The ACS LCID Project: On the origin of dwarf galaxy types: a manifestation of the halo assembly bias?

We discuss how knowledge of the whole evolutionary history of dwarf galaxies, including details on the early star formation events, can provide insight on the origin of the different dwarf galaxy types. We suggest that these types may be imprinted by the early conditions of formation rather than being only the result of a recent morphological transformation driven by environmental effects. We present precise star formation histories of a sample of Local Group dwarf galaxies, derived from colour-magnitude diagrams reaching the oldest main-sequence turnoffs. We argue that these galaxies can be assigned to two basic types: fast dwarfs that started their evolution with a dominant and short star formation event, and slow dwarfs that formed a small fraction of their stars early and have continued forming stars until the present time (or almost). These two different evolutionary paths do not map directly onto the present-day morphology (dwarf spheroidal vs dwarf irregular). Slow and fast dwarfs also differ in their inferred past location relative to the Milky Way and/or M31, which hints that slow dwarfs were generally assembled in lower density environments than fast dwarfs. We propose that the distinction between a fast and slow dwarf galaxy reflects primarily the characteristic density of the environment where they form. At a later stage, interaction with a large host galaxy may play a role in the final gas removal and ultimate termination of star formation.Comment: 7 pages, 3 figures, ApJ Letters, submitted. Comments welcom

Spiral structure in the outer galactic disk, I: the third galactic quadrant

We combine optical and radio observations to trace the spiral structure in the third quadrant of the Milky Way. The optical observations consist of a large sample of young open clusters and associations, whereas the radio observations consist of a survey of nearby and distant clouds observed in CO. Both the optical and radio samples are the largest ones thus far presented in the literature. We use this unique material to analyze the behavior of interstellar extinction and to trace the detailed structure of the third Galactic quadrant (TGQ).We find that the outer (Cygnus) grand design spiral arm is traced by stellar and CO components, while the Perseus arm is traced solely by CO and is possibly being disrupted by the crossing of the Local (Orion) arm. The Local arm is traced by CO and young stars toward l ¼ 240 and extends for over 8 kpc along the line of sight reaching the outer arm. Finally, we characterize the Galactic warp and compare the geometries implied by the young stellar and CO components.Fil: Vazquez, Ruben Angel. Universidad Nacional de La Plata. Facultad de Ciencias Astronómicas y Geofísicas; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Astrofísica La Plata. Universidad Nacional de La Plata. Facultad de Ciencias Astronómicas y Geofísicas. Instituto de Astrofísica La Plata; ArgentinaFil: May, Jorge. Universidad de Chile; ChileFil: Carraro, Giovanni. European Southern Observatory; ChileFil: Bronfman, Leonardo. Universidad de Chile; ChileFil: Moitinho, Andre. Universidad de Lisboa; PortugalFil: Baume, Gustavo Luis. Universidad Nacional de La Plata. Facultad de Ciencias Astronómicas y Geofísicas; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Astrofísica La Plata. Universidad Nacional de La Plata. Facultad de Ciencias Astronómicas y Geofísicas. Instituto de Astrofísica La Plata; Argentin

Technology Teacher: Singing the Black and Blues

Gives simple, yet authoritative answers to the questions "Why is the sky blue?" and "Why is the sky black at night?" Combines technology with its application to Earth science, astronomy, and cosmology, and does so via language arts and music! This article was originally written for and published by the International Technology Education Association in its journal 'The Technology Teacher.' It is now archived on The Space Place Web site. Educational levels: Middle school, High school

Atmospheric mass loss by stellar wind from planets around main sequence M stars

We present an analytic model for the interaction between planetary atmospheres and stellar winds from main sequence M stars, with the purpose of obtaining a quick test-model that estimates the timescale for total atmospheric mass loss due to this interaction. Planets in the habitable zone of M dwarfs may be tidally locked and may have weak magnetic fields, because of this we consider the extreme case of planets with no magnetic field. The model gives the planetary atmosphere mass loss rate as a function of the stellar wind and planetary properties (mass, atmospheric pressure and orbital distance) and an entrainment efficiency coefficient $\alpha$. We use a mixing layer model to explore two different cases: a time-independent stellar mass loss and a stellar mass loss rate that decreases with time. For both cases we consider planetary masses within the range of $1\to10$ M$_{\oplus}$ and atmospheric pressures with values of 1, 5 and 10 atm. For the time dependent case, planets without magnetic field in the habitable zone of M dwarfs with initial stellar mass losses of $\leq \dot{M}_{w} < 10^{-11}$ M$_{\odot}$ yr$^{-1}$, may retain their atmospheres for at least 1 Gyr. This case may be applied to early spectral type M dwarfs (earlier than M5). Studies have shown that late type M dwarfs (later than M5) may be active for long periods of time ($\geq 4$Gyr), and because of that our model with constant stellar mass loss rate may be more accurate. For these stars most planets may have lost their atmospheres in 1 Gyr or less because most of the late type M dwarfs are expected to be active. We emphasize that our model only considers planets without magnetic fields. Clearly we must expect a higher resistance to atmospheric erosion if we include the presence of a magnetic field.Comment: Icarus, submitted. 18 pages, 6 figure

Reaching micro-arcsecond astrometry with long baseline optical interferometry; application to the GRAVITY instrument

A basic principle of long baseline interferometry is that an optical path difference (OPD) directly translates into an astrometric measurement. In the simplest case, the OPD is equal to the scalar product between the vector linking the two telescopes and the normalized vector pointing toward the star. However, a too simple interpretation of this scalar product leads to seemingly conflicting results, called here "the baseline paradox". For micro-arcsecond accuracy astrometry, we have to model in full the metrology measurement. It involves a complex system subject to many optical effects: from pure baseline errors to static, quasi-static and high order optical aberrations. The goal of this paper is to present the strategy used by the "General Relativity Analysis via VLT InTerferometrY" instrument (GRAVITY) to minimize the biases introduced by these defects. It is possible to give an analytical formula on how the baselines and tip-tilt errors affect the astrometric measurement. This formula depends on the limit-points of three type of baselines: the wide-angle baseline, the narrow-angle baseline, and the imaging baseline. We also, numerically, include non-common path higher-order aberrations, whose amplitude were measured during technical time at the Very Large Telescope Interferometer. We end by simulating the influence of high-order common-path aberrations due to atmospheric residuals calculated from a Monte-Carlo simulation tool for Adaptive optics systems. The result of this work is an error budget of the biases caused by the multiple optical imperfections, including optical dispersion. We show that the beam stabilization through both focal and pupil tracking is crucial to the GRAVITY system. Assuming the instrument pupil is stabilized at a 4 cm level on M1, and a field tracking below 0.2$\lambda/D$, we show that GRAVITY will be able to reach its objective of 10$\mu$as accuracy.Comment: 14 pages. Accepted by A&

A new algorithm for modelling photoionising radiation in smoothed particle hydrodynamics

We present a new fast algorithm which allows the simulation of ionising radiation emitted from point sources to be included in high-resolution three-dimensional smoothed particle hydrodynamics simulations of star cluster formation. We employ a Str\"omgren volume technique in which we use the densities of particles near the line-of-sight between the source and a given target particle to locate the ionisation front in the direction of the target. Along with one--dimensional tests, we present fully three--dimensional comparisons of our code with the three--dimensional Monte-Carlo radiative transfer code, MOCASSIN, and show that we achieve good agreement, even in the case of highly complex density fields.Comment: 10 pages, 7 figures, submitted to MNRA