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CR261OID - Collinder 261 Chandra X-Ray Source Optical Counterparts Catalog

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Overview

This table contains some of the results from the first X-ray study of Collinder 261 (Cr 261), which at an age of 7 Gyr is one of the oldest open clusters known in the Galaxy. This observation with the Chandra X-Ray Observatory was aimed at uncovering the close interacting binaries in Cr 261, and reached a limiting X-ray luminosity of LX ~ 4 x 1029 erg s-1 (0.3-7 keV) for stars in the cluster. The authors detected 107 sources within the cluster half-mass radius rh, and they estimate that among the sources with LX >~ 1030 erg s-1, about 26 are associated with the cluster. They identify a mix of active binaries and candidate active binaries, candidate cataclysmic variables, and stars that have "straggled" from the main locus of CR 261 in the color-magnitude diagram. Based on a deep optical source catalog of the field, the authors estimate that Cr 261 has an approximate mass of 6500 Msun, roughly the same as the old open cluster NGC 6791. The X-ray emissivity of Cr 261 is similar to that of other old open clusters, supporting the trend that they are more luminous in X-rays per unit mass than old populations of higher (globular clusters) and lower (the local neighborhood) stellar density. This implies that the dynamical destruction of binaries in the densest environments is not solely responsible for the observed differences in X-ray emissivity.

Cr 261 was observed with the Advanced CCD Imaging Spectrometer (ACIS) on board Chandra starting 2009 November 9 14:50 UTC, for a total exposure time of 53.8 ks (ObsID 11308). The observation was made in Very Faint, Timed exposure mode, with a single frame exposure time of 3.2 s. Kharchenko et al. (2013, A&A, 558, A53) estimate that the radius of Cr 261 is ~ 14.1 arcminutes. This is considerably larger than a single ACIS chip (8 4 x 8 4 arcminute2) and therefore the authors placed the center of the cluster (J2000.0 RA = 12h 38m 06.0s, Dec = -68o 22' 01" according to Kharchenko et al. 2013) close to the I3 aimpoint so that a larger contiguous part of the cluster could be imaged (see Figure 1 in the reference paper). The CCDs used were I0, I1, I2, and I3 from the ACIS-I array, and S2 and S3 from the ACIS-S array. The authors limited the X-ray analysis to the data from chips I0, I1, I2, and I3. The S2 and S3 chips lie far from the I3 aimpoint, giving rise to large positional errors on any sources detected on them. Such large errors make it hard to identify optical counterparts, and thus to classify the sources.

The authors retrieved optical images of Cr 261 in the B and V bands from the ESO public archive. These data were taken as part of the ESO Imaging Survey (EIS; program ID 164.O-0561). The observations of Cr 261 were made using the Wide Field Imager (WFI), mounted on the 2.2 m MPG/ESO telescope at La Silla, Chile. After correcting the X-ray source positions for the (almost negligible) boresight correction (0.06 =/- 0.07 arcseconds in RA and 0.09 +/- 0.08 arcseconds in Dec), the authors matched their X-ray source list with the entire optical source list, using 95% match radii. For 89 unique X-ray sources, they found 124 optical matches; of the latter, 104 are present in both the V and B images, while for 20 there is only a V or B detection. The authors also inspected the area around each X-ray source in the WFI images by eye, and discovered that five more X-ray sources have candidate optical counterparts that are saturated and therefore missing from their optical catalog. Finally, they added to the list of candidate counterparts six optical sources that lay just outside the 95% match radius, but inside the 3-sigma radius. In total, 98 of the 151 unique X-ray sources were thus matched to one or more optical sources.

This HEASARC table contains the list of the 135 optical counterparts to 98 of the 151 X-ray sources from Table 2 of the reference paper. Information about the 151 X-ray sources is available in the HEASARC table CR261CXO (based on Table 1 of the reference paper) to which this current table has links.


Catalog Bibcode

2017ApJ...837..130V

References

A Chandra X-ray census of the interacting binaries in old open clusters -
Collinder 261
     Vats S., van den Berg M.
    <Astrophys. J. 837, 130 (2017)>
    =2017ApJ...837..130V      (SIMBAD/NED BibCode)

Provenance

This table was created by the HEASARC in June 2017 primarily based upon the machine-readable version of Table 2 from the reference paper, the catalog of optical counterparts to Chandra sources in Cr 261, that was obtained from the ApJ web site. The information on the X-ray source positions was taken from the machine-readable version of Table 1 from the reference paper that was also obtained from the ApJ web site.

Parameters

Source_Number
A unique sequential identification (CX) number for each Chandra X-ray source in the catalog, sorted in order of decreasing X-ray counts in the broad energy band (0.3 - 7 keV), i.e., the lower the source number, the more counts.

