279 research outputs found
The light-curve modulation of XY And and UZ Vir - Two Blazhko RR Lyrae stars with additional frequencies
A thorough analysis of multicolour CCD observations of two modulated
RRab-type variables, XY And and UZ Vir is presented. These Blazhko stars show
relatively simple light-curve modulation with the usual multiplet structures in
their Fourier spectra. One additional, independent frequency with
linear-combination terms of the pulsation frequency is also detected in the
residual spectrum of each of the two stars. The amplitude and phase relations
of the triplet components are studied in detail. Most of the epoch-independent
phase differences show a slight, systematic colour dependence, however, these
trends have the opposite sign in the two stars. The mean values of the global
physical parameters and their changes with Blazhko phase are determined
utilizing the Inverse Photometric Method (IPM). The modulation properties and
the IPM results are compared for the two variables. The pulsation period of XY
And is the shortest when its pulsation amplitude is the highest, while UZ Vir
has the longest pulsation period at this phase of the modulation. Despite this
opposite behaviour, the phase relations of their mean-physical-parameter
variations are similar. These results are not in accord with the predictions of
the Blazhko model of Stothers (2006, ApJ, 652, 643).Comment: Accepted for publication in MNRAS. The paper contains 7 figures and
12 tables. Tables 5, 6 and A1-A5 will be published in full online only. See
electronic data on http://konkoly.hu/24/publications/ als
The CoRoT star 105288363: strong cycle to cycle changes of the Blazhko modulation
We present the analysis of the CoRoT star 105288363, a new Blazhko RR Lyrae
star of type RRab (f0 = 1.7623 c/d), observed with the CoRoT space craft during
the second long run in direction of the galactic center (LRc02, time base 145
d). The CoRoT data are characterized by an excellent time sampling and a low
noise amplitude of 0.07 mmag in the 2-12 c/d range and allow us to study not
only the fine details of the variability of the star but also long-term changes
in the pulsation behaviour and the stability of the Blazhko cycle. We use,
among other methods, standard Fourier analysis techniques and O-C diagrams to
investigate the pulsational behavior of the Blazhko star 105288363. In addition
to the frequency pattern expected for a Blazhko RR Lyrae star, we find an
independent mode (f1 = 2.984 c/d) showing a f0/f1 ratio of 0.59 which is
similar to that observed in other Blazhko RR Lyrae stars. The bump and hump
phenomena are also analysed, with their variations over the Blazhko cycle. We
carefully investigated the strong cycle-to-cycle changes in the Blazhko
modulation (PB = 35.6 d), which seem to happen independently and partly
diametrically in the amplitude and the phase modulation. Furthermore, the
phasing between the two types of modulation is found to change during the
course of the observations.Comment: 15 pages, 8 figures, accepted for publication in MNRA
An extensive photometric study of the Blazhko RR Lyrae star MW Lyr: II. Changes in the physical parameters
The analysis of the multicolour photometric observations of MW Lyr, a large
modulation amplitude Blazhko variable, shows for the first time how the mean
global physical parameters vary during the Blazhko cycle. About 1-2 percent
changes in the mean radius, luminosity and surface effective temperature are
detected. The mean radius and temperature changes are in good accordance with
pulsation model results, which show that these parameters do indeed vary within
this order of magnitude if the amplitude of the pulsation changes
significantly. We interpret the phase modulation of the pulsation to be a
consequence of period changes. Its magnitude corresponds exactly what one
expects from the detected changes of the mean radius assuming that the
pulsation constant remains the same during the modulation. Our results indicate
that during the modulation the pulsation remains purely radial, and the
underlying mechanism is most probably a periodic perturbation of the stellar
luminosity with the modulation period.Comment: 10 figures, 2 tables. Accepted for publication in MNRA
On the stability of very massive primordial stars
The stability of metal-free very massive stars ( = 0; M = 120 - 500
\msol) is analyzed and compared with metal-enriched stars. Such zero-metal
stars are unstable to nuclear-powered radial pulsations on the main sequence,
but the growth time scale for these instabilities is much longer than for their
metal-rich counterparts. Since they stabilize quickly after evolving off the
ZAMS, the pulsation may not have sufficient time to drive appreciable mass loss
in Z = 0 stars. For reasonable assumptions regarding the efficiency of
converting pulsational energy into mass loss, we find that, even for the larger
masses considered, the star may die without losing a large fraction of its
mass. We find a transition between the - and -mechanisms for
pulsational instability at Z\sim 2\E{-4} - 2\E{-3}. For the most metal-rich
stars, the -mechanism yields much shorter -folding times, indicating
