2,204 research outputs found
HD 4915: A Maunder Minimum Candidate
We study the magnetic activity cycle of HD 4915 using the \ion{Ca}{2} H \& K
emission line strengths measured by Keck I/HIRES spectrograph. The star has
been observed as a part of California Planet Search Program from 2006 to
present. We note decreasing amplitude in the magnetic activity cycle, a pattern
suggesting the star's entry into a Magnetic Grand Minimum (MGM) state,
reminiscent of the Sun's Maunder and Dalton Minima. We recommend further
monitoring of the star to confirm the grand minimum nature of the dynamo, which
would provide insight into the state of the Sun's chromosphere and the global
magnetic field during its grand minima. We also recommend continued
observations of H \& K emission lines, and ground or space based photometric
observations to estimate the sunspot coverage.Comment: To be submitted to AAS Journals; comments welcom
Astrophysical Insights into Radial Velocity Jitter from an Analysis of 600 Planet-search Stars
Radial velocity (RV) detection of planets is hampered by astrophysical processes on the surfaces of stars that induce a stochastic signal, or "jitter," which can drown out or even mimic planetary signals. Here, we empirically and carefully measure the RV jitter of more than 600 stars from the California Planet Search sample on a star by star basis. As part of this process, we explore the activity–RV correlation of stellar cycles and include appendices listing every ostensibly companion-induced signal we removed and every activity cycle we noted. We then use precise stellar properties from Brewer et al. to separate the sample into bins of stellar mass and examine trends with activity and with evolutionary state. We find that RV jitter tracks stellar evolution and that in general, stars evolve through different stages of RV jitter: the jitter in younger stars is driven by magnetic activity, while the jitter in older stars is convectively driven and dominated by granulation and oscillations. We identify the "jitter minimum"—where activity-driven and convectively driven jitter have similar amplitudes—for stars between 0.7 and 1.7 M⊙ and find that more-massive stars reach this jitter minimum later in their lifetime, in the subgiant or even giant phases. Finally, we comment on how these results can inform future RV efforts, from prioritization of follow-up targets from transit surveys like TESS to target selection of future RV surveys
Some Bright Stars with Smooth Continua for Calibrating the Response of High Resolution Spectrographs
When characterizing a high resolution echelle spectrograph, for instance for
precise Doppler work, it is useful to observe featureless sources such as
quartz lamps or hot stars to determine the response of the instrument. Such
sources provide a way to determine the blaze function of the orders,
pixel-to-pixel variations in the detector, fringing in the system, and other
important characteristics. In practice, however, many B or early A stars do not
provide a smooth continuum, whether because they are not rotating rapidly
enough or for some other reason. In fact, we have found that published
rotational velocities and temperatures are not a specific and sensitive guide
to whether a star's continuum will be smooth. A useful resource for observers,
therefore, is a list of "good" hot stars: bright, blue stars known empirically
to have no lines or other spectral features beyond the Balmer series with
minima below 95% of the continuum.
We have compiled a list of such stars visible from Northern Hemisphere
telescopes. This list includes all stars listed in the Yale Bright Star Catalog
(Hoffleit & Jaschek 1991) as being single with V 175 km/s, and
declination > -30, and many other hot stars that we have found useful for
calibration purposes.
The list here of "bad" stars may also be of interest in studies of hot,
slowly rotating stars
Astrophysical Insights into Radial Velocity Jitter from an Analysis of 600 Planet-search Stars
Radial velocity (RV) detection of planets is hampered by astrophysical processes on the surfaces of stars that induce a stochastic signal, or "jitter," which can drown out or even mimic planetary signals. Here, we empirically and carefully measure the RV jitter of more than 600 stars from the California Planet Search sample on a star by star basis. As part of this process, we explore the activity–RV correlation of stellar cycles and include appendices listing every ostensibly companion-induced signal we removed and every activity cycle we noted. We then use precise stellar properties from Brewer et al. to separate the sample into bins of stellar mass and examine trends with activity and with evolutionary state. We find that RV jitter tracks stellar evolution and that in general, stars evolve through different stages of RV jitter: the jitter in younger stars is driven by magnetic activity, while the jitter in older stars is convectively driven and dominated by granulation and oscillations. We identify the "jitter minimum"—where activity-driven and convectively driven jitter have similar amplitudes—for stars between 0.7 and 1.7 M⊙ and find that more-massive stars reach this jitter minimum later in their lifetime, in the subgiant or even giant phases. Finally, we comment on how these results can inform future RV efforts, from prioritization of follow-up targets from transit surveys like TESS to target selection of future RV surveys
The TRENDS High-Contrast Imaging Survey. V. Discovery of an Old and Cold Benchmark T-dwarf Orbiting the Nearby G-star HD 19467
The nearby Sun-like star HD 19467 shows a subtle radial velocity (RV)
acceleration of -1.37+/-0.09 m/s/yr over an 16.9 year time baseline (an RV
trend), hinting at the existence of a distant orbiting companion. We have
obtained high-contrast adaptive optics images of the star using NIRC2 at Keck
Observatory and report the direct detection of the body that causes the
acceleration. The companion, HD 19467 B, is dK=12.57+/-0.09 mag fainter than
its parent star (contrast ratio of 9.4e-6), has blue colors J-K_s=-0.36+/-0.14
(J-H=-0.29+/-0.15), and is separated by 1.653+/-0.004" (51.1+/-1.0 AU).
