23 research outputs found
Photometric study of southern SU UMa-type dwarf novae and candidates -- III: NSV 10934, MM Sco, AB Nor, CAL 86
We photometrically observed four southern dwarf novae in outburst (NSV 10934,
MM Sco, AB Nor and CAL 86). NSV 10934 was confirmed to be an SU UMa-type dwarf
nova with a mean superhump period of 0.07478(1) d. This star also showed
transient appearance of quasi-periodic oscillations (QPOs) during the final
growing stage of the superhumps. Combined with the recent theoretical
interpretation and with the rather unusual rapid terminal fading of normal
outbursts, NSV 10934 may be a candidate intermediate polar showing SU UMa-type
properties. The mean superhump periods of MM Sco and AB Nor were determined to
be 0.06136(4) d and 0.08438(2) d, respectively. We suggest that AB Nor belongs
to a rather rare class of long-period SU UMa-type dwarf novae with low
mass-transfer rates. We also observed an outburst of the suspected SU UMa-type
dwarf nova CAL 86. We identified this outburst as a normal outburst and
determined the mean decline rate of 1.1 mag/d.Comment: 13 pages, 23 figures, to appear in MNRAS. For more information, see
http://www.kusastro.kyoto-u.ac.jp/vsnet
On the age of the magnetically active WW Psa and TX Psa members of the beta Pictoris association
There are a variety of different techniques available to estimate the ages of
pre-main-sequence stars. Components of physical pairs, thanks to their strict
coevality and the mass difference, such as the binary system analysed in this
paper, are best suited to test the effectiveness of these different techniques.
We consider the system WW Psa + TX Psa whose membership of the 25-Myr beta
Pictoris association has been well established by earlier works. We investigate
which age dating technique provides the best agreement between the age of the
system and that of the association. We have photometrically monitored WW Psa
and TX Psa and measured their rotation periods as P = 2.37d and P = 1.086d,
respectively. We have retrieved from the literature their Li equivalent widths
and measured their effective temperatures and luminosities. We investigate
whether the ages of these stars derived using three independent techniques are
consistent with the age of the beta Pictoris association. We find that the
rotation periods and the Li contents of both stars are consistent with the
distribution of other bona fide members of the cluster. On the contrary, the
isochronal fitting provides similar ages for both stars, but a factor of about
four younger than the quoted age of the association, or about 30% younger when
the effects of magnetic fields are included. We explore the origin of the
discrepant age inferred from isochronal fitting, including the possibilities
that either the two components may be unresolved binaries or that the basic
stellar parameters of both components are altered by enhanced magnetic
activity. The latter is found to be the more reasonable cause, suggesting that
age estimates based on the Li content is more reliable than isochronal fitting
for pre-main-sequence stars with pronounced magnetic activity.Comment: Accepted by Astronomy and Astrophysics on December 13, 2016. 13 pages
and 11 figure
The beta Pictoris association: Catalog of photometric rotational periods of low-mass members and candidate members
We intended to compile the most complete catalog of bona fide members and
candidate members of the beta Pictoris association, and to measure their
rotation periods and basic properties from our own observations, public
archives, and exploring the literature. We carried out a multi-observatories
campaign to get our own photometric time series and collected all archived
public photometric data time series for the stars in our catalog. Each time
series was analyzed with the Lomb-Scargle and CLEAN periodograms to search for
the stellar rotation periods. We complemented the measured rotational
properties with detailed information on multiplicity, membership, and projected
rotational velocity available in the literature and discussed star by star. We
measured the rotation periods of 112 out of 117 among bona fide members and
candidate members of the beta Pictoris association and, whenever possible, we
also measured the luminosity, radius, and inclination of the stellar rotation
axis. This represents to date the largest catalog of rotation periods of any
young loose stellar association. We provided an extensive catalog of rotation
periods together with other relevant basic properties useful to explore a
number of open issues, such as the causes of spread of rotation periods among
coeval stars, evolution of angular momentum, and lithium-rotation connection.Comment: Forthcoming article, Received: 20 June 2016 / Accepted: 09 September
2016; 40 pages, 2 figures. The online figures A1-A73 are available at CD
Photometric study of new southern SU UMa-type dwarf novae and candidates: V877 Ara, KK Tel and PU CMa
We photometrically observed three dwarf novae V877 Ara, KK Tel and PU CMa. We
discovered undisputed presence of superhumps in V877 Ara and KK Tel, with mean
periods of 0.08411(2) d and 0.08808(3) d, respectively. Both V877 Ara and KK
Tel are confirmed to belong to long-period SU UMa-type dwarf novae. In V877
Ara, we found a large decrease of the superhump period (dot(P)/P = -14.5 +/-
2.1 x 10^(-5)). There is evidence that the period of KK Tel decreased at a
similar or a more exceptional rate. Coupled with the past studies of superhump
period changes, these findings suggest that a previously neglected diversity of
phenomena is present in long-period SU UMa-type dwarf novae. The present
discovery of a diversity in long-period SU UMa-type systems would become an
additional step toward a full understanding the dwarf nova phenomenon. PU CMa
is shown to be an excellent candidate for an SU UMa-type dwarf nova. We
examined the outburst properties of these dwarf novae, and derived
characteristic outburst recurrence times. Combined with the recently published
measurement of the orbital period of PU CMa, we propose that PU CMa is the
first object filling the gap between the extreme WZ Sge-type and ER UMa-type
stars.Comment: 12 pages, 14 figures, accepted for publication in MNRA
Microlensing Event MOA-2007-BLG-400: Exhuming the Buried Signature of a Cool, Jovian-Mass Planet
We report the detection of the cool, Jovian-mass planet MOA-2007-BLG-400Lb.
