Photometry with the Hubble Space Telescope Advanced Camera for Surveys (HST ACS) reveals that the subgiant branch (SGB) of the globular cluster NGC 1851 splits into two well-defined branches. If the ...split is due only to an age effect, the two SGBs would imply two star formation episodes separated by similar to 1 Gyr. We discuss other anomalies in NGC 1851 that could be interpreted in terms of a double stellar population. Finally, we compare the case of NGC 1851 with the other two globulars known to host multiple stellar populations, and show that all three clusters differ in several important respects.
ABSTRACT We present predictions for the surface density of ultracool dwarfs (with spectral types M8-T8) for a host of deep fields that are likely to be observed with the James Webb Space Telescope. ...Based on simple thin and thick/thin disk (exponential) models, we show that the typical distance modulus is mag, which at high Galactic latitude is . Since this is a property of the density distribution of an exponential disk, it is independent of spectral type or stellar sample. Using the published estimates of the ultracool dwarf luminosity function, we show that their number counts typically peak around mag with a total surface density of arcmin−2, but with a strong dependence on galactic coordinate and spectral type. Owing to the exponential shape of the disk, the ultracool dwarfs are very rare at faint magnitudes ( mag), with typical densities of arcmin−2 (or % of the total contribution within the field). Therefore, in very narrow and deep fields, we predict there are only a few ultracool dwarfs, and hence these stars are likely not a severe contaminant in searches for high-redshift galaxies. Furthermore, the ultracool dwarfs are expected to be considerably brighter than the high-redshift galaxies, so samples near the faint end of the high-redshift galaxy population will be the purest. We present the star-count formalism in a simplified way so that observers may easily predict the number of stars for their conditions (field, depth, wavelength, etc.).
Context. The fraction of binary stars is an important ingredient to interpret globular cluster dynamical evolution and their stellar population. Aims. We investigate the properties of main-sequence ...binaries measured in a uniform photometric sample of 59 Galactic globular clusters that were observed by HST WFC/ACS as a part of the Globular Cluster Treasury project. Methods. We measured the fraction of binaries and the distribution of mass-ratio as a function of radial location within the cluster, from the central core to beyond the half-mass radius. We studied the radial distribution of binary stars, and the distribution of stellar mass ratios. We investigated monovariate relations between the fraction of binaries and the main parameters of their host clusters. Results. We found that in nearly all the clusters, the total fraction of binaries is significantly smaller than the fraction of binaries in the field, with a few exceptions only. Binary stars are significantly more centrally concentrated than single MS stars in most of the clusters studied in this paper. The distribution of the mass ratio is generally flat (for mass-ratio parameter q > 0.5). We found a significant anti-correlation between the binary fraction in a cluster and its absolute luminosity (mass). Some, less significant correlation with the collisional parameter, the central stellar density, and the central velocity dispersion are present. There is no statistically significant relation between the binary fraction and other cluster parameters. We confirm the correlation between the binary fraction and the fraction of blue stragglers in the cluster.
We studied a solar flare with pronounced quasi-periodic pulsations detected in the microwave, X-ray, and radio bands. We used correlation, Fourier, and wavelet analyses methods to examine the ...temporal fine structures and relationships between the time profiles in each wave band. We found that the time profiles of the microwaves, hard X-rays, and type III radio bursts vary quasi-periodically with a common period of 40 – 50 s. The average amplitude of the variations is high, above 30 % of the background flux level, and reaches 80 % after the flare maximum. We did not find this periodicity in either the thermal X-ray flux component or in the source size dynamics. Our findings indicate that the detected periodicity is probably associated with periodic dynamics in the injection of non-thermal electrons, which can be produced by periodic modulation of magnetic reconnection.
We present a spectral analysis of the ultraviolet-bright star vZ 1128 in M3 based on observations with the Far Ultraviolet Spectroscopic Explorer (FUSE), the Space Telescope Imaging Spectrograph ...(STIS), and the Keck High Resolution Echelle Spectrometer (HIRES). By fitting the H i, He i, and He ii lines in the Keck spectrum with non-local thermodynamic equilibrium H–He models, we obtain T
eff = 36 600 K, log g = 3.95, and log N(He)/N(H) = −0.84. The star's FUSE and STIS spectra show photospheric absorption from C, N, O, Al, Si, P, S, Fe, and Ni. No stellar features from elements beyond the iron peak are observed. Both components of the N v λ1240 doublet exhibit P Cygni profiles, indicating a weak stellar wind, but no other wind features are seen. The star's photospheric abundances appear to have changed little since it left the red giant branch (RGB). Its C, N, O, Al, Si, Fe, and Ni abundances are consistent with published values for the red giant stars in M3, and the relative abundances of C, N, and O follow the trends seen on the cluster RGB. In particular, its low C abundance suggests that the star left the asymptotic giant branch before the onset of third dredge-up.
