Table of contents

We compute the reionization histories of hydrogen and helium caused by the ionizing radiation fields produced by stars and quasars. For the quasars we use a model based on halo-merger rates that reproduces all known properties of the quasar luminosity function at high redshifts. The less constrained properties of the ionizing radiation produced by stars are modeled with two free parameters: (i) a transition redshift, z_tran, above which the stellar population is dominated by massive, zero-metallicity stars and below which it is dominated by a Scalo mass function; and (ii) the product of the escape fraction of stellar ionizing photons from their host galaxies and the star formation efficiency, f_escf_*. We constrain the allowed range of these free parameters at high redshifts on the basis of the lack of the H I Gunn-Peterson trough at z ≲ 6 and the upper limit on the total intergalactic optical depth for electron scattering, τ_es < 0.18, from recent cosmic microwave background (CMB) experiments. We find that quasars ionize helium by a redshift z ~ 4, but cannot reionize hydrogen by themselves before z ~ 6. A major fraction of the allowed combinations of f_escf_* and z_tran leads to an early peak in the ionized fraction because of the presence of metal-free stars at high redshifts. This sometimes results in two reionization epochs, namely, an early H II or He III overlap phase followed by recombination and a second overlap phase. Even if early overlap is not achieved, the peak in the visibility function for scattering of the CMB often coincides with the early ionization phase rather than with the actual reionization epoch. Consequently, τ_es does not correspond directly to the reionization redshift. We generically find values of τ_es ≳ 7%, which should be detectable by the MAP satellite.

This paper is devoted to the methods of the determination of cosmological parameters and distortions of ionization history from recent cosmic microwave background (CMB) observations. We show that the more complex models of kinetics of recombination with a few "missing" parameters describing the recombination process provide better agreement between the measured and the expected characteristics of the CMB anisotropy. In particular, we consider external sources of resonance and ionizing radiation and a model with strong clustering of the baryonic component. These factors can constrain the estimates of the cosmological parameters usually discussed. We also demonstrate that the measurements of the CMB polarization can improve these estimates and, for the precision expected from the Planck mission, allow the discrimination of a wide class of models.

We present a model-independent, spherically symmetric density estimator to be used in the cross-correlation of imaging catalogs with objects of known redshift. The estimator is a simple modification of the usual projected density estimator, with weightings that produce a spherical aperture rather than a cylindrical one.

We have created mock Sunyaev-Zel'dovich effect (SZE) surveys of galaxy clusters using high-resolution N-body simulations. To the pure surveys, we add "noise" contributions appropriate to instrument and primary cosmic microwave background anisotropies. Applying various cluster finding strategies to these mock surveys, we generate catalogs that can be compared to the known positions and masses of the clusters in the simulations. We thus show that the completeness and efficiency that can be achieved depend strongly on the frequency coverage, noise, and beam characteristics of the instruments, as well as on the candidate threshold. We study the effects of matched filtering techniques on completeness and bias. We suggest a gentler filtering method than matched filtering in single-frequency analyses. We summarize the complications that arise when analyzing the SZE signal at a single frequency and assess the limitations of such an analysis. Our results suggest that some sophistication is required when searching for "clusters" within an SZE map.

We propose two new methods for measuring tangential peculiar velocities of rich clusters of galaxies. Our first method is based on weak gravitational lensing and takes advantage of the differing images of background galaxies caused by moving and stationary gravitational potentials. Our second method is based on measuring relative frequency shifts between multiple images of a single strongly lensed background galaxy. We illustrate this method using the example of galaxy cluster CL 0024+1654.

An Abell cluster, A2152, was recently found to be forming a cluster-cluster system with another, more distant cluster whose core is almost perfectly aligned with that of A2152. We discuss the detectability of microlensing events where a single star in the source cluster behind A2152 is extremely magnified by an intracluster compact object in A2152. We show that a search with an 8 m class telescope with a wide field of view, such as the Subaru/Suprime-Cam, can probe intracluster compact objects with a wide mass range of m_co ~ 10^-5 to 10¹⁰M_☉, including ranges that have not yet been constrained by any past observations. We expect that the event rate is more biased than the foreground cluster (A2152), which would be a unique signature of microlensing, making this experiment particularly powerful. The sensitivity of this experiment for the mass fraction of compact objects would be 1%-10% in the total dark matter of the cluster, which is roughly constant against m_co, with a reasonable telescope time for large telescopes (~10 nights). Therefore, any compact objects in this mass range can be detected or rejected as the dominant component of the dark matter. About 10 events are expected if 20% of the cluster mass is in a form of compact objects with M ~ 1 M_☉, as claimed by the MACHO collaboration for the Milky Way halo. Other possibly detectable targets include intracluster stars stripped by galaxy interactions, and hypothetical very massive black holes (M ≳ 100 M_☉) produced as remnants of the first-generation stars, which might be responsible for the recently reported excess of the cosmic infrared background radiation that seems impossible to explain by normal galactic light.

We present rest-frame, R-band, galaxy luminosity function measurements for three different redshift ranges: 0.5 ≤ z ≤ 0.75, 0.75 ≤ z ≤ 1.0, and 1.0 ≤ z ≤ 1.5. Our measurements are based on photometric redshifts for ~3000 H-band-selected galaxies with apparent magnitudes 17 ≤ H ≤ 20 from the Las Campanas Infrared Survey. We show that our photometric redshifts are accurate with an rms dispersion between the photometric and spectroscopic redshifts of σ_z/(1 + z) ≈ 0.08. Using galaxies identified in the Hubble Deep Field-South and Chandra Deep Field-South regions, we find, respectively, that 7.3% ± 0.2% and 16.7% ± 0.4% of the H ≤ 20 galaxies are at z ≥ 1. We first demonstrate that the systematic uncertainty inherent in the luminosity function measurements because of uncertainties in photometric redshifts is nonnegligible and therefore must be accounted for. We then develop a technique to correct for this systematic error by incorporating the redshift error functions of individual galaxies in the luminosity function analysis. The redshift error functions account for the non-Gaussian characteristics of photometric redshift uncertainties. They are the products of a convolution between the corresponding redshift likelihood functions of individual galaxies and a Gaussian distribution function that characterizes template-mismatch variance. We demonstrate, based on a Monte Carlo simulation, that we are able to completely recover the bright end of the intrinsic galaxy luminosity function using this technique. Finally, we calculate the luminosity function separately for the total H-band-selected sample and for a subsample of early-type galaxies that have a best-fit spectral type of E/S0 or Sab from the photometric redshift analysis. The primary results of this analysis are (1) the galaxy luminosity functions are consistent with a Schechter form, (2) the evolution of the comoving luminosity density ℓ of H-band-selected galaxies is characterized by Δ log ℓ/Δ log(1 + z) = 0.6 ± 0.1 at rest frame 6800 Å, and (3) ℓ_R for color-selected, early-type galaxies exhibits moderate evolution from z ~ 1.5 to ~0.3. Specifically, ℓ_R for these red galaxies brighter than 1.0 (1.6) L_* could decrease with increasing redshift by at most a factor of 3 (6) over this redshift range after removing possible stellar brightening according to the most extreme stellar evolution scenario.

We selected 115 extremely red objects (EROs) from deep Hubble Space Telescope (HST) WFPC2 archive data combined with ground-based K-band images, with (F814W-K_s) ≥ 4, K-band signal-to-noise ratio ≥5, and a median limiting K_s magnitude of ~18.7, over a corresponding area of 228 arcmin², for a morphological study of the ERO galaxy population. The survey covered a total of ~409 arcmin² over 77 separate WFPC2 fields. This is the first complete sample of bright EROs with high-resolution HST morphologies. From a visual morphological classification, we find that 30% ± 5% of our (F814W-K_s) ≥ 4 selected sample have morphologies consistent with a pure bulge or bulge-dominated galaxy (equivalent to E/S0), while disks comprise 64% ± 7% of the sample. Only 6% of the EROs remained unclassifiable. Mergers or strongly interacting systems, which includes sources from both classes, make up 17% ± 4% of the full sample. The quantitative Medium Deep Survey profile fitting is consistent with these results. These results highlight the complex nature of optical/near-IR color selected EROs. The dominant component of our sample is composed of disks, not spheroids or strongly interacting systems like HR 10. Using Bruzual & Charlot spectral energy distribution models, we investigated population differences in EROs selected by their (I-K) versus (R-K) colors and found that I-band based surveys preferentially select systems with prolonged star formation. Real differences in the surface densities of EROs in R-band and I-band based survey may reflect this color selection effect, complicating the comparisons between and interpretations on the nature of the ERO population. We conclude that only a small fraction of EROs at z ~ 1-2 could be passively evolving elliptical galaxies formed at high redshift through a "monolithic collapse" mechanism. For the majority of EROs, even if most of their stellar mass is already in place at z ~ 1, interaction with the environment and accretion of gas still play important roles in their continuing evolution.

We present detailed numerical N-body simulations of the dynamical interaction of disk galaxies embedded in a spherical halo of dark matter and an orbiting satellite, in order to study the relationship between the kinematics and the stellar distribution of the disk. The satellite is held on a fixed, coplanar, eccentric (e = 1/3), or nearly circular (e = 0.02) orbit. It is shown that the density wave mode m = 1 has an average phase angle shift of about 30° from the positive tangential velocity wave mode m = 1 but presents greater values just after the pericentric satellite passage. It is also shown that the density m = 2 coincides (within 10° on average) with the positive tangential velocity wave mode m = 2. The pattern speeds obtained from the density and the three velocity component distributions are the same. The asymmetry parameter (A) derived from the stellar distribution agrees with the asymmetry parameter derived from the velocity field. The A(180°) mode is dominant in comparison to A(90°) when we consider the center of mass of the disk. However, this behavior is reversed when we calculate the asymmetry using the center of density of the disk or center of maximum luminosity.

