J/ApJ/869/9 Machine-learning investigation of the open cluster M67 (Gao, 2018)
A machine-learning-based investigation of the open cluster M67.
Gao X.
<Astrophys. J., 869, 9-9 (2018)>
=2018ApJ...869....9G 2018ApJ...869....9G (SIMBAD/NED BibCode)
ADC_Keywords: Clusters, open; Proper motions; Colors;
Parallaxes, trigonometric; Photometry, G band
Keywords: methods: data analysis; methods: statistical open clusters and
associations: individual: M67 parallaxes; proper motions
Abstract:
In this paper, we use a machine-learning method, random forest (RF),
to identify reliable members of the old (4Gyr) open cluster M67 based
on the high-precision astrometry and photometry taken from the second
Gaia data release (Gaia-DR2). The RF method is used to calculate
membership probabilities of 71117 stars within 2.5° of the cluster
center in an 11-dimensional parameter space, the photometric data are
also taken into account. Based on the RF membership probabilities, we
obtain 1502 likely cluster members (≥0.6), 1361 of which are
high-probability cluster members (≥0.8). Based on high-probability
memberships with high-precision astrometric data, the mean parallax
(distance) and proper-motion of the cluster are determined to be
1.1327±0.0018mas (883±1pc) and
(<µαcosδ>,<µδ>)=(-10.9378±0.0078,-2.9465
±0.0074)mas/yr, respectively. We find the cluster to have a mean
radial velocity of +34.06±0.09km/s, using 74 high-probability
cluster members with precise radial-velocity measures. We investigate
the spatial structure of the cluster, the core and limiting radius are
determined to be 4.80'±0.11' (∼1.23±0.03pc) and 61.98'±1.50'
(∼15.92±0.39pc), respectively. Our results reveal that an escaped
member with high membership probability (∼0.91) is located at a
distance of 77' (∼20pc) from the cluster center. Furthermore, our
results reveal that at least 26.4% of the main-sequence stars in M67
are binary stars. We confirm that significant mass segregation has
taken place within M67.
Description:
To identify likely cluster members of M67 in a deep and wide field, the
sample of stars was selected on the following selection criteria provided
by the second Gaia data realease (Gaia-DR2; I/345):
- the stars must lie within a radius of 2.5deg from the cluster center
- the stars must have positive parallaxes
- the stars must lie within the proper-motion range of
µαcosδ≤20mas/yr and µδ≤20mas/yr
- the stars must have G-band, BP-band, and RP-band magnitudes
According to these criteria, a sample of 71117 stars with
five-parameter astrometric solutions (positions, parallax, and proper
motions) and three band magnitudes was obtained. These sample stars
allow us to identify not only likely cluster members.
To construct a reliable training set for the random forest (RF) method
(Breiman L. 2001 Machine Learning, 45, 5), an unsupervised clustering
technique was used, and a two-component multivariate Gaussian mixture
model (GMM) (Lee+ 2012 J/MNRAS/424/2832; Gao 2018AJ....156..121G 2018AJ....156..121G), to
perform an effective cluster-field separation (full details about the
GMM method can be found in the Appendix). The GMM clustering analysis
obtained reliable cluster members if only the 7312 sample stars lying
within 60' of the cluster center and within a distance range of
500-1600pc from the Sun were taken into account. The use of the RF
method to segregate likely cluster members from the 71117 sample stars
was also attempted. A training set was constructed, including 1256
high-probability cluster members and 5720 zero-probability field
stars. To identify most likely cluster members, both the five
astrometric and six photometric parameters (G, BP, RP, BP-RP, BP-G,
and G-RP) were taken into account when using the RF method. Based on
the obtained training set, a RF with 7000 decision trees was built to
compute the membership probabilities of the 71117 sample stars in the
11-dimensional parameter space. Each decision tree randomly selected
four parameters from the 11 parameters, and made a definite decision
of whether a sample star is a cluster member or not.
Finally, 1502 likely (PRF≥0.6) and 1361 high-probability (PRF≥0.8)
cluster members were obtained.
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
table3.dat 56 1502 A catalog of fundamental information for 1502
likely cluster members with PRF≥0.6
table4.dat 123 45 *Fundamental parameters of the 45 likely escapers
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Note on table4.dat: We find that 45 high-probability (PRF≥0.8) cluster
members lie outside the tidal radius of the clusters, but only one
star (ID:608111298316195840) with a radial velocity of
+35.13±0.73km/s can be confirmed as likely escapers.
See section 5 for further discussion.
