Showing papers by "Smithsonian Astrophysical Observatory published in 1998"
TL;DR: In this paper, a color-magnitude diagram for the faint end of the Pleiades main sequence, including only stars for which high signal-to-noise spectra in the region of the lithium 6708 A absorption line have been obtained.
Abstract: We have obtained intermediate resolution spectra of 11 candidate brown dwarf members of the Pleiades open cluster using the Keck II telescope and Low-Resolution Imaging Spectrograph. Our primary goal was to determine the location of the "lithium depletion edge" in the Pleiades and hence to derive a precise age for the cluster. All but one of our 11 program objects have radial velocities appropriate for Pleiades members, have moderately strong Hα emission, and have spectral types M6-M8.5 as expected from their (R - I)C colors. We have constructed a color-magnitude diagram for the faint end of the Pleiades main sequence, including only stars for which high signal-to-noise spectra in the region of the lithium 6708 A absorption line have been obtained. These data allow us to accurately determine the Pleiades single-star lithium depletion edge at IC0 = 17.80, (R - I)C0 = 2.20, spectral type M6.5. By reference to theoretical evolutionary models, this converts fairly directly into an age for the Pleiades of τ = 125 Myr. This is significantly older than the age that is normally quoted, but does agree with some other recent estimates.
484 citations
TL;DR: The red quasar subsample was taken from the 1 Jy sample of flat-spectrum radio sources and is comprised of the 15 sources that are undetected on the POSS as discussed by the authors.
Abstract: We have searched for redshifted neutral hydrogen 21 cm absorption toward sources from the Stickel et al. "red quasar" subsample. The red quasar subsample is taken from the 1 Jy sample of flat-spectrum radio sources and is comprised of the 15 sources that are undetected on the POSS. Five of these red quasars have been searched for redshifted H I 21 cm absorption to optical depth levels of a few percent, and four show strong absorption, with neutral hydrogen column densities between 4 and 80 × 1018 × (Ts/f) cm-2. This 80% success rate for the red quasars compares to the much lower success rate of only 11% for detecting H I 21 cm absorption associated with optically selected Mg II absorption line systems. The large neutral hydrogen column densities seen toward the Stickel et al. red quasars provide circumstantial evidence supporting the dust-reddening hypothesis, as opposed to an intrinsically red spectrum for the AGN emission mechanism. The lower limits to rest-frame values of AV are between two and seven, leading to lower limits to the spin temperatures for the neutral hydrogen between 50 K and 1000 K, assuming a Galactic dust-to-gas ratio. We consider the question of biases in optically selected samples of quasars caused by dust obscuration. Overall, the data on the red quasar subsample support the models of Fall & Pei for dust obscuration by damped Lyα absorption line systems and suggest that (1) there may be a significant, but not dominant, population of quasars missing from optically selected samples because of dust obscuration, perhaps as many as 20% at the POSS limit for an optical sample with a redshift distribution similar to the 1 Jy flat-spectrum quasar sample, and (2) optically selected samples may miss about one-half the high column density quasar absorption line systems. The redshifted H I 21 cm absorption line detections presented herein are toward the sources 0108+388 at z = 0.6685, 0500+019 at z = 0.5846, and 1504+377 at z = 0.6733. No absorption is seen toward 2149+056 at z = 0.740 at a level below that seen for the three detections, although there is some uncertainty in this case as to the expected line redshift. In some systems, the absorbing gas is in the vicinity of the AGN, as either circumnuclear material or material in the general ISM of the AGN's host galaxy, as is probably the case for 0108+388 and 1504+377, and in other systems the absorption is by gas associated with galaxies cosmologically distributed along the line of sight to the quasar, as may be the case for 0500+019. The Westerbork Synthesis Radio Telescope spectrum of 1504+377 confirms the lack of H I 21 cm absorption associated with the narrow molecular absorption line system at z = 0.67150.
113 citations
TL;DR: In this article, a series of concentric arcs superposed on the two reflection nebular lobes, similar to those observed in AFGL 2688, was found to be a common phenomenon in the asymptotic giant branch (AGB) phase.
Abstract: We report Hubble Space Telescope Wide Field Planetary Camera 2 imaging of the bipolar proto-planetary nebula IRAS 17150-3224. We find a series of concentric arcs superposed on the two reflection nebular lobes, similar to those observed in AFGL 2688. This suggests that periodic mass-loss enhancement is a common phenomenon in the asymptotic giant branch (AGB) phase. The near circular shape of the arcs suggests that mass loss is spherically symmetric during the AGB phase and that the bipolar morphology develops only after the end of the AGB phase. The possible evolution in morphology from proto-planetary nebulae to planetary nebulae is discussed.
