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Angular dispersion functions are typically used to estimate the fluctuations in polarization angle around the mean magnetic field orientation in dense regions, such as molecular clouds. The technique provides accurate turbulent to regular magnetic field ratios, $\langle B_t^2\rangle^{1/2}/B_{pos}$, which are often underestimated by the classic Davis-Chandrasekhar-Fermi method. We assess the technique's suitability to characterize the turbulent and regular plane-of-sky magnetic field in low-density structures of the nearby interstellar medium (ISM), particularly when the turbulence outer scale, $\delta$, is smaller than the smallest scale observed, $\ell_{min}$. We use optical polarization maps of three intermediate-latitude fields ($|b| \gtrsim 7.\!\!^{\circ}5$) with dimensions of $0.\!\!^{\circ}3 \times 0.\!\!^{\circ}3$, sourced from the Interstellar Polarization Survey--General ISM (IPS-GI) catalog. We decomposed the HI emission detected by the Galactic All-Sky Survey (GASS) within our fields to estimate the multiphase ISM properties associated with the structure coupled to the magnetic field. We produced maps of the plane-of-sky magnetic field strength ($B_{pos}$), mass density ($\rho$), and turbulent velocity dispersion ($\sigma_{v,turb}$). In the regions with well-defined structures at $d<400$ pc, the average $B_{pos}$ ranges from ${\sim}3 \mu$G to ${\sim}9 \mu$G, depending on the method and physical properties. In the region where structures extend up to $1000$ pc, $B_{pos}$ varies from ${\sim}1 \mu$G to ${\sim}3 \mu$G. The results agree with previous estimations in the local, diffuse ISM. Finally, optical starlight polarization and thermal dust polarization at 353 GHz consistently reveal a highly regular plane-of-sky magnetic field orientation unfazed by diffuse dust structures observed at $12 \mu$m., Comment: 23 pages, 13 figures, 3 tables, accepted for publication in AJ

Starlight polarimetry, when combined with accurate distance measurements, allows for exploration of the three-dimensional structure of local magnetic fields in great detail. We present optical polarimetric observations of stars in and close to the Southern Coalsack, taken from the Interstellar Polarization Survey (IPS). Located in five fields of view approximately $0.3^{o}$ by $0.3^{o}$ in size, these data represent the highest density of optical polarimetric observations in the Southern Coalsack to date. Using these data, combined with accurate distances and extinctions based on Gaia data, we are able to characterize the magnetic field of the Coalsack and disentangle contributions to the polarization caused by the Southern Coalsack and a background structure. For the Southern Coalsack, we find an average magnetic field orientation of $\theta\sim 75^{o}$ with respect to the Galactic north pole and an average plane-of-sky magnetic field strength of approximately $B_{POS}=10$ $\mu G$, using the Davis-Chandrasekhar-Fermi (DCF) method. These values are in agreement with some earlier estimates of the Coalsack's magnetic field. In order to study the distant structure, we introduce a simple method to separate and isolate the polarization of distant stars from foreground contribution. For the distant structure, which we estimate to be located at a distance of approximately 1.3-1.5 kpc, we find an average magnetic field orientation of $\theta\sim100^{o}$ and we estimate a field strength of $B_{POS}\sim10 \ \mu G$, although this will remain highly uncertain until the precise nature of the distant structure can be uncovered., Comment: 19 pages, 6 figures, 1 table. Accepted for publication in AJ

Optical starlight can be partially polarized while propagating through the dusty, magnetized interstellar medium. The polarization efficiency describes the polarization intensity fraction per reddening unit, P$_V$/E($B-V$), related to the interstellar dust grains and magnetic field properties. The maximum value observed, [P$_V$/E$(B-V)]_{max}$, is thus achieved under optimal polarizing conditions of the interstellar medium. Therefore, the analysis of polarization efficiency observations across the Galaxy contributes to the study of magnetic field topology, small-scale magnetic fluctuations, grain-alignment efficiency, and composition. Infrared observations from $Planck$ satellite have set [P$_V$/E$(B-V)]_{max}$ to 13$\%$ mag$^{-1}$. However, recent optical polarization observations in $Planck$'s highly polarized regions showed polarization efficiency values between 13.6$\%$ mag$^{-1}$ and 18.2$\%$ mag$^{-1}$ (depending on the extinction map used), indicating that [P$_V$/E$(B-V)]_{max}$ is not well constrained yet. We used $V$-band polarimetry of the Interstellar Polarization Survey (consisting of $\sim$10500 high-quality observations distributed in 34 fields of $0.3^{\circ}\times0.3^{\circ}$) to accurately estimate the polarization efficiency in the interstellar medium. We estimated the upper limit of P$_V$/E($B-V$) with the weighted $99th$ percentile of the field. In five regions, the polarization efficiency upper limit is above 13$\%$ mag$^{-1}$. Furthermore, we found [P$_V$/E$(B-V)]_{max} = 15.8^{+1.3}_{-0.9}\%$ mag$^{-1}$ using diffuse intermediate latitude ($|b|>7.5^{\circ}$) regions with apparently strong regular Galactic magnetic field in the plane-of-sky. We studied the variations of P$_V$/E($B-V$) across the sky and tested toy models of polarization efficiency with Galactic longitude that showed some correspondence with a uniform spiral magnetic field., Comment: 23 pages, 15 figures, 2 tables, accepted for publication in AJ

