Showing papers by "Smithsonian Astrophysical Observatory published in 2001"

Disk Frequencies and Lifetimes in Young Clusters

Abstract: We report the results of the first sensitive L-band survey of the intermediate-age (2.5-30 Myr) clusters NGC 2264, NGC 2362, and NGC 1960. We use JHKL colors to obtain a census of the circumstellar disk fractions in each cluster. We find disk fractions of 52% ± 10%, 12% ± 4%, and 3% ± 3% for the three clusters, respectively. Together with our previously published JHKL investigations of the younger NGC 2024, Trapezium, and IC 348 clusters, we have completed the first systematic and homogeneous survey for circumstellar disks in a sample of young clusters that both span a significant range in age (0.3-30 Myr) and contain statistically significant numbers of stars whose masses span nearly the entire stellar mass spectrum. Analysis of the combined survey indicates that the cluster disk fraction is initially very high (≥80%) and rapidly decreases with increasing cluster age, such that one-half the stars within the clusters lose their disks in 3 Myr. Moreover, these observations yield an overall disk lifetime of ~6 Myr in the surveyed cluster sample. This is the timescale for essentially all the stars in a cluster to lose their disks. This should set a meaningful constraint for the planet-building timescale in stellar clusters. The implications of these results for current theories of planet formation are briefly discussed.

An Empirical Ultraviolet Template for Iron Emission in Quasars as Derived from I Zw 1

Abstract: We present an empirical template spectrum suitable for fitting/subtracting and studying the FeII and FeIII line emission in the restframe UV spectra of active galatic nuclei (AGNs), the first empirical UV iron template to cover the full 1250 - 3090 A range. Iron emission is often a severe contaminant in optical--UV spectra of AGNs. Its presence complicates and limits the accuracy of measurements of both strong and weak emission lines and the continuum emission, affecting studies of line and continuum interrelations, the ionization structure, and elemental abundances in AGNs. Despite the wealth of work on modeling the AGN FeII emission and the need to account for it in observed AGN spectra, there is no UV template electronically available to aid this process. The iron template we present is based on HST spectra of the Narrow Line Seyfert 1, IZw1. Its intrinsic narrow lines (~900 km/s) and rich iron spectrum make the template particularly suitable for use with most AGN spectra. The iron emission spectrum and the line identifications and measurements are presented and compared with the work of Laor et al. We illustrate the application of the derived FeII and FeIII templates by fitting and subtracting the iron emission from UV spectra of four high-z quasars and of the nearby quasar, 3C273. We briefly discuss the small discrepancies between this observed iron emission and the UV template, and compare the template with previously published ones. We discuss the advantages and limitations of the templates and of the template fitting method. We conclude that the templates work sufficiently well to be a valuable and important tool for eliminating and studying the iron emission in AGNs, at least until accurate theoretical iron emission models are developed. (Abridged)

HST STIS Observations of PG 0946+301: The Highest Quality UV Spectrum of a BALQSO

Abstract: We describe deep (40 orbits) Hubble Space Telescope (HST) Space Telescope Imaging Spectrograph observations of the broad absorption line (BAL) quasi-stellar object (QSO) PG 0946+301 and make them available to the community. These observations are the major part of a multiwavelength campaign on this object aimed at determining the ionization equilibrium and abundances (IEAs) in BALQSOs. We present simple template fits to the entire data set, which yield firm identifications for more than two dozen BALs from 18 ions and give lower limits for the ionic column densities. We find that the outflow's metallicity is consistent with being solar, while the abundance ratio of phosphorus to other metals is at least 10 times solar. These findings are based on diagnostics that are not sensitive to saturation and partial covering effects in the BALs, which considerably weakened previous claims for enhanced metallicity. Ample evidence for these effects is seen in the spectrum. We also discuss several options for extracting tighter IEA constraints in future analyses and present the significant temporal changes that are detected between these spectra and those taken by the HST Faint Object Spectrograph in 1992.

