Ultra-High-Resolution Facility Research Articles

Small-scale structure in the interstellar medium: time-varying interstellar absorption towards κ Velorum

Ultra-high spectral resolution observations of time-varying interstellar absorption towards {\kappa} Vel are reported, using the Ultra-High Resolution Facility on the Anglo-Australian Telescope. Detections of interstellar Ca I, Ca II, K I, Na I and CH are obtained, whilst an upper limit on the column density is reported for C_2. The results show continued increases in column densities of K I and Ca I since observations ~ 4 yr earlier, as the transverse motion of the star carried it ~ 10 AU perpendicular to the line of sight. Line profile models are fitted to the spectra and two main narrow components (A & B) are identified for all species except CH. The column density N(K I) is found to have increased by 82 +10-9 % between 1994 and 2006, whilst N(Ca I) is found to have increased by 32 +- 5 % over the shorter period of 2002-2006. The line widths are used to constrain the kinetic temperature to T_k,A < 671 +18-17 K and T_k,B < 114 +15-14 K. Electron densities are determined from the Ca I / Ca II ratio, which in turn place lower limits on the total number density of n_A > 7 * 10^3 cm^-3 and n_B > 2 * 10^4 cm^-3. Calcium depletions are estimated from the Ca I / K I ratio. Comparison with the chemical models of Bell et al. (2005) confirms the high number density, with n = 5 * 10^4 cm^-3 for the best-fitting model. The first measurements of diffuse interstellar bands (DIBs) towards this star are made at two epochs, but only an upper limit of < 40 % is placed on their variation over ~ 9 years. The DIBs are unusually weak for the measured E(B-V) and appear to exhibit similar behaviour to that seen in Orion. The ratio of equivalent widths of the {\lambda}5780 to {\lambda}5797 DIBs is amongst the highest known, which may indicate that the carrier of {\lambda}5797 is more sensitive to UV radiation than to local density.

Laser frequency comb techniques for precise astronomical spectroscopy

Precise astronomical spectroscopic analyses routinely assume that individual pixels in charge-coupled devices (CCDs) have uniform sensitivity to photons. Intra-pixel sensitivity (IPS) variations may already cause small systematic errors in, for example, studies of extra-solar planets via stellar radial velocities and cosmological variability in fundamental constants via quasar spectroscopy, but future experiments requiring velocity precisions approaching ~1 cm/s will be more strongly affected. Laser frequency combs have been shown to provide highly precise wavelength calibration for astronomical spectrographs, but here we show that they can also be used to measure IPS variations in astronomical CCDs in situ. We successfully tested a laser frequency comb system on the Ultra-High Resolution Facility spectrograph at the Anglo-Australian Telescope. By modelling the 2-dimensional comb signal recorded in a single CCD exposure, we find that the average IPS deviates by <8 per cent if it is assumed to vary symmetrically about the pixel centre. We also demonstrate that series of comb exposures with absolutely known offsets between them can yield tighter constraints on symmetric IPS variations from ~100 pixels. We discuss measurement of asymmetric IPS variations and absolute wavelength calibration of astronomical spectrographs and CCDs using frequency combs.

A Search for Fine Structure within the 4430 A Diffuse Interstellar Band

The dependence of the 4430 A diffuse interstellar band (DIB) on dust extinction or total gas column density indicates that this DIB, like the narrower features seen in the yellow-red portion of the spectrum, does not continue to grow in strength beyond a certain threshold. One possible interpretation is that the 4430 A DIB saturates at high column densities, suggesting that this band might consist of unresolved narrow features (such as molecular rotational lines), individually saturated, that are blended at the resolving power of previous observations. In this study we use the Ultra High Resolution Facility on the Anglo-Australian Telescope to assess that question by seeking evidence for fine structure within the 4430 A DIB at the very high spectral resolving power of Δλ/λ ≈ 106. Our target stars include lines of sight where apparent rotational line structure has been observed in other DIBs. We find no evidence of fine structure within the 4430 A DIB, suggesting that the broadening of this band is intrinsic. If so, the lack of fine structure may indicate a molecular transition followed by very rapid internal conversion or a molecular transition consisting of overlapping rotational lines. In either case, our results support the hypothesis that the apparent saturation of the band as a function of extinction is a result of the skin effect, in which the strength of DIBs stops increasing with extinction because the DIBs form primarily in the outer boundaries of interstellar clouds.

