379 research outputs found
H-alpha Activity of Old M Dwarfs: Stellar Cycles and Mean Activity Levels For 93 Low-Mass Stars in the Solar Neighborhood
Through the McDonald Observatory M Dwarf Planet Search, we have acquired
nearly 3,000 high-resolution spectra of 93 late-type (K5-M5) stars over more
than a decade using HET/HRS. This sample provides a unique opportunity to
investigate the occurrence of long-term stellar activity cycles for low-mass
stars. In this paper, we examine the stellar activity of our targets as
reflected in the H-alpha feature. We have identified periodic signals for 6
stars, with periods ranging from days to more than 10 years, and find long-term
trends for 7 others. Stellar cycles with P > 1 year are present for at least 5%
of our targets. Additionally, we present an analysis of the time-averaged
activity levels of our sample, and search for correlations with other stellar
properties. In particular, we find that more massive, earlier type (M0-M2)
stars tend to be more active than later type dwarfs. Furthermore,
high-metallicity stars tend to be more active at a given stellar mass. We also
evaluate H-alpha variability as a tracer of activity-induced radial velocity
(RV) variation. For the M dwarf GJ 1170, H-alpha variation reveals stellar
activity patterns matching those seen in the RVs, mimicking the signal of a
giant planet, and we find evidence that the previously identified stellar
activity cycle of GJ 581 may be responsible for the recently retracted planet f
(Vogt et al. 2012) in that system. In general, though, we find that H-alpha is
not frequently correlated with RV at the precision (typically 6-7 m/s) of our
measurements.Comment: Submitted to ApJ. Reflects comments from a positive refere
Precise Radial Velocities of Polaris: Detection of Amplitude Growth
We present a first results from a long-term program of a radial velocity
study of Cepheid Polaris (F7 Ib) aimed to find amplitude and period of
pulsations and nature of secondary periodicities. 264 new precise radial
velocity measurements were obtained during 2004-2007 with the fiber-fed echelle
spectrograph Bohyunsan Observatory Echelle Spectrograph (BOES) of 1.8m
telescope at Bohyunsan Optical Astronomy Observatory (BOAO) in Korea. We find a
pulsational radial velocity amplitude and period of Polaris for three seasons
of 2005.183, 2006.360, and 2007.349 as 2K = 2.210 +/- 0.048 km/s, 2K = 2.080
+/- 0.042 km/s, and 2K = 2.406 +/- 0.018 km/s respectively, indicating that the
pulsational amplitudes of Polaris that had decayed during the last century is
now increasing rapidly. The pulsational period was found to be increasing too.
This is the first detection of a historical turnaround of pulsational amplitude
change in Cepheids. We clearly find the presence of additional radial velocity
variations on a time scale of about 119 days and an amplitude of about +/- 138
m/s, that is quasi-periodic rather than strictly periodic. We do not confirm
the presence in our data the variation on a time scale 34-45 days found in
earlier radial velocity data obtained in 80's and 90's. We assume that both the
119 day quasi-periodic, noncoherent variations found in our data as well as
34-45 day variations found before can be caused by the 119 day rotation periods
of Polaris and by surface inhomogeneities such as single or multiple spot
configuration varying with the time.Comment: 15 pages, 7 figures, Accepted for publication in The Astronomical
Journa
A Second Giant Planet in 3:2 Mean-Motion Resonance in the HD 204313 System
We present 8 years of high-precision radial velocity (RV) data for HD 204313
from the 2.7 m Harlan J. Smith Telescope at McDonald Observatory. The star is
known to have a giant planet (M sin i = 3.5 M_J) on a ~1900-day orbit, and a
Neptune-mass planet at 0.2 AU. Using our own data in combination with the
published CORALIE RVs of Segransan et al. (2010), we discover an outer Jovian
(M sin i = 1.6 M_J) planet with P ~ 2800 days. Our orbital fit suggests the
planets are in a 3:2 mean motion resonance, which would potentially affect
their stability. We perform a detailed stability analysis, and verify the
planets must be in resonance.Comment: Accepted for publication in Ap
Search for radial velocity variations in eight M-dwarfs with NIRSPEC/Keck II
Context. Radial velocity (RV) measurements from near-infrared spectra have
become a potentially powerful tool to search for planets around cool stars and
sub-stellar objects. As part of a large survey to characterize M-dwarfs using
NIRSPEC at Keck II, we obtained spectra of eight late M-dwarfs (spectral types
M5.0-M8.0) during two or more observing epochs per target. These spectra were
taken with intermediate spectral resolving powers (R \sim 20,000) in the
J-band.
Aims. We search for relative radial velocity variability in these late
M-dwarfs and test the NIRSPEC capability of detecting short period brown dwarf
and massive planetary companions around low-mass stars in the J-band (\approx
1.25 micron). Additionally, we reanalyzed the data of the M8-type star vB10
(one of our targets) presented in Zapatero Osorio et al. (2009), which were
obtained with the same instrumentation as our data.
Methods. [...]
Results. For the entire M-dwarf sample, we do not find any evidence of
relative RV variations induced by a short period brown dwarf or massive
planetary companion. The typical RV precision of the measurements is between
180 and 300 m/s, which is sufficient to detect hot Neptunes around M-dwarfs.