I have assembled a diverse sample of galaxies from the literature with far-ultraviolet (FUV), optical, infrared (IR), and radio luminosities to explore the calibration of radio-derived and IR-derived star formation (SF) rates and the origin of the radio-IR correlation. By comparing the 8-1000 μm IR, which samples dust-reprocessed starlight, with direct stellar FUV emission, I show that the IR traces most of the SF in luminous ~L_* galaxies but traces only a small fraction of the SF in faint ~0.01L_* galaxies. If radio emission were a perfect SF rate indicator, this effect would cause easily detectable curvature in the radio-IR correlation. Yet, the radio-IR correlation is nearly linear. This implies that the radio flux from low-luminosity galaxies is substantially suppressed, compared to brighter galaxies. This is naturally interpreted in terms of a decreasing efficiency of nonthermal radio emission in faint galaxies. Thus, the linearity of the radio-IR correlation is a conspiracy: both indicators underestimate the SF rate at low luminosities. SF rate calibrations that take into account this effect are presented, along with estimates of the random and systematic error associated with their use.

We present the results of a Chandra/Hubble Space Telescope (HST) study of the point sources of the Virgo Cluster giant elliptical galaxy NGC 4472. We identify 144 X-ray point sources outside the nuclear region, 72 of which are located within the HST fields. The optical data show 1102 sources, of which 829 have colors consistent with being globular clusters (with only four in the restricted central 10'' region). Thirty matches are found between the two lists—these are likely to be low-mass X-ray binaries (LMXBs) associated with globular clusters, while 42 of the X-ray sources have no optical counterparts to V ≲ 25 and I ≲ 24, indicating that they are likely to be predominantly LMXBs in the field star population with a small amount of possible contamination from background active galactic nuclei. Thus approximately 40% of the X-ray sources are in globular clusters and ~4% of the globular clusters contain X-ray sources. There is suggestive evidence that the X-ray sources located in blue globular clusters may have harder X-ray spectra than those located in red globular clusters. No statistically significant differences are found between the X-ray properties of the field sources and the X-ray properties of the sources located in globular clusters. This study, along with our previous result from Paper I in this series on the similarity of the spatial profile of the field LMXBs, globular cluster LMXBs, and the globular clusters themselves, suggests that a significant fraction of the observed LMXBs in the field may be created in a globular cluster then ejected into the field by stellar interactions; however, by comparing the results for NGC 4472 with those in several other galaxies, we find tentative evidence for a correlation between the globular cluster specific frequency and the fraction of LMXBs in globular clusters, a correlation that would be most easily explained if some of the field sources were generated in situ. We show that isolated accreting very massive black holes are unlikely to be observable with current X-ray instrumentation and that these sources hence do not contaminate the LMXB population. We discuss the possibility that several point sources near the nucleus and aligned perpendicularly to the radio jet may indicate the presence of a disk wind responsible for the low radiative efficiency observed in the nucleus of this source.

We report the results of the Chandra ACIS subarcsecond resolution X-ray observation of the archetypal merger radio galaxy NGC 1316 (Fornax A). We confirm the presence of fine substructures in the hot interstellar medium (ISM). Some of these are likely to result from interaction with the radio jets, while others may be related to a complex intermingling of different phases of the ISM. We detect a low-luminosity X-ray active galactic nucleus (AGN) with L_X = 5 × 10³⁹ ergs s^-1 (in 0.3-8 keV) and a Γ = 1.7 power-law energy spectrum. We also detect 81 point sources within the 25th magnitude isophotal ellipse of NGC 1316 (L_X in the range of 2 × 10³⁷ to 2 × 10³⁹ ergs s^-1), with hard (kT ~ 5 keV) X-ray spectra, typical of X-ray binaries, and a spatial radial distribution consistent with that of the optical (i.e., stellar) surface brightness. We derive the X-ray luminosity function (XLF) of these sources, correcting for the incompleteness at the faint end caused by the presence of the diffuse emission from the hot ISM in the central regions of NGC 1316 and by the widening of the Chandra point-spread functions at increasing distance from the aim point. With these corrections, the XLF is well reproduced by a single—unbroken—power law with a slope of -1.3 down to our threshold luminosity of ~3 × 10³⁷ ergs s^-1. The hot ISM has temperatures in the 0.5-0.6 keV range, its surface brightness distribution is more centrally concentrated than that of the point sources, and its temperature appears to decrease at larger radii. These properties suggest that the ISM may be subject to partial winds. Taking into account the spectral complexity of the ISM, and the presence of unresolved low luminosity X-ray sources (which can be inferred from the spectra), we constrain the metal abundance of the hot ISM to be Z = 0.25-1.3 Z_☉ (90% confidence).

We present an X-ray investigation of the elliptical galaxy NGC 507 in the Pisces cluster. We make use of archival ROSAT HRI and Chandra data, and of previously published PSPC data, to connect the large-scale structure of the halo to the core morphology. Our analysis, based on a bidimensional double beta model of the halo surface brightness, shows that the halo core (r < 2-3r_e) and the external halo (r > 3r_e) are characterized by different dynamical properties and suggests a different origin of the two components. The halo core has a complex morphology with a main X-ray emission peak, coincident with the center of the optical galaxy, and several secondary peaks. The spatial and spectral analysis of the central peak shows that this feature is produced by denser hot gas in the galaxy core. While both homogeneous and inhomogeneous cooling flow models predict a deposited mass exceeding the observed amount, our data support the scenario in which the gas is kinetically heated by stellar mass losses. Comparison with previously published studies suggests that the core of an X-ray extended galaxy is associated with the stellar distribution and has properties similar to the X-ray halos of compact galaxies. The secondary peaks are due instead to interactions between the radio-emitting plasma and the surrounding ISM, producing density fluctuations in the hot gas. We find that the energy input by the central radio source in the ISM is large enough to prevent gas cooling and may explain the failure of the cooling flow models. The total mass profile derived from the bidimensional model shows that a significant amount of dark matter is present at large radii. The dark halo extends on cluster scales and is likely associated with the whole cluster rather than with NGC 507. This structure is typical of many X-ray-bright early-type galaxies and may explain the spatial and spectral differences with X-ray compact galaxies largely debated in the literature. The large-scale surface brightness distribution is irregular and more extended in the northeast direction. The displacement of the cluster halo from the optical galaxy and the filamentary structures observed in the halo core further suggest that the galaxy may be slowly moving within the group potential. Finally, we found that ~20% of the sources detected by Kim & Fabbiano in the NGC 507 halo are due to point sources, while the nature of the remaining population is not clear. If associated with NGC 507, they could be either accreting binaries hosting a massive black hole or density clumps of the X-ray halo.

We present Space Telescope Imaging Spectrograph spectra of the Sbc spiral galaxy NGC 4041, which were used to map the velocity field of the gas in its nuclear region. We detect the presence of a compact (r ≃ 04 ≃ 40 pc), high surface brightness, rotating nuclear disk cospatial with a nuclear star cluster. The disk is characterized by a rotation curve with a peak-to-peak amplitude of ~40 km s^-1 and is systematically blueshifted by ~10-20 km s^-1 with respect to the galaxy systemic velocity. With the standard assumption of constant mass-to-light ratio and with the nuclear disk inclination taken from the outer disk, we find that a dark point mass of (1) × 10⁷M_☉ is needed to reproduce the observed rotation curve. However, the observed blueshift suggests the possibility that the nuclear disk could be dynamically decoupled. Following this line of reasoning, we relax the standard assumptions and find that the kinematical data can be accounted for by the stellar mass provided that either the central mass-to-light ratio is increased by a factor of ~2 or the inclination is allowed to vary. This model results in a 3 σ upper limit of 6 × 10⁶M_☉ on the mass of any nuclear black hole (BH). Overall, our analysis only allows us to set an upper limit of 2 × 10⁷M_☉ on the mass of the nuclear BH. If this upper limit is taken in conjunction with an estimated bulge B magnitude of -17.7 and with a central stellar velocity dispersion of ≃95 km s^-1, then these results are not inconsistent with both the M_BH-L_sph and the M_BH-σ_* correlations. Constraints on BH masses in spiral galaxies of types as late as Sbc are still very scarce; therefore, the present result adds an important new data point to our understanding of BH demography.

We report observations of the CO J = 7 → 6 transition toward the starburst nucleus of NGC 253. This is the highest excitation CO measurement in this source to date and allows an estimate of the molecular gas excitation conditions. Comparison of the CO line intensities with a large velocity gradient, escape probability model indicates that the bulk of the × 10⁷M_☉ of molecular gas in the central 180 pc is highly excited. A model with T ~ 120 K, n ~ 4.5 × 10⁴ cm^-3, is consistent with the observed CO intensities, as well as the rotational H₂ lines observed with the Infrared Space Observatory. The inferred mass of warm, dense molecular gas is 10-30 times the atomic gas mass as traced through its [C II] and [O I] line emission. This large mass ratio is inconsistent with photodissociation region models in which the gas is heated by far-UV starlight. It is also not likely that the gas is heated by shocks in outflows or cloud-cloud collisions. We conclude that the best mechanism for heating the gas is cosmic rays, which provide a natural means of uniformly heating the full volume of molecular clouds. With the tremendous supernova rate in the nucleus of NGC 253, the cosmic-ray heating rate is at least ~800 times greater than that in the Galaxy, more than sufficient to match the cooling observed in the CO lines.

