008]' prefix (for Getman, Feigelson and Lawson, 2008),, the field code and the source number, e.g., '[GFL2008] I 1', as recommended by the CDS Dictionary of Nomenclature of Celestial Objects.

Name
The name for the X-ray source using the 'CXOU' prefix (for Chandra X-Ray Observatory, Unregistered) and the truncated J2000.0 coordinates (JHHMMSS.s+DDMMSS), in the standard way recommended by the CDS Dictionary of Nomenclature of Celestial Objects. These names were given as JHHMMSS.ss+DDMMSS.s in the original reference; the HEASARC truncated these so as to conform with the standard nomenclature for Chandra sources.

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 (0.0036 arcseconds) in the original 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 (0.0036 arcseconds) in the original table.

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.

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

Counts
The estimated net (i.e., background-subtracted) counts of the X-ray source in the total energy band (0.5 - 8 keV).

Counts_Error
The average of the upper and lower 1-sigma errors in the net total-band (0.5 - 8 keV) counts of the X-ray source. The errors were estimated by calculating the errors (Gehrels 1986, ApJ, 303, 336, equations (7) and (12)) in the total observed and background counts and then propagating those errors to the net counts.

Bck_Counts
The estimated background Counts within the PSF-based extraction area in the total energy band.

HB_Counts
The estimated net (i.e., background-subtracted) counts of the X-ray source in the hard energy band (2 - 8 keV).

PSF_Fraction
The fraction of the PSF enclosed within the source extraction region (at 1.497 keV). A reduced PSF fraction (significantly below 0.9 or 90%) may indicate that the source is in a crowded region or on the edge of the field.

SNR
The X-ray source photometric significance in the total band, as calculated by ACIS Extract assuming Poisson statistics. This is the ratio of the source's net (background-subtracted) counts to the uncertainty in that quantity.

Source_Flags
This parameter contains flags that indicate the presence of the following source anomalies:

      g = fractional time that source was on a detector
          (FRACEXPO from mkarf) is < 0.9;
      e = source on field edge;
      p = source piled up;
      s = source on readout streak.
  

Var_Flag
This parameter contains flag values that contain X-ray source variability characterization based on the K-S statistic applied to the total-band counts, coded as follows (no value is reported for sources in chip gaps or on field edges):

      a = no evidence for variability;
      b = possibly variable;
      c = definitely variable
  blank = no test is performed for sources with fewer than 4 total full-band
          counts, or for sources in chip gaps or on field edges.
  

Exposure
The effective exposure time for the X-ray source, in seconds (corrected for telescope vignetting and satellite dithering).

Median_Energy
The background-corrected median photon energy, in keV, of the X-ray source counts in the total (0.5 - 8 keV) band. The derivation of this quantity is given in Townsley et al. (2006, AJ, 131, 2164: Section 3.1).

Log_Bck_Prob
The logarithm of the Kolmogorov-Smirnov (K-S) probability that the background and source spectra are drawn from the same distribution, i.e., that the X-ray source is a background fluctuation rather than a real source. The value of this parameter has been set to -4.0 for sources with significantly different background and source spectra.

Log_NH
The log of the absorbing hydrogen column density NH derived from the spectral fit (see Overview), in H-atoms/cm2. Fitted values with log NH < 20.0 cm-2 are truncated to 20.0, because ACIS-I spectra are insensitive to differences in very low column densities.

Log_NH_Pos_Err
The upper 90%-confidence error in the value of log_nh derived from the spectral fit, in H-atoms/cm2. More significant digits are used for uncertainties < 0.1 in order to avoid large rounding errors; for consistency, the same number of significant digits is used for both lower and upper uncertainties. Uncertainties are missing when XSPEC was unable to compute them or when their values were so large that the parameter is effectively unconstrained.

Log_NH_Neg_Err
The lower 90%-confidence error in the value of log_nh derived from the spectral fit, in H-atoms/cm2. More significant digits are used for uncertainties < 0.1 in order to avoid large rounding errors; for consistency, the same number of significant digits is used for both lower and upper uncertainties. Uncertainties are missing when XSPEC was unable to compute them or when their values were so large that the parameter is effectively unconstrained.