Also, we find that the spurious RV shift in Zapatero et al. (2009) of the star
VB10 was caused by asymmetries in the instrumental profile between different
observing epochs, which were not taken into account in their analysis.Comment: A&A, 7 pages, 5 figure
A vigorous activity cycle mimicking a planetary system in HD200466
Stellar activity can be a source of radial velocity (RV) noise and can
reproduce periodic RV variations similar to those produced by an exoplanet. We
present the vigorous activity cycle in the primary of the visual binary
HD200466, a system made of two almost identical solar-type stars with an
apparent separation of 4.6 arcsec at a distance of 44+/-2 pc. High precision RV
over more than a decade, adaptive optics (AO) images, and abundances have been
obtained for both components. A linear trend in the RV is found for the
secondary. We assumed that it is due to the binary orbit and once coupled with
the astrometric data, it strongly constrains the orbital solution of the binary
at high eccentricities (e~0.85) and quite small periastron of ~21 AU. If this
orbital motion is subtracted from the primary radial velocity curve, a highly
significant (false alarm probability <0.1%) period of about 1300 d is obtained,
suggesting in a first analysis the presence of a giant planet, but it turned
out to be due to the stellar activity cycle. Since our spectra do not include
the Ca~II resonance lines, we measured a chromospheric activity indicator based
on the Halpha line to study the correlation between activity cycles and
long-term activity variations. While the bisector analysis of the line profile
does not show a clear indication of activity, the correlation between the
Halpha line indicator and the RV measurements identify the presence of a strong
activity cycle.Comment: Accepted on Astronomy and Astrophysics Main Journal 2014, 16 pages,
18 figure
The McDonald Observatory Planet Search: New Long-Period Giant Planets, and Two Interacting Jupiters in the HD 155358 System
We present high-precision radial velocity (RV) observations of four
solar-type (F7-G5) stars - HD 79498, HD 155358, HD 197037, and HD 220773 -
taken as part of the McDonald Observatory Planet Search Program. For each of
these stars, we see evidence of Keplerian motion caused by the presence of one
or more gas giant planets in long-period orbits. We derive orbital parameters
for each system, and note the properties (composition, activity, etc.) of the
host stars. While we have previously announced the two-gas-giant HD 155358
system, we now report a shorter period for planet c. This new period is
consistent with the planets being trapped in mutual 2:1 mean-motion resonance.
We therefore perform an in-depth stability analysis, placing additional
constraints on the orbital parameters of the planets. These results demonstrate
the excellent long-term RV stability of the spectrometers on both the Harlan J.
Smith 2.7 m telescope and the Hobby-Eberly telescope.Comment: 38 pages, 10 figures, 6 tables. Accepted for publication in Ap
Confirmation of the Planet Hypothesis for the Long-period Radial Velocity Variations of Beta Geminorum
We present precise stellar radial velocity measurements for the K giant star
Beta Gem spanning over 25 years. These data show that the long period low
amplitude radial velocity variations found by Hatzes & Cochran (1993) are
long-lived and coherent. An examination of the Ca II K emission, spectral line
shapes from high resolution data (R = 210,000), and Hipparcos photometry show
no significant variations of these quantities with the RV period. These data
confirm the planetary companion hypothesis suggested by Hatzes & Cochran
(1993). An orbital solution assuming a stellar mass of 1.7 M_sun yields a
period, P = 589.6 days, a minimum mass of 2.3 M_Jupiter, and a semi-major axis,
and a = 1.6 AU. The orbit is nearly circular (e = 0.02). Beta Gem is the
seventh intermediate mass star shown to host a sub-stellar companion and
suggests that planet-formation around stars much more massive than the sun may
common.Comment: 10 pages, 9 figures, Astronomy and Astrophysics, in pres
The California Planet Survey. I. Four New Giant Exoplanets
We present precise Doppler measurements of four stars obtained during the past decade at Keck Observatory by the California Planet Survey (CPS). These stars, namely, HD 34445, HD 126614, HD 13931, and Gl 179, all show evidence for a single planet in Keplerian motion. We also present Doppler measurements from the Hobby-Eberly Telescope (HET) for two of the stars, HD 34445 and Gl 179, that confirm the Keck detections and significantly refine the orbital parameters. These planets add to the statistical properties of giant planets orbiting near or beyond the ice line, and merit follow-up by astrometry, imaging, and space-borne spectroscopy. Their orbital parameters span wide ranges of planetary minimum mass (M sin i = 0.38-1.9 M(Jup)), orbital period (P = 2.87-11.5 yr), semimajor axis (a = 2.1-5.2 AU), and eccentricity (e = 0.02-0.41). HD 34445 b (P = 2.87 yr, M sin i = 0.79 MJup, e = 0.27) is a massive planet orbiting an old, G-type star. We announce a planet, HD 126614 Ab, and an M dwarf, HD 126614 B, orbiting the metal-rich star HD 126614 (which we now refer to as HD 126614 A). The planet, HD 126614 Ab, has minimum mass M sin i = 0.38 MJup and orbits the stellar primary with period P = 3.41 yr and orbital separation a = 2.3 AU. The faint M dwarf companion, HD 126614 B, is separated from the stellar primary by 489 mas (33 AU) and was discovered with direct observations using adaptive optics and the PHARO camera at Palomar Observatory. The stellar primary in this new system, HD 126614 A, has the highest measured metallicity ([ Fe/ H] = + 0.56) of any known planet-bearing star. HD 13931 b (P = 11.5 yr, M sin i = 1.88 MJup, e = 0.02) is a Jupiter analog orbiting a near solar twin. Gl 179 b (P = 6.3 yr, M sin i = 0.82 M(Jup), e = 0.21) is a massive planet orbiting a faint M dwarf. The high metallicity of Gl 179 is consistent with the planet-metallicity correlation among M dwarfs, as documented recently by Johnson & Apps.NSF AST-0702821NASA NNX06AH52GMcDonald Observator
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