J/ApJS/168/100 X-ray study of star-forming region NGC 6357 (Wang+, 2007)
An X-ray census of young stars in the massive southern star-forming complex
NGC 6357.
Wang J., Townsley L.K., Feigelson E.D., Getman K.V., Broos P.S.,
Garmire G.P., Tsujimoto M.
<Astrophys. J. Suppl. Ser., 168, 100-127 (2007)>
=2007ApJS..168..100W 2007ApJS..168..100W
ADC_Keywords: Clusters, open ; Stars, pre-main sequence ; X-ray sources ;
Photometry ; Infrared sources
Keywords: ISM: individual (NGC 6357) -
open clusters and associations: individual (Pismis 24) -
stars: formation - stars: luminosity function, mass function -
stars: pre-main-sequence - X-rays: stars
Abstract:
We present the first high spatial resolution X-ray study of the
massive star-forming region NGC 6357, obtained in a 38ks Chandra/ACIS
observation. Inside the brightest constituent of this large HII region
complex is the massive open cluster Pismis 24. It contains two of the
brightest and bluest stars known, yet remains poorly studied; only a
handful of optically bright stellar members have been identified. We
investigate the cluster extent and initial mass function and detect
∼800 X-ray sources with a limiting sensitivity of ∼1030ergs/s; this
provides the first reliable probe of the rich intermediate-mass and
low-mass population of this massive cluster, increasing the number of
known members from optical study by a factor of ∼50.
Description:
NGC 6357 was observed on 2004 July 9 with the Imaging Array of the
Advanced CCD Imaging Spectrometer (ACIS-I) on board Chandra. To
identify infrared counterparts with images that are deeper and of
higher spatial resolution than 2MASS, J, H, and K band images were
obtained on 2005 September 30 using the wide-field NIR SIRIUS
(Simultaneous 3 color Infrared Imager for Unbiased Surveys) camera on
the 1.4m IRSF (Infrared Survey Facility) telescope at the South Africa
Astronomical Observatory.
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
table1.dat 106 665 Main Chandra Catalog: basic source properties
table2.dat 106 114 Tentative source properties
table3.dat 131 40 X-ray spectroscopy for brighter sources: thermal
plasma fits
table4.dat 114 144 X-ray spectroscopy for less bright sources:
thermal plasma fits
table5.dat 90 779 Stellar counterparts and classifications
table6.dat 132 17 Known O and early-B stars in Pismis 24
table8.dat 48 100 X-ray selected candidate intermediate-mass stars
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See also:
B/mk : Catalogue of Stellar Spectral Classifications (Skiff, 2007)
II/246 : 2MASS All-Sky Catalog of Point Sources (Cutri+ 2003)
J/ApJ/574/258 : X-ray-emitting young stars in the Orion nebula
(Feigelson+, 2002)
J/ApJ/575/354 : Young stellar objects in the NGC 1333 (Getman+, 2002)
J/ApJ/614/267 : Chandra sources in Cha I North (Feigelson+, 2004)
Byte-by-byte Description of file: table1.dat table2.dat
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Bytes Format Units Label Explanations
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1- 3 I3 --- [WTF2007] Source sequence number
5- 22 A18 --- CXOUJ Chandra source identification
(HHMMSS.ss+DDMMSS.s)
24- 33 F10.6 deg RAdeg Right Ascension in decimal degrees (J2000)
35- 44 F10.6 deg DEdeg Declination in decimal degrees (J2000)
46- 48 F3.1 arcsec e_pos Positional error
50- 53 F4.1 arcmin theta Off-axis angle
55- 60 F6.1 ct Fcts Net full (0.5-8keV) extracted counts
62- 65 F4.1 ct e_Fcts The 1σ-equivalent uncertainty in Fcts,
based on Poisson statistics
67- 70 F4.1 ct Bkg Net full background extracted counts
72- 76 F5.1 ct Hcts Net hard (2-8keV) extracted counts
78- 81 F4.2 --- PSF Fraction of PSF used for source extraction (1)
83- 86 F4.1 --- Sig Photometric significance
88 A1 --- l_logPB [<] Limit flag on logPB
89- 93 F5.1 [-] logPB Log of the probability that the extracted
full band counts are solely from background
95 A1 --- Anom [geps] Source Anomaly (2)
97 A1 --- Var [abc] Source variability (3)
99-102 F4.1 ks Exp Effective exposure time (4)
104-106 F3.1 keV E Source median energy (5)
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Note (1): Reduced values for this quantity indicate source crowding.
Note (2): Note that a reduced PSF fraction (significantly below 90%) indicates
that the source is in a crowded region. The source anomaly is
defined as follows:
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.
Note (3): No test is performed for sources with fewer than 4 total
full-band counts. No value is reported for sources in chip gaps or on
field edges. The source variability is defined as follows:
a = no evidence for variability;
b = possibly variable;
c = definitely variable.
Note (4): Defined as the amount of exposure time needed for the source to
accumulate the current number of counts if placed at the Chandra aimpoint.
Note (5): In the full spectral band after background subtraction.
