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Using a simple relation between the radial expansion velocity of diffuse gas in the Central Molecular Zone (CMZ) of the Galaxy and its distance from Sgr A$^\ast$ we estimate the physical depths within the CMZ of Star $\iota$ (2MASS $J$17470898-2829561) and two Sgr B2 far-infrared continuum sources with respect to the location of Sgr A$^\ast$. To do this we use velocity profiles of mid-infrared absorption lines of H$_3^+$ and of far-infrared absorption lines of H$_2$O$^+$, OH$^+$, and $^{13}$CH$^+$. The distances to Star $\iota$ and to the Sgr B2 sources are found to be $\sim$90 pc greater than the distance to Sgr A$^\ast$. Our conclusion that Sgr B2 lies toward the rear of the CMZ is contrary to many previous models in which it has been placed shallower than Sgr A$^\ast$., Comment: 9 pages, 10 figures. Accepted by ApJ

Velocity profiles of a line of H$_3^+$ at 3.7 $\mu$m produced in warm diffuse gas have been observed toward 18 stars in the Central Molecular Zone (CMZ) of the Galaxy. Their longitude-velocity diagram indicates that the gas is radially expanding within the CMZ at speeds up to a maximum of $\sim$150 km s$^{-1}$. The current momentum and energy in the gas are $\sim 5 \times 10^8 M_\odot$ km s$^{-1}$ and $\sim 5\times 10^{53}$ erg. The motion is similar to that of the Expanding Molecular Ring (EMR) discovered in 1972 by Kaifu et al. and by Scoville. We propose that the expanding gas seen in H$_3^+$ is part of the same phenomenon, in spite of differences in estimates of density, morphology, and degree of rotation. The outward motion suggests that one or more ejection events occurred near the center of the CMZ (0.5$-$1) $\times$ 10$^6$ years ago, which may be related to creation of the recently observed microwave bubble. These observations revive the circular face-on view of the CMZ proposed in 1972, which fell out of favor after 1991 when Binney et al. proposed that a face-on view of the CMZ would show it to have an elliptical shape, with high eccentricity. While that model may apply on kiloparsec scales, we argue that it is incorrect to apply it to the much smaller CMZ. We discuss the fate of the expanding gas, which appears to be eventual infall into the center, leading to episodes of star formation and violent events associated with accretion onto Sgr~A$^\ast$., Comment: 21 pages incl. 12 figures and 1 table; accepted for publication in The Astrophysical Journal. Minor typos corrected in this version

The molecular gas in the Central Molecular Zone (CMZ) of the Galaxy has been studied using infrared absorption spectra of H$_3^+$ lines at 3.5-4.0 $\mu$m and CO lines near 2.34 $\mu$m. In addition to the previously reported spectra of these lines toward 8 stars located within 30 pc of Sgr A$^\ast$, there are now spectra toward $\sim$30 bright stars located from 140 pc west to 120 pc east of Sgr A$^\ast$. The spectra show the presence of warm ($T\sim 200$ K) and diffuse ($n < 100 $cm$^{-3}$) gas with $N$(H$_3^+$) $\sim 3 \times 10^{15} $cm$^{-2}$ on majority of sightlines. Instead of our previous analysis in which only electrons from photoionization of carbon atoms were considered, we have developed a simple model calculation in which the cosmic ray ionization of H$_2$ and H are also taken into account. We conclude: (1) Warm and diffuse gas dominates the volume of the CMZ. The volume filling factor of dense gas must be much less than 0.1 and the CMZ is not as opaque as previously considered. The X-ray emitting ultra-hot 10$^8 $K plasma, which some thought to dominate the CMZ, does not exist over extended regions. (2) The cosmic ray ionization rate is $\zeta \sim 2 \times 10^{-14} $s$^{-1}$, higher than in Galactic dense clouds and diffuse clouds by factors of $\sim$1000 and $\sim$100, respectively. If the equipartition law stands, this suggests a pervading magnetic field on the order of $\sim$100 $\mu$G., Comment: 39 pages, 7 tables, 10 figures, published at the ApJ

