A Capture Scenario for Globular Cluster Omega Centauri

We explore an accretion origin for Omega Cen by N-body modeling of the orbital decay and disruption of a Milky-Way dwarf satellite. This work is focused on studying a particular satellite model that aims to reproduce the present orbit of Omega Cen, as recently determined from absolute proper motions. The model satellite is launched from 58 kpc from the Galactic Center, on a radial, low-inclination orbit. We find that a capture scenario can produce an Omega Cen-like object with the current low-energy orbit of the cluster. Our best model is a nucleated galaxy with a Hernquist density profile that has a mass of 8 10**8 Msun, and a half-mass radius of 1.4 kpc.Comment: 15 pages, 5 figures, accepted by ApJ

The origin of globular cluster FSR 1758

Context. Globular clusters in the Milky Way are thought to have either an in situ origin, or to have been deposited in the Galaxy by past accretion events, like the spectacular Sagittarius dwarf galaxy merger.Aims. We probe the origin of the recently discovered globular cluster FSR 1758, often associated with some past merger event and which happens to be projected toward the Galactic bulge. We performed a detailed study of its Galactic orbit, and assign it to the most suitable Galactic component.Methods. We employed three different analytical time-independent potential models to calculate the orbit of the cluster by using the Gauss Radau spacings integration method. In addition, a time-dependent bar potential model is added to account for the influence of the Galactic bar. We ran a large suite of simulations via a Montecarlo method to account for the uncertainties in the initial conditions.Results. We confirm previous indications that the globular cluster FSR 1758 possesses a retrograde orbit with high eccentricity. The comparative analysis of the orbital parameters of star clusters in the Milky Way, in tandem with recent metallicity estimates, allows us to conclude that FSR 1758 is indeed a Galactic bulge intruder. The cluster can therefore be considered an old metal-poor halo globular cluster formed in situ that is passing right now in the bulge region. Its properties, however, can be roughly accounted for by also assuming that the cluster is part of some stream of extra-Galactic origin.Conclusions. We conclude that assessing the origin, either Galactic or extra-galactic, of globular clusters is surely a tantalising task. In any case, by using an Occam's razor argument, we tend to prefer an in situ origin for FSR 1758

In-Situ Star Formation in the Outskirts of the Large Magellanic Cloud: Gaia DR2 Confirmation

We explore the Gaia DR2 proper motions of six young, main-sequence stars, members of the Large Magellanic Cloud (LMC) reported by Moni Bidin et al. (2017). These stars are located in the outskirts of the disk, between 7 and 13 degrees from the LMC's center where there is very low H I content. Gaia DR2 proper motions confirm that four stars formed locally, in situ, while two are consistent with being expelled via dynamical interactions from inner, more gas-rich regions of the LMC. This finding establishes that recent star formation occurred in the periphery of the LMC, where thus far only old populations are known.Comment: Accepted for publication in Astrophysical Journal Letter

The Origins of Young Stars in the Direction of the Leading Arm of the Magellanic Stream: Abundances, Kinematics, and Orbits

We explore the origins of the young B-type stars found by Casetti-Dinescu et al.(2014) at the outskirts of the Milky-Way disk in the sky region of Leading Arm of the Magellanic Stream. High-resolution spectroscopic observations made with the MIKE instrument on the Magellan Clay 6.5m telescope for nine stars are added to the previous sample analyzed by Zhang et al. (2017). We compile a sample of fifteen young stars with well-determined stellar types, ages, abundances and kinematics. With proper motions from Gaia DR2 we also derive orbits in a realistic Milky-Way potential. We find that our previous radial-velocity selected LA candidates have substantial orbital angular momentum. The substantial amount of rotational component for these stars is in contrast with the near-polar Magellanic orbit, thus rendering these stars unlikely members of the LA. There are four large orbital-energy stars in our sample. The highest orbital-energy one has an age shorter than the time to disk crossing, with a birthplace $z=2.5$~kpc and $R_{\rm GC}\sim 28$~kpc. Therefore, the origin of this star is uncertain. The remaining three stars have disk runaway origin with birthplaces between 12 and 25 kpc from the Galactic center. Also, the most energetic stars are more metal poor ([Mg/H] =$-0.50\pm0.07$) and with larger He scatter ($\sigma_{\rm [He/H]} = 0.72$) than the inner disk ones ([Mg/H] $=0.12\pm0.36$, $\sigma_{\rm [He/H]} = 0.15$). While the former group's abundance is compatible with that of the Large Magellanic Cloud, it could also reflect the metallicity gradient of the MW disk and their runaway status via different runaway mechanisms.Comment: 58 pages, 17 figures, 7 tables. Accepted for publication in Ap

