High Resolution Echelle Spectrometer Research Articles

CHIRON—A Fiber Fed Spectrometer for Precise Radial Velocities

The CHIRON optical high-resolution echelle spectrometer was commissioned at the 1.5 m telescope at CTIO in 2011. The instrument was designed for high throughput and stability, with the goal of monitoring radial velocities of bright stars with high precision and high cadence for the discovery of low-mass exoplanets. Spectral resolution of R = 79 000 is attained when using a slicer with a total (including telescope and detector) efficiency of 6% or higher, while a resolution of R = 136 000 is available for bright stars. A fixed spectral range of 415–880 nm is covered. The echelle grating is housed in a vacuum enclosure and the instrument temperature is stabilized to ± 0.2°. Stable illumination is provided by an octagonal multimode fiber with excellent light-scrambling properties. An iodine cell is used for wavelength calibration. We describe the main optics, fiber feed, detector, exposure-meter, and other aspects of the instrument, as well as the observing procedure and data reduction.

Warm dark matter as a solution to the small scale crisis: New constraints from high redshift Lyman-αforest data

We present updated constraints on the free-streaming of warm dark matter (WDM) particles derived from an analysis of the Lyman-$\ensuremath{\alpha}$ flux power spectrum measured from high-resolution spectra of 25 $z>4$ quasars obtained with the Keck High Resolution Echelle Spectrometer and the Magellan Inamori Kyocera Echelle spectrograph. We utilize a new suite of high-resolution hydrodynamical simulations that explore WDM masses of 1, 2 and 4 keV (assuming the WDM consists of thermal relics), along with different physically motivated thermal histories. We carefully address different sources of systematic error that may affect our final results and perform an analysis of the Lyman-$\ensuremath{\alpha}$ flux power with conservative error estimates. By using a method that samples the multidimensional astrophysical and cosmological parameter space, we obtain a lower limit ${m}_{\mathrm{WDM}}\ensuremath{\gtrsim}3.3\text{ }\text{ }\mathrm{keV}$ ($2\ensuremath{\sigma}$) for warm dark matter particles in the form of early decoupled thermal relics. Adding the Sloan Digital Sky Survey Lyman-$\ensuremath{\alpha}$ flux power spectrum does not improve this limit. Thermal relics of masses 1, 2 and 2.5 keV are disfavored by the data at about the $9\ensuremath{\sigma}$, $4\ensuremath{\sigma}$ and $3\ensuremath{\sigma}$ C.L., respectively. Our analysis disfavors WDM models where there is a suppression in the linear matter power spectrum at (nonlinear) scales corresponding to $k=10h/\mathrm{Mpc}$ which deviates more than 10% from a Lambda cold dark matter model. Given this limit, the corresponding ``free-streaming mass'' below which the mass function may be suppressed is $\ensuremath{\sim}2\ifmmode\times\else\texttimes\fi{}{10}^{8}{h}^{\ensuremath{-}1}{\mathrm{M}}_{\ensuremath{\bigodot}}$. There is thus very little room for a contribution of the free-streaming of WDM to the solution of what has been termed the small scale crisis of cold dark matter.

THE FUNDAMENTAL PLANE OF DAMPED Lyα SYSTEMS

Using a sample of 100 H I - selected damped Lyman alpha (DLA) systems, observed with the High Resolution Echelle Spectrometer on the Keck I telescope, we present evidence that the scatter in the well-studied correlation between the redshift and metallicity of a DLA is largely due to the existence of a mass-metallicity relationship at each redshift. To describe the fundamental relations that exist between redshift, metallicity and mass, we use a fundamental plane description, which is described by the following equation: [M/H]=(-1.9+-0.5)+(0.74+-0.21)logdv_90-(0.32+-0.06)z. Here, we assert that the velocity width, dv_90, which is defined as the velocity interval containing 90% of the integrated optical depth, traces the mass of the underlying dark matter halo. This description provides two significant improvements over the individual descriptions of the mass-metallicity correlation and metallicity-redshift correlation. Firstly, the fundamental equation reduces the scatter around both relationships by about 20%, providing a more stringent constraint on numerical simulations modeling DLAs. Secondly, it confirms that the dark matter halos that host DLAs satisfy a mass-metallicity relationship at each redshift between redshifts 2 through 5.

