170,056 research outputs found
Bivariate least squares linear regression: towards a unified analytic formalism. II. Extreme structural models
No full textConcerning bivariate least squares linear regression, the
classical results obtained for extreme structural
models in earlier attempts (Isobe et al., 1990;
Feigelson and Babu, 1992) are
reviewed using a new formalism in terms
of deviation (matrix) traces
which, for homoscedastic data, reduce to usual
quantities leaving aside an
unessential (but dimensional) multiplicative factor.
Within the framework of classical error models,
the dependent variable relates to the independent
variable according to a variant of the usual additive model.
The classes of linear models considered are
regression lines in the limit of
uncorrelated errors in X and in Y.
The following models are
considered in detail: (Y) errors
in X negligible (ideally null) with
respect to errors in Y; (X) errors
in Y negligible (ideally null) with
respect to errors in X; (C) oblique
regression; (O) orthogonal regression;
(R) reduced major-axis regression;
(B) bisector regression.
For homoscedastic data, the results are
taken from earlier attempts and rewritten
using a more compact notation. For
heteroscedastic data, the results are
inferred from a procedure related to
functional models (York, 1966; Caimmi, 2011).
An example of astronomical
application is considered, concerning
the [O/H]-[Fe/H] empirical relations
deduced from five samples related to
different stars and/or different methods
of oxygen abundance determination.
For low-dispersion samples and assigned
methods, different regression models
yield results which are in agreement
within the errors for both
heteroscedastic and homoscedastic data,
while the contrary holds for large-dispersion
samples. In any case,
samples related to different
methods produce discrepant results,
due to the presence of (still undetected)
systematic errors, which implies no
definitive statement can be made at
present. Asymptotic expressions
approximate regression line slope
and intercept variance estimators,
for normal residuals, to a better
extent with respect to earlier
attempts. Related fractional
discrepancies are not exceeding
a few percent for low-dispersion
data, which grows up to about 10%
for large-dispersion data.
An extension of the formalism to
generic structural models
is left to a forthcoming paper
The G-dwarf problem in the Galaxy
No full textThis paper has two parts: one about observational constraints, and the other about chemical evolution models. In the first part, the empirical differential metallicity distribution (EDMD) is deduced from three different samples involving (i) local thick disk stars derived from Gliese and scaled in situ samples within the range, -1.20⩽[Fe/H]⩽-0.20 [Wyse, R.F.G., Gilmore, G., 1995. AJ 110, 2771]; (ii) 46 likely metal-weak thick disk stars within the range, -2.20⩽[Fe/H]⩽-1.00 [Chiba, M., Beers, T.C., 2000. AJ 119, 2843]; (iii) 287 chemically selected G dwarfs within 25 pc from the Sun, with the corrections performed in order to take into account the stellar scale height [Rocha-Pinto, H.J., Maciel, W.J., 1996. MNRAS 279, 447]; in addition to previous results [Caimmi, R., 2001b. AN 322, 241; Caimmi, R., 2007. NewA 12, 289] related to (iv) 372 solar neighbourhood halo subdwarfs [Ryan, S.G., Norris, J.E., 1991. AJ 101, 1865]; and (v) 268 K-giant bulge stars [Sadler, E.M., Rich, R.M., Terndrup, D.M., 1996. AJ 112, 171]. The metal-poor and metal-rich EDMD related to the thick disk shows similarities with their halo and bulge counterparts, respectively. Then the thick disk is conceived as made of two distinct regions: the halo-like and the bulge-like thick disk, and the related EDMD is deduced. Under the assumption that each distribution is typical for the corresponding subsystem, the EDMD of the thick disk, the thick + thin disk, and the Galaxy, is determined by weighting the mass. In the second part, models of chemical evolution for the halo-like thick disk, the bulge-like thick disk, and the thin disk, are computed assuming the instantaneous recycling approximation. The EDMD data are fitted, to an acceptable extent, by simple models of chemical evolution implying both homogeneous and inhomogeneous star formation, provided that star formation is inhibited during thick disk formation, with respect to the thin disk. The initial mass function (IMF) is assumed to be a universal power law, which implies the same value of the true yield in different subsystems. The theoretical differential metallicity distribution (TDMD) is first determined for the halo-like thick disk, the bulge-like thick disk, and the thin disk separately, and then for the Galaxy by weighting the mass. An indicative comparison is performed between the EDMD deduced for the disk both in presence and in absence of [O/Fe] plateau, and its counterpart computed for (vi) N=523 nearby stars within the range, -1.5<[Fe/H]<0.5, for which the oxygen abundance has been determined both in presence and in absence of the local thermodynamical equilibrium (LTE) approximation [Ramirez, I., Allende Prieto, C., Lambert, D.L., 2007. A&A 465, 271]. Both distributions are found to exhibit a similar trend, although systematic differences exist. In addition, the related empirical age-metallicity relation (EAMR) cannot be fitted by the theoretical age-metallicity relation (TAMR) predicted by the model, and the reasons for this discrepancy are explained
Bivariate least squares linear regression: Towards a unified analytic formalism. I. Functional models