Magnetic fields permeate the entire Galaxy and are essential to, for example, the regulation of several stages of the star formation process and cosmic ray transportation. Unraveling its properties, such as intensity and topology, is an observational challenge that requires combining different and complementary techniques. The polarization of starlight due to the absorption by field-aligned non-spherical dust grains provides a unique source of information about the interstellar magnetic field in the optical band. This work introduces a first analysis of a new catalog of optical observations of linearly polarized starlight in the diffuse interstellar medium (ISM), the Interstellar Polarization Survey, General ISM (IPS-GI). We used data from the IPS-GI, focusing on 38 fields sampling lines of sight in the diffuse medium. The fields are about 0.3$^{\circ}$ by 0.3$^{\circ}$ in size and each of them contains $\sim1000$ stars on average. The IPS-GI catalog has polarimetric measurements of over 40000 stars, over 18000 of which have ${P}/\sigma_{P} > 5$. We added distances and other parameters from auxiliary catalogs to over 36000 of these stars. We analyzed parameter distributions and correlations between parameters of a high-quality subsample of 10516 stars (i.e. $\sim275$ stars per field). As expected, the degree of polarization tends to increase with the extinction, producing higher values of polarization at greater distances or at lower absolute Galactic latitudes. Furthermore, we find evidence for a large-scale ordered Galactic magnetic field., Comment: 19 pages, 15 figures, accepted for publication in AJ

SW Sextantis systems are nova-like cataclysmic variables that have unusual spectroscopic properties, which are thought to be caused by an accretion geometry having part of the mass flux trajectory out of the orbital plane. Accretion onto a magnetic white dwarf is one of the proposed scenarios for these systems. To verify this possibility, we analysed photometric and polarimetric time-series data for a sample of six SW Sex stars. We report possible modulated circular polarization in BO Cet, SW Sex, and UU Aqr with periods of 11.1, 41.2 and 25.7 min, respectively, and less significant periodicities for V380 Oph at 22 min and V442 Oph at 19.4 min. We confirm previous results that LS Peg shows variable circular polarization. However, we determine a period of 18.8 min, which is different from the earlier reported value. We interpret these periods as the spin periods of the white dwarfs. Our polarimetric results indicate that 15% of the SW Sex systems have direct evidence of magnetic accretion. We also discuss SW Sex objects within the perspective of being magnetic systems, considering the latest findings about cataclysmic variables demography, formation and evolution., Comment: 29 pages, 17 figures

The discovery and characterization of Algol eclipsing binaries (EAs) provide an opportunity to contribute for a better picture of the structure and evolution of low-mass stars. However, the cadence of most current photometric surveys hinders the detection of EAs since the separation between observations is usually larger than the eclipse(s) duration and hence few measurements are found at the eclipses. Even when those objects are detected as variable, their periods can be missed if an appropriate oversampling factor is not used in the search tools. In this paper, we apply this approach to find the periods of stars cataloged in the Catalina Real-Time Transient Survey (CRTS) as EAs having unknown period (EA$_{\rm up}$). As a result, the periods of $\sim 56\%$ of them were determined. Eight objects were identified as low-mass binary systems and modeled with the Wilson \& Devinney synthesis code combined with a Monte-Carlo Markov Chain optimization procedure. The computed masses and radii are in agreement with theoretical models and show no evidence of inflated radii. This paper is the first of a series aiming to identify suspected binary systems in large surveys., Comment: 11 pages, 7 figures

We present the photometric characterization of the four iXon Ultra 888 CCD cameras of the SPARC4 instrument, which will be installed on the 1.6 m telescope of the Pico dos Dias Observatory in Brazil. We applied experimental methodologies for a systematic characterization of the read noise, electronic gain, dark current, and quantum efficiency of the CCDs. We have analyzed the statistical distribution of the read noise, and also its spatial gradient and temporal variability, where we obtained an average value of the read noise of 6.33 electrons. We applied the Janesick method to determine the electronic gain, where we obtained an average value of 3.35 e-/ADU. We have also obtained an average dark current of 0.00014 e-/pix/s for CCD internal temperature of -70 oC. We have inspected the dependency of the dark current with temperature and the spatial distribution of the dark current, where we found a variable profile in the CCD 9917. We developed an experiment using a bench mounted monochromator to obtain the spectral dependency of the quantum efficiency in the spectral range between 350 nm and 1100 nm, where we measured the quantum efficiency for each camera. The camera 9915 presents the highest quantum efficiency of 95.8 %. Our results are compared with those from the manufacturer. These experiments allow us to diagnose the performance of these CCD cameras, an important sub-system of the SPARC4 instrument. It also provides a systematic way for monitoring the aging of the CCDs.

