1,220 research outputs found
Discovery of Reflection Nebulosity Around Five Vega-like Stars
Coronagraphic optical observations of six Vega-like stars reveal reflection
nebulosities, five of which were previously unknown. The nebulosities
illuminated by HD 4881, HD 23362, HD 23680, HD 26676, and HD 49662 resemble
that of the Pleiades, indicating an interstellar origin for dust grains. The
reflection nebulosity around HD 123160 has a double-arm morphology, but no
disk-like feature is seen as close as 2.5 arcsec from the star in K-band
adaptive optics data. We demonstrate that uniform density dust clouds
surrounding HD 23362, HD 23680 and HD 123160 can account for the observed
12-100 micron spectral energy distributions. For HD 4881, HD 26676, and HD
49662 an additional emission source, such as from a circumstellar disk or
non-equilibrium grain heating, is required to fit the 12-25 micron data. These
results indicate that in some cases, particularly for Vega-like stars located
beyond the Local Bubble (>100 pc), the dust responsible for excess thermal
emission may originate from the interstellar medium rather than from a
planetary debris system.Comment: The Astrophysical Journal, in press for March, 2002 (32 pages, 13
figures
Sylvester's question and the Random Acceleration Process
Let n points be chosen randomly and independently in the unit disk.
"Sylvester's question" concerns the probability p_n that they are the vertices
of a convex n-sided polygon. Here we establish the link with another problem.
We show that for large n this polygon, when suitably parametrized by a function
r(phi) of the polar angle phi, satisfies the equation of the random
acceleration process (RAP), d^2 r/d phi^2 = f(phi), where f is Gaussian noise.
On the basis of this relation we derive the asymptotic expansion log p_n = -2n
log n + n log(2 pi^2 e^2) - c_0 n^{1/5} + ..., of which the first two terms
agree with a rigorous result due to Barany. The nonanalyticity in n of the
third term is a new result. The value 1/5 of the exponent follows from recent
work on the RAP due to Gyorgyi et al. [Phys. Rev. E 75, 021123 (2007)]. We show
that the n-sided polygon is effectively contained in an annulus of width \sim
n^{-4/5} along the edge of the disk. The distance delta_n of closest approach
to the edge is exponentially distributed with average 1/(2n).Comment: 29 pages, 4 figures; references added and minor change
A double-lined spectroscopic orbit for the young star HD 34700
We report high-resolution spectroscopic observations of the young star HD
34700, which confirm it to be a double-lined spectroscopic binary. We derive an
accurate orbital solution with a period of 23.4877 +/- 0.0013 days and an
eccentricity of e = 0.2501 +/- 0.0068. The stars are found to be of similar
mass (M2/M1 = 0.987 +/- 0.014) and luminosity. We derive also the effective
temperatures (5900 K and 5800 K) and projected rotational velocities (28 km/s
and 22 km/s) of the components. These values of v sin i are much higher than
expected for main-sequence stars of similar spectral type (G0), and are not due
to tidal synchronization. We discuss also the indicators of youth available for
the object. Although there is considerable evidence that the system is young
--strong infrared excess, X-ray emission, Li I 6708 absorption (0.17 Angstroms
equivalent width), H alpha emission (0.6 Angstroms), rapid rotation-- the
precise age cannot yet be established because the distance is unknown.Comment: 17 pages, including 2 figures and 2 tables. Accepted for publication
in AJ, to appear in February 200
A Keplerian Disk around the Herbig Ae star HD169142
We present Submillimeter Array observations of the Herbig Ae star HD169142 in
1.3 millimeter continuum emission and 12CO J=2-1 line emission at 1.5 arcsecond
resolution that reveal a circumstellar disk. The continuum emission is centered
on the star position and resolved, and provides a mass estimate of about 0.02
solar masses for the disk. The CO images show patterns in position and velocity
that are well matched by a disk in Keplerian rotation with low inclination to
the line-of-sight. We use radiative transfer calculations based on a flared,
passive disk model to constrain the disk parameters by comparison to the
spectral line emission. The derived disk radius is 235 AU, and the inclination
is 13 degrees. The model also necessitates modest depletion of the CO
molecules, similar to that found in Keplerian disks around T Tauri stars.Comment: 10 pages, 2 figures, accepted by A
Multivariate Poincar\'e series for algebras of -invariants
Let \mathcal{C}_{\mathbi{d}}, \mathcal{I}_{\mathbi{d}},
\mathbi{d}{=}(d_1,d_2,..., d_n) be the algebras of join covariants and joint
invariants of the binary forms of degrees Formulas for
computation of the multivariate Poincar\'e series
\mathcal{P}(\mathcal{C}_{\mathbi{d}},z_1,z_2,...,z_n,t) and
\mathcal{P}(\mathcal{I}_{\mathbi{d}},z_1,z_2,...,z_n) are found.Comment: 5 page
Spitzer/MIPS Observations of Stars in the Beta Pictoris Moving Group
We present Multiband Imaging Photometer for Spitzer (MIPS) observations at 24
and 70 microns for 30 stars, and at 160 microns for a subset of 12 stars, in
the nearby (~30 pc), young (~12 Myr) Beta Pictoris Moving Group (BPMG). In
several cases, the new MIPS measurements resolve source confusion and
background contamination issues in the IRAS data for this sample. We find that
7 members have 24 micron excesses, implying a debris disk fraction of 23%, and
that at least 11 have 70 micron excesses (disk fraction of >=37%). Five disks
are detected at 160 microns (out of a biased sample of 12 stars observed), with
a range of 160/70 flux ratios. The disk fraction at 24 and 70 microns, and the
size of the excesses measured at each wavelength, are both consistent with an
"inside-out" infrared excess decrease with time, wherein the shorter-wavelength
excesses disappear before longer-wavelength excesses, and consistent with the
overall decrease of infrared excess frequency with stellar age, as seen in
Spitzer studies of other young stellar groups. Assuming that the infrared
excesses are entirely due to circumstellar disks, we characterize the disk
properties using simple models and fractional infrared luminosities. Optically
thick disks, seen in the younger TW Hya and eta Cha associations, are entirely
absent in the BPMG.
