192 research outputs found
Dust Emission from Herbig Ae/Be stars - Evidence for Disks and Envelopes
IR and mm-wave emission from Herbig Ae/Be stars has produced conflicting
conclusions regarding the dust geometry in these objects. We show that the
compact dimensions of the mm-wave emitting regions are a decisive indication
for disks. But a disk cannot explain the spectral energy distribution (SED)
unless it is embedded in an extended envelope that (1) dominates the IR
emission and (2) provides additional disk heating on top of the direct stellar
radiation. Detailed radiative transfer calculations based on the simplest model
for envelope-embedded disks successfully fit the data from UV to mm wavelengths
and show that the disks have central holes. This model also resolves naturally
some puzzling results of IR imaging.Comment: 9 pages, 2 figures. accepted to ApJ
Color Transformations for the 2MASS Second Incremental Data Release
Transformation equations are presented to convert colors and magnitudes
measured in the AAO, ARNICA, CIT, DENIS, ESO, LCO (Persson standards), MSSSO,
SAAO, and UKIRT photometric systems to the photometric system inherent to the
2MASS Second Incremental Data Release. The transformations have been derived by
comparing 2MASS photometry with published magnitudes and colors for stars
observed in these systems. Transformation equations have also been derived
indirectly for the Bessell & Brett (1988) and Koornneef (1983) homogenized
photometric systems.Comment: To appear in AJ, May 200
The Vega Debris Disk -- A Surprise from Spitzer
We present high spatial resolution mid- and far-infrared images of the Vega
debris disk obtained with the Multiband Imaging Photometer for Spitzer (MIPS).
The disk is well resolved and its angular size is much larger than found
previously. The radius of the disk is at least 43" (330 AU), 70"(543 AU), and
105" (815 AU) in extent at 24, 70 and 160 um, respectively. The disk images are
circular, smooth and without clumpiness at all three wavelengths. The radial
surface brightness profiles imply an inner boundary at a radius of 11"+/-2" (86
AU). Assuming an amalgam of amorphous silicate and carbonaceous grains, the
disk can be modeled as an axially symmetric and geometrically thin disk, viewed
face-on, with the surface particle number density following an r^-1 power law.
The disk radiometric properties are consistent with a range of models using
grains of sizes ~1 to ~50 um. We find that a ring, containing grains larger
than 180 um and at radii of 86-200 AU from the star, can reproduce the observed
850 um flux, while its emission does not violate the observed MIPS profiles.
This ring could be associated with a population of larger asteroidal bodies
analogous to our own Kuiper Belt. Cascades of collisions starting with
encounters amongthese large bodies in the ring produce the small debris that is
blown outward by radiation pressure to much larger distances where we detect
its thermal emission. The dust production rate is >~10^15 g/s based on the MIPS
results. This rate would require a very massive asteroidal reservoir for the
dust to be produced in a steady state throughout Vega's life. Instead, we
suggest that the disk we imaged is ephemeral and that we are witnessing the
aftermath of a large and relatively recent collisional event, and subsequent
collisional cascade.Comment: 13 pages, 17 figures, accepted for publication in ApJ. (Figures 2,
3a, 3b and 4 have been degraded to lower resolutions.
The Rest Frame Optical Spectra of Lyman Break Galaxies: Star Formation, Extinction, Abundances, and Kinematics
We present the first results of a spectroscopic survey of Lyman break
galaxies in the near-infrared aimed at detecting the emission lines of [O II],
[O III], and Hbeta from the H II regions of star forming galaxies at z = 3.
From observations of 19 objects with the Keck and VLT telescopes, we reach
the following main conclusions. Contrary to expectations, the star formation
rates deduced from the Hbeta luminosity are on average no larger than those
implied by the stellar continuum at 1500 A; presumably any differential
extinction between rest-frame optical and UV is small compared with the
relative uncertainties in the calibrations of these two star formation tracers.