110 citations
TL;DR: In this paper, the exact Mie solution for the differential scattering cross section was used to calculate the halo properties of small-angle scatterings of X-rays by interstellar dust particles, and it was shown that the results were consistent with scattering from a mixture of bare refractory silicate and carbon grains.
Abstract: Small-angle scatterings of X-rays by interstellar dust particles create halos around X-ray sources. The halo intensity and its projected radial distribution around a source can provide important information on the spatial distribution of the dust along the line of sight to the source and on the physical properties of the scattering dust particles. Halos around X-ray point sources have been used by several authors to infer that the scattering dust particles are fluffy in nature, consisting of aggregates of smaller refractory particles with vacuum occupying a significant fraction of their volume. The nature and morphology of interstellar dust particles has recently gained new importance, since fluffy, composite dust particles have been suggested as a possible solution to the interstellar carbon "crisis." This crisis results from the discrepancy between the abundance of carbon in the interstellar medium available for creating dust and the significantly larger amount of carbon that must be in dust in order to account for the UV-optical interstellar extinction in the diffuse ISM. Previous studies of X-ray scattering have used the Rayleigh-Gans (RG) approximation to the differential scattering cross section to calculate halo properties. However, the validity of the RG approximation fails for energies below 1 keV. We use the exact Mie solution for the differential scattering cross section and find that, for these energies, the scattering becomes much less efficient than is predicted by the RG approximation. Furthermore, the effects of K and L shell absorption by atoms in the dust become important. The net effect is that the RG approximation systematically and substantially overestimates the intensity of the halo below 1 keV, relative to the Mie solution result. In particular, Mathis and coworkers used the weaker than expected halo intensity observed around Nova Cygni 1992 to conclude that interstellar dust must be fluffy. Using the Mie solution to the scattering intensity and including the effects of absorption, we find that, contrary to the conclusion of Mathis and coworkers, the halo around Nova Cygni 1992 does not require interstellar dust grains to be fluffy in nature and that the data are consistent with scattering from a mixture of bare refractory silicate and carbon grains as well.
99 citations
TL;DR: In this paper, Bracewell et al. reported mid-infrared observations using two co-mounted telescopes of the Multiple Mirror Telescope that demonstrate the viability of this technique.
Abstract: Extrasolar planets must be imaged directly if their nature is to be better understood. But this will be difficult, as the bright light from the parent star (or rather its diffracted halo in the imaging apparatus) can easily overwhelm nearby faint sources. Bracewell has proposed1 a way of selectively removing starlight before detection, by superposing the light from two telescopes so that the stellar wavefronts interfere destructively. Such a ‘nulling’ interferometer could be used in space to search for extrasolar Earth-like planets through their thermal emission and to determine through spectroscopic analysis if they possess the atmospheric signatures of life2,3,4. Here we report mid-infrared observations using two co-mounted telescopes of the Multiple Mirror Telescope that demonstrate the viability of this technique. Images of unresolved stars are seen to disappear almost completely, while light from a nearby source as close as 0.2 arcsec remains, as shown by images of Betelgeuse. With this star cancelled, there remains the thermal image of its surrounding, small dust nebula. In the future, larger ground-based interferometers that correct for atmospheric distortions (using adaptive optics) should achieve better cancellation, allowing direct detection of warm, Jupiter-size planets and faint zodiacal dust around other nearby stars5.
99 citations
TL;DR: In this paper, the authors present theoretical models of H I Lyα emission from the extended solar corona, taking into account various plasma kinetic effects that induce departures from Maxwellian velocity distributions.
Abstract: This paper presents theoretical models of H I Lyα emission from the extended solar corona, taking into account various plasma kinetic effects that induce departures from Maxwellian velocity distributions. Such phenomena as suprathermal tails, strong temperature anisotropies, and skewed or double-peaked distributions have been observed in the solar wind, and UV spectroscopy is beginning to be able to detect their signatures in the corona. For resonantly scattered lines like H I Lyα, most of the physics is contained in the frequency-dependent redistribution function. The dependence of this function on the local plasma parameters is presented analytically for four different non-Maxwellian distributions, and optically thin line intensities are computed for a representative model of the fast solar wind. Isotropic power-law κ tails in the velocity distribution should be detectable between 2 and 5 A from line center. Although existing observations that appear to have broad tails do not resemble those arising from κ-distributions, their presence is still possible. Anisotropic bi-Maxwellian distributions affect line profile shapes and total intensities via both their parallel and perpendicular components, and it is important to include an accurate description of the photon redistribution for large anisotropies. Skewness caused by a Chapman-Enskog expansion in the conductive heat flux is detectable as a unique non-Gaussian profile shape, but other types of collisionally beamed or skewed distributions may not noticeably affect the emission lines.
