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Register a new userHST/ACS Images of the GG Tauri Circumbinary Disk
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Hubble Space Telescope Advanced Camera for Surveys images of the young binary GG Tauri and its circumbinary disk in V and I bandpasses were obtained in 2002 and are the most detailed of this system to date. The confirm features previously seen in the disk including: a gap apparently caused by shadowing from circumstellar material; an asymmetrical distribution of light about the line of sight on the near edge of the disk; enhanced brightness along the near edge of the disk due to forward scattering; and a compact reflection nebula near the secondary star. New features are seen in the ACS images: two short filaments along the disk; localized but strong variations in disk intensity (gaplets); and a spur or filament extending from the reflection nebulosity near the secondary. The back side of the disk is detected in the V band for the first time. The disk appears redder than the combined light from the stars, which may be explained by a varied distribution of grain sizes. The brightness asymmetries along the disk suggest that it is asymmetrically illuminated by the stars due to extinction by nonuniform circumstellar material or the illuminated surface of the disk is warped by tidal effects (or perhaps both). Localized, time-dependent brightness variations in the disk are also seen.
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We study the population of compact stellar clusters (CSCs) in M81, using the HST/ACS images in the filters F435W, F606W and F814W covering, for the first time, the entire optical extent of the galaxy. Our sample contains 435 clusters of FWHM less than 10 ACS pixels (9 pc). The sample shows the presence of two cluster populations, a blue group of 263 objects brighter than B=22 mag, and a red group of 172 objects, brighter than B=24 mag. Based on the analysis of colour magnitude diagrams and making use of simple stellar population models, we find the blue clusters are younger than 300 Myr with some clusters as young as few Myr, and the red clusters are as old as globular clusters. The luminosity function of the blue group follows a power-law distribution with an index of 2.0, typical value for young CSCs in other galaxies. The power-law shows unmistakable signs of truncation at I=18.0 mag (M_I=-9.8 mag), which would correspond to a mass-limit of 4x10^4 M_solar if the brightest clusters are younger than 10 Myr. The red clusters have photometric masses between 10^5 to 2x10^7 M_solar for the adopted age of 5 Gyr and their luminosity function resembles very much the globular cluster luminosity function in the Milky Way. The brightest GC in M81 has M_B^0=-10.3 mag, which is ~0.9 mag brighter than w-Cen, the most massive GC in the Milky Way.
(Abridged.) We present F435W (B), F606W (Broad V), and F814W (Broad I) coronagraphic images of the debris disk around Beta Pictoris obtained with HSTs Advanced Camera for Surveys. We confirm that the previously reported warp in the inner disk is a distinct secondary disk inclined by ~5 deg from the main disk. The main disks northeast extension is linear from 80 to 250 AU, but the southwest extension is distinctly bowed with an amplitude of ~1 AU over the same region. Both extensions of the secondary disk appear linear, but not collinear, from 80 to 150 AU. Within ~120 AU of the star, the main disk is ~50% thinner than previously reported. The surface-brightness profiles along the spine of the main disk are fitted with four distinct radial power laws between 40 and 250 AU, while those of the secondary disk between 80 and 150 AU are fitted with single power laws. These discrepancies suggest that the two disks have different grain compositions or size distributions. The F606W/F435W and F814W/F435W flux ratios of the composite disk are nonuniform and asymmetric about both projected axes of the disk. Within ~120 AU, the m_F435W-m_F606W and m_F435W-m_F814W colors along the spine of the main disk are ~10% and ~20% redder, respectively, than those of Beta Pic. These colors increasingly redden beyond ~120 AU, becoming 25% and 40% redder, respectively, than the star at 250 AU. We compare the observed red colors within ~120 AU with the simulated colors of non-icy grains having a radial number density ~r^-3 and different compositions, porosities, and minimum grain sizes. The observed colors are consistent with those of compact or moderately porous grains of astronomical silicate and/or graphite with sizes >0.15-0.20 um, but the colors are inconsistent with the blue colors expected from grains with porosities >90%.
Our objective is to study the vertical dust distribution in the circumbinary ring of the binary system GG Tau and to search for evidence of stratification, one of the first steps expected to occur during planet formation. We present a simultaneous analysis of four scattered light images spanning a range of wavelength from 800 nm to 3800 nm and compare them with (i) a parametric prescription for the vertical dust stratification, and (ii) with the results of SPH bi-fluid hydrodynamic calculations. The parametric prescription and hydrodynamical calculations of stratification both reproduce the observed brightness profiles well. These models also provide a correct match for the observed star/ring integrated flux ratio. Another solution with a well-mixed, but ``exotic, dust size distribution also matches the brightness profile ratios but fails to match the star/ring flux ratio. These results give support to the presence of vertical stratification of the dust in the ring of GG Tau and further predict the presence of a radial stratification also.
A high angular resolution near-infrared polarized-intensity image of the GG Tau A binary system was obtained with the Subaru Telescope. The image shows the circumbinary disk scattering the light from the central binary. The azimuthal profile of the polarized intensity of the circumbinary disk is roughly reproduced by a simple disk model with the Henyey-Greenstein function and the Rayleigh function, indicating small dust grains at the surface of the disk. Combined with a previous observation of the circumbinary disk, our image indicates that the gap structure in the circumbinary disk orbits anti-clockwise, while material in the disk orbit clockwise. We propose a shadow of material located between the central binary and the circumbinary disk. The separations and position angles of the stellar components of the binary in the past 20 years are consistent with the binary orbit with a = 33.4 AU and e = 0.34.
[ABRIDGED] We are undertaking an imaging study of local star-forming galaxies in the Lyman-alpha (Lya) emission line using the Solar Blind Channel (SBC) of the ACS onboard the HST. Observations have been obtained in Lya and H-alpha (Ha) and 6 continuum filters between ~1500AA and the I-band. Previously (Hayes et al 2005) we demonstrated that the production of Lya line-only images in the SBC-only data-set is non-trivial and that supporting data is a requirement. We here develop various methods of continuum subtraction and assess their relative performance using a variety of spectral energy distributions (SED) as input. We conclude that simple assumptions about the behavior of the ultraviolet continuum consistently lead to results that are wildly erroneous, and determine that an SED fitting approach is essential. Moreover, a single component stellar or stellar+nebular spectrum is not always sufficient for realistic template SEDs and, in order to successfully recover the input observables, care must be taken to control contributions of nebular gas and underlying stellar populations. Independent measurements of the metallicity must first be obtained, but details of the initial mass function play only a small role. We identify the need to bin together pixels in our data to obtain signal-to-noise (S/N) of ~10 in each band before processing. At S/N=10 we can recover Lya fluxes accurate to ~30% for Lya lines with intrinsic equivalent width [W(Lya)] of 10AA. This accuracy improves to ~10% for W(Lya)=100AA. We describe the image processing applied to the observations presented in Ostlin et al. (2009) and the associated data-release. We also present simulations for an observing strategy for an alternative low-redshift SBC Lya imaging campaign using adjacent combinations of long-pass filters at slightly higher redshift.
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