Johnson BV Research Articles

A GALEX Far-ultraviolet Study of Dwarf Stars in Moving Groups

Far-ultraviolet (FUV) GALEX photometry of main-sequence stars in moving groups is used to verify that there is a relationship between age and FUV broadband magnitude among such stars. Samples of dwarf stars were compiled for twelve moving groups and a search made for both GALEX FUV magnitudes on the AB system and Johnson BV photometry. These data sets were used to calculate a (FUV − B) color index for each star, which in turn was plotted against B − V in a two-color diagram. Based on the loci that the group dwarfs define in this two-color space, there is strong evidence that among candidate members of the Hyades, HR 1614, Wolf 630, Ursa Major, Arcturus, AB Doradus, η Cephei, β Pictoris, and Tucana–Horologium moving groups there are populations of coeval stars. Evidence for coevality is weaker in the case of the ζ Herculis group, and no conclusion can be drawn for candidate stars that have previously been assigned to the Kapteyn's Star and σ Puppis groups. Polynomials of up to fifth order were fitted to the two-color loci of the dwarfs in each group. These fits were used to calculate fiducial values of (FUV − B) throughout a range in B − V color on the main sequence for most of the groups studied here. Evidence is found that at a fixed B − V there is a logarithmic dependence of (FUV − B) color on stellar age for FGK dwarfs. The relationships presented herein could potentially serve as a tool for estimating the ages of late-type Population I dwarf stars.

Far Ultraviolet Photometry of F and G Dwarfs in the Hyades, Coma Berenices, and NGC 2281 Open Clusters as Obtained by GALEX

A two-color diagram formed from GALEX FUV and Johnson BV photometry is presented for F and early G dwarfs in three open clusters: the Hyades, Coma Berenices, and NGC 2281. Denoted as (mFUV − B) versus B − V, the position of dwarf stars in such a diagram has been shown by previous work to vary with the level of chromospheric and coronal activity. The upper main sequences of the Hyades and Coma Ber clusters match quite well in this two-color diagram, consistent with their similar ages and metallicities, and effectively zero reddening. While the upper main sequence of NGC 2281 is also fairly well defined, there is some uncertainty in the reddening of this cluster, which hinders an attempt to compare it with the other two. The results of this work suggest that it may become feasible to define isochrones for Population I dwarf stars in a (mFUV − B) versus B − V diagram if a sufficient sample of stars of well-known ages can be used as calibrating sources. However, the utility of such a diagram for age-dating purposes will depend upon the additional availability of accurate reddening measurements.

Absolute Magnitudes of Seismic Red Clumps in the Kepler Field and SAGA: The Age Dependency of the Distance Scale

Abstract Red clump stars are fundamental distance indicators in astrophysics, although theoretical stellar models predict a dependence of absolute magnitudes with age. This effect is particularly strong below ∼2 Gyr, but even above this limit a mild age dependence is still expected. We use seismically identified red clump stars in the Kepler field for which we have reliable distances, masses, and ages from the SAGA survey, to first explore this effect. By excluding red clump stars with masses larger than (corresponding to ages younger than 2 Gyr), we derive robust calibrations linking intrinsic colors to absolute magnitudes in the following photometric systems: Strömgren by, Johnson BV, Sloan griz, 2MASS JHK s , and WISE . With the precision achieved we also detect a slope of absolute magnitudes in the infrared, implying that distance calibrations of clump stars can be off by up to in the infrared (over the range from 2 to 12 Gyr) if their ages are unknown. Even larger uncertainties affect optical bands, because of the stronger interdependency of absolute magnitudes on colors and age. Our distance calibrations are ultimately based on asteroseismology, and we show how the distance scale can be used to test the accuracy of seismic scaling relations. Within the uncertainties our calibrations are in agreement with those built upon local red clumps with Hipparcos parallaxes, although we find a tension, which, if confirmed, would imply that scaling relations overestimate the radii of red clump stars by 2 ± 2%. Data releases post Gaia DR1 will provide an important testbed for our results.

