Water plays a central role in the greenhouse effect in the Earth's atmosphere. The quantity of water substance (water vapor plus condensed water) in the upper troposphere and lower stratosphere (altitudes of approximately 6-18 km), is especially important to the modeling of the global climate. Of particular interest is the water substance in the vicinity of the tropopause. Empirical data concerning the mean water substance content and its spatial and temporal variability through this range of altitudes are extremely sparse; in fact, the available data are inadequate for meaningful comparison with the predictions of global climate models. The ARES program, which employs a WB-57F aircraft that can fly at a wide range of altitudes as high as 19 km, provides a powerful new resource for measuring the atmospheric water substance content through the critical range of interest. The primary instrument on board the ARES aircraft is the imaging spectrometer, which collects data in the 2-6.3 μm wavelength region. The volume density of the atmospheric water substance is determined as a function of altitude by measuring the absorption of solar or lunar irradiance at several altitude increments from 6 km to 18 km. The use of the WB-57F aircraft for these data collections allows for the measurement of geographic and seasonal variations in the atmospheric water content. Upcoming flights will examine these variations and will include an instrument upgrade package that includes an ozone meter to determine the altitude of the tropopause, as well as Lyman-α and chilled mirror hygrometers for independent verification of the water vapor densities measured by the ARES spectrophotometric data. This paper presents the results of a preliminary set of measurements obtained in May and December 1994, and describes plans for upcoming flights

The Russian American Observation Satellites (RAMOS) program is a joint US-Russian experiment designed to perform stereo real-time measurements of the background radiance and structure as seen by down and limb-looking sensors. The experiment will address environmental issues also that are well-suited for stereo observation. The RAMOS experiment consists of two dedicated satellites, an American Observational Satellite (AOS) and a Russian Observational Satellite (ROS), both in the same low Earth orbit. The targeted lifetime of the mission is one year. The complimentary sensors on board the two satellites will be used to collect stereo imagery of backgrounds of interest in the visible and infrared wavelength regions. The data base of stereo imagery will be analyzed to produce valuable information such as cloud heights, temperature profiles, the statistics of stressing clutter situations, hurricane motion and strength, and other environmental monitoring parameters. This paper will discuss the RAMOS goals and describe the roadmap in place for achieving them. Such goals include the demonstration of successful simultaneous operations and data exchange between US and Russian science teams, the overlay, registration and resolution matching of images collected by disparate sensors for constructing three structured scenes, and clutter modeling.

We have re-examined a scenario for the evolution of a binary system, initially comprising a neutron star and a low-mass giant and ending as a wide binary containing a radio pulsar and a white dwarf in a nearly circular orbit. The evolution is driven by the nuclear evolution of the giant, which results in the stable transfer of much or all of the envelope of the giant to the neutron star. The angular momentum associated with the transferred mass may spin the neutron star up to high rotation rates, yielding a 'recycled' pulsar; the white dwarf, which had been the core of the giant progenitor, remains as a fossil relic of the giant. This scenario provides a unique test of the theory of advanced stages of stellar evolution, in that it predicts the existence of a testable relationship between observable quantites: the mass, M(wd), of the white dwarf and the orbital period, P-orb, of the binary. The relationship arises because (1) stellar evolution theory predicts the existence of a rather tight relationship between the core mass, M(c), of a giant and the radius, R(g), of its envelope; and (2)in the scenario under consideration, the giant envelope is expected to fill its Roche lobe until the termination of mass transfer. The final orbital separation should thus be a well-defined function of R(g) at the end of the mass-transfer phase (i.e. at the time when the envelope of the giant is exhausted), while M(wd) will be essentially identical to the final value of M(c) at the termination of mass transfer.Using refined stellar evolution calculations, we have redetermined the most likely value of R(g) as a function of M(c) for core masses in the range 0.15 < M(c) < 1.15 M., and we have devised analytic fitting formulae to our results for both the R(g)-M(c) relation and the concomitant P-orb-M(wd) relation. We have also, for the first time, obtained a quantitative estimate of the spread in the value of R(g) at each value of M(c), resulting both from variations in the initial chemical composition and main-sequence mass of the giant and from the theoretical uncertainty in the value of the convective mixing-length parameter. We find that the maximum spread about the median Value of R(g) at any given value of M(c) is a factor of similar to 1.8, and that the corresponding maximum spread in P-orb at fixed M(wd) is a factor of similar to 2.4; smaller spreads are obtained if one or more of the parameters (e.g. the initial composition of the giant) are assumed to be known. We have compared our results against the observational parameters of 23 radio pulsars in wide, nearly circular, binary orbits with low-mass white dwarf companions. We have also examined the applicability of our results (appropriately modified for non-negligible orbital eccentricity) to two conventional wide-binary systems (Sirius and Procyon) containing more massive white dwarf companions. We find overall good agreement between our theoretical results and the available observational data; however, any comparison between theory and observation is limited by the generally large uncertainties in the masses of the white dwarfs in the binary pulsar systems and by the paucity of known systems containing more massive white dwarfs.

