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We present color-magnitude diagrams (CMDs) based on HST F555W (“V”) and F814W (“I”) observations of three old LMC clusters: NGC 2210, NGC 1786, and Reticulum. The fiducial derived from the CMD of NGC 2257, another LMC cluster, provided a good fit to the data for the new clusters. Because NGC 2257 has a similar metallicity ([Fe/H]∼ −1.8) to NGC 2210, NGC 1786, and Reticulum, the agreement between the CMDs of all four clusters indicates that they have the same age. This preliminary analysis suggests that any age differences are smaller than 2 Gyr. These new results mean that there are now 11 old LMC clusters with similar ages. An initial epoch of star cluster formation therefore happened in a short period over a large volume of space, a volume much larger than is now covered by the present-day optical LMC.
RR Lyrae variables are old (>10 Gyr) stars and, as such, they are useful probes of the earliest events of star formation in galaxies (Bernard et al. 2008, Martínez-Vázquez et al. 2016) as well as of the galaxy assembly process predicted by ΛCDM simulations of structure formation. In fact, the nature of the building-blocks of galaxies such as the Milky Way, and in particular, those of their stellar haloes, has been a matter of a substantial debate (Venn et al. 2004). Unlike other stellar tracers, RR Lyrae offer a snapshot of the stellar content present at the epoch when most of the merging action is predicted to have taken place, and thus they are ideal witnesses of this process.
We report new results from a program which is aimed at obtaining deep CCD photometry for a sample of relatively nearby globular clusters having a wide range of metallicities. The CCD cameras on the CFHT 3.6 m, CTIO 4 m and KPNO 4 m telescopes have been used over the past 4 years to obtain deep exposures in regions of a number of clusters. In order to avoid the severest crowding, all of our observations have been obtained at distances of greater than ~ 5 core radii from the cluster centers. The images have been analysed by using the DAOPHOT point-spread-function fitting routines.
This paper provides an overview of the U.S. Department of Energy’s (DOE) hydrogen and fuel cell activities within the Office of Energy Efficiency and Renewable Energy (EERE), focusing on key targets, progress towards meeting those targets, and materials-related issues that need to be addressed. The most recent, state-of-the-art data on metrics such as cost, durability, and performance of fuel cell and hydrogen technologies are presented. Key technical accomplishments to date include a 50% reduction in the modeled high volume cost of fuel cells since 2006, and an 80% cost reduction for electrolyzers since 2002. The statuses of various hydrogen production, delivery, and storage technologies are also presented along with a summary of materials-related challenges for hydrogen infrastructure technologies such as compression, dispensing, seals, pipeline materials/embrittlement, and storage materials. Specific examples and areas requiring more research are discussed. Finally, future plans including EERE’s lab consortium approach such as HyMARC (Hydrogen Storage Materials Advanced Research Consortium) and FC-PAD (Fuel Cell Performance and Durability) Consortia, are summarized.
The WFPC2 aboard the Hubble Space Telescope has been used to obtain deep images in three fields at different radial positions in the nearest Globular cluster, M4 (NGC 6121). In this paper, we discuss the white dwarf cooling sequence and show how the dynamical structure of the cluster will affect their cumulative distribution function. We also present the first discussion of our observations of the faint cluster main sequence stars.
We observed a sample of zero proper-motion stars (μ < 0.″50 cent−1) from a field previously studied by Suntzeff et al. (1986). This field is 2°.5 NW (2.6 kpc) of the center of the SMC. We obtained spectra for ~ 40 stars in the region of the Ca II infrared triplet using the CTIO Argus fiber-fed spectrograph. We also obtained Argus echelle spectra of a single order at 6300Å with R = 18000 in one run. The low-dispersion spectra were reduced to metallicities based on the Ca II equivalent widths using the Da Costa & Armandroff (1995) technique and the metallicity scale from Zinn & West (1984). The typical abundance error is 0.12 dex. For half the sample, we have echelle velocities which are accurate to 1.5 km s−1. For the rest of the sample, the low-dispersion data yield single-observation velocities accurate to about 5 km s−1 based on repeat observations.
Color-magnitude diagrams reaching from the giant branches to Vlim ~ 27.0, or about three magnitudes fainter than the turnoff, have been obtained in V and I with WFPC2 on HST for NGC 2419, Pal 3, Pal 4 and Eridanus, whose relative ages are discussed.
The Large Magellanic Cloud (LMC) is unique among galaxies in the Local Group in that it is the most massive non-spiral, is relatively gas-rich, and is actively forming stars. Determining its star-formation rate (SFR) as a function of time will be a cornerstone in our understanding of galaxy evolution. The best method of deriving a galaxy's past SFR is to compare the densities of stars in a color-magnitude diagram (CMD), a Hess diagram, with model Hess diagrams. The LMC has a complex stellar population with ages ranging from 0 to ~ 14 Gyr and metallicities from −2 ≲ [Fe/H] ≲ −0.4, and deriving its SFR and simultaneously constraining model input parameters (distance, age-metallicity relation, reddening, and stellar models) requires well-populated CMDs that span the magnitude range 15 ≤ V ≤ 24. Although existing CMDs of field stars in the LMC show tantalizing evidence for a significant burst of star formation that occurred ~ 3 Gyr ago (for examples, see Westerlund et al. 1995; Vallenari et al. 1996; Elson, et al. 1997; Gallagher et al. 1999, and references therein), estimates of the enhancement in the SFR vary from factors of 3 to 50. This uncertainty is caused by the relatively large photometric errors that plague crowded ground-based images, and the small number statistics that plague CMDs created from single Wide Field Planetary Camera 2 (WFPC2) images.