Follow-up astrometric measurements obtained over an 1.1 year time baseline
demonstrate physical association through common parallactic and proper motion.
We calculate a firm lower-limit of m>51.9^{+3.6}_{-4.3}Mjup for the companion
mass from orbital dynamics using a combination of Doppler observations and
imaging. We estimate a model-dependent mass of m=56.7^{+4.6}_{-7.2}Mjup from a
gyrochronological age of 4.3^{+1.0}_{-1.2} Gyr. Isochronal analysis suggests a
much older age of Gyr, which corresponds to a mass of
m=67.4^{+0.9}_{-1.5}Mjup. HD 19467 B's measured colors and absolute magnitude
are consistent with a late T-dwarf [~T5-T7]. We may infer a low metallicity of
[Fe/H]=-0.15+/-0.04 for the companion from its G3V parent star. HD 19467 B is
the first directly imaged benchmark T-dwarf found orbiting a Sun-like star with
a measured RV acceleration.Comment: Updated to reflect ApJ versio
The California Planet Survey IV: A Planet Orbiting the Giant Star HD 145934 and Updates to Seven Systems with Long-Period Planets
We present an update to seven stars with long-period planets or planetary
candidates using new and archival radial velocities from Keck-HIRES and
literature velocities from other telescopes. Our updated analysis better
constrains orbital parameters for these planets, four of which are known
multi-planet systems. HD 24040 b and HD 183263 c are super-Jupiters with
circular orbits and periods longer than 8 yr. We present a previously unseen
linear trend in the residuals of HD 66428 indicative on an additional planetary
companion. We confirm that GJ 849 is a multi-planet system and find a good
orbital solution for the c component: it is a planet in a 15 yr
orbit (the longest known for a planet orbiting an M dwarf). We update the HD
74156 double-planet system. We also announce the detection of HD 145934 b, a planet in a 7.5 yr orbit around a giant star. Two of our stars, HD
187123 and HD 217107, at present host the only known examples of systems
comprising a hot Jupiter and a planet with a well constrained period yr,
and with no evidence of giant planets in between. Our enlargement and
improvement of long-period planet parameters will aid future analysis of
origins, diversity, and evolution of planetary systems.Comment: 16 pages, 13 figures. Accepted for publication in Ap
Retired A Stars and Their Companions IV. Seven Jovian Exoplanets from Keck Observatory
We report precise Doppler measurements of seven subgiants from Keck
Observatory. All seven stars show variability in their radial velocities
consistent with planet-mass companions in Keplerian orbits. The host stars have
masses ranging from 1.1 < Mstar/Msun < 1.9, radii 3.4 < Rstar/Rsun < 6.1, and
metallicities -0.21 < [Fe/H] < +0.26. The planets are all more massive than
Jupiter (Msini > 1 Mjup) and have semimajor axes > 1 AU. We present
millimagnitude photometry from the T3 0.4m APT at Fairborn observatory for five
of the targets. Our monitoring shows these stars to be photometrically stable,
further strengthening the interpretation of the observed radial velocity
variability. The orbital characteristics of the planets thus far discovered
around former A-type stars are very different from the properties of planets
around dwarf stars of spectral type F, G and K, and suggests that the formation
and migration of planets is a sensitive function of stellar mass. Three of the
planetary systems show evidence of long-term, linear trends indicative of
additional distant companions. These trends, together with the high planet
masses and increased occurrence rate, indicate that A-type stars are very
promising targets for direct imaging surveys.Comment: PASP Accepted, final submission awaiting comments from the communit
K2-231 b: A sub-Neptune exoplanet transiting a solar twin in Ruprecht 147
We identify a sub-Neptune exoplanet ( R)
transiting a solar twin in the Ruprecht 147 star cluster (3 Gyr, 300 pc, [Fe/H]
= +0.1 dex). The ~81 day light curve for EPIC 219800881 (V = 12.71) from K2
Campaign 7 shows six transits with a period of 13.84 days, a depth of ~0.06%,
and a duration of ~4 hours. Based on our analysis of high-resolution MIKE
spectra, broadband optical and NIR photometry, the cluster parallax and
interstellar reddening, and isochrone models from PARSEC, Dartmouth, and MIST,
we estimate the following properties for the host star: M, R, and K. This star appears to be single, based on our modeling of the
photometry, the low radial velocity variability measured over nearly ten years,
and Keck/NIRC2 adaptive optics imaging and aperture-masking interferometry.
Applying a probabilistic mass-radius relation, we estimate that the mass of
this planet is M, which would cause a RV
semi-amplitude of m s that may be measurable with existing
precise RV facilities. After statistically validating this planet with BLENDER,
we now designate it K2-231 b, making it the second sub-stellar object to be
discovered in Ruprecht 147 and the first planet; it joins the small but growing
ranks of 23 other planets found in open clusters.Comment: 24 pages, 7 figures, light curve included as separate fil
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