Estimating Be Star Disk Radii using H-alpha Emission Equivalent Widths

We present numerical models of the circumstellar disks of Be stars, and we describe the resulting synthetic H-alpha emission lines and maps of the wavelength-integrated emission flux projected onto the sky. We demonstrate that there are monotonic relationships between the emission line equivalent width and the ratio of the angular half-width at half maximum of the projected disk major axis to the radius of the star. These relationships depend mainly upon the temperatures of the disk and star, the inclination of the disk normal to the line of sight, and the adopted outer boundary for the disk radius. We show that the predicted H-alpha disk radii are consistent with those observed directly through long baseline interferometry of nearby Be stars (especially once allowance is made for disk truncation in binaries and for dilution of the observed H-alpha equivalent width by continuum disk flux in the V-band).Comment: 12 pages, 2 figures, ApJL in pres

Observational Constraints on Interstellar Grain Alignment

We present new multicolor photo-polarimetry of stars behind the Southern Coalsack. Analyzed together with multiband polarization data from the literature, probing the Chamaeleon I, Musca, rho Opiuchus, R CrA and Taurus clouds, we show that the wavelength of maximum polarization (lambda_max) is linearly correlated with the radiation environment of the grains. Using Far-Infrared emission data, we show that the large scatter seen in previous studies of lambda_max as a function of A_V is primarily due to line of sight effects causing some A_V measurements to not be a good tracer of the extinction (radiation field strength) seen by the grains being probed. The derived slopes in lambda_max vs. A_V, for the individual clouds, are consistent with a common value, while the zero intercepts scale with the average values of the ratios of total-to-selective extinction (R_V) for the individual clouds. Within each cloud we do not find direct correlations between lambda_max and R_V. The positive slope in consistent with recent developments in theory and indicating alignment driven by the radiation field. The present data cannot conclusively differentiate between direct radiative torques and alignment driven by H_2 formation. However, the small values of lambda_max(A_V=0), seen in several clouds, suggest a role for the latter, at least at the cloud surfaces. The scatter in the lambda_max vs. A_V relation is found to be associated with the characteristics of the embedded Young Stellar Objects (YSO) in the clouds. We propose that this is partially due to locally increased plasma damping of the grain rotation caused by X-rays from the YSOs.Comment: Accepted for publication in the Astrophysical Journa

Cambio Global. Impacto de la Actividad Humana sobre el Sistema Tierra

Esta obra, dirigida al público en general y elaborada por un equipo interdisciplinar, aborda con un lenguaje claro y asequible pero sin renunciar al rigor científico, el complejo problema del cambio global, un desafío de dimensiones colosales para la humanidad. El cambio global se define en esta obra como el conjunto de cambios en los procesos fundamentales que definen el funcionamiento de la biosfera derivados de la actividad humana. Los autores describen las múltiples dimensiones del cambio global, sus causas y sus consecuencias. Se describe la transformación de la superficie del planeta Tierra por la actividad humana y sus impactos sobre los ciclos del agua, los elementos y el sistema climático, incluyendo la introducción de miles de compuestos químicos sintéticos en la biosfera. Esta exposición pretende despejar la confusión que el aluvión de noticias e informaciones, a veces contradictorias, genera en la sociedad. Los autores prestan particular atención a la definición de las estrategias de adaptación y mitigación que los distintos actores de la sociedad, desde ciudadanos a políticos, pueden desplegar para minimizar el impacto del cambio global, animándoles al mismo tiempo a aprovechar las oportunidades que estos cambios pueden conllevar

A GLIMPSE into the Nature of Galactic Mid-IR Excesses

We investigate the nature of the mid-IR excess for 31 intermediate-mass stars that exhibit an 8 micron excess in either the Galactic Legacy Infrared Mid-Plane Survey Extraordinaire or the Mid-Course Space Experiment using high resolution optical spectra to identify stars surrounded by warm circumstellar dust. From these data we determine projected stellar rotational velocities and estimate stellar effective temperatures for the sample. We estimate stellar ages from these temperatures, parallactic distances, and evolutionary models. Using MIPS [24] measurements and stellar parameters we determine the nature of the infrared excess for 19 GLIMPSE stars. We find that 15 stars exhibit Halpha emission and four exhibit Halpha absorption. Assuming that the mid-IR excesses arise in circumstellar disks, we use the Halpha fluxes to model and estimate the relative contributions of dust and free-free emission. Six stars exhibit Halpha fluxes that imply free-free emission can plausibly explain the infrared excess at [24]. These stars are candidate classical Be stars. Nine stars exhibit Halpha emission, but their Halpha fluxes are insufficient to explain the infrared excesses at [24], suggesting the presence of a circumstellar dust component. After the removal of the free-free component in these sources, we determine probable disk dust temperatures of Tdisk~300-800 K and fractional infrared luminosities of L(IR)/L(*)~10^-3. These nine stars may be pre-main-sequence stars with transitional disks undergoing disk clearing. Three of the four sources showing Halpha absorption exhibit circumstellar disk temperatures ~300-400 K, L(IR)/L(*)~10^-3, IR colors K-[24]< 3.3, and are warm debris disk candidates. One of the four Halpha absorption sources has K-[24]> 3.3 implying an optically thick outer disk and is a transition disk candidate.Comment: 17 figures. Accepted for publication in Ap