Circumstellar Disks in the IC 348 Cluster

Abstract: We report the results of the first sensitive L-band (3.4 μm) imaging survey of the young IC 348 cluster in Perseus. In conjunction with previously acquired JHK (1.25, 1.65, and 2.2 μm) observations, we use L-band data to obtain a census of the circumstellar disk population to mK = mL ≤ 12.0 in the central ~110 arcmin2 region of the cluster. An analysis of the JHKL colors of 107 sources indicates that 65% ± 8% of the cluster membership possesses (inner) circumstellar disks. This fraction is lower than those (86% ± 8% and 80% ± 7%) obtained from similar JHKL surveys of the younger NGC 2024 and Trapezium clusters, suggesting that the disk fraction in clusters decreases with cluster age. Sources with circumstellar disks in IC 348 have a median age of 0.9 Myr, while the diskless sources have a median age of 1.4 Myr, for a cluster distance of 320 pc. Although the difference in the median ages between the two populations is only marginally significant, our results suggest that over a timescale of ~2–3 Myr more than a third of the disks in the IC 348 cluster disappear. Moreover, we find that at a very high confidence level the disk fraction is a function of spectral type. All stars earlier than G appear diskless, while stars with spectral types G and later have a disk fraction ranging between 50%–67%, with the latest-type stars having the higher disk fraction. This suggests that the disks around stars with spectral types G and earlier have evolved more rapidly than those with later spectral types. The L-band disk fraction for sources with similar ages in both IC 348 and Taurus is the same within the errors, suggesting that at least in clusters with no O stars the disk lifetime is independent of environment.

Temperature and Heavy-Element Abundance Profiles of Cool Clusters of Galaxies from ASCA

Abstract: We perform a spatially resolved X-ray spectroscopic study of a set of 18 relaxed clusters of galaxies with gas temperatures below 4 keV. Spectral analysis was done using ASCA/SIS data coupled with the spatial information contained in ROSAT/PSPC and Einstein/IPC observations. We derive the temperature profiles using single-temperature fits and also correct for the presence of cold gas at the cluster centers. For all of the clusters in the sample, we derive Si and Fe abundance profiles. For a few of the clusters, we also derive Ne and S abundance profiles. We present a comparison of the elemental abundances derived at similar overdensities as well as element mass-to-light ratios. We conclude that the preferential accretion of low-entropy, low-abundance gas into the potentials of groups and cold clusters can explain most of the observed trends in metallicity. In addition, we discuss the importance of energy input from Type II supernovae on cluster scaling relations and on the relation between the observed scatter in the retainment of Type Ia supernova products with differences between the epoch of cluster formation.

Gravitational Stirring in Planetary Debris Disks

Abstract: We describe gravitational stirring models of planetary debris disks using a new multiannulus planetesimal evolution code. The current code includes gravitational stirring and dynamical friction; future studies will include coagulation, fragmentation, Poynting-Robertson drag, and other physical processes. We use the results of our calculations to investigate the physical conditions required for small bodies in a planetesimal disk to reach the shattering velocity and begin a collisional cascade. Our results demonstrate that disks composed primarily of bodies with a single size will not undergo a collisional cascade that produces small dust grains at 30–150 AU on timescales of 1 Gyr or smaller. Disks with a size distribution of bodies reach conditions necessary for a collisional cascade in 10 Myr to 1 Gyr if the disk is at least as massive as a minimum mass solar nebula and if the disk contains objects with radii of 500 km or larger. The estimated ~500 Myr survival time for these disks is close to the median age of ~400 Myr derived for nearby stars with dusty disks.

Multiple Supernova Remnant Models of the Local Bubble and the Soft X-Ray Background

Abstract: This paper examines the possibility that a series of two or three supernovae in the diffuse interstellar medium might be capable of generating a long-lived hot diffuse bubble with characteristics like those inferred for the Local Bubble surrounding the Sun. Several examples of multiple supernova remnants are explored with a one-dimensional hydrocode able to follow the nonequilibrium ionization and dust destruction. The results are generalized via scaling parameters, and a set of input parameters are identified, which should in fact lead to a bubble satisfying all known constraints on size, external pressure, external density, X-ray surface brightness and band ratios, the upper limit to X-ray emission in the 0.5-1 keV range, and the O VI absorption line characteristics. The exploratory models are a unique example of the detailed behavior of hot interstellar bubbles, and their characteristics are presented in considerable detail. Included are the distributions of temperature, density, and pressure, as well as the evolutions of those distributions with time, the evolution of bubble radius during both expansion and contraction, the evolution of the X-ray emission in observed bands and specific observed lines, a diagnostic for normalizing the X-ray surface brightness to radius, pressure, and central temperature, the radial dependence of X-ray emissivity, the full X-ray and EUV spectrum, the equivalent widths of potentially observable absorption lines, the specific characteristics of the O VI absorption (column density, centroid velocity, width), the electron column density, the emission line intensities for O VI, Fe X, and Fe XIV, the degree to which dust can be expected to have been destroyed, and the effects of incomplete dust destruction on everything else. Observations have shown that the emission lines of iron from the Local Bubble are weak or absent. This is somewhat surprising, since considerable grain destruction is expected, even if only by thermal sputtering. The result is similar to other hints that much of the iron is in its own population of dust and is very resistant to destruction. In our models, having iron alone depleted by a factor of 10 makes it much easier to accommodate the X-ray emission characteristics. Having a successful model also requires that thermal conduction within the hot bubble occurs at roughly half of its full classical rate. For substantially lower thermal conductivity, the temperature is too high to get the spectral characteristics right, and the density is too low at the highest allowed pressure to achieve the observed brightness. Our fiducial model assumes that three supernovae took place in the vicinity of the Sun over a period of 3 million years, the last having occurred a few million years ago. We show that such accidental occurrences should be sufficiently common to provide about 1 chance in 30 (with considerable uncertainty) of being found within one. Certainly supernovae occur outside of OB associations, and direct evidence for a nearby supernova some 5 million years ago that could have been responsible for the latest reheating of the Local Bubble has recently been found in the excess abundance of 60Fe in a deep ocean ferromanganese crust.

