Multiple Mirror Telescope Research Articles

Additional Observations and Analysis of the Lyman‐α Absorption Lines toward the QSO Pair Q0107−025A,B

We present further analysis and discussion of the properties of the absorption lines in the QSO pair Q0107-025A,B (zem = 0.956, 0.952; angular separation 129) based upon spectroscopy obtained with the Faint Object Spectrograph (FOS) of the Hubble Space Telescope (HST). We also present observations of the pair at shorter wavelengths taken with the HST Goddard High Resolution Spectrograph low-resolution grating, as well as Multiple Mirror Telescope spectra obtained with the intent of looking for metal-line counterparts to the Lyman-α absorption systems. The most interesting feature revealed by the GHRS spectra is a weak Lyman limit system with a redshift of zLLS = 0.3997. The Lyman limit system itself is seen only in Q0107-025B, but corresponding Lyα lines can be seen in the FOS spectra of both Q0107-025A and B indicating that lower column density gas extends out to greater distances than higher column density gas. From a sample of 5 σ lines with W0 > 0.32 Å detected in the FOS spectra, we count five systems (including a probable system) common to both spectra with velocity differences less than 150 km s-1, and six systems that are not in common to both spectra in the redshift range 0.48 < z < 0.89. From the presence of common absorption, we obtain lower limits on the radius of the absorbers of 140-160 h-1 kpc [h ≡ H0/(100 km s-1 Mpc-1); q0 = 0.5]. Using a simple maximum-likelihood analysis, we estimate a characteristic radius of 505 h-1 kpc assuming spherical absorbers, with 95% confidence lower and upper limits of 345 < R < 1520 h-1 kpc. For disklike absorbers, we derive a most probable radius of 715 h-1 kpc and 95% confidence limits of 470 < R < 2310 h-1 kpc. We also present a new statistical technique to test the relative likelihood of three geometric models. In particular, we consider spherical absorbers, with and without a distribution in size, as well as filamentary and disklike absorbers. Spherical absorbers with uniform radius can be ruled out since they cannot simultaneously reproduce the large equivalent width coincidences and anticoincidences observed. The model that best reproduce the equivalent width distribution of the observed coincident and anticoincident systems corresponds to randomly inclined disks with characteristic radius 915 h-1 kpc and 95% confidence interval 560 < R < 1270 h-1 kpc. Our results are in agreement with recent cosmological simulations that produce Lyα forest absorbers in the form of filaments and sheets with coherence lengths as great as 1 Mpc.

The Detection of the λ2175 Feature and Further Analysis of the Broad Absorption Line Profile Structure in the Gravitational Lens Candidate UM 425

We obtained Multiple Mirror Telescope spectra of the gravitational lens candidate UM 425 to compare the redshifts and line profile structures of lens components A and B, which are separated by approximately 65. The C IV λ1550 emission in both A and B exhibits broad absorption line (BAL) structure, consistent with the earlier detection of BAL structure in O VI λ1033 and N V λ1240 that was found with the International Ultraviolet Explorer in component A. Cross-correlation of the spectra of A and B using emission lines of C IV λ1550, He II λ1640, N III λ1750, C III] λ1909, and Mg II λ2800 reveals a difference in the redshifts of A and B. However, the detailed BAL profile structure found in the spectra of A and B are strikingly similar to one another, which suggests the system is lensed. The spectra of A and B also indicate significant dust extinction, which we base on the presence of the λ2175 absorption feature in the rest frame of the QSO (zQSO=1.47). This feature is commonly seen in galactic sources but is not generally observed in QSO spectra. Our spectra show the presence of the λ2175 absorption feature in spectra of both images associated with the gravitational lens UM 425. Based upon the strong similarity of BAL profile structure exhibited by UM 425A and UM 425B, particularly the presence of the λ2175 dust absorption feature in spectra of both images, we conclude that UM 425 is a gravitational lens.