We report the results of a spectroscopic investigation of the diffuse ionized gas (DIG) in M33, M81, and M51. All three galaxies have enhanced [S II]/Hα and [N II]/Hα ratios in the DIG compared to H II regions, in agreement with the predictions of photoionization models. The DIG in all three galaxies also has [O III]/Hβ ratios equal to or greater than that in neighboring H II regions. The [S II]/[N II] ratio does not vary with Hα surface brightness and is similar in both H II regions and DIG. We have tentatively detected the He I λ5876 line in the bright DIG near NGC 604, indicating a contribution from O7 and hotter stars to the ionizing spectrum in the DIG at that location. The DIG in M33 has lower [N II]/Hα ratios than M51 or the Galaxy, most likely because of low N abundance in M33. We used CLOUDY models to test the feasibility of two photoionization sources for the DIG: (1) the diluted ionizing continua of massive stars ("standard" models) and (2) the transmitted ionizing continua of density-bounded H II regions ("leaky H II region" models). The leaky H II region models produce higher [S II]/Hα, [N II]/Hα, and [O III]/Hβ ratios than standard DIG models for the same stellar temperatures. This alleviates somewhat the problem of producing high [O III]/Hβ ratios in the DIG. However, no single model can simultaneously reproduce all three line ratios. We conclude that either a combination of models is necessary or other ionization or heating mechanisms play a role in powering the DIG. Finally, we report on the spectrum of ionized gas associated with the companion to M51, NGC 5195. The gas in and around NGC 5195 has very high forbidden-line ratios and broad line widths (~ 500 km s^-1 FWHM), indicative of shock-ionized gas. At some locations the emission lines split into two velocity components, suggesting an expanding structure or outflow. The line ratios, morphology, and kinematics show that the interstellar medium in NGC 5195 is very disturbed.

Ultraviolet and optical images of the face-on spiral galaxies NGC 6753 and NGC 6782 reveal regions of strong ongoing star formation that are associated with structures traced by the old stellar populations. We use these images to construct NUV-(NUV-I₈₁₄) pixel color-magnitude diagrams (pCMDs) that reveal plumes of pixels with strongly varying near-ultraviolet (NUV) surface brightness and nearly constant I₈₁₄ surface brightness. The plumes correspond to sharply bounded radial ranges, with (NUV-I₈₁₄) at a given NUV surface brightness being bluer at larger radii. The plumes are parallel to both the reddening vector and simple model mixtures of young and old populations, thus neither reddening nor the fraction of the young population can produce the observed separation between the plumes. The images and radial surface brightness and color plots indicate that the separate plumes are caused by sharp declines in the surface densities of the old populations at radii corresponding to disk resonances. The maximum surface brightness of the NUV light remains essentially constant with radius, while the maximum I₈₁₄ surface brightness declines sharply with radius. A mid-ultraviolet (MUV) image of NGC 6782 shows emission from the nuclear ring. The distribution of points in an (MUV-NUV)-(NUV-I₈₁₄) pixel color-color diagram is broadly consistent with the simple mixture model but shows a residual trend that the bluest pixels in (MUV-NUV) are the reddest pixels in (NUV-I₈₁₄). This may be due to a combination of red continuum from late-type supergiants and [S III] emission lines associated with H II regions in active star-forming regions. We have shown that pixel mapping is a powerful tool for studying the distribution and strength of ongoing star formation in galaxies. Deep, multicolor imaging can extend this to studies of extinction and the ages and metallicities of composite stellar populations in nearby galaxies.

We use new Space Telescope Imaging Spectrograph long-slit ultraviolet spectroscopy of region A in the nearby starbursting galaxy He 2-10 to examine properties of both compact individual super-star clusters and diffuse intracluster regions. The four most luminous clusters in our slit formed coevally 4-5 Myr ago and have lower mass estimates of several × 10⁴ to a few × 10⁵M_☉. A fifth cluster is located ~200 pc away, and appears to be several megayears older. Extracted spectra of the diffuse light appear to be remarkably similar to those of the young clusters, showing P Cygni profiles from massive stars, and are very well fitted by instantaneous burst models of 4-5 Myr. Various mechanisms are considered to determine the nature of the underlying diffuse light in He 2-10. We rule out scattering of cluster light as the dominant source of the observed field. A scenario including the formation of a large number of lower mass, undetectable but coeval (compact) clusters can explain the field spectra, assuming that the clusters in the center of region A follow a power-law luminosity distribution, L^-α, with α = -2.0. This is somewhat steeper than the power-law slope of -1.7 measured by Johnson et al. for clusters in a larger portion of He 2-10 and implies very high projected (compact) cluster densities within region A. Rather, we favor a scenario where the dominant contribution to the field stellar population comes from more diffuse "scaled OB associations," which are not lacking in the most massive stars. Our observations establish that the far-UV field light in He 2-10 originates in a different stellar population than found for NGC 5253 and other nearby starbursts, where older, dissolving clusters appear to be the main mechanism responsible for creating the diffuse UV light.

Fourier decomposition is a well-established technique used in the study of stellar pulsation. However, the quality of reconstructed light curves using this method is reduced when the observed data have uneven phase coverage. We use simulated annealing techniques together with Fourier decomposition to improve the quality of the Fourier decomposition for many Optical Gravitational Lensing Experiment LMC fundamental-mode Cepheids. This method restricts the range that Fourier amplitudes can take. The ranges are specified by well-sampled Cepheids in the Galaxy and Magellanic Clouds. We also apply this method to reconstructing Cepheid light curves observed by the Hubble Space Telescope (HST). These typically consist of 12 V-band and four I-band points. We employ a direct Fourier fit to the 12 V-band points using the simulated annealing method mentioned above and explicitly derive and use Fourier interrelations to reconstruct the I-band light curve. We discuss advantages and drawbacks of this method when applied to HST Cepheid data over existing template methods. Application of this method to reconstruct the light curves of Cepheids observed in NGC 4258 shows that the derived Cepheid distance (μ₀ = 29.38 ± 0.06 mag, random error) is consistent with its geometrical distance (μ₀ = 29.28 ± 0.09 mag) derived from observations of its water maser.

We present results from a Chandra observation of the NGC 346 cluster, which is the ionizing source of N66, the most luminous H II region and the largest star formation region in the SMC. In the first part of this investigation, we have analyzed the X-ray properties of the cluster itself and the remarkable star HD 5980, but the field contains additional objects of interest. In total, 75 X-ray point sources were detected in the Chandra observation: this is 5 times the number of sources detected by previous X-ray surveys. We investigate here their characteristics in detail. Because of high foreground absorption, the sources possess rather high hardness ratios. Their cumulative luminosity function appears generally steeper than that for the rest of the SMC at higher luminosities. Their absorption columns suggest that most of the sources belong to NGC 346. Using Digitized Sky Survey data and new UBVRI imaging with the ESO 2.2 m telescope, we also discovered possible counterparts for 32 of these X-ray sources and estimated a B spectral type for a large number of these counterparts. This tends to suggest that most of the X-ray sources in the field are in fact X-ray binaries. Finally, some objects show X-ray and/or optical variability, with a need for further monitoring.

We present Far Ultraviolet Spectroscopic Explorer (FUSE) spectra for 25 O stars in the Large Magellanic Cloud. We analyze wind profiles for the resonance lines from C III, N III, S IV, P V, S VI, and O VI in the FUSE range using a "Sobolev with exact integration" method. In addition, the available data from either IUE or the Hubble Space Telescope for the resonance lines of Si IV, C IV, and N V are also modeled. Because several of the FUSE wind lines are unsaturated, the analysis provides meaningful optical depths (or equivalently, mass-loss rate times ionization fractions, q) as a function of normalized velocity, w = v/v_∞. Ratios of q (which are independent of ) determine the behavior of the relative ionization as a function of w. The results demonstrate that, with the exception of O VI in all stars and S VI in the later stars, the ionization in the winds shifts toward lower ionization stages at higher w (contrary to the expectations of the nebular approximation). This result implies that the dominant production mechanism for O VI and S VI in the late O stars differs from the other ions. Using the Vink et al. relationship between stellar parameters and mass-loss rate, we convert the measurements into mean ionization fractions for each ion, ⟨q_i⟩. Because the derived ion fractions never exceed unity, we conclude that the derived values of are not too small. However, q(P V), which is expected to be the dominant stage of ionization in some of these winds, is never greater than 0.20. This implies that either the calculated values of are too large, the assumed abundance of phosphorus is too large, or the winds are strongly clumped. The implications of each possibility are discussed. Correlations between the mean ion fractions and physical parameters such as T_eff, v_∞, and the mean wind density, ⟨ρ⟩, are examined. Two clear relationships emerge. First, as expected, the mean ionization fraction of the lower ions (C III, N III, Si IV, S IV) decreases with increasing T_eff. Second, the mean ion fraction of S VI in the latest stars and O VI in all stars increases with increasing v_∞. This reaffirms the notion, first introduced by Cassinelli & Olson, that O VI is produced nonradiatively. Finally, we discuss specific characteristics of three stars, BI 272, BI 208, and Sk -67°166. For BI 272, the ionic species present in its wind suggest it is much hotter than its available (uncertain) spectral type of O7: II-III:. In the case of BI 208, our inability to fit its observed profiles suggests that its wind is not spherically symmetric. For Sk -67°166, quantitative measurements of its line strengths confirm the suggestion by Walborn et al. that it is a nitrogen-rich O star.