Modeling the Interstellar Medium of Low Surface Brightness Galaxies: Constraining Internal Extinction, Disk Color Gradients, and Intrinsic Rotation Curve Shapes

Abstract: We use a combination of three-dimensional Monte Carlo radiative transfer techniques and multiwavelength (BRHK ,H a) imaging data to investigate the nature of the interstellar medium (ISM) in the edge-on, low surface brightness (LSB) galaxy UGC 7321. Using realistic models that incorporate multiple scattering eUects and clumping of the stars and the interstellar material, we explore the distribution and opacity of the interstellar material (gas]dust) and its eUects on the observed stellar disk luminosity pro—les, color gradients, and rotation curve shape. We —nd that UGC 7321 contains a small but nonnegligible dusty component to its ISM, yielding a B-band optical depth from disk edge to q6 e,B D 4.0 center. A signi—cant fraction (D50% ^ 10%) of the interstellar material in the innermost regions of UGC 7321 appears to be contained in a clumpy medium, indicating that LSB galaxies can support a modest, multiphase ISM structure. In spite of the clear presence of dust, we conclude that the large radial optical color gradients observed in UGC 7321 and other similar LSB spiral galaxies cannot be accounted for by dust and must result primarily from signi—cant stellar population and/or metallicity gradients. We show that realistic optical depth eUects will have little impact on the observed rotation curves of edge-on disk galaxies and cannot explain the linear, slowly rising rotation curves seen in some edge-on LSB spirals. Projection eUects create a far larger uncertainty in recovering the true underlying rotation curve shape of galaxies viewed at inclinations i Z 85i.

HST and VLT Investigations of the Fragments of Comet C/1999 S4 (LINEAR)

Abstract: At least 16 fragments were detected in images of comet C/1999 S4 (LINEAR) taken on 5 August 2000 with the Hubble Space Telescope (HST) and on 6 August with the Very Large Telescope (VLT). Photometric analysis of the fragments indicates that the largest ones have effective spherical diameters of about 100 meters, which implies that the total mass in the observed fragments was about 2 × 10 9 kilograms. The comet9s dust tail, which was the most prominent optical feature in August, was produced during a major fragmentation event, whose activity peaked on UT 22.8 ± 0.2 July 2000. The mass of small particles (diameters less than about 230 micrometers) in the tail was about 4 × 10 8 kilograms, which is comparable to the mass contained in a large fragment and to the total mass lost from water sublimation after 21 July 2000 (about 3 × 10 8 kilograms). HST spectroscopic observations during 5 and 6 July 2000 demonstrate that the nucleus contained little carbon monoxide ice (ratio of carbon monoxide to water is less than or equal to 0.4%), which suggests that this volatile species did not play a role in the fragmentation of C/1999 S4 (LINEAR).