Additional flux density measurements at 24 and 70 microns are reported for
nine Tucanae-Horologium Association member stars. Since this is <20% of the
association membership, limited analysis on the complete disk fraction of this
association is possible.Comment: Accepted for Ap
NSCAT high-resolution surface wind measurements in Typhoon Violet
NASA scatterometer (NSCAT) measurements of the western Pacific Supertyphoon Violet are presented for revolutions 478 and 485 that occurred in September 1996. A tropical cyclone planetary boundary layer numerical, model, which uses conventional meteorological and geostationary cloud data, is used to estimate the winds at 10-m elevation in the cyclone. These model winds are then compared with the winds inferred from the NSCAT backscatter data by means of a novel approach that allows a wind speed to be recovered from each individual backscatter cell. This spatial adaptive (wind vector) retrieval algorithm employs several unique steps. The backscatter values are first regrouped in terms of closest neighbors in sets of four. The maximum likelihood estimates of speed and direction are then used to obtain speeds and directions for each group. Since the cyclonic flow around the tropical cyclone is known, NSCAT wind direction alias selection is easily accomplished. The selected wind directions are then used to convert each individual backscatter value to a wind speed. The results are compared to the winds obtained from the tropical cyclone boundary layer model. The NSCAT project baseline geophysical model function, NSCAT 1, was found to yield wind speeds that were systematically too low, even after editing for suspected rain areas of the cyclone. A new geophysical model function was developed using conventional NSCAT data and airborne Ku band scatterometer measurements in an Atlantic hurricane. This new model uses the neural network method and yields substantially better agreement with the winds obtained from the boundary layer model according to the statistical tests that were used
Orogen-parallel deformation of the Himalayan mid-crust: Insights from structural and magnetic fabric analyses of the Greater Himalayan Sequence, Annapurna-Dhaulagiri Himalaya, central Nepal
The metamorphic core of the Himalaya (Greater Himalayan Sequence, GHS), in the Annapurna-Dhaulagiri region, central Nepal, recorded orogen-parallel stretching during midcrustal evolution. Anisotropy of magnetic susceptibility and field-based structural analyses suggest that midcrustal deformation of the amphibolite facies core of the GHS occurred under an oblate/suboblate strain regime with associated formation of low-angle northward dipping foliation. Magnetic and mineral stretching lineations lying within this foliation from the top of the GHS record right-lateral orogen-parallel stretching. We propose that oblate strain within a midcrustal flow accommodated oblique convergence between India and the arcuate orogenic front without the need for strain partitioning in the upper crust. Oblate flattening may have also promoted orogen-parallel melt migration and development of melt-depleted regions between km3 scale leucogranite culminations at ~50–100 km intervals along orogen strike. Following the cessation of flow, continued oblique convergence led to upper crustal strain partitioning between orogen-perpendicular convergence on thrust faults and orogen-parallel extension on normal and strike-slip faults. In the Annapurna-Dhaulagiri Himalaya, orogen-parallel stretching lineations are interpreted as a record of transition from midcrustal orogen-perpendicular extrusion to upper crustal orogen-parallel stretching. Our findings suggest that midcrustal flow and upper crustal extension could not be maintained simultaneously and support other studies from across the Himalaya, which propose an orogen-wide transition from midcrustal orogen-perpendicular extrusion to upper crustal orogen-parallel extension during the mid-Miocene. The 3-D nature of oblate strain and orogen-parallel stretching cannot be replicated by 2-D numerical simulations of the Himalayan orogen
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