136 research outputs found
Modelling the incomplete Paschen-Back effect in the spectra of magnetic Ap stars
We present first results of a systematic investigation of the incomplete
Paschen-Back effect in magnetic Ap stars. A short overview of the theory is
followed by a demonstration of how level splittings and component strengths
change with magnetic field strength for some lines of special astrophysical
interest. Requirements are set out for a code which allows the calculation of
full Stokes spectra in the Paschen-Back regime and the behaviour of Stokes I
and V profiles of transitions in the multiplet 74 of FeII is discussed in some
detail. It is shown that the incomplete Paschen-Back effect can lead to
noticeable line shifts which strongly depend on total multiplet strength,
magnetic field strength and field direction. Ghost components (which violate
the normal selection rule on J) show up in strong magnetic fields but are
probably unobservable. Finally it is shown that measurements of the integrated
magnetic field modulus are not adversely affected by the Paschen-Back
effect, and that there is a potential problem in (magnetic) Doppler mapping if
lines in the Paschen-Back regime are treated in the Zeeman approximation.Comment: 8 pages, 10 figures, to appear in MNRA
Radiative diffusion in stellar atmospheres: diffusion velocities
The present paper addresses some of the problems in the buildup of element
stratification in stellar magnetic atmospheres due to microscopic diffusion, in
particular the redistribution of momentum among the various ionisation stages
of a given element and the calculation of diffusion velocities in the presence
of inclined magnetic fields.
We have considerably modified and extended our CARAT code to provide
radiative accelerations, not only from bound-bound but also from bound-free
transitions. In addition, our code now computes ionisation and recombination
rates, both radiative and collisional. These rates are used in calculating the
redistribution of momentum among the various ionisation stages of the chemical
elements. A careful comparison shows that the two different theoretical
approaches to redistribution that are presently available lead to widely
discrepant results for some chemical elements, especially in the magnetic case.
In the absence of a fully satisfactory theory of redistribution, we propose to
use the geometrical mean of the radiative accelerations from both methods.
Diffusion velocities have been calculated for 28 chemical elements in a T_eff
= 12000K, log g = 4.00 stellar magnetic atmosphere with solar abundances.
Velocities and resulting element fluxes in magnetic fields are discussed; rates
of abundance changes are analysed for systematic trends with field strength and
field direction. Special consideration is given to the Si case and our results
are confronted in detail with well-known results derived more than two decades
ago.Comment: To be published in Astronomy & Astrophysics (accepted 02/03/2006
Modelling element distributions in the atmospheres of magnetic Ap stars
In recent papers convincing evidence has been presented for chemical
stratification in Ap star atmospheres, and surface abundance maps have been
shown to correlate with the magnetic field direction. Radiatively driven
diffusion in magnetic fields is among the processes responsible for these
inhomogeneities. Here we explore the hypothesis that equilibrium
stratifications can, in a number of cases, explain the observed abundance maps
and vertical distributions of the various elements. The investigation of
equilibrium stratifications in stellar atmospheres with temperatures from 8500K
to 12000K and fields up to 10 kG reveals considerable variations in the
vertical distribution of the 5 elements studied (Mg, Si, Ca, Ti, Fe), often
with zones of large over- or under-abundances and with indications of other
competing processes (such as mass loss). Horizontal magnetic fields can be very
efficient in helping the accumulation of elements in higher layers. A
comparison between our calculations and the vertical abundance profiles and
surface maps derived by magnetic Doppler imaging reveals that equilibrium
stratifications are in a number of cases consistent with the main trends
inferred from observed spectra. However, it is not clear whether such
equilibrium solutions will ever be reached during the evolution of an Ap star.Comment: 7 pages, 6 figures, the paper will be published in Astronomy &
Astrophysics, on November 200
Rebuilding the Cepheid Distance Scale I: A Global Analysis of Cepheid Mean Magnitudes
We develop a statistical method for using multicolor photometry to determine
distances using Cepheid variables including the effects of temperature,
extinction, and metallicity and apply it to UBVRIJHK photometry of 694 Cepheids
in 17 galaxies. We derive homogeneous distance, extinction and uncertainty
estimates for four models, starting from the standard extragalactic method and
then adding the physical effects of temperature distributions, extinction
distributions, requiring positive definite extinctions, and metallicity. While
we find general agreement with published distances when we make similar
systematic assumptions, there is a clear problem in the standard distances
because they require Cepheids with negative extinctions, particularly in low
metallicity galaxies, unless the mean LMC extinction exceeds E(B-V) > 0.25. The
problem can be explained by the physically expected metallicity dependence of
the Cepheid distance scale, where metal-poor Cepheids are hotter and fainter
than metal-rich Cepheids. For V and I we found that the mean magnitude change
is -0.14 +/- 0.14 mag/dex and the mean color change is 0.13 +/- 0.04 mag/dex,
with the change in color dominating the change in distance. The effect on Type
Ia supernova estimates of the Hubble constant is dramatic because most were
found in the metal poor galaxies with the bluest Cepheids. The Type Ia
