Volume 12, Issue 4

In the last century photometry was recognized as a powerful method for investigation of physical parameters of stars and of interstellar matter. Photometric investigations now cover the spectral range from the far ultraviolet to the far infrared. The article describes the most important landmarks in development of photometric classification of stars and the number of photometric survey projects accomplished by ground-based and space telescopes and including millions of point sources. The success of the future LSSD and Gaia projects will depend on the use of optimum photometric passbands.

Gaia will obtain multi-color photometry for astrometric and astrophysical purposes. A photometry in five broad bands and with high angular resolution will be obtained after every astrometric observation of a star, especially for the sake of correcting slight astrometric chromaticity errors of the optical system. A separate smaller telescope will obtain photometry in eleven passbands of medium width designed to serve the astrophysical analysis of the mission results. The medium-band photometry (MBP) has less angular resolution, but much longer effective integration time than the broad-band photometry (BBP).

For Dante Alighieri (1265-1321) the spiritual cosmos contained the Heavens, Earth and Hell, and it was compatible with the physical cosmos known from Aristotle (384-322 B.C.). Dante’s many references in his Divine Comedy to physical and astronomical subjects show that he wanted to treat these issues absolutely correctly. Tycho Brahe proves three hundred years later by his observations of the Stella Nova in 1572 and of comets that the spheres of heavens did not really exist. It has ever since become more and more difficult to reconcile the ancient ideas of a unified cosmos with the increasing knowledge about the physical universe. Ptolemaios derived a radius of 20 000 Earth radii for the sphere of fixed stars. This radius of the visible cosmos at that time happens to be nearly equal to the true distance of the Sun, or 14 micro-light-years. Today the radius of the visible universe is a million billion (10 15 ) times larger than Tycho Brahe believed. The lecture gave an overview of astronomical distances and their dramatic change during two and a half millennia, the following text is an extract hereof.

The fundamental problem of long-term photometric monitoring of variable stars and of multi-wavelength photometric campaigns is the problem of bringing the data to a common standard. Such homogenization can be achieved only when the measurements are made in photometric systems that are truly transformable. This fundamental problem is of a technical nature, and photometric observers, sometimes, are not aware of the problems. This frequently leads to over-interpretation of the data.

The Whole Earth Telescope (WET) saw first light in 1988. It was invented by scientists from the Astronomy Department, University of Texas at Austin. The idea was to generate a world-wide network of cooperating astronomical observatories to obtain uninterrupted time-series measurements of some variable stars. The technological goal was to resolve the multi-periodic oscillations in these objects into their individual components; the scientific goal was to construct accurate theoretical models of the target objects, constrained by their observed behavior, from which fundamental astrophysical parameters could be derived. This approach has been extremely successful, and has placed stellar seismology at the forefront of stellar astrophysics. The network is run as a single astronomical instrument with many operators, and the collaboration includes scientists from all continents on our planet, taking part in the observations, data reduction, analysis and theoretical interpretation. The expertise of Lithuanian astronomers in photometry, and their access to the observing station Mt. Maidanak in Uzbekistan, has been important for the success of the network.

CCD photometry of standard areas of the seven-color Strömvil photometric system in some open and globular clusters is described. The primary standards are being measured with a two-channel photoelectric photometer on the 1.5 meter telescope on Mt. Lemmon. CCD photometry of standard areas is in progress with the Vatican Advanced Technology Telescope on Mt. Graham and with the CASLEO 2.1 m telescope in Argentina. Two additional Strömvil projects are described.

The importance of determining the error of the flat field in CCD photometry is detailed and our methods of doing this are described. We now have reached a precision of 1-1.5 % in our photometry. Color-magnitude diagrams of the open cluster M67 (ours and Laugalys et al. 2003) are compared.

We briefly describe a method of organizing all the star field exposures taken in a complete CCD observational run. By a “CommandLog” (to be published in J. Astron. Data) one can organize all the observations and process them within IRAF in an orderly, automated manner to arrive at stellar photometry calibrated into a standard photometric system (in our case, Strömvil). During the course of this processing we determine the error of the flat and try to correct it.

A calibration of color indices of the Strömvil photometric system in terms of physical parameters of stars is presented. The calibration is based on photoelectric photometry of about 1000 stars with known effective temperatures, distances, gravities and metallicities. The observations were accomplished in 2000-2003 with the 1.5 meter telescope of the Steward Observatory. The distances were taken from the Hipparcos catalog, and T e , log g, [Fe/H]) were collected from several literature sources and averaged.

