Showing papers by "Smithsonian Astrophysical Observatory published in 2008"

Planet formation around stars of various masses : the snow line and the frequency of giant planets

Abstract: We use a semianalytic circumstellar disk model that considers movement of the snow line through evolution of accretion and the central star to investigate how gas giant frequency changes with stellar mass. The snow line distance changes weakly with stellar mass; thus, giant planets form over a wide range of spectral types. The probability that a given star has at least one gas giant increases linearly with stellar mass from 0.4 to 3 M☉. Stars more massive than 3 M☉ evolve quickly to the main sequence, which pushes the snow line to 10-15 AU before protoplanets form and limits the range of disk masses that form giant planet cores. If the frequency of gas giants around solar mass stars is 6%, we predict occurrence rates of 1% for 0.4 M☉ stars and 10% for 1.5 M☉ stars. This result is largely insensitive to our assumed model parameters. Finally, the movement of the snow line as stars 2.5 M☉ move to the main sequence may allow the ocean planets suggested by Leger et al. to form without migration.

Spitzer Observations of NGC 1333: A Study of Structure and Evolution in a Nearby Embedded Cluster

Abstract: We present a comprehensive analysis of structure in the young, embedded cluster, NGC 1333 using members identified with Spitzer and 2MASS photometry based on their IR-excess emission. A total of 137 members are identified in this way, composed of 39 protostars and 98 more evolved pre-main-sequence stars with disks. Of the latter class, four are transition/debris disk candidates. The fraction of exposed pre-main-sequence stars with disks is -->83% ? 11% , showing that there is a measurable diskless pre-main-sequence population. The sources in each of the Class I and II evolutionary states are shown to have very different spatial distributions relative to the distribution of the dense gas in their natal cloud. However, the distribution of nearest neighbor spacings among these two groups of sources are found to be quite similar, with a strong peak at spacings of 0.045 pc. Radial and azimuthal density profiles and surface density maps computed from the identified YSOs show that NGC 1333 is elongated and not strongly centrally concentrated, confirming previous claims in the literature. We interpret these new results as signs of a low velocity dispersion, extremely young cluster that is not in virial equilibrium.

Variations on Debris Disks: Icy Planet Formation at 30-150 AU for 1-3 M☉ Main-Sequence Stars

Abstract: We describe calculations for the formation of icy planets and debris disks at 30-150 AU around 1-3 M☉ stars. Debris disk formation coincides with the formation of planetary systems. As protoplanets grow, they stir leftover planetesimals to large velocities. A cascade of collisions then grinds the leftovers to dust, forming an observable debris disk. Stellar lifetimes and the collisional cascade limit the growth of protoplanets. The maximum radius of icy planets, -->rmax ≈ 1750 km, is remarkably independent of initial disk mass, stellar mass, and stellar age. These objects contain 3%-4% of the initial mass in solid material. Collisional cascades produce debris disks with maximum luminosity ~ -->2 × 10−3 times the stellar luminosity. The peak 24 μm excess varies from ~1% times the stellar photospheric flux for 1 M☉ stars to ~50 times the stellar photospheric flux for 3 M☉ stars. The peak 70-850 μm excesses are ~30-100 times the stellar photospheric flux. For all stars, the 24-160 μm excesses rise at stellar ages of 5-20 Myr, peak at 10-50 Myr, and then decline. The decline is roughly a power law, -->f t−n with -->n ≈ 0.6–1.0. This predicted evolution agrees with published observations of A-type and solar-type stars. The observed far-IR color evolution of A-type stars also matches model predictions.

Nonlinear Force-Free Modeling of Coronal Magnetic Fields. II. Modeling a Filament Arcade and Simulated Chromospheric and Photospheric Vector Fields

Abstract: We compare a variety of nonlinear force-free field (NLFFF) extrapolation algorithms, including optimization, magneto-frictional, and Grad – Rubin-like codes, applied to a solar-like reference model. The model used to test the algorithms includes realistic photospheric Lorentz forces and a complex field including a weakly twisted, right helical flux bundle. The codes were applied to both forced “photospheric” and more force-free “chromospheric” vector magnetic field boundary data derived from the model. When applied to the chromospheric boundary data, the codes are able to recover the presence of the flux bundle and the field’s free energy, though some details of the field connectivity are lost. When the codes are applied to the forced photospheric boundary data, the reference model field is not well recovered, indicating that the combination of Lorentz forces and small spatial scale structure at the photosphere severely impact the extrapolation of the field. Preprocessing of the forced photospheric boundary does improve the extrapolations considerably for the layers above the chromosphere, but the extrapolations are sensitive to the details of the numerical codes and neither the field connectivity nor the free magnetic energy in the full volume are well recovered. The magnetic virial theorem gives a rapid measure of the total magnetic energy without extrapolation though, like the NLFFF codes, it is sensitive to the Lorentz forces in the coronal volume. Both the magnetic virial theorem and the Wiegelmann extrapolation, when applied to the preprocessed photospheric boundary, give a magnetic energy which is nearly equivalent to the value derived from the chromospheric boundary, but both underestimate the free energy above the photosphere by at least a factor of two. We discuss the interpretation of the preprocessed field in this context. When applying the NLFFF codes to solar data, the problems associated with Lorentz forces present in the low solar atmosphere must be recognized: the various codes will not necessarily converge to the correct, or even the same, solution.