Far Ultraviolet Spectroscopic Explorer (FUSE) spectra of 22 Galactic halo stars are studied to determine the amount of O VI in the Galactic halo between ~0.3 and ~10 kpc from the Galactic midplane. Strong O VI λ1031.93 absorption was detected toward 21 stars, and a reliable 3 σ upper limit was obtained toward HD 97991. The weaker member of the O VI doublet at 1037.62 Å could be studied toward only six stars because of stellar and interstellar blending problems. The measured logarithmic total column densities vary from 13.65 to 14.57 with ⟨log N⟩ = 14.17 ± 0.28 (1 σ). The observed columns are reasonably consistent with a patchy exponential O VI distribution with a midplane density of 1.7 × 10^-8 cm^-3 and scale height between 2.3 and 4 kpc. We do not see clear signs of strong high-velocity components in O VI absorption along the Galactic sight lines, which indicates the general absence of high-velocity O VI within 2-5 kpc of the Galactic midplane. This result is in marked contrast to the findings of Sembach et al., who reported high-velocity O VI absorption toward ~60% of the complete halo sight lines observed by FUSE. The line centroid velocities of the O VI absorption do not reflect Galactic rotation well. The O VI velocity dispersions range from 33 to 78 km s^-1, with an average of ⟨b⟩ = 45 ± 11 km s^-1 (1 σ). These values are much higher than the value of ~18 km s^-1 expected from thermal broadening for gas at T ~ 3 × 10⁵ K, the temperature at which O VI is expected to reach its peak abundance in collisional ionization equilibrium. Turbulence, inflow, and outflow must have an effect on the shape of the O VI profiles. Kinematical comparisons of O VI with Ar I reveal that eight of 21 sight lines are closely aligned in LSR velocity (|ΔV_LSR| ≤ 5 km s^-1), while nine of 21 exhibit significant velocity differences (|ΔV_LSR| ≥ 15 km s^-1). This dual behavior may indicate the presence of two different types of O VI-bearing environments toward the Galactic sight lines. The correlation between the H I and O VI intermediate-velocity absorption is poor. We could identify the known H I intermediate-velocity components in the Ar I absorption but not in the O VI absorption in most cases. Comparison of O VI with other highly ionized species suggests that the high ions are produced primarily by cooling hot gas in the Galactic fountain flow and that turbulent mixing also has a significant contribution. The role of turbulent mixing varies from negligible to dominant. It is most important toward sight lines that sample supernova remnants like Loops I and IV. The average N(C IV)/N(O VI) ratios for the nearby halo (this work) and complete halo (Savage et al.) are similar (~0.6), but the dispersion is larger in the sample of nearby halo sight lines. We are able to show that the O VI enhancement toward the Galactic center region that was observed in the FUSE survey of complete halo sight lines (Savage et al.) is likely associated with processes occurring near the Galactic center by comparing the observations toward the nearby HD 177566 sight line to those toward extragalactic targets.

Recent measurements of the cosmic-ray (CR) antiproton flux have been shown to challenge existing CR propagation models. It was shown that the reacceleration models designed to match secondary/primary nuclei ratios (e.g., boron/carbon) produce too few antiprotons. Matching both the secondary/primary nuclei ratio and the antiproton flux requires artificial breaks in the diffusion coefficient and the primary injection spectrum, suggesting the need for other approaches. In the present paper we discuss one possibility to overcome these difficulties. Using the measured antiproton flux and B/C ratio to fix the diffusion coefficient, we show that the spectra of primary nuclei as measured in the heliosphere may contain a fresh, local, "unprocessed" component at low energies, perhaps associated with the Local Bubble, thus decreasing the measured secondary/primary nuclei ratio. The independent evidence for supernova activity in the solar vicinity in the last few Myr supports this idea. The model reproduces antiprotons, B/C ratio, and elemental abundances up to Ni (Z ≤ 28). Calculated isotopic distributions of Be and B are in perfect agreement with CR data. The abundances of three "radioactive clock" isotopes in CRs, ¹⁰Be, ²⁶Al, and ³⁶Cl, are all consistent and indicate a halo size z_h ~ 4 kpc, based on the most accurate data taken by the ACE spacecraft.

We use the Gaussian fit results of Paper I to investigate the properties of interstellar H I in the solar neighborhood. The warm and cold neutral media (WNM and CNM) are physically distinct components. The CNM spin temperature histogram peaks at about 40 K; its median, weighted by column density, is 70 K. About 60% of all H I is WNM; there is no discernible change in this fraction at z = 0. At z = 0, we derive a volume filling fraction of about 0.50 for the WNM; this value is very rough. The upper limit WNM temperatures determined from line width range upward from ~500 K; a minimum of about 48% of the WNM lies in the thermally unstable region 500-5000 K. The WNM is a prominent constituent of the interstellar medium, and its properties depend on many factors, requiring global models that include all relevant energy sources, of which there are many. We use principal components analysis, together with a form of least-squares fitting that accounts for errors in both the independent and dependent parameters, to discuss the relationships among the four CNM Gaussian parameters. The spin temperature T_s and column density N(H I) are, approximately, the two most important eigenvectors; as such, they are sufficient, convenient, and physically meaningful primary parameters for describing CNM clouds. The Mach number of internal macroscopic motions for CNM clouds is typically about 3 so that they are strongly supersonic, but there are wide variations. We discuss the historical τ₀-T_s relationship in some detail and show that it has little physical meaning. We discuss CNM morphology using the CNM pressure known from UV stellar absorption lines. Knowing the pressure allows us to show that CNM structures cannot be isotropic but instead are sheetlike, with length-to-thickness aspect ratios ranging up to about 280. We present large-scale maps of two regions where CNM lies in very large "blobby sheets." We test the McKee/Ostriker model of the interstellar medium by explicitly modeling our data with CNM cores contained in WNM envelopes. This modeling scheme works quite well for many sources and also predicts the WNM filling factor reasonably well. However, it has several deficiencies.

We have carried out an investigation of the abundance of deuterium along two extended sight lines through the interstellar medium (ISM) of the Galactic disk. The data include Far Ultraviolet Spectroscopic Explorer (FUSE) observations of HD 195965 (B1 Ib) and HD 191877 (B0 V), as well as Space Telescope Imaging Spectrograph (STIS) observations of HD 195965. The distances to HD 195965 and HD 191877, derived from spectroscopic parallax, are 794 ± 200 and 2200 ± 550 pc, respectively, making these the longest Galactic disk sight lines in which deuterium has been investigated with FUSE. The FUSE spectra contain all of the H I Lyman series transitions (and the corresponding D transitions) except Lyα. The higher Lyman lines clearly show the presence of deuterium. We use a combination of curve-of-growth analyses and line profile fitting to determine the D I abundance toward each object. We also present column densities for O I and N I toward both stars, as well as H I measured from Lyα absorption in the STIS spectrum of HD 195965. Toward HD 195965 we find D/H = (0.85) × 10^-5 (2 σ), O/H = (6.61) × 10^-4, and N/H = (7.94) × 10^-5. Toward HD 191877 we find D/H = (0.78) × 10^-5 (2 σ) and N/H = (6.76) × 10^-5. The O I column density toward HD 191877 is very uncertain. Our preferred value gives O/H = (3.09) × 10^-4, but we cannot rule out O/H values as low as O/H = 1.86 × 10^-4, so the O/H value for this sight line should be taken with caution. The D/H ratios along these sight lines are lower than the average value of (1.52 ± 0.15) × 10^-5 (2 σ in the mean) found with FUSE for the local interstellar medium (~37-179 pc from the Sun). These observations lend support to earlier detections of variation in D/H over distances greater than a few hundred parsecs. The O/H ratio toward HD 195965 is supersolar. This star is part of an OB association, so there may be local enrichment by nearby massive stars. The D/H and O/H values measured along these sight lines support the expectation that the ISM is not well mixed on distances of ~1000 pc. These observations demonstrate that although D/H studies through Lyman absorption may become impractical at d > 2500 pc and log N(H ) > 21, D/H studies in the distance range from 500 to 2500 pc may be very useful for investigating mixing and chemical evolution in the ISM.

The effectiveness of three molecular hydrogen gas tracers—the CH ²Π_1/2, J = 1/2, F = 1-1 transition, the CO(1-0) transition, and the color excess, E(B-V)—is evaluated for the translucent molecular cloud MBM 16. The CH emission line and E(B-V) are well correlated with each other but not with the CO(1-0) emission line. It is likely that in translucent molecular clouds CO is not a linear tracer of the molecular hydrogen column density because, in these objects, the CO/H₂ ratio is undergoing rapid fluctuations as a function of the molecular hydrogen column density. For translucent and diffuse molecular clouds, and possibly for the translucent envelopes of giant molecular clouds, CH observations should be used to calibrate the CO to H₂ conversion factor.

The first observations of deuterium and oxygen in the local interstellar medium (LISM) obtained with the Far Ultraviolet Spectroscopic Explorer (FUSE) can be used to search for local abundance variations. While the very limited sample of these first data may be consistent with no variations, they do offer a hint of anticorrelated variations between D/H and O/H. If confirmed by more data (which will require independently determined, accurate H I column densities), these hints suggest that observations of interstellar gas within a few kiloparsecs of the solar neighborhood will reveal clear signs of the evolution of the abundance of deuterium from there and then (the big bang) to here and now (the local interstellar medium of the Galaxy).

