American Astronomical Society Research Articles

Cosmic Expansion, Poco Adagio

AMERICAN ASTRONOMICAL SOCIETY MEETINGAUSTIN, TEXAS-- Thales, the ancient Greek astronomer, is said to have fallen down a well while gazing at the stars and not paying attention to his surroundings. Attendees at the American Astronomical Society meeting, held here from 4 to 9 January, could hardly be accused of the same fault, as they issued dozens of press releases and gave a string of press conferences. Although some results were oversold, some could be for the ages.

Soggy Cradles of Stars

AMERICAN ASTRONOMICAL SOCIETY MEETINGAUSTIN, TEXAS-- Thales, the ancient Greek astronomer, is said to have fallen down a well while gazing at the stars and not paying attention to his surroundings. Attendees at the American Astronomical Society meeting, held here from 4 to 9 January, could hardly be accused of the same fault, as they issued dozens of press releases and gave a string of press conferences. Although some results were oversold, some could be for the ages.

Superflares From Giant Planets

AMERICAN ASTRONOMICAL SOCIETY MEETINGAUSTIN, TEXAS-- Thales, the ancient Greek astronomer, is said to have fallen down a well while gazing at the stars and not paying attention to his surroundings. Attendees at the American Astronomical Society meeting, held here from 4 to 9 January, could hardly be accused of the same fault, as they issued dozens of press releases and gave a string of press conferences. Although some results were oversold, some could be for the ages.

ITAL]The[/ITAL] [ITAL]Astronomical[/ITAL] [ITAL]Journal[/ITAL] at Yale: In Context with Before and After

When the Astronomical Journal was founded by B. A. Gould, Jr., in 1849, the major topics of research were the solar system, celestial mechanics, and astrometry. During the 1880s the advent of dry-plate photography played a significant role in introducing other preferred areas for research, especially spectroscopy, astrophysics, multicolor photometry, and galaxies. In 1941, the Journal came under the jurisdiction of the American Astronomical Society, and Yale became its editorial headquarters. A comparison of the numbers of papers in the newer branches relative to the total numbers of papers published either in the Astronomical Journal or in other publications of the American Astronomical Society shows the steady increase of interest in the newer fields, regardless of the special fields of interest of the Yale editors.

Editorial

This is a very special issue of the Astronomical Journal. It celebrates the fact that it has been 150 years since the Journal was founded by Benjamin Apthorp Gould. To commemorate the event, we begin this issue with a set of four historical papers relevant to the Journal's beginnings and to its long life. The papers grew out of a special session of the 1998 June American Astronomical Society meeting that was sponsored by the Society's Historical Astronomy Division. We on the staff of the Journal are grateful to the authors and to the Division for providing a fitting celebration of our sesquicentennial year.

Report on the Progress in Stellar Evolution to 1950

At a joint meeting of the American Astronomical Society and the American Physical Society held in June of 1940, the University of Michigan astronomer Dean McLaughlin (1940) gave a review of the current understanding of stellar evolution. At the end he somewhat facetiously remarked that, “For several years I have told students that I knew all about stellar evolution in 1923, less is 1925, and nothing at all since 1930.” I would like to suggest that those dates were not chosen randomly, and in the first part of my own survey of stellar evolution to 1950, I would like to explain the significance of those dates.

The Circumstellar Shell of the Post–Asymptotic Giant Branch Star HD 56126:12C12C/12C13C Isotope Ratio and12C16O Column Density

