Dynamics of solid dispersions in oil during the lubrication of point of contacts. Part 2: Molybdenum disulfide

A Hertzian contact consisting of a steel ball in contact with a glass disk is lubricated with MoS2 dispersions and observed by optical microscopy at various slide/roll conditions. In general the behavior of MoS2 and graphite are similar. That is, the solids tend to enter the contact and form a film on the contacting surfaces whenever a rolling component of motion is used, but solid particles seldom enter the contact during pure sliding. The MoS2 has more pronounced plastic flow behavior than graphite. However, the polished steel ball is more readily scratched by MoS2 than by graphite. Under the conditions of these studies, lower friction and wear are observed with pure oil rather than with the dispersions. However under other conditions (such as different contact geometry or rougher surfaces) the solid lubricant dispersions might be beneficial

Dynamics of solid dispersions in oil during the lubrication of point contacts. Part 1: Graphite

A Hertzian contact was lubricated with dispersed graphite in mineral oils under boundary lubrication conditions. The contact was optically observed under pure rolling, combined rolling and sliding, and pure sliding conditions. The contact was formed with a steel ball on the flat surface of a glass disk. Photomicrographs are presented which show the distribution of the graphite in and around the contact. Friction and surface damage are also shown for conditions when the base oils are used alone and when graphite is added to the base oils. Under pure rolling and combined rolling and sliding conditions, it is found that, for low speeds, a graphite film can form which will separate the contacting surfaces. Under pure sliding conditions, graphite accumulates at the inlet and sweeps around the contact, but very little of the graphite passes through the contact. The accumulated graphite appears to act as a barrier which reduces the supply of oil available to the contact for boundary lubrication. Friction data show no clear short term beneficial or detrimental effect caused by addition of graphite to the base oil. However, during pure sliding, more abrasion occurs on the polished balls lubricated with the dispersion than on those lubricated with the base oil alone. All observations were for the special case of a highly-polished ball on a glass surface and may not be applicable to other geometries and materials, or to rougher surfaces

Testing evolutionary tracks of Pre-Main Sequence stars: the case of HD113449

Evolutionary tracks are of key importance for the understanding of star formation. Unfortunately, tracks published by various groups differ so that it is fundamental to have observational tests. In order to do this, we intend to measure the masses of the two components of the Pre-Main Sequence binary HD113449 by combining radial velocity measurements taken with HARPS, with infrared interferometric data using AMBER on the VLTI. The spectroscopic orbit that has already been determined, combined with the first AMBER measurement, allows us to obtain a very first estimation of the inclination of the binary system and from this the masses of the two stars. More AMBER measurements of HD 113449 are needed to improve the precision on the masses: in the ESO period P82 two new measurements are scheduled.Comment: 4 pages, 3 figures; to appear in proceedings of Cool Star 15 conference, St.Andrews 200

Photometric and spectroscopic study of the intermediate-age open cluster NGC 2355

In this paper we analyse the evolutionary status and properties of the old open cluster NGC 2355, located in the Galactic anticentre direction, as a part of the long term programme BOCCE. NGC 2355 was observed with LBC@LBT using the Bessel $B$, $V$, and $I_c$ filters. The cluster parameters have been obtained using the synthetic colour-magnitude diagram (CMD) method, as done in other papers of this series. Additional spectroscopic observations with FIES@NOT of three giant stars were used to determine the chemical properties of the cluster. Our analysis shows that NGC 2355 has metallicity slightly less than solar, with [Fe/H]$=-0.06$ dex, age between 0.8 and 1 Gyr, reddening $E(B-V)$ in the range 0.14 and 0.19 mag, and distance modulus $(m-M)_0$ of about 11 mag. We also investigated the abundances of O, Na, Al, $\alpha$, iron-peak, and neutron capture elements, showing that NGC 2355 falls within the abundance distribution of similar clusters (same age and metallicity). The Galactocentric distance of NGC~2355 places it at the border between two regimes of metallicity distribution; this makes it an important cluster for the study of the chemical properties and evolution of the disc.Comment: 20 pages, 11 figures, Accepted on MNRA

HST resolves stars in a tiny body falling on the dwarf galaxy DDO 68

We present new Hubble Space Telescope (HST) imaging of a stream-like system associated with the dwarf galaxy DDO 68, located in the Lynx-Cancer Void at a distance of D$\sim$12.65 Mpc from us. The stream, previously identified in deep Large Binocular Telescope images as a diffuse low surface brightness structure, is resolved into individual stars in the F606W (broad V) and F814W ($\sim$I) images acquired with the Wide Field Camera 3. The resulting V, I color-magnitude diagram (CMD) of the resolved stars is dominated by old (age$\gtrsim$1-2 Gyr) red giant branch (RGB) stars. From the observed RGB tip, we conclude that the stream is at the same distance as DDO 68, confirming the physical association with it. A synthetic CMD analysis indicates that the large majority of the star formation activity in the stream occurred at epochs earlier than $\sim$1 Gyr ago, and that the star formation at epochs more recent than $\sim$500 Myr ago is compatible with zero. The total stellar mass of the stream is $\sim10^{6} M_{\odot}$, about 1/100 of that of DDO~68. This is a striking example of hierarchical merging in action at the dwarf galaxy scales.Comment: ApJ in pres

