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1992Aj 104. . 203W the Astronomical Journal 203W . THE ASTRONOMICAL JOURNAL VOLUME 104, NUMBER 1 JULY 1992 104. STUDIES OF HOT B SUBDWARFS. VIII. STRÖMGREN PHOTOMETRY OF HOT, HYDROGEN-RICH SUBDWARF CANDIDATES IN THE PALOMAR-GREEN AND KITT PEAK-DOWNES SURVEYS 1992AJ F. Wesemael,1 G. Fontaine,1 P. Bergeron, and R. Lamontagne Département de Physique, Université de Montréal, C. P. 6128, Suce. A, Montréal, Québec H3C 3J7, Canada R. F. Green Kitt Peak National Observatory, National Optical Astronomy Observatories, P.O. Box 26732, Tucson, Arizona 85726-6732 Received 5 March 1992 ABSTRACT We present the results of an extensive program of photoelectric observations on the Strömgren system of 310 hot, hydrogen-rich (B and OB) subdwarf candidates in the Palomar-Green and Kitt Peak- Downes colorimetric surveys. 1. INTRODUCTION able information on the importance of this channel to the white dwarf stage (Drilling & Schönbemer 1985; Heber Colorimetric surveys, such as the Palomar-Haro- 1986), and on whether or not these stars are true disk Luyten survey (Haro & Luyten 1962), the Palomar-Green objects (e.g., Baschek & Norris 1975). survey (Green et al 1986), the Kitt Peak-Downes survey With these considerations in mind, we have carried out (Downes 1986), the Kiso survey (Noguchi et al 1980; extensive photoelectric observations in the Strömgren sys- Kondo et al 1984), and the ongoing Edinburgh-Cape tem of a large, magnitude-limited sample of subdwarf stars (Stobie et al 1987; Kilkenny et al 1991) and Montréal- identified in the PG survey. These observations represent a Cambridge-Tololo (Demers et al 1986) surveys are a significant extension of those reported by Bergeron et al valuable source of new, blue subluminous objects, above all (1984; hereafter referred to as Paper I), where Strömgren hot subdwarfs of different kinds. In the PG survey, for observations of 25 sdB stars from the Greenstein & Sargent example, the sdB stars represent the dominant population ( 1974) list were presented. Emphasis was placed, for these at all magnitudes down to the completeness limit of observations, on objects more likely to be hydrogen-rich i?pg~ 16.3, and they comprise almost half of the objects in subdwarfs. It is for these generally cooler stars that Ström- the catalog brighter than 16th magnitude. With its sample gren photometry will be the most useful, since the optical of 684 hydrogen-rich subdwarf candidates, the PG sample energy distribution of the hot sdO stars tends to be close to represents a valuable source of statistical information on the Rayleigh-Jeans limit. these objects. Follow-up photometric observations represent an inte- 2. OBSERVATIONS gral, and often essential, part of such colorimetric surveys. 21 The Selection of the Sample They can be used, first, to complement the initial spectral classification of survey objects, which, in the case of the PG In the PG spectral classifications, several internal survey, consisted of photographic and SIT spectra at ~ 10 classes are introduced to reflect the information available  resolution. They can also, if obtained for extensive sets from the low-dispersion classification spectra. We denote of color-selected candidates, help isolate new or peculiar them throughout this paper with italics, to differentiate classes of objects: in the Edinburgh-Cape Survey, for ex- them from the usual (but no more homogeneous) classifi- ample, photoelectric photometry has been used to identify cations found in the literature. Of particular interest to us a class of apparently normal B stars far above the galactic are the sdB class, where stars display the Balmer series plane. Photoelectric photometry can also be used to fix the characteristic of a high-gravity atmosphere; the sdB-0 optical magnitudes in detailed follow-up fitting of energy class, sdB stars with evidence for He I; the sd class, where distributions, as in the work of Heber (1986). Finally, a few Balmer lines are seen in lower signal-to-noise spectra; photometric data can also be used to carry out statistical the sdOA class, with spectra dominated by Balmer lines, studies of the properties of particular subsamples of the along with pronounced He I A4471 and often He I A4026; survey, such as studies of their galactic distribution, space and, finally, the sdOB class, with spectra dominated by density, and birthrate. In the particular case of the He I and He II lines, and generally showing Balmer absorp- hydrogen-rich subdwarfs, these studies could yield valu- tion. The latter is, perhaps, the most ambiguous of all of them in terms of assigning offhand a chemical composition 1 to its members. We had initially considered the possibility Guest observer, Kitt Peak National Observatory, National Optical that many of these stars might be true hydrogen-rich sub- Astronomy Observatories, operated by the Association of Universities for Research in Astronomy, Inc., under cooperative agreement with the dwarfs; Kilkenny et al (1988), on the other hand, present National Science Foundation. the Green et al sdOB stars as true helium-rich sdO stars. 