I Z[CLC]w[/CLC] 18: A New Wolf-Rayet Galaxy

We report the discovery of broad Wolf-Rayet emission lines in the Multiple Mirror Telescope (MMT) spectrum of the NW component of I Zw 18, the lowest-metallicity blue compact dwarf (BCD) galaxy known. Two broad Wolf-Rayet (W-R) bumps at the wavelengths $\lambda$4650 and $\lambda$5800 are detected indicating the presence of WN and WC stars. The total numbers of WN and WC stars inferred from the luminosities of the broad He II $\lambda$4686 and C IV $\lambda$5808 lines are equal to 17(+/-)4 and 5(+/-)2, respectively. The W-R to O stars number ratio is equal to about 0.02, in satisfactory agreement with the value predicted by massive stellar evolution models with enhanced mass loss rates. The WC stars in the northwest component of I Zw 18 can be responsible for the presence of the nebular He II $\lambda$4686 emission line, however the observed intensity of this line is several times larger than model predictions, and other sources of ionizing radiation at wavelengths shorter than 228\AA are necessary.

Spectroscopy of PKS 0528−250: New Limits on CO Absorption and Emission

We have obtained a moderate-resolution spectrum of the quasar PKS 0528-250 with the Red Channel Spectrograph on the Multiple Mirror Telescope in order to study a damped Lyα absorption-line system at z = 2.8115. We obtain a new upper limit for the CO column density for the z = 2.8108 velocity component in the z = 2.8115 damped Lyα system. The ionization of different species in this component rules out a quasar spectral energy distribution as the ionization field and implies an ultraviolet radiation field intensity a few times that of the Milky Way value. The estimated total number density is n(H) ~ 20 cm-3. The physical size for the z = 2.8108 component implied by these models is about 40 pc. The ionization of different species also suggests a structure with a hot intercloud medium associated with a H I cloud in this component, that is, most low ionized ions are from the cold medium where photoionization and photodissociation dominates. The highly ionized species may be from the intercloud medium where collisional ionization dominates. We also present newly identified Ni II absorption lines in the z = 2.1408 and z = 2.8115 damped Lyα systems. The derived depletion of nickel by dust confirms previous results that the dust-to-gas ratio in these two damped Lyα systems is about 10% of the Milky Way ratio. Millimeter-wavelength observations obtained at the NRAO 12 m telescope provide new upper limits on CO (3-2) emission in the z = 2.8115 damped Lyα system.

High Time Resolution Infrared Observations of the Crab Nebula Pulsar and the Pulsar Emission Mechanism

We present new, high signal-to-noise near-infrared observations of the Crab Nebula pulsar using the Solid State Photomultiplier instrument on the Multiple Mirror Telescope. Our observations cover the J (1.25 μm), H (1.65 μm), and K (2.2 μm) infrared wavebands and have 20 μs time resolution. Together with visible and UV observations made by the Hubble Space Telescope High-Speed Photometer, we have high time resolution observations covering over a decade in wavelength. We present the pulse profiles over this wavelength range, and we analyze the pulse shape as a function of wavelength, including the peak-to-peak phase separation, the peak full width half-maxima (FWHM), and the peak half-width half-maxima (HWHM). We also create both phase-averaged and phase-resolved color spectra of the pulsar emission. We find that the peak-to-peak phase separation shows a significant trend for an increase with wavelength, in rough agreement with models of the pulsar emission mechanism. The FWHM for peaks 1 and 2 also show a trend for increase with wavelength, again in qualitative agreement with the models. However, the HWHM for peaks 1 and 2 show significant differences in their wavelength dependences from the leading to trailing edges. This behavior is not predicted by current pulsar emission models, and the different wavelength dependences of the component HWHM values call into question the usefulness of FWHM measurements. Our spectral analyses show that the IR-UV dereddened phase-averaged color spectrum is essentially flat over more than a decade in frequency. This is in clear contrast to the X-ray and γ-ray regimes, where the spectrum is falling steeply. The color spectra of peaks 1 and 2 are also essentially flat, but the ratio of the two shows statistically significant variations from a constant value. Finally, the color spectra of peaks 1 and 2 show significant differences from the leading to trailing edges. As with the HWHM, this behavior is not predicted by current pulsar emission models.

