1,484 research outputs found
The effect of scattering on the structure and SED of protoplanetary disks
In this paper we investigate how the inclusion of scattering of the stellar
radiation into a passive flaring disk model affects its structure and spectral
energy distribution, and whether neglecting it could significantly decrease the
model reliability. In order to address these questions we construct a detailed
1+1D vertical structure model in which the scattering properties of the dust
can be varied. Models are presented with and without dust scattering, and for
different albedos and phase functions. It is found that scattering has the
effect of reducing the disk temperature at all heights, so that the disk
"shrinks", i.e., the the density at all intermediate heights decreases.
However, this effect in most cases is more than compensated by the increase of
the total extinction (absorption + scattering) cross section, so that the
surface scale height increases, and images in scattered light will see a
slightly thicker disk. The integrated infrared emission decreases as the albedo
increases, because an increasing part of the flux captured by the disk is
reflected away instead of absorbed and reprocessed. The reduction of the
infrared thermal emission of the disk is stronger at short wavelengths (near
infrared) and practically negligible at millimeter wavelengths. For relatively
low albedo (alb <~ 0.5), or for strongly forward-peaked scattering (g roughly
>0.8), the infrared flux reduction is relatively small.Comment: Accepted for publication in Astronomy & Astrophysic
A Spatially Resolved Inner Hole in the Disk around GM Aurigae
We present 0.3 arcsec resolution observations of the disk around GM Aurigae
with the Submillimeter Array (SMA) at a wavelength of 860 um and with the
Plateau de Bure Interferometer at a wavelength of 1.3 mm. These observations
probe the distribution of disk material on spatial scales commensurate with the
size of the inner hole predicted by models of the spectral energy distribution.
The data clearly indicate a sharp decrease in millimeter optical depth at the
disk center, consistent with a deficit of material at distances less than ~20
AU from the star. We refine the accretion disk model of Calvet et al. (2005)
based on the unresolved spectral energy distribution (SED) and demonstrate that
it reproduces well the spatially resolved millimeter continuum data at both
available wavelengths. We also present complementary SMA observations of CO
J=3-2 and J=2-1 emission from the disk at 2" resolution. The observed CO
morphology is consistent with the continuum model prediction, with two
significant deviations: (1) the emission displays a larger CO J=3-2/J=2-1 line
ratio than predicted, which may indicate additional heating of gas in the upper
disk layers; and (2) the position angle of the kinematic rotation pattern
differs by 11 +/- 2 degrees from that measured at smaller scales from the dust
continuum, which may indicate the presence of a warp. We note that
photoevaporation, grain growth, and binarity are unlikely mechanisms for
inducing the observed sharp decrease in opacity or surface density at the disk
center. The inner hole plausibly results from the dynamical influence of a
planet on the disk material. Warping induced by a planet could also potentially
explain the difference in position angle between the continuum and CO data
sets.Comment: 12 pages, 6 figures, accepted for publication in Ap
Empirical Constraints on Turbulence in Protoplanetary Accretion Disks
We present arcsecond-scale Submillimeter Array observations of the CO(3-2)
line emission from the disks around the young stars HD 163296 and TW Hya at a
spectral resolution of 44 m/s. These observations probe below the ~100 m/s
turbulent linewidth inferred from lower-resolution observations, and allow us
to place constraints on the turbulent linewidth in the disk atmospheres. We
reproduce the observed CO(3-2) emission using two physical models of disk
structure: (1) a power-law temperature distribution with a tapered density
distribution following a simple functional form for an evolving accretion disk,
and (2) the radiative transfer models developed by D'Alessio et al. that can
reproduce the dust emission probed by the spectral energy distribution. Both
types of models yield a low upper limit on the turbulent linewidth (Doppler
b-parameter) in the TW Hya system (<40 m/s), and a tentative (3-sigma)
detection of a ~300 m/s turbulent linewidth in the upper layers of the HD
163296 disk. These correspond to roughly <10% and 40% of the sound speed at
size scales commensurate with the resolution of the data. The derived
linewidths imply a turbulent viscosity coefficient, alpha, of order 0.01 and
provide observational support for theoretical predictions of subsonic
turbulence in protoplanetary accretion disks.Comment: 18 pages, 9 figures, accepted for publication in Ap
Vertical structure models of T Tauri and Herbig Ae/Be disks
In this paper we present detailed models of the vertical structure
(temperature and density) of passive irradiated circumstellar disks around T
Tauri and Herbig Ae/Be stars. In contrast to earlier work, we use full
frequency- and angle-dependent radiative transfer instead of the usual moment
