Search

The gravitational potential phi = GM/Rc^2 at the surface of the white dwarf G191-B2B is 10,000 times stronger than that at the Earth's surface. Numerous photospheric absorption features are detected, making this a suitable environment to test theories in which the fundamental constants depend on gravity. We have measured the fine structure constant, alpha, at the white dwarf surface, used a newly calibrated Hubble Space Telescope STIS spectrum of G191-B2B, two new independent sets of laboratory Fe V wavelengths, and new atomic calculations of the sensitivity parameters that quantify Fe V wavelength dependency on alpha. The two results obtained are: dalpha/alpha = 6.36 +/- [0.33(stat) + 1.94(sys)] X 10^{-5} and dalpha/alpha = 4.21 +/- [0.47(stat) + 2.35(sys)] X 10^{-5}. The measurements hint that the fine structure constant increases slightly in the presence of strong gravitational fields. A comprehensive search for systematic errors is summarised, including possible effects from line misidentifications, line blending, stratification of the white dwarf atmosphere, the quadratic Zeeman effect and electric field effects, photospheric velocity flows, long-range wavelength distortions in the HST spectrum, and variations in the relative Fe isotopic abundances. None fully account for the observed deviation but the systematic uncertainties are heavily dominated by laboratory wavelength measurement precision., Comment: Submitted to MNRAS 21 July 2020. Resubmitted after refereeing 27 August 2020

For around 100 years, hydrogen spectral modelling has been based on Voigt profile fitting. The semi-classical Voigt profile is based on a 2-level atom approximation. Whilst the Voigt profile is excellent for many circumstances, the accuracy is insufficient for very high column density damped Lyman-$\alpha$ absorption systems. We have adapted the quantum-mechanical Kramers-Heisenberg model to include thermal broadening, producing a new profile, the KHT profile. Interactions involving multiple discrete atomic levels and continuum terms, not accounted for in the Voigt model, generate asymmetries in the Lyman line wings. If not modelled, this can lead to significant systematics in parameter estimation when modelling real data. There are important ramifications in particular for measurements of the primordial deuterium abundance. However, the KHT model is complicated. We therefore present a simplified formulation based on Taylor series expansions and look-up tables, quantifying the impact of the approximations made. The KHT profile has been implemented within the widely-used VPFIT code., Comment: 17 pages, 7 figures, 1 table

The Cosmological Principle, the combined assumptions of cosmological isotropy and homogeneity, underpins the standard model of Big Bang cosmology with which we interpret astronomical observations. A new test of isotropy over the redshift range $2

Using newly obtained high-resolution data ($R\sim{1\times{10}^{5}}$) from the \textit{Hubble Space Telescope}, and archival UV data from the \textit{Far Ultraviolet Spectroscopic Explorer} we have conducted a detailed UV survey of the three hot, metal-polluted white dwarfs WD0455-282, WD0621-376, and WD2211-495. Using bespoke model atmospheres we measured $T_{\mathrm{eff}}$, log $g$, and photospheric abundances for these stars. In conjunction with data from Gaia we measured masses, radii, and gravitational redshift velocities for our sample of objects. We compared the measured photospheric abundances with those predicted by radiative levitation theory, and found that the observed Si abundances in all three white dwarfs, and the observed Fe abundances in WD0621-376 and WD2211-495, were larger than those predicted by an order of magnitude. These findings imply not only an external origin for the metals, but also ongoing accretion, as the metals not supported by radiative levitation would sink on extremely short timescales. We measured the radial velocities of several absorption features along the line of sight to the three objects in our sample, allowing us to determine the velocities of the photospheric and interstellar components along the line of sight for each star. Interestingly, we made detections of circumstellar absorption along the line of sight to WD0455-282 with three velocity components. To our knowledge, this is the first such detection of multi-component circumstellar absorption along the line of sight to a white dwarf., Comment: 19 pages, 23 figures, 8 tables. Accepted for publication in the Monthly Notices of the Royal Astronomical Society

White dwarf atmospheres are subjected to gravitational potentials around $10^5$ times larger than occur on Earth. They provide a unique environment in which to search for any possible variation in fundamental physics in the presence of strong gravitational fields. However, a sufficiently strong magnetic field will alter absorption line profiles and introduce additional uncertainties in measurements of the fine structure constant. Estimating the magnetic field strength is thus essential in this context. Here we model the absorption profiles of a large number of atomic transitions in the white dwarf photosphere, including first-order Zeeman effects in the line profiles, varying the magnetic field as a free parameter. We apply the method to a high signal-to-noise, high-resolution, far-ultraviolet HST/STIS spectrum of the white dwarf G191-B2B. The method yields a sensitive upper limit on its magnetic field of $B < 2300$ Gauss at the $3\sigma$ level. Using this upper limit we find that the potential impact of quadratic Zeeman shifts on measurements of the fine structure constant in G191-B2B is 4 orders of magnitude below laboratory wavelength uncertainties., Comment: 10 pages, 5 figures

The z_abs = 2.504 Lyman Limit absorption system (LLS) towards Q1009+2956 has previously been used to estimate the primordial deuterium abundance. Since the initial measurement by Burles & Tytler, this quasar has been observed extensively with the Keck telescope, providing a substantial increase in signal-to-noise (from 60 to 147 at continuum level of Ly-alpha at z_abs=2.504). We explore a set of different models for the absorption system and find that the deuterium feature is contaminated by Ly-alpha absorption from a low column density H I cloud. This significantly limits precision to which one can estimate the D/H ratio in this LLS. Our final result for this system D/H =2.48^{+0.41}_{-0.35}*10^{-5} has the same relative uncertainty of 17% as the previous estimate by Burles & Tytler despite the far higher signal-to-noise of our dataset. A weighted mean of 13 D/H measurements available in the literature (including our result) gives the primordial deuterium abundance of (D/H)_p = (2.545 +/- 0.025)*10^{-5} and the corresponding baryon density of the Universe of Omega_b h^2 = 0.02174 +/- 0.00025 marginally inconsistent with the 2015 Planck CMB data., Comment: 7 pages, 3 figures, conference proceedings from PhysicA.SPb 2017