The planet was detected in a high-magnification microlensing event (with peak
magnification A_max = 628) in which the primary lens transited the source,
resulting in a dramatic smoothing of the peak of the event. The angular extent
of the region of perturbation due to the planet is significantly smaller than
the angular size of the source, and as a result the planetary signature is also
smoothed out by the finite source size. Thus the deviation from a single-lens
fit is broad and relatively weak (~ few percent). Nevertheless, we demonstrate
that the planetary nature of the deviation can be unambiguously ascertained
from the gross features of the residuals, and detailed analysis yields a fairly
precise planet/star mass ratio of q = 0.0026+/-0.0004, in accord with the large
significance (\Delta\chi^2=1070) of the detection. The planet/star projected
separation is subject to a strong close/wide degeneracy, leading to two
indistinguishable solutions that differ in separation by a factor of ~8.5.
Upper limits on flux from the lens constrain its mass to be M < 0.75 M_Sun
(assuming it is a main-sequence star). A Bayesian analysis that includes all
available observational constraints indicates a primary in the Galactic bulge
with a mass of ~0.2-0.5 M_Sun and thus a planet mass of ~ 0.5-1.3 M_Jupiter.
The separation and equilibrium temperature are ~0.6-1.1AU (~5.3-9.7AU) and
~103K (~34K) for the close (wide) solution. If the primary is a main-sequence
star, follow-up observations would enable the detection of its light and so a
measurement of its mass and distance.Comment: 30 pages, 6 figures, Submitted to Ap
MOA-2011-BLG-293Lb: A test of pure survey microlensing planet detections
Because of the development of large-format, wide-field cameras, microlensing
surveys are now able to monitor millions of stars with sufficient cadence to
detect planets. These new discoveries will span the full range of significance
levels including planetary signals too small to be distinguished from the
noise. At present, we do not understand where the threshold is for detecting
planets. MOA-2011-BLG-293Lb is the first planet to be published from the new
surveys, and it also has substantial followup observations. This planet is
robustly detected in survey+followup data (Delta chi^2 ~ 5400). The planet/host
mass ratio is q=5.3+/- 0.2*10^{-3}. The best fit projected separation is
s=0.548+/- 0.005 Einstein radii. However, due to the s-->s^{-1} degeneracy,
projected separations of s^{-1} are only marginally disfavored at Delta
chi^2=3. A Bayesian estimate of the host mass gives M_L = 0.43^{+0.27}_{-0.17}
M_Sun, with a sharp upper limit of M_L < 1.2 M_Sun from upper limits on the
lens flux. Hence, the planet mass is m_p=2.4^{+1.5}_{-0.9} M_Jup, and the
physical projected separation is either r_perp = ~1.0 AU or r_perp = ~3.4 AU.