Aberration-corrected echelle spectrometer for measuring ultraviolet branching fractions of iron-group ions

A high-resolution echelle spectrometer with broad wavelength coverage from the UV to the IR and high sensitivity to weak lines is described. Total instrument astigmatism is suppressed through rotation of the order separation system into an orthogonal plane. A suitable choice of mirror angles avoids increased coma. This spectrometer is used to record UV through IR spectra of Fe-group ions to measure improved branching fractions. These new results reduce transition probability uncertainties and yield more accurate derived stellar abundances. Instrument design and performance, including aberration compensation, are presented.

NEW HUBBLE SPACE TELESCOPE OBSERVATIONS OF HEAVY ELEMENTS IN FOUR METAL-POOR STARS

Elements heavier than the iron group are found in nearly all halo stars. A substantial number of these elements, key to understanding neutron-capture nucleosynthesis mechanisms, can only be detected in the near-ultraviolet. We report the results of an observing campaign using the Space Telescope Imaging Spectrograph on board the Hubble Space Telescope to study the detailed heavy element abundance patterns in four metal-poor stars. We derive abundances or upper limits from 27 absorption lines of 15 elements produced by neutron-capture reactions, including seven elements (germanium, cadmium, tellurium, lutetium, osmium, platinum, and gold) that can only be detected in the near-ultraviolet. We also examine 202 heavy element absorption lines in ground-based optical spectra obtained with the Magellan Inamori Kyocera Echelle Spectrograph on the Magellan-Clay Telescope at Las Campanas Observatory and the High Resolution Echelle Spectrometer on the Keck I Telescope on Mauna Kea. We have detected up to 34 elements heavier than zinc. The bulk of the heavy elements in these four stars are produced by r-process nucleosynthesis. These observations affirm earlier results suggesting that the tellurium found in metal-poor halo stars with moderate amounts of r-process material scales with the rare earth and third r-process peak elements. Cadmium often follows the abundances of the neighboring elements palladium and silver. We identify several sources of systematic uncertainty that must be considered when comparing these abundances with theoretical predictions. We also present new isotope shift and hyperfine structure component patterns for Lu II and Pb I lines of astrophysical interest.

METALLICITY EVOLUTION OF DAMPED Lyα SYSTEMS OUT TOz∼ 5

We present chemical abundance measurements for 47 damped Lyman-alpha systems (DLAs), 30 at z>4, observed with the Echellette Spectrograph and Imager and the High Resolution Echelle Spectrometer on the Keck telescopes. HI column densities of the DLAs are measured with Voigt profile fits to the Lyman-alpha profiles, and we find an increased number of false DLA identifications with SDSS at z>4 due to the increased density of the Lyman-alpha forest. Ionic column densities are determined using the apparent optical depth method, and we combine our new metallicity measurements with 195 from previous surveys to determine the evolution of the cosmic metallicity of neutral gas. We find the metallicity of DLAs decreases with increasing redshift, improving the significance of the trend and extending it to higher redshifts, with a linear fit of -0.22+-0.03 dex per unit redshift from z=0.09-5.06. The metallicity 'floor' of ~1/600 solar continues out to z~5, despite our sensitivity for finding DLAs with much lower metallicities. However, this floor is not statistically different from a steep tail to the distribution. We also find that the intrinsic scatter of metallicity among DLAs of ~0.5 dex continues out to z~5. In addition, the metallicity distribution and the alpha/Fe ratios of z>2 DLAs are consistent with being drawn from the same parent population with those of halo stars. It is therefore possible that the halo stars in the Milky Way formed out of gas that commonly exhibits DLA absorption at z>2.