The spectrum of the $z_{\rm em} = 2.63$ quasar Q1009+2956 has been observed extensively on the Keck telescope. The Lyman limit absorption system $z_{\rm abs} = 2.504$ was previously used to measure D/H by Burles & Tytler using a spectrum with signal to noise approximately 60 per pixel in the continuum near Ly {\alpha} at $z_{\rm abs} = 2.504$. The larger dataset now available combines to form an exceptionally high signal to noise spectrum, around 147 per pixel. Several heavy element absorption lines are detected in this LLS, providing strong constraints on the kinematic structure. We explore a suite of absorption system models and find that the deuterium feature is likely to be contaminated by weak interloping Ly {\alpha} absorption from a low column density H I cloud, reducing the expected D/H precision. We find D/H = $2.48^{+0.41}_{-0.35}\times10^{-5}$ for this system. Combining this new measurement with others from the literature and applying the method of Least Trimmed Squares to a statistical sample of 15 D/H measurements results in a "reliable" sample of 13 values. This sample yields a primordial deuterium abundance of (D/H)$_{\rm p} = (2.545 \pm 0.025)\times10^{-5}$. The corresponding mean baryonic density of the Universe is $\Omega_{\rm b}h^2 = 0.02174\pm0.00025$. The quasar absorption data is of the same precision as, and marginally inconsistent with, the 2015 CMB Planck (TT+lowP+lensing) measurement, $\Omega_{\rm b}h^2 = 0.02226\pm0.00023$. Further quasar and more precise nuclear data are required to establish whether this is a random fluctuation., Comment: accepted by MNRAS, 18 pages, 12 figures, 6 tables

The primordial deuterium abundance probes fundamental physics during the Big Bang Nucleosynthesis and can be used to infer cosmological parameters. Observationally, the abundance can be measured using absorbing clouds along the lines of sight to distant quasars. Observations of the quasar PKS1937--101 contain two absorbers for which the deuterium abundance has previously been determined. Here we focus on the higher redshift one at $z_{abs} = 3.572$. We present new observations with significantly increased signal-to-noise ratio which enable a far more precise and robust measurement of the deuterium to hydrogen column density ratio, resulting in D/H = $2.62\pm0.05\times10^{-5}$. This particular measurement is of interest because it is among the most precise assessments to date and it has been derived from the second lowest column-density absorber [N(HI) $=17.9\mathrm{cm}^{-2}$] that has so-far been utilised for deuterium abundance measurements. The majority of existing high-precision measurements were obtained from considerably higher column density systems [i.e. N(HI) $>19.4\mathrm{cm}^{-2}$]. This bodes well for future observations as low column density systems are more common., Comment: 13 pages, 5 figures, 6 tables, accepted by MNRAS

Spectral lines belonging to the short-lifetime heavy radioactive elements up to Es ($Z$=99) have been found in the spectra of the Przybylski's star. We suggest that these unstable elements may be decay products of a "magic" metastable nucleus belonging to the the island of stability where the nuclei have a magic number of neutrons $N=184$. The laboratory-produced nuclei have a significantly smaller number of neutrons. To identify spectra of the $N=184$ isotopes of these nuclei and their neutron-reach superheavy decay products in astrophysical data we calculate the isotope shift which should be added to the laboratory - measured wavelenghs. The results for the isotopic shifts in the strongest optical electromagnetic transitions in No, Lr, Nh, Fl,and $Z$=120 elements are presented., Comment: 4 pages, no figures

The primordial deuterium abundance is an important tracer of the fundamental physics taking place during Big Bang Nucleosynthesis. It can be determined from absorption features along the line of sight to distant quasars. The quasar PKS1937-1009 contains two absorptions systems that have been used to measure the primordial deuterium abundance, the lower redshift one being at z_abs = 3.256. New observations of this absorber are of a substantially higher signal-to-noise and thus permit a significantly more robust estimate of the primordial deuterium abundance, leading to a D/H ratio of 2.45+/-0.28 x10^-5. Whilst the precision of the new measurement presented here is below that obtained from the recent cosmological parameter measurements by Planck, our analysis illustrates how a statistical sample obtained using similarly high spectral signal-to-noise can make deuterium a competitive and complementary cosmological parameter estimator and provide an explanation for the scatter seen between some existing deuterium measurements., Comment: 13 pages, 5 figures, matches version accepted by MNRAS

Recently, white dwarf stars have found a new use in the fundamental physics community. Many prospective theories of the fundamental interactions of Nature allow traditional constants, like the fine structure constant $\alpha$, to vary in some way. A study by Berengut et al. (2013) used the Fe/Ni V line measurements made by Preval et al. (2013) from the hot DA white dwarf G191-B2B, in an attempt to detect any variation in $\alpha$. It was found that the Fe V lines indicated an increasing alpha, whereas the Ni V lines indicated a decreasing alpha. Possible explanations for this could be misidentification of the lines, inaccurate atomic data, or wavelength dependent distortion in the spectrum. We examine the first two cases by using a high S/N reference spectrum from the hot sdO BD+28$^{\circ}$4211 to calibrate the Fe/Ni V atomic data. With this new data, we re-evaluate the work of Berengut et al. (2013) to derive a new constraint on the variation of alpha in a gravitational field., Comment: 4 pages, 2 figures: To appear in the proceedings of the "19th European White Dwarf Workshop" in Montreal, Canada, 2014

We report the detection of a strong HI 21-cm absorption system at z=0.5344, as well as a candidate system at z=0.3389, in the sight-line towards the z=2.64 quasar MG J0414+0534. This, in addition to the absorption at the host redshift and the other two intervening absorbers, takes the total to four (possibly five). The previous maximum number of 21-cm absorbers detected along a single sight-line is two and so we suspect that this number of gas-rich absorbers is in some way related to the very red colour of the background source. Despite this, no molecular gas (through OH absorption) has yet been detected at any of the 21-cm redshifts, although, from the population of 21-cm absorbers as a whole, there is evidence for a weak correlation between the atomic line strength and the optical--near-infrared colour. In either case, the fact that so many gas-rich galaxies (likely to be damped Lyman-alpha absorption systems) have been found along a single sight-line towards a highly obscured source may have far reaching implications for the population of faint galaxies not detected in optical surveys, a possibility which could be addressed through future wide-field absorption line surveys with the Square Kilometre Array., Comment: Accepted by ApJ Letters

