389 research outputs found
Negative mass corrections in a dissipative stochastic environment
We study the dynamics of a macroscopic object interacting with a dissipative stochastic environment using an adiabatic perturbation theory. The perturbation theory reproduces known expressions for the friction coefficient and, surprisingly, gives an additional negative mass correction. The effect of the negative mass correction is illustrated by studying a harmonic oscillator interacting with a dissipative stochastic environment. While it is well known that the friction coefficient causes a reduction of the oscillation frequency, we show that the negative mass correction can lead to its enhancement. By studying an exactly solvable model of a magnet coupled to a spin environment evolving under standard non-conserving dynamics we show that the effect is present even beyond the validity of the adiabatic perturbation theory.We are grateful to M Kolodrubetz for the careful reading of the manuscript and helpful comments. This work was partially supported by BSF 2010318 (YK and AP), NSF DMR-1506340 (LD and AP), AFOSR FA9550-10-1-0110 (LD and AP), ARO W911NF1410540 (LD and AP) and ISF grant (YK). LD acknowledges the office of Naval Research. YK is grateful to the BU visitors program. (2010318 - BSF; DMR-1506340 - NSF; FA9550-10-1-0110 - AFOSR; W911NF1410540 - ARO; ISF grant)Accepted manuscrip
Accretion Disks Around Young Objects. III. Grain Growth
We present detailed models of irradiated T Tauri disks including dust grain
growth with power-law size distributions. The models assume complete mixing
between dust and gas and solve for the vertical disk structure
self-consistentlyincluding the heating effects of stellar irradiation as well
as local viscous heating. For a given total dust mass, grain growth is found to
decrease the vertical height of the surface where the optical depth to the
stellar radiation becomes unit and thus the local irradiation heating, while
increasing the disk emission at mm and sub-mm wavelengths. The resulting disk
models are less geometrically thick than our previous models assuming
interstellar medium dust, and agree better with observed spectral energy
distributions and images of edge-on disks, like HK Tau/c and HH 30. The
implications of models with grain growth for determining disk masses from
long-wavelength emission are considered.Comment: 29 pages, including 11 figures and 1 table, APJ accepte
Accretion Disks Around Young Objects. II. Tests of Well-Mixed Models with Ism Dust
We construct detailed vertical structure models of irradiated accretion disks
around T Tauri stars with interstellar medium dust uniformly mixed with gas.
The dependence of the structure and emission properties on mass accretion rate,
viscosity parameter, and disk radius is explored using these models. The
theoretical spectral energy distributions (SEDs) and images for all
inclinations are compared with observations of the entire population of
Classical T Tauri stars (CTTS) and Class I objects in Taurus. In particular, we
find that the median near-infrared fluxes can be explained within the errors
with the most recent values for the median accretion rates for CTTS. We further
show that the majority of the Class I sources in Taurus cannot be Class II
sources viewed edge-on because they are too luminous and their colors would be
consistent with disks seen only in a narrow range of inclinations. Our models
appear to be too geometrically thick at large radii, as suggested by: (a)
larger far-infrared disk emission than in the typical SEDs of T Tauri stars;
(b) wider dark dust lanes in the model images than in the images of HH30 and HK
Tau/c; and (c) larger predicted number of stars extincted by edge-on disks than
consistent with current surveys. The large thickness of the model is a
consequence of the assumption that dust and gas are well-mixed, suggesting that
some degree of dust settling may be required to explain the observations.Comment: 41 pages, 13 figures, accepted in Ap
Resolving Molecular Line Emission from Protoplanetary Disks: Observational Prospects for Disks Irradiated by Infalling Envelopes
Molecular line observations that could resolve protoplanetary disks of ~100
AU both spatially and kinematically would be a useful tool to unambiguously
identify these disks and to determine their kinematical and physical
characteristics. In this work we model the expected line emission from a
protoplanetary disk irradiated by an infalling envelope, addressing the
question of its detectability with subarcsecond resolution. We adopt a
previously determined disk model structure that gives a continuum spectral
energy distribution and a mm intensity spatial distribution that are consistent
with observational constraints of HL Tau. An analysis of the capability of
presently working and projected interferometers at mm and submm wavelengths
shows that molecular transitions of moderate opacity at these wavelengths
(e.g., C17O lines) are good candidates for detecting disk lines at subarcsecond
resolution in the near future. We suggest that, in general, disks of typical
Class I sources will be detectable.Comment: 41 pages, 16 figures. To be published in The Astrophysical Journa
Spitzer Space Telescope study of disks in the young Orionis cluster
We report new Spitzer Space Telescope observations from the IRAC and MIPS
instruments of the young (~ 3 Myr) sigma Orionis cluster. We identify 336 stars
as members of the cluster using optical and near-infrared color magnitude
diagrams. Using the spectral energy distribution (SED) slopes in the IRAC
spectral range, we place objects in several classes: non-excess stars, stars
with optically thick disks(like classical T Tauri stars), class I
(protostellar) candidates, and stars with ``evolved disks''; the last exhibit
smaller IRAC excesses than optically thick disk systems. In general, this
classification agrees with the location expected in IRAC-MIPS color-color
diagrams for these objects. We find that the evolved disk systems are mostly a
combination of objects with optically thick but non-flared disks, suggesting
grain growth and/or settling, and transition disks, systems in which the inner
disk is partially or fully cleared of small dust. In all, we identify 7
transition disk candidates and 3 possible debris disk systems. As in other
young stellar populations, the fraction of disks depends on the stellar mass,
ranging from ~10% for stars in the Herbig Ae/Be mass range (>2 msun) to ~35% in
the T Tauri mass range (1-0.1 msun). We find that the disk fraction does not
decrease significantly toward the brown dwarf candidates (<0.1 msun). The IRAC
infrared excesses found in stellar clusters and associations with and without
central high mass stars are similar, suggesting that external photoevaporation
is not very important in many clusters. Finally, we find no correlation between
the X-ray luminosity and the disk infrared excess, suggesting that the X-rays
are not strongly affected by disk accretion.Comment: 44pages, 17 figures. Sent to Ap
Accounting and the Banality of Evil: Expropriation of Jewish Property in Fascist Italy (1939-1945)
Purpose – This study identifies the significant role of accounting in the expropriation of Jewish real estate after the enforcement of race laws under Benito Mussolini’s Fascist regime in Italy.
