Nearby Field Stars Research Articles

Time delay of SBS 0909+532

The time delays between the components of a lensed quasar are basic tools to analyze the expansion of the Universe and the structure of the main lens galaxy halo. In this paper, we focus on the variability and time delay of the double system SBS 0909+532A,B as well as the time behaviour of the field stars. We use VR optical observations of SBS 0909+532A,B and the field stars in 2003. The frames were taken at Cal ar Alto, Maidanak and Wise observatories, and the VR light curves of the field stars and quasar components are derived from aperture and point-spread function fitting met hods. We measure the R-band time delay of the system from the � 2 and dispersion techniques and 1000 synthetic light curves based on the observed records. One nearby field star (S BS 0909+532c) is found to be variable, and the other two nearby field stars are non-vari able sources. With respect to the quasar components, the R-band records seem more reliable and are more densely pop- ulated than the V-band ones. The observed R-band fluctuations permit a pre-conditioned measurement of the time delay. From the � 2 minimization, if we assume that the quasar emission is observed first in B and afterwards in A (in agreeme nt with basic observations of the system and the corresponding predictions), we obtain ��BA =− 45 +1 −11 days (95%

Direct Measurement of the Size of 2003 UB313 from theHubble Space Telescope

We have used the Hubble Space Telescope to directly measure the angular size of the large Kuiper belt object 2003 UB313. By carefully calibrating the point spread function of a nearby field star, we measure the size of 2003 UB313 to be 34.3$\pm$1.4 milliarcseconds, corresponding to a diameter of 2400$\pm$100 km or a size $\sim5$% larger than Pluto. The V band geometric albedo of 2003 UB313 is $86\pm7$%. The extremely high albedo is consistent with the frosty methane spectrum, the lack of red coloring, and the lack of observed photometric variation on the surface of 2003 UB313. Methane photolysis should quickly darken the surface of 2003 UB313, but continuous evaporation and redeposition of surface ices appears capable of maintaining the extreme alebdo of this body.

The Benchmark Cluster Reddening Project. I. Reddening Values for the Hyades, Coma, and Praesepe

From histories of reddening studies for the Hyades, Praesepe, and nearby field stars, it is concluded that reddening scholarship requires fundamental reform. The adopted protocol for the reformed version of scholarship includes (1) rigorous published foundations for reddening values, (2) explicit quality control of reddening techniques, (3) use of techniques with limited metallicity sensitivity, (4) explicit zero-point control for both input data and derived reddening values, (5) an explicit choice of a reddening zero point, and (6) adherence to statistical norms. It is argued that only limited use should be made of reddening values from (1) spectral types for F0 and later type stars, (2) UBV analysis of blanketed stars, and (3) certain published relations between reddening and distance. However, reddening values may be based safely on (1) polarization measurements, (2) comparisons of β and (R - I)C for F stars, and (3) Stromgren β analysis of A stars. For the Hyades and Coma, upper 95% confidence limits on E(B - V) are found to be ≤1.0 and ≤3.2 mmag, respectively. For Praesepe, the derived value of E(B - V) is 27 ± 4 mmag. The latter result is obtained after an allowance is made for the effects of an anomalous relation between b - y and (R - I)C in Praesepe A stars. A test of the possibility that differential stellar rotation causes the anomaly yields an inconclusive result. Adoption of the revised reddening value instead of the canonical value of zero yields an increase in the derived Praesepe distance modulus by 0.14 mag and a decrease in the derived cluster age by about 11%. In addition, by using published high-dispersion results, it is found that the value of [Fe/H] for Praesepe that is consistent with the derived reddening value is +0.01 ± 0.04 dex. Further results for Praesepe stars will be required for a definitive comparison of the Hyades and Praesepe metallicities.