CHANDRA-LETGS X-ray observations of Capella Temperature, density and abundance diagnostics

Abstract: We report an analysis of the X-ray spectrum of Capella from 6 to 175 Aobtained with the Low Energy Transmission Grating Spectrometer (LETGS) on board of the X-ray space observatory CHANDRA. Many emission line features appear that can be resolved much better as compared to former instruments (EUVE and ASCA). Coronal electron densities (n e ) and temperatures (T ) of brightly emitting regions are constrained by an analysis of ratios of density- and temperature-sensitive lines of helium-like ions and highly ionized iron atoms. Lines emitted by e.g., O VII & VIII, Mg X-XII, Si XII-XIV, Fe IX, X & XV-XXIII are used to derive T . Line ratios in the helium-like triplets of C V, N VI, O VII, Mg XI, and Si XIII yield T in the range 0.5-10 MK, and n e in the range 109 -1013 cm-3 . The Fe IX/X ratio yields 0.9 MK, while lines from Fe XVIII to XXII give 6-10 MK. Flux ratios of Fe XX-XXII lines indicate for the electron densities an upper limit in the range (2-5) 1012 cm-3 . From line ratios of Fe XVII and Fe XVIII we derive constraints on the optical depth τ of ~1-1.5. An emission measure distribution is derived from Fe line intensities. Results for element abundances (relative to Fe) from a 4-T model are: O and Ne/Fe about solar, N, Mg and Si/Fe ~2 solar. The results for T and n e are described in terms of quasi-static coronal loop models and it is shown that the X-ray emission originates from compact structures much smaller than the stellar radii.

Disk Frequencies and Lifetimes in Young Clusters

Abstract: We report the results of the first sensitive L-band (3.5 micron) survey of the intermediate age (2.5 - 30 Myr) clusters NGC 2264, NGC 2362 and NGC 1960. We use JHKL colors to obtain a census of the circumstellar disk fractions in each cluster. We find disk fractions of 52% +/- 10%, 12% +/- 4% and 3% +/- 3% for the three clusters respectively. Together with our previously published JHKL investigations of the younger NGC 2024, Trapezium and IC 348 clusters, we have completed the first systematic and homogenous survey for circumstellar disks in a sample of young clusters that both span a significant range in age (0.3 - 30 Myr) and contain statistically significant numbers of stars whose masses span nearly the entire stellar mass spectrum. Analysis of the combined survey indicates that the cluster disk fraction is initially very high (> 80%) and rapidly decreases with increasing cluster age, such that half the stars within the clusters lose their disks in < ~3 Myr. Moreover, these observations yield an overall disk lifetime of ~ 6 Myr in the surveyed cluster sample. This is the timescale for essentially all the stars in a cluster to lose their disks. This should set a meaningful constraint for the planet building timescale in stellar clusters. The implications of these results for current theories of planet formation are briefly discussed.

Discovery of x-ray emission from the crab pulsar at pulse minimum

Abstract: The Chandra X-ray Observatory observed the Crab Nebula and Pulsar using the Low-Energy Transmission Grating (LETG) with the High-Resolution Camera (HRC). Time-resolved zeroth-order images reveal that the pulsar emits x rays at all pulse phases. Analysis of the flux at minimum -- most likely nonthermal in origin -- places an upper limit (T(sub infinity) < 2.1 MK) on the surface temperature of the underlying neutron star. In addition, analysis of the pulse profile appears to confirm the absolute timing of the Observatory to within about 0.2 ms.

A Mid-Infrared Study of the Young Stellar Population in the NGC 2024 Cluster

Abstract: We present the results of the first broadband N (10.8 μm) survey of the NGC 2024 cluster. The mid-infrared data were combined with our previously published JHKL photometry to construct spectral energy distributions for all detected sources. The main scientific goals were to investigate the nature of the young stellar objects (YSOs) in the cluster and to examine the efficiency of detecting circumstellar disk sources from near-infrared JHKL color-color diagrams. Out of 59 sources surveyed having K-band (2.2 μm) magnitudes mK ≤ 10.5, we detected 35 (~59%) at 10 μm. Combining these detections and upper limits for the nondetections with existing JHKL data, we identify one Class I, six flat-spectrum, 28 Class II, and five Class III sources. We find a circumstellar disk fraction for NGC 2024 of ~85% ± 15%, which confirms earlier published suggestions that the majority, if not all, of the stars in the NGC 2024 cluster formed with disks, and these disks still exist at the present time. In addition, all but one of the disk sources identified in our survey lie in the infrared-excess region of the JHKL color-color diagram for the NGC 2024 cluster. This demonstrates that JHKL color-color diagrams are extremely efficient in identifying YSOs with circumstellar disks. Of the 14 sources in our survey with K-L colors suggestive of protostellar objects, ~29% are protostellar in nature, while ~7% are true Class I sources. This may be due to extinction producing very red K-L colors in Class II YSOs, thus making them appear similar in color to protostars. This suggests that caution must be applied when estimating the sizes and lifetimes of protostellar populations within star-forming regions based on K-L colors alone. A comparison of the ratio of the number of Class I and flat-spectrum sources to the number of Class II and III sources in NGC 2024, ρ Oph, and Taurus-Auriga indicates that NGC 2024 and ρ Oph have similar ages, while Taurus-Auriga is an older region of star formation, consistent with published T Tauri star ages in each region. Finally, we calculate the luminosities of the Class II sources in NGC 2024, ρ Oph, and Taurus and discuss the results.

