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

3. Results and discussion

3.1. HVC 100-7+100

The channel maps (Fig. 2; resolution  arcmin) show the monochromatic brightness distributions of this object at velocities between and  km s^-1. In calculating column densities , we have integrated from to  km s^-1, thus excluding the marginal feature around  km s^-1. The resulting column-density map (top-right panel of Fig. 2) only shows two small condensations, lying close together. In principle, we cannot exclude the possibility that HVC 100-7+100 might have an extended component, to which the synthesis instrument would not be sensitive. Comparison (Fig. 3a) of the spectrum obtained with the Jodrell Bank 76-meter single-dish telescope (Bates et al. 1991) and that measured at Westerbork, convolved to the beam of the single dish, suggests indeed that the synthesis observation does not fully recover the total flux (cf. Sect. 2.6). However, the Jodrell Bank map obtained by de Vries et al. (1997) shows no trace of any extended feature, and its morphology agrees fully with the Westerbork map. Hence, we assume that the Westerbork map in Fig. 2 is a fair approximation of the true column-density distribution in HVC 100-7+100.

Our results confirm that HVC 100-7+100 is quite small, as suspected already by Bates et al. (1991). The Westerbork map shows that the cloud consists of two condensations; both have peak column densities of order  atoms cm^-2 and velocity widths of order 15 km s^-1. At a distance of order 0.6 kpc (i.e., half the distance of the star 4 Lac), the angular diameters of about 5 arcmin correspond to about 1 pc. The HI masses in both condensations then are of order , and their average densities of order 2 atoms cm^-3. The total mass of both condensations together could at most be about , if they are at the same distance as 4 Lac: 1.2 kpc. Table 3 summarizes the estimated properties of these condensations.

From Westerbork maps of about 1 arcmin resolution, Wakker & Schwarz (1991) have found small-scale structure of similar angular sizes (1 - 5 arcmin) in several major HVCs. However, those HVCs are certain or likely to have distances of at least several kpc, hence their condensations have linear sizes of several pc; their HI column densities exceed those in HVC 100-7+100 by one or two orders of magnitude, and the masses of those condensations are much greater: of order . Hence, the condensations found here appear to be of a different nature.

The parameters of the Westerbork spectrum are listed in Table 2. Combination of Westerbork and Jodrell Bank data, as described in Sect. 2.7, yields the estimated spectrum (Spectrum 4) shown in Fig. 3b; its parameters are also given in Table 2. The best estimate for the HI column density in the direction of the star 4 Lac is  atoms cm^-2. The angle subtended by the star is, of course, only a small fraction of a second of arc. In view of the noise in the spectra of Fig. 3a and b, and the possible presence of unresolved small-scale structures, we must consider the value of uncertain by a factor of 2. Using the column densities of Fe II, Mg I, Mg II, O I and Al II measured by Bates et al. (1990) with IUE, we derive the ion abundances listed in Table 4. While the uncertainties of these values are considerable (factors 2 - 6), together they suggest that the metallicity of HVC 100-7+100 is close to the cosmic value.

Table 2. HI spectrum parameters at the star's position

Table 3. Physical properties of concentrations in HVC 100-7+100

Table 4. Abundances in HVC 100-7+100

The small mass of the cloud, and its position with respect to the star, might suggest a possible circumstellar origin. However, the HVC's velocity of  km s^-1 is very different from that of the star (4 Lac = HR 8541),  km s^-1 (Hoffleit 1962). If the HVC were part of an expanding shell around 4 Lac, one would also expect absorption at large negative velocities, around -150 km s^-1. Our data do not include such velocities. However, the Leiden-Dwingeloo Survey by Hartmann & Burton (1997) shows intense emission at these velocities over a large region of sky, undoubtedly to be identified with the well-known Outer Arm; a possible shell at -150 km s^-1 would be confused with this emission. The observed absorption at high positive velocities is inconsistent with an expanding-shell hypothesis.