Millimeter and mid-infrared observations have been made of the dense clumps of dust and gas and of young stellar objects (YSOs) associated with the bright, compact submillimeter source G79.3+0.3 P1 in the relatively nearby MSX infrared-dark cloud G79.3+0.3. The Gemini mid-infrared observations reported here indicate the presence of three YSOs within the cloud. BIMA 3 mm continuum observations show that the brightest of the YSOs is likely to be a Herbig Ae/Be star. High angular resolution molecular line observations suggest that a wind from this star may be triggering collapse in the adjacent molecular cloud. The submillimeter source G79.3+0.3 P1 itself does not contain infrared sources and may represent an earlier stage of star formation.

We report the detection of quiescent H₂ emission in the v = 1 → 0 S(1) line at 2.12183 μm in the circumstellar environment of two classical T Tauri stars, GG Tau A and LkCa 15, in high-resolution (R ≃ 60,000) spectra, bringing to four, including TW Hya and the weak-lined T Tauri star DoAr 21, the number of T Tauri stars showing such emission. The equivalent widths of the H₂ emission line lie in the range 0.02-0.10 Å, and in each case the central velocity of the emission line is centered at the star's systemic velocity. The line widths range from 9 to 14 km s^-1, in agreement with those expected from gas in Keplerian orbits in circumstellar disks surrounding K-type stars at distances ≥10 AU from the sources. UV fluorescence and X-ray heating are likely candidate mechanisms responsible for producing the observed emission. We present mass estimates from the measured line fluxes and show that the estimated masses are consistent with those expected from the possible mechanisms responsible for stimulating the observed emission. The high temperatures and low densities required for significant emission in the v = 1 → 0 S(1) line suggests that we have detected reservoirs of hot H₂ gas located in the low-density upper atmospheres of circumstellar disks of these stars.

We model the Class I source L1551 IRS 5, adopting a flattened infalling envelope surrounding a binary disk system and a circumbinary disk. With our composite model, we calculate self-consistently the spectral energy distribution of each component of the L1551 IRS 5 system, using additional constraints from recent observations by ISO, the water ice feature from observations with SpeX, the SCUBA extended spatial brightness distribution at submillimeter wavelengths, and the VLA spatial intensity distributions at 7 mm of the binary disks. We analyze the sensitivity of our results to the various parameters involved. Our results show that a flattened-envelope collapse model is required to explain simultaneously the large-scale fluxes and the water ice and silicate features. On the other hand, we find that the circumstellar disks are optically thick in the millimeter range and are inclined so that their outer parts hide the emission along the line of sight from their inner parts. We also find that these disks have lower mass accretion rates than the infall rate of the envelope.

We present deep images of the SN 1006 remnant (G327.6+14.6) with the ACIS instrument on the Chandra X-Ray Observatory. Two regions have been observed, the synchrotron-dominated northeast limb and the thermally dominated northwest limb, as well as a substantial portion of the interior of the remnant shell. The brightest features in the X-ray images correspond closely to radio features in the northeast and to Balmer-dominated filaments in the northwest. The spectra of the brighter filaments in the northeast are harder, with less prominent line emission than those in the northwest. In addition to highly elongated filaments, both images show enhanced clumps of emission well inside of the shock front that appear to be dominated by emission from oxygen. These probably arise from shock-heated ejecta, based on analysis of their X-ray spectra. We find no firm evidence for a halo of X-ray emission outside the shock to the northeast, as predicted by the Fermi shock-acceleration picture, in which relativistic electrons should be diffusing ahead of the shock. Our limits on upstream emission are less than 1.5% of the postshock levels in regions where the supernova remnant is brightest. This strongly suggests that the bright rims are flattened sheets nearly perpendicular to the plane of the sky and that the magnetic field strength jumps at the shock by a factor significantly larger than 4, as has been proposed if the shock puts significant energy into accelerating nonthermal ions. The spectra obtained of the northwest rim are all dominated by the helium-like ions of O, Ne, Mg, and Si expected from shocks with ionization (n_et) parameters of order 100 cm^-3 yr and electron temperatures of 0.5-1 keV, far lower than the postshock ion temperature implied by estimates of the shock speed obtained from the shape of the Hα line.

N66/NGC 346 is the largest and brightest H II region in the Small Magellanic Cloud and contains at least one known supernova remnant, SNR 0057-7226. Optical emission from the remnant is overwhelmed by the bright photoionized emission from the nebula, but the remnant has been detected by way of far-ultraviolet absorption (FUV) lines. Here we present data from the Far-Ultraviolet Spectroscopic Explorer (FUSE) satellite showing strong O VI and C III emission from a position at the edge of SNR 0057-7226. We also present high-resolution, long-slit Hα spectra across N66, showing high- and low-velocity emission corresponding closely to the X-ray boundaries of the supernova remnant. We use these FUV and optical data to determine the physical parameters of the shock and interaction geometry with N66. We find that ionizing photons from the many massive cluster stars nearby likely affect the ionization balance in the postshock gas, hindering the production of lower ionization and neutral species. We discuss the importance and inherent difficulty of searching for supernova remnants in or near bright H II regions and suggest that the far ultraviolet provides a viable means to discover and study such remnants.

We discuss the potential for the detection of gravitational waves from a rapidly spinning neutron star produced by supernova (SN) 1987A, taking the parameters claimed by Middleditch et al. at face value. Assuming that the dominant mechanism for spin-down is gravitational waves emitted by a freely precessing neutron star, it is possible to constrain the wobble angle, the effective moment of inertia of the precessing crust, and the crust cracking stress limit. Our analysis suggests that if the interpretation of the Middleditch data is correct, the compact remnant of SN 1987A may well provide a predictable source of gravitational waves well within the capabilities of the second Laser Interferometer Gravitational-Wave Observatory. The computational task required for the data analysis is within the capabilities of current computers if performed off-line and could be accomplished on-line using techniques such as demodulation and decimation.

We present the results of a large grid of synthetic spectra and compare them to early spectroscopic observations of SN 1993W. This supernova was discovered close to its explosion date and, at a recession velocity of 5400 km s^-1, is located in the Hubble flow. We focus here on two early spectra that were obtained approximately 5 and 9 days after explosion. We parameterize the outer supernova envelope as a power-law density profile in homologous expansion. In order to extract information on the value of the parameters, a large number of models was required. We show that very early spectra combined with detailed models can provide constraints on the value of the power-law index, the ratio of hydrogen to helium in the surface of the progenitor, the progenitor metallicity, and the amount of radioactive nickel mixed into the outer envelope of the supernova. The spectral fits reproduce the observed spectra exceedingly well. The spectral results combined with the early photometry predict that the explosion date was 4.7 ± 0.7 days before the first spectrum was obtained. The ability to obtain the metallicity from early spectra make SNe II-P attractive probes of chemical evolution in the universe, and by showing that we have the ability to pin down the parameters of the progenitor and mixing during the supernova explosion, it is likely to make SNe II-P useful cosmological distance indicators which are at the same time complementary to SNe Ia.

We present numerical simulations on the propagation of ultra-high-frequency protons with energies of 10^19.5-10²² eV in extragalactic magnetic fields over 1 Gpc. We use the Optical Redshift Survey (ORS) galaxy sample, which allows us to accurately quantify the contribution of nearby sources to the energy spectrum and the arrival distribution, as a source model. The sample is corrected, taking the selection effect and absence of galaxies in the zone of avoidance (|b| < 20^°) into account. We calculate three observable quantities—cosmic-ray spectrum, harmonic amplitude, and two-point correlation function—from our data of numerical simulations. With these quantities, we compare the results of our numerical calculations with the observation. We find that the arrival distribution of ultra-high-energy cosmic rays (UHECRs) becomes most isotropic as sources are restricted to to luminous galaxies (M_lim = -20.5). However, it is not isotropic enough to be consistent with the Akeno Giant Air Shower Array (AGASA) observation, even for M_lim = -20.5. In order to obtain a sufficiently isotropic arrival distribution, we randomly select sources more luminous than -20.5 mag from the ORS sample, which contribute to the observed cosmic-ray flux, and investigate the dependence of the results on their number. We show that three observable quantities, including the Greisen-Zatsepin-Kuz'min (GZK) cutoff of the energy spectrum, can be reproduced in the case that the number fraction ~10^-1.7 of the ORS galaxies more luminous than -20.5 mag is selected as UHECR sources. In terms of the source number density, this constraint corresponds to ~10^-6 Mpc^-3. However, since the mean number of sources within the GZK sphere is only ~0.5 in this case, the eight AGASA events above 10^20.0 eV, which do not together constitute clustered events, cannot be reproduced. On the other hand, if the cosmic-ray flux measured by the High Resolution Fly's Eye EHE Cosmic-Ray Detector (HiRes), which is consistent with the GZK cutoff, is correct and observational features about the arrival distribution of UHECRs are same as the AGASA, our source model can explain both the arrival distribution and the flux at the same time. Thus, we conclude that a large fraction of the eight AGASA events above 10²⁰ eV might originate in the top-down scenarios or that the cosmic-ray flux measured by the HiRes experiment might be better. We also discuss the origin of UHECRs below 10^20.0 eV through comparisons between the number density of astrophysical source candidates and our result (~10^-6 Mpc^-3).

A search was conducted for TeV γ-rays emitted from the direction of the ultra-high-energy cosmic ray detected by the Fly's Eye experiment with energy E ~ 3 × 10²⁰ eV. No enhancement was found at a level of 10^-10γ cm^-2 s^-1 for E > 350 GeV. A steady source of ultra-high-energy cosmic ray protons or photons would be expected to produce a γ-ray flux above this level. An upper limit was also set for the flux of TeV γ-rays from 3C 147, the most prominent active galactic nucleus in the error box.