We propose a new probe of the dependence of the fine structure constant, alpha, on a strong gravitational field using metal lines in the spectra of white dwarf stars. Comparison of laboratory spectra with far-UV astronomical spectra from the white dwarf star G191-B2B recorded by the Hubble Space Telescope Imaging Spectrograph gives limits on the fractional variation of alpha of (Delta alpha/alpha)=(4.2 +- 1.6)x10^(-5) and (-6.1 +- 5.8)x10^(-5) from Fe V and Ni V spectra, respectively, at a dimensionless gravitational potential relative to Earth of (Delta phi) ~ 5x10^(-5). With better determinations of the laboratory wavelengths of the lines employed these results could be improved by up to two orders of magnitude., Comment: 8 pages, 2 figures. v2: minor corrections

We compare the redshifts of neutral carbon and carbon monoxide in the redshifted sources in which the fine structure transition of neutral carbon, [CI], has been detected, in order to measure space-time variation of the fundamental constants. Comparison with the CO rotational lines measures gives the same combination of constants obtained from the comparison fine structure line of singly ionised carbon, [CII]. However, neutral carbon has the distinct advantage that it may be spatially coincident with the carbon monoxide, whereas [CII] could be located in the diffuse medium between molecular clouds, and so any comparison with CO could be dominated by intrinsic velocity differences. Using [CI], we obtain a mean variation of dF/F = (-3.6 +/- 8.5) x 10^-5, over z = 2.3 - 4.1, for the eight [CI] systems, which degrades to (-1.5+/- 11) x 10^-5, over z = 2.3 - 6.4 when the two [CII] systems are included. That is, zero variation over look-back times of 10.8-12.8 Gyr. However, the latest optical results indicate a spatial variation in alpha, which describes a dipole and we see the same direction in dF/F. This trend is, however, due to a single source for which the [CI] spectrum is of poor quality. This also applies to one of the two [CII] spectra previously used to find a zero variation in alpha^2/mu. Quantifying this, we find an anti-correlation between |dF/F| and the quality of the carbon detection, as measured by the spectral resolution, indicating that the typical values of >50 km/s, used to obtain a detection, are too coarse to reliably measure changes in the constants. From the fluxes of the known z > 1 CO systems, we predict that current instruments are incapable of the sensitivities required to measure changes in the constants through the comparison of CO and carbon lines. We therefore discuss in detail the use of ALMA for such an undertaking ... ABRIDGED, Comment: Accepted for publication in Section 3 - Cosmology (including clusters of galaxies) of Astronomy and Astrophysics

We have undertaken a search for millimetre-wave band absorption (through the CO and HCO+ rotational transitions) in the host galaxies of reddened radio sources (z = 0.405-1.802). Despite the colour selection (optical-near infrared colours of V - K > 5 in all but one source), no absorption was found in any of the eight quasars for which the background continuum flux was detected. On the basis of the previous (mostly intervening) H2 and OH detections, the limits reached here and in some previous surveys should be deep enough to detect molecular absorption according to their V - K colours. However, our survey makes the assumption that the reddening is associated with dust close to the emission redshift of the quasar and that the narrow millimetre component of this emission is intercepted by the compact molecular cores. By using the known millimetre absorbers to define the colour depth and comparing this with the ultra-violet luminosities of the sources, we find that, even if these assumptions are valid, only twelve of the forty objects (mainly from this work) are potentially detectable. This is assuming an excitation temperature of 10 K at z=0, with the number decreasing with increasing temperatures (to zero detectable at 100 K)., Comment: 11 pages accepted by MNRAS

We present the results of a mini-survey for associated HI 21-cm absorption at z < 0.42 with the Giant Metrewave Radio Telescope. Our targets are radio galaxies, selected on the basis that the 1216 Angstrom luminosities are below 10^23 W/Hz, above which there has never been a detection of 21-cm absorption. Of the three sources for which we obtained good data, two are unclassified active galactic nuclei (AGN) and one is type-2. Being a non-detection, the type-2 object is consistent with our previous result that 21-cm absorption in radio sources is not dictated by unified schemes of AGN. In the case of the detection, the absorption only occurs towards one of the two resolved radio lobes in PKS 1649-062. If the absorption is due to an another intervening galaxy, or cool HI gas in the intergalactic medium, covering only the south-west lobe, then, being at the same redshift, this is likely to be gravitationally bound to the optical object identified as PKS 1649-062. If the absorption is due to an inclined disk centred between the lobes, intervening the SW lobe while being located behind the NE lobe, by assuming that it covers the emission peak at 150 kpc from the nucleus, we estimate a dynamical mass of ~3 x 10^12 solar masses for the disk., Comment: 5 pages accepted by MNRAS Letters

We report the detection of a third HI 21-cm absorber in the sight-line towards the z=2.64 quasar MG J0414+0534 (4C +05.19). In addition to the absorption at the host redshift and in the z=0.96 gravitational lens, we find, through a decimetre-wave spectral scan towards this source, strong absorption at z=0.38. We believe this may be associated with "Object X", an additional feature apparent in the field of the lensing galaxy and lensed images, on the basis of its close proximity to the quasar images and the possible detection of the [OIII] doublet in a published optical spectrum. If real, the strength of the [OIII] emission would suggest the presence of an active galactic nucleus, or a gas-rich galaxy undergoing rapid star formation, either of which is consistent with the strong outflows apparent in the 21-cm spectrum. Although this is the strongest intervening 21-cm absorber yet found, simultaneous observations failed to detect any of the 18-cm OH lines at the 21-cm redshift. This suggests that, as for the lensing galaxy, this is not the primary location of the intervening material responsible for the very red colour of MG J0414+0534., Comment: 5 pages, accepted by MNRAS Letters

From a survey for redshifted HI 21-cm and OH 18-cm absorption in the hosts of a sample of radio galaxies and quasars, we detect HI in three of the ten and OH in none of the fourteen sources for which useful data were obtained. As expected from our recent result, all of the 21-cm detections occur in sources with ultra-violet continuum luminosities of L < 10^23 W/Hz. At these "moderate" luminosities, we also obtain four non-detections, although, as confirmed by the equipartition of detections between the type-1 and type-2 objects, this near-50% detection rate cannot be attributed to unified schemes of active galactic nuclei (AGN). All of our detections are at redshifts of z < 0.67, which, in conjunction with our faint source selection, biases against UV luminous objects. The importance of ultra-violet luminosity (over AGN type) in the detection of 21-cm is further supported by the non-detections in the two high redshift z ~ 3.6 - 3.8 radio galaxies, which are both type-2 objects, while having L > 10^23 W/Hz. Our 21-cm detections in combination with those previously published, give a total of eight (associated and intervening) HI absorbing sources searched and undetected in OH. Using the detected 21-cm line strengths to normalise the limits, we find that only two of these eight may have been searched sufficiently deeply in OH, although even these are marginal., Comment: 10 pages, submitted to MNRAS