A CORRELATION BETWEEN CIRCUMSTELLAR DISKS AND ROTATION IN THE UPPER SCORPIUS OB ASSOCIATION

We present projected rotational velocities for 20 early-type (B8-A9) and 74 late-type (F2-M8) members of the ~5 Myr old Upper Scorpius OB Association derived from high dispersion optical spectra obtained with the High Resolution Echelle Spectrometer (HIRES) on Keck I and the Magellan Inamori Kyocera Echelle (MIKE) on the Magellan Clay telescope. The spectroscopic sample is composed of stars and brown dwarfs with infrared signatures of circumstellar disks, both primordial and debris, and non-excess sources of comparable spectral type. We merge projected rotational velocities, accretion diagnostics, and Spitzer Space Telescope Infrared Array Camera (IRAC) and Multiband Imaging Photometer for Spitzer (MIPS) 24 micron photometry to examine the relationship between rotation and circumstellar disks. The rotational velocities are strongly correlated with spectral type, a proxy for mass, such that the median vsini for B8-A9 type stars is: 195(+/-)70 km/s, F2-K4: 37.8(+/-)7.4 km/s, K5-K9: 13.8(+21.3/-8.2) km/s, M0-M5: 16.52(+/-)5.3 km/s, and M5.5-M8: 17.72(+/-)8.1 km/s. We find with a probability of >0.99 that M-type stars and brown dwarfs having infrared excess suggestive of circumstellar disks rotate more slowly than their non-excess counterparts. A similar correlation is present among F2-K9 type stars, but only at the ~97% confidence level. Among the early-type (B8-A9) members, rotational velocities of the debris-disk and non-disk populations are indistinguishable. Considering the late-type (F2-M8) stars and brown dwarfs, we find a low fraction of slowly rotating, non-excess sources relative to younger star forming regions, suggesting that most have spun up following disk dissipation. The few late-type (F2-M5) debris disk sources, which may be representative of stars that have recently dispersed their inner disks, are evenly divided between slow and moderate rotators.

The Low-Mass Companion to the Lithium-Depleted, Spectroscopic Binary HBC 425 (St 34)

ABSTRACTWe present high angular resolution, near-infrared imaging, and spectroscopy of a low-mass companion to the lithium-depleted, double-line spectroscopic binary HBC 425 (St 34) obtained using NIRSPEC and the Keck II adaptive optics system. Positioned ∼1.23″ southeast of the primary pair, the companion, HBC 425C, is ∼2.4 mag fainter at 2.2 μm. Moderate-resolution (R ∼ 2500) J- and K-band spectroscopy reveals HBC 425C to have an M5.5 (± 0.5) spectral type. Comparisons with pre–main-sequence evolutionary models imply a mass of ∼0.09 M⊙ and ages of 8–10 Myr, assuming the nominal distance of Taurus-Auriga (∼140 pc), or ∼25 Myr if placed at ∼90 pc. We also present high-dispersion optical spectra of HBC 425 and HBC 425C obtained using the high-resolution echelle spectrometer (HIRES) on Keck I. We detect strong Li i λ6708 absorption in the spectrum of HBC 425C. Using curves of growth for the Li i λ6708 doublet, we estimate its abundance level to lie between log N(Li) = 1.9 and 3.1 dex. The spectrum of HBC 425 exhibits Ca ii H and K lines, He i λλ5876, 6678, and strong Balmer line emission, consistent with accretion. We place more restrictive upper limits on the surface abundance of lithium and find that HBC 425 retains less than ∼0.1% of its primordial abundance. The presence of lithium in the photosphere of HBC 425C does not resolve the discrepancy between isochronal and lithium-depletion ages for the primary pair. However, if lithium were depleted relative to interstellar abundance levels, even minimally, considerable support would be gained for the more advanced age of this hierarchical triple system.

