J/AJ/169/116 Gaia & 2MASS fitting for Pleiades members (Liu+, 2025)
Photometric determination of unresolved main-sequence binaries in the Pleiades:
Binary fraction and mass-ratio distribution.
Liu R., Shao Z., Li Lu
<Astron. J., 169, 116 (2025)>
=2025AJ....169..116L 2025AJ....169..116L
ADC_Keywords: Stars, double and multiple; Stars, masses; Clusters, open;
Photometry; Optical; Infrared; Magnitudes
Keywords: Open star clusters ; Binary stars ; Mass ratio ; Bayesian statistics
Abstract:
Accurate determination of binary fractions (fb) and mass ratio (q)
distributions is crucial for understanding the dynamical evolution of
open clusters. We present an improved multiband fitting technique to
enhance the analysis of binary properties. This approach enables an
accurate photometric determination of fb and q distribution in a
cluster. The detectable mass ratio can be down to the
qlim, limited by the minimum stellar mass in theoretical
models. First, we derived an empirical model for magnitudes of Gaia
DR3 and 2MASS bands that match the photometry of single stars in the
Pleiades. We then performed a multiband fitting for each cluster
member, deriving the probability density function (PDF) of its primary
mass (M1) and q in the Bayesian framework. 1154
main-sequence (MS) single stars or unresolved MS+MS binaries are
identified as members of the Pleiades. By stacking their PDFs, we
conducted a detailed analysis of binary properties of the cluster. We
found the fb of this sample is 0.34±0.02. The q distribution
exhibits a three-segment power-law profile: an initial increase,
followed by a decrease, and then another increase. This distribution
can be interpreted as a fiducial power-law profile with an exponent of
-1.0 that is determined in the range of 0.3<q<0.8, but with a
deficiency of binaries at lower q and an excess at higher q. The
variations of fb and q with M1 reveal a complex
binary distribution within the Pleiades, which might be attributed to
a combination of primordial binary formation mechanisms, dynamical
interactions, and the observational limit of photometric binaries
imposed by qlim(M1).
Description:
The Pleiades cluster is centered on α=56.601°,
δ=24.114°, with a distance ranging from 130 to 140pc
(Southworth+ 2005A&A...429..645S 2005A&A...429..645S; Melis+ 2014Sci...345.1029M 2014Sci...345.1029M) and a
half-number radius of ∼1.37° (approximately 3pc). First, sample
stars were restricted to G<19mag and located within a radius of
8.9° from the Pleiades center. For this flux-limited subset,
∼98.9% of Gaia DR3 sources possess a five-parameter astrometric
solution. We further restricted the sample to parallaxes between
6.1 and 8.6mas. This resulted in an initial sample of 6041 stars.
Notably, this sample has not been constrained by the renormalized unit
weight error (RUWE) value, because numerous unresolved binaries are
among stars with RUWE>1.4 (Table 1).
There are 97.6% sample stars having Pk>0.9 or Pk<0.1. Only a small
fraction of objects remain ambiguous. The effectiveness index of this
kinematic membership determination is Ek=0.97, which is very close
to a complete separation of members and field stars. A total of
1390 stars having Pk≥0.5 were chosen for subsequent analysis, with an
average Pk value of 0.978 (Table 4).
We assumed that the distance modulus of the Pleiades is fixed at
DMc=5.65mag (corresponding to the average modified parallax of
kinematic members: ω=7.41mas), the metallicity is fixed at
[Fe/H]=0.1 (Fu+2022, J/A+A/668/A4), and the binary mass-ratio
distribution follows a power law with exponent γq=0. Based on
these assumptions, the best-fit values for the cluster age and
extinction are log(Age/yr)=8.026 and AV=0.135mag from the MiMO
algorithm (Li+, 2022ApJ...930...44L 2022ApJ...930...44L). Then, we downloaded the model
magnitudes with these fixed or fitted parameters from the website of
the PARSEC, and defined it as the fiducial photometric model for the
Pleiades. It offers 476 sets of model magnitudes as functions of
stellar mass from 0.09 to 5.2M☉. Among these, 112 sets in the
range of 0.09-4.9M☉ were used in multiband fitting for stellar
mass and mass ratio of cluster members (Table 3).