Theories unifying gravity with other interactions suggest spatial and temporal variation of fundamental "constants" in the Universe. A change in the fine structure constant, alpha, could be detected via shifts in the frequencies of atomic transitions in quasar absorption systems. Recent studies using 140 absorption systems from the Keck telescope and 153 from the Very Large Telescope, suggest that alpha varies spatially. That is, in one direction on the sky alpha seems to have been smaller at the time of absorption, while in the opposite direction it seems to have been larger. To continue this study we need accurate laboratory measurements of atomic transition frequencies. The aim of this paper is to provide a compilation of transitions of importance to the search for alpha variation. They are E1 transitions to the ground state in several different atoms and ions, with wavelengths ranging from around 900 - 6000 A, and require an accuracy of better than 10^{-4} A. We discuss isotope shift measurements that are needed in order to resolve systematic effects in the study. The coefficients of sensitivity to alpha-variation (q) are also presented., Comment: Includes updated version of the "alpha line" list

The detection of a spatial variation of the fine-structure constant, alpha, based on study of quasar absorption systems has recently been reported. The physics that causes this alpha-variation should have other observable manifestations, and this motivates us to look for complementary astrophysical effects. In this paper we propose a method to test whether spatial variation of fundamental constants existed during the epoch of big bang nucleosynthesis. Using existing measurements of primordial deuterium abundance we find very weak indications that such a signature might exist, but the paucity of measurements precludes any firm conclusion. We also examine existing quasar absorption spectra data that are sensitive to variation of the electron-to-proton mass ratio, mu, and x = (alpha^2 mu g_p) for spatial variation., Comment: 6 pages, 1 figure

We previously reported Keck telescope observations suggesting a smaller value of the fine structure constant, alpha, at high redshift. New Very Large Telescope (VLT) data, probing a different direction in the universe, shows an inverse evolution; alpha increases at high redshift. Although the pattern could be due to as yet undetected systematic effects, with the systematics as presently understood the combined dataset fits a spatial dipole, significant at the 4.2-sigma level, in the direction right ascension 17.5 +/- 0.9 hours, declination -58 +/- 9 degrees. The independent VLT and Keck samples give consistent dipole directions and amplitudes, as do high and low redshift samples. A search for systematics, using observations duplicated at both telescopes, reveals none so far which emulate this result., Comment: 5 pages, 5 figures, published on 31st October 2011 in Physical Review Letters

Spectroscopy of redshifted radio absorption of atomic and molecular species provide excellent probes of the cold component of the gas in the early Universe which can be used to address many important issues, such as measuring baryonic content, probing large-scale structure and galaxy evolution, as well as obtaining independent measurements of various combinations of fundamental constants at large look-back times. However, such systems are currently very rare with only 80 detected in HI 21-cm and five in OH and millimetre-band species. Here we summarise the main selection criteria responsible for this and how the next generation of radio telescopes are expected to circumvent these through their wide instantaneous bandwidths and fields-of-view., Comment: 6 pages, accepted by Proceedings of Science

Previous surveys for HI 21-cm absorption in z > 0.1 radio galaxies and quasars yield a 40% detection rate, which is attributed to unified schemes of active galactic nuclei (AGN). In this paradigm absorption is only witnessed in (close to) type-2 objects, where the central obscuration is viewed (nearly) edge-on and thus absorbs the rest frame 1420 MHz emission along our sight-line. However, we find this mix of detections and non-detections to only apply at low redshift (z < 1): From a sensitive survey of eight z > 3 radio sources we find no 21-cm absorption, indicating a low abundance of cold neutral gas in (the sight-lines searched in) these objects. Analysing the spectral energy distributions of these sources, we find that our high redshift selection introduces a bias where our sample consists exclusively of quasars with ultra-violet luminosities in excess of 10e23 W/Hz. This may suggest that we have selected a class of particularly UV bright type-1 objects. Whatever the cause, it must also be invoked to explain the non-detections in an equal number of z < 0.7 sources, where we find, for the first time, the same exclusive non-detections at > 10e23 W/Hz. These objects also turn out to be quasars and, from these exclusive high UV luminosity--21-cm non-detections, it is apparent that orientation effects alone cannot account for the mix of 21-cm detections and non-detections at any redshift., Comment: 4 pages, conference proceedings for "The Central Kiloparsec, Active Galactic Nuclei and Their Hosts" to appear in Volume 79 of the Memorie della Societa Astronomica Italiana

We present the results of a z>2.9 survey for HI 21-cm and molecular absorption in the hosts of radio quasars using the GMRT and the Tidbinbilla 70-m telescope. Previously published searches, which are overwhelmingly at redshifts of z<1, exhibit a 42% detection rate (31 out of 73 sources), and the inclusion of our survey yields a 17% detection rate (2 out of 12 sources) at z>2.5. We therefore believe that our high redshift selection is responsible for our exclusive non-detections, and find that at ultra-violet luminosities of >10e23 W/Hz, 21-cm absorption has never been detected. We also find this to not only apply to our targets, but also those at low redshift exhibiting similar luminosities, giving zero detections out of a total of 16 sources over z=0.24 to 3.8. This is in contrast to the < 10e23 W/Hz sources where there is a near 50% detection rate of 21-cm absorption. The mix of 21-cm detections and non-detections is currently attributed to orientation effects, where according to unified schemes of active galactic nuclei, 21-cm absorption is more likely to occur in sources designated as radio galaxies (type-2 objects, where the nucleus is viewed through dense obscuring circumnuclear gas) than in quasars(type-1 objects, where we have a direct view to the nucleus). However, due to the exclusively high ultra-violet luminosities of our targets it is not clear whether orientation effects alone can wholly account for the distribution, although there exists the possibility that the large luminosities are indicative of a changing demographic of galaxy types. We also find that below luminosities of ~10e23 W/Hz, both type-1 and type-2 objects have a 50% likelihood of exhibiting 21-cm absorption., Comment: 21 pages, accepted by MNRAS