Another possible origin might lie in a supernova explosion. We have searched the Green (1996) catalogue for supernova remnants (SNRs) in the neighbourhood. In order to produce an absorbing cloud at  km s^-1, the SNR would have to have a shorter distance than 4 Lac. Unfortunately, for most SNRs listed by Green no distance estimate is available. However, none of Green's SNRs lies closer than seven times its own diameter to the line of sight to 4 Lac. Moreover, a velocity of  km s^-1 would be quite high for a neutral portion of an SNR. We note, in particular, that no SNRs are known in the Lacerta association around , . Also, no pulsar is known to have originated there (Blaauw, private communication). Thus, it seems unlikely that HVC 100-7+100 could be explained as part of the debris of a supernova. The origin of this cloud thus remains a puzzle.

3.2. HVC 347+35-112

Fig. 4 shows channel maps at velocities ranging from -139 to -94 km s^-1. Summation of these has yielded the column-density map in the top-right panel of Fig. 4. These maps have angular resolution of  arcmin. The fine-structure information obtained is further limited by the heavy cleaning, necessitated by incomplete u v-coverage, and by the low brightness temperatures, which remain below 0.2 K. The main concentrations lie at the edge of the primary beam, to the West and SSW of the star; those to the N and SE are marginal. The Jodrell Bank results by de Vries et al. (1997, in preparation), while in fair agreement with the Westerbork map, show weak radiation extending over a degree. The Dwingeloo survey (beam 36 arcmin) by Hulsbosch & Wakker (1988) and that by Bajaja et al. (1985) at Villa Elisa (beam 30 arcmin) show emission at similar velocities extending over a larger region. This emission was listed by Wakker & van Woerden (1991) as Complex L, which extends over 22 square degrees and has a mass of , where D is the distance in kpc. The prominent elongated feature in Fig. 5 is called HVC #132 by Wakker & van Woerden (1991), and is the brightest part of Complex L. While it is not impossible that the gas near HD 135485 seen in Fig. 4 is an isolated, small feature, it appears more likely that it is part of HVC #132.

Fig. 4. Maps of HVC 347+35-112. The top-left panel shows the synthesized beam, with maximum 1; negative contours are dashed. The top-middle panel shows the continuum; units mJy/beam. The top-right panel gives the HI column densities (or "total hydrogen"); contour values, in units of  atoms cm-2, are -0.75, -0.50, -0.25, (dashed), , , , , , (dashed). The remaining panels show monochromatic brightness distributions, at velocities spaced by 4.12 km s-1 from -138.9 to -93.6 km s-1, as given in the top-left corner of each panel; brightness temperatures shown in gray-scale, identified by scale-bar at top-right, and by contours, with values 0.057 K (dashed), 0.073 K, 0.107 K, 0.140 K, 0.173 K, 0.207 K and 0.223 K (dashed). Velocity resolution: 7.6 km s-1; angular resolution: (FWHM), shown by hatched ellipse in continuum panel. The position of the star HD 135485 is marked by an asterisk.

Fig. 5. Overview of HVC #132 (Wakker & van Woerden, 1991) and other clouds belonging to Complex L. Brightness temperatures (contour values: 0.05 K, 0.10 K, 0.20 K and 0.30 K) taken from the survey by Hulsbosch & Wakker (1988; beam FWHM, grid spacing ), except at , ; that point comes from Bajaja et al. (1985; beam, grid). Assuming profile half-widths (FWHM) of 20 km s-1, the corresponding column-density values are: 2, 4, 8 and  atoms cm-2. The positions of four stars observed by Albert et al. (1993) are marked.

The distance of these clouds is unknown. While Albert et al. (1989, 1993) had suggested a relationship of the CaII absorptions at velocities of -98 and -127 km s^-1 in HD 135485 to the high-velocity hydrogen in the region, and van Woerden (1993) had noted that this set an upper limit of 2.4 kpc on the distance of Complex L, IUE spectra by Danly et al. (1995) indicate that the absorptions are circumstellar. Thus, HVC #132 and Complex L may be beyond the star as well as in front.

Since the concentrations seen in Fig. 4 probably extend beyond the primary beam as a large, filamentary structure, and no distance constraints are available, we refrain from estimating their sizes, densities and masses.

As the Westerbork map (Fig. 4) shows no concentration at the position of the star, the best estimate for in the direction of HD 135485 follows directly from the single-dish spectrum at that position:  atoms cm^-2 (Table 2). Because the relationship between the HI structures and the CaII and UV absorptions is now severely in doubt, we cannot give metal ion abundances for Complex L.

© European Southern Observatory (ESO) 1998

Online publication: September 8, 1998
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