853 research outputs found
Non-negative curvature obstructions in cohomogeneity one and the Kervaire spheres
In contrast to the homogeneous case, we show that there are compact
cohomogeneity one manifolds, that do not support invariant metrics of
non-negative sectional curvature. In fact we exhibit infinite families of such
manifolds including the exotic Kervaire spheres. Such examples exist for any
codimension of the singular orbits except for the case where both are equal to
two, where existence of non-negatively curved metrics is known.Comment: 10 page
Far-infrared photometry of compact extragalactic sources: OJ 187 and BL Lac
The 50 and 100 micron emissions of OJ 287 were detected and upper limits for BL Lac were obtained. These first measurements of two BL Lac objects in the far-infrared show them to be similar to the few quasars previously observed in the far-infrared. In particular, there is no evidence for significant dust emission, and the lambda approximately 100 micron flux density fits on a smooth line joining the near-infrared and millimeter continuum fluxes. The implications of the results for models of the sources are discussed briefly
An infrared study of the bi-polar outflow region GGD 12-15
Infrared observations from 1 to 100 microns are presented for the region associated with a bipolar CO outflow source near the nebulous objects GGD 12 to 15. A luminous far-infrared source was found associated with a radio-continuum source in the area. This object appears to be a compact HII region around a nearly main-sequence BO star. A faint 20 micron source was also discovered at the position of an H2O maser 3O deg northwest of the HII region. This object appears to be associated with but not coincident with a 2 micron reflection nebula. This structure serves as evidence for a non-spherically symmetric, possibly disk-like dust distribution around the exciting star for the maser. This object probably powers the bi-polar CO outflow although its luminosity is less than 10% that of the star which excites the compact HII region. A number of other 2 micron sources found in the area are probably members of a recently formed cluster
Far-infrared observations of young clusters embedded in the R Coronae Austrinae and RHO Ophiuchi dark clouds
Multicolor far infrared maps in two nearby dark clouds, R Coronae Austrinae and rho Ophiuchi, were made in order to investigate the individual contribution of low mass stars to the energetics and dynamics of the surrounding gas and dust. Emission from cool dust associated with five low mass stars in Cr A and four in rho Oph was detected; their far infrared luminosities range from 2 far infrared luminosities L. up to 40 far infrared luminosities. When an estimate of the bolometric luminosity was possible, it was found that typically more than 50% of the star's energy was radiated longward of 20 micrometers. meaningful limits to the far infrared luminosities of an additional eleven association members in Cr A and two in rho Oph were also obtained. The dust optical depth surrounding the star R Cr A appears to be asymmetric and may control the dynamics of the surrounding molecular gas. The implications of the results for the cloud energetics and star formation efficiency in these two clouds are discussed
Are young stars always associated with cold massive disks? A CO and millimeter interferometric continuum survey
The results of a combined millimeter-spectral-line and continuum survey of cold far-infrared sources selected to favor embedded young stars in the Galaxy are presented. The spectral-line observations were performed with the 5 meter antenna of the University of Texas Millimeter-Wave Observatory. High resolution continuum observations were obtained with the Owens Valley (OVRO) Millimeter-Wave Interferometer. The goal of the survey was to gain insight into the mass, temperature, and distribution of cold dust which envelopes stars during the earliest stages of their evolution. The first phase of our survey involved 1.2 arcmin resolution observations of CO-12 and CO-13 emission lines toward each source. All but two sources had detectable CO emission. We found that 40% of the sources appear to be associated with star formation as evidenced by the presence of enhanced CO-12 line widths or broad wings. At least five of these objects are associated with bipolar molecular outflows. The second phase of our survey involves high resolution 2.7 mm continuum observations with 3 interferometer baselines ranging from 15 to 55 m in length. Preliminary results indicate that about 25% of the sources in our sample have detectable continuum emission on scales less than 30 arcsec. The high percentage of sources with enhanced CO-12 line widths or broad wings indicates that a significant fraction of our samples, 40%, are likely to be young stars. The lower detection percentage in the continuum observations, 25%, suggest that such objects are not always surrounded by large concentrations of gas and dust. The continuum detection percentage for actual dust emission could be lower than that given above since emission from ionized gas could be responsible for the observed 2.7 mm emission in some objects. To get an understanding of the type of object detected in our survey, a map of one of the survey sources, L1689N, has been made using the OVRO mm interferometer
Clifford algebras and new singular Riemannian foliations in spheres
Using representations of Clifford algebras we construct indecomposable
singular Riemannian foliations on round spheres, most of which are
non-homogeneous. This generalizes the construction of non-homogeneous
isoparametric hypersurfaces due to by Ferus, Karcher and Munzner.Comment: 21 pages. Construction of foliations in the Cayley plane added.