Fast X-ray transients (FXTs) with timescales from seconds to hours have been seen by numerous space instruments. Because they occur at unpredictable locations, they are difficult to observe with narrow-field instruments. Only a few hundred have been detected, although their all-sky rate is in the tens of thousands per year. We have assembled archival data from Ariel-5, HEAO 1 (A-1 and A-2), WATCH, ROSAT, and Einstein to produce a global fluence-frequency relationship for these events. Fitting the log N- log S distribution over several orders of magnitude to a simple power law, we find a slope of -1.0. The sources of FXTs are undoubtedly heterogeneous, representing several physical phenomena; the α ~ -1 power law is an approximate result of the summation of these multiple sources. Two major contributions come from gamma-ray bursts and stellar flares. These two types of progenitors are distributed isotropically in the sky; however, their individual luminosity distributions are both flatter than the -3/2 power law that applies to uniformly distributed standard candles. Extrapolating from the BATSE catalog of gamma-ray bursts (GRBs), we find that the fraction of X-ray flashes that can be the X-ray counterparts of gamma-ray bursts is a function of fluence. The exact fraction of GRB-induced X-ray counterparts is sensitive to the R_X/γ distribution, which we estimate from available experimental measurements. Certainly, most FXTs are not counterparts of standard gamma-ray bursts. The fraction of FXTs from non-GRB sources, such as magnetic stars, is greatest for the faintest FXTs. Our understanding of the FXT phenomenon remains limited and would greatly benefit from a large, homogeneous data set, which requires a wide-field, sensitive instrument.

We propose a model to explain how a gamma-ray burst can take place days or years after a supernova explosion. Our model is based on the conversion of a pure hadronic star (neutron star) into a star made at least in part of deconfined quark matter. The conversion process can be delayed if the surface tension at the interface between hadronic and deconfined quark matter phases is taken into account. The nucleation time (i.e., the time to form a critical-size drop of quark matter) can be extremely long if the mass of the star is small. Via mass accretion the nucleation time can be dramatically reduced and the star is finally converted into the stable configuration. A huge amount of energy, on the order of 10⁵²-10⁵³ ergs, is released during the conversion process and can produce a powerful gamma-ray burst. The delay between the supernova explosion generating the metastable neutron star and the new collapse can explain the delay inferred in GRB 990705 and in GRB 011211.

We follow the nuclear reactions that occur in the accretion disks of stellar-mass black holes that are accreting at a very high rate, 0.01-1 M_☉ s^-1, as is realized in many current models for gamma-ray bursts (GRBs). The degree of neutronization in the disk is a sensitive function of the accretion rate, black hole mass, Kerr parameter, and disk viscosity. For high accretion rates and low viscosity, material arriving at the black hole will consist predominantly of neutrons. This degree of neutronization will have important implications for the dynamics of the GRB-producing jet and perhaps for the synthesis of the r-process. For lower accretion rates and high viscosity, as might be appropriate for the outer disk in the collapsar model, neutron-proton equality persists, allowing the possible synthesis of ⁵⁶Ni in the disk wind. ⁵⁶Ni must be present to make any optically bright Type I supernova and, in particular, those associated with GRBs.

We report the detection of high-frequency variability in the black hole X-ray transient XTE J1650-500. A quasi-periodic oscillation (QPO) was found at 250 Hz during a transition from the hard to the soft state. We also detected less coherent variability around 50 Hz that disappeared when the 250 Hz QPO showed up. There are indications that when the energy spectrum hardened the QPO frequency increased from ~110 to ~270 Hz, although the observed frequencies are also consistent with being 1 : 2 : 3 harmonics of each other. Interpreting the 250 Hz as the orbital frequency at the innermost stable orbit around a Schwarzschild black hole leads to a mass estimate of 8.2 M_☉. The spectral results by Miller et al., which suggest considerable black hole spin, would imply a higher mass.

We discuss the solution of an accretion disk when the black hole is chosen to be rotating. We study how the fluid properties are affected for different rotation parameters of the black hole. We know that no cosmic object is static in the universe. Here the effect of the rotation of the black hole on spacetime is considered, following an earlier work of the author, where the pseudo-Newtonian potential was prescribed for the Kerr geometry. We show that, with the inclusion of rotation of the black hole, the valid disk parameter region dramatically changes, and the disk becomes unstable. Also we discuss the possibility of shocks in accretion disks around rotating black holes. When the black hole is chosen to be rotating, the sonic locations of the accretion disk get shifted or disappear, making the disk unstable. To bring it into the stable situation, the angular momentum of the accreting matter has to be reduced/enhanced (for co/counterrotating disk) by means of some physical process.

We present the results of two extensive Rossi X-Ray Timing Explorer observations of large X-ray flaring episodes from the high-mass X-ray binary pulsar LMC X-4. Light curves during the flaring episodes comprise bright peaks embedded in relatively fainter regions, with complex patterns of recurrence and clustering of flares. We identify precursors preceding the flaring activity. Pulse profiles during the flares appear to be simple sinusoids, and pulsed fractions are proportional to the flare intensities. We fit Gaussian functions to flare peaks to estimate the mean FWHM to be ~68 s. Significant rapid aperiodic variability exists up to a few hertz during the flares, which is related to the appearance of narrow, spiky peaks in the light curves. While spectral fits and softness ratios show overall spectral softening as the flare intensity increases, the narrow, spiky peaks do not follow this trend. The mean fluence of the flare peaks is (3.1 ± 2.9) × 10⁴⁰ ergs in the 2.5-25 keV energy range, with its maximum at ~1.9 × 10⁴¹ ergs. The flare peak luminosity reaches up to (2.1 ± 0.2) × 10³⁹ ergs s^-1, far above the Eddington luminosity of a neutron star. We discuss possible origins of the flares, and we also propose that inhomogeneous accretion columns onto the neutron star polar caps are responsible for the observed properties.

The evolution of the magnetorotational instability (MRI) during the transition from outburst to quiescence in a dwarf nova disk is investigated using three-dimensional MHD simulations. The shearing box approximation is adopted for the analysis so that the efficiency of angular momentum transport is studied in a small local patch of the disk: this is usually referred as to a one-zone model. To take account of the low ionization fraction of the disk, the induction equation includes both ohmic dissipation and the Hall effect. We induce a transition from outburst to quiescence by an instantaneous decrease of the temperature. The evolution of the MRI during the transition is found to be very sensitive to the temperature of the quiescent disk. As long as the temperature is higher than a critical value of about 2000 K, MHD turbulence and angular momentum transport is sustained by the MRI. However, MHD turbulence dies away within an orbital time if the temperature falls below this critical value. In this case, the stress drops off by more than 2 orders of magnitude and is dominated by the Reynolds stress associated with the remnant motions from the outburst. The critical temperature depends slightly on the distance from the central star and the local density of the disk.

We study the slow neutron capture process (s-process) in asymptotic giant branch (AGB) stars using three different stellar evolutionary models computed for a 3 M_☉, solar metallicity star. First we investigate the formation and the efficiency of the main neutron source: the ¹³C(α, n)¹⁶O reaction that occurs in radiative conditions. A tiny region rich in ¹³C (the ¹³C pocket) is created by proton captures on the abundant ¹²C in the top layers of the He intershell, the zone between the H shell and the He shell. We parametrically vary the number of protons mixed from the envelope. For high local proton-to-¹²C number ratios, p/¹²C ≳ 0.3, most of the ¹³C nuclei produced are further converted by proton capture to ¹⁴N. Besides, ¹⁴N nuclei represent a major neutron poison. We find that a linear relationship exists between the amount of ¹²C in the He intershell and the maximum value of the time-integrated neutron flux. Then we generate detailed s-process calculations on the basis of stellar evolutionary models constructed with three different codes, all of them self-consistently finding the third dredge-up, although with different efficiency. One of the codes includes a mechanism at each convective boundary that simulates time-dependent hydrodynamic overshoot. This mechanism depends on a free parameter f and results in partial mixing beyond convective boundaries, the most efficient third dredge-up, and the formation of the ¹³C pocket. For the other two codes, an identical ¹³C pocket is introduced in the postprocessing nucleosynthesis calculations. The models typically produce enhancements of heavy elements of about 2 orders of magnitude in the He intershell and of up to 1 order of magnitude at the stellar surface, after dilution with the convective envelope, thus generally reproducing spectroscopic observations. The results of the cases without overshoot are remarkably similar, pointing out that the important uncertainty in s-process predictions is the ¹³C pocket and not the intrinsic differences among different codes when no overshoot mechanism is included. The code including hydrodynamic overshoot at each convective boundary finds that the He intershell convective zone driven by the recurrent thermal instabilities of the He shell (thermal pulses) penetrates the C-O core, producing a He intershell composition near that observed in H-deficient central stars of planetary nebulae. As a result of this intershell dredge-up, the neutron fluxes have a higher efficiency, both during the interpulse periods and within thermal pulses. The s-element distribution is pushed toward the heavier s-process elements, and large abundances of neutron-rich isotopes fed by branching points in the s-process path are produced. Several observational constraints are better matched by the models without overshoot. Our study needs to be extended to different masses and metallicities and in the space of the free overshoot parameter f.