The magnetic structure of a coronal x-ray bright point

Abstract: Brown, D.S., Parnell, C.E., Deluca, E., McMullen, R. and Golub, L., 2001, The magnetic structure of a coronal X-ray bright point, Solar Physics, 201, 305-321.

CO (J = 4→3) and [C I] Observations of the Carina Molecular Cloud Complex

Abstract: We present large-area, fully sampled maps of the Carina molecular cloud complex in the CO (J = 4→3) and neutral carbon [C I] 3P1 → 3P0 transitions. These data were obtained using the 1.7 m Antarctic Submillimeter Telescope and Remote Observatory (AST/RO). The maps cover an area of approximately 3 deg2 with a uniform 1' spatial sampling. Analysis of these data, in conjunction with CO (J = 1→0) data from the Columbia CO survey and the IRAS HIRES continuum maps for the same region, suggests that the spiral density wave shock associated with the Carina spiral arm may be playing an important role in the formation and dissociation of the cloud complex, as well as in maintaining the internal energy balance of the clouds in this region. Massive stars form at the densest regions of the molecular cloud complex. The winds and outflows associated with these stars have a disrupting effect on the complex and inject mechanical energy into the parent clouds, while the UV radiation from the young stars also heats the parent clouds. The present set of data suggests, however, that massive stars alone may not account for the energetics of the clouds in the Carina region. The details of the data and the correlation among the various data sets hint at the possible role that the spiral density wave shock plays in feeding interstellar turbulence and heating molecular clouds.

Infrared Mass-to-Light Profile Throughout the Infall Region of the Coma Cluster

Abstract: Using a redshift survey of 1779 galaxies and photometry from the Two Micron All Sky Survey covering 200 deg2, we calculate independent mass and light profiles for the infall region of the Coma Cluster of galaxies. The redshift survey is complete to Ks = 12.2 (622 galaxies), 1.2 mag fainter than M at the distance of Coma. We confirm the mass profile obtained by Geller, Diaferio, and Kurtz. The enclosed mass-to-light ratio measured in the Ks band is approximately constant to a radius of 10 h-1 Mpc, where M/L = 75 ± 23 h M☉/L☉, in agreement with weak-lensing results on similar scales. Within 2.5 h-1 Mpc, X-ray estimates yield similar mass-to-light ratios (67 ± 32 h). The constant enclosed mass-to-light ratio with radius suggests that the K-band light from bright galaxies in clusters traces the total mass on scales 10 h-1 Mpc. Uncertainties in the mass profile imply that the mass-to-light ratio inside r200 may be as much as a factor of 2.5 larger than that outside r200. These data demonstrate that the K-band light is not positively biased with respect to the mass; we cannot rule out antibias. These results imply that Ωm = 0.17 ± 0.05. Estimates of possible variations in M/L with radius suggest that the density parameter is no smaller than Ωm ≈ 0.08.

A Near-Infrared Imaging Survey of the Chamaeleon I Dark Cloud

Abstract: We describe a near-infrared imaging survey covering ~1 deg2 of the Chamaeleon I dark cloud. The survey is complete for K < 15.0, H < 16.0, and J < 16.5, roughly 2 magnitudes more sensitive than previous large-scale surveys. We use the large number of background stars detected to derive an accurate near-infrared extinction law for the cloud and select new candidate members with near-infrared color excesses. We list ~100 candidates of the cloud with K ≥ 12.0, based on their positions in the J-H, H-K color-color diagram. These new stars have low luminosities (K ~ 12–16, H-K 0.5–1.5) and may have masses close to or even below the hydrogen-burning limit.

