146 research outputs found
Axial rotation and turbulence of RR ab stars: the Peterson Conundrum revisited
We calibrate and then use the relation between equivalent width (EW) and
full-width-half-maximum (FWHM) of metallic absorption lines in the spectra of
RR Lyrae stars to estimate a new upper limit of Vrot sini less than or equal to
6 km/s on their axial equatorial rotational velocities, and to derive the
variations of macroturbulent velocities in their atmospheres during pulsation
cycles. Finally, we present a simple way to estimate macroturbulent/rotational
velocity from FWHM of the cross-correlation function.Comment: 15 pages, 7 figures, 1 table. EAS Publications Series.: "New advances
in stellar physics: from microscopic to macroscopic processes", 27-31 May
2013, Roscoff, Franc
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&
No evidence of a strong magnetic field in the Blazhko star RR Lyrae
Astronomy and Astrophysics, v. 413, p. 1087-1093, 2004. http://dx.doi.org/10.1051/0004-6361%3a20031600International audienc
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
The unusually large population of Blazhko variables in the globular cluster NGC 5024 (M53)
We report the discovery of amplitude and phase modulations typical of the
Blazhko effect in 22 RRc and 9 RRab type RR Lyrae stars in NGC 5024 (M53). This
brings the confirmed Blazhko variables in this cluster to 23 RRc and 11 RRab,
that represent 66% and 37% of the total population of RRc and RRab stars in the
cluster respectively, making NGC 5024 the globular cluster with the largest
presently known population of Blazhko RRc stars. We place a lower limit on the
overall incidence rate of the Blazhko effect among the RR Lyrae population in
this cluster of 52%. New data have allowed us to refine the pulsation periods.
The limitations imposed by the time span and sampling of our data prevents
reliable estimations of the modulation periods. The amplitudes of the
modulations range between 0.02 and 0.39 mag. The RRab and RRc are neatly
separated in the CMD, and the RRc Blazhko variables are on averge redder than
their stable couterparts; these two facts may support the hypothesis that the
HB evolution in this cluster is towards the red and that the Blazhko
modulations in the RRc stars are connected with the pulsation mode switch.Comment: ACCEPTED IN MNRAS 14 pages, 9 figures and 6 table
Uncovering hidden modes in RR Lyrae stars
The Kepler space telescope revealed new, unexpected phenomena in RR Lyrae
stars: period doubling and the possible presence of additional modes.
Identifying these modes is complicated because they blend in the rich features
of the Fourier-spectrum. Our hydrodynamic calculations uncovered that a
'hidden' mode, the 9th overtone is involved in the period doubling phenomenon.
The period of the overtone changes by up to 10 per cent compared to the linear
value, indicating a very significant nonlinear period shift caused by its
resonance with the fundamental mode. The observations also revealed weak peaks
that may correspond to the first or second overtones. These additional modes
are often coupled with period doubling. We investigated the possibilities and
occurrences of mutual resonances between the fundamental mode and multiple
overtones in our models. These theoretical findings can help interpreting the
origin and nature of the 'hidden' modes may be found in the high quality light
curves of space observatories.Comment: In proceedings of "20th Stellar Pulsation Conference Series: Impact
of new instrumentation & new insights in stellar pulsations", 5-9 September
2011, Granada, Spai
Metal Abundances, Radial Velocities, and Other Physical Characteristics for the RR Lyrae Stars in The Kepler Field
Spectroscopic iron-to-hydrogen ratios, radial velocities, atmospheric parameters, and new photometric analyses are presented for 41 RR Lyrae stars (and one probable high-amplitude δ Sct star) located in the field-of-view of the Kepler space telescope. Thirty-seven of the RR Lyrae stars are fundamental-mode pulsators (i.e., RRab stars) of which sixteen exhibit the Blazhko effect. Four of the stars are multiperiodic RRc pulsators oscillating primarily in the first-overtone mode. Spectroscopic [Fe/H] values for the 34 stars for which we were able to derive estimates range from –2.54 ± 0.13 (NR Lyr) to –0.05 ± 0.13 dex (V784 Cyg), and for the 19 Kepler-field non-Blazhko stars studied by Nemec et al. the abundances agree will with their photometric [Fe/H] values. Four non-Blazhko RR Lyrae stars that they identified as metal-rich (KIC 6100702, V2470 Cyg, V782 Cyg and V784 Cyg) are confirmed as such, and four additional stars (V839 Cyg, KIC 5520878, KIC 8832417, KIC 3868420) are also shown here to be metal-rich. Five of the non-Blazhko RRab stars are found to be more metal-rich than [Fe/H] ~–0.9 dex while all of the 16 Blazhko stars are more metal-poor than this value. New P-Ø_(31)^s-[Fe/H] relationships are derived based on ~970 days of quasi-continuous high-precision Q0-Q11 long- and short-cadence Kepler photometry. With the exception of some Blazhko stars, the spectroscopic and photometric [Fe/H] values are in good agreement. Several stars with unique photometric characteristics are identified, including a Blazhko variable with the smallest known amplitude and frequency modulations (V838 Cyg)
Variable turbulent convection as the cause of the Blazhko effect - testing the Stothers model
The amplitude and phase modulation observed in a significant fraction of the
RR Lyrae variables - the Blazhko effect - represents a long-standing enigma in
stellar pulsation theory. No satisfactory explanation for the Blazhko effect
has been proposed so far. In this paper we focus on the Stothers (2006) idea,
in which modulation is caused by changes in the structure of the outer
convective zone, caused by a quasi-periodically changing magnetic field.
However, up to this date no quantitative estimates were made to investigate
whether such a mechanism can be operational and whether it is capable of
reproducing the light variation we observe in Blazhko variables. We address the
latter problem. We use a simplified model, in which the variation of turbulent
convection is introduced into the non-linear hydrodynamic models in an ad hoc
way, neglecting interaction with the magnetic field. We study the light curve
variation through the modulation cycle and properties of the resulting
frequency spectra. Our results are compared with Kepler observations of RR Lyr.
We find that reproducing the light curve variation, as is observed in RR Lyr,
requires a huge modulation of the mixing length, of the order of +/-50 per
cent, on a relatively short time-scale of less than 40 days. Even then, we are
not able to reproduce neither all the observed relations between modulation
components present in the frequency spectrum, nor the relations between Fourier
parameters describing the shape of the instantaneous light curves.Comment: 17 pages, 13 figures, accepted for publication in MNRAS; for
associated animation, see
http://homepage.univie.ac.at/radek.smolec/publications/KASC11a
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