28 research outputs found
On the Submillimeter Opacity of Protoplanetary Disks
Solid particles with the composition of interstellar dust and power-law size
distribution dn/da propto a^{-p} for a 3 lambda and 3 <
p < 4 will have submm opacity spectral index beta(lambda) = dln(kappa)/dln(nu)
approx (p-3) beta_{ism}, where beta_{ism} approx 1.7 is the opacity spectral
index of interstellar dust material in the Rayleigh limit. For the power-law
index p approx 3.5 that characterizes interstellar dust, and that appears
likely for particles growing by agglomeration in protoplanetary disks, grain
growth to sizes a > 3 mm will result in beta(1 mm) < ~1. Grain growth can
naturally account for beta approx 1 observed for protoplanetary disks, provided
that a_{max} > ~ 3 lambda.Comment: Submitted to ApJ. 17 pages, 6 figure
Diffuse interstellar bands in reflection nebulae
A Monte Carlo code for radiation transport calculations is used to compare the profiles of the lambda lambda 5780 and 6613 Angstrom diffuse interstellar bands in the transmitted and the reflected light of a star embedded within an optically thin dust cloud. In addition, the behavior of polarization across the bands were calculated. The wavelength dependent complex indices of refraction across the bands were derived from the embedded cavity model. In view of the existence of different families of diffuse interstellar bands the question of other parameters of influence is addressed in short
Silicon nanoparticles and interstellar extinction
To examine a recently proposed hypothesis that silicon nanoparticles are the
source of extended red emission (ERE) in the interstellar medium, we performed
a detailed modeling of the mean Galactic extinction in the presence of silicon
nanoparticles. For this goal we used the appropriate optical constants of
nanosized Si, essentially different from those of bulk Si due to quantum
confinement. It was found that a dust mixture of silicon nanoparticles, bare
graphite grains, silicate core-organic refractory mantle grains and three-layer
silicate-water ice-organic refractory grains works well in explaining the
extinction and, in addition, results in the acceptable fractions of UV/visible
photons absorbed by silicon nanoparticles: 0.071-0.081. Since these fractions
barely agree with the fraction of UV/visible photons needed to excite the
observed ERE, we conclude that the intrinsic photon conversion efficiency of
the photoluminescence by silicon nanoparticles must be near 100%, if they are
the source of the ERE.Comment: Latex2e, uses emulateapj.sty (included), multicol.sty, epsf.sty, 6
pages, 3 figures (8 Postscript files), accepted for publication in ApJ
Letters, complete Postscript file is also available at
http://physics.technion.ac.il/~zubko/eb.html#SNP
Calculating Cross Sections of Composite Interstellar Grains
Interstellar grains may be composite collections of particles of distinct
materials, including voids, agglomerated together. We determine the various
optical cross sections of such composite grains, given the optical properties
of each constituent, using an approximate model of the composite grain. We
assume it consists of many concentric spherical layers of the various
materials, each with a specified volume fraction. In such a case the usual Mie
theory can be generalized and the extinction, scattering, and other cross
sections determined exactly.
We find that the ordering of the materials in the layering makes some
difference to the derived cross sections, but averaging over the various
permutations of the order of the materials provides rapid convergence as the
number of shells (each of which is filled by all of the materials
proportionately to their volume fractions) is increased. Three shells, each
with one layer of a particular constituent material, give a very satisfactory
estimate of the average cross section produced by larger numbers of shells.
We give the formulae for the Rayleigh limit (small size parameter) for
multi-layered spheres and use it to propose an ``Effective Medium Theory''
(EMT), in which an average optical constant is taken to represent the ensemble
of materials.
