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Polarimetric technique is an efficient tool to study the dust properties not only in the interstellar and intra-cluster medium, but also in circumstellar environments. We present new multi-band (BV(RI)C) optical polarimetric observations of six classical Be stars, HD259431, HD62532, HD45677, HD50138, BD+56 563 and BD+56 573, to diagnose the presence of dust in their circumstellar environments. The foreground dust distribution shows the presence of dust layers at distances of ∼ 200-500 pc. Five stars show an indication of possible intrinsic polarization and (or) rotation in their polarization angles. Especially, two stars HD45677 and HD50138, which show relatively large near-infrared excess with both (J-H) and (H-K) > 0.7 mag, exhibit more polarization at shorter wavelengths than that at longer wavelengths thereby indicating the presence of small dust grains in their circumstellar disk/envelope. Four stars namely HD259431, HD62532, HD45677 and HD50138 show a significant amount of intrinsic polarization indicating the presence of copious amount of dust in an asymmetric distribution e.g., in the form of disk or toroid. In the case of two CBe stars (BD+56 563 & BD+56 573) which are located in the star cluster χ Per, the observed polarization of these two stars seems to dominated by the foreground dust polarization. [ABSTRACT FROM AUTHOR]

RE J 0317-853 is an isolated magnetic white dwarf which is hot, highly magnetized and rapidly rotating compared to the rest of the magnetic white dwarf population. Due to an alleged age discrepancy estimated by its hot nature and his colder binary counterpart LB 9802, RE J 0317-853 was assumed to be a result of a binary merger. In this work we present the parallax measurements made with the Hubble Space Telescope's Fine Guidance System and the spectro-polarimetry from Anglo-Australian Observatory in order to reassess the evolutionary history of the binary system. Through the parallax measurement we determine the radius, mass and, furthermore, with the use of evolutionary models from the literature we estimate the cooling age. Additionally, we modelled the phase resolved flux and polarisation spectra of RE J 0317-853 and for the first time our magnetic models are able to account for the formerly unexplained feature with 16% of circular polarisation. We also discuss the effect of different possible core compositions and the effect of the uncertainty of the effective temperature; the value of the latter is extremely important for a correct interpretation of the evolutionary history of RE J 0317-853. A single star origin is possible if we assume that the effective temperature is at the cooler end of the possible range from 30000 K to 50000 K; if Teff is rather at the hotter end, a binary merger scenario for RE J 0317-853 becomes more plausible. [ABSTRACT FROM AUTHOR]

Cepheids are fundamental objects astrophysically in that they hold the key to a CMB independent estimate of Hubble’s constant. A number of researchers have pointed out the possibilities of breaking degeneracies between OmegaMatter and H0 if there is a CMB independent distance scale accurate to a few percent (Hu 2005). Current uncertainties in the distance scale are about 10% but future observations, with, for example, the JWST, will be capable of estimating H0 to within a few percent. A crucial step in this process is the Cepheid PL relation. Recent evidence has emerged that the PL relation, at least in optical bands, is nonlinear and that neglect of such a nonlinearity can lead to errors in estimating H0 of up to 2 percent. Hence it is important to critically examine this possible nonlinearity both observationally and theoretically. Existing PC/PL relations rely exclusively on evaluating these relations at mean light. However, since such relations are the average of relations at different phases. Here we report on recent attempts to compare theory and observation in the multiphase PC/PL planes. We construct state of the art Cepheid pulsations models appropriate for the LMC/Galaxy and compare the resulting PC/PL relations as a function of phase with observations. For the LMC, the (V-I) period-color relation at minimum light can have quite a narrow dispersion (0.2–0.3 mags) and thus could be useful in placing constraints on models. At longer periods, the models predict significantly redder (by about 0.2–0.3 mags) V-I colors. We discuss possible reasons for this and also compare PL relations at various phases of pulsation and find clear evidence in both theory and observations for a nonlinear PL relation. [ABSTRACT FROM AUTHOR]

We observed two fields near M32 with the ACS/HRC on board the Hubble Space Telescope, located at distances of about 1.8’ and 5.4’ (hereafter Fl and F2, respectively) from the center of M32. To obtain a very detailed and deep color-magnitude diagram (CMD) and to look for short period variability, we obtained time-series imaging of each field in 32-orbit-long exposures using the F435W (B) and F555W (V) filters, spanning a temporal range of 2 days per filter. We focus on our detection of variability on RR Lyrae variable stars, which represents the only way to obtain information about the presence of a very old population (larger than 10 Gyr) in M32 from optical data. Here we present results obtained from the detection of 31 RR Lyrae in these fields: 17 in Fl and 14 in F2. [ABSTRACT FROM AUTHOR]

