Spin alignment of stars in old open clusters

Abstract

Stellar clusters form by gravitational collapse of turbulent molecular clouds, with up to several thousand stars per cluster 1 . They are thought to be the birthplace of most stars and therefore play an important role in our understanding of star formation, a fundamental problem in astrophysics 2, 3 . The initial conditions of the molecular cloud establish its dynamical history until the stellar cluster is born. However, the evolution of the cloud's angular momentum during cluster formation is not well understood 4 . Current observations have suggested that turbulence scrambles the angular momentum of the cluster-forming cloud, preventing spin alignment amongst stars within a cluster 5 . Here we use asteroseismology [6][7][8] to measure the inclination angles of spin axes in 48 stars from the two old open clusters NGC 6791 and NGC 6819. The stars within each cluster show strong alignment. Threedimensional hydrodynamical simulations of proto-cluster formation show that at least 50% of the initial proto-cluster kinetic energy has to be rotational in order to obtain strong stellarspin alignment within a cluster. Our result indicates that the global angular momentum of the cluster-forming clouds was efficiently transferred to each star and that its imprint has survived after several gigayears since the clusters formed. About half of the overall star formation in the Milky Way is occurring in the 24 most massive giant molecular clouds 1 . The star forming regions are obscured by dust, hence direct observations are limited to the infrared and radio bands 2,3 . However, open clusters can be studied in a broad range of wavelengths because they contain small amounts of interstellar gas and dust. The great advantage of studying stars in a cluster -as opposed to field stars that often originate from dissolved small stellar systems -is that they can preserve the signature of the initial conditions of the progenitor molecular cloud.It is believed that molecular clouds satisfying the Jeans instability undergo gravitational fragmentation in which the internal motions are strongly influenced by turbulence 4,9 . This suggests that the angular momentum from the progenitor cloud cannot leave any significant imprint of its action on the stars born in the cluster. However, if the stars inherit the physical properties of the molecular cloud, they should to some extent reflect its average angular momentum. To investigate the angular momentum imprint, requires measurements of the space orientation of the stellar-spin axis. Previous analyses conducted on young open clusters did not find evidence of stellar-spin alignment 5 . Asteroseismology, the study of stellar oscillations, has proven to be a powerful tool to obtain model-independent information on the inclination angle of the stellar angular momentum vector, especially for red giant stars 7,8,10,11 . Red giants are typically low-and intermediate-mass stars that have evolved off the main sequence of the stellar evolution. Most red giants oscillate and their oscillations can be analyz...

Age as a major factor in the onset of multiple populations in stellar clusters

“…12 that have ages older than 2 Gyr but no MPs, is that the global properties of a cluster (such as its mass, or initial angular momentum) can, under certain circumstances, influence the properties of its individual stars (such as the distribution of rotation speeds). Recent observations (Corsaro et al 2017) suggest that this is indeed plausible. However, this remains purely speculative at the moment.…”

Metallicity effect on stellar granulation detected from oscillating red giants in open clusters

“…We notice that two stars marked as RGB, namely KIC 2437589 in NGC 6791 and KIC 5112361 in NGC 6819, are placed in the region of the color-magnitude diagram where the corresponding RC stars of the same clusters are located. Despite their peculiar location in the diagram, both stars have a RGB evolutionary stage unambiguously determined by their oscillations (Corsaro et al 2012(Corsaro et al , 2017, with KIC 2437589 a possible evolved blue straggler Corsaro et al 2012) and KIC 5112361 a spectroscopic single lined binary (Milliman et al 2014). Corsaro et al (2012).…”

“…Corsaro et al (2012). Hydrogen-shell-burning and core-He-burning RGs are shown with filled and open symbols, respectively, with an evolutionary stage identified according to Corsaro et al (2012Corsaro et al ( , 2017, and to Molenda-Żakowicz et al (2014) for the two stars KIC 9776739 and KIC 9716090. Isochrones are shown for each cluster as solid lines (see Stello et al 2011, for more details).…”

“…We included those stars with a clear evolutionary stage determination, as discriminated using mixed mode oscillations by Corsaro et al (2012), Mosser et al (2014), Corsaro et al (2017;see also Corsaro et al, in prep.). We find in total 30 RGs for NGC 6791 and 24 for NGC 6819.…”

“…The open clusters NGC 6791, NGC 6819, and NGC 6811 have been monitored by the Kepler mission for more than four years, thus providing us with the best photometric observations currently available for the rich populations of RGs hosted by each of these clusters . Fundamental parameters such as temperature, mass, metallicity, and age, are determined for cluster stars with high reliability (e.g., Bragaglia et al 2001;Basu et al 2011;Hekker et al 2011;Stello et al 2011;Brogaard et al 2011Brogaard et al , 2012Miglio et al 2012), and the evolutionary stage of many cluster RGs is also well known from existing asteroseismic analyses (Corsaro et al 2012;Mosser et al 2014;Vrard et al 2016;Corsaro et al 2017).…”