910 research outputs found
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
Radial velocities from the N2K Project: 6 new cold gas giant planets orbiting HD 55696, HD 98736, HD 148164, HD 203473, and HD 211810
The N2K planet search program was designed to exploit the planet-metallicity
correlation by searching for gas giant planets orbiting metal-rich stars. Here,
we present the radial velocity measurements for 378 N2K target stars that were
observed with the HIRES spectrograph at Keck Observatory between 2004 and 2017.
With this data set, we announce the discovery of six new gas giant exoplanets:
a double-planet system orbiting HD 148164 ( of 1.23 and 5.16 M) and single planet detections around HD 55696 ( = 3.87 M), HD 98736 ( = 2.33 M), HD 203473 ( = 7.8
M), and HD 211810 ( = 0.67 M). These gas
giant companions have orbital semi-major axes between 1.0 and 6.2 AU and
eccentricities ranging from 0.13 to 0.71. We also report evidence for three
gravitationally bound companions with between 20 to 30 M, placing them in the mass range of brown dwarfs, around HD 148284, HD
214823, and HD 217850, and four low mass stellar companions orbiting HD 3404,
HD 24505, HD 98630, and HD 103459. In addition, we present updated orbital
parameters for 42 previously announced planets. We also report a nondetection
of the putative companion HD 73256 b. Finally, we highlight the most promising
candidates for direct imaging and astrometric detection, and find that many hot
Jupiters from our sample could be detectable by state-of-the-art telescopes
such as Gaia.Comment: Accepted by the Astronomical Journal. 75 pages, 49 figure
Empirical Limits on Radial Velocity Planet Detection for Young Stars
We report initial results from our long term search using precision radial
velocities for planetary-mass companions located within a few AU of stars
younger than the Sun. Based on a sample of >150 stars, we define a floor in the
radial velocity scatter, sigma_RV, as a function of the chromospheric activity
level R'_{HK}. This lower bound to the jitter, which increases with increasing
stellar activity, sets the minimum planet mass that could be detected. Adopting
a median activity-age relationship reveals the astrophysical limits to planet
masses discernable via radial velocity monitoring, as a function of stellar
age. Considering solar-mass primaries having the mean jitter-activity level,
when they are younger than 100 / 300 / 1000 Myr, the stochastic jitter
component in radial velocity measurements restricts detectable companion masses
to > 0.3 / 0.2 / 0.1 M_Jupiter. These numbers require a large number -- several
tens -- of radial velocity observations taken over a time frame longer than the
orbital period. Lower companion mass limits can be achieved for stars with less
than the mean jitter and/or with an increased number of observations.Comment: 5 pages, to appear the 18th Cambridge Workshop on Cool Stars, Stellar
Systems, and the Sun proceedings edited by G. van Belle & H. Harri
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
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
Accelerated Tidal Circularization Via Resonance Locking in KIC 8164262
Tidal dissipation in binary star and planetary systems is poorly understood.
Fortunately, eccentric binaries known as heartbeat stars often exhibit tidally
excited oscillations, providing observable diagnostics of tidal circularization
mechanisms and timescales. We apply tidal theories to observations of the
heartbeat star KIC 8164262, which contains an F-type primary in a very
eccentric orbit that exhibits a prominent tidally excited oscillation. We
demonstrate that the prominent oscillation is unlikely to result from a chance
resonance between tidal forcing and a stellar oscillation mode. However, the
oscillation has a frequency and amplitude consistent with the prediction of
resonance locking, a mechanism in which coupled stellar and orbital evolution
maintain a stable resonance between tidal forcing and a stellar oscillation
mode. The resonantly excited mode produces efficient tidal dissipation
(corresponding to an effective tidal quality factor ),
such that tidal orbital decay/circularization proceeds on a stellar evolution
time scale.Comment: Published in MNRAS Letters. For an interactive 3D model of the
system, go to
http://www.glowscript.org/#/user/slantburns/folder/Public/program/KIC816426
Occurrence and core-envelope structure of 1--4x Earth-size planets around Sun-like stars
Small planets, 1-4x the size of Earth, are extremely common around Sun-like
stars, and surprisingly so, as they are missing in our solar system. Recent
detections have yielded enough information about this class of exoplanets to
begin characterizing their occurrence rates, orbits, masses, densities, and
