Reprinted with permission from A.G.D. Phillip, ed. Third Conference On Faint Blue Stars, Schenectady, NY (IAU Symposium).

THE SANDAGE TWO-COLOR SURVEY OF THE GALACTIC PLANE: THE CONTINUING SEARCH FOR LOW-LUMINOSITY OBJECTS AND OTHER INTERESTING SOURCES.

Howard H. Lanning

Astronomy Programs, Computer Sciences Corporation, Space Telescope Science Institute

Michael Meakes

Space Telescope Science Institute


ABSTRACT: Examination of the Sandage two-color photographic survey of the Galactic Plane, taken in support of the UHURU X-ray satellite, is continuing. Visual inspection of the double-exposed U and B plates has produced 195 sources to date ranging in color from U-B = 0 to U-B = -1.5. Sources identified include several interesting and peculiar objects including cataclysmic variables, white dwarfs, Be stars, variable sources, etc., many of which have subsequently been observed or found to be coincident with newer X-ray and EUV spacecraft surveys. A clear potential for identification of low-luminosity stars, binary systems, old novae and other unusual objects is demonstrated. We will review in this poster paper the description and analysis of the Sandage survey plates, as well as present the current status of the overall project.


I. INTRODUCTION
During the period 1969-1972, an extensive two-color (UV and B) photographic survey of the Galactic Plane covering >4000 square degrees was obtained. This NASA funded project, headed by Dr. Allan Sandage, was taken in support of the explorer class X-ray satellite UHURU (launched in 1970) with the intent of providing optical identifications of newly detected X-ray sources. More than 100 plates were exposed by members of the project using the Palomar Observatory 48-in Schmidt telescope. The last few months of the project, a comprehensive pilot program was initiated to visually scan a small sample of the plates in an effort to identify all sources exhibiting a detectable color excess in the UV with the anticipation of discovering many new hot low-luminosity stars. Since publication of the pilot program (Lanning 1973, hereafter referred to as Paper I), two additional lists of UV-bright sources have been published (Lanning & Meakes 1994, Lanning & Meakes 1995, Papers II & III) representing a total of 195 sources identified to date. Followup studies by investigators of low-luminosity stars for a number of the UV sources ($\sim$25%) from these lists have resulted in the identification of many previously unknown and very interesting objects including two cataclysmic variables (CVs), a very hot He white dwarf, a variable Be shell star, a QSO on a high latitude plate near the X-ray source HZ Her), and several potentially interesting variable sources or possible old novae. At least 6 sources have been studied with EINSTEIN, EXOSAT, IUE, ROSAT, and EUVE. Additionally, the ROSAT group has subsequently noted, based on a search of the SIMBAD database, that 66 sources are coincident with ROSAT all-sky survey fields.

Other systematic survey projects including the high-latitude project by Green (1977,1986) and a Galactic Plane program by Downes (1986) have attempted to identify field white dwarfs by first completing a two-color photographic survey very similar to the Sandage survey. (A newer high-latitude two-color survey extension to the sourthern sky is currently in progress (Stobie et al. 1994)). Investigators participating in these projects then obtained low-resolution spectra of selected UV-excess candidates from which spectral classifications were made. Ultimately, these classifications, combined with photometric magnitudes and colors, were used to determine the space density and scale heights of white dwarfs, based on the cumulative lists of all known white dwarfs, with the goal of determining white dwarf evolutionary constraints and timescales. Both surveys were complete only to a B magnitude of $\sim$16. Because plates in the Sandage survey extend well beyond to limiting magnitudes in the range of 18.5-20.5, sources from them may be useful in further improving recent space density determinations as well as resulting in the detection of many new and interesting low-luminosity objects. The Sandage two-color survey remains one of the more productive and deepest multi-color photographic surveys of the Galactic Plane.

II. THE SURVEY - DESCRIPTION
The survey was centered on the Galactic Plane with overlapping regions covering the galactic latitudes $\pm$ 9 degrees, and extending throughout most of the northern plane l = 336o - 240o). Each Schmidt plate covers a region of approximately 43 square degrees of the sky. The plates were double-exposed on a 103a-O emulsion with one image taken in the UV using a UG1 filter and a second image taken through a GG13 filter separated by 12 arcseconds. Exposure times were selected and corrected for air mass in order to achieve a U-B = 0 for equal sized images. Therefore, examination of the paired images would be characterized by a larger image in the UV for objects similar to low-luminosity stars, normal early B stars, peculiar systems, etc. This multi-color photographic technique has been described, for example, by Haro and Herbig (1955).