Objects:
-------------------------------------------------
RA (2000) DE Designation(s)
-------------------------------------------------
03 46 37.99 +24 10 41.0 Pleiades = C 0344+239
-------------------------------------------------
File Summary:
--------------------------------------------------------------------------------
FileName Lrecl Records Explanations
--------------------------------------------------------------------------------
ReadMe 80 . This file
table1.dat 161 6041 Observational data and kinematic membership
probabilities of stars in the region of the
Pleiades
table3.dat 90 112 The fiducial photometric model and its correction
of the Gaia and 2MASS bands for the Pleiades
table4.dat 91 1390 Results of multiband fitting for stars with
Pk≥0.5
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See also:
I/357 : Gaia DR3 Part 3. Non-single stars (Gaia Collaboration, 2022)
II/246 : 2MASS All-Sky Catalog of Point Sources (Cutri+ 2003)
VII/233 : 2MASS All-Sky Extended Src Cat. (XSC) (IPAC/UMass, 2003-2006)
J/A+A/323/139 : K magnitude of Pleiades low-mass binaries (Bouvier+ 1997)
J/MNRAS/342/1241 : Low-mass stars in M45 and M44 (Pinfield+, 2003)
J/MNRAS/380/712 : ZYJHK photometry in Pleiades (Lodieu+, 2007)
J/A+A/615/A24 : Passband reconstruction from photometry (Weiler+, 2018)
J/A+A/622/A110 : Spectroscopic membership for NGC 3532 (Fritzewski+, 2019)
J/A+A/628/A66 : alpha Persei, Pleiades and Praesepe clusters (Lodieu+, 2019)
J/MNRAS/489/3093 : Gaia DR2 parallax of globular clusters (Shao+, 2019)
J/A+A/633/A99 : Gaia DR2 open clusters in the MW. II (Cantat-Gaudin+, 2020)
J/ApJ/901/91 : Sp. obs. of rapidly rot. stars in the Pleiades (Torres, 2020)
J/A+A/649/A3 : Gaia Early Data Release 3 phot. passbands (Riello+, 2021)
J/AJ/161/160 : BINOCS: NEWFIRM & IRAC IR phot. of clusters (Thompson+, 2021)
J/ApJ/921/117 : 43yr spectrosc. monitoring of Pleiades region (Torres+, 2021)
J/A+A/668/A4 : LAMOST open clusters (Fu+, 2022)
J/A+A/672/A29 : 78 Galactic Open cluster structural param. (Cordoni+, 2023)
J/A+A/675/A89 : Multiplicity fractions in Gaia open clusters (Donada+, 2023)
J/A+A/673/A114 : Improving the open cluster census. II. (Hunt+, 2023)
J/AJ/165/45 : (H-W2)-W1 vs W2-(BP-K) for 4 open clust. memb. (Malofeeva+, 2023)
J/AJ/168/156 : Pleiades members from Gaia DR3 (Chulkov+, 2024)
Byte-by-byte Description of file: table1.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 18 I18 --- GaiaDR3 Gaia DR3 (I/355) identifier
20- 25 F6.3 deg RAdeg [46/67] Gaia DR3 right ascension
(ICRS at Epoch=J2016.0)
27- 32 F6.3 deg DEdeg [15/33] Gaia DR3 declination
(ICRS at Epoch=J2016.0)
34- 40 F7.2 mas/yr pmRA [-160/612] Gaia DR3 proper motion along RA
42- 45 F4.2 mas/yr e_pmRA [0.01/1.3] Uncertainty in pmRA
47- 53 F7.2 mas/yr pmDE [-529/84] Gaia DR3 proper motion along DE
55- 58 F4.2 mas/yr e_pmDE [0.01/0.9] Uncertainty in pmDE
60- 63 F4.2 mas plx [6.14/8.6] Gaia DR3 parallax
65- 68 F4.2 mas e_plx [0.01/1.3] Uncertainty in plx
70- 75 F6.3 --- RUWE [0.6/54.5] Gaia DR3 renormalized unit weight
error
77- 81 F5.3 --- Pk [0/1] Kinematic membership probability
83- 87 F5.3 mag m-M [5.3/6.1] Distance modulus
