Subarcsecond Scales Research Articles

A deeply embedded young protoplanetary disk around L1489 IRS observed by the Submillimeter Array

Circumstellar disks are expected to form early in the process that leads to the formation of a young star, during the collapse of the dense molecular cloud core. It is currently not well understood at what stage of the collapse the disk is formed or how it subsequently evolves. We aim to identify whether an embedded Keplerian protoplanetary disk resides in the L1489 IRS system. Given the amount of envelope material still present, such a disk would respresent a very young example of a protoplanetary disk. Using the Submillimeter Array (SMA) we have observed the HCO$^+$ $J=$ 3--2 line with a resolution of about 1$''$. At this resolution a protoplanetary disk with a radius of a few hundred AUs should be detectable, if present. Radiative transfer tools are used to model the emission from both continuum and line data. We find that these data are consistent with theoretical models of a collapsing envelope and Keplerian circumstellar disk. Models reproducing both the SED and the interferometric continuum observations reveal that the disk is inclined by 40$^\circ$ which is significantly different to the surrounding envelope (74$^\circ$). This misalignment of the angular momentum axes may be caused by a gradient within the angular momentum in the parental cloud or if L1489 IRS is a binary system rather than just a single star. In the latter case, future observations looking for variability at sub-arcsecond scales may be able to constrain these dynamical variations directly. However, if stars form from turbulent cores, the accreting material will not have a constant angular momentum axis (although the average is well defined and conserved) in which case it is more likely to have a misalignment of the angular momentum axes of the disk and the envelope.

Spatially resolved mid-infrared spectroscopy of IC 5063

We present spatially resolved 10-μm spectra of the nucleus of IC 5063 that are near-diffraction-limited. The observations were obtained with T-ReCS, the mid-infrared (mid-IR) imager and spectrometer on the 8.1-m Gemini South telescope, with the slit aligned at a position angle on the sky along the direction of the cone of narrow-line emission. The spectra cover the nucleus and the inner reaches of the ionization cones at a spatial resolution of approximately 0.4 arcsec (90 pc). Individual spectra, extracted in steps in the spatial direction along the slit, reveal variations in continuum slope and silicate feature profile and depth on subarcsecond scales, illustrating in unprecedented detail the complexity of the circumnuclear regions of this galaxy at mid-IR wavelengths. The dust population in the vicinity of the narrow-line region, north-west of the nuclear position, is significantly warmer than that to the south-east of the nucleus. This is consistent with an observation of the cooler dust associated with the outer reaches of the postulated torus that obscures the type 1 nucleus in this object.

High‐Resolution Imaging of the Dust Disk around 49 Ceti

Sub-arcsecond scale Keck images of the young A1V star, 49 Ceti, resolve emission at lambda = 12.5 and 17.9 microns from a disk with long axis at PA 125 pm 10 degrees and inclination phi = 60 pm 15 degrees . At 17.9 microns, the emission is brighter and more extended toward the NW than the SE. Modeling of the mid-infrared images combined with flux densities from the literature indicate that the bulk of the mid-infrared emission comes from very small grains (a ~ 0.1 microns) confined between 30 and 60 AU from the star. This population of dust grains contributes negligibly to the significant excess observed in the spectral energy distribution. Most of the non- photospheric energy is radiated at longer wavelengths by an outer disk of larger grains (a ~ 15 microns), inner radius ~ 60 AU, and outer radius ~ 900 AU. Global properties of the 49 Cet disk show more affinity with the Beta Pic and HR 4796A disks than with other debris disks. This may be because they are all very young (t < 20 Myr), adding strength to the argument that they are transitional objects between Herbig Ae and Vega-like A stars with more tenuous circumstellar disks.

