51 research outputs found
Variability of the Be Star Population
The goal of this dissertation is to describe the behavior of the Be star population. To do this, we have studied large samples with high precision and long time baselines. The primary data type used is optical time-series photometry, but infrared and optical spectroscopy are also important. In the process of studying many hundreds of Be stars, we have characterized their diverse photometric variability, identified systems of particular interest, and established links between photometric and spectroscopic variations.Be stars and their disks have generally been characterized by the emission lines in their spectra, and especially the time variability of those spectroscopic features. They are known to also exhibit photometric variability at multiple timescales, but have not been broadly compared and analyzed by that behavior. We have taken advantage of the advent of wide-field, long-baseline, and high-cadence photometric surveys that search for transiting exoplanets to perform an analysis of brightness variations among a large number of known Be stars. The photometric data comes from the KELT transit survey, with a typical cadence of 30 minutes, baseline of up to ten years, photometric precision of about 1\%, and coverage of about 70\% of the sky. We analyze KELT light curves of 610 known Be stars in both the Northern and Southern hemispheres in an effort to study their variability in a comprehensive way. Consistent with other studies of Be star variability, we find most of the systems to be variable. We derive lower limits on the fraction of stars in our sample that exhibit features consistent with non-radial pulsation (25), disk-building events (`outbursts\u27; 36), and long-term trends in the circumstellar disk (37), and show how these are correlated with spectral sub-type. Other types of variability, such as those owing to binarity, are also explored. Simultaneous spectroscopy for some of these systems from the Be Star Spectra (BeSS) database allow us to better understand the physical causes for the observed variability, especially in cases of outbursts and changes in the disk. This sample is referred to as the ``BK sample\u27\u27 (for BeSS-KELT).In order to study the growth and evolution of circumstellar disks around classical Be stars, we analyze optical time-series photometry from the KELT survey with simultaneous infrared and visible spectroscopy from the APOGEE survey and BeSS database for a sample of 160 Galactic classical Be stars. This sample is referred to as the ``AK sample\u27\u27 (for APOGEE-KELT, since all systems have both APOGEE and KELT data). The systems studied here show variability including transitions from a disk-less to disk-possessing state (and vice versa), and persistent disks that vary in strength, being replenished at either regularly or irregularly occurring intervals. We detect disk-building events (outbursts) in the light curves of 28\% of this sample. Outbursts are more commonly observed in early- (57\%), compared to mid- (27\%) and late-type (8\%) systems. A given system may show anywhere between 0 -- 40 individual outbursts in its light curve, with amplitudes ranging up to 0.5 mag and timescales between 2 -- 1000 days. We study how both the photometry and spectroscopy change together during active episodes of disk growth or dissipation, revealing details about the evolution of the circumstellar environment. We demonstrate that photometric activity is linked to changes in the inner disk, and show that, at least in some cases, the disk growth process is asymmetrical. Observational evidence of Be star disks both growing and clearing from the inside out is presented. The duration of disk build-up and dissipation phases are measured for 70 outbursts, and we find that the average outburst takes about twice as long to dissipate as it does to build up in optical photometry. Our analysis hints that dissipation of the inner disk proceeds relatively slowly for late-type Be stars.Although both the BK and AK samples are comprised of Be stars, there are some minor differences. For each system in the AK sample, we have multiple high-resolution, infrared spectroscopic measurements, as well as optical light curves, and, in about a quarter of the sample, optical spectroscopy. Our analysis of the AK sample is mainly focused on studying disk creation, growth, and