We present a new catalogue of variable stars compiled from data taken for the University of New South Wales Extrasolar Planet Search. From 2004 October to 2007 May, 25 target fields were each observed for 1-4 months, resulting in ~87000 high precision light curves with 1600-4400 data points. We have extracted a total of 850 variable light curves, 659 of which do not have a counterpart in either the General Catalog of Variable Stars, the New Suspected Variables catalogue or the All Sky Automated Survey southern variable star catalogue. The catalogue is detailed here, and includes 142 Algol-type eclipsing binaries, 23 beta Lyrae-type eclipsing binaries, 218 contact eclipsing binaries, 53 RR Lyrae stars, 26 Cepheid stars, 13 rotationally variable active stars, 153 uncategorised pulsating stars with periods <10 d, including delta Scuti stars, and 222 long period variableswith variability on timescales of >10 d. As a general application of variable stars discovered by extrasolar planet transit search projects, we discuss several astrophysical problems which could benefit from carefully selected samples of bright variables. These include: (i) the quest for contact binaries with the smallest mass ratio, which could be used to test theories of binary mergers; (ii) detached eclipsing binaries with pre-main-sequence components, which are important test objects for calibrating stellar evolutionary models; and (iii) RR Lyrae-type pulsating stars exhibiting the Blazhko-effect, which is one of the last great mysteries of pulsating star research., Comment: 14 pages, 8 figures, 6 tables, accepted for publication in MNRAS

We present the results of a survey for 21-cm absorption in four never previously searched damped Lyman-alpha absorption systems (DLAs) with the Westerbork Synthesis Radio Telescope. We add our results to other recent studies in order to address the important issues regarding the detection of cold gas, through 21-cm absorption, in DLAs: Although, due to the DLAs identified with spiral galaxies, there is a mix of spin temperature/covering factor ratios at low redshift, two recent high redshift end points confirm that this ratio does not generally rise over the whole redshift range searched (up to z = 3.39). That is, if the covering factors of many of these galaxies were a factor of >2 smaller than for the spirals, then no significant difference in the spin temperatures between these two classes would be required. Furthermore, although it is difficult to separate the relative contributions of the spin temperature and covering factor, the new results confirm that 21-cm detections tend to occur at low angular diameter distances, where the coverage of a given absorption cross section is maximised. Finally, we also find an apparent 21-cm line strength--MgII equivalent width correlation, which appears to be due to a coupling of the velocity structure between the components that each species traces. That is, the gas seen in 21-cm absorption could be the same as that seen in optical absorption. Combined with the known equivalent width--metallicity relation, this may be manifest as a spin temperature--metallicity anti-correlation, which is non-evolutionary in origin., Comment: 11 (and a bit) pages, lots of nice figures, accepted by MNRAS

We report the detection of \HI 21-cm absorption in the $z=0.96$ early-type lensing galaxy towards MG J0414+0534 with the Green Bank Telescope. The absorption, with total $N_{\rm HI}=1.6 \times 10^{18} (T_{\rm s}/f) {\rm cm}^{-2}$, is resolved into two strong components, probably due to the two strongest lens components, which are separated by 0.4\arcsec. Unlike the other three lenses which have been detected in \HI, J0414+0534 does not exhibit strong OH absorption, giving a OH/\HI column density ratio of $N_{\rm OH}/N_{\rm HI}\lapp10^{-6}$ (for $T_{\rm s}=100$ K, $T_{\rm x}=10$ K and $f_{\rm HI}=f_{\rm OH}=1$). This underabundance of molecular gas may indicate that the extreme optical--near-IR colour ($V-K=10.26$) along the line-of-sight is not due to the lens. We therefore suggest that despite the strong upper limits on molecular absorption at the quasar redshift, as traced by millimetre lines, the extinction occurs primarily in the quasar host galaxy., Comment: Accepted by MNRAS Letters, 5 (and a bit) pages, 5 figures

We report the discovery of the first high-amplitude delta Scuti star in an eclipsing binary, which we have designated UNSW-V-500. The system is an Algol-type semi-detached eclipsing binary of maximum brightness V = 12.52 mag. A best-fitting solution to the binary light curve and two radial velocity curves is derived using the Wilson-Devinney code. We identify a late A spectral type primary component of mass 1.49+/-0.02 M_sun and a late K spectral type secondary of mass 0.33+/-0.02 M_sun, with an inclination of 86.5+/-1.0 degrees, and a period of 5.3504751+/-0.0000006 d. A Fourier analysis of the residuals from this solution is performed using PERIOD04 to investigate the delta Scuti pulsations. We detect a single pulsation frequency of f_1 = 13.621+/-0.015 c/d, and it appears this is the first overtone radial mode frequency. This system provides the first opportunity to measure the dynamical mass for a star of this variable type; previously, masses have been derived from stellar evolution and pulsation models., Comment: 7 pages, 6 figures, 2 tables, for submission to MNRAS, v2: paper size change, small typographical changes to abstract

We report the detection of 21-cm absorption at z = 0.656 towards 1622+238 (3C336). The line is very broad with a Full-Width Half Maximum (FWHM) of 235 km/s, giving a velocity integrated optical depth of 2.2 km/s. The centroid of the line is offset from that of the known damped Lyman-alpha absorption (DLA) system by 50 km/s, and if the Lyman-alpha and 21-cm absorption are due to the same gas, we derive a spin temperature of < 60 K, which would be the lowest yet in a DLA. The wide profile, which is over four times wider than that of any other DLA, supports the hypothesis that the hydrogen absorption is occurring either in the disk of a large underluminous spiral or a group of dim unidentified galaxies, associated with the single object which has been optically identified at this redshift., Comment: 5 pages, 3 figures, accepted by MNRAS Letters

Echelle spectrographs currently provide some of the most precise and detailed spectra in astronomy, the interpretation of which sometimes depends on the wavelength calibration accuracy. In some applications, such as constraining cosmological variations in the fundamental constants from quasar absorption lines, the wavelength calibration is crucial. Here we detail an algorithm for selecting thorium-argon (ThAr) emission lines for wavelength calibration which incorporates the properties of both a new laboratory wavelength list and the spectrograph of interest. We apply the algorithm to the Very Large Telescope Ultraviolet and Visual Echelle Spectrograph (UVES) and demonstrate a factor of >3 improvement in the wavelength calibration residuals (i.e. random errors) alone. It is also found that UVES spectra calibrated using a previous, widely distributed line-list contain systematic +/-30-75 m/s distortions of the wavelength scale over both short and long wavelength ranges. These distortions have important implications for current UVES constraints on cosmological variations in the fine-structure constant. The induced systematic errors are most severe for Mg/FeII quasar absorbers in the redshift range 1.2 < z < 2.3, with individual absorbers studied by recent authors containing systematic errors up to 4 times larger than quoted statistical errors., Comment: 10 pages, 11 figures (16 EPS files). Accepted by MNRAS. Supplementary material, including electronic versions of all input and output ThAr atlases and colour versions of Fig. 3(right) available at http://www.ast.cam.ac.uk/~mim/pub.html

