The Hydrogen Hole at the North Galactic Pole

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Session 35 -- ISM: Abundances and Dust
Display presentation, Wednesday, June 14, 1995, 9:20am - 6:30pm

[35.08] The Hydrogen Hole at the North Galactic Pole

T.R.Ayres (CASA/U.Colorado), J.L.Linsky (JILA/U.Colorado \& NIST), A.Diplas (CASS/UCSD)

The yellow giant 31~Comae (HD111812: G0~III) lies near the North Galatic Pole ($b^{II}= +89{\fdg}6$; $l^{II}= 114{\fdg}9$). Although 80~pc distant in the Coma Berenices open cluster, 31~Com is a strong EUV source. Recent HST/GHRS spectra of the star show surprisingly weak Mg~II $\lambda2800$ and H~I $\lambda1215$ interstellar absorptions. The neutral hydrogen column in that direction must be quite low, reminiscent of the interstellar ``void'' toward $\epsilon$~CMa discovered by the EUVE. Indeed, previous EUVE observations have shown that the lines of sight to the hot white dwarfs HZ~43 ($b^{II}= +84\deg$; $l^{II}= 54\deg$) and GD~153 ($b^{II}= +85\deg$; $l^{II}= 317\deg$) have very low columns, $N_{\rm{H}} < 1\times10^{18}$~cm$^{-2}$, suggesting another ``tunnel'' near the NGP. We modeled the GHRS spectra of 31~Com to determine whether there is a further northward extension of the HZ~43/GD~153 void.\\[-3mm]

The interstellar Mg~II h \/ and k \/ absorptions in the 31~Com spectrum are quite sharp, suggesting only a single velocity component along the line of sight (compared with several distinct clouds in the direction of $\epsilon$~CMa). Our preliminary modeling of the Mg~II, H~I, and D~I ISM features indicates that $\xi_{\rm t}$, $T$, and [D/H] are similar to the values measured previously for the lines of sight toward the nearby stars Capella and Procyon. Furthermore, the bulk velocity ($-2$~km~s$^{-1}$, heliocentric) is the same as predicted for the local cloud. The inferred neutral hydrogen column, $N_{\rm{H}} \approx 8\times10^{17}$~cm$^{-2}$, is one of the smallest ever measured directly from the Ly$\alpha$ absorption. We speculate that the majority of the neutral gas in the direction of 31~Com lies very close to the Sun in a few~pc extension of the local ``fluff'', and that the remainder of the sightline is quite rarified and probably hot. In all likelihood the HZ~43/GD~153 void is considerably larger than suspected previously.\\[-2mm]

This work was supported by NASA grants GO-5323.01-93A (HST), S-56460-D (HST), and NAG5-2274 (EUVE).

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