Elemental abundances in the Blanco 1 open cluster

High resolution spectroscopy is used to determine the detailed chemical abundances of a group of eight F- and G-type stars in the young open cluster Blanco 1. An average [Fe/H] of +0.04 +/- 0.02 (internal error) +/- 0.04 (external error) is found, considerably lower than a previous spectroscopic estimate for this cluster. The difference is due mainly to our adoption of significantly cooler temperatures which are consistent with both photometric and spectroscopic constraints. Blanco~1 exhibits sub-solar [Ni/Fe] (-0.18 +/- 0.01 +/- 0.01), [Si/Fe] (-0.09$ +/- 0.02 +/- 0.03), [Mg/Fe] (-0.14 +/- 0.02 +/- 0.03) and [Ca/Fe] (-0.09 +/- 0.03 +/- 0.03); ratios which are not observed among nearby field stars. The material from which Blanco 1 formed may not have been well mixed with interstellar matter in the galactic disc, which tallies with its current location about 240pc below the galactic plane. A simultaneous deficit of Ni and alpha elements with respect to Fe is hard to reconcile with most published models of yields from supernovae of types Ia and II. The revised abundances for Blanco 1 indicate that overall radiative opacities in its stars, and hence convective zone properties at a given mass, are similar to those in the Pleiades at approximately the same age. This can explain a previous observation that the Li depletion patterns of G- and K-type stars in the two clusters are indistinguishable. The lower overall metallicity of Blanco 1 now make it less attractive as a target for discovering transiting, short period exoplanets.

Discovery of Two Very Low Mass Binaries: Final Results of an Adaptive Optics Survey of Nearby M6.0–M7.5 Stars

We present updated results of a high-resolution, magnitude-limited (Ks < 12 mag) imaging survey of nearby low-mass M6.0-M7.5 field stars. The observations were carried out using adaptive optics at the Gemini North, VLT, Keck II, and Subaru telescopes. Our sample of 36 stars consists predominantly of nearby (≲30 pc) field stars, five of which we have resolved as binaries. Two of the binary systems, 2MASSI J0429184-312356 and 2MASSI J1847034+552243, are presented here for the first time. All five of the discovered binary systems have separations between 008 and 053 (2-9 AU) with similar mass ratios (q > 0.8, ΔKs < 1 mag). This result supports the hypothesis that wide (a > 20 AU), very low mass (VLM; Mtot < 0.19 M☉) binary systems are rare. The projected semimajor axis distribution of these systems peak at ~5 AU, and we report a sensitivity-corrected binary fraction of 9 % for stars with primaries of spectral type M6.0-M7.5 with separations ≳3 AU and mass ratios q ≳ 0.6. Within these instrumental sensitivities, these results support the overall trend that both the semimajor axis distribution and binary fraction are a function of the mass of the primary star and decrease with decreasing primary mass. These observations provide important constraints for low-mass binary star formation theories.

The structure of proper-motion diagrams for nearby stars

The structure of the proper-motion diagrams for an ensemble of nearby field stars is analyzed using a simple model. For comparison, the observed proper-motion diagram is constructed using data of the Hipparcos catalog. A complete description is given of the modification of the van Altena method, which allows stars with similar space motions to be identified based on their proper motions. In the original version of this method, the stars had to be located within one-quarter of the celestial sphere, whereas our version is free of this restriction.

Abundance Analyses of 12 Parent Stars of Extrasolar Planets Observed with the SUBARU/HDS