equations. We find that this improvement of the radiative transfer has strong
influence on the resulting vertical structure of the disk, with differences in
temperature as large as 70 %. However, the spectral energy distribution (SED)
is only mildly affected by this change. In fact, the SED compares reasonably
well with that of improved versions of the Chiang & Goldreich (CG) model. This
shows that the latter is a reasonable model for the SED, in spite of its
simplicity. It also shows that from the SED alone, little can be learned about
the vertical structure of a passive circumstellar disk. The molecular line
emission from these disks is more sensitive to the vertical temperature and
density structure, and we show as an example how the intensity and profiles of
various CO lines depend on the adopted disk model. The models presented in this
paper can also serve as the basis of theoretical studies of e.g. dust
coagulation and settling in disks.Comment: 12 pages, 15 figures, accepted for publication in A&
Polysaccharide Layer-by-Layer Coating for Polyimide-Based Neural Interfaces
Implantable flexible neural interfaces (IfNIs) are capable of directly modulating signals of the central and peripheral nervous system by stimulating or recording the action potential. Despite outstanding results in acute experiments on animals and humans, their long-term biocompatibility is hampered by the effects of foreign body reactions that worsen electrical performance and cause tissue damage. We report on the fabrication of a polysaccharide nanostructured thin film as a coating of polyimide (PI)-based IfNIs. The layer-by-layer technique was used to coat the PI surface due to its versatility and ease of manufacturing. Two different LbL deposition techniques were tested and compared: dip coating and spin coating. Morphological and physiochemical characterization showed the presence of a very smooth and nanostructured thin film coating on the PI surface that remarkably enhanced surface hydrophilicity with respect to the bare PI surface for both the deposition techniques. However, spin coating offered more control over the fabrication properties, with the possibility to tune the coating’s physiochemical and morphological properties. Overall, the proposed coating strategies allowed the deposition of a biocompatible nanostructured film onto the PI surface and could represent a valid tool to enhance long-term IfNI biocompatibility by improving tissue/electrode integration
On the Evolution and Survival of Protoplanets Embedded in a Protoplanetary Disk
We model the evolution of a Jupiter-mass protoplanet formed by the disk
instability mechanism at various radial distances accounting for the presence
of the disk. Using three different disk models, it is found that a newly-formed
Jupiter-mass protoplanet at radial distance of 5-10 AU cannot
undergo a dynamical collapse and evolve further to become a gravitational bound
planet. We therefore conclude that {\it giant planets, if formed by the
gravitational instability mechanism, must form and remain at large radial
distances during the first 10 years of their evolution}. The
minimum radial distances in which protoplanets of 1 Saturn-mass, 3 and 5
Jupiter-mass protoplanets can evolve using a disk model with and are found to be 12, 9, and 7 AU, respectively.
The effect of gas accretion on the planetary evolution of a Jupiter-mass
protoplanet is also investigated. It is shown that gas accretion can shorten
the pre-collapse timescale substantially. Our study suggests that the timescale
of the pre-collapse stage does not only depend on the planetary mass, but is
greatly affected by the presence of the disk and efficient gas accretion.Comment: 26 pages, 2 tables, 10 figures. Accepted for publication in Ap
Use of Preputial Skin as Cutaneous Graft after Nevus Excision
We report a four-year-old boy with a nevus covering all the plantar side of his second finger on the left foot. He was also affected by congenital phimosis. Surgical excision of the nevus was indicated, but the skin defect would have been too large to be directly closed. The foreskin was taken as a full-thickness skin graft to cover the cutaneous defect of the finger. The graft intake was favourable and provided a functional repair with good aesthetic characteristic
The Effects of UV Continuum and Lyman alpha Radiation on the Chemical Equilibrium of T Tauri Disks
We show in this Letter that the spectral details of the FUV radiation fields
have a large impact on the chemistry of protoplanetary disks surrounding T
Tauri stars. We show that the strength of a realistic stellar FUV field is
significantly lower than typically assumed in chemical calculations and that
the radiation field is dominated by strong line emission, most notably Lyman
alpha radiation. The effects of the strong Lyman alpha emission on the chemical
equilibrium in protoplanetary disks has previously been unrecognized. We
discuss the impact of this radiation on molecular observations in the context
of a radiative transfer model that includes both direct attenuation and
scattering. In particular, Lyman alpha radiation will directly dissociate water
vapor and may contribute to the observed enhancements of CN/HCN in disks.Comment: 14 pages, 4 figures, accepted by ApJ Letter
- …