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arXiv2008

Young massive stars and their environment in the mid-infrared at high angular resolution

We present interferometric and single-dish mid-infrared observations of a sample of massive young stellar objects (BN-type objects), using VLTI-MIDI (10 micron) and Subaru-COMICS (24.5 micron). We discuss the regions S140, Mon R2, M8E-IR, and W33A. The observations probe the inner regions of the dusty envelope at scales of 50 milli arcsecond and 0.6 arcsec (100-1000 AU), respectively. Simultaneous model fits to spectral energy distributions and spatial data are achieved using self-consistent spherical envelope modelling. We conclude that those MYSO envelopes that are best described by a spherical geometry, the commensurate density distribution is a powerlaw with index -1.0. Such a powerlaw is predicted if the envelope is supported by turbulence on the 100-1000AU scales probed with MIDI and COMICS, but the role of rotation at these spatial scales need testing.

astro-ph↗
arXiv2008

Revealing the properties of dark matter in the merging cluster MACSJ0025.4-1222

We constrain the physical nature of dark matter using the newly identified massive merging galaxy cluster MACSJ0025.4-1222. As was previously shown by the example of the Bullet Cluster (1E0657-56), such systems are ideal laboratories for detecting isolated dark matter, and distinguishing between cold dark matter (CDM) and other scenarios (e.g. self-interacting dark matter, alternative gravity theories). MACSJ0025.4-1222 consists of two merging subclusters of similar richness at z=0.586. We measure the distribution of X-ray emitting gas from Chandra X-ray data and find it to be clearly displaced from the distribution of galaxies. A strong (information from highly distorted arcs) and weak (using weakly distorted background galaxies) gravitational lensing analysis based on Hubble Space Telescope observations and Keck arc spectroscopy confirms that the subclusters have near-equal mass. The total mass distribution in each of the subclusters is clearly offset (at >4sigma significance) from the peak of the hot X-ray emitting gas (the main baryonic component), but aligned with the distribution of galaxies. We measure the fractions of mass in hot gas (0.09^{+0.07}_{-0.03}) and stars (0.010^{+0.007}_{-0.004}), consistent with those of typical clusters, finding that dark matter is the dominant contributor to the gravitational field. Under the assumption that the subclusters experienced a head-on collision in the plane of the sky, we obtain an order-of-magnitude estimate of the dark matter self-interaction cross-section of sigma/m < 4cm^2/g, re-affirming the results from the Bullet Cluster on the collisionless nature of dark matter.

astro-ph↗
arXiv2008

The HD 163296 Circumstellar Disk in Scattered Light: Evidence of Time-Variable Self-Shadowing

We present the first multi-color view of the scattered light disk of the Herbig Ae star HD 163296, based on coronagraphic observations from the Hubble Space Telescope Advanced Camera for Surveys (ACS). Radial profile fits of the surface brightness along the disk's semi-major axis indicates that the disk is not continuously flared, and extends to 540 AU. The disk's color (V-I)=1.1 at a radial distance of 3.5 arcseconds is redder than the observed stellar color (V-I)=0.15. This red disk color might be indicative of either an evolution in the grain size distribution (i.e. grain growth) and/or composition, both of which would be consistent with the observed non-flared geometry of the outer disk. We also identify a single ansa morphological structure in our F435W ACS data, which is absent from earlier epoch F606W and F814W ACS data, but corresponds to one of the two ansa observed in archival HST STIS coronagraphic data. Following transformation to similar band-passes, we find that the scattered light disk of HD 163296 is 1 mag arcsec$^{-2}$ fainter at 3.5 arcseconds in the STIS data than in the ACS data. Moreover, variations are seen in (i) the visibility of the ansa(e) structures, in (ii) the relative surface brightness of the ansa(e) structures, and in (iii) the (known) intrinsic polarization of the system. These results indicate that the scattered light from the HD 163296 disk is variable. We speculate that the inner disk wall, which Sitko et al. suggests has a variable scale height as diagnosed by near-IR SED variability, induces variable self-shadowing of the outer disk. We further speculate that the observed surface brightness variability of the ansa(e) structures may indicate that the inner disk wall is azimuthally asymmetric.