Wide-field optical monitoring with Mini-MegaTORTORA (MMT-9) multichannel high temporal resolution telescope

We describe the properties of Mini-MegaTORTORA (MMT-9) nine-channel wide-field optical sky monitoring system with subsecond temporal resolution. This instrument can observe sky areas as large as 900 deg2, perform photometry in three filters close to Johnson BV R system and polarimetry of selected objects or areas with 100–300 deg2 sizes. The limiting magnitude of the system is up to V = 11m for 0.1 s temporal resolution, and reaches V = 15m in minute-long exposures. The system is equipped with a powerful computing facility and dedicated software pipeline allowing it to perform automatic detection, real-time classification, and investigation of transient events of different nature located both in the near- Earth space and at extragalactic distances. The objects routinely detected by MMT-9 include faint meteors and artificial Earth satellites.We discuss astronomical tasks that can be solved using MMT-9, and present the results of the first two years of its operation. In particular, we report the parameters of the optical flare detected on June 25, 2016, which accompanied the gamma-ray burst GRB160625B.

USING PHOTOMETRY TO PROBE THE CIRCUMSTELLAR ENVIRONMENT OF δ SCORPII

We acquired Johnson BV photometry of the binary Be disk system delta~Scorpii during its 2009, 2010, 2011, and 2012 observing seasons and used it to probe the innermost regions of the disk. We found that several disk building events have occurred during this time, resulting in an overall brightening in the V-band and reddening of the system. In addition to these long-term trends, we found cyclical variability in each observing season on timescales between 60 and 100 days. We were able to reproduce the changes in the magnitude and colour of delta Sco using our theoretical models and found that variable mass-loss rates in the range 2.5-7.0x10^{-9} M_{sun}/yr over ~35 days can reproduce the observed increase in brightness.

The Stability of BVRI Comparison Stars Near Selected TeV Blazars

ABSTRACTWe have measured Johnson BV and Cousins RI magnitudes for comparison stars near 5 TeV blazars. We compare our values with published values, spanning 25 years in some cases, to identify those stars that are most likely proven stable. To avoid zero-point offsets mimicking long-term variability, we based our analysis on the standard deviation between measurements after a mean offset between data sets was removed. We found most stars to be stable at the 0.04 mag level. We confirm two stars as variable and identify two others as possibly being variable. In each of the five fields there are at least two stars, and typically many more, that show no evidence of variability.

BV photometry of quadrupole system ET Bootis

Abstract In this study, we present the first Johnson BV photometry of the eclipsing binary star ET Bootis, which is member of a physically connected visual pair. Analysis of times of light minima enables us to calculate accurate ephemeris of the system via O–C analysis and observed an increase in period which we believe is a result of the light-time effect in the outer visual orbit. Secondly, we determined the total brightness and color of the system in light maxima and minima. Photometric solution of the system indicates that the contribution of the visual pair to the total light is about 40% in Johnson V band. Furthermore, photometric analysis shows that the primary star in the eclipsing binary has F8 spectral type while it confirms the G5 spectral type for the visual pair. Masses of the components in eclipsing binary are M1 = 1.109 ± 0.014 M⊙ and M2 = 1.153 ± 0.011 M⊙. Absolute radii of the components are R1 = 1.444 ± 0.007 R⊙ and R2 = 1.153 ± 0.007 R⊙. Physical properties of the components leads 176 ± 7 pc distance for the system and suggests an age of 6.5 billion years.

