315 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
Long-term photometric behaviour of XZ Dra Binarity or magnetic cycle of a Blazhko type RRab star
The extended photometry available for XZ Dra, a Blazhko type RR Lyrae star,
makes it possible to study its long-term behavior. It is shown that its
pulsation period exhibit cyclic, but not strictly regular variations with
approx. 7200 d period. The Blazhko period (approx. 76 d) seems to follow the
observed period changes of the fundamental mode pulsation with dP_B/dP_0=7.7 x
10^4 gradient. Binary model cannot explain this order of period change of the
Blazhko modulation, nevertheless it can be brought into agreement with the O-C
data of the pulsation. The possibility of occurrence of magnetic cycle is
raised.Comment: 13 pages, 11 figures (submitted to A&A
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
Fitting Blazhko light curves
The correct amplitude and phase modulation formalism of the Blazhko
modulation is given. The harmonic order dependent amplitude and phase
modulation form is equivalent with the Fourier decomposition of multiplets. The
amplitude and phase modulation formalism used in electronic transmission
technique as introduced by Benk\H{o}, Szab\'o and Papar\'o (2011, MNRAS 417,
974) for Blazhko stars oversimplifies the amplitude and phase modulation
functions thus it does not describe the light variation in full detail.
The results of the different formalisms are compared and documented by
fitting the light curve of a real Blazhko star, CM UMa.Comment: 8 pages, 5 figures. 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
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
Multi-wavelength study of a new Galactic SNR G332.5-5.6
We present compelling evidence for confirmation of a Galactic supernova
remnant (SNR) candidate, G332.5-5.6, based initially on identification of new,
filamentary, optical emission line nebulosity seen in the arcsecond resolution
images from the AAO/UKST HAlpha survey. The extant radio observations and X-ray
data which we have independently re-reduced, together with new optical
spectroscopy of the large-scale fragmented nebulosity, confirms the
identification. Optical spectra, taken across five different, widely separated
nebula regions of the remnant as seen on the HAlpha images, show average ratios
of [NII]/HAlpha =2.42, [SII]/HAlpha = 2.10, and [SII] 6717/6731 = 1.23, as well
as strong [OI] 6300, 6364A and [OII] 3727A emission. These ratios are firmly
within those typical of SNRs. Here, we also present the radio-continuum
detection of the SNR at 20/13cm from observations with the Australia Telescope
Compact Array (ATCA). Radio emission is also seen at 4850 MHz, in the PMN
survey (Griffith and Wright 1993) and at 843 MHz from the SUMSS survey (Bock,
Large and Sadler 1999). We estimate an angular diameter of ~30 arcmin and
obtain an average radio spectral index of alpha = -0.6 +- 0.1 which indicates
the non-thermal nature of G332.5-5.6. Fresh analysis of existing ROSAT X-ray
data in the vicinity also confirms the existence of the SNR. The distance to
G332.5-5.6 has been independently estimated by Reynoso and Green (2007) as 3.4
kpc based on measurements of the HI lambda21 cm line seen in absorption against
the continuum emission. Our cruder estimates via assumptions on the height of
the dust layer (3.1 kpc) and using the Sigma-D relation (4 kpc) are in good
agreement.Comment: 14 pages, 18 figures. Accepted for publishing in the MNRA
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&
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
Multi-Periodic Oscillations in Cepheids and RR Lyrae-Type Stars
Classical Cepheids and RR Lyrae-type stars are usually considered to be
textbook examples of purely radial, strictly periodic pulsators. Not all the
variables, however, conform to this simple picture. In this review I discuss
different forms of multi-periodicity observed in Cepheids and RR Lyrae stars,
including Blazhko effect and various types of radial and nonradial multi-mode
oscillations.Comment: Proceedings of the 20th Stellar Pulsation Conference Series: "Impact
of new instrumentation & new insights in stellar pulsations", 5-9 September
2011, Granada, Spai
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