The increasing number of synoptic surveys made by small robotic telescopes, such as the photometric Catalina Real-Time Transient Survey (CRTS), represents a unique opportunity for the discovery of variable sources and improves the statistical samples of such classes of objects. Our goal is the discovery of magnetic Cataclysmic Variables (mCVs). These are rare objects, which probe interesting accretion scenarios controlled by the white dwarf magnetic field. In particular, improved statistics of mCVs would help to address open questions on their formation and evolution. We performed an optical spectroscopy survey to search for signatures of magnetic accretion in 45 variable objects selected mostly from the CRTS. In this sample we found 32 CVs, 22 being mCV candidates from which 13 are previously unreported as such. If the proposed classifications are confirmed, it would represent an increase of 4% in the number of known polars and 12% in the number of known IPs. A fraction of our initial sample was classified as extragalactic sources or other types of variable stars by the inspection of the identification spectra. Despite the inherent complexity in identifying a source as a mCV, variability-based selection followed by spectroscopic snapshot observations has proved to be an efficient strategy for their discoveries, being a relatively inexpensive approach in terms of telescope time., Comment: 27 pages, 7 figures, published in the AJ, accepted version, replacement version with minor changes

We present a new optical polarimetric catalog for the Small Magellanic Cloud (SMC). It contains a total of 7207 stars, located in the Northeast (NE) and Wing sections of the SMC and part of the Magellanic Bridge. This new catalog is a significant improvement compared to previous polarimetric catalogs for the SMC. We used it to study the sky-projected interstellar magnetic field structure of the SMC. Three trends were observed for the ordered magnetic field direction at position angles of $(65 \pm 10)$ deg, $(115 \pm 10)$ deg, and $(150 \pm 10)$ deg. Our results suggest the existence of an ordered magnetic field aligned with the Magellanic Bridge direction and SMC's Bar in the NE region, which have position angles roughly at $115.4$ deg and $45$ deg, respectively. However, the overall magnetic field structure is fairly complex. The trends at $115$ deg and $150$ deg may be correlated with the SMC's bimodal structure, observed in Cepheids' distances and HI velocities. We derived a value of $B_{sky} = (0.947 \pm 0.079)~\mu G$ for the ordered sky-projected magnetic field, and $\delta B = (1.465 \pm 0.069)~\mu G$ for the turbulent magnetic field. This estimate of $B_{sky}$ is significantly larger (by a factor of $\sim10$) than the line-of-sight field derived from Faraday rotation observations, suggesting that most of the ordered field component is on the plane of the sky. A turbulent magnetic field stronger than the ordered field agrees with observed estimates for other irregular and spiral galaxies. For the SMC the $B_{sky}/\delta B$ ratio is closer to what is observed for our Galaxy than other irregular dwarf galaxies., Comment: Updated to match published version (includes extra online figures)

This study confirms MLS110213:022733+130617 as a new eclipsing polar. We performed optical spectroscopic, polarimetric and photometric follow-up of this variable source identified by the Catalina Real Time Transient Survey. Using the mid-eclipse times, we estimated an orbital period of 3.787 h, which is above the orbital period gap of the cataclysmic variables. There are nine other known polars with longer orbital periods, and only four of them are eclipsing. We identified high and low-brightness states and high polarization modulated with the orbital period. The spectra are typical of polars, with strong high ionization emission lines and inverted Balmer decrement. The HeII4686 A line is as strong as Hbeta. We modelled the photometric and polarimetric bright-state light curves using the CYCLOPS code. Our modelling suggests an extended emitting region on the WD surface, with a mean temperature of 9 keV and B in the range 18 -- 33 MG, although the possibility that it could be a two-pole accretor cannot yet be ruled out. The WD mass estimated from the shock temperature is 0.67 Msol. The derived parameters are consistent with the eclipse profile. The distance was estimated as 406+- 54 pc using the Period-Luminosity-Colours method. MLS110213 populates a rare sub-group of polars, near the upper limit of the period distribution, important to understand the evolution of mCVs., Comment: 10 pages, 8 figures, accepted to be published in MNRAS on 05/12/2015

