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Register a new userStudy of Eclipsing Binary and Multiple Systems in OB Associations V: MQ Cen in Crux OB1
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The early-type massive binary MQ Cen (P$_{orb}$=3.7 d) has been investigated by means of high-resolution ($Rsim48,000$) spectral analysis and multi-band (Johnson $BVRI$ and Str{o}mgren $vby$) light curve modeling. The physical parameters of the components have been found to be $M_1= 4.26pm0.10$ M$_{odot}$, $R_1= 3.72pm0.05 $R$_{odot}$, $T_{rm eff1}=16,600pm520$ K, and $M_2= 5.14pm0.09 $M$_{odot}$, $R_2= 7.32pm0.03 $R$_{odot}$, $T_{rm eff2}=15,000pm500$ K for the primary and secondary, respectively. The orbital inclination is $i=87.0pm0.2$ deg. The distance to MQ Cen has been derived to be $d=2,460pm310$ pc which locates it in the Crux OB1 association. However, the age of MQ~Cen ($sim70$ Myr) is higher than the one reported for the Crux OB1 association ($sim$6 Myr). The derived masses are implying a spectral type of B5 for the primary and B4 for the secondary component. Nevertheless, the secondary component, which is more massive, appears to be cooler than the primary component: It has completed its lifetime on the main-sequence and it is now positioned at the turn-off point of the giant branch, meanwhile the less massive primary component is still staying on the main-sequence.
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Analyses of multi-epoch, high-resolution (~ 50000) optical spectra of seven early-type systems provided various important new insights with respect to their multiplicity. First determinations of orbital periods were made for HD 92206C (2.022 d), HD 112244 (27.665 d), HD 164438 (10.25 d), HD 123056A (~ 1314 d) and HD 123056B (< 2 d); the orbital period of HD 318015 could be improved (23.445975 d). Concerning multiplicity, a third component was discovered for HD 92206C by means of He I line profiles. For HD 93146A, which was hitherto assumed to be SB1, lines of a secondary component could be discerned. HD 123056 turns out to be a multiple system consisting of a high-mass component A (~ O8.5) displaying a broad He II 5411 A feature with variable radial velocity, and of an inner pair B (~ B0) with double He I lines. The binary HD 164816 was revisited and some of its system parameters were improved. In particular, we determined its systemic velocity to be -7 km/s, which coincides with the radial velocity of the cluster NGC 6530. This fact, together with its distance, suggests the cluster membership of HD 164816. The OB system HD 318015 (V1082 Sco) belongs to the rare class of eclipsing binaries with a supergiant primary (B0.5/0.7). Our combined orbital and light-curve analysis suggests that the secondary resembles an O9.5III star. Our results for a limited sample corroborate the findings that many O stars are actually massive multiple systems.
For understanding the process of star formation it is essential to know how many stars are formed as singles or in multiple systems, as a function of environment and binary parameters. This requires a characterization of the primordial binary population, which we define as the population of binaries that is present just after star formation has ceased, but before dynamical and stellar evolution have significantly altered its characteristics. In this article we present the first results of our adaptive optics survey of 200 (mainly) A-type stars in the nearby OB association Sco OB2. We report the discovery of 47 new candidate companions of Sco OB2 members. The next step will be to combine these observations with detailed simulations of young star clusters, in order to find the primordial binary population.
OB associations are unbound groups of young stars made prominent by their bright OB members, and have long been thought to be the expanded remnants of dense star clusters. They have been important in astrophysics for over a century thanks to their luminous massive stars, though their low-mass members have not been well studied until the last couple of decades. This has changed thanks to data from X-ray observations, spectroscopic surveys and astrometry from Gaia that allows their full stellar content to be identified and their dynamics to be studied, which in turn is leading to changes in our understanding of these systems and their origins, with the old picture of Blaauw (1964) now being superseded. It is clear now that OB associations have considerably more substructure than once envisioned, both spatially, kinematically and temporally. These changes have implications for the star formation process, the formation and evolution of planetary systems, and the build-up of stellar populations across galaxies.
OB associations are the prime star forming sites in galaxies. However the detailed formation process of such stellar systems still remains a mystery. In this context, identifying the presence of substructures may help tracing the footprints of their formation process. Here, we present a kinematic study of the two massive OB associations Cygnus OB2 and Carina OB1 using the precise astrometry from the Gaia Data Release 2 and radial velocities. From the parallaxes of stars, these OB associations are confirmed to be genuine stellar systems. Both Cygnus OB2 and Carina OB1 are composed of a few dense clusters and a halo which have different kinematic properties: the clusters occupy regions of 5-8 parsecs in diameter and display small dispersions in proper motion, while the halos spread over tens of parsecs with a 2-3 times larger dispersions in proper motion. This is reminiscent of the so-called line width-size relation of molecular clouds related to turbulence. Considering that the kinematics and structural features were inherited from those of their natal clouds would then imply that the formation of OB associations may result from structure formation driven by supersonic turbulence, rather than from the dynamical evolution of individual embedded clusters.
We discuss how contemporary multiwavelength observations of young OB-dominated clusters address long-standing astrophysical questions: Do clusters form rapidly or slowly with an age spread? When do clusters expand and disperse to constitute the field star population? Do rich clusters form by amalgamation of smaller subclusters? What is the pattern and duration of cluster formation in massive star forming regions (MSFRs)? Past observational difficulties in obtaining good stellar censuses of MSFRs have been alleviated in recent studies that combine X-ray and infrared surveys to obtain rich, though still incomplete, censuses of young stars in MSFRs. We describe here one of these efforts, the MYStIX project, that produced a catalog of 31,784 probable members of 20 MSFRs. We find that age spread within clusters are real in the sense that the stars in the core formed after the cluster halo. Cluster expansion is seen in the ensemble of (sub)clusters, and older dispersing populations are found across MSFRs. Direct evidence for subcluster merging is still unconvincing. Long-lived, asynchronous star formation is pervasive across MSFRs.
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