RR Lyrae stars play an important role as distance indicators and stellar population tracers. In this context the construction of accurate pulsation models is crucial to understand the observed properties and to constrain the intrinsic stellar parameters of these pulsators. The physical mechanism driving pulsation in RR Lyrae stars has been known since the middle of the 20th century and many efforts have been performed during the last few decades in the construction of more and more refined pulsation models. In particular, nonlinear pulsation models including a nonlocal time-dependent treatment of convection, such as the ones originally developed in Los Alamos in the seventies, allow us to reproduce all the relevant observables of stellar radial pulsation and to establish accurate relations and methods to contrain the intrinsic stellar properties and the distances of these variables. The most recent results on RR Lyrae pulsation obtained through thiese kinds of models will be presented and a few still debated problems will be discussed. [ABSTRACT FROM AUTHOR]

This review examines progress on the Pop I, fundamental-mode Cepheid distance scale with emphasis on recent developments in geometric and quasi-geometric techniques for Cepheid distance determination. Specifically I examine the surface brightness method, interferometric pulsation method, and trigonometric measurements. The three techniques are found to be in excellent agreement for distance measures in the Galaxy. The velocity p-factor is of crucial importance in the first two of these methods. A comparison of recent determinations of the p-factor for Cepheids demonstrates that observational measures of p and theoretical predictions agree within their uncertainties for Galactic Cepheids. [ABSTRACT FROM AUTHOR]

A review is presented of the past 165 years of observation of the 4-day Cepheid Polaris, including the exciting results of the last 50 years, an interval that has produced three orbital solutions for the spectroscopic binary subsystem, recently resolved by HST, parameters for the optical companion, precise measurement of the star’s trigonometric parallax and angular diameter, evidence for a rapid increase in its pulsation period, and observations of the dramatic decline and recent partial recovery of its light amplitude. There has been considerable discussion about the exact nature of the star, with potential resolutions summarized here. It is also noted that many of the star’s characteristics are shared by a small number of other Cepheids that display rapid period increases identical to those predicted for stars in the first crossing of the instability strip, small light amplitudes, and intrinsic colors typical of variables lying near the center of the strip, where Cepheids of largest amplitude normally reside. While all members of the group appear to display the canonical traits of first crossers of the instability strip, Polaris has one unique peculiarity: a brief hiatus in its monotonic period increment between 1963 and 1966 during which the pulsation period underwent a dramatic decrease. Has the average brightness of the Cepheid also increased over the last millennium? [ABSTRACT FROM AUTHOR]

The pulsation parallax method, or Baade-Wesselink method (BW), is a powerful way to measure distances to Cepheids in a pseudo-geometric way. In the quest for obtaining the most precise distance using long baseline interferometry (we reached 1.5%), we obtained two maybe not so unsuspected results. First of all, our studies show that we reached a point where the assumption that the pulsating photosphere can be approximated using static models is not valid in the context of the BW method. Secondly, we unveiled the systematic presence of Circum Stellar Envelopes (CSE) at a few stellar diameters scale, as a slight near-infrared excess, which could be an indication that mass loss is currently taking place. Not only these two results represent biases to the BW method, and deserve to be observationally studied, they also shed new light on our knowledge of the Cepheids themselves and call for extensive modeling. [ABSTRACT FROM AUTHOR]

We report observational techniques, results, and Monte Carlo population analyses for a Spitzer IRAC substellar companion search around 89 M, L, and T dwarf systems. For the typical target star distance, the survey is sensitive to T-dwarf/Y-dwarf companions with temperatures down to ∼600 K. This survey therefore represents the most sensitive population study of mid-to wide-separation (greater than ∼40 AU) substellar companions to low mass stars and brown dwarfs. Based on IRAC color-color identfication, and archival ground-based data, we find that no system shows positive evidence of a substellar companion (searchable separation ∼3 arcsec to ∼3 arcmin; projected separation ∼40–2000 AU at the median target distance). Monte Carlo population analyses reveal that, for an example 1000 K substellar population, the 40–2000 AU semi-major-axis companion fraction is <=3.3%. For an example 633 K (Y-dwarf) companion population, the companion fraction is less than ∼20%, for the 200–800 AU semi-major axis range. To the best of our knowledge, this Y-dwarf value represents the first constraints ever placed on the Y-dwarf companion population, for this semi-major axis regime. [ABSTRACT FROM AUTHOR]

