arXiv subjects

Piotr Popowski

Publications and source records attributed to Piotr Popowski.

At least 19 recordsLinked to original sources

arXiv2007

The Extinction Toward the Galactic Bulge from RR Lyrae Stars

We present mean reddenings toward 3525 RR0 Lyrae stars from the Galactic bulge fields of the MACHO Survey. These reddenings are determined using the color at minimum $V$-band light of the RR0 Lyrae stars themselves and are found to be in general agreement with extinction estimates at the same location obtained from other methods. Using 3256 stars located in the Galactic Bulge, we derive the selective extinction coefficient $R_{V,VR}=A_V/E(V-R) = 4.3 \pm 0.2$. This value is what is expected for a standard extinction law with $R_{V,BV} = 3.1 \pm 0.3$.

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arXiv2005

A test for the universality of extinction curve shape

We present an analysis of 436 lines of sight with extinction data covering wavelength range from near-infrared (NIR) to ultraviolet (UV). We use J, H, K photometry from 2MASS database, the IR intrinsic colors from Wegner (1994), and UV extinction data from Wegner (2002). We exclude 19 lines of sight (4%) from the original sample because of suspected photometric problems. We derive total to selective extinction ratios (R_V) based on the Cardelli, Clayton, and Mathis (1989; CCM) law, which is typically used to fit the extinction data for both diffuse and dense interstellar medium. We conclude that CCM law is able to fit well most of the extinction curves in our sample (71%), and we present a catalog of R_V and visual extinction (A_V) values for those cases. We divide the remaining lines of sight with peculiar extinction into two groups according to two main behaviors: a) the NIR or/and UV wavelength regions cannot be reproduced by CCM formula (14% of the entire sample), b) the NIR and UV extinction data taken separately are best fit by CCM laws with significantly different values of R_V (10% of the entire sample). We present examples of such curves. We caution that some peculiarities of the extinction curves may not be intrinsic but simply caused by faulty data. The study of the intrinsically peculiar cases could help us to learn about the physical processes that affect dust in the interstellar medium, e.g., formation of mantles on the surface of grains, evaporation, growing or shattering.

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arXiv2005

Can CCM law properly represent all extinction curves?

We present the analysis of a large sample of lines of sight with extinction curves covering wavelength range from near-infrared (NIR) to ultraviolet (UV). We derive total to selective extinction ratios based on the Cardelli, Clayton and Mathis (1989, CCM) law, which is typically used to fit the extinction data both for diffuse and dense interstellar medium. We conclude that the CCM law is able to fit most of the extinction curves in our sample. We divide the remaining lines of sight with peculiar extinction into two groups according to two main behaviors: a) the optical/IR or/and UV wavelength region cannot be reproduced by the CCM formula; b) the optical/NIR and UV extinction data are best fit by the CCM law with different values of R_V. We present examples of such curves. The study of both types of peculiar cases can help us to learn about the physical processes that affect dust in the interstellar medium, e.g., formation of mantles on the surface of grains, evaporation, growing or shattering.

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arXiv2004

Total to Selective Extinction Ratios and Visual Extinctions from Ultraviolet Data

We present determinations of the total to selective extinction ratio R_V and visual extinction A_V values for Milky Way stars using ultraviolet color excesses. We extend the analysis of Gnacinski and Sikorski (1999) by using non-equal weights derived from observational errors. We present a detailed discussion of various statistical errors. In addition, we estimate the level of systematic errors by considering different normalization of the extinction curve adopted by Wegner (2002). Our catalog of 782 R_V and A_V values and their errors is available in the electronic form on the World Wide Web.