Multi-layered models are used to compare the accuracies of several EMTs
already in the literature.Comment: 29 pages, 6 figures, accepted for publication in the Astrophysical
Journal (part 1, scheduled in Vol. 526, #1, Nov. 20
New Interstellar Dust Models Consistent with Extinction, Emission, and Abundance Constraints
We present new interstellar dust models which have been derived by
simultaneously fitting the far-ultraviolet to near-infrared extinction, the
diffuse infrared (IR) emission and, unlike previous models, the elemental
abundance constraints on the dust for different interstellar medium abundances,
including solar, F and G star, and B star abundances. The fitting problem is a
typical ill-posed inversion problem, in which the grain size distribution is
the unknown, which we solve by using the method of regularization. The dust
model contains various components: PAHs, bare silicate, graphite, and amorphous
carbon particles, as well as composite particles containing silicate, organic
refractory material, water ice, and voids. The optical properties of these
components were calculated using physical optical constants. As a special case,
we reproduce the Li & Draine (2001) results, however their model requires an
excessive amount of silicon, magnesium, and iron to be locked up in dust: about
50 ppm (atoms per million of H atoms), significantly more than the upper limit
imposed by solar abundances of these elements, about 34, 35, and 28 ppm,
respectively. A major conclusion of this paper is that there is no unique
interstellar dust model that simultaneously fits the observed extinction,
diffuse IR emission, and abundances constraints.Comment: 70 pages, 23 figures, accepted for publication in the Astrophysical
Journal Supplemen
Unveiling the Circumstellar Envelope and Disk: A Sub-Arcsecond Survey of Circumstellar Structures
We present the results of a 2.7 mm continuum interferometric survey of 24
young stellar objects in 11 fields. The target objects range from deeply
embedded Class 0 sources to optical T Tauri sources. This is the first
sub-arcsecond survey of the 2.7 mm dust continuum emission from young, embedded
stellar systems. The images show a diversity of structure and complexity. The
optically visible T Tauri stars (DG Tauri, HL Tauri, GG Tauri,and GM Aurigae)
have continuum emission dominated by compact, less than 1", circumstellar
disks. The more embedded near-infrared sources (SVS13 and L1551 IRS5) have
continuum emission that is extended and compact. The embedded sources (L1448
IRS3, NGC1333 IRAS2, NGC1333 IRAS4, VLA1623, and IRAS 16293-2422) have
continuum emission dominated by the extended envelope, typically more than 85%.
In fact, in many of the deeply embedded systems it is difficult to uniquely
isolate the disk emission component from the envelope extending inward to AU
size scales. All of the target embedded objects are in multiple systems with
separations on scales of 30" or less. Based on the system separation, we place
the objects into three categories: separate envelope (separation > 6500 AU),
common envelope (separation 150-3000 AU), and common disk (separation < 100
AU). These three groups can be linked with fragmentation events during the star
formation process: separate envelopes from prompt initial fragmentation and the
separate collapse of a loosely condensed cloud, common envelopes from
fragmentation of a moderately centrally condensed spherical system, and common
disk from fragmentation of a high angular momentum circumstellar disk.Comment: 47 Pages, 18 Figures, ApJ accepte
Envelope Emission in Young Stellar Systems: A Sub-Arcsecond Survey of Circumstellar Structure
We present modeling results for six of the eleven deeply embedded systems
from our sub-arcsecond 2.7 mm wavelength continuum interferometric survey. The
modeling, performed in the uv plane, assumes dust properties, allows for a
power-law density profile, uses a self-consistent, luminosity conserving
temperature profile, and has an embedded point source to represent a
circumstellar disk. Even though we have the highest spatial resolution to date
at these wavelengths, only the highest signal-to-noise systems can adequately
constrain the simple self-similar collapse models. Of the six sources modeled,
all six were fit with a density power-law index of 2.0; however, in half of the
systems, those with the highest signal-to-noise, a density power-law index of
1.5 can be rejected at the 95% confidence level. Further, we modeled the
systems using the pure Larson-Penston (LP) and Shu solutions with only age and
sound speed as parameters. Overall, the LP solution provides a better fit to