Deep X-ray survey of the young open cluster NGC 2516 with XMM-Newton

We report on a deep X-ray survey of the young (~ 140 Myr), rich open cluster NGC 2516 obtained with the EPIC camera on board the XMM-Newton satellite. By combining data from six observations, a high sensitivity, greater than a factor 5 with respect to recent Chandra observations, has been achieved. We detected 431 X-ray sources and 234 of them have as optical counterparts cluster stars spanning the entire NGC 2516 Main Sequence. The X-ray luminosities of cluster stars span the range log Lx (erg/s) = 28.4 - 30.8. The representative temperatures span the 0.3 - 0.6 keV (3.5 - 8 MK) range for the cool component and 1.0 - 2.0 keV (12 -- 23 MK) for the hot one; similar values are found in other young open clusters like the Pleiades, IC 2391, and Blanco 1. While no significant differences are found in X-ray spectra, NGC 2516 solar type stars are definitely less luminous in X-rays than the nearly coeval Pleiades. The comparison with a previous ROSAT survey evidence the lack of variability amplitudes larger than a factor 2 in solar type stars in a ~11 yr time scale of the cluster and thus activity cycles like in the Sun are probably absent or different by period and amplitude in young stars.Comment: 37 pages, 13 figures, to be published on A&

Identification of particles with Lorentz factor up to 10410^{4} with Transition Radiation Detectors based on micro-strip silicon detectors

This work is dedicated to the study of a technique for hadron identification in the TeV momentum range, based on the simultaneous measurement of the energies and of the emission angles of the Transition Radiation (TR) X-rays with respect to the radiating particles. A detector setup has been built and tested with particles in a wide range of Lorentz factors (from about $10^3$ to about $4 \times 10^4$ crossing different types of radiators. The measured double-differential (in energy and angle) spectra of the TR photons are in a reasonably good agreement with TR simulation predictions.Comment: 31 pages, 12 figures, paper published on Nuclear Instruments & Methods

Stellar Rotation in Young Clusters. I. Evolution of Projected Rotational Velocity Distributions

Open clusters offer us the means to study stellar properties in samples with well-defined ages and initial chemical composition. Here we present a survey of projected rotational velocities for a large sample of mainly B-type stars in young clusters to study the time evolution of the rotational properties of massive stars. The survey is based upon moderate resolution spectra made with the WIYN 3.5 m and CTIO 4 m telescopes and Hydra multi-object spectrographs, and the target stars are members of 19 young open clusters with an age range of approximately 6 to 73 Myr. We made fits of the observed lines He I 4026, 4387, 4471 and Mg II 4481 using model theoretical profiles to find projected rotational velocities for a total of 496 OB stars. We find that there are fewer slow rotators among the cluster B-type stars relative to nearby B stars in the field. We present evidence consistent with the idea that the more massive B stars (M > 9 solar masses) spin down during their main sequence phase. However, we also find that the rotational velocity distribution appears to show an increase in the numbers of rapid rotators among clusters with ages of 10 Myr and higher. These rapid rotators appear to be distributed between the zero age and terminal age main sequence locations in the Hertzsprung-Russell diagram, and thus only a minority of them can be explained as the result of a spin up at the terminal age main sequence due to core contraction. We suggest instead that some of these rapid rotators may have been spun up through mass transfer in close binary systems.Comment: 33 pages, 11 figures, accepted by Ap

Stellar Rotation in Young Clusters. II. Evolution of Stellar Rotation and Surface Helium Abundance

