142 research outputs found
Anomalous Microwave Emission from Spinning Dust and its Polarization Spectrum
Nearly twenty years after the discovery of anomalous microwave emission (AME)
that contaminates to the cosmic microwave background (CMB) radiation, its
origin remains inconclusive. Observational results from numerous experiments
have revealed that AME is most consistent with spinning dust emission from
rapidly spinning ultrasmall interstellar grains. In this paper, I will first
review our improved model of spinning dust, which treats realistic dynamics of
wobbling non-spherical grains, impulsive interactions of grains with ions in
the ambient plasma, and some other important effects. I will then discuss
recent progress in quantifying the polarization of spinning dust emission from
polycyclic aromatic hydrocarbons. I will finish with a brief discussion on
remaining issues about the origins of AME.Comment: 8 pages, 5 figures, plenary talk at the conference "Cosmology: 50
Years After CMB Discovery," Quy Nhon, Vietnam, August 201
Effect of alignment on polarized infrared emission from polycyclic aromatic hydrocarbons
Polarized emission from polycyclic aromatic hydrocarbons (PAHs) potentially
provides a new way to test basic physics of the alignment of ultrasmall grains.
In this paper, we present a new model of polarized PAH emission that takes into
account the effect of PAH alignment with the magnetic field. We first generate
a large sample of the grain angular momentum by simulating the
alignment of PAHs due to resonance paramagnetic relaxation that accounts for
various interaction processes. We then calculate the polarization level of PAH
emission features, for the different phases of the ISM, including the cold
neutral medium (CNM), reflection nebulae (RN), and photodissociation regions
(PDRs). We find that a moderate degree of PAH alignment can significantly
enhance the polarization degree of PAH emission compared to the previous
results obtained with randomly oriented . In particular, we find that
smallest, negatively charged PAHs in RN can be excited to slightly suprathermal
rotation due to enhanced ion collisional excitation, resulting in an increase
of the polarization with the ionization fraction. Our results suggest that RN
is the most favorable environment to observe polarized PAH emission and to test
alignment physics of nanoparticles. Finally, we present an explicit
relationship between the polarization level of PAH emission and the degree of
external alignment for the CNM and RN. The obtained relationship will be
particularly useful for testing alignment physics of PAHs by future
observations.Comment: 15 figures, 20 page
A dynamical constraint on interstellar dust models from radiative torque disruption
Interstellar dust is an essential component of the interstellar medium (ISM)
and plays critical roles in astrophysics. Achieving an accurate model of
interstellar dust is therefore of great importance. Interstellar dust models
are usually built based on observational constraints such as starlight
extinction and polarization, but dynamical constraints such as grain rotation
are not considered. In this paper, we show that a newly discovered effect by
Hoang et al., so-called RAdiative Torque Disruption (RATD), can act as an
important dynamical constraint for dust models. Using this dynamical
constraint, we derive the maximum size of grains that survive in the ISM for
different dust models, including contact binary, composite, silicate-core, and
amorphous carbon mantle, and compact grain model for the different radiation
fields. We find that the different dust models have different maximum size due
to their different tensile strengths, and the largest maximum size corresponds
to compact grains with the highest tensile strength. We show that the composite
grain model cannot be ruled out if constituent particles are very small with
radius 25 nm, but large composite grains would be destroyed if the
particles are large with nm. We suggest that grain internal
structures can be constrained with observations using the dynamical RATD
constraint for strong radiation fields such as supernova, nova, or star-forming
regions. Finally, our obtained results suggest that micron-sized grains perhaps
have compact/core-mantle structures or have composite structures but located in
regions with slightly higher gas density and weaker radiation intensity than
the average ISM.Comment: 12 pages, 4 figures. Accepted to ApJ. Discussion on micron-sized
grains adde
Polarization of Infrared Emission from Polycyclic Aromatic Hydrocarbons
Polarized infrared emission from polycyclic aromatic hydrocarbons (PAHs) is
important for testing the basic physics of alignment of ultrasmall grains and
