Surface Brightness Gradients Produced by the Ring Waves of Star Formation

We compute surface brightness profiles of galactic disks for outwardly propagating waves of star formation with a view to investigate the stellar populations in ring galaxies. We consider two mechanisms which can create outwardly propagating star forming rings in a purely gaseous disk --- a self-induced wave and a density wave. We show that the surface brightness profiles produced by both scenarios of ring formation are similar and are strongly sensitive to the velocity of the wave. The results of our computations are compared with the observational quantities sensitive to the young and old stellar populations in the ring galaxies A0035-335 (the Cartwheel galaxy) and VIIZw466. The best fit to the observed radial H_alpha surface brightness distribution in the Cartwheel galaxy is obtained for a wave velocity of about 90 km/s. The red continuum brightness of the ring can be fully explained by the evolving stars present in the trailing part of the wave. However the red continuum brightness in regions internal to the ring indicates that the wave of star formation propagates in a pre-existing stellar disk in the Cartwheel. The H_alpha and K-band surface brightness profiles in VIIZw466 match the values expected from stellar populations produced by a wave of star formation propagating in a purely gaseous disk very well. We conclude that VIIZw466 is probably experiencing the first event of star formation in the disk.Comment: Uses aas2pp4.sty and epsfig.sty, 15 pages To appear in Astrophysical Journal, March 10, 199

Velocity Dispersion of Dissolving OB Associations Affected by External Pressure of Formation Environment

This paper presents a possible way to understand dissolution of OB associations (or groups). Assuming rapid escape of parental cloud gas from associations, we show that the shadow of the formation environment for associations can be partially imprinted on the velocity dispersion at their dissolution. This conclusion is not surprising as long as associations are formed in a multiphase interstellar medium, because the external pressure should suppress expansion caused by the internal motion of the parental clouds. Our model predicts a few km s$^{-1}$ as the internal velocity dispersion. Observationally, the internal velocity dispersion is $\sim 1$ km s$^{-1}$ which is smaller than our prediction. This suggests that the dissipation of internal energy happens before the formation of OB associations.Comment: 6 pages. AJ accepte

Nonlinear Effects in Models of the Galaxy: 1. Midplane Stellar Orbits in the Presence of 3D Spiral Arms

With the aim of studying the nonlinear stellar and gaseous response to the gravitational potential of a galaxy such as the Milky Way, we have modeled 3D galactic spiral arms as a superposition of inhomogeneous oblate spheroids and added their contribution to an axisymmetric model of the Galactic mass distribution. Three spiral loci are proposed here, based in different sets of observations. A comparison of our model with a tight-winding approximation shows that the self-gravitation of the whole spiral pattern is important in the middle and outer galactic regions. As a first step to full 3D calculations the model is suitable for, we have explored the stellar orbital structure in the midplane of the Galaxy. We present the standard analysis in the pattern rotating frame, and complement this analysis with orbital information from the Galactic inertial frame. Prograde and retrograde orbits are defined unambiguously in the inertial frame, then labeled as such in the Poincar\'e diagrams of the non-inertial frame. In this manner we found a sharp separatrix between the two classes of orbits. Chaos is restricted to the prograde orbits, and its onset occurs for the higher spiral perturbation considered plausible in our Galaxy.Comment: 23 pages, 22 Figures. Latex. Submitted to Ap

Cosmic Ray Diffusion from the Galactic Spiral Arms, Iron Meteorites, and a possible climatic connection?

We construct a Galactic cosmic ray (CR) diffusion model while considering that CR sources reside predominantly in the Galactic spiral arms. We find that the CR flux (CRF) reaching the solar system should periodically increase each crossing of a Galactic spiral arm. We search for this signal in the CR exposure age record of Iron meteorites and confirm this prediction. We then check the hypothesis that climate, and in particular the temperature, is affected by the CRF to the extent that glaciations can be induced or completely hindered by possible climatic variations. We find that although the geological evidence for the occurrence of IAEs in the past Eon is not unequivocal, it appears to have a nontrivial correlation with the spiral arm crossings--agreeing in period and phase. Thus, a better timing study of glaciations could either confirm this result as an explanation to the occurrence of IAEs or refute a CRF climatic connection.Comment: 4 Journal pages, 2 figures, revtex4. Appearing today in Phys Rev Let

The Gaia-ESO Public Spectroscopic Survey: Motivation, implementation, GIRAFFE data processing, analysis, and final data products