Spatially Resolved Mid‐Infrared Spectroscopy of NGC 1068: The Nature and Distribution of the Nuclear Material

We present spatially resolved, near-diffraction-limited 10 μm spectra of the nucleus of the Seyfert 2 galaxy NGC 1068, obtained with Michelle, the mid-IR imager and spectrometer on the 8.1 m Gemini North Telescope. The spectra cover the nucleus and the central 60 × 04 of the ionization cones at a spatial resolution of approximately 04 (≈30 pc). The spectra extracted in 04 steps along the slit reveal striking variations in continuum slope, silicate feature profile and depth, and fine-structure line fluxes on subarcsecond scales, illustrating in unprecedented detail the complexity of the circumnuclear regions of NGC 1068 at mid-IR wavelengths. A comparison of photometry in various apertures reveals two distinct components: a compact (radius < 15 pc), bright source within the central 04 × 04 and extended, lower brightness emission. We identify the compact source with the AGN-obscuring torus, and the diffuse component with dust in the ionization cones. While the torus emission dominates the flux observed in the near-IR, the mid-IR flux measured with apertures larger than about 1'' is dominated instead by emission from the ionization cones; despite its higher brightness, the torus contributes <30% of the 11.6 μm flux in the central 12 region. Many previous attempts to determine the torus spectral energy distribution are thus likely to be significantly affected by contamination from the extended emission. The observed spectrum of the compact source is compared with clumpy torus models. The models require most of the clouds to be located within a few parsecs of the central engine, in agreement with recent mid-IR interferometric observations. We also present a UKIRT/CGS4 5 μm spectrum covering the R(0)-R(4) lines of the fundamental vibration-rotation band of 12CO. None of these lines was detected, and we discuss these nondetections in terms of the filling factor and composition of the nuclear clouds.

Signatures of restarted activity in core-dominated, triple radio sources selected from the FIRST survey

Signatures of the re-occurrence of activity in radio-loud AGNs, indicated either by the so-called double-double or X-shaped structures, have been observed in a number of radio sources. All such objects known to date have linear sizes of the order of a megaparsec. A number of the sources that are appreciably more compact than this, but that exhibit hints of a past phase of activity, were found in the VLA FIRST survey. Their structures show symmetric relic lobes straddling relatively bright, unresolved cores. Observations of the cores of 15 such structures with MERLIN at 5 GHz have shown that four of them are doubles or core-jets on the subarcsecond scale. Misalignments of between the axis of the inner structure and the line connecting the fitted maxima of the arcminute-scale relic lobes are clearly visible in three of the four sources. From these results, we can infer that a rapid repositioning of the central engine in each of these three radio sources is the most plausible interpretation of the observed morphology and that a merger is most likely the original cause of such a repositioning. In the case of TXS 1033+026, the optical image extracted from the SDSS archives clearly suggests that two objects separated by only 2.7 kpc (projected onto the sky plane) are indeed merging. The inner parts of TXS 0818+214 and TXS 1312+563 could be interpreted as double-lobed, and consequently, these sources could be of the double-double type; but further multifrequency observations are necessary to provide support for such an interpretation.

Mid-infrared, spatially resolved spectroscopy of the nucleus of the Circinus galaxy

High spatial resolution spectroscopy at 8-13 μm with T-ReCS on Gemini-S has revealed striking variations in the mid-infrared emission and absorption in the nucleus of the Circinus galaxy (hereafter Circinus) on subarcsecond scales. The core of Circinus is compact and obscured by a substantial column of cool silicate dust. Weak extended emission to the east and west coincides with the coronal line region and arises from featureless dust grains which are probably heated by line emission in the coronal emission zone. The extended emission on the east side of the nucleus displays a much deeper silicate absorption than that on the west, indicating significant columns of cool material along the line of sight and corresponding to an additional extinction of A V ∼ 25 mag. Emission bands from aromatic hydrocarbons are not subject to this additional extinction, are relatively weak in the core and in the coronal line region, and are much more spatially extended than the continuum dust emission; they presumably arise in the circumnuclear star-forming regions. These data are interpreted in terms of an inclined disc-like structure around the nucleus extending over tens of parsecs and possibly related to the inner disc found from observations of water masers by Greenhill et al..

Feeding the Monster: The Nucleus of NGC 1097 at Subarcsecond Scales in the Infrared with the Very Large Telescope