Abstract High $\mathrm{S/N}$ ratio (350–600) and high resolution ($R \sim 90000$) spectral data covering the wavelength region between 4200–8800$\,{Å}$ were obtained for 12 planet-harboring stars with the SUBARU/HDS. Detailed abundance analyses were carried out for these stars using the solar spectrum as a reference. Atmospheric parameters ($T_\mathrm{eff}, \log g$, microturbulent velocity $\xi_\mathrm{t}$, and $[\mathrm{Fe/H}]$) were determined spectroscopically using selected Fe I and Fe II lines. The abundances of 25 other elements have been determined relative to the Sun in a strictly differential manner. Three out of 12 stars (HD 82943, HD 92788, and HD 134987) are definitely metal rich, and two stars (HD 114762 and HD 190228) are metal deficient. The abundances in the remaining seven stars (HD 106252, HD 130322, HD 141937, HD 168746, HD 187123, HD 192263, HD 209458) coincide with the Sun. We find that the abundances of both volatile and refractory elements behave similarly in these stars, and do not confirm any significant dependence on the condensation temperature, $T_\mathrm{c}$. Comparisons of the $[\mathrm{X/Fe}]$ vs $[\mathrm{Fe/H}]$ relations for 19 elements between planet-harboring stars and nearby field stars reveal no distinct anomaly, except for possible enhancements of V and Co in the former group. These results suggest that the observed over-abundances of metallic elements in planet-harboring stars relative to normal disk stars in the solar neighborhood are primordial, and can be interpreted as being a consequence of the galactic chemical evolution.

The Rate of Visual Binaries among the Brightest X-Ray Stars

Using the Tycho Double Star Catalogue and the Hipparcos Catalogue, I compare the rates of occurrence of resolved visual doubles among the brightest X-ray stars from the ROSAT All-Sky Survey Bright Source Catalog (RASS-BSC) and generally in these two catalogs. A strong correlation between visual binarity and brightness in X-rays is found. The rate of binarity, quantified as the fraction of resolved components, is 7 times higher for the RASS-BSC stars than generally for all Tycho-2 stars. In order to eliminate the distance bias in the sample, I compare the rates in a distance-limited sample of Hipparcos stars within 50 pc. The comparison has shown that the fraction of visual and unresolved orbiting binaries among stars with counterparts in the RASS-BSC, hence bright in X-rays, is 2.4 times higher than among fainter X-ray stars, i.e., the ones whose X-ray brightness is below the limit of the RASS-BSC. This difference is not caused by the enhancement of the observed flux due to the combined component's radiation. Previously found in the nearby open clusters, the correlation between binarity and X-ray luminosity exists for nearby field stars as well. Possible implications of the discrepancy are discussed, including the binarity-age and binarity-rotation velocity relations. The H-R diagram of components of nearby visual binaries indicates that the red clump giants luminous in X-rays may be unresolved binaries.

Infrared photometry and polarimetry of Cygnus X-3

We present photometry and linear polarimetry of Cygnus X-3 at K (2.2 micrometers) obtained over a 5 yr period. Photometry and polarimetry at J, H, and K of nearby field stars is also presented. From an analysis of these data we find: (1) Using the x-ray ephemeris of Kitamoto et al. (ApJ, 384, 263 (1992), including the first and second derivatives of the period, the leading edge of the decline to minimum in the quiescent K light curve has not changed in phase since 1974. The duration of the minimum in the light curve has changed significantly between different epochs, becoming much broader in 1993 than it was previously. (2) In addition to an interstellar polarization component, it is likely Cyg X-3 has an intrinsic polarization component that is variable. The variations in the polarization do not show any diagnostic pattern with orbital phase. A crude analysis of the polarization suggests the intrinsic polarization of Cyg X-3 has a mean position angle of approximately 12 deg, nearly the same as the direction of the expanding radio lobes. This is consistent with circumstellar electrons scattering in an equatorial disk that is perpendicular to the lobe axis. (3) The mean position angle for the interstellar polarization in the direction of Cyg X-3 is 150 deg. This is nearly perpendicular to the axis of interstellar radio scattering seen in the extended (Very Long Baseline Inteferometry (VLBI) images. Since the position angle of interstellar polarization is the same as the projected magnetic field direction, this suggests the interstellar (not circumstellar) scattering must be taking place perpendicular to the interstellar magnetic field lines. (4) Cyg X-3 was observed at K during a flare on 1992 September 30 with a temporal resolution of 6 s. The flaring had rise and fall times of approximately 50 s with peak intensities up to 80 mJy. The flux between individual flare events never dropped to quiescent levels for the duration of our observations (approximately 2000 s).