We have made the first detection of circumstellar absorption lines of the 12C13C A 1Πu-X 1∑g+ (Phillips) system 1-0 band and the 12C16O X 1∑+ first-overtone 2-0 band in the spectrum of the post-AGB star HD 56126 (IRAS 07134+1005). All current detections of circumstellar molecular absorption lines toward HD 56126 (12C2, 12C13C, 12C14N, 13C14N, and 12C16O) yield the same heliocentric velocity of VCSE = 77.6 ± 0.4 km s-1. The 12C2, 12C13C, and 12C16O lines give rotational temperatures and integrated column densities of Trot = 328 ± 37 K, log Nint = 15.34 ± 0.10 cm-2, Trot = 256 ± 30 K, log Nint = 13.79 ± 0.12 cm-2, and Trot = 51 ± 37 K, log NInt = 18.12 ± 0.13 cm-2, respectively. The rotational temperatures are lower for molecules with a higher permanent dipole moment. Derived relative column densities ratios are 12C2/12C13C = 36 ± 13 and 12C16O/(12C2 + 12C13C) = 606 ± 230. Combined with data from a previous paper, we find relative column densities of 12C16O/(12C14N + 13C14N) = 475 ± 175 and 12C14N/13C14N = 38 ± 2. Under chemical equilibrium conditions, 12C13C is formed twice as easily as 12C2. The isotopic exchange reaction for 12C2 is too slow to significantly alter the 12C2/12C13C ratio, and the 12C2 to 12C13C ratio is a good measure of half the carbon isotope ratio: 12C/13C = 2 × 12C2/12C13C = 72 ± 26. This is in agreement with our prediction that the isotopic exchange reaction for 12C14N is efficient and our observation in Paper III of 12C14N/13C14N = 38 ± 2. A fit of the C2 excitation model of van Dishoeck & Black to the relative population distribution of C2 yields nc σ/I = 3.3 ± 1.0 × 10-14. At r ≃ 1016 cm, this translates into nc = 1.7 × 107 cm-3 and Ṁ ≃ 2.5 × 10-4 M⊙ yr-1. © 1998. The American Astronomical Society. All rights reserved.

New Constraints on the Continuum Emission Mechanism of Active Galactic Nuclei: Intensive Monitoring of NGC 7469 in the X‐Ray and Ultraviolet

We have undertaken near-continuous monitoring of the Seyfert 1 galaxy NGC 7469 in the X-ray with RXTE over a ∼30 day baseline. The source shows strong variability with a root mean square (rms) amplitude of ∼16% and peak-to-peak variations of a factor of order 2. Simultaneous data over this period were obtained in the ultraviolet (UV) using IUE, making this the most intensive X-ray UV/X-ray variability campaign performed for any active galaxy. Comparison of the continuum light curves reveals very similar amplitudes of variability but different variability characteristics, with the X-rays showing much more rapid variations. The data are not strongly correlated at zero lag. The largest absolute value of the correlation coefficient occurs for an anticorrelation between the two bands, with the X-ray variations leading the UV by ∼4 days. The largest positive correlation is for the ultraviolet to lead the X-rays by ∼4 days. Neither option appears to be compatible with any simple interband transfer function. The peak positive correlation at ∼4 days occurs because the more prominent peaks in the UV light curve appear to lead those in the X-rays by this amount. However, the minima of the light curves are near simultaneous. These observations provide new constraints on theoretical models of the central regions of active galactic nuclei. Models in which the observed UV emission is produced solely by reradiation of absorber X-rays are ruled out by our data, as are those in which the X-rays are produced solely by Compton upscattering of the observed UV component by a constant distribution of particles. New or more complex models must be sought to explain the data. We require at least two variability mechanisms, which have no simple relationship. We briefly explore means by which these observations could be reconciled with theoretical models. © 1998. The American Astronomical Society. All rights reserved.

The Challenge of Gamma Ray Burst Observations

Gamma-ray bursts, energetic emissions of high-energy photons of astrophysical origin, have posed a continuing puzzle for astronomers. As Leonard discusses in his Perspective, recent observational results are confirming that the gamma-ray bursts originate well outside our galaxy but, at the same time, offer new challenges to theorists trying to explain the mechanism of emission. Leonard discusses work recently presented at the 192nd meeting of the American Astronomical Society (7 to 12 June 1998).

The Circumstellar Shell of the Post–Asymptotic Giant Branch Star HD 56126: The12CN/13CN Isotope Ratio and Fractionation

We have detected circumstellar absorption lines of the 12CN and 13CN violet and red system in the spectrum of the post-AGB star HD 56126. From a synthetic spectrum analysis, we derive a Doppler broadening parameter of b = 0.51 ± 0.04 km s-1, 12CN/13CN = 38 ± 2, and a lower limit of 2000 on 12CN/14CN and 12C14N/12C15N. A simple chemical model has been computed of the circumstellar shell surrounding HD 56126 that takes into account the gas-phase ion-molecule reaction between CN and C+. From this we infer that this reaction leads to isotopic fractionation of CN. Taking into account the isotopic exchange reaction and the observed 12CN/13CN, we find 12C/13C ∼ 67 (for Tkin = 25 K). Our analysis suggests that 12CN has a somewhat higher rotational temperature than 13CN: Trot = 11.5 ± 0.6 and 8.0 ± 0.6 K, respectively. We identify possible causes for this difference in excitation temperature, among which is the N″ dependence of the isotopic exchange reaction. © 1998. The American Astronomical Society. All rights reserved.

