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Register a new userBridging the Ultraviolet and Optical Regions: Transformation Equations between {it GALEX} and {it UBV} Photometric Systems
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We derive transformation equations between GALEX and UBV colours by using the reliable data of 556 stars. We present two sets of equations; as a function of (only) luminosity class, and as a function of both luminosity class and metallicity. The metallicities are provided from the literature, while the luminosity classes are determined by using the PARSEC mass tracks in this study. Small colour residuals and high squared correlation coefficients promise accurate derived colours. The application of the transformation equations to 70 stars with reliable data shows that the metallicity plays an important role in estimation of more accurate colours.
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Although core helium-burning red clump (RC) stars are faint at ultraviolet wavelengths, their ultraviolet-optical color is a unique and accessible probe of their physical properties. Using data from the GALEX All Sky Imaging Survey, Gaia Data Release 2 and the SDSS APOGEE DR14 survey, we find that spectroscopic metallicity is strongly correlated with the location of an RC star in the UV-optical color magnitude diagram. The RC has a wide spread in (NUV - G)$_0$ color, over 4 magnitudes, compared to a 0.7-magnitude range in (G$_{BP}$ - G$_{RP}$)$_0$. We propose a photometric, dust-corrected, ultraviolet-optical (NUV - G)$_0$ color-metallicity [Fe/H] relation using a sample of 5,175 RC stars from APOGEE. We show that this relation has a scatter of 0.28 dex and is easier to obtain for large, wide-field samples than spectroscopic metallicities. Importantly, the effect may be comparable to the spread in RC color attributed to extinction in other studies.
Interstellar extinction in ultraviolet is the most severe in comparison with optical and infrared wavebands and a precise determination plays an important role in correctly recovering the ultraviolet brightness and colors of objects. By finding the observed bluest colors at given effective temperature and metallicity range of dwarf stars, stellar intrinsic colors, $C^0_{rm B,V}$, $C^0_{rm NUV,B}$, $C^0_{rm FUV,B}$ and $C^0_{rm FUV,NUV}$, are derived according to the stellar parameters from the LAMOST spectroscopic survey and photometric results from the $GALEX$ and APASS surveys. With the derived intrinsic colors, the ultraviolet color excesses are calculated for about 25,000 A- and F-type dwarf stars. Analysis of the color excess ratios yields the extinction law related to the $GALEX$ UV bands: $E_{{rm NUV,B}}$/$E_{{rm B,V}} = 3.77$, $E_{{rm FUV,B}}$/$E_{{rm B,V}} = 3.39$, $E_{{rm FUV,NUV}}$/$E_{{rm B,V}} = -0.38$. The results agree very well with previous works in the $NUV$ band and in general with the extinction curve derived by Fitzpatrick (1999) for $R_{rm V}=3.35$.
We present colour transformations for the conversion of the {em 2MASS} photometric system to the Johnson-Cousins $UBVRI$ system and further into the {em SDSS} $ugriz$ system. We have taken {em SDSS} $gri$ magnitudes of stars measured with the 2.5-m telescope from $SDSS$ Data Release 5 (DR5), and $BVRI$ and $JHK_{s}$ magnitudes from Stetsons catalogue and citet{Cu03}, respectively. We matched thousands of stars in the three photometric systems by their coordinates and obtained a homogeneous sample of 825 stars by the following constraints, which are not used in previous transformations: 1) the data are de-reddened, 2) giants are omitted, and 3) the sample stars selected are of the highest quality. We give metallicity, population type, and transformations dependent on two colours. The transformations provide absolute magnitude and distance determinations which can be used in space density evaluations at short distances where some or all of the {em SDSS} $ugriz$ magnitudes are saturated. The combination of these densities with those evaluated at larger distances using {em SDSS} $ugriz$ photometry will supply accurate Galactic model parameters, particularly the local space densities for each population.
SN 1572 (Tycho Brahes supernova) clearly belongs to the Ia (thermonuclear) type. It was produced by the explosion of a white dwarf in a binary system. Its remnant has been the first of this type to be explored in search of a possible surviving companion, the mass donor that brought the white dwarf to the point of explosion. A high peculiar motion with respect to the stars at the same location in the Galaxy, mainly due to the orbital velocity at the time of the explosion, is a basic criterion for the detection of such companions. Radial velocities from the spectra of the stars close to the geometrical center of Tychos supernova remnant, plus proper motions of the same stars, obtained by astrometry with the {it Hubble Space Telescope}, have been used so far. In addition, a detailed chemical analysis of the atmospheres of a sample of candidate stars had been made. However, the distances to the stars, remained uncertain. Now, the Second {it Gaia} Data Release (DR2) provides unprecedent accurate distances and new proper motions for the stars can be compared with those made from the {it HST}. We consider the Galactic orbits that the candidate stars to SN companion would have in the future. We do this to explore kinematic peculiarity. We also locate a representative sample of candidate stars in the Toomre diagram. Using the new data, we reevaluate here the status of the candidates suggested thus far, as well as the larger sample of the stars seen in the central region of the remnant.
Machine learning techniques, specifically the k-nearest neighbour algorithm applied to optical band colours, have had some success in predicting photometric redshifts of quasi-stellar objects (QSOs): Although the mean of differences between the spectroscopic and photometric redshifts is close to zero, the distribution of these differences remains wide and distinctly non-Gaussian. As per our previous empirical estimate of photometric redshifts, we find that the predictions can be significantly improved by adding colours from other wavebands, namely the near-infrared and ultraviolet. Self-testing this, by using half of the 33 643 strong QSO sample to train the algorithm, results in a significantly narrower spread for the remaining half of the sample. Using the whole QSO sample to train the algorithm, the same set of magnitudes return a similar spread for a sample of radio sources (quasars). Although the matching coincidence is relatively low (739 of the 3663 sources having photometry in the relevant bands), this is still significantly larger than from the empirical method (2%) and thus may provide a method with which to obtain redshifts for the vast number of continuum radio sources expected to be detected with the next generation of large radio telescopes.
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