J/ApJ/751/4 Radial velocities of massive stars in Cyg OB2 (Kiminki+, 2012)
An updated look at binary characteristics of massive stars in the
Cygnus OB2 association.
Kiminki D.C., Kobulnicky H.A.
<Astrophys. J., 751, 4 (2012)>
=2012ApJ...751....4K 2012ApJ...751....4K
ADC_Keywords: Stars, early-type ; Radial velocities ; Associations, stellar ;
Binaries, spectroscopic
Keywords: binaries: close - binaries: general - binaries: spectroscopic -
stars: early-type - stars: kinematics and dynamics -
techniques: radial velocities
Abstract:
This work provides a statistical analysis of the massive star binary
characteristics in the Cygnus OB2 association using radial velocity
information of 114 B3-O5 primary stars and orbital properties for the
24 known binaries. We compare these data to a series of Monte Carlo
simulations to infer the intrinsic binary fraction and distributions
of mass ratios, periods, and eccentricities. We model the distribution
of mass ratio, log-period, and eccentricity as power laws and find
best-fitting indices of α=0.1±0.5, β=0.2±0.4, and
γ=-0.6±0.3, respectively. These distributions indicate a
preference for massive companions, short periods, and low
eccentricities. Our analysis indicates that the binary fraction of the
cluster is 44%±8% if all binary systems are (artificially) assumed
to have P<1000 days; if the power-law period distribution is
extrapolated to 104 years, then a plausible upper limit for bound
systems, the binary fraction is ∼90%±10%. Of these binary (or higher
order) systems, ∼45% will have companions close enough to interact
during pre- or post-main-sequence evolution (semi-major axis ≲4.7AU).
The period distribution for P<26 days is not well reproduced by any
single power law owing to an excess of systems with periods around
3-5 days (0.08-0.31AU) and a relative shortage of systems with periods
around 7-14 days (0.14-0.62AU). We explore the idea that these
longer-period systems evolved to produce the observed excess of
short-period systems. The best-fitting binary parameters imply that
secondaries generate, on average, ∼16% of the V-band light in young
massive populations. This means that photometrically based distance
measurements for young massive clusters and associations will be
systematically low by ∼8% (0.16 mag in the distance modulus) if the
luminous contributions of unresolved secondaries are not taken into
account.
Description:
We obtained radial velocities for a total of 146 OB stars over 12
years. These 146 (given in Table 1 of Paper I, Cat. J/ApJ/664/1102)
were chosen from the UBV photometric and spectroscopic survey of
CygOB2 by Massey & Thompson (1991, Cat. J/AJ/101/1408). Nearly half of
the stars were previously identified as an OB-type and the other half
had colors consistent with classification of B3 or earlier. In this
work, we utilize 110 of the 146 Massey & Thompson (1991,
Cat. J/AJ/101/1408) stars, termed the "unbiased" sample because they
were selected on a "binary-blind" basis (i.e., no previous evidence of
a companion). The objects in the "unbiased" sample average 14
observations over 1999-2011 with a mean velocity uncertainty of
9.7km/s. Additionally, we include the published orbital information
for four systems not in our original sample, but likely members of
CygOB2: Schulte 5 from Schulte (1958ApJ...128...41S 1958ApJ...128...41S), and A36, A45,
and B17 from Comeron et al. (2002A&A...389..874C 2002A&A...389..874C). This constitutes
114 OB stars and is designated the "complete" sample.
File Summary:
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FileName Lrecl Records Explanations
--------------------------------------------------------------------------------
ReadMe 80 . This file
stars.dat 24 111 Stars used in the Monte Carlo analysis
(table added by CDS)
table1.dat 31 1488 Ephemerides for CygOB2 stars used in
Monte Carlo analysis
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See also:
J/ApJ/773/135 : Cyg OB2 cand. stars with disk multiphot. (Guarcello+, 2013)
J/A+A/543/A101 : OB stars in Cygnus OB2 association (Comeron+, 2012)
J/ApJS/202/19 : riz photometry in Cyg OB2 (Guarcello+, 2012)
J/ApJS/194/27 : Variable stars in Cygnus OB2 (Henderson+, 2011)
J/ApJ/713/871 : X-ray sources in star-forming region Cyg-OB2 (Wright+, 2010)
J/ApJS/184/84 : X-ray sources in Cyg OB2 region (Wright+, 2009)
J/A+A/486/453 : NIR phot. of early-type stars in Cyg OB2 (Comeron+, 2008)
J/A+A/464/211 : Chandra and near-IR obs. of Cyg OB2 (Albacete Colombo+, 2007)
J/ApJ/664/1102 : RV survey of Cyg OB2 association (Kiminki+, 2007)
J/ApJ/643/238 : Chandra/2MASS sources in Cygnus OB2 (Butt+, 2006)
J/ApJS/149/123 : WSRT survey of Cygnus OB2 (Setia Gunawan+, 2003)
J/MNRAS/298/753 : Variables in Cygnus OB2 (Pigulski+ 1998)
J/AJ/101/1408 : Massive stars in Cyg OB2. (Massey+ 1991)
Byte-by-byte Description of file: stars.dat
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Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 13 A13 --- SName SIMBAD name
15- 24 A10 --- Name Star designation as in table 1 (G1)
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Byte-by-byte Description of file: table1.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 10 A10 --- Name Star designation (G1)
12- 19 F8.2 d HJD Heliocentric Julian Date; HJD-2400000
21- 26 F6.1 km/s HRV Heliocentric radial velocity
28- 31 F4.1 km/s e_HRV 1σ uncertainty in HRV
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Global notes:
Note (G1): Abbreviations in name as below:
Schulte = Schulte (1958ApJ...128...41S 1958ApJ...128...41S);
A = Comeron et al. (2002A&A...389..874C 2002A&A...389..874C; <[CPR2002] ANN> in Simbad);
MT = Massey & Thompson (1991, Cat. J/AJ/101/1408; <[MT91] NNN> in Simbad).
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History:
From electronic version of the journal
References:
Kiminki et al. Paper I. 2007ApJ...664.1102K 2007ApJ...664.1102K Cat. J/ApJ/664/1102
Kiminki et al. Paper II. 2008ApJ...679.1478K 2008ApJ...679.1478K
Kiminki et al. Paper III. 2009AJ....137.4608K 2009AJ....137.4608K
Kiminki et al. Paper IV. 2012ApJ...747...41K 2012ApJ...747...41K
Kobulnicky et al. Paper VI. 2012ApJ...756...50K 2012ApJ...756...50K
(End) Greg Schwarz [AAS], Emmanuelle Perret [CDS] 11-Dec-2013