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Astron. Astrophys. 364, 712-722 (2000)

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1. Introduction

The Draco molecular cloud is one of the most interesting and enigmatic high galactic latitude molecular clouds known. It was first discovered from 21 cm observations, and was identified with an optical nebulosity in the Palomar Sky Survey (Goerigk et al. 1983). Additional observations in CO emission (Mebold et al. 1985), and more recent observations in soft X-rays with ROSAT (Herbstmeier et al. 1995) have given us greater knowledge of the cloud's composition, temperature, and kinematics, but the origin and distance to this cloud is still very uncertain. It appears as a compact and intricate "cometary cloud", approximately 5 degrees in extent, at galactic coordinates (l=90.0o,b=38.0o). The cloud is swept back into a number of dense clumps, and is surrounded by bright x-ray emission. The unusual appearance of the cloud, and its relatively large 12CO velocity (vlsr = -21 km s-1) has led many authors to conclude that it may be a molecular cloud falling into our galaxy from either an extragalactic or `galactic fountain' origin (Odenwald & Rickard 1987). In addition it has a strong X-ray shadow from ROSAT observations, and may be absorbing X-rays from beyond the local bubble (Burrows & Mendenhall 1991; Snowden et al. 1991)

The likely prospect that the Draco cloud is interacting with the halo of our galaxy near the edge of the X-ray emitting hot interstellar medium makes the determination of its distance very important for understanding the scale heights of both the neutral and hot components of the interstellar medium.

Mebold et al. (1985) attempted to derive a distance from star counts, and determined a distance of 300 [FORMULA] d [FORMULA] 800 pc. A photometric study by Goerigk & Mebold (1986) increased the minimum possible distance to d [FORMULA] 800 pc, citing the lack of reddening in stars with V [FORMULA] 14. Lillenthal et al. (1991) attempted to detect the cloud from absorption lines of Na I D toward nearby bright stars, and derived a lower limit to the distance of d [FORMULA] 180 pc. More recently, Gladders et al. (1998) has derived a distance estimate of [FORMULA] pc to the Draco cloud based on a single detection of Na I absorption from a background star.

In this work we present new CCD photometric observations of 362 stars with 14 [FORMULA] V [FORMULA] 19 to provide a more complete sample of background stars which are expected to be at distances in excess of d [FORMULA] 1200 pc. By combining this photometry with star count models which include halo and disk populations of stars, an exponentially stratified H I background, and an intervening interstellar cloud of fixed distance and extinction we can constrain estimates of the distance and extinction of the Draco cloud.

Examples of recent works which make use of similar techniques include Thoraval et al. (1997), who were able to model small scale structure in clouds using only star counts, and Andreazza et al. (1996) who report the extinction and structure of Southern dark clouds in Lupus and Corona Australius using star counts. Another study by Magnani & de Vries (1986) used digitized Palomar Sky Survey prints to derive distances to several high latitude cirrus clouds from an analysis of star counts. For our star count analysis we make use of the model of galactic structure from Bahcall & Soneira (1980), to which we have added an additional extinction term for an intervening molecular cloud at an arbitrary distance and extinction. This model was modified from the program `galaxy' available on the internet, which was reviewed by Bahcall (1986). With this model and our new photometry, we hope to provide a solid basis for future studies of this fascinating object, and to give the first upper limits for the Draco cloud distance based on deep CCD photometry.

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© European Southern Observatory (ESO) 2000

Online publication: January 29, 2001