Rotation of Horizontal Branch Stars in Globular Clusters

The rotation of horizontal branch stars places important constraints on angular momentum evolution in evolved stars and therefore rotational mixing on the giant branch. Prompted by new observations of rotation rates of horizontal branch stars, we calculate simple models for the angular momentum evolution of a globular cluster giant star from the base of the giant branch to the star's appearance on the horizontal branch. We include mass loss, and infer the accompanied loss of angular momentum for each of four assumptions about the internal angular momentum profile. These models are compared to observations of horizontal branch rotation rates in M13. We find that rapid rotation on the horizontal branch can be reconciled with slow solid body main sequence rotation if giant branch stars have differential rotation in their convective envelopes and a rapidly rotating core, which is then followed by a redistribution of angular momentum on the horizontal branch. We discuss the physical reasons why these very different properties relative to the solar case may exist in giants. Rapid rotation in the core of the main sequence precursors of the rapidly rotating horizontal branch star, or an angular momentum source on the giant branch is required for all cases if the rotational velocity of turnoff stars is less than 4 km s$^{-1}$. We suggest that the observed range in rotation rates on the horizontal branch is caused by internal angular momentum redistribution which occurs on a timescale comparable to the evolution of the stars on the horizontal branch. The apparent lack of rapid horizontal branch rotators hotter than 12 000 K in M13 could be a consequence of gravitational settling, which inhibits internal angular momentum transport. Alternative explanations and observational tests are discussed.Comment: 32 pages, 7 figures, submitted to the Astrophysical Journa

Hydrogenic Transitions in Multiply Charged Fe and Ni Ions

Ten lines in the range 3880≦λ≦5666 Å in the beam-foil spectrum of iron have been identified with specific hydrogenic transitions in Fe iv-viii. The same transitions were observed from Ni and Ar beams. Deviations from the hydrogenic wavelengths are shown to be consistent with that expected from core polarization. The absence of these lines in astrophysical sources is discussed. A wavelength table is presented for identification of hydrogenic transitions to be expected in beam-foil spectra

Simulations of Stellar Collisions Involving Pre-Main Sequence Stars

In this paper, we present the results of smoothed particle hydrodynamic (SPH) simulations of collisions between pre-main sequence stars and a variety of other kinds of stars. Simulations over a range of impact parameters and velocities were performed. We find that pre-main sequence stars tend to ``wrap themselves'' around their impactor. We discuss the probable evolutionary state of products of collisions between pre-main sequence stars and pre-main sequence, main sequence, giant branch, and compact stars. The nature of the collision product does not depend strongly on the impact parameter or the velocity of the collision.Comment: Accepted by Ap

Implications for the Formation of Blue Straggler Stars from HST Ultraviolet Observations of NGC 188

We present results of a Hubble Space Telescope far-ultraviolet (FUV) survey searching for white dwarf (WD) companions to blue straggler stars (BSSs) in open cluster NGC 188. The majority of NGC 188 BSSs (15 of 21) are single-lined binaries with properties suggestive of mass-transfer formation via Roche lobe overflow, specifically through an asymptotic giant branch star transferring mass to a main sequence secondary, yielding a BSS binary with a WD companion. In NGC 188, a BSS formed by this mechanism within the past 400 Myr will have a WD companion hot and luminous enough to be directly detected as a FUV photometric excess with HST. Comparing expected BSS FUV emission to observed photometry reveals four BSSs with WD companions above 12,000 K (younger than 250 Myr) and three WD companions with temperatures between 11,000-12,000 K. These BSS+WD binaries all formed through recent mass transfer. The location of the young BSSs in an optical color-magnitude diagram (CMD) indicates that distance from the zero-age main sequence does not necessarily correlate with BSS age. There is no clear CMD separation between mass transfer-formed BSSs and those likely formed through other mechanisms, such as collisions. The seven detected WD companions place a lower limit on the mass-transfer formation frequency of 33%. We consider other possible formation mechanisms by comparing properties of the BSS population to theoretical predictions. We conclude that 14 BSS binaries likely formed from mass transfer, resulting in an inferred mass-transfer formation frequency of approximately 67%.Comment: 13 pages, 6 figures, accepted to the Astrophysical Journa

Detection of white dwarf companions to blue stragglers in the open cluster NGC 188: direct evidence for recent mass transfer

Several possible formation pathways for blue straggler stars have been developed recently, but no one pathway has yet been observationally confirmed for a specific blue straggler. Here we report the first findings from a Hubble Space Telescope ACS/SBC far-UV photometric program to search for white dwarf companions to blue straggler stars. We find three hot and young white dwarf companions to blue straggler stars in the 7-Gyr open cluster NGC 188, indicating that mass transfer in these systems ended less than 300 Myr ago. These companions are direct and secure observational evidence that these blue straggler stars were formed through mass transfer in binary stars. Their existence in a well-studied cluster environment allows for observational constraints of both the current binary system and the progenitor binary system, mapping the entire mass transfer history.Comment: 5 pages, 3 figures, accepted for publication in The Astrophysical Journal Letter