A statistical method to determine open cluster metallicities

The study of open cluster metallicities helps to understand the local stellar formation and evolution throughout the Milky Way. Its metallicity gradient is an important tracer for the Galactic formation in a global sense. Because open clusters can be treated in a statistical way, the error of the cluster mean is minimized. Our final goal is a semi-automatic statistical robust method to estimate the metallicity of a statistically significant number of open clusters based on Johnson BV data of their members, an algorithm that can easily be extended to other photometric systems for a systematic investigation. This method incorporates evolutionary grids for different metallicities and a calibration of the effective temperature and luminosity. With cluster parameters (age, reddening and distance) it is possible to estimate the metallicity from a statistical point of view. The iterative process includes an intrinsic consistency check of the starting input parameters and allows us to modify them. We extensively tested the method with published data for the Hyades and selected sixteen open clusters within 1000pc around the Sun with available and reliable Johnson BV measurements. In addition, Berkeley 29, with a distance of about 15kpc was chosen. For several targets we are able to compare our result with published ones which yielded a very good coincidence (including Berkeley 29).

The Identification of Blue Horizontal Branch Stars Using GALEX and Other Photometry

Halo samples that have no kinematic bias have been successfully isolated by photometric observations. We show that the NUV magnitude of the GALEX All-Sky Survey can be used, together with Johnson BV magnitudes in a (NUV - V)0 versus (B - V)0 plot, to distinguish blue horizontal branch (BHB) from other A stars of the same (B - V) color for 12 < V < 18. In addition, we use SDSS gr magnitudes in a (NUV - r)0 versus (g - r)0 plot for 14 < r < 18. The faint limit will be extended by ~3 mag for the GALEX Medium-Deep Survey. Attempts to use NUV in conjunction with 2MASS magnitudes and ROTSE mr magnitudes did not prove useful. The (NUV - V)0 versus (B - V)0 plot was used to examine BHB star candidates of varying quality near the south Galactic pole. We conclude that the addition of the GALEX NUV significantly adds to the reliability with which these stars can be identified.

Astrophysical supplements to the ASCC‐2.5. I. Radial velocity data

AbstractWe present a catalogue of radial velocities of Galactic stars with high precision astrometric data CRVAD which is the result of the cross‐identification of star lists from the General Catalog of Average Radial Velocities (GCRV) and from the homogeneous All‐sky Compiled Catalogue of 2.5Million Stars (ASCC‐2.5). The CRVAD includes accurate J2000 equatorial coordinates, proper motions and trigonometric parallaxes in the Hipparcos system, Johnson's BV photometric data, spectral types, multiplicity and variability flags from the ASCC‐2.5, and radial velocities, stellar magnitudes and spectral types from the GCRV for 34553 ASCC‐2.5 stars. The CRVAD was used for the construction of a sample of standard stars with accurate astrometric, photometric and radial velocity data for the RAVE project. A second application of the CRVAD , the radial velocity determination for 292 open clusters (including 97 with previously unknown radial velocities), using their newly defined members from proper motions and photometry in the ASCC‐2.5, is briefly described. (© 2004 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Spectroscopic and photometric monitoring of southern post-AGB stars

AbstractWe present the results of contemporaneous photometric and spectroscopic monitoring of 20 post-AGB stars from Mt John University Observatory. Photometric measures were carried out using Johnson BV and Cousins RI filters, and the radial velocity measurements were acquired using spectra from an echelle spectrograph. Our programme spanned five years and the stars covered a range of spectral types from B to K in order to investigate the behaviour of post-AGB stars as they evolve away from the AGB. A number of stars proved to be variable in ways incompatible with post-AGB models and are reclassified. Periodicities are presented for a number of stars. Photometrically, HD 70379 was found to be pulsating in two modes with periods of 85 and 97 d. The radial velocities also varied, with the peak amplitude occurring when the photometry was also changing most. AI CMi presented three different types of spectra associated with photometric brightness, with varying strengths of narrow emission lines and molecular bandheads. The Hα profiles in almost all of the stars show evidence of emission which varies on time scales of days to months. The Na D line profiles are generally complex showing between 4 and 7 components due to both circumstellar and interstellar material.