The accuracy of new CCD photometry in the Vilnius system of the M67 cluster is analyzed. The observational material is obtained during six observing runs in 1994-2001 with the 1 meter telescope of the USNO Flagstaff Station. The main task was to establish CCD standards of high accuracy and to eliminate large-scale errors from our CCD photometry. We compare our results with the published CCD photometric data in other photometric systems. The comparison reveals considerable systematic errors in some datasets.

A short historical review of the development of astrophotometry in Russia and the Soviet Union is given. The input of V. Cerasky, G. Tikhov, V. B. Nikonov, Moscow University astronomers and Vilnius astronomers is described.

We present a new color index vs. absolute magnitude calibration of 2MASS JHK photometry. For the A0 to ~G5 and M segments of the main sequence information on the amount of interstellar extinction and its location in space may be obtained.

Under some assumptions, information on interstellar extinction may be obtained from modern large photometric survey data. Virtual Observatory facilities allow users to make a fast and correct cross-identification of objects from various surveys. This process yields multicolor photometry data for registered objects and makes it possible to estimate color excesses and, with a few exceptions for some areas with high total-to-selective extinction ratio, calculate interstellar extinction. A 3D extinction map then can be constructed for almost the whole sky. The method is applied to an area with the low Galactic latitude, and the results are discussed in the present paper.

The results of the study of variations of total-to-selective extinction R V near the Galactic plane (-12° ≤ b ≤+12°) are presented. It is shown that variations of R V range from 1.9 to 3.4 and it is not possible to determine its typical value for the Galaxy. The conclusion that variations of the parameter R V closely correlates with the distribution of interstellar extinction is confirmed.

ESA’s Gaia mission will provide accurate astrometric, photometric and spectroscopic data for millions of stars in the Galaxy and beyond. We discuss the prospects of using this vast amount of astrophysical information for understanding star formation histories in the Galaxy and its immediate neighbors within the Local Group.

The sensitivity of stellar spectra to C, N, O and α-process element abundances is discussed with the aim of taking this effect into account when selecting a photometric system for the Gaia orbiting observatory. On the basis of a spectrometric, photometric and theoretical study of spectra of evolved first-ascent giants and clump stars in the open cluster NGC 7789 it is demonstrated that evolutionary alterations of carbon and nitrogen abundances can cause noticeable spectral changes and, if not taken into account, may yield misleading photometric [Fe/H] determinations. Carbon features in stellar atmospheres show a particularly complex behavior being dependent on mixing processes in stars, on the stellar surface gravity and on the abundance of oxygen which can also be altered by different reasons. NH bands could serve for the evaluation of mixing processes in stars and the interpretation of carbon dominated spectral regions. Abundances of α-process elements can be evaluated photometrically by using the direct indicators - Ca II H and K lines and Mg I b triplet.

Metallicities and distances have been determined from Vilnius photometry for an in situ sample of nearly 650 stars in eight proper-motion fields at high Galactic latitudes. For half of these stars, radial velocities have been measured with the CORAVEL spectrometer, which allowed us to derive spatial velocities and Galactic orbits. In this contribution we present a status report on our results for the stellar content of the Galactic disk, with emphasis given to specific aspects of distinguishing the thick-disk stars from the old thin-disk population.

Methods used for the quantitative classification of metal-deficient stars in the Vilnius photometric system are reviewed. We present a new calibration of absolute magnitudes for dwarfs and subdwarfs, based on Hipparcos parallaxes. The new classification scheme is applied to a sample of Population II visual binaries.

The paper presents the experience, observations and results obtained with the new 50 cm photometric telescope at the Stará Lesná Observatory equipped with an SBIG ST10MXE CCD camera. The telescope time is devoted to observations of variable stars of various types. Prospects for further improvement of the instrument, observational and reduction techniques are discussed.

A new approach for the search of microvariability and high-speed phenomena in variable stars is presented. It gives a new technology of observation and analysis of the data. Statistical photometry stands on the three pillars: the digital filtering technique, the photon counting statistics based on the Mandel equation, and the integral transforms of the light curves. This technique permits to investigate low-amplitude and rapid variability of stars.

We describe the efforts of the Taiwan-America Occultation Survey (TAOS) project to develop a data acquisition and analysis scheme for fast CCD imaging photometry. The TAOS project aims to conduct a census of the Kuiper-belt objects (KBOs) by detecting chance stellar occultation events by these small bodies in the outer reach of the solar system. An array of telescopes, each with fast optics (f/2) of 0.5 m aperture and equipped with a 2K CCD camera (3 square degrees FOV), have been set up in central Taiwan to monitor a couple thousand stars simultaneously. By reading out the CCD chip sequentially a few rows of pixels at a time (pause-and-shift), it is possible to achieve stellar photometry with a sampling rate up to several hertz. Here we report how such a setup has been used to observe the SX Phoenicis type variable CY Aqr to illustrate the potential usefulness of the TAOS database in stellar variability studies.