ROCKY EXTRASOLAR PLANETARY COMPOSITIONS DERIVED FROM EXTERNALLY POLLUTED WHITE DWARFS

We report Keck High Resolution Echelle Spectrometer data and model atmosphere analysis of two helium-dominated white dwarfs, PG1225-079 and HS2253+8023, whose heavy pollutions most likely derive from the accretion of terrestrial-type planet(esimal)s. For each system, the minimum accreted mass is ~10^22 g, that of a large asteroid. In PG1225-079, Mg, Cr, Mn, Fe and Ni have abundance ratios similar to bulk Earth values, while we measure four refractory elements, Ca, Sc, Ti and V, all at a factor of ~2-3 higher abundance than in the bulk Earth. For HS2253+8023 the swallowed material was compositionally similar to bulk Earth in being more than 85% by mass in the major element species, O, Mg, Si, and Fe, and with abundances in the distinctive proportions of mineral oxides - compelling evidence for an origin in a rocky parent body. Including previous studies we now know of four heavily polluted white dwarfs where the measured oxygen and hydrogen are consistent with the view that the parents bodies formed with little ice, interior to any snow-line in their nebular environments. The growing handful of polluted white dwarf systems with comprehensive abundance measurements form a baseline for characterizing rocky exoplanet compositions that can be compared with bulk Earth.

Evaluation of grinding methods for pellets preparation aiming at the analysis of plant materials by laser induced breakdown spectrometry

It has been demonstrated that laser induced breakdown spectrometry (LIBS) can be used as an alternative method for the determination of macro (P, K, Ca, Mg) and micronutrients (B, Fe, Cu, Mn, Zn) in pellets of plant materials. However, information is required regarding the sample preparation for plant analysis by LIBS. In this work, methods involving cryogenic grinding and planetary ball milling were evaluated for leaves comminution before pellets preparation. The particle sizes were associated to chemical sample properties such as fiber and cellulose contents, as well as to pellets porosity and density. The pellets were ablated at 30 different sites by applying 25 laser pulses per site (Nd:YAG@1064nm, 5ns, 10Hz, 25Jcm−2). The plasma emission collected by lenses was directed through an optical fiber towards a high resolution echelle spectrometer equipped with an ICCD. Delay time and integration time gate were fixed at 2.0 and 4.5μs, respectively. Experiments carried out with pellets of sugarcane, orange tree and soy leaves showed a significant effect of the plant species for choosing the most appropriate grinding conditions. By using ball milling with agate materials, 20min grinding for orange tree and soy, and 60min for sugarcane leaves led to particle size distributions generally lower than 75μm. Cryogenic grinding yielded similar particle size distributions after 10min for orange tree, 20min for soy and 30min for sugarcane leaves. There was up to 50% emission signal enhancement on LIBS measurements for most elements by improving particle size distribution and consequently the pellet porosity.

