ROTATIONAL AND RADIAL VELOCITIES OF 1.3-2.2M_☉RED GIANTS IN OPEN CLUSTERS

Abstract

This study presents the rotational distribution of red giant (RG) stars in 11 old to intermediate age open clusters. The masses of these stars are all above the Kraft break, so they lose negligible amounts of their birth angular momentum (AM) during the main-sequence (MS) evolution. However, they do span a mass range with quite different AM distributions imparted during formation, with the stars less massive than ∼1.6 M arriving on the MS with lower rotation rates than the more massive stars. The majority of RGs in this study are slow rotators across the entire red giant branch regardless of mass, supporting the picture that intermediate-mass stars rapidly spin down when they evolve off the MS and develop convection zones capable of driving a magnetic dynamo. Nevertheless, a small fraction of RGs in open clusters show some level of enhanced rotation, and faster rotators are as common in these clusters as in the field RG population. Most of these enhanced rotators appear to be red clump stars, which is also true of the underlying stellar sample, while others are clearly RGs that are above or below the clump. In addition to rotational velocities, the radial velocities (RVs) and membership probabilities of individual stars are also presented. Cluster heliocentric RVs for NGC 6005 and Pismis 18 are reported for the first time.

The Ca ii triplet in red giant spectra: [Fe/H] determinations and the role of [Ca/Fe]

“…It has not been considered further. The mean velocity of the 17 stars classified as members is 19.8 ± 0.9 km s −1 (std error of mean) which agrees well with the values of 20.7 ± 0.6 and 22.1 ± 0.9 km s −1 given by Sestito et al (2008) and Carlberg (2014), respectively. The ten M67 stars observed are all known to be cluster members and the current data support that classification.…”

“…The final star is Mel66-W1580 (WEBDA id number), which Sestito et al (2008) indicate is a fast rotator. The radial velocity determined here is 50.9 ± 2.0 km s −1 while that given by Carlberg (2014) is -14 ± 17 km s −1 ; the star is probably a binary. It has not been considered further.…”

“…The mean radial velocity for these stars is 32.5 ± 0.5 km s −1 which is in excellent agreement with other determinations. For example, Carlberg (2014) give the cluster radial velocity as 33.1 ± 0.6 km s −1 . Six of the stars were also observed by Cole et al (2004).…”

Star-planet interactions

“…When this occurs, models indicate that changes at the stellar surface can be observed: a transitory and rapid increase in the luminosity (Siess & Livio 1999b), a change in the surface abundance of lithium (Carlberg et al 2010;Adamów et al 2012), or an increase in the surface rotation rate (Siess & Livio 1999a,b;Carlberg et al 2009Carlberg et al , 2010. Interestingly, some stars present observed characteristics that could be related to planet engulfments, such as the few percents of red giants (RGs) that are fast rotators (Fekel & Balachandran 1993;Massarotti et al 2008;Carlberg et al 2011;Carlberg 2014). …”

High surface magnetic field in red giants as a new signature of planet engulfment?

“…A number of past studies (Soker et al 1984;Siess & Livio 1999a,b;Villaver & Livio 2007Nordhaus et al 2010;Kunitomo et al 2011;Villaver et al 2014) have shown that the possible impact of an engulfment, planetary or otherwise, leads to changes in the stellar luminosity and radii for a short period of evolutionary time, to modifications of the surface abundances, in particular to an increase in lithium abundance (Alexander 1967;Fekel & Balachandran 1993;Sandquist et al 1998Sandquist et al , 2002Carlberg et al 2010;Adamów et al 2012), and to changes in the surface velocity (Carlberg et al 2009;Carlberg 2014).…”