Our goal is to test the feasibility to obtain accurate measurements of the continuum polarization from high-resolution spectra using the spectropolarimetric mode of ESPaDOnS. We used the new pipeline OPERA to reduce recent and archived ESPaDOnS data. A couple of standard polarization stars and several science objects were tested for the linear mode. In addition, the circular mode was tested using several objects from the archive with expected null polarization. Synthetic broad-band polarization was computed from the ESPaDOnS continuum polarization spectra and compared with published values (when available) to quantify the accuracy of the instrument. The continuum linear polarization measured by ESPaDOnS is consistent with broad-band polarimetry measurements available in the literature. The accuracy in the degree of linear polarization is around 0.2-0.3% considering the full sample. The accuracy in polarization position angle using the most polarized objects is better than 5deg. Consistent with this, the instrumental polarization computed for the circular continuum polarization is also between 0.2-0.3%. Our results suggest that measurements of the continuum polarization using ESPaDOnS are viable and can be used to study many astrophysical objects., Comment: 15 pages, 13 figures, accepted to Astronomy and Astrophysics

In this work, we present the first results of a study of a new sample of 7 polar candidates from polarimetric data obtained at the Pico dos Dias / LNA observatory. From the four polars analysed so far, we confirm the presence of high and variable polarization in 3. The data will be analysed using the code CYCLOPS., Comment: Simp\'osio de F\'isica e Astronomia do Vale do Para\'iba

We present new data that confirm MLS110213:022733+130617 as a new eclipsing polar. Some system properties were estimated., Comment: Proceedings of the XIV Latin American Regional IAU Meeting - RevMexAA

We performed an observational study of the relation between the interstellar magnetic field alignment and star formation in twenty (20) sky regions containing Bok Globules. The presence of young stellar objects in the globules is verified by a search of infrared sources with spectral energy distribution compatible with a pre main-sequence star. The interstellar magnetic field direction is mapped using optical polarimetry. These maps are used to estimate the dispersion of the interstellar magnetic field direction in each region from a Gaussian fit, sigma_B. In addition to the Gaussian dispersion, we propose a new parameter, eta, to measure the magnetic field alignment that does not rely on any function fitting. Statistical tests show that the dispersion of the magnetic field direction is different in star forming globules relative to quiescent globules. Specifically, the less organised magnetic fields occur in regions having young stellar objects., Comment: Submitted contribution to the proceedings of IAU Symposium 302: Magnetic Fields Throughout Stellar Evolution

The viewing geometry of the polar CP Tuc that better explains its optical and X-ray light curves is controversial. Previous modelling of white-light polarimetric data considered the partial self-eclipse of an extended inhomogeneous emitting region. Alternatively, phase-dependent absorption has been used to reproduce the X-ray data. This paper presents new optical polarimetric data of CP Tuc and a model that consistently explains its optical and X-ray data. The model was based on an extension of the CYCLOPS code that added X-ray bremsstrahlung emission and pre-shock region absorption to the original version, which only accounted for cyclotron emission. The new code creates the possibility of simultaneous optical and X-ray fitting. We show that self-eclipse and absorption data have distinct signatures on the X-ray spectra. Although we were able to reasonably fit the CP Tuc optical data to cases of absorption and self-eclipse, we were only able to reproduce the X-ray orbital modulation after considering the absorption in the pre-shock region. Specifically, we were unable to reproduce the X-ray observations in the self-eclipse case. We found that the primary emitting region in CP Tuc is located near the rotation pole that approximately points to the observer., Comment: 13 pages, 11 figures, accepted to be published in MNRAS

We present an analysis of eclipse timings of the post-common envelope binary NSVS 14256825, which is composed of an sdOB star and a dM star in a close orbit (P_{orb} = 0.110374 days). High-speed photometry of this system was performed between July, 2010 and August, 2012. Ten new mid-eclipse times were analyzed together with all available eclipse times in the literature. We revisited the (O-C) diagram using a linear ephemeris and verified a clear orbital period variation. On the assumption that these orbital period variations are caused by light travel time effects, the (O-C) diagram can be explained by the presence of two circumbinary bodies, even though this explanation requires a longer baseline of observations to be fully tested. The orbital periods of the best solution would be P_c ~ 3.5 years and P_d ~ 6.9 years. The corresponding projected semi-major axes would be a_c i_c ~ 1.9 AU and a_d i_d ~ 2.9 AU. The masses of the external bodies would be M_c ~ 2.9 M_{Jupiter} and M_d ~ 8.1 M_{Jupiter}, if we assume their orbits are coplanar with the close binary. Therefore NSVS 14256825 might be composed of a close binary with two circumbinary planets, though the orbital period variations is still open to other interpretations., Comment: 16 pages, 3 figures, 4 tables. Accepted for publication in ApJ