Distance limits to intermediate- and high-velocity clouds★

We present optical spectra of 403 stars and quasi-stellar objects in order to obtain distance limits towards intermediate- and high-velocity clouds (IHVCs), including new Fibre-fed Extended Range Optical Spectrograph (FEROS) observations plus archival ELODIE, FEROS, High Resolution Echelle Spectrometer (HIRES) and Ultraviolet and Visual Echelle Spectrograph (UVES) data. The non-detection of Ca ii K interstellar (IS) absorption at a velocity of −130 to −60 km s−1 towards HDE 248894 (d∼ 3 kpc) and HDE 256725 (d∼ 8 kpc) in data at signal-to-noise ratio (S/N) > 450 provides a new firm lower distance limit of 8 kpc for the anti-centre shell HVC. Similarly, the non-detection of Ca ii K IS absorption towards HD 86248 at S/N ∼ 500 places a lower distance limit of 7.6 kpc for Complex EP, unsurprising since this feature is probably related to the Magellanic System. The lack of detection of Na i D at S/N = 35 towards Mrk 595 puts an improved upper limit for the Na i column density of log (NNaD <) 10.95 cm−2 towards this part of the Cohen Stream where Ca ii was detected by Wakker et al. Absorption at ∼−40 km s−1 is detected in Na i D towards the Galactic star PG 0039+049 at S/N = 75, placing a firm upper distance limit of 1 kpc for the intermediate-velocity cloud south (IVS), where a tentative detection had previously been obtained by Centurión et al. Ca ii K and Na i D absorption is detected at −53 km s−1 towards HD 93521, which confirms the upper distance limit of 2.4 kpc for part of the IV arch complex obtained using the International Ultraviolet Explorer (IUE) data by Danly. Towards HD 216411 in Complex H a non-detection in Na D towards gas with log(⁠⁠) = 20.69 cm−2 puts a lower distance limit of 6.6 kpc towards this HVC complex. Additionally, Na i D absorption is detected at −43.7 km s−1 in the star HD 218915 at a distance of 5.0 kpc in gas in the same region of the sky as Complex H. Finally, the Na i/Ca ii and Ca ii/H i ratios of the current sample are found to lie in the range observed for previous studies of IHVCs.

KECK HIRES SPECTROSCOPY OF EXTRAGALACTIC H II REGIONS: C AND O ABUNDANCES FROM RECOMBINATION LINES

We present very deep spectrophotometry of 14 bright extragalactic H?II regions belonging to spiral, irregular, and blue compact galaxies. The data for 13 objects were taken with the High Resolution Echelle Spectrometer on the Keck I telescope. We have measured C?II recombination lines in 10 of the objects and O?II recombination lines in eight of them. We have determined electron temperatures from line ratios of several ions, especially those of low ionization potential. We have found a rather tight linear empirical relation between T e([N?II]) and T e([O?III]). We have found that O?II lines give always larger abundances than [O?III] lines. Moreover, the difference of both O++ abundance determinations?the so-called abundance discrepancy factor?is very similar in all the objects, with a mean value of 0.26 ? 0.09 dex, independent of the properties of the H?II region and of the parent galaxy. Using the observed recombination lines, we have determined the O, C, and C/O radial abundance gradients for three spiral galaxies: M33, M101, and NGC 2403, finding that C abundance gradients are always steeper than those of O, producing negative C/O gradients across the galactic disks. This result is similar to that found in the Milky Way and has important implications for chemical evolution models and the nucleosynthesis of C.

Determination of iodine via the spectrum of barium mono-iodide using high-resolution continuum source molecular absorption spectrometry in a graphite furnace

Molecular absorption spectra of the diatomic molecules AlI, GaI, InI, TlI, MgI, CaI, SrI and BaI, generated in a graphite furnace, were studied using a high-resolution echelle spectrometer with the aim of finding a simple, reliable and sensitive analytical method for the determination of iodine. Among them, the barium mono-iodide (BaI) was found to have the strongest absorption bands around 538 nm and 560 nm, each of them consisting of a series of well-resolved rotational lines with half-widths of about 40–50 pm. The strongest BaI line, the band head at 538.308 nm has been evaluated systematically for its analytical use for the determination of iodine. High concentrations of hydrochloric acid (or chloride), hydrofluoric acid (or fluoride), iron, potassium and sodium resulted in significant reduction of the BaI molecular absorption. Apart from this, no other serious spectral or non-spectral interference has been observed. Different chemical forms of iodine, such as iodide, iodate and organically bound iodine produced identical BaI absorption sensitivity. The detection limit for iodine was 600 pg, and the calibration curve was linear up to 250 ng iodine. Two real samples with different chemical forms of iodine were analyzed using the proposed method. One sample was an iodide pill with a specified iodide content of 200 mg, the other one was a thyroid hormone pill with a specified content of 63.5 mg. The results were in good or satisfactory agreement with those of independent methods, the potentiometric titration and the inductively coupled plasma time-of-flight mass spectrometry (ICP-ToF-MS); the deviations were 2% and 8% for the iodide and the thyroid hormone sample, respectively. The relative standard deviation of the analytical results ( n = 3) was below 2%.