SBS 0335−052, A Probable Nearby Young Dwarf Galaxy: Evidence Pro and Con

The results of Multiple Mirror Telescope spectrophotometry of the extremely low metallicity blue compact galaxy (BCG) SBS 0335-052 (SBS—the Second Byurakan Survey) are presented. The oxygen abundance in the central, brightest part of the galaxy is found to be 12 + log (O/H) = 7.33 ± 0.01, only slightly greater than the oxygen abundance in the most metal-deficient BCG, I Zw 18. We show that the N/O, Ne/O, S/O, and Ar/O abundance ratios in SBS 0335-052 are close to those derived for other BCGs, suggesting that heavy-element enrichment in the H II region is due to massive-star evolution. However, we find an O/Fe abundance ratio close to that in the Sun, at variance with values derived for other BCGs. The helium abundance derived from the He I 4471, 5876, and 6678 Å emission lines, taking into account collisional and fluorescent enhancement, is Y = 0.245 ± 0.006, close to the value of the primordial helium abundance, Yp = 0.243 ± 0.003, derived by Izotov, Thuan, & Lipovetsky. We detect auroral [O III] λ4363 emission in the inner part of the H II region with a diameter of 14'', or 3.6 kpc, and find that the H II region inside this diameter is hot, Te ~ 20,000 K. The oxygen abundance in this region is nearly constant [12 + log (O/H) = 7.1-7.3], with a gradual decrease to the outer part of H II region, implying effective mixing of ionized gas on short timescales. We study the distribution of the nebular He II λ4686 emission line and find that it is not produced by main-sequence O stars or Wolf-Rayet stars. Possible excitation mechanisms for this line, such as massive X-ray binaries and shocks, are discussed. We also discuss the origin of blue, underlying, extended low-intensity emission detected in SBS 0335-052 in V, R, and I images. The blue V - I and R - I color distributions suggest that a significant contribution to the extended low-intensity envelope is due to ionized gas emission. This is evidence that SBS 0335-052 is a young galaxy experiencing its very first burst of star formation. However, we find that the observed equivalent width of Hβ emission in the extended envelope is 2-3 times lower than the value expected in the case of pure gaseous emission. Furthermore, we find that the widths of Hγ and Hβ are narrower than the instrumental profiles; this could be explained by the presence of underlying stellar absorption from A stars. These findings suggest that, along with the young ( ~107 yr) blue stellar clusters in the center of the galaxy, an older stellar population of age ~108 yr may be present in the extended envelope of SBS 0335-052 having a total mass of ~107 M☉, 2 orders of magnitude smaller than the neutral gas mass but comparable with the total mass of stars in blue young stellar clusters observed in the center of the galaxy. We conclude that SBS 0335-052 is a young nearby dwarf galaxy with age ~108 yr.

The Post-Merger Elliptical NGC 1700: Stellar Kinematic Fields to Four Effective Radii

We have measured the stellar motions in the elliptical galaxy NGC 1700 along four position angles, to very large radii, using absorption features in spectra obtained with the Multiple Mirror Telescope. Our data extend the coverage of the stellar velocity field by a factor of 5 (2.5 times further in radius and twice as many PAs) beyond previous work. We have attained 10 km/s accuracy in the mean velocity out to nearly 2 effective radii (r_e), and errors are < 15% of the maximum rotation speed out to nearly 3 r_e. The lack of detectable minor-axis rotation and the nearly identical kinematics on the +/- 45 degree PAs suggest that NGC 1700 is nearly oblate for r < 2.5 r_e. Beyond this radius, twisting of the morphological and kinematic axes indicate increasing triaxiality, an intrinsic twist, or both. The velocity distribution in the low-amplitude counterrotating core is weakly skewed in the direction of rotation, arguing against a central stellar disk. The small skewness and the depression of the central velocity dispersion are consistent with the accretion of a low-mass stellar companion in a retrograde orbit. Photometric fine structure at large radii (Schweizer \& Seitzer 1992) is also indicative of a merger; a velocity reversal 50 arcsec northeast suggests a major event. However, radially increasing prograde rotation in the main body of the galaxy implies that this was not the same event responsible for the counterrotating core. The strong rotation at large R and the nearly oblate shape are consistent with N-body simulations of group mergers (Weil 1995); that all disturbances inside 2.5 r_e have phase-mixed out suggests that NGC 1700 owes its present form to a merger of 3 or more stellar systems 2 to 4 h^-1 Gyr ago.

