Wavelength Of Maximum Polarization Research Articles

BVRI polarization of stars in the direction of Stock 19

Abstract We present BVRI polarimetric measurements of nine bright stars in a 15′-square region centered on Stock 19. Seven of them satisfy the Serkowski equation for star light polarization due to interstellar dust. This allowed us to estimate the degree of maximum polarization (pmax) and the wavelength of maximum polarization (λmax). Along this line of sight, pmax ranges from 0.332% to 0.948%, and the average λmax of 0.542 ± 0.018 μm yields a total-to-selective extinction ratio RV = 3.04 ± 0.11. Four of the observed stars were previously listed as high-probability members of Stock 19. However, based on their Gaia distances, they are spread out along the line of sight and show a correlation between polarization and distance that is typical for the diffuse interstellar medium. The extinction, as obtained from dust maps, and the Planck 353 GHz polarization are low for this line of sight, additionally suggesting that these stars might not represent an actual cluster. The polarization parameters of three of the observed stars indicate the possible presence of an intrinsic polarization component, likely due to circumstellar material.

Polarization, polarizing efficiency, and grain alignment towards the direction of the cluster NGC 2345

ABSTRACT We have investigated the grain alignment and dust properties towards the direction of the cluster NGC 2345 using the multiband optical polarimetric observations. For the majority of the stars, the observed polarization is found to be due to the interstellar medium with average values of maximum polarization and wavelength corresponding to it as 1.55 per cent and 0.58 $\mu$m, respectively. This reveals a similar size distribution of dust grains to that in the general interstellar medium in the direction of NGC 2345. Alteration of dust properties near the distance of 1.2 kpc towards the direction of NGC 2345 has been noticed. The dust grains located beyond this distance are found to be aligned with the Galactic magnetic field, whereas a dispersion in the orientation of the dust grains lying in the foreground of this distance is found. Polarizing efficiency of grains in this direction is found to be close to the average efficiency for our Galaxy. The decreased grain size along with the increased polarizing efficiency towards the core region of the cluster indicates the local radiation field is higher within the cluster, which is responsible for the increased alignment efficiency of small grains. The wavelength of maximum polarization (associated with the average size of aligned grains) is also found to increase with extinction and reduces with the increase in polarizing efficiency, which can be explained by the radiative torque alignment mechanism.

The wavelength dependence of interstellar polarization in the Local Hot Bubble

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.

Interstellar polarization and extinction towards the young open cluster NGC 1502

Aims.NGC 1502 is located at the periphery of the Cam OB1 association and probably within the Orion Spur, with published distance estimates varying between 0.7 and 1.5 kpc. We combine new polarimetric observations and existinguvbyβandUBVphotometries to provide new estimates of the cluster’s parameters.Methods.We present new multi-wavelength polarization observations of 22 stars in the direction of the cluster and, using the wavelength of maximum polarization, calculate the total-to-selective extinction ratio for each of these stars. These are combined with homogenizeduvbyβandUBVphotometries to obtain the individual stellar distances and study the structure of the field.Results.We found no evidence of significant extinction variation across the cluster and obtained an average color excessE(b−y) = 0.56 ± 0.02 mag, corresponding toE(B−V) = 0.76 mag. Given the uniformity of bothpmaxandλmaxtoward the cluster and the distribution of the color excess with distance, it seems likely that the polarization in this region arises from aligned dust grains in the foreground of the cluster, not farther than 500–600 pc. Based on 20 stars located beyond 500 pc within the studied field of view, we obtained an average polarization-based total-to-selective extinction ratioR= 2.83 ± 0.14 yielding visual absorptionAV= 2.14 ± 0.16 mag. Using 11 stars withuvbyβdata that represent the main sequence of the cluster, we calculated a photometric distance of 1117−89+96pc. This estimate is fully supported by theUBV-based analysis and the new individual membership probabilities that we derive. Assuming a solar metallicity yields an age of 5 Myr. Our analysis yields controversial results regarding the membership of the stars associated with the SZ Cam system (HD 25638 and HD 25639) to the cluster.