Chemical abundances and radial velocities in the extremely metal-poor galaxy DDO 68

We present chemical abundances and radial velocities of six HII regions in the extremely metal-poor star-forming dwarf galaxy DDO 68. They are derived from deep spectra in the wavelength range 3500 - 10,000 {\AA}, acquired with the Multi Object Double Spectrograph (MODS) at the Large Binocular Telescope (LBT). In the three regions where the [O III]$\lambda$4363 {\AA} line was detected, we inferred the abundance of He, N, O, Ne, Ar, and S through the "direct" method. We also derived the oxygen abundances of all the six regions adopting indirect method calibrations. We confirm that DDO 68 is an extremely metal-poor galaxy, and a strong outlier in the luminosity - metallicity relation defined by star-forming galaxies. With the direct-method we find indeed an oxygen abundance of 12+log(O/H)=7.14$\pm$0.07 in the northernmost region of the galaxy and, although with large uncertainties, an even lower 12+log(O/H)=6.96$\pm$0.09 in the "tail". This is, at face value, the most metal-poor direct abundance detection of any galaxy known. We derive a radial oxygen gradient of -0.06$\pm$0.03 dex/kpc (or -0.30 dex $R_{25}^{-1}$) with the direct method, and a steeper gradient of -0.12$\pm$0.03 dex/kpc (or -0.59 dex $R_{25}^{-1}$) from the indirect method. For the $\alpha$-element to oxygen ratios we obtain values in agreement with those found in other metal-poor star-forming dwarfs. For nitrogen, instead, we infer much higher values, leading to log(N/O)$\sim-1.4$, at variance with the suggested existence of a tight plateau at $-1.6$ in extremely metal poor dwarfs. The derived helium mass fraction ranges from Y=0.240$\pm$0.005 to Y=0.25$\pm$0.02, compatible with standard big bang nucleosynthesis. Finally, we measured HII region radial velocities in the range 479$-$522 km/s from the tail to the head of the "comet", consistent with the rotation derived in the HI.Comment: Accepted for publication in MNRA

Pre-main sequence spectroscopic binaries suitable for VLTI observations

A severe problem of the research in star-formation is that the masses of young stars are almost always estimated only from evolutionary tracks. Since the tracks published by different groups differ, it is often only possible to give a rough estimate of the masses of young stars. It is thus crucial to test and calibrate the tracks. Up to now, only a few tests of the tracks could be carried out. However, with the VLTI it is now possible to set constrains on the tracks by determining the masses of many young binary stars precisely. In order to use the VLTI efficiently, a first step is to find suitable targets, which is the purpose of this work. Given the distance of nearby star-forming regions, suitable VLTI targets are binaries with orbital periods between at least 50 days, and few years. Although a number of surveys for detecting spectroscopic binaries have been carried out, most of the binaries found so far have periods which are too short. We thus surveyed the Chamaeleon, Corona Australis, Lupus, Sco-Cen, rho Ophiuci star-forming regions in order to search for spectroscopic binaries with periods longer than 50 days, which are suitable for the VLTI observations. As a result of the 8 years campaign we discovered 8 binaries with orbital periods longer than 50 days. Amongst the newly discovered long period binaries is CS Cha, which is one of the few classical T Tauri stars with a circumbinary disk. The survey is limited to objects with masses higher than 0.1 to 0.2 Modot for periods between 1 and 8 years. We find that the frequency of binaries with orbital periods < 3000 days is of 20+/-5 percent. The frequency of long and short period pre-main sequence spectroscopic binaries is about the same as for stars in the solar neighbourhood. In total 14 young binaries are now known which are suitable for mass determination with the VLTI

LBT/MODS spectroscopy of globular clusters in the irregular galaxy NGC 4449

We present intermediate-resolution (R$\sim$1000) spectra in the $\sim$3500-10,000 A range of 14 globular clusters in the magellanic irregular galaxy NGC 4449 acquired with the Multi Object Double Spectrograph on the Large Binocular Telescope. We derived Lick indices in the optical and the CaII-triplet index in the near-infrared in order to infer the clusters' stellar population properties. The inferred cluster ages are typically older than $\sim$9 Gyr, although ages are derived with large uncertainties. The clusters exhibit intermediate metallicities, in the range $-1.2\lesssim$[Fe/H]$\lesssim-0.7$, and typically sub-solar [$\alpha/Fe$] ratios, with a peak at $\sim-0.4$. These properties suggest that i) during the first few Gyrs NGC 4449 formed stars slowly and inefficiently, with galactic winds having possibly contributed to the expulsion of the $\alpha$-elements, and ii) globular clusters in NGC 4449 formed relatively "late", from a medium already enriched in the products of type Ia supernovae. The majority of clusters appear also under-abundant in CN compared to Milky Way halo globular clusters, perhaps because of the lack of a conspicuous N-enriched, second-generation of stars like that observed in Galactic globular clusters. Using the cluster velocities, we infer the dynamical mass of NGC 4449 inside 2.88 kpc to be M($<$2.88 kpc)=$3.15^{+3.16}_{-0.75} \times 10^9~M_\odot$. We also report the serendipitous discovery of a planetary nebula within one of the targeted clusters, a rather rare event.Comment: Accepted for publication in MNRAS; corrected typo in author lis