We present an analysis of a new, detached, double-lined eclipsing binary system with K7 Ve components, discovered as part of the University of New South Wales Extrasolar Planet Search. The object is significant in that only 6 other binary systems are known with comparable or lower mass. Such systems offer important tests of mass-radius theoretical models. Follow-up photometry and spectroscopy were obtained with the 40-inch and 2.3m telescopes at SSO respectively. An estimate of the radial velocity amplitude from spectral absorption features, combined with the orbital inclination (83.5 deg) estimated from lightcurve fitting, yielded a total mass of M=(1.041 +/- 0.06)M_sun and component masses of M_A=(0.529 +/- 0.035)M_sun and M_B=(0.512 +/- 0.035)M_sun. The radial velocity amplitude estimated from absorption features (167 +/- 3)kmps was found to be less than the estimate from the H_alpha emission lines (175 +/- 1.5)kmps. The lightcurve fit produced radii of R_A=(0.641 +/- 0.05)R_sun and R_B=(0.608 +/- 0.06)R_sun, and a temperature ratio of T_B/T_A=0.980 +/- 0.015. The apparent magnitude of the binary was estimated to be V=13.9 +/- 0.2. Combined with the spectral type, this gave the distance to the binary as 169 +/- 14 pc. The timing of the secondary eclipse gave a lower limit on the eccentricity of the binary system of 0.0025 +/- 0.0005. This is the most statistically significant non-zero eccentricity found for such a system, possibly suggesting the presence of a third companion., Comment: 6 pages, published in MNRAS (08/2006)

A correlation between the metallicity, [M/H], and rest-frame MgII equivalent width, EW, is found from 49 DLAs and strong sub-DLAs drawn from the literature over the redshift range 0.20.7A, the correlation remains at >3 sigma significance. Since the MgII2796 transition is predominantly saturated in DLAs (which always have EW greater than 0.3A), EW is far more sensitive to the kinematic spread of the metal velocity components across the absorption profile than it is to [M/H]. Thus, the observed [M/H]--EW correlation points to a strong link between the absorber metallicity and the mechanism for producing and dispersing the velocity components. We also note that approximately half of the 13 known molecular hydrogen absorbers have very high EW and very broad velocity structures which show characteristics usually associated with outflows. Follow-up ultraviolet- and blue-sensitive high-resolution spectra of high-EW systems, initially identified in low-resolution spectra, may therefore yield a large number of new H_2 discoveries., Comment: 9 pages, 2 figures (3 EPS files). Accepted by MNRAS

Current analyses of VLT/UVES quasar spectra disagree with the Keck/HIRES evidence for a varying fine-structure constant, alpha. To investigate this we introduce a simple method for calculating the minimum possible uncertainty on Delta(alpha)/alpha for a given quasar absorber. For many absorbers in Chand et al. (2004) and for the single-absorber constraint of Levshakov et al. (2006) the quoted uncertainties are smaller than the minimum allowed by the UVES data. Failure of this basic consistency test prevents reliable comparison of the UVES and HIRES results., Comment: 4 pages, 2 figures (3 EPS files). Invited contribution to the proceedings of "Precision Spectroscopy in Astrophysics", Aveiro, Portugal, Sep. 2006, eds Pasquini et al., ESO Astrophysics Symposia. v2: Corrected small typos and a reference. v3: Corrected another small typo

Nine quasar absorption spectra at 21-cm and UV rest-wavelengths are used to estimate possible variations in x=alpha^2 g_p mu, (alpha is the fine structure constant, g_p the proton g-factor and mu=me/mp the electron-to-proton mass ratio). We find ^weighted_total(=Dxxwt)=(0.63+-0.99) 10^-5 over 0.23~

We are currently undertaking a large survey for redshifted atomic and molecular absorption ... only one clear and one tentative detection were obtained: HI absorption at z = 0.097 in PKS 1555-140 and OH absorption at z =0.126 in PKS 2300-189, respectively... In order to determine why no clear molecular absorption was detected in any of the 13 sources searched, we investigate the properties of the five redshifted systems currently known to exhibit OH absorption. In four of these, molecules were first detected via millimetre-wave transitions and the flat radio spectra indicate compact background continuum sources, which may suggest a high degree of coverage of the background source by the molecular clouds in the absorber. Furthermore, for these systems we find a relationship between the molecular line strength and red optical--near infrared (V-K) colours, thus supporting the notion that the reddening of these sources is due to dust, which provides an environment conducive to the formation of molecules. Upon comparison with the V-K colours of our sample, this relationship suggests that, presuming the reddening occurs at the host galaxy redshift at least in some of the targets, many of our observations still fall short of the sensitivityrequired to detect OH absorption, although a confirmation of the ``detection'' of OH in 2300-189 could contravene this., Comment: 13 pages, loads of figures, accepted by MNRAS

In this paper we investigate the possible reasons why HI 21-cm absorption in damped Lyman-alpha systems (DLAs) has only been detected at low redshift... We suggest that the lack of 21-cm absorption detections at high redshift arises from the fact that these DLAs are at similar angular diameter distances to the background quasars (i.e. the distance ratios are always close to unity): Above z~1.6 the covering factor becomes largely independent of the DLA--QSO distance, making the high redshift absorbers much less effective at covering the background continuum emission. At low redshift, small distance ratios are strongly favoured by the 21-cm detections, whereas large ratios are favoured by the non-detections. This mix of distance ratios gives the observed mix of detections and non-detections at z<1.6.In addition to the predominance of large distance ratios and non-detections at high redshift, this strongly suggests that the observed distribution of 21-cm absorption in DLAs is dominated by geometric effects., Comment: 7 page, 6 figures, Accpeted by MNRAS

We report on the current status of the University of New South Wales Extrasolar Planet Search project, giving details of the methods we use to obtain millimagnitude precision photometry using the 0.5m Automated Patrol Telescope. We use a novel observing technique to optimally broaden the PSF and thus largely eliminate photometric noise due to intra-pixel sensitivity variations on the CCD. We have observed 8 crowded Galactic fields using this technique during 2003 and 2004. Our analysis of the first of these fields (centred on the open cluster NGC 6633) has yielded 49 variable stars and 4 shallow transit candidates. Follow-up observations of these candidates have identified them as eclipsing binary systems. We use a detailed simulation of our observations to estimate our sensitivity to short-period planets, and to select a new observing strategy to maximise the number of planets detected., Comment: 16 pages, 9 figures, version published in MNRAS Updated figures, references, and additional discussion in section 7