Near-IR images of the prototype LINER/Seyfert type 1 galaxy NGC 1097 observed with the Very Large Telescope using adaptive optics disclose with unprecedented detail a complex central network of filamentary structure spiraling down to the center of the galaxy. The structure, consisting of several spiral arms, some almost completing a revolution about the center, is most prominent within a radius of about 300 pc. Gas and dust may be channelled to the center of NGC 1097 along this central spiral. Some filaments can be traced farther out, where they seem to connect with the nuclear star-forming ring at a 0.7 kpc radius. Straight principal shocks running along the primary large-scale bar of NGC 1097, seen in the optical images as prominent dust lanes, curve into this ring, but radio polarization vectors cross the nuclear ring at a rather large angle. Here we attempt to explain this morphology in terms of three-dimensional gas flow in a barred galaxy. In our scenario, parts of the principal shock that propagate in the off-plane gas can cross the nuclear star-forming ring and excite waves inward from it. If the dispersion relation of the excited waves allows for their propagation, they will naturally take the shape of the observed central spiral. The nuclear region of NGC 1097 remains unresolved at subarcsecond scales in the near-IR, with an upper size limit of <10 pc FWHM. Thus, any putative central dusty torus or gaseous disk envisaged by the active galactic nucleus (AGN) unified schemes has to be smaller than 10 pc in diameter at near-IR wavelengths. The extinction in the region between the nuclear star-forming ring and the nucleus increases very moderately, reaching Av ~ 1 at the immediate surroundings of the nucleus. Thus, if the nuclear filaments are tracing cold dust, they contribute to a very low extinction in the line of sight and are likely to be distributed in a rather thin disk.

Polarization and kinematic studies of SS 433 indicate a continuous and decelerating jet

We present radio images of SS 433 taken in 1987 at 1.5, 5, 8 and 15 GHz using the VLA in A configuration, and taken at four more recent epochs at 5 or 1.6 GHz using MERLIN. The extremely sensitive 5-GHz VLA image appears to be inconsistent with the ballistic kinematic model predictions. However, the inclusion of a deceleration term of0.02c per period (162.375 d) or the use of a slower constant velocity of 0.225c gives a good agreement with the observations. This slower ejection velocity is not consistent with existing measurements of the jet velocity on subarcsecond scales. The images also show a depolarization region surrounding the radio core of SS 433, with an extent ∼0.5 arcsec at 5 GHz. This depolarization region is present even at our highest observing frequency of 15 GHz. The MERLIN images confirm the presence of depolarization at four separate observing epochs. The depolarizing medium is likely to be an ionized cloud of gas associated with the stellar wind of the normal star, although we cannot rule out thermal gas in the jet material co-existing with the relativistic electrons which emit via the synchrotron mechanism. Outside the depolarizing region the jets are highly linearly polarized (up to 20 per cent). We find a rotation measure of 119 ± 16 rad m -2 for the extended jets, consistent with previous lower-frequency studies. After correcting for foreground Faraday rotation we find an alignment between the implied magnetic field orientation and the kinematic locus. This can be explained by the stretching of the plasma tube along the kinematic locus as a result of differential motion. We also transform the rest-frame angles defining the apparent electric field direction to the observer's frame and show that, after allowing for Lorentz contraction, projection and light travel delays, the observer also sees the electric field perpendicular to the plasma tube. The observed alignment of the jet magnetic field and direction suggests that the jet consists of a continuous plasma tube with longitudinal magnetic field (as proposed for the jets emitted by quasars) and not of an unconnected series of plasmons.

FUSESpectroscopy of the Large Magellanic Cloud Supernova Remnant N49

Strong C III λ977 and O VI λλ1032, 1038 emission lines are detected in Far Ultraviolet Spectroscopic Explorer (FUSE) spectra of N49, the brightest optical supernova remnant in the Large Magellanic Cloud. Shocks with velocities ranging from less than 130 to more than 180 km s-1 are responsible for these emission lines and are present over the entire eastern half of the remnant. The emission lines are very broad, spanning about 700 km s-1 in all the spectra. The ratio of C III to O VI at different velocities shows that fast-moving radiative shocks are present in N49. These may be a result of instabilities, secondary shocks, and turbulent mixing. The velocity structure of O VI emission from a 4'' × 20'' region along the southeast edge of the remnant is the same as that from a larger 30'' × 30'' region, which is evidence for equally complex structure at multiple scales. An archival HST WFPC2 Hα image also shows highly filamentary morphology, with structures on subarcsecond scales. The complexity of the emission suggests that the effective interstellar extinction at far-ultraviolet wavelengths is smaller than that at optical wavelengths. The uncertainty in the far-ultraviolet extinction translates to an order of magnitude uncertainty in the intrinsic O VI intensity of N49. A comparison of the O VI 1032 and 1038 A line profiles shows that the latter is affected by absorption due to molecular hydrogen, probably associated with the remnant and its surroundings, as well as due to O I in an intermediate-velocity cloud in the Galactic halo.

