We present the orbital solutions for four OB spectroscopic binaries, MT145, GSC 03161-00815, 2MASS J20294666+4105083, and Schulte 73, and the partial orbital solution to the B spectroscopic binary, MT372, as part of an ongoing study to determine the distribution of orbital parameters for massive binaries in the Cygnus OB2 Association. MT145 is a new, single-lined, moderately eccentric (e=0.291+/-0.009) spectroscopic binary with period of 25.140+/-0.008 days. GSC 03161-00815 is a slightly eccentric (e=0.10+/-0.01), eclipsing, interacting and double-lined spectroscopic binary with a period of 4.674+/-0.004 days. 2MASS J20294666+4105083 is a moderately eccentric (e=0.273+/-0.002) double-lined spectroscopic binary with a period of 2.884+/-0.001 days. Schulte 73 is a slightly eccentric (e=0.169+/-0.009), double-lined spectroscopic binary with a period of 17.28+/-0.03 days and the first "twin" in our survey with a mass ratio of q=0.99+/-0.02. MT372 is a single-lined, eclipsing system with a period of 2.228 days and low eccentricity (e~0). Of the now 18 known OB binaries in Cyg OB2, 14 have periods and mass ratios. Emerging evidence also shows that the distribution of log(P) is flat and consistent with Oepik's Law.
Deep Dive into Five More Massive Binaries in the Cygnus OB2 Association.
We present the orbital solutions for four OB spectroscopic binaries, MT145, GSC 03161-00815, 2MASS J20294666+4105083, and Schulte 73, and the partial orbital solution to the B spectroscopic binary, MT372, as part of an ongoing study to determine the distribution of orbital parameters for massive binaries in the Cygnus OB2 Association. MT145 is a new, single-lined, moderately eccentric (e=0.291+/-0.009) spectroscopic binary with period of 25.140+/-0.008 days. GSC 03161-00815 is a slightly eccentric (e=0.10+/-0.01), eclipsing, interacting and double-lined spectroscopic binary with a period of 4.674+/-0.004 days. 2MASS J20294666+4105083 is a moderately eccentric (e=0.273+/-0.002) double-lined spectroscopic binary with a period of 2.884+/-0.001 days. Schulte 73 is a slightly eccentric (e=0.169+/-0.009), double-lined spectroscopic binary with a period of 17.28+/-0.03 days and the first “twin” in our survey with a mass ratio of q=0.99+/-0.02. MT372 is a single-lined, eclipsing system with a
The study of massive binary systems in young clusters plays a key role in our understanding of how massive stars form. In addition to providing the binary fraction for massive stars, massive binary systems provide information about their formation environment in the form of quasi-preserved parameters such as eccentricity, separation, period, and angular momentum (in the form of rotational velocities; Larson 2001). In addition, an initial mass function (IMF) composed of the secondary component masses can indicate whether the companions are randomly drawn from a Salpeter (1955) (or equivalent field star) IMF or not. This can tell us whether the binary systems may have randomly formed by way of gravitational capture after the formation process, or preferentially paired through a more complicated formation process such as competitive accretion (Bonnell et al. 1998).
Cyg OB2 provides one of the best regions for indirectly examining the formation of massive stars as it houses ∼60-70 O-type stars (Negueruela et al. 2008), including an O3If (Schulte 7;Walborn & Howarth 2000) and an O4If (Schulte 22;Comerón & Pasquali 2007), and possibly more than 2000 B stars (Knödlseder 2000). Kiminki et al. (2008) presented six new spectroscopic binary systems and summarized the then-known 11 OB binary systems in Cyg OB2. Between then and this work, three additional OB binary systems have been uncovered, including Schulte 9 (Nazé et al. 2008), 2MASS J20302730+4113253 (Stroud et al. 2009, submitted), and GSC 03161-00815 (Otero 2008a;Hanson 2003). Six of the total 14 systems, MT421, MT429, MT696, Schulte 3,Schulte 5, and GSC 03161-00815 are identified as eclipsing binaries (where notation for the first five are from Massey &Thompson 1991 andSchulte 1956). Two of the 14 systems, MT059 and MT258, are single-lined spectroscopic binaries (SB1s), and eight of the 14, MT252, MT696, MT720, MT771, Schulte 3, Schulte 8a, Schulte 9, and 2MASS J20302730+4113253, are double-lined spectroscopic binaries (SB2s). In part three of this ongoing study, we add to the 14 and present the newly uncovered spectroscopic binaries, MT145 (SB1), 2MASS J20294666+4105083 (listed as a candidate SB2 in Hanson 2003), Schulte 73 (SB2), and MT372 (SB1). We also present the first spectroscopic solution to the eclipsing system, GSC 03161-00815 (SB2). For simplicity, we will use Comerón et al. (2002) notation for GSC 03161-00815, 2MASS J20294666+4105083, and 2MASS J20302730+4113253 (A36, A45, and B17 respectively). These new systems bring the total number of OB binaries in Cyg OB2 to 18, constituting one of the highest numbers of massive binary solutions of any open cluster.
Section 2 of this work provides observational details of the new spectroscopic datasets. Section 3 discusses the measurement of radial velocities, the search for periods in the radial velocity data, and the determination of orbital elements via radial velocity curve fitting. Section 4 discusses the orbital solutions to the SB1, MT145, and the SB2s, A36, A45, and Schulte 73 (Cyg OB2 No. 73). Section 5 presents the partial solution to the SB1, MT372. Finally, Section 6 summarizes the results of the survey to date, including the total number of OB binaries uncovered in the Cyg OB2 core region, the total number of O star binary solutions, and the emerging distribution of orbital periods. Kiminki et al. (2007) & Kiminki et al. (2008) (Papers I & II) detail the observations of this survey through 2007 September. We have obtained additional datasets with the WIRO-Longslit spectrograph 1 on the Wyoming Infrared Observatory (WIRO) 2.3 m telescope and the Hydra spectrograph on the WIYN 2 3.5 m telescope. Table 1 lists the observing runs at each facility, the corresponding spectral coverages, and mean spectral resolutions.
Observations at WIYN took place over six nights on 2008 June 10-15. We used the Hydra spectrograph with the Red camera, 2 ′′ blue fibers, and the 1200 l mm -1 grating in second order to obtain four 1500 s exposures in each of two fiber configurations yielding a maximum signal-to-noise ratio (SNR) of 80:1 for the brightest stars. The spectral coverage was 3820-4500 Å at a mean resolution of R ∼4500. Copper-Argon lamps were used between each exposure to calibrate the spectra to an RMS of 0.03 Å (2 km s -1 at 4500 Å), and the typical resolution was 1.0 Å FWHM at 3900 Å and 0.82 Å FWHM at 4400 Å. Spectra were Doppler corrected to the heliocentric frame and checked against the radial velocity standards HD131156 (G8V), HD146233(G2V), HD161096(K2III), HD161797(G5IV), and HD171391(G8III) from Stefanik et al. (1999) before comparison to previous datasets.
Observations using the WIRO-Longslit spectrograph with the 1800 l mm -1 grating in first order took place over 37 nights between 2007 October 23 and 2008 September 19 to examine the Hα, He I, and He II absorption lines in suspected SB2s. Exposure times varied from 600 s to 4500 s (in multiples of 600-900 s) depending on weather conditions an
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