We present an analysis of UBVR$_{\rm C}$I$_{\rm C}$JH photometry and phase-resolved optical spectroscopy of NSVS 14256825, an HW Vir type binary. The members of this class consist of a hot subdwarf and a main-sequence low-mass star in a close orbit ($P_{\rm orb} ~ 0.1$ d). Using the primary-eclipse timings, we refine the ephemeris for the system, which has an orbital period of 0.11037 d. From the spectroscopic data analysis, we derive the effective temperature, $T_1 = 40000 \pm 500$ K, the surface gravity, $\log g_1 = 5.50\pm0.05$, and the helium abundance, $n(\rm He)/n(\rm H)=0.003\pm0.001$, for the hot component. Simultaneously modelling the photometric and spectroscopic data using the Wilson-Devinney code, we obtain the geometrical and physical parameters of NSVS 14256825. Using the fitted orbital inclination and mass ratio ($i = 82\fdg5\pm0\fdg3$ and $q = M_2/M_1 = 0.260\pm0.012$, respectively), the components of the system have $M_1 = 0.419 \pm 0.070 M_{\odot}$, $R_1 = 0.188 \pm 0.010 R_{\odot}$, $M_2 = 0.109 \pm 0.023 M_{\odot}$, and $R_2 = 0.162 \pm 0.008 R_{\odot}$. From its spectral characteristics, the hot star is classified as an sdOB star., Comment: 8 pages, 7 figures, accepted for publication in MNRAS

SOUTH POL will be a survey of the Southern sky in optical polarized light. It will use a newly designed polarimetric module at an 80cm Robotic Telescope. Telescope and polarimeter will be installed at CTIO, Chile, in late 2012. The initial goal is to cover the sky south of declination -15{\deg} in two years of observing time, aiming at a polarimetric accuracy \lesssim 0.1% down to V=15, with a camera covering a field of about 2.0 square degrees. SOUTH POL will impact areas such as Cosmology, Extragalactic Astronomy, Interstellar Medium of the Galaxy and Magellanic Clouds, Star Formation, Stellar Envelopes, Stellar explosions and Solar System, among others., Comment: To appear in "Stellar Polarimetry: from Birth to Death", Madison, WI, AIP Conf. Proc., 2012 (J. Hoffman, B. Whitney & J. Bjorkman eds.). After it is published, it will be found at http://www.aip.org

We present the basic concept of a new astronomical instrument: SPARC4 - Simultaneous Polarimeter and Rapid Camera in 4 bands. SPARC4 combines in one instrument: (i) photometric and polarimetric modes; (ii) sub-second time-resolution in photometric mode and excellent time-resolution in polarimetric mode; (iii) simultaneous imaging in four broad-bands for both modes. This combination will make SPARC4 a unique facility for ground-based optical observatories. Presently, the project is in its conceptual design phase., Comment: Poster presented at "Stellar Polarimetry: From birth to death". This article has been submitted to AIP Conference Proceedings. After it is published, it will be found at http://www.aip.org

We present time-resolved spectroscopy of the polar RBS0324(=1RXS J023052.9-684203) spanning more than one orbital cycle. The emission lines present complex and variable profiles. Radial velocities were determined for the HeII4686 and Hbeta emission lines. The curves are similar for both lines, with a semi-amplitude of about 250 km/s. The modulation of 1.5 mag of the continuum flux is compatible with those previously presented in the literature. All these characteristics are typical of polars., Comment: 5 pages, 3 figures http://www.konkoly.hu/cgi-bin/IBVS?5957

2MASS J09440940-5617117 was identified as a cataclysmic variable in 2007 by its spectrum. In this article we present optical differential photometry in the Rc band obtained using the 0.6-m telescope at Observatorio do Pico dos Dias/Brazil. The complete light curves confirm the presence of a deep eclipse. We derive an orbital period of 0.1879340(5) d. The eclipse has a width of 0.112 +- 0.003 orbital cycles., Comment: 4 pages, 5 figures, http://www.konkoly.hu/cgi-bin/IBVS?5971

From the optical to the X-ray frequencies, most of the continuum emission from AM Her systems originates in the post-shock region. Hence, using multi-spectral data can be useful to restrict the physical and geometrical properties of that region. In spite of that, distinct codes are used to model these frequency ranges. CYCLOPS is a code to model the optical cyclotron emission of polars. In this contribution, we present the first version of CYCLOPS-X, an improvement of CYCLOPS to fit simultaneously optical and X-ray data. The new code adds the bremsstrahlung as an emission process as well as the effects of X-ray absorption by the upper portion of the accretion column. As a first application of CYCLOPS-X, we present X-ray light curves using two sets of parameters provided by the optical modeling of CP Tuc. These two cases have very similar optical emission, but quite distinct light-curves in high energies. It illustrates the need of simultaneous modeling of the optical and X-ray emission to a proper description of the magnetic accretion in polars., Comment: 4 pages, poster presented at the conference on 'Physics of Accreting Compact Binaries', July 2010 in Kyoto, Japan; proceedings to be published by Universal Academy Press