The radio structure of Menzel 3

We present the first radio continuum images of the evolved late-type stellar object, Menzel 3 (Mz 3), made with multifrequency observations using the Australia Telescope Compact Array. The maximum resolutions attained are 1, 2, 4 and 7 arcsec. At each frequency investigated, the large-scale radio structure is reminiscent of that seen at infrared and optical wavelengths. In the highest-frequency image of resolution 1 arcsec, the radio brightness distribution is dominated by a compact core emission feature, which we attribute to the ionization of either the last exhalation of the superwind (single progenitor scenario) or to the ongoing stellar wind from a companion (binary scenario). At the higher frequencies investigated, we find that this core ionized outflow is asymmetric, optically thick and has a spectral index consistent with that of thermal emission. The mass loss rate associated with the core emission feature is 7 x 10 -5 M ○. yr -1 . We discuss this outflow and favour its origin as being due to a companion. Extending from either side of the compact core, at PA ∼ 120° with respect to the bipolar axis, we find evidence for wings of emission that may be due to the ionized inner surface of an equatorial collimating disc; this surface has dimensions 7 x 1.5 arcsec 2 (8800 x 1900 au 2 at 1.3 kpc). We suggest that this feature may be part of the circumbinary accretion disc suggested recently by Kastner et al. We measure low brightness temperatures, which we attribute to there being unresolved condensations on sub-arcsecond scales within the beam size.

Internal Color Properties of Resolved Spheroids in the DeepHubble Space TelescopeAdvanced Camera for Surveys Field of UGC 10214

We study the internal color properties of a morphologically selected sample of spheroidal galaxies taken from the Hubble Space Telescope (HST) Advanced Camera for Surveys (ACS) Early Release Observation program of UGC 10214 (''the Tadpole''). By taking advantage of the unprecedented high resolution of the ACS in this very deep data set, we are able to characterize spheroids at subarcsecond scales. Using the V606 and I814 bands, we construct VI color maps and extract color gradients for a sample of spheroids at I814 < 24 mag. We assess the ability of the ACS to make resolved color studies of galaxies by comparing its results with the multicolor data from the Hubble Deep Fields (HDFs). Here we report that with ACS WFC data using less than � 1/10 the exposure of the WFPC2 HDFs, it is possible to confidently carry out resolved studies of faint galaxies at similar magnitude limits. We also investigate the existence of a population of morphologically classified spheroids that show extreme variation in their internal color properties, similar to the ones reported in the HDFs. These are displayed as blue cores and inverse color gradients with respect to those accounted for from metallicity varia- tions. Following the same analysis, we find a similar fraction of early-type systems (� 30%-40%) that show nonhomologous internal colors, suggestive of recent star formation activity. We present two statistics for quantifying the internal color variation in galaxies and tracing blue cores, from which we estimate the ratio of nonhomogeneous to homogeneous internal colors as a function of redshift up to zP1:2. We find that it can be described as about constant as a function of redshift, with a small increase with redshift for the fraction of spheroids that present strong color dispersions. The implications of a constant fraction at all redshifts suggests the existence of a relatively permanent population of evolving spheroids up to zP1. We discuss the implications of this in the context of spheroidal formation. Subject headingg galaxies: elliptical and lenticular, cD — galaxies: evolution — galaxies: fundamental parameters Online material: color figures, machine-readable table

Differential Astrometry of Subarcsecond Scale Binaries at the Palomar Testbed Interferometer

We have used the Palomar Testbed Interferometer to perform very high precision differential astrometry on the 025 separation binary star HD 171779. In 70 minutes of observation, we achieve a measurement uncertainty of ≈9 μas in one axis, consistent with theoretical expectations. Night-to-night repeatability over four nights is at the level of 16 μas. This method of very narrow angle astrometry may be extremely useful for searching for planets with masses as small as 0.5MJ around a previously neglected class of stars—so-called speckle binaries. It will also provide measurements of stellar parameters such as masses and distances, useful for constraining stellar models at the 10-3 level.