Design/methodology/approach – Hannah Arendt’s understanding of government bureaucracy in 20th century totalitarian regimes informs the research which draws upon a wide range of primary sources.
Findings – Implementation of the program of expropriation was the responsibility of a government body, EGELI, which was created specifically for this purpose. The language of accounting provided the means to disguise the nature and brutality of the process and allow bureaucrats to be removed from the consequences of their actions. Accounting reports from EGELI to the Ministry of Finance confirmed each year that those who worked in EGELI were devoted to its mission as an agency of the Fascist State.
Research limitations/implications – The paper refers to the Jews expropriation from the Fascist institutions point of view while the Jews’ documents and their economic, social and human lives were not considered. It paved the way to further research about Jews discrimination and persecution in European countries other than Nazi Germany from an Accounting History viewpoint.
Originality/value – The paper is the first contribution coming from Italy about the role played by Accounting and Accountants in Jews persecution under the Fascism
PAH emission from Herbig AeBe stars
We present spectra of a sample of Herbig Ae and Be (HAeBe) stars obtained
with the Infrared Spectrograph on the Spitzer Space Telescope. All but one of
the Herbig stars show emission from polycyclic aromatic hydrocarbons (PAHs) and
seven of the spectra show PAH emission, but no silicate emission at 10 microns.
The central wavelengths of the 6.2, 7.7--8.2, and 11.3 micron emission features
decrease with stellar temperature, indicating that the PAHs are less
photo-processed in cooler radiation fields. The apparent low level of photo
processing in HAeBe stars, relative to other PAH emission sources, implies that
the PAHs are newly exposed to the UV-optical radiation fields from their host
stars. HAeBe stars show a variety of PAH emission intensities and ionization
fractions, but a narrow range of PAH spectral classifications based on
positions of major PAH feature centers. This may indicate that, regardless of
their locations relative to the stars, the PAH molecules are altered by the
same physical processes in the proto-planetary disks of intermediate-mass
stars. Analysis of the mid-IR spectral energy distributions indicates that our
sample likely includes both radially flared and more flattened/settled disk
systems, but we do not see the expected correlation of overall PAH emission
with disk geometry. We suggest that the strength of PAH emission from HAeBe
stars may depend not only on the degree of radial flaring, but also on the
abundance of PAHs in illuminated regions of the disks and possibly on the
vertical structure of the inner disk as well.Comment: 52 pages, 12 figure
A Search for Star-Disk Interaction Among the Strongest X-ray Flaring Stars in the Orion Nebula Cluster
The Chandra Orion Ultradeep Project observed hundreds of young, low-mass
stars undergoing highly energetic X-ray flare events. The 32 most powerful
cases have been modeled with the result that the magnetic structures
responsible for these flares can be many stellar radii in extent. In this
paper, we model the observed spectral energy distributions of these 32 stars in
order to determine, in detail for each star, whether there is circumstellar
disk material situated in sufficient proximity to the stellar surface for
interaction with the large magnetic loops inferred from the observed X-ray
flares. Our spectral energy distributions span the wavelength range 0.3-8 um
(plus 24 um for some stars), allowing us to constrain the presence of dusty
circumstellar material out to >10 AU from the stellar surface in most cases.
For 24 of the 32 stars in our sample the available data are sufficient to
constrain the location of the inner edge of the dusty disks. Six of these (25%)
have spectral energy distributions consistent with inner disks within reach of
the observed magnetic loops. Another four stars may have gas disks interior to
the dust disk and extending within reach of the magnetic loops, but we cannot
confirm this with the available data. The remaining 14 stars (58%) appear to
have no significant disk material within reach of the large flaring loops.
Thus, up to ~40% of the sample stars exhibit energetic X-ray flares that
possibly arise from a magnetic star-disk interaction, and the remainder are
evidently associated with extremely large, free-standing magnetic loops
anchored only to the stellar surface.Comment: Accepted to the ApJ; 26 pages, 6 tables, 6 figure
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