astro-ph↗
arXiv2008

Fragmentation of Shocked Flows: Gravity, Turbulence and Cooling

The observed rapid onset of star formation in molecular clouds requires rapid formation of dense fragments which can collapse individually before being overtaken by global gravitationally-driven flows. Many previous investigations have suggested that supersonic turbulence produces the necessary fragmentation, without addressing however the source of this turbulence. Motivated by our previous (numerical) work on the flow-driven formation of molecular clouds, we investigate the expected timescales of the dynamical and thermal instabilities leading to the rapid fragmentation of gas swept up by large-scale flows, and compare them with global gravitational collapse timescales. We identify parameter regimes in gas density, temperature and spatial scale within which a given instability will dominate. We find that dynamical instabilities disrupt large-scale coherent flows via generation of turbulence, while strong thermal fragmentation amplifies the resulting low-amplitude density perturbations, thus leading to small-scale, high-density fragments as seeds for {\em local} gravity to act upon. Global gravity dominates only on the largest scales; large-scale gravitationally-driven flows promote the formation of groups and clusters of stars formed by turbulence, thermal fragmentation, and rapid cooling.

astro-ph↗
arXiv2008

A High-Resolution Survey of Low-Redshift QSO Absorption Lines: Statistics and Physical Conditions of O VI Absorbers

Using high-resolution UV spectra of 16 low-z QSOs, we study the physical conditions and statistics of O VI absorption in the IGM at z < 0.5. We identify 51 intervening (z_{abs} << z_{QSO}) O VI systems comprised of 77 individual components, and we find 14 "proximate" systems (z_{abs} ~ z_{QSO}) containing 34 components. For intervening systems [components] with rest-frame equivalent width W_{r} > 30 mA, the number of O VI absorbers per unit redshift dN/dz = 15.6(+2.9/-2.4) [21.0(+3.2/-2.8)], and this decreases to dN/dz = 0.9(+1.0/-0.5) [0.3(+0.7/-0.3)] for W_{r} > 300 mA. The number per redshift increases steeply as z_{abs} approaches z_{QSO}, and some proximate absorbers have substantially lower H I/O VI ratios. The lower proximate ratios could be partially due to ionization effects but also require higher metallicities. We find that 37% of the intervening O VI absorbers have velocity centroids that are well-aligned with corresponding H I absorption. If the O VI and the H I trace the same gas, the relatively small differences in line widths imply the absorbers are cool with T < 10^{5} K. Most of these well-aligned absorbers have the characteristics of metal-enriched photoionized gas. However, the O VI in the apparently simple and cold systems could be associated with a hot phase with T ~ 10^{5.5} K if the metallicity is high enough to cause the associated broad Ly alpha absorption to be too weak to detect. We show that 53% of the intervening O VI systems are complex multiphase absorbers that can accommodate both lower metallicity collisionally-ionized gas with T > 10^{5} K and cold photoionzed gas.

astro-ph↗
arXiv2008

Andromeda IV: a new Local Volume very metal-poor galaxy

And IV is a low-surface brightness (LSB) dwarf galaxy at the distance of 6.1 Mpc, projecting close to M 31. In this paper the results of spectroscopy of And IV the two brightest HII regions with the SAO 6-m telescope (BTA) are presented. In both of them the faint line [OIII]4363 was detected that allowed us to determine their O/H by the classical T_e method. Their values of 12+log(O/H) are equal to 7.49+-0.06 and 7.55+-0.23, respectively. The comparison of these direct O/H determinations with the two most reliable semi-empirical and empirical methods shows their good consistency. For And IV absolute blue magnitude of M_B=-12.6, our value of O/H corresponds well to the `standard' relation between O/H and L_B for dwarf irregular galaxies (DIGs). And IV appears to be a new representative of the extremely metal-deficient gas-rich galaxies in the Local Volume. The very large range of M(HI) for LSB galaxies with close metallicities and luminosities indicates that the simple models of LSBG chemical evolution are too limited to predict such striking diversity.

