101 research outputs found
Kepler photometry of RRc stars: peculiar double-mode pulsations and period doubling
We present the analysis of four first overtone RR Lyrae stars observed with the Kepler space telescope, based on data obtained over nearly 2.5 yr. All four stars are found to be multiperiodic.
The strongest secondary mode with frequency f2 has an amplitude of a few mmag, 20–45 times lower than the main radial mode with frequency f1. The two oscillations have a period ratio of P2/P1 = 0.612–0.632 that cannot be reproduced by any two radial modes. Thus, the secondary mode is non-radial. Modes yielding similar period ratios have also recently been discovered in other variables of the RRc and RRd types. These objects form a homogenous group and constitute a new class of multimode RR Lyrae pulsators, analogous to a similar class of multimode classical Cepheids in the Magellanic Clouds. Because a secondary mode with P2/P1 ∼ 0.61 is found in almost every RRc and RRd star observed from space, this form of multiperiodicity must be common. In all four Kepler RRc stars studied, we find subharmonics of f2 at ∼1/2f2 and at ∼3/2f2. This is a signature of period doubling of the secondary oscillation, and is the first detection of period doubling in RRc stars. The amplitudes and phases of f2 and its subharmonics are variable on a time-scale of 10–200 d. The dominant radial mode also shows variations on the same time-scale, but with much smaller amplitude. In three Kepler RRc stars we detect additional periodicities, with amplitudes below 1 mmag, that must correspond to non-radial g-modes. Such modes never before have been observed in RR Lyrae variables
EPIC 201585823, a rare triple-mode RR Lyrae star discovered in K2 mission data
We have discovered a new, rare triple-mode RR Lyr star, EPIC 201585823, in the Kepler K2 mission Campaign 1 data. This star pulsates primarily in the fundamental and first-overtone radial modes, and, in addition, a third non-radial mode. The ratio of the period of the non-radial mode to that of the first-overtone radial mode, 0.616 285, is remarkably similar to that seen in 11 other triple-mode RR Lyr stars, and in 260 RRc stars observed in the Galactic bulge. This systematic character promises new constraints on RR Lyr star models. We detected subharmonics of the non-radial mode frequency, which are a signature of period doubling of this oscillation; we note that this phenomenon is ubiquitous in RRc and RRd stars observed from space, and from ground with sufficient precision. The non-radial mode and subharmonic frequencies are not constant in frequency or in amplitude. The amplitude spectrum of EPIC 201585823 is dominated by many combination frequencies among the three interacting pulsation mode frequencies. Inspection of the phase relationships of the combination frequencies in a phasor plot explains the ‘upward’ shape of the light curve. We also found that raw data with custom masks encompassing all pixels with significant signal for the star, but without correction for pointing changes, is best for frequency analysis of this star, and, by implication, other RR Lyr stars observed by the K2 mission. We compare several pipeline reductions of the K2 mission data for this star
Simulations of stellar convection, pulsation and semiconvection
We report on modelling in stellar astrophysics with the ANTARES code. First,
we describe properties of turbulence in solar granulation as seen in
high-resolution calculations. Then, we turn to the first 2D model of
pulsation-convection interaction in a cepheid. We discuss properties of the
outer and the HEII ionization zone. Thirdly, we report on our work regarding
models of semiconvection in the context of stellar physics.Comment: Astrophysical Dynamics: From Stars to Galaxies. IAU Symposium 27
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
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
The BRITE-Constellation Nanosatellite Space Mission And Its First Scientific Results
The BRIght Target Explorer (BRITE) Constellation is the first nanosatellite
mission applied to astrophysical research. Five satellites in low-Earth orbits
perform precise optical two-colour photometry of the brightest stars in the
night sky. BRITE is naturally well suited for variability studies of hot stars.
This contribution describes the basic outline of the mission and some initial
problems that needed to be overcome. Some information on BRITE data products,
how to access them, and how to join their scientific exploration is provided.
Finally, a brief summary of the first scientific results obtained by BRITE is
given.Comment: 5 pages, 1 figure, to appear in the proceedings of "Seismology of the
Sun and the Distant Stars 2016. Using Today's Successes to Prepare the
Future. Joint TASC2/KASC9 Workshop - SPACEINN/HELAS8 Conference", ed. M. J.
P. F. G. Monteir
Physical parameters and the projection factor of the classical Cepheid in the binary system OGLE-LMC-CEP-0227
A novel method of analysis of double-lined eclipsing binaries containing a
radially pulsating star is presented. The combined pulsating-eclipsing light
curve is built up from a purely eclipsing light curve grid created using an
existing modeling tool. For every pulsation phase the instantaneous radius and
surface brightness are taken into account, being calculated from the
disentangled radial velocity curve of the pulsating star and from its
out-of-eclipse pulsational light curve and the light ratio of the components,
respectively. The best model is found using the Markov Chain Monte Carlo
method.
The method is applied to the eclipsing binary Cepheid OGLE-LMC-CEP-0227
(P_puls = 3.80 d, P_orb = 309 d). We analyze a set of new spectroscopic and
photometric observations for this binary, simultaneously fitting OGLE V-band,
I-band and Spitzer 3.6 {\mu}m photometry. We derive a set of fundamental
parameters of the system significantly improving the precision comparing to the
previous results obtained by our group. The Cepheid mass and radius are M_1 =
4.165 +/- 0.032 M_solar and R_1 = 34.92 +/- 0.34 R_solar, respectively.
For the first time a direct, geometrical and distance-independent
determination of the Cepheid projection factor is presented. The value p = 1.21
+/- 0.03(stat.) +/- 0.04(syst.) is consistent with theoretical expectations for
a short period Cepheid and interferometric measurements for {\delta} Cep. We
also find a very high value of the optical limb darkening coefficients for the
Cepheid component, in strong disagreement with theoretical predictions for
static atmospheres at a given surface temperature and gravity.Comment: 16 pages, 17 figures, accepted for publication in MNRA
- …