We derive the effective temperatures and gravities of 461 OB stars in 19 young clusters by fitting the H-gamma profile in their spectra. We use synthetic model profiles for rotating stars to develop a method to estimate the polar gravity for these stars, which we argue is a useful indicator of their evolutionary status. We combine these results with projected rotational velocity measurements obtained in a previous paper on these same open clusters. We find that the more massive B-stars experience a spin down as predicted by the theories for the evolution of rotating stars. Furthermore, we find that the members of binary stars also experience a marked spin down with advanced evolutionary state due to tidal interactions. We also derive non-LTE-corrected helium abundances for most of the sample by fitting the He I 4026, 4387, 4471 lines. A large number of helium peculiar stars are found among cooler stars with Teff < 23000 K. The analysis of the high mass stars (8.5 solar masses < M < 16 solar masses) shows that the helium enrichment process progresses through the main sequence (MS) phase and is greater among the faster rotators. This discovery supports the theoretical claim that rotationally induced internal mixing is the main cause of surface chemical anomalies that appear during the MS phase. The lower mass stars appear to have slower rotation rates among the low gravity objects, and they have a large proportion of helium peculiar stars. We suggest that both properties are due to their youth. The low gravity stars are probably pre-main sequence objects that will spin up as they contract. These young objects very likely host a remnant magnetic field from their natal cloud, and these strong fields sculpt out surface regions with unusual chemical abundances.Comment: 50 pages 18 figures, accepted by Ap

An atlas of 2.4 to 4.1 microns ISO/SWS spectra of early-type stars

We present an atlas of spectra of O- and B-type stars, obtained with the Short Wavelength Spectrometer (SWS) during the Post-Helium program of the Infrared Space Observatory (ISO). This program is aimed at extending the Morgan & Keenan classification scheme into the near-infrared. Later type stars will be discussed in a seperate publication. The observations consist of 57 SWS Post-Helium spectra from 2.4 to 4.1 microns, supplemented with 10 spectra acquired during the nominal mission with a similar observational setting. For B-type stars, this sample provides ample spectral converage in terms of subtype and luminosity class. For O-type stars,the ISO sample is coarse and therefore is complemented with 8 UKIRT L'-band observations. In terms of the presence of diagnostic lines, the L'-band is likely the most promising of the near-infrared atmospheric windows for the study of the physical properties of B stars. Specifically, this wavelength interval contains the Brackett alpha, Pfund gamma, and other Pfund lines which are probes of spectral type, luminosity class and mass loss. Here, we present simple empirical methods based on the lines present in the 2.4 to 4.1 microns interval that allow the determination of: the spectral type of B dwarfs and giants to within two subtypes; the luminosity class of B stars to within two classes; the mass-loss rate of O stars and B supergiants to within 0.25 dex.Comment: 19 pages, 11 Postscript figures, accepted by A&

What are the hot R Coronae Borealis stars?

We investigate the evolutionary status of four stars: V348 Sgr, DY Cen, and MV Sgr in the Galaxy and HV 2671 in the LMC. These stars have in common random deep declines in visual brightness, which are characteristic of R Coronae Borealis (RCB) stars. RCB stars are typically cool hydrogen-deficient supergiants. The four stars studied in this paper are hotter (Teff = 15–20 kK) than the majority of RCB stars (Teff = 5000–7000 K). Although these are commonly grouped together as the hot RCB stars they do not necessarily share a common evolutionary history. We present new observational data and an extensive collection of archival and previously published data that is reassessed to ensure internal consistency. We find temporal variations of various properties on different timescales that will eventually help us to uncover the evolutionary history of these objects. DY Cen and MV Sgr have typical RCB helium abundances, which exclude any currently known post–asymptotic giant branch (post-AGB) evolutionary models. Moreover, their carbon and nitrogen abundances present us with further problems for their interpretation. V348 Sgr and HV 2671 are in general agreement with a born-again post-AGB evolution, and their abundances are similar to Wolf-Rayet central stars of planetary nebulae (PNs). The three Galactic stars in the sample have circumstellar nebulae, which produce forbidden line radiation (for HV 2671 we have no information). V348 Sgr and DY Cen have low-density, low-expansion velocity nebulae (resolved in the case of V348 Sgr), while MV Sgr has a higher density, higher expansion velocity nebula. All three stars, on the other hand, have split emission lines, which indicate the presence of an equatorial bulge but not of a Keplerian disk. In addition, the historical light curves for the three Galactic hot RCB stars show evidence for a significant fading in their maximum-light brightnesses of ~1 mag over the last 70 yr. From this we deduce that their effective temperatures increased by a few thousand degrees. If V348 Sgr is a born-again star, as we presume, this means that the star is returning from the born-again AGB phase to the phase of a central star of PN. Spectroscopically, no dramatic change is observed over the last 50 years for V348 Sgr and MV Sgr. However, there is some evidence that the winds of V348 Sgr and DY Cen have increased in strength in the last decade. HV 2671, located in the LMC, has not been analyzed in detail but at 5 Å… resolution is almost identical to V348 Sgr. Through the bolometric correction derived for V348 Sgr and the known distance, we can estimate the absolute ν magnitude of HV 2671 (Mν = -3.0 mag) and its bolometric luminosity (~6000 L⊙)