A New Method for Computing Differential Color Refraction

ABSTRACTA new method for computing differential color refraction (DCR) is presented in this paper that uses observations taken through a conventional filter as well as through a narrowband interference filter that serves as a template for the reductions. The method is tested with observations taken through Johnson BV and Cousins RI filters, and, in general, there is good agreement (excellent in all but one case) between observed and predicted values of DCR. A full description of the method is given as well as a discussion of the refraction model used to make the predictions. As expected, DCR is strongest in the B passband and weakest in the I band because of the wavelength dependence of atmospheric refraction. Furthermore, the common practice of calibrating DCR against a stellar color is investigated, finding that the procedure is justified, albeit the intrinsic scatter in the calibration can vary widely with the choice for the color. This scatter can be minimized with a judicious choice for the color system, and examples are given for the BVRI passbands considered in the paper. Nonetheless, there will always be some remaining error caused by the calibrating process, which will affect mostly star positions determined at large zenith distances. Further gains in positional accuracy are possible by choosing a passband for which the DCR is very small. Two such passbands are the Cousins I band (FWHM = 1900 Å) and the passband (FWHM = 630 Å) chosen for the USNO CCD Astrograph Catalog project. The ranges in the DCR for these passbands are, respectively, only 19 and 12 mas at a zenith distance of ZD = 60°. When calibrated against stellar color, remaining errors in the DCR calibrations for both of these passbands are expected to be under 5 mas. In the final section of the paper, numerical results are given for other possible passbands with small ranges (≤20 mas) in DCR.

ITAL]BVI[/ITAL] Time-Series Data of the Galactic Globular Cluster NGC 3201. I. RR Lyrae Stars

We present Johnson BV- and Kron-Cousins I-band time-series data collected over three consecutive nights in a region of 13 arcmin2 centered on the Galactic globular cluster (GGC) NGC 3201. The time sampling of current CCD data allowed us to derive accurate light curves and, in turn, mean magnitudes and colors for a sample of 53 RR Lyrae stars. To overcome the thorny problem of differential reddening affecting this cluster, we derived new empirical relations connecting the intrinsic B-V and V-I colors of fundamental (RRab) RR Lyrae stars to the luminosity amplitude, the metallicity, and the pulsation period. The key features of these relations are the following: (1) they rely on stellar parameters, which are not affected by reddening; (2) they supply accurate estimates of intrinsic colors across the fundamental instability strip and cover a wide metallicity range; (3) they were derived by neglecting the RR Lyrae stars that are affected by amplitude modulation. Moreover, the zero point of the E(B-V) reddening scale was empirically checked using the large sample of RR Lyrae stars in M3 from Corwin & Carney, a GGC affected by a vanishing reddening. According to these relations we estimated individual reddenings for RR Lyrae stars in our sample and the main results we found are the following: (1) The mean cluster reddening based on E(B-V) color excesses is E(B-V) = 0.30 ± 0.03. This estimate is slightly higher than the mean reddening evaluations available in the literature or based on the dust infrared map by Schlegel, Finkbeiner, & Davis, i.e., E(B-V) = 0.26 ± 0.02. Note that the angular resolution of this map is ≈6', whereas for current reddening map it is ≈1'. (2) The mean cluster reddening based on E(V-I) color excesses is E(V-I) = 0.36 ± 0.05. This estimate is only marginally in agreement with the mean cluster reddening obtained using the reddening map by von Braun & Mateo and derived by adopting cluster turnoff stars, i.e., E(V-I) = 0.25 ± 0.07. On the other hand, current intrinsic spread among individual reddenings (≈0.2 mag) agrees quite well with the estimate provided by previous authors. It is noteworthy that previous mean cluster reddenings are in very good agreement with values obtained using the empirical relations for intrinsic RR Lyrae colors provided by Kovacs & Walker. (3) According to current individual E(B-V) and E(V-I) reddenings and theoretical predictions for horizontal-branch stars, we found that the true distance modulus for this cluster is 13.32 ± 0.06 mag. This determination is somehow supported by the comparison between predicted and empirical pulsation amplitudes. (4) The comparison between present luminosity amplitudes and estimates available in the literature discloses that approximately 30% of fundamental RR Lyrae stars are affected by amplitude modulation (the Blazhko effect). This finding confirms empirical evidence originally brought out by Szeidl and by Smith.