Optical spectroscopy of very low mass stars in the Pleiades

view Abstract Citations (11) References (21) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Optical Spectroscopy of Very Low Mass Stars in the Pleiades Hamilton, Donald ; Stauffer, John R. Abstract Optical spectrophotometry for several very low mass cool stars in the Palomar Pleiades has been obtained. The optical spectra of the PP stars has been compared with the optical spectra of a nearby field star sample, indicating that all of the PPl stars are dwarfs and that the faintest star is one of the coolest dwarf stars known; this star is very similar to VB 10 in its spectroscopic properties, but with a bolometric luminosity that is 1.5 mag brighter. The other PPl stars have spectral characteristics approximately similar to Gl 905. The Balmer emission found in all observed PPl stars is far less than those of the nearby cool stars. The possible brown-dwarf nature of PPl is tied to peculiarities concerning the theoretical models and to the presumed age of PPl. It is suggested that PPl is cooler than G1 406 and is similar to VB 10 according to most spectroscopic comparisons made. Several molecular TiO band indices were defined for effective temperature discriminants greater than 2400 K. The indicator of effective temperature, known as the PC index, is one based upon the slope between two low blanketing flux-density minima. Publication: The Astronomical Journal Pub Date: May 1993 DOI: 10.1086/116559 Bibcode: 1993AJ....105.1855H Keywords: Cool Stars; Metallicity; Pleiades Cluster; Stellar Luminosity; Stellar Mass; Stellar Spectrophotometry; H Alpha Line; Titanium Oxides; Vanadium Oxides; Astrophysics; STARS:LOW-MASS full text sources ADS | data products SIMBAD (16)

8-13 µm Spectropolarimetry of Star Formation Regions

Amongst other things, mid-infrared spectropolarimetry provides information on the direction of the transverse component of a magnetic field. The polarisation is produced by dichroic emission and/or absorption by dust grains whose alignment is controlled by the ambient magnetic field. The observed position angle of the E vector allows the field direction to be determined. Here results are presented of 8-13 p,m spectropolarimetric observations of a number of star formation regions. Comparisons are made between the field direction observed and the Galactic field established by optical polarimetry of nearby field stars, and with the orientation of bipolar outflows and/or disks which are associated with some of these objects.

High-rate spectroscopic active galactic nucleus monitoring at the Wise Observatory. I - Markarian 279

view Abstract Citations (91) References (25) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS High-Rate Spectroscopic Active Galactic Nucleus Monitoring at the Wise Observatory. I. Markarian 279 Maoz, D. ; Netzer, H. ; Leibowitz, E. ; Brosch, N. ; Laor, A. ; Mendelson, H. ; Beck, S. ; Almoznino, E. ; Mazeh, T. Abstract We monitored sprectrophotometrically a sample of AGNs, and achieved a temporal sampling rate of once every 3-4 days, over a period of 5-7 months. By observing each object simultaneously with a nearby field star, in a long-slit spectroscopy mode, we obtained accurate flux measurements even during bad weather and full moon. In this paper, the first in a series, we describe our observational technique and reduction procedure in general, and discuss our results for the Seyfert 1 galaxy Mrk 279. This object brightened by 20%-50% in Hα, Hβ, and the optical continuum during the period of observation. The high sampling rate and small error allow the determination, through cross-correlation, of the lag in the emission-line response to the continuum brightening. A lag of 12 days is observed. Using Monte Carlo simulations, we estimate the size of the broad- line region (BLR) to be 12 +/- 3 light days in this object and find that the results rule out, at the 95% significance level, a BLR of 21 light-days or larger. This result is in conflict with the "standard" photoionization model of AGNs, which predicts a BLR size larger by an order of magnitude. Publication: The Astrophysical Journal Pub Date: March 1990 DOI: 10.1086/168445 Bibcode: 1990ApJ...351...75M Keywords: Active Galactic Nuclei; Astronomical Spectroscopy; Markarian Galaxies; Seyfert Galaxies; Astronomical Photometry; Charge Coupled Devices; Light Curve; Photoionization; Astrophysics; GALAXIES: PHOTOMETRY; GALAXIES: INDIVIDUAL NAME: MARKARIAN 279; GALAXIES: SEYFERT; SPECTROPHOTOMETRY full text sources ADS | data products SIMBAD (3) NED (3) Related Materials (2) Part 2: 1990ApJ...353..108N Part 3: 1993ApJ...413..445M