dissipation. Significant attention is given to systems with spectroscopic measurements that are near-contemporaneous with light curve variability. Each system in the BK sample has an optical light curve, but only about half of the sample has any spectroscopic data. Analysis of the BK sample emphasizes all types of photometric variability on all timescales, from hours to many years. The primary goal of our work with the BK sample is to generally classify photometric variability in Be stars as a population. Spectroscopic data is sometimes incorporated into the analysis of certain members of the BK sample, but is secondary to the photometric data. For these reasons, these two samples are kept separate in the text
KELT-22Ab: A Massive, Short-Period Hot Jupiter Transiting a Near-solar Twin
We present the discovery of KELT-22Ab, a hot Jupiter from the KELT-South survey. KELT-22Ab transits the moderately bright (V∼11.1) Sun-like G2V star TYC 7518-468-1. The planet has an orbital period of P = 1.3866529±0.0000027 days, a radius of R_P = 1.285^(+0.12)_(−0.071) R_J, and a relatively large mass of M_P = 3.47^(+0.15)_(−0.14) M_J. The star has R⋆ = 1.099^(+0.079)_(−0.046) R⊙, M⋆ = 1.092^(+0.045)_(−0.041) M⊙, T_(eff) = 5767^(+50)_(−49) K, log g⋆ = 4.393^(+0.039)_(−0.060) (cgs), and [m/H] = +0.259^(+0.085)_(−0.083), and thus, other than its slightly super-solar metallicity, appears to be a near solar twin. Surprisingly, KELT-22A exhibits kinematics and a Galactic orbit that are somewhat atypical for thin disk stars. Nevertheless, the star is rotating quite rapidly for its estimated age, shows evidence of chromospheric activity, and is somewhat metal rich. Imaging reveals a slightly fainter companion to KELT-22A that is likely bound, with a projected separation of 6” (∼1400 AU). In addition to the orbital motion caused by the transiting planet, we detect a possible linear trend in the radial velocity of KELT-22A suggesting the presence of another relatively nearby body that is perhaps non-stellar. KELT-22Ab is highly irradiated (as a consequence of the small semi-major axis of a/R⋆ = 4.97), and is mildly inflated. At such small separations, tidal forces become significant. The configuration of this system is optimal for measuring the rate of tidal dissipation within the host star. Our models predict that, due to tidal forces, the semi-major axis of KELT-22Ab is decreasing rapidly, and is thus predicted to spiral into the star within the next Gyr
Extreme mass ratios and fast rotation in three massive binaries
The origin of rapid rotation in massive stars remains debated, although
binary interactions are now often advocated as a cause. However, the broad and
shallow lines in the spectra of fast rotators make direct detection of binarity
difficult. In this paper, we report on the discovery and analysis of
multiplicity for three fast-rotating massive stars: HD25631 (B3V), HD191495
(B0V), and HD46485 (O7V). They display strikingly similar TESS light curves,
with two narrow eclipses superimposed on a sinusoidal variation due to
reflection effects. We complement these photometric data by spectroscopy from
various instruments (X-Shooter, Espadons, FUSE...), to further constrain the
nature of these systems. The detailed analyses of these data demonstrates that
the companions of the massive OB stars have low masses (~1Msol) with rather
large radii (2-4 Rsol) and low temperatures (<15 kK). These companions display
no UV signature, which would exclude a hot subdwarf nature, but disentangling
of the large set of X-Shooter spectra of HD25631 revealed the typical signature
of chromospheric activity in the companion's spectrum. In addition, despite the
short orbital periods (P=3-7d), the fast-rotating OB-stars still display
non-synchronized rotation and all systems appear young (<20Myr). This suggests
that, as in a few other cases, these massive stars are paired in those systems
with non-degenerate, low-mass PMS companions, implying that fast rotation would
not be a consequence of a past binary interactions in their case.Comment: accepted for publication by MNRA
KELT-22Ab: A Massive, Short-Period Hot Jupiter Transiting a Near-solar Twin
