Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userUsing the Starlight Polarization Efficiency Integral to Constrain Shapes and Porosities of Interstellar Grains
95
0
0.0
(
0
)
Ask ChatGPT about the research
No Arabic abstract
We present a new method for using the observed starlight polarization and polarized submm emission to constrain the shapes and porosities of interstellar grains. We present the modified picket fence approximation (MPFA), and verify that it is sufficiently accurate for modeling starlight polarization. We introduce the starlight polarization integral $Pi_{rm obs}$ as a measure of overall strength of the observed polarization of starlight, and the starlight polarization efficiency integral $Phi$ to characterize the effectiveness of different grain types for producing polarization of starlight. The starlight polarization integral $Pi_{rm obs}$ determines the mass-weighted alignment $langle f_{rm align}rangle$ of the grains. Approximating the aligned grains in the interstellar medium as spheroids, we use $Pi_{rm obs}/Phi$ to show that the observed starlight polarization constrains the grains to have a minimum degree of asphericity. For porosity ${cal P}=0$, the minimum axial ratio is $sim$1.4 for oblate spheroids, or $sim$1.8 for prolate spheroids. If the grains are porous, more extreme axial ratios are required. The same grains that produce the starlight polarization are able to provide the observed polarized emission at submm wavelengths, but with further limits on shape and porosity. Porosities ${cal P}>0.75$ are ruled out. If interstellar grains can be approximated by astrodust spheroids, we predict the ratio of 10$mu{rm m}$ polarization to starlight polarization $p_V$: $p(10mu{rm m})/p_V=0.222pm0.026$. For Cyg OB2-12 we predict $p(10mu{rm m})=(2.1pm0.3)%$, which should be observable.
rate research
Read More
Although interstellar grains are known to be aspherical, their actual shapes remain poorly constrained. We assess whether three continuous distributions of ellipsoidal shapes from the literature are suitable for describing the shapes of interstellar grains. Randomly-selected shapes from each distribution are shown as illustrations. The often-used Bohren-Huffman CDE includes a very large fraction of extreme shapes: fully 10% of random draws have axial ratio $a_3/a_1 > 19.7$, and 5% have $a_3/a_1 > 33$. The CDE2 distribution includes a much smaller fraction of extreme shapes, and appears to be more realistic. For each of the three CDEs considered, we derive shape-averaged cross sections for extinction and polarization in the Rayleigh limit. Finally, we describe a method for synthesizing a dielectric function for an assumed shape or shape distribution if the actual absorption cross sections per grain volume in the Rayleigh limit are known from observations. This synthetic dielectric function predicts the wavelength dependence of polarization, which can then be compared to observations to constrain the grain shape.
Interstellar grains are known to be important actors in the formation of interstellar molecules such as H$_2$, water, ammonia, and methanol. It has been suggested that the so-called interstellar complex organic molecules (iCOMs) are also formed on the interstellar grain icy surfaces by the combination of radicals via reactions assumed to have an efficiency equal to unity. In this work, we aim to investigate the robustness or weakness of this assumption by considering the case of acetaldehyde (CH$_3$CHO) as a starting study case. In the literature, it has been postulated that acetaldehyde is formed on the icy surfaces via the combination of HCO and CH$_3$. Here we report new theoretical computations on the efficiency of its formation. To this end, we coupled quantum chemical calculations of the energetics and kinetics of the reaction CH$_3$ + HCO, which can lead to the formation of CH$_3$CHO or CO + CH$_4$. Specifically, we combined reaction kinetics computed with the Rice-Ramsperger-Kassel-Marcus (RRKM) theory (tunneling included) method with diffusion and desorption competitive channels. We provide the results of our computations in the format used by astrochemical models to facilitate their exploitation. Our new computations indicate that the efficiency of acetaldehyde formation on the icy surfaces is a complex function of the temperature and, more importantly, of the assumed diffusion over binding energy ratio $f$ of the CH$_3$ radical. If the ratio $f$ is $geq$0.4, the efficiency is equal to unity in the range where the reaction can occur, namely between 12 and 30 K. However, if $f$ is smaller, the efficiency dramatically crashes: with $f$=0.3, it is at most 0.01. In addition, the formation of acetaldehyde is always in competition with that of CO + CH$_4$.
In this analysis we illustrate how the relatively new emission mechanism known as spinning dust can be used to characterize dust grains in the interstellar medium. We demonstrate this by using spinning dust emission observations to constrain the abundance of very small dust grains (a $lesssim$ 10nm) in a sample of Galactic cold cores. Using the physical properties of the cores in our sample as inputs to a spinning dust model, we predict the expected level of emission at a wavelength of 1cm for four different very small dust grain abundances, which we constrain by comparing to 1cm CARMA observations. For all of our cores we find a depletion of very small grains, which we suggest is due to the process of grain growth. This work represents the first time that spinning dust emission has been used to constrain the physical properties of interstellar dust grains.
Near-infrared polarimetric imaging observations toward the Galactic center have been carried out to examine the efficiency and wavelength dependence of interstellar polarization. A total area of about 5.7 deg$^2$ is covered in the $J$, $H$, and $K_S$ bands. We examined the polarization efficiency, defined as the ratio of degree of polarization to color excess. The interstellar medium between the Galactic center and us shows the polarization efficiency lower than that in the Galactic disk by a factor of three. Moreover we investigated the spatial variation of the polarization efficiency by comparing it with those of color excess, degree of polarization, and position angle. The spatial variations of color excess and degree of polarization depend on the Galactic latitude, while the polarization efficiency varies independently of the Galactic structure. Position angles are nearly parallel to the Galactic plane, indicating the longitudinal magnetic field configuration between the Galactic center and us. The polarization efficiency anticorrelates with dispersions of position angles. The low polarization efficiency and its spatial variation can be explained by the differences of the magnetic field directions along the line-of-sight. From the lower polarization efficiency, we suggest a higher strength of a random component relative to a uniform component of the magnetic field between the Galactic center and us. We also derived the ratios of degree of polarization $p_H/p_J$ = 0.581 $pm$ 0.004 and $p_{K_S}/p_H$ = 0.620 $pm$ 0.002. The power law indices of the wavelength dependence of polarization are $beta_{JH}$ = 2.08 $pm$ 0.02 and $beta_{HK_S}$ = 1.76 $pm$ 0.01. Therefore the wavelength dependence of interstellar polarization exhibits flattening toward longer wavelengths in the range of 1.25$-$2.14 $micron$. The flattening would be caused by aligned large-size dust grains.
The degree to which interstellar grains align with respect to the interstellar magnetic field depends on disaligning as well as aligning mechanisms. For decades, it was assumed that disalignment was due primarily to the random angular impulses a grain receives when colliding with gas-phase atoms. Recently, a new disalignment mechanism has been considered, which may be very potent for a grain that has a time-varying electric dipole moment and drifts across the magnetic field. We provide quantitative estimates of the disalignment times for silicate grains with size > approximately 0.1 micron. These appear to be shorter than the time-scale for alignment by radiative torques, unless the grains contain superparamagnetic inclusions.
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
