Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userDust emission, extinction, and scattering in LDN 1642
84
0
0.0
(
0
)
Ask ChatGPT about the research
No Arabic abstract
We study the near-infrared (NIR) scattering in LDN 1642, its correlation with the cloud structure, and the ability of dust models to simultaneously explain sub-millimetre emission, NIR extinction, and NIR scattering. We use observations from the HAWK-I instrument to measure the NIR surface brightness and extinction. These are compared with Herschel data on dust emission and, with radiative transfer modelling, with predictions calculated for different dust models. We find an optical depth ratio $tau(250,mu{rm m})/tau(J)approx 10^{-3}$, confirming earlier findings of high sub-millimetre emissivity. The relationships between the column density derived from dust emission and the NIR colour excesses is linear and consistent with the standard NIR extinction curve. The extinction peaks at $A_J=2.6,$mag, the NIR surface brightness remaining correlated with $N({rm H}_2)$ without saturation. Radiative transfer models can fit the sub-millimetre data with any of the tested dust models. However, these predict a NIR extinction that is higher and a NIR surface brightness that is lower than in observations. If the dust sub-millimetre emissivity is rescaled to the observed value of $tau(250,mu{rm m})/tau(J)$, dust models with high NIR albedo can reach the observed level of NIR surface brightness. The NIR extinction of the models tends to be higher than directly measured, which is reflected in the shape of the NIR surface brightness spectra. The combination of emission, extinction, and scattering measurements provides strong constraints on dust models. The observations of LDN 1642 indicate clear dust evolution, including a strong increase in the sub-millimetre emissivity, not yet fully explained by the current dust models.
rate research
Read More
We study the anomalous microwave emission (AME) in the Lynds Dark Nebula (LDN) 1780 on two angular scales. Using available ancillary data at an angular resolution of 1 degree, we construct an SED between 0.408 GHz to 2997 GHz. We show that there is a significant amount of AME at these angular scales and the excess is compatible with a physical spinning dust model. We find that LDN 1780 is one of the clearest examples of AME on 1 degree scales. We detected AME with a significance > 20$sigma$. We also find at these angular scales that the location of the peak of the emission at frequencies between 23-70 GHz differs from the one on the 90-3000 GHz map. In order to investigate the origin of the AME in this cloud, we use data obtained with the Combined Array for Research in Millimeter-wave Astronomy (CARMA) that provides 2 arcmin resolution at 30 GHz. We study the connection between the radio and IR emissions using morphological correlations. The best correlation is found to be with MIPS 70$mu$m, which traces warm dust (T$sim$50K). Finally, we study the difference in radio emissivity between two locations within the cloud. We measured a factor $approx 6$ of difference in 30 GHz emissivity. We show that this variation can be explained, using the spinning dust model, by a variation on the dust grain size distribution across the cloud, particularly changing the carbon fraction and hence the amount of PAHs.
We present a model for the diffuse interstellar dust that explains the observed wavelength-dependence of extinction, emission, linear and circular polarisation of light. The model is set-up with a small number of parameters. It consists of a mixture of amorphous carbon and silicate grains with sizes from the molecular domain of 0.5 up to about 500nm. Dust grains with radii larger than 6nm are spheroids. Spheroidal dust particles have a factor 1.5 - 3 larger absorption cross section in the far IR than spherical grains of the same volume. Mass estimates derived from submillimeter observations that ignore this effect are overestimated by the same amount. In the presence of a magnetic field, spheroids may be partly aligned and polarise light. We find that polarisation spectra help to determine the upper particle radius of the otherwise rather unconstrained dust size distribution. Stochastically heated small grains of graphite, silicates and polycyclic aromatic hydrocarbons (PAHs) are included. We tabulate parameters for PAH emission bands in various environments. They show a trend with the hardness of the radiation field that can be explained by the ionisation state or hydrogenation coverage of the molecules. For each dust component its relative weight is specified, so that absolute element abundances are not direct input parameters. The model is confronted with the average properties of the Milky Way, which seems to represent dust in the solar neighbourhood. It is then applied to four specific sight lines including the reflection nebula NGC2023. For these sight lines, we present linear and circular spectro-polarimetric observations obtained with FORS/VLT. Using prolate rather than oblate grains gives a better fit to observed spectra; the axial ratio of the spheroids is typically two and aligned silicates are the dominant contributor to the polarisation.