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Byte-by-byte Description of file: table3.dat table4.dat
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Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 3 I3 --- [WTF2007] Source sequence number
5- 22 A18 --- CXOUJ Chandra source identification
(HHMMSS.ss+DDMMSS.s)
24- 27 F4.2 [cm-2] e_logN ? Lower limit uncertainty on logN (1)
28 A1 --- l_logN [<] Limit flag on logN
29- 32 F4.1 [cm-2] logN Log of estimated hydrogen column density (2)
34- 37 F4.2 [cm-2] E_logN ? Upper limit uncertainty on logN (1)
39- 42 F4.2 keV e_kT ? Lower limit uncertainty on kT (1)
44 A1 --- l_kT [>] Limit flag on kT
45- 48 F4.1 keV kT Plasma energy (2)
50- 54 F5.2 keV E_kT ? Upper limit uncertainty on kT (1)
56- 59 F4.2 [cm-3] e_logEM ? Lower limit uncertainty on logEM (1)
61- 64 F4.1 [cm-3] logEM Log of the plasma emission measure (2)
66- 69 F4.2 [cm-3] E_logEM ? Upper limit uncertainty on logEM (1)
71- 75 F5.2 [10-7W] logLs Log of the soft (0.5-2keV) band luminosity
(3)
77- 81 F5.2 [10-7W] logLh Log of the hard (2-8keV) band luminosity (3)
83- 87 F5.2 [10-7W] logLhc Log of the absorption-corrected hard band
luminosity (3)
89- 93 F5.2 [10-7W] logLt Log of the total (0.5-8keV) luminosity (3)
95- 99 F5.2 [10-7W] logLtc Log of the total absorption-corrected
luminosity (3)
101-104 F4.2 --- Good ? Goodness of fit χ2ν
106-131 A26 --- Note Additional notes (4)
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Note (1): Uncertainties representing 90% confidence intervals are given;
incomplete confidence levels imply the parameters are not well
constrained, or XSPEC may have encountered some abnormality in
the error calculations.
Note (2): All fits were "wabs(apec)" in XSPEC and assumed 0.3x solar abundances.
Note (3): Luminosities were calculated assuming a distance of 2.56kpc. Units
are in erg/s.
Note (4): Additional notes:
2T = a two-temperature fit was also performed;
VT = variable abundance fit was also performed;
S = means the spectral fit is only used as a spline fit for a rough
estimate of luminosity; actual fit values are unreliable so they have
not been reported. Some confidence intervals are missing since no
error calculation is reported in XSPEC.
P = the fit does not formally satisfy the convergence criterion,
nevertheless the fit is acceptable to estimate the luminosity.
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Byte-by-byte Description of file: table5.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 3 I3 --- [WTF2007] Source sequence number
5- 22 A18 --- CXOUJ Chandra source identification
(HHMMSS.ss+DDMMSS.s)
24 A1 --- 2M [NY] A 2MASS counterpart? (No, Yes or blank) (1)
26 A1 --- SIR [NY] A SIRIUS (near-IR camera on 1.4m IRSF tel.)
counterpart? (No, Yes or blank) (1)
28 A1 --- HST [NY] A HST counterpart? (No, Yes or blank) (1)
30- 32 A3 --- Flag X-ray source flag; see section 5.5 for details
34- 35 I2 h RAh ? 2MASS Hour of Right Ascension (J2000)
37- 38 I2 min RAm ? 2MASS Minute of Right Ascension (J2000)
40- 44 F5.2 s RAs ? 2MASS Second of Right Ascension (J2000)
46 A1 --- DE- Sign of the 2MASS Declination (J2000)
47- 48 I2 deg DEd ? Degree of Declination (J2000)
50- 51 I2 arcmin DEm ? 2MASS Arcminute of Declination (J2000)
53- 56 F4.1 arcsec DEs ? 2MASS Arcsecond of Declination (J2000)
58- 61 F4.2 arcsec Offset ? 2MASS/X-ray offset
63- 68 F6.3 mag J-H ? 2MASS (J-H) color index
70- 75 F6.3 mag H-K ? 2MASS (H-Ks) color index
77- 82 F6.3 mag Kmag ? 2MASS Ks band magnitude
84- 86 A3 --- q2M 2MASS photometry quality flags (2)
88- 90 A3 --- c2M 2MASS confusion and contamination flags (3)
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Note (1): blank if source is out of the field of view of the available images.
Note (2): From Explanatory Supplement to the 2MASS All Sky Data Release
(Cutri et al. 2003, Cat. II/246). 2MASS JHKs photometry quality
flags with three characters:
A = very high significance detection (>10SNR);
B = high significance detection (>7SNR);
C = moderate significance detection (>5SNR);
D = low significance detection;
E = point spread fitting poor;
F = reliable photometric errors not available;
U = upper limit on magnitude (source not detected);
X = source is detected but no valid brightness estimate is available.
