6,230 research outputs found
Motlles naturals de microperforacions a eolianites del Pleistocè i Pliocè mallorquĂ
En aquest treball estudiem els motlles naturalis de microperforacions, actualment de LMC, en substrats de fragments de mol¡luscs. L'aragonita de les closques fou dissolt en condicions vadoses continentals, amb la consegßent posada en relleu dels motlles i de la "micritic envelope".Els materials estudiats son eolianites d'edat Riss i Pliocenes, de diversos afloraments de Mallorca. Hi hem observat quatre tipus de motlles que suposem que foren produits per l'acció perforant de quatre microorganismes diferents. S'hi analitza la història diagenètica dels motlles
SPITZER observations of the Îť Orionis cluster. II. Disks around solar-type and low-mass stars
We present IRAC/MIPS Spitzer Space Telescope observations of the solar-type and the low-mass stellar population
of the young (~5Myr) Îť Orionis cluster. Combining optical and Two Micron All Sky Survey photometry, we identify
436 stars as probable members of the cluster. Given the distance (450 pc) and the age of the cluster, our sample
ranges in mass from 2 M_â to objects below the substellar limit. With the addition of the Spitzer mid-infrared data,
we have identified 49 stars bearing disks in the stellar cluster. Using spectral energy distribution slopes, we place
objects in several classes: non-excess stars (diskless), stars with optically thick disks, stars with âevolved disksâ
(with smaller excesses than optically thick disk systems), and âtransitional diskâ candidates (in which the inner
disk is partially or fully cleared). The disk fraction depends on the stellar mass, ranging from ~6% for K-type stars (R_C â J 4). We confirm the dependence of disk
fraction on stellar mass in this age range found in other studies. Regarding clustering levels, the overall fraction of disks in the Îť Orionis cluster is similar to those reported in other stellar groups with ages normally quoted as ~5Myr
Viscous diffusion and photoevaporation of stellar disks
The evolution of a stellar disk under the influence of viscous evolution,
photoevaporation from the central source, and photoevaporation by external
stars is studied. We take the typical parameters of TTSs and the Trapezium
Cluster conditions. The photoionizing flux from the central source is assumed
to arise both from the quiescent star and accretion shocks at the base of
stellar magnetospheric columns, along which material from the disk accretes.
The accretion flux is calculated self-consistently from the accretion mass loss
rate. We find that the disk cannot be entirely removed using only viscous
evolution and photoionization from the disk-star accretion shock. However, when
FUV photoevaporation by external massive stars is included the disk is removed
in 10^6 -10^7yr; and when EUV photoevaporation by external massive stars is
included the disk is removed in 10^5 - 10^6yr.
An intriguing feature of photoevaporation by the central star is the
formation of a gap in the disk at late stages of the disk evolution. As the gap
starts forming, viscous spreading and photoevaporation work in resonance.
There is no gap formation for disks nearby external massive stars because the
outer annuli are quickly removed by the dominant EUV flux. On the other hand,
at larger, more typical distances (d>>0.03pc) from the external stars the flux
is FUV dominated. As a consequence, the disk is efficiently evaporated at two
different locations; forming a gap during the last stages of the disk
evolution.Comment: 27 pages, 11 figures, accepted for publication in Ap
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