Optimization and Performance of Adaptive Optics for Imaging Extrasolar Planets

A recent study by Angel (1994) using simplified analytical models indicated the feasibility of imaging extrasolar planets from the ground, making use of adaptive optics correction of a large telescope. We have performed detailed simulations of the method using computer codes that model propagation through atmospheric turbulence, adaptive correction, and broadband imaging. We confirm that high-resolution correction at the limit of photon noise errors reduces the halo intensity to 10-6 of the peak star flux. Our work shows how to avoid systematic errors. Thus, we find that time delays between sensing the wave front and its detection lead to persistent structure in the stellar halo, which uncorrected would prevent rapid averaging of the residual halo speckle structure. A local wave front reconstructor that extrapolates ahead in time has been devised to remove this problem. We find the chromatic differences in wave front structure are small enough that the signal-to-noise ratio can be improved by wave front sensing and imaging in separate adjacent bands. We verify that correction of amplitude scintillation is needed and the optimum level of clipping is derived. A simulated image of a twin of the solar system at 8 pc is presented for the new 6.5 m telescope and the measured turbulence at the Multiple Mirror Telescope site. The Jupiter twin shows up at the 5 σ level in a 5 hr integration.

Nine Seasons of Velocity Measurements in the Draco and Ursa Minor Dwarf Spheroidal Galaxies with the MMT Echelle

We have used the Multiple Mirror Telescope echelle spectrograph to measure 112 velocities of 42 stars in the Draco and Ursa Minor dwarf spheroidal galaxies and three velocities of three foreground stars between 1982 April and 1990 September. We used 11 A resolution spectra obtained with the MX multifiber spectrograph at the Steward 90" to find additional giant candidates; 5 UMi and 13 Draco stars were then observed at the MMT and added to the original sample of velocity members. In addition, the MX spectra were used to eliminate 74 stars in the direction of UMI and 59 stars in Draco as likely foreground dwarfs. We detected 7 velocity variables, defined as those stars whose probability of exceeding the measured X^2^ by chance is less than 1.5%. Three of these stars are Carbon (C) stars (UMi K and VA 335 and Draco C1); two have emission lines (Draco CI and UMi M). We show that the C star Draco C4, with a proper motion membership probability of 7%, has a velocity consistent with membership. It is not surprising that these C (most likely CH) stars are binaries because McClure has shown that most Galactic CH stars are in binary systems. Of the remaining 35 stars, only 4 are velocity variables, with measured velocity extrema of 29.1 km s^-1^ (UMi M), 7.2 km s^-1^ (Draco XI-2), 9.0 km s^-1^ (Draco 24), and 8.3 km s^-1^ (Draco 473). The velocity dispersions are 10.1 +/- 1.7 km s^-1^ for UMi, and 9.9 +/- 1.4 km s^-1^ for Draco. These dispersions change to 10.5 +/- 2.0 for UMi, and 8.2 +?- 1.3 for Draco if we eliminate the velocity variables. Our dispersion for UMi differs from that of Hargreaves et al. [MNRAS, 271,693 (1994b)] by 1.3σ of the combined errors. These velocities are combined with the one-component King models of Pryor &amp; Kormendy [AJ, 100,127 (1990)] to give M/L = 73 for UMi, and 77 for Draco.

Adaptive optics experiments using sodium laser guide stars

The Multiple Mirror Telescope (MMT) has been used in experiments with sodium laser guide stars to sense and correct atmospheric image distortion. The major goal was to develop and test concepts for a full adaptative optics system, to be used in 2 yr when the present array of six 1.8 m telescopes will be replaced with a single 6.5 m mirror. The guide star, produced by a continuous-wave dye laser beam projected out along the optical axis of the telescope, was as bright in the V band as a natural star of m<SUB>v</SUB> = 10.4. Our tests culminated in the first demonstration of a sodium laser guide star used to improve the image of an astronomical telescope, in this case formed by two of the six 1.8 m apertures. Two adaptive servo loops were closed simultaneously. The laser beacon provided a measure of the differential wave-front tilt between the two apertures, and a natural guide star was used to measure the overall wave-front tilt. A factor of 2 improvement in the K-band Strehl ratio was measured, and the resolution improved from 0.58 arcsec to 0.41 arcsec. The experiment demonstrated all the features needed for correction of the 6.5 m telescope to the diffraction limit using a sodium beacon. The accuracy with which the laser beacon measures the atmospheric aberration of starlight across the full 6.9 m aperture of the MMT was examined. From more extensive measurements of binary star wave fronts, we deduce that focus anisoplanatism for the 6.5 m telescope will correspond to a Strehl ratio of typically 88% at K under normal seeing conditions. In a laser-based adaptive system, a natural guide star is still required to sense overall wave-front slope. Our measurements of binary stars also yielded the image degradation to be expected from differences in the overall slopes between the wave fronts from the object of scientific interest and the natural guide star. A Strehl ratio of 80% at K was deduced for an offset of 40 arcsec, implying that good sky coverage will be possible. In general, our results are consistent with calculations based on measurements of atmospheric turbulence at the best sites. Our direct measurements over such a large aperture show clearly the effects of a finite and variable outer scale of turbulence.

