Dynamics of the Young Binary LMC Cluster NGC 1850

In this paper we have examined the age and internal dynamics of the young binary LMC cluster NGC 1850 using BV CCD images and echelle spectra of 52 supergiants. Isochrone fits to a BV color-magnitude diagram revealed that the primary cluster has an age of $\tau = 90 \pm 30$ Myr while the secondary member has $\tau = 6 \pm 5$ Myr. BV surface brightness profiles were constructed out to R $>$ 40 pc, and single-component King-Michie (KM) models were applied. The total cluster luminosity varied from L$_B$ = 2.60 - 2.65 $\times 10^6$ L$_B$\sol\ and L$_V$ = 1.25 - 1.35 $\times 10^6$ as the anisotropy radius varied from infinity to three times the scale radius with the isotropic models providing the best agreement with the data. Of the 52 stars with echelle spectra, a subset of 36 were used to study the cluster dynamics. The KM radial velocity distributions were fitted to these velocities yielding total cluster masses of 5.4 - 5.9 $\pm 2.4 \times 10^4$ M\sol\ corresponding to M/L$_B$ = 0.02 $\pm 0.01$ M\sol/L$_B$\sol\ or M/L$_V$ = 0.05 $\pm 0.02$ M\sol/L$_V$\sol. A rotational signal in the radial velocities has been detected at the 93\% confidence level implying a rotation axis at a position angle of 100\deg. A variety of rotating models were fit to the velocity data assuming cluster ellipticities of $\epsilon = 0.1 - 0.3$. These models provided slightly better agreement with the radial velocity data than the KM models and had masses that were systematically lower by a few percent. The preferred value for the slope of a power-law IMF is a relatively shallow, x = 0.29 \pmm{+0.3}{-0.8} assuming the B-band M/L or x = 0.71 \pmm{+0.2}{-0.4} for the V-band.Comment: 41 pages (figures available via anonymous FTP as described below

On the Path-Integral Derivation of the Anomaly for the Hermitian Equivalent of the Complex PTPT-Symmetric Quartic Hamiltonian

It can be shown using operator techniques that the non-Hermitian $PT$-symmetric quantum mechanical Hamiltonian with a "wrong-sign" quartic potential $-gx^4$ is equivalent to a Hermitian Hamiltonian with a positive quartic potential together with a linear term. A naive derivation of the same result in the path-integral approach misses this linear term. In a recent paper by Bender et al. it was pointed out that this term was in the nature of a parity anomaly and a more careful, discretized treatment of the path integral appeared to reproduce it successfully. However, on re-examination of this derivation we find that a yet more careful treatment is necessary, keeping terms that were ignored in that paper. An alternative, much simpler derivation is given using the additional potential that has been shown to appear whenever a change of variables to curvilinear coordinates is made in a functional integral.Comment: LaTeX, 12 pages, no figure

Detailed Chemical Abundances in NGC 5824: Another Metal-Poor Globular Cluster with Internal Heavy Element Abundance Variations

We present radial velocities, stellar parameters, and detailed abundances of 39 elements derived from high-resolution spectroscopic observations of red giant stars in the luminous, metal-poor globular cluster NGC 5824. We observe 26 stars in NGC 5824 using the Michigan/Magellan Fiber System (M2FS) and two stars using the Magellan Inamori Kyocera Echelle (MIKE) spectrograph. We derive a mean metallicity of [Fe/H]=-1.94+/-0.02 (statistical) +/-0.10 (systematic). The metallicity dispersion of this sample of stars, 0.08 dex, is in agreement with previous work and does not exceed the expected observational errors. Previous work suggested an internal metallicity spread only when fainter samples of stars were considered, so we cannot exclude the possibility of an intrinsic metallicity dispersion in NGC 5824. The M2FS spectra reveal a large internal dispersion in [Mg/Fe], 0.28 dex, which is found in a few other luminous, metal-poor clusters. [Mg/Fe] is correlated with [O/Fe] and anti-correlated with [Na/Fe] and [Al/Fe]. There is no evidence for internal dispersion among the other alpha- or Fe-group abundance ratios. Twenty-five of the 26 stars exhibit a n-capture enrichment pattern dominated by r-process nucleosynthesis ([Eu/Fe]=+0.11+/-0.12; [Ba/Eu]=-0.66+/-0.05). Only one star shows evidence of substantial s-process enhancement ([Ba/Fe]=+0.56+/-0.12; [Ba/Eu]=+0.38+/-0.14), but this star does not exhibit other characteristics associated with s-process enhancement via mass-transfer from a binary companion. The Pb and other heavy elements produced by the s-process suggest a timescale of no more than a few hundred Myr for star formation and chemical enrichment, like the complex globular clusters M2, M22, and NGC 5286.Comment: Accepted for publication in MNRAS. (26 pages, 18 figures, 9 tables including online data