the data, both in likelihood and providing the observed luminosity, but the age
of the systems required by the fits are surprising low (1000-2000 yrs). We
suggest that either there is some overall time scaling of the self-similar
solutions that invalidate the age estimates, or more likely we are at the limit
of the usefulness of these models. With our observations we have begun to reach
the stage where models need to incorporate more of the fundamental physics of
the collapse process, probably including magnetic fields and/or turbulence. In
addition to constraining collapse solutions, our modeling allows the separation
of large-scale emission from compact emission, enabling the probing of the
circumstellar disk component embedded within the protostellar envelope.Comment: 28 pages, 8 figures. Accepted for publication in Ap
Evolution of Cold Circumstellar Dust Around Solar-Type Stars
We present submillimeter (CSO 350um) and millimeter (SEST 1.2 mm, OVRO 3 mm)
photometry for 125 solar-type stars from the FEPS Spitzer Legacy program that
have masses between ~0.5 and 2.0 Msun and ages from 3 Myr to 3 Gyr. Continuum
emission was detected toward four stars with a signal to noise ratio >= 3$: the
classical T Tauri stars RX J1842.9-3532, RX J1852.3-3700, and PDS 66 with SEST,
and the debris disk system HD 107146 with OVRO. RXJ1842.9-3532 and
RXJ1852.3-3700 are located in projection nearby the R CrA molecular cloud with
estimated ages of ~10 Myr, while PDS66 is a probable member of the 20 Myr old
Lower Centaurus-Crux subgroup of the Sco-Cen OB association. The continuum
emission toward these three sources is unresolved at the 24'' SEST resolution
and likely originates from circumstellar accretion disks, each with estimated
dust masses of ~5x10**-5 Msun. Analysis of the visibility data toward HD107146
(age 80-200 Myr) indicates that the 3 mm continuum emission is centered on the
star within the astrometric uncertainties and resolved with a gaussian-fit FWHM
size of (6.5'' +/- 1.4'') x (4.2''+/-1.3''), or 185 AUx120 AU. The results from
our continuum survey are combined with published observations to quantify the
evolution of dust mass with time by comparing the mass distributions for
samples with different stellar ages. The frequency distribution of
circumstellar dust masses around solar-type stars in the Taurus molecular cloud
(age ~2 Myr) is distinguished from that around 3-10 Myr and 10-30 Myr old stars
at a significance level of ~1,5sigma and 3sigma respectively. These results
suggest a decrease in the mass of dust contained in small dust grains and/or
changes in the grain properties by stellar ages of 10-30 Myr, consistent with
previous conclusions. (abridged)Comment: 37 pages, 8 figures, accepted for publication in the Astronomical
Journa
Modeling Dust and Starlight in Galaxies Observed by Spitzer and Herschel: NGC 628 and NGC 6946
We characterize the dust in NGC628 and NGC6946, two nearby spiral galaxies in
the KINGFISH sample. With data from 3.6um to 500um, dust models are strongly
constrained. Using the Draine & Li (2007) dust model, (amorphous silicate and
carbonaceous grains), for each pixel in each galaxy we estimate (1) dust mass
surface density, (2) dust mass fraction contributed by polycyclic aromatic
hydrocarbons (PAH)s, (3) distribution of starlight intensities heating the
dust, (4) total infrared (IR) luminosity emitted by the dust, and (5) IR
luminosity originating in regions with high starlight intensity. We obtain maps
for the dust properties, which trace the spiral structure of the galaxies. The
dust models successfully reproduce the observed global and resolved spectral
energy distributions (SEDs). The overall dust/H mass ratio is estimated to be
0.0082+/-0.0017 for NGC628, and 0.0063+/-0.0009 for NGC6946, consistent with
what is expected for galaxies of near-solar metallicity. Our derived dust
masses are larger (by up to a factor 3) than estimates based on
single-temperature modified blackbody fits. We show that the SED fits are
significantly improved if the starlight intensity distribution includes a
(single intensity) "delta function" component. We find no evidence for
significant masses of cold dust T<12K. Discrepancies between PACS and MIPS
photometry in both low and high surface brightness areas result in large
uncertainties when the modeling is done at PACS resolutions, in which case
SPIRE, MIPS70 and MIPS160 data cannot be used. We recommend against attempting
to model dust at the angular resolution of PACS.Comment: To be published in Apj, September 2012. See the full version at
http://www.astro.princeton.edu/~ganiano/Papers