Carl Heiles

The Applicability of the Third Integral of Motion: Some Numerical Experiments

The problem of the existence of a third isolating integral of motion in an axisymmetric potential is investigated by numerical experiments. It is found that the third integral exists for only a limited rage of initial conditions.

The bell laboratories H I survey

We present a galactic survey which to date consists of 73,000 positions covering −5° < l < 122°, −1° < b < 1° , observed in the J=1 → 0 line of 13CO to an rms noise level of 0.15 K in 0.68 km s−1 channels, using the 7 m antenna at Crawford Hill. It is shown that the internal velocity dispersions of molecular clouds tend to vary inversely with galactocentric radius.

9286 Stars: An Agglomeration of Stellar Polarization Catalogs

We present an agglomeration of stellar polarization catalogs with results for 9286 stars. We have endeavored to eliminate errors, provide accurate (approximately arcsecond) positions, weight multiple observations of the same star sensibly, and provide reasonable distances. The catalog is available by anonymous ftp from the author.

H I shells and supershells

The entire sky has now been surveyed in the 21-cm line with an angular resolution of about 0.5 degree. In the north, above declination-20° or so, the “galactic plane” |b|<10° has been completely sampled Weaver and Williams (1973; 1974). Above declination -30° or so, the sky outside the galactic plane has been almost completely sampled (Heiles and Habing, 1974; Heiles, 1975).

The Millennium Arecibo 21 Centimeter Absorption-Line Survey. IV. Statistics of Magnetic Field, Column Density, and Turbulence

We discuss observations of the magnetic field, column density, and turbulence in the cold neutral medium (CNM). The observed quantities are only indirectly related to the intrinsic astronomical ones. We relate the observed and intrinsic quantities by relating their univariate and bivariate probability distribution functions (pdfs). We find that observations of the line-of-sight component of a magnetic field do not constrain the pdf of the total field Btot very well but do constrain the median value of Btot. In the CNM, we find a well-defined median magnetic field 6.0 ± 1.8 μG. The CNM magnetic field dominates thermal motions. Turbulence and magnetism are in approximate equipartition. We find that the probability distribution of column density N⊥(H ) in the sheets closely follows N⊥(H )-1 over a range of 2 orders of magnitude, 0.026 N⊥(H ) 2.6 × 1020 cm-2. The bivariate distributions are not well enough determined to constrain structural models of CNM sheets.

Neutral Hydrogen and the Diffuse Interstellar Medium

Neutral atomic hydrogen (HI) is an important component of the interstellar medium (ISM). The Galaxy has been estimated to contain about 4.8 x 109 M⊙ of HI (Henderson et al. 1982)*. Estimates for the total amount of H2 range from 3.5 x 109 M⊙ (Sanders et al. 1984), nearly equal to the HI estimate, to a value that is only 25% of the HI estimate (Bloemen et al. 1986). Galactic HI constitutes about 4.4% of the mass of the visible matter (Bahcall et al. 1983). The mean surface density distribution of HI is roughly constant from about 4 kpc to 20 kpc; however, HI dominates H2 in mass beyond Galactocentric radius 8 kpc (Blitz et al. 1983). Unlike H2, HI is not concentrated in a small number of giant clouds. Estimates of the “filling factor”, the fraction of the Galactic interstellar space occupied by hydrogen, range from 20% to 90%. These factors establish the preeminence of atomic hydrogen in the dynamics and evolution of the ISM.

The Atomic Component

We review the physical conditions and the distribution of the three phases which consitute the atomic component: the cold clouds, the warm, neutral medium and the warm, ionized medium. Together, these three phases occupy about ≳ 40% of the interstellar volume and contain half the interstellar mass. The size of the H I disk is comparable or exceeds that of the stellar disk. In the outer Galaxy, a spiral pattern is clearly discernable. We discuss extensively the distribution of the three phases near the sun as well as the physics and the limitations of various probes that have been used to study the atomic component. The ionization of the phases is studied in depth and we conclude that most of the interstellar electrons reside in the warm, ionized medium. We review various determinations of interstellar pressure and its scale height. Finally, we derive the filling factors of the atomic phases as a function of z. [Unless otherwise stated, R 0 = 10 kpc]

OH Zeeman observations of dark clouds

We have made measurements with the Green Bank 43 m telescope of the Zeeman effect in the 1665 and 1667 MHz lines of OH toward dark clouds. The typical 1 σ sensitivity was 3 μG. The only certain detection of a magnetic field was toward B1, for which we measured a line-of-sight component |B|cos θ=−19.1±3.9 μG. Comparison with our earlier measurement of the field toward B1 with the Arecibo telescope provided evidence for a 40% enhancement in field strength between the molecular envelope and core of the B1 cloud, which is consistent with quasi-static contraction of the cloud driven by ambipolar diffusion. Because the Zeeman effect is only sensitive to the line-of-sight component of the magnetic field, a statistical analysis of the detection and upper limits was necessary

The vertical structure of the outer milky way H I disk

We examine the outer Galactic H i disk for deviations from the b ¼ 0 � plane by constructing maps of disk surface density, mean height, and thickness. We find that the Galactic warp is well described by a vertical offset plus two Fourier modes of frequency m ¼ 1 and 2, all of which grow with galactocentric radius. Adding the m ¼ 2m ode accounts for the large asymmetry between the northern and southern warps. We use a Morlet wavelet transform to investigate the spatial and frequency localization of higher frequency modes; these modes are often referred to as ‘‘scalloping.’’ We find that the m ¼ 10 and 15 scalloping modes are well above the noise, but localized; this suggests that the scalloping does not pervade the whole disk, but only local regions. Subject headingg Galaxy: disk — Galaxy: kinematics and dynamics — Galaxy: structure — ISM: structure — radio lines: general

The Spiral Structure of the Outer Milky Way in Hydrogen

We produce a detailed map of the perturbed surface density of neutral hydrogen in the outer Milky Way disk, demonstrating that the Galaxy is a non-axisymmetric multiarmed spiral. Spiral structure in the southern half of the Galaxy can be traced out to at least 25 kiloparsecs, implying a minimum radius for the gas disk. Overdensities in the surface density are coincident with regions of reduced gas thickness. The ratio of the surface density to the local median surface density is relatively constant along an arm. Logarithmic spirals can be fit to the arms with pitch angles of 20° to 25°.