95 citations
TL;DR: In this paper, the authors incorporated the newest ISO results on the mid-infrared spectral energy distribution (SED) of galaxies into models for the number counts and redshift distributions of MIR surveys.
Abstract: In this work we incorporate the newest ISO results on the mid-infrared spectral-energy-distributions (MIR SEDs) of galaxies into models for the number counts and redshift distributions of MIR surveys.
94 citations
TL;DR: In this paper, the authors present a numerical simulation of the interaction between two line dipoles through magnetic reconnection in the lower solar atmosphere, a process believed to be the origin of many manifestations of solar activity.
Abstract: We present a numerical simulation of the interaction between two line dipoles through magnetic reconnection in the lower solar atmosphere, a process believed to be the origin of many manifestations of solar activity. This work differs from previous studies in that the field is sheared asymmetrically and that the dipoles have markedly unequal field strengths. This calculation already yielded one key discovery, denoted reconnection driven current filamentation, as described in a previous Astrophysical Journal letter. In this paper we focus on the chromospheric and coronal dynamics resulting from the shear-driven reconnection of unequal dipoles, discuss the important implications for chromospheric eruptions, compare our calculation with high-resolution Normal Incidence X-Ray Telescope observations of a surge, and contrast our results with the predictions of "fast reconnection" models.
87 citations
TL;DR: In this paper, the authors examined the validity of using observables such as X-ray activity and the amplitude of photometric variations as indicators of angular momentum loss and reported the discovery of cool, slow rotators with high amplitudes of variation.
Abstract: We present results of photometric monitoring campaigns of G, K, and M dwarfs in the Pleiades carried out in 1994-1996. We have determined rotation periods for 18 stars in this cluster. In this paper we examine the validity of using observables such as X-ray activity and the amplitude of photometric variations as indicators of angular momentum loss. We report the discovery of cool, slow rotators with high amplitudes of variation. This contradicts previous conclusions about the use of amplitudes as an alternate diagnostic of the saturation of angular momentum loss. We show that the X-ray data can be used as observational indicators of mass-dependent saturation in the angular momentum loss proposed on theoretical grounds.
84 citations
TL;DR: In this paper, the authors reported the discovery of two irregular satellites orbiting Uranus at distances of several hundred planetary radii, and the satellites are extremely faint (apparent red magnitudes mR = 20.4 and 21.9), with estimated radii of only 60 and 30 km.
Abstract: The systems of satellites and rings surrounding the giant planets in the Solar System have remarkably similar architectures1. Closest to each planet are rings with associated moonlets, then larger ‘regular’ satellites on nearly circular orbits close to the planet's equatorial plane, and finally one or more distant, small ‘irregular’ satellites on highly elliptical or inclined orbits. Hitherto, the only departure from this broad classification scheme was the satellite system around Uranus, in which no irregular satellites had been found2. Here we report the discovery of two satellites orbiting Uranus at distances of several hundred planetary radii. These satellites have inclined, retrograde orbits of moderate eccentricity that clearly identify them as irregular. The satellites are extremely faint (apparent red magnitudes mR = 20.4 and 21.9), with estimated radii of only 60 and 30 km. Both moons are unusually red in colour, suggesting a link between these objects—which were presumably captured by Uranus early in the Solar System's history—and other recently discovered bodies3 orbiting in the outer Solar System.
74 citations
TL;DR: In this article, a coronal helmet streamer was observed on 1996 July 25 using instruments aboard two solar spacecraft, the UVCS on board Solar and Heliospheric Observatory (SOHO) and the Soft X-Ray Telescope (SXT) on board Yohkoh.
Abstract: We analyze a coronal helmet streamer observed on 1996 July 25 using instruments aboard two solar spacecraft, the Ultraviolet Coronagraph Spectrometer (UVCS) on board Solar and Heliospheric Observatory (SOHO) and the Soft X-Ray Telescope (SXT) on board Yohkoh. We derive temperatures and electron densities at 1.15 R☉ from SXT/Yohkoh observations. At this height, the streamer temperature is about log T (K) = 6.28 ± 0.05, and the electron density is about log ne(cm-3) = 8.09 ± 0.26, while at 1.5 R☉ a temperature of log T (K) = 6.2 and a density of log ne(cm-3) = 7.1 are obtained by UVCS/SOHO. Within the measurement uncertainty this suggests a constant temperature from the base of the streamer to 1.5 R☉. Electron density measurements suggest that the gas in the streamer core is close to hydrostatic equilibrium. Comparison with potential field models for the magnetic field suggests a plasma β larger than 1 in the closed-field region in the streamer. In deriving electron densities and temperatures from the SXT/Yohkoh data, we include the effects of abundance anomalies on the SXT filter response. We use the elemental abundances derived from the UVCS/SOHO observations to estimate the first ionization potential and gravitational settling effects. We then give the set of abundances for the solar corona, which agrees with our observations. In addition, we analyzed the SXT data from 6 consecutive days. We found that from 1996 July 22 to July 27, the physical properties of the streamer are nearly constant. We conclude that we may be observing the same loop system over 6 days.