The Gaia-ESO Public Spectroscopic Survey is an ambitious project designed to obtain astrophysical parameters and elemental abundances for 100,000 stars, including large representative samples of the stellar populations in the Galaxy, and a well-defined sample of 60 (plus 20 archive) open clusters. We provide internally consistent results calibrated on benchmark stars and star clusters, extending across a very wide range of abundances and ages. This provides a legacy data set of intrinsic value, and equally a large wide-ranging dataset that is of value for homogenisation of other and future stellar surveys and Gaia's astrophysical parameters. This article provides an overview of the survey methodology, the scientific aims, and the implementation, including a description of the data processing for the GIRAFFE spectra. A companion paper (arXiv:2206.02901) introduces the survey results. Gaia-ESO aspires to quantify both random and systematic contributions to measurement uncertainties. Thus all available spectroscopic analysis techniques are utilised, each spectrum being analysed by up to several different analysis pipelines, with considerable effort being made to homogenise and calibrate the resulting parameters. We describe here the sequence of activities up to delivery of processed data products to the ESO Science Archive Facility for open use. The Gaia-ESO Survey obtained 202,000 spectra of 115,000 stars using 340 allocated VLT nights between December 2011 and January 2018 from GIRAFFE and UVES. The full consistently reduced final data set of spectra was released through the ESO Science Archive Facility in late 2020, with the full astrophysical parameters sets following in 2022

Holes and shells in the interstellar medium of the nearby dwarf galaxy IC 2574

Original article can be found at: http://www.journals.uchicago.edu/AJ/--Copyright American Astronomical SocietyWe present H I synthesis observations of the nearby dwarf galaxy IC 2574 (a member of the M81 group of galaxies) made with the NRAO2 Very Large Array (VLA) in its B, C, and D conÐgurations at high spatial and velocity resolution (95 pc]2.6 km s~1). In addition, we present optical broad- and narrowband images obtained with the 2.2 m telescope of the Calar Alto Observatory. The VLA H I observations show a stunning amount of detail in the form of 48 mostly expanding H I shells and holes in its neutral interstellar medium. These features range in size from about 100 pc (a limit set by the size of the beam) to about 1000 pc, dominating the appearance of the H I surface brightness map. Their dynamics clearly inÑuence the velocity Ðeld of IC 2574. In addition to well deÐned holes, some large scale ([1000 pc) coherent features are visible in the channel maps. They are probably the remainder of an older shell population. Current star formation, as traced by Ha emission, is predominantly found along the rims of the larger H I holes, suggesting propagating star formation. On linear scales of B95 pc, star formation occurs if the H I surface density reaches values higher than 1021 cm~2. The radial expansion of the H I holes (8È12 km s~1), the indicative ages (10È60 Myr) and the energy requirements for their formation (1050È1053 ergs) can be understood in terms of the combined e ects of stellar winds and multiple supernova explosions of the most massive stars formed during a recent phase of active star formation.Peer reviewe

RUNAWAY STARS AND THE FORCE PERPENDICULAR TO THE GALACTIC PLANE P.O. Lindblad

Motions of 32 Hipparcos stars with distances from the galactic plane between 400 and 3000 pc, ages less than 80 Myr and with known radial velocities have been studied. Of 25 stars younger than 40 Myr, 24 move out from the plane with velocity components perpendicular to the plane between 0 and 150 km/s. Obviously, these stars are runaway stars born in the galactic plane that have not yet reached their maximum distance from the plane. Assuming that the ejection from the plane took place soon after the birth of the star, the time it has taken the star to reach its distance from the plane must be close to the age of the star. The knowledge of the distance from the plane, the velocity at that distance and the time it has taken to reach that distance contains information about the gravitational force component perpendicular to the plane. This scheme complements the classical statistical methods for determining K z to larger values of Z. The suitability of this method to determine the force ..

The Kinematics And Nature Of Gould's Belt - A 30 Myr Old Star Forming Region

For a sample of 2440 non-supergiant Hipparcos stars belonging to Stromgren's `early group' luminosities, temperatures, ages and distances have been determined. 241 stars younger than 30 Myr with well determined ages and space velocities fall within the local flattened system, inclined 20 o to the galactic plane, called Gould's Belt. The system is dominated by a number of nearby prominent associations within a distance of 700 pc, foremost the Scorpius-Centaurus and Orion associations. We derive galactic rotation parameters for young stars of ages less than 30 Myr situated outside the Gould Belt region. This gives a flat rotation curve with a circular angular velocity at the Sun of \Omega c = 25:3 \Sigma 1:5 km s \Gamma1 kpc \Gamma1 . The kinematics of Gould's Belt differs significantly from that of stars outside the system. Besides a slight outwards motion of a few km s \Gamma1 it rotates in the same direction as the galactic rotation and expands, giving apparent values for the..

Multicentric Analysis of the Species Distribution and Antifungal Susceptibility of Cryptic Isolates from Aspergillus Section Fumigati

International audienceABSTRACT The antifungal susceptibility of Aspergillus cryptic species is poorly known. We assessed 51 isolates, belonging to seven Fumigati cryptic species, by the EUCAST reference method and the concentration gradient strip (CGS) method. Species-specific patterns were observed, with high MICs for azole drugs, except for Aspergillus hiratsukae and Aspergillus tsurutae , and high MICs for amphotericin B for Aspergillus lentulus and Aspergillus udagawae . Essential and categorical agreements between EUCAST and CGS results were between 53.3 and 93.3%