Chemical Composition of Young Stars in the Leading Arm of the Magellanic System

Chemical abundances of eight O- and B-type stars are determined from high-resolution spectra obtained with the MIKE instrument on the Magellan 6.5m Clay telescope. The sample is selected from 42 candidates of membership in the Leading Arm of the Magellanic System. Stellar parameters are measured by two independent grids of model atmospheres and analysis procedures, confirming the consistency of the stellar parameter results. Abundances of seven elements (He, C, N, O, Mg, Si, and S) are determined for the stars, as are their radial velocities and estimates of distances and ages. Among the seven B-type stars analyzed, the five that have radial velocities compatible with membership to the LA have an average [Mg/H] of $-0.42\pm0.16$, significantly lower than the average of the remaining two [Mg/H] = $-0.07\pm0.06$ that are kinematical members of the Galactic disk. Among the five LA members, four have individual [Mg/H] abundance compatible with that in the LMC. Within errors, we can not exclude the possibility that one of these stars has a [Mg/H] consistent with the more metal-poor, SMC-like material. The remaining fifth star has a [Mg/H] close to MW values. Distances to the LA members indicate that they are at the edge of the Galactic disk, while ages are of the order of $\sim 50-70$ Myr, lower than the dynamical age of the LA, suggesting a single star-forming episode in the LA. V$_{\rm LSR}$ the LA members decreases with decreasing Magellanic longitude, confirming the results of previous LA gas studies.Comment: 61 pages, 18 figures, 5 tables. Accepted for publication in Ap

Proper Motions in the Galactic Bulge: Plaut's Window

A proper motion study of a field of 20' x 20' inside Plaut's low extinction window (l,b)=(0 deg,-8 deg), has been completed. Relative proper motions and photographic BV photometry have been derived for ~21,000 stars reaching to V~20.5 mag, based on the astrometric reduction of 43 photographic plates, spanning over 21 years of epoch difference. Proper motion errors are typically 1 mas/yr and field dependent systematics are below 0.2 mas/yr. Cross-referencing with the 2MASS catalog yielded a sample of ~8,700 stars, from which predominantly disk and bulge subsamples were selected photometrically from the JH color-magnitude diagram. The two samples exhibited different proper-motion distributions, with the disk displaying the expected reflex solar motion as a function of magnitude. Galactic rotation was also detected for stars between ~2 and ~3 kpc from us. The bulge sample, represented by red giants, has an intrinsic proper motion dispersion of (sigma_l,sigma_b)=(3.39, 2.91)+/-(0.11,0.09) mas/yr, which is in good agreement with previous results, and indicates a velocity anisotropy consistent with either rotational broadening or tri-axiality. A mean distance of 6.37^{+0.87}_{-0.77} kpc has been estimated for the bulge sample, based on the observed K magnitude of the horizontal branch red clump. The metallicity [M/H] distribution was also obtained for a subsample of 60 bulge giants stars, based on calibrated photometric indices. The observed [M/H] shows a peak value at [M/H]~-0.1 with an extended metal poor tail and around 30% of the stars with supersolar metallicity. No change in proper motion dispersion was observed as a function of [M/H]. We are currently in the process of obtaining CCD UBVRI photometry for the entire proper-motion sample of ~21,000 stars.Comment: Submitted to AJ April 17th 2007. Accepted June 8th 2007. 45 pages, 14 figure