TL;DR: In this paper, the authors derived the properties of the circumstellar wind and a proposed circumstellar "disk" by fitting the observed spectral energy distribution and the V-band image by a two-dimensional radiation transfer model.
Abstract: The bipolar proto-planetary nebula IRAS 17441-2411 (the "Silkworm Nebula") was observed with the Hubble Space Telescope. By fitting the observed spectral energy distribution and the V-band image by a two-dimensional radiation transfer model, we derive the properties of the circumstellar wind and a proposed circumstellar "disk." We suggest that bipolar structures are probably more common than actually observed, because objects with similar intrinsic structures may simply appear stellar because of their orientation in the sky.
TL;DR: In this paper, the authors obtained relatively deep imaging in two colors, with limiting magnitudes of V ~ 22.5 and Ic ~ 21, of about one square degree of the Pleiades open cluster.
Abstract: We have obtained relatively deep imaging in two colors, with limiting magnitudes of V ~ 22.5 and Ic ~ 21, of about one square degree of the Pleiades open cluster. Our primary goal was to identify new candidate brown dwarf members of this ~100 Myr-old open cluster. In the process, we have also obtained V and Ic photometry for a large number of brighter proper-motion members of the cluster for which only photographic photometry had been available up until now. We identify six candidate objects whose locations in a V versus (V - I)c color-magnitude diagram are consistent with their being Pleiades members near or below the hydrogen-burning mass limit. We have obtained near-infrared photometry for several of these brown dwarf candidates, as well as for a selection of previously identified very low mass Pleiades members, and we use this new photometry to help determine whether the objects identified in the visual are indeed Pleiades members or not. Finally, we have obtained a moderate-resolution spectrum of one of the new brown dwarf candidates with the Keck LRIS spectrograph. The spectrum shows that the star has a spectral type of M7, which is compatible with its photometric colors, and that it has an Hα equivalent width of about 14 A and a marginally detected lithium 6708 A absorption equivalent width of about 0.4 A. These spectroscopic characteristics suggest that this object (MHObd1) is indeed a Pleiades member with a mass at or slightly above the hydrogen-burning mass limit.
TL;DR: In this article, the spectral energy distribution model was used to constrain the inner disk radius and the degree of Naring in a star-plus-disk system, and the parameters controlling the disk shape, size, and mass were chosen to be within theoretically and observationally derived limits.
Abstract: We describe model calculations of optical and near-infrared scattered light images expected from class II T Tauri starsEthe star-plus-disk systems. The parameters controlling the disk shape, size, and mass are chosen to be within theoretically and observationally derived limits. We restrict our models to nearly edge-on disks, since for lower inclinations the central starlight is many orders of magnitude greater than the radiation scattered in the disk. In addition to model Nux images, we calculate spectral energy dis- tributions for pole-on viewing using approximations for Nat and Nared disks. We -nd that direct imaging of edge-on disks can provide only estimates of the scale height at large distances from the central star and an estimate of the disk mass. The images are rather insensitive to the inner disk radius and the degree of Naring, provided the scale height is -xed at large radii. Spectral energy distribution modeling is required to constrain the inner disk radius and the degree of Naring. We apply our models to recent Hubble Space Telescope (HST ) images of HH 30 IRS and investigate whether the scattered light images could have been produced by starlight scattering o† the walls of jet- carved cavities in infalling envelopes associated with the embedded class I sources. We -nd that while the class I infalling envelope plus cavity model qualitatively resembles the HST images, the spatial extent of the model images is too large. Edge-on disk models appear to provide better -ts to the data and enable us to determine the disk scale height at large distances from the central star. However, the assumption of axisymmetry and uniform illumination is clearly inadequate for this variable source. In addition to producing Nux images, our radiation-transfer simulations predict the spatially resolved polar- ization structure of HH 30. We have also performed K-band simulations for HH 30 in anticipation of high-resolution infrared imaging polarimetry. Subject headings: circumstellar matter E radiative transfer E stars: individual (HH 30) E stars: preEmain-sequence
TL;DR: In this article, infrared images of the Helix Nebula centered at 6.9?m and 15?m (LW3 filter) were obtained with ISOCAM on board the Infrared Space Observatory (ISO).
