We report the results of a near-infrared survey for long-period variables in a field of view of 20× 30 arcmin2 towards the Galactic Centre (GC). We have detected 1364 variables, of which 348 are identified with those reported in Glass et al. We present a catalogue and photometric measurements for the detected variables and discuss their nature. We also establish a method for the simultaneous estimation of distances and extinctions using the period–luminosity relations for the JHK s bands. Our method is applicable to Miras with periods in the range 100–350 d and mean magnitudes available in two or more filter bands. While J band means are often unavailable for our objects because of the large extinction, we estimated distances and extinctions for 143 Miras whose H - and K s -band mean magnitudes are obtained. We find that most are located at the same distance to within our accuracy. Assuming that the barycentre of these Miras corresponds to the GC, we estimate its distance modulus to be 14.58 ± 0.02 (stat.) ± 0.11 (syst.) mag , corresponding to 8.24 ± 0.08 (stat.) ± 0.42 (syst.) kpc . We have assumed the distance modulus to the Large Magellanic Cloud to be 18.45 mag, and the uncertainty in this quantity is included in the above systematic error. We also discuss the large and highly variable extinction. Its value ranges from 1.5 mag to larger than 4 mag in except towards the thicker dark nebulae and it varies in a complicated way with the line of sight. We have identified mid-infrared counterparts in the Spitzer /IRAC catalogue of Ramírez et al. for most of our variables and find that they follow rather narrow period–luminosity relations in the 3.6–8.0 μm wavelength range. 相似文献
We aim at understanding the statistical properties of luminous sub-millimeter (submm) galaxies (SMGs) in the context of cosmological structure formation. By utilizing a cosmological N-body simulation to calculate the distribution of dark halos in the Universe, we consider the dust enrichment in individual halos by Type II supernovae (SNe II). The SN II rate is estimated under a star formation activity which is assumed to occur on a dynamical timescale in the dark matter potential. Our simple framework successfully explains the luminosity function, the typical star formation rate, and the typical dust mass of an observational SMG sample at z~3. We also examine the clustering properties of SMGs, since a positive cross correlation between SMGs and Lyα emitters (LAEs) is indeed observed by a recent observation. In the simulation, we select SMGs by FIR dust luminosity >1012L⊙, while LAEs are chosen such that the age and the virial mass are consistent with the observed LAE properties. The SMGs and LAEs selected in this way show a spatial cross correlation whose strength is consistent with the observation. This confirms that the SMGs really trace the most clustered regions at z~3 and that their luminosities can be explained by the dust accumulation as a result of their star formation activities. We extend our prediction to higher redshifts, finding that a statistical sample of submm galaxies at z≥6 can be obtained by ALMA with a 100 arcmin2 survey. With the same survey, a few submm galaxies at z~10 may be detected. 相似文献
Recently, several attempts have been made to model the wind velocity in an urban canopy in order to accurately predict the
mixing and transport of momentum, heat, and pollutants within and above the canopy on an urban scale. For this purpose, unverified
assumptions made by Macdonald (Boundary-Layer Meteorol 97:25–45, 2000) to develop a model for the profile of the mean wind
velocity within an urban canopy have been used. In the present study, in order to provide foundations for improving the urban
canopy models, the properties of the spatially-averaged mean quantities used to make these assumptions have been investigated
by performing large-eddy simulations (LES) of the airflow around square and staggered arrays of cubical blocks with the following
plan area densities: λp = 0.05, 0.11, 0.16, 0.20, 0.25, and 0.33. The LES results confirm that the discrepancy between the spatial average of wind
velocity and Macdonald’s five-point average of wind velocity can be large in both types of arrays for large λp. It is also confirmed that Prandtl’s mixing length varies significantly with height within the canopy, contrary to Macdonald’s
assumption for both types of arrays and for both small and large λp. On the other hand, in accordance with Macdonald’s assumption, the sectional drag coefficient is found to be almost constant
with height except in the case of staggered arrays with high λp. 相似文献
Depletion of Nb relative to K and La is characteristic of lavas in subduction-related magmatic arcs, as distinct from mid-ocean ridge basalts. Nb depletion is also characteristic of the continental crust. This and other geochemical similarities between the continental crust and high-Mg# andesite magmas found in arcs suggests that the continental crust may have formed by accretion of andesites. Previous studies have shown that the major element characteristics of high-Mg# andesites may be produced by melt/rock reaction in the upper mantle. In this paper, new data on partitioning of K, Nb, La and Ce between garnet, orthopyroxene and clinopyroxene in mantle xenoliths, and on partitioning of Nb and La between orthopyroxene and liquid, show that garnet and orthopyroxene have Nb crystal/liquid distribution coefficients which are much larger than those of K and La. Similar fractionations of Nb from K and La are expected in spinel and olivine. For this reason, reactions between migrating melt and large masses of mantle peridotite can produce substantial depletion of Nb in derivative liquids. Modeling shows that reaction between ascending, mantle-derived melts and mantle peridotite is a viable mechanism for producing the trace element characteristics of high-Mg# andesite magmas and the continental crust.
Alternatively, small-degree melts of metabasalt and/or metasediment in the subducting slab may leave rutile in their residue, and will thus have large Nb depletions relative to K and La [1]. Slab melts are too rich in light rare earth elements and other incompatible elements, and too poor in compatible elements, to be parental to arc magmas. However, ascending slab melts may be modified by reaction with the mantle. Our new data permit modeling of the trace element effects of reaction between small-degree melts of the slab and mantle peridotite. Modeling shows that this type of reaction is also a viable mechanism for producing the trace element characteristics of high-Mg# andesites and the continental crust. These findings, in combination with previous results, suggest that melt/rock reaction in the upper mantle has been an important process in forming the continental crust and mantle lithosphere. 相似文献
Peridotite samples recovered from IODP Site U1309 at the Atlantis Massif in the Mid-Atlantic Ridge were examined to understand
magmatic processes for the oceanic core complex formation. Original peridotite was fragmented, and the limited short peridotite
intervals are now surrounded by a huge gabbro body probably formed by late-stage melt injections. Each peridotite interval
has various petrographical and geochemical features. A spinel harzburgite in contact with gabbro shows evidence of limited
melt penetrations causing gradual compositional change, in terms of trace-element compositions of pyroxenes, as well as modal
change near the boundary. Geochemistry of clinopyroxenes with least melt effects indicates that the harzburgite is originally
mantle residue formed by partial melting under polybaric conditions, and that such a depleted peridotite is one of the components
of the oceanic core complex. Some of plagioclase-bearing peridotites, on the other hand, have more complicated origin. Although
their original features were partly overprinted by the injected melt, the original peridotites, both residual and non-residual
materials, were possibly derived from the upper mantle. This suggests that the melt injected around an upper mantle region
or into mantle material fragments. The injected melt was possibly generated at the ridge-segment center and, then, moved and
evolved toward the segment end beneath the oceanic core complex. 相似文献