The Spitzer c2d Survey of Large, Nearby, Interstellar Clouds. XI. Lupus Observed with IRAC and MIPS

Abstract: We present c2d Spitzer/IRAC observations of the Lupus I, III and IV dark clouds and discuss them in combination with optical and near-infrared and c2d MIPS data. With the Spitzer data, the new sample contains 159 stars, 4 times larger than the previous one. It is dominated by low- and very-low mass stars and it is complete down to M ≈ 0.1M⊙. We find 30-40% binaries with separations between 100 to 2000 AU with no apparent effect in the disk properties of the members. A large majority of the objects are Class II or Class III objects, with only 20 (12%) of Class I or Flat spectrum sources. The disk sample is complete down to “debris”-like systems in stars as small as M ≈ 0.2 M⊙ and includes sub-stellar objects with larger IR excesses. The disk fraction in Lupus is 70 – 80%, consistent with an age of 1 – 2 Myr. However, the young population contains 20% optically thick accretion disks and 40% relatively less flared disks. A growing variety of inner disk structures is found for larger inner disk clearings for

Hypervelocity Stars: From the Galactic Center to the Halo

Abstract: Hypervelocity stars (HVSs) traverse the Galaxy from the central black hole to the outer halo. We show that the Galactic potential within 200 pc acts as a high-pass filter preventing low-velocity HVSs from reaching the halo. To trace the orbits of HVSs throughout the Galaxy, we construct two forms of the potential which reasonably represent the observations in the range 5-105 pc: a simple spherically symmetric model and a bulge-disk-halo model. We use the Hills mechanism (disruption of binaries by the tidal field of the central black hole) to inject HVSs into the Galaxy and to compute the observable spatial and velocity distributions of HVSs with masses in the range 0.6-4 M☉. These distributions reflect the mass function in the Galactic center, properties of binaries in the Galactic center, and aspects of stellar evolution and the injection mechanism. For 0.6-4 M☉ main-sequence stars, the fraction of unbound HVSs and the asymmetry of the velocity distribution for their bound counterparts increase with stellar mass. The density profiles for unbound HVSs decline with distance from the Galactic center approximately as -->r−2 (but are steeper for the most massive stars, which evolve off the main sequence during their travel time from the Galactic center); the density profiles for the bound ejecta decline with distance approximately as -->r−3. In a survey with a limiting magnitude of -->V 23, the detectability of HVSs (unbound or bound) increases with stellar mass.

VERITAS Discovery of >200 GeV Gamma-Ray Emission from the Intermediate-Frequency-Peaked BL Lacertae Object W Comae

Abstract: We report the detection of very high-energy -ray emission from the intermediate-frequencypeaked BLLacertae object WComae (z = 0.102) by VERITAS, an array of four imaging atmospheric-Cherenkov telescopes. The source was observed between January and April 2008. A strong outburst of -ray emission was measured in the middle of March, lasting for only four days. The energy spectrum measured during the two highest flare nights is fit by a power-law and is found to be very steep, with a differential photon spectral index of = 3 .81±0.35stat±0.34syst. The integral photon flux above 200GeV during those two nights corresponds to roughly 9% of the flux from the Crab Nebula. Quasi-simultaneous Swift observations at X-ray energies were triggered by the VERITAS observations. The spectral energy distribution of the flare data can be described by synchrotron-self-Compton (SSC) or external-Compton (EC) leptonic jet models, with the latter offering a more natural set of parameters to fit the data.

The Spitzer c2d Survey of Large, Nearby, Interstellar Clouds. X. The Chamaeleon II Pre-Main-Sequence Population as Observed with IRAC and MIPS

Abstract: We discuss the results from the combined IRAC and MIPS c2d Spitzer Legacy survey observations and complementary optical and NIR data of the Chamaeleon II (Cha II) dark cloud. We perform a census of the young population in an area of similar to 1.75 deg^(2) and study the spatial distribution and properties of the cloud members and candidate pre-main-sequence (PMS) objects and their circumstellar matter. Our census is complete down to the substellar regime (M approximate to 0.03 M☉). From the analysis of the volume density of the PMS objects and candidates we find two groups of objects with volume densities higher than 25 M☉ pc^(-3) and 5-10 members each. A multiplicity fraction of about 13% +/- 3% is observed for objects with separations 0.8" < θ < 6.0" (142-1065 AU). No evidence for variability between the two epochs of the c2d IRAC data set, Δt ~ 6 hr, is detected. We estimate a star formation efficiency of 1%-4%, consistent with the estimates for Taurus and Lupus, but lower than for Cha I. This might mean that different star formation activities in the Chamaeleon clouds reflect a different history of star formation. We also find that Cha II is turning some 6-7 M☉ into stars every Myr, which is low in comparison with the star formation rate in other c2d clouds. The disk fraction of 70%-80% that we estimate in Cha II is much higher than in other star-forming regions and indicates that the population in this cloud is dominated by objects with active accretion. Finally, the Cha II outflows are discussed; a new Herbig-Haro outflow, HH 939, driven by the classical T Tauri star Sz 50, has been discovered.