Interplay of dust alignment, grain growth, and magnetic fields in polarization: lessons from the emission-to-extinction ratio

Polarized extinction and emission from dust in the interstellar medium (ISM) are hard to interpret, as they have a complex dependence on dust optical properties, grain alignment and magnetic field orientation. This is particularly true in molecular clouds. The data available today are not yet used to their full potential. The combination of emission and extinction, in particular, provides information not available from either of them alone. We combine data from the scientific literature on polarized dust extinction with Planck data on polarized emission and we use them to constrain the possible variations in dust and environmental conditions inside molecular clouds, and especially translucent lines of sight, taking into account magnetic field orientation. We focus on the dependence between \lambda_max -- the wavelength of maximum polarization in extinction -- and other observables such as the extinction polarization, the emission polarization and the ratio of the two. We set out to reproduce these correlations using Monte-Carlo simulations where the relevant quantities in a dust model -- grain alignment, size distribution and magnetic field orientation -- vary to mimic the diverse conditions expected inside molecular clouds. None of the quantities chosen can explain the observational data on its own: the best results are obtained when all quantities vary significantly across and within clouds. However, some of the data -- most notably the stars with low emission-to-extinction polarization ratio -- are not reproduced by our simulation. Our results suggest not only that dust evolution is necessary to explain polarization in molecular clouds, but that a simple change in size distribution is not sufficient to explain the data, and point the way for future and more sophisticated models.

POLARIZATION MEASUREMENTS OF HOT DUST STARS AND THE LOCAL INTERSTELLAR MEDIUM

ABSTRACT Debris discs are typically revealed through the presence of excess emission at infrared wavelengths. Most discs exhibit excess at mid- and far-infrared wavelengths, analogous to the solar system’s Asteroid and Edgeworth-Kuiper belts. Recently, stars with strong (∼1%) excess at near-infrared wavelengths were identified through interferometric measurements. Using the HIgh Precision Polarimetric Instrument, we examined a sub-sample of these hot dust stars (and appropriate controls) at parts-per-million sensitivity in SDSS g′ (green) and r′ (red) filters for evidence of scattered light. No detection of strongly polarized emission from the hot dust stars is seen. We, therefore, rule out scattered light from a normal debris disk as the origin of this emission. A wavelength-dependent contribution from multiple dust components for hot dust stars is inferred from the dispersion (the difference in polarization angle in red and green) of southern stars. Contributions of 17 ppm (green) and 30 ppm (red) are calculated, with strict 3-σ upper limits of 76 and 68 ppm, respectively. This suggests weak hot dust excesses consistent with thermal emission, although we cannot rule out contrived scenarios, e.g., dust in a spherical shell or face-on discs. We also report on the nature of the local interstellar medium (ISM), obtained as a byproduct of the control measurements. Highlights include the first measurements of the polarimetric color of the local ISM and the discovery of a southern sky region with a polarization per distance thrice the previous maximum. The data suggest that λ max, the wavelength of maximum polarization, is bluer than typical.

Observational Constraints on Interstellar Grain Alignment

We present new multicolor photo-polarimetry of stars behind the Southern Coalsack. Analyzed together with multiband polarization data from the literature, probing the Chamaeleon I, Musca, rho Opiuchus, R CrA and Taurus clouds, we show that the wavelength of maximum polarization (lambda_max) is linearly correlated with the radiation environment of the grains. Using Far-Infrared emission data, we show that the large scatter seen in previous studies of lambda_max as a function of A_V is primarily due to line of sight effects causing some A_V measurements to not be a good tracer of the extinction (radiation field strength) seen by the grains being probed. The derived slopes in lambda_max vs. A_V, for the individual clouds, are consistent with a common value, while the zero intercepts scale with the average values of the ratios of total-to-selective extinction (R_V) for the individual clouds. Within each cloud we do not find direct correlations between lambda_max and R_V. The positive slope in consistent with recent developments in theory and indicating alignment driven by the radiation field. The present data cannot conclusively differentiate between direct radiative torques and alignment driven by H_2 formation. However, the small values of lambda_max(A_V=0), seen in several clouds, suggest a role for the latter, at least at the cloud surfaces. The scatter in the lambda_max vs. A_V relation is found to be associated with the characteristics of the embedded Young Stellar Objects (YSO) in the clouds. We propose that this is partially due to locally increased plasma damping of the grain rotation caused by X-rays from the YSOs.