CP Tuc (AX J2315-592) is a polar for which two accretion geometries have been put forward. They differ in the location of the accretion column on the white-dwarf surface. In this study, we present new multiband optical photometric and polarimetric data to study the post-shock position. These data are used as input for CYCLOPS, our new code for modeling the cyclotron emission from post-shock regions of polars. CYCLOPS has a full 3-D treatment of the radiative transfer and is a powerful tool to study the physical properties of the emitting region. We find reasonable fits for the optical data of CP Tuc in both scenarios. We suggest that it should be possible to constrain the location of the base of the accreting column by modeling the X-ray light curves., Comment: 6 pages; talk given at the conference on 'Physics of Accreting Compact Binaries', July 2010 in Kyoto, Japan; proceedings to be published by Universal Academy Press

The CMa R1 star-forming region contains several compact clusters as well as many young early-B stars. It is associated with a well-known bright rimmed nebula, the nature of which is unclear (fossil HII region or supernova remnant). To help elucidate the nature of the nebula, our goal was to reconstruct the star-formation history of the CMa R1 region, including the previously unknown older, fainter low-mass stellar population, using X-rays. We analyzed images obtained with the ROSAT satellite, covering ~5 sq. deg. Complementary VRI photometry was performed with the Gemini South telescope. Colour-magnitude and colour-colour diagrams were used in conjunction with pre-main sequence evolutionary tracks to derive the masses and ages of the X-ray sources. The ROSAT images show two distinct clusters. One is associated with the known optical clusters near Z CMa, to which ~40 members are added. The other, which we name the "GU CMa" cluster, is new, and contains ~60 members. The ROSAT sources are young stars with masses down to M_star ~0.5 M_sun, and ages up to 10 Myr. The mass functions of the two clusters are similar, but the GU CMa cluster is older than the cluster around Z CMa by at least a few Myr. Also, the GU CMa cluster is away from any molecular cloud, implying that star formation must have ceased; on the contrary (as already known), star formation is very active in the Z CMa region., Comment: 19 pages, 15 figures, accepted by A&A

Polars (or AM Her systems) are cataclysmic variables without a disc due to the strong magnetic field of the white dwarf. Most of their emission comes from the region where the accretion column impacts the white dwarf and cools through cyclotron and bremsstrahlung processes. We present a new code, CYCLOPS, to model the optical emission from these systems including the four Stokes parameters. It considers a three-dimensional region with the electronic density and temperature varying following a \textit{shock-like} profile and a dipolar magnetic field. The radiative transfer is solved in steps considering the solution with non-null input radiation. The footprint of the column in the white-dwarf surface is determined by the threading region in the equatorial plane, i.e., the region from where the flow follows the magnetic lines. The extinction caused by Thomson scattering above the emitting region is optionally included. The fittings of observational data are done using a hybrid approach: a genetic algorithm is used to seek for the regions having the best models and then an amoeba code refines the search. An example of application to multi-wavelength data of V834 Cen is presented. The fit found is consistent with previous parameters estimates and is able to reproduce the features of V834 data in three wavebands., Comment: MNRAS accepted, 7 figures

Our goal is to study the physical properties of the circumstellar environment of young stellar objetcs (YSOs). In particular, the determination of the scattering mechanism can help to constrain the optical depth of the disk and/or envelope in the near infrared. We used the IAGPOL imaging polarimeter along with the CamIV infrared camera at the LNA observatory to obtain near infrared polarimetry measurements at the H band of a sample of optically visible YSOs, namely, eleven T Tauri stars and eight Herbig Ae/Be stars. An independent determination of the disk (or jet) orientation was obtained for twelve objects from the literature. The circumstellar optical depth could be then estimated comparing the integrated polarization position angle (PA) with the direction of the major axis of the disk projected in the plane of the sky. In general, optically thin disks have polarization PA perpendicular to the disk plane. In contrast, optically thick disks produce polarization PA parallel to the disks. Among the T Tauri stars, three are consistent with optically thin disks (AS 353A, RY Tau and UY Aur) and five with optically thick disks (V536 Aql, DG Tau, DO Tau, HL Tau and LkHalpha 358). Among the Herbig Ae/Be stars, two stars show evidence of optically thin disk (Hen 3-1191 and VV Ser) and two of optically thick disks (PDS 453 and MWC 297). Our results seem consistent with the fact that optically thick disks at near infrared bands are associated more likely with younger YSOs. Marginal evidence of polarization reversal is found in RY Tau, RY Ori, WW Vul, and UY Aur. On the first three cases this feature can be associated to the UXOR phenomenon. Correlations with the IRAS colours and the spectral index yielded evidence of an evolutionary segregation with the disks tend to be optically thin when they are older., Comment: 15 pages, 15 figures, accepted in Astronomy and Astrophysics

