443 research outputs found
The Lick Planet Search: Detectability and Mass Thresholds
We analyse 11 years of precise radial velocities for 76 solar type stars from
the Lick survey. Eight stars in this sample have previously reported
planetary-mass companions, all with mass (m sin i) less than 8 Jupiter masses
(MJ). For the stars without a detected companion, we place upper limits on
possible companion mass. For most stars, we can exclude companions with m sin i
> 0.7 MJ (a/AU)^1/2 for orbital radii a < 5 AU.
We use our results to interpret the observed masses and orbital radii of
planetary-mass companions. For example, we show that the finite duration of the
observations makes detection of Jupiter mass companions more and more difficult
for orbital radii beyond 3 AU. Thus it is possible that the majority of solar
type stars harbor Jupiter-mass companions much like our own, and if so these
companions should be detectable in a few years.
To search for periodicities, we adopt a "floating-mean" periodogram, which
improves on the traditional Lomb-Scargle periodogram by accounting for
statistical fluctuations in the mean of a sampled sinusoid. We discuss in
detail the normalization of the periodogram, an issue which has been of some
debate in the literature.Comment: To appear in the Astrophysical Journal (50 pages, LaTeX, including 11
figures
Assessing statistical significance of periodogram peaks
The least-squares (or Lomb-Scargle) periodogram is a powerful tool which is
used routinely in many branches of astronomy to search for periodicities in
observational data. The problem of assessing statistical significance of
candidate periodicities for different periodograms is considered. Based on
results in extreme value theory, improved analytic estimations of false alarm
probabilities are given. They include an upper limit to the false alarm
probability (or a lower limit to the significance). These estimations are
tested numerically in order to establish regions of their practical
applicability.Comment: 7 pages, 6 figures, 1 table; To be published in MNRA
On the orbital period of the cataclysmic variable RZ Leonis
In this research note we present a time-resolved study of the Balmer emission
lines of RZ Leo. From the analysis of the radial velocities we find an orbital
period of 0.07651(26) d. This is in excellent agreement with the
photometrically determined periods in quiescence and during the early stages of
superoutburst. A comparison of the recently determined superhump period gives
an excess of ~0.03, which is a typical value for an SU UMa star of this period.Comment: 3 pages, 6 figures, A&A, accepte
CCD Photometry of Faint Variable Stars in the Globular Cluster NGC 6752
We present the results of a photometric survey for variable stars in the
field of the nearby globular cluster NGC 6752. The cluster was monitored in
1996 and 1997 for a total of 54 hours with 3 different CCD cameras mounted on
the 1.0-m Swope telescope. Eleven new variables were identified: 3 SX Phe
stars, 7 contact binaries and 1 candidate detached eclipsing binary. All 3 SX
Phe variables are likely members of the cluster while only 1 out of the 7
contact binaries is a potential cluster member. As a by-product of our survey
we obtained UBV photometry for a large sample of stars in the cluster field.
Two stars with U-B \approx -1.0 and V=19.3 and V=20.6 were identified. They lie
along the extended horizontal branch of the cluster, and are likely to be faint
sdB stars from NGC 6752.Comment: 18 pages, LaTex, 9 figures (Fig. 1 not available), accepted for
publication in the Astronomical Journa
The statistical significance of the superhump signal in U Gem
Although its well determined mass ratio of q=\Msec/\Mwd=0.357\pm0.007
should avoid superoutbursts according to the thermal tidal instability model,
the prototypical dwarf nova U Gem experienced in 1985 an extraordinary long
outburst resembling very much superoutbursts observed in SU UMa systems.
Recently, the situation for the model became even worse as superhump detections
have been reported for the 1985 outburst of U Gem. The superhump signal is
noisy and the evidence provided by simple periodograms seems to be weak.