Differential Flow Speeds of Ions of the Same Element: Effects on Solar Wind Ionization Fractions

Abstract: In previous studies of solar wind ion fractions it has always been assumed that ions of the same element flow with the same flow speed. In the present paper, we investigate the effects of differential flow speeds between different ionization states of the same element on the formation of O, C, Mg, Si, and Fe ions. We show that if the difference in flow speeds of adjacent ions, vi and vi+1, is large, the discrepancy between observed in situ ion fractions and low coronal electron temperature can be significantly reduced. The calculations are carried out for a radial profile of the electron temperature chosen in agreement with Solar Ultraviolet Measurement of Emitted Radiation (SUMER) measurements of coronal dark lanes and with two radial profiles of the electron density representing lower and upper observational limits. In order for the differential flow speeds to close the gap between the ion fractions observed in situ and the ones predicted from the low electron temperature observed in the corona, the differential flow speeds have to be extremely large in the regions where the ions are formed, e.g., below 1.5RS for O ions and below 2.5RS for Si ions; and in the case of Si, the coronal electron density has to be at the upper limit of observed values. In the cases of C, O, Mg, and Fe, the lower limit on the electron density is also acceptable.

Early Chandra X-Ray Observations of η Carinae

Abstract: Subarcsecond-resolution Chandra observations of η Carinae reveal a 40'' × 70'' ring or partial shell of X-ray emission surrounding an unresolved bright central source. The spectrum of the central source is strongly absorbed and can be fitted with a high-temperature thermal continuum and emission lines. The surrounding shell is well outside the optical/infrared bipolar nebula and is coincident with the outer shell of η Car. The X-ray spectrum of the shell is much softer than that of the central source. The X-ray shell is irregular and only correlates well with optical features in which a bright X-ray knot coincides with a bright feature of the outer shell. Implications for the binary model of the central source are discussed.

X-Ray Halos and Large Grains in the Diffuse Interstellar Medium

Abstract: Recent observations with dust detectors on board the interplanetary spacecrafts Ulysses and Galileo have recorded a substantial flux of large interstellar grains with radii between 0.25 and 2.0 μm entering the solar system from the local interstellar cloud. The most commonly used interstellar grain size distribution is characterized by an a-3.5 power law in grain radii a and extends to a maximum grain radius of 0.25 μm. The extension of the interstellar grain size distribution to such large radii will have a major effect on the median grain size and on the amount of mass needed to be tied up in dust for a given visual optical depth. It is therefore important to investigate whether this population of larger dust particles prevails in the general interstellar medium or whether it is merely a local phenomenon. The presence of large interstellar grains can be inferred mainly from their effect on the intensity and radial profile of scattering halos around X-ray sources. In this Letter, we examine the grain size distribution that gives rise to the X-ray halo around Nova Cygni 1992. The results of our study confirm the need to extend the interstellar grain size distribution in the direction of this source to and possibly beyond 2 μm. The model that gives the best fit to the halo data is characterized by (1) a grain size distribution that follows an a-3.5 power law up to 0.50 μm, followed by an a-4.0 extension from 0.50 to 2.0 μm, and (2) silicate and graphite (carbon) dust-to-gas mass ratios of 0.0044 and 0.0022, respectively, consistent with solar abundance constraints. Additional observations of X-ray halos probing other spatial directions are badly needed to test the general validity of this result.

A 3 μm Survey of the Chamaeleon I Dark Cloud

Abstract: We describe an L-band photometric survey of ~ 0.5 deg2 of the Chamaeleon I dark cloud. The survey has a completeness limit of L < 11.0. Our survey detects 124 sources, including all known pre–main-sequence stars with L ≤ 11. The fraction of sources with near-IR excess emission is 58% ± 4% for K = 9–11. Cha I sources have bluer H-K and K-L colors than pre–main-sequence stars in Taurus-Auriga. These sources also have a strong correlation between EW(Hα) and K-L. Stars with K-L≤ 0.6 have weak Hα emission; stars with K-L≥ 0.6 have strong Hα emission. Because many Cha I sources are heavily reddened, this division between weak emission T Tauri stars and classical T Tauri stars occurs at a redder K-L than in Taurus-Auriga.

A Study of Hydrogen Density in Emerging Flux Loops from a Coordinated Transition Region and Coronal Explorer and Canary Islands Observation Campaign

