594 research outputs found
New period-luminosity and period-color relations of classical Cepheids: III. Cepheids in SMC
The photometric data for 460 classical, fundamental-mode Cepheids in the SMC
with log P > 0.4 measured by Udalski et al. have been analyzed for their P-C
and P-L relations, and for the variation of amplitude across the instability
strip in a similar way that was done in Papers I and II of this series. The SMC
Cepheids are bluer in (B-V) at a given period than for both the Galaxy and the
LMC. Their P-C relation in (B-V) is best fit by two lines intersecting at P=10
d. Their break must necessarily exist also in the P-L relations in B and/or V,
but remains hidden in the magnitude scatter. An additional pronounced break of
the P-L relations in B, V, and I occurs at P=2.5 d. The observed slope of the
lines of constant period in the HR diagram agrees with the theoretical
expectation from the pulsation equation. The largest amplitude Cepheids for
periods less than 13 days occur near the blue edge of the instability strip.
The sense is reversed in the period interval from 13 to 20 days, as in the
Galaxy and the LMC. The SMC P-L relation is significantly flatter than that for
the Galaxy, NGC 3351, 4321, M31, all of which have nearly the same steep slope.
The SMC P-L slope is intermediate between that of these steep slope cases and
the very shallow slope of Cepheids in the lower metallicity galaxies of NGC
3109 and Sextans A/B, consistent with the premise that the Cepheid P-L relation
varies from galaxy-to-galaxy as function of metallicity. Failure to take into
account the slope differences in the P-L relation as a function of metallicity
using Cepheids as distance indicators results in incorrect Cepheid distances.
Part of the 15% difference between our long distance scale - now independently
supported by TRGB distances - and that of the HST Key Project short scale is
due to the effect of using an inappropriate P-L relation.Comment: 9 pages, 10 figures, accepted for publication in Astronomy &
Astrophysic
Temperature Differences in the Cepheid Instability Strip Require Differences in the Period-Luminosity Relation in Slope and Zero Point
A graphical and an algebraic demonstration is made to show why the slope and
zero point of the Cepheid period-luminosity (P-L) relation is rigidly coupled
with the slope and zero point of the Cepheid instability strip in the HR
diagram. The graphical demonstration uses an arbitrary (toy) ridge line in the
instability strip, while the algebraic demonstration uses the pulsation
equation into which the observed P-L relations for the Galaxy and the LMC are
put to predict the temperature zero points and slopes of the instability
strips. Agreement between the predicted and measured instability strip slopes
argue that the observed P-L differences between the Galaxy and LMC are real. In
another proof, the direct evidence for different P-L slopes in different
galaxies is shown by comparing the Cepheid data in the Galaxy, the combined
data in NGC 3351 and NGC 4321, in M31, LMC, SMC, IC 1613, NGC 3109, and in
Sextans A+B. The P-L slopes for the Galaxy, NGC 3351, NGC 4321, and M31 are
nearly identical and are the steepest in the sample. The P-L slopes decrease
monotonically with metallicity in the order listed, showing that the P-L
relation is not the same in different galaxies, complicating their use in
calibrating the extragalactic distance scale.Comment: 15 pages, 1 table, 4 figures, submitted to The Ap
Classical Cepheid Pulsation Models. III. The Predictable Scenario
Within the current uncertainties in the treatment of the coupling between
pulsation and convection, limiting amplitude, nonlinear, convective models
appear the only viable approach for providing theoretical predictions about the
intrinsic properties of radial pulsators. In this paper we present the results
of a comprehensive set of Cepheid models computed within such theoretical
framework for selected assumptions on their original chemical composition.Comment: 24 pages, 1 latex file containing 6 tables, 10 postscript figures,
accepted for publication on Ap
Structure of the Large Magellanic Cloud from 2MASS
We derive structural parameters and evidence for extended tidal debris from
star count and preliminary standard candle analyses of the Large Magellanic
Cloud based on Two Micron All Sky Survey (2MASS) data. The full-sky coverage
and low extinction in K_s presents an ideal sample for structural analysis of
the LMC.
The star count surface densities and deprojected inclination for both young
and older populations are consistent with previous work. We use the full areal
coverage and large LMC diameter to Galactrocentric distance ratio to infer the
same value for the disk inclination based on perspective.
A standard candle analysis based on a sample of carbon long-period variables
(LPV) in a narrow color range, 1.6<J-K_s<1.7 allows us to probe the
three-dimensional structure of the LMC along the line of sight. The intrinsic
brightness distribution of carbon LPVs in selected fields implies that
\sigma_M\simlt 0.2^m for this color cut. The sample provides a {\it direct}
determination of the LMC disk inclination: .