Spectroscopic determination of the luminosity function in the galaxy clusters a2199 and virgo

Abstract: We report a new determination of the faint end of the galaxy luminosity function (LF) in the nearby clusters Abell 2199 and Virgo using data from the Sloan Digital Sky Survey (SDSS) and the Hectospec multifiber spectrograph on the MMT. The luminosity function of A2199 is consistent with a single Schechter function to Mr = ?15.6 + 5 log h 70 with a faint-end slope of ? = ?1.13 ? 0.07 (statistical). The LF in Virgo extends to Mr ?13.5 M* + 8 and has a slope of ? = ?1.28 ? 0.06 (statistical). The red sequence of cluster members is prominent in both clusters, and almost no cluster galaxies are redder than this sequence. A large fraction of photometric red-sequence galaxies lies behind the cluster. We compare our results to previous estimates and find poor agreement with estimates based on statistical background subtraction but good agreement with estimates based on photometric membership classifications (e.g., colors, morphology, surface brightness). We conclude that spectroscopic data are critical for estimating the faint end of the LF in clusters. The faint-end slope we find is consistent with values found for field galaxies, weakening any argument for environmental evolution in the relative abundance of dwarf galaxies. However, dwarf galaxies in clusters are significantly redder than field galaxies of similar luminosity or mass, indicating that star-formation processes in dwarfs do depend on environment.

The Hinode X-Ray Telescope (XRT): Camera Design, Performance and Operations

Abstract: The X-ray Telescope (XRT) aboard the Hinode satellite is a grazing incidence X-ray imager equipped with a 2048×2048 CCD. The XRT has 1 arcsec pixels with a wide field of view of 34×34 arcmin. It is sensitive to plasmas with a wide temperature range from < 1 to 30 MK, allowing us to obtain TRACE-like low-temperature images as well as Yohkoh/SXT-like high-temperature images. The spacecraft Mission Data Processor (MDP) controls the XRT through sequence tables with versatile autonomous functions such as exposure control, region-of-interest tracking, flare detection, and flare location identification. Data are compressed either with DPCM or JPEG, depending on the purpose. This results in higher cadence and/or wider field of view for a given telemetry bandwidth. With a focus adjust mechanism, a higher resolution of Gaussian focus may be available on-axis. This paper follows the first instrument paper for the XRT (Golub et al., Solar Phys. 243, 63, 2007) and discusses the design and measured performance of the X-ray CCD camera for the XRT and its control system with the MDP.

Initial Measurements of Black Hole Spin in GX 339-4 from Suzaku Spectroscopy

Abstract: We report on a deep Suzaku observation of the stellar-mass black hole GX 339-4 in outburst. A clear, strong, relativistically shaped iron emission line from the inner accretion disk is observed. The broadband disk reflection spectrum revealed is one of the most sensitive yet obtained from an accreting black hole. We fit the Suzaku spectra with a physically motivated disk reflection model, blurred by a new relativistic line function in which the black hole spin parameter is a variable. This procedure yielded a black hole spin parameter of a p. Joint modeling of these Suzaku spectra and prior XMM-Newton spectra obtained in two different 0.89 +/- 0.04 outburst phases yields a spin parameter of a = 0.93 +/- 0.01. The degree of consistency between these results suggests that disk reflection models allow for spin measurements that are not strongly biased by scattering effects. We suggest that the best value of the black hole spin parameter is a = 0.93 +/- 0.01 (statistical) +/- 0.04 (systematic). Although preliminary, these results represent the first direct measurement of nonzero spin in a stellar-mass black hole using relativistic line modeling.

The Rewards of Patience: An 822 Day Time Delay in the Gravitational Lens SDSS J1004+4112

Abstract: We present 107 new epochs of optical monitoring data for the four brightest images of the gravitational lens SDSS J1004+4112 observed between 2006 October and 2007 June. Combining this data with the previously obtained light curves, we determine the time delays between images A, B, and C. We confirm our previous measurement, that B leads A by -->Δ τBA = 40.6 ± 1.8 days, and find that image C leads image A by -->Δ τCA = 821.6 ± 2.1 days. The lower limit on the remaining delay is that image D lags image A by -->Δ τAD > 1250 days. Based on the microlensing of images A and B, we estimate that the accretion disk size at a rest wavelength of 2300 A is -->1014.8 ± 0.3 cm for a disk inclination of $cos i = f{1}{2}$ -->, which is consistent with the microlensing disk size-black hole mass correlation function given our estimate of the black hole mass from the Mg II line width of -->log MBH/M☉ = 8.4 ± 0.2. The long delays allow us to fill in the seasonal gaps and assemble a continuous, densely sampled light curve spanning 5.7 yr whose variability implies a structure function with a logarithmic slope of -->β = 0.52 ± 0.02. As C is the leading image, sharp features in the C light curve can be intensively studied 2.3 yr later in the A/B pair, potentially allowing detailed reverberation mapping studies of a quasar at minimal cost.