We present optical linear polarimetry in the line of sight to HH135/HH136. The polarimetry of the field stars reveals two populations: one corresponds to a foreground interstellar component; the other originates in the interstellar medium in the vicinity of the Herbig-Haro pair and, therefore, can be used to study the magnetic field in the star forming region. Its direction is aligned with the jet of HH135/HH136, which could be an indication that the interstellar magnetic field is important in the outflow collimation. The interstellar magnetic field magnitude was estimated to be of order 90 uG. According to recent numerical simulations, an interstellar magnetic field of such strength can be important in the definition of the outflow direction. There is also evidence that the associated dark cloud has an elongation parallel to the magnetic field. Our image polarimetry of the extended emission associated with HH135/HH136 shows a centro-symmetric pattern pointing to the knot E of HH136. Previous near infrared polarimetry traces a different illumination center, namely IRAS 11101-5829 - the probable exciting source of the system. This discrepancy can be explained if the YSO emission is completely blocked in optical wavelengths and the dominant optical source in the region is the knot E, whose nature is uncertain. A discussion of the spectral energy distributions of HH136-E and IRAS 11101-5829 is presented., Comment: Accepted for publication in The Astronomical Journal, 27 pages, 10 figures

Aims. Core-collapse supernovae may show significant polarization that implies non-spherically symmetric explosions. We observed the type II-plateau SN 2005af using optical polarimetry in order to verify whether any asphericity is present in the supernova temporal evolution. Methods. We used the IAGPOL imaging polarimeter to obtain optical linear polarization measurements in R (five epochs) and V (one epoch) broadbands. Interstellar polarization was estimated from the field stars in the CCD frames. The optical polarimetric monitoring began around one month after the explosion and lasted ~30 days, between the plateau and the early nebular phase. Results. The weighted mean observed polarization in R band was [1.89 +/- 0.03]% at position angle (PA) 54 deg. After foreground subtraction, the level of the average intrinsic polarization for SN 2005af was ~0.5% with a slight enhancement during the plateau phase and a decline at early nebular phase. A rotation in PA on a time scale of days was also observed. The polarimetric evolution of SN 2005af in the observed epochs is consistent with an overall asphericity of ~20% and an inclination of ~30 deg. Evidence for a more complex, evolving asphericity, possibly involving clumps in the SN 2005af envelope, is found., Comment: 6 pages, 5 figures, to be published A&A

We present the first optical circular and linear polarization measurements of two polar candidates from ROSAT: 1RXS J161008.0+035222 and 1RXS J231603.9-052713. We also present H band near-infrared photometry of the last object. The presence of strong circular polarization confirms them as AM Her systems. 1RXS J231603.9-052713 was observed in two different brightness states. The orbital phase dependence of the flux and polarization of 1RXS J161008.0+035222 is reasonably fitted with a simple model in which the binary is observed at a small inclination and the magnetic field axis is almost parallel to the white-dwarf rotation axis resulting in the accretion column axis being seen from top during the whole orbital revolution. The magnetic field is probably in the 10 to 20 MG range. The circular polarization of 1RXS J231603.9-052713 is complex and highly variable. The light-curves of that object have been fitted using a model which includes the white-dwarf, a heated secondary and a point-like accretion region. The secondary emission contributes significantly even in optical wavelengths. This model also reproduces the main features of the optical polarization of 1RXS J231603.9-052713. We estimate the main parameters of the binary, of the accretion region and the distance to the system. However, a more precise interpretation of this system should take in account an extended and inhomogeneous accreting region as well as non-radial accretion., Comment: MNRAS accepted

We have obtained linear polarization measurements of the Large Magellanic Cloud B[e] supergiant S 111 using optical imaging polarimetry. The intrinsic polarization found is consistent with the presence of an axisymmetric circumstellar envelope. We have additionally estimated the electron density for S 111 using data from the literature and revisited the correlation between polarization and envelope parameters of the B[e] supergiant stars using more recent IR calibration color data. The data suggest that the polarization can be indeed explained by electron scattering. We have used Monte Carlo codes to model the continuum polarization of the Magellanic B[e] supergiants. The results indicate that the electron density distribution in their envelopes is closer to a homogeneous distribution rather than an r$^{-2}$ dependence. At the same time, the data are best fitted by a spherical distribution with density contrast than a cylindrical distribution. The data and the model results support the idea of the presence of an equatorial disk and of the two-component wind model for the envelopes of the B[e] supergiants. Spectropolarimetry would help further our knowledge of these envelopes., Comment: 8 pages, 7 figures, accepted by Astronomy & Astrophysics

We present the first optical polarimetric measurements of RX J1141.3-6410 which confirm that star as a polar. The circular polarization varies between 0 and 13% with the orbital period. Halpha spectroscopy shows that this line is formed by, at least, two components: a broad and a narrow one. The phase of maximum redshift of the broad component is shifted by 0.5 with the phase of maximum circular polarization which is not usual for this class of stars. We suggest a geometrical configuration for the system which could explain the main features of the polarimetric and spectroscopic data., Comment: 6 pages + 3 figures. Accepted for publication on A&A