Close binary systems may undergo the “Common Envelope” (CE) phase when the primary star expands on the red giant branch or the asymptotic giant branch. Filling its Roche Lobe, the primary transfers mass to the companion driving it out of thermal equilibrium and causing it to expand as well. The giant core and the companion star become surrounded by a CE. When sufficient energy is deposited in the circumstellar material this will be ejected and the binary orbit will shrink further (see review by Iben 1995). Planetary nebulae (PNe) with short-period binary nuclei are considered the most probable post-CE candidates. Abell 35, Lotr 1 and Lotr 5 (the Abell 35-like objects) are the only three PNe with binary nuclei known to contain a very hot UV-bright primary and a chromospherically active, rapidly rotating, G-K companion that dominates the optical spectrum. The origin of these unusual systems is unclear and hence presents a challenge to theories of binary star evolution. Identified in 1966 by Abell, Abell 35 is possibly the largest PN known (D=1.6 pc at a distance of 360 pc, Jacoby 1981) and also the oldest (the kinematical age is 185.000 years from the small expansion velocity of 4.2 km/s, Bohuski 1974). The bright giant star BD −22° 3467 (mv = 9.6mag) lies off-center within the nebula. A white dwarf was detected at the same location in 1988 in IUE spectra obtained by Grewing and Bianchi. BD − 22° 3467 has a vsin i of 90 km/s (Vilhu et al. 1991), variable Hα and Ca II emission lines associated with chromospheric activity, and a variable light curve (P=0.76 days, Jasniewicz and Acker 1988) probably produced by the rotation of the giant star. All attempts to determine the orbital period have failed, raising doubts as to whether the nucleus of Abell 35 is a close binary at all. In pursuit of this point, we have started a radial velocity study of the giant companion.
Primordial binaries in globular clusters are important both because their properties are an integral part of the description of the stellar population and because they can strongly influence the dynamical evolution of the cluster (see Hut, this volume).
HST V, I color-magnitude diagrams (CMDs) of four outer-halo clusters, NGC 2419, Pa13, Pal4 and Eridanus, provide insight into the relative ages of old star clusters throughout the 200 kpc diameter volume sampled, and thus into the formation epoch of the Milky Way galaxy.
Fornax shows a radial age gradient. The old population is the most extended one similar to what was found in other dwarf galaxies, while younger populations are more centrally concentrated. With ≈ −1.2 dex the dominant intermediate-age population is much more metal-rich than the oldest populations traced by Fornax's globular clusters (≈ −2 dex, 15–11 Gyr). Star formation appears to have proceeded continuously with decreasing rates rather than in distinct episodes. Fornax contains the youngest population ever found in a dwarf spheroidal galaxy, 100–200 Myr, which makes its apparent lack of gas even more puzzling.
We used deep HST imaging of three LMC clusters, NGC 2257, NGC 1466 and Hodge 11, to determine their ages relative to Milky Way clusters. They all have similar ages to within ± 1 Gyr. For NGC 2257 and NGC 1466, we measured the blue straggler specific frequencies and find them to be similar to those of Galactic clusters.
Since the date of the 1935 Paris meeting two total eclipses have been successfully observed. Throughout the long path crossing Siberia and Japan the weather on June 19, 1936 on the whole about lived up to predictions. On account of widely scattered clouds neighbouring expeditions had quite different luck with the weather. In contrast, the June 8, 1937 eclipse was seen throughout the whole track under universally clear skies, which is all the more surprising for the reason that eclipse expeditions to the tropics usually fare badly with the weather. Stewart and Stokley in a ship at sea were able to observe the eclipse with a measured duration of 7 min. 6 sec., the longest period of totality in 1200 years.
In consequence of the decision made by the Fifth General Assembly of the I.A.U. I have been entrusted, from January 1936, with the direction of the Central Bureau for the International Service of Latitudes.
I am much indebted to Prof. Kimura, who preceded me as Director and to Prof. Kohlschütter, Director of the Geodetic Institute of Potsdam, for information and advice, which has been of great assistance to me; therefore I desire to acknowledge to them my deep gratitude.
The values given below are those published in each annual report of the international latitude work. They were calculated from the observations at five stations, except the last part of 1934 which was made without Kitab, because the observation books from Kitab since November 1934 have arrived too late at the Central Bureau.
The president calls attention to the large and increasing membership of Commission 12 and the policy of concentrating in it all matters relating to the sun. The result makes it comparable in breadth of field and in membership to the former Union for Co-operation in Solar Research. The main point in favour of this policy is the increased interest in the meetings of the Commission and the larger number of individuals reached compared with the meetings of small committees. One recalls the general sessions of the Solar Union in which each one present felt himself a part of the Union and in real touch with the work of different sections and after the discussions went away with fuller knowledge of what it was all about. This was a valuable result not attained to the same degree from the general sessions of the present Union, but in a measure it does follow from the meetings of the Solar Physics Committee. On the other hand the question may be raised whether or not the merging of independent commissions into subdivisions of a large commission lessens their interest to an extent not balanced by the advantages. If the present policy holds, it seems to the president that a re-organisation of Commission 12 is advisable by which more responsibility is laid upon the directors of centres. The basis of membership in the Commission may well be considered and recommendations formulated for transmission to the Executive Committee.
The majority of the members of the Commission have been good enough to send in full reports regarding subjects of interest to this Commission. Extracts from them relating to questions which may be the subject of useful discussion at the forthcoming meeting in Paris are given below; references to research undertaken during the last three years, particulars of which are readily available in recent astronomical publications, are omitted.