Planet Formation around Stars of Various Masses: Hot Super-Earths

Abstract: We consider trends resulting from two formation mechanisms for short-period super-Earths: planet-planet scattering and migration. We model scenarios where these planets originate near the snow line in "cold-finger" circumstellar disks. Low-mass planet-planet scattering excites planets to low-periastron orbits only for lower mass stars. With long circularization times, these planets reside on long-period eccentric orbits. Closer formation regions mean planets that reach short-period orbits by migration are most common around low-mass stars. Above ~1 M☉, planets massive enough to migrate to close-in orbits before the gas disk dissipates are above the critical mass for gas giant formation. Thus, there is an upper stellar mass limit for short-period super-Earths that form by migration. If disk masses are distributed as a power law, planet frequency increases with metallicity because most disks have low masses. For disk masses distributed around a relatively high mass, planet frequency decreases with increasing metallicity. As icy planets migrate, they shepherd interior objects toward the star, which grow to ~1 M⊕. In contrast to icy migrators, surviving shepherded planets are rocky. On reaching short-period orbits, planets are subject to evaporation processes. The closest planets may be reduced to rocky or icy cores. Low-mass stars have lower EUV luminosities, so the level of evaporation decreases with decreasing stellar mass.

GEM-AQ, an on-line global multiscale chemical weather modelling system: model description and evaluation of gas phase chemistry processes

Abstract: . Tropospheric chemistry and air quality processes were implemented on-line in the Global Environmental Multiscale weather prediction model. The integrated model, GEM-AQ, was developed as a platform to investigate chemical weather at scales from global to urban. The current chemical mechanism is comprised of 50 gas-phase species, 116 chemical and 19 photolysis reactions, and is complemented by a sectional aerosol module with 5 aerosols types. All tracers are advected using the semi-Lagrangian scheme native to GEM. The vertical transport includes parameterized subgrid-scale turbulence and large scale deep convection. Dry deposition is included as a flux boundary condition in the vertical diffusion equation. Wet deposition of gas-phase species is treated in a simplified way, and only below-cloud scavenging is considered. The emissions used include yearly-averaged anthropogenic, and monthly-averaged biogenic, ocean, soil, and biomass burning emission fluxes, as well as NOx from lightning. In order to evaluate the ability to simulate seasonal variations and regional distributions of trace gases such as ozone, nitrogen dioxide and carbon monoxide, the model was run for a period of five years (2001–2005) on a global uniform 1.5°×1.5° horizontal resolution domain and 28 hybrid levels extending up to 10 hPa. Model results were compared with observations from satellites, aircraft measurement campaigns and balloon sondes. We find that GEM-AQ is able to capture the spatial details of the chemical fields in the middle and lower troposphere. The modelled ozone consistently shows good agreement with observations, except over tropical oceans. The comparison of carbon monoxide and nitrogen dioxide with satellite measurements emphasizes the need for more accurate, year-specific emissions fluxes for biomass burning and anthropogenic sources. Other species also compare well with available observations.

The spitzer survey of interstellar clouds in the gould belt. i. ic 5146 observed with irac and mips

Abstract: We present observations of two areas totalling 0.57 deg^2 in the IC 5146 star-forming region at 3.6, 4.5, 5.8, 8.0, 24, and 70 μm observed with the Spitzer Space Telescope. We reexamine the issue of the distance to this cloud and conclude a value of 950 ± 80 pc is most likely. We compare source counts, colors, and magnitudes in our observed region to a subset of the SWIRE data that was processed through our pipeline. We identify more than 200 young stellar object (YSO) candidates from color-magnitude and color-color diagrams, many of which were previously unknown. We compare the colors of these YSOs to the models of Robitaille et al. and perform simple fits to the SED's to estimate properties of the circumstellar disks likely to surround the Class II and III sources. We also compare the mid-IR disk excesses to Hα emission-line data where available. We present a quantitative description of the degree of clustering, estimate the star formation efficiency, and discuss the fraction of YSOs in the region with disks relative to an estimate of the diskless YSO population. Finally, we compare the YSO distribution to the cold dust distribution mapped by SCUBA and briefly describe the diffuse emission likely due to PAHs associated with the H II region.

Direct detection of the close companion of polaris with the hubble space telescope

