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1. INTRODUCTION the Supershells Are Produced by Multiple Supernova Explo- Sions

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1. INTRODUCTION the Supershells Are Produced by Multiple Supernova Explo- Sions THE ASTROPHYSICAL JOURNAL, 476:717È729, 1997 February 20 ( 1997. The American Astronomical Society. All rights reserved. Printed in U.S.A. THE STRUCTURE OF THE GALACTIC MAGNETIC FIELD TOWARD THE HIGH-LATITUDE CLOUDS ANA I. GOMEZ DE CASTRO,1 RALPH E. PUDRITZ,2 AND PIERRE BASTIEN3 Received 1995 December 22; accepted 1996 September 5 ABSTRACT We present the results of an optical polarization survey toward the galactic anticenter, in the area 5h º a º 2h and 6¡ ¹ d ¹ 12¡. This region is characterized by the presence of a stream of high-velocity H I as well as high galactic latitude molecular clouds. We used our polarization data together with 100 km IRAS maps of the region to study the relation between the dust distribution and the geometry of the magnetic Ðeld. We Ðnd that there is a correlation between the percent polarization and the 100 km Ñux such thatP(%) ¹ (0.16 ^ 0.05)F . When the IRAS Ñux is converted into H I column densities this becomesP(%) ¹ (0.13 ^ 0.03)N 100[ 0.22, which is consistent with previous interstellar medium studies on the relation between reddening20 and polarization. This correlation indicates that our survey is as deep as IRAS and that the magnetic Ðeld geometry does not change strongly with the optical depth in the lines of sight that we have studied. The implied lower limit to the distance of our survey is 500È700 pc at galactic latitudes b \[20¡ and 100 pc at b \[50¡. Our main Ðnding is that the magnetic Ðeld is perpendicular to the H I high-velocity stream as well as the molecular cloud MBM 16 in the high-latitude region. The Ðeld is also perpendicular to the velocity gradient in the stream. Closer to the plane, the magnetic Ðeld is parallel to the dust Ðlaments that extend like a plume toward the halo. Our observations indicate that the galactic magnetic Ðeld toward the high- latitude clouds is toroidal. We propose a model in which Ñux tubes that rise out of the galactic plane become force-free and predominantly toroidal at high latitudes. The high-latitude clouds may be gas streams that are falling back toward the galactic plane within such buoyant braided ropes of magnetic Ñux. Subject headings: dust, extinction È infrared: ISM: continuum È ISM: magnetic Ðelds È ISM: molecules È polarization 1. INTRODUCTION the supershells are produced by multiple supernova explo- sions. However, many Ðlamentary, high-latitude structures I The completion of high-latitude H , IRAS, and CO consist of discrete streams of gas. While the bulk of the H I surveys has stimulated renewed interest in the question of is at low LSR velocities(o V o \ 40 km s~1), intermediate the coupling between the disk of the Milky Way and its (up too V o \ 90 km s~1LSR) and high-velocity gas (up to halo. The H I observations show that there are strong devi- o V o \ 500LSR km s~1) is found at high Galactic latitudes. ations from a uniform plane-parallel distribution of gas (see, TheseLSR infalling clouds of low column densities were dis- e.g., reviews byKulkarni & Heiles 1988; Dickey & covered in 1963 byMuller, Oort, & Raimond because of Lockman1990, hereafter DL). Large systems of arching their peculiar motions with respect to the galactic di†eren- I Ðlaments and shells are often observed. The H and the tial rotation. Since then the whole sky has been surveyed IRAS 100 km Ñuxes are well correlated(De Vries & Poole (Giovanelli 1980a; Bajajaet al. 1985 [d ¹ [10¡] and 1985; Boulanger& Perault 1988; Martin et al. 1994). In Hulsbosch& Wakker 1988 [d º [18¡]). Approximately fact, for extinctions of up to 2 mag, the 100 km brightness 18% of the sky is covered by high-velocity di†use clouds I may be considered to be a good tracer of the H column detectable in H I. They are grouped in long chains and density to within a factor of 2(Boulanger 1994). The high- streams located preferentially in two directions of the sky. A latitude molecular clouds(Magnani, Blitz, & Mundy 1985) comprehensive review on high-velocity clouds and their appear to have physical conditions that are similar to the possible origins is given byWakker & Van Woerden (1991). di†use atomic gas. These transluscent clouds have masses of Several mechanisms, including infall from the outer halo only a few hundred solar masses, and low densities (50È100 (e.g.,Mirabel 1980), have been proposed to explain the pres- cm~3). They may simply be the regions of higher column ence of gas at large distances from the galactic plane. Gas density within the atomic gas that are sufficiently self- may be injected into the halo from the galactic disk through shielding from the UV radiation