At distances of only 3.3 and 3.6 pc respectively, [variant_greek_epsilon] Eri and [variant_greek_epsilon] Ind are two of the closest solar-type stars. In addition, they both exhibit long-term radial velocity (RV) trends indicative of planetary companions at relatively large separations, making them interesting for the purpose of direct imaging. Previously, we have reported the results of a deep imaging campaign of [variant_greek_epsilon] Eri at 4 microns, which, in combination with dynamical data, led to the exclusion of any planets more massive than 3 Mjup anywhere in the system. Here, we will briefly review those results, as well as the results using the same method for [variant_greek_epsilon] Ind A, where the massive planet or low-mass brown dwarf implied by the RV trend was undetected, and similar constraints could be determined on the system parameters. [ABSTRACT FROM AUTHOR]

Because the Spitzer Space Telescope is in an Earth-trailing orbit, losing about 0.1 AU/yr, it is excellently located to perform microlens parallax observations toward the Magellanic Clouds (LMC/SMC) and the Galactic bulge. These yield the so-called “projected velocity” of the lens, which can distinguish statistically among different populations. A few such measurements toward the LMC/SMC would reveal the nature of the lenses being detected in this direction (dark halo objects, or ordinary LMC/SMC stars). Cool Spitzer has already made one such measurement of a (rare) bright red-clump source, but warm (presumably less oversubscribed) Spitzer could devote the extra time required to obtain microlens parallaxes for the more common, but fainter, turnoff sources. Warm Spitzer could observe bulge microlenses for 38 days per year, which would permit up to 24 microlens parallaxes per year. This would yield interesting information on the disk mass function, particularly old brown dwarfs, which at present are inaccessible by other techniques. Target-of-Opportunity (TOO) observations should be divided into RTOO/DTOO, i.e., “regular” and “disruptive” TOOs, as pioneered by the Space Interferometry Mission (SIM). LMC/SMC parallax measurements would be DTOO, but bulge measurements would be RTOO, i.e., they could be scheduled in advance, without knowing exactly which star was to be observed. [ABSTRACT FROM AUTHOR]

Galactic distance indicators, particularly the Fundamental Plane and the Tully-Fisher relations are discussed. These have both been widely used to measure large-scale motions of galaxies in the local universe and to extract cosmological information. However these indicators can be calibrated using nearby galaxies such as those in the local group and modelled using detailed N-body simulations and observations of stellar populations. These considerations provide a bridge between distant and nearby extragalactic objects and their stars. © 2005 American Institute of Physics [ABSTRACT FROM AUTHOR]

We show that DEBs can serve as excellent age and distance indicators for globular clusters. A short review of past surveys for eclipsing binaries in globular clusters is followed by presentation of the current status of the CASE project. A few selected systems from the CASE sample are discussed in some detail. © 2005 American Institute of Physics [ABSTRACT FROM AUTHOR]

The Baade-Wesselink method using VJK photometry was shown by Laney and Stobie (1995) to yield apparently precise results, free of obvious systematics with phase, surface gravity and microturbulence. Here we extend the empirical calibration of the p-factor [1] to a maximal sample of galactic classical Cepheids. HADS or ‘dwarf Cepheids’ have been shown to follow the same PL relation (at least for high-metallicity objects), and a HADS sample is therefore added in order to extend the period range and examine the variation of the p-factor with period. The derived pulsation parallaxes give a slope of 0.07±0.02, in good agreement with the calculations of Nardetto et al. [2]. Applying this period vs. p-factor relation to 68 galactic pulsation parallaxes gives a slope for the galactic PL relation in good agreement with the LMC PL relation. [ABSTRACT FROM AUTHOR]

We have estimated JHKs magnitudes corrected to mean intensity for LMC type II Cepheids found in the OGLE-III survey. Period-luminosity relations (PLRs) are derived in JHKs as well as in a reddening-free VI parameter. The BL Her stars (P<4 d) and the W Vir stars (P = 4 to 20 d) are co-linear in these PLRs. The slopes of the infrared relations agree with those found previously for type II Cepheids in globular clusters within the uncertainties. Using the pulsation parallaxes of V553 Cen and SW Tau, the data lead to an LMC modulus of 18.46±0.10 mag, uncorrected for any metallicity effects. We have now established the PLR of type II Cepheids as a distance indicator by confirming that (almost) the same PLR satisfies the distributions in the PL diagram of type II Cepheids in (at least) two different systems, i.e. the LMC and Galactic globular clusters, and by calibrating the zero point of the PLR. RV Tau stars in the LMC, as a group, are not co-linear with the shorter-period type II Cepheids in the infrared PLRs in marked contrast to such stars in globular clusters. We note differences in period distribution and infrared colors for RV Tau stars in the LMC, globular clusters and Galactic field. We also compare the PLR of type II Cepheids with that of classical Cepheids. [ABSTRACT FROM AUTHOR]

We report on simulations of different methods to estimate the number of pulsars for which the parallax can be measured with the SKA and to what distance a parallax can be measured. [ABSTRACT FROM AUTHOR]