Neutron stars have often been used in models of γ‐ray bursts (GRB) sources. Recently, Blaes et al. reexamined the possibility that GRB’s are caused by seismic events on old, cold, isolated neutron stars that are accreting slowly from the interstellar medium. We have further investigated this model by use of a stellar evolution code adapted to the study of neutron‐star surfaces. Our preliminary calculations reveal the evolution of the structure of the neutron‐star crust and provide an estimate of the energy stored in the mechanical structure and nonequilibrium composition of the crust. After 1010 years of slow accretion at an accretion rate of 10−16 M⊙ yr−1, the available energy is ∼1045 ergs, but there is no evidence for the development of an unstable density inversion.

SUPERNOVA 1993J in the spiral galaxy M81 is the brightest supernova since SN1987A and, like the latter, appears to be another 'peculiar' type II supernova. The available photometry1,2 of the supernova region before the explosion requires the presence of at least two supergiants (one of early spectral type and the other of late type), but the actual progenitor has yet to be identified. Here we show that the explosion of a late-type supergiant can explain the initial sharp peak in the supernova light curve, provided that the star had lost almost all of its hydrogen-rich envelope before the explosion. In our model, the secondary brightening of the supernova, approximately 10 days later, is then a consequence of the radioactive decay of Ni-56 and subsequently Co-56) produced in the explosion. The progenitor could have lost its hydrogen-rich envelope either in a strong stellar wind or, as seems more likely, through mass transfer to a companion star. In the latter case, the companion should reappear after the supernova photosphere has receded, the system having become a binary composed of a neutron star with a massive stellar companion.

We here report the results of a systematic investigation of how binary interaction affects the presupernova evolution of massive stars and the resulting supernova explosions. We summarize the various types of binary interaction and the evolutionary scenarios in which they are realized. We also present the results of a series of hydrodynamical calculations which model the supernova explosion for various progenitor types and discuss their observable characteristics.

