1,740 research outputs found
ISO LWS Spectra of T Tauri and Herbig AeBe stars
We present an analysis of ISO-LWS spectra of eight T Tauri and Herbig AeBe young stellar objects.
Some of the objects are in the embedded phase of star-formation, whereas others have cleared their environs
but are still surrounded by a circumstellar disk. Fine-structure lines of [OI] and [CII] are most likely excited by
far-ultraviolet photons in the circumstellar environment rather than high-velocity outflows, based on comparisons
of observed line strengths with predictions of photon-dominated and shock chemistry models. A subset of our
stars and their ISO spectra are adequately explained by models constructed by Chiang & Goldreich (1997) and
Chiang et al. (2001) of isolated, passively heated, flared circumstellar disks. For these sources, the bulk of the
LWS flux at wavelengths longward of 55 µm arises from the disk interior which is heated diffusively by reprocessed
radiation from the disk surface. At 45 µm, water ice emission bands appear in spectra of two of the coolest stars,
and are thought to arise from icy grains irradiated by central starlight in optically thin disk surface layers
Gas Accretion is Dominated by Warm Ionized Gas in Milky Way-Mass Galaxies at z ~ 0
We perform high-resolution hydrodynamic simulations of a Milky Way-mass
galaxy in a fully cosmological setting using the adaptive mesh refinement code,
Enzo, and study the kinematics of gas in the simulated galactic halo. We find
that the gas inflow occurs mostly along filamentary structures in the halo. The
warm-hot (10^5 K 10^6 K) ionized gases are found to
dominate the overall mass accretion in the system (with dM/dt = 3-5 M_solar/yr)
over a large range of distances, extending from the virial radius to the
vicinity of the disk. Most of the inflowing gas (by mass) does not cool, and
the small fraction that manages to cool does so primarily close to the galaxy
(R <~ 20 kpc), perhaps comprising the neutral gas that may be detectable as,
e.g., high-velocity clouds. The neutral clouds are embedded within larger,
accreting filamentary flows, and represent only a small fraction of the total
mass inflow rate. The inflowing gas has relatively low metallicity (Z/Z_solar <
0.2). The outer layers of the filamentary inflows are heated due to compression
as they approach the disk. In addition to the inflow, we find high-velocity,
metal-enriched outflows of hot gas driven by supernova feedback. Our results
are consistent with observations of halo gas at low z.Comment: 10 pages including 5 figures, submitted to Ap
The Origin and Distribution of Cold Gas in the Halo of a Milky Way-Mass Galaxy
We analyze an adaptive mesh refinement hydrodynamic cosmological simulation
of a Milky Way-sized galaxy to study the cold gas in the halo. HI observations
of the Milky Way and other nearby spirals have revealed the presence of such
gas in the form of clouds and other extended structures, which indicates
on-going accretion. We use a high-resolution simulation (136-272 pc throughout)
to study the distribution of cold gas in the halo, compare it with
observations, and examine its origin. The amount (10^8 Msun in HI), covering
fraction, and spatial distribution of the cold halo gas around the simulated
galaxy at z=0 are consistent with existing observations. At z=0 the HI mass
accretion rate onto the disk is 0.2 Msun/yr. We track the histories of the 20
satellites that are detected in HI in the redshift interval 0.5>z>0 and find
that most of them are losing gas, with a median mass loss rate per satellite of
3.1 x 10^{-3} Msun/yr. This stripped gas is a significant component of the HI
gas seen in the simulation. In addition, we see filamentary material coming
into the halo from the IGM at all redshifts. Most of this gas does not make it
directly to the disk, but part of the gas in these structures is able to cool
and form clouds. The metallicity of the gas allows us to distinguish between
filamentary flows and satellite gas. We find that the former accounts for at
least 25-75% of the cold gas in the halo seen at any redshift analyzed here.