Desorption Dynamics of Heavy Alkali Metal Atoms (Rb, Cs) off the Surface of Helium Nanodroplets

We present a combined ion imaging and density functional theory study of the dynamics of the desorption process of rubidium and cesium atoms off the surface of helium nanodroplets upon excitation of the perturbed $6s$ and $7s$ states, respectively. Both experimental and theoretical results are well represented by the pseudodiatomic model for effective masses of the helium droplet in the desorption reaction of m_eff/m_He~10 (Rb) and 13 (Cs). Deviations from this model are found for Rb excited to the 6p state. Photoelectron spectra indicate that the dopant-droplet interaction induces relaxation into low-lying electronic states of the desorbed atoms in the course of the ejection process.Comment: in press, J. Phys. Chem. A (2014

Masses for the Local Group and the Milky Way

We use the very large Millennium Simulation of the concordance $\Lambda$CDM cosmogony to calibrate the bias and error distribution of Timing Argument estimators of the masses of the Local Group and of the Milky Way. From a large number of isolated spiral-spiral pairs similar to the Milky Way/Andromeda system, we find the interquartile range of the ratio of timing mass to true mass to be a factor of 1.8, while the 5% and 95% points of the distribution of this ratio are separated by a factor of 5.7. Here we define true mass as the sum of the ``virial'' masses $M_{200}$ of the two dominant galaxies. For current best values of the distance and approach velocity of Andromeda this leads to a median likelihood estimate of the true mass of the Local Group of 5.27\times 10^{12}\msun, or $\log M_{LG}/M_\odot = 12.72$, with an interquartile range of $[12.58, 12.83]$ and a 5% to 95% range of $[12.26, 13.01]$. Thus a 95% lower confidence limit on the true mass of the Local Group is 1.81\times 10^{12}\msun. A timing estimate of the Milky Way's mass based on the large recession velocity observed for the distant satellite Leo I works equally well, although with larger systematic uncertainties. It gives an estimated virial mass for the Milky Way of 2.43 \times 10^{12}\msun with a 95% lower confidence limit of 0.80 \times 10^{12}\msun.Comment: 11 pages, 6 figures, MNRAS accepted. Added a new discussion paragraph and a new figure regarding the relative transverse velocity but conclusions unchange

Dwarf Cepheids in the Carina Dwarf Spheroidal Galaxy

We have discovered 20 dwarf Cepheids (DC) in the Carina dSph galaxy from the analysis of individual CCD images obtained for a deep photometric study of the system. These short-period pulsating variable stars are by far the most distant (~100 kpc) and faintest (V ~ 23.0) DCs known. The Carina DCs obey a well-defined period-luminosity relation, allowing us to readily distinguish between overtone and fundamental pulsators in nearly every case. Unlike RR Lyr stars, the pulsation mode turns out to be uncorrelated with light-curve shape, nor do the overtone pulsators tend towards shorter periods compared to the fundamental pulsators. Using the period-luminosity (PL) relations from Nemec et al. (1994 AJ, 108, 222) and McNamara (1995, AJ, 109, 1751), we derive (m-M)_0 = 20.06 +/- 0.12, for E(B-V) = 0.025 and [Fe/H] = -2.0, in good agreement with recent, independent estimates of the distance/reddening of Carina. The error reflects the uncertainties in the DC distance scale, and in the metallicity and reddening of Carina. The frequency of DCs among upper main sequence stars in Carina is approximately 3%. The ratio of dwarf Cepheids to RR Lyr stars in Carina is 0.13 +/- 0.10, though this result is highly sensitive to the star-formation history of Carina and the evolution of the Horizontal Branch. We discuss how DCs may be useful to search effectively for substructure in the Galactic halo out to Galactocentric distances of ~100 kpc.Comment: 20 pages of text, 7 figure

HST Studies of the WLM Galaxy. I. The Age and Metallicity of the Globular Cluster

We have obtained V and I images of the lone globular cluster that belongs to the dwarf Local Group irregular galaxy known as WLM. The color-magnitude diagram of the cluster shows that it is a normal old globular cluster with a well-defined giant branch reaching to M_V=-2.5, a horizontal branch at M_V=+0.5, and a sub-giant branch extending to our photometry limit of M_V=+2.0. A best fit to theoretical isochrones indicates that this cluster has a metallicity of [Fe/H]=-1.52\pm0.08 and an age of 14.8\pm0.6 Gyr, thus indicating that it is similar to normal old halo globulars in our Galaxy. From the fit we also find that the distance modulus of the cluster is 24.73\pm0.07 and the extinction is A_V=0.07\pm0.06, both values that agree within the errors with data obtained for the galaxy itself by others. We conclude that this normal massive cluster was able to form during the formation of WLM, despite the parent galaxy's very small intrinsic mass and size.Comment: 14 pages, 5 figures, 1 tabl