924 research outputs found
Dependence of Spiral Galaxy Distribution on Viewing Angle in RC3
The normalized inclination distributions are presented for the spiral
galaxies in RC3. The results show that, except for the bin of
-, in which the apparent minor isophotal diameters that
are used to obtain the inclinations, are affected by the central bulges, the
distributions for Sa, Sab, Scd and Sd are well consistent with the Monte-Carlo
simulation of random inclinations within 3-, and Sb and Sbc almost, but
Sc is different. One reason for the difference between the real distribution
and the Monte-Carlo simulation of Sc may be that some quite inclined spirals,
the arms of which are inherently loosely wound on the galactic plane and should
be classified to Sc galaxies, have been incorrectly classified to the earlier
ones, because the tightness of spiral arms which is one of the criteria of the
Hubble classification in RC3 is different between on the galactic plane and on
the tangent plane of the celestial sphere. Our result also implies that there
might exist biases in the luminosity functions of individual Hubble types if
spiral galaxies are only classified visually.Comment: 5 pages + 8 figures, LaTe
Confirmation of previous ground-based Cepheid P-L zero-points using Hipparcos trigonometric parallaxes
Comparisons show agreement at the 0.1-mag level between the calibration of the Cepheid period-luminosity (P-L) relation by Feast & Catchpole (FC) using the early release of Hipparcos data and four previous ground-based calibrations, three of which are either largely or totally independent of the distance to the Large Magellanic Cloud (LMC). Each of the comparisons has the sense that the FC calibration is brighter, but only at the level of ≲ 0.1 mag. In contrast, FC argue that their Hipparcos recalibration leads to a 0.2-mag revision in the distance to the LMC, and thereby to a 10 per cent decrease in the Hubble constant. We argue differently. The comparison of the Hipparcos recalibration with others should be made using only local Galactic Cepheids, not based on Cepheids in the LMC that require a set of precepts that are not germane to the direct Hipparcos recalibration. The comparison made here, using only Galactic Cepheids, gives a correction of ∽ 4 per cent or less to our value of H0 based on Type Ia supernovae, keeping all other factors and precepts the same. A second success of the Hipparcos mission is the calibration of the position of the main sequence in the Hertzsprung—Russell diagram as a function of metallicity using local subdwarfs. These data have been used by Reid and by Gratton et al. to obtain, similarly to FC, a brighter absolute magnitude of RR Lyrae stars by ∽0.3 mag from that often currently adopted. These new calibrations confirm the earlier brighter calibrations by Walker, by Sandage, and by Mazzitelli, D'Antona & Caloi, thereby reducing the ages of globular clusters by ∽30 per cent. This removes most of the cosmological time-scale problem if H0∽55 km s−1 Mpc−1. A similar conclusion, based on pulsation theory and MACHO data, has been reached by Alcock et a
RR Lyrae stars in Galactic globular clusters. VI. The Period-Amplitude relation
We compare theory and observations for fundamental RR Lyrae in the solar
neighborhood and in both Oosterhoff type I (OoI) and type II (OoII) Galactic
globular clusters (GGCs). The distribution of cluster RR_ab in the PA_V plane
depends not only on the metal abundance, but also on the cluster Horizontal
Branch (HB) morphology. On average the observed k_puls parameter, connecting
the period to the visual amplitude, increases when moving from metal-poor to
metal-rich GGCs. However, this parameter shows marginal changes among OoI
clusters with intermediate to red HB types and iron abundances -1.8<= [Fe/H]
<=-1.1, whereas its value decreases in OoII clusters with the bluer HB
morphology. Moreover, at [Fe/H]=-1.7+-0.1 the OoI clusters present redder HB
types and larger values than the OoII clusters. The RR_ab variables in
Omega Cen and in the solar neighborhood further support the evidence that the
spread in [Fe/H], at fixed k_puls, is of the order of +-0.5 dex. Synthetic HB
simulations show that the PA_V plane can provide accurate cluster distance
estimates. The RR_ab variables in OoI and in OoII clusters with very blue HB
types obey a well-defined M_V(RR)-k_puls relation, while those in OoII clusters
with moderately blue HB types present a zero-point that is ~0.05 mag brighter.
Regarding field variables, we show that with [Fe/H]=> -1.0 a unique
M_V(RR)-k_puls relation can be adopted, independently of the parent HB
morphology. Current findings suggest that the PA_V distribution does not seem
to be a robust diagnostic for the metal abundance of RR_ab variables. However,
the same observables can be used to estimate the absolute magnitude of globular
cluster and field RR_ab variables. We show that over the metallicity range
-2.4<= [Fe/H] <= 0.0 the M_V(RR)-[Fe/H] relation shows a parabolic behavior.Comment: Paper accepted on A&A, 13 pages, 18 figure
The expansion field: the value of H 0
Any calibration of the present value of the Hubble constant (H 0) requires recession velocities and distances of galaxies. While the conversion of observed velocities into true recession velocities has only a small effect on the result, the derivation of unbiased distances which rest on a solid zero point and cover a useful range of about 4-30Mpc is crucial. A list of 279 such galaxy distances within v4.5 Mpc. RRLyr star-calibrated TRGB distances of 78 galaxies above this limit give H 0=63.0±1.6 at an effective distance of 6Mpc. They compensate the effect of peculiar motions by their large number. Support for this result comes from 28 independently calibrated Cepheids that give H 0=63.4±1.7 at 15Mpc. This agrees also with the large-scale value of H 0=61.2±0.5 from the distant, Cepheid-calibrated SNeIa. A mean value of H 0=62.3±1.3 is adopted. Because the value depends on two independent zero points of the distance scale its systematic error is estimated to be 6%. Other determinations of H 0 are discussed. They either conform with the quoted value (e.g. line width data of spirals or the D n −σ method of E galaxies) or are judged to be inconclusive. Typical errors of H 0 come from the use of a universal, yet unjustified P-L relation of Cepheids, the neglect of selection bias in magnitude-limited samples, or they are inherent to the adopted model
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