A Study of the Isolated Dark Globule DC 314.8-5.1: Extinction, Distance, and a Hint of Star Formation

The southern dark globule DC 314.8-5.1 contains a reflection nebula illuminated by a normal B9 V star, HD 130079. This serendipitous association provides the opportunity to evaluate the distance to the globule with greater accuracy than would otherwise be possible, subject to accurate accounting for the effects of interstellar extinction and reddening. It is shown that the dust in the line of sight has optical properties characterized by elevated values of the ratio of extinction to reddening (RV = AV/EB-V) and the wavelength of maximum polarization (λmax), signifying growth that most probably results from grain-grain coagulation within the globule. Taking this into account yields a distance of 342 ± 50 pc, significantly lower than previous estimates that assume the standard diffuse interstellar medium average extinction law. Comparison of this result with the loci of other major sources of extinction along the line of sight suggests that the globule is isolated; it appears to not be physically associated with the adjacent Circinus molecular cloud and star formation complex G317.0-4.0. Estimates are made of the mean number density (nH 9 × 103 cm-3) and mass (30-100 M⊙) of DC 314.8-5.1. A stellar census of the region using the Two Micron All Sky Survey suggests that it is not a site of vigorous star formation, although two (out of 387) sources are identified that appear to be good candidates for young stellar object status on the basis of their near-infrared colors. A deep mid-infrared survey will be needed to determine whether DC 314.8-5.1 is, indeed, starless (with respect to indigenous birth) or a site of sedentary low-mass star formation. The globule may also prove to be a valuable laboratory for future study of the interaction of dense molecular gas and dust in a quiescent core with the relatively soft UV radiation field emanating from the embedded B9 V star.

A polarimetric view of the open cluster Lyngå6 and the Cepheid TW Normæ

We present UBVRI polarimetric observations of stars belonging to the small open cluster Lynga6 and of TW Nor, a Cepheid which can be seen near the center of the grouping. The wavelength of maximum polarization is computed and then analyzed in the context of its relation to the optical properties and characteristic particle size distribution of the grains responsible for the polarization. The amount and direction of linear polarization is also computed. Results indicate a mean polarization efficiency close to the value found for the interstellar medium. Two out of seven members of Lynga6 present indications of intrinsic polarization in their measures. We conclude that part of the polarization detected in the light from members of the cluster comes from a dust cloud located in the Local arm, at a distance of about 1 kpc. In general, polarization measurements do not support the membership of TW Nor to Lynga6.

Interstellar Extinction and Polarization in the Taurus Dark Clouds: The Optical Properties of Dust near the Diffuse/Dense Cloud Interface

Observations of interstellar linear polarization in the spectral range 0.35-2.2??m are presented for several stars reddened by dust in the Taurus region. Combined with a previously published study by Whittet et al., these results represent the most comprehensive data set available on the spectral dependence of interstellar polarization in this nearby dark cloud (a total of 27 sight lines). Extinction data for these and other reddened stars in Taurus are assembled for the same spectral range, combining published photometry and spectral classifications with photometry from the Two Micron All Sky Survey. The polarization and extinction curves are characterized in terms of the parameters ?max (the wavelength of maximum polarization) and RV (the ratio of total to selective extinction), respectively. The data are used to investigate in detail the question of whether the optical properties of the dust change systematically as a function of environment, considering stars observed through progressively more opaque (and thus progressively denser) regions of the cloud. At low visual extinctions (0 3, real changes in grain properties occur, characterized by observed RV values in the range 3.5-4.0. A simple model for the development of RV with AV suggests that RV may approach values of 4.5 or more in the densest regions of the cloud. The transition between normal extinction and dense cloud extinction occurs at AV ~ 3.2, a value coincident with the threshold extinction above which H2O-ice is detected on grains within the cloud. Changes in RV are thus either a direct consequence of mantle growth or occur under closely similar physical conditions. Dust in Taurus appears to be in a different evolutionary state compared with other nearby dark clouds, such as ? Oph, in which coagulation is the dominant physical process.

Polarimetry of the Highly Reddened Open Clusters Hogg 15 and Lyngå 14

We present UBVRI polarimetric observations of stars belonging to the highly reddened open clusters Hogg 15 and Lynga 14. The wavelength of maximum polarization is computed and then analyzed in the context of its relation to the optical properties and characteristic particle size distribution of the grains responsible for the polarization in each case. The amount and direction of the linear polarization are also computed. Results indicate that the polarization efficiency is relatively low in both cases, compared with the values attributed to the interstellar medium, and probably due to depolarization effects. Only one out of the 12 observed members in Hogg 15, and none in Lynga 14, presents indications of intrinsic polarization in its measures. There exists some intracluster dust in association with Hogg 15, with a slightly different grain size distribution when compared with the Coalsack dust itself. For Lynga 14, we conclude that the polarization detected in this case possibly comes from dust present in a nearby cloud located along the line of sight to the cluster, with some dust related to the cluster itself. The magnetic field in the direction to Hogg 15 follows the general trend of the polarization directions in the region, but this is not true for Lynga 14, where differences seem to exist.