Cosmic Web Captures Lost Matter

COSMOLOGYSAN DIEGO-- Vast hydrogen clouds filled the early universe but have since vanished, leaving the matter they contained unaccounted for. But in a talk at an American Astronomical Society meeting here earlier this month, a cosmologist predicted that observers will soon find most of the ordinary matter in the universe right under their noses. Computer simulations show that the primordial clouds condensed over time into a vast, filamentary network of ionized gas--a cosmic cobweb that now links galaxies and galaxy clusters.

Giant Survey Wallpapers the Sky

ASTRONOMYSAN DIEGO-- A roll of photographic film 10 paces long marks the first step in a survey that will reach billions of light-years into space. The Sloan Digital Sky Survey's electronic camera, mounted on a 2.5-meter telescope in New Mexico, collected its first images of stars, galaxies, and quasars on the nights of 9 and 10 May ( Science , 29 May, p. [1337][1]). Converted into a photographic scroll, just 1% of the data from the first night drew gasps last week when five members of the team unrolled it during a press conference here at a meeting of the American Astronomical Society. [1]: http://www.sciencemag.org/cgi/content/short/280/5368/1337

Single and Multiple Solar Flare Loops: Hydrodynamics and CaxixResonance Line Emission

Studies made so far with one-dimensional hydrodynamic simulations have shown that it is difficult to reproduce the soft X-ray spectral line profile observed in the early phase of solar flares. Simulated line profiles predict a dominant emission from a large blueshifted component, while observations show persistently strong stationary components. We resolve these discrepancies by utilizing a multiple-loop system instead of just a single loop for conductively heated flare simulations. Under a fixed heat input rate, we examine how the heating duration {tau}{sub heat} affects the Ca xix resonance ({ital w}) line emission from single and multiple flare loops. In the multiple-loop model, the flare energy is released into individual loops with a specified time delay, which implicitly mimics the successive formation of flare loops due to continuous reconnection. We find that whether or not {tau}{sub heat} is longer than {tau}{sub {ital c}} affects the hydrodynamic response in an individual flare loop, where {tau}{sub {ital c}} corresponds to the time when the loop is filled with evaporated plasma. The Ca xix spectral line shape is characterized by an intensity ratio of emission from evaporated plasma to emission from accumulated plasma after evaporation. This ratio is mainly determined by the parameter {tau}{submore » heat}/{tau}{sub {ital c}}. Our findings suggest that the following scenario can naturally explain the observed spectral line features. Flare energy is injected into a bundle of loops successively in two steps: in the preflare stage, {tau}{sub heat} {le} {tau}{sub {ital c}} for the inner loops, and then in the main flare stage, {tau}{sub heat} {gt} {tau}{sub {ital c}} for the outer loops. A large initial coronal density is not necessary in this scenario. {copyright} {ital {copyright} 1998.} {ital The American Astronomical Society}« less

A Spherical Non‐LTE Line‐blanketed Stellar Atmosphere Model of the Early B GiantεCanis Majoris