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arXiv2003

A Test for the Origin of Quasar Redshifts

It is commonly accepted that quasar redshifts have a cosmological character and that most of the quasars are at Gigaparsec distances. However, there are some cases where several quasars with completely different redshifts and a nearby active galaxy are aligned in a certain way or occupy a very small patch on the sky, which is claimed by some authors to be unlikely to happen by chance. Is there a small subset of quasars with non-cosmological redshifts? For quasars apparently associated with galaxies, we consider two scenarios for the origin of their redshift: 1. a standard, cosmological scenario, 2. a velocity-induced Doppler shift of a nearby object's spectrum (local, ejection scenario). We argue for a simple astrometric test which can distinguish between these two sources of quasar redshifts by constraining their proper motions. We give the predictions for the maximum possible proper motions of a quasar for the cosmological and local scenarios of the origin of their redshifts. We apply these theoretical results to the Bukhmastova (2001) catalog, which contains more than 8000 close QSO-galaxy associations. In the standard interpretation of quasar redshifts, their typical proper motions are a fraction of micro arc-second, and beyond the reach of planned astrometric missions like GAIA and SIM. On the other hand, the quasars ejected from local AGNs at velocities close to the speed of light would have proper motions 5-6 orders of magnitude larger, which would easily be measurable with future astrometric missions. The distributions of proper motions for the cosmological and local scenarios are very well separated. Moreover, the division corresponds nicely to the expected accuracy from GAIA and SIM.

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arXiv2003

The Large-Scale Extinction Map of the Galactic Bulge from the MACHO Project Photometry

We present a (V-R)-based reddening map of about 43 square degrees of the Galactic bulge/bar. The map is constructed using template image photometry from the MACHO microlensing survey, contains 9717 resolution elements, and is based on (V-R)-color averages of the entire color-magnitude diagrams (CMDs) in 4 by 4 arc-minute tiles. The conversion from the observed color to the reddening follows from an assumption that CMDs of all bulge fields would look similar in the absence of extinction. Consequently, the difference in observed color between various fields originates from varying contribution of the disk extinction summed along different lines of sight. We check that our (V-R) colors correlate very well with infrared and optical reddening maps. We show that a dusty disk obeying a cosec|b| extinction law, E(V-R) = 0.0274 cosec|b|, provides a good approximation to the extinction toward the MACHO bulge/bar fields. The large-scale (V-R)-color and visual extinction map presented here is publicly available in the electronic edition of the Journal and on the World Wide Web.

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arXiv2002

Low Microlensing Optical Depth Toward the Galactic Bar

I make a new evaluation of the microlensing optical depth toward the Galactic bar from Difference Image Analysis (DIA) of the MACHO Collaboration. First, I present supplementary evidence that MACHO field 104 located at (l,b) = (3.11,-3.01) is anomalous in terms of the event duration distribution. I argue that both the event durations and the very high optical depth of field 104 are not representative and, therefore, exclude this field as an outlier. In addition, I eliminate field 159 at (l,b) = (6.35,-4.40) based mainly on its separate location, but also on unexplained statistical properties of the event durations. The remaining six DIA fields form a very homogeneous and spatially compact set that is very suitable for averaging. The weighting of the optical depth values for these six DIA fields results in a total optical depth tau_{tot} = 2.01^{+0.34}_{-0.32} x 10^{-6} at (l,b) = (2.22,-3.18). If a fraction of all sources, f_{disk}, assumed to be in the disk, does not contribute to microlensing, then the optical depth toward the sources in the bar is tau_{bar} = 2.23^{+0.38}_{-0.35} x 10^{-6} 0.9/(1-f_{disk}). Both tau_{tot} and tau_{bar} are substantially lower than the original estimates of Alcock et al. Most of the change in the DIA-based optical depths comes from a more appropriate statistical treatment of the results in individual fields and not from the removal of fields 104 and 159. When taken together with tau_{bar} = (1.4 +/- 0.3) x 10^{-6} at (l,b) = (3.9,-3.8) as derived from clump giants, this new result suggests that the conclusions from microlensing experiments are in reasonable agreement with the expectations from infrared-based Galactic models.

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arXiv2002

Correcting Parameters of Events Based on the Entropy of Microlensing Ensemble

We entertain the idea that robust theoretical expectations can become a tool in removing hidden observational or data-reduction biases. We illustrate this approach for a specific problem associated with gravitational microlensing. Using the fact that a group is more than just a collection of individuals, we derive formulae for correcting the distribution of the dimensionless impact parameters of events, u_min. We refer to the case when undetected biases in the u_min distribution can be alleviated by multiplication of impact parameters of all events by a common constant factor. We show that in this case the general maximum likelihood problem of solving an infinite number of equations reduces to two constraints, and we find an analytic solution. Under the above assumptions, this solution represents a state in which the ``entropy'' of a microlensing ensemble is at its maximum, that is, the distribution of u_min resembles a specific, theoretically expected, box-like distribution to the highest possible extent. We also show that this technique does not allow one to correct the parameters of individual events on the event by event basis independently from each other.