Abstract: We report infrared images of the Helix Nebula centered at 6.9 ?m (LW2 filter) and 15 ?m (LW3 filter) obtained with ISOCAM on board the Infrared Space Observatory (ISO). Three fields were also measured using the ISOCAM circular variable filter (CVF). The CVF data show that the 5-16.6 ?m spectrum is dominated by the pure (v=0-0) rotational lines of molecular hydrogen from the S(7) to the S(2) transitions. The strong S(5) H2 line accounts for most of the emission detected in the LW2 filter. The only atomic lines detected are [Ne II] 12.81 ?m and [Ar III] 8.99 ?m, which are weak, and [Ne III] 15.55 ?m, which is strong and accounts for most of the emission in the LW3 filter. No emission bands or continuum of small dust particles are detected despite the carbon richness of the Helix Nebula. The H2 emission traces the individual cometary globules of the molecular envelope of the nebula, whereas the [Ne III] emission is distributed along this envelope toward the inner regions of the ionized cavity. The intensities of H2 rotational lines are accurately predicted using a rotational temperature of 900?50 K and column densities of ~3?10 -->18 cm-2. The total luminosity in the H2 lines ~4 L? (6% of the star luminosity) is much higher than predicted for photodissociation regions. The significant absence of mid-infrared dust features indicates that in this evolved planetary nebula, the molecular-sized dust particles might have been destroyed by the exposure to the radiation field from the central hot star.
TL;DR: By means of millimeter and submillimeter imaging, a massive protostellar object that coincides with a methanol maser and is not detectable in the continuum at centimeter wavelengths was identified in this article.
Abstract: By means of millimeter and submillimeter imaging, we have identified a massive protostellar object that coincides with a methanol maser and is not detectable in the continuum at centimeter wavelengths Located 84" (15 pc) southeast of the ultracompact H II (UCHII) region G3426+015, the new object G3424+013MM was discovered in a wide-field 350 μm continuum image obtained with the Submillimeter High Angular Resolution Camera (SHARC) at the Caltech Submillimeter Observatory (CSO) Interferometric imaging at 2257 and 1107 GHz continuum has determined more precisely the position and angular diameter (2"0, or 7600 AU) of the object No source was detected at that position in 12-37 μm imaging or 10 and 20 μm photometry Our observations are consistent with a cool dust core with temperature ~50 K, total gas mass 100 M_☉, and total luminosity in the range of 1600-6300 L_☉ Considering the high luminosity and lack of compact radio continuum emission, we conclude that this core probably contains a deeply embedded proto-B star
TL;DR: The Space Infrared Telescope Facility (SIRTF) is a one-meter class observatory for infrared astronomy that will be launched into high earth orbit by NASA in the late 1990s as mentioned in this paper.
Abstract: The Space Infrared Telescope Facility (SIRTF) is a one-meter class observatory for infrared astronomy that will be launched into high earth orbit by NASA in the late 1990’s. SIRTF’s three focal plane instruments will permit imaging and spectroscopy over most of the infrared spectrum with sensitivities of 100 to 10,000 times their predecessors. This paper briefly reviews SIRTF’s capabilities, science objectives, and current status.
TL;DR: In this paper, Narayanan et al. identify the dominant sources powering the outflows and argue that the results support a multiple outflow model (Narayanan & Walker) as opposed to a quadrupolar outflow scenario.
Abstract: New infrared images of Cep A East are presented that show two regions of shock-excited line emission from separate bipolar flows. We identify the dominant sources powering the outflows and argue that the results support a multiple outflow model (Narayanan & Walker) as opposed to a quadrupolar outflow scenario. The images include near-infrared broadband ( [2.158 μm], L'' [3.81 μm], and M' [4.67 μm]) and spectral line ([Fe II] emission line at 1.644 μm and H2 1-0 S[1] line at 2.122 μm) observations, as well as continuum emission, at 1.644 μm and 2.122 μm. Considering our data and other results, we present a unified, self-consistent picture of the disk and shock structure. The northern emission region appears to be the result of the ablation of a dense molecular clump (coincident with HW 6) in the path of a diverting jet from YSO HW 2 and subsequent multiple bow shocks with prompt entrainment arising from the interaction of the jet with the molecular cloud Cep A-2. The southern line emission region near HW 7 resembles the artillery shell bow shocks found in Orion and is most likely a J-type shock caused by a jet from another YSO, possibly HW 3(d)ii.
TL;DR: In this paper, the photometric and spectroscopic data indicate that the CFHT-Pl-18 system is likely a member of the Pleiades cluster, but a final confirmation will have to wait until lithium can be detected.