We have systematically investigated how binary interaction affects the presupernova evolution of massive close binaries and the resulting supernova explosions, using a Henyey-type stellar evolution code that we have modified to allow its application to binary stellar evolution calculations. With our modified code, we are able to follow the effects of mass and angular momentum loss from the binary, as well as mass transfer within the binary system. We find that a large number of binary scenarios can be distinguished, depending on the type of binary interaction and the evolutionary stage of the supernova progenitor at the time of the interaction. In general, the structure of a massive star can be affected in three fundamentally different ways: by mass loss, mass accretion, or common-envelope evolution.As a result of mass loss by Roche lobe overflow, stars can lose all of their hydrogen-rich envelopes and become helium stars, which are potential candidates for the progenitors of Type Ib supernovae. If the original masses of the binary components are nearly equal, it is possible (for reasonable assumptions about the mass and angular momentum loss rates) that the primary retains part of its hydrogen-rich envelope. In this case, the supernova progenitor would look like a more or less normal red supergiant, even though it may have lost most of its envelope, and the supernova would resemble a classical Type II-L supernova. In most cases, the system remains bound after the explosion, but it may acquire a substantial orbital eccentricity. The system may subsequently become unbound if the original secondary then evolves to become a second supernova.Mass accretion can also significantly alter the structure of the supernova progenitor, if it takes place after the main-sequence phase of the accreting star. The star may then end its life as a blue supergiant instead of a red supergiant. In this case, the resulting supernova explosion would resemble SN 1987A. At the time of the supernova explosion, the presupernova star still has a stellar or (more likely) a neutron star companion. However, since more than half of the total mass of the system is ejected in the supernova explosion, the system is likely to become unbound after the explosion.In the most dramatic case of binary interaction, in which a supernova progenitor captures its companion in a common envelope, two different outcomes are possible, depending on whether the envelope is ejected during the spiral-in phase or remains bound. If the envelope is ejected, the progenitor will become a helium star and the subsequent supernova explosion may be of the Type Ib variety. If the binary components merge completely, the final outcome would be a single star with no trace (except for possible chemical anomalies) of the original secondary. If, during the merger, a significant amount of mass is added to the envelope, the final star may again be a blue supergiant (similar to the results of the accretion scenario), and the resulting supernova would belong to the same class as SN 1987A.In order to assess the importance of the various scenarios, we performed Monte Carlo simulations to estimate the frequencies of occurrence of the individual scenarios. We find that, because of a previous binary interaction, 15%-30% of all massive stars (with initial masses greater than or similar to 8 M.) become helium stars, and another approximately 5% of all massive stars end their lives as blue supergiants rather than as red supergiants.These results may be directly applicable to Type Ib supernovae. Our estimate for the frequency of helium stars is comparable to the observed frequency of Type Ib supernovae, and hence we expect that the explosions of helium stars in binaries account for a substantial fraction of all Type Ib events.Our calculations may also help to answer one of the major puzzles about SN 1987A, namely, the question of why the apparent progenitor (Sk -69-degrees 202) was a blue supergiant rather than a red one, as had been generally expected for the precursors of Type II supernovae. In addition, binary models for SN 1987A may provide plausible explanations for a variety of other anomalies of SN 1987A, ranging from the asymmetric expansion of the ejecta and the variability of the soft X-ray flux to the barium anomaly and the "mystery spot."

It is proposed that the progenitor of SN 1987A was the product of the merger of two binary components in a common envelope that formed as a result of a dynamically unstable mass-transfer phase. Two cases are considered, one in which the primary (presupernova) star is on the first red-giant branch, and one in which it is on the asymptotic giant branch, when the common-envelope phase commences. While in the first case the merger would always have been completed before the supernova event, it is possible in the second case that the supernova progenitor exploded while the stars were still in the process of spiraling into the center of the common envelope. It is shown how the latter scenario might lead to an exotic postsupernova binary with properties that could account for the 8 hr periodicity in the 2 kHz pulse frequency of the supernova pulsar (Kristian et al., 1989). It is also shown how both scenarios may explain all of the major observational features of this supernova event, including its most striking anomalies (particularly the blue color of the apparent progenitor Sk -69 deg 202).

The MIT High Resolution X-Ray Spectroscopy experiment on the AXAF, which will study physical conditions in celestial sources by means of detailed measurements of emission and absorption features in their spectra, involves two complementary dispersive instruments: Bragg Crystal Spectrometer (BCS) and High Energy Transmission Grating (HETG). This paper discusses the principles of operation of BCS and HETG and the results that will be obtained by these instruments. Measurements of individual line strengths obtained by the AXAF spectrometers will allow the application of plasma diagnostic techniques to a study of the detailed physical conditions in celestial objects, particularly in the optically thin plasma of supernova remnants, which is particularly well suited to the application of plasma diagnostics.