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See also:
I/337 : Gaia DR1 (Gaia Collaboration, 2016)
I/345 : Gaia DR2 (Gaia Collaboration, 2018)
J/AJ/112/628 : BATC Color Survey of M67 (Fan+ 1996)
J/A+A/376/441 : Absolute proper motions of 112 open clusters (Dias+, 2001)
J/A+A/381/464 : Proper motions and membership in NGC 2548 (Wu+, 2002)
J/A+A/388/168 : Absolute proper motions of 93 open clusters (Dias+, 2002)
J/MNRAS/347/101 : Precision colour-magnitude diagram of M67 (Sandquist, 2004)
J/A+A/470/585 : NGC 2682 uvby-Hbeta photometry (Balaguer-Nunez+, 2007)
J/A+A/484/609 : BVI photometry and proper motions in M67 (Yadav+, 2008)
J/A+A/511/A56 : Abundances of five open clusters (Pancino+, 2010)
J/MNRAS/414/2602 : Automated classification of HIP variables (Dubath+, 2011)
J/MNRAS/424/2832 : Pulsars in γ-ray sources (Lee+, 2012)
J/A+A/545/A139 : Radial Velocities of stars observed in M67 (Pasquini+, 2012)
J/A+A/558/A53 : Milky Way survey of star clusters. II. (Kharchenko+,2013)
J/A+A/563/A45 : Pleiades cluster membership probabilities (Sarro+, 2014)
J/A+A/564/A79 : Proper motions of open clusters from UCAC4 (Dias+, 2014)
J/A+A/584/A44 : Photometric classification QSO from RCS-2 (Carrasco+, 2015)
J/AJ/150/97 : Rad-velocities M67. I. 1278 candidate members (Geller+,2015)
J/ApJ/809/161 : X-ray obs. and membership probabilities M37 (Nunez+, 2015)
J/ApJ/813/28 : Autoclassification of variable 3XMM sources (Farrell+, 2015)
J/other/PASA/32.36 : lambda Boo star membership evaluations (Murphy+, 2015)
J/MNRAS/456/3757 : Open-cluster density profiles (Seleznev, 2016)
J/MNRAS/463/696 : M67 solar twins chemical compositions (Liu+, 2016)
J/A+A/601/A19 : Gaia DR1 open cluster members (Gaia Collaboration+, 2017)
J/A+A/603/A85 : M67 stars radial velocities (Brucalassi+, 2017)
J/AJ/156/121 : Catalog 518 likely open cluster NGC6405 members (Gao, 2018)
J/A+A/616/A10 : 46 open clusters GaiaDR2 HR diagrams (Gaia Collab, 2018)
J/A+A/617/A15 : Membership probabilities Pleiades objects (Olivares+, 2018)
J/AJ/158/16 : Identif of RR Lyrae stars from the DES (Stringer+, 2019)
Byte-by-byte Description of file: table3.dat
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Bytes Format Units Label Explanations
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1- 18 I18 --- GaiaDR2 Gaia-DR2 identifier of the likely cluster
members
(<Gaia DR2 NNNNNNNNNNNNNNNNNNN> in Simbad)
20- 34 F15.11 deg RAdeg [130.39/135.3] Gaia DR2 right ascension (J2000)
36- 50 F15.11 deg DEdeg [9.3/14.3] Gaia DR2 declination (J2000)
52- 56 F5.3 --- PRF [0.6/1] RF cluster membership probability
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Byte-by-byte Description of file: table4.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 18 I18 --- GaiaDR2 Gaia-DR2 identifier of the likely cluster
members
(<Gaia DR2 NNNNNNNNNNNNNNNNN> in Simbad)
20- 34 F15.11 deg RAdeg [131.1/135.3] Gaia DR2 right ascension (J2000)
36- 50 F15.11 deg DEdeg [10.29/13.4] Gaia DR2 declination (J2000)
52- 57 F6.4 mas Plx [0.98/1.39] Gaia DR2 parallax (π)
59- 64 F6.4 mas e_Plx [0.02/0.38] Error on Plx
66- 72 F7.3 mas/yr pmRA [-11.7/-10.5] Gaia DR2 proper motion in RA
(µαcosδ)
74- 78 F5.3 mas/yr e_pmRA [0/0.6] Error on pmRA
80- 85 F6.3 mas/yr pmDE [-3.5/-2.4] Gaia DR2 proper motion in DEC
(µδ)
87- 91 F5.3 mas/yr e_pmDE [0/0.5] Error on pmDE
93- 99 F7.4 mag Gmag [12.5/19.2] Gaia DR2 apparent G-band magnitude
101-106 F6.4 mag G-RP [0.35/1.11] Gaia DR2 G-RP color index
108-112 F5.3 --- PRF [0.8/1] RF cluster membership probability
114-118 F5.2 km/s RV [35.13]? Gaia DR2 radial velocity
120-123 F4.2 km/s e_RV [0.73]? Error on RV
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History:
From electronic version of the journal
(End) Prepared by Coralie Fix [CDS] 29-Oct-2019