Small scale structure of the Milky Way's stellar orbit distribution

The exact processes behind the formation and evolution of galaxies are interesting puzzles in modern astrophysics. Our Galaxy offers us the unique opportunity to be studied in detail, as we can obtain the 3D positions, 3D velocities and also the chemical information on a star-by-star basis. Different Galactic surveys have advanced in the effort of studying the Milky Way. The Gaia mission in particular provides the full 6D stellar position-velocity phase-space measurements for millions of its stars. By combining Gaia with chemical information from spectroscopic surveys, we can obtain a detailed physical picture of our Galaxy. In this thesis, we set out to investigate the stellar orbit distribution of the Milky Way, while also adding their chemical information ([Fe/H]) in a chemical tagging generalization approach. We first make use of the spectroscopic information from LAMOST, in combination with parallaxes and proper motions from Gaia. We develop a method to obtain improved spectrophotometric distances (with errors less than 6%) for 150 000 main sequence stars. With more precise distances at hand, we investigate the small-scale structure in the orbit distribution of the Galactic disc for ∼ 600 000 main sequence stars in LAMOST × Gaia. Most stars disperse from their birth sites and siblings, in orbit and orbital phase, becoming ‘field stars’. We explore and provide direct observational evidence for this process in the Milky Way disc, by quantifying the probability that orbit similarity among stars implies indistinguishable metallicity. We define the orbit similarity among pairs of stars through their distance in action-angle space ∆(J, θ) and their abundance similarity by ∆[Fe/H]. By grouping such star pairs into associations with a friend-of-friends algorithm linked by ∆(J,θ), we find that hundreds of mono-abundance groups –some clusters, some spread across the sky– are over an order-of-magnitude more abundant than expected for a smooth phase-space distribution, suggesting that we are witnessing the ‘dissolution’ of stellar birth associations into the field. We finally explore a significantly larger sample of 6.2 million stars with radial velocities in Gaia, which is not limited to main sequence stars. Although this sample does not have [Fe/H] information, we are able to recover the same major groups found in the previous sample in both action and angle space. Moreover, we are able to identify other known associations by simple inspection, opening up the possibility for this method to be applied to further characterize dissolving associations across the Galaxy

Discriminating among theories of spiral structure using Gaia DR2

We compare the distribution in position and velocity of nearby stars from the Gaia DR2 radial velocity sample with predictions of current theories for spirals in disc galaxies. Although the rich substructure in velocity space contains the same information, we find it more revealing to reproject the data into action-angle variables, and we describe why resonant scattering would be more readily identifiable in these variables. We compute the predicted changes to the phase space density, in multiple different projections, that would be caused by a simplified isolated spiral pattern, finding widely differing predictions from each theory. We conclude that the phase space structure present in the Gaia data shares many of the qualitative features expected in the transient spiral mode model. We argue that the popular picture of apparently swing-amplified spirals results from the superposition of a few underlying spiral modes.Comment: Revised version accepted to appear in MNRAS. Some significant improvements. A full resolution version of Fig 4 is available from http://www.physics.rutgers.edu/~sellwood/mult_res.pd

Unbiased TGAS×\timesLAMOST distances and the role of binarity

Spectrophotometric distances to stars observed by large spectroscopic surveys offer a crucial complement to parallax distances that remain very important also after the future Gaia data releases. Here we present a probabilistic approach to modeling spectroscopic information for a subset of 4,000 main sequence stars with good parallaxes ($\sigma_\varpi/\varpi<0.1$) from the LAMOST $\times$ TGAS $\times$ 2MASS cross-match, yielding a precise spectroscopic distance estimator with uncertainties of $\sim$6% for single stars. Unlike previous approaches to this problem, we explicitly account for the individual parallax uncertainties in the model building and fully incorporate the fraction of near-equal binaries of main sequence stars, which would lead to biased distance estimates if neglected. Using this model, we estimate the distance for all (150,000) main sequence stars from LAMOST Data Release 5, without parallax information. As an application, we compute their orbital actions, where our more precise distances result in 5 times smaller action uncertainties. This illustrates how future studies of the Milky Way's orbital structure can benefit from using our model. For the fainter and more distant stars of most current spectroscopic surveys, an approach such as the one presented in this work will deliver better distances than Gaia Data Release 2.Comment: 12 pages, 13 figures. Accepted for publication in MNRA

The Age Distribution of Stellar Orbit Space Clumps

The orbit distribution of young stars in the Galactic disk is highly structured, from well-defined clusters to streams of stars that may be widely dispersed across the sky, but are compact in orbital action-angle space. The age distribution of such groups can constrain the timescales over which co-natal groups of stars disperse into the `field'. Gaia data have proven powerful to identify such groups in action-angle space, but the resulting member samples are often too small and have too narrow a CMD coverage to allow robust age determinations. Here, we develop and illustrate a new approach that can estimate robust stellar population ages for such groups of stars. This first entails projecting the predetermined action-angle distribution into the 5D space of positions, parallaxes and proper motions, where much larger samples of likely members can be identified over a much wider range of the CMD. It then entails isochrone fitting that accounts for a) widely varying distances and reddenings; b) outliers and binaries; c) sparsely populated main sequence turn-offs, by incorporating the age information of the low-mass main sequence; and d) the possible presence of an intrinsic age spread in the stellar population. When we apply this approach to 92 nearby stellar groups identified in 6D orbit space, we find that they are predominately young ($\lesssim 1$ Gyr), mono-age populations. Many groups are established (known) localized clusters with possible tidal tails, others tend to be widely dispersed and manifestly unbound. This new age-dating tool offers a stringent approach to understanding on which orbits stars form in the solar neighborhood and how quickly they disperse into the field.Comment: Accepted for publication in Ap