Near-Infrared Monitoring of the Carbon Star IRC +10 216: A High Spatial-Resolution Time Sequence of Dust-Shell Evolution

The carbon star IRC +10 216 is a long-period Asymptotic Giant Branch (AGB) star suffering from strong stellar winds (several 10-5 M⊙/yr; Loup et al. 1993) which have led to an almost complete obscuration of the star by dust. Due to the high mass-loss rate, long period of P = 649 d (Le Bertre 1992), and carbon-rich chemistry of the dust-shell, IRC +10 216 is obviously in a very advanced stage of its AGB evolution. High-resolution near-infrared imaging of IRC +10 216 has revealed that on sub-arcsecond scales (100 mas) its dust shell is clumpy, bipolar, and changing on a time scale of only ~1 yr (Weigelt et al. 1997, 1998, Haniff &amp; Buscher 1998, Osterbart et al. 2000, Tuthill et al. 2000). Since most dust shells around AGB stars are known to be spherically symmetric, whereas most proto-planetary nebulae (PPN) show an axisymmetric geometry (Olofsson 1996), it appears likely that IRC +10 216 has already entered the transition phase to the PPN stage. This suggests that the break of the dust-shell symmetry between the AGB and post-AGB phase already takes place at the end of the AGB evolution.

Stellar Companions to Stars with Planets

A combination of high-resolution and wide-field imaging reveals two binary stars and one triple star system among the sample of the first 11 stars with planets detected by radial velocity variations. High-resolution speckle or adaptive optics (AO) data probe subarcsecond scales down to the diffraction limit of the Keck 10 m or the Lick 3 m, and direct images or AO images are sensitive to a wider field, extending to 10'' or 38'', depending on the camera. One of the binary system—HD 114762—was not previously known to be a spatially resolved multiple system; additional data taken with the combination of Keck adaptive optics and NIRSPEC are used to characterize the new companion. The second binary system—τ Boo—was a known multiple with two conflicting orbital solutions; the current data will help constrain the discrepant estimates of periastron time and separation. Another target—16 Cyg B—was a known common proper motion binary, but the current data resolve a new third component, close to the wide companion 16 Cyg A. Both the HD 114762 and 16 Cyg B systems harbor planets in eccentric orbits, while the τ Boo binary contains an extremely close planet in a tidally circularized orbit. Although the sample is currently small, the proportion of binary systems is comparable to that measured in the field over a similar separation range. Incorporating the null result from another companion search project lowers the overall fraction of planets in binary systems, but the detections in our survey reveal that planets can form in binaries separated by less than 50 AU.

Sub-arcsecond atomic hydrogen absorption in the Seyfert galaxies NGC 7674 and NGC 7469

Neutral hydrogen has been studied on sub-arcsecond scales in two Seyfert galaxies, NGC 7469 and NGC 7674. The 1.4-GHz continuum emission from these sources agree with higher frequency measurements and shows linear, triple source in NGC 7674 and a compact source surrounded by a weak ring of radio emission in NGC 7469. In both of these sources we detect deep localized H I absorption in front of a single continuum component. In the case of NGC 7674 the H I absorption is only seen against the central component and has a peak optical depth of 0.21 ± 0.03, a linewidth of 114 ± 7 km s−1 and a heliocentric velocity of 8623 ± 5 km s−1. The integrated H I absorption against the central component corresponds to a column density of 5.0 ± 0.7 × 1021 atom cm−2. We interpret the lack of H I absorption against the east and west components (τ < 0.07 and 0.06) as evidence that the H I is localized within the central few hundred pc, similar to the H I observed against the nucleus of NGC 4151, and has an upper mass limit of ∼4 × 105 M⊙. We speculate that this H I may be associated with a dusty torus. Giant Meterwavelength Radio Telescope (GMRT) results show extended emission and give a rest velocity of 8699 ± 20 km s−1. The H I absorption against NGC 7469 shows a single deep feature against the strong central component. The line has a peak optical depth 0.25, a linewidth of 65 ± 6 km s−1 and a heliocentric velocity of 4843 ± 5 km s−1. The integrated column density is 3.6 ± 0.4 × 1021 atom cm−2. Unfortunately the other components in NGC 7469 are too weak to set significant limits to the H I optical depth, and hence, unlike NGC 7674, it is not possible to constrain the size of the absorbing gas.