Abstract: During an international ground-based campaign in the Canary Islands coordinated with space instruments (i.e., Transition Region and Coronal Explorer [TRACE]), we observed an active region on 1998 September 10 with high spatial and temporal resolution. New emerging flux in the central part of the active region was observed in magnetograms of the Swedish Vacuum Solar Telescope, La Palma. Emerging loops (arch-filament systems [AFSs]) are well developed in Hα and Ca II according to the observations made at the Vacuum Tower Telescope (VTT) and THEMIS telescope in Tenerife with the Multichannel Subtractive Double Pass (MSDP) spectrographs. The TRACE images obtained at 171 and 195 A show low-emission regions that are easily identified as the individual AFS. They are due to absorption by hydrogen and helium continua in the cool filament plasma. We compare two techniques of measuring the hydrogen density in the cool dense fibrils of AFSs. The first method based on TRACE observations derived the neutral hydrogen column density of the plasma absorbing coronal lines. The second one using Hα line profiles provided by the MSDP spectrographs is based on the cloud model. The results are consistent. We derive also electron density values using Hα lines that are in good agreement with those derived from the 8542 A Ca II line observed with THEMIS (Mein et al.). The three types of observations (TRACE, VTT, THEMIS) are well complementary: absorption of coronal lines giving a good approximation for the maximum value of the neutral hydrogen column density, the Hα line giving a good determination of ne, and the 8542 A Ca II line a good determination of the electronic temperature.

X-ray Halos and Large Grains in the Diffuse Interstellar Medium

Abstract: Recent observations with dust detectors on board the interplanetary spacecraft Ulysses and Galileo have recorded a substantial flux of large interstellar grains with radii between 0.25 and 2.0 mu entering the solar system from the local interstellar cloud. The most commonly used interstellar grain size distribution is characterized by a a^-3.5 power law in grain radii a, and extends to a maximum grain radius of 0.25 mu. The extension of the interstellar grain size distribution to such large radii will have a major effect on the median grain size, and on the amount of mass needed to be tied up in dust for a given visual optical depth. It is therefore important to investigate whether this population of larger dust particles prevails in the general interstellar medium, or if it is merely a local phenomenon. The presence of large interstellar grains can be mainly inferred from their effect on the intensity and radial profiles of scattering halos around X-ray sources. In this paper we examine the grain size distribution that gives rise to the X-ray halo around Nova Cygni 1992. The results of our study confirm the need to extend the interstellar grain size distribution in the direction of this source to and possibly beyond 2.0 mu. The model that gives the best fit to the halo data is characterized by: (1) a grain size distribution that follows an a^-3.5 power law up to 0.50 mu, followed by an a^-4.0 extension from 0.50 mu to 2.0 mu; and (2) silicate and graphite (carbon) dust-to-gas mass ratios of 0.0044 and 0.0022, respectively, consistent with solar abundances constraints. Additional observations of X-ray halos probing other spatial directions are badly needed to test the general validity of this result.

Mapping potential fields on the surfaces of rapidly rotating stars

Abstract: ABSTRA C T We present a technique that combines Zeeman Doppler imaging (ZDI) principles with a potential field mapping prescription in order to gain more information about the surface field topology of rapid rotators. This technique is an improvement on standard ZDI, which can sometimes suffer from the suppression of one vector component because of the effects of stellar inclination, poor phase coverage or lack of flux from dark areas on the surface. Defining a relationship between the different vector components allows information from one component to compensate for reduced information in another. We present simulations demonstrating the capability of this technique and discuss its prospects.

The Kuiper Belt and Olbers's Paradox

Abstract: We investigate the constraints that Olbers's paradox, applied to the zodiacal background as measured from space, sets on outer solar system objects. If extended to very faint limits, R ~ 40-50 mag, the steep optical number counts of Kuiper Belt objects (KBOs) at R 26 imply an infinitely bright night sky. Small KBOs with radii of r ~ 1 μm to r ~ 1 km must have a size distribution n(r) ∝ r-a, with a ~ 3.4 or smaller to satisfy the known limits on the sky-surface brightness at optical and far-infrared wavelengths. Improved limits on the measured KBO surface brightness can yield direct estimates of the albedo, temperature, and size distribution for small KBOs in the outer solar system.

The Continuing Slow Decline of AG Pegasi

Abstract: We analyze optical and ultraviolet observations of the symbiotic binary AG Pegasi acquired during 1992–1997. The bolometric luminosity of the hot component declined by a factor of 2–3 from 1980–1985 to 1997. Since 1992, the effective temperature of the hot component may have declined by 10%–20%, but this decline is comparable to the measurement errors. Optical observations of Hβ and He I emission show a clear illumination effect, where high-energy photons from the hot component ionize the outer atmosphere of the red giant. Simple illumination models generally account for the magnitude of the optical and ultraviolet emission-line fluxes. High-ionization emission lines—[Ne V], [Mg V], and [Fe VII]—suggest mechanical heating in the outer portions of the photoionized red giant wind. This emission probably originates in a low-density region ~30–300 AU from the central binary.

