Smithsonian Astrophysical Observatory

About: Smithsonian Astrophysical Observatory is a facility organization based out in Cambridge, Massachusetts, United States. It is known for research contribution in the topics: Galaxy & Stars. The organization has 1665 authors who have published 3622 publications receiving 132183 citations. The organization is also known as: SAO.

THE DISCOVERY OF BINARY WHITE DWARFS THAT WILL MERGE WITHIN 500 Myr

Abstract: We present radial velocity observations of four extremely low-mass (0.2 M {sub sun}) white dwarfs (WDs). All four stars show peak-to-peak radial velocity variations of 540-710 km s{sup -1} with 1.0-5.9 hr periods. The optical photometry rules out main-sequence companions. In addition, no millisecond pulsar companions are detected in radio observations. Thus, the invisible companions are most likely WDs. Two of the systems are the shortest period binary WDs yet discovered. Due to the loss of angular momentum through gravitational radiation, three of the systems will merge within 500 Myr. The remaining system will merge within a Hubble time. The mass functions for three of the systems imply companions more massive than 0.46 M {sub sun}; thus, those are carbon/oxygen core WDs. The unknown inclination angles prohibit a definitive conclusion about the future of these systems. However, the chance of a supernova Ia event is only 1%-5%. These systems are likely to form single R Coronae Borealis stars, providing evidence for a WD + WD merger mechanism for these unusual objects. One of the systems, SDSS J105353.89+520031.0, has a 70% chance of having a low-mass WD companion. This system will probably form a single helium-enriched subdwarf O star. All fourmore » WD systems have unusual mass ratios of {<=}0.2-0.8 that may also lead to the formation of AM CVn systems.« less

The rotational excitation of carbon monoxide by hydrogen atom impact

Abstract: A quantal study is carried out of the rotational excitation of carbon monoxide in collision with hydrogen atoms. The interaction potential at short range is constructed semi-empirically and joined to the Buckingham potential at long range. The close-coupling formulation is used to assess the reliability of the fixed-nuclei approximation, which is developed in terms of the adiabatic theory of electron-molecule scattering. The applicability of two simplified close-coupling formulations, introduced by Rabitz and by McGuire & Kouri, is examined. We found that the fixed nuclei method is unpromising at low energies and time-consuming at high energies. The two simplified close-coupling methods are capable of providing results of useful accuracy. The similarity in formulation of the fixed-nuclei and one of the close-coupling methods, both of which are body-frame treatments, and the differences in their results show that the rotational degree of freedom must be treated rigorously. The method of Rabitz is economical, and we adopted it to calculate the energy dependent rotation excitation cross sections for the scattering of H + CO. The results are presented in the form of Maxwellian-averaged rate coefficients in the temperature range of 5-150K. The efficiency with which the rotational levels of molecules are excited by impact with atomic and molecular hydrogen is a significant parameter in the quantitative interpretation of the physical conditions in which molecular absorption and emission occurs in interstellar clouds. The rotational excitation cross-sections are critical to the determination of the thermal balance of the clouds and possibly to their evolution towards the formation of protostars. Because of radiation trapping, collisions in which the rotational quantum number changes by more than unity are particularly important. For a rigid rotator model, there exists a rigorous close-coupling formulation (Arthurs & Dalgarno i960), the application of which is largely restricted to light molecules. In this paper, we use it to assess the reliability of the adiabatic approximation (Chase 1956) that has been successful in describing electron-molecule collisions

The Solar Probe Cup on the Parker Solar Probe

Abstract: The Solar Probe Cup (SPC) is a Faraday Cup instrument onboard NASA's Parker Solar Probe (PSP) spacecraft designed to make rapid measurements of thermal coronal and solar wind plasma. The spacecraft is in a heliocentric orbit that takes it closer to the Sun than any previous spacecraft, allowing measurements to be made where the coronal and solar wind plasma is being heated and accelerated. The SPC instrument was designed to be pointed directly at the Sun at all times, allowing the solar wind (which is flowing primarily radially away from the Sun) to be measured throughout the orbit. The instrument is capable of measuring solar wind ions with an energy/charge between 100 V and 6000 V (protons with speeds from $139-1072~km~s^{-1})$. It also measures electrons with an energy between 100 V and 1500 V. SPC has been designed to have a wide dynamic range that is capable of measuring protons and alpha particles at the closest perihelion (9.86 solar radii from the center of the Sun) and out to 0.25 AU. Initial observations from the first orbit of PSP indicate that the instrument is functioning well.

Variations on debris disks. ii. icy planet formation as a function of the bulk properties and initial sizes of planetesimals

Abstract: We describe comprehensive calculations of the formation of icy planets and debris disks at 30-150 AU around 1-3 M {sub sun} stars. Disks composed of large, strong planetesimals produce more massive planets than disks composed of small, weak planetesimals. The maximum radius of icy planets ranges from {approx}1500 km to 11,500 km. The formation rate of 1000 km objects-{sup P}lutos{sup -}is a useful proxy for the efficiency of icy planet formation. Plutos form more efficiently in massive disks, in disks with small planetesimals, and in disks with a range of planetesimal sizes. Although Plutos form throughout massive disks, Pluto production is usually concentrated in the inner disk. Despite the large number of Plutos produced in many calculations, icy planet formation is inefficient. At the end of the main sequence lifetime of the central star, Plutos contain less than 10% of the initial mass in solid material. This conclusion is independent of the initial mass in the disk or the properties of the planetesimals. Debris disk formation coincides with the formation of planetary systems containing Plutos. As Plutos form, they stir leftover planetesimals to large velocities. A cascade of collisions then grinds the leftovers to dust, forming an observable debris disk.more » In disks with small ({approx}<1-10 km) planetesimals, collisional cascades produce luminous debris disks with maximum luminosity {approx}10{sup -2} times the stellar luminosity. Disks with larger planetesimals produce debris disks with maximum luminosity {approx}5 x 10{sup -4} (10 km) to 5 x 10{sup -5} (100 km) times the stellar luminosity. Following peak luminosity, the evolution of the debris disk emission is roughly a power law, f {proportional_to} t {sup -n} with n{approx} 0.6-0.8. Observations of debris disks around A-type and G-type stars strongly favor models with small planetesimals. In these models, our predictions for the time evolution and detection frequency of debris disks agree with published observations. We suggest several critical observations that can test key features of our calculations.« less

Performance expectation versus reality

Abstract: The AXAF (Advanced X-ray Astrophysics Facility) high resolution mirror assembly (HRMA) now is complete and has been tested at the NASA Marshall Space Flight Center (MSFC) X-ray Calibration Facility (XRCF). The surface and alignment properties of the mirror were thoroughly measured before the x-ray test, which allowed accurate performance predictions to be performed. The preliminary analysis of the measured x-ray image distributions for all energies tested show excellent agreement with predictions made before the beginning of the test. There is a discrepancy between the measured and predicted effective areas; this typically is less than 5%, and is less than 13% for all energies measured. We present evidence that this discrepancy is due to uncertainties in the calibration of the test instrumentation, and therefore can be expected to be reduced when results from further instrument calibration tests now in progress are incorporated into the analysis. We predict that 65 - 80% (depending upon energy) of the flux from an imaged point source will be contained on a one arc second diameter aperture in flight. We expect the HRMA to more than fulfill the requirements necessary to achieve the AXAF scientific objectives.© (1997) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.