Proofs simplified and presentation improved, according to referee's
suggestions. To appear in Geom. Funct. Ana
A Mid-Infrared Imaging Survey of Embedded Young Stellar Objects in the Rho Ophiuchi Cloud Core
Results of a comprehensive, new, ground-based mid-infrared imaging survey of
the young stellar population of the Rho Ophiuchi cloud are presented. Data were
acquired at the Palomar 5-m and at the Keck 10-m telescopes with the MIRLIN and
LWS instruments, at 0.25 arcsec and 0.25 arcsec resolutions, respectively. Of
172 survey objects, 85 were detected. Among the 22 multiple systems observed,
15 were resolved and their individual component fluxes determined. A plot of
the frequency distribution of the detected objects with SED spectral slope
shows that YSOs spend ~400,000 yr in the Flat Spectrum phase, clearing out
their remnant infall envelopes. Mid-infrared variability is found among a
significant fraction of the surveyed objects, and is found to occur for all SED
classes with optically thick disks. Large-amplitude near-infrared variability,
also found for all SED classes with optically thick disks, seems to occur with
somewhat higher frequency at the earlier evolutionary stages. Although a
general trend of mid-infrared excess and NIR veiling exists proceeding through
SED classes, with Class I objects generally exhibiting K-veilings > 1, Flat
Spectrum objects with K-veilings > 0.58, and Class III objects with K-veilings
=0, Class II objects exhibit the widest range of K-band veiling values, 0-4.5.
However, the highly variable value of veiling that a single source can exhibit
in any of the SED classes in which active disk accretion can take place is
striking, and is direct observational evidence for highly time-variable
accretion activity in disks. Finally, by comparing mid-infrared vs.
near-infrared excesses in a subsample with well-determined effective
temperatures and extinction values, disk clearing mechanisms are explored. The
results are consistent with disk clearing proceeding from the inside-out.Comment: 18 pages + 5 tables + 7 figure
Detection of Molecular Hydrogen Orbiting a "Naked" T Tauri Star
Astronomers have established that for a few million years newborn stars
possess disks of orbiting gas and dust. Such disks, which are likely sites of
planet formation, appear to disappear once these stars reach ages of 5-10 times
10^6 yr; yet, >= 10^7 yr is thought necessary for giant planet formation. If
disks dissipate in less time than is needed for giant planet formation, such
planets may be rare and those known around nearby stars would be anomalies.
Herein, we report the discovery of H_2 gas orbiting a weak-lined T Tauri star
heretofore presumed nearly devoid of circumstellar material. We estimate that a
significant amount of H_2 persists in the gas phase, but only a tiny fraction
of this mass emits in the near-infrared. We propose that this star possesses an
evolved disk that has escaped detection thus far because much of the dust has
coagulated into planetesimals. This discovery suggests that the theory that
disks are largely absent around such stars should be reconsidered. The
widespread presence of such disks would indicate that planetesimals can form
quickly and giant planet formation can proceed to completion before the gas in
circumstellar disks disperses.Comment: latex 12 pages, including 1 figur
The very low-mass population of the Corona Australis and Chamaeleon II star forming regions
We present the results of a deep optical survey in the Corona Australis and
Chamaeleon II star forming regions. Our optical photometry is combined with
available near- and mid-infrared photometry to identify very low-mass candidate
members in these dark clouds. In our Chamaeleon II field, only one object
exhibits clear H-alpha emission, but the discrepancy between its optical and
near-infrared colours suggests that it might be a foreground star. We also
identify two objects without H-alpha emission that could be planetary mass
members of Chamaeleon II. In Corona Australis, we find ten stars and three
brown dwarf candidates in the Coronet cluster. Five of our new members are
identified with ISOCAM sources. Only two of them have a mid-infrared excess,
indicating the presence of an accretion disk. On the other hand, one brown
dwarf candidate has a faint close companion, seen only in our deepest I-band
image. For many of the candidates in both clouds, membership could not be
inferred from their H-alpha emission or near-infrared colours; these objects
need spectroscopic confirmation of their status
Probing the embedded YSOs of the R CrA region through VLT-ISAAC spectroscopy
Near IR spectra obtained with ISAAC at VLT, have been used to pose
constraints on the evolutionary state and accretion properties of a sample of
five embedded YSOs located in the R CrA core. This sample includes three Class
I sources (HH100 IR, IRS2 and IRS5), and two sources with NIR excesses (IRS6
and IRS3). Absorption lines have been detected in the medium resolution spectra
of all the observed targets, together with emission lines likely originating in
the disk-star-wind connected regions. We derived spectral types, veiling and
stellar luminosity of the five observed sources, which in turn have been used
to infer their mass and age adopting pre-main sequence evolutionary tracks. We
find that in HH100 IR and IRS2 most of the bolometric luminosity is due to
accretion, while the other three investigated sources, including the Class I
object IRS5a, present a low accretion activity (L_{acc}/L_{bol} < 0.2). We
observe a general correlation between the accretion luminosity, the IR veiling
and the emission line activity of the sources. A correlation between the
accretion activity and the spectral energy distribution slope is recognizable
but with the notable exception of IRS5a. Our analysis therefore shows how the
definition of the evolutionary stage of deeply embedded YSOs by means of IR
colors needs to be more carefully refined.Comment: 17 pages, 13 figures, accepted on A&
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