Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userThe efficiency and wavelength dependence of near-infrared interstellar polarization toward the Galactic center
122
0
0.0
(
0
)
Added by
Hirofumi Hatano Dr.
Publication date
2013
fields
Physics
and research's language is
English
Ask ChatGPT about the research
No Arabic abstract
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.
rate research
Read More
Near-infrared polarimetry of point sources reveals the presence of a toroidal magnetic field in the central 20 x 20 region of our Galaxy. Comparing the Stokes parameters between high extinction stars and relatively low extinction ones, we have obtained a polarization originating from magnetically aligned dust grains at the central region of our Galaxy of at most 1-2 kpc. The derived direction of the magnetic field is in good agreement with that obtained from far-infrared/submillimeter observations, which detect polarized thermal emission from dust in the molecular clouds at the Galactic center. Our results show that by subtracting foreground components, near-infrared polarimetry allows investigation of the magnetic field structure at the Galactic center. The distribution of the position angles shows a peak at around 20deg, nearly parallel to the direction of the Galactic plane, suggesting a toroidal magnetic configuration.
We describe and discuss remarkable infrared spectra, covering key portions of the $2-5$ $mu$m wavelength interval, of the probable OH/IR supergiant 2MASS J17470898$-$2829561 (2M1747), located in direction of the Sgr B molecular cloud complex within the Central Molecular Zone (CMZ) of the Galaxy. This star was originally singled out for examination based on its suitability for spectroscopy of lines of H$_3^+$ in the CMZ. Analysis of the spectra shows that 2M1747 is deeply embedded within Sgr B1, with A$_V$ $gtrsim$ 100 mag, making it the only star within Sgr B for which infrared spectra have been obtained at present, and thereby a unique infrared probe of the dense interstellar medium within the CMZ. Despite the high extinction, spectra of 2M1747 reveal a veiled photosphere in the $K$ band and circumstellar gas in the $M$ band, giving clues as to its nature. Its $ 3.5-4.0$ $mu$m spectrum contains the strongest absorption lines of H$_3^+$ observed toward any object to date. The $4.5-4.8$ $mu$m spectrum has impressively deep and wide absorption lines of interstellar CO, most of which arise in dense gas within Sgr B1. The $3-5$ $mu$m spectrum also contains several solid state absorption features, which are characteristic of both dense and diffuse clouds, and which raise questions about the identifications of some of these features. We discuss the nature of the star, the extinction to it, the extinction law for dust in the CMZ, and the identifications of the various solid-state features and where they are produced along this complex line of sight.
We present a NIR polarimetric map of the 1deg by 1deg region toward the Galactic center. Comparing Stokes parameters between highly reddened stars and less reddened ones, we have obtained a polarization originating from magnetically aligned dust grains at the central region of our Galaxy. The distribution of position angles shows a peak at the parallel direction to the Galactic plane, suggesting a toroidal magnetic field configuration. However, at high Galactic latitudes, the peak of the position angles departs from the direction of the Galactic plane. This may be a transition of a large-scale magnetic field configuration from toroidal to poloidal.
We obtained the near-infrared (NIR) high-resolution ($Requivlambda/Deltalambdasim20,000$) spectra of the seven brightest early-type stars in the Cygnus OB2 association for investigating the environmental dependence of diffuse interstellar bands (DIBs). The WINERED spectrograph mounted on the Araki 1.3m telescope in Japan was used to collect data. All 20 of the known DIBs within the wavelength coverage of WINERED ($0.91<lambda<1.36mu$m) were clearly detected along all lines of sight because of their high flux density in the NIR wavelength range and the large extinction. The equivalent widths (EWs) of DIBs were not correlated with the column densities of C$_2$ molecules, which trace the patchy dense component, suggesting that the NIR DIB carriers are distributed mainly in the diffuse component. On the basis of the correlations among the NIR DIBs both for stars in Cyg OB2 and stars observed previously, $lambdalambda$10780, 10792, 11797, 12623, and 13175 are found to constitute a family, in which the DIBs are correlated well over the wide EW range. In contrast, the EW of $lambda$10504 is found to remain almost constant over the stars in Cyg OB2. The extinction estimated from the average EW of $lambda$10504 ($A_Vsim3.6$mag) roughly corresponds to the lower limit of the extinction distribution of OB stars in Cyg OB2. This suggests that $lambda$10504 is absorbed only by the foreground clouds, implying that the carrier of $lambda$10504 is completely destroyed in Cyg OB2, probably by the strong UV radiation field. The different behaviors of the DIBs may be caused by different properties of the DIB carriers.
The properties of dust in the interstellar medium (ISM) nearest the Sun are poorly understood because the low column densities of dust toward nearby stars induce little photometric reddening, rendering the grains largely undetectable. Stellar polarimetry offers one pathway to deducing the properties of this diffuse material. Here we present multi-wavelength aperture polarimetry measurements of seven bright stars chosen to probe interstellar polarization near the edge of the Local Hot Bubble (LHB) - an amorphous region of relatively low density interstellar gas and dust extending ~70-150 pc from the Sun. The measurements were taken using the HIgh Precision Polarimetric Instrument (HIPPI) on the 3.9-m Anglo-Australian Telescope. HIPPI is an aperture stellar polarimeter with a demonstrated sensitivity of 4.3 parts-per-million (ppm). Of the stars observed two are polarized to a much greater degree than the others; they have a wavelength of maximum polarization ($lambda_{max}$) of ~550 $pm$ 20 nm - similar to that of stars beyond the LHB - and we conclude that they are in the wall of the LHB. The remaining five stars have polarizations of ~70 to 160 ppm, of these four have a much bluer $lambda_{max}$, ~350 $pm$ 50 nm. Bluer values of $lambda_{max}$ may indicate grains shocked during the evolution of the Loop I Superbubble. The remaining star, HD 4150 is not well fit by a Serkowski curve, and may be intrinsically polarized.
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