Interstellar Polarization in the Taurus Dark Clouds: Wavelength‐dependent Position Angles and Cloud Structure near TMC‐1

Systematic variations with wavelength in the position angle of interstellar linear polarization of starlight may be indicative of multiple cloud structure along the line of sight. We use polarimetric observations of two stars (HD 29647 and HD 283809) in the general direction of TMC-1 in the Taurus dark cloud to investigate grain properties and cloud structure in this region. We show the data to be consistent with a simple two-component model in which general interstellar polarization in the Taurus cloud is produced by a widely distributed cloud component with relatively uniform magnetic field orientation; light from stars close to TMC-1 suffers additional polarization arising in one (or more) subcloud(s) with larger average grain size and magnetic field directions different from the general trend. Toward HD 29647 in particular, we show that the unusually low degree of visual polarization relative to extinction is due to depolarization associated with the presence of distinct cloud components in the line of sight with markedly different magnetic field orientations. Stokes parameter calculations allow us to separate the polarization characteristics of the individual components. Results are fitted with the Serkowski empirical formula to determine the degree and wavelength of maximum polarization. Whereas λmax values in the widely distributed material are similar to the average (0.55 μm) for the diffuse interstellar medium, the subcloud in the line of sight to HD 283809, the most heavily reddened star in our study, has λmax ≈ 0.73 μm, indicating the presence of grains ~30% larger than this average. Our model also predicts detectable levels of circular polarization toward both HD 29647 and HD 283809.

Interstellar Extinction, Polarization, and Grain Alignment in the High‐Latitude Molecular Cloud toward HD 210121

We present near-infrared photometry and optical and near-infrared polarimetry for the line of sight toward HD 210121, obscured by the high-latitude cloud DBB 80. From the data, we determine remarkably small values of the ratio of total to selective extinction, RV = 2.1 ± 0.2, and the wavelength of maximum polarization, λmax = 0.38 ± 0.03 μm, as compared to average values in the diffuse Galactic interstellar medium. These values nevertheless follow the linear relationship between RV and λmax seen closer to the Galactic plane. Our results imply an abundance of small grains, which we interpret as evidence for lack of grain growth in this high-latitude cloud. This is consistent with previous observations of high molecular abundances and extremely steep far-ultraviolet extinction toward the star and also with the presence of mid-infrared emission in the foreground cloud. We show that populations of small grains responsible for extinction and polarization in DBB 80 respond to prevailing physical conditions in a correlated way. Based on its extremely short value of λmax, we predict that the line of sight to HD 210121 will be highly polarized in the ultraviolet.

A review of interstellar polarization properties and recent measurements toward the chamaeleon I and taurus dark clouds

The observed linear polarization of starlight is thought to be caused by magnetically aligned, elongated dust grains existing within interstellar clouds. Typically, the magnitude of polarization as a function of wavelength may be empirically fit with great accuracy by the Serkowski law, and comparisons of the parameters of this fit to extinction data provide information about grain sizes and environments. The linear polarization properties of the Chamaeleon I and Taurus dark clouds have been studied, using data from the literature combined with some new observations. Strong trends in the amount of polarization as a function of visual extinction ( P vs A v ) in the line of sight have been observed for both clouds. Specifically, the degree of polarization per unit extinction, or polarization efficiency P/ A v , for Taurus has been found to decrease dramatically with increasing A v the data being well fit by a power law. For Chamaeleon, the relationship between the wavelength of maximum polarization ( λ max and the ratio of total to selective extinction ( R V ) is significantly different from that shown previously for a variety of sources by Clayton and Mathis. It is suggested that these effects may be caused by the physical and chemical processes occurring in these clouds and may be used to further understand not only the grain alignment mechanisms but also the environment of dense interstellar clouds.

The magnetic field in Lynds 1457: multiband photopolarimetry.

We have performed multiband photopolarimetry toward stars behind the molecular cloud L1457 (MBM 12). This cloud is the nearest known molecular cloud (65 pc) and is thought to be contained within the local "hot bubble." The polarization shows a regular structure, indicating that the cloud is threaded by an ordered magnetic field. The wavelength dependence of the polarization seems to indicate that the grains in L1457 have higher indices of refraction than normal for interstellar clouds. However, the wavelength of maximum polarization indicates that their size distribution is close to normal.