The luminosity function at the end of the main sequence: Results of a deep, large-area, CCD survey for cool dwarfs

The luminosity function at the end of the main sequence is determined from V, R, and I data taken by the charge coupled devices (CCD)/Transit Instrument, a dedicated telescope surveying an 8.25 min wide strip of sky centered at delta = +28 deg, thus sampling Galactic latitudes of +90 deg down to -35 deg. A selection of 133 objects chosen via R - I and V - I colors has been observed spectroscopically at the 4.5 m Multiple Mirror Telescope to assess contributions by giants and subdwarfs and to verify that the reddest targets are objects of extremely late spectral class. Eighteen dwarfs of type M6 or later have been discovered, with the latest being of type M8.5. Data used for the determination of the luminosity function cover 27.3 sq. deg down to a completeness limit of R = 19.0. This luminosity function, computed at V, I, and bolometric magnitudes, shows an increase at the lowest luminosities, corresponding to spectral types later than M6- an effect suggested in earlier work by Reid & Gilmore and Legget & Hawkins. When the luminosity function is segregated into north Galactic and south Galactic portions, it is found that the upturn at faint magnitudes exists only in the southern sample. In fact, no dwarfs with M(sub I) is greater than or equal to 12.0 are found within the limiting volume of the 19.4 sq deg northern sample, in stark contrast to the smaller 7.9 sq deg area at southerly latitudes where seven such dwarfs are found. This fact, combined with the fact that the Sun is located approximately 10-40 pc north of the midplane, suggests that the latest dwarfs are part of a young population with a scale height much smaller than the 350 pc value generally adopted for other M dwarfs. These objects comprise a young population either because the lower metallicities prevelant at earlier epochs inhibited the formation of late M dwarfs or because the older counterparts of this population have cooled beyond current detection limits. The latter scenario would hold if these late-type M dwarfs are substellar. The luminosity function data together with an empirical derivation of the mass-luminosity relation (from Henry & McCarthy) are used to compute a mass function independent of theory. This mass function increases toward the end of the main sequence, but the observed density of M dwarfs is still insufficient to account for the missing mass. If the increases seen in the luminosity and mass functions are indicative of a large, unseen, substellar population, brown dwarfs may yet add significantly to the mass of the Galaxy.

Infrared to x-ray spectral energy distributions of high redshift quasars

view Abstract Citations (60) References (119) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Infrared to X-Ray Spectral Energy Distributions of High Redshift Quasars Bechtold, Jill ; Elvis, Martin ; Fiore, Fabrizio ; Kuhn, Olga ; Cutri, Roc M. ; McDowell, Jonathan C. ; Rieke, Marcia ; Siemiginowska, Aneta ; Wilkes, Belinda J. Abstract We have observed 14 quasars with z > 2.8 with the ROSAT-PSPC, and detected 12 of them, including the z = 4.11 quasar 0000-263. We present the first x-ray spectrum of a radio quiet quasar with z > 3, 1946+768. Its x-ray spectrum is consistent with a power law with spectral index α_E_= 1.8_-1.4_^+2.1^ and no evidence for absorption in excess of the galactic column [α_E_ = 1.00_-0.32_^+0.28^ assuming N_H_ = N_H_(Gal)]. A PSPC hardness ratio is used to constrain the x-ray spectral properties of the quasars for which there were less than 100 photons detected. For the radio quiet quasars, <α_E_> ~ 1.2, if one assumes that there is no absorption in excess of the galactic column. We combine the x-ray data with new ground based optical and near-IR spectrophotometry obtained at the Steward 23 m and Multiple Mirror Telescope, and data from the literature. The spectral energy distributions are compared to those of low redshift objects. For the radio quiet quasars with z > 2.5, the mean <α_ox_>~1.8. This is larger than the mean for quasars with z < 2.5, but consistent with the expected value for quasars with the high optical luminosities of the objects in this sample. For the radio-loud quasars, <α_ox_>~1.4, independent of redshift. This is smaller than the expected value for the optically luminous, high redshift objects in this sample, if they are mostly GHz peaked radio sources and hence comparable to steep-spectrum, compact radio sources at lower redshift. Finally, we compare the spectral energy distributions of two representative objects to the predicted spectrum of a thin accretion disk in the Kerr geometry, and discuss the uncertainties in deriving black hole masses and mass accretion rates. Publication: The Astronomical Journal Pub Date: August 1994 DOI: 10.1086/117076 Bibcode: 1994AJ....108..374B Keywords: Astronomical Spectroscopy; Infrared Astronomy; Quasars; Red Shift; Spectral Energy Distribution; X Ray Spectroscopy; Accretion Disks; Galaxies; Infrared Photometry; Radio Astronomy; Spectrophotometry; X Ray Astronomy; Astrophysics; INFRARED: GALAXIES; RADIO CONTINUUM: GALAXIES; ULTRAVIOLET: GALAXIES; X-RAYS: GALAXIES; QUASARS: EMISSION LINES full text sources ADS | data products SIMBAD (21) NED (21)