APEC_kT
The value for the APEC plasma temperature kT, in keV, derived from the spectral fit (see Overview). Fitted values with kT > 15 keV are truncated to 15 keV because the data often cannot discriminate between very high temperature values.

APEC_kT_Pos_Err
The upper 90%-confidence error in the value of the APEC plasma temperature, kT, derived from the spectral fit, in keV. More significant digits are used for uncertainties <= 0.1 in order to avoid large rounding errors; for consistency, the same number of significant digits is used for both lower and upper uncertainties. Uncertainties are missing when XSPEC was unable to compute them or when their values were so large that the parameter is effectively unconstrained.

APEC_kT_Neg_Err
The lower 90%-confidence error in the value of the APEC plasma temperature, kT, derived from the spectral fit, in keV. More significant digits are used for uncertainties <= 0.1 in order to avoid large rounding errors; for consistency, the same number of significant digits is used for both lower and upper uncertainties. Uncertainties are missing when XSPEC was unable to compute them or when their values were so large that the parameter is effectively unconstrained.

Log_HB_Lx_Obs
The log of the observed hard-band (2 - 8 keV) luminosity of the X-ray source, in erg/s, derived assuming a distance of 550 pc.

Log_HB_Lx_Obs_Error
The uncertainty in the logarithm of the observed hard-band (2 - 8 keV) luminosity of the X-ray source, in erg/s.

Log_HB_Lx
The log of the absorption-corrected hard-band (2 - 8 keV) luminosity of the X-ray source, in erg/s, derived assuming a distance of 550 pc.

Log_HB_Lx_Error
The uncertainty in the logarithm of the absorption-corrected hard-band (2 - 8 keV) luminosity of the X-ray source, in erg/s.

Log_Lx_Obs
The log of the observed total-band (0.5 - 8 keV) luminosity of the X-ray source, in erg/s, derived assuming a distance of 550 pc.

Log_Lx_Obs_Error
The uncertainty in the logarithm of the observed total-band (0.5 - 8 keV) luminosity of the X-ray source, in erg/s.

Log_Lx
The log of the absorption-corrected total-band (0.5 - 8 keV) luminosity of the X-ray source, in erg/s, derived assuming a distance of 550 pc.

Log_Lx_Error
The uncertainty in the logarithm of the absorption-corrected total-band (0.5 - 8 keV) luminosity of the X-ray source, in erg/s.

NOMAD_Name
The Naval Observatory Merged Astrometric Database (NOMAD) identification of the optical/IR counterpart to the X-ray source. The HEASARC has added the prefix 'NOMAD1' to the numerical part of the name given in the reference paper to conform with the recommendations of the CDS Dictionary of Nomenclature of Celestial Objects. Chandra sources were considered to have stellar counterparts when the positional coincidences were better than 1" within ~3.5' of the ACIS-I field center, and better than 2" in the outer regions of the field where the Chandra PSF deteriorates.

CXO_NOMAD_Offset
The Chandra-NOMAD positional offset, in arcseconds.

MMB2004b_Name
The source identifier from the Maheswar et al. (2004, MNRAS, 355, 1272) BVRI photometric study of the region around CG 12, in the format recommended by the Dictionary of Nomenclature of Celestial Objects, e.g., '[MMB2004b] N-F ', where N is the star number and F is the field number. Chandra sources were considered to have stellar counterparts when the positional coincidences were better than 1" within ~3.5' of the ACIS-I field center, and better than 2" in the outer regions of the field where the Chandra PSF deteriorates.

Vmag
The V magnitude of the optical counterpart to the Chandra X-ray source. When formal statistical errors are given, the V and R magnitudes are from Maheswar et al. (2004). V magnitudes for three B-type stars, 'I 57', 'I 58', and 'I 74', are from Williams et al. (1977, MNRAS, 180, 709). Otherwise, V and R magnitudes are from the NOMAD catalog, in which case all V magnitudes except for the Chandra source 'I 81' are from the YB6 (USNO unpublished) catalog, and all R magnitudes except for the Chandra source 'I 57' are from the USNO-B1.0 catalog (R magnitudes from USNO-B1.0 are second red magnitudes R2, except for the Chandra sources 'I 70' and 'II 9', for which it is the first red magnitude R1). For the Chandra source 'I 81', the V magnitude is from the Tycho-2 catalog, and for the Chandra source 'I 57', the R magnitude is from the UCAC2 catalog.