Ðeld (N º 5 ] 1020 cm~2) the agencies of supernova-driven fountain (Bregman 1980) that molecules may form (see DL). or chimney Ñows(Norman & Ikeuchi 1989). Similarly, the The dynamical state of high-latitude atomic gas has been Parker instability (which is the magnetic analogue of the intensively studied for decades. There is good evidence that Rayleigh-Taylor instability wherein heavy Ñuid is sup- ported by a light one) achieves similar e†ects. Buoyant 1 Departamento de Astronomia y Geodesia, Facultad de CC. Matema- arcades of magnetic Ñux tubes, originally layered in the ticas, Universidad Complutense de Madrid, Madrid, 28040, Spain. galactic disk, push disk gas upward as the tubes rise. The 2 Department of Physics and Astronomy, McMaster University, gas then slides back down the Ðeld lines and gathers in Hamilton, ONT L8S 4M1, Canada. magnetic valleys, (see, e.g.,Blitz & Shu 1980; Elmegreen 3 Observatoire du Mont Megantic and Departement de physique, Universite de Montreal, B. P. 6128, Succ. Centre-ville, Montreal, Quebec 1982; Matsumotoet al. 1988; and Go mez de Castro & H3C 3J7, Canada. Pudritz 1992). 717 718 GOMEZ DE CASTRO ET AL. A neglected but potentially important clue about the surveys in this region, as compiled byKlare & Neckel nature of such disk/halo coupling can be gleaned from (1977), are also useful but do not focus on polarization knowledge of the strength and structure of the galactic mag- toward the high-latitude clouds in particular. We selected netic Ðeld at high latitudes. To take but one example, if the several stellar Ðelds along the ACHV stream on the basis of Parker instability is operative then one anticipates that the H I column density from theGiovanelli (1980a) maps. streams of gas Ñow back down toward the galactic plane Moreover, the red and blue POSS plates were searched for along magnetic Ðeld lines. If gas gathers in magnetic valleys, the more reddened regions, especially close to the galactic then the denser high-latitude molecular clouds could be plane (a D 5h) where the reddening was more apparent. The regions where infalling streams converge and shock. Given Ðelds studied are indicated on the IRAS 100 km map shown that a microgauss Ðeld is sufficiently strong to a†ect the inFigure 1 (Plates 8 and 9) (data retrieved from the Cana- dynamics of the di†use interstellar medium (ISM), it is clear dian Astronomy Data Centre). The 100 km IRAS Ñux that hydromagnetic processes must be taken into account. extends like a plume from the galactic plane toward high- Motivated by these ideas, we undertook an observing galactic latitudes. The substantial deviations in N(H) from program to measure the optical polarization of selected the mean galactic model(DL) are mirrored in the IRAS stars in order to deduce the structure of the galactic mag- maps. netic Ðeld toward dynamically interesting regions, i.e., selec- We divided our map into two regions. The preÐx B is ted high-latitude gas streams. A preliminary report of our used for Ðelds in the high-latitude stream (b from [40¡ to Ðndings is found inGo mez de Castro, Pudritz, & Bastien [52¡), and A for the Ðelds close to the Orion region (1991). We selected the so-called anticenter stream or chain (b D [20¡). We measured the polarization of D10 stars per D(Wakker & van Woerden 1991) because of its promi- Ðeld up to a total of 136 targets. This survey samples stars nence, and because of the presence of both low- and high- as faint as visual magnitude 14.6 with polarization velocity atomic gas as well as high-latitude molecular P \ 0.8%. All of our target stars are in the HST Guide Star clouds (HLMCs) along its length. The anticenter stream is Catalog (GSC). Their identiÐcation, coordinates, and mag- dominated by several elongated, Ðlamentary H I clouds on nitude taken from the HST GSC I, as well as their polariza- whose borders one Ðnds some well-studied, HLMCs such as tion measured in this work, are given in Table 1. MBM 16, 12, and 9. It is known that the HLMCs in our Ðeld lie within 65È95 pc(Hobbs et al. 1989). Thus we can be 2.2. Observations conÐdent that many of the stars in the nearby Ðelds are The observations were carried out in 1990 and 1991 at behind the clouds. At negative latitudes in the anticenter the Observatories of Mont Megantic (Canada) and Calar region there are two di†erent dynamical components to the Alto (Spain). The 1990 November data were obtained at the gas; the anticenter very high velocity (ACVHV) gas at LSR Mont Megantic Observatory on a 1.6 m Ritchey-Chretien velocities of ¹ [150 km s~1, and anticenter high velocity telescope. The instrument was a two-channel photoelectric gas at LSR velocities of [90 km s~1 to [150 km s~1 polarimeter similar to the one described byAngel & Land- (Wakker& van Woerden 1991 and references therein).
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