No full textConcerning bivariate least squares linear regression, the classical approach pursued for functional models in earlier attempts (York, 1966, 1969) is reviewed using a new formalism in terms of deviation (matrix) traces which, for unweighted data, reduce to usual quantities leaving aside an unessential (but dimensional) multiplicative factor. Within the framework of classical error models, the dependent variable relates to the independent variable according to the usual additive model. The classes of linear models considered are regression lines in the general case of correlated errors in X and in Y for weighted data, and in the opposite limiting situations of (i) uncorrelated errors in X and in Y, and (ii) completely correlated errors in X and in Y. The special case of (C) generalized orthogonal regression is considered in detail together with well known subcases, namely: (Y) errors in X negligible (ideally null) with respect to errors in Y; (X) errors in Y negligible (ideally null) with respect to errors in X; (O) genuine orthogonal regression; (R) reduced major-axis regression. In the limit of unweighted data, the results determined for functional models are compared with their counterparts related to extreme structural models i.e. the instrumental scatter is negligible (ideally null) with respect to the intrinsic scatter (Isobe et al., 1990; Feigelson and Babu, 1992). While regression line slope and intercept estimators for functional and structural models necessarily coincide, the contrary holds for related variance estimators even if the residuals obey a Gaussian distribution, with the exception of Y models. An example of astronomical application is considered, concerning the [O/H]-[Fe/H] empirical relations deduced from five samples related to different stars and/or different methods of oxygen abundance determination. For selected samples and assigned methods, different regression models yield consistent results within the errors (∓σ) for both heteroscedastic and homoscedastic data. Conversely, samples related to different methods produce discrepant results, due to the presence of (still undetected) systematic errors, which implies no definitive statement can be made at present. A comparison is also made between different expressions of regression line slope and intercept variance estimators, where fractional discrepancies are found to be not exceeding a few percent, which grows up to about 20% in the presence of large dispersion data. An extension of the formalism to structural models is left to a forthcoming paper
Test of Clausius' Virial Dynamical Theory of Fundamental Plane By Homogeneous + γ-Free Two Component Galaxy Model
No full textIntroduction: the theory of the Fundamental Plane (FP) proposed by Secco (2000, 2001,2005) is based on the existence of a maximum in the Clausius' Virial potential energy (CV) of a stellar component when it is
completely embedded inside a dark matter (DM) halo. At the first order approximation the theory was developed by modeling the two-components with two power-law density profiles and two homogeneous cores. In order to test the extension of the theory to an higher order we explore the effect on an homogeneous stellar component due to a DM halo with a density profile characterized by a inner slope γfree and an outer slope -3, according to high resolution rotation curves of Sps (Garrido et al. 2004). The aim is to investigate the role of the dark to bright mass ratio m and of the halo concentration C[D] in order to produce the maximum of CV. Particular attention is devoted to the slope of the density halo profile at the maximum location, to its height in comparison with the CV value when the two component coincide, V[n.] For all the models we choose γ=0. Method: we follow the general method
proposed by Caimmi (1993) for two striated ellipsoidals with Zhao-density profiles. Virial equilibrium is described by tensor virial equations extended to two subcomponents (Caimmi & Secco,1992). The interaction terms are numerically performed for different values of m and C [D] and sequences of CV as function of the ratio baryonic to halo virial semi-axis are taken into account. Results: the special configuration at the CV maximum with all the properties discovered with the theory of first order appears if m is greater than a given threshold.The corresponding slope (in absolute value) on the halo DM profile decreases either as m increases at fixed C[D] or as C[D] decreases at fixed m. The same conspiracy between m and C[D] appears in order to obtain the highest values of V[n]. Discussion: the test is relevant in order to confirm the main results of the first order approach and then to move the description of the main features of galaxy FP toward more realistic models
Simple MCBR models of chemical evolution: An application to the thin and the thick disk
No full textSimple multistage closed-(box+reservoir) (MCBR) models of chemical evolution,
formulated in an earlier attempt, are extended to the limit of dominant gas
inflow or outflow with respect to gas locked up into long-lived stars and
remnants. For an assigned empirical differential oxygen abundance
distribution (EDOD), which can be linearly fitted, a family of theoretical
differential oxygen abundance distribution (TDOD) curves is built up with the
following prescriptions: (i) the initial and the ending points of the linear
fit are common to all curves; (ii) the flow parameter k ranges from an
extremum point to ± ∞, where negative and positive k correspond to inflow and
outflow, respectively; (iii) the cut parameter ζO ranges from an extremum
point (which cannot be negative) to the limit (ζO) ∞ related to |k|→ + ∞. For
curves with increasing ζO, the gas mass fraction locked up into long-lived
stars and remnants is found to attain a maximum and then decrease towards
zero as |k|→ + ∞ while the remaining parameters show a monotonic trend. The
theoretical integral oxygen abundance distribution (TIOD) is also expressed.