89- 94 F6.3 mag BPmag [3.5/21.7]? Gaia DR3 blue passband apparent
magnitude
96- 100 F5.3 mag e_BPmag [3e-3/0.8]? Uncertainty in BPmag
102- 107 F6.3 mag Gmag [3.6/19] Gaia DR3 G band apparent magnitude
109- 113 F5.3 mag e_Gmag [3e-3/0.04] Uncertainty in Gmag
115- 120 F6.3 mag RPmag [3.6/19]? Gaia DR3 red passband apparent
magnitude
122- 126 F5.3 mag e_RPmag [4e-3/0.81]? Uncertainty in RPmag
128- 132 F5.2 mag Jmag [3.1/16.9]? 2MASS (II/246) J band apparent
magnitude
134- 137 F4.2 mag e_Jmag [0.02/0.3]? Uncertainty in Jmag
139- 143 F5.2 mag Hmag [2.5/16.5]? 2MASS (II/246) H band apparent
magnitude
145- 149 F5.2 mag e_Hmag [0.01/10]? Uncertainty in Hmag
151- 155 F5.2 mag Ksmag [2.2/16.2]? 2MASS (II/246) Ks band apparent
magnitude
157- 161 F5.2 mag e_Ksmag [0.01/10]? Uncertainty in Ksmag
--------------------------------------------------------------------------------
Byte-by-byte Description of file: table3.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 6 F6.4 Msun Ms [0.09/4.9] Initial stellar mass
8- 13 F6.3 mag BPmag [-2/16.7] Gaia DR3 (I/355) blue passband
magnitude (1)
15- 20 F6.3 mag Gmag [-2/13.9] Gaia DR3 (I/355) G band magnitude (1)
22- 27 F6.3 mag RPmag [-2/12.4] Gaia DR3 (I/355) red passband
magnitude (1)
29- 34 F6.3 mag Jmag [-1.9/9.7] 2MASS (II/246) J band magnitude (1)
36- 41 F6.3 mag Hmag [-1.8/9.1] 2MASS (II/246) H band magnitude (1)
43- 48 F6.3 mag Ksmag [-1.8/8.8] 2MASS (II/246) Ks band magnitude (1)
50- 55 F6.3 mag DelBP [-0.7/0.05] Gaia DR3 blue passband correction
57- 62 F6.3 mag DelG [-0.04/0.03] Gaia DR3 G passband correction
64- 69 F6.3 mag DelRP [-0.04/0.04] Gaia DR3 red passband correction
71- 76 F6.3 mag DelJ [-0.12/0.51] 2MASS J band correction
78- 83 F6.3 mag DelH [-0.11/0.51] 2MASS H band correction
85- 90 F6.3 mag DelKs [-0.11/0.44] 2MASS Ks band correction
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Note (1): From the PARSEC1.2s code with [Fe/H]=0.1, log(Age)/yr=8.026
and AV=0.135 mag.
--------------------------------------------------------------------------------
Byte-by-byte Description of file: table4.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 18 I18 --- GaiaDR3 Gaia DR3 (I/355) identifier
20- 25 F6.4 Msun M1 [0.09/4.9]? Best-fit primary mass value
27- 31 F5.3 --- q [0/1]? Best-fit mass ratio value; M2/M1
33- 38 F6.4 Msun M1_16 [0.09/4.81]? The 16% quantile for the primary
mass
40- 45 F6.4 Msun M1_50 [0.09/4.9]? The 50% quantile for the primary
mass
47- 52 F6.4 Msun M1_84 [0.09/4.9]? The 84% quantile for the primary
mass
54- 58 F5.3 --- q16 [0.01/0.98]? The 16% quantile for the mass
ratio
60- 64 F5.3 --- q50 [0.03/0.99]? The 50% quantile for the mass
ratio
66- 70 F5.3 --- q84 [0.06/1]? The 84% quantile for the mass ratio
72- 76 F5.3 --- Pb [0/1]? Probability of a star being a binary (1)
78- 91 A14 --- Flag Exclusion flag (s) (2)
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Note (1): Derived from the probability density function.
Note (2): Indicates the criteria used for excluding field stars and
non-main-sequence members.
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History:
From electronic version of the journal
(End) Prepared by [AAS], Robin Leichtnam [CDS] 05-Dec-2025