Ultra-high-resolution observations of circumstellar K I and C2 around the post-AGB star HD 56126

We have used the Ultra-High-Resolution Facility (UHRF) at the AAT, operating at a resolution of 0.35 km s−1 (FWHM), to observe K I and C2 absorption lines arising in the circumstellar environment of the post-AGB star HD 56126. We find three narrow circumstellar absorption components in K I, two of which are also present in C2. We attribute this velocity structure to discrete shells resulting from multiple mass-loss events from the star. The very high spectral resolution has enabled us to resolve the intrinsic linewidths of these narrow lines for the first time, and we obtain velocity dispersions (b-values) of 0.2–0.3 km s−1 for the K I components, and 0.54 ± 0.03 km s−1 for the strongest (and best defined) C2 component. These correspond to rigorous kinetic temperature upper limits of 211 K for K I and 420 K for C2, although the b-value ratio implies that these two species do not co-exist spatially. The observed degree of rotational excitation of C2 implies low kinetic temperatures (Tk ≈10 K) and high densities (n ≈ 106 to 107 cm−3) within the shell responsible for the main C2 component. Given this low temperature, the line profiles then imply either mildly supersonic turbulence or an unresolved velocity gradient through the shell.

A spectroscopic search for molecular oxygen in the Venus middle atmosphere

The results from a new attempt to detect ground state molecular oxygen in the Venus middle atmosphere are presented. The upper limit inferred from the January 1995 observations using the Ultra High Resolution Facility (UHRF) spectrometer at the Anglo‐Australian Observatory (AAO) is equivalent to a uniform volume mixing ratio of 3×10−6 at and above the 300 mbar level. This is comparable to the upper limit inferred from observations in the early 1970s and is a factor of 10 larger than the upper limit inferred from observations in 1982. These observations indicate the observed secular decrease in the abundance of SO2 and SO from the early 1980s to 1995 did not result in an O2 abundance that could be detected within the sensitivity of the present observations. Radiative transfer calculations show the sensitivity of observations to the vertical profile of O2. These calculations show that all three spectroscopic observations (1973, 1982, and 1995) would have detected the presence of O2 if its mixing ratio near 50 km altitude had been 10−5 as was reported by in situ measurements in 1980.

Atomic and Molecular Interstellar Absorption Lines toward the High Galactic Latitude Stars HD 141569 and HD 157841 at Ultra–High Resolution

We present ultra-high-resolution (0.32 km s-1) spectra obtained with the 3.9 m Anglo-Australian Telescope (AAT) and Ultra-High-Resolution Facility (UHRF) of interstellar Na I D1, Na I D2, Ca II K, K I, and CH absorption toward two high Galactic latitude stars HD 141569 and HD 157841. We have compared our data with 21 cm observations obtained from the Leiden/Dwingeloo H I survey. We derive the velocity structure and column densities of the clouds represented by the various components and identify the clouds with ISM structures seen in the region at other wavelengths. We further derive abundances, linear depletions, and H2 fractional abundances for these clouds wherever possible. Both stars are located in regions of IRAS 100 μm emission associated with high Galactic latitude molecular clouds (HLCs): HD 141569 lies, in projection, close to MBM 37 and the Lynds dark cloud L134N, whereas HD 157841 is in the vicinity of the MBM 151. Toward HD 141569 we detect two components in our UHRF spectra: a weak, broad b = 4.5 km s-1 component at -15 km s-1, seen only in Ca II K absorption, and another component at 0 km s-1, seen in Na I D1, Na I D2, Ca II K, K I, and CH absorption. The cloud represented by the -15 km s-1 component is warm and may be located in a region close to the star. The cloud represented by the 0 km s-1 component has a Ca linear depletion δ(Ca) = 1.4 × 10-4 and shows evidence for the presence of dust, consistent with strong 100 μm emission seen in this region. The H2 fractional abundance f(H2) derived for this cloud is 0.4, which is typically what is observed toward HLCs. We conclude that this 0 km s-1 cloud is associated with MBM 37 and L134N based on the presence of dust and molecular gas (CH) and good velocity agreement with CO emission from these two clouds. This places HD 141569 beyond MBM 37 and L134N, which are estimated to be at ≈ 110 pc. In the case of the HD 157841 sight line, a total of six components are seen on our UHRF spectra in Na I D1, Na I D2, Ca II K, K I, and CH absorption. Two of these six components are seen only in a single species. The cloud represented by the components at 1.85 km s-1 has a Ca linear depletion δ(Ca) = 2.8 × 10-4, indicating the presence of dust. The f(H2) derived for this cloud is 0.45, and there is good velocity agreement with CO emission from MBM 151. To the best of our knowledge, this 1.85 km s-1 component toward HD 157841 is the first one found to have relative line widths that are consistent with pure thermal broadening only. We associate the 1.85 km s-1 cloud seen in our UHRF spectra with MBM 151 and conclude that HD 157841 must lie beyond ~200 pc, the estimated distance to MBM 151.

