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Astron. Astrophys. 338, 200-208 (1998)

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1. Introduction

1.1. Definition and properties of High-Velocity Clouds; outline of this paper

High-Velocity Clouds (HVCs) are concentrations of atomic hydrogen gas whose velocities are incompatible with simple models of differential rotation in the Galactic Disk. In practice, HVCs are generally defined in terms of a lower limit on the absolute value of the LSR velocity, e.g. 80 km s-1 (at [FORMULA]). Sizes vary from a few arcminutes to more than 10 degrees; column densities range up to 1020 atoms cm-2. For reviews of the subject see Wakker (1991) and Wakker & van Woerden (1997).

In view of their anomalous velocities, HVCs probably lie outside the Galactic Disk, in the Halo. However, even 30 years after their first discovery (Muller et al. 1963), the origins of most HVCs remain unknown. The most likely origins appear to be: infall of debris from tidal interactions between the Galaxy and the Magellanic Clouds, and circulation of gas between the Disk and the Halo, driven by violent events in the Disk (Wakker & van Woerden 1997).

A key problem in the investigation of origins is the lack of information about the distances of HVCs, and thus about their precise locations within the Milky Way. Upper and lower limits to HVC distances may, in principle, be derived from the presence or absence of absorption by metal ions at the HVC's velocity in spectra of stars at various distances. In most HVCs investigated, metal ions have indeed been found present through their absorption in the spectra of extragalactic background probes (Schwarz et al. 1995, Wakker & van Woerden 1997). However, so far only few solid distances are available (Wakker & van Woerden 1997).

For two smaller clouds, which we name HVC 100-7+100 and HVC 347+35-112, upper distance limits were reported several years ago, although for the latter cloud the limit is no longer valid (see below). For these two clouds, we have used the Westerbork Synthesis Radio Telescope in the 21-cm HI line, in order to determine their small-scale structure and measure their physical properties. Together with single-dish observations obtained at Jodrell Bank, the Westerbork maps further provide improved HI column densities on the lines of sight to the stars, and hence (for HVC 100-7+100) improved metal ion abundances.

In the remainder of this section, we summarize previous information on HVC 100-7+100 and HVC 347+35-112. Sect. 2describes the Westerbork observations and their reduction. Sect. 3discusses the results. Sect. 4gives conclusions.

1.2. HVC 100-7+100

Bates et al. (1990) found interstellar absorption lines of several ions (AlII, FeII, MgI, MgII, OI) at [FORMULA] km s-1 in IUE spectra of the star 4 Lac (= HD 212593), which lies at 1.2 kpc distance in [FORMULA], [FORMULA]. (Ultraviolet and visual absorption at [FORMULA] km s-1 had earlier been found in several stars over a region of 40 by 30 degrees (Bates et al. 1983), and a possible association with the Radio Loops II and III suggested.) At Jodrell Bank, 21-cm HI emission at [FORMULA] km s-1 was found on the position of 4 Lac (with [FORMULA] K and FWHM [FORMULA] km s-1), but not at neighbouring positions, suggesting that the absorbing cloud was quite small (Bates et al. 1991). Van Woerden (1993) noted that this cloud, to be called HVC 100-7+100, had not been detected in the Dwingeloo survey of Hulsbosch & Wakker (1988). Assuming a distance of D kpc for the cloud, he derived limits of [FORMULA] and [FORMULA] for its HI mass; the lower value followed from the Jodrell Bank observation, assuming the cloud to be small and centred on the star; the higher value was valid if the cloud was located at [FORMULA], [FORMULA], and had the largest brightness and size consistent both with the Jodrell Bank observation and with the non-detection at the four surrounding Dwingeloo grid points.

The star 4 Lac sets an upper limit of 1.2 kpc on the distance of this HVC, and hence of 140 pc on its distance from the Galactic plane (van Woerden 1993). This places HVC 100-7+100 within the generally assumed boundaries of the Galactic HI layer. On the other hand, since in this direction differential Galactic rotation leads to negative velocities, this HVC must have a velocity of at least 100 km s-1 with respect to its surroundings. These facts made this HVC a very unusual one. We undertook Westerbork observations in order to derive its size, structure, mass and further physical properties.

1.3. HVC 347+35-112

CaII K-line absorption was observed by Albert et al. (1989, 1993) at LSR velocities of -98 and -127 km s-1 in the star HD 135485 ([FORMULA], [FORMULA], distance 2.4 kpc, z height [FORMULA] kpc), but not in three neighbouring stars: HD 135230 ([FORMULA] pc), HD 135681 ([FORMULA] pc), and HD 138485 ([FORMULA] pc). Van Woerden (1993) noted that the position of HD 135485 and both absorption velocities lie within the ranges covered by HVC Complex L (Wakker & van Woerden 1991), which consists of several small clouds, together covering only 22 square degrees, and with l, b, [FORMULA] averages [FORMULA], [FORMULA], -112 km s-1. At an assumed distance of 1 kpc this HVC complex would have a mass of [FORMULA]. In what follows, we use the star's position in naming the HI observed in the field around HD 135485: HVC 347+35-112.

The distance bracket [FORMULA] kpc suggested by Albert et al. (1989) prompted us to obtain Westerbork observations in the direction of HD 135485, and derive the small-scale structure, HI column density and Ca+ abundance of this HVC. Meanwhile, Albert et al. (1993) noted that ultraviolet spectra of this star are complex, revealing possible stellar wind features and suggesting that the optical absorption lines found might be attributable to circumstellar, rather than interstellar, material. After the present study had been essentially completed, analysis of the ultraviolet spectrum of HD 135485 led Danly et al. (1995) to the firm conclusion that the high-velocity CaII absorptions must be circumstellar. We discuss the consequences of this in Sect. 3.2.

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© European Southern Observatory (ESO) 1998

Online publication: September 8, 1998