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arXiv2000

Harmonizing the RR Lyrae and Clump Distance Scales - Stretching the Short Distance Scale to Intermediate Ranges?

I explore the consequences of making the RR Lyrae and clump giant distance scales consistent in the solar neighborhood, Galactic bulge and Large Magellanic Cloud (LMC). I employ two major assumptions: 1) that the absolute magnitude - metallicity, M_V(RR) - [Fe/H], relation for RR Lyrae stars is universal, and 2) that absolute I-magnitudes of clump giants, M_I(RC), in Baade's Window are known (e.g., can be inferred from the local Hipparcos-based calibration or theoretical modeling). A comparison between the solar neighborhood and Baade's Window sets M_V(RR) at [Fe/H] = -1.6 in the range (0.59 +/- 0.05, 0.70 +/- 0.05), somewhat brighter than the statistical parallax solution. More luminous RR Lyrae stars imply younger ages of globular cluster, which would be in better agreement with the conclusions from the currently favored stellar evolution and cosmological models. A comparison between Baade's Window and the LMC sets the M_I^{LMC}(RC) in the range (-0.33 +/- 0.09, -0.53 +/- 0.09). The distance modulus to the LMC, mu^{LMC}, is between 18.24 +/- 0.08 and 18.44 +/- 0.07. Unlike M_I^{LMC}(RC), this range in mu^{LMC} does NOT depend on the adopted value of the dereddened LMC clump magnitude, I_0^{LMC}(RC). I argue that the currently available information is insufficient to select the correct distance scale with high confidence.

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arXiv2000

The Distance to the Large Magellanic Cloud

I demonstrate that the two unexpected results in the local Universe: anomalous intrinsic (V-I)_0 colors of RR Lyrae stars and clump giants in the Galactic center, and very short distances to Magellanic Clouds inferred from clump giants, can be at least partially resolved with a modified coefficient of selective extinction A_V/E(V-I). With this modification, I find a new clump-giant distance modulus to the Large Magellanic Cloud, mu_{LMC} = 18.27 +/- 0.07, which is 0.09 larger than the Udalski (1998b) result. When distance estimates from the red clump, RR Lyrae stars and the eclipsing binary HV2274 are combined, one obtains mu_{LMC} = 18.31 +/- 0.04 (internal).

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arXiv1999

Clump Distance to the Magellanic Clouds and Anomalous Colors in the Galactic Bulge

I demonstrate that the two unexpected results in the local Universe: 1) anomalous intrinsic (V-I)_0 colors of the clump giants and RR Lyrae stars in the Galactic center, and 2) very short distances to the Magellanic Clouds (LMC, SMC) as inferred from clump giants, are connected with each other. The (V-I)_0 anomaly is partially resolved by using the photometry from the phase-II of the Optical Gravitational Lensing Experiment (OGLE) rather than phase-I. The need for V- or I-magnitude-based change in the bulge (V-I)_0 is one option to explain the remaining color discrepancy. Such change may originate in a coefficient of selective extinction A_V/E(V-I) smaller than typically assumed. Application of the (V-I)_0 correction (independent of its source) doubles the slope of the absolute magnitude - metallicity relation for clump giants, so that M_I(RC) = -0.23 + 0.19[Fe/H]. Consequently, the estimates of the clump distances to the LMC and SMC are affected. Udalski's (1998c) distance modulus of mu_{LMC} = 18.18 +/- 0.06 increases to mu_{LMC} = 18.27 +/- 0.07. The distance modulus to the SMC increases by 0.12 to mu_{SMC} = 18.77 +/- 0.08. I argue that a more comprehensive assessment of the metallicity effect on M_I(RC) is needed.

