We present comprehensive catalogues of hot star candidates in the Milky Way (MW), selected from Galaxy Evolution Explorer (GALEX) far-UV (FUV; 1344-1786 Å) and near-UV (NUV; 1771-2831 Å) imaging. The FUV and NUV photometry allows us to extract the hottest stellar objects, in particular hot white dwarfs (WD), which are elusive at other wavelengths because of their high temperatures and faint optical luminosities. We generated catalogues of UV sources from two GALEX's surveys: All-Sky Imaging Survey (AIS; depth AB magnitude ∼19.9/ 20.8 in FUV/NUV) and Medium-depth Imaging Survey (MIS; depth ∼22.6/22.7 mag). The two catalogues (from GALEX fifth data release) contain 65.3/12.6 million (AIS/MIS) unique UV sources with error NUV 0.5 mag, over 21 435/1579 deg 2 . We also constructed subcatalogues of the UV sources with matched optical photometry from Sloan Digital Sky Survey (SDSS; seventh data release): these contain 0.6/0.9 million (AIS/MIS) sources with errors 0.3 mag in both FUV and NUV, excluding sources with multiple optical counterparts, over an area of 7325/1103 deg 2 . All catalogues are available online. We then selected 28 319 (AIS)/9028 (MIS) matched sources with FUV − NUV < −0.13; this colour cut corresponds to stellar T eff hotter than ∼18 000 K (the exact value varying with gravity). An additional colour cut of NUV−r > 0.1 isolates binaries with largely differing T eff s, and some intruding quasistellar objects (QSOs; more numerous at faint magnitudes). Available spectroscopy for a subsample indicates that hot-star candidates with NUV−r < 0.1 (mostly 'single' hot stars) have negligible contamination by non-stellar objects. We discuss the distribution of sources in the catalogues, and the effects of error and colour cuts on the samples. The density of hot-star candidates increases from high to low Galactic latitudes, but drops on the MW plane due to dust extinction. Our hot-star counts at all latitudes are better matched by MW models computed with an initial-final mass relation (IFMR) that favours lower final masses. The model analysis indicates that the brightest sample is likely composed of WDs located in the thin disc, at typical distances between 0.15 and 1 kpc, while the fainter sample comprises also a fraction of thick disc and halo stars. Proper motion distributions, available only for the bright sample (NUV < 18 mag), are consistent with the kinematics of a thin-disc population.

We describe the content and properties of UV source catalogs from GALEX's All-Sky Imaging Survey (AIS, 5σ depth ≈19.9(FUV)/20.8(NUV) mag, in the AB system) and Medium-depth Imaging Survey (MIS, 5σ depth ≈22.6(FUV)/22.7(NUV) mag), constructed by Bianchi L., et al.: Mon. Not. R. Astron. Soc. (2010, in press). The catalogs contain 65.3/12.6 million (AIS/MIS) unique UV sources with photometric error in NUV less than 0.5 mag, over 21 435(AIS)/1579(MIS) square degrees. Matched optical data from GSC-II provide additional B, R, I photometry for the brightest sources, and SDSS provides u g r i z photometry over 7325(AIS)/1103(MIS) square degrees overlap areas. We discuss statistical properties that are relevant for understanding sample selection biases and completeness, in potential science applications of these catalogs. The FUV (1344-1786 Å) and NUV (1771-2831 Å) photometry uniquely enable selection of the hottest stellar objects, in particular hot white dwarfs (WD), which are elusive at optical wavelengths because of their hot temperatures and faint luminosities. From the GALEX-SDSS matched sources we

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We use the Galaxy Evolution Explorer (GALEX ) Medium and All-Sky Imaging Survey ( MIS and AIS) data from the first public data release (GR1), matched to the Sloan Digital Sky Survey (SDSS) DR3 catalog, to perform source classification. The GALEX surveys provide photometry in far-and near-UV bands and the SDSS in five optical bands (u, g, r, i, z). The GR1/ DR3 overlapping areas are 363 (86) deg 2 for the GALEX AIS ( MIS), for sources within the 0.5 central area of the GALEX fields. Our sample covers mostly jbj > 30 Galactic latitudes. We present statistical properties of the GALEX-SDSS matched sources catalog, containing >2 ; 10 6 objects detected in at least one UV band. We classify the matched sources by comparing the seven-band photometry to model colors constructed for different classes of astrophysical objects. For sources with photometric errors <0.3 mag, the corresponding typical AB-magnitude limits are m FUV $ 21:5, m NUV $ 22:5 for AIS, and m FUV $ 24, m NUV $ 24:5 for MIS. At AIS depth, the number of Galactic and extragalactic objects are comparable, but the latter predominate in the MIS. On the basis of our stellar models, we estimate the GALEX surveys detect hot white dwarfs throughout the Milky Way halo (down to a radius of 0.04 R at MIS depth), providing an unprecedented improvement in the Galactic WD census. Their observed surface density is consistent with Milky Way model predictions. We also select low-redshift QSO candidates, extending the known QSO samples to lower magnitudes, and providing z % 1 candidates for detailed z % 1 follow-up investigations. SDSS optical spectra available for a large subsample confirm the classification for the photometrically selected candidates with 97% purity for single hot stars, %45% (AIS) or 31% ( MIS) for binaries containing a hot star and a cooler companion, and about 85% for QSOs.

We have modelled far‐UV, UV and optical spectra of a sample of 10 hydrogen‐rich central stars of planetary nebulae (CSPN) using stellar atmosphere codes to derive their photospheric and wind parameters. The resulting stellar temperatures range from 40 to 120 kK, well spanning the CSPN evolutionary phase and allowing certain trends to be discerned. In particular, an inhomogeneous wind structure and X‐ray emission in the wind are required to match spectral diagnostic lines in many cases. For the majority of the sample, a wind clumping factor of 0.1 ≤f≤ 0.04 is derived (mainly from the P vλλ1118, 28 and O vλ1371 lines). Such factors correspond to clump densities of ∼10–25 times that of the smooth wind density, with resulting mass‐loss rates one‐third to one‐fifth the smooth wind values, which is of significant consequence to nebular dynamics, stellar and galactic evolution. Furthermore, we find clumping to begin at small radii (∼1.2 R*), as has been found when modelling the winds of (massive) O stars. The inclusion of X‐ray fluxes, presumably from shocks, in the model atmosphere calculations is found to improve the fit of the O viλλ1032, 38 line (and other features) for stars with 55 ≤ Teff≤ 80 kK, and to be absolutely necessary to match this feature for the coolest stars in our sample (Teff≲ 45 kK). These findings suggest that shocks originating from line‐driven wind instabilities leading to the formation of clumped winds and X‐rays may be a common characteristic of CSPN, as has been found for the winds of massive O‐type stars. We also find interesting results for some individual stars. NGC 1360 (Teff≃ 105 kK) displays the signature of a (previously undetected) weak stellar wind in its O vi 1032, 38 profile, and probably has the lowest mass‐loss rate ( M⊙ yr−1) of any known CSPN. In contrast, we find the wind terminal velocity of NGC 2392 ( Teff≃ 45 kK) to be v∞≃ 300 km s−1, one of the slowest CSPN wind known, probably related to its subsolar metallicity. We have included in the model calculations many elements and high‐ionization species previously neglected in analyses of this type, providing additional wind diagnostics such as Ne viiλ973 and Ar viiλ1064. The effects of including these as well as other line‐blanketing elements are discussed.