A HIRES/KECK SPECTROSCOPIC INVESTIGATION OF THE MEASUREMENT OF SODIUM IN THE ATMOSPHERE OF HD 209458b

We present high-resolution High Resolution Echelle Spectrometer (HIRES)/Keck spectra of HD 209458, and a Monte Carlo variation on the basic method used by other workers, to look for the excess in-transit absorption in the NaD doublet at 5893 A due to the extrasolar planet. The HIRES data, binned by bandpass, allow a direct comparison with previous results. We find >3{sigma} results in most test bandpasses around the NaD doublet, including relative absorption of (-108.8 {+-} 25.7) x 10{sup -5} in the 'narrow' bandpass used by other workers. This is {approx}4.7 times larger than the 'narrow' results reported by Charbonneau et al. for HD 209458b. However, >2{sigma} absorption is detected in some weak Fe I and Ni I lines that were tested for comparison, raising concern about the uncertainties introduced by continuum-fitting and terrestrial atmosphere subtraction.

Probing feedback in protogalaxies: multiphase gas in a DLA atz≈ 2.4

We investigate the physical processes occurring in the multiphase gas of a damped Lyα absorber (DLA). We base our analysis on a high-quality Keck High-Resolution Echelle Spectrometer (HIRES) spectrum of the QSO J1211+0422 in which a DLA is detected at z≃ 2.377. There is little contamination of the high-ion (O vi, N v, C iv, Si iv) absorption, allowing us to explore the properties of the highly ionized gas and its connection to other gas phases. The metallicity ([Z/H]=−1.41 ± 0.08), H i column density [], full-width velocity (Δvneut≃ 70 km s−1) and relative abundances ([Si/Fe]=+0.23 ± 0.05 and [N/Si]=−0.88 ± 0.07) of this DLA are not unusual. However, we derive the lowest C ii* cooling rate in a DLA, lc < 10−27.8erg s−1 per H atom (3σ). Using this stringent limit, we show that the neutral gas (confined at |v| < +39 km s−1) must be warm and the star formation rate is <7.1 × 10−3M⊙ yr−1kpc−2. Surprisingly, the gas shows strong, complex absorption profiles from highly ionized gas whose kinematics appear connected to each other and the low ions. The total amount of highly and weakly ionized gas is very large with ≳1.5. At |v| ≳+39 km s−1, the gas is fully and highly ionized []. Based on ionization models, O vi and N v are generally difficult to produce by hard photons, while Si iv and C iv can be photoionized to a large extent. There is, however, no evidence of O vi-bearing gas at T∼ 106 K associated with this DLA. In contrast, there is some evidence for narrow O vi, N v and C iv components (unexplained by photoionization), implying too low temperatures (T < 105 K) for simple collisional ionization models to produce their observed column densities. Stellar feedback is a possible source for producing the high ions, but we cannot rule out accretion of non-pristine material on to the protogalaxy.

High-resolution continuum source electrothermal absorption spectrometry of AlBr and CaBr for the determination of bromine

Molecular absorption spectra of AlBr and CaBr, produced in a graphite furnace, were investigated using a high-resolution echelle spectrometer equipped with a xenon short-arc lamp as continuum source. The analytical usability of the spectra for the determination of bromine was studied. To this end, the molecular absorptions of AlBr at 278.914 nm and CaBr at 625.315 nm were evaluated. Apart from strong absorption bands of CaF around 625.3 nm, which disturb the use of CaBr, no spectral interferences were observed for both AlBr and CaBr. Regarding chemical interference with matrix substances, the molecular absorption of AlBr and CaBr is influenced in a different way. While the sensitivity of the CaBr absorption is susceptible to chloride, aluminum, potassium and sodium ions, there is no significant effect on the AlBr absorption. In contrast, the inorganic acids (nitric, phosphoric, and sulfuric) have an influence on AlBr, but not on the CaBr molecular absorption. Therefore, the two methods complement each other and each has its own application area. Regarding real samples, a salt sample from the death sea and an organic pharmaceutical were evaluated. The results were in good agreement with those derived from two independent methods and with an existing reference value. Relative standard deviations were found in the range of 5%. The limit of detection for bromine was about 2 ng for both AlBr and CaBr molecular absorption; the dynamic range was linear at least up to 250 ng Br.