Because the opacity of clouds in substellar mass object (SMO) atmospheres depends on the composition and distribution of particle sizes within the cloud, a credible cloud model is essential for accurately modeling SMO spectra and colors. We present a one-dimensional model of cloud particle formation and subsequent growth based on a consideration of basic cloud microphysics. We apply this microphysical cloud model to a set of synthetic brown dwarf atmospheres spanning a broad range of surface gravities and effective temperatures (g_surf = 1.78 × 10³-3 × 10⁵ cm s^-2 and T_eff = 600-1600 K) to obtain plausible particle sizes for several abundant species (Fe, Mg₂SiO₄, and Ca₂Al₂SiO₇). At the base of the clouds, where the particles are largest, the particle sizes thus computed range from ~5 to over 300 μm in radius over the full range of atmospheric conditions considered. We show that average particle sizes decrease significantly with increasing brown dwarf surface gravity. We also find that brown dwarfs with higher effective temperatures have characteristically larger cloud particles than those with lower effective temperatures. We therefore conclude that it is unrealistic when modeling SMO spectra to apply a single particle size distribution to the entire class of objects.

We have developed a general purpose dust radiative transfer code for an axisymmetric system, 2-DUST, motivated by the recent increasing availability of high-resolution images of circumstellar dust shells at various wavelengths. This code solves the equation of radiative transfer following the principle of long characteristic in a two-dimensional polar grid while considering a three-dimensional radiation field at each grid point. A solution is sought through an iterative scheme in which self-consistency of the solution is achieved by requiring a global luminosity constancy throughout the shell. The dust opacities are calculated through Mie theory from the given size distribution and optical properties of the dust grains. The main focus of the code is to obtain insights on (1) the global energetics of dust grains in the shell and (2) the two-dimensional projected morphologies that are strongly dependent on the mixed effects of the axisymmetric dust distribution and inclination angle of the shell. Here test models are presented with discussion of the results. The code can be supplied with a user-defined density distribution function and, thus, is applicable to a variety of dusty astronomical objects possessing the axisymmetric geometry.

We present near-infrared (IR) magnitudes for all white dwarfs (selected from the catalog of McCook & Sion) contained in the Two Micron All-Sky Survey second incremental data release (2MASS 2IDR). We show that the near-IR color-color diagram is an effective means of identifying candidate binary stars containing a WD and a low-mass, main-sequence star. The loci of single WDs and WD+red dwarf binaries occupy distinct regions of the near-IR color-color diagram. We recovered all known unresolved WD+red dwarf binaries located in the 2IDR sky coverage and also identified as many new candidate binaries (47 new candidates out of 95 total). Using observational near-IR data for WDs and M-L dwarfs, we have compared a sample of simulated WD+red dwarf binaries with our 2MASS data. The colors of the simulated binaries are dominated by the low-mass companion through the late M to early L spectral types. As the spectral type of the companion becomes progressively later, however, the colors of unresolved binaries become progressively bluer. Binaries containing the lowest mass companions will be difficult to distinguish from single WDs solely on the basis of their near-IR colors.

We examine the theoretical implications of a population of low-mass helium core white dwarfs in globular clusters. In particular, we focus on the observed population in the core of NGC 6397, where several low-mass white dwarf candidates have been identified as "nonflickerers" by Cool and collaborators. Age and mass estimates from cooling models, combined with dynamical and evolutionary considerations, lead us to infer that the dark binary companions are C/O white dwarfs rather than neutron stars. Furthermore, we find that the progenitor binaries very likely underwent an exchange interaction within the last 10⁹ yr. We examine the prospects for detecting a similar population in other globular clusters, with particular attention to the case of 47 Tuc.

Most extrasolar planets are observed to have eccentricities much larger than those in the solar system. Some of these planets have sibling planets, with comparable masses, orbiting around the same host stars. In these multiple planetary systems, eccentricity is modulated by the planets' mutual secular interaction as a consequence of angular momentum exchange between them. For mature planets, the eigenfrequencies of this modulation are determined by their mass and semimajor axis ratios. However, prior to the disk depletion, self-gravity of the planets' nascent disks dominates the precession eigenfrequencies. We examine here the initial evolution of young planets' eccentricity due to the apsidal libration or circulation induced by both the secular interaction between them and the self-gravity of their nascent disks. We show that as the latter effect declines adiabatically with disk depletion, the modulation amplitude of the planets' relative phase of periapsis is approximately invariant despite the time-asymmetrical exchange of angular momentum between planets. However, as the young planets' orbits pass through a state of secular resonance, their mean eccentricities undergo systematic quantitative changes. For applications, we analyze the eccentricity evolution of planets around υ Andromedae and HD 168443 during the epoch of protostellar disk depletion. We find that the disk depletion can change the planets' eccentricity ratio. However, the relatively large amplitude of the planets' eccentricity cannot be excited if all the planets had small initial eccentricities.

We report the detection of three extrasolar planets from the Lick and Keck observatories. The F8 V star HD 40979 has a companion with orbital period P = 263.1 ± 3 days, eccentricity e = 0.25 ± 0.05, and velocity semiamplitude K = 101.2 ± 5.6 m s^-1. The inferred semimajor axis is 0.83 AU and M sin i = 3.28M_Jup. Observations of planetary companions orbiting the G6 V star HD 12661 and the G4 IV star HD 38529 have already been published, and here we report additional, longer period companions for both of these stars. The outer companion to HD 12661 has P_c = 1444.5 ± 12.5 days, e_c = 0.20 ± 0.04, and K_c = 27.6 ± 2.5 m s^-1. Adopting a stellar mass of 1.07 M_☉, we find M_c sin i = 1.57M_Jup and a semimajor axis of 2.56 AU. The second companion to HD 38529 has P_c = 2174 ± 30 days, e_c = 0.36 ± 0.05, and K_c = 170.5 ± 9 m s^-1. The assumed mass of 1.39 M_☉ for HD 38529 yields M_c sin i = 12.7M_Jup and a semimajor axis of 3.68 AU. Photometric observations at Fairborn Observatory reveal low-amplitude brightness variations in HD 40979 and HD 38529 due to rotational modulation in the visibility of photospheric starspots, and they yield rotation periods of 7.0 and 35.7 days, respectively, very different from the planetary orbital periods. The orbital parameters of these two systems are compared with updated parameters for all of the known multiple-planet systems. Updated velocities are provided for the ϒ Andromedae system.

We analyze a unique 9 hr sequence of near-simultaneous, high-resolution and high-cadence G-band and K-line solar filtergrams, together with magnetograms of lower cadence and resolution. Our focus is on the phenomena surrounding discrete photospheric darkening "events" in internetwork G-band intensities. 72% of the darkenings are followed after 2 minutes by K-line brightenings. In the remaining cases, the darkenings are instead preceded by K-line brightenings 2 minutes earlier. Equivalent results are found when reference is shifted to K-line brightening events, although these two sets overlap by no more than 15%. The timing and coupling of the photospheric darkenings and chromospheric brightenings appear to be regulated by a preexisting 4 minute oscillation of the solar atmosphere. Other oscillations with periods in the range 1-8 minutes also are present, and in general the wave power is doubled at the time of an event. Our results favor an acoustic source for enhanced amplitudes of K-line intensity oscillations. The magnetic field acts as a passive tracer of horizontal photospheric flows that converge on the photospheric darkening events and then rebound.

We present EUV light curves, Doppler shifts, and line-broadening measurements for a flaring solar active region obtained with the Coronal Diagnostic Spectrometer (CDS) aboard the NASA ESA Solar and Heliospheric Observatory (SOHO) under conditions of (1) comprehensive temporal coverage (including the quiescent preflare, impulsive, and gradual decline phases), (2) high time resolution (9.83 s), and (3) narrow field of view (4'' × 20''). The four strong lines of O III at 599.587 Å, O V at 629.732 Å, Mg X at 624.937 Å, and Fe XIX at 592.225 Å are analyzed and provide diagnostics of plasma dynamics for 4.9 ≤ log T ≤ 6.9. Wavelengths and widths measured during the preflare and late decline phases provide standards against which Doppler shifts and excess line broadening are measured during the impulsive and early decline phases. The entire profile of all four lines is blueshifted early during the impulsive rise of the flare, but only the O III, O V, and Mg X lines subsequently exhibit multiple components and downflows. These downflows provide evidence of "warm rain" due to cooling coronal flare plasma following chromospheric evaporation during the impulsive phase. O III and O V exhibit a pronounced precursor brightening during which the Fe XIX emission emerges above the noise; this, combined with the fact that the O III and O V intensities begin their impulsive rise earlier than do those of Mg X and Fe XIX, is consistent with the transport of coronal flare energy to the chromosphere by nonthermal particle beams.

Observed isotope abundances in the energy range >15 MeV nucleon^-1 by the ERNE instrument on board Solar and Heliospheric Observatory show a group of high-energy ³He-rich solar particle events that apparently has not been identified in earlier observations. Those events are observed to start well after the flare, when the associated coronal mass ejection (CME) already extends to ~0.3 AU from the Sun. We present the first model of interplanetary reacceleration of ³He that explains main properties of the new group of solar particle events. Our numerical simulations suggest that appearance of ³He in the unusually high-energy range can be caused by a reacceleration of ~1 MeV solar ions in oblique shocks or compressions driven by not very fast CMEs associated with those events.

The enhancement factor of resonant thermonuclear reaction rates is calculated for extremely dense stellar plasmas in the liquid phase. In order to calculate the enhancement factor we use the screening potential deduced from the numerical experiment of the classical one-component plasma. It is found that the enhancement is tremendous for white dwarf densities if the ¹²C + ¹²C fusion cross sections show resonant behavior in the astrophysical energy range. We summarize our numerical results by accurate analytic fitting formulae.