astro-ph↗
arXiv2008

Numerical Color-Magnitude Diagram Analysis of SDSS Data and Application to the New Milky Way satellites

We have tested the application to Sloan Digital Sky Survey data of the software package MATCH, which fits color-magnitude diagrams (CMDs) to estimate stellar population parameters and distances. These tests on a set of six globular clusters show that these techniques recover their known properties. New ways of using the CMD-fitting software enable us to deal with an extended distribution of stars along the line-of-sight, to constrain the overall properties of sparsely populated objects, and to detect the presence of stellar overdensities in wide-area surveys. We then also apply MATCH to CMDs for twelve recently discovered Milky Way satellites to derive in a uniform fashion their distances, ages and metallicities. While the majority of them appear consistent with a single stellar population, CVn I, UMa II, and Leo T exhibit (from SDSS data alone) a more complex history with multiple epochs of star formation.

astro-ph↗
arXiv2007

Resolving the ionized wind of the post-Red Supergiant IRC +10 420 with VLTI/AMBER

The paper investigates the milli-arcsecond scale structure of the present-day mass-loss of the post-Red Supergiant IRC+10420. We use three telescopes of the VLT Interferometer in combination with the AMBER near-infrared beam combiner to measure spectrally dispersed correlated fluxes in the K-band around the Br gamma transition. The resulting visibilities are compared to the predicted visibilities of emission structures with various simple models in order to infer the size of the observed emission region. The Br gamma line is resolved by VLTI+AMBER on all three baselines, with the maximum projected baseline extending 69 meter and a P.A. ranging between 10 and 30 degrees. A differential phase between line and continuum is detected on the longest baseline. The Br gamma emission region is found to have a diameter of 3.3 milli-arcseconds (FWHM), when compared to a Gaussian intensity distribution. A uniform disk and a ring-like intensity distribution do not fit the line visibilities. Comparing the AMBER equivalent width of Br gamma with measurements from various epochs, we find that the stellar photosphere contributes about 60% of the total continuum light at 2.2 micron. The remaining 40% continuum emission is found on scales larger than the 66mas AMBER field of view. Using simple arguments, and assuming optically thick line emission, we find that the line emitting region is elongated. We briefly discuss the possibilities whether such a structure is due to a bi-polar flow or a circumstellar disk. (Abridged).

astro-ph↗
arXiv2007

Coupled nanopillar waveguides: optical properties and applications

In this paper we review basic properties of coupled periodic and aperiodic nanopillar waveguides. A coupled nanopillar waveguide consists of several rows of periodically or aperiodically placed dielectric rods (pillars). In such a waveguide, light confinement is due to the total internal reflection, while guided modes dispersion is strongly affected by the waveguide structure. We present a systematic analysis of the optical properties of coupled nanopillar waveguides and discuss their possible applications for integrated optics.

physics.optics↗
arXiv2007

Spiral Galaxies in the SAURON Survey

We discuss some recent integral field spectroscopy using the SAURON instrument of a sample consisting of 24 early-type spirals, part of the SAURON Survey, and 18 late-type spirals. Using 2-dimensional maps of their stellar radial velocity, velocity dispersion, and absorption line strength, it is now much easier to understand the nature of nearby galactic bulges. We discuss a few highlights of this work, and point out some new ideas about the formation of galactic bulges.

astro-ph↗
arXiv2007

On the X-ray and optical properties of the Be star HD 110432: a very hard-thermal X-ray emitter