The diffuse interstellar band (DIB) at 5797.1 \AA\ is simulated based on three premises: (1) The carrier of the DIB is polar as concluded by T. Oka et al. from the anomalous spectrum toward Herschel 36. (2) The sharp central feature observed by P. J. Sarre's group is the $Q$-branch of a parallel band of a prolate top. (3) The radiative temperature of the environment is $T_r$=2.73 K. A $^2 \Pi \leftarrow$ $^2\Pi$ transition of a linear radical is simulated. Results depend on 10 parameters, with the rotational constant $B$ being the most critical. Comparisons of calculated spectra with observed data constrain $B$, which in turn constrains the number of heavy atoms to 5 $\leq n \leq 7$. Upper limits based on the Her 36 spectrum and lower limits based on stability against photodissociation are also discussed. The latter is based on the assumption that the DIB molecules are produced top-down from the breakdown of dust rather than bottom-up by chemical reactions. A difficulty with this limit is that J. P. Maier's laboratory has observed many molecules within this size range, some of which have been tested astronomically. Candidates are discussed in light of many prolate tops observed by the P. Thaddeus and M. C. McCarthy microwave group., Comment: 17 pages, 5 figures

We have obtained improved spectra of key fundamental band lines of H3+, R(1,1)l, R(3,3)l, and R(2,2)l, and ro-vibrational transitions of CO on sightlines toward the luminous infrared sources GCIRS 3 and GCIRS 1W, each located in the Central Cluster of the Galactic center within several arcseconds of Sgr A*. The spectra reveal absorption occurring in three kinds of gaseous environments: (1) cold dense and diffuse gas associated with foreground spiral/lateral arms; (2) warm and diffuse gas absorbing over a wide and mostly negative velocity range, which appears to fill a significant fraction of the Galaxy's Central Molecular Zone (CMZ); and (3) warm, dense and compact clouds with velocities near +50 km s^-1 probably within 1-2 pc of the center. The absorptions by the first two cloud types are nearly identical for all the sources in the Central Cluster, and are similar to those previously observed on sightlines from Sgr A* to 30 pc east of it. Cloud type (3), which has only been observed toward the Central Cluster, shows distinct differences between the sightlines to GCIRS 3 and GCIRS 1W, which are separated on the sky by only 0.33 pc in projection. We identify this material as part of an inward extension of the Circumnuclear Disk previously known from HCN mapping. Lower limits on the products of the hydrogen ionization rate zeta and the path length L are 2.3 x 10^5 cm s^-1 and 1.5 x 10^3 cm s^-1 for the warm and diffuse CMZ gas and for the warm and dense clouds in the core, respectively. The limits indicate that the ionization rates in these regions are well above 10^-15 s^-1., Comment: 15 pages, 12 figures, 7 tables ApJ accepted

Anomalously broad diffuse interstellar bands (DIBs) at 5780.5, 5797.1, 6196.0, and 6613.6 A are found in absorption along the line of sight to Herschel 36, the star illuminating the bright Hourglass region of the H II region Messier 8. Interstellar absorption from excited CH+ in the J=1 level and from excited CH in the J=3/2 level is also seen. To our knowledge, neither those excited molecular lines nor such strongly extended DIBs have previously been seen in absorption from interstellar gas. These unusual features appear to arise in a small region near Herschel 36 which contains most of the neutral interstellar material in the sight line. The CH+ and CH in that region are radiatively excited by strong far-IR radiation from the adjacent infrared source Her 36 SE. Similarly, the broadening of the DIBs toward Herschel 36 may be due to radiative pumping of closely spaced high-J rotational levels of relatively small, polar carrier molecules. If this picture of excited rotational states for the DIB carriers is correct and applicable to most DIBs, the 2.7 degree cosmic microwave background may set the minimum widths (about 0.35 A) of known DIBs, with molecular processes and/or local radiation fields producing the larger widths found for the broader DIBs. Despite the intense local UV radiation field within the cluster NGC 6530, no previously undetected DIBs stronger than 10 mA in equivalent width are found in the optical spectrum of Herschel 36, suggesting that neither dissociation nor ionization of the carriers of the known DIBs by this intense field creates new carriers with easily detectable DIB-like features. Possibly related profile anomalies for several other DIBs are noted., Comment: 36 pages; 8 figures; accepted to ApJ; higher quality versions of figures 1 and 2 may be found at http://astro.uchicago.edu/~dwelty/Her36/