Abstract: Hubble Space Telescope Near-Infrared Camera and Multiobject Spectrometer (NICMOS) observations of six brown dwarf candidates in the Pleiades open cluster are presented. One of them, namely CFHT-Pl-18, is clearly resolved as a binary with an angular separation of 033. The very low density of contaminating background stars in our images and the photometry of the components support that this system is a physical binary rather than a chance projection. All of the available photometric and spectroscopic data indicate that the CFHT-Pl-18 system is likely a member of the Pleiades cluster, but a final confirmation will have to wait until lithium can be detected. Assuming cluster membership, we compare our NICMOS photometry with evolutionary models and find that the inclusion of the effects of dust grains is necessary for fitting the data. We estimate that the masses of the components are about 0.045 and 0.035 M☉. The binary system has a projected separation of 42 AU (for a distance of 125 pc) that is common among stellar binaries.
TL;DR: In this paper, a comparison of the radial velocities of two groups of Praesepe stars (a dF-dK sample and a dM sample) was carried out to find a possible explanation for this result.
Abstract: Randich and Schmitt [1995, A&A 298, 115] found that the coronal activity of solar-type and low mass stars in Praesepe is significantly lower than that of stars in the Hyades cluster. We have carried out several tests in order to find a possible explanation for this result. We have measured radial velocities of two groups of Praesepe stars (a dF-dK sample and a dM sample) and have measured H$\alpha$ as a chromospheric activity index for the dM sample. We conclude that the Praesepe catalog used in the X-ray analysis does not contain a significant number of non-members. The comparison of the H$\alpha$ equivalent widths for the M dwarfs in Praesepe with those in the Hyades indicates that, at least for stars in this mass range, the Praesepe stars are as active or more active than their Hyades counterparts. We have also analyzed a few ROSAT PSPC pointings of Praesepe in order to obtain a new and independent estimate of the X-ray luminosities and upper limits for a small sample of Praesepe members concluding that the small differences between the old and new upper limits are not large enough to explain the dichotomy in the X-ray properties of Praesepe and the Hyades. Therefore, our examination of the available data does not provide a clear reason to explain why the X-ray luminosity functions of the two clusters are different. Part of the explanation could be found in the binaries. Speculatively, these clusters could have different orbital period distributions, with more short period binaries among the Hyades, which would show larger coronal activity.
TL;DR: In this article, the authors investigated the effects on the scattered light images of a T T Tauri star-plus-disk system of nonaxisymmetric illumination of the circumstellar disk by hot starspots.
Abstract: The magnetic accretion model as applied to T Tauri stars predicts the formation of hot spots or rings on the stellar surface where accreting disk material impacts the stellar surface at or near the magnetic poles. When the magnetic poles are not aligned with the stellar rotation axis, or the hot spots are nonuniform, brightness variations arise as the hot spots rotate into and out of view. We have investigated the effects on the scattered light images of a T Tauri star-plus-disk system of nonaxisymmetric illumination of the circumstellar disk by hot starspots. We find that changes in the scattered light pattern occur during a stellar rotation period. These changes are observable with current high-resolution imagery and provide a further observational test of the magnetic accretion theory. The observed brightening and dimming of HH 30's scattered light disk may be explained by such a model, although further synoptic observations are required to confirm this.
TL;DR: In this paper, the mass of the Cepheid U Aql was measured using the high-resolution spectrograph on the Hubble Space Telescope (HST) to determine the orbital velocity amplitude.
Abstract: We have obtained medium-resolution spectra (λ/Δλ ~ 20,000) of the hot binary companion to the classical Cepheid U Aql with the Goddard High Resolution Spectrograph on the Hubble Space Telescope (HST). These have been used to determine the orbital velocity amplitude. Combining this with the orbital velocity amplitude of the Cepheid from the ground-based orbit and the mass of the companion inferred from its spectral type, we measure a mass of the Cepheid of 5.1 ± 0.7 M☉. We discuss the full sample of Cepheids for which we have determined masses with HST (S Mus, V350 Sgr, Y Car, and U Aql) and also SU Cyg (mass from IUE). The HST masses are in agreement with the luminosities predicted by recent evolutionary tracks with moderate overshoot. This comparison, however, may be altered by reassessment of Cepheid distances based on Hipparcos parallaxes.
01 Jan 1998
TL;DR: The Proceedings from the Ninth Cambridge Workshop on Cool Stars, Stellar Systems, and the Sun held in Florence, Italy from 3 to 6 October 1995 were published by the Astronomical Society of the Pacific in their Conference Series, Volume 109 in 1996 as discussed by the authors.
Abstract: This Grant was used to publish the Proceedings from the Ninth Cambridge Workshop on Cool Stars, Stellar Systems, and the Sun held in Florence, Italy from 3 to 6 October 1995. The Proceedings were published by the Astronomical Society of the Pacific in their Conference Series, Volume 109 in 1996. This volume was edited by Roberto Pallavicini and Andrea K. Dupree. A copy of the title page and the Table of Contents of the volume is appended.