For the galaxies in our sample, the abundance of O can only be determined to
within one order of magnitude. Even so, it seems well established that LBGs are
the most metal-enriched structures at z = 3, apart from QSOs, with abundances
greater than about 1/10 solar. They are also significantly overluminous for
their metallicities; this is probably an indication that their mass-to-light
ratios are small compared with present-day galaxies. At face value their
velocity dispersions, sigma = 50 - 115 km/s imply virial masses of about
10^{10} solar masses within half-light radii of 2.5 kpc. However, we are unable
to establish if the widths of the emission lines do reflect the motions of the
H II regions within the gravitational potential of the galaxies, even though in
two cases we see hints of rotation curves. All 19 LBGs observed show evidence
for galactic-scale superwinds; such outflows are important for regulating star
formation, distributing metals over large volumes, and allowing Lyman continuum
photons to escape and ionize the IGM.Comment: 44 pages, LaTeX, 13 Postscript Figures. Accepted for publication in
the Astrophysical Journal, Vol. 554, No. 2 (Jun 20, 2001
The Primordial Binary Population - I: A near-infrared adaptive optics search for close visual companions to A star members of Scorpius OB2
We present the results of a near-infrared adaptive optics survey with the aim
to detect close companions to Hipparcos members in the three subgroups of the
nearby OB association Sco OB2: Upper Scorpius (US), Upper Centaurus Lupus (UCL)
and Lower Centaurus Crux (LCC). We have targeted 199 A-type and late B-type
stars in the Ks band, and a subset also in the J and H band. We find 151
stellar components other than the target stars. A brightness criterion is used
to separate these components into 77 background stars and 74 candidate physical
companion stars. Out of these 74 candidate companions, 41 have not been
reported before (14 in US; 13 in UCL; 14 in LCC). Companion star masses range
from 0.1 to 3 Msun. The mass ratio distribution follows f(q) = q^-0.33, which
excludes random pairing. No close (rho < 3.75'') companion stars or background
stars are found in the magnitude range 12 < Ks < 14. The lack of stars with
these properties cannot be explained by low-number statistics, and may imply a
lower limit on the companion mass of ~ 0.1 Msun. Close stellar components with
Ks > 14 are observed. If these components are very low-mass companion stars, a
gap in the companion mass distribution might be present. The small number of
close low-mass companion stars could support the embryo-ejection formation
scenario for brown dwarfs. Our findings are compared with and complementary to
visual, spectroscopic, and astrometric data on binarity in Sco OB2. We find an
overall companion star fraction of 0.52 in this association. This paper is the
first step toward our goal to derive the primordial binary population in Sco
OB2.Comment: 27 pages, to accepted by A&
Clathrin- and Dynamin-Independent Endocytosis of FGFR3 – Implications for Signalling
Endocytosis of tyrosine kinase receptors can influence both the duration and the specificity of the signal emitted. We have investigated the mechanisms of internalization of fibroblast growth factor receptor 3 (FGFR3) and compared it to that of FGFR1 which is internalized predominantly through clathrin-mediated endocytosis. Interestingly, we observed that FGFR3 was internalized at a slower rate than FGFR1 indicating that it may use a different endocytic mechanism than FGFR1. Indeed, after depletion of cells for clathrin, internalization of FGFR3 was only partly inhibited while endocytosis of FGFR1 was almost completely abolished. Similarly, expression of dominant negative mutants of dynamin resulted in partial inhibition of the endocytosis of FGFR3 whereas internalization of FGFR1 was blocked. Interfering with proposed regulators of clathrin-independent endocytosis such as Arf6, flotillin 1 and 2 and Cdc42 did not affect the endocytosis of FGFR1 or FGFR3. Furthermore, depletion of clathrin decreased the degradation of FGFR1 resulting in sustained signalling. In the case of FGFR3, both the degradation and the signalling were only slightly affected by clathrin depletion. The data indicate that clathrin-mediated endocytosis is required for efficient internalization and downregulation of FGFR1 while FGFR3, however, is internalized by both clathrin-dependent and clathrin-independent mechanisms
A GWAS sequence variant for platelet volume marks an alternative DNM3 promoter in megakaryocytes near a MEIS1 binding site
We recently identified 68 genomic loci where common sequence variants are associated with platelet count and volume. Platelets are formed in the bone marrow by megakaryocytes, which are derived from hematopoietic stem cells by a process mainly controlled by transcription factors. The homeobox transcription factor MEIS1 is uniquely transcribed in megakaryocytes and not in the other lineage-committed blood cells. By ChIP-seq, we show that 5 of the 68 loci pinpoint a MEIS1 binding event within a group of 252 MK-overexpressed genes. In one such locus in DNM3, regulating platelet volume, the MEIS1 binding site falls within a region acting as an alternative promoter that is solely used in megakaryocytes, where allelic variation dictates different levels of a shorter transcript. The importance of dynamin activity to the latter stages of thrombopoiesis was confirmed by the observation that the inhibitor Dynasore reduced murine proplatelet for-mation in vitro
On the difference between nuclear and contraction ages
Context:Ages derived from the low mass stars still contracting onto the main
sequence often differ from ages derived from the high mass ones that have
already evolved away from it.
Aims:We investigate the general claim of disagreement between these two
independent age determinations by presenting UBVRI photometry of the young
galactic open clusters NGC 2232, NGC 2516, NGC 2547 and NGC 4755, spanning the
age range ~10-150 Myr
Methods: We derive reddenings, distances and nuclear ages by fitting ZAMS and
isochrones to color-magnitudes and color-color diagrams. To derive contraction
ages, we use four different pre-main sequence models, with an empirically
calibrated color-temperature relation to match the Pleiades cluster sequence.
Results: When using exclusively the V vs. V-I color-magnitude diagram and
empirically calibrated isochrones, there is consistency between nuclear and
contraction ages for the studied clusters. Although the contraction ages seem
systematically underestimated, in none of cases they deviate by more than one
standard deviation from the nuclear ages.Comment: 22 pages, 16 figures, accepted for publication on A&A. Nature of
replacement: A&A structured abstrac
The Circadian Neuropeptide PDF Signals Preferentially through a Specific Adenylate Cyclase Isoform AC3 in M Pacemakers of Drosophila
To synchronize a network of pacemakers in the Drosophila brain, a neuropeptide receptor specifically associates with adenylate cyclase 3 to create a “circadian signalosome.
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