Acoso hacia contratistas en una entidad pública

Los términos estudio e investigación científica no son lo mismo; entre ellos hay diferencia, mientras que investigar significa resolver un problema de carácter teórico, planteando una hipótesis, para luego verificarla en la práctica; estudiar es ejercitar el entendimiento para alcanzar o comprender algo. Por otra parte, la investigación académica en torno a la violación de derechos humanos en contra de contratistas en Colombia ha demostrado que el principal mecanismo de instigación es por medio del acoso laboral, dado que no se tienen evidencias técnicas y legales oportunas sobre intimidación, violencia física o persecución política.Requerimientos de sistema: Adobe Acrobat Reade

From birth associations to field stars: mapping the small-scale orbit distribution in the Galactic disc

Stars born at the same time in the same place should have formed from gas of the same element composition. But most stars subsequently disperse from their birth siblings, in orbit and orbital phase, becoming 'field stars'. Here we explore and provide direct observational evidence for this process in the Milky Way disc, by quantifying the probability that orbit-similarity among stars implies indistinguishable metallicity. We define the orbit similarity among stars through their distance in action-angle space, $\Delta (J,\theta)$, and their abundance similarity simply by $\Delta$[Fe/H]. Analyzing a sample of main sequence stars from Gaia DR2 and LAMOST, we find an excess of pairs with the same metallicity ($\Delta\mathrm{[Fe/H]}<0.1$) that extends to remarkably large separations in $\Delta (J,\theta)$ that correspond to nearly 1 kpc distances. We assess the significance of this effect through a mock sample, drawn from a smooth and phase-mixed orbit distribution. Through grouping such star pairs into associations with a friend-of-friends algorithm linked by $\Delta (J,\theta)$, we find 100s of mono-abundance groups with $\ge 3$ (to $\gtrsim 20$) members; these groups -- some clusters, some spread across the sky -- are over an order-of-magnitude more abundant than expected for a smooth phase-space distribution, suggesting that we are witnessing the 'dissolution' of stellar birth associations into the field.Comment: 16 pages, 13 figures. Accepted for publication in MNRA

Efecto del fertilizante acido fosforoso inyectado al pseudotallo de la planta de banano (MUSA AAA) en las variedades williams y gran enano en la region de la aguja municipio Zona Bananera

El banano es una fruta tropical originaria del sudeste asiático, probablemente de Malasia, China Meridional e Indonesia. Desde allí fue llevado a Madagascar en el siglo XV, de allí fue difundido a la costa oriental y occidental de África, aunque algunos lo sitúan en ese continente desde hace unos 8000 años. En el siglo XV los portugueses lo encontraron en la costa occidental africana, en la región de Guinea, llamándolo guineo. Hacia 1516 el padre Tomás de Berlanga lo introdujo en la isla La Española, en el Caribe, probablemente llevándolo desde las islas Canarias, donde se cultiva desde 1450. Parece ser que el comercio del banano en el mundo, realmente comenzó al final del siglo XIX. En 1915, Europa importó más de 100.000 toneladas de bananos de la variedad Gross Michel, pero posteriormente, en 1940, la llamada enfermedad de panamá diezmó las plantaciones y esta variedad ha sido reemplazada por variedades resistentes perteneciente al grupo Cavendish; aunque existen otras como las aromadas de Martinica y Guadalupe, algunas de las Islas Canarias, y las variedades largas africanas. Las musáceas en general, son cultivos permanentes que se auto reemplazan con un pequeño retoño que crece al lado de la planta que muere al ser cosechada. Las dos especies más conocidas en nuestro medio son: Musa paradisíaca que corresponde al plátano para cocción, y la Musa sapientum o banano que se consume maduro

Chemically peculiar A and F stars with enhanced s-process and iron-peak elements: stellar radiative acceleration at work

We present $\gtrsim 15,000$ metal-rich (${\rm [Fe/H]}>-0.2$dex) A and F stars whose surface abundances deviate strongly from Solar abundance ratios and cannot plausibly reflect their birth material composition. These stars are identified by their high [Ba/Fe] abundance ratios (${\rm [Ba/Fe]}>1.0$dex) in the LAMOST DR5 spectra analyzed by Xiang et al. (2019). They are almost exclusively main sequence and subgiant stars with $T_{\rm eff}\gtrsim6300$K. Their distribution in the Kiel diagram ($T_{\rm eff}$--$\log g$) traces a sharp border at low temperatures along a roughly fixed-mass trajectory (around $1.4M_\odot)$ that corresponds to an upper limit in convective envelope mass fraction of around $10^{-4}$. Most of these stars exhibit distinctly enhanced abundances of iron-peak elements (Cr, Mn, Fe, Ni) but depleted abundances of Mg and Ca. Rotational velocity measurements from GALAH DR2 show that the majority of these stars rotate slower than typical stars in an equivalent temperature range. These characteristics suggest that they are related to the so-called Am/Fm stars. Their abundance patterns are qualitatively consistent with the predictions of stellar evolution models that incorporate radiative acceleration, suggesting they are a consequence of stellar internal evolution particularly involving the competition between gravitational settling and radiative acceleration. These peculiar stars constitute 40% of the whole population of stars with mass above 1.5$M_\odot$, affirming that "peculiar" photospheric abundances due to stellar evolution effects are a ubiquitous phenomenon for these intermediate-mass stars. This large sample of Ba-enhanced chemically peculiar A/F stars with individual element abundances provides the statistics to test more stringently the mechanisms that alter the surface abundances in stars with radiative envelopes.Comment: 21 pages, 17 figures, accepted for publication in Ap