The NEAR-Shoemaker x-ray/gamma-ray spectrometer experiment: Overview and lessons learned

Abstract: — The near-Earth asteroid rendezvous (NEAR)-Shoemaker remote-sensing x-ray/gamma-ray spectrometer (XGRS) completed more than a year of operation in orbit and on the surface of 433 Eros. Elemental compositions for a number of regions on the surface of Eros have been derived from analyses of the characteristic x-ray and gamma-ray emission spectra. The NEAR XGRS detection system was included as part of the interplanetary network (IPN) for the detection and localization of gamma-ray bursts (GRBs). Preliminary results for both the elemental composition of the surface of Eros and the detection of GRBs have been obtained. In addition to the science results, the design and operation of the NEAR XGRS is considered. Significant information important for the design of future remote sensing XGRS systems has been obtained and evaluated. We focus on four factors that became particularly critical during NEAR: (1) overall spacecraft design, (2) selection of materials, (3) increase of the signal-to-noise ratio and (4) knowledge of the incident solar x-ray spectrum.

Redshifts for 2410 Galaxies in the Century Survey Region

Abstract: The Century Survey strip covers 102 deg2 within the limits 85 ≤ α ≤ 165, 290 ≤ δ ≤ 300, equinox B1950.0. The strip passes through the Corona Borealis supercluster and the outer region of the Coma cluster. Within the Century Survey region, we have measured 2410 redshifts that constitute four overlapping complete redshift surveys: (1) 1728 galaxies with Kron-Cousins Rph ≤ 16.13 covering the entire strip, (2) 507 galaxies with Rph ≤ 16.4 in right ascension range 8h32m ≤ α ≤ 10 h45m, equinox B1950.0, (3) 1251 galaxies with absorption- and K-corrected RCCDc ≤ 16.2 (where "c" indicates "corrected") covering the right ascension range 85 ≤ α ≤ 135, equinox B1950.0, and (4) 1255 galaxies with absorption- and K-corrected VCCDc ≤ 16.7 also covering the right ascension range 85 ≤ α ≤ 135, equinox B1950.0. All these redshift samples are more than 98% complete to the specified magnitude limit. We derived samples 1 and 2 from scans of the POSS1 red (E) plates calibrated with CCD photometry. We derived samples 3 and 4 from deep V and R CCD images covering the entire region. We include coarse morphological types for all the galaxies in sample 1. The distribution of (V-R)CCD for each type corresponds appropriately with the classification.

$^{13}$C I in high-mass star-forming clouds

Abstract: We report measurements of the C/C abundance ratio in the three galactic regions G 333.0-0.4, NGC 6334 A and G 351.6-1.3 from observations of the Ci P2 →P1 transition and the hyperfine components of the corresponding Ci transition near 809 GHz. These transitions were observed simultaneously with the CO 7–6 line emission at 806 GHz with the AST/RO telescope located at the South Pole. From a simultaneous fit to the Ci P2 →P1 transition and the HF components of the corresponding Ci transition and an independent estimate of an upper limit to the optical depth of the Ci emission we determine intrinsic Ci/Ci column density ratios of 23 ± 1 for G 333.0-0.4, 56 ± 14 for NGC 6334 A and 69 ± 12 for G 351.6-1.3. As the regions observed are photon dominated, we argue that the apparent enhancement in the abundance of C towards G 333.0-0.4 may be due to strong isotope-selective photodissociation of CO, outweighing the effects of chemical isotopic fractionation as suggested by models of PDRs. Towards NGC 6334 A and G 351.6-1.3 these effects appear to be balanced, similar to the situation for the Orion Bar region observed by Keene et al. (1998).

Shock Wave Emission at Miller’s Position 1 in the Cygnus Loop

Abstract: We present the far-UV spectrum (900-1800 A) of the bright filament known as Miller's (1974) position 1 in the Cygnus Loop obtained with the Hopkins Ultraviolet Telescope during the Astro-2 space shuttle mission. Longer wavelength IUE and ground-based spectra of adjoining regions were scaled to produce a combined spectrum ranging from 900 to 7500 A. We compare this spectrum with models of radiative shock waves to infer shock velocities and elemental abundances. A power-law distribution of shock velocities within the spectrograph apertures can reproduce the observed spectrum. We consider the question of whether a simpler shock structure, thermally unstable cooling, or a mixing layer could produce a similar spectrum. We also discuss the elemental abundances with particular attention to C and Si, whose lines are only available in the UV.