The Lyman-Alpha forest near 34 quasi-stellar objects with Z greater than 2.6

view Abstract Citations (206) References (125) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Lyman-Alpha Forest near 34 Quasi-stellar Objects with Z > 2.6 Bechtold, Jill Abstract Moderate-resolution spectra of 34 high-redshift QSOs were obtained at the Multiple Mirror Telescope and Palomar 5 m Telescope in order to study the properties of the Lyα forest absorption lines with z_abs_~ z_em_. For z_abs_ << z_em_, the evolution of the number of lines per unit redshift, dN/dz, is well described by the form (1 + z)gamma, with γ = 1.89 +/- 0.28 for lines with rest equivalent width greater than 0.32 A. For lines stronger than 0.16 A, γ = 1.32 +/- 0.24. These results imply slower evolution in the mean free path for absorption for the clouds than previously thought. When this sample is combined with HST results for low redshift (Bahcall et al. 1993), a single γ is an acceptable fit to the data, with γ = 1.26 +/- 0.13 for z_abs_ = 0-4.1, although and inflection at z~ 1.5 may be present. In the high- redshift sample, there is a significant trend of decreasing γ with decreasing equivalent width limit, or, alternatively, a change in equivalent width distribution with redshift. There is no significant curvature in the log (dN/dz) versus log (1 + z) relation for the strong lines, but some steepening of γ at z ~ 2.5 for the weak ones. While these findings for the weak lines are highly significant in a statistical sense, they may result from some subtle line-blending problem, rather than a real physical process. The lines with z_abs_ ~ z_em_ are significantly depleted. The strength of the deficit of lines depends weakly on QSO Lyman limit luminosity, but not redshift. The strength of the deficit also depends on the rest equivalent width of the lines, with weak lines showing a relatively weaker effect. These findings support previous suggestions that the "proximity effect" results from enhanced photoionization by the QSO's EUV radiation, rather than some property of the QSO or intergalactic medium which evolves with redshift. The simple photoionization model of Bajtlik, Duncan, & Ostriker describes the data well. If the ionizing background at the Lyman limit, Jnu_, is assumed to be constant with redshift for 1.6 < z < 4.1, and writing Jnu_ = J_-21_ x 10^-21^ ergs cm^-2^ s^-1^ Hz^-1^ sr^-1^, then J_-21_~ 3. This value is formally consistent with previous results, although about a factor of 3 larger. Hence, the factor by which QSOs fail to provide the EUV background at high redshifts (at least in this simple picture), is also increased. If the redshift dependence of Jnu_ is parameterized as Jnu_ is proportional to (1+z)^j^, then j is not well constrained owing to the small redshift interval sampled, and acceptable fits are obtained for -7 < j < 4. Additions to the simple model and the resulting uncertainties in these estimates of Jnu_ are discussed. One serious uncertainty is the neutral fraction of the clouds. If the clouds are nearly neutral, then Jnu_ must be implausibly low, J_-21_ ~ 0.01 in order to account for the proximity effect observed in this sample. In addition, if the systemic redshifts of the QSOs are systematically underestimated, then J_-21_ may be up to a factor of ~3 smaller. Given these uncertainties, the J_-21_ at z~3 derived from the proximity effect is in reasonable agreement with the expected contribution of luminous QSOs. Publication: The Astrophysical Journal Supplement Series Pub Date: March 1994 DOI: 10.1086/191937 Bibcode: 1994ApJS...91....1B Keywords: Absorption Spectra; Astronomical Models; Intergalactic Media; Luminosity; Lyman Alpha Radiation; Photoionization; Proximity Effect (Electricity); Quasars; Red Shift; Spectral Line Width; Tables (Data); Hubble Space Telescope; Mean Free Path; Reflecting Telescopes; Spectrographs; Ultraviolet Radiation; Astrophysics; GALAXIES: QUASARS: ABSORPTION LINES full text sources ADS | data products SIMBAD (76) NED (34) CDS (1) Related Materials (1) Catalog: 2002yCat..20910001B