Designing high-performance spectrographs for extremely large telescopes

To answer the increasingly challenging questions of modern astronomy, larger and larger optical telescopes have been built in the last few decades, equipped with a sizable suite of focal-plane instruments devoted to observations at all wavelengths, and spatial and spectral resolutions. As a general rule, for a given spectral resolution and under seeing-limited conditions, larger telescopes require larger instruments because spectrograph dimensions scale with the slit width, which scales with telescope diameter. For example, the High Resolution Echelle Spectrometer at the Keck telescope1 has a 1.2 × 0.3m2 mosaic grating consisting of three segments, and collimator and camera mirrors of the same order of magnitude. Some attempts to reduce the dimensions of the optics have been made, for instance for the Ultraviolet and Visual Echelle Spectrograph2 at the Very Large Telescope (VLT), where a smaller 0.8 × 0.2m2 grating has been used. If we were to linearly scale such instruments to 30–40mclass telescopes, such as the 42m European Extremely Large Telescope3 (E-ELT, illustrated in Figure 1), this would result in three to five times larger components and thus very large, heavy, and expensive instruments. Moreover, manufacturing constraints have been reached in many areas. For example, the ruling engines that are used to make diffraction gratings—the main dispersing component of a spectrograph—are limited to ruled areas of around 40 × 30cm2. We have investigated a new design approach4 that uses slicing techniques (at image or pupil planes) and ‘anamorphic’ magnification at different levels to keep instrument size and cost affordable. We can use these methods because the resolving Figure 1. Model of the E-ELT within its enclosure. The main mirror has a diameter of 42m. The COsmic Dynamics EXperiment (CODEX) instrument will be situated at a stationary location below the telescope and receive its light via an optical train of mirrors and fibers. (Courtesy: European Southern Observatory.)

An improved measurement of the flux distribution of the Ly forest in QSO absorption spectra: the effect of continuum fitting, metal contamination and noise properties

We have performed an extensive Voigt profile analysis of the neutral hydrogen (H i) and metal absorption present in a sample of 18 high-resolution, high signal-to-noise ratio quasi-stellar object (QSO) spectra observed with the Very Large Telescope Ultraviolet and Visual Echelle Spectrograph. We use this analysis to separate the metal contribution from the H absorption and present an improved measurement of the flux probability distribution function (PDF) due to H i absorption alone at (z) = 2.07,2.52 and 2.94. The flux PDF is sensitive to the continuum fit in the normalized flux range 0.8 < F < 1.0 and to metal absorption at 0.2 < F < 0.8. Our new measurements of the flux PDF due to H i absorption alone are systematically lower at 0.2 < F < 0.8 by up to 30 per cent compared to the widely used measurement of McDonald et al., based on a significantly smaller sample of Keck High Resolution Echelle Spectrometer data. This discrepancy is probably due to a combination of our improved removal of the metal absorption and cosmic variance, since variations in the flux PDF between different lines-of-sight are large. The H effective optical depth τ eff Ht at 1.7 < z < 4 is best fit with a single power law, τ eff HI = (0.0023 ± 0.0007)(1 + z) 365±0.21 , in good agreement with previous measurements from comparable data. As also found previously, the effect of noise on the flux distribution is not significant in high-resolution, high signal-to-noise ratio data.