In the 1960s, novel and increasingly powerful observational techniques opened up the field of high-energy astrophysics. Cosmology started to become an empirical science, and there was a resurgence in the study of general relativity. Martin Rees became a ...Read More

Since the discovery of cosmic X‐ray bursts in 1975, a wealth of observational information concerning this phenomenon has been obtained. X‐ray burst have rise times of ≲1 s, decay timescale of ∼3‐30 s, and intervals between bursts that are usually in the range of ∼104‐105 s. The burst spectra can often be well fitted by a blackbody spectrum from an emission region of maximum temperature ∼3×107 K and roughly constant size. If X‐ray burst sources are typically at distances of ∼10 kpc, as indicated by their concentration in the direction of the galactic center, then they have maximum luminosities of ∼1038 ergs s−1, total emitted energies of ∼1039 ergs per burst, and effective blackbody radii of ∼10 km. The observational properties of X‐ray burst sources have recently been reviewed by Lewin and Joss.1Thermonuclear flashes in the surface layers of accreting neutron stars are one of several mechanisms that have been proposed to account for X‐ray bursts2,3. Computations of the evolution of such surface layers4 demonstrate that under many circumstances, the helium‐burning shell is thermally unstable and should undergo flashes that result in the emission of X‐ray bursts from the neutron‐star photosphere. The calculated properties of these bursts are remarkably similar to those of bursts from most observed X‐ray burst sources.If the surface magnetic field of the neutron star is sufficiently strong to funnel the accretion flow onto the magnetic polar caps, then the rotation of the neutron star and its associated accretion pattern should result in the emission of periodic X‐ray pulsations. However, the magnetic funneling shold also alter the structure of the surface layers so as to tend to suppress thermonuclear flashes5. Hence, the apparent dichotomy between X‐ray burst sources and X‐ray pulsars may be readily understood.A number of theoretical and phenomenological problems remain to be resolved (see Lewin and Joss1 for a review). Among the theoretical issues currently under active investigation are the complex interactions between the helium‐burning and hydrogen‐burning shells6−10, the role of general relatistic effects11,12,10, and the conditions required to maintain the core of the neutron star in the thermal equilibrium13,10. Other considerations, such as violations of spherical symmetry and the role of dynamical effects in the outer surface layers, may also turn out to be important.

view Abstract Citations (48) References (20) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS 4U 1626-67 and the character of highly compact binary X-ray sources. Li, F. K. ; Joss, P. C. ; McClintock, J. E. ; Rappaport, S. ; Wright, E. L. Abstract We present the results of new observations of the 7.7 5 X-ray pulsar 4U 1626 - 67 with SAS 3. We confirm the presence of quasi-periodic oscillations in the X-ray intensity and demonstrate that these oscillations have a preferred time scale of ∼1 × 103 s but are not strictly periodic. We also place stringent new upper limits on orbital motion of the X-ray star for orbital periods between 10 5 and 7 hr. This evidence lends further support to a model for this source composed of a neutron star that is accreting matter from a low-mass main-sequence dwarf or degenerate dwarf companion in a highly compact binary system. If this picture is correct, it is likely that the time scale of the quasi-periodic oscillations is directly related to the orbital period. We discuss other properties of this model, which may be applicable to other galactic X-ray sources. In particular, we analyze the temporal evolution of X-ray luminosity, Lx, from such a system under the assumption that the mass transfer is driven by the decay of the orbit due to gravitational radiation, and we show that high luminosities (Lx ≳ 1036 ergs s-1) may well be attained during some phases of the binary evolution. Publication: The Astrophysical Journal Pub Date: September 1980 DOI: 10.1086/158268 Bibcode: 1980ApJ...240..628L Keywords: Binary Stars; Neutron Stars; Pulsars; Stellar Evolution; X Ray Sources; Dwarf Stars; Orbits; Power Spectra; Stellar Models; Stellar Motions; Stellar Radiation; Astrophysics full text sources ADS | data products SIMBAD (1)