Quasar absorption spectra at 21-cm and UV rest-wavelengths are used to estimate the time variation of x=alpha^2 g_p mu, where alpha is the fine structure constant, g_p the proton g factor, and m_e/m_p=mu the electron/proton mass ratio. Over a redshift range 0.24 ~< z_abs ~< 2.04, _total_weighted=(1.17+-1.01) x 10^-5. A linear fit gives d/dt{x}/x=(-1.43+-1.27) x 10^-15 yr^-1. Two previous results on varying alpha yield the strong limits Delta mu/mu=(2.31+-1.03) x 10^-5 and Delta mu/mu=(1.29+-1.01) x 10^-5. Our sample, 8x larger than any previous, provides the first direct estimate of the intrinsic 21-cm and UV velocity differences ~6 km s^-1., Comment: 12 pages, 9 figures, accepted on 10 June 2005 for publication in Physical Review Letters. Results unchanged. Minor restructuring of abstract and one reference added. Pages 1-4 identical to accepted version following referees' comments. Pages 5-12 show plots to improve understanding of this work

We investigate the practice of assigning high spin temperatures to damped Lyman-alpha absorption systems (DLAs) not detected in HI 21-cm absorption. In particular, Kanekar & Chengaulr (2003) have attributed the mix of 21-cm detections and non-detections in low redshift (z<2.04) DLAs to a mix of spin temperatures, while the non-detections at high redshift were attributed to high spin temperatures. Below z=0.9, where some of the DLA host galaxy morphologies are known, we find that 21-cm absorption is normally detected towards large radio sources when the absorber is known to be associated with a large intermediate (spiral) galaxy. Furthermore, at these redshifts, only one of the six 21-cm non-detections has an optical identification and these DLAs tend to lie along the sight-lines to the largest background radio continuum sources. For these and many of the high redshift DLAs occulting large radio continua, we therefore expect covering factors of less than the assumed/estimated value of unity. This would have the effect of introducing a range of spin temperatures considerably narrower than the current range of >9000 K, while still supporting the hypothesis that the high redshift DLA sample comprises a larger proportion of compact galaxies than the low redshift sample., Comment: Submitted to MNRAS, 11 pages, 6 figures

Theories unifying gravity with other interactions suggest spatial and temporal variation of fundamental ``constants'' in the Universe. A change in the fine structure constant, alpha, could be detected via shifts in the frequencies of atomic transitions in quasar absorption systems. Previous studies of three independent samples of data, containing 143 absorption systems spread from 2 to 10 billion years after big bang, hint that alpha was smaller 7 - 11 billion years ago. To continue this study we urgently need accurate laboratory measurements of atomic transition frequencies. In this paper, we present a current list of transitions of importance to the search for alpha variation. They are E1 transitions to the ground state in several different atoms and ions, with wavelengths ranging from around 900 - 6000 Angstroms, and require an accuracy of better than 10^(-4) Angstroms. We also discuss isotope shift measurements that are needed in order to resolve systematic effects in the study. Researchers who are interested in performing these measurements should contact the authors directly., Comment: Reformatted all-in-one table; expanded references; fixed errors

We have used the Green Bank Telescope (GBT) and Berkeley-Illinois-Maryland Association (BIMA) array to search for redshifted millimetre absorption in a sample of damped Lyman-alpha absorption systems (DLAs). This brings the number of published systems searched from 18 to 30. In 17 cases we reach $3\sigma$ limits of $\tau\leq0.1$, which is a significant improvement over the previous searches and more than sufficient to detect the 4 known redshifted millimetre absorbers ($\tau\gapp1$). While the CO rotational (millimetre) column density limits obtained are weaker than the electronic (optical) limits, they may provide useful limits below the atmospheric cut-off for the Lyman and Werner \MOLH-bands in the UV ($z_{\rm abs}\lapp1.8$). Using a model for the DLA metallicity evolution combined with assumed HCO$^+$/\MOLH ~and CO/\MOLH \~conversion ratios, we use the molecular column density limits to calculate plausible \MOLH ~molecular fraction limits. Finally, we use these results to discuss the feasibility of detecting rotational CO transitions in DLAs with the next generation of large radio telescopes., Comment: Accepted by MNRAS Supplement available from http://www.phys.unsw.edu.au/~sjc/dla-fig2.ps.gz

We have identified a new Lyman limit absorption system towards PKS1937-1009, with log N(HI)=18.25 +/- 0.02 at z=3.256. It is suitable for measuring D/H, and we find a 68.3% confidence range for D/H of 1.6^{+0.25}_{-0.30} \times 10^{-5}, and a 95.4% range of 1.6^{+0.5}_{-0.4} \times 10^{-5}. The metallicity of the cloud where D/H was measured is low, [Si/H] = -2.0 +/- 0.5. At these metallicities we expect that D/H will be close to the primordial value. Our D/H disagrees at a level of 99.4% with the predicted D/H using the baryon density calculated from the cosmic background radiation measured by WMAP, 2.60^{+0.19}_{-0.17} \times 10^{-5}. Our result also exacerbates the scatter in D/H values around the mean primordial D/H., Comment: 13 pages, accepted by MNRAS. Version 2: corrected typos. Version 3: Final accepted version. Includes new section comparing this result to other QSO D/H results

We use Big Bang Nucleosynthesis calculations and light element abundance data to constrain the relative variation of the deuteron binding energy since the universe was a few minutes old, $\delta Q = Q(BBN)-Q(present)$. Two approaches are used, first treating the baryon to photon ratio, $\eta$, as a free parameter, but with the additional freedom of varying $\delta Q$, and second using the WMAP value of $\eta$ and solving only for $\delta Q$. Including varying $Q$ yields a better fit to the observational data than imposing the present day value, rectifying the discrepancy between the $^4He$ abundance and the deuterium and $^7Li$ abundances, {\it and} yields good agreement with the independently determined $\eta_{WMAP}$. The minimal deviation consistent with the data is significant at about the 4-$\sigma$ level; $\delta Q/Q= -0.019 \pm 0.005$. If the primordial $^4$He abundance lies towards the low end of values in the literature, this deviation is even larger and more statistically significant. Taking the light element abundance data at face-value, our result may be interpreted as variation of the dimensionless ratio $X=m_s/\Lambda_{QCD}$ of the strange quark mass and strong scale: $\delta X/X=(1.1 \pm 0.3) \times 10^{-3}$. These results provide a strong motivation for a more thorough exploration of the potential systematic errors in the light element abundance data., Comment: 9 pages, 5 figures