203 Astron. J. 104 (1), July 1992 0004-6256/92/070203-13S00.90 © 1992 Am. Astron. Soc. 203 © American Astronomical Society • Provided by the NASA Astrophysics Data System 203W . 204 WESEMAEL ET AL. : HOT B SUBDWARFS 204 104. In any case, it is expected that most of the PG objects photomultipliers, as well as with diaphragms of ~ 18 arsec. included in these classes will have hydrogen atmospheres, The filter set was one of the KPNO Strömgren set (u although later reobservations at improved S/N ratio and #1179; v #1162; b #1161; and y #1176), and was used model atmosphere analyses will undoubtedly affect the throughout. The observing procedure is identical to that 1992AJ spectral classification of some stars. Thus, we endeavored used for the observations of Paper I: the photometer cycles to observe as many members as possible of the sdB, sdB-O, through the filters for a star measurement, and the tele- sd, sdOA, and sdOB classes. The limiting photographic scope then moves to a patch of empty sky for the sky magnitude of our photoelectric survey was chosen on the measurement. The total integration time was chosen for basis of two criteria. First, the requirement to have a suf- each object so as to ensure that at least 104 counts had ficiently large sample of objects for that sample to be usable accumulated above the sky in each bandpass. for statistical analyses; and second, the requirement that As in Paper I, the data were reduced with the KPNO narrowband colors be obtained within a reasonable amount reduction program kindly made available to us by J. of time on a 1.3 m telescope. We adopted a limiting mag- Barnes. We used the usual transformation equations of nitude of Bvg~ 14.6, and observed a total of 310 objects in Crawford & Barnes (1970), except that the index (u — b) the course of 55 nights of observation. The 310 objects are was employed instead of q. Thus, divided as follows: 286 are from the PG catalog, 23 are ; from the KPD survey, and the remaining object, UV 1758 y=A +y0bs+^(^-> ), ( 1 ) + 36, is a TD1 ultraviolet source for which accurate mag- (b-y)=C+D(b-y) , (2) nitudes were required for the analysis of Lamontagne et al ohs (1985). Of the 286 PG objects observed, 145 are classified m^E+Fm^te+Ab-y), (3) sdB, 36 are sdB-O, 51 are sd, 19 are sdOA, 28 are sdOB, 2 (PG 1104+243 and PG 1636+104) are classified as bina- (U-b) = G+H(U-b)ohs+I(b-y), (4) ries (bin), and 5 are classified as horizontal-branch B stars (HBB). For the latter, this was not the original classifica- where the label obs refers to magnitudes and color indices tion made by Green in the privately circulating version already corrected for extinction. In the above, mx = (v—b) which we used to select our program objects when this — (b—y). Average values of the extinction and transfor- observing project was initiated. The final classification as mation coefficients were obtained for all runs, although, in HBB stars was made on the basis of their red colors, and is one case (run of 09/1984), it was deemed preferable to use clearly justified as all five have q > +0.100 (see below). the extinction obtained from the later 12/1984 run. All Specifically excluded from our new observations were extinction and transformation coefficients are listed in Ta- ble l.2 The coefficients / and J were set to zero for all runs, 10 PG objects which satisfy the l?pg criterion, but which were already included (under other names) in the so- and the zero-point terms (A, C, E, and G) were adjusted called Greenstein-Sargent hydrogen-rich subdwarf sample nightly. Standard stars were selected among the list of B- observed in Paper I. Specifically, these are: PG 0101+039 type stars published by Crawford et al ( 1971 ). Because of (Fll), PG 0958—073 (GD 108; this object is incorrectly the rather extreme properties of our program objects com- referred to as GD 198 in Paper I), PG 1101+249 (F36), pared to those standards, we restricted our selection of PG 1114+073 (F38), PG 1202 + 608 (F55; it is also GD standard stars to the very faintest and bluest stars of that 314 and LB 2197), PG 1329+159 (F81), PG 1338 + 611 list. Hence, our standards generally satisfied simulta- (F87), PG 1424+332 (Ton 199), PG 1433 + 239 (Ton neously F>6.0, (U—B) % —0.44, and (6-y) <+0.050. 209), and PG 1512 + 244 (Ton 788). Typically, two dozen standard stars were selected for each Three additional PG stars can be found in Paper I, but run.
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