potentially offers a new way to trace magnetic fields. In this paper, a new
model of polarized PAH emission is presented, taking into account the effect of
PAH alignment with the magnetic field. The polarization level of PAH emission
features, for the different phases of the diffuse interstellar medium (ISM) is
discussed. We find that negatively charged smallest PAHs in the reflection
nebula can be excited to slightly suprathermal rotation due to enhanced ion
collisional excitation, which enhances the degree of PAH alignment and the
polarization level of PAH emission. The polarization level and polarization
angle predicted by our model including PAH alignment are supported by the first
detection of the polarization of at 11.3 m PAH feature from
MWC 1080 nebula by Zhang et al. The theoretical and observational progress
reveals that PAHs can be aligned with the magnetic field, resulting in a
moderate polarization level of spinning dust emission. Polarized infrared PAH
polarization would be useful for tracing the magnetic fields.Comment: To appears in the proceedings of the AKARI conference "The Cosmic
Wheel and the Legacy of the AKARI archive: from galaxies and stars to planets
and life", October 17-20, 2017, Tokyo, Japa
Relativistic Gas Drag on Dust Grains and Implications
We study the drag force on dust grains moving at relativistic velocities
through interstellar gas and explore its application. First, we derive a new
analytical formula of the drag force at high energies and find that it is
significantly reduced compared to the classical model. Second, we apply the
obtained drag force to calculate the terminal velocities of interstellar grains
by strong radiation sources such as supernovae and active galactic nuclei
(AGNs). We find that grains can be accelerated to relativistic velocities by
very luminous AGNs. We then quantify the deceleration of relativistic
spacecraft proposed by the Breakthrough Starshot initiative due to gas drag on
a relativistic lightsail. We find that the spacecraft's slowing down is
negligible because of the suppression of gas drag at relativistic velocities,
suggesting that the lightsail may be open for communication during its journey
to Centauri without causing a considerable delay. Finally, we show
that the damage to relativistic thin lightsails by interstellar dust is a minor
effect.Comment: ApJ, in press; 7 pages, 5 figure
Properties and alignment of interstellar dust grains toward Type Ia Supernovae with anomalous polarization curves
Recent photometric and polarimetric observations of type Ia supernovae (SNe
Ia) show unusually low total-to-selective extinction ratio () and
wavelength of maximum polarization () for several SNe
Ia, which indicates peculiar properties of interstellar (IS) dust in the SN
hosted galaxies and/or the presence of circumstellar (CS) dust. In this paper,
we use inversion technique to infer best-fit grain size distribution and
alignment function of interstellar grains along the lines of sight toward four
SNe Ia with anomalous extinction and polarization data (SNe 1986G, 2006X,
2008fp, and 2014J). We find that to reproduce low values of , a
significant enhancement in the mass of small grains of radius is
required. For SN 2014J, a simultaneous fit to observed extinction and
polarization data is unsuccessful if the entire data is attributed to IS dust
(model 1), but a good fit is obtained when accounting for the contribution of
CS dust (model 2). For SN 2008fp, our fitting results for model 1 show that, to
reproduce an extreme value of , very small
silicate grains must be aligned as efficiently as big grains. We suggest that
tiny grains in the intervening molecular cloud can be aligned efficiently by
radiative torques (RATs) from the SNe Ia. The resulting time dependence
polarization from this RAT alignment model can be tested by observing at
ultraviolet wavelengths. Our results are in favor of the existence of CS dust
in SN 2014J, but its presence in SN 2008fp remains uncertain.Comment: 11 pages, 6 figures; accepted to ApJ with minor revisio
Electromagnetic forces on a relativistic spacecraft in the interstellar medium
A relativistic spacecraft of the type envisioned by the Breakthrough Starshot
initiative will inevitably get charged through collisions with interstellar
particles and UV photons. Interstellar magnetic fields would, therefore,
deflect the trajectory of the spacecraft. We calculate the expected deflection
for typical interstellar conditions. We also find that the charge distribution
of the spacecraft is asymmetric, producing an electric dipole moment. The
interaction between the moving electric dipole and the interstellar magnetic
field is found to produce a large torque, which can result in fast oscillation