Proper Motions of the HH 111 Jet Observed with the [ITAL]Hubble Space Telescope[/ITAL

New Hα and [S II] images of the HH 111 jet taken with the Hubble Space Telescope reveal marked proper motions and morphological changes when compared with similar images obtained 4 years earlier. Knots in the jet, which are dominated by emission from nested bow shocks, generally move ballistically, with no evidence for turbulent motions even in regions where the emission has a complex morphology. These bow shocks sometimes overtake one another; the new images show this occurred in knot L about 80 years ago. Photometric variability, clearly visible for the first time at subarcsecond scales, can confuse ground-based measurements that require many years between epochs to detect reliable proper motions. With the exception of the bow shock L, whose wings expand laterally, the jet moves mainly along its long axis. Because HH 111 lies nearly in the plane of the sky, the proper motions translate accurately to space velocities, which range from 220 to 330 km s-1 with a typical uncertainty of ±5 km s-1. The fastest knots are associated with object E at the base of the visible jet, where a cooling layer is in the process of forming behind one of the shocks. Velocity differences between adjacent knots within the optically bright part of the jet are typically 40 km s-1, in line with predictions of nonmagnetic shock models based on emission-line fluxes. This agreement limits the component of the magnetic field perpendicular to the axis of the jet to be 1 mG.

On the Relation of G‐Band Bright Points to the Photospheric Magnetic Field

Cotemporal observations in the 4305 A G-band and Ca II λ3933 K-line, Fe I 6302 A magnetograms, and 6563 A Hα images are used to study the relation of G-band bright points (GBPs) to magnetic elements in the photosphere. Angular resolution of 02 and 03 is achieved in the best G-band and magnetogram images, respectively. Single magnetogram sensitivity of 120-150 gauss (Φmin ~ 1016 Mx) is achieved. Small-scale GBPs appear both in intergranular lanes and on the edges of certain bright, rapidly expanding granules. The latter class of GBPs are nonmagnetic (at the flux limit) and are a source of confusion in magnetic element studies. The large-scale pattern of GBPs and magnetic flux in plage are highly correlated; GBPs occur preferentially on the periphery of extended plage regions, which are primarily demarcated by dark or neutral-contrast regions (e.g., pores or unresolved structures). On subarcsecond scales, GBPs are cospatial (to within an average of 024) and comorphous with magnetic elements in intergranular lanes; larger flux concentrations appear more diffuse than the associated GBP groupings. The average peak flux density of GBPs in the data set is ~160 gauss. No significant trend in GBP flux density as a function of either contrast or size is found. Rings of magnetic elements on 5'' scales remain very stable for periods on the order of hours. GBPs trace the temporal evolution of magnetic elements closely: there is no indication of a lead or lag (to within the 30-90 s precision of our data) in the appearance of GBPs relative to the appearance of magnetic elements. Pore formation via the accumulation of magnetic elements at a flow-field sink is seen in the data set. Magnetic elements and granules are continually advected into pores by the photospheric flow field.

Kinematics of 1200 Kilometer per Second Jets in He 3‐1475

Spectroscopic observations of a proto-planetary nebula He 3-1475 with the Space Telescope Imaging Spectrograph reveal the kinematics of its high-velocity (1200 km s-1) jets. The jets are formed at a large (0.15 pc) distance from its central star by collimation of an asymmetric stellar wind in a pair of conical shocks seen in Wide Field Planetary Camera 2 (WFPC2) images. The jets consist of several pairs of knots symmetrically distributed with respect to the central star, with most knots exhibiting a head-tail morphology. Large (up to 650 km s-1) radial velocity gradients are seen within the knots on subarcsecond scales, with velocities decreasing from the knot heads toward their trailing tails. These large velocity gradients are a sign of efficient deceleration of jets by a much slower bipolar outflow. The inclination angle of the bipolar outflow is equal to 40°, based on Doppler shifts of the scattered stellar Hα line. Its velocity is equal to 140 km s-1 at a distance of 0.23 pc from the star and increases monotonically with the radial distance from the star. A comparison of new WFPC2 [N II] λ6584 images with older WFPC2 images reveals expansion of the jets. The measured jet proper motions in combination with their radial velocities imply that He 3-1475 is a Galactic bulge star at a distance of 8 kpc, located 800 pc above the Galactic plane. Its very high luminosity (25,000 L☉) implies that He 3-1475 must be significantly more massive than a typical asymptotic giant branch star within the Galactic bulge, perhaps because of a past mass transfer and/or a merger event in an interacting binary system.