internal structures. The Kepler mission finds the smallest planets to be most
common, as 26% of Sun-like stars have small, 1-2 R_e planets with orbital
periods under 100 days, and 11% have 1-2 R_e planets that receive 1-4x the
incident stellar flux that warms our Earth. These Earth-size planets are
sprinkled uniformly with orbital distance (logarithmically) out to 0.4 AU, and
probably beyond. Mass measurements for 33 transiting planets of 1-4 R_e show
that the smallest of them, R < 1.5 R_e, have the density expected for rocky
planets. Their densities increase with increasing radius, likely caused by
gravitational compression. Including solar system planets yields a relation:
rho = 2.32 + 3.19 R/R_e [g/cc]. Larger planets, in the radius range 1.5-4.0
R_e, have densities that decline with increasing radius, revealing increasing
amounts of low-density material in an envelope surrounding a rocky core,
befitting the appellation "mini-Neptunes." Planets of ~1.5 R_e have the highest
densities, averaging near 10 g/cc. The gas giant planets occur preferentially
around stars that are rich in heavy elements, while rocky planets occur around
stars having a range of heavy element abundances. One explanation is that the
fast formation of rocky cores in protoplanetary disks enriched in heavy
elements permits the gravitational accumulation of gas before it vanishes,
forming giant planets. But models of the formation of 1-4 R_e planets remain
uncertain. Defining habitable zones remains difficult, without benefit of
either detections of life elsewhere or an understanding of life's biochemical
origins.Comment: 11 pages, 6 figures, accepted for publication Proc. Natl. Acad. Sc
Detection of Stars Within ~0.8 in of Kepler Objects of Interest
We present an algorithm to search for the faint spectrum of a second star mixed with the spectrum of a brighter star in high resolution spectra. We model optical stellar spectra as the sum of two input spectra drawn from a vast library of stars throughout the H-R diagram. From typical spectra having a resolution of R = 60,000, we are able to detect companions as faint as 1% relative to the primary star in approximately the V and R bandpasses of photometry. We are also able to find evidence for triple and quadruple systems, given that any additional companions are sufficiently bright. The precise threshold percentage depends on the signal-to-noise of the spectrum and the properties of the two stars. For cases of non-detection, we place a limit on the brightness of any potential companions. This algorithm is useful for detecting faint orbiting companions and background stars that are angularly close to a foreground target star. The size of the entrance slit to the spectrometer, 0.87 × 3 arcsec (typically), sets the angular domain within which the second star can be detected. We analyzed Keck-HIRES spectra of 1160 California Kepler Survey objects of interest (KOI) searching for the secondary spectra, with the two goals of alerting the community to two possible host stars of the transiting planet and to dilution of the light curve. We report 63 California KOI showing spectroscopic evidence of a secondary star
Newly-Discovered Planets Orbiting HD~5319, HD~11506, HD~75784 and HD~10442 from the N2K Consortium
Initially designed to discover short-period planets, the N2K campaign has
since evolved to discover new worlds at large separations from their host
stars. Detecting such worlds will help determine the giant planet occurrence at
semi-major axes beyond the ice line, where gas giants are thought to mostly
form. Here we report four newly-discovered gas giant planets (with minimum
masses ranging from 0.4 to 2.1 MJup) orbiting stars monitored as part of the
N2K program. Two of these planets orbit stars already known to host planets: HD
5319 and HD 11506. The remaining discoveries reside in previously-unknown
planetary systems: HD 10442 and HD 75784. The refined orbital period of the
inner planet orbiting HD 5319 is 641 days. The newly-discovered outer planet
orbits in 886 days. The large masses combined with the proximity to a 4:3 mean
motion resonance make this system a challenge to explain with current formation
and migration theories. HD 11506 has one confirmed planet, and here we confirm
a second. The outer planet has an orbital period of 1627.5 days, and the
newly-discovered inner planet orbits in 223.6 days. A planet has also been
discovered orbiting HD 75784 with an orbital period of 341.7 days. There is
evidence for a longer period signal; however, several more years of
observations are needed to put tight constraints on the Keplerian parameters
for the outer planet. Lastly, an additional planet has been detected orbiting
HD 10442 with a period of 1043 days.Comment: Accepted for publication in Ap
- …