Optical Monitoring of S5 0716+714 from 1994 to 2000

We present multicolor optical (Johnson BV and Cousins RI) monitoring results of 5.3 yr for S5 0716+714. The four-band light curves showed a high duty cycle of variability. We studied its intranight variability and the quasi-periodic oscillations and found that the variability can be explained by a relativistic jet model.

The Four College Automated Photoelectric Telescope

For the past decade, astronomers from The Citadel, The College of Charleston, the University of Nevada Las Vegas, and Villanova University have operated in Southern Arizona a 0.75-m automated telescope to obtain differential Strömgren uvby, Johnson BV, and Cousins RI photometry of a wide variety of stars. Each school averages the equivalent of about 40 nights/year of high quality photometry at a cost less than using observers. By mixing our programs we obtained observations of certain important stars on nearly every clear night they can be observed. Usually we request a star be observed only once per night. Still continuous coverage can be obtained. The stars are given priorities and scheduled using ATIS. The telescope selects targets from the groups with the highest priority by choosing the one closest to the western edge of the observing window. Some data has been analyzed by undergraduate and graduate students. We use internet to send requests for observations and to retrieve data. We believe our experiences are germane to others interested in automated photometric telescopes. We are open to the possibility of collaborations with other astronomers who are obtaining photometric and spectroscopic data. (Coauthors are: L. Boyd, R.J. Dukes Jr, E.F. Guinan, G.M. McCook and D.M. Pyper, all of the U.S.A..)

Using [ITAL]BV[/ITAL] Photometry to Distinguish between Type I and Type II Cepheids

We use Johnson BV photometry of Galactic type I and type II Cepheids in a variety of diagrams to see whether the two populations can be adequately separated. The diagrams include the Fourier parameters 21, 31, 41, R21, R31, R41, the loop width in the V-(B-V) plane, the relative phasing of V and B-V extrema, and the amplitude ratios of B-V to V, all versus period. We found the Ri1 diagrams useless in this regard, while a plot of the phasing parameter versus amplitude ratio was best, closely followed by the i1 diagrams. A major difficulty was arriving at a sample of Cepheids that could be reliably taken to be definitely type II. We found it necessary to use only stars that would have been more than 2 kpc from the Galactic plane had they been type I for this sample, and even then it is uncertain whether such overlap between types as we find in our diagrams is due to inadequacy of the tests or is due to type I stars' having contaminated our type II sample. We point out evidence strongly suggesting that type I stars can be found at least up to 2 kpc from the plane. Selection effects in our sample of type II Cepheids resulted in only one BL Herculis star (short-period type II) being included, so our conclusions basically apply only to W Virginis stars.

Optical observations of eight X-ray selected BL Lacertae objects

We present optical data in the Johnson's BV and Cousins'R bands for eight X-ray selected BL Lacertae objects (BL Lacs) that have recently been pointed by the Satellite per Astronomia X \Beppo (BeppoSAX). The observations were done with the 1:05 m telescope of the Torino Astronomical Observatory and the observational periods include, or are close to, the satellite pointings. These data provide optical information on sources that have been rarely observed in the optical band. Moreover, they can be compared to the X-ray ones for a better un- derstanding of the emission properties of these objects. Variability on short time scales (a few days) was found for MS 0317.0+1834, 1ES 0347 121, and MS 0737.9+7441.

BVR photometry of comparison stars in selected blazar fields

We present photometric sequences in the Johnson's BV and Cousins' R bands in the elds of 10 BL Lacertae objects (BL Lacs) monitored at the Torino Astronomical Observatory. Data were taken during 14 photometric nights from February 1995 to May 1997. The 56 calibrated stars are brighter than V = 16 and can be used as comparison objects in order to derive the BL Lac magnitudes. Finding charts are included for the stars iden- tication.