Abstract: Polaris, the nearest and brightest classical Cepheid, is a single-lined spectroscopic binary with an orbital period of 30 yr. Using the High Resolution Channel of the Advanced Camera for Surveys on board the Hubble Space Telescope (HST) at a wavelength of ~2255 A, we have directly detected the faint companion at a separation of 017. A second HST observation 1.04 yr later confirms orbital motion in a retrograde direction. By combining our two measures with the spectroscopic orbit of Kamper and an analysis of the Hipparcos and FK5 proper motions by Wielen et al., we find a mass for Polaris Aa of 4.5+2.2 –1.4 M ☉—the first purely dynamical mass determined for any Cepheid. For the faint companion Polaris Ab we find a dynamical mass of 1.26+0.14 –0.07 M ☉, consistent with an inferred spectral type of F6 V and with a flux difference of 5.4 mag observed at 2255 A. The magnitude difference at the V band is estimated to be 7.2 mag. Continued HST observations will significantly reduce the mass errors, which are currently still too large to provide critical constraints on the roles of convective overshoot, mass loss, rotation, and opacities in the evolution of intermediate-mass stars. Our astrometry, combined with two centuries of archival measurements, also confirms that the well-known, more distant (18'') visual companion, Polaris B, has a nearly common proper motion with that of the Aa, Ab pair. This is consistent with orbital motion in a long-period bound system. The ultraviolet brightness of Polaris B is in accordance with its known F3 V spectral type if it has the same distance as Polaris Aa, Ab.

A classification of the X-ray and radio states of Cyg X-3 and their long-term correlations

Abstract: We present a detailed classification of the X-ray states of Cyg X-3 based on the spectral shape and a new classification of the radio states based on the long-term correlated behaviour of the radio and soft X-ray light curves. We find a sequence of correlations, starting with a positive correlation between the radio and soft X-ray fluxes in the hard spectral state, changing to a negative one at the transition to soft spectral states. The temporal evolution can be in either direction on that sequence, unless the source goes into a very weak radio state, from which it can return only following a major radio flare. The flare decline is via relatively bright radio states, which results in a hysteresis loop on the flux–flux diagram. We also study the hard X-ray light curve, and find its overall anticorrelation with the soft X-rays. During major radio flares, the radio flux responds exponentially to the level of a hard X-ray high-energy tail. We also specify the detailed correspondence between the radio states and the X-ray spectral states. We compare our results to those of black hole and neutron star binaries. Except for the effect of strong absorption and the energy of the high-energy break in the hard state, the X-ray spectral states of Cyg X-3 closely correspond to the canonical X-ray states of black hole binaries. Also, the radio/X-ray correlation closely corresponds to that found in black hole binaries, but it significantly differs from that in neutron star binaries. Overall, our results strongly support the presence of a black hole in Cyg X-3.

The century survey galactic halo project. iii. a complete 4300 deg 2 survey of blue horizontal branch stars in the metal-weak thick disk and inner halo

Abstract: We present a complete spectroscopic survey of 2414 2MASS-selected blue horizontal branch (BHB) candidates selected over 4300 deg2 of the sky. We identify 655 BHB stars in this non-kinematically selected sample. We calculate the luminosity function of field BHB stars, and find evidence for very few hot BHB stars in the field. The BHB stars located at a distance from the Galactic plane |Z| < 4 kpc trace what is clearly a metal-weak thick disk population, with a mean metallicity of [Fe/H] = −1.7, a rotation velocity gradient of dvrot/d|Z| = −28 ± 3.4 km s−1 in the region |Z| < 6 kpc, and a density scale height of hZ = 1.26 ± 0.1 kpc. The BHB stars located at 5 < |Z| < 9 kpc are a predominantly inner-halo population, with a mean metallicity of [Fe/H] = −2.0 and a mean Galactic rotation of −4 ± 31 km s−1. We infer the density of halo and thick disk BHB stars is 104 ± 37 kpc−3 near the Sun, and the relative normalization of halo to thick-disk BHB stars is 4 ± 1% near the Sun.

The eccentric accretion disc of the black hole A0620-00

Abstract: We present spectroscopic observations of the quiescent black hole binary A0620-00 with the 6.5-m Magellan Clay telescope at Las Campanas Observatory. We measure absorption-line radial velocities of the secondary and make the most precise determination to date (K-2 = 435.4 +/- 0.5 km s(-1)). By fitting the rotational broadening of the secondary, we refine the mass ratio to q = 0.060 +/- 0.004; these results, combined with the orbital period, imply a minimum mass for the compact object of 3.10 +/- 0.04 M-circle dot. Although quiescence implies little accretion activity, we find that the disc contributes 56 +/- 7 per cent of the light in B and V, and is subject to significant flickering. Doppler maps of the Balmer lines reveal bright emission from the gas stream-disc impact point and unusual crescent-shaped features. We also find that the disc centre of symmetry does not coincide with the predicted black hole velocity. By comparison with smoothed particle hydrodynamics (SPH) simulations, we identify this source with an eccentric disc. With high signal-to-noise ratio (S/N), we pursue modulation tomography of H alpha and find that the aforementioned bright regions are strongly modulated at the orbital period. We interpret this modulation in the context of disc precession, and discuss cases for the accretion disc evolution.