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arXiv1998

Anomaluos RR Lyrae (V-I)_0 colors in Baade's Window

We compare (V-I)_0-(V-K)_0 color-color and (V-I)_0-log P period-color diagrams for Baade's Window and local RRab Lyrae stars. We find that for a fixed log P the Baade's Window RR Lyrae stars are ~0.17 magnitudes redder in (V-I)_0 than the local RR Lyrae stars. We also show that there is no such effect observed in (V-K)_0. We argue that an extinction misestimate towards Baade's Window is not a plausible explanation of the discrepancy. Unlike Baade's Window RR Lyrae stars, the local ones follow a black-body color-color relation and are well approximated by theoretical models. We test two parameters, metallicity and surface gravity, and find that their effects are too small to explain the (V-I)_0 discrepancy between the two groups of stars. We do not provide any explanation for the anomalous (V-I)_0 behavior of the Baade's Window RR Lyrae stars. We note that a similar effect for clump giant stars has been recently reported by Paczynski and we caution that RR Lyrae stars and clump giants, often used as standard candles, can be subject to the same type of systematics.

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arXiv1998

The RR Lyrae Distance Scale

We review seven methods of measuring the absolute magnitude M_V of RR Lyrae stars in light of the Hipparcos mission and other recent developments. We focus on identifying possible systematic errors and rank the methods by relative immunity to such errors. For the three most robust methods, statistical parallax, trigonometric parallax, and cluster kinematics, we find M_V (at [Fe/H] = -1.6) of 0.77 +/- 0.13, 0.71 +/- 0.15, 0.67 +/- 0.10. These methods cluster consistently around 0.71 +/- 0.07. We find that Baade-Wesselink and theoretical models both yield a broad range of possible values (0.45-0.70 and 0.45-0.65) due to systematic uncertainties in the temperature scale and input physics. Main-sequence fitting gives a much brighter M_V = 0.45 +/- 0.04 but this may be due to a difference in the metallicity scales of the cluster giants and the calibrating subdwarfs. White-dwarf cooling-sequence fitting gives 0.67 +/- 0.13 and is potentially very robust, but at present is too new to be fully tested for systematics. If the three most robust methods are combined with Walker's mean measurement for 6 LMC clusters, V_{0,LMC} = 18.98 +/- 0.03 at [Fe/H] = -1.9, then mu_{LMC} = 18.33 +/- 0.08.

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arXiv1998

Systematics of RR Lyrae Statistical Parallax III: Apparent Magnitudes and Extinctions

We sing the praises of the central limit theorem. Having previously removed all other possible causes of significant systematic error in the statistical parallax determination of RR Lyrae absolute magnitudes, we investigate systematic errors from two final sources of input data: apparent magnitudes and extinctions. We find corrections due to each of ~0.05 mag, i.e., ~1/2 the statistical error. However, these are of opposite sign and so roughly cancel. The apparent magnitude system that we previously adopted from Layden et al. was calibrated to the photometry of Clube & Dawe. Using Hipparcos photometry we show that the Clube & Dawe system is ~0.06 mag too bright. Extinctions were previously pinned to the HI-based map of Burstein & Heiles. We argue that A_V should rather be based on new COBE/IRAS dust-emission map of Schlegel, Finkbeiner & Davis. This change increases the mean A_V by ~0.05 mag. We find M_V=0.77 +/- 0.13 at [Fe/H]=-1.60 for a pure sample of 147 halo RR Lyraes, or M_V=0.80 +/- 0.11 at [Fe/H]=-1.71 if we incorporate kinematic information from 716 non-kinematically selected non-RR Lyrae stars from Beers & Sommer-Larsen. These are 2 and 3 sigma fainter than recent determinations of M_V from main sequence fitting of clusters using Hipparcos measurements of subdwarfs by Reid and Gratton et al. Since statistical parallax is being cleared of systematic errors and since the chance of a >2 sigma statistical fluctuation is <1/20, we conclude that these brighter determinations may be in error. In the course of three papers, we have corrected 6 systematic errors whose absolute values total 0.20 mag. Had these, contrary to the expectation of the central limit theorem, all lined up one way, they could have resolved the conflict in favor of the brighter determinations. In fact, the net change was only 0.06 mag.