On the problem of secular variability in the stellar initial mass function

AbstractThe hypothesis of secular variations in the stellar initial mass function (IMF) is studied. It is found that neither the present‐day mass function of the nearby main sequence field stars nor the velocity distribution of these stars are contradictory with this hypothesis. The luminosity functions of unbiased kinematically defined age‐groups of the nearby stars also provide no strong constraints. Simple evolution models with time‐dependent IMF and star formation rate enable to fit the data of blue irregular galaxies. Some problems with an universal IMF are pointed out.

On the dust and gas associated with Sharpless 252

The stellar cluster and the massive molecular cloud associated with the H II region Sharpless 252 have been studied by means of multicolour polarization and molecular line measurements. The average wavelength of the maximum polarization λmax and polarization efficiency for the cluster stars are similar to the values observed for the nearby field stars. Two local maxima lying only 2′ apart were found in the molecular cloud core in CO, NH3, and HCO+. The excitation conditions and radial velocities associated with the maxima are different

Magnetic field structure in the Taurus dark cloud

view Abstract Citations (150) References (31) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Magnetic field structure in the Taurus dark cloud. Moneti, A. ; Pipher, J. L. ; Helfer, H. L. ; McMillan, R. S. ; Perry, M. L. Abstract Optical and infrared polarimetry of sources in the direction of the Taurus cloud are obtained in order to study the magnetic field in this cloud and its possible role in the cloud's evolution. Most of the stars are background giant stars whose light shines through the cloud and is polarized by the cloud material. The transverse component of the magnetic field, as delineated by the polarization vectors, is generally perpendicular to the galactic plane, and the stratified structure of the cloud could be due to the effect of the magnetic field during the early stages of collapse. Three of the 13 embedded stars are strongly polarized with position angles nearly perpendicular to those of nearby field stars. The polarization of these stars is most likely intrinsic, and the direction of polarization indicates that the material surrounding these stars may be magnetic i.e., that the magnetic field is frozen in this material. Publication: The Astrophysical Journal Pub Date: July 1984 DOI: 10.1086/162228 Bibcode: 1984ApJ...282..508M Keywords: Interstellar Magnetic Fields; Nebulae; T Tauri Stars; Infrared Spectra; Interstellar Extinction; Magnetic Field Configurations; Polarimetry; Visible Spectrum; Astrophysics full text sources ADS | data products SIMBAD (250)

Characteristics of Nebulosity Associated with Parkes Quasars

A sample of 13 out of 15 low redshift (0.1 ≤ z ≤ 0.6) radio-loud quasars have been resolved on large-scale (19 arcsec mm−1), sky-limited (µR ∼ 26.5 mag sec−2) Kodak IIIa-F photographs obtained with the ESO 3.6-m telescope. The QSO images were analyzed by digitally subtracting the plate background and the point-spread function defined by images of nearby (≤ 1 arcmin) field stars having magnitudes comparable to the quasars (Δm ≲ 0.3 mag). The resolved nebulosities underlying the QSO images have isophotal diameters in the range θ ∼ 7 to 40 arcsec, with surface brightnesses µR ∼ 22–26 mag sec−2, and integrated apparent red magnitudes ∼ 16–21.