We use a spherical non-LTE fully line-blanketed model atmosphere to fit the full multiwavelength spectrum, including the extreme-ultraviolet (EUV) continuum observed by the {ital Extreme} {ital Ultraviolet} {ital Explorer}, of the B2 II star {epsilon} Canis Majoris (CMa). The available spectrophotometry of {epsilon} CMa from 350 {Angstrom} to 25 {mu}m is best fitted with model parameters {ital T}{sub eff} = 21,750 K, log {ital g} = 3.5, and an angular diameter of 0.77 mas. Our best-fit model predicts a hydrogen ionizing flux, {ital q}{sub 0}, of 1.59 {times} 10{sup 21} photons cm{sup {minus}2} s{sup {minus}1} at the star`s surface and 2290 photons cm{sup {minus}2} s{sup {minus}1} at the surface of the Local Cloud. The close agreement between the model and the measured EUV flux from {epsilon} CMa is a result of the higher temperatures at the formation depths of the H i and He i Lyman continua compared with other models. The realistic model treatment of early B giants with spherical geometry and non-LTE metal line{endash}blanketing results in the prediction of significantly larger EUV fluxes compared with plane-parallel models. We find that our metal line{endash}blanketed spherical models show significantly warmer temperature structures, 1{endash}3 kK at the formation depth of themore » Lyman continua, and predict stronger EUV fluxes, up to a factor of 5 in the H i Lyman continuum, compared with plane-parallel atmospheres that have identical model parameters. In contrast, we find that spherical and plane-parallel models that do not include metal line blanketing are nearly identical. Our {ital T}{sub eff} = 21,000 K, log {ital g} = 3.2, spherical non-LTE model predicts more than twice as many hydrogen ionizing photons and over 200 times more neutral helium ionizing photons than a standard hydrostatic plane-parallel LTE model with the same stellar parameters. Our synthetic spectra are in reasonably good agreement with observed continuum and line fluxes from echelle spectra obtained with the Goddard High Resolution Spectrograph. While we find agreement between the absolute UV flux of {epsilon} CMa as measured by GHRS and our model atmosphere, these fluxes are {approximately}30{percent} higher in the UV than those measured by {ital IUE}, {ital OAO} {ital 2}, and {ital TD}-{ital 1}, in excess of the published errors in the absolute calibration of these data. {copyright} {ital {copyright} 1998.} {ital The American Astronomical Society}« less

Constraining Ω0with the Angular Size–Redshift Relation of Double‐lobed Quasars in the FIRST Survey

In previous attempts to measure cosmological parameters from the angular size-redshift ({theta}{minus}z) relation of double-lobed radio sources, the observed data have generally been consistent with a static Euclidean universe rather than with standard Friedmann models, and past authors have disagreed significantly as to what effects are responsible for this observation. These results and different interpretations may be due largely to a variety of selection effects and differences in the sample definitions destroying the integrity of the data sets, and inconsistencies in the analysis undermining the results. Using the VLA FIRST survey, we investigate the {theta}{minus}z relation for a new sample of double-lobed quasars. We define a set of 103 sources, carefully addressing the various potential problems that, we believe, have compromised past work, including a robust definition of size and the completeness and homogeneity of the sample, and further devise a self-consistent method to assure accurate morphological classification and account for finite resolution effects in the analysis. Before focusing on cosmological constraints, we investigate the possible impact of correlations among the intrinsic properties of these sources over the entire assumed range of allowed cosmological parameter values. For all cases, we find apparent size evolution of the form l{proportional_to}(1+z){sup c}, withmore » c{approx}{minus}0.8+0.4, which is found to arise mainly from a power-size correlation of the form l{proportional_to}P{sup {beta}}({beta}{approx}{minus}0.13{plus_minus}0.06) coupled with a power-redshift correlation. Intrinsic size evolution is consistent with zero. We also find that in all cases, a subsample with c{approx}0 can be defined, whose {theta}{minus}z relation should therefore arise primarily from cosmological effects. These results are found to be independent of orientation effects, although other evidence indicates that orientation effects are present and consistent with predictions of the unified scheme for radio-loud active galactic nuclei. The above results are all confirmed by nonparametric analysis. Contrary to past work, we find that the observed {theta}{minus}z relation for our sample is more consistent with standard Friedmann models than with a static Euclidean universe. Though the current data cannot distinguish with high significance between various Friedmann models, significant constraints on the cosmological parameters within a given model are obtained. In particular, we find that a flat, matter-dominated universe ({Omega}{sub 0}=1), a flat universe with a cosmological constant, and an open universe all provide comparably good fits to the data, with the latter two models both yielding {Omega}{sub 0}{approx}0.35 with 1 {sigma} ranges including values between {approximately}0.25 and 1.0, the c{approx}0 subsamples yield values of {Omega}{sub 0} near unity in these models, though with even greater error ranges. We also examine the values of H{sub 0} implied by the data, using plausible assumptions about the intrinsic source sizes, and find these to be consistent with the currently accepted range of values. We determine the sample size needed to improve significantly the results and outline future strategies for such work. {copyright} {ital 1998} {ital The American Astronomical Society}« less