of the spacecraft around the axis perpendicular to the direction of motion,
with a period of 0.5 hr. We then study the spacecraft rotation arising
from impulsive torques by dust bombardment. Finally, we discuss the effect of
the spacecraft rotation and suggest several methods to mitigate it.Comment: ApJ, in press; 9 pages, 6 figure
Subsonic Mechanical Alignment of Irregular Grains
We show that grains can be efficiently aligned by interacting with a subsonic
gaseous flow. The alignment arises from grains having irregularities that
scatter atoms with different efficiency in the right and left directions. The
grains tend to align with long axes perpendicular to magnetic field, which
corresponds to Davis-Greenstein predictions. Choosing conservative estimates,
scattering efficiency of impinging atoms and conservative ``degree of
helicity'', the alignment of helical grains is much more efficient than the
Gold-type alignment processes.Comment: 12 pages, 4 figures, typos correcte
Magnetic Properties of Dust Grains, Effect of Precession and Radiative Torque Alignment
Alignment of dust grains in astrophysical environments results in the
polarization of starlight as well as the polarization of radiation emitted by
dust. We demonstrate the advances in grain alignment theory allow the use of
linear and circular polarization to probe not only the magnetic field, but also
dust composition, the dust environment, etc. We revisit the process of grain
alignment by Radiative Torques (RATs) and focus on constraining magnetic
susceptibility of grains via observations. We discuss the possibility of
observational testing of the magnetic properties of grains as the alignment
changes from being in respect to the magnetic field to being in respect to the
radiation direction. This opens both a possibility of constraining the
uncertain parameters of the RATs theory and provides a new way of measuring
magnetic fields in interstellar medium and circumstellar regions. We provide a
detailed discussion of the precession induced both by the magnetic field and
the anisotropic radiation and revisit a number of key processes related to
magnetic response of the grains. We consider various effects that increase the
rate of magnetic relaxation both in silicate and carbonaceous grains. In
particular, we find a new relaxation process related to the change of the
amplitude of internal magnetization within a wobbling triaxial grain and
identify a range of grain sizes in which this effect can dominate the internal
alignment of angular momentum within grain axes. We show that these relaxation
processes significantly change the dynamics of grains in the presence of RATs.
We apply our analysis for observed grain alignment in special environments to
put constraints on the enhanced magnetic properties of dust grains in the cloud
near supernovae, in cometary coma, and protoplanetary disks.Comment: 39 pages, 9 figures; discussion extended; accepted to Ap
A unified model of grain alignment: radiative alignment of interstellar grains with magnetic inclusions
The radiative torque (RAT) alignment of interstellar grains with ordinary
paramagnetic susceptibilities has been supported by earlier studies. The
alignment of such grains depends on the so-called RAT parameter that
is determined by the grain shape. In this paper, we elaborate our model of RAT
alignment for grains with enhanced magnetic susceptibility due to iron
inclusions, such that RAT alignment is magnetically enhanced for which we term
MRAT mechanism. Such grains can get aligned with high angular momentum at the
so-called high-J attractor points, achieving a high degree of alignment. Using
our analytical model of RATs we derive the critical value of the magnetic
relaxation parameter to produce high-J attractor points as
functions of and the anisotropic radiation angle relative to the
magnetic field . We find that if about of total iron abundance
present in silicate grains are forming iron clusters, it is sufficient to
produce high-J attractor points for all reasonable values of . To
calculate the degree of grain alignment, we carry out numerical simulations of
MRAT alignment by including stochastic excitations from gas collisions and
magnetic fluctuations. We show that large grains can achieve perfect alignment
when the high-J attractor point is present, regardless of the values of
. Our obtained results pave the way for physical modeling of
polarized thermal dust emission as well as magnetic dipole emission. We also
find that millimeter-sized grains in accretion disks may be aligned with the
magnetic field if they are incorporated with iron nanoparticles.Comment: 21 pages, 10 figures, accepted to ApJ, Sec 8.1 revise
- …