the presence of a strong instability. We thus stress the fundamental difference
of the stability and late stages of evolution between very massive stars born
in the early universe and those that might be born today.Comment: 7 pages, 5 figures. Minor changes, more results given in Table 1,
accepted for publication in Ap
Intermediate mass stars: updated models
A new set of stellar models in the mass range 1.2 to 9 is
presented. The adopted chemical compositions cover the typical galactic values,
namely and . A comparison among
the most recent compilations of similar stellar models is also discussed. The
main conclusion is that the differencies among the various evolutionary results
are still rather large. For example, we found that the H-burning evolutionary
time may differ up to 20 %. An even larger disagreement is found for the
He-burning phase (up to 40-50 %). Since the connection between the various
input physics and the numerical algorithms could amplify or counterbalance the
effect of a single ingredient on the resulting stellar model, the origin of
this discrepancies is not evident. However most of these discrepancies, which
are clearly found in the evolutionary tracks, are reduced on the isochrones. By
means of our updated models we show that the ages inferred by the theory of
stellar evolution is in excellent agreement with those obtained by using other
independent methods applied to the nearby Open Clusters. Finally, the
theoretical initial/final mass relation is revised.Comment: 35 pages, 24 figures, 4 tables, accepted for publication in the
Astrophisycal Journa
CoRoT light curves of RR Lyrae stars. CoRoT 101128793: long-term changes in the Blazhko effect and excitation of additional modes
The CoRoT (Convection, Rotation and planetary Transits) space mission
provides a valuable opportunity to monitor stars with uninterrupted time
sampling for up to 150 days at a time. The study of RR Lyrae stars, performed
in the framework of the Additional Programmes belonging to the exoplanetary
field, will particularly benefit from such dense, long-duration monitoring. The
Blazhko effect in RR Lyrae stars is a long-standing, unsolved problem of
stellar astrophysics. We used the CoRoT data of the new RR Lyrae variable CoRoT
101128793 (f0=2.119 c/d, P=0.4719296 d) to provide us with more detailed
observational facts to understand the physical process behind the phenomenon.
The CoRoT data were corrected for one jump and the long-term drift. We applied
different period-finding techniques to the corrected timeseries to investigate
amplitude and phase modulation. We detected 79 frequencies in the light curve
of CoRoT 101128793. They have been identified as the main frequency f0, and its
harmonics, two independent terms, the terms related to the Blazhko frequency,
and several combination terms. A Blazhko frequency fB=0.056 c/d and a triplet
structure around the fundamental radial mode and harmonics were detected, as
well as a long-term variability of the Blazhko modulation. Indeed, the
amplitude of the main oscillation is decreasing along the CoRoT survey. The
Blazhko modulation is one of the smallest observed in RR Lyrae stars. Moreover,
the additional modes f1=3.630 and f2=3.159 c/d are detected. Taking its ratio
with the fundamental radial mode into account, the term f1 could be the
identified as the second radial overtone. Detecting of these modes in
horizontal branch stars is a new result obtained by CoRoT.Comment: 13 pages, 2 figures, 2 long tables. Accepted for publication in A&
The MACHO Project 9 Million Star Color-Magnitude Diagram of the Large Magellanic Cloud
We present a 9 million star color-magnitude diagram (9M CMD) of the LMC bar.
The 9M CMD reveals a complex superposition of different age and metallicity
stellar populations, with important stellar evolutionary phases occurring over
3 orders of magnitude in number density. First, we count the non-variable
supergiants, the associated Cepheids, and measure the effective temperatures
defining the instability strip. Lifetime predictions of stellar evolution
theory are tested, with implications for the origin of low-luminosity Cepheids.
The highly-evolved AGB stars have a bimodal distribution in brightness, which
we interpret as discrete old populations (>1 Gyr). The faint AGB may be
metal-poor and very old. We identify the clusters NGC 411 and M3 as templates
for the admixture of old stellar populations. However, there are indications
that the old and metal-poor field population has a red HB morphology: the RR
Lyraes lie on the red edge of the instability strip, the AGB-bump is very red,
and the ratio of AGB-bump stars to RR Lyraes is quite large. If the HB second
parameter is age, the old and metal-poor field population likely formed after
the oldest clusters. Lifetime predictions of stellar evolution theory lead us
to associate a significant fraction of the red HB clump giants with the same
old and metal-poor population producing the RR Lyraes and the AGB-bump. In this
case, compared to the age-dependent luminosity predictions of stellar evolution
theory, the red HB clump is too bright relative to the RR Lyraes and AGB-bump.