Note (3): From Explanatory Supplement to the 2MASS All Sky Data Release
(Cutri et al. 2003, Cat. II/246). 2MASS JHKs confusion and
contamination flags with three characters:
0 = source unaffected by artifacts;
b = bandmerge confusion (possible multiple source);
c = photometric confusion from nearby star;
d = diffraction spike confusion from nearby star;
p = persistence contamination from nearby star;
s = electronic stripe from nearby star.
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Byte-by-byte Description of file: table6.dat
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Bytes Format Units Label Explanations
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1- 9 A9 --- Name Source name (1)
10 A1 --- f_SpType [i] Tentative spectral type (2)
12- 22 A11 --- SpType MK spectral type (3)
24- 25 I2 h RAh Hour of Right Ascension (J2000.0) (4)
27- 28 I2 min RAm Minute of Right Ascension (J2000.0) (4)
30- 34 F5.2 s RAs Second of Right Ascension (J2000.0) (4)
36 A1 --- DE- Sign of the Declination (J2000.0) (4)
37- 38 I2 deg DEd Degree of Declination (J2000.0) (4)
40- 41 I2 arcmin DEm Arcminute of Declination (J2000.0) (4)
43- 46 F4.1 arcsec DEs Arcsecond of Declination (J2000.0) (4)
48- 52 F5.2 mag Vmag V magnitude (3)
54- 57 F4.2 mag U-B ? U-B color index (3)
58- 59 A2 --- u_U-B [::] Uncertainty flag on U-B
61- 64 F4.2 mag B-V B-V color index (3)
66- 69 F4.2 mag V-J V-J color index (5)
71- 74 F4.2 mag J-H J-H color index (5)
76- 79 F4.2 mag H-Ks H-Ks color index (5)
81- 85 F5.2 mag Ksmag 2MASS Ks magnitude (5)
87- 90 F4.2 mag AV Extinction (6)
92- 94 A3 --- Seq Sequence (7)
96-100 F5.2 [10-7W] logLX ? Log of absorption-corrected full-band
luminosity Lt,c (8)
102-105 F4.1 [cm-2] logN ? Log of X-ray-derived column density
107 A1 --- l_logLbol [>] Upper limit flag on logLbol
108-112 F5.2 [10-7W] logLbol ? Log of the bolometric luminosity (8)
114-132 A19 --- Notes Notes (9)
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Note (1): Identifications from Moffat & Vogt (1973A&AS...10..135M 1973A&AS...10..135M) and
Massey et al. (2001AJ....121.1050M 2001AJ....121.1050M).
Note (2): Not included in Massey et al. (2001AJ....121.1050M 2001AJ....121.1050M) spectroscopic
classification sample. Tentative spectral types are inferred from
color-magnitude diagram by dereddening 2MASS photometric colors to the
1Myr pre-MS isochrone.
Note (3): Spectral types from Massey et al. (2001AJ....121.1050M 2001AJ....121.1050M) and
Walborn et al. (2002AJ....123.2754W 2002AJ....123.2754W). Optical photometry from Massey
et al. (2001AJ....121.1050M 2001AJ....121.1050M). Bolometric luminosities from Massey et
al. (2001AJ....121.1050M 2001AJ....121.1050M) are recalculated using bolometric
corrections from Heap et al. (2006ApJ...638..409H 2006ApJ...638..409H) instead of
Vacca et al. (1996ApJ...460..914V 1996ApJ...460..914V).
Note (4): Coordinates are from the Catalogue of Stellar Spectral
Classifications (Skiff 2005, Cat. B/mk) and Massey et al.
(2001AJ....121.1050M 2001AJ....121.1050M).
Note (5): Infrared photometry from 2MASS (Cat. II/246).
Note (6): Visual extinction from Bohigas et al. (2004AJ....127.2826B 2004AJ....127.2826B)
AV=1.39E(V-J).
Note (7): ACIS source number for X-ray detections.
Note (8): In units of ergs/s.
Note (9): Other identifications from Neckel (1978A&A....69...51N 1978A&A....69...51N).
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Byte-by-byte Description of file: table8.dat
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Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 3 I3 --- [WTF2007] Source sequence number
5- 22 A18 --- CXOUJ Chandra source identification
(HHMMSS.ss+DDMMSS.s)
24- 28 F5.2 mag Jmag 2MASS J band magnitude
30- 34 F5.2 mag Hmag 2MASS H band magnitude
36- 40 F5.2 mag Ksmag 2MASS Ks band magnitude
42- 43 I2 mag AV The V band extinction (1)
45- 46 I2 solMass Mass Estimated mass (1)
48 A1 mag DelK [s] Indicates significant Ks band excess (2)
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Note (1): From de-reddened location along the standard interstellar
reddening vector to the 1 Myr pre-MS isochrone in the 2MASS
color-magnitude diagram, assuming a distance ∼2.56kpc and a cluster
age of 1Myr. For early B stars are especially dependent on the age and
distance assumptions (Figure 7).
Note (2): Significant Ks-band excess derived from high quality 2MASS
photometry where near-IR color excess E(H-K) is larger than
σ(H-K), the uncertainty in H-K color index.
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History:
From electronic version of the journal
(End) Greg Schwarz [AAS], Marianne Brouty [CDS] 15-Oct-2008