We report the discovery of a large-scale structure of Lyα emitters (LAEs) at z = 4.86 based on wide-field imaging with the prime-focus camera (Suprime-Cam) on the Subaru Telescope. We observed a 25' × 45' area of the Subaru Deep Field in a narrow band (NB711, λ_c = 7126 Å and FWHM = 73 Å) together with R and i'. We isolate from these data 43 LAE candidates down to NB711 = 25.5 mag using color criteria. Follow-up spectroscopy of five candidates suggests the contamination by low-z objects to be ~20%. We find that the LAE candidates are clustered in an elongated region on the sky of 20 Mpc in width and 50 Mpc in length at z = 4.86, which is comparable in size to present-day large-scale structures (we adopt H₀ = 70 km s^-1 Mpc^-1, Ω₀ = 0.3, and λ₀ = 0.7). This elongated region includes a circular region of 12 Mpc radius of higher surface overdensity (δ_Σ = 2), which may be the progenitor of a cluster of galaxies. Assuming this circular region to be a sphere with a spatial overdensity of 2, we compare our observation with predictions by cold dark matter models. We find that an Ω₀ = 0.3 flat model with σ₈ = 0.9 predicts the number of such spheres consistent with the observed number (one sphere in our survey volume) if the bias parameter of LAEs is b ≃ 6. This value suggests that the typical mass of dark halos hosting LAEs at z ≃ 5 is of the order of 10¹²M_☉. Such a large mass poses an interesting question about the nature of LAEs.

We have carried out a deep imaging survey for Hα-emitting galaxies at z ≈ 0.24 using a narrowband filter tuned with the redshifted line. The total sky area covered is 706 arcmin² within a redshift range from 0.234 to 0.252 (δz = 0.018). This corresponds to a volume of 3.9 × 10³ Mpc³ when Ω_matter = 0.3, Ω_Λ = 0.7, and H₀ = 70 km s^-1 Mpc^-1 are adopted. We obtain a sample of 348 Hα-emitting galaxies whose observed emission-line equivalent widths are greater than 12 Å. We find an extinction-corrected Hα luminosity density of 10 ergs s^-1 Mpc^-3. Using the Kennicutt relation between the Hα luminosity and the star formation rate, the star formation rate density in the covered volume is estimated to be 0.036M_☉ yr^-1 Mpc^-3. This value is higher by a factor of 3 than the local star formation rate density.

It is pointed out that the Seyfert 1 galaxy NGC 6212 is surrounded by a large number of quasi-stellar objects (QSOs), including the very active and rapidly varying radio QSO 3C 345, which lies only 47 away from the center of the galaxy. There are two pairs of QSOs with the same redshifts, including 3C 345, very close to the nucleus of NGC 6212, and the very high density of the QSOs falls off rapidly with distance from that galaxy. If 3C 345 lies at the distance of NGC 6212, the proper motions in the radio jet in 3C 345, which have been studied in detail, correspond to only mildly relativistic speeds, of about 0.33c.

Dwarf satellite galaxies undergo strong tidal forces produced by the main galaxy potential. These forces disturb the satellite and produce asymmetries in its stellar distribution, tidal tail formation, and modifications of the velocity dispersion profiles. Most of these features are observed in the Ursa Minor (UMi) dwarf spheroidal galaxy, which is one of the closest satellites of the Milky Way. These features show that UMi is being tidally disrupted and is probably not in virial equilibrium. The high-velocity dispersion of UMi would also be a reflection of this tidal disruption and is not the signature of the large dark matter content that would be deduced if virial equilibrium is assumed. In order to avoid the uncertainty produced when virial equilibrium is assumed in systems in strong tidal fields, we present a new method of calculating the mass-to-luminosity ratio of disrupted dwarf galaxies. This method is based on numerical simulations and only takes into account the shape of the dwarf density profile and the tidal tail brightness, but it does not depend on the kinematics of the dwarf. Applying this method to UMi, we obtain a mass-to-luminosity relation of 12, which is lower than the value obtained assuming virial equilibrium (M/L = 60). In addition, if UMi has a large dark matter content, it will be impossible to reproduce a tidal tail as luminous as the one observed.

We have obtained the first detection of spectral absorption lines in one of the high-velocity stars in the vicinity of the Galaxy's central supermassive black hole. Both Brγ (2.1661 μm) and He I (2.1126 μm) are seen in absorption in S0-2 with equivalent widths (2.8 ± 0.3 and 1.7 ± 0.4 Å) and an inferred stellar rotational velocity (220 ± 40 km s^-1) that are consistent with that of an O8-B0 dwarf, which suggests that it is a massive (~15 M_☉) young (less than 10 Myr) main-sequence star. This presents a major challenge to star formation theories, given the strong tidal forces that prevail over all distances reached by S0-2 in its current orbit (130-1900 AU) and the difficulty in migrating this star inward during its lifetime from farther out where tidal forces should no longer preclude star formation. The radial velocity measurements ( = -510 ± 40 km s^-1) and our reported proper motions for S0-2 strongly constrain its orbit, providing a direct measure of the black hole mass of 4.1(±0.6) × 10⁶³M_☉. The Keplerian orbit parameters have uncertainties that are reduced by a factor of 2-3 compared to previously reported values and include, for the first time, an independent solution for the dynamical center; this location, while consistent with the nominal infrared position of Sgr A*, is localized to a factor of 5 more precisely (±2 mas). Furthermore, the ambiguity in the inclination of the orbit is resolved with the addition of the radial velocity measurement, indicating that the star is behind the black hole at the time of closest approach and counterrevolving against the Galaxy. With further radial velocity measurements in the next few years, the orbit of S0-2 will provide the most robust estimate of the distance to the Galactic center.

We derive and parameterize the Galactic mass function (MF) below 1 M_☉ characteristic of both single objects and binary systems. We resolve the long-standing discrepancy between the MFs derived from the Hubble Space Telescope (HST) and from the nearby luminosity functions, respectively. We show that this discrepancy stemmed from two cumulative effects, namely, (1) incorrect color-magnitude-determined distances, due to a substantial fraction of M dwarfs in the HST sample belonging to the metal-depleted thick-disk population, as corrected recently by Zheng et al., and (2) unresolved binaries. We show that both the nearby and HST MF for unresolved systems are consistent with a fraction ~50% of M dwarf binaries, with the mass of both the primaries and the companions originating from the same underlying single MF. This implies that ~30% of M dwarfs should have an M dwarf companion and ~20% should have a brown dwarf companion, in agreement with recent determinations. The present calculations show that the so-called "brown dwarf desert" should be reinterpreted as a lack of high mass ratio (m₂/m₁ ≲ 0.1) systems and does not preclude a substantial fraction of brown dwarfs as companions of M dwarfs or for other brown dwarfs.

We have used the Very Large Array with the Pie Town antenna of the Very Long Baseline Array to image for the first time L1551 IRS 5 at 3.5 cm with an angular resolution of ~01. These observations clearly reveal the presence of a binary jet, with each component approximately centered on the 7 mm compact protoplanetary disks previously reported. As anticipated from results at various wavelengths, L1551 IRS 5 is a binary system where each star is surrounded by a disk and drives a collimated jet. The curved morphology of the northern jet, showing reflection symmetry, is suggestive of that expected for a source in a binary orbit. A region of radio emission located between the two jets could be a zone of shock interaction between the two outflows.

We have carried out a millimeter interferometric continuum survey toward seven young stellar objects (YSOs) in the MBM 12 cloud. Thermal emissions associated with two YSOs were detected above the 3 σ level at 2.1 mm, and one also showed a 1.3 mm thermal emission. Another object was marginally detected at 2.1 mm. Spectral energy distributions of the YSOs are well fitted by a simple power-law disk model. Masses of the circumstellar disks are estimated to be of the order of 0.05 M_☉. The circumstellar disks in the MBM 12 cloud have properties in common with the disks in nearby star-forming regions, in terms of disk parameters, such as disk mass, as well as an infrared excess.

It is suggested that the three-segmented morphology of the soft X-ray emission from cluster and field stars may be understood in terms of the recent classification of rotating stars into three kinds: those lying on the convective sequence, on the interface sequence, or in the gap between them.

We report the discovery of LSR 0602+3910, an L dwarf of class L1. The object was initially identified by Lépine et al. as a new high proper motion star lying close to the Galactic plane. Its Two Micron All Sky Survey value of J-K_s = 1.43 is consistent with an L dwarf, which we now confirm spectroscopically. In addition, we see a signature of Li I absorption, making LSR 0602+3910 a brown dwarf, one of the brightest known (K_s = 10.86). Among L dwarfs, it is second in brightness to the combined light of 2MASS 0746+20, a close binary system. We see no indication that LSR 0602+3910 is a binary, although high-resolution imaging will be required to confirm this. Spectroscopic and photometric distance estimates agree very well, placing LSR 0602+3910 at d = 10.6 ± 0.8 pc. LSR 0602+3910 was most likely missed in previous searches because of its proximity to the plane, the region that most searches avoided. We estimate that some 40% of bright L dwarfs are missed because of this selection effect.

In this work, we perform a global analysis of the radial velocity curve of the HD 12661 system. Orbital fits that are obtained by the genetic and gradient algorithms of minimization reveal the proximity of the system to the 6 : 1 mean motion resonance. The orbits are locked in the secular resonance with apsidal axes librating about 180°, with a full amplitude of ≃90°-180°. Our solution incorporates the mutual interaction between the companions. A stability analysis with the MEGNO (Mean Exponential Growth factor of Nearby Orbits) indicator shows that the system is located in an extended stable zone of quasi-periodic motions. These results are different from those obtained on the basis of the orbital fit published by Fischer et al.