Multi-color Light Curve Shape (MLCS) method estimate for H_0 formally rises
from 69 +/- 8 km/s Mpc to 80 +/- 6 km/s Mpc with the metallicity correction.Comment: 54 pages, 7 figures, 4 tables, submitted to Ap
Characterisation of the magnetic field of the Herbig Be star HD 200775
After our recent discovery of four magnetic Herbig stars, we have decided to
study in detail one of them, HD 200775, to determine if its magnetic topology
is similar to that of the main sequence magnetic stars. With this aim, we
monitored this star in Stokes I and V over more than two years, using the new
spectropolarimeters ESPaDOnS at CFHT, and Narval at TBL. Using our data, we
find that HD 200775 is a double-lined spectroscopic binary system, whose
secondary seems similar, in temperature, to the primary. We determine the
luminosity ratio of the system, and using the luminosity of the system found in
literature, we derive the luminosity of both stars. From our measurements of
the radial velocities of both stars we determine the ephemeris and the orbital
parameters of the system. We have fitted 30 Stokes V profiles simultaneously,
using a chi2 minimisation method, with a decentered-dipole model. The best-fit
model provides a rotation period of 4.3281 d an inclination angle of 60
degrees, and a magnetic obliquity angle of 125 degrees. The polar strength of
the magnetic dipole field is 1000 G, which is decentered by 0.05 R* from the
center of the star. The derived magnetic field model is qualitatively identical
to those commonly observed in the Ap/Bp stars, which bring strong argument in
favour of the fossil field hypothesis, to explain the origin of the magnetic
fields in the main sequence Ap/Bp stars. Our determination of the inclination
of the rotation axis leads to a radius of the primary which is smaller than
that derived from the HR diagram position. This can be explained by a larger
intrinsic luminosity of the secondary relative to the primary, due to a larger
circumstellar extinction of the secondary relative to the primary.Comment: Accepted for publication in MNRAS, 14 pages, 10 figure
The Hubble Space Telescope Extragalactic Distance Scale Key Project. X. The Cepheid Distance to NGC 7331
The distance to NGC 7331 has been derived from Cepheid variables observed
with HST/WFPC2, as part of the Extragalactic Distance Scale Key Project.
Multi-epoch exposures in F555W (V) and F814W (I), with photometry derived
independently from DoPHOT and DAOPHOT/ALLFRAME programs, were used to detect a
total of 13 reliable Cepheids, with periods between 11 and 42 days. The
relative distance moduli between NGC 7331 and the LMC, imply an extinction to
NGC 7331 of A_V = 0.47+-0.15 mag, and an extinction-corrected distance modulus
to NGC 7331 of 30.89+-0.14(random) mag, equivalent to a distance of 15.1 Mpc.
There are additional systematic uncertainties in the distance modulus of +-0.12
mag due to the calibration of the Cepheid Period-Luminosity relation, and a
systematic offset of +0.05+-0.04 mag if we applied the metallicity correction
inferred from the M101 results of Kennicutt et al 1998.Comment: To be published in The Astrophysical Journal, 1998 July 1, v501 note:
Figs 1 and 2 (JPEG files) and Fig 7 (multipage .eps file) need to be
viewed/printed separatel
The contribution of microlensing surveys to the distance scale
In the early nineties several teams started large scale systematic surveys of
the Magellanic Clouds and the Galactic Bulge to search for microlensing
effects. As a by product, these groups have created enormous time-series
databases of photometric measurements of stars with a temporal sampling
duration and accuracy which are unprecedented. They provide the opportunity to
test the accuracy of primary distance indicators, such as Cepheids, RRLyrae
stars, the detached eclipsing binaries, or the luminosity of the red clump. We
will review the contribution of the microlensing surveys to the understanding
of the physics of the primary distance indicators, recent differential studies
and direct distance determinations to the Magellanic Clouds and the Galactic
Bulge.Comment: Invited review article to appear in: `Post-Hipparcos Cosmic Candles',
A. Heck & F. Caputo (Eds), Kluwer Academic Publ., Dordrecht, in press. 21
pages; uses Kluwer's crckapb.sty LaTeX style file, enclose
The PL calibration for Milky Way Cepheids and its implications for the distance scale
The rationale behind recent calibrations of the Cepheid PL relation using the
Wesenheit formulation is reviewed and reanalyzed, and it is shown that recent
conclusions regarding a possible change in slope of the PL relation for
short-period and long-period Cepheids are tied to a pathological distribution
of HST calibrators within the instability strip. A recalibration of the
period-luminosity relation is obtained using Galactic Cepheids in open clusters
and groups, the resulting relationship, described by log L/L_sun =
2.415(+-0.035) + 1.148(+-0.044)log P, exhibiting only the moderate scatter
expected from color spread within the instability strip. The relationship is
confirmed by Cepheids with HST parallaxes, although without the need for
Lutz-Kelker corrections, and in general by Cepheids with revised Hipparcos
parallaxes, albeit with concerns about the cited precisions of the latter. A
Wesenheit formulation of Wv = -2.259(+-0.083) - 4.185(+-0.103)log P for
Galactic Cepheids is tested successfully using Cepheids in the inner regions of
the galaxy NGC 4258, confirming the independent geometrical distance
established for the galaxy from OH masers. Differences between the extinction
properties of interstellar and extragalactic dust may yet play an important
role in the further calibration of the Cepheid PL relation and its application
to the extragalactic distance scale.Comment: Accepted for Publication (Astrophysics & Space Science
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