The nature of the hard state of Cygnus X-3

Abstract: The X-ray binary Cygnus X-3 (Cyg X-3) is a highly variable X-ray source that displays a wide range of observed spectral states. One of the main states is significantly harder than the others, peaking at ∼20 keV, with only a weak low-energy component. Due to the enigmatic nature of this object, hidden inside the strong stellar wind of its Wolf‐Rayet companion, it has remained unclear whether this state represents an intrinsic hard state, with truncation of the inner disc, or whether it is just a result of increased local absorption. We study the X-ray light curves from RXTE/ASM and CGRO/BATSE in terms of distributions and correlations of flux and hardness and find several signs of a bimodal behaviour of the accretion flow that are not likely to be the result of increased absorption in a surrounding medium. Using INTEGRAL observations, we model the broad-band spectrum of Cyg X-3 in its apparent hard state. We find that it can be well described by a model of a hard state with a truncated disc, despite the low cut-off energy, provided the accreted power is supplied to the electrons in the inner flow in the form of acceleration rather than thermal heating, resulting in a hybrid electron distribution and a spectrum with a significant contribution from non-thermal Comptonization, usually observed only in soft states. The high luminosity of this non-thermal hard state implies that either the transition takes place at significantly higher L/LE than in the usual advection models, or the mass of the compact object is � 20 M� , possibly making it the most-massive black hole observed in an X-ray binary in our Galaxy so far. We find that an absorption model as well as a model of almost pure Compton reflection also fit the data well, but both have difficulties explaining other results, in particular the radio/X-ray correlation.

MMT extremely metal-poor galaxy survey. I. An efficient technique for identifying metal-poor galaxies

Abstract: We demonstrate a successful strategy for identifying extremely metal-poor galaxies. Our preliminary survey of 24 candidates contains 10 metal-poor galaxies of which 4 have , some of the lowest-metallicity blue compact galaxies known to date. Interestingly, our sample of metal-poor galaxies have systematically lower metallicity for their luminosity than comparable samples of blue compact galaxies, dwarf irregulars, and normal star-forming galaxies. Our metal-poor galaxies share very similar properties, however, with the host galaxies of nearby long-duration gamma-ray bursts (GRBs), including similar metallicity, stellar ages, and star formation rates. We use Hβ to measure the number of OB stars present in our galaxies and estimate a core-collapse supernova rate of ~10−3 yr−1. A larger sample of metal-poor galaxies may provide new clues about the environment where GRBs form and may provide a list of potential GRB hosts.

HVS 7 - a chemically peculiar hyper-velocity star ⋆

Abstract: Context. Hyper-velocity stars are suggested to originate from the dynamical interaction of binary stars with the supermassive black hole in the Galactic centre (GC), which accelerates one component of the binary to beyond the Galactic escape velocity. Aims. The evolutionary status and GC origin of the hyper-velocity star SDSS J113312.12+010824.9 (aka HVS 7) is constrained from a detailed study of its stellar parameters and chemical composition. Methods. High-resolution spectra of HVS 7 obtained with UVES on the ESO VLT were analysed using state-of-the-art NLTE/LTE modelling techniques that can account for a chemically-peculiar composition via opacity sampling. Results. Instead of the expected slight enrichments ofα-elements and near-solar iron, huge chemical peculiarities of all elements are apparent. The helium abundance is very low (<1/100 solar), C, N and O are below the detection limit, i.e they are underabundant (<1/100,.1/3 and<1/10 solar). Heavier elements, however, are overabundant: the iron group by a factor of∼10, P, Co and Cl by factors∼40, 80 and 440 and rare-earth elements and mercury even by∼10 000. An additional finding, relevant also for other chemic ally peculiar stars are the large NLTE effects on abundances of Tiii and Feii (∼0.6‐0.7 dex). The derived abundance pattern of HVS 7 is characteristic for the class of chemical peculiar magnetic B stars on the main sequence. The chemical composition and high projected rotation velocity v sin i= 55±2 km s −1 render a low mass nature of HVS 7 as a blue horizontal branch star unlikely. Conclusions. Such a surface abundance pattern is caused by atomic diffusion in a possibly magnetically stabilised, non-convective atmosphere. Hence all chemical information on the star’s plac e of birth and its evolution has been washed out. High precision astrometry is the only means to validate a GC origin for HVS 7.

HVS7: a chemically peculiar hyper-velocity star

Abstract: Context: Hyper-velocity stars are suggested to originate from the dynamical interaction of binary stars with the supermassive black hole in the Galactic centre (GC), which accelerates one component of the binary to beyond the Galactic escape velocity. Aims: The evolutionary status and GC origin of the HVS SDSS J113312.12+010824.9 (HVS7) is constrained from a detailed study of its stellar parameters and chemical composition. Methods: High-resolution spectra of HVS7 obtained with UVES on the ESO VLT were analysed using state-of-the-art NLTE/LTE modelling techniques that can account for a chemically-peculiar composition via opacity sampling. Results: Instead of the expected slight enrichments of alpha-elements and near-solar Fe, huge chemical peculiarities of all elements are apparent. The He abundance is very low (<1/100 solar), C, N and O are below the detection limit, i.e they are underabundant (<1/100, <1/3 and <1/10 solar). Heavier elements, however, are overabundant: the iron group by a factor of ~10, P, Co and Cl by factors ~40, 80 and 440 and rare-earth elements and Hg even by ~10000. An additional finding, relevant also for other chemically peculiar stars are the large NLTE effects on abundances of TiII and FeII (~0.6-0.7dex). The derived abundance pattern of HVS7 is characteristic for the class of chemical peculiar magnetic B stars on the main sequence. The chemical composition and high vsini=55+-2km/s render a low mass nature of HVS7 as a blue horizontal branch star unlikely. Conclusions: Such a surface abundance pattern is caused by atomic diffusion in a possibly magnetically stabilised, non-convective atmosphere. Hence all chemical information on the star's place of birth and its evolution has been washed out. High precision astrometry is the only means to validate a GC origin for HVS7.