Distinct features in the photometric distribution suggest several distinct
populations. We interpret this as the presence of an extended stellar component
of the LMC, which may be as thick as 14 kpc, and intervening tidal debris at
roughly 15 kpc from the LMC.Comment: 24 pages, 9 figures. Submitted to Ap
Intermediate-mass star models with different helium and metal contents
We present a comprehensive theoretical investigation of the evolutionary
properties of intermediate-mass stars. The evolutionary sequences were computed
from the Zero Age Main Sequence up to the central He exhaustion and often up to
the phases which precede the carbon ignition or to the reignition of the
H-shell which marks the beginning of the thermal pulse phase. The evolutionary
tracks were constructed by adopting a wide range of stellar masses
(\msun) and chemical compositions. In order to account for
current uncertainties on the He to heavy elements enrichment ratio, the stellar
models were computed by adopting at Z=0.02 two different He contents (Y=0.27,
0.289) and at Z=0.04 three different He contents (Y=0.29, 0.34, and 0.37). To
supply a homogeneous evolutionary scenario which accounts for young Magellanic
stellar systems the calculations were also extended toward lower metallicities
(Z=0.004, Z=0.01), by adopting different initial He abundances. We evaluated
for both solar (Z=0.02) and super-metal-rich (SMR, Z=0.04) models the
transition mass between the stellar structures igniting carbon and
those which develop a full electron degeneracy inside the CO core. This
evolutionary scenario allows us to investigate in detail the properties of
classical Cepheids. In particular, we find that the range of stellar masses
which perform the blue loop during the central He-burning phase narrows when
moving toward metal-rich and SMR structures.Comment: 25 pages, 10 figures (4 postscript + 6 gif files), 7 postscript
tables. accepted for publication on ApJ (November 2000
Targeting the Mitotic Checkpoint to Kill Tumor Cells
One of the most common hallmarks of cancer cells is aneuploidy or an abnormal number of chromosomes. This abnormal chromosome content is a consequence of chromosome missegregation during mitosis, a defect that is seen more frequently in tumor cell divisions as in normal cell divisions. In fact, a large fraction of human tumors display a chromosome instable phenotype, meaning that they very frequently missegregate chromosomes. This can cause variegated aneuploidy within the tumor tissue. It has been argued that this hallmark of cancer could be exploited in anti-cancer therapies. Here we test this hypothesis by inactivation of the mitotic checkpoint through RNAi-mediated depletion of an essential checkpoint component, Mps1. The mitotic checkpoint delays segregation of chromosomes during mitosis until all chromosomes are properly attached to the mitotic spindle. Its inactivation will therefore lead to increased segregation errors. Indeed, we show that this can lead to increased cell death in tumor cells. We demonstrate that increased cell death is associated with a dramatic increase in segregation errors. This suggests that inhibition of the mitotic checkpoint might represent a useful anti-cancer strategy
Teleology and Realism in Leibniz's Philosophy of Science
This paper argues for an interpretation of Leibniz’s claim that physics requires both mechanical and teleological principles as a view regarding the interpretation of physical theories. Granting that Leibniz’s fundamental ontology remains non-physical, or mentalistic, it argues that teleological principles nevertheless ground a realist commitment about mechanical descriptions of phenomena. The empirical results of the new sciences, according to Leibniz, have genuine truth conditions: there is a fact of the matter about the regularities observed in experience. Taking this stance, however, requires bringing non-empirical reasons to bear upon mechanical causal claims. This paper first evaluates extant interpretations of Leibniz’s thesis that there are two realms in physics as describing parallel, self-sufficient sets of laws. It then examines Leibniz’s use of teleological principles to interpret scientific results in the context of his interventions in debates in seventeenth-century kinematic theory, and in the teaching of Copernicanism. Leibniz’s use of the principle of continuity and the principle of simplicity, for instance, reveal an underlying commitment to the truth-aptness, or approximate truth-aptness, of the new natural sciences. The paper concludes with a brief remark on the relation between metaphysics, theology, and physics in Leibniz
Ligand-engaged TCR is triggered by Lck not associated with CD8 coreceptor
Producción CientíficaThe earliest molecular events in T-cell recognition have not yet been fully described, and the initial T-cell receptor (TCR)-triggering mechanism remains a subject of controversy. Here, using total internal reflection/Forster resonance energy transfer microscopy, we observe a two-stage interaction between TCR, CD8 and major histocompatibility complex (MHC)-peptide. There is an early (within seconds) interaction between CD3ζ and the coreceptor CD8 that is independent of the binding of CD8 to MHC, but that requires CD8 association with Lck. Later (several minutes) CD3ζ–CD8 interactions require CD8–MHC binding. Lck can be found free or bound to the coreceptor. This work indicates that the initial TCR-triggering event is induced by free Lck. The early signalling events that trigger initial T-cell receptor signalling are not clearly defined. Here the authors show that this occurs in two stages, the first between the CD8 coreceptor and CD3 requiring Lck association to CD8, while the second interaction requires binding of major histocompatibility molecules
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