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arXiv1998

The Hipparcos Proper Motions in Support of the Short RR Lyrae Distance Scale

In this paper we investigate whether a misestimate of proper motions could have been a source of substantial systematic errors in the statistical parallax determination of the absolute magnitude of RR Lyrae stars. In an earlier paper, we showed that the statistical parallax method is extremely robust and rather insensitive to various systematic effects. The main potential problem with this method would therefore arise from systematically bad observational inputs, primarily radial velocities and proper motions. In that paper, we demonstrated that the radial velocities have not been systematically misestimated. Here we turn our attention to proper motions. We compare three different catalogs of proper motions --- Lick, Hipparcos and the one compiled by Wan et al. (WMJ). We find that the WMJ catalog is too heterogeneous to be a reliable source. We analyze the sample of 165 halo RR Lyrae stars with either Lick or Hipparcos proper motions. For the stars with both Lick and Hipparcos proper motions we use the weighted means of reported values. Various possible biases are investigated through vigorous Monte Carlo simulations and we evaluate small corrections due to Malmquist bias, anisotropic positions of the stars on the sky, and non-Gaussian distribution of stellar velocities. The mean RR Lyrae absolute magnitude is M_V = 0.74 +/- 0.12 at the mean metallicity of the sample <[Fe/H]> = -1.60, only 0.01 mag brighter than the value obtained in the previous study which did not incorporate Hipparcos proper motions. The faint absolute magnitudes of RR Lyrae stars confirmed by this analysis gives strong support to the short distance scale.

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arXiv1998

Mathematics of Statistical Parallax and the Local Distance Scale

We present a mathematical analysis of the statistical parallax method. The method yields physical insight into the maximum-likelihood determinations of the luminosity and velocity distribution and enables us to conduct a vigorous Monte Carlo investigation into various systematic effects. We apply our analytic formalism to the RR Lyrae sample of Layden et al. The velocity distribution of RR Lyrae stars is highly non-Gaussian, with kurtoses K_π= 2.04, K_θ= 3.22 and K_z = 4.28 in the three principal directions, but this has almost no effect on either the best fit or the uncertainty of the luminosity determination. Indeed, our principal result is that the statistical parallax method is extremely robust in the face of all systematic effects that we considered. Our analysis, applied to the Layden et al. RR Lyrae sample, strictly confirms the majority of their results. The mean RR Lyrae absolute magnitude is M_V = 0.75 +/- 0.13 at the mean metallicity of the sample <[Fe/H]> = -1.61, compared to M_V = 0.71 +/- 0.12 obtained by Layden et al. Most of the difference is due to Malmquist bias which was not considered in previous studies. We also analyze a semi-independent non-kinematically selected sample of stars with metallicities at the [Fe/H] <= 1.5 taken from Layden et al. and Beers & Sommer-Larsen and obtain M_V = 0.79 +/- 0.12 at <[Fe/H]> = -1.79. Additionally, this analysis yields measurements of the radial bulk motion (4 +/- 10 km/s) and vertical bulk motion (0 +/- 6 km/s) of the halo relative to the Local Standard of Rest.

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arXiv1997

The Proper Motion of NGC 6522 in Baade's Window

We have detected seven stars with a common proper motion which are located within 2.5 arcminutes of the globular cluster NGC 6522 in the Baade's Window field of the Galactic bulge. We argue that these stars are members of the cluster, and derive a weighted mean proper motion and heliocentric radial velocity of mu_l = 1.4 +/- 0.2 mas/yr, mu_b = -6.2 +/- 0.2 mas/yr, v = -28.5 +/- 6.5 km/s. We rederive the distance to NGC 6522 (0.91 +/- 0.04 R_0, where R_0 is the Galactocentric distance) and metallicity ([Fe/H] = -1.28 +/- 0.12), making use of recent revisions in the foreground extinction toward the cluster (A_V = 1.42 +/- 0.05). We find the spatial velocity of the cluster and conclude that the cluster stays close to the Galactic center, and may have experienced significant bulge/disk shocking during its lifetime.

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arXiv1997

The Zero Point of Extinction Toward Baade's Window

We measure the zero point of the Stanek (1996) extinction map by comparing the observed (V-K) colors of 206 K giant stars with their intrinsic (V-K)_0 colors as derived from their Hβindices. We find that the zero point of the Stanek map should be changed by ΔA_V = -0.10 +/- 0.06 mag, obtaining as a bonus a three-fold reduction of the previous statistical error. The most direct way to test for systematic errors in this determination would be to conduct a parallel measurement based on the (V-K) colors of RR Lyraes (type ab).

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