An application is made to the EDOD deduced from two different samples of disk
stars, for both the thin and the thick disk. The constraints on formation and
evolution are discussed in the light of the model. The evolution is
tentatively subdivided into four stages, namely: assembling (A), formation
(F), contraction (C), equilibrium (E). The EDOD related to any stage is
fitted by all curves where 0 ≤ ζO ≤ (ζO) ∞ for inflowing gas and (ζO) ∞ ≤ ζO
≤ 1.2 for outflowing gas, with a single exception related to the thin disk (A
stage), where the range of fitting curves is restricted to 0.35 ≤ ζO ≤ (ζO)
∞. The F stage may safely be described by a steady inflow regime (k= -1),
implying a flat TDOD, in agreement with the results of hydrodynamical
simulations. Finally, (1) the change of fractional mass due to the extension
of the linear fit to the EDOD, towards both the (undetected) low-metallicity
and high-metallicity tail, is evaluated and (2) the idea of a thick disk -
thin disk collapse is discussed, in the light of the model
Perioperative anaphylaxis: epidemiology
No full textThe clinical diagnosis of an anesthesia-related immediate hypersensitivity reaction is a difficult task for clinicians. Anaphylaxis may present as cardiovascular collapse or airway obstruction, associated or not with cutaneous manifestations. Drug hypersensitivity reactions that occur during anesthesia are responsible for significant morbidity and mortality and socio-economic costs. Perioperative anaphylaxis is becoming more common, probably because of the more frequent use of anesthesia and the increasing complexity of the drugs used. However, despite increased awareness of anaphylactic reactions to drugs and compounds used in anesthesia, their incidence remains poorly defined. Moreover, current epidemiological data should be carefully evaluated since the various studies published concerned non-homogeneous populations and gave differing definitions of drug hypersensitivit
Interlaminar Fracture Toughness Of Carbon Fabric Reinforced Epoxy Composites
No full textThe Mode I and Mode II fracture behaviour of three carbon-epoxy composite laminates with different fabric reinforcement and different matrices was investigated. Standard Double Cantilever Beam (DCB) and End Notched Flexure (ENF) delamination tests were performed to determine initiation toughness and to asses the subsequent crack propagation behaviour. Various toughening mechanism, acting at the microscopic level and responsible for the stick-slip propagation behaviour observed, have been identified. The effect of temperature in a range from -60° to 165 °C was investigated
Dark matter haloes: an additional criterion for the choice of fitting density profiles
No full textSimulated dark matter haloes are fitted by self-similar, universal density profiles, where the scaled parameters depend only on a scaled (truncation) radius, Xi=R/r0, which, in turn, is supposed to be independent of the mass and the formation redshift. The further assumption of a lognormal distribution (for a selected mass bin) of the scaled radius, or concentration, in agreement with the data from a large statistical sample of simulated haloes (Bullock et al. 2001), allows (at least to a first approximation) a normal or lognormal distribution for other scaled parameters, via the same procedure which leads to the propagation of the errors. A criterion is proposed for the choice of the best fitting density profile, with regard to a set of high-resolution simulations, where some averaging procedure on scaled density profiles has been performed, in connection with a number of fitting density profiles. To this aim, a minimum value of the ratio, | x\overline{η}|/ σs,\overline{η}= |\overline{η}- η*|/σs,\overline{η}, is required to yield the best fit, where \overline{η} is the arithmetic mean over the whole set; η* is its counterpart related to the fitting density profile; σs,\overline{η} is the standard deviation from the mean; and η is a selected, scaled i.e. dimensionless parameter. The above criterion is applied to a pair of sets each made of a dozen of high-resolution simulations, FM01 (Fukushige and Makino 2001) and KLA01 (Klypin et al. 2001), in connection with two currently used fitting density profiles, NFW (e.g. Navarro et al. 1997) and MOA (e.g. Moore et al. 1999), where the dependence of the scaled radius on the mass and the formation redshift may be neglected to a first extent. With regard to FM01 and KLA01 samples, the best fits turn out to be MOA and NFW, respectively. In addition, the above results also hold in dealing with rms errors derived via the propagation of the errors, with regard to the distributions of scaled parameters. The sensitivity error of simulations is also estimated and shown to be less than the related, standard deviation, that is a necessary condition for detectability of accidental errors. Some features of the early evolution of dark matter haloes, represented by fitting density profiles, are discussed in the limit of the spherical top-hat model. Although the related matter distributions appear to be poorly representative of simulated haloes, unless the (mean) peak height is an increasing function of the mass, the results are shown to be consistent, provided considerable acquisition of angular momentum takes place during the expansion phase
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