Plates are visually scanned through a stereoscopic microscope in a raster pattern with overlapping fields to assure that all areas of the plate are covered. Blue magnitudes and U-B color differences are calibrated on each plate using identifications of local field stars or galactic clusters. All objects with images brighter in the UV are then marked on the plates. This process is more difficult for images exhibiting distortions in the blue or ultraviolet image resulting from guiding errors or merged images. Firm identifications and magnitude estimates are therefore generally concentrated on those objects with mB$\sim$10 or fainter. Automated scanning techniques attempted by Lanning using the KPNO PDS have also suffered due to extreme crowding in the galactic plane fields. Workable threshold settings limited the range and accuracy of detection. The dynamic range and ability to differentiate merged sources by the eye therefore strongly justified the return to the visual scanning method.

Upon completion of the scan of an entire plate, initial positions are determined for each source selected using overlays generated with in-house tools. Images are then extracted from the Space Telescope Science Institute collection of Guide Star digital plate scans using the GS Astrometric Support Program (GASP), or more recently via the Digitized Sky Survey Web interface. Each source is then verified from correlation with the two-color plate images. Accurate J2000 positions are determined using STSDAS/IRAF routines from the DSS images for the identified sources and marked finding charts are created.

Prior to publication, the latest SIMBAD database and EUVE and ROSAT catalogs are searched to cross reference any previously identified objects. The SIMBAD search is defined to retrieve database entries for all sources within a 5 arcmin radius of the UV positions. References and identifications for all targets returned are examined and noted as appropriate in the tables and text of the catalog in preparation. Following the SIMBAD nomenclature previously established, all newly identified sources are referred to by their Lanning ID number.

III. RESULTS TO DATE
Follow-up investigations of $\sim$25% of the published Lanning UV-bright sources have shown that at least 10% are interesting objects including CVs, white dwarfs, QSOs, variable stars, etc., cf Table 1. While Papers I-III contain a high percentage of normal early-type B stars and subdwarfs, evidence suggests a relatively high rate of white dwarf and peculiar object discovery unique to this survey. This rate of discovery is in good agreement with the discovery rate quoted by Downes (1986). However, Downes' rate is based upon follow-up studies of the predominately brighter UV sources, i.e. brighter than mB=16. The detection of interesting candidates could be much higher when including fainter sources from more extensive, deeper surveys obtained with larger aperture ground-based telescopes such as the Sandage survey. Each catalog of Lanning UV sources published to date, or in work, typically represents scans of at least three Schmidt plates and adds $\sim$70 UV candidates to the compiled list. Therefore, should the trend continue, we might expect to identify seven or more candidates worthy of more intensive ground-based and space satellite investigation from each catalog. All material from this survey published to date may be viewed on the WWW site devoted to the survey, cf http://www.stsci.edu/$\sim$lanning/index.html.


Table 1. Unusual/Interesting UV Objects Studied to Date

Type Source Name References
CVs: Lanning 90 = V1776 Cyg Shafter, Lanning, & Ulrich (1983); Garnavich, et al. (1990)
  Lanning 10 = V363 Aur Horne, Lanning, & Gomer (1982); Margon & Downes (1981)
    Szkody & Crosa (1981)
Be: Lanning 33 Margon & Downes (1981); Szkody & Crosa (1981)
DO WD: Lanning 14 = WD0640+015 Margon & Downes (1981)
QSO: Lanning 15 = QSO1656+348 Grueff & Vigotti (1972); Pauliny-Toth, et al. (1972)
QSO?: Lanning 41 Pesch (private communication)
ROSAT: Lanning 18 = RE J1847+015 Pounds, et al. (1993)
  Lanning 23 = RE J2249+583 "
Nova?: Lanning 17 Lanning (1973); Margon & Downes (1981)
Var: Lanning 37 Margon & Downes (1981)


IV. THE FUTURE
The current high interest in UV-bright objects, as well as continued interest shown in this particular survey, offers strong justification for a more dedicated and constant level of effort for this project. Interest in pursuing ground-based follow-up programs by colleagues in the field has also been expressed and will help in providing firm classifications for the UV objects investigated. Particularly interesting objects including new CVs similar to Lanning 10 (V363 Aur) and Lanning 90 (V1776 Cyg) will be of special interest for more detailed studies.


REFERENCES:
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Garnavich, P. M., Szkody, P., Mateo, M., Feinswog, L, Booth, J., Goodrich, B.,Miller, H. R., Carini, M. T., and Wilson, J. W. 1990, ApJ, 365, 696

Green, R. F. 1977, PhD Thesis

Green, R. F., Schmidt, M., Leibert, J. 1986, ApJS, 61, 305

Grueff, G. and Vigotti, M. 1972, ApJS, 61, 569

Haro, G. and Herbig, G. H. 1955, Bol. Obs. Tonantzintla y Tacubaya, 12, 33

Horne, K., Lanning, H. H., Gomer, R. H. 1982, ApJ, 252, 681

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Margon, B. and Downes, R. A. 1981, AJ, 86, 747

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Stobie, R. S., Kilkenny, D., O'Donoghue, D., Chen, A., 1994, IAU Colloq. 148

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UV Survey Intro Page