Strong constraints on possible variations in fundamental constants can be derived from HI 21-cm and molecular rotational absorption lines observed towards quasars. With the aim of forming a statistical sample of constraints we have begun a program of systematic searches for such absorption systems. Here we describe molecular rotational searches in 25 damped Lyman-alpha systems where, in many cases, we set optical depth limits an order of magnitude better than that required to detect the 4 known redshifted millimeter-wave absorbers. We also discuss the contributory factors in the detectability of HI 21-cm absorption, focusing on possible biases (e.g.low covering factors) in the currently known sample of absorbers and non-detections., Comment: 4 pages, to appear in The Neutral ISM in Starburst Galaxies, ASP Conference Series, 2003, edited by S. Aalto, S. Huettemeister, A. Pedlar

Millimetre-band scans of the frequency space towards optically dim quasars is potentially a highly efficient method for detecting new high redshift molecular absorption systems. Here we describe scans towards 7 quasars over wide bandwidths (up to 23 GHz) with sensitivity limits sufficient to detect the 4 redshifted absorbers already known. With wider frequency bands, highly efficient searches of large numbers of possibly obscured objects will yield many new molecular absorbers., Comment: 3 pages, conference proceedings to appear in Highlights of Astronomy, ASP (IAU XXV JD21, The Astrochemistry of External Galaxies)

The chemical abundances in damped Lyman-alpha systems (DLAs) show more than 2 orders of magnitude variation at a given epoch, possibly because DLAs arise in a wide variety of host galaxies. This could significantly bias estimates of chemical evolution. We explore the possibility that DLAs in which H_2 absorption is detected may trace cosmological chemical evolution more reliably since they may comprise a narrower set of physical conditions. The 9 known H_2 absorption systems support this hypothesis: metallicity exhibits a faster, more well-defined evolution with redshift than in the general DLA population. The dust-depletion factor and, particularly, H_2 molecular fraction also show rapid increases with decreasing redshift. We comment on possible observational selection effects which may bias this evolution. Larger samples of H_2-bearing DLAs are clearly required and may constrain evolution of the UV background and DLA galaxy host type with redshift., Comment: 6 pages, 1 figure, 1 table, v2: updated references, corrected typos, to appear in Recycling intergalactic and interstellar matter, IAU Symposium series, Vol. 217, 2004, edited by P.-A. Duc, J. Braine, E. Brinks

We present evidence for variations in the fine-structure constant from Keck/HIRES spectra of 143 quasar absorption systems over the redshift range 0.2 < z_abs < 4.2. This includes 15 new systems, mostly at high-z (z_abs > 1.8). Our most robust estimate is a weighted mean da/a=(-0.57 +/- 0.11)x10^{-5}. We respond to recent criticisms of the many-multiplet method used to extract these constraints. The most important potential systematic error at low-z is the possibility of very different Mg heavy isotope abundances in the absorption clouds and laboratory: {\it higher} abundances of {25,26}Mg in the absorbers may explain the low-z results. Approximately equal mixes of {24}Mg and {25,26}Mg are required. Observations of Galactic stars generally show {\it lower} {25,26}Mg isotope fractions at the low metallicities typifying the absorbers. Higher values can be achieved with an enhanced population of intermediate mass stars at high redshift, a possibility at odds with observed absorption system element abundances. At present, all observational evidence is consistent with the varying-alpha results. Another promising method to search for variation of fundamental constants involves comparing different atomic clocks. Here we calculate the dependence of nuclear magnetic moments on quark masses and obtain limits on the variation of alpha and m_q/Lambda_QCD from recent atomic clock experiments with hyperfine transitions in H, Rb, Cs, Hg+ and an optical transition in Hg+., Comment: 20 pages, 6 Figures (9 EPS files), 1 Table, to appear in Lecture Notes in Physics, Proc. 302 WE Heraeus Seminar: Astrophysics, Clocks and Fundamental Constants (16-18 June 2003: Bad Honnef, Germany). v2: Added and updated references. v3: Removed model dependent constraints in Section 3, added and updated references, surrendered to Springer-Verlag's "war on footnotes"

[Abridged] We previously presented evidence for a varying fine-structure constant, alpha, in two independent samples of Keck/HIRES QSO spectra. Here we present a detailed many-multiplet analysis of a third Keck/HIRES sample containing 78 absorption systems. We also re-analyse the previous samples, providing a total of 128 absorption systems over the redshift range 0.2

We describe initial results of a search for redshifted molecular absorption toward four millimetre-loud, optically faint quasars. A wide frequency bandwidth of up to 23 GHz per quasar was scanned using the Swedish-ESO Sub-millimetre Telescope at La Silla. Using a search list of commonly detected molecules, we obtained nearly complete redshift coverage up to z_abs=5. The sensitivity of our data is adequate to have revealed absorption systems with characteristics similar to those seen in the four known redshifted millimetre-band absorption systems, but none were found. Our frequency-scan technique nevertheless demonstrates the value of wide-band correlator instruments for searches such as these. We suggest that a somewhat larger sample of similar observations should lead to the discovery of new millimetre-band absorption systems., Comment: 8 pages, 7 EPS figures, 3 tables, accepted by MNRAS

We report a lambda > 1cm search for rotational molecular absorption towards quasars, now possible with the upgraded Australia Telescope Compact Array (ATCA). The targets were PKS 0201+113, PKS 0336-017 and Q 0537-286, where known damped Lyman-alpha absorption systems (DLAs) could cause redshifted molecular absorption in the 12mm band of the telescope. We place 3-sigma upper limits on any HCO+ 0-1 absorption features of < 30mJy per 3.4km/s channel. The non-detections could be attributed to the inherent low metallicities in DLAs leading to generally low H_2, and thus HCO+, column densities. In general, the detection of molecular rotational transitions in DLAs could be further hindered by a lower than expected CO-to-H_2 conversion ratio, whether due either to photoionization of carbon or its relative under-abundance at high redshift., Comment: 5 pages, 3 EPS figures, 2 tables, accepted by MNRAS