Progress in diffraction-limited imaging at the Multiple Mirror Telescope with adaptive optics

Low spatial frequencies of atmospheric turbulence are especially troublesome to astronomers because the phase distortions that these frequencies cause have a large amplitude. We have begun experiments at the Multiple Mirror Telescope to remove these errors with tip, tilt, and piston control of pieces of the wave front that are defined by the telescope’s six 1.8-m primary mirrors. We present long-exposure images that were recorded at the telescope with a resolution of as high as 0.08 arcsec under piston control, and 0.32 arcsec under tilt control, by use of an adaptive instrument designed to restore diffraction-limited imaging in the near infrared. Of particular importance for high-resolution imaging is the control of the piston or the mean phase errors between the segments. These errors can be calculated from the Fourier transform of the short-exposure combined-focus image, but the accuracy of the reconstructed wave front depends critically on the signal-to-noise ratio of the data. We present a theoretical analysis of the effects of photon and detector read noise on the derived piston errors and computer simulations of wave-front reconstructor algorithms. We find that a Wiener filter combined with nonlinear weighting of the piston errors minimizes the impact of noise. Finally, we summarize expected improvements to our system and discuss the application of these techniques to forthcoming large telescopes.

Interpretation of out-of-focus star images in terms of wave-front curvature

The wave-front-curvature-sensing method used by F. Roddier for the University of Hawaii’s curvature-adaptive-optics system is derived from the interpretation in 1979 of the out-of-focus images obtained by use of the Multiple Mirror Telescope. I summarize those original Multiple Mirror Telescope observations and the simple interpretation given in terms of wave-front curvature by use of geometrical optics at that time.

An Adaptive Steerable Imaging Array

We propose an optimized design for the redeployment of the Multiple Mirror Telescope (MMT) primary optics as a 25 m Adaptive Steerable Imaging Array (ASIA) providing direct focal plane image formation. The design incorporates concepts of active telescope alignment and adaptive optics proven in the existing MMT. Basing predictions on contemporary radio telescope mount technology, 45 m to 100 m optical baselines may be practical.

THE PERFORMANCE OF PARTIAL ADAPTIVE CORRECTION AT THE MULTIPLE MIRROR TELESCOPE

ABSTRACT In this paper we investigate the relative merits of the uses of an adaptive mirror at the Multiple Mirror Telescope (MMT) for improving imaging performance. This study, using simulated data for near-infrared seeing conditions, investigates the benefits of both wavefront piston- and tilt-correction for each of the sub-apertures thereby predicting the performance of the recently installed six-element adaptive mirror. We also investigate how the post-detection correction technique of peak-tracking improves the resulting image quality as well as the improvement in the performance of speckle imaging. The overall imaging performance is measured from the increase in Strehl ratio. The diffraction-limited imaging performance, not necessarily given by the Strehl ratio, is determined from the high-spatial frequency content of the images. This analysis both demonstrates the necessity of atmospheric compensation for interferometric imaging and investigates the adaptive performance of a monolithic aperture using a relatively small number of ~r0 sub-apertures.