Placing constraints on cloud formation mechanisms allows us to better
understand how galaxies accrete gas and fuel star formation at z=0.Comment: 13 pages, 8 figures. Accepted for publication in Ap
Haptic Cushion: Automatic Generation of Vibro- tactile Feedback Based on Audio Signal for Immersive Interaction with Multimedia
This paper presents a haptic display providing audio-based vibrotactile feedback to enhance the immersive feeling of the user who interacts with multimedia content. The newly developed display has two main features, i) an automatic transformation algorithm and ii) a vibrotactile actuator. The proposed algorithm automatically transforms auditory signals into vibrotactile patterns in real-time by extracting principal frequencies from acoustic unit sequences and superposing vibration waves. The actuator was designed based on the structure of the voice coil linear motor to operate effectively over a wide range of vibration frequencies. Experiments were carried out to evaluate characteristics of the implemented system and demonstrate the effectiveness of the proposed approach
Dual Pair Correspondence in Physics: Oscillator Realizations and Representations
We study general aspects of the reductive dual pair correspondence, also known as Howe duality. We make an explicit and systematic treatment, where we first derive the oscillator realizations of all irreducible dual pairs: , , , , , and . Then, we decompose the Fock space into irreducible representations of each group in the dual pairs for the cases where one member of the pair is compact as well as the first non-trivial cases of where it is non-compact. We discuss the relevance of these representations in several physical applications throughout this analysis. In particular, we discuss peculiarities of their branching properties. Finally, closed-form expressions relating all Casimir operators of two groups in a pair are established
Photoionization of High Altitude Gas in a Supernova-Driven Turbulent Interstellar Medium
We investigate models for the photoionization of the widespread diffuse
ionized gas in galaxies. In particular we address the long standing question of
the penetration of Lyman continuum photons from sources close to the galactic
midplane to large heights in the galactic halo. We find that recent
hydrodynamical simulations of a supernova-driven interstellar medium have low
density paths and voids that allow for ionizing photons from midplane OB stars
to reach and ionize gas many kiloparsecs above the midplane. We find ionizing
fluxes throughout our simulation grids are larger than predicted by one
dimensional slab models, thus allowing for photoionization by O stars of low
altitude neutral clouds in the Galaxy that are also detected in Halpha. In
previous studies of such clouds the photoionization scenario had been rejected
and the Halpha had been attributed to enhanced cosmic ray ionization or
scattered light from midplane H II regions. We do find that the emission
measure distributions in our simulations are wider than those derived from
Halpha observations in the Milky Way. In addition, the horizontally averaged
height dependence of the gas density in the hydrodynamical models is lower than
inferred in the Galaxy. These discrepancies are likely due to the absence of
magnetic fields in the hydrodynamic simulations and we discuss how
magnetohydrodynamic effects may reconcile models and observations.
Nevertheless, we anticipate that the inclusion of magnetic fields in the
dynamical simulations will not alter our primary finding that midplane OB stars
are capable of producing high altitude diffuse ionized gas in a realistic
three-dimensional interstellar medium.Comment: ApJ accepted. 17 pages, 7 figure
Dependence of Interstellar Turbulent Pressure on Supernova Rate
Feedback from massive stars is one of the least understood aspects of galaxy
formation. We perform a suite of vertically stratified local interstellar
medium (ISM) simulations in which supernova rates and vertical gas column
densities are systematically varied based on the Schmidt-Kennicutt law. Our
simulations have a sufficiently high spatial resolution (1.95 pc) to follow the
hydrodynamic interactions among multiple supernovae that structure the ISM. At
a given supernova rate, we find that the mean mass-weighted sound speed and
velocity dispersion decrease as the inverse square root of gas density,
indicating that both thermal and turbulent pressures are nearly constant in the
midplane, so the effective equation of state is isobaric. In contrast, across
our four models having supernova rates that range from one to 512 times the
Galactic supernova rate, the mass-weighted velocity dispersion remains in the
range 4-6 km/s. Hence, gas averaged over ~100 pc regions follows an effective
equation of state that is close to isothermal. Simulated H I emission lines
have widths of 10-18 km/s, comparable to observed values. In our highest
supernova rate model, superbubble blow-outs occur, and the turbulent pressure
on large scales is >~4 times higher than the thermal pressure. We find a tight
correlation between the thermal and turbulent pressures averaged over ~100 pc
regions in the midplane of each model, as well as across the four ISM models.
We construct a subgrid model for turbulent pressure based on analytic arguments
and explicitly calibrate it against our stratified ISM simulations. The subgrid
model provides a simple yet physically motivated way to include supernova
feedback in cosmological simulations.Comment: 13 pages incl. 8 figures; accepted for publication in ApJ; contains a
new model of starburst galaxy showing superbubble blow-ou
Type-Ia Supernova-driven Galactic Bulge Wind
Stellar feedback in galactic bulges plays an essential role in shaping the
evolution of galaxies. To quantify this role and facilitate comparisons with
X-ray observations, we conduct 3D hydrodynamical simulations with the adaptive
mesh refinement code, FLASH, to investigate the physical properties of hot gas
inside a galactic bulge, similar to that of our Galaxy or M31. We assume that
the dynamical and thermal properties of the hot gas are dominated by mechanical
energy input from SNe, primarily Type Ia, and mass injection from evolved stars
as well as iron enrichment from SNe. We study the bulge-wide outflow as well as
the SN heating on scales down to ~4 pc. An embedding scheme that is devised to
plant individual SNR seeds, allows to examine, for the first time, the effect
of sporadic SNe on the density, temperature, and iron ejecta distribution of
the hot gas as well as the resultant X-ray morphology and spectrum. We find
that the SNe produce a bulge wind with highly filamentary density structures
and patchy ejecta. Compared with a 1D spherical wind model, the non-uniformity
of simulated gas density, temperature, and metallicity substantially alters the
spectral shape and increases the diffuse X-ray luminosity. The differential
emission measure as a function of temperature of the simulated gas exhibits a
log-normal distribution, with a peak value much lower than that of the
corresponding 1D model. The bulk of the X-ray emission comes from the
relatively low temperature and low abundance gas shells associated with SN
blastwaves. SN ejecta are not well mixed with the ambient medium, at least in
the bulge region. These results, at least partly, account for the apparent lack
of evidence for iron enrichment in the soft X-ray-emitting gas in galactic
bulges and intermediate-mass elliptical galaxies.[...]Comment: 37 pages, 19 figures, submitted to MNRAS; comments are welcom
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