Absorption spectra toward Herschel 36 for the A^1Pi <-- X^1Sigma transitions of CH+ in the J=1 excited rotational level and the A^2Delta <-- X^2Pi transition of CH in the J=3/2 excited fine structure level have been analyzed. These excited levels are above their ground levels by 40.1 K and ~25.7 K and indicate high radiative temperatures of the environment, 14.6 K and 6.7 K, respectively. The effect of the high radiative temperature is more spectacular in some diffuse interstellar bands (DIBs) observed toward Her 36; remarkable extended tails toward red (ETR) were observed. We interpret these ETRs as due to a small decrease of rotational constants upon excitation of excited electronic states. Along with radiative pumping of a great many high-J rotational levels, this causes the ETRs. In order to study this effect quantitatively, we have developed a model calculation in which the effects of collision and radiation are treated simultaneously. The simplest case of linear molecules is considered. It has been found that the ETR is reproduced if the fraction of the variation of the rotational constant, beta = (B'-B)/B, is sufficiently high (3-5%) and the radiative temperature is high (T_r > 50 K). Although modeling for general molecules is beyond the scope of this paper, the results indicate that the prototypical DIBs at 5780.5, 5797.1, and 6613.6 A which show the pronounced ETRs are due to polar molecules sensitive to the radiative excitation. The requirement of high beta favors relatively small molecules with 3-6 heavy atoms. DIBs at 5849.8, 6196.0, and 6379.3 A which do not show the pronounced ETRs are likely due to non-polar molecules or large polar molecules with small beta., Comment: 35 pages; 5 figures; accepted to ApJ; corrected minor errors in eqns. 3, 6, and 8 and fig. 4; conclusions unchanged

Observational and theoretical evidence suggests that high-energy Galactic cosmic rays are primarily accelerated by supernova remnants. If also true for low-energy cosmic rays, the ionization rate near a supernova remnant should be higher than in the general Galactic interstellar medium (ISM). We have searched for H3+ absorption features in 6 sight lines which pass through molecular material near IC 443---a well-studied case of a supernova remnant interacting with its surrounding molecular material---for the purpose of inferring the cosmic-ray ionization rate in the region. In 2 of the sight lines (toward ALS 8828 and HD 254577) we find large H3+ column densities, N(H3+)~3*10^14 cm^-2, and deduce ionization rates of zeta_2~2*10^-15 s^-1, about 5 times larger than inferred toward average diffuse molecular cloud sight lines. However, the 3 sigma upper limits found for the other 4 sight lines are consistent with typical Galactic values. This wide range of ionization rates is likely the result of particle acceleration and propagation effects, which predict that the cosmic-ray spectrum and thus ionization rate should vary in and around the remnant. While we cannot determine if the H3+ absorption arises in post-shock (interior) or pre-shock (exterior) gas, the large inferred ionization rates suggest that IC 443 is in fact accelerating a large population of low-energy cosmic rays. Still, it is unclear whether this population can propagate far enough into the ISM to account for the ionization rate inferred in diffuse Galactic sight lines., Comment: 14 pages, 3 figures, 4 tables

The formation of CH+ in the interstellar medium has long been an outstanding problem in chemical models. In order to probe the physical conditions of the ISM in which CH+ forms, we propose the use of CH3+ observations. The pathway to forming CH3+ begins with CH+, and a steady state analysis of CH3+ and the reaction intermediary CH2+ results in a relationship between the CH+ and CH3+ abundances. This relationship depends on the molecular hydrogen fraction, f_H2, and gas temperature, T, so observations of CH+ and CH3+ can be used to infer the properties of the gas in which both species reside. We present observations of both molecules along the diffuse cloud sight line toward Cyg OB2 No. 12. Using our computed column densities and upper limits, we put constraints on the f_H2 vs. T parameter space in which CH+ and CH3+ form. We find that average, static, diffuse molecular cloud conditions (i.e. f_H2>0.2, T~60 K) are excluded by our analysis. However, current theory suggests that non-equilibrium effects drive the reaction C+ + H_2 --> CH+ + H, endothermic by 4640 K. If we consider a higher effective temperature due to collisions between neutrals and accelerated ions, the CH3+ partition function predicts that the overall population will be spread out into several excited rotational levels. As a result, observations of more CH3+ transitions with higher signal-to-noise ratios are necessary to place any constraints on models where magnetic acceleration of ions drives the formation of CH+., Comment: 7 pages, 3 figures, 2 tables, accepted for publication in ApJ