Polaris the Cepheid Returns: 4.5 Years of Monitoring from Ground and Space

Abstract: We present the analysis of 4.5 years of nearly continuous observations of the classical Cepheid Polaris, which comprise the most precise data available for this star. We have made spectroscopic measurements from ground and photometric measurements from the WIRE star tracker and the SMEI instrument on the Coriolis satellite. Measurements of the amplitude of the dominant oscillation (P = 4 days), which go back more than a century, show a decrease from AV = 120 to 30 mmag around the turn of the millennium. It has been speculated that the reason for the decrease in amplitude is the evolution of Polaris toward the edge of the instability strip. However, our new data reveal an increase in the amplitude by ~30% from 2003 to 2006. It now appears that the amplitude change is cyclic rather than monotonic and most likely the result of a pulsation phenomenon. In addition, previous radial velocity campaigns have claimed the detection of long-period variation in Polaris (P > 40 days). Our radial velocity data are more precise than previous data sets, and we find no evidence for additional variation for periods in the range 3-50 days with an upper limit of 100 m s−1. However, in the WIRE data we find evidence of variation on timescales of 2-6 days, which we interpret as being due to granulation.

X-ray tail in ngc 7619

Abstract: We present new observational results of NGC 7619, an elliptical galaxy with a prominent X-ray tail and a dominant member of the Pegasus group. With Chandra and XMM-Newton observations, we confirm the presence of a long X-ray tail in the southwest direction; moreover, we identify for the first time a sharp discontinuity of the X-ray surface brightness in the opposite (northeast) side of the galaxy. The density, temperature, and pressure jump at the northeast discontinuity suggest a Mach number ~1, corresponding to a galaxy velocity of ~500 km s−1, relative to the surrounding hot gas. Spectral analysis of these data shows that the iron abundance of the hot gaseous medium is much higher (1-2 solar) near the center of NGC 7619 and in the tail extending from the core than in the surrounding regions (≤1/2 solar), indicating that the gas in the tail is originated from the galaxy. The possible origin of the head-tail structure is either ongoing ram pressure stripping or sloshing. The morphology of the structure is more in line with a ram pressure stripping phenomenon, while the position of NGC 7619 at the center of the Pegasus I group, and its dominance, would prefer sloshing.

WMAP5 and the Cluster Mass Function

Abstract: The recently revised cosmological constraints from the 5 year WMAP data ameliorate previous tension between cosmological constraints from the microwave background and from cluster abundances. We demonstrate that the revised estimates of cosmological parameters are in excellent agreement with the mass function of X-ray clusters in the Sloan Digital Sky Survey. Velocity segregation between galaxies and the underlying dark matter could cause virial mass estimates to be biased, causing the mass scale of the mass function to be offset from the true value. Modest velocity segregation (?gxy/?DM = 1.13 -->?0.05+0.06) is sufficient to match the mass function to the 5 year WMAP results. However, when the new WMAP results are combined with constraints from supernovae and baryon acoustic oscillations, there is no need for velocity segregation (?gxy/?DM = 1.05 ? 0.05). This result agrees with expectations for velocity segregation from state-of-the-art numerical simulations of clusters. Together with the improved agreement between the new WMAP results and recent cosmic shear measurements, this result demonstrates that the amplitude of large-scale structure in the nearby universe matches that predicted from the structure seen in the microwave background. The new constraint we place on velocity segregation in clusters indicates that virial mass estimates for clusters are reasonably accurate. This result suggests that future cluster surveys will be able to probe both cosmological parameters and fundamental cluster physics.

Searching for Narrow Emission Lines in X-ray Spectra: Computation and Methods

Abstract: The detection and quantification of narrow emission lines in X-ray spectra is a challenging statistical task. The Poisson nature of the photon counts leads to local random fluctuations in the observed spectrum that often result in excess emission in a narrow band of energy resembling a weak narrow line. From a formal statistical perspective, this leads to a (sometimes highly) multimodal likelihood. Many standard statistical procedures are based on (asymptotic) Gaussian approximations to the likelihood and simply cannot be used in such settings. Bayesian methods offer a more direct paradigm for accounting for such complicated likelihood functions, but even here multimodal likelihoods pose significant computational challenges. The new Markov chain Monte Carlo (MCMC) methods developed in 2008 by van Dyk and Park, however, are able to fully explore the complex posterior distribution of the location of a narrow line, and thus provide valid statistical inference. Even with these computational tools, standard statistical quantities such as means and standard deviations cannot adequately summarize inference and standard testing procedures cannot be used to test for emission lines. In this paper, we use new efficient MCMC algorithms to fit the location of narrow emission lines, we develop new statistical strategies for summarizing highly multimodal distributions and quantifying valid statistical inference, and we extend the method of posterior predictive p-values proposed by Protassov and coworkers to test for the presence of narrow emission lines in X-ray spectra. We illustrate and validate our methods using simulation studies and apply them to the Chandra observations of the high-redshift quasar PG 1634+706.