Vmag_Error
The uncertainty in the V magnitude, where available.

Rmag
The R magnitude of the optical counterpart to the Chandra X-ray source. When formal statistical errors are given, the V and R magnitudes are from Maheswar et al. (2004). V magnitudes for three B-type stars, 'I 57', 'I 58', and 'I 74', are from Williams et al. (1977, MNRAS, 180, 709). Otherwise, V and R magnitudes are from the NOMAD catalog, in which case all V magnitudes except for the Chandra source 'I 81' are from the YB6 (USNO unpublished) catalog, and all R magnitudes except for the Chandra source 'I 57' are from the USNO-B1.0 catalog (R magnitudes from USNO-B1.0 are second red magnitudes R2, except for the Chandra sources 'I 70' and 'II 9', for which it is the first red magnitude R1). For the Chandra source 'I 81', the V magnitude is from the Tycho-2 catalog, and for the Chandra source 'I 57', the R magnitude is from the UCAC2 catalog.

Rmag_Error
The uncertainty in the R magnitude, where available.

Best_PM_RA
The 'best' proper motion in the direction of RA (pmRA * cosDec), in milliarcseconds per year, of the optical counterpart to the Chandra X-ray source, as taken from the catalogs compiled in NOMAD.

Best_PM_RA_Error
The mean error in the 'best' proper motion in the direction of RA, in milliarcseconds per year, of the optical counterpart to the Chandra X-ray source, as taken from the catalogs compiled in NOMAD.

Best_PM_Dec
The 'best' proper motion in the direction of Declination (pmDec), in milliarcseconds per year, of the optical counterpart to the Chandra X-ray source, as taken from the catalogs compiled in NOMAD.

Best_PM_Dec_Error
The mean error in the 'best' proper motion in the direction of Declination, in milliarcseconds per year, of the optical counterpart to the Chandra X-ray source, as taken from the catalogs compiled in NOMAD.

TwoMASS_Name
The 2MASS identification of the optical/IR counterpart to the X-ray source, viz., '2MASS JHHMMSSss+DDMMSSs'. The HEASARC added the standard prefix '2MASS J' to the numerical (position-based) part of the name which was given in Table 4 of the reference paper. Chandra sources were considered to have stellar counterparts when the positional coincidences were better than 1" within ~3.5' of the ACIS-I field center, and better than 2" in the outer regions of the field where the Chandra PSF deteriorates.

CXO_2MASS_Offset
The Chandra-2MASS sources positional offset, in arcseconds.

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

Jmag_Error
The uncertainty in the 2MASS J magnitude of the infared counterpart to the X-ray source.

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

Hmag_Error
The uncertainty in the 2MASS H magnitude of the infared counterpart to the X-ray source.

Ks_Mag
The 2MASS Ks magnitude of the infared counterpart to the X-ray source.

Ks_Mag_Error
The uncertainty in the 2MASS Ks magnitude of the infared counterpart to the X-ray source.

TwoMASS_Flags
The 2MASS 'ph_qual' photometric quality (bytes 1-3) and 'cc_flg' confusion-contamination (bytes 4-6) flags for the J, H and Ks bands, respectively, where, for the quality, 'A'. 'B', 'C' and 'D' indicate decreasing quality of measurements and 'U' means that the quoted value is an upper limit, while the meaning of the cc_flg values (as well as a fuller explanation of ph_qual) is given in the Explanatory Supplement to the 2MASS All Sky Data Release at http://www.ipac.caltech.edu/2mass/releases/allsky/doc/sec2_2a.html .


Contact Person

Questions regarding the CG12CXO database table can be addressed to the HEASARC User Hotline.

Page Author: Browse Software Development Team
Last Modified: 8-Oct-2010