Last, the surface density profile of RR Lyraes is fit by an exponential,
favoring a disk-like rather than spheroidal distribution. We conclude that the
age of the LMC disk is probably similar to the age of the Galactic disk.
(ABRIDGED)Comment: to appear in the Astronomical Journal, 49 pages, 12 figures,
aaspp4.st
Period and light curve fluctuations of the Kepler Cepheid V1154 Cyg
We present a detailed period analysis of the bright Cepheid-type variable
star V1154 Cygni (V =9.1 mag, P~4.9 d) based on almost 600 days of continuous
observations by the Kepler space telescope. The data reveal significant
cycle-to-cycle fluctuations in the pulsation period, indicating that classical
Cepheids may not be as accurate astrophysical clocks as commonly believed:
regardless of the specific points used to determine the O-C values, the cycle
lengths show a scatter of 0.015-0.02 days over the 120 cycles covered by the
observations. A very slight correlation between the individual Fourier
parameters and the O-C values was found, suggesting that the O - C variations
might be due to the instability of the light curve shape. Random fluctuation
tests revealed a linear trend up to a cycle difference 15, but for long term,
the period remains around the mean value. We compare the measurements with
simulated light curves that were constructed to mimic V1154 Cyg as a perfect
pulsator modulated only by the light travel time effect caused by low-mass
companions. We show that the observed period jitter in V1154 Cyg represents a
serious limitation in the search for binary companions. While the Kepler data
are accurate enough to allow the detection of planetary bodies in close orbits
around a Cepheid, the astrophysical noise can easily hide the signal of the
light-time effect.Comment: published in MNRAS: 8 pages, 7 figure
An extensive photometric study of the Blazhko RR Lyrae star MW Lyr: I. Light curve solution
We have obtained the most extensive and most accurate photometric data of a
Blazhko variable MW Lyr during the 2006-2007 observing seasons. The data within
each 0.05 phase bin of the modulation period (P_m=1/f_m) cover the entire light
cycle of the primary pulsation period (P_0=1/f_0), making possible a very
rigorous and complete analysis. The modulation period is found to be 16.5462 d,
which is about half of that was reported earlier from visual observations.
Previously unknown features of the modulation have been detected. Besides the
main modulation frequency f_m, sidelobe modulation frequencies around the
pulsation frequency and its harmonics appear at +/- 2 f_m, +/- 4 f_m, and +/-
12.5 f_m separations as well. Residual signals in the prewhitened light curve
larger than the observational noise appear at the minimum-rising branch-maximum
phase of the pulsation, which most probably arise from some stochastic/chaotic
behaviour of the pulsation/modulation. The Fourier parameters of the mean light
curve differ significantly from the averages of the Fourier parameters of the
observed light curves in the different phases of the Blazhko cycle.
Consequently, the mean light curve of MW Lyrae never matches its actual light
variation. The Phi_21, Phi_31 phase differences in different phases of the
modulation show unexpected stability during the Blazhko cycle. A new
phenomenological description of the light curve variation is defined that
separates the amplitude and phase (period) modulations utilising the phase
coherency of the lower order Fourier phases.Comment: Accepted for publication in MNRAS. 15 pages, 14 figures, and 7
printed tables (2 of them available in electronic form
Amorphous alumina in the extended atmosphere of Alpha Orionis
In this paper we study the extended atmosphere of the late-type supergiant
Alpha Orionis. Infrared spectroscopy of red supergiants reveals strong
molecular bands, some of which do not originate in the photosphere but in a
cooler layer of molecular material above it. Lately, these layers have been
spatially resolved by near and mid-IR interferometry. In this paper, we try to
reconcile the IR interferometric and ISO-SWS spectroscopic results on Alpha
Orionis with a thorough modelling of the photosphere, molecular layer(s) and
dust shell. From the ISO and near-IR interferometric observations, we find that
Alpha Orionis has only a very low density water layer close above the
photosphere. However, mid-IR interferometric observations and a narrow-slit
N-band spectrum suggest much larger extra-photospheric opacity close to the
photosphere at those wavelengths, even when taking into account the detached
dust shell. We argue that this cannot be due to the water layer, and that
another source of mid-IR opacity must be present. We show that this opacity
source is probably neither molecular nor chromospheric. Rather, we present
amorphous alumina (Al2O3) as the best candidate and discuss this hypothesis in
the framework of dust-condensation scenarios.Comment: 15 pages, 18 figures, accepted for publication in A&
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