We have analyzed spectra from FUSE, COS, GHRS, and Keck HIRES of the UV-bright star Barnard 29 in M13 (NGC 6205). By comparing the photospheric abundances derived from multiple ionization states of ...C, N, O, Si, and S, we infer an effective temperature = 21,400 400 K. Balmer-line fits yield a surface gravity = 3.10 0.03. We derive photospheric abundances of He, C, N, O, Mg, Al, Si, P, S, Cl, Ar, Ti, Cr, Fe, Ni, and Ge. Barnard 29 exhibits an abundance pattern typical of the first-generation stars in M13, enhanced in oxygen and depleted in aluminum. An underabundance of C and an overabundance of N suggest that the star experienced nonconvective mixing on the red giant branch (RGB). We see no evidence of significant chemical evolution since the star left the RGB; in particular, it did not undergo third dredge-up. Previous workers found that the star's far-UV spectra yield an iron abundance about 0.5 dex lower than its optical spectrum, but the iron abundances derived from all of our spectra are consistent with the cluster value. We attribute this difference to our use of model atmospheres without microturbulence, which is ruled out by careful fits to optical absorption features. We derive a mass M*/M = 0.45-0.55 and luminosity = 3.26-3.35. Comparison with stellar-evolution models suggests that Barnard 29 evolved from a zero-age horizontal branch star of mass M*/M between 0.50 and 0.55, near the boundary between the extreme and blue horizontal branches.
We present the results of a Hubble Space Telescope NICMOS imaging survey of 22 T-type field brown dwarfs. Five are resolved as binary systems with angular separations of 0".05-0".35, and ...companionship is established on the basis of component F110W- F170M colors (indicative of CH4 absorption) and low probabilities of background contamination. Prior ground-based observations show 2MASS 1553+1532AB to be a common proper-motion binary. The properties of these systems--low multiplicity fraction (12 super(+7 -4) % resolved, as corrected for sample selection biases), close projected separations (r = 1.8-5.0 AU) and near-unity mass ratios--are consistent with previous results for field brown dwarf binaries. Three of the binaries have components that span the poorly understood transition between L dwarfs and T dwarfs. Spectral decomposition analysis of one of these, SDSS 1021-0304AB, reveals apeculiar flux reversal between its components, as its cooler T5 secondary is 630% brighter at 1.05 and 1.27 km than its T1 primary. This system, 2MASS 0518-2828AB, and SDSS 1534+1615AB all demonstrate that the J-band brightening observed between late-type L to mid-type T dwarfs is an intrinsic feature of this spectral transition, albeit less pronounced than previously surmised. We also find that the resolved binary fraction of L7 to T3.5 dwarfs is twice that of other L and T dwarfs, an anomaly that can be explained by a relatively rapid evolution of brown dwarfs through the L/T transition, perhaps driven by dynamic (nonequilibrium) depletion of photospheric condensates.
The Hubble Space Telescope (HST) Treasury Program on the Orion Nebula Cluster (ONC) has used 104 orbits of HST time to image the Great Orion Nebula region with the Advanced Camera for Surveys (ACS), ...the Wide-Field/Planetary Camera 2 (WFPC2), and the Near-Infrared Camera and Multi-Object Spectrograph (NICMOS) instrument in 11 filters ranging from the U band to the H band equivalent of HST. The program has been intended to perform the definitive study of the stellar component of the ONC at visible wavelengths, addressing key questions like the cluster initial mass function, age spread, mass accretion, binarity, and cirumstellar disk evolution. The scanning pattern allowed us to cover a contiguous field of approximately 600 arcmin super(2) with both ACS and WFPC2, with a typical exposure time of approximately 11 minutes per ACS filter, corresponding to a point source depth AB(F435W) = 25.8 and AB(F775W) = 25.2 with 0.2 mag of photometric error. We describe the observations, data reduction, and data products, including images, source catalogs, and tools for quick look preview. In particular, we provide ACS photometry for 3399 stars, most of them detected at multiple epochs; WFPC2 photometry for 1643 stars, 1021 of them detected in the U band; and NICMOS JH photometry for 2116 stars. We summarize the early science results that have been presented in a number of papers. The final set of images and the photometric catalogs are publicly available through the archive as High Level Science Products at the STScI Multimission Archive hosted by the Space Telescope Science Institute.
We determine projected rotation velocities in DAZ white dwarfs, for the first time using the rotational broadening of the Ca II K line. The results confirm previous findings that white dwarfs are ...very slow rotators, and set even more stringent upper limits of typically less than 10 km s-1. The few exceptions include 3 stars known or suspected to be variable ZZ Ceti stars, where the line broadening is very likely not due to rotation. The results demonstrate that the angular momentum of the core cannot be preserved completely between main sequence and final stage.
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