We show that survey data alone predict this solution and are able to
characterize the planet, but the Delta chi^2 is much smaller (Delta chi^2~500)
than with the followup data. The Delta chi^2 for the survey data alone is
smaller than for any other securely detected planet. This event suggests a
means to probe the detection threshold, by analyzing a large sample of events
like MOA-2011-BLG-293, which have both followup data and high cadence survey
data, to provide a guide for the interpretation of pure survey microlensing
data.Comment: 29 pages, 6 figures, Replaced 7/3/12 with the version accepted to Ap
Extreme Magnification Microlensing Event OGLE-2008-BLG-279: Strong Limits on Planetary Companions to the Lens Star
We analyze the extreme high-magnification microlensing event
OGLE-2008-BLG-279, which peaked at a maximum magnification of A ~ 1600 on 30
May 2008. The peak of this event exhibits both finite-source effects and
terrestrial parallax, from which we determine the mass of the lens, M_l=0.64
+/- 0.10 M_Sun, and its distance, D_l = 4.0 +/- 0.6. We rule out Jupiter-mass
planetary companions to the lens star for projected separations in the range
0.5-20 AU. More generally, we find that this event was sensitive to planets
with masses as small as 0.2 M_Earth ~= 2 M_Mars with projected separations near
the Einstein ring (~3 AU).Comment: 25 pages, 7 figures, submitted to Ap
MOA-2009-BLG-387Lb: A massive planet orbiting an M dwarf
We report the discovery of a planet with a high planet-to-star mass ratio in
the microlensing event MOA-2009-BLG-387, which exhibited pronounced deviations
over a 12-day interval, one of the longest for any planetary event. The host is
an M dwarf, with a mass in the range 0.07 M_sun < M_host < 0.49M_sun at 90%
confidence. The planet-star mass ratio q = 0.0132 +- 0.003 has been measured
extremely well, so at the best-estimated host mass, the planet mass is m_p =
2.6 Jupiter masses for the median host mass, M = 0.19 M_sun. The host mass is
determined from two "higher order" microlensing parameters. One of these, the
angular Einstein radius \theta_E = 0.31 +- 0.03 mas, is very well measured, but
the other (the microlens parallax \pi_E, which is due to the Earth's orbital
motion) is highly degenate with the orbital motion of the planet. We
statistically resolve the degeneracy between Earth and planet orbital effects
by imposing priors from a Galactic model that specifies the positions and
velocities of lenses and sources and a Kepler model of orbits. The 90%
confidence intervals for the distance, semi-major axis, and period of the
planet are 3.5 kpc < D_L < 7.9 kpc, 1.1 AU < a < 2.7AU, and 3.8 yr < P < 7.6
yr, respectively.Comment: 20 pages including 8 figures. A&A 529 102 (2011
Frequency of Solar-Like Systems and of Ice and Gas Giants Beyond the Snow Line from High-Magnification Microlensing Events in 2005-2008
We present the first measurement of planet frequency beyond the "snow line"
for planet/star mass-ratios[-4.5<log q<-2]: d^2 N/dlog q/dlog
s=(0.36+-0.15)/dex^2 at mean mass ratio q=5e-4, and consistent with being flat
in log projected separation, s. Our result is based on a sample of 6 planets
detected from intensive follow-up of high-mag (A>200) microlensing events
during 2005-8. The sample host stars have typical mass M_host 0.5 Msun, and
detection is sensitive to planets over a range of projected separations
(R_E/s_max,R_E*s_max), where R_E 3.5 AU sqrt(M_host/Msun) is the Einstein
radius and s_max (q/5e-5)^{2/3}, corresponding to deprojected separations ~3
times the "snow line". Though frenetic, the observations constitute a
"controlled experiment", which permits measurement of absolute planet
frequency. High-mag events are rare, but the high-mag channel is efficient:
half of high-mag events were successfully monitored and half of these yielded
planet detections. The planet frequency derived from microlensing is a factor 7
larger than from RV studies at factor ~25 smaller separations [2<P<2000 days].
However, this difference is basically consistent with the gradient derived from
RV studies (when extrapolated well beyond the separations from which it is
measured). This suggests a universal separation distribution across 2 dex in
semi-major axis, 2 dex in mass ratio, and 0.3 dex in host mass. Finally, if all
planetary systems were "analogs" of the Solar System, our sample would have
yielded 18.2 planets (11.4 "Jupiters", 6.4 "Saturns", 0.3 "Uranuses", 0.2
"Neptunes") including 6.1 systems with 2 or more planet detections. This
compares to 6 planets including one 2-planet system in the actual sample,
implying a first estimate of 1/6 for the frequency of solar-like systems.Comment: 42 pages, 10 figure
Survey of Period Variations of Superhumps in SU UMa-Type Dwarf Novae
We systematically surveyed period variations of superhumps in SU UMa-type
dwarf novae based on newly obtained data and past publications. In many
systems, the evolution of superhump period are found to be composed of three
distinct stages: early evolutionary stage with a longer superhump period,
middle stage with systematically varying periods, final stage with a shorter,
stable superhump period. During the middle stage, many systems with superhump
periods less than 0.08 d show positive period derivatives. Contrary to the
earlier claim, we found no clear evidence for variation of period derivatives
between superoutburst of the same object. We present an interpretation that the
lengthening of the superhump period is a result of outward propagation of the
eccentricity wave and is limited by the radius near the tidal truncation. We
interpret that late stage superhumps are rejuvenized excitation of 3:1
resonance when the superhumps in the outer disk is effectively quenched. Many
of WZ Sge-type dwarf novae showed long-enduring superhumps during the
post-superoutburst stage having periods longer than those during the main
superoutburst. The period derivatives in WZ Sge-type dwarf novae are found to
be strongly correlated with the fractional superhump excess, or consequently,
mass ratio. WZ Sge-type dwarf novae with a long-lasting rebrightening or with
multiple rebrightenings tend to have smaller period derivatives and are
excellent candidate for the systems around or after the period minimum of
evolution of cataclysmic variables (abridged).Comment: 239 pages, 225 figures, PASJ accepte