TL;DR: In this paper, the authors used the ROSAT Position Sensitive Proportional Counter (PSPC) to obtain a 13 ks exposure and the ROSat HRI to obtain an exposure of 42.5 ks in the S0 galaxy NGC 1380.
Abstract: We used the ROSAT Position Sensitive Proportional Counter (PSPC) to obtain a 13 ks exposure and the ROSAT HRI to obtain a 42.5 ks exposure of the faint S0 galaxy NGC 1380. We show that the PSPC spectrum can be modeled as a thermal spectrum with kT ~ 0.5 keV, which is consistent with emission from a hot interstellar medium. We do not detect any emission from a hard component; the 90% upper limit is consistent with the trend for the scaled-up luminosities of X-ray–emitting point sources. The radial profiles from the PSPC and HRI data support the spectral fitting, because each profile is more sharply concentrated than the optical light (which reflects the stellar distribution), in contrast to other E/S0 galaxies. The PSPC radial profile is consistent with emission from a point source, because of the larger point-spread function; the HRI profile reveals the presence of extended hot gas. We also extend the LX-LB plane, described by Matsumoto et al. in the range 10.8 < log LB < 11.3, to log LB ~ 10.5. We extend this plane separately for the hard discrete and the soft thermal components and show that the scaling relation for the hard component continues to at least log LB ~ 10.5.
01 Jan 1998
TL;DR: The Propulsive Small Expendable Deployer System (ProSEDS) space experiment as mentioned in this paper demonstrated the use of an electrodynamic tether propulsion system, which is capable of orbit raising, lowering and inclination changes.
Abstract: The Propulsive Small Expendable Deployer System (ProSEDS) space experiment will demonstrate the use of an electrodynamic tether propulsion system. The flight experiment is a precursor to the more ambitious electrodynamic tether upper stage demonstration mission which will be capable of orbit raising, lowering and inclination changes-all using electrodynamic thrust. ProSEDS which is planned to fly in 2000, will use the flight proven Small Expendable Deployer System (SEDS) to deploy a tether (5km bare wire plus 15 km spectra) from a Delta II upper stage to achieve approximately 0.4N drag thrust, thus demonstrating deorbit thrust. The experiment will use a predominantly 'bare' tether for current collection in lieu of endmass collector and insulated tether approach used on previous missions. ProSEDS will utilize tether-generated current to provide limited spacecraft power. In addition to the use of this technology to provide orbit transfer of payloads and upper stages from LEO to higher orbits it may also be an attractive option for future missions to Jupiter and any other planetary body with a magnetosphere.
TL;DR: In this article, images of the cluster of embedded young stars designated NGC 7538 IRS 1, 2, and 3 were presented in the J (1.25 ) and H(1.64 ) broad bands; the 1.64?m line of [Fe II]; the 2.12?m and 2.25?m lines of H2; and the 3.29?m unidentified dust feature.
Abstract: ?????We present images of the cluster of embedded young stars designated NGC 7538 IRS 1, 2, and 3 in the J (1.25 ?m), H (1.64 ?m), and K (2.16 ?m) broad bands; the 1.64 ?m line of [Fe II]; the 2.12 ?m and 2.25 ?m lines of H2; the 2.17 ?m Brackett ? H-recombination line; and the 3.29 ?m unidentified dust feature. The NGC 7538 IRS 2 compact H II region, as seen in Brackett ? line emission, possesses a cometary morphology. We find evidence for a shell-like distribution of [Fe II] line emission along the periphery of the compact H II region. This morphology and the observed intensity ratio I([Fe II] 1.64 ?m)/I(Br?) imply that the [Fe II] line emission emanates from shocked stellar wind material. We observe arcs of H2 line emission immediately exterior to the [Fe II] line emission shell and suggest that these arcs delineate molecular cloud material shocked via collision with the stellar wind. We observe larger arcs with centers of curvature displaced from each other and IRS 2, in broad-band-continuum, H2-line, and 3.29 ?m feature emission, and suggest that these arcs represent molecular gas (either shocked or UV-excited) and dust swept up during distinct episodes of stellar mass ejection. We submit that the morphological and energetic evidence favors a stellar wind bow shock interpretation, in which the motion of the central O star with respect to the surrounding molecular cloud, in conjunction with the star's high-velocity stellar wind and episodic mass ejection, gives rise to the observed general cometary morphology and circum-H II region emission.
TL;DR: In this paper, the authors employ a Monte Carlo radiation transfer code to investigate the multi-wavelength photopolarimetric variability arising from a spotted T Tauri star surrounded by a dusty circumstellar disk.
