J/ApJ/787/109 Age estimates for NGC 2024 and ONC stars (Getman+, 2014)
Core-halo age gradients and star formation in the Orion nebula and NGC 2024
young stellar clusters.
Getman K.V., Feigelson E.D., Kuhn M.A.
<Astrophys. J., 787, 109 (2014)>
=2014ApJ...787..109G 2014ApJ...787..109G (SIMBAD/NED BibCode)
ADC_Keywords: Clusters, open ; Stars, ages ; Stars, distances ; X-ray sources
Keywords: stars: early-type - stars: formation - stars: pre-main sequence
Abstract:
We analyze age distributions of two nearby rich stellar clusters, the
NGC 2024 (Flame Nebula) and Orion Nebula cluster (ONC) in the Orion
molecular cloud complex. Our analysis is based on samples from the
MYStIX survey and a new estimator of pre-main sequence (PMS) stellar
ages, AgeJX, derived from X-ray and near-infrared photometric data.
To overcome the problem of uncertain individual ages and large spreads
of age distributions for entire clusters, we compute median ages and
their confidence intervals of stellar samples within annular
subregions of the clusters. We find core-halo age gradients in both
the NGC 2024 cluster and ONC: PMS stars in cluster cores appear
younger and thus were formed later than PMS stars in cluster
peripheries. These findings are further supported by the spatial
gradients in the disk fraction and K-band excess frequency. Our age
analysis is based on AgeJX estimates for PMS stars and is
independent of any consideration of OB stars. The result has important
implications for the formation of young stellar clusters. One basic
implication is that clusters form slowly and the apparent age spreads
in young stellar clusters, which are often controversial, are (at
least in part) real. The result further implies that simple models
where clusters form inside-out are incorrect and more complex models
are needed. We provide several star formation scenarios that alone or
in combination may lead to the observed core-halo age gradients.
Description:
The MYStIX project has produced rich stellar samples of 31784
probable members of 20 OB-dominated young star-forming regions at
distances 0.4-3.6 kpc using sensitive X-ray, NIR, and mid-infrared
photometric catalogs (Feigelson et al. 2013, J/ApJS/209/26). Based on
the MYStIX data, G14 developed a new PMS stellar age estimator, AgeJX.
This estimator is based on J-band stellar photospheric emission and on
X-ray emission from coronal magnetic flaring, and relies on an
assumption of a nearly universal LX-M relationship for ~<5 Myr old PMS
stars (Section 3.1 in G14).
Objects:
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RA (ICRS) DE Designation(s)
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05 35 17.3 -05 23 28 Orion Nebula cluster = M 42
05 41 43 -01 50.5 NGC 2024 = [BDB2003] G206.47-16.32
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File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
table1.dat 76 512 AgeJX Estimates for NGC 2024 and ONC Stars
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See also:
J/ApJ/582/382 : Chandra X-ray sources in Orion Nebula Cluster
(Flaccomio+, 2003)
J/ApJ/598/375 : Chandra X-ray observations of NGC 2024 (Skinner+, 2003)
J/ApJS/160/319 : COUP: observations and source lists (Getman+, 2005)
J/ApJS/160/353 : Membership of the Orion nebula population (Getman+, 2005)
J/ApJS/160/450 : X-ray emission in Orion Nebula young stars (Flaccomio+, 2005)
J/ApJ/646/1215 : Low-mass stars and brown dwarfs in NGC 2024 (Levine+, 2006)
J/ApJ/671/605 : NGC 2264 and ONC PMS stars in the infrared (Cieza+, 2007)
J/ApJS/209/27 : MYStIX: the Chandra X-ray sources (Kuhn+, 2013)
J/ApJS/209/30 : MYStIX project: Bayesian matching (Naylor+, 2013)
J/ApJS/209/31 : The MYStIX IR-Excess Source catalog (MIRES) (Povich+, 2013)
J/ApJS/209/32 : Probable young stars in the MYStIX project (Broos+, 2013)
J/ApJ/787/107 : Spatial structure of young stellar clusters. I. (Kuhn+, 2014)
J/ApJ/787/108 : Age estimates for massive SFR stellar populations
(Getman+, 2014)
Byte-by-byte Description of file: table1.dat
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Bytes Format Units Label Explanations
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1- 8 A8 --- CID Cluster identifier (NGC 2024, ONC)
10- 27 A18 --- MPCM MYStIX source IAU designation
(HHMMSS.ss+DDMMSS.s)
29- 37 F9.6 deg RAdeg Right Ascension in decimal degrees (J2000)
39- 47 F9.6 deg DEdeg Declination in decimal degrees (J2000)
49- 52 F4.2 pc R Angular distance from cluster center
54- 58 F5.2 [10-7W] logLX Log intrinsic 0.5-8 keV X-ray luminosity (1)
60- 62 F3.1 Myr Age Age estimate from Getman et al.
2014ApJ...787..108G 2014ApJ...787..108G (2)
64- 67 A4 --- DFlag Disk flag (3)
69- 76 A8 --- AFlag Annular region stratum (4)
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Note (1): In units of erg/s and from Broos et al. (2013, J/ApJS/209/32).
Note (2): Individual age estimates of 5 Myr indicate that ages run into the
truncation limit of 5 Myr (Getman et al. 2014ApJ...787..108G 2014ApJ...787..108G).
Note (3): For NGC 2024: 'DSK' and 'NOD' - disk-bearing and disk-free young
stellar objects; the classification is adopted from the MIRES catalog
of Povich et al. (2013, J/ApJS/209/31) based on NIR and Spitzer IRAC
photometry. For ONC: 'Ks' and 'noKs' - PMS stars with and without
Ks-excess (Section 3.4); the classification is adopted from
Getman et al. (2005, J/ApJS/160/319) based on NIR photometry.
Note (4): In units of core radii from Figure 1.
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History:
From electronic version of the journal
(End) Prepared by [AAS], Tiphaine Pouvreau [CDS] 29-Jun-2017