We discuss the design and performance of TEXES, the Texas Echelon Cross Echelle Spectrograph. TEXES is a mid-infrared (5-25 µm) spectrograph with several operating modes: high-resolution, cross-dispersed with a resolving power of R = λ/δλ ≈ 100,000, 0.5% spectral coverage, and a ∼ 1.5 × 8 ′′ slit; medium-resolution long-slit, with R ≈ 15,000, 0.5% coverage, and a ∼ 1.5×45 ′′ slit; low-resolution long-slit, with δλ ≈ .004 µm, 0.25 µm coverage, and a ∼ 1.5 × 45 ′′ slit; and source acquisition imaging with 0.33 ′′ pixels and a 25 × 25 ′′ field of view on a 3 m telescope. TEXES has been used at the McDonald Observatory 2.7m and the IRTF 3m telescopes, and has proven to be both sensitive and versatile.Subject headings: infrared: general -instrumentation: spectrographs -techniques: spectroscopic Observatory (ISO) short wavelength spectrometer with R = λ/δλ ≈ 2000. SIRTF, with R ≈ 600 and much better sensitivity than can be achieved from the ground, will continue this study. However, neither of these spacecraft spectrometers was optimized for studies of narrow gas-phase lines.Mid-infrared ionic lines have been studied in a number of sources by several ground-based instruments, as well as by ISO. These lines provide information similar to that obtained from visible wavelength forbidden and recombination lines, but can be studied in much more obscured sources. With widths ≈ 10 − 100 km s −1 (the latter in external galaxies), these lines are best studied by instruments with R ∼ 10 4 . Our previous instrument, Irshell (Lacy et al. 1989), was optimized for the study of these lines. ISO greatly expanded on the earlier ground-based work, as will SIRTF, although the lower resolution on the space-based mid-IR spectrometers is insufficient for detailed kinematic studies.Less work has been done on molecular lines in the mid-infrared. In molecular clouds these lines have widths of only a few km s −1 , requiring R ≥ 100,000 for optimal sensitivity and to obtain kinematic information in line profiles. Although a few instruments have achieved such high resolution in the mid-infrared (heterodyne spectrometers, e.g. Mumma et al. 1982, and Fourier transform spectrometers, e.g. Ridgway & Brault 1984), the sensitivity of these instruments was sufficient for the study of only a few of the brightest objects in the sky. Considerable work has been done with ISO (e.g. Lahuis & van Dishoeck 2000) concentrating on the many lines of molecular Q-branches. Using Irshell, with R ≈ 10,000, we observed interstellar C 2 H 2 and CH 4 , as well as several other molecules, but could not resolve the individual lines except in regions of shocks or rapid outflows. As a result, we were limited to measurements of equivalent widths of often saturated lines. From this experience we concluded that whereas space-based spectrometers will improve on our observations of solid-state and ionic lines, at least in cases where high spatial resolution is not required, a high spectral resolution and high sensitivity ground-based (or airborne) spectrograph was required to fur...

A spectral survey of IRC +10216 has been carried out in the range 11Y14 m with a spectral resolution of about 4 km s À1 . We have identified a forest of lines in six bands of C 2 H 2 involving the vibrational states from the ground to 3 5 and in two bands of HCN, involving the vibrational states from the ground up to 2 2 . Some of these transitions are observed also in H 13 CCH and H 13 CN. We have estimated the kinetic, vibrational, and rotational temperatures and the abundances and column densities of C 2 H 2 and HCN between 1R Ã and 300R Ã ('1:5 ; 10 16 cm) by fitting about 300 of these rovibrational lines. The envelope can be divided into three regions with approximate boundaries at 0.019 00 (the stellar photosphere), 0.1 00 (the inner dust formation zone), and 0.4 00 (outer dust formation zone). Most of the lines might require a large microturbulence broadening. The derived abundances of C 2 H 2 and HCN increase by factors of 10 and 4, respectively, from the innermost envelope outward. The derived column densities for both C 2 H 2 and HCN are '1:6 ; 10 19 cm À2 . Vibrational states up to 3000 K above ground are populated, suggesting pumping by near-infrared radiation from the star and innermost envelope. Low rotational levels can be considered under LTE, while those with J > 20Y30 are not thermalized. A few lines require special analysis to deal with effects like overlap with lines of other molecules.

Photodissociation regions, where UV radiation dominates the energetics and chemistry of the neutral gas, contain most of the mass in the dense interstellar medium of our galaxy. Observations of H 2 rotational and ro-vibrational lines reveal that PDRs contain unexpectedly large amounts of very warm (400-700 K) molecular gas. Theoretical models have difficulty explaining the existence of so much warm gas. Possible problems include errors in the heating and cooling functions or in the formation rate for H 2 . To date, observations of H 2 rotational lines smear out the structure of the PDR. Only by resolving the hottest layers of H 2 can one test the predictions and assumptions of current models.Using the Texas Echelon Cross Echelle Spectrograph (TEXES) we mapped emission in the H 2 v = 0-0 S(1) and S(2) lines toward the Orion Bar PDR at 2 ′′ resolution. We also observed H 2 v = 0-0 S(4) at selected points toward the front of the PDR. Our maps cover a 12 ′′ by 40 ′′ region of the bar where H 2 ro-vibrational lines are bright. The distributions of H 2 0-0 S(1), 0-0 S(2), and 1-0 S(1) line emission agree in remarkable detail.The high spatial resolution (0.002 pc) of our observations allows us to probe the distribution of warm gas in the Orion Bar to a distance approaching the