Resolution of quasar images

view Abstract Citations (110) References (32) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Resolution of quasar images. Wyckoff, S. ; Wehinger, P. A. ; Gehren, T. Abstract Thirteen low redshift (z approximately less than 0.6) quasars have been resolved on large-scale, sky-limited photographs, and the images were analyzed by digitally removing the plate background and the point-spread function defined by images of nearby field stars, having magnitudes comparable to those of the quasars. The nebulosities around the quasars were found to have an average metric diameter of approximately 90 + or - 30 kpc and an integrated absolute magnitude of approximately -21.8 + or - 0.8. An expression is presented for the average intensity profile of the underlying nebulosity. Correlations (between the isophotal diameters of the resolved nebulosity and quasar redshifts; and the integrated apparent magnitudes of the underlying nebulosity and isophotal diameters) indicate roughly constant diameters and surface brightnesses for the nebulosities associated with the quasars. The physical and statistical properties of the nebulosities surrounding the quasars support the hypothesis that quasars are the luminous nuclei of distant galaxies. Publication: The Astrophysical Journal Pub Date: August 1981 DOI: 10.1086/159086 Bibcode: 1981ApJ...247..750W Keywords: Astronomical Photometry; Galactic Nuclei; Galactic Structure; Image Resolution; Quasars; Red Shift; Digital Techniques; Hubble Diagram; Imaging Techniques; Isophotes; Point Spread Functions; Stellar Magnitude; Astronomy full text sources ADS | data products SIMBAD (15) NED (15)

An investigation of previously derived Hyades, Coma, and M67 reddenings

view Abstract Citations (49) References (40) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS An investigation of previously derived Hyades, Coma, and M 67 reddenings. Taylor, B. J. Abstract New Hyades polarimetry and field star photometry were obtained to check the Hyades reddening, which was found to be nonzero in a previous study (Taylor, 1978). The new Hyades polarimetry implies essentially zero reddening. Four photometric techniques which are assumed to be insensitive to blanketing are used to compare the Hyades to nearby field stars and are found to yield essentially zero reddening. A simultaneous solution for the Hyades, Coma, and M67 reddenings is made, and the results are E(B-V) = 3 plus or minus 2(sigma) mmag, -1 plus or minus 3(sigma) mmag, and 46 plus or minus 6(sigma) mmag, respectively. Publication: The Astronomical Journal Pub Date: March 1980 DOI: 10.1086/112669 Bibcode: 1980AJ.....85..242T Keywords: Astronomical Photometry; Galactic Clusters; Polarimetry; Red Shift; Astrophysics; Coma; Astrophysics full text sources ADS | data products SIMBAD (44) NED (1)

On the Reddening and the Effective Temperature of HD 101065

The reddening in the direction of the peculiar star HD 101065 is evaluated using UBV observations of 60 nearby field stars and an increase of EB–V with distance modulus found. Using this, infrared measurements of Hyland et al. (1975), who neglected reddening and found a spectral type of F5–F6, are re-discussed. Their effective temperature of HD 101065 could be too low by as much as 1000 K, depending upon the star's distance. The strengths of Hα and Hβ and the general appearance of the star's spectrum are then examined and it is concluded that the effective temperature is nearer 7000 K.

The Helium-to-Hydrogen Ratio of Stars in Young Clusters and Associations

A photoelectric narrow-band index, I(4026), of the He I λ 4026 line has been observed with an echelle spectrometer for B-type stars in young clusters and associations.Model-atmosphere computations of the Strömgren [c1] index, the Hβ index, and I(4026) as functions of Teff, g and the helium-to-hydrogen ratio have been used to derive these atmospheric parameters from the observed indices for each star.For the OriOBIb, LacOBI and Sco-Cen associations 11 out of 48 stars in the temperature range from 14 000 to 20 000 K are found to be helium poor in the sense that the derived helium-to-hydrogen ratios lie in the range from 0.05 down to very low values. In the temperature range from 20 000 to 30 000 K no helium poor stars are found, but two stars in OriOBIb are helium rich. Both of these stars show rather strong variations of the I(4026) index.All stars in h + χ Per and CepOBIII are apparently helium deficient compared to nearby field stars.