4,077 research outputs found
Time-resolved multicolour photometry of bright B-type variable stars in Scorpius
The first two of a total of six nano-satellites that will constitute the
BRITE-Constellation space photometry mission have recently been launched
successfully. In preparation for this project, we carried out time-resolved
colour photometry in a field that is an excellent candidate for BRITE
measurements from space. We acquired 117 h of Stromgren uvy data during 19
nights. Our targets comprised the Beta Cephei stars Kappa and Lambda Sco, the
eclipsing binary Mu 1 Sco, and the variable super/hypergiant Zeta 1 Sco. For
Kappa Sco, a photometric mode identification in combination with results from
the spectroscopic literature suggests a dominant (l, m) = (1, -1) Beta
Cephei-type pulsation mode of the primary star. The longer period of the star
may be a rotational variation or a g-mode pulsation. For Lambda Sco, we recover
the known dominant Beta Cephei pulsation, a longer-period variation, and
observed part of an eclipse. Lack of ultraviolet data precludes mode
identification for this star. We noticed that the spectroscopic orbital
ephemeris of the closer pair in this triple system is inconsistent with eclipse
timings and propose a refined value for the orbital period of the closer pair
of 5.95189 +/- 0.00003 d. We also argue that the components of the Lambda Sco
system are some 30% more massive than previously thought. The binary light
curve solution of Mu 1 Sco requires inclusion of the irradiation effect to
explain the u light curve, and the system could show additional low amplitude
variations on top of the orbital light changes. Zeta 1 Sco shows long-term
variability on a time scale of at least two weeks that we prefer to interpret
in terms of a variable wind or strange mode pulsations.Comment: 7 pages, 7 figures, 3 Tables, accepted by A&
Indications of a Large Fraction of Spectroscopic Binaries Among Nuclei of Planetary Nebulae
Previous work indicates that about 10% of planetary-nebula nuclei (PNNi) are
photometrically variable short-period binaries with periods of hours to a few
days. These systems have most likely descended from common-envelope (CE)
interactions in initially much wider binaries. Population-synthesis studies
suggest that these very close pairs could be the short-period tail of a much
larger post-CE binary population with periods of up to a few months. We have
initiated a radial-velocity (RV) survey of PNNi with the WIYN 3.5-m telescope
and Hydra spectrograph, which is aimed at discovering these intermediate-period
binaries. We present initial results showing that 10 out of 11 well-observed
PNNi have variable RVs, suggesting that a significant binary population may be
present. However, further observations are required because we have as yet been
unable to fit our sparse measurements with definite orbital periods, and
because some of the RV variability might be due to variations in the stellar
winds of some of our PNNi.Comment: 11 pages, 1 table, no figures. Accepted by the Astrophysical Journal
Letter
The asteroseismological potential of the pulsating DB white dwarf stars CBS 114 and PG 1456+103
We have acquired 65 h of single-site time-resolved CCD photometry of the
pulsating DB white dwarf star CBS 114 and 62 h of two-site high-speed CCD
photometry of another DBV, PG 1456+103. The pulsation spectrum of PG 1456+103
is complicated and variable on time scales of about one week and could only
partly be deciphered with our measurements. The modes of CBS 114 are more
stable in time and we were able to arrive at a frequency solution somewhat
affected by aliasing, but still satisfactory, involving seven independent modes
and two combination frequencies. These frequencies also explain the discovery
data of the star, taken 13 years earlier. We find a mean period spacing of 37.1
+/- 0.7 s significant at the 98% level between the independent modes of CBS 114
and argue that they are due to nonradial g-mode pulsations of spherical degree
l=1. We performed a global search for asteroseismological models of CBS 114
using a genetic algorithm, and we examined the susceptibility of the results to
the uncertainties of the observational frequency determinations and mode
identifications (we could not provide m values). The families of possible
solutions are identified correctly even without knowledge of m. Our optimal
model suggests Teff = 21,000 K and M_* = 0.730 M_sun as well as log(M_He/M_*) =
-6.66, X_O = 0.61. This measurement of the central oxygen mass fraction implies
a rate for the ^12C(alpha,gamma)^16O nuclear reaction near S_300=180 keV b,
consistent with laboratory measurements.Comment: 10 pages, 10 embedded figures, 3 embedded tables. Accepted for
publication in MNRA
Discovery and analysis of p-mode and g-mode oscillations in the A-type primary of the eccentric binary HD 209295
We have discovered both intermediate-order gravity mode and low-order
pressure mode pulsation in the same star, HD 209295. It is therefore both a
Gamma Doradus and a Delta Scuti star, which makes it the first pulsating star
to be a member of two classes.