Abstract: We employ a Monte Carlo radiation transfer code to investigate the multi- wavelength photopolarimetric variability arising from a spotted T Tauri star surrounded by a dusty circumstellar disk. Our aim is to assess the ability of the magnetic accretion model to explain the observed photopolarimetric variability of classical T Tauri stars, and to identify potentially useful observational diagnostics of T Tauri star/disk/spot parameters. We model a range of spot sizes, spot latitudes, inner disk truncation radii, and system inclination angles, as well as multiple disk and spot geometries. We find that the amplitude, morphology, and wavelength dependence of the photopolarimetric variability predicted by our models are generally consistent with existing observations; a flared disk geometry is required to reproduce the largest observed polarization levels and variations. Our models can further explain stochastic polarimetric variability if unsteady accretion is invoked, in which case irregular -- but correlated -- photometric variability is predicted, in agreement with observations.
Potentially useful observational diagnostics of system parameters of interest are discussed. We also investigate the reliability of modeling spot parameters via analytic fits to multi-band photometric variations.
TL;DR: In this paper, a sample of relatively bright, short-and intermediate-period (P=1-10d) Type II Cepheids in the Galactic field have been observed spectroscopically with an intermediate-resolution (λ/Δλ=11000) spectrograph.
Abstract: A sample of relatively bright, short- and intermediate-period (P=1–10 d) Type II Cepheids in the Galactic field have been observed spectroscopically with an intermediate-resolution (λ/Δλ=11000) spectrograph. The wavelength region was 6500–6700 A, including the Hα line and some photospheric iron lines. The signal-to-noise ratio (S/N) was usually between 50 and 100, depending on weather conditions and the brightness of target stars. Radial velocities were determined by cross-correlating the Cepheid spectra with those of selected IAU velocity standard stars having F–G spectral types. The internal error of the velocity determination process was calculated to be about 1 km s−1. Hα emission and strong line splitting were observed in BL Her during the expansion phase, but no similar phenomenon was detected in any other stars in this programme, except for AU Peg which has an unusual Hα line showing a P Cygni-like profile. The velocity curve agrees well with recent CORAVEL measurements. The velocity gradients in Cepheid atmospheres are studied using the Hα minus metallic velocities. Similar data are collected from the literature. It seems that having large velocity differences (vHα−vmetal>40 km s−1) is a characteristic feature of the very short-period (P 10 d) Cepheids. Between these period regions the Cepheid atmospheres exhibit smaller velocity differences. Most of the Type II Cepheids observed in the present study fall into this latter category. There might be a tendency for classical Cepheids of intermediate period to have larger maximum velocity differences.
01 Mar 1998
TL;DR: In this paper, a single-stage system for transferring payloads from LEO to GEO was proposed, and the first detailed analyses of this proposal, its extension to a two stage system and the likely implementation of the operational system were presented.
Abstract: The projected traffic to geostationary earth orbit (GEO) is expected to increase over the next few decades. At the same time, the cost of delivering payloads from the Earth's surface to low earth orbit (LEO) is projected to decrease, thanks in part to the Reusable Launch Vehicle (RLV). A comparable reduction in the cost of delivering payloads from LEO to GEO is sought. The use of in-space tethers, eliminating the requirement for traditional chemical upper stages and thereby reducing the launch mass, has been identified as such an alternative. Spinning tethers are excellent kinetic energy storage devices for providing the large delta vee's required for LEO to GEO transfer. A single-stage system for transferring payloads from LEO to GEO was proposed some years ago. The study results presented here contain the first detailed analyses of this proposal, its extension to a two-stage system, and the likely implementation of the operational system.
19 Nov 1998
TL;DR: The Smithsonian Astrophysical Observatory uses the HRMA X-ray Detection System (HXDS) to calibrate the High-Resolution Mirror Assembly of the Advanced Xray Astrophysics Facility AXAF as discussed by the authors.
Abstract: The Smithsonian Astrophysical Observatory uses the HRMA X- ray Detection System (HXDS) to calibrate the High-Resolution Mirror Assembly of the Advanced X-ray Astrophysics Facility AXAF. Apart from two high-purity-germanium solid-state detectors (SSDs) with good energy resolution and very high efficiency at higher energies, the detection system comprises seven flow proportional counters (FPCs) and one microchannel-plate High-Speed Imager. For the lower energy range, the FPCs are more appropriate. They have been calibrated at the radiometry laboratory of the Physikalisch- Technische Bundesandstalt, using the electron storage ring BESSY. For the determination of the absolute quantum efficiency two methods have been applied. First, the detector response was measured in the lower energy range 0.1 keV to 1.7 keV at several discrete energies using monochromatized radiation. The absolute photon flux has been determined by Si n-on-p photodiodes, calibrated against a cryogenic electrical-substitution radiometer used as primary detector source standard BESSY, which can be calculated very accurately. Combining both measurements the determination of the detection efficiency over the entire desired spectral range was possible with a typical relative uncertainty around 1 percent to 2 percent in the central energy range.