Structure and physical properties of the rapidly evolving dusty envelope of IRC + 10216 reconstructed by detailed two-dimensional radiative transfer modeling

We present the first detailed, two-dimensional radiative transfer model of the dusty envelope around the carbon star IRC + 10216. Our goal was to find a self-consistent model of the star and its envelope which takes into account as many observational constraints as possible. The model reproduces very well the entire beam-matched spectral energy distribution of IRC + 10216 from optical to centimeter wavelengths (at several phases of stellar luminosity), observed intensity profiles of the object at 1.25, 2.2, 10.5, 50, 100 ${\mu}$m, and 1.3 mm, a 10.5 ${\mu}$m lunar occultation intensity profile, our high-resolution $J, H$, K , and $H\!-\!K$ bispectrum speckle-interferometry images, and visibilities in $J, H, K, L, M$, and N bands. For the adopted distance of 130 pc, the model of IRC + 10216 implies that the object changes its luminosity between 13 000 and 5200 $L_{\odot}$, its effective temperature between 2800 and 2500 K, and its radius between 500 and 390 $R_{\odot}$. There is a dense non-spherical dust shell around the star, with outflow cavities at position angle PA${ \approx }$20°. The southern cavity with a full opening angle of 36° is tilted toward us by 40° from the plane of sky, causing the observed bipolar appearance of the object on a subarcsecond scale. If the envelope's outflow velocity of 15 km s -1 applies to the material making up the dense core, then just ${\sim }$15 years ago the star was losing mass at a rate of $9{ }10^{-5}$ $M_{\odot}$ yr -1 . Dust exists in the envelope of IRC + 10216 everywhere from the stellar photosphere up to a distance of 3 pc from the star. The total mass of the envelope lost by the central star is 3 $M_{\odot}$ and the dust-to-gas mass ratio is 0.004. The total optical depth $\tau_{V}$ toward the star in the visual is 40, in the polar cavities it is 10. The innermost parts of the envelope are optically thick even at 10.7 ${\mu}$m due to a strong resonance absorption of silicon carbide grains at that wavelength. In addition to SiC dust, the model contains inhomogeneous grains made of a mixture of SiC and incompletely amorphous carbon with thin [ Mg 0.5 Fe 0.5 ] S mantles. This is the simplest dust mixture required to fit all observations of IRC + 10216 and to correctly interpret the well-known 11.3 ${\mu}$m and 27 ${\mu}$m emission bands. The dust model found in this study can also be successfully applied to many other carbon stars exhibiting broad emission features in the 10.3-12.6 ${\mu}$m and 25-37 ${\mu}$m wavelength regions. An important and firm result of our modeling is that the brightest compact peak observed in IRC + 10216 is not the direct light from the underlying central star. In contrast to previous suggestions, the brightest southern component, labeled A in our high-resolution near-infrared images [CITE], is only the radiation emitted and scattered in the optically thinner southern cavity of the bipolar dense shell moving away from the central star. The carbon star is at the position of the fainter component B in our H and K images, which is 0 fraction of arcsecond21 away from A along the symmetry axis. Direct stellar light (component B) is not seen at all in the Hubble Space Telescope 0.8 ${\mu}$m and 1.1 ${\mu}$m images, being absorbed by the dense dusty material. The even fainter components C and D in the H and K images are probably due to smaller deviations of the dense shell from the spherical shape. IRC + 10216 seems to have entered a phase immediately before moving off the asymptotic giant branch and started developing asymmetries in its envelope.

Constraints on the Warm Dark Matter Model from Gravitational Lensing

Formation of sub-galactic halos is suppressed in warm dark matter (WDM) model due to thermal motion of WDM particles. This may provide a natural resolution to some puzzles in standard cold dark matter (CDM) theory such as the cusped density profiles of virialized dark halos and the overabundance of low mass satellites. One of the observational tests of the WDM model is to measure the gravitationally lensed images of distant quasars below sub-arcsecond scales. In this Letter, we report a comparison of the lensing probabilities of multiple images between CDM and WDM models using a singular isothermal sphere model for the mass density profiles of dark halos and the Press-Schechter mass function for their distribution and cosmic evolution. It is shown that the differential probability of multiple images with small angular separations down to 10 milliarcseconds should allow one to set useful constraints on the WDM particle mass. We discuss briefly the feasibility and uncertainties of this method in future radio surveys (e.g. VLBI) for gravitational lensing.