Spectral shape of the UV ionizing background and O VI absorbers at z $\mathsf{\sim 1.5}$ towards HS 0747+4259

Aims.We report on high resolution spectra of the bright QSO HS 0747+4259 (zem = 1.90, ) observed to search for intermediate redshift absorption systems.Methods.The spectra were obtained by means of the Space Telescope Imaging Spectrograph (STIS) at the Hubble Space Telescope (HST) and the High Resolution Echelle Spectrometer (HIRES) at the W. M. Keck telescope.Results.We identify 16 systems in the range 1.07 1.87. Among them, six systems with zabs = 1.46–1.8 exhibit a sufficient number of lines of different ionic transitions to estimate the shape of the ionizing radiation field in the range 1 Ryd Ryd. All recovered UV ionizing spectra are characterized by the enhanced intensity at Ryd compared to the model spectrum of Haardt & Madau (1996, ApJ, 461, 20). This is in line with the observational evidence of a deficiency of strong Lyα absorbers with N() cm-2 at . The UV background shows significant local variations: the spectral shape estimated at differs from that obtained at and 1.73. A possible cause of these variations is the presence of a QSO/AGN at close to the line of sight. No features favoring the input of stellar radiation to the ionizing background are detected, limiting the escape fraction of the galactic UV photons to .

Beryllium Abundances in F and G Dwarfs in Praesepe and Other Young Clusters from Keck HIRES Observations

The study of both Be and Li gives useful clues about stellar internal structure. Of particular interest is the study of these light elements in open clusters, which have a known age and metallicity. In this paper we present a study of Be abundances in 10 F-type stars in Praesepe and a comprehensive discussion about Be abundances in other open clusters: Hyades, Pleiades, α Per, Coma, and UMa. We have made observations of the doublet of Be II around 3130 A in Praesepe stars, using the Keck I telescope and the High Resolution Echelle Spectrometer (HIRES). Beryllium abundances were derived from the spectra using the spectrum synthesis method. We find four stars with definite Be depletion in the temperature range of the Li dip like we found in our previous cluster studies, notably for the Hyades and Coma clusters. Putting all the clusters together, we confirm the existence of a Be dip in a narrow temperature range for F stars. Beryllium depletion in this dip is less pronounced than Li depletion. For the cooler stars there is little or no Be depletion, even though there are large depletions of Li. For stars that have little or no Li depletion, A(Li) ≥ 3.0, the ratio Li/Be is 75 ± 4.6, compared to the meteoritic ratio of 77.6. For stars cooler than ~5900 K there appears to be little or no Be depletion, and the mean A(Be) is 1.30 ± 0.02. For these cooler stars within a given cluster there is no evidence for intrinsic star-to-star differences in A(Be), with the possible exception of the cool Pleiades stars. In the temperature range of the Li-Be dip, a strong correlation exists between Li and Be, consistent with the theory of rotationally induced mixing. Moreover, the slopes of the Li versus Be correlations are different depending on the temperature range. For the full sample of 42 stars between 5900 and 6650 K the slope is 0.43 ± 0.05 [where A(Li) is the abscissa]. The slope is 0.48 ± 0.08 for 6300 K < Teff < 6650 K and 0.30 ± 0.05 for 5900 K < Teff < 6300 K. For the Li plateau stars (the cooler subset), the slope is smaller as the impact of the increasing surface convection zone affects the mixing, thus depleting more Li relative to Be. The different behavior in Be depletion for clusters of different ages is consistent with the idea of slow mixing related to rotation during the main-sequence phase of evolution. The range in metallicity in this sample of clusters is only 0.2 dex, so it is difficult to discern any influence of metallicity on the Li-Be relationship; however, the mean A(Be) in the cooler Hyades stars (with [Fe/H] = +0.13) is 1.35 ± 0.02, which is higher than that for the Coma stars (with [Fe/H] = -0.09) of 1.26 ± 0.02 by 0.09 dex.