A short description of participation of the Lithuanian astronomers in the Whole Earth Telescope program is given.

A short description is presented of the instrumentation used at the Molėtai Observatory (Lithuania) for the Whole Earth Telescope collaboration and an overview of the observation campaigns in which the Vilnius WET group took part.

It is demonstrated how the length of a continuous time-series of photometric data and their gaps affect the power spectrum of a Fourier Transform of the data. It is shown that second order effects of the atmospheric extinction - the dependence of the extinction coefficient on spectral type of a star and on the air mass - cannot be ignored. The ignoring of these effects results in aliases at frequencies lower than 200 μHz. A modification of the extinction correction procedure is proposed.

A technique for discovering variable stars based on the calculation of the correlation coefficients is proposed. Applications of the technique are shown on the results of numerical experiments and on the Hipparcos photometric data.

The large variety of long-term (secondary) variations is demonstrated on the basis of a 30 year long photographic red magnitude time-series for 18 carbon stars in Cygnus based on the observations carried out with the Baldone Schmidt telescope.

We investigate the effect on U,B,V magnitudes of the removal of emission lines from the spectra of some symbiotic stars and novae during their nebular phases. We approach this problem by the precise reconstruction of the composite UV/optical continuum and the line spectrum. The corrections ΔU, ΔB and ΔV are determined from the ratio of fluxes with and without emission lines. We demonstrate the effect for symbiotic nova V1016 Cyg during its nebular phase. We find that about 68%, 78% and 66% of the observed flux in the U, B and V filters is radiated in the emission lines. The effect should be taken into account before using the observed color indices of emission-line objects for diagnosis of their radiation in the continuum.

Long-term photographic, photoelectric and recent CCD photometry of the classical nova V723 Cas and symbiotic novae V1329 Cyg, PU Vul, V1016 Cyg and HM Sge were used to find their orbital periods. The arguments in favor of the presence of the third components in these systems are given. Physical processes, responsible for the brightness variations, are discussed.

The J-H, H-K diagram based on the 2MASS Sky Survey for 140 carbon stars in Cygnus, which have been photometrically monitored with the Baldone Schmidt telescope, is discussed.

A M V ,B−V diagram for metal-deficient, kinematically selected thick disk and halo visual binary stars with known orbits is presented. The magnitude differences Δm between the components are analyzed.

The period changes of the SC-type Mira variable LX Cygni are analyzed.

Three medium-band photometric systems proposed for the Gaia space mission are intercompared in determining color excesses for stars of spectral classes from O to M at V = 18 mag. A possibility of obtaining a three-dimensional map of the interstellar extinction is discussed.

The results of search for the optimal location of ultraviolet passbands for an extra-atmospheric photometric system are given. The addition of passbands near the well-known extinction bump at 218 nm to the photometric system makes it possible to determine interstellar extinction with greater precision than with the photometric system without UV passbands.

We present recent photometric observations of Z And, BF Cyg, and AX Per made in the standard Johnson UBVR system. The main characteristics of their light behavior can be summarized as follows: Z And: our observations cover the recent active phase, from its maximum in 2000 December to the latest measurements (2003 September). BF Cyg: the minimum of the recent wave in the star’s brightness was the deepest ever observed; in 2003 February a short-term flare developed in the light curve. AX Per: from 1995 October the light curve displays signatures of a quiescent phase of the star; in 2003 May a 0.5 mag flare was detected.

We present a preliminary study of a fragment of the radial color profile of the spiral galaxy M 31 in terms of 2-D model accounting for internal extinction in the disk. The two stellar population disk model was assumed. The old dust-free disk population is represented by the double exponential law, and the young disk population, well mixed with the dust, resides in spiral arms of various scale-heights. We find a good agreement among the radial color B-R profiles produced by this simple model and the profile measured around the spiral arm S4 of M 31.

The errors of Gaia photometry originating from tolerancies of the light filter mean wavelengths and half-widths are investigated.

We briefly describe the dual-star photometer designed by P. Kalv, a former director of Tallinn Observatory (1934-2002) for multicolor photometry of close binaries and intrinsic variable stars.

The results of the first testing of a new CCD camera of the Molėtai Observatory are given. The linearity and the flat field corrections of good accuracy are determined by using shifted star field exposures.

We give overview of the CCD instrumentation and data reduction techniques used at the Tartu Observatory. The first results from photometric observations of the peculiar variable V838 Mon are presented.

An overview of the early history of the Astronomical Observatory of Vilnius University from its foundation in 1753 to 1878 is presented.