The star is a single-lined spectroscopic binary with an orbital period of
3.10575 d and an eccentricity of 0.352. Weak pulsational signals are found in
both the radial velocity and line-profile variations, allowing us to show that
the two highest-amplitude Gamma Doradus pulsation modes are consistent with l=1
and |m|=1.
In our 280 h of BVI multi-site photometry we detected ten frequencies in the
light variations, one in the Delta Scuti regime and nine in the Gamma Doradus
domain. Five of the Gamma Doradus frequencies are exact integer multiples of
the orbital frequency. This observation leads us to suspect they are tidally
excited. Results of theoretical modeling (stability analysis, tidal excitation)
were consistent with the observations.
We could not detect the secondary component of the system in infrared
photometry, suggesting that it may not be a main-sequence star. Archival data
of HD 209295 show a strong ultraviolet excess, the origin of which is not
known. The orbit of the primary is consistent with a secondary mass of M > 1.04
Msun indicative of a neutron star or a white dwarf companion.Comment: 18 pages, 18 figures, accepted for publication in MNRAS, shortened
abstrac
The pulsating DA white dwarf star EC 14012-1446: results from four epochs of time-resolved photometry
The pulsating DA white dwarfs are the coolest degenerate stars that undergo
self-driven oscillations. Understanding their interior structure will help to
understand the previous evolution of the star. To this end, we report the
analysis of more than 200 h of time-resolved CCD photometry of the pulsating DA
white dwarf star EC 14012-1446 acquired during four observing epochs in three
different years, including a coordinated three-site campaign. A total of 19
independent frequencies in the star's light variations together with 148
combination signals up to fifth order could be detected. We are unable to
obtain the period spacing of the normal modes and therefore a mass estimate of
the star, but we infer a fairly short rotation period of 0.61 +/- 0.03 d,
assuming the rotationally split modes are l=1. The pulsation modes of the star
undergo amplitude and frequency variations, in the sense that modes with higher
radial overtone show more pronounced variability and that amplitude changes are
always accompanied by frequency variations. Most of the second-order
combination frequencies detected have amplitudes that are a function of their
parent mode amplitudes, but we found a few cases of possible resonantly excited
modes. We point out the complications in the analysis and interpretation of
data sets of pulsating white dwarfs that are affected by combination
frequencies of the form f_A+f_B-f_C intruding into the frequency range of the
independent modes.Comment: 14 pages, 6 figures, 6 tables. MNRAS, in pres
Asteroseismological studies of three Beta Cephei stars: IL Vel, V433 Car and KZ Mus
We have acquired between 127 and 150 h of time-resolved multicolour
photometry for each of the three Beta Cephei stars IL Vel, V433 Car and KZ Mus
over a time span of four months from two observatories. All three objects are
multiperiodic with at least three modes of pulsation. Mode identification from
the relative colour amplitudes is performed. We obtain unambiguous results for
the two highest-amplitude modes of IL Vel (both are l=1) and the three
strongest modes of KZ Mus (l=2,0 and 1), but none for V433 Car. Spectroscopy
shows the latter star to be a fast rotator (v sin i = 240 km/s), whereas the
other two have moderate v sin i (65 and 47 km/s, respectively). We performed
model calculations with the Warsaw-New Jersey stellar evolution and pulsation
code. We find that IL Vel is an object of about 12 Msun in the second half of
its main sequence evolutionary track. Its two dipole modes are most likely
rotationally split components of the mode originating as p1 on the ZAMS; one of
these modes is m=0. V433 Car is suggested to be an unevolved 13 Msun star just
entering the Beta Cephei instability strip. KZ Mus seems less massive (about
12.7 Msun) and somewhat more evolved, and its radial mode is probably the
fundamental one. In this case its quadrupole mode would be the one originating
as g1, and its dipole mode would be p1. It is suggested that mode
identification of slowly rotating Beta Cephei stars based on photometric colour
amplitudes is reliable; we estimate that a relative accuracy of 3% in the
amplitudes is sufficient for unambiguous identifications. Due to the good
agreement of our theoretical and observational results we conclude that the
prospects for asteroseismology of multiperiodic slowly rotating Beta Cephei
star are good.Comment: 16 pages, 16 figures, MNRAS, in pres
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