A study of the Chamaeleon I dark cloud and T-association – VI. Interstellar polarization, grain alignment and magnetic field

We present new measurements of optical and near-infrared linear polarization towards 39 field stars reddened by dust in the Chamaeleon I dark cloud. New and previously published data are combined in a detailed investigation of the wavelength dependence of interstellar polarization in the cloud. The observations are well represented by the Serkowski formula, with values of the wavelength of maximum polarization (lambda(max)) in the range 0.47-0.75 mum. The highest values of lambda(max) are found in lines of sight that intercept the dense central region of the cloud. The ratio of total to selective extinction (R(V) = A(V)/E(B-V)) is only weakly correlated with lambda(max), suggesting a degree of independence among the populations of grains responsible for optical extinction and polarization. We show that the ratio of polarization to reddening is unusually high in Cha I, with p(max)/A(V) congruent-to 4.5 per cent mag-1 in some lines of sight, indicating a remarkable degree of alignment efficiency in comparison to other dark clouds. The mean direction of the magnetic field in the plane of the sky is perpendicular to the long axis of the cloud and parallel to the external magnetic field in the Galactic neighbourhood of Chamaeleon, suggesting that the cloud formed by uniform collapse along field lines. Our observations provide important constraints on theoretical models for both grain alignment and the relation between magnetic field, cloud collapse and star formation.

An investigation of grain properties in the Rho Ophiuchi dark cloud

We present a comprehensive study of dust grains in the p Ophiuchi dark cloud. By combining new polarimetric, optical photometric, and spectral classification data for 134 stars, cloud material is probed to an absorption depth of A V ≤ 5-7 mag. For stars with R determined from the color-difference method, a mean value of R = 3.99 ± 0.18 at a mean extinction of A V = 3.25 mag is found. R scales with the wavelength of maximum polarization at the rate R = 5.7λ max , consistent with results from other studies. Grain size, as deduced from λ max , increases with optical depth in a manner that is consistent with a simple model of turbulence-driven coagulative grain growth

The wavelength dependence of interstellar polarization in the Large Magellanic Cloud

view Abstract Citations (32) References (22) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The wavelength dependence of interstellar polarization in the LMC. Clayton, G. C. ; Martin, P. G. ; Thompson, I. Abstract The wavelength dependence of interstellar polarization in the Large Magellanic Cloud has been studied. It was found that the data were fitted well by the empirical Serkowski relation used for Galactic stars. The range in λmax, the wavelength of maximum polarization, is also like that found in the Galaxy. The grains in both galaxies have similar optical polarization properties. Publication: The Astrophysical Journal Pub Date: February 1983 DOI: 10.1086/160663 Bibcode: 1983ApJ...265..194C Keywords: Interstellar Extinction; Magellanic Clouds; Milky Way Galaxy; Optical Polarization; Polarized Light; Stellar Spectrophotometry; Infrared Radiation; Interstellar Matter; Stellar Spectra; Wavelengths; Astrophysics full text sources ADS | data products SIMBAD (25) NED (1)

Interstellar grain size - A look at the deviant four

A linear polarimeter has been used to examine three of four stars (BR 545, 885, and 929 in Orion) that do not appear to fit either a simple relationship between IR excess and wavelength of maximum polarization or the simple grain-growth models of McMillan (1978). The data obtained are shown to resolve the discrepancies in the cases of BR 545 and BR 929. The polarization of BR 885 is found to be quite complex, and the data indicate that interstellar reddening, possibly caused by two large intracluster dust clouds in the line of sight, is the source of the IR excess and polarization. It is concluded that the available interstellar data for these three stars are consistent with models of accretion of nonabsorbing mantles on dust-grain cores.

Diffuse interstellar band formation in dense clouds

Data are presented which show that the strengths of the diffuse interstellar bands at 4430, 5780, and 5797 A tend to be deficient with respect to color excess in the lines of sight to stars lying behind dense interstellar clouds. In addition, diffuse-band data for ten stars in the well-studied rho Ophiuchi cloud complex are presented, and systematic variations of band strengths with optical depth are discussed. It is found that the band strength per grain in the line of sight apparently decreases with increasing grain size, taking the ratio of total-to-selective absorption and the wavelength of maximum polarization as indicators of the latter. It is concluded that diffuse-band formation probably takes place most efficiently in the outer regions of interstellar clouds. This may be consistent with a diffuse-band origin in small grains which are coated by mantles in cloud interiors; or in some molecule or molecules which are most abundant in the outer portions of interstellar clouds.