The Anomalous Molecular Abundances of Comet P/Wolf-Harrington

Narrowband filter photometry of Comet P/Wolf-Harrington was performed at Lowell Observatory during the comet's 1984 and 1990/1991 apparitions. A single spectrophotometric observation was also obtained in 1991 using the Multiple Mirror Telescope at Mt. Hopkins. Production rates of OH, CN, C 2, C 3, NH, and NH 2 were derived from these data sets, and the quantity Aƒρ—a measure of the dust production—was also calculated. Relative abundances, expressed as ratios of production rates, were compared to those measured in other, "typical," comets. Comet Wolf-Harrington is shown to be depleted by over an order of magnitude in its pure carbon species when compared to OH (a measure of the water abundance) and to CN. Both NH and NH 2 are also depleted, but apparently by a lesser amount. Similar abundance anomalies have been seen in Comet P/Giacobini-Zinner, but they are more extreme in Wolf-Harrington. Taken together, these data imply that a nonnegligible fraction of comets (or their constituent components) formed at a different temperature and, therefore, a different location and/or time than did the majority of comets.

Direct 75 milliarcsecond images from the Multiple Mirror Telescope with adaptive optics

We report results from an adaptive optics system designed to provide imaging at the diffraction limit of resolution in the near-infrared at the Multiple Mirror Telescope (MMT). For the present experiment, the aperture consisted of five of the six primary mirrors of the MMT, operating as a coherently phased array. The largest components of the atmospherically induced wave-front aberration are the fluctuations in mean phase between the segments. These errors were derived in real time from the Fourier transform of short-exposure stellar images at 2.2 microns and corrected at an image of the telescope pupil with piston motion from a segmented adaptive mirror. At a correction rate of 43 Hz, this level of adaptive control resulted in an integrated image with a clear diffraction-limited component of 0.075 arcsec FWHM. This stabilized component is present directly in the light arriving at the detector and is not the result of postprocessing. We discuss future improvements to our adaptive wave-front control and its application to astronomical observations.

THE BLUE STRAGGLERS OF M67 AND OTHER OPEN CLUSTERS

ABSTRACT Results are presented of a study aimed at establishing the frequency of spectroscopic binaries in a sample of blue stragglers, belonging to five old- and intermediate-age open clusters: NGC 752, NGC 2360, NGC 2420, NGC 2682 or M67, and NGC 7789. The observational material, which for the M67 subsample spans 10 years, consists of high-resolution, low signal-to-noise echelle spectra obtained with equivalent systmes at the Multiple Mirror Telescope and at the 1.5-m Tillinghast Reflector, both atop Mt. Hopkins (Arizona). In order to obtain reliable radial velocities for these early-type and/or rapidly rotating objects with the cross-correlation method, synthetic spectra matching the observed spectra were employed as templates. The goodness of the grid of computed spectra wsa estimated to be excellent judging from the degree of agreement in high-resolution, high signal-to-noise observed spectra of the Sun, Procyon, and Sirius with their theoretical counterparts. Using this technique it was possible to determine for the first time the spectroscopic orbit for the M67 blue straggler Fagerholm 190 (P 4.2 days). It is proposed that this is a case where mass transfer between the components has been involved, that the system is still in the final stages of mass transfer, and that the original primary--now the secondary--is an abnormal subgiant evolving towards a white dwarf. The rather large orbital eccentricity (e 0.21) remains an open question, since according to current theories a system with such a period should have been circularized by now. If the mass-transfer process that made F190 into a blue straggler started recently, by using the additional information provided by the turnoff mass we can put very strict limits on the original and current masses of both components. Very long-period variations (several thousand days) were detected in three other blue stragglers of M67; since only one of these has a preliminary orbit available, it is not clear whether these systems could also be explained by the mass-transfer mechanism, or a binary-binary interaction scenario should be invoked. Three large-amplitude spectroscopic binaries were discovered among the blue stragglers of the remaining clusters. Some blue stragglers do not show any signifianct variation at all. It is found that the contamination of the blue-straggler sample by field stars is significant in the case of NGC 7789, and to a lesser extent also for NGC 2420. The frequency of spectroscopic binaries among the blue stragglers studied exceeds 40%, approximately twice the current value accepted for the field or normal cluster members. Blue stragglers tend to be more concentrated towards the center of the clusters than single stars, where the frequency of binaries is expected to increase due to mass segregation; in that case, the derived value would be within the expected range. Nevertheless, more than one mechanism seems to be required to account for all the blue stragglers.