To elucidate the dust properties in the SMC we have for the first time measured linear polarization in five colors in the optical region of the spectrum for a sample of reddened stars. For two of these stars, for which there were no existing UV spectrophotometric measurements, but for which we measured a relatively large polarization, we have also obtained data from the International Ultraviolet Explorer (IUE) in order to study the extinction. The main results are: (1) the wavelength of maximum polarization, $\lambda_{max}$, in the SMC is typically smaller than that in the Galaxy; (2) however, AZV 456, which shows the UV extinction bump, has a $\lambda_{max}$ typical of that in the Galaxy, its polarization curve is narrower, its bump is shifted to shorter wavelengths as compared to the Galaxy and its UV extinction does not conform to the Galactic analytical interpolation curve based on the ratio of total to selective extinction; (3) the 'typical', monotonic SMC extinction curve can be best fit with amorphous carbon and silicate grains; (4) the extinction towards AZV456 may only be explained by assuming a larger gas-to-dust ratio than the observed N(HI)/A(V) value, with a small amount of the available carbon in graphite form; (5) from an analysis of both the extinction and polarization data and our model fits it appears that the SMC has typically smaller grains than those in the Galaxy., Comment: To appear in the ApJ, 50 pages, latex file

The typical extinction curve for the Small Magellanic Cloud (SMC), in contrast to that for the Galaxy, has no bump at 2175 A and has a steeper rise into the far ultraviolet. For the Galaxy the interpretation of the extinction and, therefore, the dust content of the interstellar medium has been greatly assisted by measurements of the wavelength dependence of the polarization. For the SMC no such measurements existed. Therefore, to further elucidate the dust properties in the SMC we have for the first time measured linear polarization with five colors in the optical region of the spectrum for a sample of reddened stars. For two of these stars, for which there were no existing UV spectrophotometric measurements, but for which we measured a relatively large polarization, we have also obtained data from the International Ultraviolet Explorer (IUE) in order to study the extinction. We also attempt to correlate the SMC extinction and polarization data. The main results are: the wavelength of maximum polarization, lambda(sub max), in the SMC is typically smaller than that in the Galaxy; however, AZC 456, which shows the UV extinction bump, has a lambda(sub max) typical of that in the Galaxy, but its polarization curve is narrower and its bump is shifted to shorter wavelengths as compared to the Galaxy; and from an analysis of both the extinction and polarization data it appears that the SMC has typically smaller grains than those in the Galaxy. The absence of the extinction bump in the SMC has generally been thought to imply a lower carbon abundance in the SMC compared to the Galaxy. We interpret our results to mean that te size distribution of the interstellar grains, and not only the carbon abundance, is different in the SMC as compared to the Galaxy. In Paper 2 we present dust model fits to these observations.

We study the dust in the Small Magellanic Cloud using our polarization and extinction data (Paper 1) and existing dust models. The data suggest that the monotonic SMC extinction curve is related to values of lambda(sub max), the wavelength of maximum polarization, which are on the average smaller than the mean for the Galaxy. On the other hand, AZV 456, a star with an extinction similar to that for the Galaxy, shows a value of lambda(sub max) similar to the mean for the Galaxy. We discuss simultaneous dust model fits to extinction and polarization. Fits to the wavelength dependent polarization data are possible for stars with small lambda(sub max). In general, they imply dust size distributions which are narrower and have smaller mean sizes compared to typical size distributions for the Galaxy. However, stars with lambda(sub max) close to the Galactic norm, which also have a narrower polarization curve, cannot be fit adequately. This holds true for all of the dust models considered. The best fits to the extinction curves are obtained with a power law size distribution by assuming that the cylindrical and spherical silicate grains have a volume distribution which is continuous from the smaller spheres to the larger cylinders. The size distribution for the cylinders is taken from the fit to the polarization. The 'typical', monotonic SMC extinction curve can be fit well with graphite and silicate grains if a small fraction of the SMC carbon is locked up in the grain. However, amorphous carbon and silicate grains also fit the data well. AZV456, which has an extinction curve similar to that for the Galaxy, has a UV bump which is too blue to be fit by spherical graphite grains.

We discuss simultaneous dust model fits to our extinction and polarization data for the Small Magellanic Cloud (SMC) using existing dust models. Dust model fits to the wavelength dependent polarization are possible for stars with small lambda(sub max). They generally imply size distributions which are narrower and have smaller average sizes compared to those in the Galaxy. The best fits for the extinction curves are obtained with a power law size distribution. The typical, monotonic SMC extinction curve can be well fit with graphite and silicate grains if a small fraction of the SMC carbon is locked up in the grains. Amorphous carbon and silicate grains also fit the data well.