(ABRIDGED) - HD 110432 is the first proposed, and best studied, member of a growing group of Be stars with X-ray properties similar to gamma Cas. These stars exhibit hard-thermal X-ray emissions (kT ~> 7 keV) that are variable on all measurable timescales. In this work we present X-ray spectral and timing properties of HD 110432 from three XMM-Newton observations in addition to new optical spectroscopic observations. Its X-ray spectrum, complex and timing dependent, is well described in each observation by three thermal plasmas with temperatures ranging between 0.4-0.7, 3-6, and 21-37 keV. Thus, HD 110432 has the hottest thermal plasma of any known Be star. A sub-solar iron abundance (~ 0.3-0.5 x ZFe_sun) is derived for the hottest plasma. The star has a moderate 0.2-12 keV luminosity of ~ 5 x 10^32 erg/s. Recurrent flare-like events on time scales as short as ~ 10 seconds are superimposed over a slowly ~ 5-10 x 10^3 seconds varying basal flux, followed by similarly rapid hardness variabilities. There is no evidence for coherent oscillations, and an upper limit of ~ 2.5% is derived on the pulsed fraction for short pulsations from 0.005 to 2.5 Hz. In the optical region the strong and quasi-symmetrical profile of the Halpha line (EW \~ -60 Angs.) as well as the detection of several metallic lines in emission strongly suggest a dense and/or large circumstellar disk. HD 110432 has several properties reminiscence of the cataclysmic variables such as a very hot X-ray temperature and its detailed spectral features. This suggests that it might be a Be star harbouring an accreting white dwarf. On the other hand, accumulating evidence of magnetic activities in the literature supports the surface-disk of the star as being the X-ray site.

astro-ph↗
arXiv2007

The environs of the HII region Gum31

We analyze the distribution of the interstellar matter in the environs of the \hii region Gum 31, excited by the open cluster NGC 3324, located in the complex Carina region, with the aim of investigating the action of the massive stars on the surrounding neutral material. We use 21cm-line data, radio continuum images at 0.843, 2.4 and 4.9 GHz, $^{12}${\bf CO(1-0)} observations, and IRAS and MSX infrared data. Adopting a distance of 3 kpc for the \hii\ region and the ionizing cluster, we have derived an electron density of 33$\pm$3 cm$^{-3}$ and an ionized mass of (3.3$\pm$1.1)$\times10^3$ M$_{\odot}$ based on the radio continuum data at 4.9 GHz. The \hi 21-cm line images revealed an \hi shell surrounding the H {\sc ii} region. The \hi structure is 10.0$\pm$1.7 pc in radius, has a neutral mass of 1500$\pm$500 M$_{\odot}$, and is expanding at 11 km s$^{-1}$. The associated molecular gas amounts to (1.5$\pm$0.5)$\times10^5$ M$_{\odot}$, being its volume density of about 500 cm^{-3}. This molecular material probably represents the remains of the cloud where the young open cluster NGC 3324 was born. The distributions of the ionized and molecular material, along with that of the emission in the MSX band A suggest that a photodissociation region has developed at the interface between the ionized and molecular gas. We conclude that either the massive stars in the open cluster have weak stellar winds or the stellar winds have blown during a very short period of time to create an interstellar bubble in an interstellar medium as dense as observed. IRAS, MSX, and 2MASS point sources projected onto the molecular envelope are compatible with protostellar candidates, showing the presence star forming regions. The expansion of the \hii region has triggered stellar formation in the molecular shell.

astro-ph↗
arXiv2007

Propagation of Ultra-high-energy Protons in Cosmic Magnetic Fields

We simulate the arrival distribution of ultra-high-energy (UHE) protons by following their propagation processes in several strengths of a structured extragalactic magnetic field (EGMF). Comparing our result to observational one by Akeno Giant Air Shower Array, we constrain the number density of UHE cosmic ray sources with the small-scale anisotropy. As a result, the source number density is $\sim 10^{-5} {\rm Mpc}^{-3}$ with uncertainty of about an order of magnitude due to the small number of observed events. This hardly depends on our structured EGMF strength. We also investigate future prospects for this approarch. The near future observations, such as Pierre Auger Observatory, can distinguish $10^{-6} {\rm Mpc}^{-3}$ accurately from the more source densities. Number of events to discriminate between $10^{-4}$ and $10^{-5} {\rm Mpc}^{-3}$ is dependent on the EGMF strength.

