75 research outputs found
Photometric variability of the Herbig Ae star HD 37806
The more massive counterparts of T Tauri stars, Herbig Ae/Be stars, are known
to vary in a complex way with no variability mechanism clearly identified. We
attempt to characterize the optical variability of HD~37806 (MWC 120) on time
scales ranging between minutes and several years. A continuous, one-minute
resolution, 21 day-long sequence of MOST (Microvariability & Oscillations of
STars) satellite observations has been analyzed using wavelet, scalegram and
dispersion analysis tools. The MOST data have been augmented by sparse
observations over 9 seasons from ASAS (All Sky Automated Survey), by previously
non-analyzed ESO (European Southern Observatory) data partly covering 3 seasons
and by archival measurements dating back half a century ago. Mutually
superimposed flares or accretion instabilities grow in size from about 0.0003
of the mean flux on a time scale of minutes to a peak-to-peak range of <~0.05
on a time scale of a few years. The resulting variability has properties of
stochastic "red" noise, whose self-similar characteristics are very similar to
those observed in cataclysmic binary stars, but with much longer characteristic
time scales of hours to days (rather than minutes) and with amplitudes which
appear to cease growing in size on time scales of tens of years. In addition to
chaotic brightness variations combined with stochastic noise, the MOST data
show a weakly defined cyclic signal with a period of about 1.5 days, which may
correspond to the rotation of the star.Comment: Accepted for publication by Astron. & Astroph. 8 pages, 9 figures.
For some reason Fig.5 incorrectly shows in arXiv: Contours OK, gray scale no
Discovery of Bright Galactic R Coronae Borealis and DY Persei Variables: Rare Gems Mined from ACVS
We present the results of a machine-learning (ML) based search for new R
Coronae Borealis (RCB) stars and DY Persei-like stars (DYPers) in the Galaxy
using cataloged light curves from the All-Sky Automated Survey (ASAS) Catalog
of Variable Stars (ACVS). RCB stars - a rare class of hydrogen-deficient
carbon-rich supergiants - are of great interest owing to the insights they can
provide on the late stages of stellar evolution. DYPers are possibly the
low-temperature, low-luminosity analogs to the RCB phenomenon, though
additional examples are needed to fully establish this connection. While RCB
stars and DYPers are traditionally identified by epochs of extreme dimming that
occur without regularity, the ML search framework more fully captures the
richness and diversity of their photometric behavior. We demonstrate that our
ML method can use newly discovered RCB stars to identify additional candidates
within the same data set. Our search yields 15 candidates that we consider
likely RCB stars/DYPers: new spectroscopic observations confirm that four of
these candidates are RCB stars and four are DYPers. Our discovery of four new
DYPers increases the number of known Galactic DYPers from two to six;
noteworthy is that one of the new DYPers has a measured parallax and is m ~ 7
mag, making it the brightest known DYPer to date. Future observations of these
new DYPers should prove instrumental in establishing the RCB connection. We
consider these results, derived from a machine-learned probabilistic
classification catalog, as an important proof-of-concept for the efficient
discovery of rare sources with time-domain surveys.Comment: 18 pages, 2 new figures, accepted for publication in Ap
Variability of Luminous Stars in the Large Magellanic Cloud Using 10 Years of ASAS Data
Motivated by the detection of a recent outburst of the massive luminous blue
variable LMC-R71, which reached an absolute magnitude M_V = -9.3 mag, we
undertook a systematic study of the optical variability of 1268 massive stars
in the Large Magellanic Cloud, using a recent catalog by Bonanos et al. (2009)
as the input. The ASAS All Star Catalog (Pojmanski 2002) provided well-sampled
light curves of these bright stars spanning 10 years. Combining the two
catalogs resulted in 599 matches, on which we performed a variability search.
We identified 117 variable stars, 38 of which were not known before, despite
their brightness and large amplitude of variation. We found 13 periodic stars
that we classify as eclipsing binary (EB) stars, eight of which are newly
discovered bright, massive eclipsing binaries composed of OB type stars. The
remaining 104 variables are either semi- or non-periodic, the majority (85)
being red supergiants. Most (26) of the newly discovered variables in this
category are also red supergiants with only three B and four O stars.Comment: 23 pages, 10 figures and 3 tables; published in A
The catalog of short periods stars from the ''Pi of the Sky'' data
Based on the data from the ''Pi of the Sky'' project we made a catalog of the
variable stars with periods from 0.1 to 10 days. We used data collected during
a period of two years (2004 and 2005) and classified 725 variable stars. Most
of the stars in our catalog are eclipsing binaries - 464 (about 64%), while the
number of pulsating stars is 125 (about 17%). Our classification is based on
the shape of the light curve, as in the GCVS catalog. However, some stars in
our catalog were classified as of different type than in the GCVS catalog. We
have found periods for 15 stars present in the GCVS catalog with previously
unknown period.Comment: New Astronomy in prin
AD Leonis: Radial Velocity Signal of Stellar Rotation or Spin–Orbit Resonance?