Stargazing From the End of the Earth

SOUTH POLE-- Although the thin, still, bone-dry air and constant winter darkness here offer observing conditions better than practically anywhere else short of outer space, it wasn9t known until recently if it would even be possible to operate a telescope over the winter. Many astronomers now contend that, since it was founded in 1991, the Center for Astrophysics in Antarctica (CARA), a $28 million test-bed for astronomers to build and operate instruments in the unforgiving antarctic environment, has succeeded in demonstrating that this is a viable place to do cutting-edge astronomy. At an American Astronomical Society meeting in San Diego in June, CARA staff members will make a pitch to the broader community to support an International Antarctic Observatory (IAO). But unless IAO boosters overcome several hurdles, the biggest one being funding, the grand plans for an observatory could come to naught. Realizing that the National Science Foundation alone won9t foot a bill totaling at least $100 million for all the telescopes they want, proponents will try to put together international coalitions under IAO9s umbrella to pay for individual instruments. The project would have to compete for logistical support with the new station. And CARA9s own image as a clubby organization that doesn9t welcome outside ideas, together with the problems it has experienced with some of its telescopes, could also hamper the bid for a new observatory.

Hubble Sees All the Light There Is

WASHINGTON, D.C.-- The most penetrating view of the cosmos ever, recorded by the Hubble Space Telescope, may have revealed nearly all sources of visible light in the universe. At a meeting of the American Astronomical Society here last week, astronomers announced an analysis suggesting that little if any light shines from just beyond the Hubble9s range of vision. However, many galaxies may be veiled from Hubble9s view by far-off dust clouds.

Intruder in a Star's Dust Glimpsed

ASTRONOMYWASHINGTON, D.C.-- Images from the Hubble Space Telescope reveal pronounced warps in a dramatic disk of dust circling Beta Pictoris, a young star 63 light-years away. Researchers who unveiled the images here at a meeting of the American Astronomical Society last week said the bulges may betray the existence of at least one planet. But others suggested that gravitational nudges from a passing star or a large dim companion might account for the distortions.

Electronic Publishing: Now and the Future

Electronic publishing of journals has already arrived, and is established. The Astrophysical Journal and Letters, Astronomy and Astrophysics Supplements and New Astronomy have been publishing a parallel electronic edition since the beginning of 1997. Astronomy and Astrophysics will follow in 1998, and the Monthly Notices of the Royal Astronomical Society are being developed. The American Astronomical Society has been the leader in developing electronic publishing in astrophysics. They have led not only by being first, but also by the high standards of their electronic publication.One of the stories that school children in Great Britain learn at an early age is about King Canute II. Canute was a Danish king who ruled part of what is now Great Britain from 1016 to 1035. He is famous because he sat bythe sea as the tide was coming in and commanded the tide to go back. It didnot.Electronic publication is going to bring many changes to the way research is done and published. There are obvious advantages and disadvantages. There are certainly problems which we have not yet realized. The purpose of this Joint Discussion is to stimulate a discussion of what the international astrophysical community would like from electronic publishing. Please do not say that you want electronic publishing to go away, because then you will be behaving like King Canute. But the publishers of journals and the bodies responsible for maintaining the quality of publication are feeling their way into completely new territory. To exploit electronic publishing to the full, and to minimize the disadvantages, discussion in the community is needed.Electronic publishing will bring great connectivity into journals. While looking at one paper, a click on the mouse will immediately connect with another paper published in another journal. Large tables, theoretical or observational, will be read in the same way from databases. An electronic publication will longer be static. It can be modified to refer to papers published later, it can contain movies or computer programs that can be executed online.

The RR Lyrae Population of the Galactic Bulge from the MACHO Database: Mean Colors and Magnitudes

Mean colors and magnitudes of RR Lyrae stars in 24 fields toward the Galactic bulge from the MACHO database are presented. Accurate mean reddenings are computed for these fields on the basis of the mean colors. The distribution along the line of sight of the RR Lyrae population is examined on the basis of the mean magnitudes, and it is shown that the bulk of the RR Lyrae population is not barred. Only the RR Lyrae stars in the inner fields closer to the Galactic center (l{lt}4{degree}, b{gt}{minus}4{degree}) show evidence for a bar. The red giant clump stars in the MACHO fields, however, clearly show a barred distribution, confirming the results of previous studies. Given the different spatial distribution, the RR Lyrae stars and the clump giants trace two different populations. The RR Lyrae variables would represent the inner extension of the Galactic halo in these fields. {copyright} {ital 1998} {ital The American Astronomical Society}