We present the discovery of KELT-22Ab, a hot Jupiter from the KELT-South survey. KELT-22Ab transits the moderately bright (V∼11.1) Sun-like G2V star TYC 7518-468-1. The planet has an orbital period of P = 1.3866529±0.0000027 days, a radius of R_P = 1.285^(+0.12)_(−0.071) R_J, and a relatively large mass of M_P = 3.47^(+0.15)_(−0.14) M_J. The star has R⋆ = 1.099^(+0.079)_(−0.046) R⊙, M⋆ = 1.092^(+0.045)_(−0.041) M⊙, T_(eff) = 5767^(+50)_(−49) K, log g⋆ = 4.393^(+0.039)_(−0.060) (cgs), and [m/H] = +0.259^(+0.085)_(−0.083), and thus, other than its slightly super-solar metallicity, appears to be a near solar twin. Surprisingly, KELT-22A exhibits kinematics and a Galactic orbit that are somewhat atypical for thin disk stars. Nevertheless, the star is rotating quite rapidly for its estimated age, shows evidence of chromospheric activity, and is somewhat metal rich. Imaging reveals a slightly fainter companion to KELT-22A that is likely bound, with a projected separation of 6” (∼1400 AU). In addition to the orbital motion caused by the transiting planet, we detect a possible linear trend in the radial velocity of KELT-22A suggesting the presence of another relatively nearby body that is perhaps non-stellar. KELT-22Ab is highly irradiated (as a consequence of the small semi-major axis of a/R⋆ = 4.97), and is mildly inflated. At such small separations, tidal forces become significant. The configuration of this system is optimal for measuring the rate of tidal dissipation within the host star. Our models predict that, due to tidal forces, the semi-major axis of KELT-22Ab is decreasing rapidly, and is thus predicted to spiral into the star within the next Gyr
Trumpler 16-26: A New Centrifugal Magnetosphere Discovered via SDSS/APOGEE H-band Spectroscopy
We report the discovery of a new example of the rare class of highly
magnetized, rapidly rotating, helium enhanced, early B stars that produce
anomalously wide hydrogen emission due to a centrifugal magnetosphere (CM). The
star is Trumpler 16-26, a B1.5 V member of the Trumpler 16 open cluster. A CM
was initially suspected based on hydrogen Brackett series emission observed in
SDSS/APOGEE -band spectra. Similar to the other stars of this type, the
emission was highly variable and at all times remarkable due to the extreme
velocity separations of the double peaks (up to 1300 km s.) Another clue
lay in the TESS lightcurve, which shows two irregular eclipses per cycle when
phased with the likely 0.9718115 day rotation period, similar to the behavior
of the well known CM host star Ori E. To confirm a strong magnetic
field and rotation-phase-locked variability, we initiated a follow-up campaign
consisting of optical spectropolarimetry and spectroscopy. The associated data
revealed a longitudinal magnetic field varying between and kG
with the period found from photometry. The optical spectra confirmed rapid
rotation ( km s), surface helium enhancement, and wide,
variable hydrogen emission. Tr16-26 is thus confirmed as the 20
known, the fourth most rapidly rotating, and the faintest CM host star yet
discovered. With a projected dipole magnetic field strength of
kG, Tr16-26 is also among the most magnetic CM stars
New Beta Cephei Stars from the KELT Project
We present the results of a search for Galactic β Cephei stars, which are massive pulsating stars with both pressure modes and mixed modes. Thus, these stars can serve as benchmarks for seismological studies of the interiors of massive stars. We conducted the search by performing a frequency analysis on the optical light curves of known O- and B-type stars with data from the Kilodegree Extremely Little Telescope exoplanet survey. We identify 113 β Cephei stars, of which 86 are new discoveries, which altogether represent a 70% increase in the number currently known. An additional 97 candidates are identified. Among our targets, we find five new eclipsing binaries and 22 stars with equal frequency spacings suggestive of rotational splitting of nonradial pulsation modes. Candidates for runaway stars among our targets and a number of interesting individual objects are discussed. Most of the known and newly discovered β Cephei stars will be observed by the Transiting Exoplanet Survey Satellite mission, providing by far the most comprehensive observational data set of massive main-sequence pulsating stars of sufficient quality for detailed asteroseismic studies. Future analysis of these light curves has the potential to dramatically increase our understanding of the structure of stellar interiors and the physical processes taking place therein
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