AD Leonis is a nearby magnetically active M dwarf. We find Doppler variability with a period of 2.23 days, as well as photometric signals: (1) a short-period signal, which is similar to the radial velocity signal, albeit with considerable variability; and (2) a long-term activity cycle of 4070 ± 120 days. We examine the short-term photometric signal in the available All-Sky Automated Survey and Microvariability and Oscillations of STars (MOST) photometry and find that the signal is not consistently present and varies considerably as a function of time. This signal undergoes a phase change of roughly 0.8 rad when considering the first and second halves of the MOST data set, which are separated in median time by 3.38 days. In contrast, the Doppler signal is stable in the combined High-Accuracy Radial velocity Planet Searcher and High Resolution Echelle Spectrometer radial velocities for over 4700 days and does not appear to vary in time in amplitude, phase, period, or as a function of extracted wavelength. We consider a variety of starspot scenarios and find it challenging to simultaneously explain the rapidly varying photometric signal and the stable radial velocity signal as being caused by starspots corotating on the stellar surface. This suggests that the origin of the Doppler periodicity might be the gravitational tug of a planet orbiting the star in spin–orbit resonance. For such a scenario and no spin–orbit misalignment, the measured v sin i indicates an inclination angle of 15°̣5 ± 2°̣5 and a planetary companion mass of 0.237 ± 0.047 M Jup
Substellar-mass companions to the K-dwarf BD +14 4559 and the K-giants HD 240210 and BD +20 2457
We present the discovery of substellar-mass companions to three stars by the
ongoing Penn State - Toru\' n Planet Search (PTPS) conducted with the 9.2-m
Hobby-Eberly Telescope. The K2-dwarf, BD +14 4559, has a 1.5 M companion
with the orbital period of 269 days and shows a non-linear, long-term radial
velocity trend, which indicates a possible presence of another planet-mass body
in the system. The K3-giant, HD 240210, exhibits radial velocity variations
that require modeling with multiple orbits, but the available data are not yet
sufficient to do it unambiguously. A tentative, one-planet model calls for a
6.9 M planet in a 502-day orbit around the star. The most massive of the
three stars, the K2-giant, BD +20 2457, whose estimated mass is 2.81.5
M, has two companions with the respective minimum masses of 21.4 M
and 12.5 M and orbital periods of 380 and 622 days. Depending on the
unknown inclinations of the orbits, the currently very uncertain mass of the
star, and the dynamical properties of the system, it may represent the first
detection of two brown dwarf-mass companions orbiting a giant. The existence of
such objects will have consequences for the interpretation of the so-called
brown dwarf desert known to exist in the case of solar-mass stars.Comment: 28 pages, 4 tables, 10 figures. Submitted to Ap
The HATNet and HATSouth Exoplanet Surveys
The Hungarian-made Automated Telescope Network (HATNet) has been in operation
since 2003, with the key science goal being the discovery and accurate
characterization of transiting extrasolar planets (TEPs) around bright stars.
Using six small, 11\,cm\ aperture, fully automated telescopes in Arizona and
Hawaii, as of 2017 March, it has discovered and accurately characterized 67
such objects. The HATSouth network of telescopes has been in operation since
2009, using slightly larger, 18\,cm diameter optical tubes. It was the first
global network of telescopes using identical instrumentation. With three
premier sites spread out in longitude (Chile, Namibia, Australia), the HATSouth
network permits round-the-clock observations of a 128 square arcdegree swath of
the sky at any given time, weather permitting. As of this writing, HATSouth has
discovered 36 transiting exoplanets. Many of the altogether ~100 HAT and
HATSouth exoplanets were the first of their kind. They have been important
contributors to the rapidly developing field of exoplanets, motivating and
influencing observational techniques, theoretical studies, and also actively
shaping future instrumentation for the detection and characterization of such
objects.Comment: Invited review chapter, accepted for publication in "Handbook of
Exoplanets", edited by H.J. Deeg and J.A. Belmonte, Springer Reference Work
Discovery of the 2010 Eruption and the Pre-Eruption Light Curve for Recurrent Nova U Scorpii
We report the discovery by B. G. Harris and S. Dvorak on JD 2455224.9385
(2010 Jan 28.4385 UT) of the predicted eruption of the recurrent nova U Scorpii
(U Sco). We also report on 815 magnitudes (and 16 useful limits) on the
pre-eruption light curve in the UBVRI and Sloan r' and i' bands from 2000.4 up
to 9 hours before the peak of the January 2010 eruption. We found no
significant long-term variations, though we did find frequent fast variations
(flickering) with amplitudes up to 0.4 mag. We show that U Sco did not have any
rises or dips with amplitude greater than 0.2 mag on timescales from one day to
one year before the eruption. We find that the peak of this eruption occurred
at JD 2455224.69+-0.07 and the start of the rise was at JD 2455224.32+-0.12.
From our analysis of the average B-band flux between eruptions, we find that
the total mass accreted between eruptions is consistent with being a constant,
in agreement with a strong prediction of nova trigger theory. The date of the
next eruption can be anticipated with an accuracy of +-5 months by following
the average B-band magnitudes for the next ~10 years, although at this time we
can only predict that the next eruption will be in the year 2020+-2.Comment: Astronomical Journal submitted, 36 pages, 3 figures, full table
A low-mass planet candidate orbiting Proxima Centauri at a distance of 1.5 AU
Copyright © 2020 The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science. No claim to original U.S. Government Works. Distributed under a Creative Commons Attribution NonCommercial License 4.0 (CC BY-NC).Our nearest neighbor, Proxima Centauri, hosts a temperate terrestrial planet. We detected in radial velocities evidence of a possible second planet with minimum mass m c sin i c = 5.8 ± 1.9 M ⊕ and orbital period P c = 5.21 - 0.22 + 0.26 years. The analysis of photometric data and spectro-scopic activity diagnostics does not explain the signal in terms of a stellar activity cycle, but follow-up is required in the coming years for confirming its planetary origin. We show that the existence of the planet can be ascertained, and its true mass can be determined with high accuracy, by combining Gaia astrometry and radial velocities. Proxima c could become a prime target for follow-up and characterization with next-generation direct imaging instrumentation due to the large maximum angular separation of ~1 arc second from the parent star. The candidate planet represents a challenge for the models of super-Earth formation and evolution.Peer reviewedFinal Published versio
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