astro-ph↗
arXiv2007

Lensing, reddening and extinction effects of MgII absorbers from z=0.4 to z=2

Using a sample of almost 7000 strong MgII absorbers with 0.4 < z < 2.2 detected in the SDSS DR4 dataset, we investigate the gravitational lensing and dust extinction effects they induce on background quasars. After carefully quantifying several selection biases, we isolate the reddening effects as a function of redshift and absorber rest equivalent width, W_0. We find the amount of dust to increase with cosmic time as (1+z)^(-1.1 +/- 0.4), following the evolution of cosmic star density or integrated star formation rate. We measure the reddening effects over a factor 30 in E(B-V) and we find the dust column density to be proportional to W_0^(1.9 +/- 0.2), which provides an important scaling for theoretical modeling of metal absorbers. We also measure the dust-to-metals ratio and find it similar to that of the Milky Way. In contrast to previous studies, we do not detect any gravitational magnification by MgII systems. We measure the upper limit μ<1.10 and discuss the origin of the discrepancy. Finally, we estimate the fraction of absorbers missed due to extinction effects and show that it rises from 1 to 50% in the range 1<W_0<6 Angstrom. We parametrize this effect and provide a correction for recovering the intrinsic distribution of absorber rest equivalent widths.

astro-ph↗
arXiv2007

Detailed Models of super-Earths: How well can we infer bulk properties?

The field of extrasolar planets has rapidly expanded to include the detection of planets with masses smaller than that of Uranus. Many of these are expected to have little or no hydrogen and helium gas and we might find Earth analogs among them. In this paper we describe our detailed interior models for a rich variety of such massive terrestrial and ocean planets in the 1-to-10 earth-mass range (super-Earths). The grid presented here allows the characterization of the bulk composition of super-Earths detected in transit and with a measured mass. We show that, on average, planet radius measurements to better than 5%, combined with mass measurements to better than 10% would permit us to distinguish between an icy or rocky composition. This is due to the fact that there is a maximum radius a rocky terrestrial planet may achieve for a given mass. Any value of the radius above this maximum terrestrial radius implies that the planet contains a large (> 10%) amount of water (ocean planet).

astro-ph↗
arXiv2007

Periodic accretion from a circumbinary disk in the young binary UZ Tau E

Close pre-main-sequence binary stars are expected to clear central holes in their protoplanetary disks, but the extent to which material can flow from the circumbinary disk across the gap onto the individual circumstellar disks has been unclear. In binaries with eccentric orbits, periodic perturbation of the outer disk is predicted to induce mass flow across the gap, resulting in accretion that varies with the binary period. This accretion may manifest itself observationally as periodic changes in luminosity. Here we present a search for such periodic accretion in the pre-main-sequence spectroscopic binary UZ Tau E. We present BVRI photometry spanning three years; we find that the brightness of UZ Tau E is clearly periodic, with a best-fit period of 19.16 +/- 0.04 days. This is consistent with the spectroscopic binary period of 19.13 days, refined here from analysis of new and existing radial velocity data. The brightness of UZ Tau E shows significant random variability, but the overall periodic pattern is a broad peak in enhanced brightness, spanning more than half the binary orbital period. The variability of the H-alpha line is not as clearly periodic, but given the sparseness of the data, some periodic component is not ruled out. The photometric variations are in good agreement with predictions from simulations of binaries with orbital parameters similar to those of UZ Tau E, suggesting that periodic accretion does occur from circumbinary disks, replenishing the inner disks and possibly extending the timescale over which they might form planets.

astro-ph↗
arXiv2007

The Magnetic Field Structure of the LMC 2 Supershell: NGC 2100

We present U,B,V,R,I imaging polarimetry of NGC 2100 and its surrounding environment, which comprise a part of the LMC 2 supershell. The morphology of the observed position angle distribution provides a tracer of the projected magnetic field in this environment. Our polarization maps detail regions exhibiting similarly aligned polarization position angles, as well as more complex position angle patterns. We observe regions of coherent fields on spatial scales of 42 x 24 pc to 104 x 83 pc, and infer projected field strengths of $\sim14-30μ$G. We propose that the superposition of global outflows from the LMC 2 environment, as well as outflows created within NGC 2100, produce the unique field geometry in the region.

astro-ph↗