Ultra-high-resolution observations of interstellar C2, CH and CN towards Ophiuchi and HD 169454

We have used the Ultra-High-Resolution Facility (UHRF) at the Anglo-Australian Telescope (AAT), operating at a resolution of 0.33 km s -1 (FWHM), to observe lines arising from the five lowest rotational levels of interstellar C2 towards r Ophiuchi and HD 169454. We have also obtained new observations of CH and CN towards the latter star. The very high spectral resolution has enabled us to perform a rotational excitation analysis for both of the closely spaced velocity components (separation 1.1 ± 0.1 kms-1) known to be present towards r Oph, and, contrary to earlier results based on an analysis of only two lines, we show that these components have very similar physical conditions. In the case of HD 169454, we confirm the presence of two very closely spaced velocity components (separation 0.7 ± 0.1 km S-1) in both the C2 and the CN lines. We have determined the turbulent velocities directly from the line profiles. In all cases we find values in the range 0.23-0.45kms-1, and there is no convincing evidence for supersonic turbulence in any of the identified velocity components.

First results from the UHRF: ultra-high-resolution observations of atomic interstellar lines towards ζ Ophiuchi

We present ultra-high-resolution observations of optical interstellar lines towards zeta Oph obtained during the commissioning phase of the new Ultra-High-Resolution Facility (UHRF) at the AAT. The UHRF has achieved its design resolving power of R approximate to 10(6), and has very clearly resolved hyperfine splitting in three of the 11 identified Na I D-line velocity components towards zeta Oph. In addition, the intrinsic line profiles of the 3s-4p Na I doublet at 3302 Angstrom have been measured for the first time. 12 velocity components are identified in the interstellar Ca II K-line absorption profile, of which seven have definite velocity counterparts in the Na I profile. Three of the Na I velocity components have b-values which correspond to a gas temperature of about 200 K, for the case of pure thermal broadening. The strongest component in the sightline (-14.8 km s(-1)) was found to have a b-value of 0.6(-0.1)(+0.2) km s(-1), corresponding to a temperature of 500 K. If a true gas kinetic temperature of 54 K is assumed to apply to this component, from the H-2 rotational excitation temperature previously measured for the zeta Oph sightline, then the observed Na I and Fe I line profiles both imply an rms turbulent velocity of 0.4 km s(-1) within the cloud, just less than the local sound speed. The observed Ca II linewidths are all significantly larger than those of the corresponding Na I components. The Ca II components are interpreted as arising from warm neutral material (intercloud medium) surrounding cooler clouds, while half of the Na I components arise from cool clouds and half arise from warm intercloud material. The measured Na I/Ca II column density ratios, and the inferred temperatures, are consistent with a model in which calcium is heavily depleted on to grains in cool clouds but is restored to the gas phase by impact desorption in the warm intercloud medium.

The Ultra-High-Resolution Facility at the Anglo-Australian Telescope

The Ultra-High-Resolution Facility (UHRF) has been commissioned at the coude focus of the Anglo-Australian Telescope (AAT), and provides a resolving power of R≈106. This is the highest spectral resolution ever obtained by an optical astronomical spectrograph, and is intended mainly for astrophysical studies of cool interstellar clouds. This paper describes the development of this ambitious project, which includes diffraction-limited optical components, very tight specification for the optical configuration, and a new type of image slicer which allows ultra-high-resolution work towards faint stars. Astrophysical results from the first observing runs are presented to demonstrate the performance in terms of both resolution and throughput

First results from the UHRF: ultra-high-resolution observations of interstellar CH, Formula and CN towards Formula Ophiuchi

We present ultra-high-resolution observations (R>700 000) of interstellar CH, CH+ and CN towards ζ Oph obtained with the newly commissioned Ultra-High-Resolution Facility (UHRF) at the AAT. The molecular lines have all been fully resolved, and we discuss their implications for this well-studied molecular sightline. The CH and CN lines each consist of two narrow (b≈0.5 km s−1) components separated by 1.13±0.10 km s−1⁠; an additional broad (b=1.8 km s−1) component is present in CH. We show that the very broad (b=2.1 km s−1) CH+ line profile is consistent with the formation of this molecule in warm haloes surrounding the cold cores responsible for the narrow CH and CN components.

Ultra-high-resolution observations of Ca K line variations in the Formula Pictoris disc

We present observations of the β Pictoris circumstellar Ca II K line, obtained with the new Ultra-High-Resolution facility (UHRF) at the Anglo-Australian Telescope. The resolving power was R≳900000⁠, and these data therefore comprise the highest resolution observations yet obtained of this object. Observations were obtained on three nights in 1993 (May 11, August 26 and August 29), and significant temporal variability was observed, including previously unobserved changes in the profile of the main circumstellar component.