30 research outputs found
The benefits of the orthogonal LSM models
In the last few decades both the volume of high-quality observing data on
variable stars and common access to them have boomed; however the standard used
methods of data processing and interpretation have lagged behind this progress.
The most popular method of data treatment remains for many decades Linear
Regression (LR) based on the principles of Least Squares Method (LSM) or
linearized LSM. Unfortunately, we have to state that the method of linear
regression is not as a rule used accordingly namely in the evaluation of
uncertainties of the LR parameters and estimates of the uncertainty of the LR
predictions.
We present the matrix version of basic relations of LR and the true estimate
of the uncertainty of the LR predictions. We define properties of the
orthogonal LR models and show how to transform general LR models into
orthogonal ones. We give relations for orthogonal models for common polynomial
series.Comment: 5 pages, 2 figures, submitted to Odessa Astronomical Publications,
vol. 20, 200
Visual and ultraviolet flux variability of the bright CP star Aur
Chemically peculiar stars of the upper part of the main sequence show
periodical variability in line intensities and continua, modulated by the
stellar rotation, which is attributed to the existence of chemical spots on the
surface of these stars. The flux variability is caused by the changing
redistribution rate of the radiative flux predominantly from the
short-wavelength part of the spectra to the long-wavelength part, which is a
result of abundance anomalies. We study the nature of the multi-spectral
variability of one of the brightest chemically peculiar stars, Aur. We
predict the flux variability of Aur from the emerging intensities
calculated for individual surface elements of the star taking into account
horizontal variation of chemical composition derived from Doppler abundance
maps. The simulated optical variability in the Str\"omgren photometric system
and the ultraviolet flux variability agree well with observations. The IUE flux
distribution is reproduced in great detail by our models. The resonance lines
of magnesium and possibly also some lines of silicon are relatively weak in the
ultraviolet domain, which indicates non-negligible vertical abundance gradients
in the atmosphere. We also derive a new period of the star,
d, from all available photometric and magnetic measurements and show that the
observed rotational period is constant over decades. The ultraviolet and visual
variability of Aur is mostly caused by silicon bound-free absorption
and chromium and iron line absorption. These elements redistribute the flux
mainly from the far-ultraviolet region to the near-ultraviolet and optical
regions in the surface abundance spots. The light variability is modulated by
the stellar rotation. The ultraviolet domain is key for understanding the
properties of chemically peculiar stars. (abridged)Comment: 12 pages, accepted for publication in Astronomy & Astrophysic
Study of Eclipsing Binary and Multiple Systems in OB Associations: I. Ori OB1a - IM Mon
All available photometric and spectroscopic observations were collected and
used as the basis of a detailed analysis of the close binary IM Mon. The
orbital period of the binary was refined to 1.19024249(0.00000014) days. The
Roche equipotentials, fractional luminosities (in (B, V) and H_p bands) and
fractional radii for the component stars in addition to mass ratio q,
inclination i of the orbit and the effective temperature T_eff of the secondary
cooler less massive component were obtained by the analysis of light curves. IM
Mon is classified to be a detached binary system in contrast to the contact
configuration estimations in the literature. The absolute parameters of IM Mon
were derived by the simultaneous solutions of light and radial velocity curves
as M_1,2=5.50(0.24)M_o and 3.32(0.16)M_o, R1,2=3.15(0.04)R_o and 2.36(0.03)R_o,
T_eff1,2=17500(350) K and 14500(550) K implying spectral types of B4 and B6.5
ZAMS stars for the primary and secondary components respectively. The modelling
of the high resolution spectrum revealed the rotational velocities of the
component stars as V_rot1=147(15) km/s and V_rot2=90(25) km/s. The photometric
distance of 353(59) pc was found more precise and reliable than Hipparcos
distance of 341(85) pc. An evolutionary age of 11.5(1.5) Myr was obtained for
IM Mon. Kinematical and dynamical analysis support the membership of the young
thin-disk population system IM Mon to the Ori OB1a association dynamically.
Finally, we derived the distance, age and metallicity information of Ori OB1a
sub-group using the information of IM Mon parameters.Comment: 26 pages, 5 figures and 6 tables, accepted for publication in
Publication of the Astronomical Society of Japa
Transient jets in the symbiotic prototype Z Andromedae
We present development of the collimated bipolar jets from the symbiotic
prototype Z And that appeared and disappeared during its 2006 outburst. In 2006
July Z And reached its historical maximum at U ~ 8.0. During this period, rapid
photometric variations with Dm ~ 0.06 mag on the timescale of hours developed.