Therefore and because of the importance for our understanding of superoutbursts
and superhumps, we determine the statistical significance of the recently
published detection of superhumps in the AAVSO light curve of the famous long
1985 outburst of U Gem. Using Lomb-Scargle periodograms, analysis of variance
(AoV), and Monte-Carlo methods we analyse the 160 visual magnitudes obtained by
the AAVSO during the outburst and relate our analyse to previous superhump
detections. The 160 data points of the outburst alone do not contain a
statistically significant period. However, using additionally the
characteristics of superhumps detected previously in other SU UMa systems and
searching only for signals that are consistent with these, we derive a
significance for the superhump signal. The alleged appearance of an
additional superhump at the end of the outbursts appears to be statistically
insignificant. Although of weak statistical significance, the superhump signal
of the long 1985 outburst of U Gem can be interpreted as further indication for
the SU UMa nature of this outburst. This further contradicts the tidal
instability model as the explanation for the superhump phenomenon.Comment: 7 pages, 7 figures, accepted for publication in A&
Fast Calculation of the Lomb-Scargle Periodogram Using Graphics Processing Units
I introduce a new code for fast calculation of the Lomb-Scargle periodogram,
that leverages the computing power of graphics processing units (GPUs). After
establishing a background to the newly emergent field of GPU computing, I
discuss the code design and narrate key parts of its source. Benchmarking
calculations indicate no significant differences in accuracy compared to an
equivalent CPU-based code. However, the differences in performance are
pronounced; running on a low-end GPU, the code can match 8 CPU cores, and on a
high-end GPU it is faster by a factor approaching thirty. Applications of the
code include analysis of long photometric time series obtained by ongoing
satellite missions and upcoming ground-based monitoring facilities; and
Monte-Carlo simulation of periodogram statistical properties.Comment: Accepted by ApJ. Accompanying program source (updated since
acceptance) can be downloaded from
http://www.astro.wisc.edu/~townsend/resource/download/code/culsp.tar.g
Low Luminosity States of the Black Hole Candidate GX~339--4. II. Timing Analysis
Here we present timing analysis of a set of eight Rossi X-ray Timing Explorer
(RXTE) observations of the black hole candidate GX 339-4 that were taken during
its hard/low state. On long time scales, the RXTE All Sky Monitor data reveal
evidence of a 240 day periodicity, comparable to timescales expected from
warped, precessing accretion disks. On short timescales all observations save
one show evidence of a persistent f approximately equal to 0.3 Hz QPO. The
broad band (10^{-3}-10^2 Hz) power appears to be dominated by two independent
processes that can be modeled as very broad Lorentzians with Q approximately
less than 1. The coherence function between soft and hard photon variability
shows that if these are truly independent processes, then they are individually
coherent, but they are incoherent with one another. This is evidenced by the
fact that the coherence function between the hard and soft variability is near
unity between 0.005-10 Hz but shows evidence of a dip at f approximately equal
to 1 Hz. This is the region of overlap between the broad Lorentzian fits to the
PSD. Similar to Cyg X-1, the coherence also drops dramatically at frequencies
approximately greater than 10 Hz. Also similar to Cyg X-1, the hard photon
variability is seen to lag the soft photon variability with the lag time
increasing with decreasing Fourier frequency. The magnitude of this time lag
appears to be positively correlated with the flux of GX 339-4. We discuss all
of these observations in light of current theoretical models of both black hole
spectra and temporal variability.Comment: To Appear in the AStrophysical Journa
Analysis of RR Lyrae Stars in the Northern Sky Variability Survey
We use data from the Northern Sky Variability Survey (NSVS), obtained from
the first generation Robotic Optical Transient Search Experiment (ROTSE-I), to
identify and study RR Lyrae variable stars in the solar neighborhood. We
initially identified 1197 RRab (RR0) candidate stars brighter than the ROTSE
median magnitude V = 14. Periods, amplitudes, and mean V magnitudes are
determined for a subset of 1188 RRab stars with well defined light curves.
Metallicities are determined for 589 stars by the Fourier parameter method and
by the relationship between period, amplitude, and [Fe/H]. We comment upon the
difficulties of clearly classifying RRc (RR1) variables in the NSVS dataset.
Distances to the RRab stars are calculated using an adopted
luminosity-metallicity relation with corrections for interstellar extinction.
The 589 RRab stars in our final sample are used to study the properties of the
RRab population within 5 kpc of the Sun. The Bailey diagram of period versus
amplitude shows that the largest component of this sample belongs to Oosterhoff
type I. Metal-rich ([Fe/H] > -1) RRab stars appear to be associated with the
Galactic disk. Our metal-rich RRab sample may include a thin disk as well as a
thick disk population, although the uncertainties are too large to establish
this. There is some evidence among the metal-rich RRab stars for a decline in
scale height with increasing [Fe/H], as was found by Layden (1995). The
distribution of RRab stars with -1 < [Fe/H] < -1.25 indicates that within this
metallicity range the RRab stars are a mixture of stars belonging to halo and
disk populations.Comment: 68 pages, 26 figures, 9 tables, accepted to A
Limits on I-band microvariability of the Galactic Bulge Miras
We search for microvariability in a sample of 485 Mira variables with high
quality I-band light curves from the second generation Optical Gravitational
Lensing Experiment (OGLE-II). Rapid variations with amplitudes in the ~0.2-1.1
mag range lasting hours to days were discovered in Hipparcos data by de Laverny
et al. (1998). Our search is primarily sensitive to events with time-scales of
about 1 day, but retains a few percent efficiency (per object) for detecting
unresolved microvariability events as short as 2 hours. We do not detect any
candidate events. Assuming that the distribution of the event time profiles is
identical to that from the Hipparcos light curves we derive the 95% confidence
level upper limit of 0.038 per year per star for the rate of such events (1 per
26 years per average object of the ensemble). The high event rates of the order
of 1 per year per star implied by the Hipparcos study in the H_P band are
excluded with high confidence by the OGLE-II data in the I band. Our
non-detection could still be explained by much redder spectral response of the
I filter compared to the H_P band or by population differences between the
bulge and the solar neighborhood. In any case, the OGLE-II I-band data provide
the first limit on the rate of the postulated microvariability events in Mira
stars and offer new quantitative constraints on their properties. Similar
limits are obtained for other pulse shapes and a range of the assumed
time-scales and size-frequency distributions.Comment: Accepted for publication in Ap
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