We present the results based on the optical $R$-band polarization observations of 280 stars distributed towards the dark globule LDN,1225. {it Gaia} data release 2 parallaxes along with the polarization data of $sim$200 stars have been used to (a) constrain the distance of LDN,1225 as 830$pm$83~pc, (b) determine the contribution of interstellar polarization (ISP), and (c) characterize the dust properties and delineate the magnetic field (B-field) morphology of LDN,1225. We find that B-fields are more organized and exhibit a small dispersion of 12$degr$. Using the $^{12}$CO molecular line data from the Purple Mountain Observatory (PMO), along with the column density, dispersion in B-fields, we estimate B-field strength to be $sim$56,$pm$,10,$mu$G, magnetic to turbulence pressure to be $sim$3,$pm$,2, and the mass-to-magnetic flux ratio (in units of critical value) to be~$<$,1. These results indicate the dominant role of B-fields in comparison to turbulence and gravity in rendering the cloud support. B-fields are aligned parallel to the low-density parts (traced by $^{12}$CO map) of the cloud, in contrast they are neither parallel nor perpendicular to the high-density core structures (traced by $^{13}$CO and C$^{18}$O maps). LDN,1225 hosts two 70,$mu$m sources which seem to be of low-mass Class 0 sources. The total-to-selective extinction derived using optical and near-infrared photometric data is found to be anomalous ($R_{V}$~$=$~3.4), suggesting dust grain growth in LDN,1225. Polarization efficiency of dust grains follows a power-law index of $-$0.7 inferring that optical polarimetry traces B-fields in the outer parts of the cloud.
The large majority of extinction sight lines in our Galaxy obey a simple relation depending on one parameter, the total-to-selective extinction coefficient, Rv. Different values of Rv are able to match the whole extinction curve through different environments so characterizing normal extinction curves. In this paper more than sixty curves with large ultraviolet deviations from their best-fit one parameter curve are analyzed. These curves are fitted with dust models to shed light into the properties of the grains, the processes affecting them, and their relations with the environmental characteristics. The extinction curve models are reckoned by following recent prescriptions on grain size distributions able to describe one parameter curves for Rv values from 3.1 to 5.5. Such models, here extended down to Rv=2.0, allow us to compare the resulting properties of our deviating curves with the same as normal curves in a self-consistent framework, and thus to recover the relative trends overcoming the modeling uncertainties. Such curves represent the larger and homogeneous sample of anomalous curves studied so far with dust models. Results show that the ultraviolet deviations are driven by a larger amount of small grains than predicted for lines of sight where extinction depends on one parameter only. Moreover, the dust-to-gas ratios of anomalous curves are lower than the same values for no deviating lines of sight. Shocks and grain-grain collisions should both destroy dust grains, so reducing the amount of the dust trapped into the grains, and modify the size distribution of the dust, so increasing the small-to-large grain size ratio. Therefore, the extinction properties derived should arise along sight lines where shocks and high velocity flows perturb the physical state of the interstellar medium living their signature on the dust properties. (Abridged version)
We report new dust polarization results of a nearly edge-on disk in the HH 212 protostellar system, obtained with ALMA at ~ 0.035 (14 au) resolution in continuum at lambda ~ 878 um. Dust polarization is detected within ~ 44 au of the central source, where a rotationally supported disk has formed. The polarized emission forms V-shaped structures opening to the east and probably west arising from the disk surfaces and arm structures further away in the east and west that could be due to potential spiral arms excited in the outer disk. The polarization orientations are mainly parallel to the minor axis of the disk, with some in the western part tilting slightly away from the minor axis to form a concave shape with respect to the center. This tilt of polarization orientations is expected from dust self-scattering, e.g., by 50-75 um grains in a young disk. The polarized intensity and polarization degree both peak near the central source with a small dip at the central source and decrease towards the edges. These decreases of polarized intensity and polarization degree are expected from dichroic extinction by grains aligned by poloidal fields, but may also be consistent with dust self-scattering if the grain size decreases toward the edges. It is possible that both mechanisms are needed to produce the observed dust polarization, suggesting the presence of both grain growth and poloidal fields in the disk.
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