Proper Motion Study of the Magellanic Clouds using SPM material

Absolute proper motions are determined for stars and galaxies to V=17.5 over a 450 square-degree area that encloses both Magellanic Clouds. The proper motions are based on photographic and CCD observations of the Yale/San Juan Southern Proper Motion program, which span over a baseline of 40 years. Multiple, local relative proper motion measures are combined in an overlap solution using photometrically selected Galactic Disk stars to define a global relative system that is then transformed to absolute using external galaxies and Hipparcos stars to tie into the ICRS. The resulting catalog of 1.4 million objects is used to derive the mean absolute proper motions of the Large Magellanic Cloud and the Small Magellanic Cloud; (\mu_\alpha\cos\delta,\mu_\delta)_{LMC}=(1.89,+0.39)\pm (0.27,0.27)\;\;\{mas yr}^{-1} and (\mu_\alpha\cos\delta,\mu_\delta)_{SMC}=(0.98,-1.01)\pm (0.30,0.29)\;\;\{mas yr}^{-1}. These mean motions are based on best-measured samples of 3822 LMC stars and 964 SMC stars. A dominant portion (0.25 mas yr$^{-1}$) of the formal errors is due to the estimated uncertainty in the inertial system of the Hipparcos Catalog stars used to anchor the bright end of our proper motion measures. A more precise determination can be made for the proper motion of the SMC {\it relative} to the LMC; (\mu_{\alpha\cos\delta},\mu_\delta)_{SMC-LMC} = (-0.91,-1.49) \pm (0.16,0.15)\;\;\{mas yr}^{-1}. This differential value is combined with measurements of the proper motion of the LMC taken from the literature to produce new absolute proper-motion determinations for the SMC, as well as an estimate of the total velocity difference of the two clouds to within $\pm$54 kms$^{-1}$.Comment: 50 pages (referee format), 13 figures. Accepted for publication in A

Biodegradation of Polyvinyl Alcohol-Based Binary Composites

Исследовано поведение бинарных композитов (БК), включающих поливиниловый спирт (ПВС) марки 1799, с различным содержанием микроцеллюлозы (МЦ) в воде и биохимических средах: почве, компосте, водно-иловой смеси. Результаты эксперимента подчеркивают сложный характер биодеградации исследуемых БК в естественных условиях. Введение МЦ порядка 60 об. % в полимерную матрицу ПВС сопровождается образованием пористой структуры, способствующей проникновению воды и компонентов биохимических сред в объем материала, при этом равномерное распределение волокон МЦ создает эффект армирующего наполнителя и позволяет сохранять прочность даже при длительном вымачивании БК. Достижение «нулевой» прочности (т. е. такое состояние образца, которое не позволяет провести испытание, так как материал разрушается в руках) при экспозиции в воде наблюдается через 2 сут, 14 сут, 2 мес. для БК с содержанием МЦ 0, 80–60, 40–10 об. % соответственно. Через 6 месяцев инкубации в почвенной среде индекс деструкции БК составил 0.89, 0.87, 0.95, 0.96 при степени наполнения 10, 20, 40, 80 об. % соответственно, при этом наблюдалось отсутствие фрагментации образцов. Предложен метод компьютерной цветометрии в динамике процесса биодеградации наполненных полисахаридами термопластов для оценки степени проникновения компонентов биохимческих сред в объем материалаThe behavior of binary composites (BC) containing polyvinyl alcohol (PVA‑1799) and various amounts of microcellulose (MC) in water and biochemical media (soil, compost, water-silt mixture) is studied. The results of the experiment emphasize the complex nature of the biodegradation of the studied composites under natural conditions. Introduction of microcellulose into the polymer matrix of PVA about 60 vol.% is accompanied by the formation of a porous structure that promotes the penetration of water and components of biochemical media into the volume of the material. At the same time, the uniform distribution of the MC fibers creates the effect of a reinforcing filler and allows you to maintain strength even with prolonged soaking of the BC. The achievement of «zero» strength at exposure in water is observed after 2 days, 14 days, 2 months. for BC with a content of MC 0, 80–60, 40–10 vol.%, respectively. After 6 months of incubation in the soil environment, the index of destruction of composites was 0.89, 0.87, 0.95, 0.96 with a degree of filling with microcellulose of 10, 20,40, 80 vol.%, respectively, while there was a lack of fragmentation of the samples. A method of computer colorometry in the dynamics of biodegradation of polysaccharide-filled thermoplastics is proposed to assess the degree of penetration of components of biochemical media into the volume of the materia