Name
The name of the X-ray source using the 'CXOU' prefix (for Chandra X-ray Observatory Unregistered source) and the J2000.0 coordinates of the source in the standard "JHHMMSS.s+DDMMSS" format.

RA
The Right Ascension of the X-ray source in the selected equinox as found by wavdetect, i.e., not of the optical counterpart. This was given in J2000 decimal degree coordinates to a precision of 10-6 degrees in the original table.

Dec
The Declination of the X-ray source in the selected equinox as found by wavdetect, i.e., not of the optical counterpart. This was given in J2000 decimal degree coordinates to a precision of 10-6 degrees in the original table.

LII
The Galactic Longitude of the X-ray source, i.e., not of the optical counterpart.

BII
The Galactic Latitude of the X-ray source, i.e., not of the optical counterpart.

Optical_ID
The identification number of the optical counterpart to the Chandra X-ray source, taken from the authors' optical source catalog, except for five X-ray sources, viz. CX 7, CX 30, CX 40, CX 100, and CX 129, for which the IDs are their UCAC4 catalog IDs. These stars were saturated in the optical images used by the authors to construct their source catalog, and for these 5 stars their photometry was obtained from the UCAC4 catalog.

Optical_Id_Flag
This parameter can contain flags with values as indicated below containing notes on the optical counterparts:

      a = photometry of the source may be dubious due to image artifacts
          (CX81, CX65) or due to low S/N ratio (CX2, CX24, CX121);
      c = the optical counterpart lies just outside the 95% match radius,
          but within the 3{sigma} match radius.
  

Offset
The angular offset, in arcseconds, between the X-ray source and the specified optical counterpart.

Vmag
The optical (usually V-band) magnitude of the counterpart to the Chandra X-ray source (taken in most cases from the authors.optical catalog of Cr 26), except in a few cases (indicated by v_mag_flag values of 'b') where the B-band magnitude of the optical counterpart is given.

Vmag_Flag
This flag parameter can have non-blank values indicating the following notes concerning the quoted optical magnitude of the optical counterpart:

      b = The quoted magnitude is a B magnitude.
      d = The value was obtained from Mazur et al. (1995, MNRAS, 273, 59).
  

BV_Color
The (B-V) color of the counterpart to the Chandra X-ray source.

BV_Color_Flag
This flag parameter can have non-blank values indicating the following notes concerning the quoted B-V color of the optical counterpart:

      d = The value was obtained from Mazur et al. (1995, MNRAS, 273, 59).
      e = The magnitude is a B Magnitude obtained from the USNO B1.0 catalog
          (Monet et al. 2003, AJ, 125, 984).
  

Mazur_Source_Number
The short-period binary identifier of the optical counterpart to the Chandra source given by Mazur et al. (1995, MNRAS, 273, 59).

Orbital_Period
The orbital period of the short-period binary counterpart to the Chandra source, in days, as given by Mazur et al. (1995, MNRAS, 273, 59).

Orbital_Period_Flag
This flag parameter is set to '?' to indicate that the quoted orbital period of the short-period binary counterpart is uncertain or not known (if no orbital period value is given).

Var_Type
The variable type of the optical counterpart, if it is a short-period binary, as mentioned in Mazur et al. (1995, MNRAS, 273, 59): EA (Algol-type), EB (beta Lyr-type) or EW (contact binary type).

Lx
The unabsorbed 0.3-7 keV X-ray luminosity of the Chandra source, in erg s-1, assuming the source lies at the distance of the cluster, viz. 2.5 kpc.

Log_Fx_Over_Fv
The logarithm of the ratio of the unabsorbed X-ray (0.3-7 keV) flux to the V-band flux of the specified optical counterpart, assuming the X-ray spectrum, is that of a 2 keV MeKaL model and a neutral hydrogen column of 1.9 x 1021 cm-2.

Broad_Type
The broad object classification of the counterpart to the X-ray source, as follows:

       CV? = Candidate cataclysmic variable;
     AB(?) = Active binary (candidate);
       SSG = Sub-subgiant;
       BSS = Blue straggler star;
       YSS = Yellow straggler star;
       Unc = Uncertain classification;
        NM = Non-member.
  

Class
The HEASARC Browse object classification of the counterpart to the X-ray source, based on the var_type parameter value, if given, else on the broad_type parameter value. Notice that all objects with broad_type values of 'AB' for active binary has been given class values of 'RS CVn'.


Contact Person

Questions regarding the CR261OID database table can be addressed to the HEASARC User Hotline.

Page Author: Browse Software Development Team
Last Modified: 15-Jun-2017