Simultaneously, high-velocity satellite components appeared on both sides of
the H-alpha and H-beta emission line profiles. They were launched
asymmetrically with the red/blue velocity ratio of 1.2 - 1.3. From about
mid-August they became symmetric. Their spectral properties indicated ejection
of bipolar jets collimated within an average opening angle of 6.1 degrees. We
estimated average outflow rate via jets to dM(jet)/dt ~
2xE10-6(R(jet)/1AU)**(1/2) M(Sun)/year, during their August - September
maximum, which corresponds to the emitting mass in jets, M(jet, emitting) ~
6xE-10(Rjet)/1AU)^{3/2} M(Sun). During their lifetime, the jets released the
total mass of M(jet, total) approx 7.4x1E-7 M(Sun). Evolution in the rapid
photometric variability and asymmetric ejection of jets around the optical
maximum can be explained by a disruption of the inner parts of the disk caused
by radiation-induced warping of the disk.Comment: 31 pages, 9 figures, 2 tables, accepted for Ap
Study of Eclipsing Binary and Multiple Systems in OB Associations II. The Cygnus OB Region: V443 Cyg, V456 Cyg and V2107 Cyg
Three presumably young eclipsing binary systems in the direction of the
Cygnus OB1, OB3 and OB9 associations are studied. Component spectra are
reconstructed and their orbits are determined using light curves and spectra
disentangling techniques. V443 Cyg and V456 Cyg have circular orbits, while the
light curve of V2107\,Cyg imposes a slightly eccentric orbit
(. V443 Cyg harbours F-type stars, and not young early-A stars
as previously suggested in the literature based on photometry solely. It
appears to be situated in the foreground (distance kpc) of the
young stellar populations in Cygnus. V456 Cyg, at a distance of
kpc consists of a slightly metal-weak A--type and an early--F star. The age of
both systems, on or very near to the main sequence, remains uncertain by an
order of magnitude. V2107 Cyg is a more massive system ( and
) at kpc and, also kinematically, a strong
candidate-member of Cyg OB1. The more massive component is slightly evolved and
appears to undergo non-radial -type pulsations. The Doppler signal
of the secondary is barely detectable. A more extensive study is important to
fix masses more precisely, and an asteroseismological study would then become
appropriate. Nevertheless, the position of the primary in the HR-diagram
confines the age already reasonably well to Myr, indicating for Cyg
OB1 a similar extent of star formation history as established for Cyg OB2.Comment: 27 pages, including 9 figures and 6 tables, accepted for publication
in Astronomical Journa
Revisiting the Rigidly Rotating Magnetosphere model for Ori E - II. Magnetic Doppler imaging, arbitrary field RRM, and light variability
The initial success of the Rigidly Rotating Magnetosphere (RRM) model
application to the B2Vp star sigma OriE by Townsend, Owocki & Groote (2005)
triggered a renewed era of observational monitoring of this archetypal object.
We utilize high-resolution spectropolarimetry and the magnetic Doppler imaging
(MDI) technique to simultaneously determine the magnetic configuration, which
is predominately dipolar, with a polar strength Bd = 7.3-7.8 kG and a smaller
non-axisymmetric quadrupolar contribution, as well as the surface distribution
of abundance of He, Fe, C, and Si. We describe a revised RRM model that now
accepts an arbitrary surface magnetic field configuration, with the field
topology from the MDI models used as input. The resulting synthetic Ha emission
and broadband photometric observations generally agree with observations,
however, several features are poorly fit. To explore the possibility of a
photospheric contribution to the observed photometric variability, the MDI
abundance maps were used to compute a synthetic photospheric light curve to
determine the effect of the surface inhomogeneities. Including the computed
photospheric brightness modulation fails to improve the agreement between the
observed and computed photometry. We conclude that the discrepancies cannot be
explained as an effect of inhomogeneous surface abundance. Analysis of the UV
light variability shows good agreement between observed variability and
computed light curves, supporting the accuracy of the photospheric light
variation calculation. We thus conclude that significant additional physics is
necessary for the RRM model to acceptably reproduce observations of not only
sigma Ori E, but also other similar stars with significant stellar
wind-magnetic field interactions.Comment: 16 pages, 17 figures, accepted for publication in MNRA
Understanding the rotational variability of K2 targets. HgMn star KIC 250152017 and blue horizontal branch star KIC 249660366
Ultraprecise space photometry enables us to reveal light variability even in
stars that were previously deemed constant. A large group of such stars show
variations that may be rotationally modulated. This type of light variability
is of special interest because it provides precise estimates of rotational
rates. We aim to understand the origin of the light variability of K2 targets
that show signatures of rotational modulation. We used phase-resolved
medium-resolution XSHOOTER spectroscopy to understand the light variability of
the stars KIC~250152017 and KIC~249660366, which are possibly rotationally
modulated. We determined the atmospheric parameters at individual phases and
tested the presence of the rotational modulation in the spectra. KIC 250152017
is a HgMn star, whose light variability is caused by the inhomogeneous surface
distribution of manganese and iron. It is only the second HgMn star whose light
variability is well understood. KIC 249660366 is a He-weak, high-velocity
horizontal branch star with overabundances of silicon and argon. The light
variability of this star is likely caused by a reflection effect in this
post-common envelope binary.Comment: 8 pages, accepted for publication in Astronomy & Astrophysic