Infrared absorption lines of H3+, including the metastable R(3,3)l line, have been observed toward eight bright infrared sources associated with hot and massive stars located in and between the Galactic Center Cluster and the Quintuplet Cluster 30 pc to the east. The absorption lines with high velocity dispersion arise in the Galaxy's Central Molecular Zone (CMZ) as well as in foreground spiral arms. The temperature and density of the gas in the CMZ, as determined from the relative strengths of the H3+ lines, are T=200-300K and n=50-200cm^-3. The detection of high column densities of H3+ toward all eight stars implies that this warm and diffuse gaseous environment is widespread in the CMZ. The products of the ionization rate and path length for these sight lines are 1000 and 10 times higher than in dense and diffuse clouds in the Galactic disk, respectively, indicating that the ionization rate, zeta, is not less than 10^-15 s^-1 and that L is at least on the order of 50 pc. The warm and diffuse gas is an important component of the CMZ, in addition to the three previously known gaseous environments: (1) cold molecular clouds observed by radio emission of CO and other molecules, (2) hot (T=10^4-10^6K) and highly ionized diffuse gas (n_e=10-100cm^-3) seen in radio recombination lines, far infrared atomic lines, and radio-wave scattering, and (3) ultra-hot (T=10^7-10^8K) X-ray emitting plasma. Its prevalence significantly changes the understanding of the environment of the CMZ. The sight line toward GC IRS 3 is unique in showing an additional H3+ absorption component, which is interpreted as due to either a cloud associated with circumnuclear disk or the "50 km s^-1 cloud" known from radio observations. An infrared pumping scheme is examined as a mechanism to populate the (3,3) metastable level in this cloud., Comment: 15 pages, 5 figures, 4 tables Accepted for publication in the Astrophysical Journal

Using high resolution infrared spectroscopy we have surveyed twenty sightlines for H3+ absorption. H3+ is detected in eight diffuse cloud sightlines with column densities varying from 0.6x10^14 cm^-2 to 3.9x10^14 cm^-2. This brings to fourteen the total number of diffuse cloud sightlines where H3+ has been detected. These detections are mostly along sightlines concentrated in the Galactic plane, but well dispersed in Galactic longitude. The results imply that abundant H3+ is common in the diffuse interstellar medium. Because of the simple chemistry associated with H3+ production and destruction, these column density measurements can be used in concert with various other data to infer the primary cosmic-ray ionization rate, zeta_p. Values range from 0.5x10^-16 s^-1 to 3x10^-16 s^-1 with an average of 2x10^-16 s^-1. Where H3+ is not detected the upper limits on the ionization rate are consistent with this range. The average value of zeta_p is about an order of magnitude larger than both the canonical rate and rates previously reported by other groups using measurements of OH and HD. The discrepancy is most likely due to inaccurate measurements of rate constants and the omission of effects which were unknown when those studies were performed. We believe that the observed column density of H3+ is the most direct tracer for the cosmic-ray ionization rate due to its simple chemistry. Recent models of diffuse cloud chemistry require cosmic-ray ionization rates on the order of 10^-16 s^-1 to reproduce observed abundances of various atomic and molecular species, in rough accord with our observational findings., Comment: Accepted to ApJ, 35 pages, 5 figures, 5 tables

Using an absorption line from the metastable (J, K) = (3, 3) level of H3+ together with other lines of H3+ and CO observed along several sightlines, we have discovered a vast amount of high temperature (T ~ 250 K) and low density (n ~ 100 cm-3) gas with a large velocity dispersion in the Central Molecular Zone (CMZ) of the Galaxy, i.e., within 200 pc of the center. Approximately three fourths of the H3+ along the line of sight to the brightest source we observed, the Quintuplet object GCS 3-2, is inferred to be in the CMZ, with the remaining H3+ located in intervening spiral arms. About half of H3+ in the CMZ has velocities near ~ - 100 km s-1 indicating that it is associated with the 180 pc radius Expanding Molecular Ring which approximately forms outer boundary of the CMZ. The other half, with velocities of ~ - 50 km s-1 and ~ 0 km s-1, is probably closer to the center. CO is not very abundant in those clouds. Hot and diffuse gas in which the (3, 3) level is populated was not detected toward several dense clouds and diffuse clouds in the Galactic disk where large column densities of colder H3+ have been reported previously. Thus the newly discovered environment appears to be unique to the CMZ. The large observed H3+ column densities in the CMZ suggests an ionization rate much higher than in the diffuse interstellar medium in the Galactic disk. Our finding that the H3+ in the CMZ is almost entirely in diffuse clouds indicates that the reported volume filling factor (f ≥ 0.1) for n ≥ 104 cm-3 clouds in the CMZ is an overestimate by at least an order of magnitude., Comment: 33 pages, 5 figures, 3 tables