Molecular gas in NUclei of GAlaxies (NUGA) - VIII. The Seyfert 2 NGC 6574

Abstract: Within the frame of the NUclei of GAlaxies (NUGA) project, we have determined the distribution and kinematics of the molecular gas within the central kpc with high spatial resolution (100-150 pc), for a sample of active galaxies. The goal is to study the gas-fueling mechanisms in AGN. We present interferometric observations of 12 CO(1-0) and 12 CO(2-1) line emission from the Seyfert 2 galaxy NGC 6574, obtained with the IRAM Plateau de Bure Interferometer (PdBI). These data have been combined with 30 m mapping data in these lines to correct for the flux resolved by the interferometer. At an angular resolution of 0.7" (=110 pc), the 12 CO(2-1) emission is resolved into an inner disk with a radius of 300 pc. The molecular gas in NGC 6574 is primarily distributed in four components: nucleus, bar, spiral arms - winding up into a pseudo-ring - and an extended underlying disk component. For the overall galaxy host, we find a 12 CO(2-1) to 12 CO(1-0) line ratio of ∼0.3 indicative of cold or sub-thermally excited gas. For the nucleus, this ratio is close to unity, indicating emission from dense and warm molecular gas. Modeling the gas kinematics with elliptical orbits shows that the molecular gas in the differentially rotating disk of NGC 6574 is strongly influenced by the presence of a stellar bar. The nuclear component shows an extension toward the southeast that may be an indication of the lopsidedness of the nuclear gas distribution. We computed the gravity torques exerted from the stellar bar on the gas, deriving the gravitational potential from near-infrared images, and weighting the torques by the CO distribution. We find negative torques for the gas inside the ring, since the gas aligned with the bar has a slight advance phase shift, leading the bar. This means that gas is flowing in towards the center, at least down to 400 pc in radius, which can explain the observed high nuclear gas concentration. This concentration corresponds to a possible inner Lindblad resonance of the bar, according to the measured rotation curve. The gas has been piling up in this location quite recently, since no startburst has been observed yet.

Swift observations of IBL and LBL objects

Abstract: Context. BL Lacs are an enigmatic class of active galactic nuclei (AGNs), characterized by the non-thermal continuum typically attributed to synchrotron and inverse Compton emission. Depending on the frequency location of the maxima of these components, they are subdivided into three subclasses LBLs, IBLs, and HBLs. We present the results of a set of observations of eight BL Lac objects of LBL and IBL type performed by the XRT and UVOT detectors onboard the Swift satellite between January 2005 and November 2006. Aims. We are mainly interested in measuring the spectral parameters, and particularly the steepness between the UV and the X-ray band, useful for determining the classification of these sources. We compare the behavior of these sources with previous XMM-Newton, BeppoSAX observations and with historical data in the X-ray and in the optical band. We are also interested in classifying the sources in our sample on the basis of the Swift observations and comparing them with their classification presented in literature. Methods. We performed X-ray spectral analysis of observed BL Lac objects using a simple powerlaw and in a few cases the logparabolic model. We also combined the UV emission with the low energy X-ray data to describe their spectral energy distribution. Results. We used Swift observational data to classify sources in our sample and derived parameters of their spectral energy distribution. Conclusions. We found that for the IBLs X-rays low states show features of the high energy component, usually interpreted as due to inverse Compton emission. Sources in our sample exhibit a range of temporal UV and X-ray behaviors, some objects having clear and neat correlated UV and X-ray variations (e.g. ON231) and other objects showing no clear (e.g. AO 0235+164) UV and X-ray correlation. Finally, we also note that our estimates of spectral curvature are in the range of that measured for the High frequency peaked BL Lac objects (HBLs).

The milli-arc-second structure imager (MASSIM): a new concept for a high angular resolution x-ray telescope

Abstract: MASSIM, the Milli-Arc-Second Structure Imager, is a mission that has been proposed for study within the context of NASA's Astrophysics Strategic Mission Concept Studies program. It uses a set of achromatic diffractive-refractive Fresnel lenses on an optics spacecraft to focus 5-11 keV X-rays onto detectors on a second spacecraft flying in formation 1000 km away. It will have a point-source sensitivity comparable with that of the current generation of major X-ray observatories (Chandra, XMM-Newton) but an angular resolution some three orders of magnitude better. MASSIM is optimized for the study of jets and other phenomena that occur in the immediate vicinity of black holes and neutron stars. It can also be used for studying other astrophysical phenomena on the milli-arc-second scale, such as those involving proto-stars, the surfaces and surroundings of nearby active stars and interacting winds. We describe the MASSIM mission concept, scientific objectives and the trade-offs within the X-ray optics design. The anticipated performance of the mission and possible future developments using the diffractive-refractive optics approach to imaging at X-ray and gamma-ray energies are discussed.